Revealing the Structural-Redox Interplay in the High-Voltage Spinel LiNi0.5Mn1.5O4 Cathode

The evolution of transition metal oxide cathodes with cycling is governed by a subtle interplay between redox processes and (local) structural distortions. Here, we use first-principles calculations to monitor the electronic and structural evolution of the spinel LiNi0.5Mn1.5O4 (LNMO) cathode at various stages of charge. In its P4332 ordered ground state, redox-active NiO6 octahedra are spatially isolated by redox-inactive Mn4+ cations, providing a clean platform to disentangle local redox and structural phenomena. By combining analyses of magnetic moments, Bader charges, and charge-density difference maps, we show that redox processes are localized on Ni, yet trigger significant transition metal-oxygen rehybridization extending to both Ni-O and Mn-O networks. Changes in oxygen charge are traced to polarization driven by evolving Ni-O bonding, rather than to anion redox activity. Concurrent structural changes include a stepwise volumetric contraction of the NiO6 octahedra with increasing oxidation state, and Jahn-Teller distortions that are sensitively modulated by the local Li/vacancy environment. Beyond these effects, we identify additional distortion modes, including pinching deformations in both NiO6 and MnO6 polyhedra, which exhibit clear and systematic trends with transition metal oxidation. Together, these results provide an atomic-level picture of coupled redox and lattice distortion processes in LNMO, revealing how Li/vacancy configurations and transition metal-oxygen rehybridization govern electrochemical behavior. The mechanisms uncovered here point to design principles that are likely to be equally critical in more complex oxide cathodes.