Silicon/Wolfram Carbide@Graphene Composite: Enhancing Conductivity and Structural Stability in Amorphous-Silicon for Lithium Storage Performance
Authors: Sun Wei , Hu Renzong , Zhu Min 
Published: February 2020

Lithium ion batteries (LIBs) are very important power sources in a variety of applications, including portable electronic devices and home electronics, and are expanding to electric vehicles nowadays. The rapid development of these applications demands for making LIB smaller and lighter, namely, requires incorporation of higher gravimetric and volumetric capacity materials to replace the currently commercialized graphite anode[1]. Silicon (Si) is the most attractive anode material due to its high theoretical capacity of 3579 mAh g-1 at room temperature, which is an order higher in magnitude than that of graphite (372 mAh g-1)[2]. However, the poor cycleability, which is caused by structural failure and pulverization due to the huge volume change (>300%) of Si during the Li+insertion and removal, hinders its practical application.
In order to enhance the cycling performance, one of the promising ways is reducing the particle size of Si into nano-crystallite or amorphous structure to minimize the expansion, and then dispersing the as-obtained Si in the carbon matrix which mostly functioned as an elastic network with high e-/Li+ conductivity[3]. In contrst to the rather complicated deposition synthesis, ball milling (BM) is considered to be an effective approach to obtain nanosized powder materials, which is also favorable for large scale production. In this work, we report an amorphous-Si/WC@graphene (SW@G) composite anode with improved performance had been designed and produced by a simple two-step BM process. [4] It should be noted that the rigid wolframcarbide (WC) particles were expected to assist refining of the common micro-sized Si to the amorphous state gradually in milling process.
Resemble a concrete structure, the SW@G composite consisted of multiple-scale WC particles which were uniform distributed in amorphous Si (a-Si) matrices, with outside coating by few layers of graphene (Fig.1). The greatly enhaced structural stability of SW@G composite can be attributing to the following reasons: (i) the lithiation and the subsequent volume expansion of a-Si would have no preferred orientations in the SW@G; (ii) the multiple-scale WC particles not only act as the gravel to strengthen the inside structure to withstand the pulverizations and aggregations of Si during cycling, but also provides proper electric contact between the Si particles due to their high intrinsic electronic conductivity; (iii) like the outside cement layers of concrete, the GNs layers work as shell of the whole SW@G composite, cloud much improve the electronic conductivity among the SW@G particles, as well as help to form a stable SEI film to keep the inside Si from directly exposing to electrolyte. In these viewpoints, this concrete-like structure could lead to a high structure stability and consequently a good cycleability, as well as a superior rate performance for the SW@G composite.

Acknowledgements
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This work was supported by the National Science Foundation of China (Projects 51201065 and 51231003), by the Guangdong National Science Foundation (Project S2013010012487), and by the Doctorate Foundation of the Ministry of Education (Projects 20120172120007).

References

1. B. Wang, M. Liang, L. Zhi, Nano letters, 2013, 13, 5578.
2. W. J. Zhang, J. Power Sources 2011, 196, 13.
3. W. Sun, R. Z. Hu, M. Zhu, J. Power Sources 2008, 53, 3377.
4. W. Sun, R. Z. Hu, M. Zhu, Electrochim. Acta 2016, 191, 462

Figure 1