Researchers from UCLouvain, KU Leuven (SOLVOMET & Materials Engineering) and international partners have demonstrated a new fluorine-free electrolyte for calcium metal batteries, offering a safer and more sustainable alternative to conventional fluorinated systems. The study introduces calcium bis(methanesulfonimide) (Ca(MSI)₂), a hydrogen-rich analogue of Ca(TFSI)₂, which avoids fluorine decomposition products such as electronically resistive CaF₂. Instead, Ca(MSI)₂ forms a uniform, organic-rich SEI that enables highly stable calcium plating and stripping over 1600 hours at room temperature—an important advance for next-generation multivalent energy-storage technologies.
The team shows that Ca(MSI)₂, dissolved in dimethylacetamide (DMAc), delivers lower overpotentials, improved Ca²⁺-ion transport, and more homogeneous metal deposition than the benchmark Ca(TFSI)₂ electrolyte. Detailed XPS and SEM analyses (pp. 12–15) highlight the absence of fluorine in the interphase when using Ca(MSI)₂, in marked contrast to Ca(TFSI)₂, which forms a fluorine-rich, highly resistive CaF₂-dominated surface layer. These interfacial improvements directly translate into electrochemical gains, positioning MSI-based salts as credible candidates for sustainable, high-performance Ca-battery electrolytes.
Beyond performance, Ca(MSI)₂ introduces a simpler and safer synthesis route compared to fluorinated salts, which typically require energy-intensive fluorination steps and generate environmentally persistent by-products. By demonstrating that fluorine-free electrolytes can still deliver competitive conductivity, stable cycling and favourable SEI chemistry, this work provides a significant step forward in designing low-impact electrolyte systems for multivalent metal batteries.
Reference
A. Kachmar et al. “A Fluorine-Free Electrolyte for Calcium Metal Batteries,” EES Batteries, 2026. DOI: 10.1039/D5EB00162E.
Acknowledgements
This work was supported by the Excellence of Science Programme (EOS 40007515), M-ERA.NET/FRS-FNRS (COBRA), F.R.S.–FNRS fellowships, the European Research Council (ERC, Grant 101089281), and the Spanish Severo Ochoa Programme.
