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

Abstract

The application discloses a lithium iron phosphate positive electrode material, a preparation method thereof and an electrochemical device. The preparation method comprises the following steps: mixing a phosphorus source, an iron source, a lithium source and a first carbon source in a dry manner 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 to obtain a secondary mixture; and performing secondary sintering on the secondary mixture to obtain the lithium iron phosphate positive electrode material. The dry mixing and grinding are synergistically used, the number of grinding and spray drying is reduced, the process is optimized, the energy consumption is reduced, the particle size of the secondary mixture is accurately controlled by using the grinding process, and the compaction density and the electrical performance of the lithium iron phosphate positive electrode material are simultaneously improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of new energy, in particular to a lithium iron phosphate positive electrode material, a preparation method thereof, and an electrochemical device. BACKGROUND

[0002] The lithium iron phosphate positive electrode 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 the lithium iron phosphate battery limits its development in the power battery market. The most effective method to improve the energy density of the lithium iron phosphate battery is to improve the compaction density of the lithium iron phosphate positive electrode material.

[0003] Generally, the compaction density of the lithium iron phosphate positive electrode material is improved by promoting particle growth through increasing the sintering temperature. However, this method will cause the electrical performance of the positive electrode material to decrease. In order to achieve the simultaneous improvement of the electrical performance and the compaction density, the industry currently usually adopts the process of wet mixing, wet grinding, spray drying, high-temperature sintering, airflow crushing, and iron removal and packaging by sieving to produce the lithium iron phosphate positive electrode material. In the preparation process, even secondary or tertiary mixing, grinding, and sintering are required, and the process is complex and the flow is cumbersome, with low production capacity and high power consumption. SUMMARY

[0004] In view of this, in order to solve at least one of the above technical problems, the embodiments of the present application provide a preparation method of a lithium iron phosphate positive electrode material.

[0005] In addition, the embodiments of the present application also provide a lithium iron phosphate positive electrode material prepared by the foregoing preparation method and an electrochemical device using the lithium iron phosphate positive electrode material.

[0006] The embodiments of the present application provide a preparation method of a lithium iron phosphate positive electrode material, which comprises the following steps:

[0007] Mixing a phosphorus source, an iron source, a lithium source, and a first carbon source in a dry manner to obtain a primary mixture;

[0008] Sintering the primary mixture once to obtain a lithium iron phosphate precursor;

[0009] Mixing and grinding the lithium iron phosphate precursor, a second carbon source, and a solvent to obtain a secondary mixture; and

[0010] Sintering the secondary mixture twice to obtain the lithium iron phosphate positive electrode material.

[0011] In some possible embodiments, the dry mixing adopts a plowshare mixer.

[0012] In some possible embodiments, the conditions of the plowshare mixer mixing comprise at least one of the following features:

[0013] (1) the mixing time is 0.5h-4.0h;

[0014] (2) the frequency of the main motor of the plough is 25Hz-50Hz;

[0015] (3) the frequency of the flying knife of the plough is 10Hz-50Hz.

[0016] In some possible embodiments, the particle size D50 of the secondary mixed material is 0.4μm-0.6μm;

[0017] 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.

[0018] In some possible embodiments, the particle size distribution of the secondary mixed material satisfies the following relationship: 0.90≤(D90-D10) / D50≤1.70.

[0019] In some possible embodiments, the first carbon source accounts for 4%-8% of the mass percentage of the primary mixed material;

[0020] The second carbon source accounts for 4%-20% of the mass percentage of the secondary mixed material.

[0021] 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 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.

[0022] 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;

[0023] 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.

[0024] The application further provides a lithium iron phosphate positive electrode material, which is prepared by the method for preparing a lithium iron phosphate positive electrode material.

[0025] The application further provides an electrochemical device, comprising 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 aforementioned lithium iron phosphate positive electrode material.

[0026] Compared with the prior art, the preparation method of the lithium iron phosphate positive electrode material provided by the application is prepared by the synergistic effect of dry mixing and wet grinding. The preparation of the primary mixture adopts dry mixing instead of wet grinding, so that uniform mixing of raw materials is 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. The preparation of the secondary mixture is prepared by cooperating with the wet grinding process, 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 application has a simple process, can realize synchronous improvement of the compaction density and electrical performance of the positive electrode material, has higher production capacity, and has lower cost. The method can effectively solve the production capacity bottleneck of the lithium iron phosphate positive electrode material. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 The process flow chart of the preparation method of the lithium iron phosphate positive electrode material provided by the application is shown.

