All Electrochemical Process for Synthesis of Si Coating on TiO2 Nanotubes As Durable Negative Electrode Material for Li-Ion Batteries

Abstract

Although silicon is a promising negative electrode material for Li-ion batteries, its practical application has been hindered by the huge volume expansion resulting poor cyclic stability. It has been demonstrated that ultra-thin (< 50 nm) amorphous Si shown an excellent cyclic stability. However, the energy density delivered from the ultra-thin film is not sufficient for high energy application and increasing the thickness leads to the fracture of the film. Therefore, it is a key to design a conductive substrate with high surface area to enhance adhesion and Si active mass before reaching the fracture thickness. In this work, amorphous silicon (a-Si) film electrodeposited on self-organized TiO2 nanotubes is investigated as durable negative electrode material for Li-ion batteries. The nanostructured composite electrodes were fabricated by a two-step cost effective electrochemical process. Firstly, TiO2 nanotube arrays were synthesized by anodizing of Ti foil. Subsequently, thanks to room temperature ionic liquid, conformal Si film was electrochemically coated on the TiO2 nanotubes for the fabrication of nanostructured a-Si/TiO2 nanotube composite negative electrodes. Compared to other strategies where Si is used as a form of thin films, the TiO2 nanotube arrays provide several advantages such as a strong mechanical support to buffer Si volume expansion, a high surface area to increase Si active mass, and the direct contact of the nanotubes with the Ti current collector facilitates 1D electron transport and avoids the need of adding inactive binders or conductive additives. The influence of the Si loading as well as the crystallinity of the TiO2 nanotubes have been studied in terms of capacity and cyclic stability of the composite electrode. For an optimized Si loading, it is shown that the amorphous state for the TiO2 nanotubes enables to get stable lithiation and delithiation with a total areal charge capacity of about 0.32 mA h cm-2 with improved capacity retention of about 84% after 50 cycles. The specific capacity contribution of Si in the composite is estimated to be about 900 mAh/g after 50th cycle. However, the Si deposited on crystalline TiO2 nanotubes showed poor cyclic stability independently from the Si loading. Fig. 1 SEM images of TiO2 nanotubes formed by anodizing Ti foil (left) and after Si coating of the nanotubes via electrodeposition from room temperature ionic liquids (right). Figure 1