Polymerized Ionic-Liquid Electrolytes with Borate Mixed-Anion Chemistry: Structure, Ion Transport, and Thermal Properties

Solid-state sodium batteries (SSSBs) are emerging as next-generation energy storage devices due to their low cost, high safety, and high-energy density. Nevertheless, one of the main challenges in developing SSSBs stems from choosing an ideal solid electrolyte that offers high conductivity, high chemical stability, good mechanical integrity, and superior cycling performance. This study focuses on the role of a weakly coordinating borate-based sodium salt i.e. sodium difluoro(oxalate)borate–Na[DFOB], in polymerized ionic liquids (poly(ILs)). The results reveal that Na[DFOB] salt content has a key role in the crystallinity and ionic conductivity of the membranes. Notably, the addition of only 2 wt % of borate salt in a ternary electrolyte comprised of poly(diallyldimethylammonium) bis(fluorosulfonyl)imide ([PDADMA][FSI]) mixed with Na[FSI] is sufficient to enhance ionic properties. In comparison to the reference electrolyte i.e. [PDADMA][FSI]–Na[FSI], the addition of 2 wt % Na[DFOB] significantly increased the sodium transport number of the polymer electrolyte. Furthermore, solid-state NMR and FTIR spectroscopies demonstrate coordination between the sodium cation and all the different anions in the Na[DFOB] containing system. In addition, it exhibits an apparent oxidative onset shift to higher voltages under the stated LSV conditions that may suggest the electrolyte can sustain higher voltages, positioning these electrolytes as strong candidates for solid-state sodium batteries. We also investigated the advantages of a mixed-anion system in the presence of common impurities found in electrolyte systems, specifically chloride ions originating from the poly(IL) synthesis. Notably, the mixed-anion system exhibits a significantly improved tolerance to these impurities compared to the single-anion reference electrolyte.