The MeLuBatt project, funded by the German Federal Ministry of Education and Research (BMBF) under the grant number 03XP0110E, investigated calcium‑based metal‑air batteries with the aim of overcoming the limitations that have prevented calcium and magnesium systems from reaching commercial viability. The research was carried out in a consortium that included the University of Bonn, the Zentrum für Sonnenenergie und Wasserstoff‑Forschung Baden‑Württemberg (ZSW) in Ulm, and several industrial partners. The consortium’s tasks were divided into distinct work packages: development of new cell chemistries for Ca/O₂ and Mg/O₂ batteries, optimization of electrolytes, detailed electrochemical characterization of metal deposition and oxygen reactions, and comparative analysis of common challenges across metal‑air systems. The project ran for three years, with key milestones at 12, 24, and 36 months; the first two milestones were achieved successfully, culminating in a demonstration of an optimized metal‑air cell.
On the technical side, the study established that reversible calcium deposition is feasible on inexpensive, non‑precious substrates such as copper, stainless steel, nickel, and aluminum. The overpotential required for plating was found to be –240 mV versus the Ca²⁺/Ca couple, a value only 70 mV more negative than the theoretical minimum, indicating efficient electroplating. Cyclic voltammetry of calcium in tetra‑fluoroborate and bis(trifluoromethanesulfonyl)imide electrolytes revealed that the onset potential for the oxygen reduction reaction (ORR) is largely independent of the working electrode material, but the current density and passivation behavior differ markedly between gold and platinum. On platinum, the ORR proceeds mainly through the formation of solid CaO₂, whereas on gold the soluble Ca(O₂)₂ species dominate. This mechanistic divergence was confirmed by the appearance of distinct peaks in the voltammograms and by the higher cathodic currents observed on platinum, which also led to a larger amount of deposited reaction products.
The influence of solvent donor number (DN) on ORR performance was quantified. Solvents with higher DN, such as dimethyl sulfoxide (DN = 29.8), facilitated greater solvation of the reaction intermediates, resulting in higher cathodic currents and reduced electrode passivation. Conversely, solvents with lower DN, like adiponitrile (DN = 13.8), produced lower currents and more pronounced surface blocking. Rotating ring‑disk electrode experiments further demonstrated that in a DMSO‑based electrolyte, 18.8 % of the ring current relative to the disk current was achieved at 2000 rpm, corresponding to a theoretical efficiency of 25.6 %. This indicates that 73.4 % of the ORR products can be re‑oxidized at the ring, confirming the reversibility of the process. In contrast, a MEIM‑based electrolyte showed permanent electrolyte decomposition, underscoring the importance of solvent choice.
The project also clarified the kinetics of calcium nucleation and growth. Current‑time transients recorded at –280 mV versus Ca/Ca²⁺ on a copper electrode displayed an initial rise due to double‑layer charging, followed by a decline as stable calcium nuclei formed, and a subsequent increase as growth proceeded. Analysis using the Scharifker–Hill model allowed differentiation between progressive and instantaneous nucleation mechanisms, with the experimental data aligning with 3‑dimensional progressive nucleation under diffusion control.
Overall, the MeLuBatt consortium delivered a comprehensive understanding of calcium deposition, oxygen reaction pathways, and solvent effects in calcium‑air batteries. The findings provide a solid foundation for further optimization of electrolytes and electrode materials, bringing calcium‑based metal‑air systems closer to practical application.