O3-Type Na 2/3 Ni 1/3 Ti 2/3 O 2 Layered Oxide as a Stable and High-Rate Anode Material for Sodium Storage.

Sodium-ion batteries (SIBs) are currently the most promising candidates for large-scale energy storage devices owing to their low cost and abundant resources. Titanium-based layered oxides have attracted widespread attention as promising anode materials due to delivering a safe potential of about 0.7 V (vs Na + /Na) and a small volume contraction during cycles; P2-type Ti-based layered oxides are typically reported, due to the challenging synthesis of the O3-type counterpart resulting from the high percentage of unstable Ti 3+ . Herein, we report an anomalous O3-Na 2/3 Ni 1/3 Ti 2/3 O 2 layered oxide as an ultrastable and high-rate anode material for SIBs. The anode material delivers a reversible capacity of 112 mA h g -1 after 300 cycles at 0.1 C, a good capacity retention rate of 91% after 1400 cycles at 2 C, and, in particular, a capacity of 52 mA h g -1 even at a high rate of 20 C (1780 mA g -1 ). Furthermore, the in situ X-ray diffraction monitoring reveals no phase transitions and almost zero strain both underlie the good long-cycle stability. The measured high apparent Na + diffusion coefficient (2.06 × 10 -10 cm 2 s -1 ) and the low migration energy barrier (0.59 eV) from density functional theory calculations are responsible for the superior rate capability. Our results promise advanced high-performance O3-type Ti-based layered oxides as promising anode materials toward application for SIBs.