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

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 prepare and characterize PVDF-HFP composite membranes swollen with a solution of LiTFSI in ether-functionalized pyrrolidinium-imide (PYRA12O1). The membranes are filled in with two different types of silica: i) mesoporous SiO2 (SBA-15) and a commercial nano-size one (HiSilTM T700). The ionic conductivity and the electrochemical properties of the gel electrolytes are studied in terms of nature and microstructure of the filler. The cycling behavior of Li/composite GPE/LiFePO4 cells at room temperature is also presented. 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/LiFePO4 cells with PVDF-PYRA12O1 films containing 10 wt% of SBA-15 show good charge/discharge capacity, coulombic efficiency close to unity, and low capacity losses at medium C-rates during 180 cycles.