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

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. The electrically coupled composite electrolyte takes the merits of cross‐linked gel polymer and a well‐tuned antioxidant additive (4‐trifluoromethylphenylboronic acid, TFPBA). Based on the electrical coupling effect, polymer chains with electronegative groups can immobilize TFPBA to enhance its oxidative stability. The TFPBA with electron acceptor capability also can spontaneously adsorb onto the surface of the cathodes to participate in the formation of the CEI film containing favorable fluoroboron complexes and NaF, and play a key role in fixing PF6− to inhibit their excessive decomposition. As a result, the cell integrating the high‐voltage P2/O3 cathode and well‐tailored gel polymer electrolyte exhibits an impressive long‐term performance.