Mesoporous Spheres Comprised of Rutile TiNbO₄ Particles for Na Storage Materials

We prepared rutile-type TiNbO₄ by a sol-gel method to investigate the relationship between the calcination temperature in the synthesis and its nanostructure and evaluated the influence of the nanostructure on the electrode properties as a Na-ion battery anode. The crystallite size of TiNbO₄ and the size of the diffusion path along its c-axis were increased with increasing calcination temperature. We revealed that the TiNbO₄ was comprised of aggregated single-crystalline primary nanoparticles to form mesoporous secondary particles of spherical marimo shape. At a high current rate of 5C, the electrode of TiNbO₄ calcined at 700 °C exhibited the highest reversible capacity of 170 mA h g^-1. It is suggested that TiNbO₄ at 700 °C has a relatively wide diffusion path to allow high-speed Na⁺ insertion-extraction and that its smaller crystallite size improves the utilization rate of the active material.