Improvement in the low temperature performance of gelled polymer lithium-ion cells

Improved low temperature performance of gelled polymer lithium-ion cells was achieved by optimizing both electrolyte formulations and processes of gelled polymer electrolyte preparation and electrode coating. In the electrolyte system, LiPF6 in ethylene carbonate (EC)/propylene carbonate (PC), the highest ionic conductivity value was found when the LiPF6 concentration was around 1.0 M and the EC:PC ratio was around 1:1 (w/o) over a temperature range of −20 to 20 °C. Low temperature electrochemical performance showed that cells with lower EC content electrolytes delivered finite discharge capacities under group-special-mobile (GSM) conditions, compared with nearly zero discharge capacities for cells with higher EC content electrolytes. Electrochemical impedance study showed that cells with higher EC content electrolytes had much larger impedance at low temperature than cells with lower EC content electrolytes. The best low temperature electrochemical performance was obtained when electrodes were paste-coated and the gelled polymer electrolyte was cured under ultra violet radiation. Cells prepared this way were shown to have good interfacial properties both at room temperature and at −20 °C.