In situ Studies on Structural Evolution during Cycling

2 In situ Studies on Structural Evolution during Cycling 

In addition to in situ XRD, powerful in situ TEM techniques for monitoring electrochemical reactions in real time have recently been developed. These experiments involve the use of a specialized dual-probe electrical biasing TEM holder on which a nanoscale electrochemical cell is fabricated, as shown in the schematic in 

This series of images shows that volume expansion of this 112 -oriented NW primarily occurs in a radial fashion. This NW/ Cu structure does not bend after lithiation bending would be expected if the SiNW changed in length while the Cu layer did not 

In previous sections, it was demonstrated that the lithiation of crystalline Si proceeds via a two-phase reaction in which lithiated Si (Li (Si) is interfaced with un-lithiated crystalline Si. In SiNWs, this is manifested as a crystalline Si core that shrinks as the lithiated shell grows. Interestingly, a few different experimental studies have shown that the direction of lithiation and volume expansion of crystalline Si nanostructures is strongly influenced by the crystallography of the shrinking crystalline Si core. In one set of experiments. Si nanopillars with three different axial orientations were tested. In order to test the effect of lithiation on crystallographic orientation. 

Finally, we note that the preceding discussion on expansion anisotropy has focused on the lithiation of crystalline Si nanostructures. The lithiation of a-Si is a different process altogether it is a single-phase reaction in which the concentration of Li is governed by diffusion. The volume expansion during lithiation of amorphous structures has been shown to be more isotropic. This has important consequences for the stress state and fracture properties of amorphous vs. crystalline Si nanostructures, as will be examined later. 

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