 Average discharge voltage of the P-type-layered oxides cathode materials is one of the key factors that determine the sodium-ion batteries (SIBs) performance, especially for the high energy density performance. Inspired by the strategy of lithium and magnesium doping in the transition metal layer to promote the anionic oxygen redox and the synergistic effect of composite materials for SIBs, we develop a layered P2/P3-Na0.75Li0.2Mg0.05Al0.05Mn0.7O2 composite cathode material enabled by the anionic oxygen redox. The synthesized composite delivers a reversible capacity of 180 mAh g−1 between 2.0 and 4.5 V at 0.2 C and keep ≈ 3.1 V stable average voltage above 2.5 V the discharge cutoff voltage. With a combination of the electrochemical performance, hard and soft X-ray absorption spectroscopy, we demonstrate that the irreversible anionic oxygen activity in the high-voltage region results in the capacity fading and the irreversible cationic manganese activity in the low-voltage region is responsible for the voltage decay. These findings provide a simple and effective method for designing low-voltage decay and high-energy density Na-deficient layered oxide composite.