Lithium iron phosphate positive electrode material and preparation method thereof and lithium ion battery

Abstract

The present invention relates to the field of positive electrode materials, and discloses a lithium iron phosphate positive electrode material and a preparation method thereof and a lithium ion battery. The expression of the lithium iron phosphate positive electrode material is LiFe 1‑x M x PO 4 / C, wherein 0<x≤0.05; M is selected from at least one element of Mg, Al, Zr, Ti, Co, V, Mn, Zr, W, Sn, Nb and Mo; wherein the particle size distribution of the lithium iron phosphate positive electrode material satisfies: (D90-D10) / D50=1-2.17; the content of magnetic material in the lithium iron phosphate positive electrode material is 850-900 weight ppm. The lithium iron phosphate positive electrode material of the present invention has less impurities, small primary particle size, good dispersibility, narrow particle size distribution range, and low content of magnetic material. When used as a lithium battery positive electrode material, it is beneficial to improve the low temperature performance and rate performance of the lithium ion battery, and at the same time can reduce the self-discharge of the lithium ion battery, and improve the high temperature storage performance and cycle performance of the lithium ion battery.

Description

Technical Field

[0001] The present invention relates to the field of positive electrode materials, and in particular to a lithium iron phosphate positive electrode material and a preparation method thereof, and a lithium ion battery. Background Art

[0002] Lithium iron phosphate cathode material and ternary cathode material (NCM and NCA) are the two main cathode materials used in lithium-ion batteries. Based on their respective characteristics, their application fields are clearly differentiated. Lithium iron phosphate cathode material has good safety and long cycle life, so it is widely used in the field of large buses, while ternary cathode material has high energy density and is widely used in the field of passenger cars.

[0003] The structure of lithium iron phosphate belongs to the olivine structure of the orthorhombic system. The volume change of lithium insertion and extraction during the charge and discharge process is very small (6.8%), so it has extremely excellent cycle performance, but its poor electronic and ionic conductivity also leads to its poor low temperature and rate performance, which is difficult to meet the requirements of power batteries for low temperature and rate. In view of the poor low temperature performance and rate performance of lithium iron phosphate positive electrode materials, many improvements have been made and certain progress has been made. However, the low temperature performance and rate performance of lithium iron phosphate positive electrode materials are still far behind those of ternary positive electrode materials. At the same time, due to the iron impurities introduced during the production process, the content of magnetic substances in lithium iron phosphate positive electrode materials is relatively high, which makes the battery self-discharge serious and even has safety problems. This is also a key problem that needs to be solved urgently.

[0004] The preparation methods of lithium iron phosphate mainly include solid phase method and hydrothermal synthesis method (referred to as "wet method"). The solid phase method is to mix lithium salt, phosphorus source, iron source and carbon source evenly by mechanical mixing, and then obtain lithium iron phosphate positive electrode material by spraying, sintering and gas crushing process. This method is currently adopted by most positive electrode manufacturers because of its mature technology and low cost. The prepared lithium iron phosphate has high capacity and high compaction. However, the lithium iron phosphate synthesized by the solid phase method has problems of poor low temperature and rate performance due to its large particle size. Compared with the solid phase method, the wet synthesis method has great advantages in low temperature and rate performance due to the small size of the obtained lithium iron phosphate. However, the lithium iron phosphate prepared by the wet synthesis technology has more impurities, more influencing factors in the synthesis process, poor consistency, so the capacity is low, and due to the presence of impurities, the content of magnetic substances is high, and the high temperature and cycle performance are poor. Compared with the solid-phase method, the particle size of lithium iron phosphate produced by wet synthesis is smaller, but the primary particles are mostly above 160nm and the agglomeration is relatively serious. Although the low-temperature and rate performance are improved compared with the solid-phase method, it is still difficult to meet the requirements of power batteries. Summary of the invention

[0005] The purpose of the present invention is to overcome the problems of poor low temperature performance, poor rate performance and high content of magnetic substances in lithium iron phosphate positive electrode materials in the prior art, and to provide a lithium iron phosphate positive electrode material and a preparation method thereof and a lithium ion battery.

[0006] In order to achieve the above-mentioned object, the first aspect of the present invention provides a lithium iron phosphate positive electrode material, wherein the expression of the lithium iron phosphate positive electrode material is LiFe 1-x M x PO4 / C, wherein 0<x≤0.05; M is at least one element selected from Mg, Al, Zr, Ti, Co, V, Mn, Zr, W, Sn, Nb and Mo;

[0007] The particle size distribution of the lithium iron phosphate positive electrode material satisfies: (D90-D10) / D50=1-2.17; the content of magnetic substance in the lithium iron phosphate positive electrode material is 850-900 ppm by weight.

[0008] A second aspect of the present invention provides a method for preparing a lithium iron phosphate positive electrode material, the method comprising:

[0009] (1) in the presence of a solvent, mixing lithium salt, phosphoric acid and iron salt in an inert atmosphere and then performing a hydrothermal reaction to obtain a first slurry;

[0010] (2) washing the first slurry to obtain a second slurry;

[0011] (3) adding an organic carbon source, a dispersant and a grain growth inhibitor to the second slurry, and performing sand milling under an inert atmosphere to obtain a third slurry;

[0012] (4) uniformly mixing the third slurry and the lithium supplementing agent, and then spray drying to obtain a precursor;

[0013] (5) Sintering the precursor, and subjecting the sintered product to gas crushing to obtain a lithium iron phosphate positive electrode material.

