Electrolyte Design Resisting Anionic Redox-Induced Oxygen Attacks for Robust Layered Sodium Cathodes.

Abstract

Anionic redox reactions (ARR) are an effective strategy to enhance the reversible capacity of layered oxide cathode materials, thereby enabling high-energy-density sodium-ion batteries (SIBs). However, ARR generates highly reactive and unstable oxygen species that may trigger nucleophilic attacks and compromise cathode-electrolyte interface (CEI) stability, causing structural degradation and rapid capacity fading. In this study, we introduce dimethyl 2-fluoromalonate (2MDF) as a multifunctional cosolvent into a conventional carbonate electrolyte to stabilize ARR in Na_0.67Li_0.24Mn_0.64Mg_0.06Ti_0.06O₂ (NLMMT) cathode. 2MDF, characterized by its low coordination ability, preferential oxidation characteristics, and fluorinated ester groups, simultaneously modulates Na⁺ solvation, promotes formation of a robust inorganic-rich CEI, and suppresses nucleophilic attack by reactive oxygen species. Density functional theory calculations and electrochemical analyses reveal that 2MDF forms weaker interactions with reactive oxygen species than conventional carbonates, enabling superior interfacial stability and preferential sacrificial oxidation. Consequently, the NLMMT cathode exhibits a high reversible capacity of 255.3 mAh g^-1 at 20 mA g^-1 and outstanding cycling stability, retaining 72% capacity after 400 cycles at 400 mA g^-1 in the voltage range of 1.5-4.5 V. This study establishes a rational electrolyte design strategy that balances oxidative stability and nucleophilic resistance, providing general guidance for achieving reversible ARR in high-energy SIBs.