Cellular porous polyvinylidene fluoride composite membranes for lithium-ion batteries

Macroporous polyvinylidene fluoride (PVdF) membranes were prepared by a phase inversion method and evaluated as battery separators. Two totally different morphologies (cellular and finger-like) were obtained by coagulating PVdF solutions with two different solvents. The cellular membranes were formed immediately by precipitating the PVdF solution with a latent solvent (acetone) in water, while the finger-like membranes were precipitated from the PVdF solution with a true solvent (N-methyl-2-pyrrolidone). The incorporation of a silica filler decreased the ionic resistance of the PVdF membranes of both morphologies. However, the cellular membranes showed better mechanical properties and enabled higher ionic conductivities than the finger-like ones, especially when the silica loading was low. Compared with a conventional untreated polyolefin separator, the porous PVdF membranes showed good wettability by a liquid electrolyte. After being activated with a commercial LiPF6–ethylene carbonate–dimethyl carbonate electrolyte, the PVdF membranes were tested for their applications in lithium-ion batteries. Coin cells with these PVdF membranes exhibited stable cycling performance and good rate capability at room temperature. However, the cellular membranes are preferred over the finger-like ones because they offer higher mechanical performance, and can be processed into flat membranes more easily.