Fluorinated Carbonate Electrolyte with Superior Oxidative Stability Enables Long-Term Cycle Stability of Na_2/3Ni_1/3Mn_2/3O₂ Cathodes in Sodium-Ion Batteries

Layered transition metal oxides are promising cathode materials for sodium-ion batteries applicable for low-cost energy storage systems. However, their cycle stability needs to be substantially improved to meet the requirements of practical applications. Specifically, the issues related to electrolyte stability and the formation of an unstable cathode–electrolyte interface (CEI) remain unsolved. Herein, it is shown that an electrolyte with high fluorine content may induce a robust fluorinated CEI on Na_2/3Ni_1/3Mn_2/3O₂ cathode, a representative transitional metal oxide, which can efficiently passivate its surface and suppress continuous electrolyte decomposition during cycling. As a result, the cells deliver a remarkably improved rate capability and cycle stability. Density functional simulations further validate the superior stability of fluorinated electrolyte on cathodes with low highest occupied molecule orbital energy and high dissociation energy barriers. This finding demonstrates the favorable role of fluorinated electrolytes for improving the long-term cycle stability of Na_2/3Ni_1/3Mn_2/3O₂ cathode toward grid-scale applications.