The critical role of configurational flexibility in facilitating reversible reactive metal deposition from borohydride solutions

Abstract

Development of calcium metal batteries has been historically frustrated by a lack of electrolytes capable of supporting reversible calcium electrodeposition. In this paper, we report the study of an electrolyte consisting of Ca(BH₄)₂ in tetrahydrofuran (THF) to gain important insight into the role of the liquid solvation environment in facilitating the reversible electrodeposition of this highly reactive, divalent metal. Through interrogation of the Ca²⁺ solvation environment and comparison with Mg²⁺ analogs, we show that an ability to reversibly electrodeposit metal at reasonable rates is strongly regulated by dication charge density and polarizability. Our results indicate that the greater polarizability of Ca²⁺ over Mg²⁺ confers greater configurational flexibility, enabling ionic cluster formation via neutral multimer intermediates. Increased concentration of the proposed electroactive species, CaBH₄⁺, enables rapid and stable delivery of Ca²⁺ to the electrode interface. This work helps set the stage for future progress in the development of electrolytes for calcium and other divalent metal batteries.