CO2-regulated, ultrahigh-concentration aqueous electrolytes for high energy and power of supercapacitors

Abstract

Aqueous supercapacitors are recognized for their high power density, ultralong cycle life, and reliable safety. However, their widespread applications are impeded by low energy density arising from the narrow electrochemical stability windows of aqueous electrolytes. Although the use of high-concentration aqueous electrolytes can mitigate this issue, sluggish ion diffusion within these electrolytes results in decreased capacitance and power performance of supercapacitors. Herein, we propose the simultaneous achievement of high energy and power density of supercapacitors using carbon dioxide (CO2)-regulated high-concentration aqueous electrolytes. The introduction of CO2 enables greater capacitance in sodium perchlorate (NaClO4)-containing electrolyte than sodium nitrate (NaNO3)- and sodium bis (fluorosulfonyl)imide (NaFSI)-containing electrolytes. This is attributed to the increased H+ concentration and the unique interaction between CO2 and ClO4−, leading to a denser counter-ion arrangement on the electrode surface. Notably, the CO2-regulation strategy is effective in a nearly-saturated 17 mol kg−1 NaClO4 electrolyte, showing an increase in energy density of up to 27.8 %, while maintaining a high 2.2 V operating voltage and a long >20 000 cycle life of supercapacitors. This study offers a distinct insight into the regulation of high-concentration aqueous electrolytes for comprehensive enhancement of supercapacitor performance.