High-capacity and high-compaction-density lithium iron phosphate material as well as preparation method and application thereof

Abstract

The invention discloses a high-capacity and high-compaction-density lithium iron phosphate material as well as a preparation method and application thereof, and belongs to the technical field of lithium ion battery materials. The preparation method of the material comprises the following steps: uniformly mixing a lithium source, a carbon source, iron phosphate A with the metal element doping amount of 150-500ppm, iron phosphate B with the metal element doping amount of 5000-7000ppm and water to obtain mixed slurry; grinding and drying the mixed slurry to obtain a powder material; and sintering the powder material in a protective atmosphere to obtain a target product. According to the preparation method, raw materials with different properties are selected, grain composition is formed by utilizing growth activity differentiation of the raw materials, and high powder compacted lithium iron phosphate is prepared through one-step material mixing and sintering, so that the powder compacted density of the material is greatly improved, the electrochemical performance of the material is ensured, and the material has high capacity; meanwhile, the method has the advantages of being simple in technological process and low in industrialization difficulty.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium ion battery materials, and specifically relates to a high-capacity, high-density lithium iron phosphate material and a preparation method and application thereof. Background Art

[0002] Lithium-ion batteries have the advantages of high energy density, long life, and environmental friendliness. They have been widely used in portable devices, vehicle-mounted equipment, and electrochemical energy storage. With the continuous expansion of the application of lithium iron phosphate, the requirements for the energy density of lithium iron phosphate materials are getting higher and higher. As far as the commercial lithium iron phosphate materials are concerned, the energy density still needs to be improved. The powder compaction density of lithium iron phosphate materials, as a reference indicator for measuring the energy density of materials, affects the energy density of lithium-ion batteries. At present, the mainstream processes are divided into high-temperature solid-phase method and liquid-phase method. The high-temperature solid-phase method can increase the powder compaction density of the material by increasing the material size, but the large-sized particles make it difficult for lithium ions to be deintercalated, and at the same time, it will cause the polarization to increase during the charge and discharge process, thereby reducing the specific capacity, specific energy and cycle performance of the material. The patent application with publication number CN114314550A adopts a secondary spray process, which uses a method of particle grading of large-particle lithium iron phosphate and small-particle lithium iron phosphate to improve the specific energy and powder compaction density of lithium iron phosphate. The compaction density of the finished material is as high as 2.57g / cc, but because the particle size of the large-particle lithium iron phosphate is 15-30μm and the particle size of the small particle is 5-15μm, the lithium ion diffusion channel cannot be shortened, which leads to a low specific capacity, 0.1C is only 151mAh / g, and the material specific energy cannot be fully released. The liquid phase method has a nanostructure, and the particles grow more uniformly, with excellent carbon coating effect, which can improve the charge and discharge performance of the material and ensure the cycle capacity. However, it is easy to agglomerate, and it is difficult to form a good particle gradation. At the same time, the liquid phase method has a complex production process, and the equipment requirements are relatively harsh. It is difficult to industrialize, so it is difficult to obtain lithium iron phosphate with a high compaction density. For example, the patent application with the authorization announcement number CN103259015B prepares a small-sized lithium iron phosphate precursor by the liquid phase method, and then batch mixes it with the lithium iron phosphate material and performs sintering heat treatment, which accurately controls the particle size and carbon content of the material, and further improves the gram capacity of the material, but its compaction density can only reach 2.40 g / cc. Summary of the invention

[0003] Aiming at the problem that it is difficult to balance the high capacity and high tap density of lithium iron phosphate in the prior art, the present invention provides a lithium iron phosphate material with high capacity and high tap density, its preparation method and application. By selecting raw materials with different properties and utilizing the difference in growth activity of the raw materials to form particle gradation, high powder tap density lithium iron phosphate is prepared through one-step mixing and sintering, greatly improving the powder tap density of the material and ensuring the electrochemical performance of the material, making it have high capacity. At the same time, this method has the advantages of simple process flow and low industrialization difficulty.

