Stability Promotion\nof Carbon-Based Quasi-Reference\nElectrodes in Nonaqueous Calcium Ion Electrolytes

The high demands of energy storage systems nowadays accelerate\nthe development of alternatives to lithium ion batteries, especially\ncalcium-based batteries. Nevertheless, the incompatibility between\nmetallic calcium and most organic electrolytes hinders the research\nprogress of calcium-based batteries due to the lack of suitable reference\nelectrodes. Although carbon-based quasi-reference electrodes (QREs)\nhave been widely employed as reference electrodes in Ca<sup>2+</sup>-containing electrolytes, promoting the potential stability of such\nQREs remains a challenge. Herein, we thoroughly elucidate all the\npossible factors, including the selection of carbon materials, the\nfabrication process of QREs, as well as the effects of solvents and\nanions, solvation structure, and electrolyte concentration, impacting\nthe stability of carbon-based QREs. The donor number of solvents is\nthe most critical factor affecting the potential shift of QREs when\nweakly coordinated calcium salts are used in the solutions, where\nall the inert properties of organic solvents, including donor number,\ndielectric constant, and surface area, contribute to the solvation\nstructure in Ca<sup>2+</sup>-containing electrolytes examined by Raman\nspectroscopy. This research offers a new viewpoint on controlling\nthe stability of QREs in the nonaqueous calcium ion electrolyte system,\ndelivering very important information for future research on calcium\nion batteries.