Identifying interface-dominated behavior and cell configuration effects on the electrochemistry of calcium foil anodes

Abstract

Fundamental research and practical assembly of rechargeable calcium (Ca)-based batteries will benefit from the ability to use Ca metal anodes to provide a sufficient Ca²⁺ reservoir for full cell electrochemistry. When Ca metal comes in contact with an organic electrolyte, a native solid electrolyte interphase (SEI) forms which dramatically alters the electrode, and in some instances fully passivates it. Considering that Ca deposition and dissolution is widely believed to be a surface-film-controlled process, understanding the role of the interface on Ca electrochemistry is paramount. This study examines the electrochemical signatures of several Ca interfaces in a current benchmark electrolyte, Ca(BH₄)₂ in 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 a persistent and history-dependent layer from its prior handling, which manifests as reduction/oxidation overpotentials and characteristic interface-derived CV features. The initial surface-dependent behavior diminishes as Ca is continuously cycled, but formation of the native CaH₂ interface can return the CV to interface-dominated behavior. Imparting a synthetic, nanoscale-thickness CaF₂ layer prior to cell assembly can successfully decouple the Ca anode from the electrolyte; however, access points for more-active Ca plating/stripping form throughout cycling, progressively breaking through the film, and the interface is not chemically stable after several days.

CVs are compared in both three-electrode glass cell and two-electrode coin cell configurations, where the cell configuration is also shown to have significant effects on the resulting electrochemistry. The electrochemical signature of the native passivation layer is still present during the first cycle in coin cells, but high current density and cycle life are achievable with moderate cycling parameters.