Issue 2 - Volume MA2021-02 - ECS Meeting Abstracts (2024)

The discovery of layered, spinel, and polyanion oxide cathodes for lithium-ion batteries in the 1980s has aided a significant increase in the cell voltage and energy density of lithium-ion batteries. Among them, the layered LiMO₂ oxides are at the forefront for portable electronics and electric vehicle applications. Although 40 years have passed by after their initial discovery, we are still in the process of learning and understanding the intricacies of the solid-state chemistry associated with them, particularly at high nickel contents. Cycle, thermal, and air instabilities are the major hurdles in employing ultrahigh-nickel layered oxide cathodes in practical cells. Phase transitions, cracks, aggressive surface reactivity with the organic electrolyte or ambient air, transition-metal-ion dissolution and migration to the anode, and consequent degradation of the anode are some of the major challenges. Cationic doping and surface conditioning are pursued to overcome some of the challenges, but largely on a trial and error process.

This presentation will focus on a solid-state chemistry perspective of the intricacies of layered oxide cathodes with high-nickel contents with various dopants. A fundamental understanding of the following factors will be provided: the role of different dopants in terms of their site occupancy and bulk vs. surface decoration, the dominant degradation factors, the origin of transition-metal ion dissolution, role of electrolyte additives, etc. The understanding is developed based on specifically designed cathode compositions and in-depth characterizations of the cathodes and anodes after extensive cycling, employing a variety of advanced characterization methodologies, such as in-situ x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), time of flight – secondary ion mass spectrometry (TOF-SIMS), and high-resolution transmission electron microscopy (HR-TEM). The understanding developed is used to design and scale-up robust high-nickel layered oxide cathodes with low or no expensive cobalt in them.

Layered cathode materials are the enabler of today's Li-ion industry. I will review some of the fundamental relations between electronic structure, cation ordering, and electrochemical performance in layered Li, Na and K-compounds. Diffusion in layered cathodes occurs through a divacancy mechanism which enables the low-energy Li passage through the tetrahedral sites that connect the octahedral positions in the Li layers. This diffusion mechanism provides high rate capability to layered cathodes, but relies on the Li-slab spacing to remain large upon electrochemical cycling. Any migration of transition metals into the Li-layer tends to contract the slab spacing and reduce rate capability. This limits practical layered materials to the NMC chemistry as Ni, Co and Mn4+ are the only ions that have a strong enough octahedral ligand-field stabilization to remain in the transition metal layer upon cycling. This problem is less pronounced for Na-layered oxides as the larger slab spacing makes occupancy of the tetrahedral and octahedral site in the Na-layer much less favorable for transition metals. But layered oxides with large alkali ions such as Na+ and K+ suffer from more sloped voltage profiles with limits their usable energy density.

Finally, I will show how the chemical diversity of Li-ion cathodes can be extended to a much broader set of elements by enabling Li-excess disordered rocksalts (DRX). These materials diffuse lithium ions through a statistical network of low-energy migration environments, and have recently been shown to have very high capacity and rate performance.

This talk is dedicated to Prof. Claude Delmas and three of his magnificent study fields, namely, those on "Chimie Douce" (soft chemistry) [1], LiNiO₂ [2] and Ni(OH)₂ derivatives [3].

Though LiNiO₂ has been extensively studied as a powerful substitute for LiCoO₂, its use as the source of Chimie Douce has been limited. This study shows possibilities to use LiNiO₂ for obtaining metastable phases that are hardly obtained by high-temperature calcination. The NiO₂ templet for Chimie Douce was obtained by the disproportionation reaction of LiNiO₂ using proton as the catalyst [4,5]. Alkaline ion insertion to the NiO₂ templet was employed using OH^- as the reductant [6,7]. Low temperature heating is also effective for obtaining new polymorphs.

The NiO₂ templet has the nickel oxidation state of 3.8±0.1 and has the CdCl₂ structure (O3) unless the Cdl₂ structure (O1) is formed by using strong acids [8]. The Pourbaix diagram suggests that the NiO₂ templet is stable only in the low pH regions and can be reduced in other regions. Treating NiO₂ with LiOH leads to the formation of Li_0.5NiO₂ that is essentially the same as that obtained by electrochemical reactions. On the other hand, NaOH and KOH treatments typically produce M_0.3NiO₂•0.5H₂O (P3), which corresponds to highly crystalline γ-NiOOH. The significant increase of the interlayer distance (Fig. 1) is caused by the water uptake on the Na⁺/K⁺ insertion. It is noteworthy that strongly solvated Li⁺ is inserted with desolvation while weakly solvated Na⁺/K⁺ are inserted as solvated.

The P3 phases as rechargeable electrodes were inactive in non-aqueous lithium cells due to water decomposition on discharging, but were active in aqueous KOH electrolytes. Operando XRD analysis indicated that the cycle performance was better than that of the hydrated nickel hydroxide obtained by the conventional co-precipitation method, though the strong XRD peaks of the P3 phase weakened during discharging [9]. The longer interlayer distance of the discharged product suggests further water uptake on discharging. The use of Ni⁴⁺/ Ni³⁺/ Ni²⁺ redox couples is attractive for high energy aqueous rechargeable cells.

In addition, we propose new proton insertion chemistry as a part of Chimie Douce application [10], which will be detailed in the presentation.

References

[1] C. Delmas, Y. Borthomieu, C. Faure, A. Delahaye and M. Figlarz, Solid State Ionics., 32/33, 104 (1989).

[2] L Croguennec, C. Pouillerie and C. Delmas, J. Electrochem. Soc., 147, 1314 (2000).

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[4] J.C. Hunter, J. Solid State Chem., 39, 142 (1981).

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[6] W. Li, R. McKinnon and J.R. Dahn, J. Electrochem. Soc., 141, 2310 (1994).

[7] H. Arai, M. Tsuda, M. Hayashi, H. Ohtsuka and Y. Sakurai, Electrochim. Acta, 50, 1821 (2005).

[8] H. Arai, M. Tsuda, K. Saito, M. Hayashi, K. Takei and Y. Sakurai, J. Solid State Chem., 163, 340 (2002).

[9] K. Imai, A. Ikezawa, S. Sato and H. Arai, Battery Symposium in Japan, 1H05 (2019).

[10] T. Nishizawa, A. Ikezawa and H. Arai, Battery Symposium in Japan, 3B09 (2020).

Figure 1

The energy density of Li-ion batteries can be improved by storing charge at high voltages through the oxidation of oxide ions in the cathode material. However, oxidation of O²⁻ triggers irreversible structural rearrangements in the bulk and an associated loss of the high voltage plateau, which is replaced by a lower discharge voltage, and a loss of O₂ accompanied by densification at the surface. Understanding the O-redox process and the nature of oxidised oxygen has proved very challenging.

Using a range of techniques including XAS, RIXS, STEM, NMR, diffraction and DFT, applied across a wide range of alkali metal rich transition metal oxides, reveals that O^2- is oxidised to O₂.^1-4 The O₂ is either evolved from the surface or trapped in voids formed in the bulk by reorganisation of vacancies within the structure. Although O₂ can be reduced back to O^2-, the process is not energetically reversible, explaining the much lower voltage on discharge compare with the first charge (approx. 1eV less), the phenomenon of so-called voltage hysteresis. Furthermore, it is possible to suppress O₂ formation, trapping hole states on O^2- and obtaining energetic (voltage) and structural reversibility.⁴ Such behaviour points the way towards high energy density cathodes for Li-ion batteries.

1. House, R. A. et al. First cycle voltage hysteresis in Li-rich 3d cathodes associated with molecular O₂ trapped in the bulk. Nature Energy8, 777–785 (2020).

2. House, R. A. et al. The role of O₂ in O-redox cathodes for Li-ion batteries. Nature Energy 1–9 (2021).

3. Boivin, E. et al. The Role of Ni and Co in Suppressing O‐Loss in Li‐Rich Layered Cathodes. Advanced Functional Materials 2003660 (2020).

4. House, R. A. et al. Superstructure control of first-cycle voltage hysteresis in oxygen-redox cathodes. Nature577, 502–508 (2020).

As an important alternative for Li-based battery technology, rechargeable zinc metal batteries promise attractive advantages including safety, high volumetric energy density and low cost; however, such benefits cannot be unlocked unless Zn reversibility meets stringent commercial viability^1-2.

As part of the Joint Center for Energy Storage Research (JCESR), a DOE Energy Innovation Hub led by Argonne National Laboratory and focused on advancing battery science and technology, ARL team is building new understanding and improvement strategies for Zn metal anode reversibility. In this presentation, we will revisit the quantification of Zn reversibility and report a few remarkable improvements on Zn reversibility with novel electrolyte chemistries. The microscopic characterizations combined with molecular dynamics simulation and density functional theory calculations were used to identify the electrolyte bulk structures, correlated interphasial chemistries and their functionalities as the key factors responsible for dictating reversible Zn chemistry.

Reference

1. E. Blanc, D. Kundu, L. F. Nazar, Joule2020, 4, 771–799.

2. Ma, M. A. Schroeder, O. Borodin, T. P. Pollard, M. S. Ding, C. Wang, K. Xu, Nature Energy 2020, 5, 743-749.

Novel approaches for the study of disordered rocksalt oxyfluoride intercalation cathodes

Raynald Giovine,^a Yuefan Ji,^a Ashlea Patterson,^a Emily Foley,^a Zhengyan Lun,^c Daniil Kitchaev,^a,b Bin Ouyang,^d Jinhyuk Lee,^c,e Yuan Yue,^f Yang Ha,^g Wanli Yang,^g Wei Tong,^f Gerbrand Ceder,^c,d Raphaële Clément^a

a. Materials Department, University of California, Santa Barbara, CA 93106.

b. Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139.

c. Department of Materials Science and Engineering, University of California, Berkeley, CA 94720.

d. Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720.

e. Department of Mining and Materials Engineering, McGill University, Montreal, QC, H3A 0C5, Canada.

f. Energy Storage and Distributed Resources Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720.

g. Advanced Light Source Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720.

Disordered rocksalt oxides and oxyfluorides (DRX) have recently emerged as a promising class of lithium-ion cathodes, with initial capacities up to 300 mAh/g and initial energy densities up to 1000 Wh/kg.[1] The presence of extensive disorder and the ability to partially replace O by F result in unique Li⁺ diffusion and redox properties. Yet, the structure of such compounds is particularly difficult to characterize, as a result of the intrinsic disorder on the cation lattice, the small particle size required for reasonable rate performance, and the inability of x-ray and neutron diffraction to distinguish between O and F.

In this presentation, I will show that high resolution ex situ solid-state NMR is uniquely suited for the study of cation short-range order[2] and structural degradation mechanisms resulting in rapid capacity fade upon cycling, and to assess the solubility limit of F into the rocksalt structure. Our findings indicate that the limited F solubility in disordered rocksalts depends on cathode composition and does not impede their performance, that one major source of impedance build up during cycling is degradation of common Li-ion electrolyte salts at high voltage,[3] as well as metal dissolution, and that cation short-range order can be eliminated through the formation of 'high entropy' compounds.[4] These insights allow us to establish links between composition and electrochemical properties and establish design rules for next-generation intercalation-type Li-ion cathodes.

References

[1] R. J. Clément, Z. Lun, and G. Ceder, Energy Environ. Sci., 13(2), 345–373 (2020).

[2] R. J. Clément, D. Kitchaev, J. Lee, and G. Ceder, Chem. Mater., 30, 6945–6956 (2018).

[3] Y. Yue^#, Y. Ha,^# R. Giovine,^# R. J. Clément, W. Yang, W. Tong, in preparation.

[4] Z. Lun, B. Ouyang, D.-H. Kwon, Y. Ha, E. E. Foley, T.-Y. Huang, Z. Cai, H. Kim, M. Balasubramanian, Y. Sun, J. Huang, Y. Tian, H. Kim, B. D. McCloskey, W. Yang, R. J. Clément, H. Ji, and G. Ceder, Nat. Mater., 20(2), 214–221 (2021).

In the past decade, lithium-enriched compounds, Li₂MO₃ (M = Mn^4+, Ru⁴⁺ etc.), have been extensively studied for high-capacity positive electrode materials of Li-ion batteries. Large reversible capacities originate from charge compensation by anions species (anionic redox) coupled with partial cationic redox of transition metal ions. Recently, many cation-disordered rocksalt oxides have been proposed as a new series of electrode materials, which utilize reversible anionic redox. Nevertheless, insufficient electrode kinetics for the cation-disordered rocksalt system limits its use for practical applications. One simple strategy is synthesizing nanosized materials to overcome a problem of electrode kinetics (for electrons, holes and ions), and electrode kinetics are significantly improved through nanosizing even for a non-lithium-excess and stoichiometric system.¹⁾ Moreover, Nanosized Li/Na-excess oxides also deliver large reversible capacities at room temperature,^2-4) which shows much better electrode kinetics compared with as-prepared samples.

From these findings, we discuss the advantages/disadvantages of "nanostructured" electrode materialsto develop high energy advanced Li-ion batteries.

References

1) Sato et al., and N. Yabuuchi., Journal of Materials Chemistry A, 6, 13943 (2018).

2) Kobayashi et al., and N. Yabuuchi, Small, 15, 1902462 (2019).

3) Kobayashi et al., and N. Yabuuchi, Materials Today, 37, 43 (2020).

4) Sawamura et al., and N. Yabuuchi, ACS Central Science, 6, 2326 (2020).

Lithium-rich nickel manganese oxides (LLRNMOs) experience a large degree of irreversible charge capacity due to an electrochemically induced phase transformation ^[1–4]. This transformation has been linked to electrolyte decomposition accompanied by outgassing, which limits the commercial viability of this class of materials. Past studies documented that after treating cathodes of similar structure (such as layered LiNiO₂, and NMC333) with acidic solutions, the resultant materials resembled untreated electrochemically cycled materials^[5–8]. In this study, LLRNMOs were exposed to the acidic solutions of H₃PO₄, H₂SO₄, HCl, and HNO₃. To the best of our knowledge, this study marks the first time that a lithium-excess cobalt-free cathode was exposed to acidic solutions. X-ray diffraction (XRD) of samples revealed that all acid treatments resulted in (003) shoulder peaks consistent with charged samples ^[9]. Relithiation of one of these samples resulted in the loss of this shoulder peak. By contrast, XRD of post-acid treatment samples saw patterns with preserved superlattice peaks between 20-25° 2θ, while electrochemically cycled samples do not^[10]. Besides these commonalities, XRD patterns suggested the different acid treatments were inducing different phase changes in the materials. Most noticeably, H₃PO₄ treated samples' XRD patterns suggested that treatment induced the formation of other secondary phases. X-ray fluorescence (XRF) of samples was conducted to determine the Mn/Ni ratio of samples, verifying that the varied acidic treatments are inducing equally varied phase transformations. Scanning electron microscopy (SEM) was conducted on the samples, and it was found that only the H₃PO₄-treated materials had altered morphologies. Our results demonstrate that these chemically induced phase transformations are a poor substitute for the electrochemically induced phase transformation. The possible reasons and consequences of this are discussed.

