Interplay between Intermediate Anionic and Cationic Species and the Reflection to Voltage Hysteresis of Oxygen-Redox Battery Electrodes: A Holistic Picture

Ligand-to-metal charge transfer (LMCT) is conceived as a universal theory to account for voltage hysteresis in oxygen-redox battery electrodes. However, the influence of oxygen anionic species on mediating LMCT and its reflection to voltage hysteresis remain poorly understood. Herein, we demonstrate a close interplay between the chemical states of oxidized oxygen species, the cationic species, and the kinetics of LMCT and forcefully identify their influence on the magnitude of voltage hysteresis. Combining electrochemical/spectroscopic evidence and first-principles calculations, we clarify two distinct kinds of dynamic LMCT processes─that is, the formation of trapped molecular O₂ accompanied by the reduction of Ni⁴⁺/Ni³⁺ to Ni²⁺ (fast LMCT) during relaxation in Li-rich cation-disordered rock-salt (DRX) Li_1.3Ni_0.27Ta_0.43O₂ with extremely large voltage hysteresis, the formation of O-O dimers, and the partial reduction of Mn⁴⁺ to Mn³⁺ (slow LMCT) in DRX-Li_1.3Mn_0.4Ta_0.3O₂ with medium hysteresis. We further validate the maintenance of both cationic (Mn⁴⁺) and anionic (O^-•) species during relaxation in Na₂Mn₃O₇, reconciling its nonhysteretic behavior to the absence of LMCT. This study highlights the critical role of intermediate anionic species in mediating LMCT and provides a causal explanation of various voltage hysteresis signatures of oxygen-redox materials.