Lithium iron phosphate positive electrode material, and preparation method therefor and use thereof

Abstract

The present invention relates to the technical field of lithium-ion batteries, and specifically discloses a lithium iron phosphate positive electrode material, and a preparation method therefor and the use thereof. The preparation method comprises: mixing polyethylene glycol, glucose, a dopant, lithium carbonate and FePO4, and then sanding same; taking a sanded sample in S1 for particle size detection, discharging the sample after the particle size is qualified, and drying same; and sintering the dried material in S2 under the protection of nitrogen, grinding the sintered material, taking a ground sample to particle size detection, and discharging same after the particle size is qualified, so as to obtain a finished lithium iron phosphate product. By regulating and controlling the sanding size of an iron phosphate precursor and the grind size of a lithiated material, the specific discharge capacity, rate performance and cycle performance of lithium iron phosphate and the compaction density of the material are comprehensively improved; and the lithium iron phosphate positive electrode material prepared by using the method can be used in the production of lithium-ion batteries.

Description

Lithium iron phosphate positive electrode material and its preparation method and application Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and in particular to a lithium iron phosphate positive electrode material and a preparation method and application thereof. Background Art

[0002] Lithium iron phosphate materials have occupied a place in the battery market due to their excellent safety performance and low price. Batteries are divided into power batteries (which require higher energy density, that is, the positive electrode material must have a higher compaction density) and energy storage batteries. Power batteries are mostly used in electric bicycles, electric vehicles, etc. In order to facilitate portability, they require a higher energy density of the material (that is, more capacity per unit weight or unit volume can be volatilized), and their cycle performance is poor. Therefore, power batteries require the positive electrode material used to have a higher compaction density and focus on the rate performance of the positive electrode material. However, due to the lower electronic conductivity (10 -9 S / cm) and ionic conductivity (10 -13 -10 -16 S / cm) limits its high-current charge and discharge capabilities. To address this issue, various approaches have been proposed to improve the rate performance of lithium iron phosphate materials. Nano-sizing is a particularly effective method. The preparation of nano-LFP particles effectively shortens the ion diffusion distance, thereby improving LFP rate performance. While nano-sizing can effectively improve the rate performance of LFP materials, it also brings a series of problems, such as reduced compaction density, which affects the energy density of the battery.

[0003] However, unlike power batteries, most energy storage devices do not require movement, so energy density is not a direct goal for energy storage lithium batteries. Energy storage battery equipment emphasizes battery capacity, especially requiring a longer service life (the lifespan of new energy vehicles, i.e., power batteries, is generally 5-8 years, while the lifespan of energy storage battery projects is generally expected to be greater than 10 years. The cycle life of power batteries is 1000-2000 times, while the cycle life of energy storage lithium batteries is generally required to be greater than 3500 times.). Therefore, energy storage batteries have high requirements for the cycle performance of battery positive electrode materials while meeting a certain energy density. Summary of the Invention

[0004] The present invention aims to solve, at least to some extent, one of the technical problems in the related art. To this end, one object of the present invention is to provide a lithium iron phosphate cathode material, a preparation method, and an application thereof. The lithium iron phosphate cathode material has a high compaction density and, when applied to lithium-ion batteries, can improve the discharge capacity, rate capability, and cycle performance of the lithium-ion batteries.

[0005] In one aspect, the present invention provides a lithium iron phosphate cathode material doped with a dopant, wherein the high-valent dopant element in the dopant accounts for 1000-2500 ppm by mass of the lithium iron phosphate cathode material. The high-valent dopant element can inhibit particle growth, thereby shortening the lithium ion transmission path, increasing the discharge specific capacity, and improving the energy density of the material.

[0006] In addition, the lithium iron phosphate positive electrode material according to the above embodiment of the present invention may also have the following additional technical features:

[0007] In some embodiments of the present invention, under a pressure of two tons, the compaction density of the lithium iron phosphate cathode material satisfies: 2.3 g / cm 3 ≤Compacted density≤2.4g / cm 3 , high energy density and high cycle performance.

[0008] In some embodiments of the present invention, the dopant is any one of TiO2, V2O5, and NbO2.

[0009] In a second aspect of the present invention, a preparation method is provided for preparing the above-mentioned lithium iron phosphate positive electrode material. According to an embodiment of the present invention, the method comprises the following steps:

[0010] S1, mixing polyethylene glycol, glucose, dopant, lithium carbonate and FePO4 and then sand-milling;

[0011] S2, take the sample after sand grinding in S1 and conduct particle size test. After the particle size meets the requirements, discharge the sample and dry it;

[0012] S3, taking the dried material in S2 and sintering it under the protection of inert gas, grinding the sintered material, taking the ground sample for particle size detection, and discharging the material after the particle size meets the requirements to obtain a finished lithium iron phosphate product;

[0013] In S2, the particle size satisfies: 400nm≤d50≤600nm;

[0014] In S3, the particle size satisfies: 1.855≤N=L*R / d50≤2.250;

[0015] Where, L=(d85-d15) / (d85+d15), R=(D90-D10) / D50;

[0016] d15, d50 and d85 represent the particle sizes corresponding to the cumulative volume distribution percentages of the sand-milled material in S2 reaching 15%, 50% and 85%, respectively; D10, D50 and D90 represent the particle sizes corresponding to the cumulative volume distribution percentages of the ground material in S3 reaching 10%, 50% and 90%, respectively.

