| In response to the increasing energy crisis and environmental protection requirements, developing low cost, green environmental protection, performance security, and high specific energy power battery is the research hotspot currently. For lithium ion batteries, graphite is the most widely used material as anode so far. The theoretical capacity of graphite is about372mAh/g. Once new material with higher capacity is developed, energy density of lithium ion batteries will be improved.On the basis of reviewing the developments of lithium ion battery and relative materials in detail, with Zn2SnO4anode materials as objects of the research, the Zn2SnO4anode materials were prepared using different methods such as hydrothermal method, low-temperature solid-phase reaction method and high temperature ball milling method. Pure Zn2SnO4anode materials exhibits higher capacity than graphite However, the poor capacity retention of these new materials during cycling as well as the large irreversible capacity during the first discharge/charge cycle has limited its commercially application The main purpose of this paper was to improve the electrochemical performance of Zn2SnO4.The structure, morphology and electrochemical properties of the as-prepared products are investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) and electrochemical measurements. A mechanism of Li storage on the Zn2SnO4and characterizations of lithium insertion/desertion into the electrode as well as the capacity fading were studied. The approaches for improving the cycle life were researched.The main research contents and results are as follows:1. Nanosized Zn2SnO4powders were successfully prepared by a hydrothermal method using SnCl4·5H2O, ZnCl2and N2H4·H2O as reactants. The effect on the product purity and crystallinity were studied with respect to the hydrothermal reaction temperature and N2H4-H2O concentration. The powders with uniform size around30nm can be obtained at0.5M or0.6M of N2H4·H2O solution at180℃. Results on the electrochemical properties nanosized Zn2SnO4powders showed that N2H4-H2O concentration, voltage windows, current density for charge and discharge effect on their specific capacities, cyclability. The powders obtained at0.5M of N2H4H2O solution at180℃showed the best electrochemical properties: its first discharge and charge capacity were about2612.4and1015.9mAh/g, respectively, with a capacity retain of673.1mAh/g in the40th cycle at a constant current density of60mA/g in the voltage range of0.05～3.0V.In addition, the structure and morphology of the as-prepared product and specimens taken from the electrodes after charging-discharging cycles are analyzed by X-ray diffraction, scanning electron microscopy, respectively. Three phases, inversed spinel Zn2SnO4, Zn and Sn, were detected at the charged state. It was found that most Li-Sn and Li-Zn alloy can be reversibly decomposed. In the case of the discharged state, the diffraction peaks of inversed spinel Zn2SnO4disappear, but Zn and Sn exist. It proved that in the discharge process the Zn2SnO4particles were reduced to Zn and Sn. It can be seen that pulverization and agglomeration occur between the active particles after40th cycles, which would result in poor contact between the active particles and increase the electrode resistance. In particular, it was found that solid electrolyte interphase (SEI) film in a cycle of continuous thickening and dense during cycling. The interface processes of lithium insertion into nanosized Zn2SnO4electrode was studied by A.C. impedance at different discharge states.2. Compound Zn2Sn1-xTixO4(x=0,0.1,0.2and0.3) were synthesized by a hydrothermal method. The influences of different doping concentration on the structure, morphology and electrochemical properties of the material were investigated and the optimum doping amount of Ti was determined. The results showed that the crystal structure of Zn2SnO4was not changed by proper doping. Electrochemical tests indicated that the addition of Ti could significantly improve the cycling performance of Zn2Sn1-xTixO4although there was a slight capacity loss at the initial discharge. Among the Zn2Sn1-xTixO4materials, the material with x=0.1showed the best cyclability and highest capacity. Its first discharge capacity was about1589.8mAh/g, with a capacity retain of456mAh/g in the40th cycle at a constant current density of100mA/g in the voltage range of0.05～3.0V.3. The composite Zn2SnO4/C was synthesized by carbothermic reduction. The influences of carbon contents were investigated. The results showed that the carbon component could buffer the volume expansion of Li-Sn and Li-Zn alloys and provide a highly conductive medium for electron transfer, improving the cycle capability of anode material Zn2SnO4. The optimum conditions were that the amount of glucose was15%and sintering at600℃for2h. The material synthesized under optimum conditions have well-developed crystal structure. It showed a better electrochemical performance than pure phase Zn2SnO4, with a much higher capacity (563.5mAh/g) after40cycles.4. The composite Zn2Sn1-xTix04/C(x=0,0.1,0.2and0.3) were then prepared through a carbothermic reduction process using the as-prepared Zn2Sn1-xTixO4and glucose as reactants. The influences of different doping