Abstract:

By utilizing the advantages of good conductivity and one-dimensional fiber structure of carbon nanotubes (CNTs), nitrogen-doped carbon nanotubes (NCNTs) formed through the thermal conversion of polypyrrole-coated carbon nanotubes (PPy@CNTs) was utilized as an enhanced conducting additive. A composite of LiFePO₄ coated with carbon layers (C@LFP) and NCNTs has been prepared to improve the electrochemical performance of commercial LiFePO₄(LFP). The C@LFP was prepared through the pyrolysis of sucrose. Then the NCNTs-C@LFP and the nitrogen-doped CNTs (NCNTs) were uniformly distributed between the C@LFP particles. X-ray diffractometer and field emission scanning electron microscope (SEM) were used to analyze the crystal structure and microstructure of the samples. NCNTs are uniformly distributed between LFP particles and the preparation process did not change the crystal structure of LiFePO₄. The analysis of the pore structure of the sample by a physical adsorption instrument showed that the nanocarbon modification increases the specific surface area and the volume of small mesopores with the size of 2～5 nm of the NCNTs-C@LFP, which can facilitate the ions migration. Electrochemical tests showed that the carbon coating could improve the electronic conductivity of LFP particles and the NCNTs enhance the conductivity between the LFP particles. The charge transfer impedance of NCNTs-C@LFP shows a decrease of five times as that of LFP. The capacity of NCNT@C@LFP reaches 165 mA·h/g at a 0.1 C rate, which increases by 32% compared with the LFP (125 mA·h/g). The capacity retention rate of NCNTs-C@LFP is 94% after 100 cycles at 1.0 C. The results indicate that the electrochemical performance of LFP can be improved efficiently through constructing the conducting networks by the carbon coating with two-dimensional structure and NCNTs with one-dimensional structure.

Key words: lithium ion battery, LiFePO₄, carbon coated, nitrogen doped CNTs