Lithium iron phosphate positive electrode material, preparation method and application

Abstract

The invention discloses a lithium iron phosphate positive electrode material, a preparation method and application, and the preparation method comprises the following steps: carrying out spray drying on slurry containing iron phosphate, a lithium source, a carbon source, polyvinylidene fluoride, a surfactant and water to obtain a spherical spray material; pressing the spherical spraying material to obtain a pressed material; and putting the pressed material into a bowl, and sintering in a protective atmosphere to obtain the lithium iron phosphate positive electrode material. Carboxymethyl cellulose and polyvinylidene fluoride are introduced into the slurry, through the synergistic effect of intermolecular hydrogen bonds and Van der Waals' force, balance of strong internal binding and surface lubricity of the lithium iron phosphate spray material can be achieved, and through cooperation with spray granulation, the fine powder rate of the spherical spray material can be reduced, and the bulk density can be increased; after pressing, the bowl loading amount can be increased, sintering powder spraying can be inhibited, and cost reduction and yield increase are achieved by improving efficiency and reducing energy consumption and material loss; in addition, the prepared LFP also has better hydrophobicity.

Description

Technical Field

[0001] The present application relates to the technical field of lithium iron phosphate materials, and more specifically, to lithium iron phosphate positive electrode materials, preparation methods, and applications. Background Art

[0002] There are several technical problems that need to be solved in the production process of lithium iron phosphate materials. First, the lithium iron phosphate materials prepared by traditional spray drying process have poor particle flowability and low tap density (usually only 1.2g / cm 3 ) and other issues have limited the amount of pots loaded, typically between 5kg and 9kg per pot. This not only restricts capacity expansion but also significantly increases production costs. One of the key technical challenges currently faced is how to increase pot loading capacity based on existing kiln equipment, thereby increasing production capacity.

[0003] Secondly, during the sintering process of the powder after centrifugal spraying, the lithiation reaction and carbonization reaction stages are prone to produce "boiling powder spraying" due to the thermal expansion of the powder, which not only causes 5%-10% material loss, but also leads to sagger pollution, seriously affecting production efficiency and product quality.

[0004] In view of this, this application is hereby filed. Summary of the Invention

[0005] The purpose of this application is to provide a lithium iron phosphate positive electrode material, a preparation method and an application, to solve the problem that the low filling amount of lithium iron phosphate material restricts production capacity and is prone to "boiling powder spraying" phenomenon causing material loss.

[0006] This application is implemented as follows:

[0007] In a first aspect, the present application provides a method for preparing a lithium iron phosphate positive electrode material, comprising:

[0008] spray drying a slurry comprising iron phosphate, a lithium source, a carbon source, polyvinylidene fluoride, a surfactant and water to obtain a spherical spray material;

[0009] Compressing the spherical spray material to obtain a compressed material;

[0010] After the pressed material is placed in a pot, it is sintered under a protective atmosphere to obtain the lithium iron phosphate positive electrode material.

[0011] In an optional embodiment, the surfactant includes at least one of carboxymethyl cellulose, sodium lauryl sulfate, and cetyltrimethylammonium bromide;

[0012] and / or, the mass ratio of the surfactant to the polyvinylidene fluoride in the slurry is 1:(18-22);

[0013] And / or, the mass fraction of polyvinylidene fluoride in the slurry is 0.034%-0.07%.

[0014] And / or, the preparation of the slurry comprises: adding polytetrafluoroethylene to an aqueous solution of a surfactant to dissolve the polytetrafluoroethylene to obtain a mixed solution, and then mixing and sand-milling the mixed solution with iron phosphate, a lithium source, and a carbon source to obtain the slurry;

[0015] In an optional embodiment, the sanding step separately sands to obtain abrasive A with an average solid particle size of 1-1.5 μm and abrasive B with an average solid particle size of 0.3-0.5 μm, and the abrasive A and abrasive B are mixed in a mass ratio of 3 / 7-4 / 6 to obtain the slurry.

[0016] In an optional embodiment, the solid content of the slurry is 30%-40%;

[0017] And / or, the molar ratio of iron, lithium and carbon in the slurry is 1:(1-1.1):(0.05-0.1).

[0018] In an optional embodiment, the air inlet temperature of the equipment used for spray drying is 210°C-230°C, and the air outlet temperature is 90°C-110°C.

