Fundamental experimental studies of amorphous silicon thin film anodes for lithium -ion applications

In an effort to understand more about the electrochemical properties of silicon with respect to lithium, as well as the role of the interface in anchoring the active material, thin films of silicon deposited on copper foil have been synthesized and characterized. RF sputtering has been chosen as the silicon deposition method due to the ease of deposition, availability of deposition equipment, good reproducibility, and ability to deposit amorphous Si (a-Si). Initial research indicated that near-theoretical reversible capacities can be achieved for at least 30 cycles in the case of 250 nm thick a-Si films. However, upon subsequent cycling, the films failed via a delamination and pile-up mechanism. Although similar delamination type failures in Sn thin films have been observed by other researchers, no detailed study of the interfacial properties has been reported. In this research, emphasis has been placed on the electrochemical and interfacial characterization of the a-Si:Cu thin film anode system.;Information about the electrochemical related properties of a-Si thin films has been gained by studying the crystal structure, microstructure, and electrochemical performance. X-ray diffraction (XRD) of the Si thin films sputter deposited on Cu indicates that the as-prepared electrodes are composed of a-Si and polycrystalline Cu. XRD did not reveal the presence of any crystalline interfacial phase. Transmission electron microscopy (TEM) results also confirm the amorphous nature of the as-deposited silicon. Cross-sectional microscopy (X-TEM) was used to confirm the thickness of the as-deposited film in the case of the 250 nm a-Si sample. High-resolution cross-sectional microscopy (X-HRTEM) was also conducted to obtain a better insight into the Cu/a-Si interface. The X-HRTEM results show a clean interface between the Cu and Si, with no evidence of the presence of any interfacial phase. Electrochemical cycling results of a-Si films ranging in thickness from 50 nm to 1 mum show very promising results, with the best sample (250 nm) exhibiting a high, reversible capacity of ∼3500 mAh/g for 30 cycles (theoretical capacity ∼4000 mAh/g). SEM and EDAX results on the Si films before and after cycling have shown that large-scale morphological changes occur in the active material as a result of lithium alloying and de-alloying. (Abstract shortened by UMI.).