Preparation method and application of lithium iron phosphate

Abstract

The invention discloses a preparation method and application of lithium iron phosphate, and belongs to the field of lithium ion battery positive electrode materials. The preparation method comprises the following steps: (1) pre-sintering a lithium iron phosphate precursor; (2) uniformly mixing the pre-sintered lithium iron phosphate precursor with a lithium source, a phosphorus source, a doping element and a carbon source, grinding, and carrying out spray drying to obtain a spray material; and (3) carrying out sintering treatment on the spray material, and carrying out crushing and demagnetizing treatment after cooling to obtain the lithium iron phosphate. The pre-sintering temperature in the step (1) is 500-750 DEG C, and the pre-sintering time is 4-8 hours. The lithium source in the step (2) is one or more of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate and lithium phosphate; the phosphorus source is one or more of phosphoric acid, lithium dihydrogen phosphate and iron phosphate; the carbon source is at least two of glucose, cane sugar, polyethylene glycol and starch. The lithium iron phosphate provided by the invention can achieve the effects of low specific surface area, high compaction density and excellent electrochemical performance.

Description

TECHNICAL FIELD

[0001] The present application relates to a lithium ion battery cathode material, in particular to a preparation method and application of lithium iron phosphate. BACKGROUND

[0002] As a kind of lithium ion battery cathode material, lithium iron phosphate has many advantages such as high specific capacity, safety, non-toxicity, excellent cycle life, etc.Currently, the main methods for preparing lithium iron phosphate are solid phase synthesis, coprecipitation and microwave synthesis. 2 / g. Lithium iron phosphate cathode material prepared from lithium iron phosphate precursor has simple process, good batch stability, can effectively improve the yield of product and reduce cost. Due to the characteristics of small particle size, irregular morphology and easy grinding of the precursor, fine powder is easily formed in the process of preparing lithium iron phosphate cathode material in the later end, which leads to too large specific surface area and affects the processing performance of the product. SUMMARY

[0003] The purpose of the present application is to provide a preparation method of lithium iron phosphate with low specific surface area, high compaction density and excellent electrochemical performance.

[0004] Another purpose of the present application is to provide the application of lithium iron phosphate obtained by the above method in preparing lithium ion battery cathode.

[0005] TECHNICAL SCHEME The preparation method of lithium iron phosphate according to the present application comprises the following steps:

[0006] (1) pre-sintering lithium iron phosphate precursor;

[0007] (2) mixing the pre-sintered lithium iron phosphate precursor with lithium source, phosphorus source, doping element and carbon source, and then spray drying to obtain spray material;

[0008] (3) sintering the spray material and then performing post-treatment to obtain lithium iron phosphate.

[0009] In step (1), the pre-sintering temperature is 500-750℃, and the pre-sintering time is 4-8h; the chemical formula of lithium iron phosphate precursor is Li x Fe(PO4) y , 0.91≤x≤1.04, 0.95≤y≤1.01, more preferably 0.95≤x≤1.00, 0.98≤y≤1.01.

[0010] Preferably, the lithium source in step (2) is one or more of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium phosphate; the phosphorus source is one or more of phosphoric acid, lithium dihydrogen phosphate, iron phosphate; the carbon source is at least two of glucose, sucrose, polyethylene glycol, starch; the doping element is at least one of Mg, Ti, B, V, Zr or Nb, and is further preferably Ti, and the titanium content is 2800-3200 ppm; the inlet air temperature of the spray drying is 200-220 DEG C, and the outlet air temperature is 95-110 DEG C.

[0011] Preferably, after mixing in step (2), the mixture is ground, and the grinding process is as follows: after mixing all raw materials with water for 20-30 min, the mixture is coarsely ground for 30-60 min, and then finely ground for 120-180 min; the solid content of the raw materials is 30-50 wt%. If the solid content is low, the production efficiency is affected; if the solid content is high, the viscosity of the slurry is high, and the grinding process is affected.

[0012] Preferably, in the sintering process in step (3), the maximum temperature is 760-820 DEG C, and the holding time is 8-10 h; the post-treatment includes a crushing process, and after the crushing process, the crushed particle size D50 is 1.0-1.5 mu m.