[0014] A third aspect of the present invention provides a lithium iron phosphate positive electrode material prepared by the method described above.

[0015] A fourth aspect of the present invention provides a lithium-ion battery comprising the lithium iron phosphate positive electrode material as described above.

[0016] Through the above technical solution, the present invention has the following advantages:

[0017] (1) The lithium iron phosphate positive electrode material of the present invention has less impurities, small primary particle size, good dispersibility, and a narrow particle size distribution range. When used as a positive electrode material for a lithium battery, it is beneficial to improve the low temperature performance and rate performance of a lithium ion battery;

[0018] (2) The lithium iron phosphate positive electrode material of the present invention has a low content of magnetic substances. When used as a positive electrode material for a lithium ion battery, it is beneficial to reduce the self-discharge of the lithium ion battery and improve the high temperature storage performance and cycle performance of the lithium ion battery;

[0019] (3) The method for preparing lithium iron phosphate positive electrode material of the present invention uses a dispersant and a grain growth inhibitor in combination with sand grinding. The three act synergistically. On the one hand, it ensures that the small-sized particles produced in the sand grinding process do not agglomerate, and at the same time prevents their agglomeration growth in the subsequent sintering process, avoiding their formation of molten large particles. On the other hand, the bulk ion doping of the grain growth inhibitor improves the electronic conductivity and ionic conductivity, which is beneficial to achieving an improvement in rate performance. In addition, by adding a lithium supplement, the content of impurities and magnetic substances in the lithium iron phosphate positive electrode material is effectively reduced, which is beneficial to improving the self-discharge performance, high-temperature storage performance and cycle performance of the lithium iron phosphate positive electrode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a scanning electron microscope (SEM) image of the lithium iron phosphate positive electrode material prepared in Example 1 of the present invention, with a magnification of 100k;

[0021] Figure 2 This is a scanning electron microscope (SEM) image of the lithium iron phosphate positive electrode material prepared in Example 2 of the present invention, with a magnification of 100k;

[0022] Figure 3 This is a scanning electron microscope (SEM) image of the lithium iron phosphate positive electrode material prepared in Example 3 of the present invention, with a magnification of 100k;

[0023] Figure 4 This is a scanning electron microscope (SEM) image of the lithium iron phosphate positive electrode material prepared in Comparative Example 1 of the present invention, with a magnification of 100k. DETAILED DESCRIPTION

[0024] The endpoints and any values ​​of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0025] The first aspect of the present invention provides a lithium iron phosphate positive electrode material, wherein the expression of the lithium iron phosphate positive electrode material is LiFe 1-x M xPO4 / C, wherein 0<x≤0.05; M is at least one element selected from Mg, Al, Zr, Ti, Co, V, Mn, Zr, W, Sn, Nb and Mo;

[0026] The particle size distribution of the lithium iron phosphate positive electrode material satisfies: (D90-D10) / D50=1-2.17; the content of magnetic material in the lithium iron phosphate positive electrode material is 850-900 ppm by weight. The lithium iron phosphate positive electrode material has small particle size, good dispersibility and narrow particle size distribution range.

[0027] In some embodiments of the present invention, based on the total weight of the lithium iron phosphate positive electrode material, the carbon content in the lithium iron phosphate positive electrode material is 1-3.5 wt %, preferably 1.4-3.2 wt %.

[0028] In some embodiments of the present invention, the particle size distribution of the lithium iron phosphate positive electrode material satisfies: (D90-D10) / D50=1-2.17, preferably, the particle size distribution of the lithium iron phosphate positive electrode material satisfies: (D90-D10) / D50=1-1.55. The lithium iron phosphate positive electrode material has less impurities, small primary particle size, good dispersibility, and narrow particle size distribution range. When used as a lithium battery positive electrode material, it is beneficial to improve the low temperature performance and rate performance of lithium ion batteries.

[0029] In some embodiments of the present invention, the content of magnetic substance in the lithium iron phosphate positive electrode material is 850-900 ppm by weight, preferably 850-885 ppm by weight. The lithium iron phosphate positive electrode material has a low content of magnetic substance. When used as a positive electrode material for a lithium ion battery, it is beneficial to reduce the self-discharge of the lithium ion battery and improve the high temperature storage performance and cycle performance of the lithium ion battery. The content of magnetic substance involved in the present invention is obtained by testing using a JSII-G1 magnetic material analyzer.

[0030] In some embodiments of the present invention, M is selected from at least one element of Mg, Al, Zr, Ti, Co, V, Mn, Zr, W, Sn, Nb and Mo. Preferably, M is selected from at least one element of Ti, V and Nb.

