Solvent-Mediated Electrolyte Design for Calcium Metal Batteries

Current electrolytes for calcium batteries (CaBs) rely on cumbersome salt synthesis, hindering research and development. As a subclass of CaBs, calcium metal batteries (CMBs) could potentially offer high energy density due to their use of a calcium anode. However, realizing this advantage remains difficult, largely due to calcium’s electrochemical instability. To address these challenges, we introduce a family of electrolytes made entirely from commercially accessible Ca-salts and solvent mixtures and further demonstrate stable cycling of symmetric Ca||Ca cells using only a solvent mixture, without added salt (i.e., not being an electrolyte on its own). Notably, this cycling stability extends to CMB full cells using low salt concentration electrolytes (e.g., 0.1 M Ca(OTf)₂ in NMA:TMP), and similar full cell performance is also achieved using other combinations of salts and solvent mixtures. Extensive electrochemical testing confirms stable cycling under diverse and challenging conditions. Overall, our findings reframe electrolyte design principles and pave the way for practically useful CMB cells.