[0017] According to the method for preparing the lithium iron phosphate positive electrode material of the embodiment of the present invention, on the one hand, the appropriate particle size distribution can improve the compaction density of the finished lithium iron phosphate product, and on the other hand, the high-valent doping elements (Ti, V, Nb, etc.) can inhibit particle growth, thereby shortening the lithium ion transmission path, improving the discharge specific capacity, and increasing the energy density of the material.

[0018] In addition, the preparation method of the lithium iron phosphate positive electrode material according to the above embodiment of the present invention may also have the following additional technical features:

[0019] In some embodiments of the present invention, in S1, the molar ratio of Li element in lithium carbonate and P element in FePO4 is (1.003-1.006):1, the mass ratio of polyethylene glycol / FePO4 is (5.8-6.5):1, and the mass ratio of glucose / FePO4 is (5.8-6.5):1.

[0020] In some embodiments of the present invention, in S1, sand grinding is wet grinding with water in a sand grinder, the speed of the sand grinder is 900-1100 rpm, and the sand grinding time per round is 6-10 min;

[0021] In S2, the sample after being sanded by the sand mill for 10-25 cycles is taken for particle size measurement using a laser particle size analyzer.

[0022] In some embodiments of the present invention, in S2, a spray dryer is used for drying, and the spray dryer pressure is set to 0.4-0.5 MPa, the air inlet temperature is set to 145-150°C, and the air outlet temperature is set to 90-95°C.

[0023] In some embodiments of the present invention, in S3, sintering includes the following steps: taking the dried material in S2 and placing it in a graphite crucible, sintering it in a box-type atmosphere furnace at 780-800°C under nitrogen protection, with a heating rate of 1.4-1.6°C / min, and keeping warm for 10-12 hours.

[0024] In some embodiments of the present invention, in S3, a jet mill is used for grinding, and the jet mill pressure is set to 0.4 MPa, the feeding frequency is 6-8 Hz, and the classification frequency is 130-150 Hz.

[0025] In the third aspect of the present invention, the present invention provides a lithium-ion battery, which, according to an embodiment of the present invention, includes the above-mentioned lithium iron phosphate positive electrode material. Therefore, the lithium-ion battery has good discharge specific capacity, rate and cycle performance.

[0026] Additional aspects and advantages of the present invention will be set forth in part in the description which follows and, in part, will be obvious from the description which follows, or may be learned by practice of the present invention. DETAILED DESCRIPTION

[0027] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments with reference to the following drawings, in which:

[0028] FIG1 is a diagram of the finished product of Example 1 provided by the present invention;

[0029] FIG2 is a finished product diagram of Comparative Example 4 provided by the present invention.

[0030] The following describes embodiments of the present invention in detail, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and are not to be construed as limiting the present invention.

[0031] In one aspect, the present invention provides a lithium iron phosphate cathode material doped with a dopant, wherein the high-valent dopant element in the dopant accounts for 1000-2500 ppm by mass of the lithium iron phosphate cathode material. The high-valent dopant element can inhibit particle growth, thereby shortening the lithium ion transmission path, increasing the discharge specific capacity, and improving the energy density of the material.

[0032] The dopant can be selected from high-valent doping elements, such as any one of TiO2, V2O5, and NbO2, and the specific doping ratio is preferably such that the high-valent doping element in the dopant accounts for 1000-2500ppm of the mass of the lithium iron phosphate positive electrode material.

[0033] Furthermore, under a pressure of two tons, the compaction density of the lithium iron phosphate cathode material meets the following requirements: 2.3g / cm 3 ≤Compacted density≤2.4g / cm 3 , high energy density and high cycle performance. Lithium iron phosphate positive electrode materials with this compaction density have higher energy density and high cycle performance.

[0034] In a second aspect of the present invention, a preparation method is provided for preparing the above-mentioned lithium iron phosphate positive electrode material. According to an embodiment of the present invention, the method comprises the following steps:

[0035] S1, mixing polyethylene glycol, glucose, dopant, lithium carbonate and FePO4 and then sand-milling;

[0036] S2, take the sample after sand grinding in S1 and conduct particle size test. After the particle size meets the requirements, discharge the sample and dry it;

[0037] S3. The dried material in S2 is sintered under the protection of an inert gas (preferably nitrogen, which is low in cost), and the sintered material is ground. A sample of the ground material is taken for particle size testing. After the particle size is qualified, the material is discharged to obtain a finished lithium iron phosphate product.

