Solvent-Mediated Electrolyte Design for Calcium Metal Batteries

Current electrolytes for calcium batteries (CaBs) rely on complex 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, which complicates the development of practical CMBs. To address these challenges, we introduce a new family of electrolytes made entirely from commercially accessible Ca-salts and solvent mixtures. We demonstrate that Ca metal exhibits stable cycling in 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, as demonstrated using a low-concentration electrolyte (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 redefine electrolyte design principles for CMBs and pave the way for practically useful cells.