Stability Promotion of Carbon-Based Quasi-Reference Electrodes in Nonaqueous Calcium Ion Electrolytes

The high demands of energy storage systems nowadays accelerate the development of alternatives to lithium ion batteries, especially calcium-based batteries. Nevertheless, the incompatibility between metallic calcium and most organic electrolytes hinders the research progress of calcium-based batteries due to the lack of suitable reference electrodes. Although carbon-based quasi-reference electrodes (QREs) have been widely employed as reference electrodes in Ca2+-containing electrolytes, promoting the potential stability of such QREs remains a challenge. Herein, we thoroughly elucidate all the possible factors, including the selection of carbon materials, the fabrication process of QREs, as well as the effects of solvents and anions, solvation structure, and electrolyte concentration, impacting the stability of carbon-based QREs. The donor number of solvents is the most critical factor affecting the potential shift of QREs when weakly coordinated calcium salts are used in the solutions, where all the inert properties of organic solvents, including donor number, dielectric constant, and surface area, contribute to the solvation structure in Ca2+-containing electrolytes examined by Raman spectroscopy. This research offers a new viewpoint on controlling the stability of QREs in the nonaqueous calcium ion electrolyte system, delivering very important information for future research on calcium ion batteries.