Abstract: Solid electrolytes are the most promising candidate for replacing liquid electrolytes due to their safety and chemical stability advantages. However, a single inorganic or organic solid electrolyte cannot meet the requirements of commercial all-solid-state batteries (ASSBs), which motivates the composite polymer electrolyte (CPE). Herein, a CPE of boron nitride nanofiber (BNNF) with a high specific surface area, rich pore structure, and poly (ethylene oxide) (PEO) are reported. Anions strongly adsorb on the surface of BNNF through electrostatic interactions based on oxygen vacancies, promoting the dissociation of lithium salts at the two-phase interface. The three-dimensional (3D) BNNF network provides three advantages in the CPE, including (i) improving ionic conductivity through strong interaction between polymers and fillers, (ii) improving mechanical properties through weaving a robust skeleton, and (iii) improving stability through a rapid and uniform thermal dispersion pathway. Therefore, the CPE with BNNF delivers high ionic conduction of 4.21 × 10^-4 S cm^-1 at 60 °C and excellent cycling stability (plating/stripping cycles for 2000 h with a low overpotential of ∼40 mV), which results in excellent electrochemical performance of LiFePO₄ (LFP) full cell assembled with CPE-5BNNF-1300 (152.7 mAh g^-1 after 200 cycles at 0.5 C, and 134.8 mAh g^-1 at 2.0 C). Furthermore, when matched with high-voltage LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622), it also exhibits an outstanding rate capacity of 120.4 mAh g^-1 at 1.0 C. This work provides insight into the BNNF composite electrolyte and promotes its practical application for ASSBs.

Key words: Boron nitride nanofiber, Three-dimensional network, Oxygen vacancy, Thermal dispersion, Composite polymer electrolyte