Preparation Of Si-based Nanofilm Via Magnetron Sputtering Towards Under Standing Electrochemical Property In Lithium-ion Battery Applications

The silicon materials possess inexpensive and environment friendly advantages,it have high theoretical capacity 3579 mAh/g and low potential(<0.4 V vs.Li/Li+)in insertion and extraction process of Li+.However,Si-based materials possess large volume effect reach to 300%lead to rapidly fading capacity and poor cycle life.The Ge and carbon materials have high electronic/ion conductivity and mechanical toughness,so the composites materials is prepared by the silicon combined with the Ge,carbon materials,and arising synergistic effects.It has a great significance on the research of Si-based anode materials of lithium ion battery with high energy density,which has been hotspots of research.1.The pure Si and Si/Ge films thickness are optimized and electrochemistry performance measured.The Si film optimal thickness is selected by adjusting the time of sputtering on the copper foil.Subsequent,the Ge film optimal thickness is chosen by change the deposition time.The results show that the Si and Ge film optimal thickness are 75 and 27 nm,respectively.The Si/Ge(75/27 nm)electrode exhibits reversible specific capacity 930 mAh/g after 500 cycles at a 4 A/g current density with the voltage range of 0.01-2.0 V,and the capacity retention of 78%.It also demonstrated that the average per cycle lost capacity is 0.53 mAh/g.The electrochemical impedance spectroscopy(EIS)measure shows that the charge transfer impedance decrease about 50%,and the Li+ diffusion coefficient is enhanced approximatively three times.Meanwhile,the X-ray photoelectron spectroscope measure shows that the Li2O and Li4SiO4 are the mainly irreversible products for whole processes.2.In order to improve the Si/Ge electrode cycling stability,this electrode is modified by carbon film.Firstly,the study of the carbon layer deposit place in the Si/Ge electrode.We design three structures of Si/Ge/C,Si/C/Ge and Si/C/Ge/C multilayer films.The results show that the Si/C/Ge/C multilayer film specific capacity is 1614 mAh/g at 0.8A/g after 200 cycles,and average per cycle fading capacity is 0.89 mAh/g(less than 1.29 mAh/g of Si/Ge electrode).It suggests that the carbon layer possess minimizing the volume strain and the Si/C/Ge/C electrode has optimal cycling stability contrasts other electrodes.The CV measure shows that the Si/Ge and Si/C/Ge/C electrodes possess difference potential of SEI film,which are 0.47 V and 0.8 V,respectively,this due to different electrode interface react with electrolyte.Second,the study of the pure Si,Si/Ge and Si/C/Ge/C film electrodes cycling stability at large rate after 400 cycles.This result in displays the Si/C/Ge/C multilayer film electrode has optimal capacity retention of 90.7%.3.In order to improve the electrode energy density,the Graphene/Si/C is obtained via sputtering Si and carbon on copper substrate modified by graphene.The Raman characterization shows monolayer graphene,and the SEM images show the electrode surface roughness and partly fold.The cycling performance measure shows that the Graphene/Si/C electrode reversible capacity is 557 mAh/g after 1000 cycles in the 2 A/g current density,with the capacity retention of 44%.The average per cycle fading capacity is 1.64 mAh/g after 220 cycles,but subsequent cycles,the average per cycle fading capacity is 0.822 mAh/g,this suggests that the average per cycle fading capacity reduce about 50%and the electrode become more stabilized than before.Moreover,the reversible specific capacity of electrode is remained at 1150 mAh/g after 350 cycles while the Si/C/Ge/C thin film electrode only 1071 mAh/g.The Rs decrease to 1.3 ?less than 2.6 ? of Si/C/Ge/C electrode,this due to the Cu substrate is modified by graphene improved electron conductivity.