Lithium iron phosphate positive electrode material and preparation method and application thereof
By using different iron phosphate precursors and controlled sintering processes, lithium iron phosphate cathode materials with suitable particle size and carbon coating were prepared, solving the problems of increased slurry viscosity and poor processing performance in the slurry preparation process of lithium iron phosphate materials, and achieving good processability and electrochemical performance under high areal density.
Patent Information
- Application Number
- CN202511400575.7
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-09-28
- Publication Date
- 2025-11-04
AI Technical Summary
Existing lithium iron phosphate materials exhibit rapid viscosity increases and poor fluidity during the slurry preparation process, making it difficult to achieve high areal density coating. Furthermore, the coating process is prone to tape breakage and cracking after rolling, affecting processing performance.
Two lithium iron phosphate intermediates were prepared using different iron phosphate precursors. By controlling the sintering temperature and heating rate, and combining the carbon source mass ratio, lithium iron phosphate cathode materials with suitable particle size and carbon coating were prepared, and the slurry viscosity was controlled and the fluidity was improved.
This study achieves good processability of lithium iron phosphate cathode material under high areal density, suppresses the increase of slurry viscosity, prevents cracks during coating and rolling processes, and improves the compaction density and electrochemical performance of the material.
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Figure CN120887401A_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The present application relates to the technical field of secondary batteries, in particular to a lithium iron phosphate positive electrode material and a preparation method and application thereof. BACKGROUND
[0002] The positive electrode material of a lithium ion battery can be roughly divided into four technical systems of lithium cobaltate material, lithium manganate material, lithium iron phosphate material and ternary material, among which the lithium iron phosphate material and the ternary material account for a higher use proportion. The ternary material has a higher specific capacity, but has poor safety and stability; the lithium iron phosphate material has the advantages of high stability, good cycle performance and high safety, and the disadvantage is a lower energy density. The reasons for the lower energy density of the lithium iron phosphate material include a low theoretical specific capacity and a low available compaction density. After improvement, the energy density of the current lithium iron phosphate material has increased, and the 1 / 3C discharge capacity can reach more than 140 mAh / g, and the compaction density of the pole piece can reach more than 2.6 g / cc. In order to further break through the energy density of the lithium iron phosphate material on this basis, the existing technology begins to try to increase the number of nanoparticles and the amount of carbon coating, but this leads to a higher specific surface area of the lithium iron phosphate material, a faster increase in the viscosity of the slurry during the homogenization process, and an impact on the pole piece coating; and in order to improve the energy density of the lithium iron phosphate battery, a higher surface density is usually selected when coating the lithium iron phosphate positive electrode slurry, the viscosity of the lithium iron phosphate slurry increases rapidly, the flowability is poor, and it is not easy to process to achieve a high surface density; and a large number of nanoparticles can lead to easy breaking of the belt during coating and cracks after rolling. SUMMARY
[0003] Therefore, the purpose of the present application is to provide a lithium iron phosphate positive electrode material and a preparation method and application thereof, which can improve the flowability of the lithium iron phosphate material after being made into a slurry, control the increase in the viscosity of the slurry during the homogenization process, reduce the occurrence of cracks after coating the lithium iron phosphate positive electrode slurry and rolling, and make the lithium iron phosphate positive electrode material have good processability at a high surface density.
[0004] To this end, the present application provides the following technical solutions.
[0005] The present application provides a preparation method of a lithium iron phosphate positive electrode material, comprising the following steps: (1) mixing a first precursor, a lithium source, a carbon source and a solvent to obtain a first slurry, and grinding, drying and sintering to obtain a first lithium iron phosphate intermediate; (2) mixing the first lithium iron phosphate intermediate, a carbon source and a solvent to obtain a second slurry, and grinding and drying to obtain a coating material A; (3) mixing a second precursor, a lithium source, a carbon source and a solvent to obtain a third slurry, and grinding, drying and sintering to obtain a second lithium iron phosphate intermediate; (4) mixing the second lithium iron phosphate intermediate, the carbon source and the solvent to obtain a fourth slurry, and grinding and drying the fourth slurry to obtain a coating material B; The first precursor and the second precursor use different iron phosphate precursors respectively. (5) mixing the coating material A and the coating material B, and performing first sintering, crushing and second sintering to obtain the lithium iron phosphate positive electrode material. The second sintering temperature is greater than the first sintering temperature. The preparation method of the lithium iron phosphate positive electrode material satisfies the relationship: 1.1≤(C2×S2) / (C1×S1)≤2. C1 is a ratio of the mass of the carbon source to the mass of the first lithium iron phosphate intermediate in step (2); C2 is a ratio of the mass of the carbon source to the mass of the second lithium iron phosphate intermediate in step (4); S1 is a value of the heating rate of sintering in step (1), in units of ℃ / min; and S2 is a value of the heating rate of sintering in step (3), in units of ℃ / min.
