Preparation method and application of low-magnetic foreign matter and high-compaction lithium iron phosphate

Abstract

The invention discloses a preparation method and application of low-magnetic foreign matter and high-compaction lithium iron phosphate. The preparation method comprises the following steps: S1, mixing a lithium source, an iron source, a phosphorus source, a carbon source and water, grinding, drying, sintering in an inert atmosphere at 770-830 DEG C, and cooling to obtain a lithium iron phosphate material; and S2, placing the lithium iron phosphate material obtained in the step S1 in an inert atmosphere for secondary sintering, controlling the oxygen content to be less than 5ppm, and controlling the secondary sintering temperature to be 700-830 DEG C, so as to obtain the low-magnetic foreign matter and high-compaction lithium iron phosphate. The content of magnetic foreign matter iron phosphide in the prepared low-magnetic foreign matter and high-compaction lithium iron phosphate is lower than 50 ppb, the compaction density reaches 2.75 cm < 3 > / g or above, the 0.1 C discharge specific capacity is 163 mAh / g or above, and the 1C discharge specific capacity is 150 mAh / g or above.

Description

Technical Field

[0001] The present invention relates to the technical field of battery positive electrode materials, and in particular to a preparation method and application of low-magnetic foreign matter and high-density lithium iron phosphate. Background Art

[0002] With the continuous development of the new energy market, olivine-structured lithium iron phosphate has obvious cost advantages and is safer and more reliable, becoming one of the most widely used battery materials. The diversification of application scenarios has put forward higher requirements on the mass energy density and volume energy density of lithium iron phosphate positive electrode materials. At present, the gram capacity of lithium iron phosphate is close to the theoretical capacity, and there is not much room for improvement in mass energy density, but there is a lot of room for improvement in compaction density. Increasing compaction density is an effective means to increase the volume energy density of lithium iron phosphate.

[0003] The most common method to increase the compaction density is to achieve the increase in compaction density by mixing and grading the size of lithium iron phosphate particles, but the use of two particle sizes is difficult to operate and the uniformity of mixing is difficult to ensure. In addition, the sintering temperature can be controlled to prepare high-density lithium iron phosphate. The compaction density of lithium iron phosphate can be increased by high-temperature sintering, but lithium iron phosphate is easily reduced to weakly magnetic foreign matter iron phosphide by CO produced by carbon source cracking at high temperatures. A high content of magnetic foreign matter will cause serious self-discharge of lithium iron phosphate batteries and even puncture the battery diaphragm, causing the battery to fail completely. At present, permanent magnets and electromagnetic iron removers are usually used for demagnetization, but electromagnetic demagnetization is difficult to eliminate weakly magnetic iron phosphide, so how to prepare high-density lithium iron while reducing the content of magnetic foreign matter iron phosphide is an urgent problem to be solved in the industry.

[0004] Chinese patent CN116161638A discloses dispersing lithium iron phosphate in a dilute acid solution and adding a lithium source, a phosphorus source and a carbon source for secondary sintering to remove magnetic foreign matter, such as iron phosphide. It mainly uses acid to dissolve the magnetic foreign matter in the lithium iron phosphate. However, the addition of the acid solution is likely to produce side reactions that affect the performance of the lithium iron phosphate. At the same time, the addition of the carbon source will also produce weakly magnetic iron phosphide during the sintering process, and the demagnetization effect of the lithium iron phosphate is poor. Summary of the invention

[0005] The present invention mainly aims to solve the technical problem of excessively high iron phosphide content of magnetic foreign matter in high-density lithium iron phosphate. The primary purpose of the present invention is to provide a method for preparing low-magnetic foreign matter and high-density lithium iron phosphate.

[0006] Another object of the present invention is to provide an application of the above-mentioned low-magnetic foreign matter and high-density lithium iron phosphate.

[0007] The above-mentioned purpose of the present invention is achieved through the following technical solutions:

[0008] The present invention provides a method for preparing low-magnetic foreign matter and high-density lithium iron phosphate, comprising the following steps:

[0009] S1. The lithium source, phosphorus source, iron source, carbon source and water are mixed and wet-ground, dried and sintered at 770 to 830 ° C in an inert atmosphere, and cooled to obtain a lithium iron phosphate material;

[0010] S2. The lithium iron phosphate material obtained in S1 is placed in an inert atmosphere for secondary sintering, the oxygen content is controlled to be less than 5ppm, and the sintering temperature is 700-830°C.

