Investigation of Voltage-Dependent Electrochemical Behavior in Ni-Fe-Mn Layered Cathodes for Sodium-Ion Batteries

Sodium-ion batteries are considered as a promising alternative to lithium-ion batteries in large-scale applications. We propose a new P2-type layered oxide as an excellent intercalation cathode material for sodium-ion batteries. However, Fe-Mn-based layered oxides suffer from structural changes such as the direct phase transition of P2-O2, which causes power performance degradation. In this work, we substitute redox active elements to increase reversible capacity, and various experiments demonstrated reversible capacity can be enhanced. We compare P2-type Na_0.7Fe_0.4Mn_0.6O₂ synthesized via a solid state synthesis method with P2- type Na_0.7Fe_0.4-xNi_xMn_0.6O₂ (x=0, 0.1, 0.2) with partial substitution of Ni in the Fe site. Phase changes during electrochemical reactions are important for understanding the relationship between layered structure and electrochemical properties. In-situ x-ray diffraction reveals the phase transition mechanism of Fe-Mn-based layered oxides with P2 stacks. We also find that partial substitution of Ni, an active element, can significantly improve the battery performance by smoothing the electrochemical charge/discharge profile. The P2-type Na_0.7Ni_0.2Fe_0.2Mn_0.6O₂ cathode exhibits a high specific discharge capacity of ~162.8 mAh g^-1 at 20 mA g^-1. The smooth and continuous structural changes resulted in improved cycling performance, including reduced voltage attenuation during cycling. Partial substitution of Ni can improve cycling performance, including reduced voltage attenuation during cycling. These results suggest that structural stability is one of the most efficient ways to improve capacity and charge/discharge performance in P2 Fe-Mn-based layered oxides.