First principles study of structure and lithium storage in inorganic nanotubes

Abstract

The exact structure of layered inorganic nanotubes is difficult to determine, but this information is vital to using atomistic calculations to predict nanotube properties. A multi-walled nanotube with a circular cross section will have either a mostly incoherent interface or a large amount of tensile strain to accommodate a coherent interface, but a polygonal cross section could result in a coherent interface with considerably less strain. An energy component model is parameterized with atomistic calculations to compare nanotubes with a circular and polygonal cross section. The model shows that for TiS2 nanotubes with some chiralities the radius at which a polygonal shape becomes energetically favorable is approximately 15 A. Due to the higher strain energy and lower interfacial energy the critical radius for polygonal formation of MoS2 nanotubes is 36 A. Both of these values are below the typical radius of TiS2 and MoS 2 nanotubes seen experimentally, indicating that for certain chiralities polygonal nanotubes should form. We also investigate the potential of inorganic nanotubes as energy storage materials. First principles calculations on curved surfaces and distorted slabs are used to analyze the effect of curvature and stacking on voltage and diffusion properties. The effect is qualitatively and quantitatively dependent on the material and structure. The Li voltage on the surface of TiS2 nanotubes decreases with a decreasing radius whether lithium is inside or outside of the nanotube. On the surface of MoS2, the voltage decreases with decreasing radius when Li is inside the tube, but increases with decreasing radius when Li is outside the tube.

(cont.) The activation barrier for lithium diffusion increases with decreasing radius whether Li is outside or inside the nanotube while the barrier decreases in either case for MoS 2. When the stacking is disordered the lithium voltage and activation barrier between TiS2 layers decreases, although the decrease in voltage is not as large as the decrease in activation barrier because the stable lithium site changes from the octahedral site to the tetrahedral site at some stacking arrangements.