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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.

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.

Abstract: This paper reports on Si core/C shell nanofibers as an anode material for Li-ion batteries. The Si core/C shell nanofibers are synthesized using co-axial electrospinning of Si nanoparticles in styrene-co-acrylonitrile core solution and poly(acrylonitrile) shell solution combined with subsequent thermal treatments. The microstructure and elemental composition of the nanofibers are systematically examined using FE-SEM, TEM and WAXD for confirmation of the crystalline Si core/turbostratic C shell structure. In situ observation of contact-lithiation of the Si core/C shell nanofibers is carried out to investigate their mechanical durability during the electrochemical reactions. The electrochemical performances of the nanofibers as an anode are characterized by galvanostatic charge–discharge test. Due to simple processing but a well-developed core/shell structure and good electrochemical performance, the Si core/C shell nanofibers fabricated in this study offer the potential to develop enhanced Si/C nanostructured composite anodes. [Copyright &y& Elsevier]

Abstract: This work reports on hollow carbon nanofibers (HCNFs) as anode materials for Li-ion batteries. Various HCNFs are synthesized using co-axial electrospinning of styrene-co-acrylonitrile (core) and poly(acrylonitrile) (shell) solutions and subsequent thermal treatments. The microstructures of HCNFs are examined using SEM, Raman spectroscopy, WAXD, and HR-TEM. The effect of the carbonization temperature on their turbostratic carbon structures and electrochemical properties is systematically investigated. As the carbonization temperature increases, both crystallite thickness and length significantly increases while the initial irreversible capacity decreases. These predictable microstructure and electrochemical performance of HCNFs provide important insight for the design of novel nanostructured anode materials such as Si or Sn encapsulated HCNFs. [Copyright &y& Elsevier]

This report describes a new twist-spinning process for the manufacture of piezoelectric yarn without the need for additional poling processes. Poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) nanofibers were first electrospun and organized into a web structure. Using rotational and translational motors, the nanofibers of the web were pulled and stretched and, finally, twisted into piezoelectric yarns. The crystallinity, β phase ratio, and mechanical and piezoelectric properties of these twist-spun piezoelectric yarns were characterized to assess the effects of the twist-spinning operation on their microstructure and performance. Twist-spun piezoelectric yarns that had undergone high degrees of stretching exhibited enhancements in both crystallinity and β phase ratio by 83% and 12%, respectively. In contrast, high stretching reduced the tensile strength and modulus of the yarns due to small surface angles. The twist-spun piezoelectric yarn with the highest β phase ratio and lowest modulus attained in this study yielded a piezoelectric potential and piezoelectric voltage constant of 500 mV and 0.412 mVm/N, respectively. Unlabelled Image • Poling-free spinning process was designed for piezoelectric nanofiber yarns. • PVDF-TrFE nanofibers were enhanced in both crystallinity (by 83%) and β phase ratio (by 12%) after yarn spinning process. • Piezoelectric yarn showed high mechanical (tensile strength of ~25 MPa) and piezoelectric (potential of 500 mV and g 33 of 0.40) properties. [ABSTRACT FROM AUTHOR]