Studies Of Lithium Storage Properties In Graphene-based Metal Oxides

Lithium-ion battery（LIB） as a kind of effective electrochemical energy storage device has been widely used in human daily life such as a variety of portable electronics, electric bicycles and electrical vehicles. In comparison to other types of batteries, LIB has many advantages including higher energy density（120-170 Wh kg-1） and longer cycle life. However, its power density is still relatively low to be used alone in some high-power driving devices such as electrical vehicles or is not satisfactory in some conventional uses, for examples, the rapid charging of mobile phones or laptops. It is highly demanded to further improve the power density of LIB while still remaining its high energy density but it is very challenging. The principle of LIB is the lithium ions intercalation and de-intercalation at the electrode/electrolyte interface during charge/discharge process. This greatly relies on a fast electrode kinetics that involves the interfacial Faraday resistance, the mass transport rate of lithium ions in the porous electrode materials, robust material structure for long cycle life and electrical conductivity of the electrode for electron transport. Therefore, it is critical to both chemically and physically tailor the electrode material possessing high conductivity, high specific surface area and rational nanostructure materials to shorten the lithium ion migration length while are robust for long cycle life. In these respects, graphene is a very promising candidate material because of its superior conductivity, mechanical flexibility, chemical stability and high specific surface area.Following research works are proposed and performed in this MS program research:1. This MS thesis briefly reviews the current status and challenges of LIB including the development history, working principle, characteristics, composition structure and the advantages/disadvantages of the battery. The progress and main achievements are briefly discussed. In particular, several anode materials are compared. Single metal oxide anode material have a large volume expansion during charge/discharge cycles resulting in partial pulverization for poor contact between electrode material and current collector. Graphene nanocomposite can be used to effectively improve the physical and chemical properties including conductivity, specific surface area, contact area of the active materials and material utilization for high cycling stability and rate capability.2. Experimental methods to characterize the physical properties and electrochemical properties of the lithium ion batteries are discussed in detail.3. One-step hydrothermal method is used to successfully grow the flower like zinc molybdate, carbon coated zinc molybdate, zinc molybdate/graphene composites on Ti sheet. XRD and SEM measurements further confirm that the obtained zinc molybdate is triclinic and flower like structure made of nanometer thickness of nanorod clusters. The lithium storage properties of the three materials were compared. The initial capacity of the ZnMoO4/G, ZnMoO4/C, ZnMoO4 are 2398 mAh g-1, 1970 mAh g-1, 1864 mAh g-1, respectively. In the constant current charge/discharge test, ZnMoO4/G rate capacity is the best among the ZnMoO4 and ZnMoO4/C. After 110 cycles, ZnMoO4/G shows discharge capacity of 649 mAh g-1, ZnMoO4, ZnMo O4/C are 283 mAh g-1 and 359 mAh g-1 respectively, indeed greatly improving the ZnMoO4 cycle performance. The analysis of AC impedance spectra reveal a reduced charge-transfer resistance and improved conductivity of the electrode material after composing graphene with metal oxides, mainly promoting the migration of the lithium ion for improved lithium storage performance. The largest slope of the impedance of ZnMoO4/G indicates its superior Li+ diffusion speed. In addition, the electrode grown on the Ti sheet can be directly used as a lithium ion battery electrode without binder and conductive agent, simplifying the assembly process of battery and improving the practical application.4. SnO₂ hollow nanospheres are successfully prepared by a simple one-step hydrothermal method following by surface modification of APTS, and then adding GO to tailor graphene-encapsulated SnO₂（SnO₂-G） hollow nanospheres. Examinations of XRD, SEM and TEM confirm that the products are affirmative rutile structure SnO₂. SnO₂ is hollow nanosphere with sizes between 300-500 nm. The tailored graphene-encapsulated oxide also shows a hollow nanosphere without significant size changes although its surface is not smooth, textured and folded. In addition, the edge has a layer of carbon structure. Studies of the lithium storage performance of SnO₂ and SnO₂-G show a discharge capacity of 585 mAh g-1 at current density of 1A/g after 70 cycles for SnO₂-G, while only 306 mAh g-1 for SnO₂. AC impedance analysis reveals that the charge-transfer resistance of the SnO₂-G（74.69Ω） is smaller than SnO₂（113.1Ω）, indication the graphene encapsulation can promote the lithium storage performance.In brief, the thesis works reveal that grapheme modification, assembly and composition can be used to delicately tailor the chemical composition and physical structures for unique electrochemistry properties of electrode materials, which can significantly improve the electrocatalytic activity, electrode conductivity and the migration of the lithium ion for improved lithium storage performance.