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Innovation through structural design has been an effective mean for the realization of today's batteries. Our ability to master the atomic scale through the identification of host structures and chemistries able to intercalate lithium ions at convenient voltages has been rewarded by the deployment of Li-ion technology and the ushering in of the portable electronics era. A massive effort is now devoted to the improvement of the selected group of materials that have reached commercialization and the development of new materials and chemistries. However, the development of the next generation of energy storage materials requires an unprecedented ability to understand and control all levels of organization of matter and its coupling with function, including disorder and defects, which have often been dismissed as deleterious to performance. However, if understood and controlled, can provide a depth of control and utility to design better materials.

Progress in the characterization of disorder is providing unprecedented insights into defect structures. In particular, recent models and tools applied to X-ray scattering techniques, often in complement with electron microscopy or solid-state Nuclear Magnetic Resonance, offer an accessible window for the observation and accurate parametrization of complex microstructural features. Through different examples related to energy storage materials, several of these models will be shown. These comprise those included in X-ray Rietveld refinement programs to extract structural descriptors related to point defects, anti-sites or anti-phase domains; as well as the FAULTS program,which allows to quantitatively describe planar disorder such as stacking faults. The ability to characterize the dynamic evolution of such phenomena using operando techniques will also be discussed.

Positive electrode materials such as layered lithium nickel, manganese and cobalt oxides (NMC)^1,2 in Li-ion batteries typically store charge by relying on the redox activity of transition metal species, which is accompanied by the intercalation and deintercalation of Li ions into and out the host structure. Anionic redox in positive electrode materials can provide additional charge storage beyond the conventional metal redox. However, the physical origin of observed anion redox as well as the requirements for the reversible anionic redox activity remain under debate, hindering rational design of new electrode materials leveraging reversible anionic redox.

In this talk, we first focus on understanding the cationic and anionic redox process in the positive electrode materials upon lithium deintercalation using X-ray absorption and emission spectroscopy (XAS and XES), X-ray photoelectron spectroscopy (XPS), coupled with density functional theory (DFT) calculations. We show electronic signatures of oxygen-oxygen coupling, direct evidence central to lattice oxygen redox (O^2-/(O₂)^n-), in charged Li_2-xRuO₃ after Ru oxidation (Ru⁴⁺/Ru⁵⁺) upon first-electron removal with lithium de-intercalation. This lattice oxygen redox of Li_2-xRuO₃ was accompanied by bulk Ru reduction.³ This observed redox trend is in stark contrast of the observations in charged Ni-rich NMC upon charging. In Ni-rich NMC positive electrodes, nickel oxidation is primarily responsible for the charge capacity up to removing ~0.7 Li, beyond which is followed by Ni reduction near the surface (up to 100 nm) due to oxygen release, where there is no significant bulk metal reduction observed.⁴ The uniqueness of Ru-based system lies in the highly covalent nature of Ru-O bond, which stabilizes the O^2-/(O₂)^n- intermediates, forbidding further oxygen release.

We further discuss that a strong metal-oxygen covalency is needed to enhance a reversible anionic redox.⁵ Differential electrochemical mass spectrometry (DEMS) was employed to monitor the oxygen release and quantify the reversibility of anionic redox of Li₂Ru_0.75M_0.25O₃ (M= Ti, Cr, Mn, Fe, Ru, Sn, Pt, Ir) upon first charge. We show through X-ray absorption spectroscopy that more ionic substituents and reduced metal-oxygen covalency introduce irreversible oxygen redox, accompanied with easier distortion of M-O octahedron and smaller barrier for forming oxygen dimer within the octahedron. We propose a universal electronic structure descriptor that could control bulk anionic redox, leading to a rational design strategy to enhance the cycling stability of Ni-rich NMC⁶ as well as Li-rich positive electrodes. Our study has laid a solid foundation for future high-throughput screening of novel and affordable metal oxides for battery and electrocatalysis applications.

Reference

(1) Noh, H.-J.; Youn, S.; Yoon, C. S.; Sun, Y.-K. Comparison of the Structural and Electrochemical Properties of Layered Li [NixCoyMnz] O2 (X= 1/3, 0.5, 0.6, 0.7, 0.8 and 0.85) Cathode Material for Lithium-Ion Batteries. Journal of power sources2013, 233, 121–130.

(2) Jung, R.; Metzger, M.; Maglia, F.; Stinner, C.; Gasteiger, H. A. Oxygen Release and Its Effect on the Cycling Stability of LiNixMnyCozO2 (NMC) Cathode Materials for Li-Ion Batteries. Journal of The Electrochemical Society2017, 164 (7), A1361–A1377.

(3) Yu, Y.; Karayaylali, P.; Nowak, S. H.; Giordano, L.; Gauthier, M.; Hong, W.; Kou, R.; Li, Q.; Vinson, J.; Kroll, T. Revealing Electronic Signatures of Lattice Oxygen Redox in Lithium Ruthenates and Implications for High-Energy Li-Ion Battery Material Designs. Chemistry of Materials2019, 31 (19), 7864–7876.

(4) Yu, Y.; Karayaylali, P.; Giordano, L.; Corchado-García, J.; Hwang, J.; Sokaras, D.; Maglia, F.; Jung, R.; Gittleson, F. S.; Shao-Horn, Y. Probing Depth-Dependent Transition-Metal Redox of Lithium Nickel, Manganese, and Cobalt Oxides in Li-Ion Batteries. ACS Applied Materials & Interfaces2020.

(5) Yu, Y.; Karayaylali, P.; Sokaras, D.; Giordano, L.; Kou, R.; Sun, C.-J.; Maglia, F.; Jung, R.; Gittleson, F. S.; Shao-Horn, Y. Towards Controlling the Reversibility of Anionic Redox in Transition Metal Oxides for High-Energy Li-Ion Positive Electrodes. Energy & Environmental Science2021, 14 (4), 2322–2334.

(6) Yu, Y.; Zhang, Y.; Giordano, L.; Zhu, Y. G.; Maglia, F.; Jung, R.; Gittleson, F. S.; Shao-horn, Y. Enhanced Cycling of Ni-Rich Positive Electrodes by Fluorine Modification. Journal of The Electrochemical Society2021.

Li and Na intercalation compounds exhibit a wide variety of electronic, structural and phase transformation phenomena. Very different phases and electronic properties can be realized within the same layered transition metal oxide by simply changing the guest cation from Li to Na. For example, Na intercalation compounds exhibit many more ordering reactions and structural phase transformations than their Li counterparts. In this talk we will focus on layered Li and Na intercalation compounds that start out as O3. The larger size of Na leads to the stabilization of P3 upon Na extraction. Our first-principles statistical mechanics studies have revealed that the P3 host tends to stabilize an intriguing family of Na-vacancy orderings that consist of well-ordered domains separated by anti-phase boundaries. The family of ordered phases forms a devil's staircase whereby the concentration can be varied by changing the density of anti-phase boundaries. A systematic study of diffusion mechanisms in the devil's staircase of ordered P3 phases shows that Na transport is mediated by the migration of anti-phase boundaries rather than conventional cation-vacancy exchanges as occurs in Li-intercalation compounds.

We will also discuss a variety of intriguing redox mechanisms in Li and Na layered intercalation compounds that arise either from p-bonded complexes involving oxygen and transition metals or complexes made of metal-metal bonds. An important example is Na₂Mn₃O₇, which can be charged in a Na-ion battery in spite of the fact that Mn has a formal oxidation state of 4+. The 'excess' capacity is enabled by a p-redox centered that is delocalized over a ring of six Mn p-bonded to six O surrounding a Mn vacancy. Similar molecular-like complexes that can host redox centers are also present in Mo containing layered Li-intercalation compounds.

Anionic redox in Li-rich and Na-rich transition metal oxides (A-rich-TMOs) has emerged as a new paradigm to increase the energy density of rechargeable batteries. [1,2] However, most A-rich-TMOs reported so far with high energy density due to anionic redox are prone to structural instabilities that prevent their use in practical applications. [3] Several alternatives have been proposed to limit or suppress these instabilities, among them the increase of M–O bond covalency, [4] the chemical substitution of M for d⁰ metals, [5] the control of cationic migration, [6] or the use of cation-disordered rocksalt structures. [7] The unified picture developed in this introduces the number of holes per oxygen (h^O) as another critical parameter to sustain a reversible anionic capacity. From a representative set of materials reported in the literature, h^O = 1/3 seems to be the critical value to avoid O₂ release and achieve fully reversible anionic redox. [8] Measurable quantities such as ΔCT, x stoichiometry and disorder are therefore sufficient to determine h^O and to infer the electrochemical behaviour of A[AxM1−x]O₂ electrodes. Within this general framework, the best candidates for high energy density are those exhibiting the an hom*ogeneous O-network (no cation disorder), a (1-x)e- cationic capacity and an anionic capacity limited to h^O ≤ 1/3 on each individual O-sublattice. Tridimensional structures should be preferred over layered structures to prevent structural instability at low A content and take advantage of the full theoretical capacity. Ordered structures should favour hom*ogeneous O networks and prevent one O sublattice to uptake a critical h^O > x. Very few TMOs should satisfy these requirements, in particular when they also have to meet industrial specifications such as cost, toxicity and natural abundance. Overall, strategies devoted to the activation of anionic redox as a lever to improve the energy density of electrode materials are risky, as they implicitly question the general chemical rules to guarantee structural stability of oxide-materials. The triptych of high potential, high capacity and high structural stability still appears out of our current reach and calls for trade-offs.

References:

[1] Koga, H. et al. Reversible oxygen participation to the redox processes revealed for Li1.20Mn0.54Co0.13Ni0.13O2. J. Electrochem. Soc. 160, A786–A792 (2013).

[2] Sathiya, M. et al. Reversible anionic redox chemistry in high-capacity layered-oxide electrodes. Nat. Mater. 12, 827–835 (2013).

[3] Assat, G. et al. Fundamental interplay between anionic/cationic redox governing the kinetics and thermodynamics of lithium-rich cathodes. Nat. Commun. 8, 2219 (2017).

[4] Pearce, P. E. et al. Evidence for anionic redox activity in a tridimensional- ordered Li-rich positive electrode β-Li2IrO3. Nat. Mater. 16, 580–587 (2017).

[5] Yabuuchi, N. et al. High-capacity electrode materials for rechargeable lithium batteries: Li3NbO4-based system with cation-disordered rocksalt structure. Proc. Natl Acad. Sci. USA 112, 7650–7655 (2015).

[6] Gent, W. E. et al. Coupling between oxygen redox and cation migration explains unusual electrochemistry in lithium-rich layered oxides. Nat. Commun. 8, 2091-1–12 (2017).

[7] Urban, A., Matts, I., Abdellahi, A. & Ceder, G. Computation design and preparation of cation-disordered oxides for high-energy-density Li-ion batteries. Adv. Energy Mater. 6, 1600488-1–1600488-8 (2016).

[8] Ben Yahia, M.; Vergnet, J., Saubanère, M., Doublet, M.-L., Unified picture of anionic redox in Li/Na-ion batteries. Nat. Mat. 18, 496-502 (2019).

Metal anodes offer significantly higher capacities than graphite and are therefore central to strategies to develop advanced rechargeable battery chemistries that meet range and performance targets for electric vehicles. Although closer than ever, lithium (Li) anodes still cannot meet the >99.9% Coulombic efficiency (CE) consistently needed for >1,000 cycle life. This shortfall arises from uncontrolled reactivity at the solid electrolyte interphase (SEI), leading to inhom*ogeneous plating and stripping, continuous electrolyte consumption and loss of active Li from the cell. In spite of much recent progress in liquid electrolyte development, the lack of precise understanding of chemistry, structure and function of the SEI still hinders attempts to rationally design an improved interface and bridge the remaining gap in CE.

To help inform such efforts, our work is developing techniques to gain insights into SEI phases and reveal interplays between their chemistry, structure and function. We developed an approach to isolate and synthesize ionic phases relevant to the native Li SEI at nanoscale thickness directly on Li metal. These interfaces are then interrogated via targeted electrochemical and spectroscopy techniques to reveal their transport properties, Li⁺ exchange kinetics and chemical reactivity in different electrolytes, providing insight into how such phases may function in a native SEI. I will then discuss our efforts using operando gas chromatography and titration-based chromatography to quantify the evolution of organic phases during initial SEI formation and over cycling. These analytical techniques, which allow for unprecedented chemical resolution of SEI-forming processes, help to provide missing information about the fragile organic phases that elude most conventional solid-state characterization and support an increasingly quantitative description of SEI chemical dynamics. By applying these tools to high- and low-CE electrolytes alike, we are identifying descriptors that can support continued development of advanced electrolytes and additives.

Recently, we are also translating these efforts to study Ca metal anodes, which remain in much earlier stages of research due to particularly severe challenges originating with the Ca SEI. I will describe some key similarities and differences between Li and Ca that are informing our approach to Ca interface characterization, Ca foil testing and CE benchmarking in leading electrolytes developed recently. Early findings are revealing how Ca²⁺ coordination can dramatically influence the activity – or lack thereof – of Ca plating/stripping, which directly relates to the nature of the formed interface.

Na- ion layered oxide as cathodes have attracted both the scientific community and industry. They offer high gravimetric energy density, tunability of properties and ease of synthesis. This leads to both interesting science and low cost applied materials.

Layered oxides are generally classified by the nomenclature introduced by Delmas et. al.¹ which depends on the alkali environment and stack of layers in the unit cell. The same team also used layered oxides for the first-time as potential positive electrode for reversible sodium storage.²

In this talk we present our contributions on (i) O3, (ii) P3 and (iii) modified P3 type layered oxides as cathode materials for Na-ion batteries.

O3-type layered sodium transition metal oxides (Na_xMO₂, M = transition metal ions) are one of the most attractive cathode materials considering their capacity. However, the use of O3 phases is limited due to their low redox voltage and several associated phase changes which are unfavorable for long cycling.^3-5In this presentation, we show the effect of Cu, Ni and Ti doping in the O3 structure and relate the observed local environments to their storage performances. Cu²⁺ Jahn-Teller distortion leads to irreversibility and dictates the amount of Cu²⁺ that can be doped in the layer. Our results show that 10-12% of Cu-doping is optimal. In order to further examine the effect of local structures, we doped Ni²⁺ and achieved excellent structural reversibility which favors high-rate performance and cycling stability.⁶

Na-sufficient Fe and Mn based P3 layered oxide is able to accommodate higher than 0.67 moles of Na-ions.^7,8 We present the effects of voltage limits on the storage performances and highlight associated phase changes during electrochemical cycling. XANES data confirm the charge compensation mechanism during electrochemical cycling. Full cell data further elucidate the sufficiency of Na-ions in the P3 structure.⁸

Lastly, the storage performances of P3-Na_0.9Fe_0.5Mn_0.5O₂ are compared with those for modified P3 phase, P3 (major)/ O3 (minor) layered oxide. The phase changes during cycling and comprehension of charge compensation mechanism from XANES of this modified P3 structure are reported.

