Lithium insertion/deinsertion of boron doped graphitic carbons synthesized by different procedure Original Research Article

The lithium insertion/deinsertion behaviour for a series of pure and boron doped graphites prepared from coal tar derived pitch has been studied. Materials treated with boron were pitch-coke and graphitic carbon produced by heat treatment at 1000 and 2900 °C, respectively. The powdered samples were mixed with 2 wt% of nanocrystalline boron and annealed at 2300 °C for 0.5 h under argon. Additionally, post-treatment of samples in hydrogen at 700 °C for 1 h was applied. X-ray diffraction (XRD) analysis was performed. Lithium intercalation/deintercalation was monitored using voltammetry and galvanostatic methods. The results show that boron is a very effective catalyst of pitch-coke graphitization during heat treatment at 2300 °C. When added to graphitic carbon, boron induces a slight improvement of structural ordering which has been established by preceding heat treatment at 2900 °C, but the final effect is inferior to that of heat treatment at 3100 °C without boron. Boron seems to have no positive effect on lithium insertion, whatever the procedure used. For all the graphitized materials lithium insertion/deinsertion occurs within a narrow potential range (0−0.5 V) with reversible capacity Xrev between 0.75 and 0.83 (where X stands for Li insertion degree in the LixC6 compound, X=1 corresponds to 372 mAh/g). Post-treatment of graphitic materials in hydrogen at 700 °C has a profitable effect on lithium insertion/deinsertion, giving the formation of stable passivating layer at first cycle and a decrease of irreversible capacity. Graphite produced by heat treatment at 2900 °C followed by treatment in hydrogen gives the most promising electrochemical characteristics (Xrev=0.83 and Xirr=0.10).