Electrochemical Signatures of Interface-Dominated Behavior in the Testing of Calcium Foil Anodes

Fundamental research and practical assembly of rechargeable calcium (Ca) batteries will benefit from an ability to use Ca foil anodes. Given that Ca electrochemistry is considered a surface-film-controlled process, understanding the interface's role is paramount. This study examines electrochemical signatures of several Ca interfaces in a benchmark electrolyte, Ca(BH₄)₂/tetrahydrofuran (THF). Preparation methodologies of Ca foils are presented, along with Ca plating/stripping through either pre-existing, native calcium hydride (CaH₂), or pre-formed calcium fluoride (CaF₂) interfaces. In contrast to earlier work examining Ca foil in other electrolytes, Ca foils are accessible for reversible electrochemistry in Ca(BH₄)₂/THF. However, the first cyclic voltammetry (CV) cycle reflects persistent, history-dependent behavior from prior handling, which manifests as characteristic interface-derived features. This behavior diminishes as Ca is cycled, though formation of a native interface can return the CV to interface-dominated behavior. CaF₂ modification enhances such interface-dominance; however, continued cycling suppresses such features, collectively indicating the dynamic nature of certain Ca interfaces. Cell configuration is also found to significantly influence electrochemistry. With appropriate preparation of Ca foils, the signature of interface-dominated behavior is still present during the first cycle in coin cells, but higher current density compared to three-electrode cells along with moderate cycle life are readily achievable.