[0028] Figure 2 The structure schematic diagram of the electrochemical device of the lithium iron phosphate positive electrode material provided by the application is shown.

[0029] Figure 3 The scanning electron microscope image of the lithium iron phosphate precursor in Example 1 of the application is shown.

[0030] Figure 4 The scanning electron microscope image of the lithium iron phosphate positive electrode material in Example 1 of the application is shown.

[0031] Figure 5 The particle size distribution diagram of the lithium iron phosphate positive electrode material in Example 1 of the application is shown.

[0032] Figure 6 The scanning electron microscope image of the lithium iron phosphate positive electrode material in Comparative Example 1 of the application is shown.

[0033] Figure 7 The scanning electron microscope image of the lithium iron phosphate positive electrode material in Comparative Example 2 of the application is shown. DETAILED DESCRIPTION

[0034] With reference to the accompanying drawings on which the drawings in the embodiments of the present application are shown, the technical solutions in the embodiments of the present application will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments of the present application, all other embodiments obtained by a person of ordinary skill in the art without creative effort belong to the protection scope of the present application.

[0035] In view of this, please refer to Figure 1 The embodiments of the present application provide a preparation method of a lithium iron phosphate positive electrode material, which comprises the following steps:

[0036] In step S1, a phosphorus source, an iron source, a lithium source and a first carbon source are dry mixed to obtain a primary mixture.

[0037] Specifically, the phosphorus source, the iron source, the lithium source and the first carbon source are dry mixed in a certain proportion to obtain a primary mixture with uniform mixing. Dry mixing can realize 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 and preparation of lithium iron phosphate, optimize the process and reduce energy consumption. Meanwhile, the first carbon source in the primary mixture can prepare a lithium iron phosphate precursor through thermal reduction in the next sintering process, and promote the formation of lithium iron phosphate crystal structure.

[0038] In some embodiments, the dry mixing can be performed by a plowshare mixer. The plowshare mixer can effectively mix the materials by generating strong shearing and stirring force in the materials through the plowshare-shaped blades. Compared with high-efficiency mixers, vibration mixers, drum mixers or V-shaped mixers, the plowshare mixer can generate better shearing force to break and disperse the materials, which is conducive to further controlling the particle size of the materials.

[0039] In some embodiments, the mixing time of the plowshare 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 exemplarily can be 0.5h, 1h, 1.5h, 2h, 2.5h, 3h, 3.5h or 4h, etc.

[0040] In some embodiments, the frequency of the plowshare main motor can be 25Hz-50Hz. The frequency of the plowshare main motor determines the rotation speed of the plowshare mixer, thereby affecting the stirring intensity of the plowshare 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 plowshare main motor can be further 35Hz-50Hz, and exemplarily can be 25Hz, 30Hz, 35Hz, 40Hz, 45Hz or 50Hz, etc.

[0041] In some embodiments, the frequency of the plough blade flying knife can be 10 Hz-50 Hz. The frequency of the flying knife determines the rotation speed of the flying knife, thereby affecting the dispersion degree and mixing effect of the material. The flying knife frequency of 10 Hz-50 Hz can refine the material particles and further improve the dispersibility of the material. The frequency of the plough blade flying knife can further be 10 Hz-30 Hz, for example, 10 Hz, 15 Hz, 20 Hz, 25 Hz, 30 Hz, 35 Hz, 40 Hz, 45 Hz, or 50 Hz, etc.

[0042] In some embodiments, the first carbon source can include at least one of glucose, sucrose, starch, and polyethylene glycol, etc.

[0043] In some embodiments, the mass percentage of the first carbon source in the primary mixture can be 4%-8%. The low carbon source content of 4%-8% can effectively improve the crystal quality and consistency of the lithium iron phosphate precursor. The mass percentage of the first carbon source in the primary mixture can further be 4%-6%, for example, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, or 8%, etc.

[0044] In some embodiments, the phosphorus source can include at least one of iron phosphate and lithium dihydrogen phosphate, etc.

[0045] In some embodiments, the iron source can include at least one of iron oxide and iron phosphate, etc.

[0046] In some embodiments, the lithium source can include at least one of lithium carbonate, lithium hydroxide, and lithium dihydrogen phosphate, etc.

