Reversible anionic redox reactions represent a transformational change for creating advanced high-energy-density positive-electrode materials for lithium-ion batteries. The activation mechanism of these reactions is frequently linked to ligand-to-metal charge transfer (LMCT) processes, which have not been fully validated experimentally due to the lack of suitable model materials. Here we show that the activation of anionic redox in cation-disordered rock-salt Li1.17Ti0.58Ni0.25O2 involves a long-lived intermediate Ni3+/4+ species, which can fully evolve to Ni2+ during relaxation. Combining electrochemical analysis and spectroscopic techniques, we quantitatively identified that the reduction of this Ni3+/4+ species goes through a dynamic LMCT process (Ni3+/4+–O2− → Ni2+–On−). Our findings provide experimental validation of previous theoretical hypotheses and help to rationalize several peculiarities associated with anionic redox, such as cationic–anionic redox inversion and voltage hysteresis. This work also provides additional guidance for designing high-capacity electrodes by screening appropriate cationic species for mediating LMCT.
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This research used resources of the Advanced Photon Source, a US Department of Energy (DOE) Office of Science User Facility, operated for the DOE Office of Science by Argonne National Laboratory under contract number DE-AC02-06CH11357. We are grateful to J. Freeland, T. Wu and G. Sterbinsky for their help during mail-in XAS measurements at the Advanced Photon Source. J.C. and I.R. acknowledge support from the National Science Foundation, under grant number DMR-1809372. K.K. acknowledges support from the National Science Foundation, under grant number CBET-1800357. A.M.A. and A.V.M. are grateful to the Russian Science Foundation for financial support (grant 20-13-00233). Access to TEM facilities was granted by the Advance Imaging Core Facility of Skoltech. HAXPES experiments (proposal no. 99210184) were performed on the GALAXIES beamline at the SOLEIL Synchrotron, France. The Ni K-edge XAS was collected on the ROCK beamline at the SOLEIL Synchrotron through a rapid access for urgent need. We are grateful to J. Sottmann and J.-P. Rueff for their assistance during the HAXPES experiments. J.-M.T and B.L. acknowledge funding from the European Research Council (ERC) (FP/2014)/ERC Grant-Project 670116-ARPEMA.
The authors declare no competing interests.
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Li, B., Kumar, K., Roy, I. et al. Capturing dynamic ligand-to-metal charge transfer with a long-lived cationic intermediate for anionic redox. Nat. Mater. (2022). https://doi.org/10.1038/s41563-022-01278-2