concentration on the structure, morphology and electrochemical properties of the material were investigated and the optimum doping amount of Ti was determined. Comparing to the pure Zn2SnO4, some improved electrochemical properties were obtained for Zn2Sn1-XTiXO4, Zn2SnO4/C and composite Zn2Sn1-XTixO4/C. The composite Zn2Sno.9Ti0.1O4/C showed the best electrochemical properties, and its first discharge capacity was about1551.8mAh/g, with a capacity retain of496.6mAh/g in the100th cycle. The larger capacity and better cyclability of the composite Zn2Sn0.8Ti0.2O4/C electrode were attributed to composite structure consisting of active Zn, Sn and TiO2, and inactive LiTiO2in the carbon matrix during the cycling process.5. Nanosized Zn2SnO4powder was synthesized by high temperature ball milling method. The influences of claiming temperature on the structure, morphology and electrochemical properties of the material were investigated. In the high temperature ball milling process, the cooperation action of milling and temperature, and then significantly increased the activation energy of the reaction system to promote the reaction quickly. Compared with low-temperature solid-phase reaction, the high temperature ball milling method not only decreased the solid synthesis temperature from650to550℃, but also enhanced the electrochemical properties. The high temperature ball milling method thus opens a new path to solid state synthesis, especially for the lithium ion battery materials.6. The composite Zn2SnO4/PANI was synthesized through in-situ polymerization method and doped three kinds of different inorganic/organic acids. The affects such as the type of acid and dopant, aniline concentration on conductivity capabilities and electrochemical properties were studied. The results showed that the electrical conductivity was enhanced obviously due to the introduction of PANI to Zn2SnO4. The first discharge capacity of pure Zn2SnO4was about1662.4mAh/g, with a capacity retain of255.1mAh/g in the70th cycle at a constant current density of100mA/g in the voltage range of0.05～3.0V. Comparing to the pure some improved electrochemical properties were obtained for composite Zn2SnO4/PANI. Its first discharge capacity was about1675.5mAh/g, with a capacity retain of401.4mAh/g in the70th cycle.The kinetics behaviors of Zn2SnO4electrode were studied by means of chronoamperometry measurements. It is found that diffusion coefficient of lithium (DL) increase with Li intercalation into Zn2SnO4electrode. In addition, the diffusion cofficient of lithium of Zn2SnO4powders prepared by low-temperature solid-phase reaction and high temperature ball milling were4.26x10-12cm2·s-1and1.46×10-11cm2·s-1, respectively.The innovations of this paper are as follows:1. The structure and morphology of the as-prepared product and specimens taken from the electrodes after charging-discharging cycles are analyzed by X-ray diffraction, scanning electron microscopy, respectively. It was found that pulverization and agglomeration occur between the active particles after cycles and solid electrolyte interphase (SEI) film in a cycle of continuous thickening and dense during cycling.2. The composite Zn2SnO4/C was synthesized by carbothermic reduction. It showed that the carbon component could buffer the volume expansion of Li-Sn and Li-Zn alloys and provide a highly conductive medium for electron transfer, improving the cycle capability of anode material Zn2SnO4.3. Nanosizeds Zn2Sn1-xTixO4powders were prepared by a hydrothermal method. In the charge-discharge cycles, the structure of Ti-O bond separated active centers (Sn or Zn) from each other, which avoided or reduced the aggregation of metal Zn and Sn. In addition, it could reduce the extent of poor conductive contact caused by the pulverization of electrode materials, and then improve the reversibility of electrode reaction. As a result, the cycle performance of anode material was improved.4. Novel composite Zn2Sn1-xTix04/C(x=0,0.1,0.2and0.3) materials were prepared. Comparing to the pure Zn2SnO4, some improved electrochemical properties were obtained for Zn2Sn1-xTixO4, Zn2Sn4/C and composite Zn2Sn1-xTixO4/C. The composite Zn2Sno.9Ti0.1O4/C showed the best electrochemical properties, and its first discharge capacity was about1551.8mAh/g, with a capacity retain of496.6mAh/g in the100th cycle.5. The larger capacity and better cyclability of the composite Zn2Sn0.8Ti0.2O4/C electrode were attributed to composite structure consisting of active Zn, Sn and TiO2, and inactive LiTiO2in the carbon matrix during the cycling process. 6. Nanosized Zn2SnO4powder was first synthesized by high temperature ball milling method.In the high temperature ball milling process, the cooperation action of milling and temperature, and then significantly increased the activation energy of the reaction system to promote the reaction quickly. Compared with low-temperature solid-phase reaction, the high temperature ball milling method not only decreased the solid synthesis temperature from650to550℃, but also enhanced the electrochemical properties. The high temperature ball milling method thus opens a new path to solid state synthesis, especially for the lithium ion battery materials. |
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