Chain-Elongated Ionic Liquid Electrolytes for Low Self-Discharge All-Solid-State Supercapacitors at High Temperature

High power and good stability enable supercapacitors to work efficiently at high temperatures. However, the high-temperature-induced excessive ion transfer of the electrolyte would lead to severe self-discharge behavior, which has often been overlooked but can be highly detrimental. In this study, solid electrolytes consisting of poly(ethylene oxide) (PEO), bentonite clay, and ionic liquids (IL)-PEO-clay@[EMIM][BF₄ ] (PCE), PEO-clay@[BMIM][BF₄ ] (PCB), and PEO-clay@[HMIM][BF₄ ] (PCH) lead to dramatic decreases in self-discharge when used in all-solid-state supercapacitors at high temperature of 70 °C, which correlate with chain elongation (i. e., [EMIM⁺ ]<[BMIM⁺ ]<[HMIM⁺ ]). Benefiting from both cation adsorption and high-temperature stabilization by bentonite clay, PCH-based supercapacitors (IL=[HMIM][BF₄ ]) deliver an extremely low self-discharge rate, with only a 30.7 % voltage drop over 10 h at 70 °C (44.5 % for 38 h), which is much lower than that of traditional liquid supercapacitors (63.7 % drop over 10 h at 70 °C). This improvement in high-temperature self-discharge behavior is found to be from the decrease in diffusion-controlled faradaic process. Based on the longer-chain [HMIM⁺ ], soft-packaged supercapacitors exhibit a low self-discharge rate and work consistently at 70 °C. This chain-elongation strategy provides a new possibility for the suppression of self-discharge behavior in supercapacitors and further aids long-term energy storage by supercapacitors at high temperatures.