Plasticizing Li single-ion conductors with low-volatility siloxane copolymers and oligomers containing ethylene oxide and cyclic carbonates

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 (T_g) very well for the random polysiloxane-based copolymer plasticizers (11000 < M < 13000) while the smaller oligomer plasticizers have T_g 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 T_g. 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 T_g, increasing dielectric constant and providing better solvation of Li⁺. The best oligomeric plasticizer only has T_g 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.