Phosphonium-based deep eutectic solvents: Physicochemical properties and application in Zn-air battery

Electrolytes play a significant role in metal-air batteries and influence the battery's performance. Deep eutectic solvents (DESs) are viable and attractive candidates as high-performance electrolytes in batteries. This study investigated the temperature-dependent physicochemical properties of several phosphonium-based DESs such as the freezing point, density, viscosity, surface tension, and conductivity. The viscosity of the DES electrolyte is correlated as a function of molar conductivity using Walden's rule. Subsequently, the electrochemical characterizations of a Zn-air battery with the DES electrolyte, such as the cell voltage, power density, and current limit are studied. The electrochemical and physicochemical profiles of the phosphonium-based DESs possess dependence on the salt-to-hydrogen bond donor (HBD) molar ratio and appear viable for further exploration in metalair battery applications. To the best of our knowledge, this is the first study that investigates the efficacy of the phosphonium-based DESs as the electrolyte in Zn-air batteries.