Method for reducing magnetism of high-magnetism substance lithium iron phosphate

Abstract

The invention discloses a magnetism reducing method of high-magnetism lithium iron phosphate, and belongs to the technical field of preparation of lithium ion battery positive electrode materials. The process comprises the following steps of: uniformly mixing a high-magnetism lithium iron phosphate unqualified product generated in the conventional lithium iron phosphate industrial production process or electromagnetic slag generated by demagnetizing in the crushing process as a raw material with a powdery composite organic carbon source, putting the mixture into a bowl again, setting the highest temperature of a kiln to be 450-750 DEG C, heating again in a nitrogen atmosphere, and carrying out secondary sintering, so as to obtain the high-magnetism lithium iron phosphate product. The magnetic substance content of the material is remarkably reduced, so that the performance requirement of the lithium battery positive electrode material is met, the carbon layer can be repaired, the rate capacity and other performance can be improved, and the application prospect is good.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium ion battery positive electrode material preparation, and in particular relates to a method for demagnetizing highly magnetic lithium iron phosphate. Background Art

[0002] Lithium iron phosphate, chemical formula is LiFePO 4 (LFP for short) is an inorganic compound with an orthorhombic olivine structure, mainly used in various lithium-ion batteries. As an electrode material for lithium-ion batteries, lithium iron phosphate has a low tap density and a low voltage platform, but its high safety, environmental protection, and long life are very significant, which is difficult to match with other positive electrode materials, especially its safety is highly respected. In addition, the chemical structure of lithium iron phosphate is stable, and it will not produce free oxygen even in the case of overcharge, and it will not react with the electrolyte. Therefore, lithium iron phosphate batteries are also truly safe lithium-ion batteries at present.

[0003] However, in order to further improve battery safety and prevent potential problems such as battery short circuit and fire, the market has increasingly stringent requirements on the magnetic content of lithium iron phosphate cathode materials. Unfortunately, during the production process, some materials inevitably exceed the magnetic content due to some abnormal problems. These materials become waste because they cannot meet market demand, which in turn increases production costs and waste volume. Summary of the invention

[0004] In order to solve the problems raised by the background technology, the purpose of the present invention is to provide a method for demagnetizing highly magnetic lithium iron phosphate. The present invention reduces the magnetic content of the lithium iron phosphate positive electrode material by mixing the lithium iron phosphate waste that cannot be sold due to high magnetic content in the industrial production process with a powdered composite organic carbon source and then heating it again for secondary sintering. The process method provided by the present invention is simple and highly operable. On the basis of effectively reducing the magnetic content of the positive electrode material, it can also improve the performance such as rate capacity.

[0005] In order to effectively reduce the magnetic content of lithium iron phosphate waste, the present invention adopts the following technical solution: The present invention provides a method for demagnetizing lithium iron phosphate, a highly magnetic material, comprising the following steps: Step 1: Take highly magnetic lithium iron phosphate waste or electromagnetic slag and process it into powder for later use; Step 2: Adding composite organic carbon to the lithium iron phosphate waste residue powder obtained in step 1 and mixing them evenly to obtain a mixed powder; Step 3: gradually heating the mixed powder obtained in step 2 to 450-750° C. for sintering under a protective atmosphere, and obtaining a demagnetized lithium iron phosphate product after taking it out of the furnace.

[0006] Furthermore, the highly magnetic lithium iron phosphate waste or electromagnetic slag in step 1 has a magnetic content of not less than 1 ppm, a carbon content of 1.2%-1.5%, a resistivity of not more than 30 Ω·cm, and an initial discharge of not less than 150 mAh / g.

[0007] Furthermore, the compacted density of the lithium iron phosphate waste powder obtained in step 1 is not less than 2.40 g / cm 3 , D50 is 1.0-1.5 μm.

[0008] Furthermore, the composite organic carbon in step 2 consists of glucose, sodium methoxide and needle coke.

[0009] Furthermore, the composite organic carbon is composed of glucose, sodium methoxide, and oil-based needle coke mixed in a mass ratio of 10:(3-5):(1-2).

[0010] Furthermore, the amount of the composite organic carbon added in step 2 is 1%-2% of the mass of the lithium iron phosphate waste slag powder.

[0011] Furthermore, the heating rate of the sintering in step 3 is 1-15°C / min; and the sintering time is 4-8 h.

[0012] Furthermore, the magnetic content of the demagnetized lithium iron phosphate product obtained in step 3 is less than 1 ppm.

