Next generation battery salt sourcing—the example of calcium bis(fluorosulfonyl)imide (Ca(FSI)2)

Journal article, 2026

High and uniform quality of commercially sourced materials is crucial for establishing trust and comparability across scientific studies. This is particularly true in the area of next generation batteries, as low-quality materials could wrongfully discourage a technology/chemistry from future use. Electrolyte salts are a class of materials that have historically been laden with such problems. Herein, the basic properties and performance of five commercial Ca(FSI)2 salts are compared, focusing on thermal stability and phase transitions, along with vibrational spectroscopy techniques to assess the purity of the salts. Calcium battery electrolytes are prepared by dissolving the salts in dimethylacetamide and subsequently tested in both symmetric (Ca||Ca) and full cells (Ca||PTCDI). Notably, there are quite some differences between fundamental basic properties between all the five Ca(FSI)2 salts, however, this is not really reflected in their electrochemical performance, which could be interpreted as rather reassuring. Nevertheless, it must be stressed that the differences may also be electrochemically significant for other calcium battery (electrolyte) designs and/or operating conditions.