A lithium iron phosphate material and its preparation method, a lithium iron phosphate cathode slurry, and a lithium iron phosphate battery.

Abstract

This invention discloses a method for preparing lithium iron phosphate material. Flaky iron phosphate with a particle size D50 of 10-20 μm and ellipsoidal iron phosphate with a particle size D50 of 1-7 μm are mixed with a lithium source, a carbon source, an activator, and deionized water at a mass ratio of 100:(24-27):(7-15):(0.5-2):(150-250), and milled to obtain slurry X and slurry Y. Slurry X and slurry Y are then mixed at a mass ratio of (1-8):(1-8) and dried to obtain a precursor, which is then sintered, cooled, and pulverized. Compared with existing technologies, the lithium iron phosphate material prepared by this method has higher compaction density and excellent low-temperature discharge performance. This invention also discloses a method for preparing lithium iron phosphate cathode slurry. This method has a short slurry preparation time, high production efficiency, uniform and consistent slurry dispersion, and stable processing performance, effectively solving the particle agglomeration problem easily caused by existing slurry preparation methods.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery technology, specifically relating to a lithium iron phosphate material and its preparation method, a lithium iron phosphate cathode slurry, and a lithium iron phosphate battery. Background Technology

[0002] Lithium iron phosphate (LFP) materials possess advantages such as abundant raw material sources, non-toxicity, environmental friendliness, high safety, high specific capacity, stable cycle performance, and low cost, making them considered ideal cathode materials for lithium-ion batteries. However, LFP itself has poor conductivity, resulting in a low lithium-ion diffusion coefficient and poor low-temperature rate discharge performance. In the fields of power batteries and energy storage, high energy density is required, necessitating LFP materials with high compaction density while maintaining excellent low-temperature rate discharge performance. Therefore, there is an urgent need to develop a LFP material with both high compaction density and excellent low-temperature performance.

[0003] When using lithium iron phosphate (LFP) materials to prepare lithium iron phosphate (LFP) batteries, the first step is slurry preparation. Slurry preparation is a crucial step in the LFP battery production process, as the uniformity of dispersion of each component in the slurry directly affects the consistency of subsequent processing performance and electrical performance. Current cathode slurry mixing methods mainly include dry mixing and wet mixing. In dry mixing, the components are stirred and dispersed in a double planetary mixer. However, this method has dispersion dead zones, and the viscosity and fineness of the slurry vary at different locations within the container. Furthermore, the dispersion time is relatively long, resulting in a wide particle size distribution of the LFP cathode material and small conductive agent particles. This makes it impossible for conventional adhesives to effectively adhere all particles, causing the prepared cathode sheet to easily shed powder or even fall off. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of the existing technology by providing a lithium iron phosphate material and its preparation method. This invention utilizes iron phosphate raw materials with different particle sizes, surface morphologies, and properties, and adjusts the addition ratio. After sand milling and mixing of particles of varying sizes, lithium iron phosphate is prepared by sintering. This results in a lithium iron phosphate material with high compaction density and excellent low-temperature discharge performance.

[0005] Another objective of this invention is to provide a method for preparing lithium iron phosphate cathode slurry. In terms of selecting the binder, both high molecular weight binders are selected to improve adhesion, and low molecular weight binders are selected to improve slurry viscosity, improve the dispersibility of small particles, and prevent agglomeration. During the slurry preparation process, the dispersibility of the slurry is further improved by circulating the components through a homogenizing pump system, while shortening the slurry preparation time, resulting in a uniformly dispersed lithium iron phosphate cathode slurry without particle agglomeration.