[0038] Specifically, in S2 and S3, the criteria for judging whether the particle size is qualified are:

[0039] In S2, the particle size satisfies: 340nm≤d50≤600nm;

[0040] In S3, the particle size satisfies: 1.855≤N=L*R / d50≤2.250;

[0041] Where, L=(d85-d15) / (d85+d15), R=(D90-D10) / D50;

[0042] d15, d50 and d85 represent the particle sizes corresponding to the cumulative volume distribution percentages of the sand-milled material in S2 reaching 15%, 50% and 85%, respectively; D10, D50 and D90 represent the particle sizes corresponding to the cumulative volume distribution percentages of the ground material in S3 reaching 10%, 50% and 90%, respectively.

[0043] Based on the above examples, it can be seen that in order to prepare 2.3g / cm 3 ≤Compacted density≤2.4g / cm 3 The lithium iron phosphate positive electrode material of the present invention adopts a dopant that can inhibit particle growth, and regulates the sand grinding particle size of the iron phosphate precursor and the crushing particle size of the material after lithiation. Specifically, the sand grinding particle size is controlled within a certain range, and the positive electrode material particles synthesized after sintering are relatively uniform. In this way, during the charge and discharge process, the primary particles of the positive electrode material can almost reach the state of lithium ion insertion and deintercalation at the same time, and overcharge or over-discharge of some particles will not occur, thereby stabilizing the structure of the positive electrode material and improving the discharge specific capacity and cycle performance of the positive electrode material.

[0044] Furthermore, in S1, the initial mixing ratio follows the following proportion:

[0045] The molar ratio of Li element in lithium carbonate and P element in FePO4 = (1.003-1.006):1, the mass ratio of polyethylene glycol / FePO4 = (5.8-6.5):1, and the mass ratio of glucose / FePO4 = (5.8-6.5):1.

[0046] Within the above-mentioned ratio range, the overall performance of the positive electrode material is better. When the ratio value is too large, it will inhibit the growth of particles, resulting in lower compaction and poor cycle performance; when the ratio value is too small, the carbon content coated on the surface of the finished product is less, affecting the conductivity of the positive electrode material.

[0047] Furthermore, in S1, sand milling is performed using a sand mill with rotating blades. Since sand milling is wet grinding, water needs to be added to the sand mill for wet grinding. The amount of water added is adjusted based on the feed rate of the sand mill. Generally, the solid mass content is preferably controlled at 35-45%. The sand mill speed is controlled at 900-1100 rpm, and the sand milling time per round is 6-10 minutes.

[0048] During the first sampling of the sand mill, combined with the change of the material particle size over time during sand milling, the sample after 10-25 rounds of sand milling is generally tested for particle size using a laser particle size analyzer. Once the particle size meets the conditions, sand milling can be stopped; if it does not meet the conditions, sand milling can be continued until the particle size is qualified.

[0049] The compaction density of lithium iron phosphate products can be improved through appropriate particle size distribution. The particle size of the iron phosphate precursor will also affect the uniformity of the particle size of the lithium iron phosphate material, thereby affecting the compaction density and electrical properties of the final product. As the compaction density increases, its energy density will also increase. However, when the compaction density is higher than a certain value, the discharge specific capacity of the positive electrode material is low and the cycle performance is poor, so comprehensive consideration is needed.

[0050] Furthermore, in S2, a spray dryer may be used for drying, and the spray dryer pressure is set to 0.4-0.5 MPa, the air inlet temperature is set to 145-150° C., and the air outlet temperature is set to 90-95° C. Other drying equipment with higher efficiency may also be used.

[0051] Furthermore, in S3, sintering includes the following steps: taking the dried material in S2 and placing it in a graphite crucible, sintering it in a box-type atmosphere furnace at 780-800°C under nitrogen (nitrogen purity ≥99.999%) protection, with a heating rate of 1.4-1.6°C / min and keeping warm for 10-12 hours.

[0052] Furthermore, in S3, the grinding can be performed using a jet mill, and the jet mill pressure is set to 0.4 MPa, the feeding is set to 6-8 Hz, and the classification is set to 130-150 Hz.

[0053] In the third aspect of the present invention, the present invention provides a lithium-ion battery, which, according to an embodiment of the present invention, includes the above-mentioned lithium iron phosphate positive electrode material. Therefore, the lithium-ion battery has good discharge specific capacity, rate and cycle performance.

[0054] In a specific embodiment, a lithium-ion battery is prepared using a lithium iron phosphate positive electrode material. First, a positive electrode sheet including the lithium iron phosphate positive electrode material needs to be prepared. The positive electrode sheet is prepared in the following manner:

[0055] The lithium iron phosphate positive electrode material, polyvinylidene fluoride glue and N-methyl pyrrolidone are mixed uniformly in a mass ratio of (90-92): (3-5): (3-5): (1-3) to obtain a slurry; the slurry is then coated on an aluminum foil with a thickness of 20 μm, and vacuum dried and roll-pressed to form a positive electrode sheet;

[0056] Then, a lithium metal sheet was used as the negative electrode, and the electrolyte was a 1.15 M LiPF6 / EC:DMC solution. This solution, consisting of a 1:1 volume ratio of EC (ethylene carbonate) and DMC (dimethyl carbonate) as the solvent and LiPF6 (lithium hexafluorophosphate) as the solute, was prepared to a 1.15 mol / l LiPF6 solution. The lithium-ion battery was then assembled.

