| The dissertation presents strategies on improving the performance attributes of nano-structured anodes in lithium ion batteries. Two anode materials were tested - silicon and graphene. While silicon has an exceptionally high theoretical capacity of 4200 mAh/g, ~1 order of magnitude higher than commercial graphitic anodes (372 mAh/g), silicon suffers from drastic limitations owing to its inherent structural and electrochemical instability. This in turn impacts the cycle life and lowers the commercial viability of silicon anodes. In order to overcome the aforementioned issues in silicon anodes, a polymer (parylene) coating and a nano-scale spiraling structure has been proposed. The spiraling structure allows scalability of the silicon anodes while ensuring retention of porosity and lithium diffusion characteristics, allowing mass scalability of silicon up to 1 mg/cm2. The con-formal parylene coating on the other hand induces a passivation layer at the electrode-electrolyte interface, thereby improving the electrochemical stability of silicon anodes and allowing excellent cycling stability with a steady capacity greater than 1000 mAh/g over 50 charge/discharge cycles.;An alternate class of anode materials - graphene was also proposed. Graphene is a 2-D sheet of sp2 hybridized carbon with a very high surface area, exceptional mechanical strength and excellent electrical conductivity and is therefore an attractive material for application as anodes in lithium ion batteries. However, in order to achieve performance characteristics with graphene anodes that are significantly better than their commercial graphitic counterparts, it is necessary to optimize the graphene synthesis process. Thermal and photo-thermal reduction of graphene oxide were proposed to create a porous gra-phene network. A novel mechanism of defect-induced plating was observed in thermally and photo-thermally reduced graphene anodes, previously unknown among researchers investigating graphene anodes in lithium ion batteries. This defect-induced plating facilitated a significantly improved charge storage mechanism, allowing the graphene anodes to provide capacities in excess of 900 mAh/g, 2-3 times better than commercial graphitic anodes. In addition, the porous graphene network allowed excellent electrolyte wettability and lithium diffusion characteristics that allowed the lithium ions to transport through the graphene sheets and interact with the active sites faster, thereby allowing power densities as high as 30 kW/kg to be achieved. Photo-thermally and thermally reduced graphene anodes were tested extensively to assess the entire range of perfor-mance metrics including repeatability, reliability, safety and cycle life.;Finally, the mechanism of lithium-graphene interaction, based on a defect-induced lithium metal plating in graphene, was extended to synthesize a new class of electrode materials. It was observed that in porous graphene structures, lithium metal plating occurred within the nano-pores and were strictly contained within these nano-pores. It was confirmed through extensive cycling and characterization that the lithium metal plating in graphene is indeed benign and does not lead to the formation of dendrites, therefore assuring the safety characteristics of the graphene-lithium electrode. The advantage of the graphene-lithium electrode, when used as the source of lithium ions in a lithium ion battery (conventionally referred to as the cathode), is that it allows for a much higher capacity to be achieved. Lithium metal is known to be able to provide capacities as high as 3840 mAh/g. However, pure lithium metal is not used in lithium ion batteries due to safety concerns associated with dendritic outgrowths. Instead, commer-cial cathodes use complex chemical matrices such as lithium cobalt oxides, phospho-olivines such as lithium iron phosphates or spinels such as lithium manganese oxides. However, commercial cathodes have a much lower capacity, ranging from 150-200 mAh/g while the graphene-lithium composite could provide capacities as high as 850 mAh/g. Therefore, the graphene-lithium composite electrode provides an opportunity to drastically improve the achievable capacity of lithium ion batteries. Importantly, the graphene-lithium composite electrodes also eliminate toxic and expensive metals like cobalt, nickel, aluminum and copper, commonly used in present-day lithium ion batteries, therefore enabling a lighter, cheaper and environmentally sustainable next-generation lithium ion battery. |
