Tin-based Compounds As Thin Film Anode Materials Prepared By R.F.Magnetron Sputtering For Lithium Ion Battery

Lithium-ion batteries have now become the main power supply of mobile phones, Mp3players, laptops, digital cameras, video cameras and other portable devices for their significant advantages such as high energy density, long cycle life and environmental friendliness. In recent years, the rapid development of electric tools, electric bicycle, electric vehicles provides good application prospect for lithium ion batteries, while put forward higher requirements for its performance including energy density, cycle life, environmental compatibility as well as price and safety. The theoretical lithium storage capacity of carbon negative electrode materials is relatively low(372mAh/g), unable to meet the demands of high power and large capacity. Therefore many non-carbon anode materials have been investigated, among these materials, metal tin draw lots of attentions due to its large theoretical capacity, which is up to994mAh/g. However, it suffers large volume expansion-contraction during lithiation and delithiation processes, resulting in poor capacity retention. Tin-based nano materials (TCO) proved to be one of hot research topics because of its stationary response platform, low potential, considerable cycle performance and capacity characteristics. Up to now, tremendous reports about lithium electrochemical behavior of tin-based anode materials mostly concentrated on SnO2、carbon-coated SnO2nanocomposites and transition metal/metal oxide doped SnO2composites with structure and morphology improved. In order to buffer volume expansion of the alloying reaction, we propose new tin-based compounds with certain framework, which enables lithium ions freely intercalate and deintercalate. It could come back to initial state after recharging to limit potential. Here, we employ radio frequency (r.f.) magnetron sputtering to fabricate amorphous Sn2P2O7and SnC2O4thin films as anode materials.Sn2P2O7thin film electrodes were first prepared by radio frequency sputtering on the stainless steel (ss) substrates under the pressure of Ar ambient gas. The XPS and HRTEM&SAED measurements suggested that the as-deposited thin films were composed of amorphous Sn2P2O7. Nanostructured thin films of Sn2P2O7exhibited a high reversible capacity around887mAh/g with excellent cycling performance, corresponding to12.8Li per Sn2P2O7. This value is far larger than that reported by Lee et al.(520mAh/g) and the theoretical one (572mAh/g). CV curve showed that multi-steps’ reactions were involved, furthermore, a new pair of redox peaks which correspond to reversible reaction:Sn2P2O7+4Li++4e-<->2Sn+Li3PO4+LiPO3was found above the potential of1.5V. The highly reversible electrochemical reaction mechanism of Sn2P2O7with lithium was revealed by using ex situ TEM&SAED, XPS measurements.Similarly, the SnC2O4thin films were deposited directly on the stainless steel substrate by r.f. magnetron sputtering at room temperature. Considering the oxalate substance generated in charging and discharging processes may be dissolved in the electrolyte, we coated a layer of LiPON(inorganic solid state thin-film electrolyte-lithium phosphorus oxynitride) on the surface of SnC2O4film in order to improve the cycling performance. LiPON with good stability ensures SnC2O4indirect contact with electrolyte and keeps high ionic conductivity.Li storage capacities and cycling performance of LiPON-coated SnC2O4film were investigated by Galvanostatic cycling measurements. The capacity of the initial discharge was found to be775.1mAh g-1. After20cycles, a reversible capacity of438.1mAh/g was still obtained. CV curve of LiPON-coated SnC2O4was different from that without LiPON. These redox peaks corresponded to reactions SnC2O4+2Li++2e<-> Li2C2O4+Sn and tin-lithium alloying process respectively. Our results demonstrated that oxalic ion acted as inert matrix and played the role in buffering the volume change during alloying and dealloying reactions between Sn and Li. These results indicated that Sn2P2O7and SnC2O4film electrode were promise anode materials for lithium ion battery.