[0031] A second aspect of the present invention provides a method for preparing a lithium iron phosphate positive electrode material, the method comprising:

[0032] (1) in the presence of a solvent, mixing lithium salt, phosphoric acid and iron salt in an inert atmosphere and then performing a hydrothermal reaction to obtain a first slurry;

[0033] (2) washing the first slurry to obtain a second slurry;

[0034] (3) adding an organic carbon source, a dispersant and a grain growth inhibitor to the second slurry, and performing sand milling under an inert atmosphere to obtain a third slurry;

[0035] (4) uniformly mixing the third slurry and the lithium supplementing agent, and then spray drying to obtain a precursor;

[0036] (5) Sintering the precursor, and subjecting the sintered product to gas crushing to obtain a lithium iron phosphate positive electrode material.

[0037] Although in the prior art, the rate performance of lithium iron phosphate cathode materials can be improved to a certain extent by in-situ carbon coating and metal doping, it is difficult to form effective in-situ coating and ion doping during the wet synthesis process. The main reasons are: 1. If a carbon source is added during the wet synthesis process, it is difficult to carbonize in situ because the reaction temperature is not high. In the subsequent washing process, the carbon source is water-soluble and will still be washed away; 2. Metal ion doping can only diffuse into the interior of the material through the subsequent sintering process, and the reaction conditions in the hydrothermal synthesis stage are not sufficient to allow metal ions to enter the interior of the material. In addition, although sand milling can further reduce the particle size of the hydrothermally synthesized material, the reduction in particle size will also lead to agglomeration, causing the material to agglomerate and grow in the subsequent sintering process to form large molten particles, which is not conducive to the improvement of rate performance.

[0038] The inventors of the present invention have found that by using a dispersant and a grain growth inhibitor in combination with sand milling, the three work synergistically, on the one hand, ensuring that the small-sized particles produced during the sand milling process do not agglomerate, while preventing their agglomeration growth during the subsequent sintering process, and avoiding their formation of molten large particles, on the other hand, the bulk ion doping of the grain growth inhibitor improves the electronic conductivity and ionic conductivity, which is conducive to achieving an improvement in rate performance. In addition, a carbon source is added during the sand milling process, because the carbon source is water-soluble, so molecular-level mixed coating can be achieved.

[0039] In some embodiments of the present invention, the amounts of the lithium salt, phosphoric acid and iron salt are such that the molar ratio of lithium, phosphorus and iron is 2.8-3.2:1:1.

[0040] In some embodiments of the present invention, based on the dry basis of the second slurry, the amount of the organic carbon source is 8-16wt%; the amount of the dispersant is 0.5-5wt%; the amount of the grain growth inhibitor is 0.1-6wt%, preferably 0.3-5wt%.

[0041] In some embodiments of the present invention, based on the dry basis of the third slurry, the amount of the lithium supplement agent is 0.2-5.5wt%, preferably 0.5-5wt%.

[0042] In the present invention, "based on the dry basis of the second slurry" means that the calculation is based on the mass of the powder in the second slurry, and "based on the dry basis of the third slurry" means that the calculation is based on the mass of the powder in the third slurry. The mass of the powder in the second slurry is calculated by multiplying the mass of the second slurry and the solid content of the second slurry; the mass of the powder in the third slurry is calculated by multiplying the mass of the third slurry and the solid content of the third slurry. The solid content of the second slurry and the third slurry can be obtained by testing with a moisture tester at 160°C.

[0043] In some embodiments of the present invention, in step (1), the lithium salt is a soluble lithium salt, for example, the lithium salt can be selected from at least one of lithium chloride, lithium sulfate, lithium hydroxide and lithium nitrate.

[0044] In some embodiments of the present invention, in step (1), the iron salt can be selected from at least one of ferrous sulfate, ferrous chloride, ferrous nitrate and ferrous oxalate.

[0045] In some embodiments of the present invention, in step (3), the organic carbon source can be selected from sucrose, water-soluble phenolic resin, glucose, polyethylene glycol 6000 (PEG6000), hydroxymethyl cellulose, polyacrylamide (C3H5NO) n , weight average molecular weight of 5 million to 12 million g / mol), starch and polyvinyl alcohol (weight average molecular weight of 200,000 to 700,000 g / mol).

[0046] In some embodiments of the present invention, the dispersant is selected from at least one of sodium dodecyl sulfate, polyethylene glycol 200 (PEG200), polyethylene glycol 400 (PEG400), sodium dodecyl sulfate, methyl amyl alcohol, sodium tripolyphosphate, sodium hexametaphosphate and sodium pyrophosphate. Adding the dispersant during the sand milling process can effectively inhibit the secondary agglomeration of small particles, which is beneficial to improving the low temperature and rate performance of the lithium iron phosphate cathode material.

[0047] In some embodiments of the present invention, the grain growth inhibitor is selected from at least one of magnesium chloride, magnesium nitrate, aluminum nitrate, zirconium nitrate, zirconium oxide, tetraethyl titanate, ethyl titanate, cobalt acetate, cobalt nitrate, vanadium pentoxide, ammonium metavanadate, manganese nitrate, manganese chloride, manganese sulfate, niobium pentachloride, tungsten disulfide, tin chloride, tin oxide, molybdenum sulfide and molybdenum oxide; preferably, the concentration of the grain growth inhibitor is 0.3-5wt%. The grain growth inhibitor added during the sand milling process can play a "crystal rolling" role in the subsequent sintering process, inhibiting the growth of particles, and providing metal ions entering the lithium iron phosphate lattice, thereby improving the electronic and ionic conductivity of the lithium iron phosphate positive electrode material, which is beneficial to improving the low temperature and rate performance of the lithium iron phosphate positive electrode material.

