Applicability of Molybdite as an Electrode Material in Calcium Batteries: A Structural Study of Layer-type CaxMoO₃

Calcium batteries could be an alternative to lithium analogues, but this technology is still in its infancy. It is previously known that layered-type molybdite (α-MoO3) can intercalate hydrated calcium ions in aqueous electrolyte, and this intercalation process increases the interlayer spacing. In this work, we have found that α-MoO3 is electrochemically active in calcium cell with nonaqueous electrolyte. The mechanism of intercalation has been explored by using XRD, Raman, and XPS. The layered structure of α-MoO3 is preserved upon electrochemical intercalation of unsolvated calcium, and the perovskite-type structure of CaMoO3 is not formed. The experimental length of the lattice parameter perpendicular to the slab increases from 13.85 to 14.07 Å in the first stages of intercalation. This limited increase can be optimum for achieving good electrochemical cycling. The model for calcium intercalation in the interlayer space was optimized by theoretical calculations based on the density functional theory. The resulting experimental reversible capacity is about 80–100 mA h g–1, and the average voltage is ca. 1.3 V vs Ca. Further improvement of the electrolyte composition and particle size and morphology could render molybdite as a suitable electrode for rechargeable calcium batteries. The slow diffusion of calcium ion, the side-reactions, and the competing conversion reaction could be drawbacks, particularly at deep discharge and low voltages.