Electrospun polar-nanofiber PVDF separator for lithium–sulfur batteries with enhanced charge storage capacity and cycling durability

Abstract

Lithium–sulfur batteries (LSBs) provide a high theoretical specific energy up to 5 times higher than that of lithium-ion batteries (2600 vs. ∼500 W h kg⁻¹) but their commercialization is restricted by intrinsic problems such as polysulfide shutting and dendrite growth at the Li metal anode during cycling. A polar, non-ionic, free-standing, electrospun, predominantly β-phase, polyvinylidene fluoride (PVDF) nanofiber separator membrane has been designed and realised in order to mitigate the polysufide shuttling effect. The β-PVDF sheet, with its inherent polarity, can chemically immobilize the polysulfides and suppress the formation of dendrites in LSBs. The fibrous separator obtained displays the combined advantages of being ultrathin (35 μm thick) and lightweight (0.85 mg cm⁻²), with very high porosity (79.5%), high wettability (95%), and high electrolyte affinity. In cells that were chosen deliberately to use a commercial cathode and to differ only in the separator used, the nanofibrous polar PVDF film showed high effective ionic conductivity, when LSB-electrolyte impregnated, at nearly double that of the commercially available polypropylene (PP) separator. After 100 GDC cycles at 0.05C, LSBs with polar PVDF separator and the commerical cathode material showed charge storage capacities of approximately 800 mA h g⁻¹, that being 200 mA h g⁻¹ greater than with otherwise identical cells containing a current commercial polyolefin-based separator.