[0057] The present invention is described below with reference to specific examples. It should be noted that these examples are merely illustrative and do not limit the present invention in any way. Example 1

[0058] (1) The blades of the sand mill rotate and feed the materials. Water is added to the sand mill according to the solid content of 40%. Polyethylene glycol, glucose, TiO2, lithium carbonate, and FePO4 are put into the sand mill B tank in order (wherein the mass ratio of Li element to P element is 1.005:1, the mass ratio of polyethylene glycol to FePO4 is 6.0:1, the mass ratio of glucose to FePO4 is 6.0:1, and the doped Ti accounts for 2000ppm of the final lithium iron phosphate product);

[0059] (2) Start sand grinding, set the number of sand grinding circles to 25 circles (about 300±25nm), the sand grinding machine speed to 1000rpm, and the sand grinding time per circle to 8min. After sand grinding for 25 circles, take a sample and measure the particle size using a laser particle size analyzer. The measured values ​​are d15=0.252mm, d50=0.345mm, and d85=0.475mm, and then discharge the material;

[0060] (3) The spray dryer pressure is 0.4 MPa, the air inlet temperature is 145-150°C, and the air outlet temperature is 90-95°C;

[0061] (4) The material from step 3 was placed in a graphite crucible and sintered in a box-type atmosphere furnace at 780°C under nitrogen (nitrogen purity ≥ 99.999%), with a heating rate of 1.5°C / min and a holding time of 12 h. The grinding pressure was 0.4 MPa, the feed rate was 7 Hz, and the grading rate was 140 Hz. After pulverization, the particle size was measured using a laser particle size analyzer, with the results of D10 = 0.44 µm, D50 = 1.17 µm, and D90 = 3.36 µm. Example 2

[0062] (1) The blades of the sand mill rotate and feed the materials. Water is added to the sand mill according to the solid content of 40%. Polyethylene glycol, glucose, TiO2, lithium carbonate, and FePO4 are put into the sand mill B tank in order (wherein the mass ratio of Li element to P element is 1.005:1, the mass ratio of polyethylene glycol to FePO4 is 6.0:1, the mass ratio of glucose to FePO4 is 6.0:1, and the doped Ti accounts for 2000ppm of the final lithium iron phosphate product);

[0063] (2) Start sand grinding, set the number of sand grinding circles to 23 circles (about 300±25nm), the sand grinding machine speed to 1000rpm, and the sand grinding time per circle to 8min. After sand grinding for 23 circles, take a sample and measure the particle size using a laser particle size analyzer. The measured values ​​are d15=0.269mm, d50=0.374mm, and d85=0.523mm, and then discharge the material;

[0064] (3) The spray dryer pressure is 0.4 MPa, the air inlet temperature is 145-150°C, and the air outlet temperature is 90-95°C;

[0065] (4) The material from step 3 was placed in a graphite crucible and sintered in a box-type atmosphere furnace at 780°C under nitrogen (nitrogen purity ≥ 99.999%), with a heating rate of 1.5°C / min and a holding time of 12 h. The grinding pressure was 0.4 MPa, the feed rate was 9 Hz, and the grading rate was 150 Hz. After pulverization, the particle size was measured using a laser particle size analyzer, and the results were D10 = 0.40 µm, D50 = 0.94 µm, and D90 = 2.58 µm. Example 3

[0066] (1) The sand mill blades rotate to feed the materials. The sand mill is filled with 41 kg of water. Polyethylene glycol, glucose, TiO2, lithium carbonate, and FePO4 are put into the sand mill B tank in order (wherein the mass ratio of Li element to P element is 1.005:1, the mass ratio of polyethylene glycol to FePO4 is 6.0:1, the mass ratio of glucose to FePO4 is 6.0:1, and the doped Ti accounts for 2000 ppm of the final lithium iron phosphate product);

[0067] (2) Start sand grinding, set the number of sand grinding circles to 25 circles (about 300±25nm), the sand grinding machine speed to 1000rpm, and the sand grinding time per circle to 8min. After sand grinding for 25 circles, take a sample and measure the particle size using a laser particle size analyzer. The measured values ​​are d15=0.253mm, d50=0.348mm, and d85=0.480mm, and then discharge the material;

[0068] (3) The spray dryer pressure is 0.4 MPa, the air inlet temperature is 145-150°C, and the air outlet temperature is 90-95°C;

[0069] (4) The material from step 3 was placed in a graphite crucible and sintered in a box-type atmosphere furnace at 780°C under nitrogen (nitrogen purity ≥ 99.999%) protection, with a heating rate of 1.5°C / min and a holding time of 12 h. The grinding pressure was 0.4 MPa, the feed rate was 9 Hz, and the grading rate was 150 Hz. After pulverization, the particle size was measured using a laser particle size analyzer, and the results were D10 = 0.43 µm, D50 = 0.94 µm, and D90 = 2.56 µm. Example 4