[0006] C1: a ratio of the mass of the carbon source to the mass of the first lithium iron phosphate intermediate in step (2) = the mass of the carbon source / the mass of the first lithium iron phosphate intermediate in step (2).
[0007] C2: a ratio of the mass of the carbon source to the mass of the second lithium iron phosphate intermediate in step (4) = the mass of the carbon source / the mass of the second lithium iron phosphate intermediate in step (4).
[0008] In the sintering process in step (1), the heating rate is S1 ℃ / min to the sintering temperature, and then a holding treatment is performed.
[0009] In the sintering process in step (3), the heating rate is S2 ℃ / min to the sintering temperature, and then a holding treatment is performed.
[0010] Optionally, in the first slurry, the amount of the first precursor and the lithium source is such that the molar ratio of Li to Fe is 1.01-1.05, and the molar ratio of Fe to P is 0.95-0.98. The molar ratio of Fe to P in the commonly used iron phosphate precursor is generally between 0.95-0.98, and P is slightly excessive, so the first slurry can satisfy the molar ratio of Fe to P.
[0011] Optionally, in the first slurry, the mass of the carbon source is 2%-6% of the mass of the first precursor.
[0012] Optionally, the mass percentage of the lithium iron phosphate is ≥90% based on the mass of the first lithium iron phosphate intermediate.
[0013] Optionally, the iron phosphate precursor includes any one of an iron method iron phosphate precursor, an ammonium method iron phosphate precursor and a sodium method iron phosphate precursor.
[0014] The preparation method of the lithium iron phosphate positive electrode material provided by the application can use ammonium method iron phosphate precursor, iron method iron phosphate precursor, and sodium method iron phosphate precursor, which can be self-made or purchased, and the corresponding preparation method is a common preparation method. Typically, the preparation method of the ammonium method iron phosphate precursor includes: using ferrous sulfate and monoammonium phosphate as raw materials, adding solvent water to the raw materials, mixing and stirring under heating conditions, reacting, aging, obtaining dihydrate iron phosphate, performing solid-liquid separation, washing and drying, and calcining to obtain iron phosphate; the preparation method of the iron method iron phosphate precursor includes: using iron powder, iron block, and aqueous phosphoric acid as raw materials, mixing and dissolving the raw materials, and then performing aging reaction, solid-liquid separation, washing and drying, and calcining to obtain iron phosphate products; and the preparation method of the sodium method iron phosphate precursor includes: using phosphoric acid, ferrous sulfate, sodium hydroxide, and / or sodium carbonate as raw materials, mixing and heating the raw materials to react, aging, performing solid-liquid separation, washing and drying, and calcining to obtain iron phosphate products. Due to the differences in raw materials, pH, stirring speed, reaction temperature, and solid content during the preparation process, the morphology and crystal structure of the iron phosphate prepared by different methods are different. The physicochemical parameters of the iron phosphate prepared by different methods can be close, but the morphology is different. The primary particles of the iron method iron phosphate precursor are flaky; the primary particles of the sodium method iron phosphate precursor are small and spherical and are closely combined; and the primary particles of the ammonium method iron phosphate precursor are spherical and are loosely combined.
[0015] Optionally, in the third slurry, the amount of the second precursor and the lithium source is such that the molar ratio of Li to Fe is 1.01-1.05, and the molar ratio of Fe to P is 0.95-0.98.
[0016] Optionally, in the third slurry, the mass of the carbon source is 2%-6% of the mass of the second precursor.
[0017] Optionally, the mass percentage of lithium iron phosphate is ≥90% based on the mass of the second lithium iron phosphate intermediate. In the first lithium iron phosphate intermediate and the second lithium iron phosphate intermediate, the mass percentage of lithium iron phosphate structure is controlled to be ≥90%, which can prevent uneven carbon coating caused by the reaction of trivalent iron and the carbon source in the subsequent sintering step, thereby assisting in improving the material performance.
[0018] Optionally, in step (2), C1 is 2%-5%.
[0019] Optionally, in step (4), C2 is 5%-8%.
[0020] Optionally, S1 is 1-5 ℃ / min.
[0021] Optionally, S2 is 1-5 ℃ / min.
[0022] Optionally, in step (5), the mass ratio of the coating material A to the coating material B is 1:9-9:1.
[0023] Optionally, the temperature of the first sintering is 400-700°C, the holding time is 6-15h, and the heating rate is 1-5°C / min.
[0024] Optionally, the temperature of the second sintering is 700-820°C, the holding time is 3-6h, and the heating rate is 1-5°C / min.
[0025] Optionally, in step (5), the particle size of the obtained material after the crushing is less than 10μm.
[0026] Optionally, in step (1) or step (3), the sintering temperature is 500-750°C, and the holding time is 3-10h.
[0027] Optionally, in step (1), step (2), step (3) or step (4), the grinding includes sand grinding.