[0004] To achieve the above object, the technical solution adopted by the present invention is as follows: The present invention provides a preparation method of a lithium iron phosphate material with high capacity and high tap density, comprising the following steps: Mix lithium source, carbon source, iron phosphate A with a metal element doping amount of 150 - 500 ppm, iron phosphate B with a metal element doping amount of 5000 - 7000 ppm and water evenly to obtain a mixed slurry; preferably, the molar ratio of lithium to iron in the mixed slurry is (1 - 1.06):1; the molar ratio of iron to phosphorus in the mixed slurry is (0.960 - 0.975):1. When the molar ratio of iron to phosphorus exceeds 0.975, the sintering activity will be greatly reduced, it is difficult to form particle gradation, and the tap density will drop significantly; when the molar ratio of iron to phosphorus is lower than 0.960, the growth activity of iron phosphate will be greatly enhanced. Under this element doping amount condition, it is impossible to effectively inhibit particle growth, resulting in the failure of growth activity gradation and a significant drop in electrical performance.

[0005] The mixed slurry is ground to obtain a finished slurry, and after the finished slurry is dried, a powder material is obtained; preferably, the D50 of the finished slurry is 0.5 - 0.6 μm; the drying is spray drying; The powder material is sintered in a protective atmosphere and then crushed to obtain the target product. Preferably, the sintering temperature is 775 - 820 °C.

[0006] As a preferred technical solution, the lithium source is at least one of lithium carbonate, lithium hydroxide, and lithium fluoride; the carbon source is at least one of glucose, polyacrylonitrile, sucrose, and polyethylene glycol; the metal element is Ti, V or Co; As a preferred technical solution, the mass ratio of iron phosphate A to iron phosphate B is (0.5 - 2):1. Due to different metal doping amounts, iron phosphate A and iron phosphate B exhibit different particle growth activities at the same sintering temperature, thus forming large and small particles of different sizes. Specifically, iron phosphate A is the source of large particles of lithium iron phosphate, and iron phosphate B is the source of small particles of lithium iron phosphate. Among them, large particles pay more attention to improving the tap density, and small particles pay attention to improving the rate performance and capacity performance of the material. By adjusting the ratio of iron phosphate A and B in the present invention, the proportion of large and small particles of lithium iron phosphate can be accurately adjusted, thereby adjusting the grading effect of the material, and further controlling the balance of the tap density and electrical properties of lithium iron phosphate.

[0007] The present invention also provides a lithium iron phosphate material with high capacity and high tap density, which is prepared by the preparation method as described above. This lithium iron phosphate material has a high tap density and high electrical properties, and has good application prospects in lithium-ion batteries.

[0008] Compared with the prior art, the present invention has the following beneficial effects: The preparation method of the lithium iron phosphate material provided by the present invention selects two iron phosphates with different metal element doping amounts. Due to the difference in metal element doping amounts, their particle growth activities are different. By one-step mixing and sintering, particles of different particle sizes are prepared. Among them, small particle materials enter the gaps between large particle materials to achieve the grading of particle sizes, and a lithium iron phosphate material with high tap density and high capacity is obtained.

[0009] The metal elements in iron phosphate A and iron phosphate B used in the present invention are doped into the reaction raw materials during the preparation of iron phosphate, which belongs to the front-end doping type of iron phosphate. Compared with the prior art method of blending iron phosphate with a dopant containing metal elements (also known as back-end doping), since the doped metal is located in the iron phosphate in the raw materials used in the present invention, the metal element distribution is more uniform, and the performance improvement effect of the obtained product is more obvious.

[0010] The preparation method provided by the present invention belongs to the one-step grading method. By one-time mixing and sintering, a product with excellent performance is prepared. This preparation method has a simple synthesis process and low industrialization difficulty, and has good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 SEM image of the lithium iron phosphate material prepared in Example 1 of the present invention; Figure 2 Particle size distribution map of the lithium iron phosphate material prepared in Example 1 of the present invention; Figure 3 Charge-discharge curve of the lithium iron phosphate material prepared in Example 1 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention will be further described below in conjunction with embodiments, so that those skilled in the art can better understand the present invention and be able to implement it, but the embodiments given are not intended to limit the present invention.

