In Situ Observation of Epitaxial Li-Si-Nanostructure Formation on Si(111)

Silicon is considered as anode material for Li-ion batteries due to its high theoretical capacity with a value of 4200 mAh/g for Li22Si5. The associated volume change exceeding 300 % during de-/lithiation of silicon is considered as the major problem for application with a possible solution in using nanostructured material. Various details of the formation of Li-Si phases are presently under discussion like the formation of amorphous vs. crystalline structures or the Li-Si phase order during lithiation.

To shed light on the relevant processes during the Li-Si phase formation a clean Si substrate was exposed under ultra-high condition in a molecular beam epitaxy (MBE) environment to an atomic lithium flux. During the experiment the Si substrate temperature was gradually reduced from initial 700°C down to room temperature. Simultaneously, the onset of the nucleation and the formation of Li-Si phases were observed by in situ synchrotron x-ray diffraction (XRD), azimuthal reflection high-energy electron diffraction (ARHEED), and line-of-sight quadrupole mass spectrometry (QMS) of the desorbing Li. Initial only Si surface reconstructions are observed involving only the top most atomic layer. At lower temperatures, Li-Si nanostructures are formed. First, crystalline Li12Si7 appears, followed by the formation of Li21Si5 phases. Both phases grow epitaxially. The epitaxial relationship is Si[111]||Li12Si7[010] and Si[11-2]||Li12Si7[100] as well as  Si[111]||Li21Si5[111] and Si[01-1] ||Li21Si5[01-1]. Further confirmation besides the XRD data is gained by investigating the Li-Si surfaces. The Li21Si5(111) surface exhibits an approximately nine times larger (1x1) surface unit cell than the unreconstructed Si(111) surface. Ex situ investigations of various intermediate stages of the Li-Si phase formation by atomic force microscopy show nanostructures which formed on the substrate surface with a height of about 10 nm and 100 nm diameter much smaller than usually reported in Si anode material. It demonstrates that the formation of the Li-Si phases depends on the Li kinetics as well as the Si release mechanism from the crystalline substrate and its diffusion.

Acknowledgements: This work was supported by the Leibniz-Gemeinschaft under the project No. SAW-2011-PDI-230. The experiments are carried out at the  synchrotron radiation facility Bessy II (Helmholtz-Zentrum Berlin, Germany).