[0047] 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 proportion of the material, it is beneficial to generate a high-purity lithium iron phosphate positive electrode material and reduce the risk of incomplete reaction of the material. 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), for example, 1:1:1.015, 1.02:1:1.015, 1.039:1:1.032, or 1.045:1:1.036, etc.

[0048] In some embodiments, the doping material can be added to the primary mixture for dry mixing, and 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, so as to further improve the comprehensive performance of the lithium iron phosphate positive electrode material.

[0049] 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, and the appropriate 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, and exemplarily can be 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, or 0.45%, etc.

[0050] In step S2, the primary mixture is subjected to primary sintering to obtain a lithium iron phosphate precursor.

[0051] Specifically, the primary mixture is subjected to primary sintering to generate a crystal phase structure of lithium iron phosphate, thereby obtaining a lithium iron phosphate precursor.

[0052] In some embodiments, the temperature of the primary sintering can be 600°C to 750°C, which is beneficial to promote the formation of the lithium iron phosphate crystal phase and control the growth of the particles. The temperature can further be 650°C to 750°C, and exemplarily can be 600°C, 650°C, 750°C, or 750°C, etc.

[0053] In some embodiments, the time of the primary sintering can be 3h to 15h, which can ensure that the sintering reaction is fully carried out. The time can further be 5h to 12h, and exemplarily can be 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h, 13h, 14h, or 15h, etc.

[0054] In some embodiments, the heating rate of the primary sintering can be 2°C / min to 5°C / min, which is beneficial to uniform heating, avoids excessive thermal stress, and can also prevent the caking phenomenon caused by rapid heating. The heating rate can further be 3°C / min to 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.

[0055] In some embodiments, the atmosphere of the first 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 precursor and improve crystallinity.

[0056] In some embodiments, the particle size D50 of the lithium iron phosphate precursor can be 2.0-8.0 μm, further can be 3.0-6.0 μm, and exemplarily can be 2.0 μm, 3.0 μm, 4.0 μm, 4.5 μm, 5.0 μm, 5.5 μm, 6.0 μm, 7.0 μm or 8.0 μm, etc.

[0057] In step S3, the lithium iron phosphate precursor, the second carbon source and the solvent are mixed and ground to obtain a second mixture.

[0058] Specifically, the lithium iron phosphate precursor, the second carbon source and the solvent are mixed and ground to effectively control the particle size of the second mixture, thereby improving the tap density of the lithium iron phosphate. In addition, since the dry mixing pretreatment is adopted in step S1, the grinding process can achieve the grading effect of the particle size only once, thereby reducing the process steps such as grinding and drying, simplifying the process and reducing the energy consumption.

[0059] The second carbon source in the second mixture mainly functions to form a coating layer on the surface of the lithium iron phosphate cathode material in the subsequent sintering process and control the particle size of the lithium iron phosphate cathode material.

[0060] In some embodiments, the particle size D50 of the second mixture can be 0.4-0.6 μm, and the lithium iron phosphate precursor obtained after the first sintering has a large particle size. The grinding can accurately control the particle size of the second mixture. The particle size distribution of the second mixture in the above range forms a particle size grading, thereby further improving the tap density of the lithium iron phosphate cathode material. Meanwhile, the second 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 cathode material and further improves the comprehensive performance of the lithium iron phosphate cathode material. Further, the particle size D50 of the second mixture can be 0.5-0.6 μm, and exemplarily can be 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.

[0061] 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 40-70% of the total mass of the second mixture.

[0062] In some embodiments, the grinding can be performed in two steps, in which the second mixture is first ground to a particle size D50 of 1.0-2.0 μm, and then the product of the first step is further ground to a particle size D50 of 0.4-0.6 μm. By performing the grinding in two steps, the grinding conditions can be optimized in each step, the over-grinding phenomenon can be reduced, the particle size can be precisely controlled, and the energy consumption and grinding efficiency can be further reduced. For example, the grinding can be performed by first introducing the material into a sand mill, performing coarse grinding (adding zirconium beads with a diameter of 0.6 mm) to a particle size of 1.0-2.0 μm, and then performing fine grinding (adding zirconium beads with a diameter of 0.3 mm) to a particle size of 0.4-0.6 μm.

[0063] In some embodiments, the particle size distribution of the second mixture 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, or other values within the above range, which are not limited herein. When the particle size distribution of the second mixture is within the above range, there are large particles and small particles in the second mixture, and the large particles and 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, thereby further improving the tap density of the lithium iron phosphate positive electrode material.

