Preparation And Properties Of Silicon Carbon Anode For Lithium Ion Battery

The Silicon/graphite/carbon (Si/G/C) composite was prepared by pyrolyzing pitch at 900℃in this paper. Then the Si/G/C composites were modified by methods of two-step coating, element B doping, ethching of Cu current collector etched and dipping of Li2CO3 solution. The surface morphology and structure of the composites were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Also, the electrochemical properties were investigated by constant current charge/discharge test, cyclic voltammetry (CV) experiments and electrochemical impedance spectroscopy (EIS) analysis.The cycle performance of Si/G/C composites was studied with different components. It was found that the composites with Si content less than 25mass% showed better performance. The composite consist of 15mass% Si and 35mass% graphite revealed the best cycle performance, whose first reversible capacity is 621.3mAh/g and 398mAh/g at 40th. The composites prepared by two-step coating method showed better performance with the capacity retention of 66.6% for the composites of 20mass%Si and 20mass%graphite within 40 cycles, while the one-step coating method owned the capacity retention of 58.1%. The comparison between PVDF and SBR system showed that SBR as binder can make a low capacity fade rate than the PVDF binder.When B element was added to the Si/G/C (25mass%Si and 50mass% graphite) composite, it was found that the capacity increased firstly then descended with the increasement of B element. And the composite with 7.5mass% B showed the best properties with the first discharge/charge capacity of 1531mAh/g and 1120.6mAh/g respectively, and the capacity fade decreased after 25 cycles and was stable at 460 mAh/g after 40 cycles with the capacity retention of 41%, which is better than the undoped composites.The properties of the composites on the modified Cu current collector were also investigated by etching method with different time. For the composites of 25mass%Si and 25mass%graphite, the results demonstrated that the Cu foil etched in 3min had the similar properties with the no etched Cu foil, and the one etched with 6min owned the best performance with the capacity retention of 58.2% after 40 cycles, while the no etched one was 45.9%. Further the EIS showed that the impedance of 6min etched Cu foil was much smaller after 40 cycles.The composite was also modified by dipping in the saturated Li2CO3 solution and 0.5mass% Li2CO3 doping. These two methods demonstrated that when doped by Li2CO3, the initial capacity was higher but the cycling performance was worse than the undoped composites. On the other hand, although the initial capacity was less than the undipped composites, the composites dipped in the Li2CO3 solution showed better cycliblity.