[0013] In addition, in the preparation processes of the following embodiments, unless otherwise specified, they are all conventional means in the prior art in this field. Therefore, they will not be described in detail; the raw materials used in the present invention are all commercially available products and can be obtained through purchase; among them, the iron phosphates with different doping amounts of metal elements are all purchased from Tongling Nayuan Materials Technology Co., Ltd.

[0014] Example 1 A preparation method of a lithium iron phosphate material with high capacity and high tap density includes the following steps: Step (1): Take lithium carbonate and iron phosphate with a Li:Fe molar ratio of 1.03:1, glucose accounting for 8.7% of the mass of iron phosphate, and 1.5% of polyethylene glycol, and mix them evenly to obtain a mixed slurry; among them, the iron phosphate includes iron phosphate A and iron phosphate B, and their iron to phosphorus molar ratio is 0.965. The doping amount of Ti element in iron phosphate A is 150 ppm, the doping amount of Ti element in iron phosphate B is 6500 ppm, and the mass ratio of iron phosphate A to iron phosphate B is 5:5; Step (2): Grind the mixed slurry through a sand mill. After the particle size of the slurry meets the standard, a finished slurry is obtained, and the particle size range of the finished slurry is 0.5 - 0.55 μm. The obtained finished slurry is dried and granulated through a spray dryer to obtain a dried powder material; Step (3): Under nitrogen protection, place the sample obtained in step (2) in a box furnace and perform high-temperature treatment at 785 °C for 10 h to obtain a lithium iron phosphate material with high capacity and high tap density; Example 2 The difference between the preparation method provided in this example and the preparation method of Example 1 is that the doping amount of Ti element in iron phosphate A in step (1) is 300 ppm, and the sintering temperature in step (3) is 795 °C.

[0015] Example 3 The difference between the preparation method provided in this example and the preparation method of Example 1 is that the doping amount of metal elements in iron phosphate B in step (1) is 5500 ppm, and the sintering temperature in step (3) is 775 °C.

[0016] Example 4 The difference between the preparation method provided in this example and the preparation method of Example 1 is that the doping amount of metal elements in iron phosphate B in step (1) is 7000 ppm, and the sintering temperature in step (3) is 815 °C.

[0017] Example 5 The preparation method provided in this example is different from the preparation method of Example 1 in that the doping amount of metal elements in iron phosphate A in step (1) is 300 ppm, and the doping amount of metal elements in iron phosphate B is 5500 ppm.

[0018] Example 6 The preparation method provided in this example is different from the preparation method of Example 2 in that the doping amount of metal elements in iron phosphate B in step (1) is 7000 ppm, and the sintering temperature in step (3) is 820 °C.

[0019] Example 7 The preparation method provided in this example is different from the preparation method of Example 1 in that the mass ratio of iron phosphate A to iron phosphate B in step (1) is 6:4.

[0020] Example 8 The preparation method provided in this example is different from the preparation method of Example 1 in that the mass ratio of iron phosphate A to iron phosphate B in step (1) is 4:6.

[0021] Example 9 The preparation method provided in this example is different from the preparation method of Example 1 in that the doping element of iron phosphate A and iron phosphate B in step (1) is V.

[0022] Example 10 The preparation method provided in this example is different from the preparation method of Example 1 in that the doping element of iron phosphate A and iron phosphate B in step (1) is Co.

[0023] Example 11 The preparation method provided in this example is different from the preparation method of Example 1 in that the doping amount of Ti element in iron phosphate A in step (1) is 500 ppm, and the sintering temperature in step (3) is 805 °C.

[0024] Comparative Example 1 The preparation method provided in this comparative example is different from the preparation method of Example 1 in that the doping amount of metal elements in iron phosphate A in step (1) is 800 ppm, and the sintering temperature in step (3) is 825 °C. Other processes are the same as those in Example 1.