[0064] In some embodiments, the second carbon source can include at least one of glucose, sucrose, starch, and polyethylene glycol. For example, it can be a mixture of glucose and polyethylene glycol.

[0065] In some embodiments, the second carbon source can account for 4-20% of the mass percentage of the second mixture. The carbon content of 4-20% can help 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 second carbon source can further account for 8-18% of the mass percentage of the first mixture. For example, it can be 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, or 20%, or other values.

[0066] In some embodiments, a doping material can also be added to the second mixture. 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.

[0067] 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, and a suitable 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, and exemplarily can be 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, or 0.3%, etc.

[0068] In some embodiments, after the lithium iron phosphate precursor is mixed with the second carbon source and ground, the secondary mixed material can also be subjected to spray drying. The inlet temperature of the spray drying can be 220°C to 260°C, and the outlet temperature can be 85°C to 110°C. Spray drying is conducive to the volatilization of impurities and the liquid medium added in the wet grinding, and 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, while a lower outlet temperature helps to reduce liquid residue.

[0069] Step S4: The secondary mixed material is subjected to secondary sintering to obtain a lithium iron phosphate positive electrode material.

[0070] Specifically, the secondary mixed material is subjected to secondary sintering, and then subjected to airflow crushing, sieving, and iron removal after post-processing to obtain a 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; 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 can occur in the primary sintering or sanding process; in addition, can also improve the stability of the lithium iron phosphate positive electrode material, thereby reducing 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.

[0071] In some embodiments, the temperature of the secondary sintering can be 700°C to 800°C, which is conducive to further improving the particle crystallinity of the lithium iron phosphate positive electrode material in the above temperature range. The temperature can further be 750°C to 800°C, and exemplarily can be 700°C, 720°C, 750°C, 780°C, or 800°C, etc.

[0072] In some embodiments, the time of the secondary sintering can be 3h to 15h, and the above sintering time is conducive to the full progress of the sintering reaction. The time can further be 5h to 12h, and exemplarily can be 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h, 13h, 14h, or 15h, etc.

[0073] In some embodiments, the heating rate of the secondary sintering can be 2-5℃ / min, which is conducive to uniform heating, avoids excessive thermal stress, and prevents the phenomenon of caking caused by rapid heating. The heating rate can further be 3-5℃ / min, and can exemplarily be 2℃ / min, 2.5℃ / min, 3℃ / min, 3.5℃ / min, 4℃ / min, 4.5℃ / min or 5℃ / min, etc.

[0074] 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, and is helpful to prevent the oxidation of the lithium iron phosphate positive electrode material and improve the crystallinity.

[0075] Compared with the prior art, the preparation method of the lithium iron phosphate positive electrode material provided by the embodiments of the present application has the following beneficial effects:

[0076] 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 which adopts twice wet grinding, the low-energy dry mixing is used to replace the high-energy wet grinding, the number of grinding and spray drying is reduced, the production process is simplified, and the production efficiency is improved. At the same time, the wet grinding is used to accurately control the particle size of the secondary mixed material, and the synchronous improvement of the compaction density and electrical performance of the lithium iron phosphate positive electrode material is realized.

[0077] 2. The dry mixing is carried out by using a plowshare mixer, the shear force and stirring force generated by the plowshare mixer can be used to better mix, crush and disperse the materials, which is conducive to further controlling the particle size of the materials.

[0078] 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 the energy consumption and improve the grinding efficiency by step-by-step grinding.

[0079] 4. The first carbon source is added to the primary mixed material to promote the formation of lithium iron phosphate crystal structure; the second carbon source is added to the secondary mixed material to form a coating layer on the surface of the lithium iron phosphate positive electrode material during the sintering process in step S4, which is also conducive to controlling 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.

[0080] 5. The preparation method of the present 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 conducive to the large-scale batch production of the lithium iron phosphate positive electrode material.

[0081] The application further provides a lithium iron phosphate positive electrode material prepared by the preparation method of the positive electrode material.

[0082] In some embodiments, the lithium iron phosphate positive electrode material has a tap density of 2.52 g / cm 3 ~ 2.57 g / cm 3 A higher tap density is conducive to the improvement of the energy density of the lithium iron phosphate positive electrode material. The tap density of the lithium iron phosphate positive electrode material can be 2.55 g / cm 3 ~ 2.57 g / cm 3 , for example, 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 , etc.

