Abstracts Category : Other

Electrochemical Performance of Si-based Material : C Composite Anode for Lithium ion Batteries

by Qin Si

The volume effects of silicon upon Li insertion and extraction can be effectively suppressed by designing a composite microstructure containing ultrafine silicon which is uniformly dispersed in a ductile conducting carbonaceous matrix with electron/ion conductivity. By this way, in the part 1, several types of Si-based composites have been developed by means of pyrolysis process in accompany with adjunction of CNF in the liquid electrolyte. After only the pyrolysis process, obtained Si/C composite shows fairly rapid capacity decay, although the Si/C composite have better capacity retention than only Si during cycling. The pyrolized carbon matrix alone is not sufficient to accommodate the large volume change of silicon during the cycling, which leads to the cracking and crumbling of the Si/C particles. The loss in electronic contact with the Si particles therefore results in the gradual degradation of the electrode. We found that the CNFs homogeneously dispersed on the Si/C mixture surface form a secondary conductive matrix due to the ability of the CNFs to wrap the Si/C particles, thus holding them together and keeping the electronic contact. With the formation of the secondary conductive matrix, these Si particles are kept in contact within two-layer matrixes of the disordered carbon and the CNFs and thus the capacity of the electrode is largely maintained. Moreover, a new type of the composite was also prepared through the other route, where CNF was covered with pyrolytic carbon derived from PVC. CNF, Si and PVC were mixed and then heated, so that Si and CNF were covered with the PVC-derived pyrolytic carbon. The new composite material has a porous structure. Such a structure offers more free space to buffer the volume changes of LixSi during the Li insertion and extraction process. However, when the particle size is down to below 100nm, silicon active particles are very easy to reunion during the Li-insertion and extraction to form the electrochemical sintering. Moreover, nanometer silicon powder shows a large surface area, so the increase in direct contact with the electrolyte leads to a large irreversible capacity and reduces the coulomb efficiency. In addition, nanometer silicon powder is produced mainly by the laser method, so the preparation costs are very high. An inexpensive micron size of SiO powder was pulverized using HEMM and then mixed with CNF. The electrode performance of the SiO/CNF composite anode was investigated as a function of the milling time and was discussed with respect to the change of the Si valence by the milling time. The high irreversible capacity at the first cycle was compensated by chemically precharging with a lithium thin film attached to the composite electrode. The SiO was ball-milled for 12 h with CNF to produce a composite electrode material that exhibited excellent cycling performance. A reversible capacity of approximately 700 mAh g-1 was observed after 200 cycles. Several factors were considered for the contribution of the improved cycling performance. Firstly, by the microAdvisors/Committee Members: , .