Tailoring the Bulk Structure and Surface Chemistry of Ni-Rich NCM811 Cathodes via Polyanion Incorporation for Enhanced Electrochemical Performance up to 4.5 V

In this study, we investigate the effect of PO₄³⁻ polyanion incorporation on the physicochemical and electrochemical properties of Ni-rich layered Li(Ni_0.8Co_0.1Mn_0.1)O₂ (P_x-NCM811, where x = 0.0, 0.3, and 0.5) cathode materials. A coprecipitation method with controlled polyanion injection was employed to ensure homogeneous distribution of PO₄³⁻ within the precursor particles. Comprehensive structural and morphological analyses confirmed that PO₄³⁻ incorporation led to reduced primary particle size and the formation of a compact, densely packed microstructure, particularly for the P_0.3-NCM811 cathode material. X-ray diffraction (XRD) and Rietveld refinement analyses revealed lattice expansion along the c-axis, while X-ray photoelectron spectroscopy (XPS) analysis demonstrated suppressed Ni²⁺ accumulation and surface stabilization via Li₃PO₄ formation. Electrochemical evaluation showed that P_0.3-NCM811 exhibited superior initial discharge capacity (~227 mA h g⁻¹), Coulombic efficiency (~92.7%), rate capability, and cycling stability, with approximately 86.7% capacity retention after 100 cycles at 1.0 C. Electrochemical impedance spectroscopy (EIS) further confirmed lower surface film and charge transfer resistances, as well as enhanced Li⁺ diffusion kinetics in the polyanion-modified cathodes. Differential capacity analysis indicated improved structural reversibility during phase transitions for P_0.3-NCM811, with reduced polarization and minimal H2 → H3 transition-induced degradation. These results demonstrate that PO₄³⁻ polyanion incorporation is a promising strategy to stabilize the structure and improve the electrochemical performance of Ni-rich layered oxide cathodes under high-voltage operation (4.5 V vs. Li/Li⁺).