(412i) Controlling the Spatial Distribution and Orientation of Carbon Nanotubes and Graphene Nanoribbons in Polymer Nanofibers for Future Application in Li-Ion Battery Anode

Spatial distribution and orientation of nanoparticles (NPs) in polymer matrices has a vital effect on intrinsic properties of advanced polymer nanocomposites. Here, we demonstrate that without surface modification, we can control the spatial orientation and dispersion of 1-D and spherical NPs in polymer matrices by applying a circumferentially uniform air flow through the sheath layer of a coaxial nozzle during gas-assisted electrospinning (GAES). The ability to tune the dispersion and orientation of various NPs was demonstrated by varying the air flow rate in GAES of 10 wt.% aqueous solution of Poly(vinyl alcohol). First, carbon nanotubes (CNTs) and their unzipped counterpart graphene nanoribbons (GNRs), and later spherical SiO₂ and Si were investigated in the current study.

To quantify the dispersion, TEM images of microtomed longitudinal sections of NFs were examined using the dispersion area analysis (DAA), calculating the area occupied by NPs relative to the total NF area. Then, the fast Fourier transform (FFT) was performed to obtain the highest probability separation between individual NPs and/or agglomerates. Finally, the alignment analysis (AA) was carried out to study the orientation of 1-D NPs in the flow direction as a response to higher deformation.

Our analysis indicated a 90% and 350% improvement in dispersion area and a 50% and 75% decrease in separation between NPs and/or agglomerates, with the application of additional extensional air flow, for CNTs/GNRs and spherical NPs respectively. This behavior is well explained through a rigorous time scale analysis that related deformation and diffusion rates at the initial stage of fiber formation and revealed the existence of two dispersion regimes. During the first, the diffusion-enhanced dispersion mechanism is leading and competing with the deformation-induced dispersion mechanism (0.2 < Pe < 6.77). However, it was empirically observed that at the intermediate assisting air flow rate (Pe = 6.77), there is an apparent increase in dispersion area and a sharp decrease in the most probable separation, which is the evidence of the second dispersion regime, where deformation-induced dispersion is a dominant mechanism. At the same intermediate air flow rate there was a 60% improvement in the alignment of 1-D NPs with the flow direction. This behavior can be well comprehended by studying the orientational component of NP motion in polymeric fluid, the time scale of which will significantly decrease when the additional deformation is applied. CNTs have higher rigidity and would faster and more uniformly respond to deformation and align with the flow. On the other hand, more flexible GNRs tend to inhomogeneously respond to additional deformation, aligning along the flow direction at lower air flow rates, however forming coils at higher.

Our experimental observations are further supported by the results from the Li-ion battery performance study. We tested the life cycle of half-cells of anodes based on Si NP/CNT/nanofibers at 0.2 A/g for 100 cycles, fabricated via ES and GAES. It is evident that GAES fibers with the same composition have more than 600 mAh/g improvement in capacity over the life of the battery that is a direct result of enhanced NP distribution. Finally, we performed a CGMD simulation for different planar elongational rates and concentrations of 1-D and spherical NPs. Similar to the experimental observations there is a significant improvement in NP dispersion and a good organization towards the extensional direction with the increase of elongation rates.