Lithium iron phosphate positive electrode material and preparation method therefor, and electrochemical device

Abstract

A lithium iron phosphate positive electrode material, a preparation method therefor, and an electrochemical device. The preparation method comprises: dry-mixing a phosphorus source, an iron source, a lithium source and a first carbon source to obtain a primary mixture; performing primary sintering on the primary mixture to obtain a lithium iron phosphate precursor; mixing the lithium iron phosphate precursor, a second carbon source and a solvent and grinding same to obtain a secondary mixture; and performing secondary sintering on the secondary mixture to obtain a lithium iron phosphate positive electrode material. The present application utilizes a cooperative effect of dry mixing and grinding, wherein the dry mixing can reduce the number of times of grinding and spray drying, thereby optimizing the process and reducing the energy consumption; and the grinding process can accurately control the particle size of the secondary mixture, thereby achieving synchronous improvement in the compaction density and electrical properties of the lithium iron phosphate positive electrode material.

Description

Lithium iron phosphate cathode material, preparation method thereof, and electrochemical device

[0001] Cross-reference to Related Applications

[0002] The present application claims priority to the Chinese patent application No. 202411345629.X, filed on September 25, 2024, and entitled "Lithium iron phosphate cathode material, preparation method thereof, and electrochemical device", the content of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0003] The present application relates to the field of new energy, in particular to a lithium iron phosphate cathode material, a preparation method thereof, and an electrochemical device. BACKGROUND

[0004] Lithium iron phosphate cathode material has been widely used in power batteries and energy storage fields due to its excellent safety, cost-effectiveness, structural stability, and long cycle life. However, the energy density of lithium iron phosphate batteries limits its development in the power battery market. The most effective way to improve the energy density of lithium iron phosphate batteries is to improve the tap density of lithium iron phosphate cathode material.

[0005] Generally, the tap density of lithium iron phosphate cathode material is improved by promoting particle growth through increasing sintering temperature. However, this method leads to the decline of the electrical performance of the cathode material. In order to achieve the simultaneous improvement of electrical performance and tap density, the industry currently uses the process of wet mixing, wet grinding, spray drying, high-temperature sintering, airflow crushing, and iron removal and packaging by sieving to produce lithium iron phosphate cathode material. In the preparation process, even secondary or tertiary mixing, grinding, and sintering are required, which is complex and tedious, and has low production capacity and high power consumption. SUMMARY

[0006] In view of this, in order to solve at least one of the above technical problems, the present application provides a preparation method of a lithium iron phosphate cathode material.

[0007] In addition, the present application also provides a lithium iron phosphate cathode material prepared by the aforementioned preparation method and an electrochemical device using the lithium iron phosphate cathode material.

[0008] The present application provides a preparation method of a lithium iron phosphate cathode material, which comprises the following steps: dry mixing a phosphorus source, an iron source, a lithium source, and a first carbon source to obtain a primary mixture; sintering the primary mixture to obtain a lithium iron phosphate precursor; mixing and grinding the lithium iron phosphate precursor, a second carbon source, and a solvent to obtain a secondary mixture; and sintering the secondary mixture to obtain the lithium iron phosphate cathode material.

[0009] In some possible embodiments, the dry mixing is performed by using a ploughshare mixer.

[0010] In some possible embodiments, the conditions of the ploughshare mixer mixing include at least one of the following features: (1) the mixing time is 0.5h-4.0h; (2) the frequency of a ploughshare main motor is 25Hz-50Hz; (3) the frequency of a ploughshare flying knife is 10Hz-50Hz.

[0011] In some possible embodiments, the particle size D50 of the secondary mixed material is 0.4μm-0.6μm; the grinding is performed in two steps, in the first step, the secondary mixed material is ground to a particle size D50 of 1.0μm-2.0μm, and in the second step, the grinding product of the first step is further ground to a particle size D50 of 0.4μm-0.6μm.

[0012] In some possible embodiments, the particle size distribution of the secondary mixed material satisfies the following relationship: 0.90≤(D90-D10) / D50≤1.70.

[0013] In some possible embodiments, the first carbon source accounts for 4%-8% of the mass percentage of the primary mixed material; and the second carbon source accounts for 4%-20% of the mass percentage of the secondary mixed material.

[0014] In some possible embodiments, a doping material is further added to the primary mixed material and / or the secondary mixed material, and the doping material includes at least one of titanium dioxide, tetrabutyl titanate, magnesium oxide, magnesium acetate, magnesium hydroxide, magnesium nitrate, zirconium nitrate, zirconium hydroxide, zirconium oxide and niobium pentoxide.

[0015] In some possible embodiments, the conditions of the primary sintering include: a sintering temperature of 600℃-750℃, a sintering time of 3h-15h, and a heating rate of 2℃ / min-5℃ / min.

[0016] The conditions of the secondary sintering include: a sintering temperature of 700℃-800℃, a sintering time of 3h-15h, and a heating rate of 2℃ / min-5℃ / min.

[0017] The application also provides a lithium iron phosphate positive electrode material prepared by the method.

[0018] The application also provides an electrochemical device, which includes a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the separator is arranged between the positive electrode sheet and the negative electrode sheet, and the positive electrode sheet includes the lithium iron phosphate positive electrode material.

