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

Abstract

A lithium iron phosphate material, and a preparation method therefor and the use thereof. The lithium iron phosphate material comprises a lithium iron phosphate particle A, a lithium iron phosphate particle B and a lithium iron phosphate particle C, wherein the particle A is formed by agglomerating primary particles A1, and the average particle size of the primary particles A1 is 50-300 nm; the particle B is formed by agglomerating primary particles B1, and the average particle size of the primary particles B1 is 600-1000 nm; the particle C is formed by means of agglomerating primary particles C1, and the average particle size of the primary particles C1 is 1200-1500 nm; and the mass ratio of the particle A to the particle B to the particle C is (4-7):(1-2):(2-4). The lithium iron phosphate material has the advantages of high compaction density and good electrochemical performance, and the preparation method therefor is simple.

Description

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

[0001] The present invention specifically relates to lithium iron phosphate materials and preparation methods and applications thereof. Background Art

[0002] Currently, lithium-ion battery cathode materials mainly include lithium iron phosphate and ternary materials. Lithium iron phosphate has a stable structure that will not collapse even under high temperature or overcharge conditions, ensuring its good cycle performance and safety, thus having broad application prospects. However, the battery system it comprises has a low energy density, which limits its application in power batteries. In order to improve the energy density of lithium iron phosphate batteries, it is necessary to increase the capacity, voltage platform or compaction density of lithium iron phosphate. However, the capacity and voltage platform of lithium iron phosphate on the market are currently close to their theoretical values, and there is little room for improvement. Therefore, the energy density of lithium iron phosphate can be improved by increasing its compaction density.

[0003] There are two main methods for improving the compaction density of lithium iron phosphate materials in the prior art. One method is to mix large particles with small particles. For example, Chinese patent CN108011104A discloses a high compaction density lithium iron phosphate positive electrode material and its preparation method, which uses two kinds of particle slurries, large and small, and mixes the large particle slurry and the small particle slurry in a certain ratio during the grinding stage, and then respectively undergoes drying and heat treatment to obtain high compaction density lithium iron phosphate. Chinese patent CN107256968A also discloses a high compaction density lithium iron phosphate and its preparation method, which is to mix a mixed iron phosphate raw material of large and small particles with a lithium source, and obtain a lithium iron phosphate precursor with iron phosphate as the skeleton through a one-step grinding, and then calcine to obtain high compaction density lithium iron phosphate. This method makes it difficult to mix large and small particles evenly, and the gaps between large particles are difficult to be effectively filled by small particles, so that the degree of improvement in compaction density is limited.

[0004] Another method is to improve the sintering process during the preparation process. For example, CN102275887A discloses a method for preparing a high-capacity and high-compacted-density lithium iron phosphate material, which includes mixing a lithium source, an Fe3+ source, lithium iron phosphate, a dopant, and an organic carbon source, followed by spray granulation, pre-calcination, sand milling, spray drying, sintering, and grinding to obtain a lithium iron phosphate product. Chinese patent CN109192948B discloses a high-compacted-density iron phosphate and its preparation method, which obtains a compacted density of 2.7g / cm3 through three-stage sintering. This method has a complex sintering process and a cumbersome preparation process.

[0005] Therefore, developing a lithium iron phosphate material with high compaction density, simple preparation method and good performance is of great significance to this field.

[0006] Summary of the Invention

[0007] The present invention primarily aims to overcome the limitations of prior art lithium iron phosphate materials, such as their limited compaction density and complex preparation methods, by providing a lithium iron phosphate material, its preparation method, and its application. The lithium iron phosphate material provided by the present invention has the advantages of high compaction density, good electrochemical performance, and a simple preparation method.

[0008] In order to overcome the above-mentioned defects, the present invention provides the following technical solutions.

[0009] The present invention provides a lithium iron phosphate material, which includes lithium iron phosphate particles A, lithium iron phosphate particles B and lithium iron phosphate particles C; wherein,

[0010] The lithium iron phosphate particles A are formed by agglomerating primary particles A1, and the average particle size of the primary particles A1 is 50nm-300nm; the lithium iron phosphate particles B are formed by agglomerating primary particles B1, and the average particle size of the primary particles B1 is 600nm-1000nm. The lithium iron phosphate particles B contain a dopant content of 0.05-0.2%; the lithium iron phosphate particles C are formed by agglomerating primary particles C1, and the average particle size of the primary particles C1 is 1200nm-1500nm. The lithium iron phosphate particles C contain a dopant content of 0.2-0.5%.