[0019] And / or, the Dv50 of the spherical spray material is 10 μm-30 μm,

[0020] In an optional embodiment, the equipment used in the pressing step is a dry granulator. In the pressing step, the parameters of the dry granulator meet at least one of the following characteristics:

[0021] a. The oil pressure of the dry granulator is 5MPa-25MPa;

[0022] b. The gap between the pressure rollers is 0.5mm-1.2mm;

[0023] c. Feed speed is 5Hz-25Hz;

[0024] d. The speed of the main pressure roller is 5Hz-30Hz;

[0025] e. The pressure roller is provided with a silicon carbide coating;

[0026] f. The dry granulator is connected to the bowl loading machine via a conveying pipe, and the conveying pipe is provided with an inlet section, a contraction section, a throat section, a diffusion section and a discharge section in sequence along the direction of material conveying.

[0027] In an optional embodiment, the ratio of the inner diameters of the inlet section, the throat section, and the discharge section is 75-85:25:75-85;

[0028] and / or, the angle between the inner wall of the contraction section and the axis is ≤15°;

[0029] and / or, the angle between the inner diameter of the diffuser section and the axis is ≤5°;

[0030] And / or, the ratio of the length to the diameter of the throat section is 5-8.

[0031] In an optional embodiment, the average particle size of the pressed material is 1 mm to 3 mm;

[0032] And / or, the tap density of the pressed material is 1.1 g / cm 3 -1.7g / cm 3 ;

[0033] And / or, the bulk density of the pressed material is 0.8 g / cm 3 -0.95g / cm 3 .

[0034] And / or, the filling amount is 12Kg-14Kg.

[0035] In an optional embodiment, the heating rate of the sintering step is 2°C / min-5°C / min;

[0036] and / or, the holding temperature of the sintering step is 800° C.-820° C.;

[0037] and / or, the holding time of the sintering step is 8h-12h;

[0038] And / or, the method further includes crushing the sintered material.

[0039] In a second aspect, the present application provides a lithium iron phosphate positive electrode material prepared by the method described in the aforementioned embodiment, wherein the DV50 of the lithium iron phosphate positive electrode material is 1 μm to 1.5 μm;

[0040] And / or, the compaction density of the lithium iron phosphate positive electrode material is 2.5 g / cm 3 2.6 g / cm 3 ;

[0041] And / or, the water absorption rate of the lithium iron phosphate positive electrode material is ≤0.7%.

[0042] In a third aspect, the present application provides a lithium-ion battery comprising the lithium iron phosphate positive electrode material described in the aforementioned embodiment.

[0043] This application has the following beneficial effects:

[0044] In this application, the carbon source usually has a certain viscosity, but the viscosity is weak. Surfactants and polyvinylidene fluoride are introduced into the slurry. The two work synergistically through intermolecular hydrogen bonds and van der Waals forces to achieve a balance between strong internal bonding and surface lubricity of the lithium iron phosphate spray material. Combined with the means of spray granulation, it is beneficial to reduce the fine powder rate of the spherical spray material and increase the bulk density.

[0045] During the pressing process, the presence of a small amount of fine powder in the spherical spray material can match with the large particles, and squeeze them into denser particles during the pressing process, further improving the sintering grading effect of the material. The particles formed after sintering and crushing are smaller and have a higher density, which is beneficial to increase the filling amount while suppressing sintering powder spraying, thereby achieving cost reduction and production increase by improving efficiency, reducing energy consumption and material loss.

[0046] In this application, polyvinylidene fluoride not only improves the bonding performance of the LFP spray material, but also allows LFP to inherit this hydrophobic property due to its own hydrophobic properties. However, polyvinylidene fluoride is insoluble in water, so a surfactant needs to be introduced. On the one hand, it is beneficial to the solubility of polyvinylidene fluoride in water, and on the other hand, it can prevent particle agglomeration and improve the uniformity of the slurry. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following is a brief introduction to the drawings required for use in the embodiments. It should be understood that the following drawings only show certain embodiments of the present application and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without creative work.

[0048] Figure 1 It is a structural diagram of the transmission pipeline;

[0049] Figure 2 This is a SEM image of the lithium iron phosphate cathode material prepared in Example 1;

[0050] Figure 3 This is the XRD pattern of the lithium iron phosphate positive electrode material prepared in Example 1.

[0051] Diagram: 1- inlet section; 2- contraction section; 3- throat section; 4- diffusion section; 5- discharge section. DETAILED DESCRIPTION

[0052] To make the purpose, technical solutions and advantages of the examples of the present application clearer, the technical solutions in the examples of the present application will be described clearly and completely below. Where specific conditions are not specified in the examples, conventional conditions or conditions recommended by the manufacturer are used. Where the manufacturer of the reagents or instruments is not specified, they are all conventional products that can be purchased commercially.

