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 Ga-doped O3-type NaZnNiGaMnTiO cathode materials are prepared, and the impact of Ga doping on their bulk/interface properties and electrochemical performance is systematically examined. Ga incorporation enhances the structural ordering of the layered framework and widens Na transport pathways, thereby reducing Na transport barrier. The Ga-doped material demonstrates superior structural reversibility and mechanical stability compared to the pristine counterpart during cycling. As evidenced by the density functional theory calculations, Ga 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, NaZnNiGaMnTiO exhibits a higher capacity retention after 100 cycles at 100 mA g (86.4 % vs. 68.1 %) and better rate capability at 2000 mA g (94.1 mAh g vs. 80.0 mAh g) than NaZnNiMnTiO. This work provides valuable insights into the role of Ga in high-voltage O3-type layered oxides and offers guidance for the design of high-entropy cathode materials for SIBs.