Fabrication,Modification And Electrochemical Performance Of Silicon-based Anode Materials For Lithium-ion Battery

Silicon is the ideal anode material for the future lithium-ion battery,because of high theoretical capacity(～4200 m Ah g^-1,which is more than 10 times the capacity of the traditional commercial graphite carbon anode).Moreover,it also has the advantages of low lithium potential and abundant crustal reserves.However,the huge volume change（>300%）will lead to the powder and fall off of electrode materials in the process of Si_xLi alloying,as well as resulting in the low initial coulomb efficiency,poor conductivity and short cycle life of the battery,which seriously limits its commercial application.At present,most of the commercial silicon anode materials are nano-silicon particles with a high price and short cycle.Based on the above problems,low-temperature molten salt,compound doping,sol-gel and surface coating methods were adopted in this article,aiming at improving the electrochemical performance of silicon-based anode materials.A variety of physical and chemical characterization methods were used to systematically research materials.The details are as follows:（1）To alleviate the volume expansion effect of silicon materials and improve the cycle life of LIBs,crystalline silicon with nano-porous hollow structure（NP H-Si）was prepared by the improved St?ber method combined with the low-temperature molten salt reduction in this study.The structure of NP H-Si was characterized by XRD,SEM,TEM and XPS.The experimental results show that it has good crystallinity and the size is evenly distributed at 400～500 nm.The initial coulomb efficiency was as high as 91%at0.1 A g^-1.Compared to commercial silicon,its specific discharge capacity is 1642 m Ah g^-1after 300 cycles at 0.5 A g^-1.The rate performance test shows that it has good cycle stability under different current densities of 0.1,0.5,1.0,3.0,and 5.0 A g^-1.When the current density jumps to 0.1 A g^-1,the discharge specific capacity also recovers to 2534.4m Ah g^-1.The excellent performance confirmed the technical feasibility of designing porous hollow silicon particles in LIBs.（2）The micron silicon-carbon composite was prepared through the secondary granulation,which effectively improved the conductivity and rate performance of silicon anode.The nano-silicon was embedded in the graphite interlayer by mechanical ball milling,and the polydopamine was coated on the surface of the material by in-situ polymerization.Finally,the graphite was doped with carbon-coated silicon microspheres（Si/G@C）was obtained after carbonization at high temperature.The physical analysis method like XRD,SEM,TEM,XPS and so on was taken,and the results showed that nano-silicon was better embedded in the graphite interlayer and the amorphous carbon material after high-temperature carbonization cross-links graphite with silicon material into microspheres with a diameter of about 2μm.Compared with Si@C and Si/G to further illustrate the electrochemical excellence of this composite,constant current charge-discharge were performed at 0.5 A g^-1after 500th cycles,the specific discharge capacity remained at 2730 m Ah g^-1.Cycling for 10th at current densities of 0.1,0.3,0.5,1.0,and 2.0 A g^-1,and the capacity retention rate was 87%when it finally recovered to0.1 A g^-1.It has a certain reference value for the practical production of silicon carbon composites.（3）In order to further improve the safety performance of the battery,Si/Ti O₂/GO@C composites with core-shell structure were prepared by sol-gel/hydrothermal method.Ti O₂was deposited on the surface of silicon particles by the sol-gel method,and GO was encapsulated by the hydrothermal method.The core-shell structure was verified by TEM and BET.TGA analysis shows that the composite has good thermal stability（ΔH=3.8 KJ/g）.The results of the electrochemical performance test show that after 800 cycles at 0.5 A g^-1,which discharge specific capacity is 2147 m Ah g^-1.Even after 300 cycles of 2 A g^-1which still has 1421.7 m Ah g^-1,and the capacity retention rate of up to 70%.The Si/Ti O₂/GO@C composite material prepared by this scheme has important guiding significance for the safety research of lithium-ion batteries.