… Si anodes suffer from rapid decay of capacity upon cycling; this can be attributed to the structural degradation, loss of electrical contact, and the unstable solid electrolyte interphase (SEI) on the silicon surface caused by the dramatic volume change of Si (∼300%) during lithiation-delithiation cycling. To address these issues, one popular and effective tactic is to couple nanostructured silicon with a second phase (e.g., carbon). The core of this strategy is fundamentally to create effective and robust three-dimensional transport networks for both electrons and lithium ions. As the exciting materials formulations and electrode prototypes developed based upon elaborately designed carbonaceous matrices have contributed to significant improvements in some properties of silicon anodes at low mass loadings, it remains an unmet goal to harness the potential of silicon in LIB anodes, specifically in terms of gravimetric capacity, volumetric capacity, areal capacity, rate capability, and cyclic stability at the same time.

Graphene, a single-layer carbon sheet with a hexagonal packed lattice structure, has been touted to be a remarkable carbonaceous structural platform to afford the exertion of various functional nanostructured materials, on account of its unique two-dimensional structure, excellent electronic conductivity, superior mechanical flexibility, good chemical stability, and high theoretical surface area. To date, numerous novel approaches have been explored to utilize graphene to address the challenges of silicon anodes and, thus, to improve their lithium storage capabilities. Most of them are based on the hybridization of graphene sheets and nano-structured silicon of different dimensionalities at the materials unit scale. However, the thus-resulted hybrids always require additional components (e.g., binders and conductive additives) that are required in constructing a conventional electrode. The substantial need of such extra substances not only adversely affects the designated electrochemical properties of the hybrids but also unfavorably dilutes the electrode performance including both volumetric capacity and gravimetric capacity, as the total weight and volume of the electrode must be counted from a practical viewpoint.

… it is highly desirable to exploit new material/electrode design principles to combine graphene and silicon, so as to overcome the above-mentioned challenges of silicon anodes. We believe that the similar circumstances hold as well for building the systems of graphene and other electrode materials encountering large volume changes. In this report, we propose a novel material/electrode design formula, and develop an engineered self-supporting electrode configuration where, graphene sheets (G) are oriented and bridged by silicon nanoparticle-templated carbon (TC) hinges, thus forming silicon-nanoparticle-impregnated assemblies of templated carbon-bridged oriented graphene (denoted as TCG-Si).

—Zhou et al.

The templated carbon-bridged oriented graphene assemblies (TCG) form a robust bicontinuous network to facilitate the electron and lithium ion transport throughout the electrode even at high areal mass loadings; the TCG assemblies also enable a substantially high tap density of impregnated silicon (1.3 g cm^–3).

Electrochemical characteristics. (a) Gravimetric capacity and Coulombic efficiency of the TCG-Si and control electrodes (G-Si, SiNP). The current rate is 0.2 A g−1 for the first cycle and 2 A g−1 for later cycles; all the gravimetric capacities reported are on the basis of the total electrode weight. (b) Volumetric capacity achievable at the annotated current rates and cycles for the TCG-Si and G-Si electrodes comparing with some representative Si anodes reported in the literatures as noted in the paper. Their respective gravimetric capacity based on the total electrode weight is superimposed with gray columns as well. (c) Rate capabilities for the TCG-Si electrodes of different thicknesses (6−28 μm), as well as a 5 μm thick G-Si electrode. (d) Areal capacities of the TCG-Si electrodes with different areal mass loadings (0.8−3.7 mg cm−2), where the annotated current rates are used for testing. Credit: ACS, Zhou et al. Click to enlarge.

We note that a Si anode with this combined level of performance has rarely been described. This study expands the potential of graphene in improving silicon anode performances and will propagate new and viable battery material/electrode design formulas and opportunities for energy storage systems with high-energy and high-power characteristics.

—Zhou et al.

Resources

• Min Zhou, Xianglong Li, Bin Wang, Yunbo Zhang, Jing Ning, Zhichang Xiao, Xinghao Zhang, Yanhong Chang, and Linjie Zhi (2015) “High-Performance Silicon Battery Anodes Enabled by Engineering Graphene Assemblies” Nano Letters doi: 10.1021/acs.nanolett.5b02697