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Carbon Coated Lithium Iron Phosphate And Lithium Titanate,MnO2@carbon Polyhedral Electrode Preparation Of Materials And Performance Study Of Lithium Ion Batteries

Posted on:2020-04-15Degree:MasterType:Thesis
Country:ChinaCandidate:X Y PengFull Text:PDF
GTID:2381330572961827Subject:Engineering
Abstract/Summary:PDF Full Text Request
Cathode and Anode materials are the main factors affecting the performance of lithiumion batteries.The positive electrode material LiFePO4 has a neat olivine structure.From the structural point of view,it is advantageous for deintercalation of lithium ions,reversible capacity up to 170 mAh/g,abundant resource reserves,low price,non-toxic and harmless,etc.The positive electrode material is a very good choice,but the LiFePO4 material itself has poor ion and electron conductivity,and the lithium ion diffusion coefficient is low,which causes the performance of the battery to be greatly affected;the negative electrode material Li4Ti5O12 has a spinel structure.It has high battery safety,"zero volume"change during charging and discharging,high energy density and high coulombic efficiency,but the material ion diffusion coefficient is low,ion and electron conductivity are poor;MnO2 is used as the anode material of lithium battery.The advantages of high theoretical specific capacity,abundant resource reserves,and low price have attracted the attention of researchers.However,due to the low self-conductivity of materials and large volume changes,the material yield performance is poor.In order to improve the properties of materials such as LiFePO4,Li4Ti5O12 and MnO2,they are modified to improve the properties of the materials,and they are applied to the research of full cells.The main research contents of this paper are as follows:?1?The LiFePO4 material was prepared by hydrothermal method and was in the form of olivine.A layer of amorphous carbon was coated on the surface of LiFePO4 material by dopamine polymerization carbonization.The thickness of the carbon layer was about 5 nm.The material was subjected to electrochemical performance tests such as cyclic voltammetry,constant current charge and discharge,rate performance and cycle performance.Cyclic voltammetry showed that the LiFePO4/C composite had sharp oxidation/reduction peaks and reduced polarization.It shows that the coating of carbon material improves the electrical conductivity of the composite and enhances the electrochemical activity.The constant current charge and discharge test observed a longer charge and discharge platform,and the electrochemical reaction was very stable.In the cycle performance test,the composite has a first discharge specific capacity of 194.44 mAh/g,which is much higher than the theoretical capacity of LiFePO4 material;it is then maintained at a stable high specific capacity level.After200 cycles,the discharge specific capacity of the battery was 111.1 mAh/g,showing a high specific capacity and good cycle stability.The electrochemical performance of LiFePO4/C composites under full battery was further studied,and the effect of positive and negative active material capacity ratio on the performance of all-cell lithium battery was clarified.The full battery with a 1.2 times capacity ratio remains at a high specific capacity level for 200 charge and discharge cycles.The average discharge specific capacity is as high as 134.3 mAh/g.The full battery with a capacity ratio of 1.5 times has the smallest change in capacity at 200 cycles,showing the best cycle stability.?2?Li4Ti5O12?LTO?material was successfully prepared and carbon coated with chemical deposition and calcination to prepare LTO/C composite.The LTO material is spherical in shape,and the outer layer is uniformly coated with carbon material,and the thickness of the carbon layer is 5 nm to 10 nm.Electrochemical testing of constant current charge and discharge and cycle performance was carried out as a negative electrode material for lithium batteries.The constant current charge-discharge curve can clearly observe the long charging and discharging platform.The curve coincidence is high in the three cycles,and the internal reaction of the material is very stable.In the cycle performance test,the initial discharge specific capacity of the composite material is 186.9 mAh/g,charging.With a specific capacity of 204.9 mAh/g,the Coulomb efficiency is as high as 109.6%,which is higher than the theoretical capacity of the LTO material and exhibits a high initial coulombic efficiency.The charge-discharge ratio has been maintained at a high and stable level.After 200 cycles,the battery has a specific discharge capacity of 177.8 mAh/g,showing a high specific capacity and good cycle stability.The effects of LTO/C composites with 5%mass fraction of positive and negative electrodes on the electrochemical performance of LiFePO4/C and graphite composite cells were investigated.The comparison of cycle performance test and coulombic efficiency shows that the incorporation of LTO/C composites plays a certain role in the improvement of initial coulombic efficiency.Especially the incorporation of LTO/C composites in the negative electrode material is the most significant increase in the pre-Coulomb efficiency of the whole battery.?3?MnO2 nanosheets were grown on ZIF-67 derived amorphous C-compressed polyhedron by water bath method,and samples were analyzed by energy dispersive X-ray spectroscopy?EDS?,field emission scanning electron microscopy?FESEM?and transmission electron microscopy?TEM?.The phase composition and microscopic morphology,and the performance of lithium batteries were studied by constant current charge and discharge and cyclic voltammetry.The results show that the MnO2 nanosheet array has been successfully grown on ZIF-67-derived amorphous C polyhedron.The nanosheet is about 150 nm high and about 5-15 nm thick.In the current density 0.1 C,amorphous C polyhedron@MnO2 nanosheet array composite The material has a first discharge specific capacity of 944.6 mAh/g,and the discharge specific capacity remains at 625 mAh/g after 200 cycles;this indicates that the composite has a high specific capacity and good cycle performance,and is a viable lithium.Battery anode material.
Keywords/Search Tags:Lithium iron phosphate, Lithium titanate, Dopamine, Manganese dioxide, ZIF-67, Lithium ion half-cell, Lithium ion full battery
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