Structural Battery Electrolytes Based on a Cross‐Linked Methacrylate Polymer and a Protic Ionic Liquid: Is There an Optimal Composition?

Within the development of structural batteries, finding the optimal electrolyte composition, that is, one that offers both high ionic conductivity and mechanical stiffness, is essential. Structural batteries are multifunctional composites able to store electrical energy within load‐bearing elements of devices. Their use results in a significant mass reduction, thereby improving fuel efficiency and enabling a shift to sustainable energy. In this work, structural battery electrolytes consisting of a methacrylate‐based polymer, 1‐ethylimidazolium bis(trifluoromethylsulfonyl)imide protic ionic liquid, and a lithium salt are investigated. Interestingly, the transport properties of the confined liquid electrolyte seem primarily limited by the percolation of the polymer network. Furthermore, upon confinement, a decrease in the glass transition temperature of the polymer phase and weaker intermolecular interactions are observed, which correlate to faster local dynamics. The self‐diffusivity of the Li ions keeps high with respect to the other diffusing ions and tends to decouple from the anions upon increased temperature. The composite sample with 50 wt% of liquid electrolyte shows an ionic conductivity of ≈0.1 mS cm −1 with a shear storage modulus of ≈150 MPa and was thus selected for proof‐of‐concept tests by electrochemical methods. Overall, this comprehensive study highlights the versatility of these biphasic systems for various applications.