Composite Electrolytes Based on Poly(Ethylene Oxide) and Lithium Borohydrides for All-Solid-State Lithium–Sulfur Batteries

Complex hydride LiBH4 is a promising solid-state electrolyte (SSE) for rechargeable batteries, owing to its great compatibility with the lithium metal anode and good mechanical properties. However, LiBH4 only exhibits high ionic conductivity (10–3 S cm–1) with the hexagonal structure at temperatures above 117 °C. To overcome this obstacle, composite poly(ethylene oxide)–hydride electrolytes were synthesized by a solution-casting method. PEO–Li4(BH4)3I (EO/Li+ = 10) delivers the highest ionic conductivity of 4.09 × 10–4 S cm–1 with high Li+ transference number of 0.45 at 70 °C. The addition of nano-SiO2 as an inorganic filler enhances the dendrite-free ability of electrolyte membranes dramatically and prolongs the cycle life of the Li symmetric battery from <100–360 h. In order to suppress the shuttling effect of polysulfide and volume expansion in lithium–sulfur (Li–S) batteries, sulfurized pyrolyzed poly(acrylonitrile) was selected as the cathode material for all-solid-state batteries, which presented a remarkable cycling stability of 232 (967 for sulfur content) mA h g–1 after 75 cycles. These attempts not only decrease the application temperature of LiBH4 but also broaden the utilization of hydride in SSEs.