[0053] The present application provides a method for preparing a lithium iron phosphate (LFP) positive electrode material, comprising:

[0054] spray drying a slurry comprising iron phosphate, a lithium source, a carbon source, polyvinylidene fluoride, a surfactant and water to obtain a spherical spray material;

[0055] Compressing the spherical spray material to obtain a compressed material;

[0056] After the pressed material is placed in a pot, it is sintered under a protective atmosphere to obtain the lithium iron phosphate positive electrode material.

[0057] In some embodiments of the present application, the lithium source may be lithium carbonate or lithium hydroxide, and the carbon source may be glucose or sucrose.

[0058] Most of the carbon sources used in this application have a certain viscosity, but the viscosity is relatively weak. Surfactants and polyvinylidene fluoride are introduced into the slurry. The two work synergistically through intermolecular hydrogen bonds and van der Waals forces to achieve a balance between strong internal bonding and surface lubricity of the lithium iron phosphate spray material. Combined with the means of spray granulation, it is beneficial to reduce the fine powder rate of the spherical spray material and increase the bulk density.

[0059] The presence of a small amount of remaining fine powder in the spherical spray material can be matched with the large particles, and squeezed into more compact particles during the pressing process, further improving the sintering grading effect of the material. The particles formed after sintering and crushing are smaller and have a higher density, which is beneficial to increase the filling amount while inhibiting sintering powder spraying, thereby achieving cost reduction and production increase by improving efficiency, reducing energy consumption and material loss.

[0060] It should be noted that in this application, polyvinylidene fluoride not only improves the bonding performance of the LFP spray material, but also makes LFP inherit this hydrophobic property due to its own hydrophobic properties. However, polyvinylidene fluoride is insoluble in water, so a surfactant needs to be introduced. On the one hand, it is beneficial to the solubility of polyvinylidene fluoride in water, and on the other hand, it can prevent particle agglomeration and improve the uniformity of the slurry.

[0061] In an optional embodiment, the surfactant includes at least one of carboxymethyl cellulose, sodium lauryl sulfate, and cetyltrimethylammonium bromide. In some embodiments, the surfactant is cetyltrimethylammonium bromide.

[0062] In an optional embodiment, the mass ratio of the surfactant to polyvinylidene fluoride in the slurry is 1:(18-22), such as 1:18, 1:19, 1:20, 1:21, 1:22; sufficient surfactant is present to ensure that the polyvinylidene fluoride can be dissolved, and at the same time, it is beneficial to improve the uniformity of particle dispersion in the slurry.

[0063] In an optional embodiment, the mass fraction of polyvinylidene fluoride in the slurry is 0.034%-0.07%, such as 0.034%, 0.04%, 0.05%, 0.06%, and 0.07%. An appropriate amount of polyvinylidene fluoride can ensure the bonding effect on the particles while reducing the impact on the electrical properties of the lithium iron phosphate positive electrode material.

[0064] In an optional embodiment, the slurry is prepared by adding polytetrafluoroethylene to an aqueous solution of a surfactant to dissolve the polytetrafluoroethylene to obtain a mixed solution, and then mixing and sand-milling the solution with iron phosphate, a lithium source, and a carbon source to obtain the slurry. During the slurry preparation process, the order of adding the materials is very important for the dissolution of the polytetrafluoroethylene. Preparing the aqueous solution of the surfactant first and then adding the polytetrafluoroethylene can produce a uniform mixed aqueous solution of the surfactant and polyvinylidene fluoride. Conversely, if the polyvinylidene fluoride is directly added to water and then the surfactant is added, the polyvinylidene fluoride will not dissolve in the water.

[0065] In an optional embodiment, the sum of the mass fractions of the surfactant and polyvinylidene fluoride in the mixed solution is 40%-50%, ensuring that the surfactant and polyvinylidene fluoride are dissolved to obtain a uniform mixed solution.

[0066] In an optional embodiment, the sanding step separately sands to obtain abrasive A with an average solid particle size of 1-1.5 μm and abrasive B with an average solid particle size of 0.3-0.5 μm, and the abrasive A and abrasive B are mixed in a mass ratio of 3 / 7-4 / 6 to obtain the slurry.

[0067] In an optional embodiment, the solid content of the slurry is 30%-40%, such as 30%, 35%, or 40%. This solid content does not cause nozzle clogging, is easy to atomize, and does not lead to excessively long drying time, which is beneficial to increasing the density of the spherical atomized material.

[0068] And / or, the molar ratio of iron, lithium and carbon in the slurry is 1:(1-1.1):(0.05-0.1), such as 1:1:0.1, 1:1.02:0.09, 1:1.04:0.08, 1:10.6:0.07, 1:1.08:0.06, 1:1.10:0.05.

[0069] In an optional embodiment, the average particle size of the particles in the slurry is 0.4 μm-0.6 μm, such as 0.4 μm, 0.5 μm, and 0.6 μm. Small-sized raw material particles are beneficial to improving the uniformity of the slurry, and thus are beneficial to obtaining spherical atomized materials with higher component uniformity.

