Calcium Metal Batteries with Long Cycle Life Using a Hydride‐Based Electrolyte and Copper Sulfide Electrode

As potential alternatives to Li‐ion batteries, rechargeable Ca metal batteries offer advantageous features such as high energy density, cost‐effectiveness, and natural elemental abundance. However, challenges, such as Ca metal passivation by electrolytes and a lack of cathode materials with efficient Ca(2+) storage capabilities, impede the development of practical Ca metal batteries. To overcome these limitations, the applicability of a CuS cathode in Ca metal batteries and its electrochemical properties are verified herein. Ex situ spectroscopy and electron microscopy results show that a CuS cathode comprising nanoparticles that are well dispersed in a high‐surface‐area carbon matrix can serve as an effective cathode for Ca(2+) storage via the conversion reaction. This optimally functioning cathode is coupled with a tailored, weakly coordinating monocarborane‐anion electrolyte, namely, Ca(CB(11)H(12))(2) in 1,2‐dimethoxyethane/tetrahydrofuran, which enables reversible Ca plating/stripping at room temperature. The combination affords a Ca metal battery with a long cycle life of over 500 cycles and capacity retention of 92% based on the capacity of the 10th cycle. This study confirms the feasibility of the long‐term operation of Ca metal anodes and can expedite the development of Ca metal batteries.