[0013] The beneficial effects of the present invention are: The invention mixes unqualified lithium iron phosphate products with high magnetic properties produced in the conventional lithium iron phosphate industrial production process or electromagnetic slag produced in the demagnetization process with a powdered composite organic carbon source and re-fills the mixture. The maximum temperature of the kiln is set to 450°C-750°C. The kiln is heated again in a nitrogen atmosphere for secondary sintering, so that the particles undergo secondary crystallization during the heating process, and part of the unreduced Fe in the high magnetic lithium iron phosphate waste after the primary sintering is removed. 3+ Reduction to Fe 2+ , and the residual Li + The reaction continues, and some magnetic substances such as iron phosphide are combined and recrystallized to form lithium iron phosphate materials. At the same time, the selected composite organic carbon can repair and further improve the carbon layer destroyed during the heating process, forming a lithium iron phosphate material with a more complete structure, rounded particles and complete carbon layer, thereby improving the physical and chemical properties of the material and meeting the requirements for shipping high-quality low-magnetic finished products. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 SEM images of the lithium iron phosphate products before and after calcination in Examples 1-6 of the present invention. DETAILED DESCRIPTION

[0015] In order to make the purpose, technical scheme and advantages of the present invention clearer, the technical scheme of the present invention will be clearly and completely described below in conjunction with the embodiments. If the specific conditions are not specified in the embodiments, they are carried out according to the conventional conditions or the conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments is not specified, they are all conventional products that can be purchased commercially.

[0016] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more related listed items.

[0017] Example 1 A method for demagnetizing highly magnetic lithium iron phosphate, comprising the following steps: 1. The high magnetic lithium iron phosphate waste material used in this embodiment has the appearance of black powder, the primary particles are spherical, the magnetic content is 15.512 ppm, the carbon content is 1.34%, the resistivity is 14.8 Ω·cm, and the first discharge is 161.3 mAh / g; the waste material is processed and ground (D50 is 1.001 μm, compacted density is 2.44 g / cm 3 ) for later use.

[0018] 2. Add 1.5% by weight of composite organic carbon (glucose, sodium methoxide, and oil-based needle coke mixed in a mass ratio of 10:4:1) to the obtained lithium iron phosphate waste residue powder and mix evenly to obtain a mixed powder for later use.

[0019] 3. The obtained mixed powder is transferred into a furnace, heated to 450°C at a rate of 10°C / min under a nitrogen atmosphere, and sintered for 4 hours. After sintering, the mixed powder is taken out of the furnace to obtain demagnetized lithium iron phosphate.

[0020] Embodiment 2-6 With reference to the step parameters of Example 1, the sintering parameters of step 3 were adjusted to sinter at 550°C for 4 h, sinter at 650°C for 4 h, sinter at 750°C for 4 h, sinter at 450°C for 8 h, and sinter at 750°C for 8 h, respectively, to obtain Examples 2-6.

[0021] Figure 1 The SEM images of the finished lithium iron phosphate products before and after the second sintering of Examples 1-6 of the present invention are shown. It can be seen that the primary particles of the material after the second sintering after adding the selected composite carbon source of the present invention are more rounded, and there are some agglomerates that are not completely melted in the first sintering material. After the second sintering, more remaining agglomerates are melted and converted into more large single crystals. Some primary particles absorb energy again and the particles are more rounded after the second crystallization, and the carbon layer coating is more complete, which shows that the material structure is more perfect after sintering.

[0022] Comparative Example 1 Refer to the step parameters of Example 5, except that the sintering parameters of step 3 are changed to sintering at 400°C for 12 h.

[0023] Comparative Example 2 Refer to the step parameters of Example 5, the difference is that in step 2, glucose is used instead of composite organic carbon to be mixed with lithium iron phosphate waste residue powder.

[0024] Comparative Example 3 Refer to the step parameters of Example 5, the difference is that in step 2, the composite organic carbon composed of glucose and sodium methoxide mixed in an equal mass ratio is mixed with the lithium iron phosphate waste residue powder.

[0025] Comparative Example 4 Refer to the step parameters of Example 5, except that in step 2, the amount of composite organic carbon added is adjusted to 0.8% of the mass of the lithium iron phosphate waste residue powder.

[0026] The performance of the products prepared in the examples and comparative examples was tested (testing standard GB / T 41704-2022), and the results are shown in Table 1.

