Tertiary sulfonium/quaternary ammonium-containing silsesquioxane nanoparticles with lithium salts as potential hybrid electrolytes

Solid and semisolid electrolytes consisted of ionic liquids, that can be used in lithium batteries to enhance their safety and performance, have attracted increasing attention, owing to the heavy demand for clean energy and a sustainable society. In this paper, we report the design, synthesis, and study of the chemical–physical properties of tertiary sulfonium/quaternary ammonium-based silsesquioxane nanoparticles (SQ-NPs) with lithium salt additives. Hydrolytic condensation of sulfide-containing triethoxysilane prepared from 3-(methylthio)propyl acrylate (MTPA) and (3-aminopropyl)triethoxysilane yielded MTPA-containing SQ-NP. The cationization of MTPA-containing SQ-NP and subsequent anion exchange reaction with lithium salts (lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide (LiFSI)) produced viscous MTPA(S+)-SQ[TFSI−] and MTPA(S+)-SQ[FSI−], respectively. Adopting the same procedure, 2-(dimethylamino)ethyl acrylate (DMAEA) was used as a tertiary amine-containing starting material to prepare DMAEA(N+)-SQ[TFSI−] and DMAEA(N+)-SQ[FSI−]. Among the four SQ-NPs, MTPA(S+)-SQ[FSI−] exhibited the highest ionic conductivity (3.32 × 10−4 S cm−1 at 55 °C), and the lowest Tg (−24.8 °C) and viscosity (2.75 × 103 Pa∙S), while maintaining high thermal stability (Td5 = 217 °C). The maximum ionic conductivities of 2.24 × 10−5 and 3.48 × 10−3 S cm−1 at 25 and 90 °C, respectively, were observed in MTPA(S+)-SQ[FSI−] mixed with 30 wt% Li salt (LiTFSI). These results reveal the potential of the tertiary sulfonium/quaternary ammonium-based hybrids, showing the synergistic effects of combining the sulfonium/ammonium-based ionic liquids and multifunctional SQ-NPs (<5 nm), as semisolid-state electrolytes for the future development of lithium ion batteries and related applications.