Working Mechanism of Chlorine‐Containing Electrolyte in Conversion‐Type Cathode Materials for Rechargeable Magnesium‐Ion Batteries

ABSTRACT Rechargeable magnesium‐ion batteries (RMBs) have garnered significant attention due to their high theoretical capacity, low cost, and outstanding safety. However, the detailed working mechanism of RMBs cathode, especially conversion‐type cathode, in chloride‐containing electrolytes remains poorly understood. Convincing experimental evidence is still lacking to clearly elucidate the reaction pathway. Herein, electrochemical quartz crystal microbalance (EQCM) was employed to track the dynamic mass changes of cathode during the cycling. Combined with comprehensive spectroscopic characterizations and electrochemical analyses, main active species reacting with transition metal chalcogenide cathode materials was unambiguously identified as [MgCl] + , instead of Mg 2+ . Through systematic screening, the optimal electrolyte formulation was identified, and the CoS cathode delivers an excellent specific capacity of 260 mAh g −1 at a current density of 100 mA g −1 . This study reveals the charge‐carrier of RMBs cathode and provides a solid theoretical foundation for the electrolyte design for RMBs.