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 H-1, C-13 and Si-29 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 (11 000 < M < 13 000) 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 x 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.