Nanocrystalline Germanium

The large irreversible capacity observed on the initial cycle may be attributed to a partial reduction of the native surface oxide and the formation of an SEI layer, as previously discussed for the silicon system. The peaks in the differential capacity [350 mV (insertion) and 500 mV (extraction)] in the first cycle disappear after the first few cycles and are indicative of the solid-electrol5de reaction to form the SEI layer. The first cycle capacity loss is approximately 70% of what was observed in the Li-Si system. The lower irreversible capacity on the first cycle is attributed to a reduced native oxide layer, which is only a few monolayers thick. 

The reversible cycling of up to 3.8 lithium atoms per germanium atom in the nanocrystalline electrode is considerably larger than the capacities measured in the analogous silicon system, which exhibited a reversible capacity of 1.1 lithium atoms per silicon atom. The enhanced lithium uptake is attributed to the higher diffiisivity of lithium in germanium at room temperature (Dce = 400 ZJsi). 

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Germanium is an important elemental semiconductor which exhibits an indirect band gap of 0.67 eV at room temperature in the microcrystalline phase. In contrast to the microcrystalline element, nanocrystalline germanium is a direct semiconductor and a promising material in the optoelectronic industry. The... 

Transition-metal nanoparticles in imidazolium ionic liquids recycable catalysts for biphasic hydrogenation reactions. Journal of the American Chemical Society, Vol.124, No.l6, (April 2002), pp. 4228-4229, ISSN 0002-7863 Endres, F. Bukowski, M. Hempelmann, R. Natter, H. (2003). Electrodeposition of nanocrystalline metals and alloys from ionic liquids. Angewandte Chemie, International Edition, Vol.42, No.29, 0uly 2003), pp., 3428-3430, ISSN 1521-37732003 Endres, F. Abedin, S. Z. (2002). Electrodeposition of stable and narrowly dispersed germanium nanoclusters from an ionic liquid. Chemical Commununication, No. 8, pp. 892 - 893, pp. 1359-7345, ISSN 1359-7345... 

Electrodes of silicon and germanium amorphous films were prepared by depositing the material directly onto a copper substrate. The electrochemical cells were prepared and cycled under conditions previously described for the nanocrystalline materials. The thin films exhibited excellent electrochemical performance as demonstrated in Fig. 2.10a and c, with voltage profiles obtained from q cles 1, 25, and 50 for silicon and germanium amorphous films, respectively. Plots of the differential capacity are shown in Fig. 2.10b and d, respectively. 

These results broadly demonstrate the utility of nanoscale electrodes, which yield greater capacities and cycle life than their bulk counterparts. We showed that improved electrochemical performance in the alloy electrodes could be achieved by preparing the electrodes in an amorphous rather than nanocrystalline state. Finally, we demonstrated that silicon and germanium are viable lithium electrodes and, when prepared with the proper nanostructure and morphology, can be cycled 50 times with little capacity loss. 

Graetz J, Ahn CC, Yazami R, Fultz B (2004) Nanocrystalline and thin film Germanium electrodes with high lithium capacity and high rate capabilities. J Electrochem Soc 151 ... 

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