[0011] Wherein, the dopant is at least one of a titanium compound, a vanadium compound, a strontium compound and a niobium compound; the content of the dopant is the atomic ratio of the dopant to the lithium iron phosphate particles B and the lithium iron phosphate particles C respectively;

[0012] The mass ratio of the lithium iron phosphate particles A, the lithium iron phosphate particles B and the lithium iron phosphate particles C is (4-7):(1-2):(2-4).

[0013] In the present invention, the mass ratio of the lithium iron phosphate particles A, the lithium iron phosphate particles B, and the lithium iron phosphate particles C may be (4-6.5):(1.5-2):(2-3.5).

[0014] In some specific embodiments, the mass ratio of the lithium iron phosphate particles A, the lithium iron phosphate particles B, and the lithium iron phosphate particles C may be 5:2:3, 4:2:4, or 7:1:2.

[0015] In the present invention, the niobium compound is preferably niobium oxide.

[0016] In the present invention, the D10 particle size of the lithium iron phosphate material may be ≥0.35 μm, for example, 0.35 μm, 0.36 μm, 0.40 μm, 0.45 μm or 0.51 μm.

[0017] In the present invention, the D50 particle size of the lithium iron phosphate material may be 0.5-1.5 μm, for example, 1.09 μm, 1.12 μm, 1.17 μm, 1.23 μm or 1.36 μm.

[0018] In the present invention, the D99 particle size of the lithium iron phosphate material may be ≤25 μm, for example, 5.98 μm, 5.65 μm, 5.37 μm, 5.08 or 4.97 μm.

[0019] In the present invention, the carbon content of the lithium iron phosphate material may be 1.0%-2.0%, preferably 1.5%-2.0%, for example 1.57%, 1.51%, 1.70%, 1.61% or 1.51%, where the percentage is the mass of the carbon element to the total mass of the lithium iron phosphate material.

[0020] In the present invention, the iron content of the lithium iron phosphate material may be 33%-35%, such as 33.54%, 33.43%, 33.52%, 33.47% or 33.64%, where the percentage is the mass of the iron element as a percentage of the total mass of the lithium iron phosphate material.

[0021] In the present invention, the phosphorus content of the lithium iron phosphate material may be 18%-20%, such as 19.84%, 19.87% or 19.72%, where the percentage is the mass percentage of phosphorus element to the total mass of the lithium iron phosphate material.

[0022] In the present invention, the doping element content of the lithium iron phosphate material may be 0.005-0.03%, such as 0.085%, 0.0104%, 0.0057%, 0.0085% or 0.0090%, where the percentage is the atomic percentage of the dopant content in the lithium iron phosphate material.

[0023] In the present invention, the specific surface area of ​​the lithium iron phosphate material can be 10-20m 2 / g, for example 12.74m 2 / g, 10.80m 2 / g, 13.92m 2 / g, 13.41m 2 / g or 11.78m 2 / g.

[0024] In the present invention, the carbon content in the lithium iron phosphate particles A may be 1.6%-2.0%, for example, 1.9%, where the percentage is the percentage of the mass of the carbon element to the total mass of the lithium iron phosphate particles A.

[0025] In the present invention, the specific surface area of ​​the lithium iron phosphate particles A can be 15m 2 / g-25m 2 / g, for example 17m2 / g, 20m 2 / g or 23m 2 / g.

[0026] In the present invention, the average particle size of the primary particles A1 may be 50-260 nm, for example, 200 nm.

[0027] In the present invention, the dopant content in the lithium iron phosphate particles A is 0-0.005%, for example, 0% or 0.005%.

[0028] In the present invention, the carbon content in the lithium iron phosphate particles B may be 1.3%-1.6%, for example, 1.5%, where the percentage is the percentage of the mass of the carbon element to the total mass of the lithium iron phosphate particles B.

[0029] In the present invention, the specific surface area of ​​the lithium iron phosphate particles B can be 10m 2 / g-15m 2 / g, for example 11m 2 / g、13m 2 / g or 15m 2 / g.

[0030] In the present invention, the average particle size of the primary particles B1 is preferably 700-1000 nm, for example 800 nm or 900 nm.

[0031] In the present invention, the dopant content in the lithium iron phosphate particles B is 0.1%-0.2%, for example, 0.18%.

[0032] In the present invention, the carbon content in the lithium iron phosphate particles C may be 1.0%-1.3%, for example, 1.1%, where the percentage is the percentage of the mass of the carbon element to the total mass of the lithium iron phosphate particles C.

[0033] In the present invention, the specific surface area of ​​the lithium iron phosphate particles C can be 8m 2 / g-12m 2 / g, for example 8m 2 / g、10m 2 / g or 12m 2 / g.

[0034] In the present invention, the average particle size of the primary particles C1 is preferably 1300-1500 nm, for example 1500 nm.

