A two-pronged approach:Bulk-phase substitution and second-phase synergism to enhance the anion redox stability of layered oxide cathode for sodium-ion batteries

As a supplement to lithium-ion batteries, sodium-ion batteries have great application potential in large-scale energy storage equipment, vehicle start-stop power supplies, and low-speed electric vehicle power. P3-Na_xLi_yMn_1-yO₂ (NLMO), has received widespread attention due to its higher charge voltage and discharge capacity. However, poor cycle stability and rapid voltage platform decay caused by the release of oxygen during charging hinder its practical application. This paper reports a new NLMO modification strategy, which effectively improves the cycling stability of NLMO by substituting Zr⁴⁺ for Mn⁴⁺ in the bulk phase to suppress the O²⁻/Oⁿ⁻ redox couple activity and stabilize the ZrO₂ second phase lattice. Under the combined action of substitution and second phase, the capacity retention rate of NLMO@Zr0.15 was 85.4 % after 100 cycles at a rate of 5 C. The results show that Zr⁴⁺ changes the coordination environment and electronic structure of O in the crystal lattice, prompting further splitting of Mn t_{2 g}, thereby increasing the proportion of Mn 3d orbitals near the Fermi level. During the first discharge process, due to the charge balancing effect, Mn³⁺/Mn⁴⁺ replaced the electrochemical activity of O²⁻/Oⁿ⁻, obtaining a specific capacity of up to 161 mAh g⁻¹. This internal and external improvement idea provides a new direction for solving the poor cycle stability of NLMO high specific energy sodium-ion battery cathode materials and improving the specific capacity.