[0025] Comparative Example 2 The preparation method provided in this comparative example is different from the preparation method of Example 1 in that the doping amount of metal elements in iron phosphate B in step (1) is 3500 ppm, and the sintering temperature in step (3) is 770. Other processes are the same as those in Example 1.

[0026] Comparative Example 3 The preparation method provided in this comparative example is different from that of Example 1 in that the proportion of Iron Phosphate A in step (1) is 100%. Other processes are the same as those in Example 1.

[0027] Comparative Example 4 The preparation method provided in this comparative example is different from that of Example 1 in that the proportion of Iron Phosphate B in step (1) is 100%, and the sintering temperature in step (3) is 835 °C. Other processes are the same as those in Example 1.

[0028] Comparative Example 5 The preparation method provided in this comparative example is different from that of Example 1 in that the mass ratio of Iron Phosphate A to Iron Phosphate B in step (1) is 3:7, and the sintering temperature in step (1) is 810 °C. Other processes are the same as those in Example 1.

[0029] Comparative Example 6 The preparation method provided in this comparative example is different from that of Example 1 in that the iron phosphate used in step (1) is undoped iron phosphate, that is, the doping amount of Ti element in iron phosphate is 0. Other processes are the same as those in Example 1.

[0030] Comparative Example 7 The preparation method provided in this comparative example is different from that of Example 1 in that the iron phosphate used in step (1) does not contain Ti element, that is, the doping amount of Ti element in iron phosphate is 0. It adopts a post-doping process, that is, the doping substance is not doped into the iron phosphate in advance, but is directly mixed with the iron phosphate, that is, an additional metal element dopant TiO2 is added, and the addition amount is that the mass ratio of Ti to iron phosphate is 3500 ppm. Other processes are the same as those in Example 1.

[0031] Comparative Example 8 The preparation method of this comparative example is different from that of Example 1 in that the iron-to-phosphorus molar ratio of iron phosphate in step (1) is 0.945. Other processes are the same as those in Example 1.

[0032] Comparative Example 9 The preparation method of this comparative example is different from that of Example 1 in that the iron-to-phosphorus molar ratio of iron phosphate in step (1) is 0.985. Other processes are the same as those in Example 1.

[0033] Application Example Using the lithium iron phosphate materials prepared in the above embodiments and comparative examples as the positive active materials, button cells were assembled as follows: The lithium iron phosphate material was mixed with conductive carbon black and the binder polyvinylidene fluoride in a mass ratio of 80:10:10, and N-methylpyrrolidone was used as the solvent to prepare a positive electrode sheet; a lithium sheet was selected as the negative electrode, and the electrolyte was a 1 mol / LiPF6 EC / DEC / DMC solution (volume ratio 1:1:1) to assemble a button cell.

[0034] Performance Test Figure 1 This is the SEM image of the lithium iron phosphate material prepared in Example 1 of the present invention. It can be seen that the lithium iron phosphate material shows an obvious effect of particle size grading.

[0035] Figure 2 This is the particle size distribution map of the lithium iron phosphate material prepared in Example 1 of the present invention. From Figure 2 it can be seen the specific particle distribution of the lithium iron phosphate material. There are obvious large and small particles. The abscissa corresponds to the particle size, and the ordinate is the corresponding volume percentage. It can be seen from the figure that the large particles reach the maximum volume density at 4-5 μm, accounting for about 30%-40%, and the small particles reach the maximum volume density at 0.7-0.8 μm, accounting for about 50-60%.

[0036] Figure 3 This is the charge-discharge curve of the battery made of the lithium iron phosphate material prepared in Example 1 of the present invention. Figure 3 It can be seen the charge and discharge performance of the battery at different rates, specifically showing its excellent electrical performance.

