Na/Vacancy Disordered P2-NaCoTiO: 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 NaCoTiO (x < 0.2) cathode with an extended potential range higher than 4.4 V to present a high specific capacity of 166 mAh g. 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. NaCoO 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, NaCoTiO exhibits excellent capacity retention (115 mAh g) even after 100 cycles, whereas NaCoO exhibits negligible capacity retention (<10 mAh g) at 4.5 V cutoff conditions. NaCoTiO also exhibits outstanding rate capabilities of 108 mAh g at a current density of 1000 mA g (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.