1Osaka University, Osaka, Osaka, Japan
A liquid electrolyte is an indispensable component in rechargeable batteries, which functions as an ion-conductive and electron-insulating medium between positive and negative electrodes. An ever-increasing demand for better batteries (with high voltage, high capacity, fast charging, and high safety) has set extraordinarily high standards for electrolyte materials, which has driven many researchers to work on the development of new liquid electrolytes.
The nature of electrolyte solutions is dominated by three factors: salts, solvents, and their mixing ratios (salt concentrations). Conventionally, the selection of salts and solvents has been the most important in designing lithium-ion battery electrolytes. On the other hand, salt concentration has been simply optimized to approximately 1 mol/L based on maximized ionic conductivity. Generally, further increasing salt concentration over the conventional 1 mol/L increases the viscosity and decrease the ionic conductivity, both of which are unfavorable for battery electrolytes. However, various unusual functions have been discovered at high salt concentrations of over 3 mol/L, including i) widened potential window, ii) fast electrode reactions, iii) high safety, iv) wide liquidus temperature range, and vi) prevention of Al corrosion, etc. As a result, concentrated nonaqueous and aqueous solutions are emerging as a new class of liquid electrolytes for advanced batteries. Importantly, these unique functions result from peculiar ion-solvent coordination structure achieved at high salt concentrations.
In this talk, I will introduce various unusual functions of concentrated electrolytes and discuss the mechanism from the viewpoint of their unique ion-solvent coordination structure. Furthermore, I will show a new aspect of battery electrolytes, Li+ chemical potential or Li electrode potential, as a descriptor of charge-discharge efficiency of advanced batteries.