A P2/O3 mixed phase cathode integrates the high structural stability of P2-type domains with the high capacity of O3-type configurations, solving the critical rate-stability trade-off in sodium-ion batteries. By combining thermal diffusion regulation with a high-entropy strategy, researchers successfully constructed an interlocking biphasic cathode $Na_{0.67}Ni_{0.33}Fe_{0.1}Mn_{0.3}Cu_{0.1}Ti_{0.2}O_2$ (NFMCT) that suppresses catastrophic phase transitions and oxygen release above 4.0 V. The mechanical robustness of this biphasic architecture was experimentally evaluated using the IEST SPFT2000 Single-Particle Mechanical Tester, confirming enhanced particle fracture resilience and multi-stage stress mitigation.
Sodium-ion batteries hold great promise as next-generation energy storage systems due to their similar working mechanism to lithium-ion batteries, lower cost, and abundant sodium resources. The typical P2-type $Na_{0.67}Ni_{0.33}Mn_{0.67}O_2$ cathode material is easy to synthesize and can deliver relatively high specific capacity under high-voltage conditions. However, when the voltage exceeds 4.0 V, this material undergoes an irreversible P2-OP4/O2 phase transition, resulting in significant volume changes and structural collapse. In addition, the generation of non-bonding oxygen at high voltage induces severe cation migration and lattice distortion. In contrast, O3-type materials are generally more stable at high voltages, but their rate capability and air stability are insufficient due to their typically higher sodium content and narrower diffusion channels. Therefore, how to balance the stability, capacity, and rate performance of these layered oxides remains a critical challenge that urgently needs to be addressed.
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 $Na_{0.67}Ni_{0.33}Fe_{0.1}Mn_{0.3}Cu_{0.1}Ti_{0.2}O_2$ (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.
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 an elevated calcination temperature of 900°C in a multi-element system, Cu (the element with the largest atomic radius) diffuses most sluggishly, leading to localized segregation. To maintain charge balance, additional $\text{Na}^+$ ions preferentially locate in the Cu-rich regions, thereby forming local Na-rich O3 domains.
By adjusting the extent of thermal diffusion—specifically an optimal holding time of 15 hours—the mixed-phase ratio can be tuned to an optimal O3:P2 ratio of approximately 80:20. The experimental data demonstrate 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 X-ray Diffraction (XRD) reveals that the interlocking effect of the mixed phase effectively suppresses the severe P2 → O2 transformation at high voltages. During continuous charge/discharge cycling, the NFMCT cathode undergoes a highly reversible and relatively mild sequence: $\text{P2/O3} \rightarrow \text{P2/P3} \rightarrow \text{OP4/OP2}$, causing significantly less irreversible lattice damage.
Both Density Functional Theory (DFT) DOS calculations and operando mass spectrometry/Raman spectroscopy show that the introduction of Ti and Cu increases the binding energy of the Ni–O bond. The high-entropy configuration enhances the anchoring of oxygen atoms, effectively inhibiting oxygen loss and unwanted anionic redox activity at high voltages.
3.3 Excellent Electrochemical Capacity and Rate Capability
Thanks to the high-entropy-induced exposure of favorable crystal facets and a distinctive strip-like supporting morphology, the material possesses widened diffusion channels for sodium ions.
Specific Capacity: In the voltage range of 2.0–4.3 V, NFMCT delivers a high specific capacity of $152.3\text{ mAh g}^{-1}$ at 0.2C.
Cycling Stability: After 200 cycles at 5C, capacity retention reaches $89.1\%$.
Ultra-High Rate Performance: After 1000 cycles at an ultra-high rate of 10C (2.0–4.0 V), capacity retention remains at $82.0\%$.
These metrics decisively overcome the rate-capability deficiencies traditionally associated with conventional O3-type cathodes.
Figure 3. Electrochemical performance of the P2/O3 biphasic cathode.
Outstanding (82.0% retention after 1000 cycles at 10C)
Superior Multi-stage fracture resistance confirmed via single-particle compression.
Solves the structural/capacity trade-off using a dual lattice regulation and high-entropy strategy.
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, demonstrating practical fast-charging potential. The material also exhibits excellent air stability: after 15 days of exposure at $50\%$ relative humidity, the P2/O3 structure remains intact with negligible electrochemical decay, resulting from its tailored low-sodium character that suppresses $\text{Na}_2\text{CO}_3$ formation.
To evaluate microstructural robustness, single-particle compression tests were conducted using the IEST SPFT2000 Single-Particle Mechanical Tester. The force-displacement profiles reveal a characteristic multi-stage fracture behavior enabled by the interlocking biphasic structure. This structural resilience prevents catastrophic particle pulverization and intergranular crack propagation under localized mechanical strain during cycling.
Figure 4. Comprehensive performance evaluation of the high-entropy P2/O3 mixed phase cathode.
4. Conclusion
This study demonstrates that exploiting thermal diffusion kinetics of Cu within a multi-element high-entropy system induces sodium ion redistribution, yielding a robust P2/O3 biphasic cathode. This material architecture suppresses phase transitions, maintains oxygen lattice stability, and enhances mechanical robustness at high operating voltages, providing clear design guidelines for low-cost, long-life, and 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).
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What is a P2/O3 mixed phase in sodium-ion battery cathodes?
A P2/O3 mixed phase is an engineered biphasic crystal structure combining P2 and O3 packing sequences. This interlocking configuration harnesses the fast ionic transport of P2 domains alongside the high capacity of O3 domains, minimizing structural stress during high-voltage cycling.
How does a high-entropy strategy improve layered oxide cathodes?
A high-entropy strategy incorporates multiple transition metal elements into a single crystallographic site. The resulting high configurational entropy enhances transition metal-oxygen bonding energy, suppresses adverse phase transitions, and prevents oxygen loss at cutoff voltages above 4.0 V.
What is the primary difference between P2-type and O3-type layered cathodes?
P2-type cathodes provide superior rate capability and structural stability but suffer from lower initial sodium content. In contrast, O3-type cathodes offer higher specific capacity but suffer from sluggish diffusion kinetics and poor air stability. A biphasic P2/O3 architecture balances these trade-offs.
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Evaluating cathode particle mechanical properties requires high force resolution and precision displacement control at the micron scale. IESTSPFT2000 meets these requirements by measuring individual particle crushing strength, yield modulus, and deformation curves under controlled load.
How does thermal diffusion regulation affect the synthesis of high-entropy layered oxides?
Thermal diffusion regulation controls calcination holding time (e.g., 15 hours at 900°C) to exploit differences in elemental diffusion kinetics. Slower-diffusing elements like Cu segregate locally, directing sodium ions to form stable, interlocking P2/O3 mixed phase domains.