References:

1. Delmas, C., Fouassier, C. & Hagenmuller, P. Structural classification and properties of the layered oxides. Physica B+C 99, 81–85 (1980).

2. Delmas, C., Braconnier, J., Fouassier, C. & Hagenmuller, P. Electrochemical intercalation of sodium in NaxCoO2 bronzes. Solid State Ionics 3–4, 165–169 (1981).

3. Sathiya, M. et al. A Chemical Approach to Raise Cell Voltage and Suppress Phase Transition in O3 Sodium Layered Oxide Electrodes. Adv. Energy Mater. 8, 1702599 (2018).

4. Yoshida, H., Yabuuchi, N. & Komaba, S. NaFe0.5Co0.5O2 as high energy and power positive electrode for Na-ion batteries. Electrochemistry Communications 34, 60–63 (2013).

5. Ding, F. et al., A Novel Ni-rich O3-Na[Ni0.60Fe0.25Mn0.15]O2 Cathode for Na-ion Batteries. Energy Storage Materials 11 (2020).

6. Tripathi, A., Rudola, A., Gajjela, S. R., Xi, S. & Balaya, P. Developing an O3 type layered oxide cathode and its application in 18650 commercial type Na-ion batteries. J. Mater. Chem. A 7, 25944–25960 (2019).

7. Risthaus, T. et al. P3 Na0.9Ni0.5Mn0.5O2 Cathode Material for Sodium Ion Batteries. Chem. Mater. 8 (2019).

8. Tripathi, A., Xi, S., Gajjela, S. R. & Balaya, P. Introducing Na-sufficient P3-Na 0.9 Fe 0.5 Mn 0.5 O 2 as a cathode material for Na-ion batteries. Chem. Commun. 56, 10686–10689 (2020).

Iron(Fe)-based layered cathode materials have attracted much attention for sodium ion batteries because iron is earth-abundant, low-cost, and environmentally benign element [1]. These are important considerations as the battery field moves forward in sustainable chemistries. Layered NaFeO₂ cathode is electrochemically active in a sodium-ion battery from the Fe³⁺/Fe⁴⁺ redox reaction, which provides a high operating voltage (~3.3 V) and energy density (~300 mWh/g). [2] However, the material suffers from a significant degradation process that is not totally understood. The electrochemically oxidized Na_1-xFeO₂ experiences irreversible structural change by Fe migration from octahedral to tetrahedral sites, as well as a deleterious chemical side reaction between unstable Fe⁴⁺ species and the electrolyte. [3] The detrimental chemical reaction leads to the growth of a highly resistive SEI layer on the cathode surface. The consequence of this high interfacial impedance results in poor electrochemical cycling performance. Therefore, one must understand first the chemical makeup and reactivity of the NaFeO₂ cathode/electrolyte interface in order to improve it.

Herein, we investigate the electrolyte effect on the parasitic side reaction of Na_1-xFeO₂ cathode and improve the chemical stability of the cathode-electrolyte interface by electrolyte modification. Simple addition of FEC (fluoroethylene carbonate) additive with NaPF₆ standard salt suppresses the parasitic reaction and dramatically improves the reversible capacity and coulombic efficiency. We will present these findings and discuss its implications on Fe-based layered oxide cathode for future development.

Reference

[1] X. Wang, S. Roy, Q. Shi, Y. Li, Y. Zhao and J. Zhang, J. Mater. Chem. A, 2021,9, 1938-1969

[2] N. Yabuuchi, M. Yano, H. Yoshida, S. Kuze, and S. Komaba, Journal of The Electrochemical Society, 160 (5) A3131-A3137 (2013)

[3] E. Lee, D. E. Brown, E. E. Alp, Y. Ren, J. Lu, J-J. Woo, and C. S. Johnson, Chem. Mater. 2015, 27, 6755−6764

Layered AMO₂ oxides have been exhaustively researched over the past 40 years. This process was accelerated by the advent and commercial success of the Li-ion battery. With many AMO₂ chemistries approaching their theoretical limit in terms of performance, it is important to continue the search for new material compositions and structures. This search will continue to advance our understanding of the solid state and intercalation chemistry in novel cathode materials for design principles that can aid in making new chemistries feasible or result in the discovery of new solid state electrochemical phenomenon.

The nickel-tellurate system has been seldom explored for lithium and sodium positive electrode materials. The presence of Te⁶⁺ in the structure allows for a wide range of compositions within a structural family. The cation ratio of A⁺:Ni²⁺ can be varied to tailor the amount of vacancies in the structure. In this study, novel compositions in the A⁺-Ni²⁺-Te⁶⁺-O (A=Li or Na) phase space have been synthesized and characterized with X-ray diffraction, solid-state nuclear magnetic resonance spectroscopy, and electrochemical techniques to elucidate the structure-electrochemical property relationships in these materials. Experimental characterization methods are coupled with density functional theory (DFT) calculations to probe the local atomistic structure and reaction mechanisms. Our findings have led to the identification of unusual structural features for both lithium and sodium cathode materials. For the layered sodium nickel-tellurates, the introduction of excess Na into the MO₂ layers led to superior electrochemical performance via the suppression of correlated Na⁺-ion motion and MO₂ layer gliding in the layered Na_xMO₂ structure.^[1] For lithium, the structural nuances have led to poor electrochemical performance, but can nevertheless be used as guiding principles for the exploration of novel cathode chemistries.^[2]

References:

[1] N.S. Grundish, I. D. Seymour, Y. Li, J.-B. Sand, G. Henkelman, C. Delmas, and J.B. Goodenough, "Structural and Electrochemical Consequences of Sodium in the Transition-Metal Layer of O'3-Na₃Ni_1.5TeO₆", Chem. Mater. 2020, 32, 23, 10035-10044.

[2] N. S. Grundish, I. D. Seymour, G. Henkelman, and J. B. Goodenough, "Electrochemical Characterization of Three Li₂Ni₂TeO₆ Structural Polymorphs", Chem. Mater. 2019, 31, 22, 9379-9388.

In the last decade, many interesting cathode and anode materials have been proposed for Na-ion batteries^1-20. Among them, layered oxide cathodes and disordered carbon anodes would be the focus of the first generation of commercial Na-ion batteries. In this talk, firstly, I will introduce a series of Na-Cu-Fe-Mn-O cathodes on the basis of our finding of electroactivity of Cu²⁺/Cu³⁺ redox couple in Na containing layered oxides ^6-8 and Na_x[Li_1-yMn_y]O₂ cathode based on reversible oxygen redox couple^19,20. Secondly, I will present a series of carbon anodes from anthracite and biomass^14-18. Thirdly, prototype Na-ion cells with capacity of 1-12 Ah were fabricated and and their electrochemical and safety performance of Na-ion cells will be presented. Most impressively, the cycle life of such cells can reach 4800 times at a high rate of 2C. Finally, the first 100kWh Na-ion battery energy storage system was demonstrated and their technical details will be disclosed.

References:

1. Pan, H. L.; Hu, Y.-S.*; Chen, L. Q. Energy & Environmental Science 2013, 6, 2338-2360.

2. Zhao, C.L.; ...; Hu, Y.-S.* Science 2020, 370, 708-711.

3. Zhao, C.L.; ...; Hu, Y.-S.* JACS 2020, 142, 5742-5750.

4. Sun, Y.; Zhao, L.; Pan, H. L.; Lu, X.; Gu, L.*; Hu, Y.-S.*; et al. Nature Communications 2013, 4, 1870.

5. Wang, Y. S.; ...; Hu, Y.-S.*; et al. Nature Communications 2013, 4, 2365.

6. Xu, S. Y.; Wu, X. Y.; Li, Y. M.; Hu, Y.-S.*; Chen, L.-Q. Chinese Physics B 2014, 23, 118202.

7. Li, Y. M.; Yang, Z.; Xu, S.; Mu, L.; Gu, L.*; Hu, Y.-S.*; et al., Advanced Science 2015, 2, 1500031.

8. Mu, L. Q.; Xu, S.; Li, Y.; Hu, Y.-S.*; Li, H.; Chen, L.; Huang, X., Advanced Materials 2015, 27, 6928.

9. Wang, Y.; Xiao, R.; Hu, Y.-S.*; Avdeev, M.*; Chen, L., Nature Communications 2015, 6, 6954.

10. Wang, Y.; ...; Hu, Y.-S.*; et al. Nature Communications 2015, 6, 6401.

11. Wang, Y.; ...; Hu, Y.-S.*; et al., Advanced Energy Materials 2015, 5, 1501005.

12. Xu, S.; ...; Hu, Y.-S.*; et al., Advanced Energy Materials 2015, 5, 1501156.

13. Wu, Y.; ...; Hu, Y.-S.*; et al., Science Advances 2015, 1, e1500330.

14. Li, Y. M.; Lu, Y.; Hu, Y.-S.* et al.; Energy Storage Materials 2017, 7, 130.

15. Li, Y. M.; Hu, Y.-S.* et Energy Storage Materials 2016, 5, 191.

16. Lu, Y.; ...; Hu, Y.-S.* Advanced Energy Materials 2018, 8, 1800108.

17. Zhao, C.; Wang, Q.; Lu, Y.X.*; ...; Hu, Y.-S.* Science Bulletin, 2018, 63, 1125.

18. Meng, Q.; Lu, Y.X.*; ...; Hu, Y.-S.* ACS Energy Letters 2019, 4, 2608-2612.

19. Rong, X.; Liu, J.; Hu, E.; ... ; Hu, Y.-S.*, et Joule 2018, 2, 2348-2363.

20. 20. Rong, X.; Hu, E.; Lu, Y.; ...; Hu, Y.-S.*, et al., Joule 2019, 3, 503-517.

Yong-Sheng Hu is a full professor at the Institute of Physics, Chinese Academy of Sciences. He received his Ph.D. in Condensed Matter Physics from IoP-CAS with Prof. Liquan Chen in 2004, and then moved to Max Planck Institute for Solid State Research as Postdoc and Principal researcher. After a short stay at the University of California at Santa Barbara, he joined IoP-CAS in 2008 and is working on advanced materials for long-life stationary batteries and their energy storage mechanism, particularly focusing on Na based batteries. His recent original contributions include: propose the use of "cationic potential" to predict the O and P stacking structures; discover the electroactivity of Cu²⁺/Cu³⁺ redox couple in sodium containing oxides and design a series of air-stable and Co-/Ni-free Na-Cu-Fe-Mn-O cathode materials for Na-ion batteries; propose a superior low-cost amorphous carbon made from anthracite as an anode for Na-ion batteries; design zero-strain anode materials for sodium-ion batteries; propose a "Solvent-in-Salt" electrolyte; etc. He has published over 200 internationally refereed SCI publications including Science、Nature Mater.、Nature Energy、Joule、Nature Commun.、Science Adv.、etc, which have been cited over 26000 times according to ISI web of science with an H-index of 86. He was selected as a Thomson Reuters Highly Cited Researchers from 2014 to 2020. He became the senior Editor of ACS Energy Letters from October of 2018. He also received several awards and honors, such as The National Science Fund for Distinguished Young Scholars, The 14th China Youth Science and Technology Award, Tajima Prize, Fellow of The Institute of Physics (UK), Fellow of The Royal Society of Chemistry, etc.

Figure 1

Utilizing the reversible sulfur redox reaction in layered chalcogenides opens new approaches for the development of new battery cathode materials with higher capacities for sodium-ion batteries. However, only limited number of studies on the nature of anionic redox chemistry in layered chalcogenides has been reported in the literature. We have designed and synthesized a series of layered chalcogenide cathode materials NaCrS_1-xSe_x and investigated their sodium storage performances. Among them, NaCrSeS exhibits a unique charge/discharge feature with a quite small polarization of 0.15 V and more than 90% high coulombic efficiency. At the same time, a very high charge capacities of 115.5mAh g^-1 (0.80 Na⁺/CrSSe) is achieved at a charging rate as high as 27.8 C. The charge compensation mechanisms and structural evolution of these cathode materials have been investigated using synchrotron based multi-model characterization techniques such as ex situ x-ray absorption spectroscopy, x-ray diffraction and x-ray pair distribution function analysis combined with DFT calculation. The results show that the charge compensation in NaCrSSe cathode is mainly through the dual anionic-redox reaction with the reversible formation of (S)^n– and (Se)^n– as well as (S/Se)₂^m– species during the Na intercalation/deintercalation processes. These results will provide valuable information for developing new anion redox based cathode materials with high-capacity and fast kinetics.

Acknowledgment: The work at Brookhaven National Laboratory was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Vehicle Technology Office of the U.S. Department of Energy through the Advanced Battery Materials Research (BMR) Program under contract DE-SC0012704. This research used resources at beamlines 7-BM (QAS) and 28-ID-2 (XPD) of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704.

With the rapid development of sodium-ion batteries in recent years, the prospect of replacing lithium-ion batteries for some applications like personal electronics is more and more promising. The abundant reserve of sodium element and the cheap transition metals for layered oxides are the decisive advantages to solve the cost of electricity storage and environmental pollution, which may make massive energy storage accessible. During the past ten years, layered oxides of the form Na_xFe_yMn_zO₂ have received a lot of attention with high specific capacities and eco-friendly transition metal selection. Since the radius of sodium ion is significantly larger than that of lithium ion, the configurations of the sodium layered oxides are more complex, which allows sodium ions to be located in either octahedral (O-type) or prismatic (P-type) sites. Meanwhile, the electrochemical properties of sodium layered oxides vary dramatically with the composition. Despite this, a relatively small number of compositions have been studied to date in the Na-Fe-Mn-O system. Herein, we apply high-throughput methods to systematically analyze both structures and battery performance across the entire Na_xFe_yMn_zO₂ system.

The high-throughput methods established elsewhereare used here to make and characterize 64 different materials simultaneously. In total, materials at 304 different compositions were made for this study, all of which were examined by X-ray diffraction (XRD) and 184 compositions were also studied by high-throughput electrochemistry as they are particularly interesting as potential battery materials. Qualitative phase identification based on the XRD is performed as well as quantitative fitting of the data to extract phase compositions and lattice parameters. Each of P2-, P3- and O3-type sodium layered oxides are identified in the phase diagram and their solid solution regions were determined and will be discussed in detail. High-throughput cyclic voltammetry (CV) is used to obtain the charge/discharge voltages and specific capacities for 15 cycles thereby allowing some discrimination of the quality of extended cycling. These methods enable a rapid and highly precise screening across very wide compositions. Figure 1 shows both the experimental setup used for key steps in the synthesis and also a map of specific capacity across the phase diagram. Such maps will be correlated to the structural phase diagram obtained by XRD in order to extract meaningful structure-property relationships. Moreover, high-throughput XPS is used to study the surface stability after over 6 months of storage in air. Interestingly, some compositions show a greater tendency to form carbonates and have consequences on the electrochemical performance.

