Fabrication of a porous polymer electrolyte from poly(vinylidene fluoride-hexafluoropropylene) via one-step reactive vapor-induced phase separation for lithium-ion battery

Gel polymer electrolyte with high ionic conductivity and high lithium ion transference number for lithium-ion batteries attracts unremitting pursuit of scientists. For that, we develop a novel strategy of one-step reactive vapor-induced phase separation (RVIPS) to fabricate a porous polymer electrolyte (PPE) membrane from poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP). The polymer and a salt of lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) were first mixed in acetone, then the solution was cast under an ammonia water atmosphere to evaporate solvent. The porous microstructure of the obtained PPE membrane was carefully analyzed and was compared with that of the PPE membrane prepared under water vapor of a usual VIPS. Moreover, the electrochemical properties of the PPE membranes were measured. Our results indicate that the reactive vapor of ammonia water induced dehydrofluorination of PVDF-HFP to result in its cross-linking. The cross-linking together with the plasticizer effect of the incorporated LiTFSI suppresses the crystallization of PVDF-HFP, while enhancing the flexibility of the polymer chain segments. As a result, the PPE membrane containing 10% LiTFSI displays interconnected pores and takes up 240% liquid electrolyte, so as to possess an ionic conductivity as high as 1.32 x 10(-3) S cm(-1) at 30 C-o. In addition, battery tests demonstrate a high-rate ability and excellent cycle performance of the PPE membrane fabricated via the RVIPS. An initial capacity of 128 mAh? g(-1), 97.7% capacity retention and 99.9% Coulombic efficiency at 1C after 120 cycles of the cell equipped with the PPE membrane undoubtedly indicate its promising potential.