Reversible cationic-anionic redox in disordered rocksalt cathodes enabled by fluorination-induced integrated structure design

Abstract

Cation-disordered rocksalt oxides (DRX) have been identified as promising cathode materials for high energy density applications owing to their variable elemental composition and cationic-anionic redox activity. However, their practical implementation has been impeded by unwanted phenomena such as irrepressible transition metal migration/dissolution and O2/CO2 evolution, which arise due to parasitic reactions and densification-degradation mechanisms during extended cycling. To address these issues, a micron-sized DRX cathode Li1.2Ni1/3Ti1/3W2/15O1.85F0.15 (SLNTWOF) with F substitution and ultrathin LiF coating layer is developed by alcohols assisted sol–gel method. Within this fluorination-induced integrated structure design (FISD) strategy, in-situ F substitution modifies the activity/reversibility of the cationic-anionic redox reaction, while the ultrathin LiF coating and single-crystal structure synergistically mitigate the cathode/electrolyte parasitic reaction and densification-degradation mechanism. Attributed to the multiple modifications and size effect in the FISD strategy, the SLNTWOF sample exhibits reversible cationic-anionic redox chemistry with a meliorated reversible capacity of 290.3 mA h g−1 at 0.05C (1C = 200 mA g−1), improved cycling stability of 78.5% capacity retention after 50 cycles at 0.5 C, and modified rate capability of 102.8 mA h g−1 at 2 C. This work reveals that the synergistic effects between bulk structure modification, surface regulation, and engineering particle size can effectively modulate the distribution and evolution of cationic-anionic redox activities in DRX cathodes.