[0070] In an optional embodiment, the air inlet temperature of the equipment used for spray drying is 210°C-230°C, such as 210°C, 220°C, and 230°C, and the air outlet temperature is 90°C-110°C, such as 90°C, 100°C, and 110°C, which is conducive to obtaining spherical spray materials with appropriate moisture content. If the temperature rises, the moisture content is too low, which will result in an increase in the fine powder rate and a decrease in the bulk density; if the temperature drops, the moisture content is too high, which will lead to a decrease in the electrical properties of the prepared lithium iron phosphate product.

[0071] And / or, the Dv50 of the spherical spray material is 10 μm-30 μm, such as 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, which is beneficial for obtaining a compressed material with higher density in the subsequent compression step.

[0072] In an optional embodiment, the equipment used in the pressing step is a dry granulator. In the pressing step, the parameters of the dry granulator meet at least one of the following characteristics:

[0073] a. The oil pressure of the dry granulator is 5MPa-25MPa, such as 5MPa, 10MPa, 15MPa, 20MPa, 25MPa;

[0074] b. The gap between the pressure rollers is 0.5mm-1.2mm, such as 0.5mm, 0.6mm, 0.8mm, 1.0mm, 1.2mm;

[0075] c. Feed speed is 5Hz-25Hz, such as 5MPa, 10MPa, 15MPa, 20MPa, 25MPa;

[0076] d. The speed of the main roller is 5Hz-30Hz, such as 5MPa, 10MPa, 15MPa, 20MPa, 25MPa, 30Hz;

[0077] e. The pressure roller is provided with a silicon carbide coating to prevent the material from sticking to the pressure roller;

[0078] f. The dry granulator is connected to the bowl loading machine via a delivery pipe, and the delivery pipe is provided with an inlet section 1, a contraction section 2, a throat section 3, a diffusion section 4 and a discharge section 5 along the direction of material delivery. The specific structure is as follows. Figure 1 shown.

[0079] The dry granulator used in this application can be a commercially available dry granulator. Under the above conditions, it is advantageous to obtain a compressed material with a higher density while avoiding excessive pressure that increases internal cracks in the granules and reduces electrical properties. The conveying pipeline is similar to a Venturi tube, which can prevent shear forces generated by the high-speed flow of the granules from damaging the granules and affecting the filling capacity.

[0080] In an optional embodiment, the ratio of the inner diameters of the inlet section 1, the throat section 3 and the discharge section 5 is 75-85:25:75-85; for example, the inner diameters of the inlet section 1, the throat section 3 and the discharge section 5 are Φ80 mm, Φ25 mm and Φ80 mm respectively;

[0081] and / or, the angle between the inner wall of the contraction section 2 and the axis is ≤15°, such as 15°, 14°, 13°, 12°, 11° and 10°;

[0082] and / or, the angle between the inner diameter of the diffuser section 4 and the axis is ≤5°, such as 5°, 4° and 3°;

[0083] And / or, the ratio of the length to the diameter of the throat section 3 is 5-8, such as 1, 6, 7 and 8.

[0084] The reduction in the inner diameter of the throat section 3 in the conveying pipeline can speed up the flow rate of the material. However, if the inner diameter is too small and the material flow rate is too fast, the probability of particle collision will increase. Therefore, the inner diameter of the throat section 3 needs to be reasonably set; further, the length of the throat section 3 should not be too short so that it can be stable and smooth and reduce the turbulent kinetic energy of the material; in addition, the angle between the inner wall of the contraction section 2 and the diffusion section 4 and the axis should not be too large to avoid the pressure change too fast to increase the probability of particle collision, which is beneficial to reduce internal cracks in the particles. If necessary, more than two discharging units can be arranged in sequence behind the throat section 3, and each discharging unit includes a diffusion section and a sub-discharging section arranged in sequence to restore the pressure step by step to prevent the sudden drop in particle pressure and inertial collision. Among them, the sub-discharging section farthest from the throat section is the discharging section 5.

[0085] In an optional embodiment, the average particle size of the pressed material is 1 mm to 3 mm, such as 1 mm, 2 mm, or 3 mm;

[0086] And / or, the tap density of the pressed material is 1.1 g / cm 3 -1.7g / cm 3 , such as 1.1g / cm 3 , 1.2g / cm 3 , 1.3g / cm 3 , 1.4g / cm 3 , 1.5g / cm 3 , 1.6g / cm 3 , 1.7g / cm 3 ;

[0087] And / or, the bulk density of the pressed material is 0.8 g / cm 3 -0.95g / cm 3 , such as 0.8g / cm 3 , 0.82g / cm 3 、0.84g / cm 3, 0.86g / cm 3 、0.88g / cm 3 , 0.9g / cm 3 , 0.92g / cm 3 、0.94g / cm 3 , 0.95g / cm 3 .

