Chemistry of Calcium battery

This PhD thesis addresses the fundamental components of calcium-based batteries, i.e., electrolytes, cathodes, and anodes, through a systematic and integrated investigation aimed at advancing both the understanding and the feasibility of this emerging energy storage technology. The electrolyte studies focused on the development and evaluation of two novel calcium salts, Ca FPB and CaB₁₂H₁₂, as well as on the adoption of Ca(TFSI)₂ in glyme- and phosphate-based solvents. Ca-FPB demonstrated high electrochemical stability and reversible Ca plating/stripping, though its complex, moisture-sensitive synthesis limits scalability. CaB₁₂H₁₂ exhibited promising reversibility in specific solvent/additive combinations but with restricted stability windows. These studies highlighted the critical role of electrolyte formulation in enabling stable interphases and reproducible electrochemistry. To support accurate electrode testing, a pseudo-capacitive three-electrode setup was also developed, providing a stable environment without the complications of metallic calcium. On the cathode side, intercalation materials such as the Prussian Blue Analogue MF21 were investigated. MF21 demonstrated reversible Ca²⁺ intercalation with structural retention, though capacity fading in full-cell configurations underscored persistent interfacial issues. Tests in the pseudo-capacitive setup confirmed improved stability, at the expense of capacity, pointing to complex electrolyte–electrode interactions that remain to be fully understood. Anode investigations comprised both intercalation- and alloy-type materials. TiO₂-nanotubes in the anatase phase showed modest but reversible Ca²⁺ intercalation while maintaining structural integrity, suggesting potential for divalent ion storage. Alloy-type anodes were studied in detail, with Ca–Zn and Ca–Sn systems prepared via different synthetic approaches. A systematic screening demonstrated that arc melting is the most effective route for producing homogeneous alloys, outperforming annealing and ball milling. Comparative studies revealed superior cycling stability for Zn-rich Ca Zn alloys, leading to the identification of CaZn₂ as the most promising formulation, further characterized structurally and electrochemically. In parallel, Ca–Sn alloys were preliminarily evaluated, showing initial activity but also rapid deactivation linked to volume expansion and unstable SEI growth. Overall, this work provides a comprehensive assessment of the limitations and prospects of calcium based batteries. It establishes methodological guidelines for electrolyte formulation, electrode testing, and alloy synthesis, while identifying CaZn₂ as a benchmark alloy composition for further studies. Although commercialization of Ca-based batteries remains distant, the results presented here contribute to understanding the fundamental processes governing their performance and to laying the groundwork for the rational design of next-generation multivalent energy storage technologies.