[0013] In the lithium iron phosphate, the molar ratio of lithium, iron and phosphorus is (1.00-1.05):1:(1.00-1.05). If the lithium content is too low, the lithium ion transmission is affected, and the capacity is affected; if the lithium content is too high, the particle growth is inhibited, and the tap density is low. If the iron content is too low, the capacity of the positive electrode material is poor; if the iron content is too high, the tap density of the material is low.

[0014] The main elements of the coating layer of the lithium iron phosphate are one or more of C, O, P, Li, Ti, V and Mg.

[0015] Invention principle: The carbon-uncoated lithium iron phosphate precursor is easily ground into many small particles in the sand milling stage, and after sintering, many small powders are generated, resulting in an excessively large specific surface area. In this method, the lithium iron phosphate precursor is pre-sintered, and after the first sintering of the precursor, the first particles grow into ellipsoidal or spherical particles, the grinding time is increased, the particle size is relatively uniform, and many small particles are not formed. After sand milling, spray drying and high-temperature sintering, the particles grow uniformly, the specific surface area is reduced, and the tap density is improved.

[0016] Advantages: Compared with the prior art, the lithium iron phosphate prepared by the method has the following advantages: (1) the lithium iron phosphate has a low specific surface area and a high tap density, and the specific surface area is less than 13 m 2 / g, and the tap density can reach 2.52 g / cm3 The material has higher energy density, and the lower specific surface area also makes the material have good processing performance; (2) The lithium iron phosphate prepared in the application has a lower carbon layer thickness of the lithium iron phosphate due to the preferable carbon coating, so that the lithium iron phosphate has better electrochemical performance on the basis of higher compaction density, and the 0.1C charge specific capacity is greater than or equal to 160 mAh / g, and the 0.1C discharge specific capacity is greater than or equal to 158 mAh / g in the voltage range of 2.0V-3.75V; (3) The lithium iron phosphate positive electrode material prepared by using the method has higher production efficiency and yield, and thus has greater price advantage, which is beneficial to saving production cost. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 SEM image of the lithium iron phosphate precursor pretreated for Example 1;

[0018] Figure 2 SEM image of the lithium iron phosphate prepared for Example 1;

[0019] Figure 3 SEM image of the lithium iron phosphate precursor pretreated for Example 2;

[0020] Figure 4 SEM image of the lithium iron phosphate prepared for Example 2;

[0021] Figure 5 SEM image of the lithium iron phosphate precursor pretreated for Example 3;

[0022] Figure 6 SEM image of the lithium iron phosphate prepared for Example 3. DETAILED DESCRIPTION

[0023] The technical solutions of the application will be further described below in combination with examples.

[0024] The lithium iron phosphate precursor in the application is provided by Guizhou Weite High-tech Energy Technology Co., Ltd. In the following examples, the lithium iron phosphate precursor is only taken as an example of Li 0.96 Fe(PO4) 1.01 , and other chemical formulas meeting the condition of Li x Fe(PO4) y (0.91≤x≤1.04, 0.95≤y≤1.01) can also be realized.

[0025] Example 1

[0026] The preparation method of the lithium iron phosphate in the application comprises the following steps:

[0027] (1) precursor pre-sintering treatment: 6 kg of lithium iron phosphate precursor Li 0.96 Fe(PO4)1.01 The pre-processed lithium iron phosphate precursor was obtained by placing the calcined product in a crucible, placing it in a box furnace, and sintering under nitrogen at a rate of 5°C / min to 500°C and maintaining the temperature for 8h. The SEM image of the pre-processed lithium iron phosphate precursor is shown in FIG. 1. Figure 1

[0028] (2) Preparation of lithium iron phosphate spray material: 2 kg of the pre-processed lithium iron phosphate precursor, 46.54 g of lithium carbonate, 78.90 g of phosphoric acid, 110.56 g of glucose, 50.32 g of polyethylene glycol (6000), 4.4 g of titanium dioxide, and 3500 g of deionized water were uniformly mixed for 30 min, and then ground. First, a coarse grinder was used to control the D50 to 1.2 μm, and then a sand mill was used to control the D50 to 0.45 μm. Finally, spray drying was performed at an inlet temperature of 210°C and an outlet temperature of 95°C to obtain the spray material.