[0011] The inventor has found that the iron phosphide in lithium iron phosphate is mainly produced by the reaction of reducing gas formed by the decomposition of lithium iron phosphate and carbon source in a kiln at a temperature higher than 770°C. The present invention places the high-density lithium iron phosphate obtained after the first sintering in a kiln, controls the oxygen content to be less than 5ppm, and performs secondary sintering at a temperature of 700-830°C, so that the magnetic foreign matter iron phosphide in the lithium iron phosphate reacts with the side reaction product Li3PO4 contained in the lithium iron phosphate material to generate LiFePO4 and is removed. At the same time, secondary sintering within this temperature range can also make the particles grow further, so that the density between particles is increased, thereby further increasing the compaction density of lithium iron phosphate. If the temperature of the secondary sintering is too low, it is not conducive to the reaction of converting iron phosphide into lithium iron phosphate, resulting in an excessively high content of magnetic foreign matter in the lithium iron phosphate product. If the temperature of the secondary sintering is too high, the lithium iron phosphate is prone to excessive agglomeration, which reduces its electrochemical performance.

[0012] Furthermore, the secondary sintering temperature is 770-830°C.

[0013] By regulating the secondary sintering temperature within this range, the obtained lithium iron phosphate has a higher compaction density and a lower content of magnetic foreign matter, iron phosphide.

[0014] Furthermore, in step S1, the molar ratio of the lithium source, the iron source and the phosphorus source is Li:Fe:P=1-1.05:1:1-1.07.

[0015] Furthermore, the lithium source in step S1 is one or more of lithium carbonate and lithium hydroxide.

[0016] Furthermore, the phosphorus source in step S1 is one or more of phosphoric acid, monoammonium phosphate, diammonium phosphate, and iron phosphate.

[0017] Furthermore, the iron source in step S1 is one or more of ferrous oxalate, ferric phosphate, iron oxide, ferric pyrophosphate, and ferrous dihydrogen phosphate.

[0018] Furthermore, the carbon source in step S1 is one or more of sucrose, glucose, starch, PEG, ascorbic acid, and cellulose.

[0019] Furthermore, the amount of the carbon source added in step S1 is 5% to 15% of the molar amount of the iron source.

[0020] Furthermore, the sintering time in step S1 is 6 to 14 hours.

[0021] Furthermore, water is added in step S1 to control the solid content in the solution to 30% to 60%.

[0022] Furthermore, the wet grinding is performed using a sand mill and the particle size of the ground solid particles is controlled to be 0.2-0.6 μm. Controlling the particle size of the ground solid particles within this range is beneficial to increasing the compaction density of the lithium iron phosphate material obtained after calcination.

[0023] Furthermore, the high-density lithium iron phosphate material obtained in step S1 is subjected to preliminary demagnetization by using a magnetic rod to absorb magnetic foreign matter, thereby removing magnetic foreign matter such as iron element and iron oxide in the lithium iron phosphate, thereby obtaining a high-density lithium iron phosphate material after preliminary demagnetization, and then analyzing it using ICP to determine the iron phosphide content.

[0024] Furthermore, in step S2, a phosphorus source and / or a lithium source may be added for sintering.

[0025] Furthermore, the phosphorus source in step S2 is one or more of P2O5, NH4H2PO4, (NH4)2HPO4, and H3PO4.

[0026] Furthermore, the lithium source in step S2 is one or more of lithium hydroxide, lithium dihydrogen phosphate, and lithium phosphate.

[0027] Furthermore, the lithium iron phosphate material in step S1 contains iron phosphide impurities, and the molar ratio of the phosphorus source and / or lithium source to the iron phosphide in step S2 is 1:1 to 1.5.

[0028] In the present invention, a phosphorus source and / or a lithium source can be added during the secondary sintering to reduce iron phosphide in step S2. The addition of the phosphorus source can promote the forward reaction of the iron phosphide to generate lithium iron phosphate, and the added lithium source can react with the iron phosphide to generate lithium iron phosphate, thereby further reducing the content of iron phosphide in the lithium iron phosphate.

[0029] Furthermore, the inert atmosphere in step S2 may be nitrogen and / or argon.

[0030] Furthermore, the sintering time in step S2 is 8 to 16 hours.

[0031] Furthermore, the compaction density of the low magnetic foreign matter and high compaction lithium iron phosphate obtained by the above preparation method is ≥2.75g / cm 3 ; Magnetic foreign matter content ≤50ppb.

