Key Takeaways & Executive Findings
- •• • The synergistic high sodium and Li/Mg co-doping strategy achieves 82.3% capacity retention after 500 cycles at 1C, a 16.9% improvement over the undoped P2-type cathode (65.4%), directly addressing the cycling stability bottleneck for commercial SIBs. • • The co-doped cathode delivers a reversible capacity of 112 mAh/g at 5C, demonstrating superior rate capability due to the open prismatic Na+ diffusion channels and reduced Na+/vacancy ordering, which is critical for high-power applications. • • In-situ XRD analysis confirms that the co-doping suppresses detrimental P2-O2 phase transitions during cycling, maintaining structural integrity and reducing volume strain, which is essential for long-term operational reliability. • • The high sodium content (x > 0.7) elevates the TM(3d-eg*) energy level, lowering the energy barrier for electron extraction and enhancing reaction kinetics, as evidenced by reduced polarization and improved electrochemical performance.
Abstract
Sodium-ion batteries (SIBs) are emerging as a cost-effective alternative to lithium-ion batteries due to the abundance of sodium resources. Among cathode materials, P2-type layered oxides (Na_xTMO_2) offer high ionic conductivity and rate capability but suffer from low initial sodium content and Na+/vacancy ordering, leading to structural degradation and capacity fading. This study proposes a synergistic strategy combining high sodium content with Li/Mg co-doping to enhance the cycling stability of P2-type cathodes. The high sodium content increases the sodium reservoir, reducing the depth of desodiation for a given capacity, while Li/Mg co-doping mitigates Na+/vacancy ordering and stabilizes the crystal structure. The optimized cathode exhibits significantly improved cycling performance, retaining 82.3% of its initial capacity after 500 cycles at 1C, compared to 65.4% for the undoped counterpart. Furthermore, the co-doped material demonstrates enhanced rate capability, delivering 112 mAh/g at 5C, and suppressed phase transitions, as evidenced by in-situ X-ray diffraction. This work provides a rational design pathway for high-performance P2-type cathodes, addressing key bottlenecks in SIB commercialization.
1. Introduction
P2-type layered oxide cathodes for sodium-ion batteries (SIBs) have attracted significant attention due to their high theoretical capacity and open diffusion channels for Na+. However, their commercial viability is hampered by low initial sodium content (x < 0.7), which leads to structural degradation upon deep desodiation, and Na+/vacancy ordering that traps Na+ and increases diffusion barriers. These issues result in poor cycling stability and rate capability, preventing P2-type cathodes from meeting the stringent demands of grid-scale energy storage and electric vehicles.
Existing mitigation strategies, such as transition metal doping, often sacrifice specific capacity by substituting electrochemically active elements. This study introduces a synergistic approach that simultaneously increases sodium content and co-dopes with Li and Mg. High sodium content provides a larger sodium reservoir, reducing the depth of desodiation for a given capacity, while Li/Mg co-doping suppresses Na+/vacancy ordering and stabilizes the crystal lattice. This dual strategy addresses the core bottlenecks of P2-type cathodes, offering a pathway to high-energy, long-life SIBs without compromising capacity.
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Yuheng Gao, Ping Zhang, Guohua Zhang, Jianwei Wu, Jiwei Ma, Yunhui Huang, Renyuan Zhang (2026). Boosting the cycling stability of P2-type layered oxide cathodes via a synergistic high sodium and Li/Mg co-doping strategy. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-026-4208-8
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Frequently Asked Questions
What is the specific mechanism by which Li/Mg co-doping suppresses Na+/vacancy ordering in P2-type cathodes?
Li/Mg co-doping introduces immobile ions in the transition metal layers, which disrupt the long-range ordering of Na+ and vacancies. This reduces the trapping of Na+ and lowers the diffusion energy barrier, as evidenced by improved rate capability (112 mAh/g at 5C) and reduced polarization.
How does increasing sodium content affect the average valence state of transition metals and the electrochemical kinetics?
Higher sodium content requires TM ions to adopt a lower average valence state to maintain electroneutrality. This elevates the TM(3d-eg*) energy level, reducing the energy gap to the conduction band and facilitating electron extraction during charging, thereby improving reaction kinetics and lowering the energy barrier.
What are the long-term cycling stability metrics of the co-doped cathode compared to the undoped baseline?
The co-doped cathode retains 82.3% of its initial capacity after 500 cycles at 1C, whereas the undoped cathode retains only 65.4%. This represents a 16.9% improvement, demonstrating enhanced structural stability and reduced capacity fade.
Does the Li/Mg co-doping strategy affect the specific capacity or energy density of the cathode?
The co-doping strategy maintains a high reversible capacity, delivering 112 mAh/g at 5C, which is comparable to or better than undoped P2-type cathodes. The high sodium content ensures a sufficient sodium reservoir, and the co-doping does not significantly sacrifice capacity, making it a viable approach for high-energy applications.
What are the scalability and cost implications of this co-doping strategy for industrial production?
The synthesis process for Li/Mg co-doped P2-type cathodes is similar to conventional solid-state methods, requiring no complex equipment. The raw materials (Li, Mg, Na, Mn, etc.) are abundant and low-cost, making the strategy economically feasible for large-scale production. The improved cycling stability also reduces lifecycle costs, enhancing the economic viability of SIBs.
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