Figure 1. Steps in the high-throughput electrochemical methods used to study 64 samples at once: a) a well plate containing sol-gel precursors in various ratios to map out a region of the phase diagram, (b) the samples after sintering at 850 °C, (c) combinatorial cell used to collect the CVs shown in (d), and (e) specific capacities extracted from the CVs plotted on the Na-Fe-Mn-O pseudo-ternary Gibbs triangle. Compositions P2-Na_2/3Fe_1/2Mn_1/2O₂ and O3-Na_2/3Fe_1/2Mn_1/2O₂ are labeled on the ternary plot.

References

1. Liu, Q.; Hu, Z.; Chen, M.; Zou, C.; Jin, H.; Wang, S.; Chou, S. L.; Dou, S. X., Recent progress of layered transition metal oxide cathodes for sodium‐ion batteries. Small 2019,15 (32), 1805381.

2. Yabuuchi, N.; Kajiyama, M.; Iwatate, J.; Nishikawa, H.; Hitomi, S.; Okuyama, R.; Usui, R.; Yamada, Y.; Komaba, S., P2-type Na x [Fe 1/2 Mn 1/2] O 2 made from earth-abundant elements for rechargeable Na batteries. Nature materials 2012,11 (6), 512.

3. Adhikari, T.; Hebert, A.; Adamic, M.; Yao, J.; Potts, K.; McCalla, E., Development of High-Throughput Methods for Sodium-Ion Battery Cathodes. ACS combinatorial science 2020,22 (6), 311.

Figure 1

Alkali-ion batteries have revolutionized modern life through enabling the widespread application of portable electronic devices. The call for adopting renewable energy in many applications will also see an increase in the demand of alkali-ion batteries, specially to account for the intermittent nature of the renewable energy sources. However, the advancement of batteries for such technologies will require innovation on the forefront of materials development as well as fundamental understanding on the physical and chemical processes from atomic to device length scales. In this presentation, we focus on cathode materials development and discovery as well as fundamental understanding through multiscale advanced synchrotron spectroscopy and microscopy. Multiscale electrochemical properties of cathode materials are unraveled through complementary characterizations and design principles are developed for stable cathode materials for Na-ion and Li-ion batteries. We present tuning nano/mesoscale elemental distribution of transition metal on each individual cathode particle as a pathway toward stable cathode materials.¹ Surface-to-bulk redox chemistry of cathode particles is elucidated,² and structural and chemical complexities of Na layered cathodes are explored as a means of designing stable cathode materials.³ We show that local transition metal‒oxygen symmetry induced structural and chemical processes majorly dictate the stability of oxygen redox in layered cathodes.^4,5 The critical role of crystal defects on the stability of layered cathodes is unraveled. Finally, design principles of cathode materials for application in extreme environment such as outer space and nuclear reactors is presented.⁶

Reference

1. M. M. Rahman et al., Energy Environ. Sci., 11, 2496–2508 (2018).

2. M. M. Rahman et al., J. Phys. Chem. C, 123, 11428–11435 (2019).

3. M. M. Rahman et al., ACS Mater. Lett., 1, 573–581 (2019).

4. M. M. Rahman et al., ChemRxiv (2021) https://chemrxiv.org/engage/chemrxiv/article-details/60c75903ee301c953ec7b821.

5. M. M. Rahman and F. Lin, Matter, 4, 490–527 (2021).

6. M. M. Rahman et al., Nat. Commun., 11, 4548 (2020).

Polyanionic materials (phosphates in particular) are of special interest as positive electrodes for Li-Ion or Na-ion batteries since they offer competitive electro-chemical performances compared to sodiated or lithiated transition metal oxides [1,2]. They are based upon stable 3D frameworks, which provide long-term structural stability and demonstrate a unique variety of atomic arrangements in their crystal structures. Recent electrochemical and structural investigations of vanadium-based phosphate compounds (LiVPO₄O-LiVPO₄F, Na₃V₂(PO₄)₂F₃, Na₃V₂(PO₄)₃ .....) revealed promising perspectives [3-6].

The NASICON structural family with its large panel of compositions, Na_xMM'(PO₄)₃ (0 < x < 4 ; M,M' = Ti, Fe, V, Cr, Mn) is among the most widely investigated, due to its specific three-dimensional framework structure, stable long-term cycling ability and high Na⁺ mobility [1, 7]. Among them, the vanadium phosphate Na₃V₂(PO₄)₃ [8] is of particular interest. The crystal chemistry of these compositions is very rich and we will present several new structures that we determined, from pristine powders or for intermediate compositions spotted by operando X-Ray diffraction.

By substitution of Vanadium with Aluminum, towards the new composition Na₃AlV(PO₄)₃, a higher operating voltage is reached at 3.9 V vs. Na/Na⁺ for the V^4+/5+ redox couple [9]. Similar phenomena were spotted for Na₃FeV(PO₄)₃ and Na₂TiV(PO₄)₃ [10-11]. Recently, Mn²⁺ was used as a substituting ion to enhance the capacity of the Na₃V₂(PO₄)₃ cathode materials [12-13]. Single phase Na₄MnV(PO₄)₃ powders were synthesized and studied structurally and electrochemically in details. Na₄MnV(PO₄)₃ can be charged at 156 mAh/g towards the new composition NaMnV(PO₄)₃ but tricky redox phenomena were spotted thanks to operando XAS spectroscopy and apparent structural irreversibility occurs when vanadium is oxidized from V⁴⁺ to V⁵⁺. Even more recently, we succeeded in synthesizing Fe-substituted Na₄FeV(PO₄)₃ that allows the reversible extraction of close to 3 Na⁺ (for two transition metals). We will report on its crystal structure and on that of Na₃FeV(PO₄)₃ for which new Na⁺ order-disorder phenomena have been spotted [14].

References

[1] C. Masquelier, L. Croguennec ; Chemical Reviews, 113(8), 6552-6591 (2013)

[2] P. Adelhelm, M. Casas-Cabanas, L. Croguennec, I. Hasa, A. Koposov, S. Mariyappan, C. Masquelier, D. Saurel, J. Power Sources, 482, 228872 (2021)

[3] E. Boivin, J. N. Chotard, C. Masquelier, L. Croguennec, Molecules, 26(5), 1428 (2021)

[4] T. Broux, F. Fauth, N. Hall, M. Bianchini, T. Bamine, J.-B. Leriche^, E. Suard, D. Carlier, L. Simonin, C. Masquelier & L. Croguennec, Small Methods, 3, 1800215 (2019)

[5] F. Chen, V. Kovrugin, R. David, J. N. Chotard, O. Mentré, F. Fauth & C. Masquelier, Small Methods, 3, 1800218 (2019)

[6] B. Singh, Z. Wang, S. Park, G. Sai Gautam, J.N. Chotard, L. Croguennec, D. Carlier, A. K. Cheetham, C. Masquelier & P. Canepa, J. Mater. Chem. A, 9(1), 281-292 (2021)

[7] C. Delmas, A. Nadiri and J. L. Soubeyroux, Solid State Ionics, 28–30, 419–423 (1988)

[8] J.N. Chotard, G. Rousse, R. David, O. Mentré, C. Masquelier, Chem. Mater., 27(17),5982-5987 (2015)

[9] F. Lalère, V. Seznec, M. Courty, J. N. Chotard & C. Masquelier, J. Mater. Chem. A, 3, 16198-16205 (2015)

[10] F. Lalère, V. Seznec, M. Courty, J. N. Chotard & C. Masquelier, J. Mater. Chem. A, 6, 6654-6659 (2018)

[11] W. Zhou; L. Xue; X. Lü; H. Gao; Y. Li; S. Xin; G. Fu; Z. Cui; Y. Zhu; J. B. Goodenough, Nano letters, 16 (12) (2016)

[12] F. Chen, V. Kovrugin, R. David, J. N. Chotard, O. Mentré, F. Fauth & C. Masquelier, Small Methods, 1800218 (2018)

[13] M. Zakharkin, O. Drozhzhin, I. Tereshchenko, D. Chernyshov, A. Abakumov, E. Antipov, K. Stevenson, Appl. Energy Materials, 1(11), 5842 (2018)

[14] S. Park, J. N. Chotard, D. Carlier, I. Moog, M. Courty, M. Duttine, F. Fauth, A. Iadecola, L. Croguennec, C. Masquelier, Chem. Mater., submitted (2021)

The rapid progress in mass-market applications of metal-ion batteries intensifies the research of electrode materials for Na-ion and K-ion batteries as a possible alternative to the already matured Li-ion technology. Similar to the Li-ion intercalation systems, the Na and K-based mixed oxides and polyanion materials are extensively scrutinized as potential cathodes with the aim to enhance the specific energy, durability and rate capability. Whereas the layered oxides are characterized by greater volumetric energy density, the polyanion materials usually exhibit better cycling and thermal stability and higher C-rate capability due to covalently bonded structural frameworks in these compounds. The polyanion compounds also demonstrate an extra dimension in their crystal chemistry which significantly expands the search space for the materials with better electrochemical performance. Further advantages are expected from combining different anions (such as (XO₄)^p- and F^-) in the anion sublattice.

The fluoride-phosphates AMPO₄F (A=Li, Na, K; M=V and Ti) with the KTP-type structure are considered now as perspective cathode materials for the metal-ion batteries. The "VPO₄F" framework featured outstanding rate capability and capacity retention. Another quality of this framework is supporting reversible Na⁺, K⁺ and even Rb⁺ ions de/insertion sustaining the host structure. The ion diffusion coefficients obtained by PITT were the lowest for Li⁺ and the highest for K⁺, with the latter anticipating high-power applications of KVPO₄F in K-ion batteries. Moreover, the titanium redox activity traditionally considered as "reducing" can be upshifted to near-4V electrode potentials in the "TiPO₄F" framework thus providing a playground to design sustainable and cost-effective titanium-containing positive electrode materials with promising electrochemical characteristics.

An overview of the research on these fluoride-phosphates will be presented with a special emphasis on the interrelation between chemical composition, synthesis conditions, crystal structure peculiarities and their electrochemical properties.

This work was supported by the Russian Science Foundation (grant No. 17-73-30006).

The properties of lithium ion battery cathodes strongly depend on the diffusion of lithium ions during charge/discharge process. Then, direct visualization of lithium site is required to understand the mechanism of the diffusion of lithium ions. In this study, aberration (Cs) corrected STEM were applied to directly observe the {010} surface, which corresponded to perpendicular to the 1-D diffusion orientation, of the olivine Li_xFePO₄. The morphology of the interface between Li-rich and Li-poor phases of Li_xFePO₄ after chemical delithiation were observed with atomic resolution at fit intervals during half a year. It was found that orientation of boundary layers at the FePO₄/Li_2/3FePO₄ interface gradually changed from lower index planes to higher index planes. This indicates that intermediate phase plays an important role in healing crystal cracking by allowing the interface to remain coherent so that Li ions can diffuse back into regions depleted during delithiation. The mechanism of the lithiation/delithiation from and to the surface will be discussed based on the observation results.

Lithium lanthanum titanate (LLTO) is expected to apply for an electrolyte in the all-solid-state Li-ion battery because of its high Li-ion conductivity in the bulk. However, it has been reported that Li-ion conductivity is strongly suppressed at the grain boundaries (GBs) in polycrystalline materials. It is therefore needed to understand the atomistic mechanism of the reduction of Li-ion conductivity at individual GBs in order to design suitable LLTO polycrystals. In this study, two different GBs, Σ5 and Σ13, were prepared by fabricating the bicrystals, and their structures were observed by Cs corrected STEM and atomic force microscopy (AFM). Then, the charge states, Li-ion conductivities, atomic and electronic structures at the respective Σ5 and Σ13 GBs of LLTO were systematically and quantitatively investigated. It was found that the Σ5 GB has no significant influence on Li-ion conductivity, but the Σ13 GB shows the significant reduction of the Li-ion conductivity. We further found that Σ13 GB is positively charged by the formation of large amount of oxygen vacancies at the GB. Li-ion depletion layers is considered to be formed at the Σ13 GB, which causes the significant reduction of Li-ion conductivity, to compensate such positive charge at the GB.

References

• Kobayashi, C.A.J.Fisher, T.Kato,Y.Ukyo,T.Hirayama and Y.Ikuhara, Nano Lett. 16, 5409 (2016).

• Kobayashi, A. Kuwabara, C. A.J. Fisher, Y. Ukyo and Y. Ikuhara,Nat. Commun., 9, 2863 (2018)

• Sasano, R. Ishikawa, I. Sugiyama, T. Higashi, T. Kimura, Y. H. Ikuhara, N. Shibata and Y. Ikuhara, Applied Physics Express, 10, 061102 (2017)

• Sasano, R. Ishikawa, K. Kawahara, T. Kimura, Y. H. Ikuhara, N. Shibata and Y. Ikuhara, Applied Physics Letters, 116, 043901 (2020)

Sodium (Na)-ion batteries are among the most promising devices to supersede the present Li-ion batteries in heavy-duty and smart grid applications. This is mostly encouraged by the lower cost of N-metal and its worldwide availability[1], as well as the possibility of replacing expensive copper current collectors by aluminium ones. In the recent years, many electrode materials have been discovered and designed to enhance the energy density of Na-ion batteries. Among those electrode materials, the Natrium Superionic CONductor (NaSICON)[2] prototype structure are known for their wide range of electrochemical potentials, high ionic conductivities, and most importantly their structural and thermal stabilities [3–5].

In this talk we will explored the chemical space of NaSICON structured materials, Na_xMM'(PO₄)₃ where M, M'(Ti, V, Cr, Mn, Fe, Co & Ni), using ab initio density functional theory (DFT) and thermodynamic approaches [6]. We have analysed the thermodynamic stabilities, Na⁺ intercalation voltages and corresponding redox pairs for 28 combinations of M & M' and the full Na concentration range (i.e., 1 ≤ x ≤ 4). This study enabled us to pinpoint anomalies in exiting experimental reports as well as to identify new NaSICON compositions that can enable the extractions of three Na ions (e.g., Na_xMn₂(PO₄)₃ and Na_xVCo(PO₄)₃, etc). Furthermore, the calculated quaternary phase diagrams Na-Mn-P-O could show the stability of Na_xMn₂(PO₄)_3, while analogues phase diagrams, i.e., Na-Ni-P-O and Na-Co-P-O explained the origin of the suspected instability of Ni and Co-based NaSICONs. We realized that the presence of Jahn-Teller active transition metals ions enhances the voltages but limits the practicality of entire chemical space for cathodic applications.

[1] D. Larcher and J. M. Tarascon, Nat. Chem., 2015, 7, 19.