[0029] (3) Preparation of lithium iron phosphate: The spray material obtained in step (2) was placed in a graphite crucible and placed in a box furnace for sintering. The temperature was increased to 400°C at a rate of 5°C / min and maintained for 6h, and then increased to 780°C and maintained for 10h. Nitrogen was used to protect the sintering process. After cooling, the product was broken and demagnetized to obtain lithium iron phosphate. The SEM image of the lithium iron phosphate is shown in FIG. 2. The lithium iron phosphate prepared in this example had a titanium content of 3000 ppm, a particle size D50 of 1.22 μm, a particle size D99 of 7.59 μm, and a molar ratio of lithium, iron, and phosphorus of 1.025:1:1.023. Figure 2

[0030] Example 2

[0031] The same as in Example 1 will not be repeated, and the difference is that the pre-sintering temperature in step (1) is 600°C.

[0032] The lithium iron phosphate prepared in this example had a titanium content of 3000 ppm, a particle size D50 of 1.45 μm, a particle size D99 of 9.83 μm, and a molar ratio of lithium, iron, and phosphorus of 1.023:1:1.015.

[0033] Example 3

[0034] The same as in Example 1 will not be repeated, and the difference is that the pre-sintering temperature in step (1) is 700°C.

[0035] The lithium iron phosphate prepared in this example had a titanium content of 3000 ppm, a particle size D50 of 1.35 μm, a particle size D99 of 10.26 μm, and a molar ratio of lithium, iron, and phosphorus of 1.030:1:1.022.

[0036] Example 4 ​​

[0037] The same as example 1 is not described again, the difference lies in: the highest temperature of sintering in step (3) is 800℃.

[0038] The lithium iron phosphate prepared in this example has titanium content of 3000ppm, particle size D50 of 1.31μm, particle size D99 of 8.06μm, and molar ratio of lithium, iron and phosphorus of 1.015:1:1.021.

[0039] Example 5

[0040] The same as example 1 is not described again, the difference lies in: the highest temperature of sintering in step (3) is 820℃.

[0041] The lithium iron phosphate prepared in this example has titanium content of 3000ppm, particle size D50 of 1.21μm, particle size D99 of 8.79μm, and molar ratio of lithium, iron and phosphorus of 1.010:1:1.032.

[0042] Comparative Example 1

[0043] The same as example 1 is not described again, the difference lies in: step (1) is removed, only step (2)-(3) is carried out.

[0044] The lithium iron phosphate prepared in this example has titanium content of 3000ppm, particle size D50 of 0.89μm, particle size D99 of 6.05μm, and molar ratio of lithium, iron and phosphorus of 1.031:1:1.025.

[0045] Comparative Example 2

[0046] The same as example 1 is not described again, the difference lies in: 110.56g of glucose in step (2) is replaced by 95.48g of sucrose.

[0047] The lithium iron phosphate prepared in this example has titanium content of 3000ppm, particle size D50 of 1.15μm, particle size D99 of 8.24μm, and molar ratio of lithium, iron and phosphorus of 1.025:1:1.027.

[0048] Comparative Example 3

[0049] The same as example 1 is not described again, the difference lies in: 78.89g of phosphoric acid in step (2) is replaced by 39.445g of phosphoric acid and 39.335g of ammonium dihydrogen phosphate, and molar ratio of lithium, iron and phosphorus is 1.033:1:1.016.

[0050] The lithium iron phosphate prepared in this example has titanium content of 3000ppm, particle size D50 of 1.01μm, particle size D99 of 5.89μm.

[0051] Comparative Example 4

[0052] The same as Example 1 is not repeated, the difference is that the highest temperature of sintering in step (3) is 830℃.

[0053] The lithium iron phosphate prepared in the present comparative example has a titanium content of 3000ppm, a particle size D50 of 1.33μm, a particle size D99 of 9.24μm, and a molar ratio of lithium, iron and phosphorus of 1.005:1:1.006.

[0054] Comparative Example 5

[0055] The same as Example 1 is not repeated, the difference is that the highest temperature of pre-sintering in step (1) is 400℃.

[0056] The lithium iron phosphate prepared in the present comparative example has a titanium content of 3000ppm, a particle size D50 of 0.93μm, a particle size D99 of 7.35μm, and a molar ratio of lithium, iron and phosphorus of 1.028:1:1.022.

