Intercalation of Ca into a Highly Defective Manganese Oxide at Room Temperature

The utilization of oxide frameworks as intercalation cathodes for non-aqueous Ca-ion batteries potentially unlocks a new energy storage system that delivers high energy density. However, the slow kinetics of Ca<sup>2+</sup> in oxide electrodes strongly handicaps their activity and reversibility at room temperature. Here, nanocrystals of layered MnO<sub>x</sub> containing a high concentration of atomic defects and lattice water are shown to have remarkable electrochemical activity towards Ca<sup>2+</sup>, amounting to a capacity of ~130 mAh/g at room temperature. Multimodal characterization revealed the notable degree of intercalation by probing the structural, compositional and redox changes undertaken by the defective MnO<sub>x</sub> nanocrystals. The results suggest that the existence of atomic defects and lattice water played a role in improving Ca<sup>2+</sup> diffusivity in the oxide. These outcomes reaffirm the prospects for functional Ca-ion batteries using oxide cathodes under moderate conditions.