[0048] In some embodiments of the present invention, the lithium supplement agent is selected from at least one of lithium dihydrogen phosphate, lithium monohydrogen phosphate and lithium phosphate. In the method for preparing the lithium iron phosphate positive electrode material, by adding the lithium supplement agent, the impurity phase in the lithium iron phosphate positive electrode material can be reduced and the content of its magnetic substance can be reduced. The principle is: the sand milling process can re-expose the inside of the hydrothermally synthesized lithium iron phosphate material. Under the action of the lithium supplement agent, the iron impurity phase inside the material can be re-converted into the lithium iron phosphate material through the subsequent sintering process, which effectively reduces the content of the impurity phase and magnetic substance contained in the material, which is beneficial to improve the self-discharge performance, high temperature storage performance and cycle performance of the lithium iron phosphate positive electrode material.

[0049] In some embodiments of the present invention, in step (1), the solvent is preferably water; preferably, the lithium salt, phosphoric acid and iron salt are mixed evenly under an inert atmosphere. The inert gas that can be used in the present invention includes but is not limited to nitrogen or argon.

[0050] In some embodiments of the present invention, the conditions of the hydrothermal reaction are not particularly limited, and can refer to conventional process conditions in the art. For example, the conditions of the hydrothermal reaction may include: a temperature of 130-200° C. and a time of 2-8 h.

[0051] In some embodiments of the present invention, the first slurry is a lithium iron phosphate slurry containing a reaction mother liquor obtained after a hydrothermal reaction. Preferably, the pH of the first slurry is 5.5-7.

[0052] In some embodiments of the present invention, in step (2), the washing specifically comprises: removing the reaction mother liquor in the first slurry, and then washing the centrifuged product with a dilute saline solution;

[0053] The diluted salt solution is selected from at least one of lithium chloride, lithium nitrate, lithium dihydrogen phosphate, lithium monohydrogen phosphate and ammonium dihydrogen phosphate aqueous solutions; preferably, the concentration of the diluted salt solution is 0.01-2wt%. The concentration is calculated based on the mass of the lithium iron phosphate powder. The washing can be a single wash or multiple step washes.

[0054] In some embodiments of the present invention, in step (3), sand grinding can reduce the primary particle size of the lithium iron phosphate positive electrode material, which is beneficial to improving its low temperature and rate performance. In order to further improve the low temperature and rate performance of the lithium iron phosphate positive electrode material, preferably, the sand grinding conditions include: a linear speed of 5-15 m / s and a time of 0.5-12 h.

[0055] In some embodiments of the present invention, the primary particle size of the lithium iron phosphate positive electrode material can be controlled by controlling the linear speed and time of sand milling; the secondary agglomeration of small-sized lithium iron phosphate particles can be prevented by adding a dispersant; and the growth and adhesion and agglomeration of particles can be inhibited in the subsequent sintering process by adding a grain growth inhibitor.

[0056] In some embodiments of the present invention, the inert atmosphere is a nitrogen atmosphere or a helium atmosphere.

[0057] In some embodiments of the present invention, preferably, in step (4), the spray drying is to evaporate the water in the third slurry by flash evaporation, thereby obtaining secondary spherical particles assembled from primary particles.

[0058] In some embodiments of the present invention, the spray drying conditions are not particularly limited. For example, the spray drying conditions may include: a solid content of 30-50 wt%, a temperature of 90-105° C., and a time of 5-12 h.

[0059] In some embodiments of the present invention, the sintering conditions are not particularly limited, and can refer to conventional process conditions in the art. For example, the sintering conditions may include: a temperature of 650-760° C. and a time of 5-10 hours. Preferably, the sintering is performed under an inert atmosphere. The inert gas that can be used in the present invention includes, but is not limited to, nitrogen or argon.

[0060] In some embodiments of the present invention, the sintered product is a secondary spherical particle assembled from primary particles; the gas crushing is to break up the secondary spherical particles of the sintered product to obtain dispersed primary particles of lithium iron phosphate positive electrode material.

[0061] The third aspect of the present invention provides a lithium iron phosphate positive electrode material prepared by the method as described above. The expression of the lithium iron phosphate positive electrode material is LiFe 1-x M x PO4 / C, wherein 0<x≤0.05; M is selected from at least one element of Mg, Al, Zr, Ti, Co, V, Mn, Zr, W, Sn, Nb and Mo; wherein the particle size distribution of the lithium iron phosphate positive electrode material satisfies: (D90-D10) / D50=1-2.17; the content of magnetic substance in the lithium iron phosphate positive electrode material is 850-900 weight ppm.

[0062] Preferably, the particle size distribution of the lithium iron phosphate positive electrode material satisfies: (D90-D10) / D50=1-1.55; the content of magnetic substance in the lithium iron phosphate positive electrode material is 850-885 ppm by weight.

