Cation-Anion Regulation Engineering in a Flame-Retardant Electrolyte toward Safe Na-Ion Batteries with Appealing Stability.

Great electrochemical stability and intrinsic safety are of critical significance in realizing large-scale applications of Na-ion batteries (NIBs). Unfortunately, the notorious decomposition of the electrolyte and undesirable side reactions on the cathode-electrolyte interphase (CEI) pose major obstacles to the practical implementation of NIBs. Besides, the flammability of traditional carbonate-based electrolytes raises increasing safety concerns about the batteries. Herein, a flame-retardant all-fluorinated electrolyte is proposed to achieve an anion-aggregated inner solvation shell by modulating cation-anion interactions through a low-coordination number cosolvent. The more electrochemically antioxidant fluorinated solvents and anion-dominated interfacial chemistry contribute to the construction of both mechanically and chemically stable F-rich CEI. Such thin, homogeneous interphase effectively inhibits the parasitic reaction, strengthens the interfacial stability, and enables fast Na diffusion kinetics on the interface. When employing this electrolyte, the NaNiFeMnTiO (NFMT) cathode delivers remarkable discharge capacity up to 169.7 mAh g, with stable cycling at 1C for 500 cycles. Impressively, NFMT//hard carbon pouch cells with such electrolyte also achieve a steady operation for 100 cycles at 0.5C with 86.8% capacity remaining. This study offers a practical reference for developing high-performance and flame-retardant electrolytes.