| Rechargeable lithium-ion battery has become the power source of portable electronic devices in the past .But the shortage of lithium resources and large-scale demand for it has promoted the researchers to find alternatives to lithium-ion batteries,that is sustainable sodium ion Battery (SIB). Although sodium has the advantages of rich resources and cheap price, low dynamic performance and low energy density restrict its electrochemical properties. Among present sodium ion battery electrode materials, polyanionic electrode materials have attracted widespread attention.Based on the induced effect and the stronger electronegativity, sulfate polyanion materials achieve a higher redox potential. Therefore,the study of sulfate electrode materials has expanded the study scope of polyanion electrode materials.In this thesis, we adopted a simple low-temperature method to synthesis krohnkite type Na2Fe(SO4)2·2H2O. We construct a three-dimensional layered nanostructure for sulfates .Hydrous sulfate nanoparticles with a diameter of 50 ?100 nm were anchored on graphene nanosheets and stacked layer by layer to build a three- dimensional conductive network. We applied them to sodium ion battery system as well as lithium-ion battery system and found that the layered composites exhibit a better electronic conductivity and sodium - lithium ion intercalation capacity than the pristine one. It shows a reversible capacity of 71 mAh/g (vs.Na+/Na) and 69 mAh / g (vs. Li+/Li), respectively. In addition, layered composite materials also demonstrate the high rate capability. In 5C rate, the capacity retention rate is 81% in sodium ion battery system and 70% in lithium-ion battery system. Na2Fe(SO4)2·2H2O as the first dihydrated deintercalation compound for sodium ion battery materials, it also paves the way for the study of sulfate electrode materials .We incorporate sulphate materials with various forms of carbon, including 0D(acetylene carbon, AC), 1D (carbon nanotube, CNT) and 2D (graphene nanosheet, GA)(carbon content was controlled in the range of 0 to 30%). A comprehensive assessment is obtained based on their physicochemical properties and electrochemical properties: AC-matrix composites with a high specific surface and a porous structure have high ion diffusion capacity and high rate performance,but at the same time it suffers from high moisture sensitivity and low cycle characteristics. In contrast, with compacted structure,GA- based composites are easier to the rapid electrons transfer and inhibit the hydration reaction , in the same manner,its rate capability and ion diffusion capability is suppressed.Considering carbon utilization efficiency and chemical properties,We also get another comprehensive conclusion : when the carbon content is at the range of 1?10 wt.%,it is reasonable for all of the carbon structure network.In this thesis,a simple and effective method is employed to synthesis Na2Fe2(SO4)3/SWNT nanomaterials. The SEM and TEM images show Na2Fe2(SO4)3 nanoparticles with the size of 50 ?100nm were wrapped by sheet-walled nanotubes to form spindle -shaped nanoparticles. BET Test shows its porous structural and high surface area 64.135 m2/g as well as large pore volume 0.16cm3/g. Single-walled carbon nanotubes with three-dimensional porous conductive frame structure, which can ensure the high efficiency of electron / ion transfer and fast sodium ion intercalation / deintercalation capacity and it also benefit to electrochemical performance for sulphate. When the current density is increased to more than 1C,layered composite displays a significant higher capacity than the original sample. At 1C and 5C,the capacity of layered composite is 85.3 mAh/g and 74.9mAh/g,while the capacity of pristine material is only 80.1mAh/g and 52.3mAh/g.Morever,Na2Fe2(SO4)3/SWNT nanoparticles exhibit a high operating voltage ?3.8 V (vs.Na+/ Na) and good cycle stability,after 100 cycles the capacity retention rate was 92% at 5C. |
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