Poly(vinylidene fluoride) separators with dual-asymmetric structure for high-performance lithium ion batteries

Dual-asymmetric poly(vinylidene fluoride) (PVDF) separators have been fabricated by thermally induced phase separation with dimethyl sulfone (DMSO2) and glycerol as mixed diluents. The separators have a porous bulk with large interconnected pores (~1.0 μm) and two surfaces with small pores (~30 nm). This dual-asymmetric porous structure endows the separators with higher electrolyte uptake amount and rapider uptake rate, as well as better electrolyte retention ability than the commercialized Celgard 2400. The separators even maintain their dimensional stability up to 160 °C, at which temperature the surface pores close up, leading to a dramatic decrease of air permeability. The electrolyte filled separators also show high ion conductivity (1.72 mS∙cm―1) at room temperature. Lithium iron phosphate (LiFePO4)/lithium (Li) cells using these separators display superior discharge capacity and better rate performance as compared with those from the commercialized ones. The results provide new insight into the design and development of separators for high-performance lithium ion batteries with enhanced safety.