Modulating composite polymer electrolyte by lithium closo-borohydride achieves highly stable solid-state battery at 25°C

Rational composite design is highly important for the development of high-performance composite polymer electrolytes (CPEs) for solid-state lithium (Li) metal batteries. In this work, Li closo-borohydride, Li2B12H12, is introduced to poly(vinylidene fluoride)-Li-bis-(trifluoromethanesulfonyl) imide (PVDF-LiTFSI) with a bound N-methyl pyrrolidone plasticizer to form a novel CPE. This CPE shows superb Li+ conduction properties, as evidenced by its conductivity of 1.43 × 10−4 S cm−1 and Li+ transference number of 0.34 at 25°C. Density functional theory calculations reveal that Li2B12H12, which features electron-deficient multicenter bonds, can facilitate the dissociation of LiTFSI and enhance the immobilization of TFSI to improve the Li+ conduction properties of the CPE. Moreover, the fabricated CPE exhibits excellent electrochemical, thermal, and mechanical stability. The addition of Li2B12H12 can help form a protective layer at the anode/electrolyte interface, thereby preventing unwanted reactions. The above benefits of the fabricated CPE contribute to the high compatibility of the electrode. Symmetric Li cells can be stably cycled at 0.2 mA cm−2 for over 1200 h, and Li∥LiFePO4 cells can deliver a reversible specific capacity of 140 mA h g−1 after 200 cycles at 1 C at 25°C with a capacity retention of 98%.