[0028] Optionally, in step (1), step (2), step (3) or step (4), the grinding is to a particle size D50 of 200-1000nm in the slurry.
[0029] Optionally, in step (1), step (2), step (3) or step (4), the drying includes spray drying. Typically and non-limitingly, the outlet temperature of the spray drying is 85-95°C, and the inlet temperature is 150-170°C.
[0030] Optionally, the first slurry and the third slurry further comprise an additive, and the additive comprises a doping element; optionally, in the first slurry, the additive is added in an amount such that the mass proportion of the doping element in the first lithium iron phosphate intermediate is 1000-3000ppm; optionally, in the third slurry, the additive is added in an amount such that the mass proportion of the doping element in the second lithium iron phosphate intermediate is 3000-6000ppm.
[0031] Optionally, the additive comprises at least one of titanium dioxide, magnesium oxide, aluminum oxide, vanadium pentoxide, ammonium metavanadate, and metatitanic acid.
[0032] Optionally, the doping element comprises at least one of titanium element, magnesium element, aluminum element, and vanadium element.
[0033] Optionally, in step (1), step (2), step (3) or step (4), the carbon source comprises at least one of sucrose, glucose, polyvinyl alcohol, polyethylene glycol, and starch.
[0034] Optionally, in step (1) or step (3), the lithium source includes at least one of lithium carbonate, lithium hydroxide, and lithium dihydrogen phosphate.
[0035] Optionally, in steps (1), (2), (3), or (4), the solvent may include water.
[0036] In the preparation method of lithium iron phosphate cathode material provided by the present invention, the amount of solvent added is such that the first slurry, the second slurry, the third slurry, and the fourth slurry reach a certain solid content, which is convenient for subsequent grinding, drying and other steps. Typically, without limitation, the solid content of the first slurry, the second slurry, the third slurry, and the fourth slurry are all 35% to 50%.
[0037] This invention provides a lithium iron phosphate cathode material prepared by the above-described preparation method.
[0038] The present invention provides a positive electrode sheet comprising the above-mentioned lithium iron phosphate positive electrode material.
[0039] The present invention also provides a secondary battery, including the above-mentioned positive electrode, and further including a negative electrode, a separator and an electrolyte.
[0040] In summary, the beneficial effects achieved by this invention are as follows: The method for preparing lithium iron phosphate cathode material provided by this invention specifies the preparation of two lithium iron phosphate intermediates, and specifies that the precursors used to prepare the two lithium iron phosphate intermediates are different iron phosphate precursors. It specifies that the corresponding coating materials A and B of the lithium iron phosphate intermediates are mixed and then sintered twice to obtain the lithium iron phosphate cathode material. It also specifies that the preparation method of lithium iron phosphate cathode material satisfies the following relationship: 1.1≤(C2×S2) / (C1×S1)≤2; where C1 is the ratio of the mass of carbon source to the mass of the first lithium iron phosphate intermediate in step (2); C2 is the ratio of the mass of carbon source to the mass of the second lithium iron phosphate intermediate in step (4); S1 is the value of the heating rate of sintering in step (1), in °C / min; S2 is the value of the heating rate of sintering in step (3), in °C / min. The lithium iron phosphate cathode material prepared by this method exhibits excellent particle flowability after being prepared into a cathode slurry. It effectively suppresses the uncontrollable increase in slurry viscosity during homogenization and prevents tape breakage during coating and surface cracking of the electrode sheet after rolling under high areal density conditions, thus improving the processability of the material at high areal density. Furthermore, the lithium iron phosphate cathode material prepared by this method has a compacted density ≥2.65 g / cc, powder resistivity ≤10 Ω·cm, 0.1C capacity ≥155 mAh / g, 1C capacity ≥145 mAh / g, and slurry viscosity <10000 mPa·s after 6 hours of homogenization, with no cracks in the electrode sheet.
[0041] The heating rate of sintering in step (1), and the mass percentage of the carbon source to the first lithium iron phosphate intermediate in step (2) can control the particle size of the obtained coating material A; the heating rate of sintering in step (3), and the mass ratio of the carbon source to the second lithium iron phosphate intermediate in step (4) can control the particle size of the obtained coating material B. Specifically, the heating rate of sintering in step (1) and step (3) can control the particle size of the first lithium iron phosphate intermediate and the second lithium iron phosphate intermediate, the slower the heating rate, the longer the material is at high temperature, and the larger the particle. The mass percentage of the carbon source to the lithium iron phosphate intermediate in step (2) and step (4) can control the thickness of the carbon coating layer, at the same time, the carbon coating layer can also inhibit the further growth of the particle in the subsequent step. Satisfying formula 1.1≤(C2×S2) / (C1×S1)≤2 can make the obtained lithium iron phosphate positive electrode material have appropriate particle size difference and carbon coating effect, help to achieve reasonable particle size distribution, and further improve the compaction density. By strictly controlling the multiple sintering process, the sintering heating rate, the sintering temperature, and the carbon content, the sphericity of the lithium iron phosphate particle is also improved, the specific surface area is reduced, and the flowability of the material is improved; the material has good processability on the basis of high energy density, prevents the rapid increase of slurry viscosity in the homogenizing process, and prevents the generation of scratches and cracks in the electrode plate manufacturing process.
