Silicon is very promising anode materials not only for boosting the energy density of lithium-ion batteries (LIBs) but for replacing the dendrite forming lithium metal anodes in lithium metal free Li-O₂and Li-S batteries [1-3]. However, it is challenging to design silicon-based anodes exhibiting stable cycling behavior, high volumetric and specific capacity, and low volume expansion.

In this study, we proposed that a sol–gel process can serve as a facile platform for composite materials of high lithium storage capacity. These composite materials consist of silicon nanoparticles (SiNPs) coated with carbon, a small graphene network interconnecting these nanoparticles to form condensed micro-sized particles, and a large fraction of void spaces between nanoparticles and graphene networks in these composites. Since, Si/C composites are internally wired with graphene networks, they are denoted as Si/C-IWGNs hereafter. The Si/C-IWGN samples were simply prepared by carbonizing the composite gels formed in aqueous mixtures consisting of SiNPs and resorcinol (R)–formaldehyde (F) as the carbon precursor and a small amount of graphene oxide (GO) in one-pot reactions. Various Si/C-IWGN samples were prepared with different contents of SiNPs (40 or 50 wt%) and graphene (1, 5 and 10 wt%). Two types of gelation catalysts (C), Na₂CO₃ or NH₄OH, with different concentrations (R/C ratio = 100–500 in molar) were used for forming composite gels. Various electrochemical responses of Si/C-IWGNs, such as cycling stability, volumetric as well as specific capacity, coulombic efficiency, and electrode thickness increase, are thoroughly compared with those of the following reference samples: (1) control composites, Si/C composites, which were prepared without GO addition in the gel formation process and (2) commercial graphite. Finally, we have demonstrated a hybrid strategy to develop a reliable and low cost anode material consisting of mixtures of high capacity Si/CIWGNs and commercial graphite.

The characterization and electrochemical responses of this novel composite suggest that graphene networks in Si/C-IWGNs not only provide electrical networks but also generate void spaces (Fig. 1a~c), which are very effective in absorbing large volume expansions of Si to the level comparable to that of commercial graphite (Fig. 1d, inset). Thereby, graphene networks effectively tackled the issue of common electrode pulverization and ensured the cycling stability of Si-based anodes (Fig. 1d) [4]. Compared to commercial graphite, a properly designed Si/C-IWGN composite material exhibited about 420% increase in specific capacity and about 141% increase in volumetric capacity, which was determined on the basis of electrode weight or volume. We devised a reliable and cost-effective hybrid anode using a mixture of Si/C-IWGN and commercial graphite. Si–Gr is a hybrid that offers the capacity level (800–1000 mAh g^-1) required to improve the energy density of current LIB cells. Moreover, the volumetric capacity of the Si–Gr electrode is 161% higher than that obtained from commercial graphite [4]. The Si–Gr hybrid also has excellent cycling stability. We conclude that the preparation of Si/C-IWGNs is a scalable process. Moreover, owing to the remarkable electrochemical responses of Si/C-IWGNs, it could be used in manufacturing energy storage devices of high energy density and long cycle life.

[1] N.Liu, Z. Lu, J. Zhao, M.T. McDowell, H.-W. Lee, W. Zhao, and Yi Cui, Nat. Nanotechnol., 9, 187-192 (2014).

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

[3] S.-K. Lee, S.-M. Oh, E. Park, B. Scrosati, J. Hassoun, M.-S. Park, Y.- J. Kim, H. Kim, I. Belharouak, and Y.-K. Sun, Nano Lett., 15, 2863-2868 (2015).

[4] 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).

Figure 1. (a) SEM image, (b) TEM image, (c) SEM image of particle cross-section and (d) cycling stability of Si/C and Si/C-IWGNs (Inset shows electrode thickness changes).