| NC (Needle coke) was heat-treated with different HTTmax (the maximum heated-treatment temperature). The microstructures of these samples were characterized by their densities, XRD spectra and Raman spectra. Their charging-discharging performances were investigated by galvanostatic charging-discharging experiments and powder microelectrode cyclic voltammetry experiments. The relationship between their charging-discharging performances and the microstructures was discussed. The compatibilities of sample NC2700 with six kinds of electrolytes were investigated too. The compositions of the solid electrolyte interphase(SEI) films formed during the first charging process were analyzed by FTIR spectra. The relationship between the SEI films and the compatibilities of samples with electrolytes was examined. The influences of other factors (the granularity of NC2700, the content of PTFE and the content of acetylene black) on the charging-discharging performances of NC2700 were also investigated by the orthogonal method through galvanostatic charging-discharging experiments. The experimental results are as follows:1.The microstructures and the charging-discharging performances of NC samples relate to HTTmax. When HTTmax<1500 ℃, the microstructures and the charging-discharging performances of NC samples remain unchanged due to the previous calcination under temperature 1400℃-1500℃. When HTTmax <2100℃, the graphite microcrystals have not appeared. The mechanism of storing' lithium-ions is to insert lithium ions in the micropores of the samples. The charging-discharging curves look like the letter "V" and have no flat plateaus due to the different sizes of the micropores. With the increasing of HTTmax, the micropores in NC samples become smaller and fewer and the charging-discharging capacities decrease. When HTTmax = 2100℃, the charging-discharging capacity reaches the minimum due to the minimum micropores and the small and few graphite microcrystals in NC samples. When HTTmax >2100℃, the graphite microcrystals grow rapidly, and the charging-discharging capacities of the samples increase too. The mechanism of storing lithium-ions converts to the intercalation of the lithium ions between the graphene layers of the graphite microcrystals. The charging-discharging curves of the samples look like the letter "U" and have low potential flat plateaus. When HTT.ax-2700℃, the charging-discharging characteristic of NC sample in 1mol/L LiClO4/EC+DEC (1:1) electrolyte reaches the optimum with discharging capacity 314.3mAh/g and charging-discharging efficiency 95.7% in the 3rd cycle. When HTTmax>2700℃, the charging-discharging performances of NC become worse since the exfoliation of graphene layers in the graphite microcrystals due to the decrease of sp3 bonds which can prevent the solvate-d lithium ions from intercalating into graphene layers.2. Using either LiClO4 or LiPF6 as the lithium salt solute, the charging-discharging performances in EC-based solvent system are always much better than in PC based solvent system. For EC-based system, the charging-discharging efficiency using the solute LiClO4 is about 5% higher than using the solute LiPF6. The chemical compositions of SEI films formed on the interfaces of NC2700 samples in different electrolytes during the first charging process are mainly Li2CO3 and LiOCO2R, but their textures are different. The SEI films formed inEC-based electrolytes are thin and compact, which can prevent the solvated lithium ions from cointercalating into graphene layers; the irreversible capacities are small. The compatibilities with NC2700 are good. However, the SEI films formed in PC-based electrolytes are thick but defective, which could not effectively prevent solvated lithium ions from intercalation; the irreversible capacities in PC-based electrolytes are large. The compatibilities with NC2700 are very bad.3. In the process of the manufacture of the electrode, the granularity of NC2700 (A), the contents of PTFE (B) and acetylene black(C) influence the charging-dischargi... |
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