In a paper published in the Journal of the American Chemical Society, they report that their approach resulted in a material with excellent cycling performance and rate capability in comparison with Ge@C nanoparticles.

Among the strategies that have been used to try to improve the performance of germanium-based electrodes has been the use of Ge nanoparticles (NPs) coated with carbon shells to improve cycling performance. Decreasing the size of Ge particles to the nanoscale can mitigate the physical strains during the Li uptake/release process, so the volume change causes less cracking and particle pulverization, the authors note. Further, the carbon shell with moderate kinetic properties toward Li ion and electron transport also plays a structural buffering role in minimizing the mechanical stress induced by the volume change of Ge.

This strategy leads to enhanced capacity in initial several cycles but is naturally not meant to achieve long cycle life and high power demands because of the unavoidable agglomeration of small NPs. It is still a big challenge to enhance both the cyclability and rate capability of Ge anode materials.

...One of the most promising strategies for tackling the aggregation problem of electrode materials is to confine them within individual carbon shells and then to distribute the wrapped NPs within carbon shells onto the surface of graphene, thus forming mixed conducting 3D networks, which have been shown to be highly efficient for Li storage. If this double protection strategy of active electrode materials could be realized, high-performance anode materials would be expected.

Herein we propose and realize a double protection strategy for improving both the cyclability and rate capability of Ge anodes. Ge NPs are first covered by a carbon nanocoating layer, avoiding direct exposure of the NPs to the electrolyte. Elastic reduced graphene oxide (RGO) networks are then introduced for dispersing, embedding, and electrically wiring the Ge@C core−shell NPs. The power of this concept is demonstrated by the facile synthesis of the Ge@C/RGO nanocomposite, which shows much-improved specific capacity, cycling performance, and rate capability in comparison with pristine Ge@C NPs when used as an anode material for Li ion batteries.

—Xue et al.

Galvanostatic discharge−charge testing of the nanocomposite found initial discharge and charge specific capacities of 1,803 and 938 mA h g⁻¹. Although a large irreversible capacity loss was observed in the first cycle, the reversibility of the capacity was significantly improved, with an average columbic efficiency of >99% for up to 50 cycles after the second cycle.

After 50 cycles under a current density of 50 mA g⁻¹, the Ge@C/RGO nanocomposites retained a reversible capacity of ∼940 mAh g⁻¹, which is ∼3 times higher than the theoretical capacity of graphite, whereas Ge@C NPs tested for comparison showed a specific capacity lower than 490 mAh g⁻¹.

The authors attributed the cycling performance and rate capability of their nanocomposite to the electronically conductive and elastic RGO networks as well as the carbon shells and the small size of the Ge NPs.

This double protection strategy could offer an effective and general approach to improving the cyclability and rate capability of high-capacity electrode materials with large volume variations and low electrical conductivities in the battery area.

—Xue et al.

Resources

• Ding-Jiang Xue, Sen Xin, Yang Yan, Ke-Cheng Jiang, Ya-Xia Yin, Yu-Guo Guo, and Li-Jun Wan (2012) Improving the Electrode Performance of Ge through Ge@C Core–Shell Nanoparticles and Graphene Networks. Journal of the American Chemical Society doi: 10.1021/ja211266m