Entropy modulation strategy of P2-type layered transition metal oxide cathodes for sodium-ion batteries with a high performance

P2-type Na0.67Ni0.33Mn0.67O2 is a promising cathode for sodium-ion batteries due to its high theoretical capacity, facile fabrication, low cost, and environmental friendliness. However, it suffers from an undesirable phase transformation and severe capacity fading above 4.2 V. In this work, we propose a strategy to improve the performance of the P2-type layered transition metal oxide cathode by adjusting the configurational entropy of the material and the synergistic effect of zinc and magnesium co-doping. We use X-ray diffraction (XRD), X-ray photoelectron spectroscopy, and in situ XRD technologies to reveal the change in configurational entropy of the material structure and to explore the influence of related factors. The structural and thermal stability was improved through modification with zinc and magnesium co-doping, compared to Na0.67Mn0.67Ni0.37O2. Meanwhile, the P2 to O2 phase transition of the material under high voltage was inhibited. When tested in combination with a sodium-metal anode in a coin cell configuration, Na0.67Mn0.67Ni0.21Mg0.06Zn0.06O2 exhibited improved rate capability (69.24 mA h g-1 at 10C) and excellent cycling stability (82.1% capacity retention after 200 cycles at 1C). This work presents a route to rationally design cathode materials with entropy modulation to improve the performance of cathode materials for sodium-ion batteries. Configuration entropy is increased by doping with multiple cations, whereby the material defects and active sites are increased and phase transition is inhibited at high voltage. Meanwhile, the sodium-ion diffusion rate was improved with the co-doping strategy.