Highly Enhanced Cycleability from High Crystalline Biaxially Aligned CNT Web for Li-Air Cathode Applications

Abstract

In recent years, Li-O2 batteries have attracted a great deal of attention due to the high theoretical capacity for applications as electronic vehicles. Carbon nanotubes are widely used as cathode materials for Li-O2 batteries because of their high conductivity, high surface area and low density. However, due to their hydrophobic surface properties and the high van der Waals interaction between their walls, they tend to aggregate on their own, failing full use of the CNT surface. Therefore, it is necessary to engineer the nanoscale space between the CNTs such that each CNT aligns with each other and the nanoscale spacing lies between them. In this way, it is possible for the ionic liquid electrolyte and oxygen to access individual CNTs and enable uniform Li2O2 formation. In this research, we developed a one-step process for biaxially oriented, highly crystalline CNT fabrics using a combination of floating catalyst chemical deposition (FCCVD) and one-step direct spinning technology. A highly crystalline DWCNT with a diameter of 6 nm was synthesized by FCCVD using ferrocene, thiophene, and methane, as precursors, and as-spun CNTs were woven into fabric by 2-directional spinning process. The intensity ratio between D and G bands (ID/IG) of 0.06 showed high crystallinity of CNT surface. The microscopic architectures of randomly oriented CNT fabric and biaxially oriented CNT fabric were investigated using a high-resolution scanning electron microscope (See Figure 1.(a) and (b)). The biaxial orientation was reconfirmed using a polarized Raman spectroscopy. In order to investigate the effect of the microscopic structure on the performance of the Li-O2 battery, we have assembled Li-O2 cells using a random CNT fabric and a biaxially aligned CNT fabric as a cathode. In the random CNT cathode, the charging overpotential gradually increases, leading to rapid decay of capacity upon cycling (Fig. 1(c)). The increase in overpotential is typically associated with the accumulation of insulating byproducts due to parasitic reactions involving the electrolyte or the carbon electrode. However, in the biaxially aligned CNT electrode, a much improved cycle stability and small charging overpotentials are demonstrated (Fig. 1.(d)). This dramatically improved cycling performance might result from the increase in the surface area corresponding to mesopores, providing more effective sites capable of carrying discharge products such as Li2O2. In addition, Li-O2 cathode using biaxially aligned CNT fabric can offer open framework in which O2 and electrolytes can access to the individual CNTs, facilitating reversible Li2O2 formation and removal, and consequently exhibiting enhanced cyclability. Figure 1