EnhancedHigh-Voltage Stability in P2–Na_0.66Ni_0.33Mn_{0.66–<i>x</i>}Ru_<i>x</i>O₂ Cathodes via Ru-Dopingfor Improved ElectrochemicalPerformance

P2-type Na_0.67Ni_0.33Mn_0.66O₂ has been widely studied as a sodium-ion battery cathode material due to its higher theoretical capacity and operating voltage. However, Na⁺/vacancy ordering, irreversible phase transition, and oxidation of lattice oxygen cause rapid capacity fading when cycled at higher voltages. Herein, we employ Ru-doping for Mn, with the objective that robust Ru 4d and O 2p covalent bonds can enhance structural stability, suppress irreversible oxygen release, and hence lead to stable capacity. A series of Ru-doped P2–Na_0.66Ni_0.33Mn_{0.66–<i>x</i>}Ru_<i>x</i>O₂ (<i>x</i> = 0, 0.1, 0.2, 0.3) cathode materials are synthesized, and their electrochemical performance are analyzed in the voltage range of 2–4.5 V. Multiple voltage plateaus observed in charge/discharge profiles become smoothened with the increased Ru concentration, showing that Na⁺/vacancy ordering is effectively suppressed by Ru-doping. In situ X-ray diffraction (XRD) studies have revealed that the detrimental high-voltage P2–O2 phase transition that is normally observed for pristine composition is replaced by a more reversible P2–OP4 transition, which enhances the structural stability. The Ru-doped samples show improved cycling stability, with the Na_0.66Ni_0.33Mn_0.36Ru_0.3O₂ cathode delivering an initial capacity of 150 mAh g^–1 and a retaining capacity of 108 mAh g^–1 after 150 cycles when cycled between 2 and 4.5 V at a 0.2C rate, while the capacity of the pristine (undoped) sample dropped to 35 mAh g^–1 after 120 cycles. Also, Ru-doping enhanced the rate capability and diffusion coefficient values. This work paves new insights into the high-voltage stabilization of P2-type cathodes for sodium-ion batteries.