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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 SiH 4 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 LiFePO 4 -based cathode), ∼97% capacity retention has been obtained after 100 cycles @ 341 μA/cm 2 . 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.

Tin (Sn)-based anodes for sodium (Na)-ion batteries possess higher Na-storage capacity and better safety aspects compared to hard carbon -based anodes but suffer from poor cyclic stability due to volume expansion/contraction and concomitant loss in mechanical integrity during sodiation/ desodiation. To address this, the usage of nanoscaled electrodeactive particles and nanoscaled-carbon-based buffers has been explored, but with compromises with the tap density, accrued irreversible surface reactions, overall capacity (for "inactive" carbon), and adoption of non-scalable/complex preparation routes. Against this backdrop, anode-active "layered" bismuth (Bi) has been incorporated with Sn via a facile-cum-scalable mechanicalmilling approach, leading to individual electrode-active particles being composed of well-dispersed Sn and Bi phases. The optimized carbon-free Sn−Bi compositions, benefiting from the combined effects of "buffering" action and faster Na transport of Bi, to go with the greater Na-storage capacity and lower operating potential of Sn, exhibit excellent cyclic stability (viz., ∼83−92% capacity retention after 200 cycles at 1C) and rate capability (viz., no capacity drop from C/5 to 2C, with only ∼25% drop at 5C), despite having fairly coarse particles (∼5−10 μm). As proven by operando synchrotron X-ray diffraction and stress measurements, the sequential sodiation/desodiation of the components and, concomitantly, stress build-ups at different potentials provide "buffering" action even for such "active−active" Sn−Bi compositions. Furthermore, the overall stress development upon sodiation of Bi has been found to be significantly lower than that of Sn (by a factor of ∼3.8), which renders Bi promising as a "buffer" material, in general. Dissemination of such complex interplay between electrode-active components during electrochemical cycling also paves the way for the development of high-performance, safe, and scalable "alloyingreaction"-based anode materials for Na-ion batteries and beyond, sans the need for ultrafine/nanoscaled electrode particles or "inactive" nanoscaled-carbon-based "buffer" materials.

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