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

Abstract 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][BF 4 ] (PCE), PEO‐clay@[BMIM][BF 4 ] (PCB), and PEO‐clay@[HMIM][BF 4 ] (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][BF 4 ]) 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.