Impact of high-entropy doping on phase transition and ion diffusion mechanisms in O3-type sodium-ion layered oxide cathodes

Sodium-ion batteries (SIBs) hold great promise for large-scale energy storage, but the phase instability and transition-metal valence state fluctuations of layered oxide cathodes severely limit their cycling stability and rate capability. In this work, eleven high-entropy layered oxide cathodes with the general formula NaMn x Fe y Ni 1–x–y TMzO ₂ (T = Zn, Cu, Co, Al, Li, etc.) were synthesized to investigate the impact of high-entropy doping on phase-transition behavior and ion diffusion mechanisms. Two representative compositions of N 0.25 FM-ZCCAL 0.03 and N 0.3 FM-ZCCAL 0.02 were compared in detail. Although N 0.3 FM-ZCCAL 0.02 delivered a higher initial capacity (164.46 mAh g −1 ), its cycling retention (57.98%) was significantly inferior to that of N 0.25 FM-ZCCAL 0.03 (77.73%). Ex situ XRD, XPS, GITT, and EIS analyses collectively reveal that both N 0.25 FM-ZCCAL 0.03 and N 0.3 FM-ZCCAL 0.02 benefit from the synergistic effect of multivalent cations (Zn 2+ , Cu 2+ , Al 3+ , Li + , and Co 2+ ), which effectively suppress the Mn 3+ /Mn 4+ redox fluctuation, stabilize the Fe 3+ state, and alleviate Jahn–Teller distortion. Despite these shared advantages, the two compositions exhibit markedly different structural evolution behaviors. Specifically, N 0.25 FM-ZCCAL 0.03 undergoes a highly reversible O3 → P3 → OP2 phase transition, whereas N 0.3 FM-ZCCAL 0.02 experiences multiphase coexistence and NiO precipitation, leading to irreversible structural degradation. The formation of NiO can be attributed to the interplay between nickel over-oxidation and structural instability during high-voltage cycling. Moreover, N 0.25 FM-ZCCAL 0.03 exhibits a higher Na + diffusion coefficient (5.68 × 10 –9 cm 2  s −1 ) and lower charge-transfer resistance (8.2 Ω), demonstrating markedly enhanced electrochemical kinetics compared to N 0.3 FM-ZCCAL 0.02 . This work highlights the critical role of high-entropy doping in regulating phase transitions and electronic structures, providing mechanistic guidance for the rational design of high-performance sodium-ion battery cathodes.