Li-ion battery separator membranes based on poly(vinylidene fluoride-trifluoroethylene)/carbon nanotube composites

Porous polymer membranes of poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) with different types and contents of carbon nanotubes (MWCNT) were prepared by solvent casting at room temperature. The influence of MWCNT on the morphological, thermal, mechanical and electrochemical properties of the composite membranes was evaluated. It was found that the presence of MWCNT in the polymer matrix does not affect the pore distribution but increases the average pore diameter from 9 for the pure polymer up to 60 μm for the composite samples. The overall porosity increases to ~ 82% and the electrolyte uptake to 275% for the 0.5 wt.% MWCNT filler content composite membrane. Vibration bands, thermal and mechanical properties are not significantly affected by the MWCNT content and type, the degree of crystallinity decreasing with increasing filler content. The overall electrochemical behavior of the composites is improved by the presence of the MWCNT: the room temperature ionic conductivity increases from ~ 5 × 10− 7 for the pure polymer to ~ 2 × 10− 6 S/cm for the composites and becomes more stable against temperature variations. The diffusion coefficient reach a maximum value of 3.84 × 10− 5 cm2/s for membranes with 0.2 wt.% of MWCNT. The composite membranes present a stable electrochemical operation window between − 2.0 and 4.0 V, independently of MWCNT content and type. Concluding, the inclusion of MWCNT fillers up to 0.5 wt.% in a P(VDF-TrFE) matrix improves the mechanical and electrochemical properties of this porous membrane showing adequate performance for battery separator applications.