Sodium-ion migration in secondary battery cathode material Na₄Co₃(PO₄)₂P₂O₇: A first-principles molecular dynamics study

Abstract

Sodium-ion batteries promise to be a low-cost, environmentally-friendly alternative to lithium-ion batteries, as the latter present a number of problems in terms of safety, cost, and limited mineral resources that need to be overcome for batteries to be used widely in vehicles and stationary energy storage systems. Newly developed cathode material Na₄Co₃(PO₄)₂P₂O₇ is attracting attention for use in sodium-ion secondary batteries because of its high rate capability, high capacity, and high voltage compared to other candidate materials. We performed first-principles molecular dynamics simulations of the system Na_xCo₃(PO₄)₂P₂O₇ for 0 < x < 4 using the GGA+U formalism of density functional theory, examining in detail the Na-ion migration mechanism and other atomic-level features. Local electronic and crystal structure changes during Na removal and insertion confirm that Na ions migrate via a 3D conduction pathway with low activation barriers of 0.10–0.17 eV within the orthorhombic lattice. Na ions spend most of their time migrating in the 2D main channels, occasionally traversing the short distance between channels in the a direction. These low values help explain why this material is able to support high charge/discharge rates, making it a promising cathode material for Na-ion battery systems.