Amorphous Silicon and Germanium

Structural phase transitions that occur during lithiation are typically undesirable since they often lead to slow kinetics and poor cycle life. In that respect, the electrochemically driven amorphization of silicon is advantageous since it allows lithiation to occur while b3q)assing multiple crystallographic transitions. However, the electrochemical properties of silicon and germanium could likety 

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. 

For the silicon film, an initial discharge capacity of 3500 mAh/g, and charge capacity of 2500 mAh/g were measured, which yields a columbic efficiency of 71% for the first cycle. Upon subsequent cycling, the electrode exhibited a rather stable specific capacity 2000 mAh/g with a columbic efficiency of 98% on cycle 9. After 20 cycles, the amorphous thin film exhibited a mean capacity loss of only 8 mAh/g per cycle. The differential capacity plot (Fig. 2.10b) 

The stability of the amorphous nanofilms during cycling is rather surprising. 

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Ikeda T, Kobayashi T, Takata M, Takayama T, Sakata M (1998) Charge density distributions of strontium titanate obtained by the maximum entropy method. Solid State Ion 108 151-157 Imai M, Mitamura T, Yaoita K, Tsuji K (1996) Pressure-induced phase transition of crystalline and amorphous silicon and germanium at low temperatures. High Pressure Res 15 167-189... 

D.E. Polk. Stmctural model for amorphous silicon and germanium. J. Non-Cryst. Solids 5, 365-376 (1971). 

Van Wieringen, A., Warmoltz, N. On the permeation of hydrogen and helium in single crystal silicon and germanium at elevated temperatures. Physica22, 849 (1956) Volkert, C.A. Stress and plastic flow in silicon during amorphization by ion bombardment. J. Appl. Phys. 70, 3521 (1991)... 

Fig. 6.4 Low temperature kj for various type I clathrates [35, 40-44], as well as data for amorphous Si02 dashed curve) and room temperature Kmin for elemental silicon and germanium (stars). The solid black line indicates a 1/...

Fig. 5.23. Resistance of amorphous carbon, silicon, and germanium film plotted logarithmically against (after Morgan and Walley (1971)).

There is some evidence, albeit preliminary, that this explanation may not be the entire story and that hydrog may perhaps play a more direct role in the recombination processes. In amorphous alloys containing both silicon and germanium (a-Si peaks of the PL... 

DR Clarke, MC Kroll, PD Kirchner, RF Cook, BJ Hockey. Amorphization and conductivity of silicon and germanium induced by indentation. Phys Rev Lett 60 2156-2159, 1988. 

Other commercial glass systems include fluoride-based glasses chalcogenide and chal-cohalide glasses the amorphous semiconductors, silicon and germanium and glassy metals. 

Amorphous thin films of silicon and germanium were prepared by physical vapor deposition using a bell jar thermal evaporator (Fig. 2.4b]. A charge of elemental material was evaporated under a... 

Figure 2.11 Gravimetric capacity of bulk, nanociystalline, and amorphous films of silicon and germanium over 50 cycles. The dotted line represents the theoretical capacity for the commercially available lithium-intercalated graphite.

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. 

Miller, E., Rocheleau, R., Khan, S. 2004. A hybrid multijunction photoelectrode for hydrogen production fabricated with amorphous silicon/germanium and iron oxide thin films. Int J Hydrogen Energy 29 907-914. 

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