Syntheses, Structure And Performance Of Silicon Composite As Anode Materials For Lithium-ion Batteries

Si-based anode materials have been attracted much attention in recent years due to its advantages of high abundance, low intercalation potential and the highest theoretical capacity among the present anode materials. Moreover, the micron/submicron-sized silicon powders, which possesses characteristics of high tap density, high volumetric energy density, simple syntheses and low cost, are promising anode materials for next generation lithium-ion batteries (LIBs). However, Si has low electronic conductivity, and meanwhile its large volume change (>280%) during lithiation/delithiation processes results in a rapid capacity degradation, especially for micron/submicron-sized silicon anode. Such shortcomings prevent silicon from the commercial application in LIBs.In order to solve the low cycling stability of micron/submicron size silicon powder, which results from its low electrical conductivity and pulverization during lithiation/delithiation processes due to its large volume change, we explore three ways mainly, including introduction of vapor grown carbon fiber (VGCF) as conductive agent in the particulate Si; in-situ introduction of high conductive carbon and low active SiOx buffer phases through combining ball milling Si with citric acid and acetylene black, assisted with spry dry and pyrolysis.. Effects of the preparation methods and their parameters on the structure and electrochemical performance of the Si anode materials are systematically investigated by means of X-ray diffraction, transmission electron microscopy and electrochemical testing of galvanostatic charge-discharge.The addition of VGCF in the Si powder constructs an 3D conductive network on the outside of the silicon powder. It can not only improve the conductivity of Si anode, but also buffer the volume change of Si during lithiation/delithiation process, which finally resulted in markedly improved cycling stability of the electrode and better cycle performance of silicon anode materials. The Si anode with average particle size of 2 μm shows a capacity of 1520 mAh/g after 100 cycles, possessing a capacity retention of 45%.A micro/submicron-sized Si@SiOx@C composite with high SiOx content and low carbon content was prepared through effectively combining mechanical ball milling Si with citric acid assisted with a pyrolysis of the citric acid. The particle size of Si was reduced during the milling, more over, bi-phases of SiOx and carbon were generated on the surface of the Si particles, forming a Si@SiOx@C composite with micro/submicron size, which shows significantly improved cycling stability. After optimization of the pyrolysis temperature, the composite prepared from 650℃ shows much favorable electrochemical property. An initial coulomb efficiency of 74.2%, a capacity of 776 mAh/g and capacity retention of 74.6% after 200 cycles at a discharge/charge current density of 300 mA/g are obtained when no extra conductive agent is added in the anode preparation.Based on the preparation of the Si@SiOx@C composite, acetylene black (AB) was added as conductive agent during the milling process. A Si@SiOx@C/AB composite with total carbon content of ca.17 wt% is synthesized assisted with a spry dry process for the milled mixture. The Si@SiOx@C/AB composite prepared at 650℃ of pyrolysis for the introduced citric acid shows an initial coulomb efficiency of 73.7%, a capacity of 855 mAh/g and capacity retention of 84.0% after 200 cycles at a discharge/charge current density of 300 mA/g.