Preparation And Performance Of Si Thin Films For Lithium-ion Battery Anode By MPCVD

As the much more mature energy storage device in today’s world,lithium ion battery has been widely used in military and civilian small electrical appliances.,such as mobile phones,laptops and other portable electronic devices.And now,LIB is more and more used in electric vehicle industry.Because of the advantages of high capacity,high battery voltage,wide working temperature range and long cyclic life,the research and development of LIB greatly satisfy the demand of energy storage devices.The structure of lithium ion batteries includes electrodes,diaphragm,casting,electrolyte and binder.The demand for battery energy density and power density is increasing day by day,and it is the most remarkable method to develop the electrode material with higher capacity to solve this problem.The traditional lithium-ion batteries use carbon materials as anode,including carbon nanotubes,carbon films,carbon nanoparticles and other forms.Although carbon materials has stable electrochemical performance as anode of LIB,the theoretical capacity(372mAh/g)restricts the battery capacity.Silicon has the highest theoretical capacity(4200mAh/g)in known active materials,so it is the most promising substitute for carbon and graphite as new anode active material for LIB.But during battery charging and discharging cycle processes,silicon anode presented serious volume expansion and contraction(～300%),which results in cracking and disintegration of the electrode,with severe fade of capacity.And the bare fresh surface constantly generates solid electrolyte interphase(SEI).To effectively solve these problems,it needs mature preparation technology,which better utilize silicon superior electrochemical performance.This paper uses MPCVD(microwave plasma chemical vapor deposition)to deposit silicon thin film on the surface of copper foil as direct electrode,which effectively avoids bad influence between active material and copper foil caused by binder and other materials.MPCVD can generate big and uniform plasma field.The preparation process of direct electrode is simple.The parameters can be easily controlled,therefore we can optimize structure and performances of samples by contrast experiments.It is easily apply to market by scaling up machine.In experiment,we use 5 vol.%silane(diluted with argon)as deposition gas,argon and hydrogen as discharge gas,to prepare high-capacity silicon-based lithium ion battery anode.In this paper,the influence of gas flow ration,deposition time and microwave power on the microstructure and electrochemical properties of silicon thin film has been investigated.SEM,EDS,Raman are used to microstructure and composition analysis,while though constant current charge-discharge and CV test to finish the characterization of capacity,initial Coulombic efficiency and cycle performances.The results showed that when silane flux is 50sccm,silicon thin film anode has the highest capacity,in the 50mA/g current density of 50mA/g,the capacity is 1500mAh/g;when microwave power is 1.0kW,in the current density of 100mA/g,battery capacity is 1146.4mAh/g;when deposition time is 3h,in the current density of 100mA/g,battery capacity is 1478.5mAh/g.Deposited silicon film with optimum parameter(50sccm silane flux,1.0kW power,3h deposition time),SEM result shows that silicon thin film has uniform particle morphology with a particle size of 0.2～0.3μm.And particles completely cover copper foil substrate.Raman result shows that the films prepared were silicon films,and the main component are amorphous silicon and crystalline silicon.The battery capacity is 3190.6mAh/g in the current density of 100mA/g.