Effect of pyrolysis temperature of 3D graphene/carbon nanotubes anode materials on yield of carbon nanotubes and their electrochemical properties for Na-ion batteries

A novel graphene/carbon nanotube (GN-CNT) hybrid was synthesized by thermal pyrolysis of urea on the surface of graphene at 800, 900 and 1000 °C. The effects of pyrolysis temperature on yield of carbon nanotubes and electrochemical properties of the GN-CNT hybrid were investigated carefully. The production rate of pyrolysis gas was increased as temperature rising. The GN-CNT hybrid reached the largest surface area and the generation of a mass of pores at the pyrolysis temperature of 900 °C due to the carbon nanotubes cracking during pyrolysis at 1000 °C. The properties of favorable 3D architecture, the generation of a mass of pores during the release of carbon nitride gases and outstanding mechanical flexibility, showed pronounced effects on the fast and steady transfer of electrons and sodium-ion. Consequently, the GN-CNT hybrid pyrolysized at 900 °C exhibit a very high reversible capacity of up to 269.14 mAh g⁻¹ after 100 cycles at a current density of 300 mA g⁻¹. Even up to 5 A g⁻¹, a rate capacity of 195.37 mA h g⁻¹ can be obtained after 700 cycles.