Moss-like Hierarchical Architecture Self-Assembled by Ultrathin Na 2 Ti 3 O 7 Nanotubes: Synthesis, Electrical Conductivity, and Electrochemical Performance in Sodium-Ion Batteries.

Nanocrystalline layer-structured monoclinic Na 2 Ti 3 O 7 is currently under consideration for usage in solid state electrolyte applications or electrochemical devices, including sodium-ion batteries, fuel cells, and sensors. Herein, a facile one-pot hydrothermal synthetic procedure is developed to prepare self-assembled moss-like hierarchical porous structure constructed by ultrathin Na 2 Ti 3 O 7 nanotubes with an outer diameter of 6-9 nm, a wall thickness of 2-3 nm, and a length of several hundred nanometers. The phase and chemical transformations, optoelectronic, conductive, and electrochemical properties of as-prepared hierarchically-organized Na 2 Ti 3 O 7 nanotubes have been studied. It is established that the obtained substance possesses an electrical conductivity of 3.34 × 10 -4 S/cm at room temperature allowing faster motion of charge carriers. Besides, the unique hierarchical Na 2 Ti 3 O 7 architecture exhibits promising cycling and rate performance as an anode material for sodium-ion batteries. In particular, after 50 charge/discharge cycles at the current loads of 50, 150, 350, and 800 mA/g, the reversible capacities of about 145, 120, 100, and 80 mA∙h/g, respectively, were achieved. Upon prolonged cycling at 350 mA/g, the capacity of approximately 95 mA∙h/g at the 200th cycle was observed with a Coulombic efficiency of almost 100% showing the retention as high as 95.0% initial storage. At last, it is found that residual water in the un-annealed nanotubular Na 2 Ti 3 O 7 affects its electrochemical properties.