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][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.

Different shapes based on ionic liquid leading to a two-stage discharge process

“…Two stages can be found in the self-discharge of experiments on [C 2 mim][BF 4 ] (Fig. 7d) conducted by Yang and coworkers 39,54 and in the fast-discharging process of experiments on [C 2 mim][BF 4 ] (Fig. 8a) conducted by Svyatoslav Kondrat and coworkers.…”

“…8a) conducted by Svyatoslav Kondrat and coworkers. 38 There are some commonalities between self-discharge and fast discharge: both processes rapidly release energy in the rst stage and then slowly decline in the second stage, which is 39 The prefix "PC" represents the mixtures of poly(ethylene oxide) (PEO) and bentonite clay, and "H"/"B" and "E" represent ILs before the parentheses.…”

“…The latter is a schematic diagram from earlier studies of a self-discharge experiment by Yang and coworkers. 39 The addition of poly(ethylene oxide) (PEO) and bentonite clay into the same ILs ([C 2 mim][BF 4 ]) caused only slight changes, which can be seen from the small differences between the red and green lines in Fig. 7d.…”

“…Therefore, the adsorption capacity on bentonite clay of [C 6 mim] + is far greater than that of [C 4 mim] + and [C 2 mim] + . 39 However, Mattia Belotti and coworkers explained this difference with chain elongation of cations by their projected dipole moments and strong hydrogen bonding in an electric field, 19 and they did not attribute it to any materials that could adsorb the cations. This result is more reasonable since most of the [C n mim] + is closer to the negative electrode at the first stage of discharge rather than being in contact with bentonite clay in the middle of the electrolyte.…”

“…(a) Represent the cumulative number of p-, i-and j-shaped cations with n ¼ 2, (b) is for that with n ¼ 4, (c) is for that with n ¼ 6. (d) Schematic diagram of the self-discharging process from experimental results by Yang 39. The prefix "PC" represents the mixtures of poly(ethylene oxide) (PEO) and bentonite clay, and "H"/"B" and "E" represent ILs before the parentheses.…”