To prepare a safe electrolyte for lithium ion batteries, two groups of novel low-volatility plasticizers combining pendant cyclic carbonates and short ethylene oxide chains have been successfully synthesized, as confirmed by 1H, 13C and 29Si NMR spectroscopy. The Fox equation describes the composition dependence of the glass transition temperature (Tg) very well for the random polysiloxane-based copolymer plasticizers (11000 < M < 13000) while the smaller oligomer plasticizers have Tg as much as 20 K lower than the Fox equation prediction because of their lower molecular weight (450 < M < 700). The Landau-Lifshitz mixing rule describes the dielectric constant of the random polysiloxane-based copolymer plasticizers at all temperatures above Tg. Mixing with 20 wt% polysiloxane tetraphenyl borate-Li ionomer (14 mol% borate and 86 mol% cyclic carbonate) increases conductivity relative to the neat ionomer by lowering Tg, increasing dielectric constant and providing better solvation of Li+. The best oligomeric plasticizer only has Tg 10 K lower than the Fox prediction but has dielectric constant 30% larger than expected by the Landau-Lifshitz mixing rule, owing to a surprisingly low viscosity, resulting in ambient conductivity 2 × 10-5 S cm-1. For both groups of plasticizers, the fraction of cyclic carbonates relative to ethylene oxide governs the magnitude and temperature dependence of the ionic conductivity.
All Science Journal Classification (ASJC) codes
- Renewable Energy, Sustainability and the Environment
- Materials Science(all)