[0063] Preferably, based on the total weight of the lithium iron phosphate positive electrode material, the carbon content in the lithium iron phosphate positive electrode material is 1-3.5 wt %, preferably 1.4-3.2 wt %.

[0064] Preferably, M is at least one element selected from Ti, V and Nb.

[0065] A fourth aspect of the present invention provides a lithium ion battery, comprising the lithium iron phosphate cathode material as described above. The lithium iron phosphate cathode material of the present invention exhibits excellent low temperature performance, rate performance, high temperature performance and cycle performance in a lithium ion battery.

[0066] In some embodiments of the present invention, the low-temperature capacity retention rate of the lithium iron phosphate positive electrode material at -20°C and 1C is 65-80%, preferably 70-80%, and more preferably 70-75%.

[0067] In some embodiments of the present invention, the discharge capacity retention rate of the lithium iron phosphate positive electrode material at a rate of 15C is 75-86%, preferably 78-86%, and more preferably 81-84%.

[0068] In some embodiments of the present invention, after the lithium iron phosphate positive electrode material is stored at 60°C for 7 days, the high temperature capacity residual rate is 90-99%, preferably 92-99%, and more preferably 95-98.5%; the high temperature capacity recovery rate is 96.5-102%, preferably 99.5-102%, and more preferably 100-101%.

[0069] In some embodiments of the present invention, after the lithium iron phosphate positive electrode material is cycled 500 times at a rate of 1C, the cycle capacity retention rate is above 95.5%, preferably above 96%, and more preferably 96.8-97.5%.

[0070] The present invention will be described in detail below through examples.

[0071] In the following examples and comparative examples, unless otherwise specified, all raw materials used are commercially available products.

[0072] Unless otherwise specified, the room temperature in the present invention refers to 25±2°C.

[0073] In the following examples and comparative examples, the relevant parameters were obtained by the following test methods:

[0074] (1) SEM morphology test: obtained by scanning electron microscope S-4800 of Hitachi, Japan;

[0075] (2) Particle size distribution: Smileview software was used to perform statistics on the SEM images with a magnification of 100k using a scanning electron microscope S-4800 produced by Hitachi, Japan. The short side dimensions of all particles were measured, and the obtained data were imported into the Mintab software for statistical analysis to obtain the D10, D50 and D90 values. The particle size distribution value was then calculated based on these values.

[0076] (3) Magnetic substance content: JSII-G1 magnetic substance analyzer was used for testing. The test conditions were as follows: 10 g of sample was placed in a measuring tube for direct measurement;

[0077] (4) XPS spectrum: obtained by testing with an Escalab 250Xi X-ray photoelectron spectrometer purchased from Thermo Scientific and equipped with Thermo Avantage V5.926 software, wherein the test conditions include: analysis area: circular with a diameter of about 800 μm, information depth: about 10 nm; atomic number percentage detection lower limit: 0.1%; test environment: temperature 23.6°C, relative humidity 50%, vacuum degree 5.0×10 -8 Torr; voltage is 15kV, current is 23mA, power is 350W; X-ray scanning angle is 45.0°;

[0078] (5) Electrochemical performance test:

[0079] In the following examples and comparative examples, the electrochemical performance of the lithium iron phosphate positive electrode material was tested using a 2025 button cell.

[0080] The preparation process of 2025 button battery is as follows:

[0081] Preparation of pole piece: lithium iron phosphate positive electrode material, acetylene black and polyvinylidene fluoride (PVDF) were mixed with an appropriate amount of N-methylpyrrolidone (NMP) in a mass ratio of 100:1.5:2.5 to form a uniform positive electrode slurry. The positive electrode slurry was coated on aluminum foil and dried at 100°C for 12 hours to form a positive electrode piece with a diameter of 14 mm. The loading amount of the lithium iron phosphate positive electrode material was 5-10 mg / cm 2 .

[0082] Battery assembly: In an argon-filled glove box with a water content and oxygen content of less than 5ppm, the positive electrode, separator, negative electrode and electrolyte were assembled into a 2025 button cell and left to stand for 6 hours. The negative electrode used a metal lithium sheet with a diameter of 17mm and a thickness of 1mm; the separator used a polyethylene porous membrane (Celgard 2325) with a thickness of 25μm; and the electrolyte used an equal mixture of 1mol / L LiPF6, ethylene carbonate (EC) and diethyl carbonate (DEC).

[0083] Electrochemical performance test:

[0084] In the following examples and comparative examples, the electrochemical performance of 2025 button batteries was tested using Shenzhen Xinwell battery testing system.

[0085] Low temperature capacity retention rate: At room temperature, the battery is charged to 3.8V at a constant current of 0.1C, and then the constant voltage is cut off at 3.8V to a current of 0.1C, and then the constant current is discharged to 2.5V at 0.1C. After one cycle, the battery is charged to 3.8V at a constant current of 0.5C, and then the constant voltage is cut off at 3.8V to a current of 0.1C, and the charging specific capacity of the battery at room temperature and 0.5C is obtained; then the battery is placed in a -20℃ freezer, and the constant current is discharged to 2.0V at 1C, and the discharge specific capacity of the battery at -20℃ and 1C is obtained; the ratio of the discharge specific capacity of the battery at -20℃ and 1C to the charging specific capacity of the battery at room temperature and 0.5C is the low temperature capacity retention rate of the battery;

[0086] Discharge capacity retention rate: CCCV charge to 3.8V at 0.2C rate, cut-off current is 0.02C, then CC discharge to 2.5V at 15C rate. The ratio of the discharge capacity at 15C rate to the discharge capacity at 0.2C rate is the discharge capacity retention rate at 15C rate.

