This paper reports on a simple and effective method for improving the electrochemical performance of silicon nanoparticle-core/carbon-shell (Si-core/C-shell) nanofibers. Instead of increasing the encapsulation amount of Si nanoparticles, additional conductive paths between the Si nanoparticles were formed by incorporating a small percentage of multi-walled carbon nanotubes (MWNTs) (e.g., 5 wt% with respect to Si) into the Si nanoparticle core. The electrical conductivity of a single Si-core/C-shell nanofiber was measured by a four-point probe using four nano-manipulators, which showed a more than five times increase according to MWNT addition. A galvanostatic charge-discharge test demonstrated that a small amount of MWNTs greatly improved the electrochemical performance of the Si-core/C-shell nanofibers (e.g., a 25.1% increase in the Li-ion storage capability) due to the enhanced participation of Si through the additional conductive paths formed between the Si nanoparticles.
In the foreseeable future, there will be a sharp increase in the demand for flexible Li-ion batteries. One of the most important components of such batteries will be a freestanding electrode, because the traditional electrodes are easily damaged by repeated deformations. The mechanical sustainability of carbon-based freestanding electrodes subjected to repeated electrochemical reactions with Li ions is investigated via nanotensile tests of individual hollow carbon nanofibers (HCNFs). Surprisingly, the mechanical properties of such electrodes are improved by repeated electrochemical reactions with Li ions, which is contrary to the conventional wisdom that the mechanical sustainability of carbon-based electrodes should be degraded by repeated electrochemical reactions. Microscopic studies reveal a reinforcing mechanism behind this improvement, namely, that inserted Li ions form irreversible face-centered-cubic (FCC) crystals within HCNF cavities, which can reinforce the carbonaceous matrix as strong second-phase particles. These FCC Li crystals formed within the carbon matrix create tremendous potential for HCNFs as freestanding electrodes for flexible batteries, but they also contribute to the irreversible (and thus low) capacity of HCNFs.
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