Ionogels as Polymer Electrolytes for Lithium–Metal Batteries: Comparison of Poly(ethylene glycol) Diacrylate and an Imidazolium-Based Ionic Liquid Crosslinker

We investigated the influence of neutral and charged crosslinkers on the physicochemical properties of ionogels (IGs) as well as their applicability in lithium–metal batteries (LMBs). Specifically, IGs with varying compositions of covalent crosslinker, that is, poly(ethylene glycol) diacrylate (PEGDA; charge neutral) or 1-(p-vinylbenzyl)-3-vinyl imidazolium bis(trifluoromethane sulfonyl)imide ([VBVI]TFSI; charged), ionic liquid monomer, ionic liquid, and lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) were synthesized by photopolymerization. Furthermore, the mechanical properties and ionic conductivities (ICs) of the samples were investigated by a unique setup allowing for simultaneous rheology and broadband dielectric spectroscopy. In general, higher mechanical stability and lower ICs were obtained with increasing content of ionic liquid monomer and crosslinker. Furthermore, VBVI-based IGs showed superior mechanical shear strength (G′ = 4 × 105 vs 105 Pa), while PEGDA-based IGs displayed higher ICs (e.g., for 30/70-V50-20 vs 30/70-P50-20: 0.17 vs 0.25 mS cm–1 at 25 °C). As the IC however includes the diffusion of all ions present, the Li+-transference number (tLi+) was determined to judge the actual Li+ conductivity and thus the potential performance in LMBs. Here, tLi+ was found to be 3–7 times higher for the VBVI-based IGs in comparison to the PEGDA-based ones despite their lower ICs. These results were corroborated by lithium plating/stripping experiments with alternating current densities (CDs). Here, higher CDs for VBVI-based IGs were found in comparison to their PEGDA-based counterpart (0.8 vs 0.1 mA cm–2), therefore demonstrating the improved Li+ transport. Finally, the compatibility of the IGs with the lithium metal was investigated by long-term plating/stripping experiments (53 full cycles, 106 h) at 30 °C. Our results highlight (1) the significance of tLi+ measurements in comparison to solely the ICs to judge the performance of IGs in LMBs and reveal (2) the superiority of VBVI-based IGs as a polymer electrolyte compared to conventional PEGDA-based IGs.