[0035] In the present invention, the dopant content in the lithium iron phosphate particles C may be 0.3%-0.5%, for example, 0.35%.

[0036] In the present invention, the preparation method of the lithium iron phosphate particles A can be conventional in the art, and preferably the lithium iron phosphate particles A are prepared by the following method:

[0037] S1. reacting the iron source with the phosphoric acid mixture I to obtain a first product after completion of the reaction;

[0038] reacting a mixture II of an organic acid, a lithium source, and a carbon source to obtain a second product after the reaction is completed; the order of preparing the first product and the second product is not limited;

[0039] S2. Grind and sinter the mixture of the first product and the second product to obtain lithium iron phosphate particles A.

[0040] The molar ratio of the iron element in the iron source to the phosphorus element in the phosphoric acid is preferably (0.95-1.01):1, for example 0.99:1.

[0041] The molar ratio of the lithium element in the lithium source to the phosphorus element in the phosphoric acid is preferably (1.00-1.05):1, for example, 1.02:1.

[0042] The amount of the carbon source is preferably 50%-100% of the mass of the iron source.

[0043] Wherein, at least one of the mixture I and the mixture II preferably further comprises a dopant; the molar ratio of the dopant to the phosphorus element in the phosphoric acid is (0-0.005):1, for example, 0.01:1.

[0044] The amount of the organic acid is preferably 70%-170% of the mass of the iron source, for example 118%.

[0045] The sintering temperature is preferably 650°C-700°C, for example 680°C.

[0046] The sintering time is preferably 10 h to 30 h, for example 10 h.

[0047] The average particle size of the slurry obtained by the grinding may be 50 nm-260 nm, for example, 200 nm.

[0048] In the present invention, the preparation method of the lithium iron phosphate particles B can be conventional in the art, and preferably includes the following steps:

[0049] S1. The mixture of an iron source and phosphoric acid is reacted I to obtain a first product after completion of the reaction; the mixture of an organic acid, a lithium source and a carbon source is reacted II to obtain a second product after completion of the reaction; the order of preparation of the first product and the second product is not limited;

[0050] wherein at least one of the mixture I and the mixture II further comprises a dopant;

[0051] S2. Grind and sinter the mixture of the first product and the second product to obtain lithium iron phosphate particles B.

[0052] The molar ratio of the iron element in the iron source to the phosphorus element in the phosphoric acid is preferably (0.97-10.1):1, for example, 0.99:1.

[0053] The molar ratio of the lithium element in the lithium source to the phosphorus element in the phosphoric acid is preferably (1.00-1.05):1, for example, 1.03:1.

[0054] The amount of the carbon source is preferably 30%-50% of the mass of the iron source.

[0055] The molar ratio of the dopant to the phosphorus element in the phosphoric acid is preferably (0.01-0.013):1, for example, 0.01:1.

[0056] The amount of the organic acid is preferably 50%-120% of the mass of the iron source, for example 118%.

[0057] The sintering temperature is preferably 700°C-740°C, for example 700°C.

[0058] The sintering time is preferably 15 hours to 20 hours, for example 15 hours.

[0059] The average particle size of the slurry obtained by the grinding is preferably 700 nm-1000 nm, for example 800 nm.

[0060] In the present invention, the preparation method of the lithium iron phosphate particles C can be conventional in the art, and preferably includes the following steps:

[0061] S1. The mixture of an iron source and phosphoric acid is reacted I to obtain a first product after completion of the reaction; the mixture of an organic acid, a lithium source and a carbon source is reacted II to obtain a second product after completion of the reaction; the order of preparation of the first product and the second product is not limited;

[0062] wherein at least one of the mixture I and the mixture II further comprises a dopant;

[0063] S2. Grind and sinter the mixture of the first product and the second product to obtain lithium iron phosphate particles A.

[0064] The molar ratio of the iron element in the iron source to the phosphorus element in the phosphoric acid is preferably (0.97-1.01):1, for example, 0.985:1.

[0065] The molar ratio of the lithium element in the lithium source to the phosphorus element in the phosphoric acid is preferably (1.00-1.05):1, for example, 1.035:1.

[0066] The molar ratio of the dopant to the phosphorus element in the phosphoric acid is preferably (0.02-0.05):1, for example, 0.03:1.

[0067] The amount of the organic acid is preferably 70%-120% of the mass of the iron source, for example 118%.

[0068] The sintering temperature is preferably 740°C-780°C, for example 740°C.

[0069] The sintering time is preferably 10 h to 30 h, for example 25 h.

[0070] The average particle size of the slurry obtained by the grinding may be 1300 nm-1500 nm, for example, 1500 nm.

