Additive-level sodium improves calcium metal plating/stripping efficiency by controlling deposit morphology and interphase chemistry

Reversible Ca metal deposition remains one of the major challenges in the development of rechargeable Ca metal batteries. Among proposed electrolyte design strategies, alkali-metal incorporation has emerged as a promising approach for improving Ca plating/stripping. In this work, Na[B(hfip) 4 ] (hfip = hexafluoroisopropoxy) was introduced as an additive into a Ca[B(hfip) 4 ] 2 /DME electrolyte, and its influence on Ca plating/stripping was evaluated using electrochemical measurements and SEM-EDX characterization of the deposits. Additive-level Na incorporation resulted in reduced polarization, increased Coulombic efficiency, and improved cycling stability, with the Ca:Na = 95:5 electrolyte exhibiting the best performance and a maximum Coulombic efficiency of 83%. Comparison of two commercially available carbon-coated Al current collectors revealed that substrate properties influence the electrochemical response, with graphitic domains supporting additional reversible redox processes attributed to intercalation of solvated cations. Galvanostatic measurements at different deposited capacities and current densities showed that both parameters strongly influence electrochemical performance and deposit morphology. Lower deposited capacities and higher current densities promoted smaller, more uniformly distributed deposits and improved cycling stability, whereas higher deposited capacity resulted in deposit agglomeration and accelerated performance degradation. XPS analysis further demonstrated that Na incorporation alters the interphase chemistry, decreasing Ca–F and carbonate contributions while increasing the Ca–O contribution and introducing Na-containing species. Overall, the results demonstrate that additive-level Na improves the reversibility of Ca plating/stripping while maintaining Ca-based deposition.