[0019] Compared with the prior art, the preparation method of the lithium iron phosphate positive electrode material provided by the embodiment of the application is prepared through the synergistic effect of dry mixing and wet grinding. In the preparation of the primary mixture, dry mixing instead of wet grinding is adopted, so that uniform mixing of raw materials can be achieved without adding a solvent, the number of grinding and spray drying in the entire preparation process is reduced, the production process is simplified, the production efficiency is improved, and the energy consumption and production cost are reduced. In the preparation of the secondary mixture, the wet grinding process is adopted, so that the particle size of the secondary mixture can be accurately controlled, the propagation path of lithium ions is shortened, the compaction density and electrical performance of the lithium iron phosphate positive electrode material are improved, and the energy density of the lithium iron phosphate positive electrode material is improved. The preparation method of the lithium iron phosphate positive electrode material provided by the embodiment of the application is simple in process, can realize the synchronous improvement of the compaction density and electrical performance of the positive electrode material, has high production capacity, is low in cost, and can effectively solve the production capacity bottleneck of the lithium iron phosphate positive electrode material. BRIEF DESCRIPTION OF DRAWINGS

[0020] FIG. 1 is a process flow diagram of the preparation method of the lithium iron phosphate positive electrode material provided by the embodiment of the application.

[0021] FIG. 2 is a structural schematic diagram of an electrochemical device of the lithium iron phosphate positive electrode material provided by the embodiment of the application.

[0022] FIG. 3 is a scanning electron microscope (SEM) image of a lithium iron phosphate precursor in Example 1 of the application.

[0023] FIG. 4 is a scanning electron microscope (SEM) image of a lithium iron phosphate positive electrode material in Example 1 of the application.

[0024] FIG. 5 is a particle size distribution diagram of the lithium iron phosphate positive electrode material in Example 1 of the application.

[0025] FIG. 6 is a scanning electron microscope (SEM) image of a lithium iron phosphate positive electrode material in Comparative Example 1 of the application.

[0026] FIG. 7 is a scanning electron microscope (SEM) image of a lithium iron phosphate positive electrode material in Comparative Example 2 of the application. DETAILED DESCRIPTION

[0027] The technical solutions in the embodiments of the application will be described clearly and completely below with reference to the drawings in the embodiments of the application. Obviously, the described embodiments are only some of the embodiments of the application, rather than all the embodiments. Based on the embodiments of the application, any other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the application.

[0028] In view of this, referring to FIG. 1, the embodiment of the application provides a preparation method of a lithium iron phosphate positive electrode material, which comprises the following steps:

[0029] Step S1, dry mixing the phosphorus source, iron source, lithium source and the first carbon source to obtain a first mixture.

[0030] Specifically, the phosphorus source, iron source, lithium source and the first carbon source are dry mixed in a certain proportion to obtain a first mixture with uniform mixing. Dry mixing can achieve efficient mixing of materials without liquid medium (or solvent), and can also break and uniformly disperse larger particles. As an efficient and simple pretreatment step, dry mixing can effectively reduce the number of grinding and drying processes in the production of lithium iron phosphate, optimize the process and reduce energy consumption. Meanwhile, the first carbon source in the first mixture can prepare lithium iron phosphate precursor through thermal reduction in the next sintering process, promoting the formation of lithium iron phosphate crystal structure.

[0031] In some embodiments, the dry mixing can be performed using a plow mixer. The plow mixer can effectively mix the materials by generating strong shearing and stirring force in the materials through the plow-shaped blades. Compared with high-efficiency mixers, vibration mixers, drum mixers or V-shaped mixers, the plow mixer can generate better shearing force to break and disperse the materials, which is beneficial to further control the particle size of the materials.

[0032] In some embodiments, the mixing time of the plow mixer can be 0.5h-4.0h, which can further improve the mixing effect of the materials. The mixing time can be further 2h-4h, and can be exemplarily 0.5h, 1h, 1.5h, 2h, 2.5h, 3h, 3.5h or 4h, etc.

[0033] In some embodiments, the frequency of the plow main motor can be 25Hz-50Hz. The frequency of the plow main motor determines the rotation speed of the plow mixer, thereby affecting the stirring intensity of the plow and the flowability of the materials. The frequency of 25Hz-50Hz can increase the stirring intensity and further improve the mixing effect. The frequency of the plow main motor can be further 35Hz-50Hz, and can be exemplarily 25Hz, 30Hz, 35Hz, 40Hz, 45Hz or 50Hz, etc.

[0034] In some embodiments, the frequency of the plow flying knife can be 10Hz-50Hz. The frequency of the flying knife determines the rotation speed of the flying knife, thereby affecting the dispersion degree and mixing effect of the materials. The frequency of 10Hz-50Hz can refine the material particles and further improve the dispersibility of the materials. The frequency of the plow flying knife can be further 10Hz-30Hz, and can be exemplarily 10Hz, 15Hz, 20Hz, 25Hz, 30Hz, 35Hz, 40Hz, 45Hz or 50Hz, etc.

[0035] In some embodiments, the first carbon source can include at least one of glucose, sucrose, starch, polyethylene glycol, and the like.

[0036] In some embodiments, the first carbon source can account for 4% to 8% of the mass of the primary mixture. A low carbon source content of 4% to 8% can effectively improve the crystal quality and consistency of the lithium iron phosphate precursor. The first carbon source can further account for 4% to 6% of the mass of the primary mixture. Exemplarily, the first carbon source can account for 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, or 8%, and the like, of the mass of the primary mixture.

