Degradation Mechanisms of Fluorinated Disordered Rocksalt Cathodes: Effects of Electrolyte Chemistry and Discharge Voltage

Cation-disordered rock-salt (DRX) oxides have emerged as a promising class of high energy density cathodes for next-generation lithium-ion batteries. Fluorination has been widely employed to tune the redox chemistry and structural stability of these materials, leading to enhanced electrochemical performance. However, the degradation mechanisms of fluorinated DRX (F-DRX) cathodes during electrochemical cycling remain poorly understood. Here, we investigate the degradation behavior of an F-DRX cathode cycled in a conventional carbonate-based electrolyte and a localized high-concentration electrolyte (LHCE). By correlating electrochemical performance with interfacial and bulk structural evolution, we elucidate the role of electrolyte chemistry and deep discharge voltage in governing structural transformation and degradation. The results reveal more pronounced surface and bulk structural changes in the conventional LiPF6-carbonate electrolyte relative to LiFSI-LHCE, indicating accelerated degradation in the former. Lowering the discharge cutoff voltage from 2.0 to 1.5 V further promotes interfacial degradation in both electrolyte systems. This work demonstrates that the LHCE offers superior compatibility with F-DRX cathodes, enabling stable cycling by mitigating surface and bulk structural degradation. These insights clarify the interplay between electrolyte chemistry and voltage window in F-DRX degradation and highlight the critical importance of advanced electrolyte designs for unlocking the full potential of DRX cathodes.