Fe doping mechanism of Na_0.44MnO₂ tunnel phase cathode electrode in sodium-ion batteries

Due to its stability and low cost, the tunnel-style sodium-manganese oxide (Na_0.44MnO₂) material is deemed a popular cathode choice for sodium-ion rechargeable batteries. However, the Jahn-Teller effect caused by Mn³⁺ in the material results in poor capacity and cycling stability. The purpose of this experimental study is to partially replace Mn³⁺ with Fe³⁺, in order to reduce the Jahn-Teller effect of the material during charging and discharging process. The results of Raman spectroscopy and X-ray photoelectron spectroscopy confirmed that the content of Mn³⁺ decreased after Fe³⁺ doping. Electrochemical studies show that the Na_0.44Mn_0.994Fe_0.006O₂ cathode has better rate performance (exhibits a reversible capacity of 87.9 mAh/g at 2 C) and cycle stability in sodium-ion batteries. The diffusion coefficient of sodium ions increases by Fe³⁺ doping. The excellent rate performance and capacity improvement are verified by density functional theory (DFT) calculation. After doping, the band gap decreases significantly, and the results show that the state density of O 2p increases near the Fermi level, which promotes the oxidation-reduction of oxygen. This work provides a straightforward approach to enhance the performance of Na_0.44MnO₂ nanorods, and this performance improvement has guiding significance for the design of other materials in the energy storage domain.