[0037] In some embodiments, the phosphorus source can include at least one of iron phosphate and lithium dihydrogen phosphate.

[0038] In some embodiments, the iron source can include at least one of iron oxide and iron phosphate.

[0039] In some embodiments, the lithium source can include at least one of lithium carbonate, lithium hydroxide, and lithium dihydrogen phosphate.

[0040] In some embodiments, the molar ratio of Li:Fe:P in the phosphorus source, the iron source, and the lithium source can be (1.0-1.05):1:(1.015-1.036). By controlling the proportions of the materials, it is beneficial to generate a high-purity lithium iron phosphate positive electrode material and reduce the risk of incomplete reaction of the materials. The molar ratio of Li:Fe:P in the phosphorus source, the iron source, and the lithium source can further be (1.02-1.04):1:(1.015-1.036). Exemplarily, the molar ratio of Li:Fe:P in the phosphorus source, the iron source, and the lithium source can be 1:1:1.015, 1.02:1:1.015, 1.039:1:1.032, or 1.045:1:1.036, and the like.

[0041] In some embodiments, a doping material can be added to the primary mixture for dry mixing. The doping material can include at least one of titanium dioxide, tetrabutyl titanate, magnesium oxide, magnesium acetate, magnesium hydroxide, magnesium nitrate, zirconium nitrate, zirconium hydroxide, zirconium oxide, and niobium pentoxide. The addition of the doping material is beneficial to improve the electrochemical performance, structural stability, and thermal stability of the lithium iron phosphate positive electrode material, thereby further improving the comprehensive performance of the lithium iron phosphate positive electrode material.

[0042] In some embodiments, the mass of the metal in the doping material can account for 0.15% to 0.45% of the mass of the primary mixture. A suitable doping amount is beneficial to further optimize the performance of the lithium iron phosphate positive electrode material. The mass of the metal in the doping material can further account for 0.3% to 0.45% of the mass of the primary mixture. Exemplarily, the mass of the metal in the doping material can account for 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, or 0.45%, and the like, of the mass of the primary mixture.

[0043] Step S2, the primary mixture is subjected to primary sintering to obtain the lithium iron phosphate precursor.

[0044] Specifically, the primary mixture is subjected to primary sintering to generate the crystal phase structure of lithium iron phosphate, thereby obtaining the lithium iron phosphate precursor.

[0045] In some embodiments, the temperature of the primary sintering can be 600-750°C, which is conducive to promoting the formation of the lithium iron phosphate crystal phase and controlling the growth of the particles. The temperature can further be 650-750°C, and can exemplarily be 600°C, 650°C, 750°C, or 750°C, etc.

[0046] In some embodiments, the time of the primary sintering can be 3-15h, which can ensure that the sintering reaction is fully carried out. The time can further be 5-12h, and can exemplarily be 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h, 13h, 14h, or 15h, etc.

[0047] In some embodiments, the temperature rising rate of the primary sintering can be 2-5°C / min, which is conducive to uniform heating, avoids excessive thermal stress, and can also prevent the caking phenomenon caused by rapid temperature rising. The temperature rising rate can further be 3-5°C / min, and can exemplarily be 2°C / min, 2.5°C / min, 3°C / min, 3.5°C / min, 4°C / min, 4.5°C / min, or 5°C / min, etc.

[0048] In some embodiments, the atmosphere of the primary sintering can be an inert gas atmosphere, which can include at least one of nitrogen, argon, and helium, and is helpful to prevent the oxidation reaction of the lithium iron phosphate precursor and improve the crystallinity.

[0049] In some embodiments, the particle size D50 of the lithium iron phosphate precursor is 2.0-8.0μm, and can further be 3.0-6.0μm. The particle size D50 can exemplarily be 2.0μm, 3.0μm, 4.0μm, 4.5μm, 5.0μm, 5.5μm, 6.0μm, 7.0μm, or 8.0μm, etc.

[0050] Step S3, the lithium iron phosphate precursor, the second carbon source, and the solvent are mixed and ground to obtain a secondary mixture.

[0051] Specifically, the lithium iron phosphate precursor, the second carbon source and the solvent are mixed uniformly and ground to effectively control the particle size of the secondary mixture, thereby improving the compaction density of the lithium iron phosphate. In addition, due to the dry mixing pretreatment in step S1, the grinding process only needs to be carried out once to achieve the grading effect of the particle size, thereby reducing the process steps such as grinding and drying, simplifying the process and reducing the energy consumption.

[0052] The second carbon source in the secondary mixture mainly plays the following roles: forming a coating layer on the surface of the lithium iron phosphate positive electrode material in the subsequent sintering process, and controlling the particle size of the lithium iron phosphate positive electrode material.

[0053] In some embodiments, the particle size D50 of the secondary mixture is 0.4 μm to 0.6 μm. After the first sintering, the lithium iron phosphate precursor particles are large, and the grinding can accurately control the particle size of the secondary mixture. The particle size distribution of the secondary mixture in the above range forms a particle size grading, thereby further improving the compaction density of the lithium iron phosphate positive electrode material. At the same time, the secondary mixture with the particle size in the above range shortens the lithium ion transmission path, which is beneficial to the improvement of the rate performance of the lithium iron phosphate positive electrode material, and further improves the comprehensive performance of the lithium iron phosphate positive electrode material. The particle size D50 of the secondary mixture can be further 0.5 μm to 0.6 μm, and can be exemplarily 0.4 μm, 0.42 μm, 0.45 μm, 0.48 μm, 0.5 μm, 0.52 μm, 0.55 μm, 0.58 μm or 0.6 μm, etc.