[0006] A third objective of this invention is to provide a lithium iron phosphate battery.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] A method for preparing lithium iron phosphate material includes the following steps:

[0009] (1) Flake iron phosphate with a particle size D50 of 10-20μm is mixed with lithium source, carbon source, activator and deionized water in a mass ratio of 100:(24-27):(7-15):(0.5-2):(150-250) and then sand-milled to obtain slurry X;

[0010] (2) Ellipsoidal iron phosphate with a particle size D50 of 1-7 μm is mixed with lithium source, carbon source, activator and deionized water in a mass ratio of 100:(24-27):(7-15):(0.5-2):(150-250) and then sand-milled to obtain slurry Y;

[0011] (3) Mix slurry X and slurry Y evenly at a mass ratio of (1-8):(1-8) and dry to obtain lithium iron phosphate precursor;

[0012] (4) The lithium iron phosphate precursor is sintered, cooled to room temperature and pulverized to obtain lithium iron phosphate material.

[0013] In steps (1) and (2), the lithium source is selected from any one of lithium phosphate, lithium hydroxide, lithium chloride, lithium carbonate, lithium dihydrogen phosphate, or lithium acetate; the carbon source is selected from any one of maltose, potato starch, glucose, cellulose, sucrose, citric acid, fructose, or starch; and the activator is selected from any one of polyethylene glycol (PEG), polyvinyl alcohol (PVA), polypropylene glycol (PPG), or CTAB.

[0014] Furthermore, in step (1), the rotation speed of the sand mill is 600 to 1400 rpm, and the sand mill particle size is controlled to be 1 to 1.5 μm.

[0015] Furthermore, in step (2), the rotation speed of the sand mill is 600 to 1400 rpm, and the sand mill particle size is controlled to be 0.2 to 0.7 μm.

[0016] Furthermore, in step (3), the drying temperature is 120–350°C.

[0017] Furthermore, in step (4), the sintering is carried out in a nitrogen atmosphere at a temperature of 500–800°C for 3–6 hours.

[0018] Furthermore, in step (4), the pulverization refers to pulverizing to a particle size D50 of 0.5 to 5.5 μm.

[0019] The lithium iron phosphate material prepared by the above method.

[0020] A method for preparing lithium iron phosphate cathode slurry: The above-mentioned lithium iron phosphate material is stirred and dispersed with a conductive agent and a binder in one step, and then a solvent is added, and the mixture is stirred and dispersed in a second step and homogenized to obtain lithium iron phosphate cathode slurry;

[0021] The mass ratio of the lithium iron phosphate material to the conductive agent, binder, and solvent is 100:(2-5):(2.5-3.5):(70-90); the conductive agent is selected from one or more of Ketjen Black, SFG-6, Super-P, acetylene black, KS-6, or KS-15 in any proportion; the solvent is N-methylpyrrolidone (NMP); the binder is a mixture of high molecular weight polyvinylidene fluoride with a molecular weight greater than 1 million and low molecular weight polyvinylidene fluoride with a molecular weight of 300,000-500,000 in a mass ratio of (1:1-2).

[0022] Furthermore, the stirring speed for the first stirring and dispersion is 10-35 rpm, and the dispersion speed is 150-400 rpm; the stirring speed for the second stirring and dispersion is 10-35 rpm, and the dispersion speed is 800-1400 rpm; the homogenization is performed using a homogenizing pump, and the speed of the homogenizing pump during homogenization is 800-3000 rpm.

[0023] A lithium iron phosphate battery includes a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode is prepared using the aforementioned lithium iron phosphate positive electrode slurry.

[0024] The beneficial effects of this invention are:

[0025] 1) This invention discloses a lithium iron phosphate material and its preparation method. By selecting iron phosphate raw materials with different particle sizes, surface morphologies and properties and adjusting the addition ratio, the materials are mixed and sintered after sand milling, so that the prepared lithium iron phosphate material has high compaction density and excellent low-temperature discharge performance.

[0026] 2) This invention discloses a method for preparing lithium iron phosphate cathode slurry. This method has a short slurry preparation time, good bonding effect, high production efficiency, more uniform slurry dispersion, good consistency, and stable processing performance, effectively solving the problem of particle agglomeration that is easily caused by existing slurry preparation methods. Attached Figure Description

[0027] Figure 1 This is a SEM image of the lithium iron phosphate cathode sheet prepared using the lithium iron phosphate cathode slurry of Example 2.

