Alkyl Ammonium Halides Solubilizer‐Mediated Interfacial Engineering Enables Reversible Divalent Metal Batteries

ABSTRACT The development of reversible calcium and magnesium metal batteries is plagued by the formation of ion‐blocking solid electrolyte interphases (SEIs) on metal surfaces. CaI 2 phase is recognized as a beneficial component for facilitating Ca 2+ transport, but low solubility of CaI 2 electrolyte results in CaI 2 ‐deficient SEIs and impedes Ca 2+ migration kinetics. Herein, a family of alkyl ammonium iodide (RNH 3 I) solubilizers is reported to reorganize Ca 2+ solvation structure, forming soluble I − ‐rich complex to effectively promote CaI 2 salt dissolution. The elevated I − concentration in CaI 2 /RNH 3 I electrolytes enriches CaI 2 species within the SEIs, while electrochemical reduction of RNH 3 + cations generates a Ca 3 N 2 component with a favorable migration barrier. Thanks to an enhanced Ca 2+ migration kinetics in CaI 2 /Ca 3 N 2 hybrid SEIs, the developed electrolytes realizes an over 20‐fold reduction in voltage polarization compared to blank CaI 2 counterpart, enabling stable cycling of reversible Ca metal batteries. Furthermore, this solubilization strategy could be readily extended to Mg metal batteries with electrochemical performance enhancements, demonstrating a universal electrolyte design concept for engineering divalent metal batteries’ interface.