[0042] The preparation method of the lithium iron phosphate positive electrode material provided by the application, the lithium iron phosphate precursor includes any one of iron method lithium iron phosphate precursor, ammonium method lithium iron phosphate precursor, and sodium method lithium iron phosphate precursor. The morphology and structure of the iron method lithium iron phosphate precursor, the ammonium method lithium iron phosphate precursor, and the sodium method lithium iron phosphate precursor are different, and the lithium iron phosphate obtained after sintering of the precursors is also different in particle size distribution and capacity. The application improves the rationality of the particle size distribution by using different precursors, thereby improving the compaction density. BRIEF DESCRIPTION OF DRAWINGS
[0043] In order to more clearly illustrate the specific embodiments of the application or the technical solutions in the prior art, the drawings needed in the following description of the specific embodiments or the prior art will be briefly introduced. Obviously, the drawings in the following description are some embodiments of the application, and other drawings can also be obtained by those skilled in the art without creative labor.
[0044] Figure 1 The SEM image of the ammonium method lithium iron phosphate precursor is shown in the following figure; Figure 2 The SEM image of the iron method lithium iron phosphate precursor is shown in the following figure; Figure 3 The SEM image of the sodium method lithium iron phosphate precursor is shown in the following figure; Figure 4The image of the positive electrode sheet coated with the lithium iron phosphate positive electrode material prepared in Example 1 is shown in Figure 1. Figure 5 The image of the positive electrode sheet coated with the lithium iron phosphate positive electrode material prepared in Example 2 is shown in Figure 2. Figure 6 The image of the positive electrode sheet coated with the lithium iron phosphate positive electrode material prepared in Comparative Example 1 is shown in Figure 3. DETAILED DESCRIPTION
[0045] The technical solutions of the present application will be described clearly and completely below with reference to the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.
[0046] The specific experimental steps or conditions not mentioned in the embodiments can be performed according to the conventional experimental steps or conditions described in the literature in the art. The reagents or instruments used are not specified by the manufacturer, and are conventional reagent products that can be obtained by purchase.
[0047] Experimental reagents Polyethylene glycol: polyethylene glycol 2000; Ammonium method iron phosphate precursor: iron-phosphorus ratio 0.96, specific surface area 9.4 m 2 / g, D50 = 15 μm; an example of the SEM image is shown in Figure 1. Figure 1 ; Iron method iron phosphate precursor: iron-phosphorus ratio 0.971, specific surface area 8.1 m 2 / g, D50 = 9 μm; an example of the SEM image is shown in Figure 2. Figure 2 ; Sodium method iron phosphate precursor: iron-phosphorus ratio 0.966, specific surface area 7.2 m 2 / g, D50 = 21 μm; an example of the SEM image is shown in Figure 3. Figure 3 .
[0048] Example 1 The present embodiment provides a lithium iron phosphate positive electrode material and a preparation method thereof, specifically comprising the following steps: (1) mixed ammonium method iron phosphate precursor 200 kg, lithium carbonate 50.1 kg, glucose 6 kg, polyethylene glycol 3 kg, titanium dioxide 0.8 kg and deionized water 400 kg, stirred, sand grinding at a speed of 1000 rpm, spray drying, to obtain the first spray material; the inlet air temperature of spray drying is 160℃, the outlet air temperature is 90℃ (if there is no special instruction, the inlet air temperature and outlet air temperature of spray drying are used, and will not be given one by one). The first spray material is heated to 650℃ at a heating rate of 2℃ / min, and the holding time is 6h, sintering, to obtain the first lithium iron phosphate intermediate, wherein the mass fraction of the doping element titanium is 2350ppm (ICP is used for detection, the same below). After mixing the first lithium iron phosphate intermediate 200 kg with 10 kg of glucose and 300 kg of deionized water, sand grinding at a speed of 1000 rpm, spray drying, to obtain the coating material A; control the sand grinding to the D50 of the particles in the slurry is 600nm (the particle size is detected by Malvern 3000 laser particle size analyzer, the same below).