[0028] Figure 2 This is a SEM image of a lithium iron phosphate cathode sheet prepared using the lithium iron phosphate cathode slurry of Comparative Example 2. Detailed Implementation

[0029] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. The embodiments are only for the purpose of helping to understand the present invention, but the implementation of the present invention is not limited thereto.

[0030] Example 1

[0031] A method for preparing lithium iron phosphate material includes the following steps:

[0032] (1) 7 kg of flaky ferric phosphate (purchased from Tongling Nayuan Materials) with a particle size D50 of 15 μm, 1.82 kg of lithium carbonate, 0.574 kg of glucose, 0.039 kg of polyvinyl alcohol (PVA1792) and 14.2 kg of deionized water were added to a mixing tank and stirred evenly. Then, a horizontal sand mill was used to grind the mixture at a speed of 900 rpm, and the grinding particle size was controlled to be 1-1.5 μm to obtain slurry X.

[0033] (2) 3 kg of ellipsoidal iron phosphate (purchased from Tinci Materials) with a particle size D50 of 5 μm, 0.78 kg of lithium carbonate, 0.246 kg of glucose, 0.017 kg of polyvinyl alcohol (PVA1792) and 6.086 kg of deionized water were added to a mixing tank and stirred evenly. Then, a horizontal sand mill was used to grind the mixture at a speed of 900 rpm, and the grinding particle size was controlled at 0.2-0.7 μm to obtain slurry Y.

[0034] (3) After mixing 23.633 kg of slurry X and 10.129 kg of slurry Y evenly, the mixture was dried at 250°C using a spray dryer to obtain lithium iron phosphate precursor;

[0035] (4) The lithium iron phosphate precursor was placed in a box furnace and sintered at 700°C for 4 hours under a nitrogen atmosphere. After the sintering was completed, it was cooled to room temperature and pulverized at 35 Hz using an air jet mill. The particle size D50 after pulverization was controlled at 0.5-5.5 μm to obtain lithium iron phosphate material, which was marked as Y3.

[0036] Preparation of lithium iron phosphate cathode slurry: 10 kg of lithium iron phosphate material (Y3), 0.280 kg of Ketjen Black, 0.123 kg of KS-6, 0.166 kg of high molecular weight polyvinylidene fluoride (PVDF) (molecular weight 1.05 million) and 0.166 kg of low molecular weight polyvinylidene fluoride (PVDF) (molecular weight 350,000) were stirred and dispersed once in a double planetary mixer (stirring speed 22 rpm, dispersion speed 200 rpm, stirring time 30 min). Then, 7.459 kg of NMP was added, and a second stirring and dispersion was carried out (stirring speed 25 rpm, dispersion speed 1200 rpm, stirring time 3 h). Homogenization was carried out using a homogenizer (homogenizer speed 1500 rpm) to control the solid content of the slurry to 59%, thus obtaining the lithium iron phosphate cathode slurry.

[0037] Example 2

[0038] A method for preparing lithium iron phosphate material includes the following steps:

[0039] (1) 3 kg of flaky ferric phosphate (purchased from Tongling Nayuan Materials) with a particle size D50 of 15 μm, 0.78 kg of lithium carbonate, 0.246 kg of glucose, 0.017 kg of polyvinyl alcohol (PVA1792) and 6.086 kg of deionized water were added to a mixing tank and stirred evenly. Then, a horizontal sand mill was used to grind the mixture at a speed of 900 rpm, and the grinding particle size was controlled to be 1-1.5 μm to obtain slurry X.

[0040] (2) 7 kg of ellipsoidal iron phosphate (purchased from Tinci Materials) with a particle size D50 of 5 μm, 1.82 kg of lithium carbonate, 0.574 kg of glucose, 0.039 kg of polyvinyl alcohol (PVA1792) and 14.2 kg of deionized water were added to a mixing tank and stirred evenly. Then, a horizontal sand mill was used to grind the mixture at a speed of 900 rpm, and the grinding particle size was controlled at 0.2-0.7 μm to obtain slurry Y.

