High-Voltage Sodium Layered Cathode Stabilized by Bulk Complex-Composition Doping to Surface Phosphate Coating Design

Layered oxides are considered promising cathode materials for sodium-ion batteries (SIBs) due to their high energy density, flexible compositions, and low cost. However, they encounter significant challenges, such as multiphase transitions and structural instability at high voltages, which limit their large-scale practical application. In this study, we employed a dual modification strategy involving complex composition doping and phosphate coating to fabricate the Na_0.67Ni_0.255Mn_0.645(TiMgCuZn)_0.1O₂@phosphate cathode (D-NNM). The lattice distortion induced by complex composition doping optimizes the overall properties of the cathode, while the phosphate coating forms a robust electrode interface through stable P-O bonds. This comprehensive modification strategy stabilizes phase transitions and interfacial structure, thereby enhancing Na⁺ transport and mitigating mechanical degradation and surface reactions at high voltages. Consequently, D-NNM exhibited an initial capacity of 136.9 mA·h·g^-1 with an average potential of 3.45 V and maintained 85% capacity after 60 cycles at 4.4 V, twice that of the pristine cathode. D-NNM demonstrated faster Na⁺ diffusion kinetics at high voltage without any significant particle cracks observed even after 50 cycles. This strategy offers comprehensive protection for layered oxides from bulk to surface and provides insights into the design of high energy density cathodes for SIBs.