[0027] Table 1 Magnetic content and physical and chemical properties of materials

[0028] As can be seen from Table 1, the process of the present invention can effectively reduce the magnetic content of the material while improving the rate capacity performance; when the constant temperature time is 4 h, the compaction change of the material after secondary sintering is small, but the magnetic content gradually decreases with the increase of temperature and the capacity performance is also improved compared with the original waste material; when the temperature is ≥650℃, the magnetic content is reduced to <1 ppm, which meets the current requirements of the lithium iron phosphate product industry. When the constant temperature time is 8 h, sintering at 450℃ can meet the requirements of magnetic content; when the temperature is ≥750℃ and the constant temperature time is 8 h, the material resistance begins to increase, which may affect the product rate performance, so it is not suitable to use a higher temperature for sintering; and when the sintering temperature is low (400℃), even if the sintering treatment time is extended, the efficiency of reducing the magnetic content is low, and it is not adopted for cost considerations. In addition, the composition and ratio of the composite organic carbon not only affect the demagnetization effect to a certain extent, but also can adjust the capacity and rate performance of the material; the composite organic carbon selected by the present invention significantly improves the rate capacity performance of the material, while the conventional organic carbon source has little effect on the rate performance of the material. The present invention reheats the high magnetic lithium iron phosphate waste for secondary sintering, so that the particles undergo secondary crystallization during the heating process, and part of the unreduced Fe in the high magnetic lithium iron phosphate waste after the primary sintering is removed. 3+ Reduction to Fe 2+ , and the residual Li +Continue to react and recrystallize to form lithium iron phosphate, which may contain some magnetic substances such as iron phosphide. At the same time, composite organic carbon is used to repair and improve the carbon layer destroyed in the secondary heating process, so that the degree of graphitization is higher, forming a lithium iron phosphate material with a more complete structure, rounded particles and complete carbon layer, maintaining the physical and chemical properties of the material, and meeting the requirements for the shipment of high-quality low-magnetic finished products. The present invention effectively utilizes waste materials in the industry. Through the process method of the present invention, not only can the high-magnetic lithium iron phosphate unqualified products be demagnetized, but also the material performance is further improved. While saving energy and protecting the environment, high-quality low-magnetic lithium iron phosphate products are produced, which greatly reduces the overall process cost of the industrial production of lithium iron phosphate and has good market competitiveness.

[0029] The embodiments described above only express several preferred embodiments of the present invention, and the descriptions thereof are relatively specific and detailed, but are not intended to limit the present invention. It should be noted that for those skilled in the art, the present invention may also have various changes and modifications, and any modifications, equivalent substitutions, improvements, etc. made within the concept and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for demagnetizing highly magnetic lithium iron phosphate, characterized in that: The steps include: Step 1: Take highly magnetic lithium iron phosphate waste or electromagnetic slag and process it into powder for later use; Step 2: Adding composite organic carbon to the lithium iron phosphate waste residue powder obtained in step 1 and mixing them evenly to obtain a mixed powder; Step 3: gradually heating the mixed powder obtained in step 2 to 450-750° C. for sintering under a protective atmosphere, and obtaining a demagnetized lithium iron phosphate product after taking it out of the furnace.

2. The method for demagnetizing lithium iron phosphate, a highly magnetic material, according to claim 1, characterized in that: The highly magnetic lithium iron phosphate waste or electromagnetic slag in step 1 has a magnetic content of not less than 1 ppm, a carbon content of 1.2%-1.5%, a resistivity of not more than 30 Ω·cm, and an initial discharge of not less than 150 mAh / g.

3. The method for demagnetizing lithium iron phosphate, a highly magnetic material, according to claim 1, characterized in that: The compacted density of the lithium iron phosphate waste powder obtained in step 1 is not less than 2.40 g / cm 3 , D50 is 1.0-1.5 μm.

4. The method for demagnetizing lithium iron phosphate, a highly magnetic material, according to claim 1, characterized in that: In step 2, the composite organic carbon consists of glucose, sodium methoxide and needle coke.

5. The method for demagnetizing lithium iron phosphate, a highly magnetic material, according to claim 4, characterized in that: The composite organic carbon is composed of glucose, sodium methoxide and oil-based needle coke in a mass ratio of 10:(3-5):(1-2).

6. The method for demagnetizing lithium iron phosphate, a highly magnetic material, according to claim 1, characterized in that: The amount of the composite organic carbon added in step 2 is 1%-2% of the mass of the lithium iron phosphate waste slag powder.

7. The method for demagnetizing lithium iron phosphate, a highly magnetic material, according to claim 1, characterized in that: The sintering temperature rise rate in step 3 is 1-15°C / min, and the sintering time is 4-8 h.

8. The method for demagnetizing lithium iron phosphate, a highly magnetic material, according to claim 1, characterized in that: The magnetic content of the demagnetized lithium iron phosphate product obtained in step 3 is less than 1 ppm.

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