[0087] High temperature capacity remaining rate and high temperature capacity recovery rate: the battery is fully charged at 0.1CCCV, and then stored in a 60°C oven for 7 days. After being taken out, it is discharged to 2.5V at 0.1C, then charged to 3.8V at 0.1CCCV, and then discharged to 2.5V. The ratio of the first discharge capacity of the battery after high temperature storage to the discharge capacity before storage is the high temperature capacity remaining rate, and the ratio of the discharge capacity of the last cycle to the discharge capacity before storage is the high temperature capacity recovery rate of the battery.

[0088] Cycle capacity retention rate: At room temperature, after the battery is charged and discharged 500 times at a 1C rate, the ratio of the 500th discharge capacity to the first discharge capacity is the cycle capacity retention rate.

[0089] Example 1

[0090] (1) under nitrogen atmosphere, lithium hydroxide, phosphoric acid and ferrous chloride were added to deionized water in a stoichiometric ratio (the molar ratio of lithium element, phosphorus element and iron element was 3:1:1), and stirred evenly to obtain a first mixture, and then the first mixture was transferred to a high-pressure reactor, reacted at 160° C. for 5 hours, and obtained a first slurry;

[0091] (2) centrifuging the first slurry to separate and remove the reaction mother liquor to obtain a centrifugal product, and then washing the centrifugal product with a lithium chloride solution having a concentration of 0.05 wt %, and repeating the process once to obtain a second slurry;

[0092] (3) under nitrogen atmosphere, 11 wt % of glucose, 2.5 wt % of niobium pentachloride and 2 wt % of PEG200 were added to the second slurry on a dry basis (i.e., based on the mass of the powder in the second slurry), and the mixture was sand-milled for 5 h at a linear speed of 10 m / s to obtain a third slurry;

[0093] (4) adding 1.32 wt % of lithium dihydrogen phosphate to the third slurry on a dry basis (i.e., based on the mass of the powder in the third slurry) and mixing the mixture uniformly to obtain a second mixture, and spray drying the second mixture to obtain a precursor;

[0094] (5) The precursor is placed in a nitrogen atmosphere and sintered at 720°C for 8 hours. The sintered product is subjected to gas crushing to obtain lithium iron phosphate positive electrode material LiFe 0.99 Nb 0.01 PO4 / C.

[0095] Example 2

[0096] (1) under nitrogen atmosphere, lithium hydroxide, phosphoric acid and ferrous sulfate were added to deionized water in a stoichiometric ratio (the molar ratio of lithium element, phosphorus element and iron element was 3:1:1), and stirred evenly to obtain a first mixture, and then the first mixture was transferred to a high-pressure reactor, reacted at 160° C. for 5 hours, and obtained a first slurry;

[0097] (2) centrifuging the first slurry to separate and remove the reaction mother liquor to obtain a centrifugal product, and then washing the centrifugal product with a lithium dihydrogen phosphate solution with a concentration of 0.05 wt%, repeating the process once to obtain a second slurry;

[0098] (3) Under nitrogen atmosphere, 11 wt % of glucose, 1.5 wt % of vanadium pentoxide and 2 wt % of PEG400 were added to the second slurry on a dry basis (i.e., based on the mass of the powder in the second slurry), and the mixture was sand-milled for 5 h at a linear speed of 10 m / s to obtain a third slurry;

[0099] (4) adding 2.26 wt % of lithium dihydrogen phosphate to the third slurry on a dry basis (i.e., based on the mass of the powder in the third slurry) and mixing the mixture uniformly to obtain a second mixture, and spray drying the second mixture to obtain a precursor;

[0100] (5) The precursor is placed in a nitrogen atmosphere and sintered at 720°C for 8 hours. The sintered product is gas-crushed to obtain lithium iron phosphate positive electrode material LiFe 0.975 V 0.025 PO4 / C.

