| Anthracite-based microcrystalline graphite in Jarud Banner of Inner Mongolia has a high quality.Through an ultrafine pulverization,the grains are slight and uniform,with an isotropic distribution.So the specific capacity of microcrystalline graphite is higher than theoretical specific capacity of graphite anode materials.However the high impurity content in natural microcrystalline graphite results in the poor structural stability and electrochemical performance of lithium-ion batteries.Therefore,in order to overcome these problems,the deep processing of microcrystalline graphite is needed to enhance the purity and change the structure.In this paper,ultrafine microcrystalline graphite powder is used as raw material.We systematically study the purification process of microcrystalline graphite and the effects of coating,oxidation and composite modification on microcrystalline graphite structure and lithium storage performance at normal and low temperature.Reveal the purification and modification mechanism of microcrystalline graphite.Based on the first principle calculation and experimental research,the structural characteristics of microcrystal graphite before and after modification are obtained,and the diffusion behavior of lithium ion in microcrystal graphite is observed.The process of emulsion kerosene flotation and mixing with acid is used to purify the natural microcrystalline graphite superfine powder.After emulsion kerosene flotation,the fixed carbon of microcrystalline graphite increases from 49.5%to 93.5%.The microcrystalline graphite PMG4 purified by HF-H2SiF6 method has the best electrochemical performance in lithium-ion battery.This purified microcrystalline graphite has a fixed carbon content of 99.0%.The first reversible specific capacity of PMG4 is 477.4 mAh·g-1,and the first coulomb efficiency is 61.3%.After fifty cycles,the reverse capacity retention rate is 10.2%,much higher than that of the flotation sample,and the impedance value is significantly reduced.The structure analysis shows that the impurity content of the microcrystalline graphite after purification is significantly reduced,and the layer spacing of the microcrystalline graphite after purification is increased due to the oxidation of the acid.Different mass fraction asphalt carbon coating is used to modify properties of PMG4.The results show that the mass fraction of pitch carbon is 9%,the electrochemical performance is best.The first reversible specific capacity is 299.1 mAh·g-1,and the first coulomb efficiency is 75.4%,which is 14.1%higher than that of uncoating.Moreover,cyclic and rate properties are better,meanwhile the impedance value decreases significantly.Compared with microcrystal graphite,asphalt carbon has higher disorder and smaller size,which makes the carbon atoms of microcrystal graphite rotating or translating arbitrarily,the plane size of layer decreasing,and the interlayer spacing increasing.Hummers oxidation method is used to modify properties of PMG4 to obtains PMGO1,then PMGO1 is coated with 9%pitch carbon to obtain PMGO2.The results show that the first reversible specific capacity of PMGO1 is 560.2 mAh·g-1,which is 82.8 mAh·g-1 higher than that of PMG4.The first coulomb efficiency is 62.2%,which is 0.9%higher than that of PMG4.The rate performance of PMGO2 is the best.After oxidation,the oxygen-containing functional groups of samples increase,and the functional groups recombine,so the layer spacing of microcrystal graphite increases.Due to the oxidation,the PMGO1 boundary appears fold structure,and local pores appear.When the asphalt carbon is coated,the pore structure of PMGO2 basically disappears and the coating surface is smoother due to the filling effect of asphalt carbon.Based on the First Principle,the structure of microcrystal graphite before and after asphalt carbon coating or oxidation,the optimized total energy,interlayer spacing,binding energy,diffusion energy barrier and differential charge density are calculated from microscopic angle.The results show that in the same structure,different asphalt coating position,different oxidation position and different lithium storage position,the optimized total energy is lower,the structure is more stable.The larger interlayer spacing is,the lower the diffusion energy barrier is,which indicates that lithium ion is easily diffuse in microcrystal graphite.The binding energy is lower,lithium ion deintercalation is easier in microcrystal graphite.The charge distribution density indicates the combination of lithium and microcrystal graphite.A new composite material of oxidized microcrystal graphite-nanosilicon-stannic hydroxide is prepared by mechanical grinding methods,when PMGO1 is composited modification.The results show that the first reversible specific capacity of the composite is 590.2 mAh·g-1,and the reversible capacity retention rate is 62.8%after 50 cycles,whitch is 2.3%higher than that of PMGO1.When the current density is 800 mAh·g-1,the rate performance is 32.3%higher than that of PMGO1.So the cycle and rate performance are well.Due to the influence of silicon,the crystallinity of the composite increases.Spherical particles of nanosilicon and micro-nano size tin hydroxide distribute on the PMGO1 surface of irregular polygons.We study lithium storage properties of microcrystalline graphite before and after purified and oxidized modification at low temperature.The results show that PMGO1 has the best cycle performance at 0?.The first reversible specific capacity is 297.7 mAh·g-1.After 50 cycles,the reversible capacity retains 95.7%of the first reversible specific capacity.At-20?,the cycle performance of PMGO1+Si+Sn(OH)4 is the best.The first reversible capacity is 119.1 mAh·g-1.After 50 cycles,the reversible capacity is 96.6%of the first reversible specific capacity,and the rate performance of PMGO1 is the best.With the decrease of temperature,the impedance value of the same anode material increases,and the reversible capacity and rate performance decrease gradually.There are 69 graphs,21 tables and 155 references in this paper. |
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