[0049] (2) mixed iron method iron phosphate precursor 200 kg, lithium carbonate 50.1 kg, glucose 6 kg, polyethylene glycol 3 kg, titanium dioxide 0.8 kg, vanadium pentoxide 0.5 kg and deionized water 400 kg, stirred, sand grinding at a speed of 1000 rpm, spray drying, to obtain the second spray material. The second spray material is heated to 600℃ at a heating rate of 2℃ / min, and the holding time is 6h, sintering, to obtain the second lithium iron phosphate intermediate, wherein the total mass fraction of the doping elements titanium and vanadium is 3700ppm. After mixing the second lithium iron phosphate intermediate 200 kg with 10 kg of glucose, 2 kg of polyethylene glycol and 300 kg of deionized water, sand grinding, spray drying, to obtain the coating material B; control the sand grinding to the D50 of the particles in the slurry is 260nm.
[0050] (3) mixed the coating material A and the coating material B according to the mass ratio of 1:1, heated to 500℃ at a heating rate of 3℃ / min, and the holding time is 8h, one sintering, to obtain the one sintered material; the one sintered material is crushed to a particle size of <10μm, heated to 800℃ at a heating rate of 4℃ / min, and the holding time is 5h, two sintering, the two sintered material is crushed to a particle size of <10μm, to obtain the lithium iron phosphate positive electrode material.
[0051] In example 1, (C2×S2) / (C1×S1)=(((10+2) / 200)×2) / ((10 / 200)×2)=1.2.
[0052] Example 2 The embodiment provides a lithium iron phosphate positive electrode material and a preparation method thereof. Compared with the embodiment 1, the only difference is that the equivalent sodium method lithium iron phosphate precursor is used to replace the ammonium method lithium iron phosphate precursor in the step (1), and the mass ratio of the coating material A to the coating material B is 1:9 in the step (3).
[0053] Embodiment 3 The embodiment provides a lithium iron phosphate positive electrode material and a preparation method thereof. Compared with the embodiment 1, the only difference is that the equivalent sodium method lithium iron phosphate precursor is used to replace the ammonium method lithium iron phosphate precursor in the step (1), and the mass ratio of the coating material A to the coating material B is 1:9 in the step (3).
[0054] Embodiment 4 The embodiment provides a lithium iron phosphate positive electrode material and a preparation method thereof, which specifically comprises the following steps. (1) 200 kg of the iron method lithium iron phosphate precursor, 51.3 kg of lithium carbonate, 2 kg of sucrose, 2 kg of polyethylene glycol, 0.35 kg of titanium dioxide and 400 kg of deionized water are mixed and stirred, sand milling is performed at a rotating speed of 1000 rpm, spray drying is performed, a first spray material is obtained, the first spray material is raised to 500 DEG C at a temperature rising speed of 2 DEG C / min, and sintering is performed for 10 h, a first lithium iron phosphate intermediate is obtained, wherein the total mass proportion of the doping elements titanium and vanadium is 1000 ppm (ICP is used for detection). 200 kg of the first lithium iron phosphate intermediate, 4 kg of glucose and 300 kg of deionized water are mixed, sand milling is performed at a rotating speed of 1000 rpm, spray drying is performed, and coating material A is obtained; the sand milling is controlled until the D50 of particles in the slurry is 500 nm (the particle size is detected by a Malvern 3000 laser particle size instrument, and the same applies below).
[0055] (2) 200 kg of the ammonium method lithium iron phosphate precursor, 49.5 kg of lithium carbonate, 4 kg of glucose, 0.6 kg of titanium dioxide, 0.45 kg of vanadium pentoxide and 400 kg of deionized water are mixed and stirred, sand milling is performed at a rotating speed of 1000 rpm, spray drying is performed, a second spray material is obtained. The second spray material is raised to 750 DEG C at a temperature rising speed of 1 DEG C / min, and sintering is performed for 3 h, a second lithium iron phosphate intermediate is obtained, wherein the total mass proportion of the doping elements titanium and vanadium is 3000 ppm. 200 kg of the second lithium iron phosphate intermediate, 10 kg of glucose and 300 kg of deionized water are mixed and sand milled, spray drying is performed, and coating material B is obtained; the sand milling is controlled until the D50 of particles in the slurry is 200 nm.
[0056] (3) The coating material A and the coating material B are mixed in a mass ratio of 1:1, and sintered once at a temperature of 400℃ for 15h at a temperature increasing rate of 1℃ / min, to obtain a first sintered material. The first sintered material is crushed to a particle size of less than 10μm, and sintered twice at a temperature of 700℃ for 6h at a temperature increasing rate of 1℃ / min, to obtain a second sintered material. The second sintered material is crushed to a particle size of less than 10μm, to obtain the lithium iron phosphate positive electrode material.
[0057] In Example 4, (C2×S2) / (C1×S1) = ((10 / 200)×1) / ((4 / 200)×2) = 1.25.