[0070] (1) The sand mill blades rotate to feed the materials. The sand mill is filled with 41 kg of water. Polyethylene glycol, glucose, TiO2, lithium carbonate, and FePO4 are put into the sand mill B tank in order (wherein the mass ratio of Li element to P element is 1.005:1, the mass ratio of polyethylene glycol to FePO4 is 6.0:1, the mass ratio of glucose to FePO4 is 6.0:1, and the doped Ti accounts for 2000 ppm of the final lithium iron phosphate product);

[0071] (2) Start sand grinding, set the number of sand grinding circles to 22.5 circles (about 300±25nm), the sand grinding machine speed to 1000rpm, and the sand grinding time per circle to 8min. After sand grinding for 22.5 circles, take a sample and measure the particle size using a laser particle size analyzer. The measured values ​​are d15=0.278mm, d50=0.384mm, and d85=0.539mm, and then discharge the material;

[0072] (3) The spray dryer pressure is 0.4 MPa, the air inlet temperature is 145-150°C, and the air outlet temperature is 90-95°C;

[0073] (4) The material from step 3 was placed in a graphite crucible and sintered in a box-type atmosphere furnace at 780°C under nitrogen (nitrogen purity ≥ 99.999%), with a heating rate of 1.5°C / min and a holding time of 12 h. The grinding pressure was 0.4 MPa, the feed rate was 10 Hz, and the grading rate was 145 Hz. After pulverization, the particle size was measured using a laser particle size analyzer, and the results were D10 = 0.37 µm, D50 = 0.73 µm, and D90 = 2.84 µm. Example 5

[0074] (1) The sand mill blades rotate to feed the materials. The sand mill is filled with 41 kg of water. Polyethylene glycol, glucose, TiO2, lithium carbonate, and FePO4 are put into the sand mill B tank in order (wherein the mass ratio of Li element to P element is 1.005:1, the mass ratio of polyethylene glycol to FePO4 is 6.0:1, the mass ratio of glucose to FePO4 is 6.0:1, and the doped Ti accounts for 2000 ppm of the final lithium iron phosphate product);

[0075] (2) Start sand grinding, set the number of sand grinding circles to 24 circles (about 300±25nm), the sand grinding machine speed to 1000rpm, and the sand grinding time per circle to 8min. After sand grinding for 24 circles, take a sample and measure the particle size using a laser particle size analyzer. The measured values ​​are d15=0.262mm, d50=0.360mm, and d85=0.497mm, and then discharge the material;

[0076] (3) The spray dryer pressure is 0.4 MPa, the air inlet temperature is 145-150°C, and the air outlet temperature is 90-95°C;

[0077] (4) The material from step 3 was placed in a graphite crucible and sintered in a box-type atmosphere furnace at 780°C under nitrogen (nitrogen purity ≥ 99.999%), with a heating rate of 1.5°C / min and a holding time of 12 h. The grinding pressure was 0.4 MPa, the feed rate was 9 Hz, and the grading rate was 145 Hz. After pulverization, the particle size was measured using a laser particle size analyzer, and the results were D10 = 0.41 µm, D50 = 0.98 µm, and D90 = 2.80 µm.

[0078] Comparative Example 1

[0079] (1) The sand mill blades rotate to feed the materials. The sand mill is filled with 41 kg of water. Polyethylene glycol, glucose, TiO2, lithium carbonate, and FePO4 are put into the sand mill B tank in order (wherein the mass ratio of Li element to P element is 1.005:1, the mass ratio of polyethylene glycol to FePO4 is 6.0:1, the mass ratio of glucose to FePO4 is 6.0:1, and the doped Ti accounts for 2000 ppm of the final lithium iron phosphate product);

[0080] (2) Start sand grinding, set the number of sand grinding circles to 8 circles (about 300±25nm), the sand grinding machine speed is 1000rpm, and the sand grinding time per circle is adjusted to 8min. After sand grinding for 8 circles, take a sample and measure the particle size using a laser particle size analyzer. The measured values ​​are d15=0.382mm, d50=0.601mm, and d85=1.010mm, and then discharge the material;

[0081] (3) The spray dryer pressure is 0.4 MPa, the air inlet temperature is 145-150°C, and the air outlet temperature is 90-95°C;

[0082] (4) The material from step 3 was placed in a graphite crucible and sintered in a box-type atmosphere furnace at 780°C under nitrogen (nitrogen purity ≥ 99.999%), with a heating rate of 1.5°C / min and a holding time of 12 h. The grinding pressure was 0.4 MPa, the feed rate was 9 Hz, and the grading rate was 160 Hz. After pulverization, the particle size was measured using a laser particle size analyzer, and the results were D10 = 0.40 µm, D50 = 0.94 µm, and D90 = 2.03 µm.

