1. School of Chemistry, Chemical Engineering and Life Science, State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China;2. School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China
Abstract:
Aqueous electrochemical energy storage (EES) devices have inherent advantages, such as high safety, environmental-friendliness, and low cost, exhibiting significant potential for application in future smart grids, portable/wearable electronics, and other fields. However, the low thermodynamic decomposition voltage of water (1.23 V) results in a narrow electrochemical stability window (ESW) of the aqueous electrolyte, limiting the selection of electrode materials. Therefore, aqueous alkali metal-ion batteries (AABs) have a low operating voltage and energy density. Considering the diverse application of AABs, the operation of AABs under extreme temperature conditions faces critical challenges. At a low temperature, the electrolyte freezes easily owing to the high freezing point of water (0 ℃); the ionic conductivity of the electrolyte decreases significantly, and the charge/discharge polarization increases. Therefore, AABs generally have a low capacity, poor rate performance, and low energy/power densities, and are unable to operate normally. At a high temperature, the water activity improves, and the side reaction of water decomposition intensifies. Hence, the cycle performance of AABs deteriorates, and the battery exhibits safety issues, such as expansion and thermal runaway. In recent years, significant research has been conducted to overcome the shortcomings of the aqueous EES, inspiring further research and development of future high-performance aqueous EESs. Reducing the water activity and increasing the hydrogen evolution reaction (HER) or oxygen evolution reaction (OER) overpotential are effective strategies to widen the ESW of aqueous electrolytes, which are mostly realized by utilizing high concentration salts, additives, and co-solvents. Using salt additives or organic co-solvents to break the intermolecular hydrogen bonds of water and reduce the interfacial charge transfer resistance are effective strategies to improve the low-temperature performance of AABs. Additionally, salt additives/co-solvents with high thermal stability can form strong hydrogen bonds with water, effectively improving the water retention and reducing the water activity, which ensure the enhanced electrochemical performance of the AABs at high temperatures. This review systematically summarizes the research progress of electrolyte design for AABs with a high voltage/wide operating temperature range. From the perspective of thermodynamics and kinetics, various strategies to widen the ESW and operating temperature range of the electrolyte as well as the relevant mechanisms are introduced. Potential concepts for designing high-voltage aqueous electrolytes with operation ability at a wide temperature range are proposed, and the development direction of high-performance AABs is presented.
