Structure and ionic dynamics of Mn- and Fe-based Ca-ion battery electrode materials from molecular simulations

Abstract

Divalent intercalating metal ions, such as Mg²⁺, Zn²⁺, and Ca²⁺, in metal-ion batteries have garnered significant research interest with their remarkable capacity enhancement due to two electron transfer and low cost compared to lithium-ion batteries. The remarkable diffusivity and structural stability of some positive electrode materials, potentially significant for multivalent batteries, are the main reasons for using them in commercial batteries. In this work, we choose four materials with relatively fast ionic mobility, low energy diffusive barrier, and increased specific capacity. Calcium intercalation in positive electrode materials with the chemical formula Ca_xM_yO_z, where M is the transition metal element (Fe, Mn), is investigated using density functional theory and classical molecular dynamics simulations. We have reported the structural, electronic, and transport properties of four cost-effective compounds such as Ca₄Mn₂O₇, CaMn₃O₆, and CaFe_2+nO_4+n(n = 0.25 and 0). First principle calculations reveal the atomistic structure and local chemical environment. The cation–anion interactions in these materials are analyzed by calculating radial distribution functions. The diffusion properties of Ca²⁺ ions and conductivity in these solid materials were calculated using interatomic potential parameters in classical molecular dynamics, which reveal the ionic dynamics of these materials.