[0054] In some embodiments, in step S3, the grinding is wet grinding, and a liquid medium (or solvent) is added to disperse the lithium iron phosphate precursor and the second carbon source, which is beneficial to further obtain a uniform particle distribution and reduce the particle size. Further, the liquid medium can be water, and the mass of the liquid medium can be 40% to 70% of the total mass of the secondary mixture.

[0055] In some embodiments, the grinding can be carried out in two steps. In the first step, the secondary mixture is ground to a particle size D50 of 1.0 μm to 2.0 μm, and in the second step, the ground product of the first step is further ground to a particle size D50 of 0.4 μm to 0.6 μm. By carrying out the grinding in two steps, the grinding conditions can be optimized in each stage, the over-grinding phenomenon can be reduced, the particle size can be accurately controlled, and the energy consumption can be further reduced and the grinding efficiency can be improved. Exemplarily, the grinding can be carried out by introducing the material into a sand mill, and first carrying out coarse grinding (adding zirconium beads with a diameter of 0.6 mm) to 1.0 μm to 2.0 μm, and then carrying out fine grinding (adding zirconium beads with a diameter of 0.3 mm) to 0.4 μm to 0.6 μm.

[0056] In some embodiments, the particle size distribution of the secondary mixed material can satisfy the following relationship: the particle size distribution satisfies 0.90≤(D90-D10) / D50≤1.70. Specifically, it can be 0.90, 0.95, 1.1, 1.2, 1.35, 1.52, or 1.63, and of course, it can also be other values within the above range, which is not limited herein. The particle size distribution of the secondary mixed material is within the above range, and there are large particles with large particle sizes and small particles with small particle sizes in the secondary mixed material. The large particles and the small particles can cooperate with each other to form a particle size gradation, and the small particles can fill the pores between the large particles to further improve the compaction density of the lithium iron phosphate positive electrode material through the packing effect.

[0057] In some embodiments, the second carbon source can include at least one of glucose, sucrose, starch, and polyethylene glycol. Exemplarily, it can be a mixture of glucose and polyethylene glycol.

[0058] In some embodiments, the mass percentage of the second carbon source in the secondary mixed material can be 4% to 20%. The carbon source content of 4% to 20% is helpful to form a uniform carbon layer coating, reduce the generation of free carbon, improve the conductivity of the lithium iron phosphate positive electrode material, and further improve the particle size of the lithium iron phosphate positive electrode material. The mass percentage of the second carbon source in the primary mixed material can further be 8% to 18%. Exemplarily, it can be 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, or 20%, and the like.

[0059] In some embodiments, a doping material can also be added to the secondary mixed material. The doping material can include at least one of titanium dioxide, tetrabutyl titanate, magnesium oxide, magnesium acetate, magnesium hydroxide, magnesium nitrate, zirconium nitrate, zirconium hydroxide, zirconium oxide, and niobium pentoxide. The addition of the doping material in this step can further inhibit the abnormal growth of the particles, further control the particle size of the lithium iron phosphate positive electrode material, and improve the uniformity of the carbon coating in the subsequent sintering step.

[0060] In some embodiments, the mass of the metal in the doping material added in this step can account for 0.05% to 0.3% of the mass of the secondary mixed material. The appropriate doping amount is conducive to further controlling the particle size of the lithium iron phosphate positive electrode material and improving the coating effect. The mass of the metal in the doping material can further account for 0.1% to 0.2% of the mass of the secondary mixed material. Exemplarily, it can be 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, or 0.3%, and the like.

[0061] In some embodiments, the lithium iron phosphate precursor can be mixed with the second carbon source and ground, and then the secondary mixture can be further subjected to spray drying. The inlet temperature of the spray drying can be 220-260°C, and the outlet temperature can be 85-110°C. The spray drying is conducive to the volatilization of impurities and liquid medium added in the wet grinding. Setting the range of the inlet temperature and the outlet temperature helps to control the volatilization speed and the drying degree of the liquid medium and impurities in the spray drying process. A higher inlet temperature can accelerate volatilization, and a lower outlet temperature can help reduce liquid residue.

[0062] Step S4: The secondary mixture is subjected to secondary sintering to obtain the lithium iron phosphate positive electrode material.

[0063] Specifically, the secondary mixture is subjected to secondary sintering, and then post-processing, airflow crushing, sieving, and iron removal are performed to obtain the lithium iron phosphate positive electrode material. The secondary sintering can be carbon-coated on the basis of the lithium iron phosphate precursor to improve the electrical conductivity of the lithium iron phosphate positive electrode material. It can also improve the particle crystallinity of the lithium iron phosphate positive electrode material, thereby solving the problem of abnormal growth of primary particles that may occur in the primary sintering or sanding process. In addition, it can also improve the stability of the lithium iron phosphate positive electrode material and reduce the risk of poor stability of the lithium iron phosphate positive electrode material due to poor stability of the raw material iron source and phosphorus source. This step can effectively control the size distribution of the lithium iron phosphate positive electrode material particles, reduce crystalline defects, and improve overall performance.

