A "Water-in-ionic Liquid" Electrolyte for Aqueous Supercapacitors with High Energy Density and High Power Density

Description

Electrochemical double-layer capacitors (EDLCs) are promising energy storage devices for applications that simultaneously require high power output and a long lifespan. In commercial EDLCs, organic electrolytes have been widely used by virtue of their broad electrochemical windows that endow a high operation voltage and thereof an increased energy density. Nevertheless, the flammability and volatility of organic electrolytes bring in safety concerns, and the critical purity requirement of organic electrolytes inevitably makes the fabrication complex, thereby increasing the cost of EDLCs. Moreover, the relatively sluggish ion transportation in organic electrolytes also limits further improvements in the power density of EDLCs. Thus, developing new electrolytes concurrently featured with wide electrochemical windows, high ionic conductivities, reduced cost, and superior safety is desirable for the further advance of EDLC technology.In this project, we will develop a "water-in-ionic liquid" electrolyte with a broad electrochemical window and rapid ion transportation to improve the energy density, power density, and cycle stability of EDLCs. This project is based on the followingrationales: 1) ionic liquids are compounds completely comprising cations and anions that may have strong interactions with water molecules; 2) the unique solvation structures induced by the strong interactions between the cations/anions of ionic liquids and water molecules are expected to suppress the decomposition of water, and thus broaden the electrochemical windows of electrolytes; 3) the introduction of water into ionic liquids is favorable to the dissociation of ionic liquids, which decreases the viscosity of electrolytes and thus modulate their ionic conductivity. In the project, a series of ionic liquids with different combinations of cations (e.g., pyrrolidinium- and imidazolium-based cations) and anions (e.g., sulfonate-, tetrafluoroborate-, and imide-based anions) will be used to prepare electrolytes, and the impacts of water incorporation in ionic liquids on the solvation structures, electrochemical windows, and ionic conductivities of the electrolytes will be studied. Moreover, prototypes of EDLCs will be fabricated to demonstrate the practical applications of the screened electrolytes. The implementation of the proposed project will lead to further understanding, and insight into, the design and development of new electrolyte systems for high-performance EDLCs, as well as other aqueous energy storage devices such as Li-, Na-, and Zn-ion supercapacitors and batteries.