Synthesis And Electrochemical Property Of Nano α-Fe₂O₃ And FeF₃·（H₂O）_0.33 Micro-sphere

Rechargeable lithium-ion batteries have been widely used due to their excellent characteristics of high energy density, long life time without memory effects, low self-discharge rate, high open circuit voltage and environment friendly. The electrochemical performance of cathode materials limited the development of rechargeable lithium-ion batteries. Ferric fluorides and iron oxides attract growing interest due to their high open-circuit voltage, low cost of raw materials and environment friendly characteristics. Ferric fluorides and iron oxides were usually prepared by high temperature solid-state method, sol-gel method and hydrothermal method, etc. All of these synthesis methods usually have disadvantages of complicated process and high energy cost, etc. Iron oxides were prepared by direct oxidation of iron powder in reflux reaction system. In addition, ferric fluorides were prepared by solution-precipitation method. This process works at relatively low reaction temperatures with simpler operations and lower production cost and is friendly to the environments, which facilitated the preparation of iron oxides and iron fluorides.In this work, iron oxides with different structures were prepared by direct oxidation of iron powder. The products were characterized by X-ray diffractometer, scanning electron microscopy, and BET measurements. The effect of pH on the crystal structures and micromorphologies and the corresponding adsorption capacity for As(III) of products were investigated. The representative samples of nano-sized a-Fe2O3 were used as the anode materials of rechargeable lithium bettary, its discharge mechanism and electrochemical performance was studied. In another work, ferric fluorides with different content of crystalline water including FeF3·(H2O)0.33 and β-FeF3-3H2O were prepared via three different chemical synthesis method, such as solvent-thermal method, solution-precipitation method, and reverse micelle method. Their crystal structures and micromorphologies were characterized by XRD and SEM. Three products which prepared via three different chemical synthesis method that were used as cathode material and their electrochemical performance was compared. It was observed that FeF3-(H20)0.33 micro-sphere with mesopore structure were prepared via solution-precipitation method. The influence of the content of crystalline water and mesoporous structure on the electrochemical performance of ferric fluorides were investigated. The main topics and results of the research are summarized as fellows:1. Nano-sized ferric oxides including akaganite, maghemite, ferrihydrite, and hematite with the specific surface area of 309.4,77.6,75.4, and 213.9 m2 g"1, respectively, were synthesized through facile reflux treatment of 0.5 g iron powder and 20 mL NaCIO solution (available chlorine content≥8%) at 50℃ for 12 h by adjusting the pH 2-4、6、8、10 in reaction system, respectively. As for anode material of rechargeable lithium battery, nano-sized a-Fe2O3 showed larger lithium storage capacity of 4795 mAh/g at the first cycle, which was much higher the theory capacity of Fe2O3 possibly due to the formation of SEI film on the surface of active material, the intercalation of lithium ions in acetylene black, and the newly formed Li5FeO4 in charge-discharge process. After 10 and 15 cycles, the discharge capacity decreased to 565 and 431 mAh/g, respectively. These synthesized nano-sized ferric oxides exhibited excellent adsorption capacity for As(Ⅲ), which reached 89.8,79.2,78.4, and 63.3 mg g-1. The difference in adsorption capacity was likely due to the influence of specific surface area and crystal structures, which affect their adsorption mechanism.2. FeF3(H20)0.33 was hydrothermally synthesized using a mixed solution of 0.02 mol Fe(NO3)3, hydrofluoric acid, ethanol, water and surfactant oleic acid at 120℃ for 12 h. FeF3(H20)o 33 could also be prepared by the precipitation reaction of iron ions and fluoride ions in ethnol organic solvents with adding surfactant of polyethylene clycol. β-FeF3·3H2O was fabricated by mixing reversed micellar solution containing iron ions and fluoride ions for 2 h, and dodecyltrimethylammonium chloride worked as surfactant in this process. Ferric fluorides prepared via three above chemical synthesis methods were used as cathode materials and showed excellent electrochemical performance. The specific surface area of FeF3-(H20)0.33 synthesized by solvent-thermal method reached 11.4 m2/g, the initial discharge capacity was 212.8 mAh/g, and kept 147.2 mAh/g after 50 cycles with the capacity retention of 76.9%. The specific surface area of FeF3-(H2O)0.33 synthesized by solution-precipitation method was 47.1 m2/g, the initial discharge capacities was 262.0 mAh/g, and kept 159.1 mAh/g after 100 cycles with the capacity retention of 70.1%. The specific surface area of β-FeF3·3H2O synthesized by reverse micelle method was 39.2 m2/g, and the initial discharge capacities was 223.7 mAh/g, and kept 157.1 mAh/g after 50 cycles with the capacity retention of 74.6%. It was observed that FeF3·(H20)0.33 synthesized by solution-precipitation method exhibited the best electrochemical peoformance, which was likely due to that (i) tunnel structure in the synthesized FeF3·(H2O)0.33 cathode facilitated the storage of lithium and migrations, and the crystal water with appropriate content could stable the crystal structures during the insertion and deinsertion process. (ii) micro-sized spheres with uniform size improved the contact area of electrolyte and active materials, and decreased the contact and reaction resistance. (iii) mesoporous structures did favor to the electrolyte penetration and improved the migration rate of lithium-ionlithium ions, and ionic conductivity was increased.