A method for preparing carbon microspheres and their application as lithium-ion battery electrode material

Abstract

The present invention utilizes a method for preparing mesophase carbon microspheres in an organic system, which is different from the traditional thermal polycondensation method, emulsification method and suspension method. Instead, it utilizes the good dispersing effect of molten salt on condensed aromatic hydrocarbons such as asphalt and naphthalene to propose a method for preparing carbon microspheres in an inorganic system, which is molten salt. The carbon microspheres prepared by this method can be separated from the polymerization system by simple water washing and acid washing, and the range of raw materials to be selected is wide. The prepared carbon microspheres have a typical mesophase structure, regular morphology, and a diameter between 1 and 10 μm. After high-temperature carbonization, they are applied to lithium-ion battery negative electrode materials and exhibit high reversible capacity, and a discharge specific capacity of 250 to 350 mA at a current density of 100 mA / g.

Description

Technical Field

[0001] The present invention relates to the field of lithium ion battery negative electrode materials, and in particular to a method for preparing pitch-based carbon microspheres in a molten salt system and an application of the microspheres as lithium ion battery electrode materials. Technical Background

[0002] Asphalt is a heavy residue obtained after distillation in the petrochemical and coal chemical industries. Its complex composition, especially coal tar pitch, contains a high amount of ash and is difficult to dissolve in many common organic solvents, severely limiting its processing. Mesophase pitch, an important carbon material precursor prepared from asphalt, is a key raw material. When coal tar pitch or petroleum tar pitch is heated to a certain temperature in an oxygen-free environment, the polycyclic aromatic hydrocarbon molecules in the asphalt begin to dehydrogenate and condense, gradually growing into large planar molecules. At this point, the planar molecules of varying sizes spontaneously arrange themselves into an anisotropic structure and precipitate from the parent phase, forming a new phase called mesophase pitch. Initially, these new phases, driven by surface tension, assume spherical shapes called mesophase globules. As the degree of polymerization increases, the globules grow larger, eventually forming a fused mesophase pitch.

[0003] Mesophase pitch has become a high-quality precursor for many functional carbon materials due to its excellent properties. In particular, mesophase carbon microspheres, after graphitization, are excellent lithium-ion battery anode materials. Currently, the main methods for preparing mesophase carbon microspheres include: thermal polycondensation [Honda H, Yamada Y, Oi S et al. Shapes of Meso-Carbon Microbeads [J]. Tanso, 1973 (72): 3–7.], emulsion method [Song Qianqian. Preparation of mesophase pitch carbon microspheres by emulsion method and study on electrochemical properties [D]. Beijing University of Chemical Technology, 2013.] and suspension method [Wang Wenzhi. Study on preparation of mesophase pitch microspheres by suspension method [D]. Northwestern Polytechnical University, 2006.]. The thermal polycondensation method involves directly heat-treating the raw asphalt or heat-treating it after adding a catalyst. However, its disadvantage is that the resulting mesophase spheres often quickly begin to melt and grow into bulk mesophase. Because the mesophase spheres are formed within the asphalt bulk, and because the reacted asphalt contains a high content of heavy components that are often difficult to dissolve in organic solvents such as pyridine, it is difficult to completely separate the resulting mesophase spheres from the isotropic parent asphalt. The emulsion method grinds the mesophase asphalt as a raw material, mixes it with an emulsifier such as silicone oil, and heats it to liquefy the mesophase asphalt and form spheres under the action of surface tension. The suspension method uses soluble mesophase asphalt as a raw material, dissolves it in a solvent such as pyridine, and then evaporates the solvent to precipitate the mesophase asphalt into spheres. The disadvantages of preparing mesophase carbon microspheres using the emulsion and suspension methods are high raw material requirements and complex processes.

[0004] Currently, there are three main types of carbon materials prepared using molten salts. The first type is to directly put the carbon source into the molten salt for carbonization, such as [Liu X, Giordano C, Antonietti M.A Facie Molten-Salt Route to Graphene Synthesis[J]. Small, 2014, 10(1):193-200.], where glucose is put into a mixed molten salt of lithium chloride and potassium chloride, and a small amount of graphene is obtained after carbonization at 800°C. The second type is to prepare carbon materials by electrolytically corroding graphite electrodes using molten salt as a medium, such as [Kamali AR, Fray D J. Towards largescale preparation of carbon nanostructures in molten LiCl[J]. Carbon, 2014, 77:835-845.], where graphite is used as both cathode and anode and molten anhydrous lithium chloride is used as the electrolyte to prepare carbon nanospheres and carbon nanotubes. The third type uses molten salt as an electrolyte and uses an inert electrode to electrolyze and capture carbon dioxide in the air as a carbon source to prepare carbon materials. For example, [Jiawen, Ren, Fang-Fang, et al. One-Pot Synthesis of Carbon Nanofibers from CO2[J]. Nano Letters, 2015.] uses lithium carbonate-lithium oxide mixed molten salt as an electrolyte to deposit graphene and multilayer graphite on the cathode through electrolysis.