[0083] In some embodiments, the lithium iron phosphate positive electrode material has a resistivity of 22 Ω·cm ~ 40 Ω·cm. A lower internal resistance of the lithium iron phosphate positive electrode material helps to improve the power density, charge-discharge efficiency and thermal stability of the prepared battery.

[0084] Referring to FIG. 1, Figure 2 The application provides an electrochemical device (for example, a battery), which includes a positive electrode sheet 10, a negative electrode sheet 20, a separator film 30 and an electrolyte 40. The separator film 30 is arranged between the positive electrode sheet 10 and the negative electrode sheet 20, and the positive electrode sheet 10 includes the aforementioned positive electrode material.

[0085] Compared with the prior art, the electrochemical device 100 provided by the application adopts the aforementioned positive electrode material. Since the positive electrode sheet of the electrochemical device 100 contains the aforementioned lithium iron phosphate positive electrode material with high tap density 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.

[0086] The aforementioned lithium iron phosphate positive electrode material and the preparation method thereof are further described below through specific embodiments.

[0087] Embodiment 1

[0088] Step 1, 100 kg of iron phosphate, 25 kg of lithium carbonate, 6 kg of glucose and 525.5 g of titanium dioxide were added into a plow mixer, the molar ratio of Li:Fe:P was 1.039:1:1.032, and the mixture was mixed in the plow mixer for 0.5-4.0 h, the frequency of the plow main motor was 40 Hz, and the frequency of the flying knife was 15 Hz, to obtain a primary mixture.

[0089] Step 2, the primary mixture was sintered under the protection of nitrogen atmosphere, and the temperature was increased to 650℃ at a rate of 2.5℃ / min, and the temperature was kept for 7.5 h, and finally the temperature was cooled, to obtain a lithium iron phosphate precursor.

[0090] Step 3, 100 kg of the 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 were mixed and stirred uniformly, and the mixture was introduced into a sand mill for coarse grinding (zirconium bead diameter 0.6 mm), the primary grinding particle size D50 was 1.68 μm, and then the mixture was introduced into a sand mill for fine grinding (zirconium bead diameter 0.3 mm), the grinding particle size D50 was 0.56 μm, and (D90-D10) / D50 was 1.35, to obtain a secondary mixture. The secondary mixture was then spray dried, the inlet air temperature of the spray dryer was 240℃, and the outlet air temperature was 95℃.

[0091] Step 4, the dried secondary mixture was sintered under the protection of inert atmosphere, and the temperature was increased to 780℃ at a rate of 2.5℃ / min, and the temperature was kept for 7.5 h, and then the temperature was cooled, and finally the mixture was airflow pulverized and screened to remove iron, to obtain a lithium iron phosphate positive electrode material.

[0092] Example 2

[0093] 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, and (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.

[0094] Example 3

[0095] 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, and (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.

[0096] Comparative Example 1

[0097] 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 were mixed uniformly, wherein the molar ratio of Li:Fe:P was 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, 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 first mixture. The first mixture was then spray dried, the spray drying inlet temperature was 220°C, and the outlet temperature was 90°C.

[0098] Step 2, the dried first mixture was sintered under the protection of inert atmosphere, the temperature was increased to 780°C at a rate of 2.5°C / min, and the temperature was maintained for 7.5 h, then cooled, finally airflow pulverization and iron removal by sieving were carried out, to obtain a lithium iron phosphate positive electrode material.

[0099] Comparative Example 2

[0100] 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 were mixed uniformly, wherein the molar ratio of Li:Fe:P was 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, 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 first mixture. The first mixture was then spray dried, the spray drying inlet temperature was 220°C, and the outlet temperature was 90°C.

[0101] Step 2, the dried first mixture was sintered under the protection of inert atmosphere, the temperature was increased to 780°C at a rate of 2.5°C / min, and the temperature was maintained for 7.5 h, then cooled, finally airflow pulverization and iron removal by sieving were carried out, to obtain a lithium iron phosphate positive electrode material.

[0102] 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 were mixed and stirred uniformly, the mixture was introduced into a sand mill for coarse grinding (zirconium bead diameter 0.6 mm), the first grinding particle size D50=1.68 μm, then introduced into a sand mill for fine grinding (zirconium bead diameter 0.3 mm), the grinding particle size D50=0.57 μm, (D90-D10) / D50=1.07, to obtain a second mixture. The second mixture was then spray dried, the spray drying inlet temperature was 220°C, and the outlet temperature was 90°C.