[0058] Example 5 The present embodiment provides a lithium iron phosphate positive electrode material and a preparation method thereof, which specifically comprises the following steps: (1) 200kg of ammonium method lithium iron phosphate precursor, 49.5kg of lithium carbonate, 8kg of glucose, 4kg of polyethylene glycol, 1kg of titanium dioxide, and 400kg of deionized water are mixed and stirred, and sand milling is performed at a speed of 1000rpm. Spray drying is performed to obtain a first spray material. The first spray material is sintered at a temperature of 500℃ for 10h at a temperature increasing rate of 4℃ / min, to obtain a first lithium iron phosphate intermediate. The total mass fraction of the doping elements titanium and vanadium is 3000ppm. The first lithium iron phosphate intermediate is mixed with 10kg of glucose and 300kg of deionized water, and sand milling is performed at a speed of 1000rpm. Spray drying is performed to obtain a coating material A. The sand milling is controlled to a D50 of 1000nm of particles in the slurry.
[0059] (2) 200kg of iron method lithium iron phosphate precursor, 51.3kg of lithium carbonate, 12kg of polyethylene glycol, 1kg of titanium dioxide, 1.15kg of vanadium pentoxide, and 400kg of deionized water are mixed and stirred, and sand milling is performed at a speed of 1000rpm. Spray drying is performed to obtain a second spray material. The second spray material is sintered at a temperature of 750℃ for 3h at a temperature increasing rate of 5℃ / min, to obtain a second lithium iron phosphate intermediate. The total mass fraction of the doping elements titanium and vanadium is 6000ppm. The second lithium iron phosphate intermediate is mixed with 10kg of glucose, 6kg of polyethylene glycol, and 300kg of deionized water, and sand milling is performed. Spray drying is performed to obtain a coating material B. The sand milling is controlled to a D50 of 500nm of particles in the slurry.
[0060] (3) The coating material A and the coating material B are mixed in a mass ratio of 1:1, and sintered once at 700°C for 6h with a temperature rising rate of 5°C / min, to obtain a first sintered material; the first sintered material is crushed to a particle size of less than 10μm, and sintered twice at 820°C for 3h with a temperature rising rate of 5°C / min, and the second sintered material is crushed to a particle size of less than 10μm, to obtain the lithium iron phosphate positive electrode material.
[0061] In Example 5, (C2 x S2) / (C1 x S1) = (((10+6) / 200) x 5) / ((10 / 200) x 4) = 2.
[0062] Comparative Example 1 This comparative example provides a lithium iron phosphate positive electrode material and a preparation method thereof, which is different from Example 1 only in that the equivalent iron method iron phosphate precursor is used to replace the ammonium method iron phosphate precursor in step (1).
[0063] Comparative Example 2 This comparative example provides a lithium iron phosphate positive electrode material and a preparation method thereof, which is different from Example 1 only in that the temperature rising rate in step (1) is changed to 3°C / min. In this comparative example, (C2 x S2) / (C1 x S1) = (((10+2) / 200) x 2) / ((10 / 200) x 3) = 0.8 < 1.1.
[0064] Comparative Example 3 This comparative example provides a lithium iron phosphate positive electrode material and a preparation method thereof, which is different from Example 1 only in that the first lithium iron phosphate intermediate is mixed with 10kg glucose, 2kg polyethylene glycol and 300kg deionized water in step (1).
[0065] In this comparative example, (C2 x S2) / (C1 x S1) = (((10+2) / 200) x 2) / (((10+2) / 200) x 2) = 1 < 1.1.
[0066] Comparative Example 4 This comparative example provides a lithium iron phosphate positive electrode material and a preparation method thereof, which is different from Example 1 only in that the first lithium iron phosphate intermediate is mixed with 3kg glucose, 2kg polyethylene glycol and 300kg deionized water in step (1).
[0067] In this comparative example, (C2 x S2) / (C1 x S1) = (((10+2) / 200) x 2) / (((3+2) / 200) x 2) = 2.4 > 2.
[0068] Comparative Example 5 The comparative example provides a lithium iron phosphate positive electrode material and a preparation method thereof. Compared with Example 1, the difference is only that step (3) is: mixing coating material A and coating material B at a mass ratio of 1:1, sintering at a temperature rising rate of 3 ℃ / min to 800 ℃, and keeping the temperature for 8 h, crushing to a particle size of <10 μm, and obtaining the lithium iron phosphate positive electrode material.
[0069] Test Example 1 The lithium iron phosphate positive electrode materials prepared in the examples and comparative examples are used to prepare corresponding positive electrode slurries, specifically including the following steps: using the lithium iron phosphate positive electrode materials prepared in the examples and comparative examples as positive electrode active materials, homogenizing the positive electrode active materials: SP (superconducting carbon black): PVDF (polyvinylidene fluoride) at a mass ratio of 90:5:5, stirring for 2 h until the materials are uniformly dispersed, taking part of the slurry to stand in a beaker, and recording the initial viscosity, 3 h viscosity, 6 h viscosity, and 12 h viscosity. The obtained data are shown in Table 1.
