Preparation Of Transition Metal Oxides/Carbon Composite Materials And Their Electrochemical Properties Studies

Transition metal oxides（TMOs） can be used as electrode materials for lithium-ion batteries or electrochemical capacitors because their reversible redox reactions that happened under some conditions, which could generate significant pseudocapacitance or lithium storage capacity. Unfortunately, two major shortcomings limit the application of TMOs as the electrode materials. One is the poor conductivity causing a high internal resistance of energy storage devices.Another is the unsatisfied cycling performance originating from the structure collapse of active materials during charge-discharge（i.e. lithiation-delithiation）.Carbon coating and doping are useful ways to solve these problems nowadays.Meanwhile, not only can carbon coating enhance the electronic conductivity of the TMOs, but also buffer crystalline volume change in the process of charging and discharging. As a result, electrochemical properties of the TMOs are effectively improved. In recent years, TMO/carbon composite materials with various morphology have been prepared by hydrothermal method. In this work, the hydrothermal methodology was adopted to prepare TMO/carbon composite materials.Physicochemical properties of the obtained composites were characterized by some modern techniques such as SEM, XRD, TEM, XPS, TG, BET and EDX. In addition,electrochemical properties of them were evaluated by galvanostatic charging-discharging measurements, cyclic voltametry（CV） and electrochemical impedance spectroscopy（EIS）. The research contents are listed as following:（1） Carbon encapsulated Mn-doped V₂O₅ quasi spheres(Mn_0.1V₂O₅@C) have been successfully prepared by a hydrothermal treatment followed by high-temperature calcination. These quasi spheres were built of nanorods that embedded uniformly in a porous carbon matrix. The porous structure facilitated penetrations of electrolyte into the quasi spheres leading to forming 3D ion-conduct network while the porous carbon matrix resulting in the 3D electron-conduct network. Owing to such a unique morphology, the obtained composite material possessed an excellent rate capability over the potential rang of 4.0-2.0 V（vs. Li/Li+）. It could deliver 164 m Ah g-1 at 20 C rate, which is about 62% of the discharge capacity at 0.1 C rate（265 m Ah g-1）. Also they exhibited excellent cycle performance（retaining 94% of the initial capacity after500 cycles at 5C rate）. The results show that doping cation into inter-layers and carbon coating can significantly improve the electrochemical properties of V₂O₅.（2） Mo O₂@C composite materials were synthesized by a hydrothermal treatment followed by a high-temperature calcination. They appeared as many hollow tubes whose inner diameter is 2 μm and outer diameter 4.7 μm. The tube length was about10 μm while the tube wall thickness was 1.3 μm. The BET surface area and the BJH pore size were around 7.12 m2 g-1 and 32.26 nm, respectively. Electrochemical measurement results showed that the as-prepared Mo O₂@C composite material could deliver an initial capacity of 1041 m Ah g-1 at 0.05 C rate within 0.02-3.00 V（vs Li/Li+）accompanying with a coulomb efficiency of 66%. After 45 times cycle, the capacity retention was about 65% of the second discharge capacity. At the rates of 0.1C, 0.5C,1C and 2C, the first discharge capacity is 442, 341, 245 and 137 m Ah g-1,respectively. Compared to pure Mo O₂, the as-prepared Mo O₂@C hollow tubes had better electrochemical performances.（3） Mo O₃（K_x）@C rod-shaped composite materials were prepared via hydrothermal combination with heat treatment method. The length and wideness of the rod was 3-5 μm and 1 μm, respectively. It was found that carbon content of Mo O₃（K_x）@C composites was about 10%, which determined by TG test in air atmosphere. When the K/Mo molar ratio was 0.1%, namely Mo O₃（1K）@C material, it could deliver an initial capacity of 231 m Ah g-1 at 0.05 C rate, and 118 m Ah g-1 at 2C rate within the potenyial range of 1.5-4.0 V, respectively. The Mo O₃（1K）@C and undoped sample retained 66% and 39% of their own initial capacities after 80 cycles at the rate of 0.05 C, respectively. The results suggested that K+ doping combination with carbon coating can improve the electrochemical performance of Mo O₃.