Engineering the Local Chemistry through Fe Substitution in Layered P2-Na0.7Ni0.2Co0.2Mn0.6O2 for High-Performance Sodium-Ion Batteries

Abstract

Sodium-ion batteries (SIBs) are considered promising alternatives to lithium-ion batteries (LIBs) for large-scale applications. Layered transition metal oxides are mainly used as cathode materials to enhance energy density and electrochemical performances. In this study, we compare Mn-based P2-type Na_0.7Ni_0.2Co_0.2Mn_0.6O₂ (NCM) with partially Fe-substituted Na_0.7Ni_0.2Co_0.2Mn_0.5Fe_0.1O₂ (NCMF) via facile solid-state synthesis. Interestingly, Fe-substitution improves not only structural stability but also Na⁺ diffusion kinetics. It is found that the P2-O2 phase transition at high voltage region is mitigated with smaller volume change and enhanced oxygen redox reaction as demonstrated by in-situ X-ray diffraction and ex-situ X-ray photoelectron spectroscopy. In addition, density functional theory calculations exhibit that NCMF expedites Na⁺ diffusion and reduces the site energy difference between Na_f and Na_e by decreasing Na occupancy in the Na_f site, which is located right below the transition metal ions. As a result, the NCMF electrode delivers a high initial energy density of 601.5 Wh kg^-1 with an average discharge voltage of 3.05 V (V vs. Na⁺/Na). It also shows a high discharge capacity of 168.15 mAh g^-1 at 0.5 C with excellent capacity retention of 68.7% after 100 cycles within a wide voltage range of 1.5-4.5 V. These findings provide a significant impact of Na site occupancy difference for improving electrochemical performance and structural stability as a rational method for the commercialization of SIBs.