Effect of the degree of porosity on the performance of poly(vinylidene fluoride-trifluoroethylene)/poly(ethylene oxide) blend membranes for lithium-ion battery separators

Porous polymer membranes based on poly(vinylidene fluoride-trifluoroethylene)/poly(ethylene oxide) copolymers, P(VDF-TrFE)/PEO, are prepared through the, from partial to total, elimination of PEO, leading to interconnected micropores in the polymer blends. Electrolyte uptake, thermal and mechanical properties depend on the amount of PEO present in the polymer blend. Further, the degree of crystallinity of PEO and the elastic modulus (E´) of the polymer blend decrease with increasing PEO removal. Electrical properties of the polymer blend membranes are influenced by the porosity and are dominated by diffusion. The temperature dependence of ionic conductivity follows the Arrhenius behavior. It is the highest for the membranes with a volume fraction of pores of 44% (i.e, 90% PEO removal), reaching a value of 0.54 mS.cm-1 at room temperature. Battery performance was determined by assembling Li/C-LiFePO4 swagelok cells. The polymer blends with 90% PEO removal exhibit rate (124 mAhg-1 at C/5 and 47 mAhg-1 at 2C) and cycling capabilities suitable for lithium ion battery applications.This work is funded by FEDER funds through the “Programa Operacional Factores de Competitividade–COMPETE” and by national funds from FCT–Fundação para a Ciência e a Tecnologia, in the framework of the strategic project Strategic Projects PEST-C/FIS/UI607/2013, PEST-C/QUI/UI0686/2013 and grants SFRH/BD/68499/2010 (C.M.C.), SFRH/BD/90313/2012 (A.G.) and SFRH/BD/66930/2009 (J.N.P.). The authors thank funding from “Matepro–Optimizing Materials and Processes”, ref. NORTE-07-0124-FEDER-000037”, co-funded by the “Programa Operacional Regional do Norte” (ON.2–O Novo Norte), under the “Quadro de Referência Estratégico Nacional”(QREN), through the “Fundo Europeu de Desenvolvimento Regional” (FEDER). JLGR acknowledges the support of the Ministerio de Economía y Competitividad, MINECO, through the MAT2013-46467-C4-1-R project (including FEDER finantial support). CIBER-BBN is an initiative funded by the VI National R&D&i Plan 2008– 2011, Iniciativa Ingenio 2010, Consolider Program, CIBER Actions and financed by the Instituto de Salud Carlos III with assistance from the European Regional Development Fund