| In recent years,energy shortage and environmental pollution have triggered a huge demand for sustainable energy resources and energy storage devices.Among them,lithium-ion battery is favored by people because of its excellent performance in the field of energy storage.However,as the most commonly used graphite material for the negative electrode of lithium ion batteries,in recent years,due to the impact of raw materials and energy costs in the graphitization process,the price of high performance artificial graphite rises year by year,and natural graphite,as a strategic material,has been listed as a global supply risk material,which restricts the wider application of lithium ion batteries.Therefore,the search for low-cost graphite supply channels such as graphite waste material recycling has become a research focus.In fact,graphite materials are widely used in chemical,machinery,electronics industries and national defense.Especially in the super hard industry,graphite is the main precursor of synthetic diamond.However,more than 15 billion carats of diamonds are produced annually worldwide,while more than 100,000 to 200,000 tons of graphite waste is produced.At present,due to the lack of final use market,waste graphite is landfill and incineration,which will seriously threaten public health and environmental safety.Therefore,we consider that if these graphite wastes after high temperature and high pressure and metal-based catalyst treatment can be transformed into anode materials for lithium ion batteries in some ways that meet the market needs,it will be an important supplement to the current graphite anode materials market.However,no studies have been reported on this subject.Based on this,the graphite waste(DSG)generated in the synthetic process of industrial grade diamond was studied as follows.1.In order to obtain graphite in line with national standards,the purification method of residual metal catalyst in graphite waste is studied.On this basis,the relationship between the morphology,structure and electrochemical properties of natural graphite and graphite waste before and after impurity removal was studied.(1)The metal catalyst impurities in DSG are effectively removed through the combination of magnetic separation and electrolysis,so that the purity of purified graphite(MDSG)can reach 99.7%.From the characterization results of ICP-OES,it can be seen that the content of Fe impurity in MDSG meets the national standard of graphite carbon and can meet the requirements of lithium ion battery anode material.In addition,we found that DSG and MDSG have many defects,and some surfaces have through-hole structure.The edge of MDSG has a layer of flocculent about 10-100nm.(2)For the electrochemical performance test of natural flake graphite(NFG),DSG and MDSG,the three samples have similar specific capacities at 0.1C current density.However,the specific capacities of DSG and MDSG were respectively 3-4 times higher than NFG at4C-5C.At 10C,the specific capacity difference between NFG and DSG and MDSG is more than 3 times.The good performance of DSG and MDSG may be caused by the Li~+through hole structure,which reduces the transmission distance.Therefore,MDSG is suitable for charging and discharging at high rate.2.MDSG was oxidized in air environment,the relationship between its morphology,structure and electrochemical performance was studied,and the appropriate oxidation modification temperature was found.(1)It was found that the mass loss of MDSG was about 7.6wt%at 480℃and 3h,while the mass loss was more than 56wt%at 580℃because oxygen acted as a strong oxidant.Therefore,MDGD-580 is not suitable for mass production.In addition,compared with DSG and MDSG,the end face of DSG-480 becomes smooth and the surface shows more irregular through-hole structure,especially the end flocculent of DSG-480 sample disappears.The disappearance and defects of the end flocculent may be caused by oxygen attack of graphite under high temperature environment.(2)The electrochemical test showed that the initial discharge capacity of the sample electrode of MDGD-480 at 0.1C was 325 m Ah/g and 321 m Ah/g respectively,which was slightly higher than that of NFG.However,the MDGD-480 provides a high capacity of 125m Ah/g after 1200 cycles at 5 C,more than 7 times that of NFG.In addition,the ratio of the initial potential of NFG to MDSG-480 at 1C and 2C is 1.45 and 2.4,especially about 3.7 at5C.The good performance is due to the electrolyte stored in the through hole to shorten the migration distance of Li~+into the graphite layer and reduce the transport distance of lithium ions in the electrode.3.The relationship between the morphology,structure and electrochemical properties of MDSG coated polytetrafluoroethylene(PTFE)and sodium alginate(SA)was studied.(1)At 400℃argon,MDSG was carbonized with PTFE in different proportions(denoted as MDSG@PTFE-5,MDSG@PTFE-10 and MDSG@PTFE-15,respectively).With the increase of coating ratio,the graphitization degree of the three samples decreased.According to the results of IR,C-F bond exists in the sample.SEM showed that the surface morphologies of the three samples changed from dispersed to tight spherical structure to netted spherical structure with the increase of coating amount,and the specific surface area and pore volume showed an upward trend.(2)The electrochemical performance test of the three samples shows that the first coulomb efficiency of the three samples is 82.4%,88.2%and 71.6%respectively at 0.1C.The reason why the first Coulomb efficiency increases first and then decreases may be related to the fact that C-F bond is more stable than C-C bond.In addition,the three samples were provided 255,210,and 175 m Ah/g,respectively,after 500 cycles at 1C,and MDSG@PTFE-10 was more stable than the other two samples.(3)Under 1000℃argon,MDSG was carbonized with different proportions of SA(denoted as MDSG@SA-10,MDSG@SA-20 and MDSG@SA-30,respectively).With the increase of coating ratio,the graphitization degree of the three samples decreased.SEM test showed that the morphology of the three samples changed from sheet structure to network structure with the increase of coating amount,and the specific surface area and pore volume showed an upward trend.(4)The electrochemical performance test shows that the first coulomb efficiency of the three samples is 83.5%,83.3%and 64.7%respectively,and the first coulomb efficiency increases first and then decreases,which may be related to the rise of specific surface area and pore volume.In addition,the three samples were provided 325,310,and 270 m Ah/g,respectively,after 500 cycles at 1C,and MDSG@SA-20 was more stable than the other two samples. |
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