[0064] In some embodiments, the temperature of the secondary sintering can be 700-800°C. Within the above temperature range, it is conducive to further improving the particle crystallinity of the lithium iron phosphate positive electrode material. The temperature can further be 750-800°C, and exemplarily can be 700°C, 720°C, 750°C, 780°C, or 800°C, etc.

[0065] In some embodiments, the time of the secondary sintering can be 3-15h. The above sintering time is conducive to the full progress of the sintering reaction. The time can further be 5-12h, and exemplarily can be 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h, 13h, 14h, or 15h, etc.

[0066] In some embodiments, the heating rate of the secondary sintering can be 2-5°C / min, which is conducive to uniform heating, avoids excessive thermal stress, and can also prevent caking caused by rapid heating. The heating rate can further be 3-5°C / min, and exemplarily can be 2°C / min, 2.5°C / min, 3°C / min, 3.5°C / min, 4°C / min, 4.5°C / min, or 5°C / min, etc.

[0067] In some embodiments, the atmosphere of the secondary sintering can be an inert gas atmosphere, which can include at least one of nitrogen, argon and helium, to help prevent oxidation of the lithium iron phosphate positive electrode material and improve crystallinity.

[0068] Compared with the prior art, the method for preparing the lithium iron phosphate positive electrode material provided in the embodiments of the application has the following beneficial effects:

[0069] 1. By the synergistic effect of dry mixing and wet grinding, compared with the preparation method of the high-density lithium iron phosphate positive electrode material in the industry using twice wet grinding, the method uses low-energy dry mixing instead of high-energy wet grinding, reduces the number of grinding and spray drying, simplifies the production process, and improves the production efficiency; at the same time, the wet grinding can accurately control the particle size of the secondary mixed material, and realize the synchronous improvement of the compaction density and electrical performance of the lithium iron phosphate positive electrode material.

[0070] 2. The dry mixing can be performed by a plowshare mixer, which can generate shear force and stirring force to better mix, crush and disperse the materials, which is beneficial to further control the particle size of the materials.

[0071] 3. The grinding step can control the particle size distribution of the secondary mixed material, realize the particle size grading of large and small particles, and further improve the compaction density of the lithium iron phosphate positive electrode material; the grinding step can also further reduce energy consumption and improve grinding efficiency through step-by-step grinding.

[0072] 4. By adding a first carbon source to the primary mixed material, the formation of the lithium iron phosphate crystal structure is promoted; by adding a second carbon source to the secondary mixed material, a coating layer is formed on the surface of the lithium iron phosphate positive electrode material during the sintering process in step S4, which is also beneficial to control the particle size of the lithium iron phosphate positive electrode material. The addition amount of the first carbon source and the second carbon source can also be adjusted to further improve the effect of the first carbon source and the second carbon source.

[0073] 5. The preparation method provided in the application has simple process, high production capacity and low cost, which can effectively solve the production bottleneck of the lithium iron phosphate positive electrode material, and is beneficial to large-scale batch production of the lithium iron phosphate positive electrode material.

[0074] The embodiments of the application also provide a lithium iron phosphate positive electrode material, which is prepared by the above-mentioned preparation method of the positive electrode material. Compared with the prior art, the lithium iron phosphate positive electrode material provided in the embodiments of the application has good electrochemical performance and compaction density, and low cost.

[0075] In some embodiments, the compaction density of the lithium iron phosphate positive electrode material can be 2.52g / cm 3 ~ 2.57g / cm 3, and the higher compaction density is beneficial to the improvement of the energy density of the lithium iron phosphate cathode material. The compaction density of the lithium iron phosphate cathode material can be further 2.55 g / cm 3 ~ 2.57 g / cm 3 , and can be exemplarily 2.52 g / cm 3 , 2.53 g / cm 3 , 2.54 g / cm 3 , 2.55 g / cm 3 , 2.56 g / cm 3 , or 2.57 g / cm 3 .

[0076] In some embodiments, the lithium iron phosphate cathode material can have a resistivity of 22 Ω·cm ~ 40 Ω·cm, and the lower internal resistance of the lithium iron phosphate cathode material is helpful to improve the power density, the charge-discharge efficiency and the thermal stability of the prepared battery.

[0077] Referring to FIG. 2, the embodiment of the present application provides an electrochemical device (for example, a battery), which comprises a positive electrode sheet 10, a negative electrode sheet 20, a separator film 30 and an electrolyte 40, wherein the separator film 30 is arranged between the positive electrode sheet 10 and the negative electrode sheet 20, and the positive electrode sheet 10 comprises the aforementioned positive electrode material.

[0078] Compared with the prior art, the electrochemical device 100 provided by the embodiment of the present application adopts the aforementioned positive electrode material. Since the positive electrode sheet of the electrochemical device 100 contains the aforementioned lithium iron phosphate cathode material with high compaction and low resistance, the electrochemical device 100 has good energy density, power density, charge-discharge efficiency and thermal stability, and is safe, efficient and low in cost.

[0079] The aforementioned lithium iron phosphate cathode material and the preparation method thereof are further described below through specific embodiments.

[0080] Embodiment 1

[0081] Step 1: 100 kg of iron phosphate, 25 kg of lithium carbonate, 6 kg of glucose and 525.5 g of titanium dioxide are added into a plow mixer, wherein the molar ratio of Li:Fe:P is 1.039:1:1.032, and the first mixture is obtained by mixing in the plow mixer for 0.5-4.0 h, the frequency of the plow main motor is 40 Hz, and the frequency of the flying knife is 15 Hz.

