Enhanced Durability of Ca Metal Battery with Dual Salt: Synergistic Effect on Solid Electrolyte Interphase and...

The use of Ca metal in battery technology is a promising approach owing to its high energy density and sustainability. However, the increased battery resistance during extended cycling significantly narrows its application range. This study aimed to improve the long-term stability of Ca deposition by employing a dual-salt strategy based on calcium monocarborane, Ca(CB₁₁H₁₂)₂, which demonstrated favorable Ca deposition characteristics as a single-salt electrolyte. The addition of LiBr to Ca(CB₁₁H₁₂)₂ resulted in a low battery resistance even after 200 h of cycling in contrast to the single-salt electrolyte whose resistance gradually increased. The dual-salt effect was examined by investigating solvation structures and electrolyte decomposition products. The introduction of Li cations into Ca(CB₁₁H₁₂)₂ significantly altered the solid electrolyte interphase composition, effectively mitigating the increase in resistance during cycling. Furthermore, the inclusion of LiBr salt induced substantial changes in the solvation structures, reducing the number of solvent molecules surrounding Ca²⁺ ions. This transformation was accompanied by a noticeable decrease in the amount of CaCO₃ among the electrolyte decomposition products and simultaneous increase in the polymer-based solid electrolyte interphase. The application of the dual-salt electrolyte comprising Ca(CB₁₁H₁₂)₂ and LiBr demonstrated robust cycling stability over extended periods in two-electrode cells utilizing Ca metal anodes and anthraquinone-based organic cathodes. The capacity retention remained at 75% after 200 cycles, indicating the highest performance observed among the previously reported batteries containing Ca metal anodes and organic cathodes in two-electrode cell systems. This study highlights the efficacy of the dual-salt approach based on the stability of Ca(CB₁₁H₁₂)₂ and its exceptional ability to enhance the long-term stability of Ca metal deposition, thereby significantly improving the practical application prospects of Ca-based batteries.