Towards Dry and Contaminant Free Ca(BF4)2 Based Electrolyte for Ca Metal Anode Based Batteries

Divalent metal batteries have gained much attention recently given the high gravimetric capacities of calcium and magnesium metal electrodes (1340 and 2000 mAh/g respectively), and thus, holding promise as high energy density next-generation battery technologies. Although calcium metal possesses lower reduction potential (-2.76 V vs SHE) than magnesium (-2.38 V vs SHE), the development of calcium metal batteries has been hampered by the lack of electrolytes allowing for reversible electrodeposition. After the first demonstration of reversible calcium plating/stripping in and organic electrolyte based on Ca(BF 4 ) 2 in carbonate solvents [1], the attention devoted to calcium-based batteries have substantially increased. Recently, other electrolyte systems allowing for reversible plating/stripping were reported [2–4]. One of the crucial parameters to consider in the devolvement of divalent metal batteries is the presence of undesired impurities, particularly water, as it will affect greatly the surface chemistry of the calcium metal anode and prevent electrodeposition. As normally alkaline-earth metal salts are more hygroscopic that their alkaline counterparts, obtaining ultra-dry electrolytes is a key challenge which needs to be addressed. In the case of commercial Ca(BF 4 ) 2 , most likely synthesised in aqueous solution, the water content is usually as high as 30wt%. In the present study we evaluate different drying methods and report on their effect on the stability of the BF 4 - anion. The performance of the differently dried electrolytes with regard to the calcium electrodeposition is evaluated and discussed. An anhydrous synthetic route of Ca(BF 4 ) 2 will also be presented and compared to the commercially available salt. References 1. A. Ponrouch, C. Frontera, F. Bardé, and M. R. Palacín, Nat. Mater. , 15 , 169–172 (2016). 2. D. Wang et al., Nat. Mater. , 17 , 16–20 (2017). 3. Z. Li, O. Fuhr, M. Fichtner, and Z. Zhao-Karger, Energy Environ. Sci. (2019). 4. A. Shyamsunder, L. E. Blanc, A. Assoud, and L. F. Nazar, ACS Energy Lett. , 4 , 2271–2276 (2019).