First-Principles and Experimental Investigation of ABO₄ Zircons as Calcium Intercalation Cathodes

Identifying next-generation batteries with multivalent ions, such as Ca²⁺ is an active area of research to meet the increasing demand for large-scale, renewable energy storage solutions. Despite the promise of higher energy densities with multivalent batteries, one of their main challenges is addressing the sluggish kinetics in cathodes that arise from stronger electrostatic interactions between the multivalent ion and host lattice. In this paper, zircons are theoretically and experimentally evaluated as Ca cathodes. A migration barrier as low as 113 meV is computationally found in YVO₄, which is the lowest Ca²⁺ barrier reported to date. Low barriers are confirmed across 18 zircon compositions, which are related to the low coordination change and reduced interstitial site preference of Ca²⁺ along the diffusion pathway. Among the four materials (BiVO₄, YVO₄, EuCrO₄, and YCrO₄) that were synthesized, characterized, and electrochemically cycled, the highest initial capacity of 81 mA h/g and the most reversible capacity of 65 mA h/g were achieved in YVO₄ and BiVO₄, respectively. Despite the facile migration of multivalent ions in zircons, density functional theory predictions of the unstable, discharged structures at higher Ca²⁺ concentrations (Ca_{<i>x</i>>0.25}ABO₄), the low dimensionality of the migration pathway, and the defect analysis of the B site atom can rationalize the limited intercalation observed upon electrochemical cycling.