[2] J. B. Goodenough , H. Y. P. Hong and J. A. Kafalas, Mater. Res. Bull., 1976, 11 , 203

[3] W. Zhou , L. Xue , X. Lü , H. Gao , Y. Li , S. Xin , G. Fu , Z. Cui , Y. Zhu and J. B. Goodenough , Nano Lett., 2016, 16 , 7836

[4] J. Wang , Y. Wang , D. Seo , T. Shi , S. Chen , Y. Tian , H. Kim and G. Ceder , Adv. Energy Mater., 2020, 10 , 1903968

[5] Z. Deng , G. Sai Gautam , S. K. Kolli , J.-N. Chotard , A. K. Cheetham , C. Masquelier and P. Canepa , Chem. Mater., 2020, 32 , 7908

[6] Baltej Singh, Z. Wang, S. Park, G. S. Gautam, J. N. Chotard, L. Croguennec, D. Carlier, A. K. Cheetham, C. Masquelier and P. Canepa. J. Mater. Chem. A, 9, 281-292 (2021)

Vanadium phosphate positive electrode materials attract great interest in the field of Alkali-ion (Li, Na and K-ion) batteries due to their ability to store several electrons per transition metal. These multi-electron reactions (from V²⁺ to V⁵⁺) combined with the high voltage of corresponding redox couples (e.g., 4.0 V vs. Na⁺/Na for V³⁺/V⁴⁺ in Na₃V₂(PO₄)₂F₃) could allow the achievement of the 1 kWh/kg milestone at the positive electrode level in Alkali-ion batteries. However, a massive divergence in the voltage reported for the V³⁺/V⁴⁺ and V⁴⁺/V⁵⁺ redox couples as a function of crystal structure is noticed. Moreover, vanadium phosphates that operate at high V³⁺/V⁴⁺ voltages are usually unable to reversibly exchange several electrons in a narrow enough voltage range.

During this talk, through the review of redox mechanisms and structural evolutions occuring upon electrochemical operation of selected widely studied materials, we will identify the crystallographic origin of this trend: the distribution of PO₄ groups around vanadium octahedra, that allows or prevents the formation of the vanadyl distortion (O . . . V⁴⁺=O or O . . . V⁵⁺=O).^1-4 While the vanadyl entity massively lowers the voltage of the V³⁺/V⁴⁺ and V⁴⁺/V⁵⁺ couples, it considerably improves the reversibility of these redox reactions. Therefore, anionic substitutions, mainly O^2- by F^-, have been identified as a strategy allowing for combining the beneficial effect of the vanadyl distortion on the reversibility with the high voltage of vanadium redox couples in fluorine rich environments.

Acknowledgements

This study is performed in the frame of the French RS2E and European Alistore-ERI networks on battery research. The authors thank Région Nouvelle Aquitaine, the French National Research Agency (STORE-EX Labex Project ANR-10-LABX-76-01) and the European Union's Horizon 2020 research and innovation program under grant agreement No

875629 (NAIMA) for their financial support.

References

1. Boivin, E.; Chotard, J.-N.; Masquelier, C.; Croguennec, L., Towards Reversible High-Voltage Multi-Electron Reactions in Alkali-Ion Batteries Using Vanadium Phosphate Positive Electrode Materials. Molecules2021, 26, 1428.

2. Boivin, E.; Iadecola, A.; Fauth, F.; Chotard, J.N.; Masquelier, C.; Croguennec, L. Redox Paradox of Vanadium in Tavorite LiVPO4F_1-yO_y. Chem. Mater.2019, 31, 7367–7376.

3. Nguyen, L H.B.; Broux, T.; Sanz Camacho, P.; Denux, D.; Bourgeois, L.; Belin, S.; Iadecola, A.; Fauth, F.; Carlier, D.; Olchowka, J.; Masquelier, C.; Croguennec, L. Stability in water and electrochemical properties of the Na₃V₂(PO₄)₂F₃ – Na₃(VO)₂(PO₄)₂F solid solution. Energy Storage Materials2019, 20, 324-334.

4. Nguyen, L.H.B.; Iadecola, A.; Belin, S.; Olchowka, J.; Masquelier, C.; Carlier, D.; Croguennec, L. A Combined Operando Synchrotron X-ray Absorption Spectroscopy and First-Principles Density Functional Theory Study to Unravel the Vanadium Redox Paradox in the Na₃V₂(PO₄)₂F₃-Na₃V₂(PO₄)₂FO₂ Compositions. J. Phys. Chem. C2020, 124, 23511–23522.

Alkali-ion intercalation compounds are commonly used cathode materials for rechargeable batteries, including Li-, Na-, and K-ion batteries. In our previous works, we developed KVPO₄F as a high voltage cathode material for K-ion batteries.[1, 2] The KVPO₄F framework has large void spaces that can accommodate facile K ion intercalation. In this present work, we investigated KVPO₄F as a host structure for versatile alkali ion (Li, Na, and K) intercalation after fully removing the K ions.[3] Interestingly, our work finds that the voltage for Na insertion is even higher than that for Li insertion in K_xVPO₄F (x~0) host structure, in contrast to the common belief that Li intercalation voltage is always higher than Na intercalation. The lower Li intercalation voltage is likely attributed to unstable Li site in a large cavity in K_xVPO₄F (x~0), making less stable discharged product upon Li insertion vs. Na insertion. In addition, we discovered that Li intercalation is much more sluggish than Na and K intercalation in K_xVPO₄F (x~0). Since Li ion is too small compared to the cavity in the K_xVPO₄F (x~0) structure, Li ions are under-coordinated in the transition state. Therefore, Li ion migration barrier becomes much higher than Na and K migration. This finding indicates that large cavity size (or channel size) is not always good for fast alkali ion migration and we need to finely tune the cavity size suitable for each intercalating ion species.

References

[1] H. Kim et al. A New Strategy for High‐Voltage Cathodes for K‐Ion Batteries: Stoichiometric KVPO₄F. Adv. Energy Mater. 8, 1801591 (2018)

[2] H. Kim et al. Origin of Capacity Degradation of High-Voltage KVPO4F Cathode. J. Electrochem. Soc. 167, 110555 (2020)

[3] H. Kim et al. Investigation of Alkali‐Ion (Li, Na, and K) Intercalation in K_xVPO₄F (x ∼ 0) Cathode. Adv. Funct. Mater. 29, 1902392 (2019)

Recently, alkali metal hydrate melts, which are molten hydrated salts at room temperature, have attracted attention as electrolytes realizing high-voltage aqueous batteries.¹ K⁺ ion has a smaller Stokes radius than that of Li⁺ ion due to weaker interaction with water molecules. Thus, K hydrate melt electrolytes demonstrate higher ionic conductivity than the Li ones.¹ Our group and Ko et al. reported a new K hydrate melt mixing two anions, bis(fluorosulfonyl)amide (FSA⁻) and trifluoromethanesulfonate (OTf⁻).^2,3 However, electrochemical performance and reaction mechanism of K⁺ ion insertion materials in the hydrate melt is not fully understood, and the demonstration of high-voltage aqueous KIBs is still challenging. In this study, we investigate electrochemical performance of potential K⁺ insertion materials, such as Prussian blue analogues and organic compounds, for high-voltage aqueous K-ion battery (KIB).

A series of K(FSA)_1-x(OTf)_x·nH₂O were prepared at room temperature to check formation of hydrate melt. The electrochemical stability windows were examined by linear sweep voltammetry (LSV). Pt and Al foils were used as working electrodes to evaluate anodic and cathodic stability, respectively. Galvanostatic charge/discharge tests of K₂Fe_0.5Mn_0.5[Fe(CN)₆] and 3,4,9,10-perylenetetracarboxylicdiimide (PTCDI), which is a potential positive electrode and negative electrode, respectively, for aqueous KIBs,⁴ were conducted with three-electrode cells assembled with activated carbon as a counter electrode. An aqueous K-ion full cell was fabricated by combining the positive electrode and negative electrode in the weight ratio of 1:2.1.

Among the prepared K(FSA)_1-x(OTf)_x·nH₂O solution, K salts monohydrate melt of K(FSA)_0.6(OTf)_0.4·1.0H₂O contains the least H₂O content. Figure 1a displays the LSV curves in the electrolytes of K(FSA)_0.6(OTf)_0.4·1.0H₂O and the endmembers of KFSA·1.8H₂O and KOTf·2.8H₂O. KFSA·1.8H₂O and KOTf·2.8H₂O had a potential window of 2.67 V and 2.01 V, respectively. On the other hand, K(FSA)_0.6(OTf)_0.4·1.0H₂O exhibited that of 2.89 V, which was much wider than KFSA·1.8H₂O and KOTf·2.8H₂O. In KOTf·2.8H₂O, K₂Fe_0.5Mn_0.5[Fe(CN)₆] electrode delivered a low initial capacity of 53 mAh g^-1 at the initial cycle and capacity degradation over 50 cycles (Fig. 1b, top). In contrast, the electrode delivered a higher initial capacity of 124 mAh g^-1 in K(FSA)_0.6(OTf)_0.4·1.0H₂O and better cycle performance (Fig. 1b, bottom) than that in KOTf·2.8H₂O. Similar to the positive electrode, the PTCDI electrode exhibited continuous capacity degradation in KOTf·2.8H₂O (Fig. 1c, top), whereas the electrode delivered an excellent cycle performance in K(FSA)_0.6(OTf)_0.4·1.0H₂O over 200 cycles (Fig. 1c, bottom). Figure 1d compares charge/discharge curve of PTCDI//K₂Fe_0.5Mn_0.5[Fe(CN)₆] full cells filled with KOTf·2.8H₂O and K(FSA)_0.6(OTf)_0.4·1.0H₂O. The K(FSA)_0.6(OTf)_0.4·1.0H₂O cell demonstrated a much better cycle performance over 50 cycles than that in KOTf·2.8H₂O. The PTCDI//K₂Fe_0.5Mn_0.5[Fe(CN)₆] full cell exhibited average voltage of 1.5 V. Moreover, K(FSA)_0.6(OTf)_0.4·1.0H₂O electrolyte realized 2 V-class aqueous KIBs by selecting a proper negative electrode material. In addition, we will discuss charge/discharge mechanism including insertion species of K₂Fe_0.5Mn_0.5[Fe(CN)₆] and PTCDI.

References:

[1] Q. Zheng, A. Yamada, et al., Angew. Chem., Int. Ed., 58, 14202 (2019) .

[2] R.Takahashi, S. Komaba, et al., ECSJ Spring Meet.,3I19(2020).

[3] S. Ko, A. Yamada, et al., Electrochem. Commun., 116, 1067645(2020) .

[4] L. Jiang, Y. Hu, et al., Nat. Energy, 4, 495 (2019) .

Figure 1

State-of-the-art batteries for electric vehicles are limited to specific energies below 300 Wh kg^-1, shortcoming the high requirements for extended EV driving ranges [1]. Due to their high specific capacity and operative voltage, Li-rich layered oxide (LRLO) positive electrode materials are expected to enable very high specific energies, beyond 350 Wh kg^-1 at the electrode stack level even without cobalt being present in their formulation [2]. More specifically, Li_1.2Ni_0.2Mn_0.6O₂ (LRNM) offers a large specific capacity (250 mAh g^-1) and therefore, higher energy density than any other existing Co-free cathode material.However, its practical use is impeded by the low first cycle Coulombic efficiency, the rather low rate capability and, most critically, the pronounced capacity and voltage fading upon cycling [3]. Herein, we demonstrate as the design of the cathode-electrolyte interphase (CEI) using appropriate electrolytes strongly reduces these drawbacks enabling Li metal and Li-ion cells showing excellent long-term cycling performance due to mitigated the capacity fading and voltage decay.

References

1. X. Zeng, M. Li, D. Abd El‐Hady, W. Alsh*tari, A. S. Al‐Bogami, J. Lu and K. Amine, Adv. Energy Mater. 2019, 9, 1900161.

2. R. Schmuch, R. Wagner, G. Hörpel, T. Placke and M. Winter, Nat. Energy. 2018, 3, 267.

3. J. Zheng, S. Myeong, W. Cho, P. Yan, J. Xiao, C. Wang, J. Cho and J.-G. Zhang, Adv. Energy Mater.2017, 7, 1601284.

Ideally, the redox activity in Li-ion batteries, which is associated with storage and release of Li ions, is distributed hom*ogeneously throughout the electrode, thereby minimizing detrimental processes while maximizing battery performance. However, it appears that every electrode material displays inhom*ogeneous redox activity which is expected to play a dominant role in battery performance parameters such as cycle life, efficiency and rate performance.¹ For example, the occurrence of localized redox activity in layered electrodes can accelerate degradation reactions through local strain and/or decomposition^1-2. Further, inhom*ogeneous reactions render estimating the true state of charge challenging and can introduce history effects³ that make optimization of battery performance via a battery management system problematic. Inhom*ogeneous redox reactions in Li-ion battery electrodes can occur on different length scales, starting at the nm length in individual electrode particles progressing up to the dimensions of complete composite electrodes (tens of μm) and can have various origins. Most directly, inhom*ogeneous reactions in electrodes stem from charge transport limitations, generally determined by the electrode morphology (including electrode thickness, porosity and tortuosity).^4-6 A second origin for inhom*ogeneous reactions can be a difference in insertion potential, through mixing electrode materials⁵ or due to a distribution of nanoparticle sizes.⁷ Lastly, the nature of the phase transition can also give rise to inhom*ogeneous reactions. For example, the particle-by-particle transformation mechanism in LiFePO₄ has been associated with a hysteresis effect⁸ and has been suggested to give rise to a memory effect that can influence the observed potential.³ Despite the above, limited research has been performed on this subject to date and many questions remain, mainly due to the difficulty to probe these heterogeneities in realistic battery geometries and during cycling conditions.

Here we present a study of the phase transformation of ten's of individual NMC crystallites under operando cycling conditions in pouch cells using microbeam diffraction (ID11, ESRF, France). Due to the small, bright and sub-micron sized synchrotron beam diffraction rings observed with powder diffraction fall apart in individual spots each representing individual NMC crystallites in the positive electrode. In this way the phase transition behaviour through time of ten's of individual NMC crystallites is monitored at the same time while varying the electrochemical conditions.^9,10 Figure 1 shows snapshots of the (108) and (110) reflection of individual NMC111 crystallites during a full C/4 charge-discharge cycle, demonstrated the anticipated continuous shift. For each individual crystallite the evolution of the lattice parameters provide insight in the distribution of the composition and local potential of each crystallite, as well as the individual transformation rate. In addition the average transformation rate can be translated in the active particle fraction and average local current density. By studying this for the series NMC111, NMC622 and NMC811, we gain insight in the heterogeneous redox activity and the impact of composition, relevant for both rate performance and cycle life of this class of intercalation cathodes.

Figure 1. Following the phase transformation of single NCM111 crystallites through the (108) and (110) reflection during C/4 charge and discharge.

1. Zhang et al. Journal of Materials Chemistry A 2019,7 (41), 23628-23661.

2. Lin et al. Nature Communications 2014,5 (1), 3529.

3. Sasaki et al. Nature Materials 2013,12 (6), 569-575.

4. Strobridge et al. Chemistry of Materials 2015,27 (7), 2374-2386.

5. Sasaki et al. Advanced Science 2015,2 (7), 1500083.

6. Zhang et al. Advanced Energy Materials 2015,15, 1500498.

7. Van der Ven et al. Electrochemistry Communications 2009,11 (4), 881-884.

8. Dreyer et al. Nature Materials 2010,9 (5), 448-453.

9. van Hulzen et al. Font. Energy Res. 2018,6 (59). 5.