[0070] Table 1
[0071]
[0072] As can be seen from Table 1, the positive electrode material prepared by using the preparation method provided by the application in the examples has a relatively slow viscosity increase after being placed for a period of time, and has good processing performance. The positive electrode material prepared in the comparative examples has a high viscosity increase after being placed for a period of time, and the viscosity can reach about 300% of the initial viscosity or even higher after being placed for 12 h, which affects the subsequent processing and use.
[0073] Test Example 2 The lithium iron phosphate positive electrode materials prepared in the examples and comparative examples are used to measure the tap density and resistivity of the samples by using an NITTOSEIKO ANALYTECH brand MCP-T700 type automatic powder resistivity and tap density test system. The resistivity test uses 4, 8, 12, 16, and 20 kN pressure in turn, and the volume resistivity under 20 kN pressure is taken as the final value. The obtained data are shown in Table 2.
[0074] The specific surface area test adopts the BET method, the reference national standard is GB / T19587-2017, and the equipment used is a 3H-2000 series analyzer produced by Belsize Technology Co., Ltd.
[0075] As can be seen from Table 2, the positive electrode material prepared in the examples has high tap density, low resistivity, and small specific surface area. The positive electrode material prepared in the comparative examples has a significantly lower tap density than the examples, and the resistivity and specific surface area are significantly higher. It is the excessively high specific surface area that causes the viscosity to increase rapidly after being placed for a period of time.
[0076] Test Example 3 Take the lithium iron phosphate positive electrode material prepared in the examples, comparative examples, and prepare the positive electrode slurry according to the method in test example 1. Take part of the positive electrode slurry, coat on the 20 μm thick aluminum foil, and make the positive electrode sheet with a surface density of 23 mg / cm 2 , and bake in an 80℃ oven for 2 hours, take out the electrode sheet to observe the electrode sheet state, and then roll, die cut, and punch into a positive electrode sheet.
[0077] The corresponding positive electrode sheet image of example 1 is shown in Figure 4 , and the corresponding positive electrode sheet image of example 2 is shown in Figure 5 It can be seen that the state of the two electrode sheets is good and there is no crack. The corresponding positive electrode sheet image of comparative example 1 is shown in Figure 6 , and the surface of the electrode sheet is cracked and the state is poor.
[0078] Test Example 4 Take the positive electrode sheet prepared in test example 3, use lithium sheet as negative electrode, use PE material as separator, drop 80 μL of electrolyte (the electrolyte uses Xuzhou lithium ion battery electrolyte, model LBC3401A60), and use R2032 button cell shell for button cell assembly.
[0079] Test the button cell prepared above using a blue electricity tester; place the prepared button cell in a 25℃ high temperature oven for charge and discharge test, the voltage range is 2.0V~3.75V, 0.1C charge and discharge for 2 cycles for activation, constant current and constant voltage charging at 0.5C, the cutoff current is 0.05C, discharge at 1C, and calculate the capacity retention rate after 100 cycles. The first 0.1C activation discharge capacity is the 0.1C capacity, and the first 1C discharge capacity is the 1C capacity. The data obtained is shown in table 2.
[0080] Table 2
[0081]
[0082] From table 2, it can be seen that the 0.1C and 1C capacities of the positive electrode material of the example are both high, and the 100 cycle capacity retention rate is 98% and above, and the electrical performance is excellent. The capacity and 100 cycle capacity retention rate of the positive electrode material of the comparative example are both worse than the example. The positive electrode material prepared using the preparation method given in the application has good electrical performance and processing performance.
[0083] Test Example 5 The first lithium iron phosphate intermediate and the second lithium iron phosphate intermediate prepared in Examples 1, 4 and 5 were respectively measured for the proportion of lithium iron phosphate structure mass, XRD testing was performed using a Bruker D2 PHASER X-ray diffractometer, the testing conditions were referred to the national standard GB / T 30904-2014, and data analysis was performed using TOPAS software. The obtained data is shown in Table 3.
[0084] Table 3
[0085]
[0086] As can be seen from Table 3, the preparation method in the present application successfully realizes control of the proportion of lithium iron phosphate structure mass of the first lithium iron phosphate intermediate and the second lithium iron phosphate intermediate in the control examples to be above 90%, which helps to control the carbon coating to be more uniform in the subsequent sintering step.
[0087] Obviously, the above examples are merely examples for clear illustration, and are not intended to limit the embodiments. Based on the above description, other different forms of changes or variations can also be made by those of ordinary skill in the art. It is not necessary or possible to exhaust all the embodiments. The obvious changes or variations derived therefrom are still within the protection scope of the present application.