[0041] (3) After mixing 10.129 kg of slurry X and 23.633 kg of slurry Y evenly, the mixture was dried at 250°C using a spray dryer to obtain the lithium iron phosphate precursor.

[0042] (4) The lithium iron phosphate precursor was placed in a box furnace and sintered at 700°C for 4 hours under a nitrogen atmosphere. After the sintering was completed, it was cooled to room temperature and pulverized at 35 Hz using an air jet mill. The particle size D50 after pulverization was controlled at 0.5-5.5 μm to obtain lithium iron phosphate material, which was marked as Y7.

[0043] Preparation of lithium iron phosphate cathode slurry: 10 kg of lithium iron phosphate material (Y7), 0.280 kg of Ketjen Black, 0.123 kg of KS-6, 0.166 kg of high molecular weight polyvinylidene fluoride (PVDF) (molecular weight 1.05 million) and 0.166 kg of low molecular weight polyvinylidene fluoride (PVDF) (molecular weight 350,000) were stirred and dispersed once in a double planetary mixer (stirring speed 22 rpm, dispersion speed 200 rpm, stirring time 30 min). Then, 7.459 kg of NMP was added, and a second stirring and dispersion was carried out (stirring speed 25 rpm, dispersion speed 1200 rpm, stirring time 3 h). Homogenization was carried out using a homogenizer (homogenizer speed 1500 rpm) to control the solid content of the slurry to 59%, thus obtaining the lithium iron phosphate cathode slurry.

[0044] Example 3

[0045] The preparation of lithium iron phosphate material is the same as in Example 1.

[0046] Preparation of lithium iron phosphate cathode slurry: 10 kg of lithium iron phosphate material (Y3), 0.280 kg of Ketjen Black, 0.123 kg of KS-6, 0.133 kg of high molecular weight polyvinylidene fluoride (PVDF) (molecular weight 1.05 million) and 0.199 kg of low molecular weight polyvinylidene fluoride (PVDF) (molecular weight 350,000) were stirred and dispersed once in a double planetary mixer (stirring speed 22 rpm, dispersion speed 200 rpm, stirring time 30 min). Then, 7.459 kg of NMP was added, and a second stirring and dispersion was carried out (stirring speed 25 rpm, dispersion speed 1200 rpm, stirring time 3 h). Homogenization was carried out using a homogenizer (homogenizer speed 1500 rpm) to control the solid content of the slurry to 59%, thus obtaining the lithium iron phosphate cathode slurry.

[0047] Comparative Example 1

[0048] A method for preparing lithium iron phosphate material includes the following steps:

[0049] (1) 10 kg of ellipsoidal iron phosphate (purchased from Tinci Materials) with a particle size D50 of 5 μm, 2.6 kg of lithium carbonate, 0.82 kg of glucose, 0.056 kg of polyvinyl alcohol (PVA1792) and 20.286 kg of deionized water were added to a mixing tank and mixed evenly. Then, a horizontal sand mill was used to grind the mixture at a speed of 900 rpm, and the grinding particle size was controlled at 0.2-0.7 μm to obtain a slurry.

[0050] (2) The slurry was dried at 250°C using a spray dryer to obtain the precursor;

[0051] (3) The precursor was placed in a box furnace and sintered at 700°C for 4 hours under a nitrogen atmosphere. After the sintering was completed, it was cooled to room temperature and pulverized at 35 Hz using an air jet mill. The particle size D50 after pulverization was controlled at 0.5-5.5 μm to obtain lithium iron phosphate material, which was marked as Y10.

[0052] Preparation method of lithium iron phosphate cathode slurry: 10 kg of lithium iron phosphate material (Y10), 0.280 kg of Ketjen Black, 0.123 kg of KS-6, 0.166 kg of high molecular weight polyvinylidene fluoride (PVDF) (molecular weight 1.05 million) and 0.166 kg of low molecular weight polyvinylidene fluoride (PVDF) (molecular weight 350,000) were stirred and dispersed once in a double planetary mixer (stirring speed 22 rpm, dispersion speed 200 rpm, stirring time 30 min). Then, 7.459 kg of NMP was added, and a second stirring and dispersion was carried out (stirring speed 25 rpm, dispersion speed 1200 rpm, stirring time 3 h). Homogenization was carried out using a homogenizer pump (homogenizer pump speed 1500 rpm) to control the solid content of the slurry to 59%, thus obtaining the lithium iron phosphate cathode slurry.

