Utilizing manganese hexacyanoferrate (MFCN) as the cathode, the battery intercalates Ca reversibly in a dry nonaqueous electrolyte with tin as the anode with a capacity of about 80 mAh/g.

This insertion at approximately 3.4 V vs Ca/Ca²⁺, reported in their paper in the ACS journal Chemical Materials,causes only the Mn oxidation state to change. This implies a theoretical capacity of 100 mAh/g (excluding the mass of Ca). Through modification of the Prussian blue type structure and improvements in the cell design, capacity could theoretically be doubled to about 200 mAh/g if both transition metals undergo redox, the authors said.

The authors noted that the preferred anode would be Ca metal, but currently no electrolyte has been shown to reversibly plate and strip Ca metal with high Coulombic efficiency.

There is an approximately 50% reduction in capacity after 35 cycles… This capacity fade could be due to a number of factors including the electrolyte being absorbed by the graphite foil, delamination of the tin anode due to volume expansion, lack of passivation on the tin causing it to dissolve into the electrolyte, and an increase in cell resistance due to surface film formation. It is clear that for a more practical battery new electrolytes are needed to improve passivation and surface layer formation, along with enabling a high Coulombic efficiency Ca metal anode; however, these results are the first proof of concept for a functioning reversible nonaqueous calcium battery system.

—Lipson et al.

Resources

• Albert L. Lipson, Baofei Pan, Saul H. Lapidus, Chen Liao, John T. Vaughey, and Brian J. Ingram (2015) “Rechargeable Ca-Ion Batteries: A New Energy Storage System” Chemistry of Materials 27 (24), 8442-8447 doi: 10.1021/acs.chemmater.5b04027