Enhanced Separator Properties By Thermal Curing of Poly(ethylene glycol)Diacrylate-Based Gel Polymer Electrolytes for Lithium-Ion Batteries

Porous polyethylene (PE) or nonwoven poly(vinylidene fluoride) (PVdF) separator-supported gel polymer electrolytes are realized by thermal polymerization of a precursor solution consisting of poly(ethylene glycol)diacrylate (PEGDA) and an electrolyte solution (1 M LiPF₆ in an equal-volume mixture of ethylene carbonate and dimethyl carbonate). The polymerization conditions are optimized to include a PEGDA content of 3 wt.% in the precursor solution and subsequent heat treatment at 80^oC for 10 min. Even though the gelled PEGDA electrolyte has a lower ionic conductivity than the electrolyte solution, a Li­_xCoO₂/graphite full-cell that has a gel electrolyte with optimized PEGDA content on the PVdF separator achieves a battery performance superior to the one with PE. The best battery performances achieved are a high discharge capacity (116 mAh g^-1), a good high-rate capability (95 mAh g^-1 at 5.0 C-rate), and a high capacity retention ratio (90%) after the 100th cycle. This enhancement is due to the incorporation of a polar electrolyte solution that is entrapped by the polar PEGDA matrix within the nonwoven PVdF separator, which is a more suitable host that is able to well absorb and preserve the gel electrolyte. Figures below are (left) a schematic diagram of gelation procedures from the full-cell fabrication to its heat treatment via a droplet insertion of the precursor solution and (right) cycle performance of the Li_xCoO₂/graphite full-cells at 1.0 C-rate, using the PEGDA-containing gel polymer electrolytes on PE and PVdF separators.