posted on 2024-02-06, 19:34authored byNicholas Vallana, Eleonora Carena, Nicole Ceribelli, Lorenzo Mezzomo, Giovanni Di Liberto, Michele Mauri, Chiara Ferrara, Roberto Lorenzi, Livia Giordano, Riccardo Ruffo, Piercarlo Mustarelli
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.