Synergistic Induction of P2/O3 Mixed Phase ByThermal Diffusion Regulation and High‑Entropy Strategy for High‑Performance Layered Cathodes

1. Article Information

Wenhao Qiu , Mingjie Dong, Ziyi Zhan, Wenhai Ji, Ping Miao, Wujun Peng, Wei Xu, Xinxin Teng, Kejun Zhang, Ziwei Chen*, Qinghua Zhang*, Synergistic inducting P2/O3 mixed phase by thermal diffusion regulation and high-entropy strategy for high performance layered cathode, Chemical Engineering Journal, 2026, 534, 175314.

1. Research Background

Sodium-ion batteries attract growing attention as a low-cost, resource-abundant alternative to lithium-ion systems. Conventional P2-type Na₀.₆₇Ni₀.₃₃Mn₀.₆₇O₂ offers facile synthesis and decent capacity at high voltage. Yet irreversible P2-OP4/O2 phase transitions above 4.0 V trigger severe volume changes and structural collapse. Non-bonding oxygen release further promotes cation migration and lattice distortion. In contrast, O3-type phases provide better high-voltage stability but suffer from sluggish rate performance and poor air stability due to higher sodium content and narrower diffusion channels. Balancing structural stability, capacity, and rate capability in layered oxide cathodes remains a critical challenge.

2. Work Summary

Researchers at Zhejiang University have introduced an innovative approach that combines thermal diffusion regulation with a high-entropy strategy. This synergy induces inhomogeneous Cu diffusion and uneven Na distribution, successfully constructing a P2/O3 mixed phase material with the composition Na₀.₆₇Ni₀.₃Fe₀.₁Mn₀.₃Cu₀.₁Ti₀.₂O₂ (NFMCT). The interlocking effect of the biphasic structure, reinforced by high-entropy stabilization, effectively suppresses detrimental phase transitions and anion redox activity at high voltages, leading to markedly improved structural stability. This work provides a new design paradigm for layered oxide cathodes with superior overall performance. The results were published in Chemical Engineering Journal under the title “Synergistic inducting P2/O3 mixed phase by thermal diffusion regulation and high‑entropy strategy for high performance layered cathode”. Wenhao Qiu is the first author; Ziwei Chen and Qinghua Zhang are co‑corresponding authors.

Figure 1. Schematic illustration of using calcination time to control non‑uniform distribution of high‑entropy elements and thereby induce a P2/O3 mixed phase.

3. Key Findings

3.1 Mechanism of P2/O3 Mixed‑Phase Induction via Thermal Regulation

At 900 °C in a multi‑element system, Cu (the element with the largest atomic radius) diffuses most sluggishly, leading to its segregation. To maintain charge balance, additional Na⁺ ions preferentially locate in the Cu‑rich regions, thereby forming local Na‑rich O3 domains. By adjusting the extent of thermal diffusion (e.g., an optimal holding time of 15 hours), the mixed‑phase ratio can be tuned – in this case an O3:P2 ratio of approximately 80:20. This demonstrates that a Na‑lean mixed‑phase material can be reliably obtained through thermal diffusion regulation and the high‑entropy effect.

Figure 2. Investigation of calcination time and high‑entropy element distribution for mixed‑phase control.

3.2 Structural Stabilization and Phase‑Transition Suppression (Interlocking and High‑Entropy Effects)

Operando XRD reveals that the interlocking effect of the mixed phase effectively suppresses the severe P2→O2 transformation at high voltage. During charge/discharge, NFMCT undergoes a highly reversible and relatively mild sequence: P2/O3 → P2/P3 → OP4/OP2, which causes much less irreversible lattice damage. Both density functional theory (DOS calculations) and experiments (operando mass spectrometry, Raman spectroscopy) show that the introduction of Ti and Cu increases the binding energy of the Ni–O bond, while the high‑entropy configuration enhances the anchoring of oxygen. Consequently, oxygen loss and anionic redox activity at high voltage are significantly inhibited.

3.3 Excellent Electrochemical Capacity and Rate Capability

Thanks to the high‑entropy‑induced exposure of favorable crystal facets and a distinctive morphology (strip‑like supporting structures on the lateral surfaces), the material possesses wider diffusion channels for sodium ions. In the voltage range of 2.0–4.3 V, NFMCT delivers a high specific capacity of 152.3 mAh g⁻¹ at 0.2 C. Even more impressively, after 200 cycles at 5 C, the capacity retention reaches 89.1%; after 1000 cycles at an ultra‑high rate of 10 C (2.0–4.0 V), the retention remains 82.0%. These figures convincingly overcome the long‑standing rate‑capability deficiency of conventional O3‑type cathodes.

Figure 3. Electrochemical performance of the P2/O3 biphasic cathode.

3.4 Full-Cell Performance, Air Stability, and Mechanical Properties

Full cells pairing the P2/O3 biphasic cathode with hard carbon anodes exhibit 93.1% capacity retention after 50 cycles at 5C, highlighting practical fast-charging potential. The material also demonstrates excellent air stability: after 15 days of exposure at 50% relative humidity, the P2/O3 structure remains intact with negligible electrochemical decay, thanks to its low-sodium character that suppresses Na₂CO₃ formation. Single-particle compression tests further reveal multi-stage fracture behavior enabled by the interlocking biphasic structure, which enhances mechanical resilience and prevents catastrophic failure under stress.

Figure 4. Comprehensive performance evaluation of the high-entropy P2/O3 mixed phase cathode.

4. Conclusion

This work cleverly exploits differences in the thermal diffusion kinetics of Cu within a multi‑element high‑entropy system to induce redistribution of sodium ions, thereby successfully producing a high‑performance P2/O3 biphasic cathode. The strategy not only suppresses detrimental phase transitions but also maintains oxygen stability and mechanical strength at high voltage. It provides important design guidelines for developing low‑cost, long‑life, fast‑charging sodium‑ion batteries.

5. Acknowledgements

This research received funding from the Zhejiang Provincial Natural Science Foundation (Grant No. LQN25B060007), the National Natural Science Foundation of China (12505345), the Quzhou Science and Technology Plan Project (2024K011), the Zhejiang Provincial Key R&D Project (2024C01056), the Guangdong Innovative and Entrepreneurial Team Project (2021ZT09C539), and the Institute of Zhejiang University – Quzhou Research Fund (IZQ2024RCZX011). We also thank IEST Instrument. for providing single‑particle compression testing support, and the China Spallation Neutron Source (CSNS) high‑resolution neutron diffractometer (TREND) for neutron beam time.