Characterization and performance of modified 1 M LIPF6: EC: DEC electrolyte for Lithium ion cells

Abstract

In this study, three systems of liquid electrolyte have been investigated. These are 1 M LiPF6:EC:DEC (v/v=1/2) as a baseline electrolyte, Methyl acetate added with baseline electrolyte and ethyl propionate added with baseline electrolyte. The room temperature ionic conductivity for the highest conducting samples in the baseline electrolyte, baseline electrolyte added with 28.6 vol.% of methyl acetate and baseline electrolyte added with 28.6 vol.% of ethyl propionate systems is (5.42 ± 0.20) mS cm-1, (9.62 ± 0.20) mS cm−1 and (8.05 ± 0.16) mS cm−1 respectively. At 50 oC and -30 oC the ionic conductivity of baseline electrolyte is 8.59 mS cm-1 and 0.66 mS cm-1 respectively. Ionic conductivity of the baseline electrolyte added with 28.6 vol.% methyl acetate at 50 oC and -30 oC is 11.4 mS cm-1 and 3.08 mS cm-1 respectively. For baseline electrolyte added with 28.6 vol.% ethyl propionate, the conductivity is 12.6 mS cm-1 and 1.09 mS cm-1 at 50 oC and -30 oC respectively. Linear sweep voltammetry shows that baseline electrolytes with additives decompose at a lower potential compared to baseline electrolyte. However, the decomposition potential is outside the working potential of a liquid lithium ion electrolyte. Fourier transform infrared spectroscopy (FTIR) spectra of methyl acetate and ethyl propionate added baseline electrolytes showed small shifting to lower wavenumbers. Due to the change in conductivity with additive content and with temperature, methyl acetate and ethyl propionate may serve as plasticizers or lubricants that may lead to higher ionic mobility and conductivity. The cells using baseline electrolyte containing 28.6 vol. % of methyl acetate showed the best performance at temperatures -30 oC, -20 oC and 25 oC. This is followed by the cells using 28.6 vol.% of ethyl propionate added electrolyte. However, at 30 oC, the cells using 28.6 vol.% of ethyl propionate added electrolyte has higher capacity than the cells using 28.6 vol.% of methyl acetate added electrolyte.