Role of the surface in Li insertion into nanowires of TiO2-B

Nanostructured TiO2-B is considered to be one of the most promising new anode materials for Li rechargeable batteries, exhibiting highly desirable properties of high capacity, good reversibility, and exceptionally fast Li-insertion. Recently, it has been demonstrated that nanowires of TiO2-B may act as capacitors for Li-ions in a wide range of Li concentrations. In this paper, we examine the sites and energetics of Li absorption, the barriers for Li-self-diffusion and the low-energy pathways from the surface into the interior through advanced simulation techniques. The simulations show that the relative stability of the sorption sites is affected by the surface, which constrains the structural anisotropic expansion of lithiated TiO2-B. We pinpoint the mechanisms responsible for exceptionally fast transport of Li ions and uncover the crucial role of the high surface area with an abundance of absorption sites. Each of the surface absorption sites is connected to the interior through a low-energy radial pathway, which brings an absorbed Li ion to its trapping site almost independently, contributing to pseudocapacitive behavior of TiO2-B nanowires with respect to Li-ions. We show that the curvature of the surface restricts the intercalation performance of nanowires and predict that (stretched) nanoribbons will exhibit superior properties. Copyright (C) 2010 John Wiley & Sons, Ltd.