With an ever-expanding application of energy storage systems, lithium-ion batteries (LIBs) have attracted a significant scientific and industrial attention during last decades. However, the current LIBs based on lithium metal oxides or phosphate cathodes and graphitic carbon anodes have intrinsic limitation in energy density. To further improve the specific and volumetric energy densities of current LIBs, graphite anode has to be replaced with a high capacity alternative. Thus, silicon has received a great attention due to its high theoretical capacity (3572 mAh/g at RT as a form of Li₁₅Si₄) [1] and relatively low working potential. However, the huge volume expansion (~300%) of silicon associated with lithiation and delithiation during cycling leads to electrode pulverization causing the common rapid capacity fading. Another issue associated with silicon is its low electrical conductivity. Therefore, composites of silicon with conducting carbons could be a practical solution. Among many classes of carbon, graphene has been identified as an attractive supporting material that may address the critical issues associated with Si anodes [2, 3]. For practical application of silicon-graphene assemblies in LIB anodes, it is desirable to form a dense particulate assembly which can give high volumetric capacity and be readily adaptable in industrial coating process [4, 5].

In this study, we employed an electrostatic self-assembly process between amine-functionalized SiNPs (Si-APTMS) and graphene oxide (GO) in an acidic aqueous solution (pH=2) followed by carbon coating process to form dense particulate Si/GS/C assemblies. For carbon coating, two different carbon sources such as poly-vinylidenefluoride (PVdF) or sucrose were employed to prepare the Si/GS/C-P and Si/GS/C-S samples, respectively. For comparison, a Si/GS was prepared by a direct carbonization of Si/GO assembly. The characteristic features of the Si/GS and Si/GS/C samples were assessed with elemental analysis (EA), FT-IR, TGA, SEM, TEM, XRD and XPS etc. The electrochemical responses of samples were obtained with galvanostatic cycling test, and cyclic voltammetry and impedance tests using CR-2032 coin-type cells with a Li-foil as the reference electrode.

The TEM image in Fig. 1a shows that SiNPs are all attached on GO surface in the Si/GO assembly. Different from the Si/GS, the Si/GS/C-S formed dense particulate composites (Fig. 1b and c), in which SiNPs are well distributed between graphene sheets and the whole structure become dense with additional carbon coating. As compared in Fig. 1d, the Si/GS (80% Si) showed very poor cycling stability at 500 mA g^-1. On the other hand, the Si/GS/C-P (60% Si) and Si/GS/C-S (50% Si) showed much enhanced cycling stability at 500 mA g^-1. In particular, the Si/GS/C-S prepared by an in-situ carbon-gel coating in solution of Si/GO assembly delivered reversible capacities over 1300 mAh g^-1 at 100 mA g^-1 and over 900 mAh g^-1 at 500 mA g^-1 without capacity fading. The excellent cycling performance of the Si/GS/C-S can be attributed to its structural features. First, the volume variations of SiNPs are effectively damped by flexible and highly conducting graphene sheets encapsulating them. Second, additional carbon coating surrounding the Si/GS assemblies enhances the coherence between SiNPs and graphene in comparison to the Si/GS obtained by direct carbonization of Si/GO assembly without carbon coating.

[1] M. N. Obrovac, L. Christensen, Electrochem Solid State Lett. 7, A93 (2004).

[2] J. K. Lee, K. B. Smith, C. M. Hayner, and H. H. Kung, Chem. Commu., 46 ,2025-2027 (2010).

[3] R. Raccichini, A. Varzi, S. Passerini, and B. Scrosati, Nat. Mater., 14, 271-279 (2015).

[4] C. Chae, H.-J. Noh, J. K. Lee, B. Scrosati, and Y.-K. Sun, Adv. Funct. Mater,, 24, 3036-3042 (2014).

[5] J. Kim, C. Oh, C. Chae, D.-H. Yeom, J. Choi, N. Kim, E.-S. Oh, and J. K. Lee, J. Mater. Chem. A., 3,18684-18695 (2015).

Fig.1. TEM image of (a) Si/GO assembly and (b) Si/GS/C-S, (c) SEM image of dense particulate form of Si/GS/C-S, and d) cycling performances of Si/GS, Si/GS/C-P and Si/GS/C-S (Inset shows TGA profiles of samples).