[0088] And / or, the filling amount is 12Kg-14Kg, such as 12Kg, 13Kg, 14Kg.

[0089] The pressed material has a high tap density and loose density, and with a reasonable particle size, it is beneficial to reduce the porosity and increase the filling volume.

[0090] In an optional embodiment, the heating rate of the sintering step is 2°C / min-5°C / min, such as 2°C / min, 3°C / min, 4°C / min, 5°C / min;

[0091] and / or, the holding temperature of the sintering step is 800° C.-820° C., such as 800° C., 810° C., 820° C.;

[0092] and / or, the holding time of the sintering step is 8 h to 12 h, such as 8 h, 9 h, 10 h, 11 h, 12 h;

[0093] And / or, the method further includes crushing the sintered material.

[0094] The pressed material is placed in a pot and then sintered. Since the tap density and loose density of the pressed material are relatively large, and the pressed material contains polyvinylidene fluoride and carbon source, both of which have a certain viscosity, it is beneficial to reduce sintering powder spraying.

[0095] The present application also provides a lithium iron phosphate cathode material prepared by the method of the aforementioned embodiment, wherein the DV50 of the lithium iron phosphate cathode material is 1 μm to 1.5 μm, such as 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, and 1.5 μm;

[0096] And / or, the compaction density of the lithium iron phosphate positive electrode material is 2.5 g / cm 3 -2.6g / cm 3 , such as 2.52g / cm 3 , 2.54g / cm 3 , 2.56g / cm 3 , 2.58g / cm 3 , 2.6g / cm;

[0097] And / or, the water absorption rate of the lithium iron phosphate positive electrode material is ≤0.7%, such as 0.7%, 0.6%, 0.5%, or 0.4%.

[0098] The lithium iron phosphate positive electrode material prepared in the present application has a higher compaction density and lower water absorption rate, which is beneficial to improving the electrical performance of the battery using it.

[0099] An embodiment of the present application further provides a lithium-ion battery, comprising the lithium iron phosphate positive electrode material described in the aforementioned embodiment.

[0100] The features and performance of the present application are further described in detail below with reference to the embodiments.

[0101] Example 1:

[0102] This embodiment provides a method for preparing a lithium iron phosphate positive electrode material, which specifically includes:

[0103] A lithium source and a carbon source are added to the ferric phosphate raw material according to the molar ratio of iron, lithium and carbon of 1:1.04:0.08, wherein the lithium source is lithium carbonate and the carbon source is glucose. A mixed solution of cetyltrimethylammonium bromide and polyvinylidene fluoride is added, wherein the mass ratio of cetyltrimethylammonium bromide to polyvinylidene fluoride is 1:20, and the total mass of cetyltrimethylammonium bromide and polyvinylidene fluoride accounts for 0.15% of the total mass of the ferric phosphate raw material, lithium source and carbon source. Water is then added to premix a slurry with a solid content of 35%. The obtained slurry is ground to obtain an abrasive A with an average particle size of 1.2 μm and an abrasive C with an average particle size of 0.4 μm, which are mixed in a mass ratio of 3:7 to obtain a graded ingredient.

[0104] The graded materials are spray-dried, and the air inlet temperature of the spray-drying process is controlled to be 220° C. and the air outlet temperature is controlled to be 105° C. to obtain a spherical spray material with a water content of 3-4 wt %.

[0105] The spherical spray material is pneumatically conveyed to a dry granulator. The parameters of the dry granulator are: oil pressure of 10 MPa, gap of 0.7 mm, feed speed of 25 Hz, and roller main machine speed of 15 Hz. After being granulated by the dry granulator, it is transported to a bowl loading machine through a horizontal pneumatic conveying pipeline. An inlet section 1, a contraction section 2, a throat section 3, a diffusion section 4, and a discharge section 5 are sequentially arranged along the direction of material conveying. The inner diameters of the inlet section 1, throat section 3, and discharge section 5 are Φ80 mm, Φ25 mm, and Φ80 mm, respectively. The lengths of the inlet section 1, throat section 3, and discharge section 5 are 150 mm, 125 mm, and 560 mm, respectively. The angle between the inner wall of the contraction section 2 and the axis is ≤15°, and the angle between the inner diameter of the diffusion section 4 and the axis is ≤5°. Figure 1 shown.