[0082] Step 2: the first mixture is sintered under the protection of nitrogen atmosphere, and the temperature is increased to 650℃ at a speed of 2.5℃ / min, and the temperature is kept for 7.5 h, and finally the temperature is cooled down to obtain the lithium iron phosphate precursor.

[0083] Step 3, 100 kg of the above lithium iron phosphate precursor, 3.5 kg of glucose, 6.0 kg of polyethylene glycol (molecular weight 6000), 333.7 g of titanium dioxide and 136 kg of water are mixed and stirred uniformly, and the mixture is introduced into a sand mill for coarse grinding (zirconium bead diameter 0.6 mm), the first grinding particle size D50 = 1.68 μm, and then introduced into a sand mill for fine grinding (zirconium bead diameter 0.3 mm), the grinding particle size D50 = 0.56 μm, (D90-D10) / D50 = 1.35, to obtain a secondary mixture. The secondary mixture is then spray dried, the spray drying inlet temperature is 240°C, and the outlet temperature is 95°C.

[0084] Step 4, the dried secondary mixture is subjected to secondary sintering under the protection of an inert atmosphere, the temperature is increased to 780°C at a rate of 2.5°C / min, and the temperature is maintained for 7.5 hours, and then cooled. Finally, airflow crushing and iron removal by sieving are performed to obtain a lithium iron phosphate positive electrode material.

[0085] Example 2

[0086] The specific process of the preparation process is referred to Example 1, and the difference from Example 1 is that the particle size D50 of the secondary mixture after fine grinding in Step 3 is 0.42 μm, (D90-D10) / D50 is 0.91, and the rest of the preparation method of the lithium iron phosphate positive electrode material is basically the same as that of Example 1, which will not be described in detail here.

[0087] Example 3

[0088] The specific process of the preparation process is referred to Example 1, and the difference from Example 1 is that the particle size D50 of the secondary mixture after fine grinding in Step 3 is 0.68 μm, (D90-D10) / D50 is 1.37, and the rest of the preparation method of the lithium iron phosphate positive electrode material is basically the same as that of Example 1, which will not be described in detail here.

[0089] Comparative Example 1

[0090] Step 1, 100 kg of iron phosphate, 25 kg of lithium carbonate, 8.5 kg of glucose, 2.6 kg of polyethylene glycol (molecular weight 6000), 525.5 g of titanium dioxide and 142 kg of water are mixed uniformly, wherein the molar ratio of Li:Fe:P is 1.039:1:1.032, introduced into a sand mill for coarse grinding (zirconium bead diameter 0.6 mm), the first grinding particle size D50 = 1.86 μm, and then introduced into a sand mill for fine grinding (zirconium bead diameter 0.3 mm), the grinding particle size D50 = 0.46 μm, (D90-D10) / D50 = 1.30, to obtain a primary mixture. The primary mixture is then spray dried, the spray drying inlet temperature is 220°C, and the outlet temperature is 90°C.

[0091] Step 2, the dried primary mixture is sintered under inert atmosphere protection, the temperature is raised to 780℃ at a rate of 2.5℃ / min, and the temperature is maintained for 7.5 hours, and then the temperature is lowered, and finally, the lithium iron phosphate positive electrode material is obtained by air flow crushing and iron removal by sieving.

[0092] Comparative Example 2

[0093] Step 1, 100 kg of iron phosphate, 25 kg of lithium carbonate, 6 kg of glucose, 525.5 g of titanium dioxide and 142 kg of water are uniformly mixed, the molar ratio of Li:Fe:P is 1.039:1:1.032, introduced into a sand mill for coarse grinding (zirconium bead diameter 0.6 mm), the primary grinding particle size D50 is 1.86 μm, and then introduced into a sand mill for fine grinding (zirconium bead diameter 0.3 mm), the grinding particle size D50 is 1.0 μm, to obtain a primary mixture. The primary mixture is then spray dried, the spray drying inlet temperature is 220℃, and the outlet temperature is 90℃.

[0094] Step 2, the dried primary mixture is sintered under inert atmosphere protection, the temperature is raised to 780℃ at a rate of 2.5℃ / min, and the temperature is maintained for 7.5 hours, and then the temperature is lowered, and finally, the lithium iron phosphate positive electrode material is obtained by air flow crushing and iron removal by sieving.

[0095] Step 3, 100 kg of the above lithium iron phosphate precursor, 3.5 kg of glucose, 6.0 kg of polyethylene glycol (molecular weight 6000), 333.7 g of titanium dioxide and 136 kg of water are uniformly mixed and stirred, the mixture is introduced into a sand mill for coarse grinding (zirconium bead diameter 0.6 mm), the primary grinding particle size D50 is 1.68 μm, and then introduced into a sand mill for fine grinding (zirconium bead diameter 0.3 mm), the grinding particle size D50 is 0.57 μm, (D90-D10) / D50 is 1.07, to obtain a secondary mixture. The secondary mixture is then spray dried, the spray drying inlet temperature is 220℃, and the outlet temperature is 90℃.

