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<p>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<sub>x</sub>Fe<sub>y</sub>Ni<sub>1–x–y</sub>TMzO<sub>₂</sub> (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<sub>0.25</sub>FM-ZCCAL<sub>0.03</sub> and N<sub>0.3</sub>FM-ZCCAL<sub>0.02</sub> were compared in detail. Although N<sub>0.3</sub>FM-ZCCAL<sub>0.02</sub> delivered a higher initial capacity (164.46 mAh g<sup>−1</sup>), its cycling retention (57.98%) was significantly inferior to that of N<sub>0.25</sub>FM-ZCCAL<sub>0.03</sub> (77.73%). Ex situ XRD, XPS, GITT, and EIS analyses collectively reveal that both N<sub>0.25</sub>FM-ZCCAL<sub>0.03</sub> and N<sub>0.3</sub>FM-ZCCAL<sub>0.02</sub> benefit from the synergistic effect of multivalent cations (Zn<sup>2+</sup>, Cu<sup>2+</sup>, Al<sup>3+</sup>, Li<sup>+</sup>, and Co<sup>2+</sup>), which effectively suppress the Mn<sup>3+</sup>/Mn<sup>4+</sup> redox fluctuation, stabilize the Fe<sup>3+</sup> state, and alleviate Jahn–Teller distortion. Despite these shared advantages, the two compositions exhibit markedly different structural evolution behaviors. Specifically, N<sub>0.25</sub>FM-ZCCAL<sub>0.03</sub> undergoes a highly reversible O3 → P3 → OP2 phase transition, whereas N<sub>0.3</sub>FM-ZCCAL<sub>0.02</sub> 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<sub>0.25</sub>FM-ZCCAL<sub>0.03</sub> exhibits a higher Na<sup>+</sup> diffusion coefficient (5.68 × 10<sup>–9</sup> cm<sup>2</sup>&#xa0;s<sup>−1</sup>) and lower charge-transfer resistance (8.2 Ω), demonstrating markedly enhanced electrochemical kinetics compared to N<sub>0.3</sub>FM-ZCCAL<sub>0.02</sub>. 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.</p>

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

  • 期刊论文

  • 发表日期：

    2026年1月6日

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  • Jingyuan Guo,
  • Yuan Ha,
  • Cheng Wei,
  • Ran Gu,
  • Xing Chen,
  • Chaoyi Zhou,
  • Zhimin Li

摘要

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 NaMnxFeyNi1–x–yTMzO₂ (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 N0.25FM-ZCCAL0.03 and N0.3FM-ZCCAL0.02 were compared in detail. Although N0.3FM-ZCCAL0.02 delivered a higher initial capacity (164.46 mAh g−1), its cycling retention (57.98%) was significantly inferior to that of N0.25FM-ZCCAL0.03 (77.73%). Ex situ XRD, XPS, GITT, and EIS analyses collectively reveal that both N0.25FM-ZCCAL0.03 and N0.3FM-ZCCAL0.02 benefit from the synergistic effect of multivalent cations (Zn2+, Cu2+, Al3+, Li+, and Co2+), which effectively suppress the Mn3+/Mn4+ redox fluctuation, stabilize the Fe3+ state, and alleviate Jahn–Teller distortion. Despite these shared advantages, the two compositions exhibit markedly different structural evolution behaviors. Specifically, N0.25FM-ZCCAL0.03 undergoes a highly reversible O3 → P3 → OP2 phase transition, whereas N0.3FM-ZCCAL0.02 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, N0.25FM-ZCCAL0.03 exhibits a higher Na+ diffusion coefficient (5.68 × 10–9 cm2 s−1) and lower charge-transfer resistance (8.2 Ω), demonstrating markedly enhanced electrochemical kinetics compared to N0.3FM-ZCCAL0.02. 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.