Preparation of poly(vinylidene fluoride) lithium-ion battery separators and their compatibilization with ionic liquid - a green solvent approach

Battery separator membranes for lithium-ion batteries have been developed based on poly(vinylidene fluoride), PVDF, and using the “green solvent” - N,N’-dimethylpropyleneurea (DMPU). This separator presents a porous microstructure with a degree of porosity of 20% and pore size below 1 µm, showing excellent mechanical properties. Further, its ionic conductivity, tortuosity and MacMullin number values are 0.1 mS.cm-1, 4 and 82, respectively. This PVDF separator membrane was evaluated in Li/C–LiFePO4 half-cells with electrolyte solution showing good cyclability and rate capability, with a discharge value after 50 cycles of 56 mAh.g-1 at C, corresponding to 50% of the capacity retention. Its electrochemical performance soaked in ionic liquid presents a discharge capacity of 45 mAh.g -1 at C/5-rate and capacity retention of 60 %, respectively. Thus, this separator is an appropriate candidate for environmentally friendlier and safer lithium-ion batteries.This work was supported by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UID/FIS/04650/2013. The authors thank FEDER funds through the COMPETE 2020 Programme and National Funds through FCT under the projects PTDC/CTM-ENE/5387/2014 and UID/CTM/50025/2013 and grants SFRH/BD/90313/2012 (A.G.), and SFRH/BPD/112547/2015 (C.M.C.). The authors acknowledge funding by the Spanish Ministry of Economy and Competitiveness (MINECO) through the project MAT2016-76039- C4-3-R (AEI/FEDER, UE) and from the Basque Government Industry Department under the ELKARTEK program.