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Preparation Of MnO2/Ni?OH?2 And CoNi2S4 Nanosheet Arrays And Their Performances For Lithium And Lithium-Sulfur Batteries

Posted on:2019-02-02Degree:MasterType:Thesis
Country:ChinaCandidate:Y L ZhaoFull Text:PDF
GTID:2321330542473686Subject:Engineering
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With the rapid economic and social development in our country,the issue of energy utilization and environmental protection has become even more important.In recent years,lithium-ion battery as a new type of secondary battery,by the scientific research workers of all ages.Due to their high theoretical capacity,transition metal oxides have become the research hotspots of electrode materials.They are typically represented by MnO2,Co3O4,Ni?OH?2,NiO and RuO.However,the low conductivity and the serious volume effect of the transition metal oxide lead to the low specific capacity of the material.The structure of the electrode material determines the electrochemical performance of the lithium ion battery.The nano-scale electrode material facilitates the electrolyte Full contact with the active material,shorten the ion transmission path,forming a stable structure of the physical structure.It can also be doped with other materials to form a composite material to improve the conductivity of the transition metal oxide to further optimize its electrochemical properties.Therefore,in this paper,MnO2/Ni?OH?2 nanosheet arrays were prepared by water bath method and CoNi2S4 nanosheet arrays were prepared by two-step hydrothermal method.Lithium-sulfur batteries are predicted to be the most promising lithium secondary batteries due to the theoretical discharge energy density of up to 2600 Wh kg-1 due to the specific discharge capacity of 1675 mAh g-1 for sublimated sulfur.However,the problems of the electrical insulation of sublimation sulfur and lithium sulfide,the growth of lithium dendrites and the shuttle effect caused by lithium polysulfide have hindered the industrialized production of lithium-sulfur batteries.In this thesis,MnO2/Ni?OH?2 and CoNi2S4 nano-sheets were coated on S nanocapsules by melt impregnation method to prepare the positive electrode material of lithium-sulfur battery.It is expected to improve the electrochemical performance of the lithium-sulfur battery and to study its structure and electrochemical properties analysis.The main contents of this thesis are as follows:?1?The MnO2/Ni?OH?2 nanosheet arrays were successfully prepared on the surface of nickel foam by water bath method.After 100 cycles,the average specific capacity of the lithium battery was maintained at 1052.2 mAh g-1,showing excellent cycling performance High specific capacity.An integrated electrode sheet is formed between the nanosheet and the foam nickel substrate,and the nanosheets are interlaced with each other to form a 3D network structure,thereby improving the structural strength of the material and helping to improve the cycling performance of the material.MnO2 and Ni?OH?2 are uniformly mixed and grow into nanosheets in the form of nano-domains,pores between 150 and 300 nm are formed between the nanosheets,and both sides of the nanosheets are in full contact with the electrolyte to shorten the diffusion path of Li+Improve the ionic conductivity of the composite material.The specific capacity retention of MnO2/Ni?OH?2nanosheets at more than 80%was maintained at different current densities,indicating that MnO2/Ni?OH?2 nanosheet arrays have excellent rate performance.?2?The CoNi2S4 nanosheets were directly grown on the surface of nickel foam by a two-step hydrothermal method.The purity of CoNi2S4 synthesized by XRD was higher than that of CoNi2S4nanosheets,120 nm,increasing the contact area with the electrolyte,CoNi2S4 nanosheets completely coated on the nickel foam surface.At different current densities,the Coulombic efficiencies of the first 50 cycles of CoNi2S4 remain above 95%.When the current density is increased to 2000 mA g-1,the specific discharge capacity of the material remains at 814.9 mAh g-1.A higher coulomb efficiency and reversibility.The charge and discharge curves of CoNi2S4nanocomposites were found to be 1506.2/1592.3 mAh g-1 at the first charge and discharge cycles,and 1168.3 mAh g-1 at the 50th charge and discharge cycles respectively,indicating that the material has a higher ratio capacity.The CV curves reveal that the CoNi2S4 nanomaterials have a high degree of reversibility.?3?The surface of MnO2/Ni?OH?2 composite nanosheets was coated with a sublimation sulfur layer with a thickness of20 nm by melt impregnation.Nanoplates formed between the pore size of100 nm pores,can increase the infiltration between the electrolyte and the composite material,making lithium-ion and charge transfer faster,after the electrochemical properties of the test found that the current density of 200 mA g-1.During the last 58 cycles,the battery had an average discharge capacity of506 mAh g-1 and showed a stable specific capacity at various current densities(200-1000 mA g-1)Material obtained higher rate performance and cycle stability.?4?The surface of CoNi2S4 composite was coated with a layer of30 nm sublimation sulfur by melt impregnation method.The phase characterization of the CoNi2S4 composite by XRD showed that the surface of CoNi2S4 composite was coated with sulfur.The SEM results showed that sulfur was uniformly adhered to the CoNi2S4 composite,The surface of the nanosheet forms a novel nanosheet structure of S-CoNi2S4-S with a thickness of about 100 nm,which has good binding capacity for sublimation of sulfur.At a current density of 200 mA g-1,the initial discharge capacity of the lithium-sulfur battery reached 579.7 mAh g-1.After 60 cycles,the battery maintained a Coulometric efficiency of 97.6%.
Keywords/Search Tags:Ni?OH?2, MnO2, Sulfur cobalt nickel, Lithium-ion Battery, Lithium-sulfur Battery, Nanosheets
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