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

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 P6₃/mmc) 10% Ti-doped Na_2/3Mn_2/3Ni_1/3O2 is carried out in different voltage ranges (1.5-4.0 V, 2.0-4.0 V, and 2.0-4.5 V). Ti⁴⁺ 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_2/3Mn_0.567Ti_0.100Ni_1/3O₂ (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) evidences the Ti⁴⁺-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_2/3Mn_2/3Ni_1/3O₂.