Host–Guest Interactions and Transport Mechanism in Poly(vinylidene fluoride)-Based Quasi-Solid Electrolytes for Lithium Metal Batteries

All-solid-state lithium metal batteries (SS-LMBs) are expected to meet the strong requirements of the automotive sector in terms of performance and safety. Among the different solid electrolytes, poly(vinylidene fluoride) (PVDF)-based systems offer good performance in terms of ionic conductivity and stability at the anodic interface. However, despite the high polymer permittivity (ϵ′ ≈ 10-11) which should allow efficient salt dissociation, there is growing evidence that the ionic transport requires the presence of a non-negligible amount of residual, or permanent, solvent in the membrane. In this paper, we study the Li+ transport mechanism in a model system consisting of poly(vinylidene fluoride-co-hexafluoropropylene) (PFDF-HFP), lithium bis(fluorosulfonyl)imide (LiFSI) salt, and dimethylformamide (DMF) as permanent solvent, combining a large set of experimental techniques (thermal analysis, NMR, IR and Raman spectroscopy, impedance spectroscopy) and accurate density functional theory (DFT) modeling. We show that Li+-DMF interactions are predominant in these quasi-solid electrolytes (QSEs) and are the basis of the effective ion transport mechanism. Permanent solvent amounts on the order of [DMF]/[Li+] ∼ 2-3 are required to make QSEs able to practically work in a real environment.