Multifunctional role of gallium-doping in O3-type layered-oxide cathodes for sodium-ion batteries: Enhancing...

Charging O3-type layered-oxide cathodes to a high cutoff voltage of 4.3 V (vs. Na⁺/Na) can enhance the energy density of sodium-ion batteries (SIBs). However, the irreversible oxygen redox reaction at high voltages often leads accelerated capacity degradation. Herein, a series of Ga³⁺-doped O3-type Na_0.9Zn_0.07Ni_0.38-0.5xGa_xMn_0.45-0.5xTi_0.1O₂ cathode materials are prepared, and the impact of Ga³⁺ doping on their bulk/interface properties and electrochemical performance is systematically examined. Ga³⁺ incorporation enhances the structural ordering of the layered framework and widens Na⁺ transport pathways, thereby reducing Na⁺ transport barrier. The Ga³⁺-doped material demonstrates superior structural reversibility and mechanical stability compared to the pristine counterpart during cycling. As evidenced by the density functional theory calculations, Ga³⁺ doping modulates the O 2p state near the Fermi level, mitigating the charge compensation mechanism of lattice oxygen, oxygen vacancy formation, and electrolyte decomposition at high voltages. Consequently, within the voltage range of 2.2-4.3 V, Na_0.9Zn_0.07Ni_0.35Ga_0.06Mn_0.42Ti_0.1O₂ exhibits a higher capacity retention after 100 cycles at 100 mA g^-1 (86.4 % vs. 68.1 %) and better rate capability at 2000 mA g^-1 (94.1 mAh g^-1 vs. 80.0 mAh g^-1) than Na_0.9Zn_0.07Ni_0.38Mn_0.45Ti_0.1O₂. This work provides valuable insights into the role of Ga³⁺ in high-voltage O3-type layered oxides and offers guidance for the design of high-entropy cathode materials for SIBs.