Host–Guest Interactions and Transport Mechanism in Poly(vinylidene fluoride)-Based Quasi-Solid Electrolytes for Lithium Metal Batteries
Nicholas Vallana, Eleonora Carena, Nicole Ceribelli, Lorenzo Mezzomo, Giovanni Di Liberto, Michele Mauri, Chiara Ferrara, Roberto Lorenzi, Livia Giordano, Riccardo Ruffο, Piercarlo Mustarelli
Abstract
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.