Durable Cu-doped P3-type Na0.62Mn0.75Cu0.19O2 cathodes for high-capacity sodium-ion batteries

Abstract

Sodium-ion batteries (SIBs) are currently receiving considerable attention for their prospective use in next-generation energy storage technology. Still, a primary constraint that impedes their practical application is the limited efficacy of the cathode materials. Mn-based layered oxides experience significant capacity degradation due to structural changes during cycling. Herein, this research presents a comprehensive study of the novel P3-Na_0.62Mn_0.75Cu_0.19O₂ material, elucidating the effects of partial Cu-doping on its structural and electrochemical characteristics. Our investigation employs operando X-ray diffraction (XRD), demonstrating a stable single-phase reaction during the battery's cycling operation, accordingly preventing the P3–O3 phase transition. Furthermore, operando differential electrochemical mass spectrometry (DEMS) demonstrates the absence of irreversible O₂ evolution, hence affirming the stability of reversible oxygen redox processes in this material. Ex situ X-ray absorption near edge structure (XANES) study reveals substantial contributions from the Cu²⁺/Cu³⁺, Mn³⁺/Mn⁴⁺, and O²⁻/Oⁿ⁻ redox pairs to the overall capacity of the battery. The findings have been confirmed by X-ray photoelectron spectroscopy (XPS), which not only supports the results from the XANES investigation but also significantly enhances them. Additionally, the oxygen redox processes have been established by the obvious widening apparent in the O K-edge XANES spectra and the detection of peroxo-like oxygen species in the XPS spectra when the battery is charged to 4.7 V. The electrochemical properties of the P3-Na_0.62Mn_0.75Cu_0.19O₂ material have been extensively investigated, demonstrating high capacity (212.2 mA h g⁻¹ at 20 mA g⁻¹) and excellent rate performance due to the incorporation of electrochemically active Cu²⁺ ions. Finally, a full-cell of P3-Na_0.62Mn_0.75Cu_0.19O₂ with commercial hard carbon could achieve exceptional rate capability. This systematic approach highlights the key significance of Cu-doping for boosting electrochemical performance by promoting stable oxygen redox activities.