[0053] Comparative Example 2

[0054] The preparation of lithium iron phosphate material is the same as in Example 2, but the preparation of lithium iron phosphate cathode slurry is different.

[0055] Preparation method of lithium iron phosphate cathode slurry: 10 kg of lithium iron phosphate material (Y7), 0.280 kg of Ketjen Black, 0.123 kg of KS-6, and 0.332 kg of high molecular weight polyvinylidene fluoride (PVDF) (molecular weight 1.05 million) were stirred and dispersed once in a double planetary mixer (stirring speed 22 rpm, dispersion speed 200 rpm, stirring time 30 min). Then, 7.459 kg of NMP was added, and a second stirring and dispersion was carried out (stirring speed 25 rpm, dispersion speed 1200 rpm, stirring time 4.5 h). The solid content of the slurry was controlled at 59% to obtain lithium iron phosphate cathode slurry.

[0056] Performance testing:

[0057] 1. Low-temperature discharge performance test

[0058] Test method: The lithium iron phosphate positive electrode slurry prepared in Example 1 and Comparative Example 1 was coated on 15mm thick aluminum foil, dried and rolled to obtain lithium iron phosphate positive electrode sheets. The corresponding graphite negative electrode slurry was coated on copper foil, rolled and dried to obtain negative electrode sheets. LiPF6 / EC+DMC (volume ratio 1:1) was used as electrolyte. A 15μm PE ceramic separator was used and then wound to form a bare cell. The bare cell was placed in a steel shell and assembled into a cell. The electrolyte was injected into the steel shell. After sealing, pre-charging, formation and capacity testing, lithium iron phosphate lithium-ion battery was obtained. Then the discharge capacity of lithium iron phosphate lithium-ion battery at different temperatures was tested and the discharge efficiency D at different temperatures was calculated. Discharge efficiency D = discharge capacity at different temperatures / discharge capacity at 25℃ * 100%.

[0059] The test results are shown in Table 1:

[0060] Table 1

[0061]

[0062] As can be seen from Table 1, Example 1 exhibits superior discharge performance at different temperatures.

[0063] 2. The lithium iron phosphate cathode slurries prepared in Example 2 and Comparative Example 2 were coated onto 15 mm aluminum foil, dried, and then subjected to scanning electron microscopy to obtain lithium iron phosphate cathode sheets.

[0064] Figure 1 The image shows a SEM image of a lithium iron phosphate cathode sheet prepared using the lithium iron phosphate cathode slurry of Example 2. It can be seen that the particle size distribution on the electrode surface is uniform, the interparticle gaps are relatively obvious, and there is no agglomeration. Figure 2The image shows a SEM image of a lithium iron phosphate cathode sheet prepared using the lithium iron phosphate cathode slurry of Comparative Example 2. It can be seen that there are multiple particle agglomerations on the surface of the electrode sheet, and the dispersion effect is not ideal. This indicates that the slurry preparation method of the present invention can prepare a uniformly dispersed cathode slurry without agglomeration in a short time.

[0065] 3. The compaction density of the lithium iron phosphate material in Example 3 was tested.

[0066] Test method: The lithium iron phosphate cathode slurry prepared in Example 3 was coated on a 15mm thick aluminum foil and dried to obtain a lithium iron phosphate cathode sheet. The cathode sheet was rolled by a roller press and the compaction density of the cathode sheet was tested. Compaction density = areal density / (thickness of cathode sheet after rolling - thickness of aluminum foil).

[0067] Test results: The compacted density was measured to be 2.61 g / cm³. 3 After rolling, the electrode sheets do not become brittle or fall off when folded in half.

[0068] The above description is only a specific embodiment of the present invention, and any modifications and changes to the present invention fall within the protection scope and disclosure scope of the present invention.

