Small‐Molecular Crowding Electrolyte Enables High‐Voltage and High‐Rate Supercapacitors

Aqueous energy storage devices attract increased attention due to their high safety, low cost, and easy maintenance. However, the low energy density caused by the narrow electrochemical stability window (ESW) of aqueous electrolytes severely restricts their widespread applications. Herein, a new type of “small‐molecule crowding” electrolyte of 95EG‐H 2 O (95 wt% ethylene glycol [EG]) is proposed for the first time. Significant enhancement of water molecular stability is accomplished through the engineering of a hydrogen bond network. The small‐molecular crowding agent (EG) not only expands the ESW to 3.2 V, but also endows the electrolyte with low viscosity. As a proof‐of‐concept device, the symmetry carbon‐based supercapacitor using the newly developed electrolyte exhibits a so far record‐high operating voltage of 2.8 V, a high energy density of 58.7 Wh kg −1 at a power density of 1.4 and 30.3 Wh kg −1 at a power density of 42 kW kg −1 , and a durable lifespan exceeding 20 000 cycles at a current density of 5 A g −1 .