Calcium–Oxygen Batteries as a Promising Alternative to Sodium–Oxygen Batteries

The development of a reversibly working metal–O₂ battery is an ongoing challenge. While lithium and sodium have been extensively used to study the oxygen reduction reaction in aprotic solvents, calcium did not receive attention for use in a metal–O₂ battery. This paper aims at examining the oxygen reduction and evolution reaction in Ca²⁺-containing dimethyl sulfoxide at different electrode materials via the use of differential electrochemical mass spectrometry and rotating ring disc electrodes. The measurements reveal that superoxide is the main product of the oxygen reduction at various electrode materials. However, at gold electrodes the transfer of two electrons per oxygen molecule is observed. The superoxide is soluble and can be reoxidized almost completely from the solution to evolve oxygen, leading to roughly 90% true Coulombic efficiency. Furthermore, the oxygen evolution from species deposited during the oxygen reduction has been observed resulting in an overall reversibility of 95%. Our results combined with recent advances in the deposition of calcium emphasize the relevance of calcium for use in a future Ca–O₂ battery.