Asymmetrically Architectured PVDF Membranes with Enhanced Ion Conduction for Lithium-Metal Battery

Background: The instability of lithium-electrolyte interface is the major challenge limiting the practical applications of Li-metal anode as well as Li-metal batteries. To stabilize the anode surface, an asymmetric polyvinylidene fluoride (PVDF) membrane consisted of a bi-continuously nanoporous dense layer and a packing of micro-sized spherulites is proposed. Methods: The asymmetric PVDF membrane is served as the separator in lithium metal batteries, and the unique structure is prepared by nonsolvent induced phase separation (NIPS) method with triethyl phosphate as the solvent. Significant Findings: The asymmetric PVDF separator presents an outstanding ionic conductivity of 1.92 mS cm⁻¹ due to its high porosity and electrolyte uptake. Most significantly, the transference number of such separator reaches 0.73, which can be attributed to the enrich β-phase PVDF crystalline in the nanoporous dense layer. The battery performance in Li/LiFePO₄ cells also exhibits a good capacity retention of 91% after 100 cycles at 0.5 C. Moreover, a remarkable rate capability with specific capacity of 92 mAh g⁻¹ at 10 C is achieved, which is almost double of commercial polypropylene separator. These results demonstrate the promising potential for asymmetric PVDF membranes in lithium battery applications.