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

Area of Science:

• Materials Science
• Electrochemistry
• Energy Storage

Background:

• Supercapacitors offer high power and stability at elevated temperatures.
• High-temperature operation can exacerbate electrolyte ion transfer, leading to detrimental self-discharge.
• Mitigating self-discharge is crucial for reliable high-temperature supercapacitor performance.

Purpose of the Study:

• To investigate the impact of solid electrolytes on high-temperature self-discharge in supercapacitors.
• To develop a strategy for suppressing self-discharge using poly(ethylene oxide)-clay-ionic liquid composites.
• To correlate cation chain length with self-discharge suppression at elevated temperatures.

Main Methods:

• Fabrication of solid electrolytes based on poly(ethylene oxide) (PEO), bentonite clay, and various ionic liquids (ILs): PEO-clay@[EMIM][BF4] (PCE), PEO-clay@[BMIM][BF4] (PCB), and PEO-clay@[HMIM][BF4] (PCH).
• Assembly of all-solid-state supercapacitors using these electrolytes.
• Evaluation of self-discharge rates at 70°C and correlation with ionic liquid cation chain length ([EMIM+] < [BMIM+] < [HMIM+]).
• Analysis of the underlying mechanisms, including cation adsorption and diffusion-controlled faradaic processes.

Main Results:

• PEO-clay-IL solid electrolytes significantly reduced self-discharge in supercapacitors at 70°C compared to liquid electrolytes.
• Supercapacitors utilizing PCH (with [HMIM][BF4]) exhibited the lowest self-discharge rate, with only a 30.7% voltage drop over 10 hours.
• The observed improvement in self-discharge behavior is attributed to decreased diffusion-controlled faradaic processes, enhanced by bentonite clay's cation adsorption and stabilization effects.
• Soft-packaged supercapacitors based on longer-chain cations demonstrated consistent performance and low self-discharge at 70°C.

Conclusions:

• The chain elongation strategy using specific ionic liquids in PEO-clay electrolytes effectively suppresses high-temperature self-discharge in supercapacitors.
• Bentonite clay plays a key role in cation adsorption and high-temperature stabilization, further reducing self-discharge.
• This approach offers a promising pathway for developing robust supercapacitors for long-term energy storage applications at elevated temperatures.