Conducting Polymer-Coated Carbon Nanotubes As High Rate Anode Material for Rechargeable Aqueous Electrolyte Battery

Among various energy storage systems, the rechargeable battery has been proposed as one of the most promising strategy for grid-scale energy storage because of flexible power and energy characteristics, high energy conversion efficiency, and low maintenance. And, their compact size makes them well suited for use at distributed location to control the output fluctuations in local power generation.

Lithium-ion batteries are being considered as one of the most promising power sources for smart grids because of their high energy density and long cycling life. However, safety and fast charging performance are still two challenging issues to be solved for their use as large-scale systems. An attractive approach to circumvent this problem is to use an aqueous electrolyte for lithium-ion batteries, which adopt a ‘rocking-chair’ concept similar to the organic lithium-ion battery. In the mid 1990s, an aqueous lithium-ion battery was proposed that uses VO₂ as the anode and LiMn₂O₄ as the cathode. By this combination, the safety problem of the organic electrolyte is fundamentally resolved. Furthermore, since the rigorous assembly conditions are avoided, the cost is reduced greatly, compared with the non-aqueous lithium-ion battery.

In this study, polypyrrole-coated carbon nanotube(CNT) was prepared by chemical oxidation polymerization of pyrrole on the surface-functionalized carbon nanotube in the presence of FeCl₃. As increasing the concentration of pyrrole, the thickness of polypyrrole layer on the surface of CNT was gradually increased.

Polypyrrole was very stable during the lithium and sodium doping/dedoping processes in the aqueous solution without evolution of hydrogen. When the polymers were dispersed on the surface of carbon, the discharge capacity was highly improved due to the enhanced surface area and decreased diffusion length. In the charging-discharging experiments, the decay of capacity was greatly decreased although the charging/discharging rate was increased to 10C. The conducting polymer-coated CNT showed very promising charge-discharge characteristics at high-rate region in the aqueous sodium electrolyte.