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

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 Na0.9Zn0.07Ni0.38-0.5xGaxMn0.45-0.5xTi0.1O2 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, Na0.9Zn0.07Ni0.35Ga0.06Mn0.42Ti0.1O2 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 Na0.9Zn0.07Ni0.38Mn0.45Ti0.1O2. 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.