Salt dissociation and localized high-concentration solvation at the interface of a fluorinated gel and polymer solid electrolyte

Low salt dissociation and the unstable [Li(N,N-dimethylformamide (DMF))_x]⁺ solvent structure in poly(vinylidene fluoride) (PVDF)-based solid polymer electrolyte (SPE) remarkably restricts the high throughput ion transport and interfacial stability. Here, we designed a hybrid electrolyte (denoted as HFGP-SE) composed of fluorinated gel solid electrolyte (FG-SE) and poly(vinylidene fluoride-co-hexafluoropropylene) (PVHF)-based solid polymer electrolyte (PVHF-SPE). We found that in the HFGP-SE, the interface of FG-SE and PVHF-SPE effectively promotes lithium salt dissociation and creates a localized high-concentration (LHC) solvation structure. The developed HFGP-SE shows high ionic conductivity (0.84 mS cm⁻¹) and a remarkably improved lithium transference number (t_Li+ = 0.87). Meanwhile, the controlled LHC solvation structure formed at the interface between FG-SE and PVHF-SPE supports the formation of inorganic-rich solid electrolyte interphases (SEIs) derived from anions, allowing for stable lithium deposition and ultra-stable plating/stripping performance for over 1200 hours at a current density of 0.5 mA cm⁺⁻². Additionally, HFGP-SE supported stable cycling in 4.5 V class Li||NCM811 full cells under practical conditions, with a 50 μm thick lithium metal anode and cathodes with a mass loading of 12 mg cm⁻², achieving an areal capacity >2 mA h cm⁻². This work proposes a novel strategy using interfaces existing in hybrid solid electrolytes to significantly enhance lithium salt dissociation, fast ion transport, and interfacial stability of solid-state electrolytes for lithium metal batteries. © 2024 The Royal Society of Chemistry.