Towards a Reversible Calcium Anode through Electrolyte and Interface Engineering for Calcium Metal Batteries
Authors: Melemed Aaron Max , Gallant Betar M. 
Published: September 2023

Lithium-ion batteries have played an important role in decarbonizing the transportation sector, but broader utilization is currently limited by practically-attainable energy densities, safety concerns, and supply chain issues. Batteries based on calcium (Ca) metal anodes offer higher theoretical energy densities, as well as significant potential improvements in scalability and sustainability. The field of Ca metal batteries is currently in its early stages, however, due to a limited number of electrolytes that can reversibly plate and strip Ca. Two important challenges to overcome are (1) the formation of passivating solid electrolyte interphases (SEI) between Ca and the electrolyte that prevent Ca2+ transport, and (2) Ca2+-anion interactions in the electrolyte that promote cluster formation and hinder electrochemistry.1,2 This talk will explore research performed in both aspects of Ca battery development and identify strategies for future advancements. First, the electrochemical signatures of three model interfaces on Ca foil electrodes were investigated in a baseline electrolyte using cyclic voltammetry;3 the long-term cycling behavior was governed to a greater extent by electrolyte properties than the initial surface layers. Subsequent efforts have focused on electrolyte engineering, and specifically, on studying the impact that the Ca2+ coordination environment has on reversible Ca plating and stripping. For example, strong coordination between Ca2+ and the anion TFSI- has been demonstrated to deactivate Ca plating behavior via TFSI- decomposition.4,5 However, competition between coordinating molecules can be exploited to favor specific Ca2+ speciation; preferential coordination between Ca2+ and BH4
- displaces bidentate TFSI---Ca2+, unlocking Ca plating behavior in tetrahydrofuran.5 Further studies into preferential coordination are ongoing in order to better identify the connections between Ca2+ coordination, SEI formation, and anode performance. We hope to identify specific electrolyte design criteria that can advance the functionality of Ca electrolytes and ultimately examine the feasibility of Ca as a novel battery chemistry.

1 Aurbach, D., Skaletsky, R. & Gofer, Y. The Electrochemical Behavior of Calcium Electrodes in a Few Organic Electrolytes. Journal of The Electrochemical Society
138, 3536-3545 (1991).

2 Arroyo-de Dompablo, M. E., Ponrouch, A., Johansson, P. & Palacín, M. R. Achievements, Challenges, and Prospects of Calcium Batteries. Chemical Reviews (2019).

3 Melemed, A. M. & Gallant, B. M. Electrochemical Signatures of Interface-Dominated Behavior in the Testing of Calcium Foil Anodes. Journal of The Electrochemical Society
167, 140543 (2020).

4 Hahn, N. T. et al. Influence of Ether Solvent and Anion Coordination on Electrochemical Behavior in Calcium Battery Electrolytes. ACS Applied Energy Materials
3, 8437-8447 (2020).

5 Melemed, A. M., Skiba, D. A. & Gallant, B. M. Toggling Calcium Plating Activity and Reversibility through Modulation of Ca2+ Speciation in Borohydride-Based Electrolytes. The Journal of Physical Chemistry C
126, 892-902 (2022).