[0101] Example 3

[0102] (1) under nitrogen atmosphere, lithium hydroxide, phosphoric acid and ferrous sulfate were added to deionized water in a stoichiometric ratio (the molar ratio of lithium element, phosphorus element and iron element was 3:1:1), and stirred evenly to obtain a first mixture, and then the first mixture was transferred to a high-pressure reactor, reacted at 160° C. for 5 hours, and obtained a first slurry;

[0103] (2) centrifuging the first slurry to separate and remove the reaction mother liquor to obtain a centrifugal product, and then washing the centrifugal product with a lithium dihydrogen phosphate solution with a concentration of 0.05 wt%, repeating the process once to obtain a second slurry;

[0104] (3) Under nitrogen atmosphere, 11 wt % of glucose, 2.5 wt % of tetrabutyl titanate and 2 wt % of PEG200 were added to the second slurry on a dry basis (i.e., based on the mass of the powder in the second slurry), and the mixture was sand-milled for 5 h at a linear speed of 10 m / s to obtain a third slurry;

[0105] (4) adding 1.69 wt % of lithium dihydrogen phosphate to the third slurry on a dry basis (i.e., based on the mass of the powder in the third slurry) and mixing the mixture uniformly to obtain a second mixture, and spray drying the second mixture to obtain a precursor;

[0106] (5) The precursor is placed in a nitrogen atmosphere and sintered at 720°C for 8 hours. The sintered product is gas-crushed to obtain lithium iron phosphate positive electrode material LiFe 0.99 Ti 0.01 PO4 / C.

[0107] Example 4

[0108] A lithium iron phosphate positive electrode material was prepared according to the method of Example 1, except that an equal molar amount of manganese chloride was used to replace the niobium pentachloride in step (3) to obtain a lithium iron phosphate positive electrode material LiFe 0.99 Mn 0.01 PO4 / C.

[0109] Example 5

[0110] The lithium iron phosphate positive electrode material was prepared according to the method of Example 1, except that the amount of PEG200 (dispersant) used was 0.4 wt % based on the dry basis of the second slurry to obtain the lithium iron phosphate positive electrode material LiFe 0.99 Nb 0.01 PO4 / C.

[0111] Example 6

[0112] The lithium iron phosphate positive electrode material was prepared according to the method of Example 2, except that the amount of vanadium pentoxide (grain growth inhibitor) used was 0.25 wt % based on the dry basis of the second slurry to obtain the lithium iron phosphate positive electrode material LiFe 0.996 V 0.004 PO4 / C.

[0113] Comparative Example 1

[0114] A lithium iron phosphate positive electrode material was prepared according to the method of Example 1, except that no dispersant and grain growth inhibitor were added in step (3), thereby obtaining a lithium iron phosphate positive electrode material LiFePO4 / C.

[0115] Comparative Example 2

[0116] A lithium iron phosphate positive electrode material was prepared according to the method of Example 1, except that no dispersant and grain growth inhibitor were added in step (3), and no sand grinding was performed, to obtain a lithium iron phosphate positive electrode material LiFePO4 / C.

[0117] Comparative Example 3

[0118] A lithium iron phosphate positive electrode material was prepared according to the method of Example 1, except that no sand grinding was performed in step (3), to obtain a lithium iron phosphate positive electrode material LiFe 0..99 Nb 0.01 PO4 / C.

[0119] Comparative Example 4

[0120] A lithium iron phosphate positive electrode material was prepared according to the method of Example 1, except that no lithium supplement was added in step (4), to obtain a lithium iron phosphate positive electrode material LiFe 0.99 Nb 0.01 PO4 / C.

[0121] Test Case

[0122] (1) Morphology test

[0123] The present invention tests the scanning electron microscope images of the lithium iron phosphate positive electrode materials prepared in the above embodiments and comparative examples, and exemplarily provides the SEM images of the lithium iron phosphate positive electrode materials prepared in embodiments 1-3 and comparative example 1. The results are as follows: Figure 1-4 As shown, from Figure 1-4 It can be seen that the primary particle size of the lithium iron phosphate material of the present invention is small and the particle size distribution is uniform.

[0124] (2) Physical property test

[0125] The present invention tests the particle size distribution and magnetic content of the lithium iron phosphate positive electrode materials prepared in the above-mentioned embodiments and comparative examples. The specific test results are shown in Table 1.

[0126] Table 1

[0127] serial number D10 D50 D90 (D90-D10) / D50 Magnetic substances / weight ppm Example 1 34 57 104 1.23 885 Example 2 23 49 98 1.53 850 Example 3 42 65.2 110 1.04 865 Example 4 38 58 137 1.71 900 Example 5 35 61 152 1.92 890 Example 6 36 62 135 1.60 888 Comparative Example 1 15 108 350 3.10 1165 Comparative Example 2 35 98 400 3.72 2950 Comparative Example 3 85 115 225 1.21 1205 Comparative Example 4 34 56 105 1.27 2498

[0128] It can be seen from the results in Table 1 that the method for preparing lithium iron phosphate positive electrode material of the present invention can obtain lithium iron phosphate positive electrode material with small particle size, good dispersibility, narrow particle size distribution and low magnetic material content by combining dispersant, grain growth inhibitor, lithium supplement agent and sand milling.

[0129] (3) Electrochemical performance test

[0130] The present invention tests the electrochemical properties of the lithium iron phosphate positive electrode materials prepared in the above embodiments and comparative examples, including: low-temperature capacity retention rate at -20°C and 1C, discharge capacity retention rate at 15C rate, high-temperature capacity residual rate and high-temperature capacity recovery rate stored at 60°C for 7 days, and cycle capacity retention rate after 500 cycles at 1C rate. The specific test results are shown in Table 2.