[0083] Comparative Example 2

[0084] (1) The sand mill blades rotate to feed the materials. The sand mill is filled with 41 kg of water. Polyethylene glycol, glucose, TiO2, lithium carbonate, and FePO4 are put into the sand mill B tank in order (wherein the mass ratio of Li element to P element is 1.005:1, the mass ratio of polyethylene glycol to FePO4 is 6.0:1, the mass ratio of glucose to FePO4 is 6.0:1, and the doped Ti accounts for 2000 ppm of the final lithium iron phosphate product);

[0085] (2) Start sand grinding, set the number of sand grinding circles to 10 circles (about 300±25nm), the sand grinding machine speed to 1000rpm, and the sand grinding time per circle to 8min. After sand grinding for 10 circles, take a sample and measure the particle size using a laser particle size analyzer. The measured values ​​are d15=0.36mm, d50=0.556mm, and d85=0.942mm, and then discharge the material;

[0086] (3) The spray dryer pressure is 0.4 MPa, the air inlet temperature is 145-150°C, and the air outlet temperature is 90-95°C;

[0087] (4) The material from step 3 was placed in a graphite crucible and sintered in a box-type atmosphere furnace at 780°C under nitrogen (nitrogen purity ≥ 99.999%), with a heating rate of 1.5°C / min and a holding time of 12 h. The grinding pressure was 0.4 MPa, the feed rate was 10 Hz, and the grading rate was 130 Hz. After pulverization, the particle size was measured using a laser particle size analyzer, and the results were D10 = 0.37 µm, D50 = 0.76 µm, and D90 = 4.68 µm.

[0088] Comparative Example 3

[0089] (1) The sand mill blades rotate to feed the materials. The sand mill is filled with 41 kg of water. Polyethylene glycol, glucose, TiO2, lithium carbonate, and FePO4 are put into the sand mill B tank in order (wherein the mass ratio of Li element to P element is 1.005:1, the mass ratio of polyethylene glycol to FePO4 is 6.0:1, the mass ratio of glucose to FePO4 is 6.0:1, and the doped Ti accounts for 2000 ppm of the final lithium iron phosphate product);

[0090] (2) Start sand grinding, set the number of sand grinding circles to 12 circles (about 300±25nm), the sand grinding machine speed to 1000rpm, and the sand grinding time per circle to 8min. After sand grinding for 25 circles, take a sample and measure the particle size using a laser particle size analyzer. The measured values ​​are d15=0.347mm, d50=0.525mm, and d85=0.86mm, and then discharge the material;

[0091] (3) The spray dryer pressure is 0.4 MPa, the air inlet temperature is 145-150°C, and the air outlet temperature is 90-95°C;

[0092] (4) The material from step 3 was placed in a graphite crucible and sintered in a box-type atmosphere furnace at 780°C under nitrogen (nitrogen purity ≥ 99.999%), with a heating rate of 1.5°C / min and a holding time of 12 h. The grinding pressure was 0.4 MPa, the feed rate was 9 Hz, and the grading rate was 135 Hz. After pulverization, the particle size was measured using a laser particle size analyzer, and the results were D10 = 0.39 µm, D50 = 0.97 µm, and D90 = 3.80 µm.

[0093] Comparative Example 4

[0094] (1) The sand mill blades rotate to feed the materials. The sand mill is filled with 41 kg of water. Polyethylene glycol, glucose, TiO2, lithium carbonate, and FePO4 are put into the sand mill B tank in order (wherein the mass ratio of Li element to P element is 1.005:1, the mass ratio of polyethylene glycol to FePO4 is 6.0:1, the mass ratio of glucose to FePO4 is 6.0:1, and the doped Ti accounts for 2000 ppm of the final lithium iron phosphate product);

[0095] (2) Start sand grinding, set the number of sand grinding circles to 27 circles (about 300±25nm), the sand grinding machine speed to 1000rpm, and the sand grinding time per circle to 8min. After sand grinding for 27 circles, take a sample and measure the particle size using a laser particle size analyzer. The measured values ​​are d15=0.212mm, d50=0.320mm, and d85=0.415mm, and then discharge the material;

[0096] (3) The spray dryer pressure is 0.4 MPa, the air inlet temperature is 145-150°C, and the air outlet temperature is 90-95°C;

[0097] (4) The material from step 3 was placed in a graphite crucible and sintered in a box-type atmosphere furnace at 780°C under nitrogen (nitrogen purity ≥ 99.999%), with a heating rate of 1.5°C / min and a holding time of 12 h. The grinding pressure was 0.4 MPa, the feed rate was 9 Hz, and the grading rate was 140 Hz. After pulverization, the particle size was measured using a laser particle size analyzer, and the results were D10 = 0.41 µm, D50 = 0.99 µm, and D90 = 2.78 µm.

