Electrically Coupled Electrolyte Engineering Enables High Interfacial Stability for High‐Voltage Sodium‐Ion Batteries

Abstract Sodium‐ion batteries (SIBs) suffer from severe capacity decay as the harmful substances caused by the violent decomposition of electrolyte under high voltages continue to erode the cathodes. Therefore, the design of high‐voltage electrolyte and construction of robust cathode–electrolyte interface (CEI) are critical for long‐life SIBs. Herein, an electrically coupled composite electrolyte that takes the merits of cross‐linked gel polymers and s well‐tuned antioxidant additive (4‐trifluoromethylphenylboronic acid, TFPBA) is proposed. Through an electrical coupling effect, TFPBA can be anchored by the cross‐linked polymer framework to immobilize the PF₆⁻ anion and adsorb onto cathode surface spontaneously, both of which promote the formation of a robust CEI layer to facilitate Na⁺ transportation and suppress subsequent side reactions and corrosive cracking. As a result, the cells integrating high‐voltage P2/O3 cathode and well‐tailored gel polymer electrolyte achieve stable cycling over 550 cycles within 1.8–4.2 V with a capacity retention of 71.0% and a high‐rate discharge capacity of 77.4 mAh g⁻¹ at 5 C. The work paves the way for the development of functionalized quasi‐solid electrolyte for practical next generation high‐voltage SIBs.