[0131] Table 2

[0132]

[0133]

[0134] It can be seen from the results in Table 2 that the method for preparing lithium iron phosphate positive electrode material of the present invention can obtain lithium iron phosphate positive electrode material with excellent rate performance, low temperature performance and high temperature performance by combining dispersant, grain growth inhibitor, lithium supplement agent and sand grinding.

[0135] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be regarded as the contents disclosed by the present invention and belong to the protection scope of the present invention.

Claims

1. A lithium iron phosphate positive electrode material, characterized in that: The expression of the lithium iron phosphate positive electrode material is LiFe 1- x M x PO4 / C, wherein 0<x≤0.05; M is at least one element selected from Ti, V and Nb; The particle size distribution of the lithium iron phosphate positive electrode material satisfies: (D90-D10) / D50=1-1.55; the content of magnetic substance in the lithium iron phosphate positive electrode material is 850-885 wt ppm; based on the total weight of the lithium iron phosphate positive electrode material, the carbon content in the lithium iron phosphate positive electrode material is 1.4-3.2 wt %; The lithium iron phosphate positive electrode material is a primary particle; the low-temperature capacity retention rate of the lithium iron phosphate positive electrode material under -20°C and 1C conditions is 70-80%.

2. A method for preparing the lithium iron phosphate positive electrode material as claimed in claim 1, characterized in that: The method includes: (1) In the presence of a solvent, a lithium salt, a phosphoric acid and an iron salt are mixed uniformly under an inert atmosphere and then subjected to a hydrothermal reaction to obtain a first slurry; the amounts of the lithium salt, the phosphoric acid and the iron salt are such that the molar ratio of the lithium element, the phosphorus element and the iron element is 2.8-3.2:1:1; the conditions of the hydrothermal reaction include a temperature of 130-200° C. and a time of 2-8 h; (2) washing the first slurry to obtain a second slurry; (3) adding an organic carbon source, a dispersant and a grain growth inhibitor to the second slurry, and performing sand milling under inert atmosphere to obtain a third slurry; the grain growth inhibitor is selected from at least one of tetraethyl titanate, ethyl titanate, vanadium pentoxide, ammonium metavanadate and niobium pentachloride; (4) uniformly mixing the third slurry and the lithium supplement agent, and then spray drying to obtain a precursor; (5) Sintering the precursor, and subjecting the sintered product to gas crushing to obtain a lithium iron phosphate positive electrode material.

3. The method according to claim 2, wherein: The amount of the organic carbon source is 8-16 wt % based on the dry basis of the second slurry; The amount of the dispersant is 0.5-5 wt % based on the dry basis of the second slurry; The amount of the grain growth inhibitor is 0.1-6 wt % based on the dry basis of the second slurry; Based on the dry basis of the third slurry, the amount of the lithium supplement agent is 0.2-5.5 wt %.

4. The method according to claim 3, wherein: The grain growth inhibitor is used in an amount of 0.3-5 wt % based on the dry basis of the second slurry; The amount of the lithium supplement agent is 0.5-5 wt % based on the dry basis of the third slurry.

5. The method according to any one of claims 2 to 4, wherein: The lithium salt is selected from at least one of lithium chloride, lithium sulfate, lithium hydroxide and lithium nitrate; The iron salt is selected from at least one of ferrous sulfate, ferrous chloride, ferrous nitrate and ferrous oxalate.

6. The method according to any one of claims 2 to 4, wherein: The organic carbon source is selected from at least one of sucrose, water-soluble phenolic resin, glucose, polyethylene glycol 6000, hydroxymethyl cellulose, polyacrylamide, starch and polyvinyl alcohol, wherein the weight average molecular weight of the polyacrylamide is 5 million to 12 million g / mol, and the weight average molecular weight of the polyvinyl alcohol is 200,000 to 700,000 g / mol; The dispersant is selected from at least one of sodium dodecyl sulfonate, polyethylene glycol 200, polyethylene glycol 400, sodium dodecyl sulfate, methyl amyl alcohol, sodium tripolyphosphate, sodium hexametaphosphate and sodium pyrophosphate.

7. The method according to any one of claims 2 to 4, wherein: The lithium supplement is selected from at least one of lithium dihydrogen phosphate, lithium monohydrogen phosphate and lithium phosphate.

8. The method according to any one of claims 2 to 4, wherein: In step (2), the washing specifically comprises: removing the reaction mother liquor in the first slurry, and then washing the centrifuged product with a dilute saline solution; the dilute saline solution is selected from at least one of aqueous solutions of lithium chloride, lithium nitrate, lithium dihydrogen phosphate, lithium monohydrogen phosphate and ammonium dihydrogen phosphate.

9. The method according to claim 8, wherein: The concentration of the dilute saline solution is 0.01-2 wt %.

10. The method according to any one of claims 2 to 4, wherein: In step (3), the sand grinding conditions include: a linear speed of 5-15 m / s and a time of 0.5-12 h.

11. The method according to any one of claims 2 to 4, wherein: In step (4), the spray drying conditions include: solid content of 30-50 wt%, temperature of 90-105°C, and time of 5-12h; In step (5), the sintering conditions include: temperature of 650-760°C and time of 5-10 h.

12. A lithium ion battery, characterized in that: Comprising the lithium iron phosphate positive electrode material as claimed in claim 1.

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