Multivalent-ion batteries are receiving significant attention as potential post-lithium ion energy storage devices, such as Mg, Ca, etc.^1-6 Among them, calcium has a potential much closer to Li (0.17 V vs. Li) and its natural abundance in the Earth’s crust make calcium cells potentially attractive. However, the major problem of calcium anodes is their tendency to reduce the organic electrolyte solvents forming passivating surface films such as CaO and CaCO₃ that inhibit further plating.^2,8-9 Recent studies showed calcium deposition under elevated temperatures, 75 to 100 °C, however with significant side reactions.⁵

Here, our recent studies show that calcium can be plated and stripped at room temperature with capacities of 1 mAh cm^-2 at a rate of 1 mA cm^-2, with low polarization (100 mV), in excess of 50 cycles and with low side reactions, Fig.1. The key role of the electrolyte, Ca(BH₄)₂ in tetrahydrofuran (THF), will be discussed.

Figure 1. Galvanostatic Ca plating/stripping load curves in Ca(BH₄)₂ in THF at the rate of 1 mA cm^-2. Inset shows variation of coulombic efficiency with cycle number.

References:

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