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 Na_0.95Ni_0.4Fe_0.15Mn_0.3Ti_0.15O₂ (NFMT) cathode delivers remarkable discharge capacity up to 169.7 mAh g^-1, 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.