Low-valence lithium doping modulates the Mn3+/Mn4+ redox couple and stabilizes the P2 framework for high-rate sodium-ion cathodes

P2-type Na 0.65 Mn 0.75 Ni 0.25 O 2 (NMNO) has been widely investigated as a promising cathode material for sodium-ion batteries (SIBs) owing to its high specific capacity and excellent structural stability. However, deterioration of cycling stability and Na + diffusion kinetics occurs in the P2-NMNO cathode due to the irreversible P2–to–O2 phase transformation, reinforced Na + /vacancy ordering, Jahn–Teller effect and so on during electrochemical processes, especially at high voltages. In the quest to improve P2 cathodes for SIBs, the Na 0.65 Mn 0.75 Ni 0.25-x Li x O 2 ( x = 0, 0.05, 0.1, 0.2) with Li substitution for Ni was studied. This deliberate substitution is found to regulate Na + site occupancy, disrupt cation ordering to enable faster ion diffusion, and effectively optimize the Mn 3+ /Mn 4+ redox couple, thereby suppressing the detrimental phase transition. As a synergistic result of these effects, the Li + -doped NMNO-Li0.1 cathode exhibits good electrochemical performance: it delivers 92.1 % capacity retention after 200 cycles at an ultrahigh rate of 20 C and achieves an increased Na + diffusion coefficient compared to the pristine material.