10. Zhang et al. Nature Commun. 2015,6

Figure 1

New positive electrode materials operating at higher voltage (such as LiNi_0.5Mn_1.5O₄ - LNMO), hold great promise for the next generation of high energy lithium-ion batteries (LIB), but show a rapid degradation of their performance upon cycling, which limits their immediate development. One major problem associated to the high operational voltage of such electrode materials, is the pronounced oxidation of standard electrolytes used in LIBs and the concomitant dissolution of the LNMO material and the possible impact on the negative electrode (cross-talking). Through the development of a set of operando diagnostic techniques, this work aims at establishing during the charge/discharge cycles the correlation between the structural changes in LNMO, the interfacial processes (electrolyte oxidation, formation of Cathode Electrolyte Interphase: CEI, LNMO transition metal dissolution) and the cross-talk process depending on the electrodes/separator assembly.

We will introduce in this presentation our recent developments on operando SHINERS¹ (Shell-Isolated Nanoparticles-Enhanced Raman Spectroscopy) to track the dynamic of the interfacial processes² at LNMO electrodes³. We will also present how temporally and spatially resolved confocal fluorescence and X-Ray spectroscopy measurements implemented operando on the edge of the electrode assembly can be used to quantify the LNMO dissolution upon cycling. The electrolyte composition and the role of electrolyte additives to stabilize the high-voltage cathode / electrolyte interface will be discussed.

1. Li, J.F., Huang, Y.F., Ding, Y., Yang, Z.L., Li, S.B., Zhou, X.S., Fan, F.R., Zhang, W., Zhou, Z.Y., Wu, D.Y., et al. (2010). Shell-isolated nanoparticle-enhanced Raman spectroscopy. Nature 464, 392–395.

2. Gajan, A., Lecourt, C., Torres Bautista, B.E., Fillaud, L., Demeaux, J., and Lucas, I.T. (2021). Solid Electrolyte Interphase Instability in Operating Lithium-Ion Batteries Unraveled by Enhanced-Raman Spectroscopy. ACS Energy Lett., 1757–1763.

3. Dumaz, P., Rossignol, C., Mantoux, A., Sergent, N., and Bouchet, R. (2020). Kinetics analysis of the electro-catalyzed degradation of high potential LiNi0,5Mn1,5O4 active materials. Journal of Power Sources 469, 228337.

Figure 1

We have developed a family of vanadium ferrite (VFe₂Ox) aerogels that exhibit promising performance for Li-ion storage [1]. These materials are synthesized by a simple epoxide-driven sol–gel route that yields aerogels with high surface area (200–300 m² g^–1) and no discernible crystallinity by powder X-ray diffraction. Subsequent calcination at 300°C is required to decompose residual organics from the synthesis, but even after such thermal treatment, the VFe₂Ox aerogels retain their disordered nature. Because of their high surface area and lack of defined crystal structure, VFe₂O_x aerogels exhibit the electrochemical characteristics of pseudocapacitive materials―broad voltammetric envelopes and sloping discharge curves; fast charge–discharge response; and low charge–discharge voltage hysteresis―but with capacity levels commensurate with many battery-type materials. We find that substituting ~10 at% of the V sites with nominally electroinactive Al³⁺ enhances specific capacity by >25% from 100 mAh g^–1 for VFe₂Ox to 128 mAh g^–1 for Al-VFe₂Ox, while also improving capacity retention at high current density. X-ray photoelectron spectroscopy (XPS) shows that incorporation of Al³⁺ induces the formation of V⁴⁺ sites that likely increase local electronic conductivity in Al-VFe₂Ox. Ex-situ XPS analysis of materials electrochemically conditioned to various charge–discharge states reveals distinct differences in the utilization of V and Fe redox sites for VFe₂Ox versus Al-VFe₂Ox. First principles modeling shows the effect of Al-doping on V and Fe oxidation as a function of dopant concentration and site location within the amorphous material. Lessons learned from this paper will inform future studies on rational substituents and targeted defective compositions for high-rate lithium-ion charge storage.

1. Chervin, Christopher N., et al. "Defective by design: vanadium-substituted iron oxide nanoarchitectures as cation-insertion hosts for electrochemical charge storage." Journal of Materials Chemistry A22 (2015): 12059-12068.

2. Chervin, Christopher N., et al. "Enhancing Li-ion capacity and rate capability in cation-defective vanadium ferrite aerogels via aluminum substitution." RSC Adv., under review, March 2021.

The Li−Ni−O phase diagram contains a variety of compounds, most of which are electrochemically active in Li-ion batteries [1-2]. LiNiO₂ (LNO) is particularly well-known due to its behavior as cathode active material, potentially excellent but in practice plagued by several instability issues, which ultimately led to elemental substitutions of Ni with Co, Mn and Al towards NCM and NCA materials. A significant amount of understanding on LNO and on the elemental substitution it can allow were developed in the group of Claude Delmas [3]. In this contribution, I will briefly review the Li−Ni−O phase diagram, its history and the special role LNO occupies in it, with a focus on its synthesis, structure and decomposition [4-6]. Additionally, Li-rich phases of composition nearing Li₂NiO₃ will be discussed [7-9]. I will report a facile solid-state method to prepare Li₂NiO₃ and other Li-rich Ni oxides of composition Li_1+xNi_1−xO₂ (0 ≤ x ≤ 0.33) [10]. I will also detail their crystal and electronic structure, exhibiting a highly oxidized Ni state and defects of various nature (Li−Ni disorder, stacking faults, oxygen vacancies). Moreover, the use of Li₂NiO₃ as a cathode active material has been investigated. It shows remarkably high specific capacity, which however fades quickly. While we demonstrate that the initial capacity is due to irreversible O₂ release, such process stops quickly in favor of more classical reversible redox mechanisms that allow cycling the material for >100 cycles. After the severe oxygen loss (∼15−20%) and prolonged cycling, the Bragg reflections of Li₂NiO₃ disappear. Analysis of the diffracted intensities suggests the resulting phase is a disordered rock salt-type material with high Li content, close to Li_0.5Ni_0.5O, never reported to date and capable of Li diffusion. Our findings demonstrate that the Li−Ni−O phase diagram has not been fully investigated yet, especially concerning the preparation of new promising materials by out-of-equilibrium methods.

Bibliography

[1] Dahn, J., Vonsacken, U., Michal, C. A., Structure and Electrochemistry of Li_1+-yNiO₂ and a New Li₂NiO₂ Phase with the Ni(OH)₂ Structure, Solid State Ionics, 1991, 44, (1-2), 87-97

[2] Goodenough, J. B.; Wickham, D. G.; Croft, W. J. Some Magnetic and Crystallographic Properties of the System Li_x⁺Ni_1−2x⁺²Ni_x⁺⁺⁺O. J.Phys. Chem. Solids 1958, 5 (1−2), 107−116.

[3] Delmas, C., Menetrier, M., Croguennec, L., Saadoune, I., Rougier, A., Pouillerie, C., Prado, G., Grune, M., Fournes, L., An overview of the Li(Ni,M)O₂ systems: syntheses, structures and properties, Electrochimica Acta, 1999, 45, (1-2), 243-253

[4] Bianchini, M.; Roca-Ayats, M.; Hartmann, P.; Brezesinski, T.; Janek, J. There and Back Again-The Journey of LiNiO₂ as a Cathode Active Material. Angew. Chem., Int. Ed. 2019, 58, 10434−10458.

[5] Delmas, C., Croguennec, L., Layered Li(Ni,M)O₂ Systems as the Cathode Material in Lithium-Ion Batteries, MRS Bullettin, 2002, 27 (8), 608-612

[6] Bianchini, M.; Fauth, F.; Hartmann, P.; Brezesinski, T.; Janek, J, An in situ structural study on the synthesis and decomposition of LiNiO₂, J. Mat. Chem. A, 2020, 8 (4), 1808-1820.

[7] Migeon, H.; Courtois, A.; Zanne, M.; Gleitzer, C. Preparation and Study of Li₂NiO_3−y (y less-than-or-equal-to 0.135). Rev. Chim. Minér. 1976, 13 (1), 1−8.

[8] Stoyanova, R.; Zhecheva, E.; Alcántara, R.; Tirado, J. L.; Bromiley, G.; Bromiley, F.; Boffa Ballaran, T. Lithium/nickel mixing in the transition metal layers of lithium nickelate: high-pressure synthesis of layered Li[Li_xNi_1−x]O₂ oxides as cathode materials for lithium-ion batteries. Solid State Ionics 2003, 161 (3), 197−204

[9] Tabuchi, M.; Kuriyama, N.; Takamori, K.; Imanari, Y.; Nakane, K. Appearance of Lithium-Excess LiNiO₂ with High Cyclability Synthesized by Thermal Decomposition Route from LiNiO₂ - Li₂NiO₃ Solid Solution. J. Electrochem. Soc. 2016, 163 (10), A2312−A2317.

[10] Bianchini M.; Schiele, A.; Schweidler S.; Sicolo, S.; Fauth, F.; Suard, E.; Indris, S.; Mazilkin, A.; Nagel, P.; Schuppler, S.; Merz, M.; Hartmann, P.; Brezesinski, T.; Janek, J.; From LiNiO₂ to Li₂NiO₃: Synthesis, Structures and Electrochemical Mechanisms in Li-Rich Nickel Oxides, Chem. Mater. 2020, 32, 9211−9227

Figure 1

In order to push further the development of next generation Li-ion batteries (LiBs), high energy density, good capacity retention and increased safety are key features that need to be taken into account when designing energy storage devices. To this date, commercial batteries for portable devices mainly use graphite as anode. However, with the advent of electric vehicles and intelligent power grids, there is an increasing demand for high capacity materials to meet the energy and power requirement of these applications. More recently, Si anode has received considerable attention because of its natural abundance and its ability to deliver high energy density, with a gravimetric capacity of 3579 mAhg^-1 which is almost 10 times that of graphite (372 mAhg^-1) [1-2].

Bearing in mind that commercial LiBS use carbonate electrolytes which are recognized to pose safety issues and have been identified to suffer from degradation during charge-discharge cycles, we explored in this study the performance of Si anode in ionic liquid electrolytes based on phosphonium and pyrrolidinium cations, systems that are currently the most studied in the field of Li batteries [3-4]. Ionic liquid electrolytes are an appealing alternative to carbonates mainly because of their low volatility and better stability.

In the recent years, good electrochemical behavior has been observed in ionic liquids with high salt concentration [3-5]. In this work, we were able to demonstrate better electrochemical performance of Si anode in ionic liquid electrolytes with 3.2 M LiFSI compared to conventional carbonate electrolytes in terms of capacity and capacity retention at 50°C. Our recent study using superconcentrated electrolytes revealed the superior electrochemical behaviour of triethyl(methyl)phosphonium bis(fluorosulfonyl)imide (P₁₂₂₂FSI) in a half-cell investigation of Si negative electrodes (Fig. 1). This result was linked to the stability of the electrolyte upon cycling and to improved lithium ion transport which is facilitated at high Li salt content [6]. Moreover, initial investigation of the SEI using NMR and XPS spectroscopy revealed that less degradation products are formed upon cycling in ionic liquid electrolytes compared to usual carbonates electrolytes. In the present work, we report the importance of tuning the Si/IL electrolyte interface as a function of IL cation nature and salt concentration. Experimental techniques such as ⁷Li, ¹⁹F MAS-NMR, SEM, and STEM-EDX were used to determine the SEI composition and morphology in order to further investigate its influence on the electrochemical performance as well as the relationship of the electrolyte chemistry (P₁₂₂₂FSI, N-methyl-N-propylpyrrolidinium (C_3mpyr) FSI-based electrolytes and carbonate-based electrolyte) to the SEI properties. In addition, computational methods and differential capacitance (DC) experiments using AC impedance were applied to gain a better understanding of the SEI formation by examining the structuring at the electrode/electrolyte interphase.

References:

1. Lestriez, B., Jean, M. & Moreau, P. A low-cost and high performance ball-milled Si-based negative electrode for high-energy Li-ion batteries Energy &. (2013). doi:10.1039/C3EE41318G

2. Karkar, Z. et al. Electrochimica Acta Threshold-like dependence of silicon-based electrode performance on active mass loading and nature of carbon conductive additive. Electrochim. Acta215, 276–288 (2016).

3. Girard, G. M. A. et al. Electrochemical and physicochemical properties of small phosphonium cation ionic liquid electrolytes with high lithium salt content . 111, 8706–8713 (2015).

4. Kerr, R. et al. High-Capacity Retention of Si Anodes Using a Mixed Lithium/Phosphonium Bis( fl uorosulfonyl)imide Ionic Liquid Electrolyte. (2017). doi:10.1021/acsenergylett.7b00403

5. Forsyth, M. et al. Electrochimica Acta Inorganic-Organic Ionic Liquid Electrolytes Enabling High Energy-Density Metal Electrodes for Energy Storage. Electrochim. Acta220, 609–617 (2016).

6. K. Araño, et al. Understanding the Superior Cycling Performance of Si Anode in Highly Concentrated Phosphonium -based IonicLiquid Electrolyte. J. Electrochem. Soc 167 (2020) 120520

Figure 1

Since the inception of the 1st open cell nanobattery study, significant progress has been made in the field of in situ nano electrochemistry. Many new techniques are emerging, which include combining transmission electron microscopy ― scanning probe microscopy (TEM-SPM) with environmental TEM (ETEM) to enable metal ― gas battery studies; combining TEM-SPM with microelectromechanical system (MEMS) heating devices to enable high temperature battery studies; using large diameter carbon nanotuble as an electrochemical reaction cell to enable in situ liquid cell studies. Our latest results on Li/Na-O₂/CO₂ batteries, Li/Na-S/Se batteries as well as quantitative measurements of the mechanical properties of Li and Na dendrites will be highlighted [1]. Fig. 1 shows an example of measuring the mechanical property of lithium whiskers. Progress on the in situ studies of solid state batteries will also be presented.

1. Liqiang Zhang, Yongfu Tang, Jianyu Huang et al., Science 330, (2010) 1515; Nature Nano 15, (2020) 94; ACS Nano 14, (2020) 13232; Nano Lett. 18, (2018) 3723; Eng. Env. Sci. 4 (2011) 3844; 14(2021) 602; Adv. Mater. 1900608, 2019; ACS Energy Lett. 5, (2020) 2546; Materials Today 42, (2021) 137.