[0032] The present invention also provides the use of the low-magnetic foreign matter and high-density lithium iron phosphate in lithium ion batteries.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] The present invention performs two sintering processes in the process of preparing low-magnetic foreign matter and high-density lithium iron phosphate. First, the temperature is controlled to be 770-830°C for a sintering process and then cooled to obtain a high-density lithium iron phosphate material. Then, the material is placed at a temperature of 700-830°C for a second sintering process to convert the iron phosphide in the lithium iron phosphate material obtained by the first sintering into lithium iron phosphate, significantly reducing the iron phosphide content in the lithium iron phosphate material. At the same time, the compaction density can be further improved, and finally low-magnetic foreign matter and high-density lithium iron phosphate are obtained. The method of the present invention is simpler and easier to implement than the prior art, and has a good effect on removing iron phosphide, which can reach a level of <50ppb, and a high compaction density of 2.75g / cm 3 The above, 0.1C discharge specific capacity is above 163mAh / g, 1C discharge specific capacity is above 150mAh / g. DETAILED DESCRIPTION

[0035] The content of the present invention is further described below in conjunction with specific embodiments, but the embodiments do not limit the present invention in any form. Unless otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art. Unless otherwise specified, the reagents, methods and equipment used in the present embodiment are conventional reagents, methods and equipment in the art.

[0036] Example 1

[0037] A method for preparing low-magnetic foreign matter and high-density lithium iron phosphate, comprising: (1) mixing lithium carbonate, iron phosphate and glucose at a molar ratio of Li:Fe:P of 1.02:1:1.05, wherein the amount of glucose added is 10%, and water is added to control the solid content to 50% and the mixture is evenly mixed; then wet grinding is performed using a sand mill so that the solid particles have a particle size D 50 The particle size is 0.2-0.6 μm, and then dried, sintered at 800 °C for 12 h, cooled, and pulverized by air flow to obtain a powder with a compaction density of 2.60 g / cm 3 (2) The lithium iron phosphate material obtained in step (1) is subjected to a magnetic rod to absorb magnetic foreign matter to obtain a high-density lithium iron phosphate material after preliminary demagnetization, and the iron phosphide content is obtained by ICP quantitative analysis. (3) The product obtained in step (2) is placed in a nitrogen atmosphere, the oxygen content is controlled to be less than 5ppm, and the temperature is raised to 800°C for secondary sintering for 10 hours to obtain low-magnetic foreign matter and high-density lithium iron phosphate, and its compaction density is 2.79g / cm 3 .

[0038] Example 2

[0039] A method for preparing low-magnetic foreign matter and high-density lithium iron phosphate, the difference from Example 1 is only that the secondary sintering temperature in step (3) is 770°C.

[0040] Example 3

[0041] A method for preparing low-magnetic foreign matter and high-density lithium iron phosphate, the difference from Example 1 is only that the secondary sintering temperature in step (3) is 740°C.

[0042] Example 4

[0043] A method for preparing low-magnetic foreign matter and high-density lithium iron phosphate, the difference from Example 1 is only that the secondary sintering temperature in step (3) is 700°C.

[0044] Example 5

[0045] A method for preparing low-magnetic foreign matter and high-density lithium iron phosphate, the difference from Example 1 is only that the secondary sintering temperature in step (3) is 830°C.

[0046] Example 6

[0047] A method for preparing low-magnetic foreign matter and high-density lithium iron phosphate, the difference from Example 1 is only that the secondary sintering time in step (3) is 8 hours.

[0048] Example 7

[0049] A method for preparing low-magnetic foreign matter and high-density lithium iron phosphate, the difference from Example 1 is only that the secondary sintering time in step (3) is 16 hours.

[0050] Example 8

[0051] A method for preparing low-magnetic foreign matter and high-density lithium iron phosphate, comprising: (1) mixing lithium carbonate, iron phosphate and glucose at a molar ratio of Li:Fe:P of 1.02:1:1.05, adding water to control the solid content to 50% and mixing evenly, and then using a sand mill to wet grind the solid particles to a particle size of D 50 The particle size of the powder is 0.20-0.6 μm, and then it is dried, sintered at 800 °C for 12 hours, cooled, and pulverized by air flow to obtain a powder with a compaction density of 2.68 g / cm 3(2) The lithium iron phosphate material obtained in step (1) is subjected to a magnetic bar to absorb magnetic foreign matter to obtain a high-density lithium iron phosphate material after preliminary demagnetization, and the iron phosphide content is obtained by ICP quantitative analysis. (3) The product obtained in step (2) is mixed with LiH2PO4, placed in a nitrogen atmosphere, the oxygen content is controlled to be less than 5ppm, and a secondary sintering is performed at a temperature of 800°C for 8h to obtain a low-magnetic foreign matter and high-density lithium iron phosphate, wherein the molar ratio of LiH2PO4 to the iron phosphide in the lithium iron phosphate material is 1:1.2.