[0037] The lithium iron phosphate materials prepared in the above embodiments and comparative examples were tested for powder tap density; the assembled button cells were tested for electrochemical performance. The test method was: the test temperature was 25 °C, the test charging method was constant current and constant voltage charging, and the voltage range was 2.0-3.75 V. The test results are shown in Table 1 below: Table 1 Electrochemical Performance Test Results

[0038] It can be seen from the data comparison in Table 1 that the comprehensive performance of the lithium iron phosphate prepared by this method in terms of compaction and electrical performance is far better than the comparative samples, and the improvement is obvious. In summary, the following rules can be seen: (1) Under the same preparation process, different iron-phosphorus molar ratios and metal doping contents of iron phosphate can ensure that the growth sizes of large and small particles of the material are within a reasonable range. This is because different metal doping amounts result in different particle growth activities. Beyond the reasonable range, the tap density or electrical performance of the material will deteriorate sharply. (2) Under the same preparation process, a suitable ratio of iron phosphate can form a good particle size distribution. Due to the different proportions of large and small particles, it is difficult to distinguish the tap density and electrical properties. By adjusting the ratio of different iron phosphates, the particle size distribution effect of large and small particles can be well controlled, thus obtaining a lithium iron phosphate material with high tap density. (3) Under the same preparation process, when using iron phosphate doped with metal elements at the front end, it shows better tap density and electrical properties compared to that doped with metal elements at the back end. This is because the doping uniformity at the front end is better, and the doping effect is superior to that at the back end. Therefore, the improvement effect of tap density and electrical properties by front-end doping is better.

[0039] (4) Under the same preparation process, lithium iron phosphates doped with different metal elements can be obtained with basically the same tap density and electrical properties. Therefore, this preparation process has good universality.

[0040] It can be seen from this that under the preparation scheme and process parameters defined in the present invention, the obtained lithium iron phosphate material with high tap density has better powder tap density and electrochemical properties than the materials in the comparative examples. Therefore, the lithium iron phosphate cathode material prepared by the method of the present invention has higher powder tap density and discharge capacity.

[0041] The above embodiments are the preferred embodiments of the present invention, but the embodiments of the present invention are not limited by the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention shall be equivalent replacement methods and shall be included in the protection scope of the present invention.

Claims

1. A preparation method of a lithium iron phosphate material with high capacity and high tap density, characterized in that: It includes the following steps: Mix a lithium source, a carbon source, iron phosphate A with a metal element doping amount of 150 - 500 ppm, iron phosphate B with a metal element doping amount of 5000 - 7000 ppm, and water evenly to obtain a mixed slurry; the molar ratio of iron to phosphorus in the mixed slurry is (0.960 - 0.975):1; After the mixed slurry is ground and dried, a powder material is obtained; After the powder material is sintered in a protective atmosphere and then crushed, the target product is obtained.

2. The preparation method of the lithium iron phosphate material with high capacity and high tap density according to claim 1, characterized in that: The lithium source is at least one of lithium carbonate, lithium hydroxide, and lithium fluoride.

3. The preparation method of the lithium iron phosphate material with high capacity and high tap density according to claim 1, characterized in that: The carbon source is at least one of glucose, polyacrylonitrile, sucrose, and polyethylene glycol.

4. The preparation method of the lithium iron phosphate material with high capacity and high tap density according to claim 1, characterized in that: The metal element is Ti, V, or Co.

5. The preparation method of the lithium iron phosphate material with high capacity and high tap density according to claim 1, characterized in that: The mass ratio of iron phosphate A to iron phosphate B is (0.5 - 2):

1.

6. The preparation method of the lithium iron phosphate material with high capacity and high tap density according to claim 1, characterized in that: The molar ratio of lithium to iron in the mixed slurry is (1 - 1.06):

1.

7. The preparation method of the lithium iron phosphate material with high capacity and high tap density according to claim 1, characterized in that: The sintering temperature is 775 - 820 °C.

8. A lithium iron phosphate material with high capacity and high tap density, characterized in that: It is prepared by using the preparation method described in any one of claims 1 to 7.

9. A lithium ion battery, characterized in that: The lithium-ion battery contains the lithium iron phosphate material described in claim 8.

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