Metal–oxygen decoordination stabilizes anion redox in Li-rich oxides

Abstract

Reversible high-voltage redox chemistry is an essential component of many electrochemical technologies, from (electro)catalysts to lithium-ion batteries. Oxygen-anion redox has garnered intense interest for such applications, particularly lithium-ion batteries, as it offers substantial redox capacity at more than 4 V versus Li/Li+ in a variety of oxide materials. However, oxidation of oxygen is almost universally correlated with irreversible local structural transformations, voltage hysteresis and voltage fade, which currently preclude its widespread use. By comprehensively studying the Li2−xIr1−ySnyO3 model system, which exhibits tunable oxidation state and structural evolution with y upon cycling, we reveal that this structure–redox coupling arises from the local stabilization of short approximately 1.8 Å metal–oxygen π bonds and approximately 1.4 Å O–O dimers during oxygen redox, which occurs in Li2−xIr1−ySnyO3 through ligand-to-metal charge transfer. Crucially, formation of these oxidized oxygen species necessitates the decoordination of oxygen to a single covalent bonding partner through formation of vacancies at neighbouring cation sites, driving cation disorder. These insights establish a point-defect explanation for why anion redox often occurs alongside local structural disordering and voltage hysteresis during cycling. Our findings offer an explanation for the unique electrochemical properties of lithium-rich layered oxides, with implications generally for the design of materials employing oxygen redox chemistry.

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Fig. 1: Reversible multivalent iridium redox in Li2-xIrO3.
Fig. 2: Hybridized Ir–O redox in Li2−xIrO3.
Fig. 3: Irreversible electrochemistry, structural disordering, and redox behaviour of Li2−xIr1−ySnyO3.
Fig. 4: Computational predictions of M–O decoordination and Ir=O/O–O stabilized anion redox.
Fig. 5: Proposed electronic mechanism of cation migration and LMCT-mediated anion redox in LISO.

Data availability

All experimental data within the article and its Supplementary Information will be made available upon reasonable request to the authors.

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Acknowledgements

This research was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies, Battery Materials Research Program, US Department of Energy (DOE). W.E.G. was supported additionally by the Advanced Light Source Doctoral Fellowship and the Siebel Scholars programme. K.L. was supported additionally by the Kwanjeong Education Foundation Fellowship. Use of the ALS was supported by the Office of Science, Office of Basic Energy Sciences, of the US DOE under contract no. DE-AC02-05CH11231. Use of the SSRL, SLAC National Accelerator Laboratory, was supported by the Office of Science, Office of Basic Energy Sciences, of the US DOE under contract no. DE-AC02-76SF00515. Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the US DOE under contract no. DE-AC02-05CH11231. Part of this work was performed at the Stanford Nano Shared Facilities, supported by the National Science Foundation under award ECCS-1542152. This research used resources of the APS, an Office of Science User Facility operated for the US DOE Office of Science by Argonne National Laboratory, and was supported by the US DOE under contract no. DE-AC02-06CH11357, and the Canadian Light Source and its funding partners. The computational work was funded by the NorthEast Center for Chemical Energy Storage, an Energy Frontier Research Center, supported by the US DOE, Office of Science, Office of Basic Energy Sciences under award no. DE-SC0012583. G.C. also thanks the China Automotive Battery Research Institute and the General Research Institute for NonFerrous Metals for financial support on oxygen redox in cathode materials. W.E.G. thanks Ariel Jacobs for insightful discussions on metal–oxygen bonding interactions. The authors thank Karena Chapman (APS) for valuable comments on X-ray total scattering analysis.

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J.H., W.E.G., W.C.C. and M.F.T. conceived the study. J.H. carried out materials synthesis, characterization and testing. J.H. and K.L. performed ex situ and operando synchrotron measurements including XRD, PDF, XAS, sXAS and RIXS. J.H. and W.E.G. measured ex situ STXM and RIXS spectra. W.E.G., J.W. and W.Y. processed and analysed spectroscopic data. K.L., J.H., K.H.S., D. Passarello and M.F.T. performed the structural analyses. K.L., J.H., C.J.T., M.F.T. and W.C.C. designed and constructed settings for in situ synchrotron measurements. P.X., D.-H.S. and G.C. conducted DFT calculations. J.W., K.H.S., D.N., C.-J.S. and K.L. configured synchrotron end stations. P.M.C. provided constructive advice for experiments. J.H., W.E.G., D. Prendergast, W.C.C. and M.F.T. devised the oxygen redox model. J.H., W.E.G., G.C., W.C.C. and M.F.T. wrote the manuscript and all authors revised the manuscript.

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Correspondence to Gerbrand Ceder or Michael F. Toney or William C. Chueh.

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Hong, J., Gent, W.E., Xiao, P. et al. Metal–oxygen decoordination stabilizes anion redox in Li-rich oxides. Nature Mater 18, 256–265 (2019). https://doi.org/10.1038/s41563-018-0276-1

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