Thesis title: Chemistry of Calcium batteries
This PhD thesis addresses the fundamental components of calcium-based batteries, namely 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.
Electrolyte studies focused on the development and evaluation of two novel calcium salts, Ca-FPB and CaB₁₂H₁₂, as well as on the use of Ca(TFSI)₂ in glyme- and phosphate-based solvents. Ca-FPB exhibited high electrochemical stability and reversible Ca plating/stripping, but its complex and moisture-sensitive synthesis limits scalability. CaB₁₂H₁₂ showed promising reversibility in specific solvent/additive combinations, though with restricted stability windows. These investigations highlighted the critical role of electrolyte formulation in enabling stable interphases and reproducible electrochemistry. To enable accurate evaluation of electrode materials, 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, but capacity fading in full-cell configurations underscored persistent interfacial challenges. Tests in the pseudo-capacitive setup confirmed improved stability, albeit with reduced capacity, pointing to complex electrolyte–electrode interactions that remain to be fully understood.
On the anode side, two classes of materials were explored. TiO₂ nanotubes in the anatase phase exhibited modest but reversible Ca²⁺ intercalation while maintaining structural integrity, suggesting potential for divalent ion storage. Alloy-type anodes were examined in detail, with Ca–Zn and Ca–Sn systems synthesized through different routes. A systematic screening demonstrated that arc melting is the most effective method 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 rapid deactivation, likely linked to interfacial and compositional instabilities.
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.