[0103] Step 4, the dried secondary mixture is sintered again under the protection of inert atmosphere, and the temperature is increased to 750°C at a rate of 2.5°C / min, and the temperature is kept for 7.5 hours, and then the temperature is decreased, and finally the airflow crushing and iron removal by sieving are performed to obtain the lithium iron phosphate positive electrode material.

[0104] The process parameters of the above examples and comparative examples are shown in Table 1.

[0105] 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 sanding and spraying is calculated.

[0106] 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.

[0107] Table 1

[0108]

[0109] The lithium iron phosphate positive electrode materials obtained in Examples 1-3 and Comparative Examples 1-2 are tested as follows, and the corresponding test results are obtained.

[0110] 1. The lithium iron phosphate positive electrode materials obtained in Examples 1-3 and Comparative Examples 1-2 are tested for pH value, specific surface area, carbon content, powder compaction density, resistivity, and electrical performance, and the results are shown in Table 2.

[0111] (1) Specific surface area: nitrogen adsorption-desorption method is used to test the specific surface area of the object at liquid nitrogen temperature. The equilibrium adsorption amount of nitrogen on the surface of the object is related to the specific surface area, and the specific surface area can be tested by combining the change rule of the adsorption amount with the relative pressure in the adsorption process.

[0112] (2) Carbon content: infrared analysis method is used to test the carbon content of the lithium iron phosphate positive electrode material. Carbon-sulfur analyzer is used to burn the sample in a high-temperature oxygen-rich state. The carbon element contained in the sample is oxidized to carbon dioxide, which enters the infrared detector with the carrier gas. The carbon content is calculated by quantitatively calculating the change of infrared absorption wavelength intensity of carbon dioxide signal.

[0113] (3) Powder compaction density: the method for testing the powder compaction density of the lithium ion battery positive electrode material is tested according to the method specified in the standard drafted by the National Non-ferrous Metals Standardization Technical Committee.

[0114] (4) Powder resistivity test: the method for testing the powder resistivity of the lithium ion battery positive electrode material is tested according to the method specified in the standard drafted by the National Non-ferrous Metals Standardization Technical Committee.

[0115] (5) The electrical performance test: the lithium iron phosphate cathode material in examples 1-3 and comparative example 1-2 were made into positive electrode sheet, and then assembled into button cell with metal lithium as negative electrode material. The button cell was tested in the voltage range of 2.0-3.75V.

[0116] The test procedure of the first charge-discharge efficiency included: under the condition of 25℃, the lithium ion battery was charged to 3.75V at 0.1C current, and then charged to the current decreased to 0.05C at constant voltage, the charging stopped, and the first charge capacity was recorded; then discharged to the cut-off voltage of 2.0V at 0.1C, and the first discharge capacity was recorded, the 0.1C specific capacity of the cathode material was calculated according to 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%.

[0117] Then the above charging and discharging process was repeated once at 1C current, and the 2nd charge capacity and 2nd discharge capacity were recorded, and the 1C specific capacity of the cathode material was calculated according to the 2nd discharge capacity.

[0118] Table 2

[0119]

[0120] Firstly, the electrical performance and the tap density of the lithium iron phosphate cathode material in examples 1-3 were effectively improved. The tap density of the lithium iron phosphate cathode material powder in examples 1-3 was greatly improved compared with the process of comparative example 1 with only one wet grinding, and the tap density of the lithium iron phosphate cathode material powder in examples 1-3 was 2.55g / cc, 2.52g / cc and 2.57g / cc respectively, while the tap density of the lithium iron phosphate cathode material powder in comparative example 1 was only 2.38g / cc; in addition, the electrical resistivity of the lithium iron phosphate cathode material powder in examples 1-3 was less than that in comparative example 1. The tap density, electrical resistivity and electrical performance of examples 1-3 were close to those of comparative example 2, but the process of comparative example 2 adopted twice grinding, which was complex, high energy consumption and low productivity. Different from comparative example 2, the dry mixing was adopted in examples 1-3, which could reduce the production energy consumption and cost.

[0121] Among them, the overall comprehensive performance such as the tap density, electrical resistivity and electrical performance of example 1-2 was better than that of example 3, because the particle size D50 in example 1-2 was 0.4-0.6μm, while the particle size D50 of the secondary mixed material in example 3 was 0.68μm, although the tap density increased slightly, but the electrical performance decreased due to the increase of lithium ion transmission path.

