| Carbon atom wires (CAW) were produced from the pyrolysis of starch. Their morphology and structure had been characterized by use of TEM, HRTEM, IR, Ramam, EEIS and some other means.CAW were firstly reported to be applied as a cathode of the lithium-ion secondary batteries in the dissertation. The electrochemical features of the CAW cathode were studied by means of the intermittent galvanostatic titration, cyclic voltammetry, constant-current charging-discharging experiments and AC impedance measurements. The mechanism in the intercalation-deintercalation processes in the CAW electrode and the related chemical diffusion coefficient of Li+ were determined.To improve the performance of the CAW electrode, having better reversibility and reducing its irreversible capacity, some modifications such as surface treatment and compounded with some other elements were investigated.To produce CAW, starch pyrolysed at 500-800C in the presence of a ferric metal as a catalyst in the atmosphere of Ar / H2, in a tube-furnace. The residence of the catalyst in the product was removed with 6M HCI. In the image of HRTEM, CAW appeared in the shape of winding lines with the diameter of 2.0 A, nearing to the diameter of a carbon atom (1.54A). Raman and EELS revealed the sp-bonded structure and IR indicated the presence of OH and -COOH groups in CAW.Along with the lithium-ion intercalated continually, the chemical diffusion coefficient of the lithium-ion in CAW decreased firstly and then inclined to a steady value. The diffusion coefficient was much greater than that of other carbon materials normally used in lithium-ion secondary batteries at present.The cyclic voltammetry and constant-current charging-discharging experiments showed no current-peaks or platforms appeared on the corresponding curves, indicating no stage-compounds of Li/C produced during the intercalation processes.The constant-current charging-discharging experiments for the CAW cathode showed a rather large irreversible capacity in the first cycle and the progressive decrease of theirreversible capacity and the improvement of reversibility with the increase of cycles.In the AC impedance spectra for the CAW cathode, the first semicircle in the high-frequency region was related to the contact resistance between the electrode materials and the current collector; the second semicircle in the mid-frequency region corresponded to the resistance of the interphase between the electrode and electrolyte solution; the last semicircle or line in the low-frequency region was for the resistance of the interphase between the counter electrode of Li and electrolyte solution or the diffusion of lithium-ion in the CAW electrode.Though the reversibility and the recycle performance for the CAW cathode need to be improved further, its first specific capacity is as great as 627 mAh g"1, much more than that of normal carbon materials normally used in lithium-ion secondary batteries at present, whose maximum specific capacity is 372 mAh g"1. Considering the great value of the chemical diffusion coefficient of the lithium-ion as well, CAW are potential ideal cathode materials for the lithium-ion secondary batteries.Treated by concentrated nitric acid, much more amounts of the alcoholic hydroxyl groups and carboxylic acid groups on CAW were produced. The alcoholic hydroxyl groups and carboxylic acid groups could help to form a solid film arising from the reduction of the solvent and the electrolyte on the interphase between the electrode and electrolyte solution in the process along with the lithium-ion intercalation. The solid film can reduce the co-intercalation of solvent molecules and inhibit their decomposition. Therefore, the nitric acid treatment can reduce the irreversible capacity, increase the reversible capacity and improve the performance of the CAW electrode.The discharging specific capacity in the first cycle of the CAW electrode compounded with Co was smaller than that of the pure CAW electrode without any treatment, but its charging specific capacity was much m... |
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