| On the basis of a brief summary of the investigation and application of lithium ion batteries and a review of the investigations and development of tin-based anode materials, the thesis aims to introduce carbonaceous materials in the tin-based anode materials system, preparing Sno2/C and SnO2/Sn/C composites anode materials. Such combination of active and less active are hopeful to increase the cycle stability of the tin-based anode for lithium ion batteries. Citric acid and stannic chloride were used as raw materials to fabricate nanostructured SnO2/C or SnO2/Sn/C composites by a sol-gel method. In addition, acetylene black, nanostructured SnO2 and Sn powder were used to synthesize SnO2/C composites and Sn/C composites by high-energy ball milling.The effect of the fabrication technique and its parameters on the structure and electrochemical properties and the correlation of the structure and electrochemical properties of composites were investigated by XRD, SEM, TEM, element analysis, Raman spectrum, etc. and eletrochemical testing of galvanostatic charge-discharge, CV. The key factors that influence the cycle stability of composites and its mechanism were also discussed.The results of the present investigations show that, at the synthesis condition of citric acid and stannic chloride in the mole ratio of 2:1, and dwelled under the calcination temperature ranging from 500 to 800℃for 4h in nitrogen atmosphere, SnO2/C composites were synthesized. The SnO2 particles are ultrafine and dispersed in the amorphous carbon. In the calcination temperature range of 500-700℃, the carbon content maintains around 36-39 wt.%. When the calcination temperature increased to 900 and 1000℃, partial SnO2 was reduced by the in situ formed carbon forming nano-Sn particles, and the carbon content was increased to 49 wt.% when the calcination temperature was 900℃. With the increasing of the calcination temperature, the charge and discharge capacities decreased as well as its initial irreversible capacities. However, the initial coulombic efficiencies for the composites are very close, around 64-69%. Moreover, after initial several cycles, the reversible capacity of the SnO2/C composites calcined at temperature range of 600-800℃turned to be stable and possessed around 380-400 mAh/g after 60 cycles. For the SnO2/Sn/C composites, as there are higher carbon content and improved crystalline order of the carbon, the cycle stability was further improved, but the capacity was somewhat reduced. Compared with commercial nano-SnO2 anode, the irreversible capacity of SnO2/C and SnO2/Sn/C composites anode was greatly reduced and the cycle stability was effectively improved. The ultrafine SnO2 and Sn particles suffered less strain during the insertion and extraction of lithium ion, and the amorphous carbon matrix can also accommodate more volume expansion/contraction of the SnO2 and Sn particles during cycle. Meanwhile, the conductive carbon provided a better electronic connection of the SnO2 and Sn particles. These are the main factors for the improvement of the cycle stability of the composites. In addition, increasing the addition of citric acid also favors the improvement of the cycle stability of the SnO2/C composites, however, the capacity was reduced.At the calcination parameters of 700℃×4 h, reduction atmosphere (mixture of nitrogen and hydrogen with the hydrogen content ranging from 10 vol%to 30 vol%), considerable amount of crystalline Sn formed in the calcination process (a gel precursor with citric acid and stannic chloride in the mole ratio of 2:1 was used), forming SnO2/Sn/C composites. The composites reveal good cycle stability. The capacity retention of 62-65%was obtained after 50 cycles. The carbon content was increased with the increase of hydrogen in the nitrogen. But as the carbon matrix has a large irreversible capacity, the initial coulombic efficiencies of the composites were not sound, a bit lower than 60%. Compared with the composites synthesized at nitrogen, the composites synthesized at the mixture gases of nitrogen and hydrogen show a lower capacity due to the higher carbon content. Additionally, the product synthesized under oxygen was mainly composed of SnO2 and shows low cycle stability.SnO2/C and Sn/C composites were formed by ball milling nano-sized SnO2 and Sn powders with acetylene black, respectively. Weight ratios of 7:3 and 6:4 for SnO2 or Sn to acetylene black were used. Compared with the pristine SnO2 and Sn anodes, the composites display much better cycle stability. Moreover, increasing the content of the acetylene black also favors the improvement of the cycle stability of the composites, but the capacity was reduced. |
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