[0057] According to the corresponding standards, the specific surface area (GB / T 19587-2017), the compaction density under 3T pressure (GB / T 24533-2010), the carbon content (GB / T 20123-2006), and the electrical performance in the voltage range of 2.0-3.75V (GB / T 33822-2017, JJF 1910-2021) of each sample were tested, and the results are shown in Table 1.

[0058] Electrical performance test procedure: the lithium iron phosphate sample prepared in the present application, conductive carbon black (SP) conductive agent, polyvinylidene fluoride (PVDF) and appropriate amount of N-methyl pyrrolidone (NMP) were mixed uniformly by a high-speed mixer, wherein the mass ratio of the sample, SP conductive agent and PVDF was 90:5:5. Then an automatic coating machine was used to coat on an aluminum foil, the coated aluminum foil was baked in an oven and cut into small round pieces of the required size and weighed; with pure lithium sheet as the negative electrode sheet, the positive electrode shell, the negative electrode shell, the positive electrode sheet, the lithium sheet, the separator and the electrolyte were assembled into a button cell according to the requirements, and the button cell was hung on a battery test system for testing after standing.

[0059] Table 1 Performance test results of each sample

[0060] Sample Compacted density (g / cm 3 )]> Carbon content (%) Specific surface area (m 2 / g)]]> 0.1 C charge specific capacity (mAh / g) 0.1 C discharge specific capacity (mAh / g) 0.1 C efficiency (%) Example 1 2.45 1.21 12.40 163.40 160.25 98.07 Example 2 2.43 1.23 11.25 161.75 159.83 98.81 Example 3 2.42 1.12 10.05 160.26 158.33 98.80 Example 4 2.48 1.21 11.80 161.28 159.62 98.97 Example 5 2.52 1.19 11.50 161.51 158.33 98.03 Comparative Example 1 2.43 1.46 17.46 160.50 158.66 98.85 Comparative Example 2 2.41 1.39 16.79 159.23 155.86 97.88 Comparative Example 3 2.46 1.34 15.96 159.85 159.31 99.66 Comparative Example 4 2.47 0.99 13.05 159.56 150.52 94.33 Comparative Example 5 2.40 1.22 14.06 160.74 159.45 99.20 .

[0061] According to Figure 1 、 3As shown in Figure 5, with the increase of pre-sintering temperature, the particles melt and grow, resulting in an increase in the proportion of large particles in the finished product and a decrease in specific surface area. According to the data in Table 1, it can be found that a single pre-sintering treatment of the lithium iron phosphate precursor (Examples 1-5, Comparative Example 1) can effectively reduce the specific surface area of ​​lithium iron phosphate, resulting in lithium iron phosphate with a specific surface area below 13 m². 2 / g, correspondingly, lithium iron phosphate exhibits superior electrochemical performance, primarily because the uncoated precursor is easily ground into many fine particles during the sand milling stage. After sintering, this results in numerous fine powders, leading to an excessively large specific surface area. In contrast, after sintering the precursor at 500℃, the particles grow into ellipsoidal or spherical shapes. Increased grinding time results in more uniform particle size, preventing the formation of numerous fine particles. Subsequent sintering leads to more uniform particle growth and a reduced specific surface area. This is consistent with... Figure 1-2 morphology Figure 1 To.

[0062] Within a certain range, the higher the temperature of the pre-sintering treatment of lithium iron phosphate precursor (Examples 1-3), the lower the specific surface area and the lower the 0.1C discharge capacity of the obtained lithium iron phosphate cathode material. When the pre-sintering temperature is 500℃, the obtained lithium iron phosphate exhibits excellent specific surface area, compaction density, and electrochemical performance.

[0063] When the sintering temperature of the sprayed material is gradually increased (Examples 1, 4-5), the resulting lithium iron phosphate particles grow significantly, forming a clear particle size distribution, increasing compaction density, decreasing carbon content, decreasing specific surface area, and slightly decreasing electrochemical performance; when the temperature is increased to 830℃ (Comparative Example 4), the electrochemical performance decreases more significantly.

