Degradation analysis and doping modification optimization for high-voltage P-type layered cathode in sodium-ion batteries

Advancing high-voltage stability of layered sodium-ion oxides represents a pivotal avenue for their progress in energy storage applications. Despite this, a comprehensive understanding of the mechanisms underpinning their structural deterioration at elevated voltages remains insufficiently explored. In this study, we unveil a layer delamination phenomenon of Na_0.67Ni_0.3Mn_0.7O₂ (NNM) within the 2.0–4.3 V voltage, attributed to considerable volumetric fluctuations along the c-axis and lattice oxygen reactions induced by the simultaneous Ni³⁺/Ni⁴⁺ and anion redox reactions. By introducing Mg doping to diminished Ni–O antibonding, the anion oxidation-reduction reactions are effectively mitigated, and the structural integrity of the P2 phase remains firmly intact, safeguarding active sites and precluding the formation of novel interfaces. The Na_0.67Mg_0.05Ni_0.25Mn_0.7O₂ (NMNM-5) exhibits a specific capacity of 100.7 mA h g⁻¹, signifying an 83% improvement compared to the NNM material within the voltage of 2.0–4.3 V. This investigation underscores the intricate interplay between high-voltage stability and structural degradation mechanisms in layered sodium-ion oxides.