[0106] Entering the roller kiln, the sintering temperature is 820℃, the heating rate is 5℃ / min, and the sintering time is 10h. Then the sintered material is jet milled to obtain the SEM image of the lithium iron phosphate positive electrode material. Figure 2 As shown, XRD pattern is as follows Figure 3 shown.

[0107] Example 2:

[0108] This embodiment provides a method for preparing a lithium iron phosphate positive electrode material, which specifically includes:

[0109] A lithium source and a carbon source are added to the ferric phosphate raw material according to the molar ratio of iron, lithium and carbon of 1:1.04:0.08, wherein the lithium source is lithium carbonate and the carbon source is glucose. A mixed solution of cetyltrimethylammonium bromide and polyvinylidene fluoride is added, wherein the mass ratio of cetyltrimethylammonium bromide to polyvinylidene fluoride is 1:18, and the total mass of cetyltrimethylammonium bromide and polyvinylidene fluoride accounts for 0.18% of the total mass of the ferric phosphate raw material, the lithium source and the carbon source. Water is then added to premix a slurry with a solid content of 40%. The obtained slurry is ground to obtain an abrasive A with an average particle size of 1.2 μm and an abrasive C with an average particle size of 0.4 μm, the mass ratio of the two being 4:6, and the graded ingredients are mixed.

[0110] The graded materials are spray-dried, and the air inlet temperature of the spray drying is controlled to be 210° C. and the air outlet temperature is controlled to be 100° C. to obtain a spherical spray material with a water content of 2-3 wt %.

[0111] The spherical spray material passes through Figure 1 The pneumatic conveying pipeline shown above transports the material to a dry granulator. The dry granulator parameters are: oil pressure of 5 MPa, gap of 0.7 mm, feed speed of 10 Hz, and roller speed of 5 Hz. After granulation in the dry granulator, the material is gravity-fed to a loader and then to a roller kiln. The sintering temperature is 800°C, the heating rate is 2°C / min, and the sintering time is 8 hours. The sintered material is then pulverized by air jet milling.

[0112] Example 3:

[0113] This embodiment provides a method for preparing a lithium iron phosphate positive electrode material, which specifically includes:

[0114] A lithium source and a carbon source are added to the ferric phosphate raw material according to the molar ratio of iron, lithium and carbon of 1:1.04:0.08, wherein the lithium source is lithium carbonate and the carbon source is glucose. A mixed solution of cetyltrimethylammonium bromide and polyvinylidene fluoride is added, wherein the mass ratio of cetyltrimethylammonium bromide to polyvinylidene fluoride is 1:22, and the total mass of cetyltrimethylammonium bromide and polyvinylidene fluoride accounts for 0.12% of the total mass of the ferric phosphate raw material, the lithium source and the carbon source. Water is then added to premix a slurry with a solid content of 30%. The obtained slurry is ground to obtain an abrasive A with an average particle size of 1.2 μm and an abrasive C with an average particle size of 0.4 μm, respectively, with a mass ratio of 3.5:6.5. The two are mixed to obtain a graded ingredient.

[0115] The graded materials are spray-dried, and the air inlet temperature of the spray-drying process is controlled to be 230° C. and the air outlet temperature to be 95° C. to obtain a spherical spray material with a water content of 3-4 wt %.

[0116] The spherical spray material passes through Figure 1 The pneumatic conveying pipeline shown above transports the material to a dry granulator. The dry granulator parameters are: oil pressure of 25 MPa, gap of 1.2 mm, feed speed of 20 Hz, and roller speed of 10 Hz. After granulation in the dry granulator, the material is gravity-fed to a loader and then to a roller kiln. The sintering temperature is 810°C, the heating rate is 3°C / min, and the sintering time is 9 hours. The sintered material is then pulverized by air jet milling.

[0117] Comparative Example 1:

[0118] This embodiment provides a method for preparing a lithium iron phosphate positive electrode material. The main difference from Example 1 is that the slurry does not contain polyvinylidene fluoride.

[0119] Comparative Example 2:

[0120] This embodiment provides a method for preparing a lithium iron phosphate positive electrode material. The main difference from Example 1 is that the slurry does not contain cetyltrimethylammonium bromide, polyvinylidene fluoride cannot be dissolved, and lithium iron phosphate positive electrode material cannot be prepared.

[0121] Example 4:

[0122] This embodiment provides a method for preparing a lithium iron phosphate positive electrode material. The main difference from Example 1 is that the mass of polyvinylidene fluoride in the slurry remains unchanged, and the mass ratio of hexadecyltrimethylammonium bromide to polyvinylidene fluoride is 1:30.

[0123] Example 5:

[0124] This embodiment provides a method for preparing a lithium iron phosphate positive electrode material. The main difference from Example 1 is that the mass of polyvinylidene fluoride in the slurry is doubled, and the mass ratio of hexadecyltrimethylammonium bromide to polyvinylidene fluoride is maintained at 1:20.

