Multifunctional role of gallium-doping in O3-type layered-oxide cathodes for sodium-ion batteries: Enhancing bulk-to-surface stability

Abstract

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.