Evolution and function of residual solvent in polymer-Li₂B₁₂H₁₂ composite solid electrolyte

Composite solid electrolytes (CSEs) containing polymer matrices and inorganic fillers hold promise for the next generation of solid-state batteries. However, the role of residual solvents in CSEs remains controversial. This study investigated the evolution and function of the residual solvent in a polymer-Li₂B₁₂H₁₂ CSE. A partial reaction occurred between Li₂B₁₂H₁₂ and solvent N, N-dimethylformamide (DMF), which produced dimethylaminomethanol (DMAM) in the CSE. Density functional theory calculations have revealed that DMAM forms stronger hydrogen bonds with polyvinylidene fluoride chains than DMF, which can have a plasticizing effect on the polymer matrix, leading to lower crystallinity and quicker segment motion. Therefore, this CSE exhibited improved Li-ion conducting properties, enabling the stable cycling of Li||LiFePO₄ solid-state batteries. This study provided insights into the role of residual solvents in CSEs.