Calcium batteries: A step toward a high-energy density battery using an abundant, non-toxic alkali-earth metal

07/26/2021

A designer electrolyte from concept to testing

Illustration of the calcium monocarborane electrolyte. The yellow, green, brown, and blue spheres represent calcium, carbon, boron, and hydrogen atoms, respectively. The icosahedrons highlight individual monocarborane units.

© 2021 Kazuaki Kisu and Shin-ichi Orimo

A collaboration between AIMR, the Institute for Materials Research at Tohoku University, and the Laboratory of Materials for Renewable Energy at École Polytechnique Fédérale de Lausanne has designed, synthesized, and tested a new fluorine-free calcium monocarborane (Ca[CB11H12]2) electrolyte1. With improved conductivity and electrochemical stability, the new electrolyte is a significant step towards realizing rechargeable calcium batteries.

Calcium batteries are prime candidates for replacing their lithium-ion counterparts because of calcium’s abundance, non-toxicity, lower reduction potential compared to Mg or Al, and higher (Ca2+) ionic charge capacity compared to that of Li+.

However, the lack of a suitable electrolyte hinders the realization of calcium batteries with the above advantages.

Currently, cutting-edge model calcium batteries use fluorine-containing, weakly-coordinating electrolytes (e.g., Ca(BF4)2 or Ca(B[hfip]4)2 (hfip = hexafluoroisopropyloxy) in organic solvents) to maximize Ca2+ conductivity at the expense of electrochemical stability. Moreover, the presence of fluorine also results in the formation of a passivating CaF2 film that prevents the reversible calcium plating/stripping at the anode surface.

To address these challenges, the team led by Kazuaki Kisu (currently affiliated to the Institute for Materials Research) and Shin-ichi Orimo from AIMR has designed a new electrolyte using monocarborane counterions (icosahedrons in Figure). Like the Ca(BF4)2 example, the new counterion is a weakly coordinating anion that maximizes Ca2+ conductivity; but unlike Ca(BF4)2, it is free from CaF2 film forming.

“The monocarborane cluster is a stable, weakly coordinating counterion, but it is not soluble in many solvents,” says Kisu. “We overcame this problem by using a mixture of desired solvents at specific ratios.”

In this way, the team has synthesized the new electrolyte using a known scalable route. Preliminary tests of the electrolyte performances show promising results including high conductivity, wide electrochemical window, and reversible calcium plating/stripping without CaF2 film formation.

“The test results from the new monocarborane electrolyte open a new path for other multivalent rechargeable-battery systems,” says Orimo. “We are currently exploring the inclusions of other metals such as magnesium and aluminum.”

(Author: Patrick Han)

References

  1. Kisu, K., Kim, S., Shinohara, T., Zhao, K., Züttel, A. & Orimo, S. Monocarborane cluster as a stable fluorine-free calcium battery electrolyte. Scientific Reports 11, 7563 (2021). | article

This research highlight has been approved by the author of the original article and all empirical data contained within has been provided by said author.