On the Relevance of the Polar β-Phase of Poly(vinylidene fluoride) for High Performance Lithium-Ion Battery Separators

Separator membranes based on poly(vinylidene fluoride), PVDF, poly(vinylidene fluoride-co-trifluoroethylene), PVDF-TrFE, poly(vinylidene fluoride-co-hexafluoropropylene), PVDF-HFP, and poly(vinylidene fluoride-co-chlorotrifluoroethylene), PVDF-CTFE, were prepared by a solvent casting method using N,N-dimethylformamide (DMF) as solvent. In all cases, the same polymer/solvent ratio and solvent evaporation temperature were used. For all membranes, a porous microstructure is achieved with a degree of porosity larger than 50%. The β-phase content as well as degree of crystallinity were different for each membrane, which were lower for the copolymer membranes when compared with PVDF. On the other hand, the observed ionic conductivity values, electrolyte uptake, tortuosity, and MacMullin number were similar for all membranes. The electrochemical performance of the separator membranes was evaluated in a Li/C–LiFePO4 half-cell configuration showing good cyclability and rate capability for all membranes. Among all the separator membranes, PVDF-TrFE demonstrated the best electrochemical performance, with a discharge capacity value of 87 mAh·g–1 after 50 cycles with a capacity retention of 78% at 2C. Finally, the correlation between the β-phase content in the membranes and the cycling performance was demonstrated (which was significant at high C rates): larger β-phase contents, leading to higher polarity, facilitates faster lithium ion migration within the separator for similar microstructures.