Surface Coating Mediated Swelling and Fracture in Lithiated Core-Shell Nanowires

Addressing the stability challenges in silicon-­‐based anode materials is necessary for achieving high capacity lithium-­ion batteries. Recent studies have shown that surface coatings favorably influence the cycling performance of such materials. These surface coatings enhance the current collection efficiency and promote the formation of a stable solid electrolyte interface, and act as a mechanical constraining layer. Despite the potential benefits, little knowledge is available on the impact of such coatings on the lithiation behavior and the reverse. This work details on the effect of conformal metallic Ni coatings on the lithiation behavior of crystalline Si nanopillars. The fabricated composite structures have different morphological parameters: Ni shell thicknesses of 40 nm, 80 nm and 120 nm with a Si core diameter of 170 nm, 330 nm and 480 nm. Pristine nanopillars display anisotropic swelling along directions and fracture sites along directions consistent with previous literature reports. The Ni coated structures exhibit a different swelling behavior with two distinct fracture regimes. For Ni thickness lower than 80 nm, the anisotropic behavior is preserved but fracture sites are present along directions. With thicker coatings, an anisotropy reduction is observed together with a single randomly oriented fracture. A combined thermodynamic-­mechanical model corroborated these observations. The proposed theoretical framework can be extended to other types of coatings or nanostructures and provide guidelines for tailoring electrode materials to overcome the drawbacks around Si-based anodes