Effect of interlayer spacing on sodium ion insertion in nanostructured titanium hydrogeno phosphates/carbon nanotube composites

Abstract

In sodium ion batteries, the ease of insertion and extraction of sodium ions in the electrode materials is one of the key parameters for the overall performance. In this article, the electrochemical sodium ion insertion in layered titanium hydrogeno phosphates (TiP) has been studied. In this material, the interlayer spacing and the particle morphology can be controlled by the choice of synthesis methods. Both nanostructured TiP and the coarse grained bulk counterpart were synthesized and properties were compared. While the specific capacity of nanostructured TiP materials was found to be not sensitive to the interlayer spacing, the specific capacity of coarse grained bulk TiP materials was significantly increased as the interlayer spacing was increased with the intercalation of water molecules in the layered host structure. These results indicate that interlayer spacing may not be the primary factor for Na-ion diffusion in nanostructured materials, where many interstitials are available for Na-ion diffusion. It is shown that nanostructured TiP materials can deliver excellent rate capability, and long term cycle stability with stable reversible capacity without the need of interlayer spacing expansion. The electrochemical properties of nanostructured materials were further enhanced when prepared as composites with carbon nanotubes that enhance the overall conductivity of the electrode materials.