Enhancing anionic redox stability via oxygen coordination configurations

Anionic redox in Li-rich cathode materials with disordered crystal structures has potential to increase battery energy density. However, capacity fading due to anionic redox-induced structural transformation hinders practical implementation. To address this challenge, it is crucial to understand the influence of the anion coordination structure on redox reversibility. By comprehensively studying the spinel-like Li_1.7Mn_1.6O_3.7F_0.3 and layered Li₂MnO₃ model systems, we found that tetrahedral oxygen exhibits higher kinetic and thermodynamic stability than octahedral oxygen in Li_1.7Mn_1.6O_3.7F_0.3 and Li₂MnO₃, effectively suppressing aggregation of oxidized anions. Electronic structure analysis showed that the 2p lone-pair states in tetrahedral oxygen lie deeper than those in octahedral oxygen. The Li-O-TM bond angle in a polyhedron is identified as a characteristic parameter to correlate anionic redox stability. TM substitutions using Co³⁺, Ti⁴⁺ and Mo⁵⁺ could effectively regulate the Li-O-Mn bond angle and anionic active electronic state. Our finding that anionic redox stability is influenced by the polyhedral structure offers new opportunities for designing high-energy-density Li-rich cathode materials.