[0125] Example 6:

[0126] This embodiment provides a method for preparing a lithium iron phosphate positive electrode material. The main difference from Example 1 is that no grading is performed, and the slurry with a solid content of 35% is directly ground once and then spray-dried.

[0127] Example 7:

[0128] This embodiment provides a method for preparing a lithium iron phosphate positive electrode material. The main difference from Example 1 is that the air outlet temperature is adjusted to 115° C. to control the moisture content of the material to 2%.

[0129] Comparative Example 3:

[0130] This comparative example provides a method for preparing a lithium iron phosphate positive electrode material, which is mainly different from Example 1 in that no granulator is used for granulation.

[0131] Example 8:

[0132] This embodiment provides a method for preparing a lithium iron phosphate positive electrode material. The main difference from Example 1 is that an equal mass of sodium lauryl sulfate is used to replace hexadecyltrimethylammonium bromide.

[0133] Example 9:

[0134] This embodiment provides a method for preparing a lithium iron phosphate positive electrode material. The main difference from Example 1 is that after being granulated by a dry granulator, the material passes through a vertical pipe and is transferred to a loader under the action of gravity.

[0135] The filling amount, finished product loss rate and prepared lithium iron phosphate positive electrode materials in the above embodiments and comparative examples were tested. The results are shown in Table 1. The test methods and conditions are as follows.

[0136] Dv50: tested by dry particle size tester;

[0137] Filling amount: Specify the filling weight corresponding to the filling height.

[0138] Finished product loss rate: the ratio of the weight difference before and after sintering to the weight before sintering.

[0139] Tap density: Use a tap density meter to place the powder in a measuring cylinder loaded with a tapping device. After fixing, vibrate at 250 times / minute for 12 minutes, and measure the ratio of the powder mass to volume in the measuring cylinder.

[0140] Compaction density: Use Sansi vertical and horizontal compaction density instrument to apply 3T pressure, measure its volume, and calculate the ratio of mass to volume as the compaction density.

[0141] 0.1C discharge capacity: The charge and discharge test was conducted using a blue-electric test system with a voltage range of 2.0-3.75V.

[0142] Water absorption rate: Measure the moisture content m1 (wt%) of the lithium iron phosphate positive electrode material sample and the moisture content m2 (wt%) after being placed at a temperature of 25°C and a humidity of 40% for 2 days. Water absorption rate = (m2-m1) / m1.

[0143] Table 1

[0144]

[0145]

[0146] According to Table 1, Example 1 is compared with Comparative Example 1 to see that without the addition of polyvinylidene fluoride, the amount of material loaded may be less due to more fine powder during the granulation process, and the addition of polyvinylidene fluoride can effectively improve the water absorption of the LFP material. It is found from the comparison of Example 1 and Comparative Example 3 that without granulation, the amount of material loaded is less, and the dusting is more serious. By the combination of granulation and fine powder, the morphology in Example 1 is a spherical morphology, and presents a size particle gradation, and small particles can fill the gap between large particles. This effect is the effect of nano-fine powder and millimeter particles mixed during granulation. In the granulation process, millimeter-scale particles are granulated because the internal extrusion force small particles will not grow up, and the fine powder and millimeter particle surface will grow up because there is no binding force thereof. Grading is formed after pulverization, which greatly improves the compaction density of the material.

[0147] Example 4 Compared with Example 1, the amount of hexadecyltrimethylammonium bromide is reduced, which will cause the polyvinylidene fluoride to dissolve for a long time, affecting the normal production rhythm of the production line; Example 5 Compared with Example 1, it can be seen that the amount of polyvinylidene fluoride added is increased, the slurry viscosity increases, the spherical spray material particle size becomes larger, and the amount of hexadecyltrimethylammonium bromide is increased while increasing polyvinylidene fluoride. The excess of hexadecyltrimethylammonium bromide can cause excessive foaming of the slurry, and at the same time, it can cause the high viscosity to damage the spray system, and also affect the electrochemical properties of the material. Example 6 Compared with Example 1, it can be seen that due to the lack of grading, the compaction and loose density of the pressed material are reduced, which in turn leads to a reduction in the amount of filling. Example 7 Compared with Example 1, the water content of the spherical spray material is reduced, resulting in a reduction in the amount of filling, an increase in the finished product loss rate, and the electrochemical performance of the prepared lithium iron phosphate is also reduced. Example 8 Compared with Example 1, the surfactant is replaced with sodium lauryl sulfate, and the filling amount, lithium iron phosphate compaction density and electrical properties are reduced. Compared with Example 1, Example 9 uses a conveying pipe with a constant inner diameter to discharge materials under the action of gravity, and the filling capacity and electrical performance are reduced.

