Structure and Performance of Na_xMn_0.85Al_0.1Fe_0.05O₂ (0.7 ≤ x ≤ 1.0) Composite Materials for Sodium-Ion Batteries

P2 and O3 structures are two important sodium manganese oxide phases for sodium-ion batteries; however, encounter Na-deficient and poor rate performance, respectively. Herein, a systematic study of Na_xMn_0.85Al_0.1Fe_0.05O₂ (0.7 ≤ x ≤ 1.0) materials is performed by employing solid-state NMR, X-ray diffraction, and electrochemical analysis, to provide an in-depth understanding on the structure and the correlated performance for the rational design. The interlayer spacing of α-NaMnO₂ broadens, and the content of distorted O3 structures (α- and β-NaMnO₂) increases with raising Na content. It is exhibited that the NaMn_0.85Al_0.1Fe_0.05O₂ composite material presents better rate and cycling performance than P2-type Na_0.7Mn_0.85Al_0.1Fe_0.05O₂, delivering a capacity of 87 mAh g^–1 at 5 C. Significantly, the determinants of performance are further discussed, which reveal that diffusion coefficient is probably not the decisive factor restricts the rate performance of O3 and composite materials. The phase transition relaxation and the interfacial charge transfer resistance should be seriously addressed for further improvement.