Preparation And Electrochemical Performance Of Micron-level Silica-based Anode Materials For Lithium Ion Batteries

At present,lithium-ion batteries（LIBs）are widely used in consumer products such as mobile phones,notebook computer and electric vehicles owing to its high energy density,high operating voltage and long cycle life.However,with the rapid development of electronic products,electric vehicles,energy storage and other fields,people have put forward higher requirements for the energy density,service life and reliability of lithium-ion batteries.The theoretical specific capacity of commercial graphite anode and other indicators have been difficult to meet these requirements,so it is urgent to develop new anode materials.The silicon monoxide is considered as a potential high energy density anode material as result of its high theoretical specific capacity and moderate volume change.But the silicon monoxide has been confined to large-scale commercial application as result of the low conductivity and the first Coulombic efficiency,so that the electrochemical properties of silicon monoxide should be improved by means of nanocrystallization,surface coating and prelithium.Starting from the preparation process of silicon monoxide matrix composites suitable for large-scale production,the pyrolysis temperature and carbon content of carbon coated silicon monoxide as well as the technological conditions of different adding methods of graphite in the preparation of composite materials were investigated respectively,and the effect of the addition of a small amount of metal Sn on the electrochemical performance of SiO was also studied.Specific research work is as follows:SiO/C composites were prepared by high temperature pyrolysis using bitumen as carbon precursor and micron scale SiO.The effects of pyrolysis temperature and carbon coating amount on the electrochemical performance of SiO/C composites were investigated.The best electrochemical property performed by the 700-SiO/C material which obtained during 700℃,the first reversible capacity is 1492.3 m Ah g^-1,and the capacity retention rate is 59.2%after 50 cycles.This is mainly attributed to the difference in the amount of carbon residue and the composition of asphalt pyrolysis at different temperatures.GITT test shows that the presence of carbon layer can improve the diffusion coefficient of lithium ions,consequently,improving the rate performance of the material.The optimization study of carbon coating amount showed that as the higher carbon content had greater effect on the constriction of the volume expansion and surface integrity of the electrode,it would reduce the specific capacity of the material.When the SiO:asphalt was 5:1（w/w）,the electrode structure of the 5-SiO/C composite which composed of SiO and asphalt（w/w=5:1）was complete and the electrochemical performance was the best.The material provides the first reversible capacity of1488.4 m Ah g^-1and the initial coulombic efficiency of 72.09%SiO/Sn（w/w=4:1）composite was prepared by simple ball milling.The first reversible capacity of the SiO/Sn（w/w=4:1）composite which was prepared by simple ball milling is 1575.4 m Ah g^-1and the first Coulomb efficiency is 76.61%.This can be attributed to the fact that effective improvement of the electrical conductivity and lithium ion diffusion coefficient of the composite for the addition of metal Sn.Sn was involved in the formation of SEI film on the surface of SiO/Sn electrode as result of XPS analysis,which helped to reduce the impedance of SEI film and further improve the electrochemical performance of the composites.The SiO/Sn/C（8:2:1）composites prepared by forming carbon layer on the surface of SiO/Sn showed better electrochemical performance,showing the first reversible capacity of 1599.5 m Ah g^-1and the inital coulombic efficiency of 89.2%.SiO/Sn/C was combined with graphite（80%wt of graphite）to prepare a silicon oxide matrix composite with excellent cyclic performance meeting the design capacity.The first reversible capacity was 601.5 m Ah g^-1,and the first Coulomb efficiency was 88.6%.The capacity retention rate is up to 94.5%after 200 cycles,showing a good prospect of industrial application.