[0096] Step 4, the dried secondary mixture is sintered under inert atmosphere protection, the temperature is raised to 750℃ at a rate of 2.5℃ / min, and the temperature is maintained for 7.5 hours, and then the temperature is lowered, and finally, the lithium iron phosphate positive electrode material is obtained by air flow crushing and iron removal by sieving.

[0097] The process parameters of the above examples and comparative examples are shown in Table 1.

[0098] The energy consumption test method is as follows: according to the theoretical output of the lithium iron phosphate positive electrode material, the power consumption (kWh / t) generated by sand milling and spraying is calculated;

[0099] The gas consumption test method is as follows: according to the theoretical output of the lithium iron phosphate positive electrode material, the gas consumption (m 3 / t) generated by spraying is calculated.

[0100] Table 1

[0101] The lithium iron phosphate positive electrode materials obtained in Examples 1-3 and Comparative Examples 1-2 were subjected to the following tests, and the corresponding test results were obtained.

[0102] 1. The lithium iron phosphate positive electrode materials obtained in Examples 1-3 and Comparative Examples 1-2 were subjected to tests of pH value, specific surface area, carbon content, powder compaction density, resistivity, and electrical performance, and the results are shown in Table 2.

[0103] (1) Specific surface area: The equilibrium adsorption amount of nitrogen gas on the surface of an object is related to its specific surface area and other characteristics at liquid nitrogen temperature, and the specific surface area can be tested by combining the law of adsorption amount change with relative pressure during the adsorption process.

[0104] (2) Carbon content: The carbon content of the lithium iron phosphate positive electrode material was tested by infrared analysis. A carbon-sulfur analyzer was used, and the sample was burned in a high-temperature oxygen-rich state. The carbon elements contained therein were oxidized to carbon dioxide, and entered the infrared detector with the carrier gas. The carbon element content was calculated by quantitatively calculating the change in infrared absorption wavelength intensity of the carbon dioxide signal.

[0105] (3) Powder compaction density: The test was performed in accordance with the method for determining the powder compaction density of the positive electrode material of a lithium ion battery specified in the standard drafted by the National Non-ferrous Metals Standardization Technical Committee.

[0106] (4) Powder resistivity test: The test was performed in accordance with the method for determining the powder resistivity of the positive electrode material of a lithium ion battery specified in the standard drafted by the National Non-ferrous Metals Standardization Technical Committee.

[0107] (5) Electrical performance test: The lithium iron phosphate positive electrode materials in Examples 1-3 and Comparative Examples 1-2 were made into positive electrode sheets, and a button cell was assembled with metal lithium as the negative electrode material. The button cell obtained was tested in the voltage range of 2.0-3.75 V.

[0108] The test steps of the first charge-discharge efficiency included: under the condition of 25°C normal temperature, the lithium ion battery was charged to 3.75 V at a current of 0.1C, and then charged at a constant voltage until the current decreased to 0.05C, the charging stopped, and the first charge capacity was recorded. Then, it was discharged to the cut-off voltage of 2.0 V at a current of 0.1C, and the first discharge capacity was recorded. The 0.1C specific capacity of the positive electrode material was calculated based on the first discharge capacity, and the first charge-discharge efficiency of the battery was calculated according to the following formula, i.e. the first efficiency: first charge-discharge efficiency = (first discharge capacity / first charge capacity)*100%.

[0109] Then the above charging and discharging process is repeated once at 1C current, and the 2nd charging capacity and the 2nd discharging capacity are recorded, and the 1C specific capacity of the positive electrode material is calculated based on the 2nd discharging capacity.

[0110] Table 2

[0111] Firstly, the electrical performance and the compaction density of the lithium iron phosphate positive electrode material in Examples 1-3 are effectively improved. Since the dry mixing is performed once and the wet grinding is performed once in Examples 1-3, compared with the process of wet grinding only once in Comparative Example 1, the powder compaction density of the lithium iron phosphate positive electrode material in Examples 1-3 is greatly improved, specifically, the powder compaction density of the lithium iron phosphate positive electrode material in Examples 1-3 is 2.55 g / cc, 2.52 g / cc and 2.57 g / cc respectively, while the powder compaction density of the lithium iron phosphate positive electrode material in Comparative Example 1 is only 2.38 g / cc; in addition, the electrical resistivity of the lithium iron phosphate positive electrode material powder in Examples 1-3 is less than that in Comparative Example 1. The powder compaction density, the electrical resistivity and the electrical performance of Examples 1-3 are close to those of Comparative Example 2, but the process of Comparative Example 2 adopts twice grinding, which is complex, high in energy consumption and low in productivity. Different from Comparative Example 2, the dry mixing is adopted in Examples 1-3, which can reduce the production energy consumption and cost.

[0112] Among them, the overall comprehensive performance such as the powder compaction density, the electrical resistivity and the electrical performance of Example 1-2 is better than that of Example 3, which is because the particle size D50 in Example 1-2 is in the range of 0.4 μm to 0.6 μm, while the particle size D50 of the secondary mixed material in Example 3 is 0.68 μm, although the powder compaction density is slightly increased, but the electrical performance is reduced due to the increase of lithium ion transmission path.

[0113] Secondly, the carbon content of the lithium iron phosphate positive electrode material in Examples 1-3 is in the appropriate range, which is beneficial to the uniform distribution of the carbon coating layer, thereby improving the electrical conductivity and the structural stability of the lithium iron phosphate positive electrode material.