Claims

1. A method for producing a lithium iron phosphate material, characterized by, Comprising the following steps: (1) Stir and mix the flaky iron phosphate with particle size D50 of 10-20 μm, lithium source, carbon source, active agent, deionized water according to the mass ratio of 100:(24~27):(7~15):(0.5~2):(150~250), then sand mill to obtain slurry X; (2) Stir and mix the ellipsoidal iron phosphate with particle size D50 of 1-7 μm, lithium source, carbon source, active agent, deionized water according to the mass ratio of 100:(24~27):(7~15):(0.5~2):(150~250), then sand mill to obtain slurry Y; (3) Mix slurry X and slurry Y uniformly according to the mass ratio of (1~8):(1~8), then dry to obtain lithium iron phosphate precursor; (4) Sinter the lithium iron phosphate precursor, cool to room temperature after completion, and crush to obtain lithium iron phosphate material; In steps (1) and (2), the lithium source is selected from any one of lithium phosphate, lithium hydroxide, lithium chloride, lithium carbonate, lithium dihydrogen phosphate or lithium acetate; the carbon source is selected from any one of maltose, potato powder, glucose, cellulose, sucrose, citric acid, fructose or starch; the active agent is selected from any one of polyethylene glycol, polyvinyl alcohol, polypropylene glycol or CTAB; In step (1), the rotation speed of sand mill is 600~1400 rpm, and the sand mill particle size is controlled to be 1~1.5 μm; In step (2), the rotation speed of sand mill is 600~1400 rpm, and the sand mill particle size is controlled to be 0.2~0.7 μm.

2. The method for preparing lithium iron phosphate material as described in claim 1, characterized in that, In step (3), the drying temperature is 120~350℃.

3. The method of claim 1, wherein the lithium iron phosphate material is prepared by the steps of: mixing lithium carbonate, iron oxide, and phosphoric acid; and heating the mixture at a temperature of 600 to 800°C for 2 to 10 hours. In step (4), the sintering is carried out in nitrogen atmosphere, the sintering temperature is 500~800℃, and the time is 3~6 h.

4. The method for preparing the lithium iron phosphate material according to any one of claims 1 to 3, characterized in that, In step (4), the crushing refers to crushing to a particle size D50 of 0.5~5.5 μm.

5. The lithium iron phosphate material prepared by the method of any one of claims 1 to 4.

6. A method of preparing a lithium iron phosphate cathode slurry, characterized by, Stir and disperse the lithium iron phosphate material of claim 5, conductive agent and binder once, then add solvent, stir and disperse twice and homogenize to obtain lithium iron phosphate positive electrode slurry; The mass ratio of the lithium iron phosphate material, conductive agent, binder and solvent is 100:(2~5):(2.5~3.5):(70~90); the conductive agent is selected from one or more than two of ketjen black, SFG-6, Super-P, acetylene black, KS-6 or KS-15 in any proportion; the solvent is N-methyl pyrrolidone; the binder is a mixture of high molecular weight polyvinylidene fluoride with a molecular weight greater than 1 million and low molecular weight polyvinylidene fluoride with a molecular weight of 30-500 thousand according to the mass ratio of (1:1~2).

7. The method of claim 6, wherein the lithium iron phosphate cathode slurry is prepared by the steps of: mixing the lithium iron phosphate, the binder, and the solvent to form a mixture; and adding the dispersant to the mixture. The stirring rotation speed of the first stirring and dispersion is 10~35 rpm, and the dispersion rotation speed is 150~400 rpm; the stirring rotation speed of the second stirring and dispersion is 10~35 rpm, and the dispersion rotation speed is 800~1400 rpm; the homogenization is carried out by using a homogenizer, and the rotation speed of the homogenizer is 800~3000 rpm.

8. A lithium iron phosphate battery comprising a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte, characterized by, The positive electrode plate is prepared by using the lithium iron phosphate positive electrode slurry of claim 6 or 7. The positive electrode plate is prepared by using the lithium iron phosphate positive electrode slurry of claim 6 or 7.

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