Si–graphite composites as anode materials for lithium secondary batteries
Introduction
Si has been projected as a candidate for anode material in lithium secondary batteries because of its high theoretical capacity (4200 mAh g−1). Even though it has much higher theoretical capacity than the graphite (372 mAh g−1), there are many obstacles to be commercialized. The major problems are the volume change (expansion/contraction) during the alloying (charge) and de-alloying (discharge) reaction with Li+ ions (420%) and the low electrical conductivity [1], [2], [3], [4], [5]. Due to the volume change, Si particles are isolated from electron conducting paths in the electrode resulted to lose Li+ storage ability eventually. To resolve this problem, different types of approach has been investigated such as reducing particle size to nano-level [6], [7], dispersing Si in other materials matrix [8], [9], [10], [11], [12], [13], and fabricating Si in the form of thin films [14], [15], [16], [17]. Carbon materials are used common matrix because of its effect on increasing the electrical conductivity, reducing the volume change, and contributing the cell capacity [18], [19], [20].
In this paper, we report the characteristics of two different types of Si–C composites including the preparation, morphology verification, and electrochemical performance. One is Si particles placed on the graphite surface (type A) and the other is Si particles embedded in the graphite (type B).
Section snippets
Sample preparation
The Si–C composites were made from natural flake graphite (FG) and Si particles (Sigma–Aldrich, 325 mesh). The average diameter of FG particle was 200 μm and overall particle shape was thin and flat. Si powder was ball-milled to reduce the size down to 100 nm. The morphologies of FG and Si are shown in Fig. 1. For the type A, spherical graphite (SG) was prepared by shape modification method using rotational impact blending machine, which was useful method to synthesize spherical graphite having
Results and discussion
The powder morphology of Si–C composites is shown in Fig. 2. Type A has rounded shape while type B has mixed shapes of flake and round. The average size of rounded particle is about 15 μm. The enlarged surface images of types A and B (Fig. 2c and d) shows quite different morphologies which the type A has a smooth surface while the type B has a rough one. Different morphology is attributed to synthesis methods applied to type A and type B. For type A, the rounded shape graphite was coated with Si
Conclusion
Si powders located in different sites in Si–C composites (type A: surface coated; type B: inner-particle embedded) are successfully synthesized and tested as an anode material for lithium secondary batteries. Electrochemical characterization revealed that the polarization is not increased by Si but the capacity of graphite is decreased in type A. In type B, the damaged graphite by agglomerated Si particles induces the capacity loss during cycles because of losing electrical contact.
Acknowledgement
This work was supported by the division of advanced batteries in NGE program (project No. = 10016453).
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