Na Battery Electrolytes Prepared by Dissolution of Commercial Polymers in NaPF₆-Diglyme

The incorporation of small amounts of polymers into liquid electrolytes to produce gel electrolytes has many benefits. Decreasing, or even avoiding flow reduces accidents caused by leaks, allows for simpler and more flexible geometries and configurations, stabilizes electrochemical cycling, and may even permit the separate recycling of each cell component. Direct mixing of polymers with liquid electrolytes is very frequently possible, and it is a sustainable and scalable procedure that avoids evaporation stages and produces thermoreversible materials. In this work, some of the polymers most commonly used for gel electrolyte formationpolyvinylidene fluoride (PVDF), polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP), polyvinyl chloride (PVC), poly-(methyl methacrylate) (PMMA), and poly-(ethylene oxide) (PEO)are employed to produce gels with the electrolyte NaPF₆ 1.1 m in diglyme, and their behavior and performance are described. All these polymers are soluble in the electrolyte, and the ultrahigh molecular weight (UHMW) PEO and the halogenated polymers are able to form self-standing gels at concentrations ranging from 5 to 10 wt %. The fluorinated polymer gels left standing in an Ar glovebox progressively become colored, turning almost black after one month, showing a lack of long-term chemical stability. PVC 10 wt % and UHMW PEO 5 wt % gels, however, become hard gels with no macroscopic phase separation and remain stable for periods of months. Their electrochemical stability against Na electrodes was tested, showing that the PVC gel is not stable, and only the UHMW PEO 5 wt % gels were further tested with Prussian white (PW) cathodes. The PEO gel electrolyte, without any physical separator, showed equivalent performance in PW||Na half-cells as the baseline electrolyte with a glass fiber separator, while also being self-standing and solid-like in consistency. The PEO gel electrolytes also showed applicability to PW||HC full cells. Moreover, after electrochemical testing, the coin cells with PEO gels were easily disassembled, and their components were recovered.