Polyimide-Wrapped Carbon Nanotube As High Capacity and Rate Anode for Aqueous Rechargeable Na-Ion Batteries

Rechargeable lithium-ion batteries (LIBs) are rapidly extended from portable device to electric vehicle and smart grids owing to their high energy density and long cycling life. However, its potential for large-scale applications in electric vehicles and energy storage is hampered by the high production costs and finite supply of lithium. With this background, an attractive approach to circumvent this problem is to use an aqueous electrolyte for Na-ion batteries (NIBs). Researchers hope to overcome this problem by replacing it with sodium, which is cheaper, nontoxic, and easier to process and almost infinitely abundant. In this study, pyromellitic dianhydride (PMDA)-derived polyimide deposited on the carbon nanotubes were used as anode of aqueous electrolyte battery. That is, polyimide was directly polymerized on the surface of CNT. SEM and TEM revealed that the polyimide was homogeneously wrapped the surface of the CNTs and formed a uniform shell. This suggests that the interactions between polyimide molecules and CNTs overcame the Van der Waals interaction between CNTs, which generally otherwise would result in separate growth or aggregates of polyimide. The charge-discharge characteristics were investigated by using 3-electrode beaker cell with 2M Na 2 SO 4 solution. The reduction and oxidation of polyimide are accompanied by the association and disassociation of Na + ions with oxygen as described in Fig. 1. Each formula unit is able to transfer four electrons through two steps, which may allow a high theoretical specific capacity, 280.4 mAh g -1 . The polyimide-wrapped carbon nanotube exhibited much superior high-rate and long cycling performance to conventional inorganic anode. The calculation of diffusion coefficient and impedance study revealed that these high performances were attributed to the advantages of hierarchical core-shell structure. The conductive CNT core offered continuous electron transport pathways and thin polyimide shell provides rapid Na + ion diffusion pathways. Thus, the bicontinuous electron/ion transport channel greatly facilitate the ion diffusion kinetics and reduce the charge transport resistance, resulting in the excellent rate capability. Figure 1