[0005] The method for preparing mesophase carbon microspheres invented in this patent is characterized by utilizing the dispersion phenomenon of polycyclic aromatic hydrocarbons such as asphalt in some mixed molten salts to achieve an effect similar to dissolution, so that the asphalt is polymerized in the molten salt to form a mesophase and precipitate into mesophase carbon microspheres. The whole process is simple to operate. Mesophase carbon microspheres can be generated by simply mixing the raw asphalt and mixed salt in a certain proportion and then passing a certain heating procedure. Finally, the solidified salt and carbon microspheres can be separated by washing the obtained product with deionized water and acid washing. Carbon microspheres with a diameter of about 5 microns are obtained, and they maintain a good morphology. Summary of the Invention

[0006] In view of the above problems of the prior art, the present invention aims to provide a method for preparing pitch-based carbon microspheres under a new system, which is prepared according to the following method:

[0007] Step 1: Mix an appropriate amount of raw asphalt with a certain proportion of mixed molten salt to obtain a mixture, and charge the mixture into a reactor under an inert atmosphere.

[0008] Step 2: Heat the reactor prepared in step 1 to a certain temperature and keep it warm for a certain time; after the insulation is completed, take out the reacted material.

[0009] Step 3: The material obtained in step 2 is washed with deionized water, acid-washed for a certain period of time, filtered, washed, and dried to obtain the final carbon microsphere product.

[0010] The raw material asphalt described in the present invention is selected from medium-temperature coal pitch, naphthyl pitch, petroleum pitch, mesophase pitch, and naphthalene; one of the mixed molten salts is selected from aluminum chloride, and the other is one of lithium chloride, zinc chloride, sodium chloride, potassium chloride, ferric chloride, magnesium chloride, and calcium chloride; and the acid is selected from one of hydrochloric acid, nitric acid, and sulfuric acid.

[0011] A further preferred embodiment of the present invention involves determining an appropriate holding temperature and holding time. A temperature that is too low prevents the raw materials from polymerizing and precipitating into spheres. A temperature that is too high causes the mixed salt to sublime violently, leading to turbulence and reduced sphericity. A holding time that is too short results in an insufficient reaction, resulting in only a small fraction of the product being spheres, with the remainder being irregular particles. A holding time that is too long causes fusion between the spheres, reducing sphericity. Therefore, a holding temperature of 350-600°C and a holding time of 0.5-20 hours are selected.

[0012] In addition, the present invention also provides a lithium ion battery electrode material prepared from pitch-based carbon microspheres.

[0013] The present invention is different from the traditional thermal polycondensation method, emulsion method and suspension method for preparing mesophase carbon microspheres. Instead, it uses a molten salt system to disperse the raw materials in the molten salt on a microscopic scale, and polymerize and precipitate into spheres in the molten salt. Due to the isolation of the molten salt, the precipitated small spheres are not easy to merge, thereby obtaining carbon microspheres with good sphericity. The raw materials used in this method are ordinary asphalt instead of mesophase asphalt, and the prepared carbon microspheres are easy to separate from the parent phase. The prepared carbon microspheres have a diameter distribution of 1 to 10 microns and a regular morphology. After high-temperature carbonization, they show excellent performance in lithium-ion battery electrode materials; they can maintain a specific capacity of 250 to 350 mAh / g after 50 cycles at a current density of 100 mAh / g. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Attachment Figure 1 This is the SEM image of the product obtained by keeping medium-temperature coal tar pitch in a mixed molten salt of aluminum chloride and sodium chloride at 450℃ for 8h.

[0015] Attachment Figure 2 This is an SEM image of the cross section of carbon microspheres obtained by medium-temperature coal tar pitch in a mixed molten salt of aluminum chloride and sodium chloride.

[0016] Attachment Figure 3This is the SEM image of the product obtained by keeping medium-temperature coal tar pitch in a mixed molten salt of aluminum chloride and sodium chloride at 450℃ for 4h.

