Abstract:

Cathode materials play a critical role in determining the energy density, cycle life, and cost-effectiveness of sodium-ion batteries (SIBs). Among various candidates, P2-type layered oxide cathodes exhibit superior high-rate charge/discharge performance due to their open Na⁺ diffusion channels, making them particularly suitable for applications requiring rapid power delivery, such as starter batteries and grid frequency regulation. However, while the conventional P2-type Na_0.67Ni_0.33Mn_0.67O₂ (P2-NNMO) cathode demonstrates high energy density, the strong O²⁻–O²⁻ electrostatic repulsion within the transition metal layer during high-voltage charging induces an irreversible P2 → O2 phase transition accompanied by approximately 20% volume strain. This results in severe lattice distortion and structural collapse. Additionally, oxygen oxidation at high voltages contributes to charge compensation, reducing electrochemical reaction reversibility and accelerating structural degradation. Consequently, the P2-NNMO cathode suffers from rapid capacity decay and poor cycling stability, hindering its practical application. To overcome these challenges, we developed a multi-element doping strategy to design a P2-Na_0.67Zn_0.05Ni_0.23Fe_0.1Mn_0.57Ti_0.05O₂ (P2-NZNFMTO) layered oxide cathode. The synergistic doping of Zn²⁺, Ti⁴⁺, and Fe³⁺ enables concurrent optimization of structural and electrochemical properties. Specifically, Zn²⁺ doping enhances the O²⁻–Na⁺–O²⁻ electrostatic interaction, promoting the formation of local "Na⁺ pillars" within the Na layer to mitigate volume variation and phase transitions during cycling. Ti⁴⁺ doping disrupts Na⁺/vacancy ordering, significantly improving Na⁺ diffusion kinetics. The incorporation of Zn²⁺, Ti⁴⁺, and Fe³⁺ also alleviates the Jahn-Teller distortion associated with Ni²⁺/Ni³⁺, enhancing cycling performance while reducing material costs. The P2-NZNFMTO cathode exhibits exceptional electrochemical performance, demonstrating 96.7% capacity retention after 100 cycles at 1C rate with a high cut-off voltage of 4.3 V. Even at a high rate of 3C, it maintains over 85% capacity retention after 300 cycles. In-situ X-ray diffraction (XRD) and galvanostatic intermittent titration technique (GITT) analyses confirm its excellent structural stability and rapid Na⁺ transport capability at high voltages. This multi-element synergistic doping strategy establishes a novel design principle and theoretical foundation for developing high-voltage, long-cycle-life, and high-power SIB cathodes.

Key words: Sodium-ion battery, High rate, Multi-element doping, Cathode materials, High-voltage