Research On The Anode Materials Of Lithium Ion Batteries Based On Physical Vapor Deposition-Nanocrystalline Silicon

Nano silicon-carbon anodes have the qualities of high specific capacity and appropriate potential of lithiation/delithiation,and they have the greatest possibility to obtain high-performance lithium ion batteries if matched with triple-system cathode materials.However,nowadays,the preparation of nano silicon-carbon-based materials is high-cost,complicated as well as their unstable electrochemical performances,etc.So the deep researches on the preparation of nano silicon-carbon anode materials,interfacial phenomena during lithiation/delithiation process and the corresponding electrochemical performances are full of significantly theoertical and practical interests.Based on the physical vapor deposition-nanocrystalline silicon of electron beam evaporation,the present research focuses on the preparation of silicon nanoparticles and nano silicon-carbon(graphite)anode materials,reveals the rules of microstructure evolutions during the lithiation/delithiation process,and characterizes the electrochemical performances of anode materials,such as cycling and rate performancs,thus the corresponding capacity fading mephanisms are analyzed and the optimization solutions are proposed.All these researches provide theoretical and technological guides,which are benefit for preparing high-performance anode materials of lithium ion batteries.According to the systematically research,important conclusions and achievements are made as follows:(1)Silicon nanoparticles can be controllably prepared through electron beam evaporation and milling.The physical vapor deposition-nanocrystalline silicon is porous,and the average size of the crystal is 33 nm.The microstructure and growth rule of the physical vapor deposition-nanocrystalline silicon are researched,and argon is theoretically proposed to control the migration route and flux of the vapor silicon.When the argon flow is parallel to the surface of molten silicon and at the velocity of 1.16 m·s-1,not only the evaporation flux of vapor silicon is increased,but also the vapor silicon is intensively transferred to the outlet.Under the ethyl alcohol aid,the physical vapor deposition-nanocrystalline silicon is milled to prepare silicon nanoparticles with media size of 50 nm.The nanoparticles mainly are amorphous silicon and only a few of crystalline silicon,with the multivalence-silicon oxide layer.(2)The structural stability and evolution of Li 15Si4 alloy in the nano silicon-carbon anode materials are investigated,and the corresponding mechanisms are elucidated,which provide theoretical guides for the recommbination of silicon nanoparticles and carbon to achieve anode materials of more stable electrochemical performances.Microstructure and phase changes drastically take place in the core-shell type silicon nanoparticles-carbon anode materials during the lithiation/delithiation process,namely,silicon and crabon fracture for the silicon volume swelling,and the lithium ions accumulate in the local position due to the diffusion kinetics of lithium ion lag behind the diffusion kinetics of electron,significantly decreasing the electrode potential in the local position.The accumulation in the local position is severer when the current density increases,inducing the heterogeneous growth of Li15Si4 alloy in the low electrode potential-local position,though the anode voltage is high.In consequence,it destroys the lithiation/delithiation stability of the electrode materials.(3)According to the evolution of Li15Si4 alloy and interfacial phenomena of the electrode,the anode materials are modified and redesigned.The encapsulated structure silicon nanoparticles-carbon anode materials(silicon,57.0 wt.%and media size of 125 nm)are prepared by self-assembly method.Thanks to the enhanced intergination between the silicon nanoparticles and carbon layer by the silane coupling agent,the anodes' electrochemical performances are excellent.The initial delithiation specific capacity is 1178 mAh-g-1 at the current density of 0.1 A·g-1,and the capacity maintains the 84%of initial capacity after 100 cycles;The initial delithiation specific capacity is 837 mAh·g-1 at the current density of 1 A·g-1,and the capacity maintains the 85%of initial capacity after 800 cycles;The rate performance is good,and the delithiation specific capacity is especially as high as 404 mAh·g-1 at the current density of 10 A·g-1.(4)Based on the nano silicon-carbon anode materials mentioned above,in order to investigate the influence of silicon nanoparticles size on the electrochemical performances when high-content graphite,the encapsulated structure silicon nanoparticles-carbon/graphite anode materials(10.5 wt.%silicon)at silicon nanoparticles of different sizes are prepared.Thanks to the silicon nanoparticles with media size of 51 nm highly alleviate the volumetric strain,the electronic connection amongst the active materials and current collector and the delithiation reversibility of Li15Si4 alloy both are enhanced.As a result,the anodes deliver excellent electrochemical performances.The initial specific capacity is 505 mAh·g-1 at the current density of 0.1 A·g-1,and it maintains the 86.3%of initial capacity after 500 cycles;The initial specific capcity is 308 mAh·g-1 at the current density of 1 A·g-1,and it maintains the 91.5%of initial capacity after 500 cycles.After series of cycles with high and low current densities,the anodes' capacity can completely recover compared with the initial capacity,exhibiting a quite excellent rate performance.The prepared technologies for the silicon nanoparticles and nano silicon-carbon(graphite)anode materials are facile and environmental friendly.Additionally,combined with characterizations,numerical simulation and theoretical analysis,the research deeply investigates the electrochemical performances of nano silicon-carbon(graphite)anodes and corresponding mechanisms by focusing on microstructure change and Li 15Si4 alloy evolution(such as growth and delithiation)of the anode materials during the lithiation/delithiation process,which provides a feasible guide line for the preparation of silicon-carbon material-system anodes.