We investigated the effect of aluminum coating layers and of the support growth substrates on the electrochemical performance of silicon nanowires (SiNWs) used as negative electrodes in lithium ion battery half-cells. Extensive TEM and SEM analysis was utilized to detail the cycling induced morphology changes in both the Al-SiNW nanocomposites and in the baseline SiNWs. We observed an improved cycling performance in the Si nanowires that were coated with 3 and 8 wt.% aluminum. After 50 cycles, both the bare and the 3 wt.% Al coated nanowires retained 2600 mAh/g capacity. However beyond 50 cycles, the coated nanowires showed higher capacity as well as better capacity retention with respect to the first cycle. Our hypothesis is that the nanoscale yet continuous electrochemically active aluminum shell places the Si nanowires in compression, reducing the magnitude of their cracking/disintegration and the subsequent loss of electrical contact with the electrode. We combined impedance spectroscopy with microscopy analysis to demonstrate how the Al coating affects the solid electrolyte interface (SEI). A similar thickness alumina (Al2O3) coating, grown via atomic layer deposition (ALD), was shown not to be as effective in reducing the long-term capacity loss. We demonstrate that an electrically conducting TiN barrier layer present between the nanowires and the underlying stainless steel current collector leads to a higher specific capacity during cycling and a significantly improved coulombic efficiency. Using TiN the irreversible capacity loss was only 6.9% from the initial 3581 mAh/g, while the first discharge (lithiation) capacity loss was only 4%. This is one of the best combinations reported in literature.
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