Calcium metal negatrodes are interesting candidates to post lithium ion batteries, as calcium is highly abundant and has a low oxidation potential.
Due to the easily passivating nature of Ca the highest priority is to generate Ca2+-conducting artificial interphases. Ca dissolves in liquid ammonia. Upon evaporation of the solvent the metallic complex [Ca(NH3)6] forms and upon evacuation, metallic Ca is retrieved. We use this process to generate clean surfaces on Ca electrodes as well as a fine and highly reactive Ca powder. The powder can be directly deposited onto the current collector material. Thereby, thin deposits of Ca can be obtained. This is especially reasonable, as Ca is hard to process, making foils expensive and of inferior surface quality. The clean surfaces will be used to generate and study artificial SEIs, unbiased by native impurities. Good interphases seem to combine inorganic as well as organic compounds. The latter mainly form from the solvent. The inorganic part might be introduced from the wide range of (partially fluoroalkylated) acids towards borohydride. Highly reactive Ca powder might allow the synthesis of pure interphase materials to be routinely studied with bulk analytical methods.
The fluorine-free liquid electrolyte Ca[AlCl4]2 ∙ x SO2 does not contain environmentally persistent compounds. This system is very interesting, as it delivers extraordinarily high conductivities with an available, benign and cheap counter anion. The amount of solvent per ion would be diminished and the electrolyte is light-weight. The system exhibits a slight and manageable overpressure at rt. We think this and related systems are well qualified to hinder dissolution of polymer electrodes as well.