(Localized) Highly Concentrated Electrolytes for Calcium Batteries

Abstract The concept of non‐aqueous highly concentrated electrolytes (HCEs) for modern rechargeable batteries has recently evolved further by also adding a non‐coordinating solvent, i. e., a diluent, to create localized HCEs (LHCEs). LHCEs rely on a charge carrier design similar to that of HCEs in synergy with tailored macroscopic properties, especially reduced viscosity. LHCEs have now been extensively investigated for monovalent Li⁺ and Na⁺ based batteries, but here we investigate both HCEs and LHCEs for divalent Ca²⁺ conducting systems. Here we systematically map both molecular and macroscopic features as function of composition of Ca(TFSI)₂ (calcium bis(trifluoromethane)sulfonimide) in PC (propylene carbonate) based HCEs as well as the corresponding LHCEs created using TTE (1,1,2,2‐tetrafluoroethyl‐2,2,3,3‐tetrafluoropropyl ether) as diluent. Some unique HCE properties arise already at ca. 2.00 m, which is at a lower salt concentration than for monovalent systems, and in addition the local structure of the HCE can be maintained even within a nominal 0.45 m LHCE (starting from a 3.26 m parent HCE). The combined observations made at molecular and macro levels pave the way for further optimization of important physico‐chemical properties, proper design of electrochemical investigations, and eventually a better understanding of how to best improve the desolvation kinetics at e. g., the electrolyte/electrode interfaces of a Ca metal anode.