(Localized) Highly Concentrated Electrolytes for Calcium Batteries

Journal article, 2023

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.