Multifunctional Na<sub>2</sub>TiO<sub>3</sub> Coating-Enabled High-Voltage and Capacitive-like Sodium-Ion Storage of Na<sub>0.44</sub>MnO<sub>2</sub>
Yuge Cao, Meijing Xiao, Wujie Dong, Tianxun Cai, Yusha Gao, Hui Bi, Fuqiang Huang
Abstract
Sodium-ion batteries, as an attractive option for large-scale energy storage, still face the problems of low energy density and unsatisfactory rate performance. Among various cathodes, the tunnel-type Na 0.44 MnO 2 with large S-shaped Na + transport tunnels is one of the promising cathode materials for fast and robust sodium-ion storage, yet suffering from Mn dissolution and structural collapse. Herein, a Na-rich layered oxide Na 2 TiO 3 is first constructed as a multifunctional coating layer on the surface of the Na 0.44 MnO 2 nanorod. Na 2 TiO 3 not only acts as an Na + reservoir, but also serves as a protective layer to prevent Na 0.44 MnO 2 from electrolyte etching. Besides, the derived Ti-doped Na 0.44 MnO 2 transition layer supplies additional Na + diffusion pathways along the radial direction of the nanorod with a short migration distance. The optimized 3 wt % Na 2 TiO 3 -coated Na 0.44 MnO 2 exhibits enhanced an initial capacity of 127 mAh g –1 at 2–4.5 V. In addition, it shows an ultra-high capacitive-like capacity ratio of 96.7%, hence delivering an excellent rate performance of 80.2 mAh g –1 at 20C. Long-term cycling tests indicate splendid stability against high voltage, achieving 97.7% capacity retention at 20C after 900 cycles. This work provides an effective strategy to improve the rate performance and high-voltage stability of Na 0.44 MnO 2 for high energy and power density batteries.