First-principles investigation of NaX(Mn,Ti)O2 cathode material for rechargeable sodium-ion batteries

Owing to the abundant supply of sodium and comparable performance to lithium-ion batteries, rechargeable sodium-ion batteries have attracted significant attention for large-scale electric energy-storage applications and smart grids. This study investigated the structural and electrical properties of Ti-doped NaMnO 2 using the density functional theory. The exchange-correlation functional in the generalized gradient approximation, as given by the Perdew–Burke–Ernzerhof functional, was used to calculate the total energies. The spin-polarized calculations were performed in sodiated and de-sodiated systems to determine the structural and electronic properties of the 2 × 2 × 2 NaMnO 2 and Na X (Mn,Ti)O 2 supercells. The dopants induced the volume expansion. The lowest conduction band and highest valence band originate primarily from the 3d orbital of the Mn atom and transition metal dopants, which are responsible for electronic conductivity. Na X (Mn, Ti)O 2 exhibits a reduced band gap compared to NaMnO 2 due to the effect of the Ti dopant. Furthermore, all independent elastic constants for the Na X (Mn, Ti)O 2 structures met the mechanical stability requirement of the orthorhombic lattice system, and the formation energy values were found to be negative, suggesting that the structures are stable with the predicted voltage window of between 3.410 and 4.132 V. This study also investigates the thermodynamic properties of a doped system, revealing that the systems remain stable during battery operation, reducing structural damage risk.