Si Nanowires Grown on Cu Substrates via the Hot-Wire-Assisted Vapor–Liquid–Solid Method for Use as Anodes for Li-Ion Batteries

Silicon is a high-capacity and safer next-generation anode material for Li-ion batteries, with challenges from rapid capacity fade due to colossal volume changes during Li alloying/de-alloying. Nanostructured Si is deemed to address the above issue, with the possible usage of Si nanowires (SiNWs) on copper substrates (sans any binder or conducting additive) offering the highest performance in terms of anode capacity. However, the direct growth of SiNW on copper current collector foils is challenging and not reported earlier. Against this backdrop, we demonstrate here, for the first time, the successful growth of SiNW, with controllable features, on battery-grade copper substrates via a hot-wire-assisted vapor–liquid–solid (VLS) route. The usage of Sn as a nanotemplate has allowed bringing down the growth temperature to 400 °C, with the SiH4 pressure and growth duration being other crucial parameters controlling various features of SiNWs, such as length, diameter, aspect ratio, effective crystalline core-to-amorphous shell ratio, morphology of the shell, and orientation with respect to the substrate. The emphasis here is on the variations of different morphological features of these nanowires with changes in process conditions as these are bound to have important implications for various electronic applications. One such application that we explore is their usage as an anode in Li-ion batteries. In the Li "half" cell, the free-standing SiNWs on copper foil exhibit reversible Li-storage capacities of ∼3556 mAh/g @ C/5 and ∼2462 mAh/g @ 1C while retaining ∼89% of the capacity after 100 cycles @ 1C. In the Li-ion "full" cell (with a home-made LiFePO4-based cathode), ∼97% capacity retention has been obtained after 100 cycles @ 341 μA/cm2. The superior electrochemical performance as an anode, the scalability of the growth technique, and the ability to tune the SiNW characteristics open up the possibility of industrial-scale application of the as-grown SiNWs on copper foil.