Synthesis and Nitrogen-Plasma Treatment of Carbon Nanotube/Graphene Composites As Anode Materials for Lithium-Ion Batteries

Abstract

Lithium ion batteries compared to other batteries (such as Ni-Cd, Lead-Acid, and Ni-MH) possess higher energy densities (100 – 150 Wh/kg), higher voltage, and lower maintenance [1]. Hybrid CNTs and graphene nanostructures applied in lithium ion batteries were directly grown by ambient pressure chemical vapor deposition (CVD); methane was introduced to form graphene on copper foils at 950 oC, then Fe catalysts were deposited on graphene/copper foils by e-beam evaporation, and ethylene was next introduced to grow pillar CNTs on graphene/copper foil at 750 oC [2]. To increase the accessibly specific surface area and the stability as well as conductivity between the carbon nanotube bundles as well as the nickel foam, three dimensional few layer graphene/multi-walled carbon nanotube architectures were fabricated on oxygen-plasma treated nickel foam coated with Fe catalysts by e-beam evaporation through a one-step ambient pressure CVD process using a mixture of acetylene and hydrogen [3]. N-doped multiwall carbon nanotubes prepared by a plasma enhanced chemical vapor deposition process contain wall defects through which lithium ions can diffuse so as to occupy a large portion of the interwall space as storage regions and then improve the Li storage capability [4]. N-doped graphene nanosheets prepared by heat treatment (800 oC and 2 h) of graphite oxide under an ammonia atmosphere exhibited a high reversible capacity due to N-doping inducing a large number of defects on the graphene layer as well as forming a disordered carbon structure (further enhancing Li intercalation properties) and enhanced cycling stability, which demonstrated the N-doped graphene nanosheets to be a promising candidate for anode materials of lithium ion batteries [5]. The effects of the electron-deficiency (making the defect graphenes have an electron-accepting tendency) of N-doped (pyridinic, pyrrolic, and graphitic) graphenes on their application in lithium ion batteries were investigated and the pyridinic graphene possessed the best as anode materials of lithium ion batteries, while graphitic graphene would be the weakest of the three defect structures [6]. To simultaneously synthesize carbon nanotubes and graphene on nickel foam without additional catalysts, it is tried to grow using one-step ambient pressure chemical vapor deposition (CVD) at different temperatures (700, 800, and 900 oC). Next, in order to add nitrogen-doped defects to the surface of carbon nanotube/graphene composites, it is modified by RF (radio frequency) nitrogen-plasma at different power levels (50, 100, and 150 W) and time periods (5, 15, and 30 min). Carbon nanotubes and graphene are simultaneously synthesized by CVD at 800 oC. Furthermore, the specific capacity (618.32 mAh/g) reached a maximum at the better nitrogen-plasma treatment conditions (power = 100 W and time = 15 min) due to the highest percentage of the pyridinic defect structure which is the most suitable for Li storage with a high storage capacity [6]. Moreover, it also shows higher electrochemical stability after carbon nanotube/graphene composites being treated by nitrogen-plasma.