| Transition metal oxides have drawn significant attention of researchers since they were first reported in the Nature by Poizot and his collaborators in 2000 because of their excellent electrochemical properties as anode materials for lithium-ion batteries (LIBs). Transition metal oxides as LIBs anode material have the advantages of high theoretical capacity and natural abundance. However, their potential applications in practical LIBs were limited due to their intrinsic low electrical conductivity and large volume expansion, leading to rapid structural damage and shorten using life of LIBs in the discharge-charge process.In this dissertation, submillimeter sized GO was prepared by an electrochemically assisted improved Hummers method. The composite material nMnO2-srGO with modified structure and properties was prepared with needle-like MnO2 and large-scale GO through one-step electrophoretic deposition. When evaluated as anodes for LIBs, nMn02-srGO composite material deliver excellent electrochemical properties. Reduced graphene oxide-based ternary composite tin-iron oxide (SnFe2O4/rGO) was synthesized by one-step electrophoretic deposition and revealed excellent electrochemical properties when using as anodes for LIBs. The specific research contents are as follows:Graphite sheets were used as electrodes. The aqueous H2SO4 solution was served as electrolyte. The expanded graphite flakes with particle size larger than 300 ?m were synthesized by electrochemical intercalation at a constant DC voltage condition. The expanded graphite flake with an interlayer distance of 0.64 nm reveals prominent properties, a low atomic percentage of O (11.82%), and high C/O ratio (5.9). The ID/IG ration of D peak and G peak is 0.21. The presence of sulphate ions with 3.79% sulfur element content was in favor of electrochemical intercalation reaction in the next improved Hummers method. Large sized GO was prepared using the expanded graphite sheets by an improved Hummers method. Test results indicated that GO exhibited excellent properties. The lateral size of GO reaches 300 ?m, and about 90% of GO flakes had less than five layers by calculating the layers of 50 GO flakes in the HR-TEM image. Meanwhile, GO displays a high degree of oxidation,low C/O ratio (2.11), and high oxygenated C content (59.37%).Based on the large sized GO, the needle-like MnO2-submillimeter sized reduced graphene oxide (nMn02-srGO) hybrid films were fabricated through one-step electrophoretic deposition. When evaluated as anodes for LIBs, the nMnO2-srGO composites with a content of 76.9 wt.% Mn02 deliver an excellent electrochemical properties. The specific performance is as follows:The cycling performance test of the nMn02-srGO composites shows a first discharge of 1850.7 mA h·g-1 at a current density of 0.1 A·g-1 and a high capacity of approximately 1652.2 mA h·g-1 at a current density of 0.1 A·g-1 after 200 cycles. The specific capacity presents obvious self-enhancement phenomenon during the whole cycling process. During the rate performance test, the discharge capacity reaches 946.1, 877.7, 795.9, 744.4, 707.3, and 616.8 mA h·g-1 at the current density of 0.1, 0.2, 0.5, 1, 2, and 4 A·g-1,respectively. The capacity retention is 65.1% at current densities from 0.1 to 4 A·g-1. When current density returns to 0.1 A·g-1, the specific capacity of the composite reaches a higher value of 1064.5 mA h · g-1 than the specific capacity (946.1 mA h·g-1) in the first 10 cycles, and presents obvious self-enhancement phenomenon. By studying the effect of GO's lateral dimensions on electrochemical performance of nMn02-srGO composites that used as LIBs anode electrode, we indicated the electrochemical performance of nMn02-srGO composites decrease gradually with the decrease of lateral dimensions of GO, and demonstrated the rationality of electrophoretic deposition method that we selected to synthesize nMn02-srGO composite materal.The reduced graphene oxide-based ternary composite tin-iron oxides(SnFe2O4/rGO) was synthesized by one-step electrophoretic deposition using large sized GO, Fe(NO3)3·9H20, and SnCl2·2H20 as raw material. The research results show that the structure of SnFe204/rGO hybrid films is the hierarchical porous structures and is model with "rG0-SnFe204 particles-rGO”. SnFe2O4/rGO hybrid films reveal excellent electrochemical properties when made into anodes for LIBs. By studying the effect of initial addition mass of Fe(NO3)3·9H20, and SnCl2·2H20 on structure and properties of SnFe2O4/rGO, the SnFe2O4/rGO hybrid films reveal the best electrochemical properties of LIBs when the initial addition mass of Fe(NO3)3·9H20 and SnCl2·2H20 is 202 and 56.5 mg, respectively. The cycling performance of the SnFe2O4/rGO hybrid films shows that the first discharge is 1184.6 mA h·g-1 at a current density of 0.1 A·g-1, thereby providing a coulombic efficiency of 83.1%. The SnFe2O4/rGO composites deliver a high capacity of approximately 1018.5 mA h·g-1 at a current density of 0.1 A·g-1 after 200 cycles. During the rate performance test, the capacity retention is up to 61.2% at current densities from 0.1 to 4 A·g-1, which indicates the structure stability of SnFe204/rG0 hybrid films is good at high current density. By studying the effect of deposition density on electrochemical performance of SnFe204/rG0 hybrid films that used as LIBs anode electrode, the results shows that the deposition density of SnFe204/rG0 has no obvious effect on the cycle performance and rate performance of LIBs, but the internal resistance,especially the SEI film resistance and charge transfer resistance of the LIBs are obviously increased with the increase of the deposition density. By means of studying the effect of GO's lateral dimensions on electrochemical performance of LIBs, the result reveals that the specific capacity of SnFe204/rGO composites decrease gradually with the decrease of lateral dimensions of GO, and demonstrated the rationality of electrophoretic deposition method that we selected to synthesize SnFe2O4/rGO composite materal. |
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