[0148] The above description is merely a preferred embodiment of the present application and is not intended to limit the present application. Various modifications and variations are possible for those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present application shall be included within the scope of protection of the present application.

Claims

1. A method for preparing a lithium iron phosphate positive electrode material, characterized in that: include: spray drying a slurry comprising iron phosphate, a lithium source, a carbon source, polyvinylidene fluoride, a surfactant and water to obtain a spherical spray material; Compressing the spherical spray material to obtain a compressed material; After the pressed material is placed in a pot, it is sintered under a protective atmosphere to obtain the lithium iron phosphate positive electrode material.

2. The method for preparing the lithium iron phosphate positive electrode material according to claim 1, characterized in that: The surfactant includes at least one of carboxymethyl cellulose, sodium lauryl sulfate, and cetyltrimethylammonium bromide; and / or, the mass ratio of the surfactant to the polyvinylidene fluoride in the slurry is 1:(18-22); And / or, the mass fraction of polyvinylidene fluoride in the slurry is 0.034%-0.07%. And / or, the preparation of the slurry includes: adding polytetrafluoroethylene to an aqueous solution of a surfactant to dissolve the polytetrafluoroethylene to obtain a mixed solution, and then mixing and sand-milling the mixed solution with iron phosphate, a lithium source, and a carbon source to obtain the slurry.

3. The method for preparing the lithium iron phosphate cathode material according to claim 2, wherein: The sanding step obtains abrasive A with an average solid particle size of 1-1.5 μm and abrasive B with an average solid particle size of 0.3-0.5 μm, respectively. The abrasive A and the abrasive B are mixed in a mass ratio of 3 / 7-4 / 6 to obtain the slurry.

4. The method for preparing the lithium iron phosphate positive electrode material according to claim 1, characterized in that: The solid content of the slurry is 30%-40%; And / or, the molar ratio of iron, lithium and carbon in the slurry is 1:(1-1.1):(0.05-0.1).

5. The method for preparing the lithium iron phosphate positive electrode material according to claim 1, characterized in that: The air inlet temperature of the equipment used for spray drying is 210℃-230℃, and the air outlet temperature is 90℃-110℃. And / or, the Dv50 of the spherical spray material is 10 μm-30 μm.

6. The method for preparing a lithium iron phosphate cathode material according to claim 1, wherein: The equipment used in the pressing step is a dry granulator. In the pressing step, the dry granulator meets at least one of the following characteristics: a. The oil pressure of the dry granulator is 5MPa-25MPa; b. The gap between the pressure rollers is 0.5mm-1.2mm; c. Feed speed is 5Hz-25Hz; d. The speed of the main pressure roller is 5Hz-30Hz; e. The pressure roller is provided with a silicon carbide coating; f. The dry granulator is connected to the bowl loading machine via a conveying pipe, and the conveying pipe is provided with an inlet section, a contraction section, a throat section, a diffusion section and a discharge section in sequence along the direction of material conveying.

7. The method for preparing a lithium iron phosphate cathode material according to claim 6, wherein: The ratio of the inner diameters of the inlet section, throat section and discharge section is 75-85:25:75-85; and / or, the angle between the inner wall of the contraction section and the axis is ≤15°; and / or, the angle between the inner diameter of the diffuser section and the axis is ≤5°; And / or, the ratio of the length to the diameter of the throat section is 5-8.

8. The method for preparing a lithium iron phosphate cathode material according to claim 1, wherein: The average particle size of the pressed material is 1 mm to 3 mm; And / or, the tap density of the pressed material is 1.1 g / cm 3 -1.7g / cm 3 ; And / or, the bulk density of the pressed material is 0.8 g / cm 3 -0.95g / cm 3 . And / or, the filling amount is 12Kg-14Kg. and / or, the heating rate of the sintering step is 2°C / min-5°C / min; and / or, the holding temperature of the sintering step is 800° C.-820° C.; and / or, the holding time of the sintering step is 8h-12h; And / or, the method further includes crushing the sintered material.

9. A lithium iron phosphate positive electrode material prepared by the method of claim 1, characterized in that: The Dv50 of the lithium iron phosphate positive electrode material is 1 μm to 1.5 μm; And / or, the compaction density of the lithium iron phosphate positive electrode material is 2.5 g / cm 3 -2.6g / cm 3 ; And / or, the water absorption rate of the lithium iron phosphate positive electrode material is ≤0.7%.

10. A lithium ion battery, characterized in that: Including the lithium iron phosphate positive electrode material according to claim 9.

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