Application of bis(trifluoromethanesulfonyl)imide lithium-N-methyl-N- butylpiperidinium-bis(trifluoromethanesulfonyl)imide-poly(vinylidene difluoride-co-hexafluoropropylene) ionic liquid gel polymer electrolytes in Li/LiFePO₄ batteries at different temperatures

Ionic liquid gel polymer electrolyte (ILGPE) based on N-methyl-N- butylpiperidinium-bis(trifluoromethanesulfonyl)imide (PP14TFSI), bis(trifluoromethanesulfonyl)imide lithium salt (LiTFSI) and poly(vinylidene difluoride-co-hexafluoropropylene) (P(VdF-HFP)) is prepared for lithium ion batteries by solution casting. The ILGPE is characterized by scanning electron microscopy (SEM), thermogravimetry (TG) and electrochemical tests. The results indicate that the addition of PP14TFSI to the P(VdF-HFP)-LiTFSI polymer electrolyte leads to 3-dimensional network structures, and the ILGPE is stable below 150 °C and will not decompose until 310 °C without considering the volatilizing of the residual solvent. The ionic conductivity increases with the temperature rising, and the ionic conductivity is 0.23 × 10^-3 S cm^-1 and 2.1 × 10^-3 S cm^-1 at 20 °C and 80 °C, respectively. The electrochemical window is -0.15 to 4.85 V vs. Li⁺/Li, which is wide enough to be used with common electrode materials. Battery tests show that the ILGPE is stable even at 80 °C when operated in Li/LiFePO₄ batteries, and Li/LiFePO₄ batteries with the gel polymer electrolyte exhibit good cycling performance at 60 °C at 1C current rate. The discharge capacity was 131.01 mAh g^-1 after 100 cycles with 90% capacity retention.