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

Abstract 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.