| Lithium-ion batteries are widely used throughout the world for mostly portable electronic devices and mobile phones and shows great potential for larger applications like the electric vehicle. Unfortunately where lithium-ion batteries are now, they lack the required level of energy storage to meet such applications. Silicon nanoparticles have shown great potential as anode material to replace our currently and widely used graphite material. Silicon has shown to have a high theoretical gravimetric capacity of approximately 4200 mAh/g compared to graphite's 372 mAh/g. Though it has amazing capacity it suffers from rapid degradation with each cycle due to its volume expansion of approximately 400% during lithiation putting strain on the material and causes it to break down and create poor electrical contact among other problems. I worked to create a silicon/carbon composite using our labs novelty graphene tubes to reduce the damage of the volume expansion and improve overall performance of the silicon anode material. I worked with optimizing electrode and coin cell procedure to get the best performance of any materials tested. I vary synthesis procedure for the graphene tubes to optimize performance or to create a structure to meet the needs of our experiment while also testing different size silicon particles. The silicon and graphene tubes are incorporated using two methods of in situ, where the silicon is incorporated during the synthesis of the graphene tubes, and physical mixing, where the pre-synthesized graphene tubes are physically mixed together with the silicon by mortar. My findings show that pure graphene tubes can be synthesized in a number of ways that can be more cost effective without altering the overall performance of the battery. I also found that silicon of smaller particle sizes have better electrochemical stability than larger ones, but are more vulnerable to side reactions in comparison. Unfortunately the results of the silicon/graphene tube composite hasn't been favorable. Though there have been some good results the use of graphene tubes hasn't been able to reduce the damage of the silicon expansion in both in situ and physical mixing method. With the in situ process, the main problem is that when graphitizing the graphene tubes in a furnace with the silicon incorporated a side reaction with the silicon and the carbon forms silicon carbide. Silicon carbide is inert to lithiation reducing overall battery performance. Certain factors contributed to this include, but not limited to, temperature of graphitization, carbon precursor for the graphene tubes, and the size of the silicon particle. With the physical mixing, it appears the degree of capacity fading of the material is unaffected by the inclusion of graphene tubes. Though the stability of the composite is higher with more graphene, the overall capacity suffers from the lack of silicon which is the dominant capacity contributor. As of now the logical conclusion may be that graphene tube isn't the carbon structure we hoped would solve the problem, but there could be other ways to implement it besides the in situ process. |
