Ti-Substitution Facilitating Anionic Redox and Cycle Stability in a P2-Type Na2/3Mn2/3Ni1/3O₂ Na-Ion Battery Cathode

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P2-type layered oxides have attracted tremendous attention as the leading candidate for the cathode material in Na-ion batteries owing to their ease of synthesis and facile Na-ion diffusion. In this work, an in-depth investigation of the electrochemical behavior of P2-type (space group P 6 3 / mmc ) 10% Ti-doped Na 2/3 Mn 2/3 Ni 1/3 O 2 is carried out in different voltage ranges (1.5–4.0 V, 2.0–4.0 V, and 2.0–4.5 V). Ti 4+ doping is found to disrupt the Na-ion/vacancy ordering and increase the Na–O2 layer spacings, which results in improved rate performance (∼68 mAh g –1 at 5C in the 2.0–4.0 V range). In the 2.0–4.5 V range, Na 2/3 Mn 0.567 Ti 0.100 Ni 1/3 O 2 (NMNT) exhibits a reduced initial specific discharge capacity of 140 mAh g –1 and significantly improved capacity retention of 71% after 100 cycles due to enhanced reversibility of anionic redox. Better charge–discharge cycling stability of NMNT (80% capacity retention at 0.33C in 1.5–4.0 V range) is evidence of the Ti 4+ -induced disruption of cooperative Jahn–Teller distortion. Galvanostatic intermittent titration results confirm higher Na + diffusion coefficients in NMNT. Interestingly, a marginally higher cathode–electrolyte interphase resistance in NMNT is endorsed by electrochemical impedance measurements, while the overall cell resistance and the charge-transfer resistance are much lower (by ∼45% and ∼56.7%, respectively) than in Na 2/3 Mn 2/3 Ni 1/3 O 2 .