Utilizing both cationic and anionic oxygen redox reactions is regarded as an important approach to exploit high-capacity layered cathode materials with earth abundant elements. It has been popular strategies to effectively elevate the oxygen redox activities by Li-doping to introduce unhybridized O 2p orbitals in Na_x MnO₂ -based chemistries or enabling high covalency transition metals in P2-Na_0.66 Mn_x TM_{1- x} O₂ (TM = Fe, Cu, Ni) materials. Here, the effect of Li doping on regulating the oxygen redox activities P2-structured Na_0.66 Ni_0.25 Mn_0.75 O₂ materials is investigated. Systematic X-ray characterizations and ab initio simulations have shown that the doped Li has uncommon behavior in modulating the density of states of the neighboring Ni, Mn, and O, leading to the suppression of the existing oxygen and Mn redox reactivities and the promotion of the Ni redox. The findings provide a complementary scenario to current oxygen redox mechanisms and shed lights on developing new routes for high-performance cathodes.