Fig. 1 In-situ ETEM-AFM characterization of stress generation during Li dendrite growth. (a) Schematic of the ETEM-AFM setup used for imaging and measurement of Li dendrite growth. An arc-discharged CNT was attached to a conducting AFM, and this assembly was used as cathode; the scratched Li metal on the top of a sharp tungsten needle was used as anode, and the naturally formed Li₂O on the Li surface as a solid electrolyte. (b) A TEM image showing an AFM cantilever was approaching the counter electrode of Li metal. (c) A TEM image showing a CNT was attached to a flattened AFM tip. (d) Time-lapse TEM images of the Li dendrite growth. A nano-sized Li ball nucleated from the CNT, Li₂O and gas triple point (1863 s). As the Li ball grew to about 1.26 μm in size, the dendrite emerged underneath the ball (2028 s) which pushed the AFM cantilever up. When the dendrite reached 4.08 mm in length, it collapsed (2177 s) due to the axial compression by the AFM tip. The blue dotted line indicates a fixed reference position, and the red arrow indicates the upward displacement of the AFM tip.

Figure 1

Tetrel (Si, Ge, Sn) clathrates are host-guest structures that display novel electrochemical reactions with Li and Na due to their unique cage structure. One area of interest for Tetrel clathrates is the possibility of topotactic Li/Na insertion into the clathrate framework due to the open cage structure. The type II clathrate Na_24-x(Si,Ge)₁₃₆ is unique because the structure can be obtained without guest atoms in the clathrate framework. Figure 1 shows a crystal model schematic of the two unique polyhedral cages that comprise the type II Si clathrate structure: the dodecahedra (Si₂₀, grey) and the hexakaidecahedra (Si₂₈, blue). The structure usually forms with Na guest atoms (yellow) in the center of these cages, but when heated (300 – 500 °C ) under vacuum, the Na atoms diffuse and evaporate out of the structure resulting in a guest free clathrate, Si₁₃₆.¹ Once Na is removed the structure, Li intercalation into the vacant cages becomes possible.²

During the lithiation of the guest-free type II Si clathrate (Na₁Si₁₃₆), there is plateau at 0.30 V vs Li/Li⁺ which corresponds to Li intercalation into the vacant cages of the clathrate structure.^2,3 With synchrotron powder X-ray diffraction, we confirm that Li intercalation is occurring in a topotactic fashion into the framework and identify the Li positions in the two types of cages. In the Si₂₀ cage, Li is found near the center of the cages with a slightly split position, while Li in the Si₂₈ cage is found very off-center, coordinated off the hexagonal faces. Density functional theory (DFT) calculations corroborate the Li positions in both cages. Next, we demonstrate that by using a voltage cutoff of 0.26 V, reversible Li insertion is possible as evidenced by the reproducible voltage profile. Due to the very low volume expansion (0.23 %) and reasonable capacity and voltage (230 mAh/g at 0.30 V), the type II Si clathrate has possible applications as an anode for Li-ion rechargeable batteries.

In contrast to the room temperature intercalation of Li, we find that electrochemical deintercalation of Na from the type II structure requires high temperatures (350-450 °C) and overpotentials to achieve. We demonstrate electrochemical desodiation using a novel high temperature approach using a Na beta-alumina solid electrolyte.⁴ By using DFT to calculate the migration barriers for Na and Li, we demonstrate that the large differences between Li and Na intercalation originates from the much higher migration barriers for Na (1.0 - 2.0 eV) in the type II structure when compared to Li (0.2 eV). Based on these results, design rules for intercalation into Tetrel frameworks will be discussed and other promising open Tetrel frameworks highlighted.

Figure 1: Crystal model schematic of the two types of polyhedral in the type II Si clathrate structure: the dodecahedra (grey) and the hexakaidecahedra (blue). Si atoms are orange and Na atoms are yellow.

References

(1) Krishna, L.; Baranowski, L. L.; Martinez, A. D.; Koh, C. A.; Taylor, P. C.; Tamboli, A. C.; Toberer, E. S. Efficient Route to Phase Selective Synthesis of Type II Silicon Clathrates with Low Sodium Occupancy. CrystEngComm2014, 16, 3940–3949.

(2) Langer, T.; Dupke, S.; Trill, H.; Passerini, S.; Eckert, H.; Pöttgen, R.; Winter, M. Electrochemical Lithiation of Silicon Clathrate-II. J. Electrochem. Soc.2012, 159, A1318–A1322.

(3) Dopilka, A.; Weller, J. M.; Ovchinnikov, A.; Childs, A.; Bobev, S.; Peng, X.; Chan, C. K. Structural Origin of Reversible Li Insertion in Guest‐Free, Type II Silicon Clathrates. Adv. Energy Sustain. Res.2021, 2000114.

(4) Dopilka, A.; Childs, A.; Bobev, S.; Chan, C. K. Solid-State Electrochemical Synthesis of Silicon Clathrates Using a Sodium-Sulfur Battery Inspired Approach. J. Electrochem. Soc.2021, 168, 020516.

Figure 1

Hard carbons are considered as preferred negative electrodes in the development of sodium- and potassium-ion batteries because of low working potential, high capacity, as well as good cycling stability. A further advantage is that they can be made from renewable precursors, in this case lignin. The structure changes of the precursors during synthesis of the hard carbons have been extensively studied^1–3. The alkali ions insertion mechanism broadly accepted by the community is the insertion into the carbonaceous structure in the slope region, and a pore filling mechanism in the plateau region of the galvanostatic profile. The diffusion of sodium ions and potassium ions into hard carbon electrodes has been investigated using Galvanostatic Intermittent Titration Technique (GITT) combined with analytical models^4,5. However, a physics-based model can be used instead in order to analyze the experimental data and investigate potassium- and sodium ion transport into hard carbon electrodes in greater details.

In this work a physics-based single particle model combined with GITT experimental data has been used to investigate the effects of carbonization temperature and state of charge on potassium ions diffusion into an electrode made from pure lignin based hard carbon fibers. Electrospun fiber mat electrodes carbonized in the temperature range from 800 °C to 1700 °C have been investigated. The parameters extracted from the physics-based model have been used to shed further light on the insertion mechanism. Finally, the study was extended to sodium ions to investigate the impact of various alkali ions.

The results show that diffusion coefficients as well as electrochemical rate constants depend of carbonization temperature and state of charge. The physics-based model cannot reproduce the experimental data well by only considering diffusion, and a second time constant needs to be considered in the model. The coupling of parameters and open circuit voltage (OCV) curves extracted from the GITT experimental data supports that the slope region refers to the insertion into the carbonaceous structure and the plateau region refers to the pore filling mechanism.

References

1. Morikawa, Y., Nishimura, S., Hashimoto, R., Ohnuma, M. & Yamada, A. Mechanism of Sodium Storage in Hard Carbon: An X‐Ray Scattering Analysis. Adv. Energy Mater.10, 1903176 (2020).

2. Kubota, K. et al. Structural Analysis of Sucrose-Derived Hard Carbon and Correlation with the Electrochemical Properties for Lithium , Sodium , and Potassium Insertion Structural Analysis of Sucrose-Derived Hard Carbon and Correlation with the Electrochemical Properties fo. (2020) doi:10.1021/acs.chemmater.9b05235.

3. Buiel, E. R., George, A. E. & Dahn, J. R. Model of micropore closure in hard carbon prepared from sucrose. Carbon N. Y.37, 1399–1407 (1999).

4. Jian, Z., Xing, Z., Bommier, C., Li, Z. & Ji, X. Hard Carbon Microspheres: Potassium-Ion Anode Versus Sodium-Ion Anode. Adv. Energy Mater.6, 1501874 (2016).

5. Li, Y., Hu, Y. S., Titirici, M. M., Chen, L. & Huang, X. Hard Carbon Microtubes Made from Renewable Cotton as High-Performance Anode Material for Sodium-Ion Batteries. Adv. Energy Mater.6, 1–9 (2016).

Figure 1

As a possible post lithium-ion battery technology, metal-air batteries have revived interest recently. Among the different types of metal-air batteries, rechargeable zinc-air battery (ZAB) is a promising electrochemical energy storage device with the advantages of potential low cost, high safety, environmental friendliness and high energy density. However, current ZAB still suffer from the strongly alkaline electrolyte, which is chemically unstable towards the active cathode material (ambient air) and to a large extent causes electrochemical irreversibility at the Zn metal anode. Zn metal in alkaline environment suffers from formation of high surface area dendrites, well known from Li metal anodes, non-uniform electrodeposition/-dissolution, and persistent corrosion that consumes electrolyte. On the cathode side, the reaction between alkaline electrolytes and CO₂ in air produces insoluble carbonate salts, which irreversibly consumes electrolyte, and also physically clogs and chemically deactivates the porous air cathode. Since the redox reaction of O₂ occurs via a sluggish 4e^-(electron) oxygen reduction reaction (ORR) in conventional alkaline electrolytes, bi-functional catalysts have to be used on the cathode.

Herein, specific attention is given to the obstacles caused by the conventional alkaline electrolytes with the focus on the fundamental understanding in battery chemistry. Recently, we demonstrated a previous-unknown reversible zinc peroxide (ZnO₂)/O₂ chemistry for the rechargeable ZAB.[1] By comparing the hydrophobicity of anions of Zn salts, we selected the hydrophobic trifluoromethanesulfonate (OTf^-) anion with a large size as a constituent of the electrolyte solute. Comprehensive characterization and simulations identified the critical role of hydrophobic OTf^- anions in dictating the electrochemical double layer structure that favors the formation of ZnO₂ and suppression of H₂O-involved reactions. Leveraging the high reversibility of both air cathode and Zn metal anode in the Zn(OTf)₂ electrolyte, the Zn-air full cell demonstrated excellent cycling performance in ambient air despite a simple cell structure. Such tailoring of interfacial structures via electrolyte properties provides a solution to the electrochemical irreversibility that has been plaguing not only alkaline ZABs, but essentially all metal-air batteries for centuries, especially those with promising high theoretical energy densities using materials with abundance, but being only feasible in alkaline electrolytes as either primary or mechanically rechargeable batteries.

Keywords: Energy Storage, Zn-air batteries, Electrolytes, Battery Chemistry

References

[1] Wei Sun, Fei Wang, Bao Zhang, Mengyi Zhang, Verena Küpers, Xiao Ji, Claudia Theile, Peter Bieker, Kang Xu, Chunsheng Wang, Martin Winter, A rechargeable zinc-air battery based on zinc peroxide chemistry, Science 371, (2021) 46-51.

The development of new energy storage technologies is essential for the advancement of a society increasingly dependent on energy supplies. In this context, sodium-ion batteries (SIBs) have proven to be the key to revolutionizing the global energy market, especially for large-scale stationary energy storage and light electromobility [1]. However, the achievement of the full potential of SIBs technology remains a challenge and efforts should be focused on the design of more efficient high energy density and high cycle life sodium-based positive electrodes. In this scenario, sodium-based layered oxide cathodes with general formula Na_xMO₂ (M = transition metal/s) are an excellent choice due to their electrochemical performance, environmental friendliness and scalability [2,3]. Prof. Claude Delmas pioneered the study of the electrochemical intercalation processes of this family of compounds, being one of the first researchers to exploit their potential in SIBs. Although the virtues of this family of materials as cathodes are clear, there are also several drawbacks such as irreversible structural phase transitions, strong Na⁺-vacancy ordering tendencies or poor cyclability that should be addressed to ensure the optimum operation of this material in a commercial battery.

One of the peculiarities of these cathode materials lies in the diverse structures that they can adopt, being the P2 and O3 structures the most interesting from the electrochemical point of view. P2-type phases present higher rate performance and capacity retention than O3-type phases. However, they are only stable with sodium contents ≤ 0.85, which translates into higher first irreversible capacity. In O3-type phases, on the other hand, it is possible to reach the fully sodiated stoichiometry, delivering thus higher capacities but at the expense of considerable retention nested in O3-P3 phase transitions, which alter the diffusion mechanism of Na⁺ ions giving rise to a large energy barrier that Na⁺ ions must overcome.

This talk will present the main advances on the development of sodium layered oxide cathodes, as well as the most successful strategies for overcoming current challenges, such as doping with electrochemically active and inactive elements [4], surface coatings [5], the use of sacrificial salts [6] or the synergetic P2/O3 combination effect [7]. Finally, the anionic redox effect and the controversy in the explanation of the extra capacity observed in certain de-sodiated transition metal oxides will also be discussed.

1. Goikolea, T. Rojo, et al. Adv. Energy Mater.2020, 10, 2002055

2. H. Han, T. Rojo, et al., Energy Environ. Sci.2015, 8, 81–102

3. Ortiz-Vitoriano, T. Rojo, et al., Energy Environ. Sci.2017, 10, 1051–1074

4. Gonzalo, T. Rojo, et al. Enegy Storage Mater. 2021, 34, 682–707

5. Zarrabeitia, T. Rojo, et al. J. Mater. Chem. A, 2019, 7, 14169–14179

6. J. Fernández-Ropero, D. Shanmukaraj et al. ACS Appl. Mater Interfaces, 2021, 13, 11814–11821

7. Bianchini, T. Rojo, et al. J. Mater. Chem. A, 2018, 6, 3552–3559.

The recent proliferation of renewable energy generation offers mankind hope, with regard to combatting global climate change. However, reaping the full benefits of these renewable energy sources requires the ability to store and distribute any renewable energy generated in a cost-effective, safe, and sustainable manner. As such, sodium-ion batteries (NIBs) have been touted as an attractive storage technology due to their elemental abundance, promising electrochemical performance and environmentally benign nature. Moreover, new developments in sodium battery materials have enabled the adoption of high-voltage and high-capacity cathodes free of rare earth elements such as Li, Co, Ni, offering pathways for low-cost NIBs that match their lithium counterparts in energy density while serving the needs for large-scale grid energy storage.

In this presentation, metrics of energy density, cost, and lifetime are compared across various battery chemistries, where NIBs are surmised as front runners to meet the needs of the grid storage market. Fundamental obstacles toward commercialization include electrolyte composition, anode performance, electrode-electrolyte interfacial stability, safety hazards, and sustainable recyclability are analyzed, along with discussions for potential solutions to tackle them. To truly enable NIBs for grid storage, it would require the scientific community to shift development efforts beyond the academic level toward applied research, supported by investments and inputs from the industry to enable a concerted push toward practical cell/pack level testing and evaluation similar to what LIBs have achieved over the past four decades. Ultimately, today's NIBs may or may not be the perfect solution for every challenge faced by grid-scale energy storage, but it will certainly have far-reaching impacts in enabling renewable energy storage and distribution to improve our electrical grid's resilience.

Transition metal (T_M) oxides are a fascinating class of materials, whose properties can be suitably tuned in a variety of ways; such as by selecting T_M-ions/dopants having preferred electronic configurations, engineering the crystallographic site occupancy by dopants, controlling/modifying the degree of covalence of T_M-O bonds, modifying lattice spacing(s), tuning phase assemblage etc. Such modifications done from the fundamental perspectives influence the performances of T_M-oxides for a variety of applications, including their widespread usage as electrode-active materials in alkali metal-ion batteries.