[0098] Comparative Example 5

[0099] (1) The sand mill blades rotate to feed the materials. The sand mill is filled with 41 kg of water. Polyethylene glycol, glucose, TiO2, lithium carbonate, and FePO4 are put into the sand mill B tank in order (wherein the mass ratio of Li element to P element is 1.005:1, the mass ratio of polyethylene glycol to FePO4 is 6.0:1, the mass ratio of glucose to FePO4 is 6.0:1, and the doped Ti accounts for 2000 ppm of the final lithium iron phosphate product);

[0100] (2) Start sand grinding, set the number of sand grinding circles to 7 circles (about 300±25nm), the sand grinding machine speed is 1000rpm, and the sand grinding time per circle is adjusted to 8min. After sand grinding for 25 circles, take a sample and measure the particle size using a laser particle size analyzer. The measured values ​​are d15=0.425mm, d50=0.620mm, and d85=1.201mm, and then discharge the material;

[0101] (3) The spray dryer pressure is 0.4 MPa, the air inlet temperature is 145-150°C, and the air outlet temperature is 90-95°C;

[0102] (4) The material from step 3 was placed in a graphite crucible and sintered in a box-type atmosphere furnace at 780°C under nitrogen (nitrogen purity ≥ 99.999%), with a heating rate of 1.5°C / min and a holding time of 12 h. The grinding pressure was 0.4 MPa, the feed rate was 9 Hz, and the grading rate was 150 Hz. After pulverization, the particle size was measured using a laser particle size analyzer, and the results were D10 = 0.42 µm, D50 = 0.97 µm, and D90 = 2.59 µm.

[0103] By comparing FIG1 and FIG2 , it can be seen that the particle size of the finished product prepared in Example 1 is relatively uniform, while the particle size of the finished product prepared in Comparative Example 4 is uneven and the particle gradation is obvious.

[0104] Compaction density test: For Examples 2 to 5 and Comparative Examples 1 to 3, the compaction density was measured using a compaction density meter at a pressure of 2 tons. The results are shown in Table 1.

[0105] Table 1

[0106]

[0107] Then, using the positive electrode materials synthesized in Examples 1 to 5 and Comparative Examples 1 to 3, respectively, the positive electrode active material, carbon black, PVDF (polyvinylidene fluoride) glue (PVDF solid content 6.25%), and NMP (N-methylpyrrolidone) were mixed uniformly in a mass ratio of 90:4:4:2 to form a slurry. This slurry was coated on a 20 μm thick aluminum foil, vacuum dried, and roller-pressed to form a positive electrode sheet. A lithium metal sheet was used as the negative electrode, and the electrolyte was a 1.15 M LiPF6 / EC:DMC (volume ratio 1:1%). A lithium-ion battery was then assembled.

[0108] Battery charging test: The above lithium-ion batteries were tested at 25°C using a Blue Power battery testing system with a test voltage range of 2V to 3.75V. The battery was tested at 0.1C initial charge and discharge capacity, as well as 50 cycles of 0.1C charge and 1C discharge. The test results are shown in Table 2.

[0109] Table 2

[0110]

[0111] It can be seen from Examples 1-5 that when 0.340 μm ≤ d50 ≤ 0.600 μm and the N = L*R / d50 value satisfies: 1.855 ≤ N ≤ 2.250, the compaction density of the corresponding finished product is higher, and the 0.1C and 1C discharge specific capacities and cycle performance are better;

[0112] In Comparative Examples 1 to 3, N is not within the protection range, the compaction density is low, the discharge specific capacity is low, and the cycle performance is poor. This is because the particle grading of the positive electrode materials in Comparative Examples 1 to 3 is not suitable, resulting in poor contact between the positive electrode material particles, thereby affecting the electrical properties of the positive electrode materials;

[0113] The d50 and N values ​​of Comparative Examples 4 and 5 are both out of range, the compaction density of the synthetic material is out of the appropriate range, the discharge specific capacity of the positive electrode material is low and the cycle performance is poor. Specifically, if the sanding particle size d50 in step (2) is too low, the sanding material activity is relatively high, and the particles are easily fused together during the sintering process to form a portion of particles with a larger primary particle size. After the particles are crushed in step (4), a portion of the large particles are not crushed, resulting in a longer lithium ion transfer path inside the particles during the charge and discharge process, thereby reducing the discharge specific capacity. In addition, there are still some small particles in the final product. The obvious grading of large and small particles leads to overcharge and overdischarge of some particles during the charge and discharge process, thereby destroying the structure of the positive electrode material and reducing the discharge specific capacity and cycle performance of the positive electrode material. If the sanding particle size d50 in step (2) is too large, the sanding material activity is low, and the resulting material after sintering is incompletely crystallized, which will also lead to the problem of low discharge specific capacity and poor cycle performance of the positive electrode material.

[0114] By comparing comparative examples 1-3, it can be seen that when 1.855≤N≤2.250, the compaction density of the corresponding finished product is higher, and the discharge capacity, rate and cycle performance of the battery are better; for comparative examples 1 to 3, N is not within the protection range, the compaction density is low, and the electrical performance is poor.

[0115] By comparing Examples 1-5 and Comparative Examples 1-3, it can be seen that when N is not within the range, the particle grading of the positive electrode material is inappropriate, resulting in poor contact between the positive electrode material particles, thereby affecting the electrical properties of the positive electrode material, low compaction density, low discharge specific capacity, and poor cycle performance.

