Free-standing polymer electrolyte for all-solid-state lithium batteries operated at room temperature

This study reports a networked solid polymer electrolyte (N-SPE) containing no solvent, ionic liquid, oligomer, or semisolid additives for lithium-ion batteries (LIBs). The N-SPE comprises a lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt as well as a polymer framework constructed using cage-like polyhedral oligomeric silsesquioxane (POSS) and serving as hubs to network poly(ethylene oxide-co-polypropylene oxide) (P(EO-co-PO)) chains. The networking prevents polymer chain twisting that hinders ion transport. Raman analysis indicates that the POSS hubs improve the dissociation of LiTFSI and localize TFSI⁻ anions. The N-SPE exhibits a low glass transition temperature of −43 °C, a high 25 °C ionic conductivity of 1.1 × 10⁻⁴ S cm⁻¹, and a small activation energy of 3.5 kJ mol⁻¹ for ion conduction. The localization of TFSI⁻ results in a high lithium transference number of 0.62, which is determined to be beneficial to Li⁺ transport. By incorporating the N-SPE into the LiFePO₄ cathode and using a free-standing N-SPE membrane, this study assembles a Li|N-SPE|LiFePO₄ battery, which delivers a high capacity of 160 mAh g⁻¹ at 25 °C and exhibits excellent charge−discharge cycling stability. The free-standing feature of the N-SPE makes roll-to-roll assembly of LIBs readily scalable for industrial applications.