[0122] Secondly, the carbon content of the lithium iron phosphate positive electrode material in embodiments 1-3 is in a suitable range, which is beneficial to the uniform distribution of the carbon coating layer, thereby improving the conductivity and structural stability of the lithium iron phosphate positive electrode material.

[0123] In addition, scanning electron microscopy (SEM) observation was performed on the lithium iron phosphate precursor and the lithium iron phosphate positive electrode material in embodiment 1 and the lithium iron phosphate positive electrode material obtained in comparative examples 1-2.

[0124] Figure 3 The SEM image of the lithium iron phosphate precursor in embodiment 1 is shown in FIG. 1. Figure 4 The SEM image of the lithium iron phosphate positive electrode material in embodiment 1 is shown in FIG. 2. Figure 5 The particle size distribution graph of the lithium iron phosphate positive electrode material in embodiment 1 is shown in FIG. 3. Figure 3 、 Figure 4 and Figure 5 It can be seen that the particle size of the lithium iron phosphate precursor after dry mixing and sintering is large, and after wet grinding, the particle size distribution appears bimodal, realizing particle size grading, and the particle size grading effect of the lithium iron phosphate positive electrode material obtained by sintering in embodiment 1 is good, which is beneficial to the improvement of the tap density of the lithium iron phosphate positive electrode material.

[0125] Figure 6 The SEM image of the lithium iron phosphate positive electrode material in comparative example 1 is shown in FIG. 4. Figure 4 It can be seen that the particle size grading effect of the lithium iron phosphate positive electrode material in embodiment 1 is better, which is due to the particle size grading after wet grinding in embodiment 1, and the tap density of large and small particles is improved after secondary sintering. The number of particles with a particle size of more than 1 μm after one sintering in comparative example 1 is small, and the particle size grading effect is poor, so the tap density is low.

[0126] Figure 7 The SEM image of the lithium iron phosphate positive electrode material in comparative example 2 is shown in FIG. 5. Figure 4 Compared with comparative example 2, the particle size and the large and small particle grading effect are similar, but the preparation method of comparative example 2 consumes more energy and has lower productivity.

[0127] It can be understood that the above embodiments are only exemplary embodiments 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 skilled 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 preparation method comprises the following steps: mixing a phosphorus source, an iron source, a lithium source and a first carbon source in a dry manner to obtain a first mixture, wherein the dry mixing is performed by using a plow mixer, the frequency of a main motor of the plow mixer is 25-50 Hz, and the frequency of a flying knife of the plow mixer is 10-50 Hz; sintering the first 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 second mixture, wherein the particle size D50 of the second mixture is 0.4-0.6 μm, and the particle size distribution of the second mixture satisfies the following relationship: 0.90≤(D90-D10) / D50≤1.70; and sintering the second mixture to obtain the lithium iron phosphate positive electrode material.

2. The method of claim 1, wherein the lithium iron phosphate cathode material is prepared by the steps of: The mixing time of the plow mixer is 0.5-4.0 h. ​ 3. The method of claim 1, wherein the lithium iron phosphate cathode material is prepared by the steps of: The grinding is performed in two steps, wherein the second mixture is first ground to a particle size D50 of 1.0-2.0 μm in the first step, and the grinding product of the first step is further ground to a particle size D50 of 0.4-0.6 μm in the second step. ​ 4. The method of claim 1, wherein the lithium iron phosphate cathode material is prepared by the steps of: The mass percentage of the first carbon source in the first mixture is 4-8%. ​ The mass percentage of the second carbon source in the second mixture is 4-20%.

5. The method for preparing lithium iron phosphate cathode material according to claim 1, characterized in that, The first mixture and / or the second mixture further comprises a doping material, wherein 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.

6. The method of claim 1, wherein the lithium iron phosphate cathode material is prepared by the steps of: The sintering conditions of the first sintering include a sintering temperature of 600-750 ℃, a sintering time of 3-15 h and a heating rate of 2-5 ℃ / min; ​ The sintering conditions of the second sintering include a sintering temperature of 700-800 ℃, a sintering time of 3-15 h and a heating rate of 2-5 ℃ / min.

7. A lithium iron phosphate cathode material, characterized in that, 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 1-6.

8. An electrochemical device, characterized by, The battery 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 7.

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