Solid-state Li/LiFePO4 polymer electrolyte batteries incorporating an ionic liquid cycled at 40 °C

The cycle behaviour and rate performance of solid-state Li/LiFePO4 polymer electrolyte batteries incorporating the N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR13TFSI) room temperature ionic liquid (IL) into the P(EO)20LiTFSI electrolyte and the cathode have been investigated at 40 °C. The ionic conductivity of the P(EO)20LiTFSI + PYR13TFSI polymer electrolyte was about 6 × 10−4 S cm−1 at 40 °C for a PYR13+/Li+ mole ratio of 1.73. Li/LiFePO4 batteries retained about 86% of their initial discharge capacity (127 mAh g−1) after 240 continuous cycles and showed excellent reversible cyclability with a capacity fade lower than 0.06% per cycle over about 500 cycles at various current densities. In addition, the Li/LiFePO4 batteries exhibited some discharge capability at high currents up to 1.52 mA cm−2 (2 C) at 40 °C which is very significant for a lithium metal-polymer electrolyte (solvent-free) battery systems. The addition of the IL to lithium metal-polymer electrolyte batteries has resulted in a very promising improvement in performance at moderate temperatures.