Unraveling the Synergistic Effect of Mg and Ti Codoping to Realize an Ordered Structure and Excellent Performance for Sodium-Ion Batteries.

Layered cathodes have been recognized as potential advanced candidates for sodium-ion batteries (SIBs), but the poor electrochemical performance has seriously hindered their further development. Herein, an ordered Na 2/3 [Ni 2/9 Mg 1/9 Mn 5/9 Ti 1/9 ]O 2 (NMMT) is designed and investigated as a high-performance cathode for SIBs through the synergistic effect of Mg and Ti codoping. Compared to the single Mg- or Ti-doped materials, NMMT clearly exhibits superstructure ordering diffraction peaks, and neutron diffraction further confirms that the diffraction peaks can be well indexed by a larger supercell P 6 3 , rather than the common unit cell P 6 3 / mmc by X-ray diffraction (XRD). High-resolution transmission electron microscopy also approves the ordering arrangement. This material shows an obvious capacity activation process during the first cycles, thus delivering 113 mA h g -1 specific capacity at 0.1 C (close to the theoretical value). Excellent rate capability even at 15 C and cycling stability after 500 cycles between 2.0 and 4.3 V can also be achieved, indicating that an ordered cathode is still promising. Besides, a single-phase reaction mechanism is revealed by ex situ/in situ XRD experiments. This study offers some insights into the material design and characterization of layered oxide cathodes for high-performance SIBs in the future.