High-Voltage Reactivity and Long-Term Stability of Cation-Disordered Rocksalt Cathodes

High-voltage reactivity and long-term cycling stability are investigated on a fluorinated Li-excess cation-disordered rocksalt (DRX) cathode, Li1.2Mn0.625Nb0.175O1.95F0.05 (LMNOF), using combined soft X-ray absorption spectroscopy at F and O K-edges and Li-7 and F-19 solid-state nuclear magnetic resonance spectroscopy. In a F-free system [polyethylene (PE) binder and lithium perchlorate (LiClO4) salt], a finite fluorine loss from the surface of LMNOF particles is observed, while bulk fluorine anions remain stable during cycling. We also reveal the decomposition and re-formation of carbonate species during the charging and discharging of the LMNOF cathode, respectively, in the F-free system (PE/LiClO4). In sharp contrast, these carbonate reactions do not occur when cycled with PVDF/LiPF6; instead, a major degradation product of LiF is observed. Through a comprehensive study of DRX electrodes using different binder and electrolyte salt combinations, the main degradation process is identified as electrolyte decomposition on the surface of DRX particles, rather than bulk structural changes. The highlight of this work is that the high-voltage stability of the LMNOF cathode is strongly correlated to the nature of other cell components such as the binder and electrolyte, suggesting the mitigation of surface reactions, combined with an optimal cell design, plays an important role in improving the cycling performance of the DRX cathodes.