Optimized TiO Subcompounds and Elastic Expanded MXene Interlayers Boost Quick Sodium Storage Performance

Abstract To develop quick‐charge sodium‐ion battery, it is significant to optimize insertion‐type anode to afford fast Na + diffusion rate and excellent electron conductivity. First‐principles calculations reveal the TiO subcompound superiority for Na + diffusion following Ti(II)O > Ti(III)O > Ti(IV)O. Hence, in situ growth of amorphous TiO subcompounds with rich oxygen defects based on Ti 3 C 2 T x ‐MXene is developed. Meanwhile, the composite presents expanded MXene interlayer spacing and much enhanced conductivity. The synergistic effect of enhanced electron/ion conduction gives a high capacity of 107 mAh g −1 at 50 A g −1 , which gives 50% and 150% increasements compared with one counterpart without valence adjustment and another one without MXene expansion. It only needs 20 s (at 30 A g −1 ) to complete the discharge/charge process and obtains a capacity of 144.5 mAh g −1 , which also shows a long‐term cycling stability at quick‐charge mode (121 mAh g −1 after 10000 cycles at 10 A g −1 ). The enhanced performance comes from fast electron transfer among TiO subcompounds contributed by rich‐defect amorphous TiO 2–x , and a reversible change of elastic MXene with interlayer spacing between 1.4 and 1.9 nm during Na + insertion/extraction process. This study provides a feasible route to boost the kinetics and develop quick‐charge sodium‐ion battery.