On the Parameters Affecting Calcium Plating and Stripping from Organic Electrolytes – Cases of Electrolyte Optimization

Among the different next-generation battery technologies, the ones based on divalent cations have recently gained particular attention due to their higher raw material availability and potentially higher energy density when compared with Li-ion. Such is the case of calcium, being the 5 th most abundant element in the earth crust and having a standard redox potential only 170 mV above metallic lithium. However, the development of a secondary calcium-metal battery had been hampered by the lack of organic electrolytes allowing for reversible plating and stripping, which was only recently reported by our group and others employing different electrolyte formulations. The different electrolyte components – salt(s), solvent(s) and additive(s) – are expected to play a significant role in the operation of a calcium-metal battery. Here we will show how the solvation structure of the cation evolves in relation to the solvent employed and the concentration of the salt. Combining conductivity measurements with Raman spectroscopy data, a high tendency to form contact ion pairs is evidenced, which significantly affects the mobility of the ions in the liquid media. The use of different anions and their bonding strength with Ca 2+ in solution will be discussed. Combined, the anion and solvent characteristics dictate the solvation of the cation, which in turn can affect the Ca plating and stripping kinetics. Additionally, the presence of electrolyte contaminants will be explored. The crucial yet very challenging drying of electrolyte containing Ca(BF 4 ) 2 in carbonate solvents will be presented. The difficulties to obtain anhydrous and contaminant free solutions from commercially available salts will be discussed and two alternative routes for anhydrous synthesis of the target salt will be presented.