[0116] By comparing Examples 1-5 and Comparative Examples 4-5, it can be seen that when both d50 and N are not within the range, the compaction density of the synthetic material is not within the appropriate range, the discharge specific capacity of the positive electrode material is low, and the cycle performance is poor. Specifically, if the sanding particle size d50 in step (2) is too low, the sanding material activity is relatively high, and the particles are easily fused together during the sintering process to form a portion of particles with a larger primary particle size. After the particles are crushed in step (4), some large particles are not crushed, resulting in a longer lithium ion transfer path inside the particles during the charge and discharge process, thereby reducing the discharge specific capacity. In addition, some small particles are still present in the final product, and the obvious gradation of large and small particles leads to overcharge and overdischarge of some particles during the charge and discharge process, thereby destroying the structure of the positive electrode material and reducing the discharge specific capacity and cycle performance of the positive electrode material. If the sanding particle size d50 is too large, the sanding material activity is low, and the resulting material after sintering is incompletely crystallized, which will also lead to the problem of low discharge specific capacity and poor cycle performance of the positive electrode material.

[0117] In the description of this specification, the reference terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" mean that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine different embodiments or examples described in this specification and features of different embodiments or examples without contradiction.

[0118] Although the embodiments of the present invention have been shown and described above, it will be understood that the above embodiments are illustrative and are not to be construed as limitations on the present invention. A person skilled in the art may change, modify, replace and modify the above embodiments within the scope of the present invention.

Claims

1. A lithium iron phosphate positive electrode material, characterized in that: The lithium iron phosphate positive electrode material is doped with a dopant, and the high-valent doping element in the dopant accounts for 1000-2500 ppm of the mass of the lithium iron phosphate positive electrode material.

2. The lithium iron phosphate positive electrode material according to claim 1, characterized in that: Under two tons of pressure, the compaction density of the lithium iron phosphate cathode material meets the following requirements: 2.3g / cm 3 ≤Compacted density≤2.4g / cm 3 .

3. The lithium iron phosphate positive electrode material according to claim 1, characterized in that: The dopant is any one of TiO2, V2O5, and NbO2.

4. A preparation method, characterized in that: For preparing the lithium iron phosphate positive electrode material according to any one of claims 1 to 3, the preparation method comprises the following steps: S1, mixing polyethylene glycol, glucose, dopant, lithium carbonate and FePO4 and then sand-milling; S2, take the sample after sand grinding in S1 for particle size test, and discharge and dry it after the particle size is qualified; S3, taking the dried material in S2 and sintering it under the protection of inert gas, grinding the sintered material, taking the ground sample for particle size detection, and discharging the material after the particle size is qualified to obtain a lithium iron phosphate finished product; In S2, the particle size satisfies: 400nm≤d50≤600nm; In S3, the particle size satisfies: 1.855≤N=L*R / d50≤2.250; Among them, L=(d85-d15) / (d85+d15), R=(D90-D10) / D50; d15, d50 and d85 represent the particle sizes corresponding to the cumulative volume distribution percentages of the sand-milled materials in S2 reaching 15%, 50% and 85%, respectively; D10, D50 and D90 represent the particle sizes corresponding to the cumulative volume distribution percentages of the ground materials in S3 reaching 10%, 50% and 90%, respectively.

5. The preparation method according to claim 4, characterized in that: In S1, the molar ratio of Li element in lithium carbonate and P element in FePO4 = (1.003-1.006):1, the mass ratio of polyethylene glycol / FePO4 = (5.8-6.5):1, and the mass ratio of glucose / FePO4 = (5.8-6.5):

1.

6. The preparation method according to claim 4, characterized in that: In S1, sand grinding is wet grinding with water in a sand mill, the sand mill speed is 900-1100 rpm, and the sand grinding time per round is 6-10 min; In S2, the sample after being sanded by a sand mill for 10 to 25 times is taken for particle size measurement using a laser particle size analyzer.

7. The preparation method according to claim 4, characterized in that: In S2, a spray dryer is used for drying, and the spray dryer pressure is set to 0.4-0.5MPa, the air inlet temperature is set to 145-150°C, and the air outlet temperature is set to 90-95°C.

8. The preparation method according to claim 4, characterized in that: In S3, sintering includes the following steps: taking the dried material in S2 and placing it in a graphite sagger, sintering it in a box-type atmosphere furnace at 780-800°C under nitrogen protection, with a heating rate of 1.4-1.6°C / min and keeping the temperature for 10-12h.

9. The preparation method according to claim 4, characterized in that: In S3, a jet mill is used for grinding, and the jet mill pressure is set to 0.4 MPa, the feeding frequency is 6-8 Hz, and the classification frequency is 130-150 Hz.

10. A lithium ion battery, characterized in that: The invention comprises the lithium iron phosphate positive electrode material according to claims 1 to 3 or the lithium iron phosphate positive electrode material prepared by the preparation method according to any one of claims 4 to 9.

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