Synthesis, characterization, and electrochemical properties of nanostructured materials for enhanced lithium storage properties

Abstract

The morphology, porosity, and electronic properties of material play a key role in improving the performance and durability of lithium batteries such as Lithium-ion battery (Li-ion), Lithium-sulfur battery (Li-S), and Lithium metal-air battery (Li-M). Improved lithium storage properties can be obtained through the design of the morphology of active material and carbon support by using a various synthetic method. In this thesis, novel nanostructured materials have been synthesized and characterized as active materials for lithium storage reaction in lithium battery systems. Firstly, α-Fe2O3 hollow spheres in different sizes were successfully synthesized via a glycerol emulsion template method. The sizes of α-Fe2O3 hollow sphere could be easily controlled by adjusting glycerol amount over the Fe precursor during hydrothermal synthesis process. Thin carbon layer was subsequently coated on the surface of α-Fe2O3 hollow spheres via a hydrothermal treatment with glucose for the carbonization process. In which, the carbon phases could serve as a conductive layer for the efficient charge transfer, but also as a buffer layer for the accommodation of volume vibration of the electrode material during cycling. The stability and cycling performance were also significantly improved by the presence of carbon layer, indicating that the C/α-Fe2O3 hollow sphere could be a promising metal oxide anode material for the lithium-ion batteries. Secondly, an anode material system of Co3O4 nanowires composited with reduced graphene oxide (Co3O4 NWs/rGO) and protected by N-doped carbon layer (Co3O4 NWs/rGO@NC) was prepared. The N-doped carbon layer could serve as stress relief matter to reduce volume changes of the Co3O4 NWs/rGO during repeated charge/discharge cycles, and also enhance the reaction kinetics as a highly conductive layer between the Co3O4 NWs and electrolyte. The Co3O4 NWs/rGO@NC electrode delivered a high discharge capacity of ca. 995 mAh g-1 at 0.1 C after 65 cycles, and showed a much better rate performance of 428 mAh g-1 even at a high current rate of 5 C as compared to those of Co3O4 NWs/rGO electrode (203 mAh g-1). The demonstrated electrochemical properties suggested that the N-doped carbon coating on the composite of Co3O4 NWs and rGO could significantly enhance the durability and rate capability of the anode material for high performance lithium-ion batteries. Lastly, novel three-dimensional (3D) honeycomb-like N-doped carbon nanowebs (HCNs) have been synthesized through a facile aqueous solution route for use as a cathode material in lithium-sulfur batteries. The Li2S@HCNs cathode delivers a high discharge capacity of ca. 815 mAh g-1 after 65 cycles at 0.1 C, along with a superior rate capacity of ca. 568 mAh g-1 even at 2 C. The outstanding electrochemical rate performance is ascribed to their unique 3D honeycomb-like nanoweb structure, consisting of nanowires derived from polypyrrole. These properties greatly enhance the electrochemical reaction kinetics by providing efficient electron pathways and hollow channels for electrolyte transport. Nitrogen doping in the carbon nanowebs also considerably improves the chemisorption properties by tuning affinity between sulfur and oxygen functional groups on the carbon framework. The simple synthesis strategy and the resulting unique electrode structure could present a new avenue in nanostructure research for high performance lithium-sulfur batteries.