Claims
1. A method for preparing a lithium iron phosphate cathode material, characterized in that, Includes the following steps: (1) The first precursor, lithium source, carbon source and solvent are mixed to obtain the first slurry, which is then ground, dried and sintered to obtain the first lithium iron phosphate intermediate; (2) The first lithium iron phosphate intermediate, carbon source and solvent are mixed to obtain the second slurry, which is then ground and dried to obtain the coating material A; (3) The second precursor, lithium source, carbon source and solvent are mixed to obtain the third slurry, which is then ground, dried and sintered to obtain the second lithium iron phosphate intermediate; (4) The second lithium iron phosphate intermediate, carbon source and solvent are mixed to obtain the fourth slurry, which is then ground and dried to obtain the coating material B; The first precursor and the second precursor each use different iron phosphate precursors; (5) Mix coating material A and coating material B, sinter for the first time, crush, and sinter for the second time to obtain lithium iron phosphate cathode material; The second sintering temperature is greater than the first sintering temperature. The preparation method of lithium iron phosphate cathode material satisfies the following relationship: 1.1≤(C2×S2) / (C1×S1)≤2; Wherein, C1 is the ratio of the mass of the carbon source to the mass of the first lithium iron phosphate intermediate in step (2); C2 is the ratio of the mass of the carbon source to the mass of the second lithium iron phosphate intermediate in step (4); S1 is the value of the heating rate of sintering in step (1), in °C / min; S2 is the value of the heating rate of sintering in step (3), in °C / min.
2. The preparation method according to claim 1, characterized in that, In the first slurry, the amounts of the first precursor and the lithium source are such that the molar ratio of Li to Fe is 1.01~1.05 and the molar ratio of Fe to P is 0.95~0.
98. And / or, in the first slurry, the mass of the carbon source is 2% to 6% of the mass of the first precursor; And / or, based on the mass of the first lithium iron phosphate intermediate, the mass percentage of lithium iron phosphate is ≥90%; And / or, the iron phosphate precursor includes any one of the following: iron-based iron phosphate precursor, ammonium-based iron phosphate precursor, and sodium-based iron phosphate precursor.
3. The preparation method according to claim 1, characterized in that, In the third slurry, the amounts of the second precursor and the lithium source are such that the molar ratio of Li to Fe is 1.01~1.05 and the molar ratio of Fe to P is 0.95~0.
98. And / or, in the third slurry, the mass of the carbon source is 2% to 6% of the mass of the second precursor; And / or, based on the mass of the second lithium iron phosphate intermediate, the mass percentage of lithium iron phosphate is ≥90%.
4. The preparation method according to any one of claims 1 to 3, characterized in that, In step (2), C1 is 2%~5%; And / or, in step (4), C2 is 5%~8%; And / or, S1 is 1~5℃ / min; And / or, S2 is 1~5℃ / min.
5. The preparation method according to claim 1, characterized in that, In step (5), the mass ratio of coating material A to coating material B is 1:9 to 9:1; And / or, the temperature of the first sintering is 400~700℃, the holding time is 6~15h, and the heating rate is 1~5℃ / min; And / or, the temperature of the second sintering is 700~820℃, the holding time is 3~6h, and the heating rate is 1~5℃ / min; And / or, in step (5), the particle size of the material obtained after pulverization is <10μm.
6. The preparation method according to any one of claims 1 to 5, characterized in that, In step (1) or step (3), the sintering temperature is 500~750℃ and the holding time is 3~10h; And / or, in step (1), step (2), step (3) or step (4), the grinding includes sand grinding; And / or, in step (1), step (2), step (3) or step (4), the grinding is to grind until the particle size D50 of the particles in the slurry is 200~1000nm; And / or, in step (1), step (2), step (3) or step (4), the drying includes spray drying; And / or, the first slurry and the third slurry further include additives, the additives including dopant elements; optionally, in the first slurry, the amount of additive added is such that the mass percentage of the dopant element in the first lithium iron phosphate intermediate is 1000~3000ppm; optionally, in the third slurry, the amount of additive added is such that the mass percentage of the dopant element in the second lithium iron phosphate intermediate is 3000~6000ppm.
7. The preparation method according to claim 6, characterized in that, The additives include at least one of titanium dioxide, magnesium oxide, aluminum oxide, vanadium pentoxide, ammonium metavanadate, and metatitanic acid. And / or, the doping element includes at least one of titanium, magnesium, aluminum, and vanadium; And / or, in step (1), step (2), step (3) or step (4), the carbon source includes at least one of sucrose, glucose, polyvinyl alcohol, polyethylene glycol, and starch; And / or, in step (1) or step (3), the lithium source includes at least one of lithium carbonate, lithium hydroxide, and lithium dihydrogen phosphate; And / or, in step (1), step (2), step (3) or step (4), the solvent includes water.
8. A lithium iron phosphate cathode material prepared by the preparation method according to any one of claims 1 to 7.
9. A positive electrode sheet, characterized in that, Including the lithium iron phosphate cathode material as described in claim 8.
10. A secondary battery, characterized in that, It includes the positive electrode as described in claim 9, and also includes a negative electrode, a separator, and an electrolyte.