Enhanced High-Voltage Stability in P2–Na0.66Ni0.33Mn0.66–xRuxO₂ Cathodes via Ru-Doping for Improved Electrochemical Performance

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P2-type Na 0.67 Ni 0.33 Mn 0.66 O 2 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 0.66 Ni 0.33 Mn 0.66– x Ru x O 2 ( x = 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 0.66 Ni 0.33 Mn 0.36 Ru 0.3 O 2 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.