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  • Si/C Composites for Battery Materials
    Nov 06, 2018 | ACS MATERIAL LLC

    Lithium-ion batteries (LIB’s) are well-suited for fully electric and hybrid electric vehicles due to their high specific energy and energy density in comparison to other rechargeable cell options, however, their suitability depends on the quality of the anode material within. Silicon Carbide (Si/C) composites are a semi conductive material where silicon is highly dispersed within a carbon matrix. Si/C composites exhibit not only acceptable faradaic yield at the first cycle, but also large capacity and good rechargeability. These are essential and highly desirable properties making Si/C composites worth considering for use as anode material within lithium-ion batteries. 

    Introduction

    Lithium-ion batteries have been widely used as power sources in all types of modern portable equipment and they show great promise for electric transportation systems, including electric vehicles (EVs) and hybrid electric vehicles (HEVs).1-3 So far, many kinds of anode material for these Lithium-ion batteries have been investigated, such as graphitic carbons,4 disordered carbons,5 tin-based materials, nitrides, phosphides, and oxides. However, only graphitic carbons are commercially available today due to their excellent charge and discharge cycling behavior. That being noted, however, the theoretical Li-storage capacity of graphite is limited to 372 mAh/g.6 Most recently, there has been considerable interest in developing silicon anode material for lithium-ion batteries due to its high theoretical capacity of 4200 mAh/g and low electrochemical potential versus Li/Li+.7-8 One drawback associated with silicon anode material is the very large volume change during the alloying/dealloying process, potentially resulting in pulverization of the electrode materials and poor cycle performance.9 To overcome this problem, tremendous efforts have been made to investigate Si/C material for battery anodes since 1999.10 The mechanical properties of graphitic or amorphous carbon make it able to buffer the volume expansion of Si while the high conductivity of C can efficiently complement the high lithiation capacity of Si.

    Synthesis

    When graphiticor amorphous carbons are used as matrices, Si/C composites showsome promising properties such as high specific capacity and good cycling stability. Carbon plays an important role in electrode reliability because of its smallvolume change, high electronic conductivity, and its ability to limit volume expansion of Si during electrochemical cycling.

    The preparation of Si/C compositesbegins with first dissolving Polyvinyl chloride (PVC) in 1,2-epoxyproane. Nanosized silicon (<100nm) and fine
    graphite (1-2 µm) powders are added into the PVC solution and homogeneously mixed under ultrasonic action. When petroleum pitch is used as the precursor, a tetrahydrofuran solvent is used. The solvent is evaporated while stirring to get a solid blend. The blend precursor is then gradually heated to 900°C. After pyrolysis at 900°C for 2 hours, the furnace is cooled. The resulting Si/C composite samples are ground and sifted. The composition of the composites is estimated on the basis of no weight loss of graphite and silicon during pyrolysis.     The typical graphic description of prepared Si/C compositesis shown in Figure 1, using carbon black as a matrix.11 

     

    Fig.1

    Figure 1. Graphic description of the preparation of Si/C composites.11

     

    There are two types of Si/C compositesavailable on our ACS Material online store: Type A with a Si content of 8wt% and Type B with 18wt% Si content. The XRD pattern for our Type B Si/C Composite Anode Material can be seen below in Figure 2. From the chart, one can see peaks at 28.5°, 47.5°, 56.2° and 76.5° which can be attributed to crystalline reflections of the Si. From this, one can conclude that there is no phase of inert silicon carbide (SiC) observed for the Si/C Composite.

    Fig. 2

    Figure 2. XRD patterns of our ACS Material Type B Si/C Composite Anode Material.

    Applications

    The main challenge in seeking the perfect anode materials to be used within lithium-ionbatteries is the strong volume effect, which is the main reason forthe poor cycling performance of the silicon insertion host. The smallhost particle size can enhance dimensional and cyclingstability,12sowe have attmpted to disperse and embed nanosized silicon particlesin a carbon matrix with low volume expansion for lithium insertion.By this method, the total volume change of the electrode material canbe controlled at a rational level.

    Electrode fabrication and electrochemical testing for Si/C composites as negative electrode material for LIBsare described as follows.To prepare Si/C composite powders forelectrode material 86% active material, 6% carbon black, and8% polyvinylidene fluoride (PVdF)binder in n-methylpyrrolidone(NMP)are homogeneously mixed under ultrasonic action. The resulting slurry is coated on Ni foam (ɸ1.25 cm). After the solventevaporates, the electrode is dried under vacuum at 120°C andfinally pressed with 4 MPa. Electrochemical properties of the activematerials are evaluated via coin cells containing1 MLiPF6/ethylene carbonate (EC)+ dimethyl carbonate(DMC) (1:1 in volume)electrolyte and a lithium sheet acts as the counterelectrode. The charge and discharge current for testing is set to 125 mA/g and the voltagecutoff is controlled between 0.02 and 1.5 V.It is worth noting that the electrochemical and cycling performanceof a particular Si/C compositealso depends on the design and assembly of the final lithium-ionbattery.

    Conclusion

    Si is a promising negative electrode material forboosting the high energy density of LIBs because of itshigh specific capacity. However, significant challengesneed to be overcome before Si negative electrodes canbe utilized in practical LIBs. In order to overcome potential drawbacks occurringduring the  lithium insertion/extraction, designing the appropriate Si/C composite as negative electrodematerialfor LIBs is key.Although much progress has been made regarding the study of Si/C composites as negative electrode material,there are stillmany topics left unstudied in the field. Further research is needed to improve the initialcoulomb efficiency of LIBs based on Si/C negativeelectrodes because it will be critical for practical applications. Currently, it is quite expensive to prepare Si/C nanocompositesby the methods known today. This means it is necessary for us tocontinue working on developing new methods with low cost and high possibility for up-scaling.

    ACS Material Products:

    Battery Materials

    References

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