First-principles Study of P2-Type Na_xNiO₂ and Na_xNi_0.75M_0.25O₂ (M = Fe, Cu, Mn) Cathode Materials for Sodium-Ion Battery

The development of affordable, high-efficiency sodium-ion batteries is primarily dependent on the advancement of cathode materials. These materials need to exhibit a high cell voltage, significant storage capacity, and quick diffusion of sodium ions to fulfill the requirements for efficient and eco-friendly energy storage systems. In this vein, density functional theory (DFT) calculation has become instrumental in advancing the study of battery materials. This study presents a first-principles investigation of P2-type Na_xNiO₂ and Na_xNi_0.75M_0.25O₂ (M = Cu, Fe, Mn) cathode materials for sodium-ion batteries (SIBs), focusing on Na content variation and its impact on the battery performance. For NaNiO₂, we replaced part of the expensive Ni element with lower-cost Cu, Fe, and Mn in hopes of reducing costs and improving material performance. By employing density functional theory (DFT), we explore the relationship between lattice constants, cell volume, enthalpy of formation, and cell voltage, and how these factors influence sodium ion insertion/extraction. We provide insights into the diffusion paths and activation energies for Na ions, and assess the influence of transition metal (TM) substitution on the structural stability and electrochemical properties of the materials. Additionally, the study delves into the electronic structure, highlighting how Cu and Fe integration refines the band gap of the spin-down bands. The findings reveal that certain transition metal substitutions can enhance performance, offering a pathway to optimize sodium-ion battery electrode materials.