[0017] Attachment Figure 4 This is the SEM image of the product obtained after naphthalene was kept at 400℃ in a mixed molten salt of aluminum chloride and sodium chloride for 6 hours and carbonized at 1500℃ for 2 hours. DETAILED DESCRIPTION

[0018] The present invention is described in detail below with reference to the accompanying drawings and embodiments:

[0019] Example 1

[0020] Mix 0.5 g of medium-temperature coal tar, 6.09 g of sodium chloride, and 13.91 g of aluminum chloride and place them in a reactor. Place the reactor under an inert atmosphere to ensure an inert atmosphere. Place the reactor in a heating device and heat it to 450°C at a rate of 5°C per minute and maintain it for 8 hours. After the reaction is complete, remove the reaction product, dissolve it in deionized water, and filter it. The filtered product is then acid-washed by soaking it in dilute hydrochloric acid for 24 hours. After acid washing, filter it again and rinse it four times in a filtration device with deionized water. Finally, dry the filtered product to obtain the final product. It is a regular sphere with a size between 1 μm and 10 μm.

[0021] As attached Figure 1 The SEM images show that the obtained carbon microspheres have high sphericity and uniform diameter distribution.

[0022] As attached Figure 2 The SEM images shown show that the cross-sectional texture of the carbon microspheres exhibits a typical mesophase carbon microsphere structure.

[0023] The obtained product was placed in a high temperature carbonization furnace and heated in an argon atmosphere for carbonization at a heating rate of 5 °C min -1 The temperature was raised to 1500℃ and kept for 2h before the process ended, and the electrochemical properties of the carbonized product were tested.

[0024] The electrochemical performance test results show that it still has a specific capacity of 273.5 mAh / g after 50 cycles at a current density of 100 mA / g.

[0025] Example 2

[0026] Mix 0.5 grams of medium-temperature coal tar, 6.09 grams of sodium chloride and 13.91 grams of aluminum chloride evenly and put them into a reactor. Install the reactor under an inert atmosphere to ensure that the atmosphere inside the reactor is an inert atmosphere. Place the reactor in a heating device and heat it to 450°C at a rate of 5°C per minute and keep it warm for 4 hours. After the reaction is completed, take out the reaction product, dissolve it with deionized water, and filter it. The filtered product is soaked in dilute hydrochloric acid for 24 hours for pickling. After the pickling is completed, filter it again and rinse it with deionized water in the filtration device 4 times. Finally, dry the filtered product to obtain the final product. The obtained product is as shown in the attached Figure 3 shown

[0027] As attached Figure 3 The SEM image shows the morphology of the product pitch-based carbon microspheres.

[0028] The obtained product was placed in a high temperature carbonization furnace and heated in an argon atmosphere for carbonization at a heating rate of 5 °C min -1 The temperature was raised to 1500℃ and kept for 2h before the process ended, and the electrochemical properties of the carbonized product were tested.

[0029] The electrochemical performance test results show that it still has a specific capacity of 275.6 mAh / g after 50 cycles at a current density of 100 mA / g.

[0030] Example 3

[0031] Mix 1 gram of medium-temperature coal tar, 6.09 grams of sodium chloride and 13.91 grams of aluminum chloride evenly and put them into the reactor. Place the reactor under an inert atmosphere to ensure that the atmosphere inside the reactor is inert. Preheat the heating device to 550°C, place the reactor in the heated heating device and keep it warm for 0.5 hours before ending the reaction. After the reaction is completed, take out the reaction product, dissolve it in deionized water and filter it, and soak the filtered product in dilute hydrochloric acid for 24 hours for acid washing. After the acid washing is completed, filter it again and rinse it in the filtration device with deionized water 4 times. Finally, dry the filtered product to obtain the final product. Place the obtained product in a high-temperature carbonization furnace and heat it in an argon atmosphere for carbonization at a heating rate of 5°C min -1 The temperature was raised to 1500℃ and kept for 2h before the process ended, and the electrochemical properties of the carbonized product were tested.

[0032] The electrochemical performance test results show that it still has a specific capacity of 266.4 mAh / g after 247 cycles at a current density of 100 mA / g.

[0033] Example 4

[0034] Mix 0.5 g of naphthalene, 6.09 g of sodium chloride and 13.91 g of aluminum chloride and place them in a reactor. Place the reactor under an inert atmosphere to ensure that the atmosphere inside the reactor is inert. Place the reactor in a heating device and heat it up at a rate of 5°C per minute, heat it to 400°C and keep it warm for 6 hours. After the reaction is completed, take out the reaction product, dissolve it in deionized water, and filter it. The filtered product is then soaked in dilute hydrochloric acid for 24 hours for acid washing. After the acid washing is completed, filter it again and rinse it 4 times in a filtration device with deionized water. Finally, dry the filtered product to obtain the final product. The obtained product is placed in a high-temperature carbonization furnace and heated in an argon atmosphere for carbonization at a heating rate of 5°C min -1 The temperature was raised to 1500℃ and kept for 2h to obtain the final product. The product morphology is shown in the attached figure. Figure 4 shown.

