TiO2-core/Sn-shell Nanotube Arrays Based on Monolithic NegativeElectrode for Li-ion Batteries

Abstract

A TiO2nanotube (TONT) with an average pore diameter, length, and wall thickness of about 120 nm,400 nm, and 20 nm, respectively was grown anodically on a Ti substrate. Sn nanophase is then coatedthrough the entire area of the TONT by radio-frequency (RF) magnetron sputtering technique. The porediameter of Sn coated TONT (denoted as Sn/TONT) arrays were decreased to 50 nm, while the wall thick-ness was increased to about 50 nm. Further, to make a conformal coating of the Sn layer through TONTarrays, while considering that the melting temperature of Sn is below 232◦C, the post-thermal treatmentunder different ambients (pure Ar gas vs. 5% H2added Ar gas (designated as H2/Ar gas)) was performed at350◦C for 3 h. Compared with pure Ar annealing, H2/Ar annealing suppresses the formation of the SnOxphase, as confirmed by X-ray photoelectron spectroscopy (XPS) analysis, accelerating the formation ofthe pure Sn phase. The electrochemical properties of bare TONT and Sn/TONT arrays were comparedusing a cyclic voltammogram, showing that the TONT arrays with the pure Sn material participate inelectrochemical lithiation and delithiation during the cycling process from the advent of the oxidationand reduction peaks in a certain potential range. Based on the active functions of the two materials, thecapacity (0.042 mAh/cm2) of Sn/TONT arrays is increased relative to that (0.025 mAh/cm2) of bare TONTarrays and the capacity retention of Sn/TONT was maintained to be 75% at 5 C.