Lithium ion conducting PVdF-HFP composite gel electrolytes based on N-methoxyethyl- N-methylpyrrolidinium bis(trifluoromethanesulfonyl)-imide ionic liquid

Abstract

Blends of PVdF-HFP and ionic liquids (ILs) are interesting for application as electrolytes in plastic Li batteries. They combine the advantages of the gel polymer electrolytes (GPEs) swollen by conventional organic liquid electrolytes with the nonflammability, and high thermal and electrochemical stability of ILs. In this work we prepared and characterized PVdF-HFP composite membranes swollen with a solution of LiTFSI in ether-functionalized pyrrolidinium-imide ionic liquid (PYRA 12O1TFSI). The membranes were filled in with two different types of silica: (i) mesoporous SiO 2 (SBA-15) and (ii) a commercial nano-size one (HiSil™ T700). The ionic conductivity and the electrochemical properties of the gel electrolytes were studied in terms of the nature of the filler. The thermal and the transport properties of the composite membranes are similar. In particular, room temperature ionic conductivities higher than 0.25 mS cm −1 are easily obtained at defined filler contents. However, the mesoporous filler guarantees higher lithium transference numbers, a more stable electrochemical interface and better cycling performances. Contrary to the HiSil™-based membrane, the Li/LiFePO 4 cells with PVdF-HFP/PYRA 12O1TFSI–LiTFSI films containing 10 wt% of SBA-15 show good charge/discharge capacity, columbic efficiency close to unity, and low capacity losses at medium C-rates during 180 cycles.