Dual-crosslinked polyionic liquid polymer electrolyte with optimal balance of ion conductivity and mechanical properties for solid-state lithium metal battery

Polyionic liquids (PILs) are considered promising candidates for next-generation solid polymer electrolytes (SPEs) because they combine the electrochemical stability of ionic liquids with the processability/flexibility of polymers. However, the practical application of PILs-based SPEs are limited by their unsatisfactory mechanical properties in parallel with their insufficient ionic conductivity. Here, a double crosslinked PIL-based SPE with an optimal balance of ionic conductivity and mechanical properties is constructed by ultraviolet (UV) initiated polymerization. Among, the ionic liquid 1- (4- vinylbenzyl)- 3- butylimidazolium bis(trifluoromethanesulfonyl) imide ([VBBIM][TFSI]) as a monomer, poly(ethylene glycol) diacrylate (PEGDA) and polyhedral oligomeric sesquisiloxane (POSS) as a functional crosslinker. As expected, the synthesized PIL-based SPE (POSS-PIL PEGDA-1, 1 wt% POSS) shows a high ionic conductivity of 1.8 x 10-4 S cm-1 (at 30 degrees C), a satisfactory tensile strength of 0.64 MPa, an ultrahigh thermal degradation temperature around 320 degrees C and a wide electrochemical stability window about 5.0 V versus Li/Li+. As a concept proof, the POSS-PIL-PEGDA-1 SPE can effectively inhibit lithium dendrites growth, and the assembled Li||LiFePO4 solid-state lithium metal battery achieves a high discharge capacity and a good cycling performance. This strategy is attractive for studying the trade-off between mechanical properties and ionic conductivity of crosslinked PIL-based SPEs.