Face-Centered-Cubic Lithium Crystals Formed in Mesopores of Carbon Nanofiber Electrodes

Abstract

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.

Hierarchical porous carbon nanofibers via electrospinning

“…3c-e. Surprisingly, the mechanical properties of such electrodes were 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 [70].…”

“…The embedded and de-aggregated SnO 2 nanoparticles in the carbon phase, which were less than 10 nm in size, provided a huge number of reaction sites for lithium ions and ensured complete alloying with them [69]. Recently, Lee et al [70] investigated the mechanical sustainability of a carbon based anode subjected to repeated electrochemical reactions with Li ions via nanotensile tests of individual hollow CNFs, as shown in Figs. 3c-e.…”

Facile method to improve initial reversible capacity of hollow carbon nanofiber anodes

“…It should be noted that the thermal treatment temperature is the most effective parameter to control the carbon quality [18]. On the other hand, we found that the lithium face-centered-cubic crystals were irreversibly nucleated and grew into the mesopores present in the HCNFs as the electrochemical reactions progressed [19]. Thus, it can be hypothesized that reduction of the mesopores can reduce the initial irreversible capacity.…”

“…In this regard, Si nanoparticles (<100 nm) were purchased from Sigma Aldrich and incorporated into the SAN core solution before electrospinning. The detailed coaxial electrospinning process can be found in literature [19].…”

“…The microstructural changes during the CVD process were evaluated using XRD and Raman spectroscopy [19]. The CVD-HCNFs and HCNFs showed the typical XRD peaks of a carbonaceous microstructure: (0 0 2), (1 0), and (1 1 0) ( Fig.…”