Pronounced Role of Lithium‐Controlling Polymer in Water‐Processable/Halogen‐Free All‐Solid‐State Electrolytes for Lithium Supercapacitors

Polymeric solid-state electrolytes (SSEs) with environmentally friendly processes deliver safer and cleaner energy storage devices without fires and leakages than conventional liquid electrolytes. Here, water-processable halogen-free polymeric SSEs are demonstrated with high ion conductivity (≈6 mS cm^-1), prepared from aqueous solutions consisting of branched poly(ethylene imine) (bPEI), lithium hydroxide (LiOH), and poly(4-styrene sulfonic acid) (PSSA). The bPEI:LiOH:PSSA (PLP) SSEs with various PSSA molar ratios are applied to asymmetric supercapacitors with graphite-based anodes and indium tin oxide (ITO) counter electrodes. The PSSA molar ratio strongly affected the ion conductivity of PLP SSEs, leading to a maximum at PSSA = 40 mol%, owing to the role of PSSA in controlling the size of LiOH domains for better Li⁺ transport pathways. The enhanced ion conductivity enabled PLP-supercapacitors to build a high potential of 2.24 V at PSSA = 40 mol%, compared to 1.64 V at 0 mol%, upon galvanostatic charge/discharge at a current density of 0.2 mA g^-1. The endurance test shows that the supercapacitors with the PLP SSEs (PSSA = 40 mol%) can function stably with high capacitance retention (96.2%) for more than 5000 cycles, and ≈80% capacitance retention at 80 °C, supporting their practical use in high-safety supercapacitors and batteries.