X-Ray Imaging of Nanostructured Silicon Thin-Film Anodes for Lithium-Ion Batteries

Abstract

Nanostructured silicon is one of the most promising anode materials for high energy density lithium ion batteries. However, the degradation that occurs as a result of a large volume change during the lithiation/delithiation cycle hinders commercial application. The mechanism of degradation, the charge capacity, and the charge/discharge performance of silicon anodes are strong functions of microstructure and nanostructure. Therefore, it is important to develop methods of characterising silicon anodes at these structural length scales. The objective of this study is to explore X-ray based methods that can potentially be applied for in operando studies of model Li-ion batteries during charge and discharge cycles. This is of great importance for the rational design of future high performance silicon anodes. A process for fabricating nonoporous, thin film silicon was developed. The process, based on magnetron sputtering of phosphorous doped silicon and reactive ion etching, was shown to produce nanoporous films with structure that depends on the reactive ion etching parameters. This process was integrated into a scheme for fabricating microscale Li-ion batteries that are compatible with optical and X-ray microscopy. After determining that phase contrast imaging using laboratory sources can not provide the sensitivity required to characterise variations in thin-film anode structures, we evaluated the potential of synchrotron based ptychographic Fresnel coherent diffractive imaging (FCDI) using soft x-rays. In this experiment, we studied the nanostructure of the amorphous silicon film before and after deposition of a LiPF6 electrolyte solution. The coherent diffraction intensity was weak but clearly showed characteristics consistent with the expected nanostructure of the Si film. The characteristic nanostructure of the dry silicon film in coherent diffraction patterns was not observed after electrolyte deposition. While the cause for this difference is not fully understood and remains the subject for future work, the role of the electrolyte in reducing sensitivity to the nanoscale structure of the silicon, and the possibility of it inducing structural changes upon contact are considered. Quantitative phase and magnitude images of representative areas of the dry Si anode were successfully reconstructed from the ptychographic FCDI data. These results indicate that FCDI using synchrotron radiation has the required sensitivity for studying changes in the nanostructure of thin-film silicon anodes and we use the data obtained to evaluate the potential for in operando studies of dynamic processes occurring during electrochemical operation.