Enabling Fast Ionic Conductivity and Stable Interfaces of Composite Polymer Electrolytes by Incorporating Borohydride‐Oxide Dual Fillers for Solid State Lithium Metal Batteries

Abstract Poly ethylene oxide (PEO) composite polymer electrolytes (CPEs) are one of the most promising candidates for all‐solid‐state batteries with high energy density, flexibility and safety. However, the applications of PEO with practicability have been refrained from its poor tensile strength, limited Li‐ion migration and ionic conductivity. In this work, the compact and stable flexible CPEs are prepared by PEO matrix with dual‐fillers of LiBH4 and Al2O3, where Al2O3 with Lewis acid sites can weaken the complexation of Li+ and PEO as well as enhance the dissociation of Li salts. Meanwhile LiBH4 acts as fast ion conductor to provide free Li+ at the interfaces between fillers and PEO. Benefiting from their synergistic effects, both ionic conductivity and interface stability between electrolyte and anode of CPEs are improved greatly while the lithium dendrites is also inhibited. As a result, the PEO/Lithium bis(trifluoromethanesulfonyl)imide(LiTFSI)/(4%LiBH4/4%γ‐Al2O3) CPEs exhibit a high ionic conductivity of 0.3 mS cm−1 and the Li‐Li symmetrical battery can cycle for 800 h at 60 °C.