The Effects of Native Oxide Surface Layer on the Electrochemical Performance of Si Nanoparticle-Based Electrodes

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Abstract
This study controllably reduces the silicon dioxide (SiO2) layer on Si nanoparticles and evaluates its effect on the performance of Si nanoparticle-based electrodes in Li-ion batteries. Various thicknesses of this native oxide are present on Si nanoparticles generated by chemical vapor deposition (CVD) due to the process conditions and exposure to oxygen during storage. This layer can be effectively reduced by hydrofluoric acid (HF) etching, which results in improved electrochemical performance over as-received samples. As-received Si sample has a higher first-cycle capacity loss than that of the etched Si samples, when the capacity loss is normalized to the surface area of the Si particles. Spectroscopic analysis reveals that when the Si electrode is held at a low potential, the oxide layer can be converted to a more stable silicate form due to the irreversible consumption of lithium species in the cell. The thick SiO2 surface layer also isolates the Si core from lithium-ion alloying; therefore, the as-received Si nanoparticles deliver a lower specific capacity than their etched counterpart. Incomplete lithiation of the as-received Si particles is confirmed by transmission electron microscopy, which shows that nanocrystalline Si domains remain after cycling. The surface insulating effects of SiO2 also cause high impedance in the Si electrode.

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