Correlated Mg-Ion and Electron Transport in Polyanionic Co and Ni Silicate Electrodes: A Paddle Wheel-like Rotation-Induced Process

Abstract

This study investigates polyanionic-based olivine MgMSiO₄ (M = Co and Ni) electrode materials using first-principles density functional theory (DFT) and classical molecular dynamics (CMD). The structural, electronic, and ionic dynamics of these orthosilicate materials are explored. Our findings demonstrate that polyanionic [SiO₄]^4– tetrahedra facilitate Mg²⁺ ionic mobility by forming channels and enabling the paddle-wheel mechanism through their coupled reorientation dynamics. Ionic diffusion studies reveal low-energy barriers for Mg²⁺ ions, indicating favorable ionic transport in these materials. Here, we establish a linear correlation between relaxation time and ionic conductivities. This can fill the gap in understanding how ions can move independently or decouple with the anionic polyhedra in the solid lattice. The correlated dynamics of cations and anions is crucial for controlling the ionic transport properties. Additionally, minimal volume changes during intercalation–deintercalation are observed, attributed to the stabilizing effect of strong Si–O bonds, which weaken M–O bonds through an inductive effect. These bonds help to maintain structural stability during battery operation. The average voltages are 2.85 V for MgCoSiO₄ and 3.27 V for MgNiSiO₄, making them promising candidates for MIB cathodes. Olivine silicates act as polaronic conductors, where the material properties influence charge carriers. DFT results reveal the electronic structure and polaron formation with spin polarization, providing detailed insight into the behavior of free polarons. This work will help us understand polyanionic cathode materials for high-capacity rechargeable batteries at the atomic level.