Mechanistic Insights into the Electrochemical Calcium-Tin Alloying Reaction.

Alloy-type anodes have shown excellent electrochemical performance in various ion-battery systems, such as lithium and sodium, owing to their high theoretical capacities and operating potentials close to those of the corresponding alkali metals. Extending this concept to multivalent systems, calcium shares many of the advantageous electrochemical characteristics while offering additional benefits such as high natural abundance and lower cost. However, the reaction mechanisms governing calcium alloying remain poorly understood. Here, we investigate the electrochemically driven alloying behavior of calcium with tin using bulk Sn foils and nanostructured Sn coatings on stainless steel in half-cell configuration to evaluate intrinsic reactivity, transport properties, and kinetic limitations. Electrochemical techniques were combined with XPS, ToF-SIMS, and a simulation-based model to characterize phase formation and reaction dynamics. The results reveal the formation of Ca-Sn alloy. Two distinct apparent diffusion regimes were identified: an initial, relatively fast process associated with near-surface calcium insertion, followed by a slower solid-state diffusion regime with significantly lower apparent diffusion coefficient. The findings highlight the strongly surface-confined nature of calcium alloying in tin and provide quantitative insight into a combined kinetic- and diffusion-controlled process governing calcium alloy-type anodes.