Synthesis and characterizations of SnO2 nanoparticles for high-performance Lithium-ion Batteries

Abstract

Energy storage devices are becoming increasingly essential in modern society with the rapid development of novel techniques such as electric vehicles and smartphones. Not only should the high energy capacity be considered, but also lightweight, cost-efficiency and environmental friendliness. Compared to conventional lead-acid cells which are toxic and cumbersome, lithium-ion batteries show great potential for the next generation energy storage devices due to high theoretical capacity and lightness. Generally, the biggest challenge for traditional lithium-ion batteries is low energy density from the carbon-based anode. Recently, lots of researchers have focused on the silicon-based anode which has the largest theoretical specific capacity, but is still limited by the short cycle life and poor electron conductivity. Thus, transition metals and their oxides have attracted much attention due to their relatively better cyclic performance and conductivity. Tin oxide (SnxO) is one of the typical transition metals which has high mass specific capacity (792 mAh/g), more than double that of commercial carbon-based anodes. In this project, nano SnO2 has been synthesized with the sol-gel method with the precursor of SnCl2·H2O and the size of the product was controlled by the amount of the ammonium hydroxide. The intermediate product after sol-gel was proven was the "pyramid" tin hydroxide oxide phase with XRD analysis. Besides, EDX based on SEM also proved there is no chloride residue after the first sol-gel process which shows this method can be an alternative way for practical applications due to the low-cost tin resource. The terminal product after heat treatment was proven as SnO2 with the preferred orientation (110) and is used for the anodes of batteries. The cyclic voltammetry method was used for the analysis of the electrochemical behaviours of the samples. The conversion and alloying reactions were proven to start at 0.7 V and 0.2 V with CV result. The discharging specific capacity of anodes based on our nano SnO2 was around 800 mAh/g which was much higher than commercial ones tested by the cycle test. After 100 cycles, the capacity of the sample can just keep about 50 mAh/g and the impedance was still around 400 ohms. Therefore, additional experiments using a hydrothermal method, have been conducted to improve the electrochemical performance of nano SnO2. Although thes initial specific capacity of the carbon coating samples decreased from 800 mhA/g to 600 mAh/g, the cycle life increased significantly to where the capacity can keep around 50% after 50 cycles. Besides, the impedance of the carbon coating anode signicantly declined from 400 ohms to less than 200 ohms due to the coated carbon with high electron conductivity.