Interface Engineering via Metal-coating of Silicon Nanostructured Thin Films for Reducing Anode Pulverization

Si is a promising anode material for Li-ion batteries due to its high specific capacity compared to conventional graphite and metal oxides; however, the pulverization during the lithiation/de-lithiation reduces the Coulombic efficiency and cycle stability. Si nanostructures with an increased surface-to-volume ratio, such as porous Si (PS) and Si nanowires (SiNWs), are reported to improve the electrochemical performance of the cell. The work explains the surface engineering of the ultrathin (≤ 5 nm) Ag and Cu coating on the PS and SiNW array thin films fabricated by electrochemical anodization and metal-assisted chemical etching for the anode. The surface and cross-sectional morphology of the ultra-thin metal coatings were investigated by an electron microscope equipped with a focused ion beam to prepare lamellae. The interstitial gaps on the SiNW array nucleate the metal atoms and promote growth at the interstitial gaps rather than uniform distribution on the surface, like PS film. Fourier transform infrared spectroscopy shows the hydride passivation of the Si nanostructures, which prevents surface oxidation. X-ray diffraction spectra detect CuO and Ag₂O formation after metal coating, which is validated by X-ray photoelectron spectroscopy. X-ray reflectivity with critical angle measurement through reflectivity fringes determines the thickness of the Ag and Cu as (5.8 ± 1.6) nm and (4.4 ± 0.4) nm, respectively. The deposited Ag and Cu densities are 5.25 g cm⁻³ and 6.28 cm⁻³, respectively.