In the context of the upcoming Na-ion battery system, O3-type 'layered' Na-T_M-oxides are promising as cathode-active materials due to their inherently high initial Na-content (as compared to the P2 counterparts). However, they suffer from instabilities caused due to multiple phase transformations during Na-removal/insertion and sensitivity to air/moisture. Against this backdrop, with the help of a dopant, having d⁰ electronic configuration (viz., no octahedral site preference energy), we have been able to tune the composition and structural features to suppress the phase transitions upon Na-removal/insertion and also improve the air/water-stability in significant terms; so much so that long-term cyclic stability has been achieved with health/environment-friendly 'aqueous processed' electrodes (sans, usage of toxic/hazardous/expensive chemicals like NMP and PVDF) [1]. As will be explained in more elaborate terms during the talk, the changes in structural features, which have led to such outstanding water-stability, include differential contraction/dilation of the Na-'inter-slab'/T_M-'slab' spacing and partial occupancy of the dopant at tetrahedral sites of the structure.

On the anode front, successful development of Na-ion battery system necessitates looking beyond hard carbon based anodes, which possess safety hazards due to the Na-insertion potential being a bit too close to the Na-plating potential. In this context, Na-titanates promise to be 'safe' anode materials; but suffer from cyclic instability [2]. Against this backdrop, we have developed carefully tuned bi-phase Na-titanate based electrodes, having Na₂Ti₃O₇ and Na₂Ti₆O₁₃ as the primary and secondary phases, respectively. The as-developed phase assemblage has been able to address the cyclic instability of single-phase Na-titanate, leading to long-term cyclic stability even at high current densities (up to 50C!) [3]. These are important steps towards the development of health/environment-friendly, cost-effective, safe and high-performance Na-ion batteries.

Acknowledgement to RRCAT, Indore, India, for enabling the usage of Synchrotron facility.

Parts of the concerned works have been published as (i.e., the associated publications);

1. B. S. Kumar, A. Pradeep, A. Dutta, and A. Mukhopadhyay; J. Mater. Chem. A 8 (2020) 18064

2. H. S. Bhardwaj, T. Ramireddy, A. Pradeep, M. K. Jangid, V. Srihari, H. K. Poswal, and A. Mukhopadhyay; ChemElectroChem 5[8] (2018) 1219

3. A. Pradeep, B. S. Kumar, A. Kumar, V. Srihari, H. K. Poswal, and A. Mukhopadhyay; Electrochim. Acta 362 (2020) 137122

Lithium-ion batteries currently power our world, but as the demand is being ramped up in electric vehicles (EV) and the electric grid, the reliance on scarce resources like lithium and cobalt has become an increasingly serious issue. One proposed solution is sodium-ion batteries, which benefit greatly from the natural abundance of sodium and the unique ability to utilize iron or copper redox at the cathode. However, deeper understanding of the redox mechanism and consequent electrochemical performance in Na-ion cathodes is needed, especially regarding the possibility of oxygen redox and voltage fade during cycling. Here we focus on P2- and P3-structured (Na_2/3X_1/3Mn_2/3O₂) cathodes where X can be Cu or derivatives with partial lithium substitution, to elucidate how structure and lithium substitution influence the copper redox both in initial cycle and the following cycles. Synchrotron-based x-ray spectroscopy, microscopy, and scattering techniques provide insights into the redox mechanism, chemical species distribution, and phase transitions during the cycling.

Acknowledgment: The work at Brookhaven National Laboratory was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Vehicle Technology Office of the U.S. Department of Energy through the Advanced Battery Materials Research (BMR) Program under contract DE-SC0012704. This research used resources of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. Partial support for A. Ronne was provided by an NSF NRT Award in Quantitative Analysis of Dynamic Structures (DGE 1922639) as a fellowship.

The use of alkali metal electrodes is widely considered to be an enabler for the next generation of high energy-density rechargeable batteries [1]. In all-solid-state systems, the most critical interface appears to be that between the alkali metal and the solid electrolyte, from which metal-filled cracks can initiate and grow into single-crystal, polycrystal, and glassy electrolytes alike [2] under sufficiently high electrochemical stress. However, failure can be mitigated by softening of the metal electrode, whether through increases in temperature (including melting) or changes in composition (including changing alkali metals [3]). Here, we discuss semi-solid metal electrode design approaches in which a minor liquid phase fraction is deliberately introduced to produce a self-healing function that enables high current densities [3]. A bulk semi-solid electrode approach is demonstrated using Na–K alloys with controlled liquid fraction between the state-of-charge limits; these show potassium ion critical current densities (using the a K-β''-alumina electrolyte) that exceed 15 mA cm‒2. An interfacial wetting approach uses a thin interfacial film of Na–K liquid between a Li metal electrode and an LLZTO solid electrolyte; here the critical current density is doubled, and cyclable areal capacities exceed 3.5 mAh cm‒2. Moreover, evidence from both approaches suggest that void formation in solid metal electrodes during cycling at practical current densities (>0.5 mA/cm2) [4], manifested as impedance growth at the metal-solid electrolyte interface, can be largely mitigated through these semi-solid design strategies.

Support from the US Department of Energy, Office of Basic Energy Science, through award no. DE-SC0002633 (J. Vetrano, Program Manager), is gratefully acknowledged.

[1] Albertus, P., Babinec, S., Litzelman, S. & Newman, A. Nat. Energy 3, 16–21 (2018).

[2] Porz, L. et al. Adv. Energy Mater. 7, 1701003 (2017).

[3] Park, R.J-Y. et al. Nat Energy 6, 314–322 (2021).

[4] Kasemchainan, J. et al. Nat. Mater. 18, 1105–1111 (2019).

Figure 1

Lithium excess layered cathode materials are a promising cathode candidate because the higher discharge capacity. Among them, Li₂MnO₃ has the highest Li content and understanding the reaction mechanism during cycling is critical for using the material. We fabricated the cathode films and studied the reaction mechanisms by in-situ and methods using surface X-ray diffraction and operando HAXPES analyses. Experiments revealed a structural change to a high-capacity phase that proceeded gradually with cycling. First-principles calculations suggested that the activated phase has O1 stacking. We propose a mechanism: charging to a high voltage at a low Li concentration induces an irreversible transition to a phase detrimental to cycling that could, but not necessarily, be accompanied by the dissolution of Mn and/or the release of O into the electrolyte.

The all-solid-state batteries (ASSBs) using solid electrolytes are excellent candidates for the next generation of commercial devices for energy storage. Among these, the perovskite-type family of Li-ion conducting oxides Li_3xLa_2/3−xTiO₃ (LLTO) is one of the most studied ^[1] and it is shown as the most promising option for solid electrolytes because of its high bulk conductivity (10⁻³ S·cm⁻¹) ^[2], negligible electronic conductivity ^[3] high stability, wide electrochemical window (larger than 4 V) ^[4] and easy preparation. ^[5] However, there are still many challenges to be solved: (i) the high impedance associated with grain boundaries which reduce the overall lithium ionic conductivity below 10^-5 S/cm at 298 K; (ii) and the instability of LLTO in direct contact with metal Li, ^[6],[7] or graphite electrodes.^[8],[9],[10]

In this contribution, we have focused on the substitution of B-site of LLTO perovskite, La_1/2+1/2xLi_1/2-1/2xTi_1-xAl_xO₃ (0 ≤ x ≤ 1) studying the influence of vacancy distribution on percolative phenomena. Along this series, (1/2Li⁺ + Ti⁴⁺) cations were substituted by (1/2La³⁺ +Al³⁺), preserving both the charge balance and the nominal A-site vacancies (n_A = 0). Accordingly, the main goal of this study is to confirm the influence of effective vacancies and their distribution, instead of nominal vacancies, on the Li conduction mechanism. The presence of order/segregation must influence the conductivity of these materials and by performing Broad band Electric Spectroscopy (BES), which is a powerful technique for unravelling the complexities of a polycrystalline ceramic, the analysis of the different contributions to the electric response of this complex materials can be achieved^[11],[12].

Indeed, here the aim is to study the correlation between structural features, determined by HRTEM/STEM, and relaxation phenomena characterizing the electric response of these perovskites by means of the BES. To better elucidate the interplay between structural features, conductivity and relaxation phenomena from the electric response of these materials, Density Functional Theory (DFT) and dynamic molecular modelling simulations studies are carried out.

Taking all together, it is shown that the charge migration processes occurring along the different conductivity pathways for Li⁺ long range migration phenomena are very effective when in these perovskites: a) a sufficient density of charge carriers is present; and b) in the Ti⁴⁺ based BO₆ backbone octahedra, Al³⁺ ions acts as a defect, thus enhancing the dynamics characterizing their relaxation modes. This latter effect is obviously dependent on the existence of an efficient coupling between the host medium relaxations of the inorganic network and the relaxation events characterizing the long range migration processes of lithium conductivity pathways.

In conclusion, this study permits to shed light precisely into the conduction mechanism in Li_3xLa_2/3−xTiO₃ –type perovskites.

Acknowledgements

This work has been supported by the European Union's Horizon 2020 research and innovation program under grant agreement No 829145 (FETOPEN-VIDICAT) and by the Agencia Española de Investigación /Fondo Europeo de Desarrollo Regional (FEDER/UE) for funding the projects PID2019-106662RBC43 and C44. V. Di Noto thanks the University Carlos III of Madrid for the "Cátedras de Excelencia UC3M-Santander" (Chair of Excellence UC3M-Santander).

References

[1] Z. Zhang, et al., New horizons for inorganic solid state ion conductors, Energy Environ. Sci., 11 (2018) 1945-1976.

[2] Y. Inaguma, et al. High ionic-conductivity in lithium lanthanum titanate, Solid State Commun. 86 (1993) 689-693.

[3] O. Bohnke, et al., Solid State Ionics, 91 (1996) 21 -31.

[4] O. Bohnke, et al., Solid State Ionics, 188 (2011), 144-147.

[5] R. Murugan, et al., Fast lithium ion conduction in garnet-type Li₇La₃Zr₂O₁₂ Angew. Chem. Int Ed. Engl., 46 (2007) 7778-7781.

[6] H. T. T. Le, et al., J. Mater. Chem. A, 3 (2015) 22421-22431.

[7] K. Chen, et al., Solid State Ionics, 235 (2013) 8-13.

[8] C. H. Chen, and K. Amine, Ionic conductivity, lithium insertion and extraction of lanthanum lithium titanate. Solid State Ionics, 51 (2001) 144.

[9] Y. Inaguma, et al., Lithium ion conductivity in the perovskite-type LiTaO3-SrTiO3 solid solution, Solid State Ionics 79 (1995) 91.

[10] C. W. Ban, and G. M. Choi, The effect of sintering on the grain boundary conductivity of lithium lanthanum titanates, Solid State Ionics 140 (2001) 285.

[11] V. Di Noto, et al., Broadband dielectric spectroscopy: a powerful tool for the determination of charge transfer mechanisms in ion conductors, in: P. Knauth, M.L. Di Vona (Eds.), Solid State Prot. Conduct. Prop. Appl. Fuel Cells, John Wiley & Sons, Ltd, 2012, p. 426, https://doi.org/10.1002/9781119962502.ch5.

[12] K. Vezzù et al. , J. Am. Chem. Soc., 2020, 142, 801-814 https://pubs.acs.org/doi/10.1021/jacs.9b09061.

With a growing need for clean water worldwide, interest in capacitive deionization (CDI) is growing as a potentially low energy desalination methodology. Current commercial technologies are using activated charcoal at both electrodes, but these materials suffer from low capacity and poor durability. Inspired by the recent advances in sodium-ion batteries^1,2, several studies have proposed to use intercalation materials to increase the desalination potential of electrodes^3,4. Among the proposed materials for desalination batteries, hexacyanoferrate Prussian blue materials (PBs) are especially interesting because of their open framework structure⁵ that allow insertion of Na with high reversibility and fast kinetics.

This study focused on the development of heterostructured NaM_xN_y(HCF) PBs with M and N being Ni, Mn or Fe, with an optimization for performance and durability in 1M NaCl electrolyte. Materials were synthesized using a simple, cost-effective and scalable coprecipitation synthesis process at room temperature. Different compositions and electrode architecture were tested and salt absorption above 75 mg/g of electrode at rates above 100 mg/g/min will be reported.

References

[1]. H. S. Hirsh, Y. Li, D. H. S. Tan, M. Zhang, E. Zhao, Y. S. Meng, Adv. Energy Mater., 2020, 10, 2001274. doi.org/10.1002/aenm.202001274

[2]. V. Palomares, P. Serras, I. Villaluenga, K. B. Hueso, J. Carretero-Gonzalez, T. Rojo, Energy Environ. Sci., 2012, 5, 5884. doi.org/10.1039/C2EE02781J

[3]. J. Lee, S. Kim, J. Yoon, ACS Omega, 2017, 2, 1653. doi.org/10.1021/acsomega.6b00526

[4]. P. Hu, W. Peng, B. Wang, D. Xiao, U. Ahuja, J. Rethore, K. E. Aifantis, ACS Energy Lett. 2020, 5, 100. doi.org/10.1021/acsenergylett.9b02410

[5]. T. Huang, G. Du, Y. Qi, J. Li, W. Zhong, Q. Yang, X. Zhang, M. Xu, Inorg. Chem. Front., 2020, 7, 3938. doi.org/10.1039/D0QI00872A

Several studies have emerged that seek to solve the energy deficit that we face as a society, where energy storage plays an important role. In this regard, Ion-Sodium batteries (Ion-Na) represent an excellent alternative to the Ion-Li batteries currently on the market. These new Ion-Na batteries have adequate charge densities and at a lower cost. Taking this issue into account, there is great interest in studying the composition, structure, and morphology of cathodes as active materials to optimize this type of battery. [i],[ii].

In this study, the synthesis and characterization of sodium rhodizonate nanostructures to be used as cathode material in Sodium-ion batteries was carried out. In a first stage, a two-level factorial design was used as an exploratory stage to evaluate the statistical significance of the variables involved in the synthesis: i) temperature, ii) reaction time, iii) molar ratio of precursors, and iv) addition time. The nanostructuring of the sodium rhodizonate crystals was carried out by the ultrasonic crystallization method and was characterized by X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). The electrochemical characterization was performed using cyclic voltammetry, galvanostatic charge/discharge profiles, and cyclability. The results show that the nanostructuring of the material causes an increase in the storage capacity of sodium ions in the sodiation/desodiation process. This shows a strong size-dependence in the Na + ion intercalation process. Finally, these results will allow the generation of causal relationships that lead to the optimization of the electronic properties of alternative Na-ion batteries.

[i] Minah Lee, Jihyun Hong, Jeffrey Lopez, Yongming Sun, Dawei Feng, Kipil Lim, William C. Chueh, Michael F. Toney, Yi Cui & Zhenan Bao. Nature Energy, volume 2, pages 861–868 (2017).

[ii] R.-E. Dinnebier, H. Nuss, M. Jansen, Disodium rhodizonate: a powder diffraction study, Acta Crystallographica E61 (2005) m2148–m2150.