Influence of the porosity degree of poly(vinylidene fluoride-co-hexafluoropropylene) separators in the performance of Li-ion batteries

Polyvinylidenedifluoride–hexafluoropropylene, (P(VdF–HFP))-based polymer electrolytes, as separators for lithium batteries, were prepared through different polymer/solvent (N,N-dimethylformamide, DMF) ratios and physicochemically investigated. Scanning electron microscopy measurements have shown a homogeneously distributed porosity within the membranes, with moderately tortuous pathways, resulting in a liquid uptake up to 77 wt.% with respect to the overall weight and conduction values above 10−3 S cm−1 at room temperature. Prolonged cycling tests, performed on Li/Sn–C and Li/LiFePO4 half-cells based on P(VdF–HFP) polymer electrolyte separator membranes, have evidenced nominal capacities ranging from 70% to 90% of the theoretical value with very good capacity retention and charge/discharge efficiency close at 100% even at high current rates. A capacity decay is observed at high current regime, associated to the diffusion phenomena occurring within the electrode and the polymer electrolyte separator membrane.This work is funded by Federacion Espanola de Enfermedades Raras funds through the "Programa Operacional Factores de Competitividade-COMPETE" and by national funds from FCT-Fundacao para a Ciencia e a Tecnologia, in the framework of the strategic project Strategic Project PEST-C/FIS/UI607/2011 and PEst-C/QUI/U10686/2011. The authors also thank funding from Matepro-Optimizing Materials and Processes, ref. NORTE-07-0124-FEDER000037, co-funded by the "Programa Operacional Regional do Norte" (ON.2 - O Novo Norte), under the "Quadro de Referencia Estrategico Nacional" (QREN), through the "Fundo Europeu de Desenvolvimento Regional" (FEDER), and grants SFRH/BD/66930/2009 (J.N.P.) and SFRH/BD/68499/2010 (C.M.C.). G.B.A. thank the Italian Institute of Technology for the financial support. The authors thank Solvay for kindly supplying the high quality materials