Study of SEI Components Enabling Calcium Metal Plating and Stripping

Due to serious concerns about future availability and price increase of lithium minerals, several alternatives have been investigated to substitute lithium-ion batteries in high demand applications. Divalent-cation chemistries, such as calcium and magnesium, promise a high energy capacity relying on much more abundant elements (5th and 8th, respectively, in the earth crust) than lithium (25 th ). Until recently, the development of such battery concepts was focused on electrolytes that do not form passivation films (solid electrolyte interphases - SEI) on the metal surface [1]. These electrolytes, however, suffer from a low stability towards oxidation and thus, limit the output voltage of the assembled cells. Previous works from our group demonstrated calcium plating and stripping in SEI-forming electrolytes at moderate temperatures [2]. Specifically, redox activity of calcium metal was observed in a Ca(BF 4 ) 2 electrolyte dissolved in a mixture of cyclic carbonates, while no activity was observed when TFSI was used as anion. The current study focuses on understanding the chemical composition differences between the SEI layers formed in the two systems analysed. A systematic characterization of the SEI formed on the Ca metal anode in various electrolyte formulations will be presented and the most suitable SEI compounds in terms of divalent cation mobility will be discussed. References [1] J. Muldoon, C. B. Bucur and T. Gregory, Chem. Rev., 2014, 114, 11683–11720. [2] A. Ponrouch, C. Frontera, F. Bardé and M. R. Palacín, Nat. Mater., 2016, 15, 169–172.