Calcium electrolyte could be used for batteries
Scientists from Tohoku University have developed a fluorine-free calcium (Ca) electrolyte based on a hydrogen (monocarborane) cluster that could potentially realise rechargeable Ca batteries. The researchers say their new material, achieved by designing the coordination structure of Ca cation with a weakly coordinating anion and mixed solvents, shows markedly improved electrochemical performances such as high conductivity and high electrochemical stabilities.
Current lithium-ion (Li-ion) batteries have some drawbacks: they are approaching their demand limits of theoretical energy density and cost, and lithium is not naturally plentiful. Calcium, on the other hand, is the fifth most abundant element in the Earth’s crust and its metal anode has low reduction potential (-2.87 V vs standard hydrogen electrode (SHE)) and volumetric capacities of 2072 mAh cm-3, giving it comparable cell voltage and energy density to those of Li. Calcium batteries are therefore cost-effective and possess high battery performance, making them a suitable substitute for Li batteries.
That said, many challenges remain before Ca batteries can be realised due to a lack of suitable electrolytes that possess reductive/oxidative stabilities and high ionic conductivities. Moreover, realising a fluorine-free system is desirable for practical Ca electrolytes because a fluorine-containing electrolyte intrinsically induces CaF2 formation on the electrode, which hinders Ca diffusion as well as the Ca plating and stripping processes.
The Tohoku University researchers, led by Kazuaki Kisu and Shin-ichi Orimo from the Advanced Institute for Materials Research (AIMR), utilised the hydrogen cluster anion because of its high reductive and oxidative stability, which allows for a wide potential window and demonstrates excellent stability against metal anodes such as lithium, sodium and magnesium.
“A design that incorporates a hydrogen cluster into a Ca electrolyte had not been proposed yet,” Kisu said. “We were pleased that this turned out suitable for a Ca battery.”
The researchers say their Ca electrolyte, described in the journal Scientific Reports, is a major breakthrough for Ca batteries. According to Kisu, “We expect the development of a promising electrolyte candidate based on complex hydrides compatible with Ca batteries will create future opportunities for exploring other related complex hydride compounds such as multivalent electrolytes.”
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