[0064] In the preparation of lithium iron phosphate spray material, carbon source and phosphoric acid are required. According to data from Comparative Examples 1-3, using different carbon sources and reducing the amount of phosphoric acid added can reduce the degree of particle melting, resulting in more uniform carbon coating and thus lowering the specific surface area of ​​the lithium iron phosphate cathode material. The results of the prepared lithium iron phosphate cathode materials show that when sucrose is used to replace glucose, the specific surface area decreases slightly, but remains above 14 μm. 2 / g, with even worse electrochemical performance, may be due to the more uneven carbon coating on the particle surface after the use of sucrose, which leads to an increase in lithium ion transport distance and a decrease in electrochemical performance.

[0065] according to Figure 1-6 It can be seen that during the pre-sintering stage, as the sintering temperature increases, the melting of small particles intensifies, increasing from a minimum of 100-200 nm to 400-500 nm. The lithium iron phosphate particles prepared by this method have a relatively smooth surface, no obvious floating carbon, good carbon coating, and relatively concentrated particle size.

[0066] In the present application, if the lithium iron phosphate precursor is not pre-sintered, the sanding time is short to reach the required particle size, the proportion of small particles of the finished product is large after spray sintering, the particle size is small, the specific surface area is large, the material is easy to agglomerate, the hygroscopicity is strong, more binder is needed in the preparation of slurry in the later stage, the viscosity of the slurry is large, and the coating process is more difficult to control.

[0067] Application Example

[0068] The lithium ion battery cathode can be prepared by using the lithium iron phosphate cathode material prepared in any one of Embodiments 1-5, and the lithium iron phosphate cathode material prepared in Embodiment 1 is further preferred.

[0069] As shown in Table 1, the lithium iron phosphate cathode material prepared by the method has a 0.1C charge specific capacity of ≥160 mAh / g and a 0.1C discharge specific capacity of ≥158 mAh / g in the voltage range of 2.0V-3.75V. If the charge capacity of the lithium iron phosphate cathode material is too low, the overall energy density of the material is low. The charge and discharge specific capacities in the above range are beneficial to the first circle discharge performance of the battery, and are also beneficial to compensate for the loss of active lithium in the first circle, and the energy density is better.

Claims

1. A method for preparing lithium iron phosphate, characterized in that, Includes the following steps: (1) Pre-sintering of lithium iron phosphate precursor; (2) The pre-sintered lithium iron phosphate precursor is mixed with lithium source, phosphorus source, doping element and carbon source and then spray dried to obtain spray material; (3) The spray material is sintered and then post-treated to obtain lithium iron phosphate.

2. The preparation method according to claim 1, characterized in that, The pre-sintering temperature in step (1) is 500-750℃, and the pre-sintering time is 4-8h.

3. The preparation method according to claim 1, characterized in that, The lithium source mentioned in step (2) is one or more of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, and lithium phosphate; the phosphorus source is one or more of phosphoric acid, lithium dihydrogen phosphate, and iron phosphate; the carbon source is at least two of glucose, sucrose, polyethylene glycol, and starch; and the doping element is at least one of Mg, Ti, B, V, Zr, or Nb.

4. The preparation method according to claim 3, characterized in that, The doping element mentioned in step (2) is Ti, and the titanium content is 2800-3200ppm.

5. The preparation method according to claim 4, characterized in that, In step (2), after mixing, the materials are ground. The grinding process is as follows: after mixing all the raw materials with water for 20-30 minutes, coarse grinding is performed for 30-60 minutes, and then fine grinding is performed for 120-180 minutes.

6. The preparation method according to claim 5, characterized in that, The solid content of the raw material is 30-50 wt%.

7. The preparation method according to claim 1, characterized in that, The inlet air temperature of the spray drying in step (2) is 200℃-220℃, and the outlet air temperature is 95℃-110℃.

8. The preparation method according to claim 1, characterized in that, The highest temperature during the sintering process described in step (3) is 760-820℃, and the holding time is 8-10h.

9. The preparation method according to claim 1, characterized in that, The post-processing described in step (3) includes crushing, after which the crushed particle size D50 is 1.0μm-1.5μm.

10. The application of lithium iron phosphate prepared by the method of claim 1 in the preparation of a cathode for a lithium-ion battery.

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