Effect of propylene carbonate on the low temperature performance of Li-ion cells

The effect of propylene carbonate (PC) on the low temperature performance of Li-ion cells was studied by comparing two electrolytes of 1 m LiPF6 3:7 ethylene carbonate (EC):ethylmethyl carbonate (EMC) and 1 m LiPF6 1:1:3 PC:EC:EMC. We found that addition of PC to the electrolytes can significantly improve the low temperature performance of Li-ion cells. However, use of PC will cause a slight decrease in the ionic conductivity of the electrolytes owing to its high viscosity, especially at low temperatures. This result suggests that ionic conductivity of the electrolytes would not be the main limitation to the low temperature operation of the Li-ion cells. Analysis of the impedance spectra of fully charged Li-ion cells shows that at 20 °C these two cells without and with PC, respectively, have solid electrolyte interface (SEI) of similar resistance, however, the presence of PC can substantially decrease its dependence on the temperature. Therefore, the improved low temperature performance using PC can be ascribed to the relatively higher conductivity of the SEI film on the interfaces between electrolyte and electrodes.