[0114] In addition, the scanning electron microscopy (SEM) observation is performed on the lithium iron phosphate precursor and the lithium iron phosphate positive electrode material in Example 1 and the lithium iron phosphate positive electrode material obtained in Comparative Examples 1-2.

[0115] FIG. 3 is the SEM image of the lithium iron phosphate precursor in Example 1, FIG. 4 is the SEM image of the lithium iron phosphate positive electrode material in Example 1, and FIG. 5 is the particle size distribution graph of the lithium iron phosphate positive electrode material in Example 1. It can be known from FIGS. 3, 4 and 5 that the particle size of the lithium iron phosphate precursor after dry mixing and sintering is large, the particle size distribution appears bimodal after wet grinding, the particle size grading is realized, the particle size grading effect of the lithium iron phosphate positive electrode material in Example 1 obtained by sintering is good, which is beneficial to the improvement of the compaction density of the lithium iron phosphate positive electrode material.

[0116] Figure 6 is an SEM image of the lithium iron phosphate positive electrode material in Comparative Example 1. Compared with Figure 4, it can be seen that the particle size grading effect of the lithium iron phosphate positive electrode material in Example 1 is better, because the particle size grading is formed after wet grinding in Example 1, and the size particle grading compaction is improved after secondary sintering. The number of particles above 1 μm of the primary particles after primary sintering in Comparative Example 1 is less, and the particle size grading effect is poor, so the compaction is lower.

[0117] Figure 7 is an SEM image of the lithium iron phosphate positive electrode material in Comparative Example 2. Compared with Figure 4, the particle size and size particle grading effect are similar, but the preparation method of Comparative Example 2 consumes more energy and has lower productivity.

[0118] It can be understood that the above embodiments are only exemplary embodiments adopted for illustrating the principles of the present application, and the present application is not limited thereto. Various modifications and improvements can be made by those of ordinary skill in the art without departing from the spirit and essence of the present application, and these modifications and improvements are also considered to be within the protection scope of the present application.

Claims

1. A method for preparing a lithium iron phosphate cathode material, characterized in that, The application relates to a preparation method of a lithium iron phosphate positive electrode material. The phosphorus source, the iron source, the lithium source and the first carbon source are dry mixed to obtain a primary mixture; The primary mixture is sintered to obtain a lithium iron phosphate precursor; The lithium iron phosphate precursor, a second carbon source and a solvent are mixed and ground to obtain a secondary mixture; The secondary mixture is sintered to obtain the lithium iron phosphate positive electrode material. The dry mixing is carried out by using a ploughshare mixer.

2. The method of claim 1, wherein the lithium iron phosphate cathode material is prepared by the steps of: The ploughshare mixer mixing conditions include at least one of the following features: ​ 3. The method of claim 2, wherein the lithium iron phosphate cathode material is prepared by the steps of: (1) the mixing time is 0.5h-4.0h; ​ (2) the ploughshare main motor frequency is 25Hz-50Hz; (3) the ploughshare flying knife frequency is 10Hz-50Hz. The particle size D50 of the secondary mixture is 0.4mu m-0.6mu m; 4. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, The grinding is carried out in two steps, the first step is to grind the secondary mixture to a particle size D50 of 1.0mu m-2.0mu m, and the second step is to continue grinding the grinding product of the first step to a particle size D50 of 0.4mu m-0.6mu m. The particle size distribution of the secondary mixture satisfies the following relationship: 0.90 <= (D90-D10) / D50 <= 1.

70.

5. The method of claim 1, wherein the lithium iron phosphate cathode material is prepared by the steps of: mixing lithium carbonate, iron oxide, and phosphorous pentoxide; and heating the mixture to a temperature of 600-800°C for 2-10 hours. The mass percentage of the first carbon source in the primary mixture is 4%-8%; 6. The method of claim 1, wherein the lithium iron phosphate cathode material is prepared by the steps of: mixing lithium carbonate, iron oxide, and phosphorous pentoxide; and heating the mixture to a temperature of 600-800°C for 2-10 hours. The mass percentage of the second carbon source in the secondary mixture is 4%-20%. The primary mixture and / or the secondary mixture further comprises a doping material, and the doping material comprises at least one of titanium dioxide, tetrabutyl titanate, magnesium oxide, magnesium acetate, magnesium hydroxide, magnesium nitrate, zirconium nitrate, zirconium hydroxide, zirconium oxide and niobium pentoxide.

7. The method for preparing lithium iron phosphate cathode material according to claim 1, characterized in that, The sintering conditions of the primary sintering include: the sintering temperature is 600 DEG C-750 DEG C, the sintering time is 3h-15h, and the heating rate is 2 DEG C / min-5 DEG C / min; 8. The method for preparing lithium iron phosphate cathode material according to claim 1, characterized in that, The sintering conditions of the secondary sintering include: the sintering temperature is 700 DEG C-800 DEG C, the sintering time is 3h-15h, and the heating rate is 2 DEG C / min-5 DEG C / min. The lithium iron phosphate positive electrode material is prepared by the preparation method of the lithium iron phosphate positive electrode material according to any one of claims 1-8.

9. A lithium iron phosphate cathode material, characterized in that, The application further relates to a lithium ion battery, which comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the separator is arranged between the positive electrode sheet and the negative electrode sheet, and the positive electrode sheet comprises the lithium iron phosphate positive electrode material according to claim 9.

10. An electrochemical device, characterized by, ​

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