Poly(vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-CTFE) lithium-ion battery separator membranes prepared by phase inversion

Abstract

Separator membranes based on poly(vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-CTFE) were prepared by a solvent casting technique based on its phase diagram in N,N-dimethylformamide (DMF) solvent. The microstructure of the PVDF-CTFE separator membranes depends on the initial position (temperature and concentration) of the solution in the phase diagram of the PVDF-CTFE/DMF system. A porous microstructure is achieved for PVDF-CTFE membranes with solvent evaporation temperatures up to 50 °C for a polymer/solvent relative concentration of 20 wt%. The ionic conductivity of the separator depends on the degree of porosity and electrolyte uptake, the highest room temperature value being 1.5 mS cm⁻¹ for the sample with 20 wt% of polymer concentration and solvent evaporation temperature at 25 °C saturated with 1 mol L⁻¹ lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) in propylene carbonate (PC). This PVDF-CTFE separator membrane in Li/C–LiFePO₄ half-cell shows good cyclability and rate capability, showing a discharge value after 50 cycles of 92 mA h g⁻¹ at 2C, which is still 55% of the theoretical value. PVDF-CTFE separators are thus excellent candidates for high-power and safe lithium-ion battery applications.