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 Li1.7Mn1.6O3.7F0.3 and layered Li2MnO3 model systems, we found that tetrahedral oxygen exhibits higher kinetic and thermodynamic stability than octahedral oxygen in Li1.7Mn1.6O3.7F0.3 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.