Surface Coating Mediated Swelling and Fracture in Lithiated Core-Shell Silicon Nanostructures

Despite tremendous effort in designing better Si electrodes based on nanostructured materials, composite and architectures as well as on novel binders and electrolyte additives, sustaining long cycle life at deep lithiation levels is still troublesome[1]. Recent studies have shown that surface coatings can positively impact the cycling stability of the silicon anodes by improving the current collection efficiency, by enabling the formation of a stable solid electrolyte interphase (SEI - and related to that, faster surface kinetics) and by acting as a potential mechanical clamping layer[2]. Despite intensive research, little knowledge is available about the electrochemical and especially mechanical stability of such layers provided that they have to sustain the 300% volume expansion of Si upon full lithiation. A key comprehension feature for the surface coated Si structures is the mechanical stability of such layers: will they fracture or not upon full lithiation? If cracks in the coating layer appear and propagate, much of the designed advantages are simply lost.

Here, we will detail on the effect of metallic Ni coating on the lithiation behavior of crystalline Si nanostructures. To have a better understanding of the Ni shell influence upon Si lithiation we have fabricated composite structures with different morphological parameters: Ni shell thickness of 40nm, 80nm and 120nm; Si core diameter of 170nm, 330nm and 480nm. The used Si nanopillars have a perfect circular symmetry and the Ni layer forms a conformal coating of uniform thickness along radial and axial directions (Fig. 1). The composite structure is considered isotropic at equilibrium.

The lithiation behavior is shown in Figure 1. Consistent with previous reports, pristine pillars display anisotropic swelling with preferential elongation along <110> planes[3]. For coated pillars, different swelling behavior and related to that, fracture regimes have been identified (Fig. 1). At small coating thickness, preferential fracture sites are attributed to high stress localization and plastic strain accumulation in the Ni coating resulting from anisotropic swelling of Si.  For thick coatings the mechanical confinement induced by the Ni layer levels off the swelling anisotropy and, correspondingly, the plastic strain and stress concentrations, resulting in randomly oriented cracks that originate at the shell surface and propagate radially along the nanowire axis.

The observed anomalous lithiation and fracture of the Si@Ni structures are explained within a combined thermodynamical - mechanical framework. The models we develop are extended to other type of coatings or nanostructures leading to correct understand of the capacity fading and ultimately, to a better design of high capacity anodes[4].

[1] McDowell M. T. et al. Adv Mater 2013, 25, 4966.

[2] Choi N.-S. et al. J Mater Chem 2011, 21, 9825.

[3] Lee S. W. et al. Proc Natl Acad Sci 2012, 109, 4080; Liu X. H. et al. Nano Lett 2011, 11, 3312.

[4] Vlad A. et al. Proc Natl Acad Sci 2012, 109, 15168; Sandu G et al. submitted.