Na⁺/Vacancy Disordered P2-Na_0.67Co_1–<i>x</i>Ti<i>_x</i>O₂: High-Energy and High-Power Cathode Materials for Sodium Ion Batteries

Although sodium ion batteries (NIBs) have gained wide interest, their poor energy density poses a serious challenge for their practical applications. Therefore, high-energy-density cathode materials are required for NIBs to enable the utilization of a large amount of reversible Na ions. This study presents a P2-type Na_0.67Co_1–<i>x</i>Ti<i>_x</i>O₂ (<i>x</i> < 0.2) cathode with an extended potential range higher than 4.4 V to present a high specific capacity of 166 mAh g^–1. A group of P2-type cathodes containing various amounts of Ti is prepared using a facile synthetic method. These cathodes show different behaviors of the Na⁺/vacancy ordering. Na_0.67CoO₂ suffers severe capacity loss at high voltages due to irreversible structure changes causing serious polarization, while the Ti-substituted cathodes have long credible cycleability as well as high energy. In particular, Na_0.67Co_0.90Ti_0.10O₂ exhibits excellent capacity retention (115 mAh g^–1) even after 100 cycles, whereas Na_0.67CoO₂ exhibits negligible capacity retention (<10 mAh g^–1) at 4.5 V cutoff conditions. Na_0.67Co_0.90Ti_0.10O₂ also exhibits outstanding rate capabilities of 108 mAh g^–1 at a current density of 1000 mA g^–1 (7.4 C). Increased sodium diffusion kinetics from mitigated Na⁺/vacancy ordering, which allows high Na⁺ utilization, are investigated to find in detail the mechanism of the improvement by combining systematic analyses comprising TEM, in situ XRD, and electrochemical methods