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

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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.

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