[0035] As attached Figure 4 The SEM image shown is the morphology of naphthyl carbon microspheres.

[0036] Example 5

[0037] Mix 0.5 g of petroleum asphalt, 6.09 g of sodium chloride and 13.91 g of aluminum chloride and put them into a reactor. Place the reactor under an inert atmosphere to ensure that the atmosphere inside the reactor is inert. Place the reactor in a heating device and heat it at a rate of 5°C per minute, heat it to 400°C and keep it warm for 6 hours. After the reaction is completed, take out the reaction product, dissolve it in deionized water and filter it, and soak the filtered product in dilute hydrochloric acid for 24 hours for acid washing. After the acid washing is completed, filter it again and rinse it in deionized water in the filtration device 4 times. Finally, dry the filtered product to obtain the final product. Place the obtained product in a high-temperature carbonization furnace and heat it in an argon atmosphere for carbonization at a heating rate of 5°C min -1 The product was placed in a high-temperature carbonization furnace and heated in an argon atmosphere for carbonization at a heating rate of 5°C min -1 The temperature was raised to 1500℃ and kept for 2h before the process ended, and the electrochemical properties of the carbonized product were tested.

[0038] The electrochemical performance test results show that it still has a specific capacity of 275.8 mAh / g after 247 cycles at a current density of 100 mA / g.

[0039] Example 6

[0040] Mix 0.5 grams of petroleum asphalt, 7.17 grams of potassium chloride and 12.83 grams of aluminum chloride and put them into the reactor. Place the reactor under an inert atmosphere to ensure that the atmosphere inside the reactor is inert. Place the reactor in a heating device and heat it at a rate of 5°C per minute, heat it to 450°C and keep it warm for 6 hours. After the reaction is completed, take out the reaction product, dissolve it in deionized water and filter it, and soak the filtered product in dilute hydrochloric acid for 24 hours for acid washing. After the acid washing is completed, filter it again and rinse it in deionized water in the filtration device 4 times. Finally, dry the filtered product to obtain the final product. The obtained product is placed in a high-temperature carbonization furnace and heated in an argon atmosphere for carbonization at a heating rate of 5°C min -1 The product was placed in a high-temperature carbonization furnace and heated in an argon atmosphere for carbonization at a heating rate of 5°C min -1 The temperature was raised to 1500℃ and kept for 2h before the process ended, and the electrochemical properties of the carbonized product were tested.

[0041] The electrochemical performance test results show that it still has a specific capacity of 315.8 mAh / g after 250 cycles at a current density of 100 mA / g.

[0042] The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited to the described embodiments. Those skilled in the art may make various equivalent modifications or substitutions without departing from the spirit of the present invention. These equivalent modifications or substitutions are all included in the scope defined by the claims of this application.

Claims

1. A method for preparing carbon microspheres in a molten salt system, characterized in that Prepared by the following method: Step 1: Mix an appropriate amount of raw asphalt with a certain proportion of mixed molten salt to obtain a mixture, and place the mixture into a reactor under an inert atmosphere; Step 2: Heat the reactor prepared in step 1 to a certain temperature and keep it warm for a certain period of time; after the insulation is completed, take out the reacted material; Step 3: The material obtained in step 2 is washed with deionized water and acid for a certain period of time, filtered, washed to remove the solidified mixed molten salt, and then dried to obtain the final carbon microsphere product; Wherein, one of the mixed molten salts is selected from aluminum chloride, and the other is one of lithium chloride, zinc chloride, sodium chloride, potassium chloride, ferric chloride, magnesium chloride, and calcium chloride.

2. The method for preparing carbon microspheres in a molten salt system according to claim 1, wherein: The raw material is selected from coal-based pitch, petroleum-based pitch, naphthyl pitch, naphthalene, anthracene, and phenanthrene.

3. The method for preparing carbon microspheres in a molten salt system according to claim 1, wherein: The reaction temperature is 350°C to 600°C.

4. The method for preparing carbon microspheres in a molten salt system according to claim 1, wherein: The reaction time is 0.5 to 20 hours.

5. A negative electrode material for a lithium ion battery prepared from the carbon microspheres obtained according to any one of claims 1 to 4.

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