[0052] Example 9

[0053] A method for preparing low-magnetic foreign matter and high-density lithium iron phosphate, which differs from Example 8 only in that: step (3) is: mixing the product obtained in step (2) with phosphorus pentoxide, placing it in a nitrogen atmosphere, controlling the oxygen content to be less than 5ppm, raising the temperature to 800°C for secondary sintering for 8h, and obtaining low-magnetic foreign matter and high-density lithium iron phosphate, wherein the molar ratio of phosphorus pentoxide to iron phosphide in the lithium iron phosphate material is 1:1.2.

[0054] Comparative Example 1

[0055] A method for preparing low-magnetic foreign matter and high-density lithium iron phosphate, which differs from Example 1 only in that the secondary sintering temperature in step (3) is 670°C.

[0056] Performance Testing

[0057] Compaction density test: The lithium iron phosphate obtained in the embodiment and the comparative example was tested using a compaction density meter at a test pressure of 3 tons. The results are shown in Table 1.

[0058] Test of iron phosphide content: ICP was used to quantitatively detect and analyze the lithium iron phosphate before and after demagnetization in the embodiment and the comparative example. The results are shown in Table 1.

[0059] Capacity performance test: low magnetic foreign matter, high compaction lithium iron phosphate: PVDF: conductive carbon black are mixed in a mass ratio of 8:1:1 to form a slurry, which is then coated on aluminum foil to make a positive electrode sheet. The negative electrode is a lithium sheet to form a button battery. When the charge and discharge cut-off voltage is in the range of 2.0 to 3.75V, the 1C and 0.1C discharge specific capacities are tested as shown in Table 1.

[0060] Table 1

[0061]

[0062] As shown in Table 1, it can be seen from the examples that the compaction density of the low magnetic foreign matter and high compaction lithium iron phosphate prepared by the present invention can reach 2.75 g / cm 3The content of magnetic foreign matter Fe2P is less than 50ppb, the 0.1C discharge capacity is above 163mAh / g, and the 1C discharge capacity is above 150mAh / g.

[0063] It can be seen from Examples 8 to 9 that, compared with Example 6, adding a phosphorus source or a lithium source during the secondary sintering of the lithium iron phosphate material containing the magnetic foreign matter iron phosphide can further reduce the content of the magnetic foreign matter iron phosphide in the lithium iron phosphate material within the same sintering time.

[0064] It can be seen from Comparative Example 1 that the sintering temperature is too low during the secondary sintering process, which is not conducive to the conversion of iron phosphide into lithium iron phosphate, resulting in a higher content of iron phosphide in the lithium iron phosphate after the secondary sintering. At the same time, the compaction density of the lithium iron phosphate material obtained after the first sintering is limited, resulting in a lower compaction density of the lithium iron phosphate obtained after the secondary sintering.

[0065] The above embodiments are preferred experimental methods of the present invention, but the implementation methods of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A method for preparing low magnetic foreign matter and high compaction lithium iron phosphate, characterized in that: The following steps are involved: S1. The lithium source, iron source, phosphorus source, carbon source and water are mixed, ground, dried and sintered at 770 to 830 ° C in an inert atmosphere, and then cooled to obtain a lithium iron phosphate material; S2. The lithium iron phosphate material obtained in S1 is placed in an inert atmosphere for secondary sintering, the oxygen content is controlled to be less than 5ppm, and the secondary sintering temperature is 700-830°C to obtain the low-magnetic foreign matter and high-density lithium iron phosphate.

2. The preparation method according to claim 1, characterized in that: The secondary sintering temperature is 770-830°C.

3. The preparation method according to claim 1, characterized in that: In the step S2, a phosphorus source and / or a lithium source may be added before the secondary sintering.

4. The preparation method according to claim 3, characterized in that: The phosphorus source is one or more of P2O5, NH4H2PO4, (NH4)2HPO4, and H3PO4.

5. The preparation method according to claim 3, characterized in that: The lithium source is one or more of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, and lithium phosphate.

6. The preparation method according to claim 3, characterized in that: The lithium iron phosphate material in step S1 contains iron phosphide impurities, and the molar ratio of the phosphorus source and / or lithium source to the iron phosphide in step S2 is 1:1-1.

5.

7. The preparation method according to claim 1, characterized in that: The secondary sintering time is 8 to 16 hours.

8. Low magnetic foreign matter and high-density lithium iron phosphate prepared by the preparation method according to any one of claims 1 to 7.

9. The low magnetic foreign matter and high compaction lithium iron phosphate according to claim 8, characterized in that: The compaction density of the low magnetic foreign matter and high compaction lithium iron phosphate is ≥2.75g / cm 3 ; Magnetic foreign matter content ≤50ppb.

10. Use of the low-magnetic foreign matter and high-density lithium iron phosphate according to any one of claims 8 to 9 in lithium-ion batteries.

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