[0071] The preparation method of the lithium iron phosphate particles of the present invention is to first mix the raw materials to form a non-lithium iron phosphate mixture, and then crystallize the lithium iron phosphate in a subsequent sintering process, which is beneficial to the control of the size of the lithium iron phosphate primary particles. The present invention prepares lithium iron phosphate particles of different primary particle sizes by adjusting the formula or sintering process, and mixes them according to a certain ratio. The powder compaction density can be significantly improved, while the electrical performance is basically unaffected. According to such a particle size distribution, the powder compaction density remains unchanged when the particle size distribution is shifted left and right. When the distribution curve shifts to the left, the electrical performance will be significantly improved. When the distribution curve moves to the right, the electrical performance will decrease. The position of the overall offset also needs to be comprehensively considered based on the processing performance.

[0072] In the preparation methods of the lithium iron phosphate particles A, lithium iron phosphate particles B and lithium iron phosphate particles C described above:

[0073] The iron source may be an iron-containing substance commonly used in the art, preferably iron powder and / or an iron-containing compound, and the iron-containing compound is preferably ferric oxide.

[0074] The lithium source may be a conventional lithium-containing compound in the art, preferably one or more of lithium hydroxide monohydrate, lithium carbonate, lithium dihydrogen phosphate, lithium phosphate and lithium acetate.

[0075] Among them, the use of the dopant can improve the electrical conductivity of the lithium iron phosphate particles themselves, and secondly, the use of the dopant can also reduce the particle size of the lithium iron phosphate particles; through the above two effects, the addition of the dopant can make the lithium iron phosphate particles have better electrical properties.

[0076] The organic acid may be an organic acid commonly used in the art, such as at least one of oxalic acid, acrylic acid, tartaric acid and malic acid.

[0077] The carbon source can be any conventional carbon-containing compound in the art, preferably glucose, glucose derivatives, organic acids, organic acid derivatives, phenolic resin, polyethylene, polyethylene glycol, polyvinyl alcohol, polyvinyl alcohol derivatives, polyacrylic acid, polyacrylic acid derivatives, heterocyclic polymers or polycondensates containing N or O elements.

[0078] The glucose derivative preferably includes at least one of glucose, sucrose, starch and cyclodextrin.

[0079] The organic acid preferably includes at least one of formic acid, acetic acid, oxalic acid, citric acid, tartaric acid and malic acid.

[0080] The polyacrylic acid derivative preferably includes polyacrylate.

[0081] The heterocyclic polymer containing N or O elements preferably includes polyvinyl pyrrolidone.

[0082] The present invention also provides a method for preparing the lithium iron phosphate material as described above, wherein the lithium iron phosphate particles A, lithium iron phosphate particles B and lithium iron phosphate particles C as described above are mixed to prepare the lithium iron phosphate material.

[0083] In the present invention, the preparation method of the lithium iron phosphate particles A can be as described above.

[0084] In the present invention, the preparation method of the lithium iron phosphate particles B can be as described above.

[0085] In the present invention, the preparation method of the lithium iron phosphate particles B can be as described above.

[0086] In the present invention, the mass ratio of the lithium iron phosphate particles A, the lithium iron phosphate particles B, and the lithium iron phosphate particles C can be as described above.

[0087] The present invention also provides the use of the lithium iron phosphate material as described above as a positive electrode active material in the field of lithium ion batteries.

[0088] On the basis of conforming to the common sense in this field, the above-mentioned preferred conditions can be arbitrarily combined to obtain the preferred embodiments of the present invention.

[0089] The reagents and raw materials used in the present invention are commercially available.

[0090] The positive progress effect of the present invention is:

[0091] The lithium iron phosphate material provided by the present invention can achieve a synergistic effect by combining particles of different particle sizes, and at the same time has excellent compaction density and electrical properties, and the processing performance can be maintained or improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0092] FIG1 is a SEM electron microscope image of the lithium iron phosphate material prepared in Example 1.

[0093] FIG2 is a TEM image of the lithium iron phosphate material prepared in Example 1.

[0094] FIG3 is a particle size distribution diagram of the lithium iron phosphate material prepared in Example 1. DETAILED DESCRIPTION

[0095] The present invention is further illustrated by way of examples below, but the present invention is not limited to the scope of the examples. Experimental methods in the following examples where specific conditions are not specified were performed according to conventional methods and conditions, or selected according to the product specifications.

[0096] Example 1

[0097] The proportions of the raw materials are shown in Table 1:

[0098] Table 1

[0099] (1) Preparation of lithium iron phosphate particles A:

[0100] S1. Add 5.17 kg of iron powder to 11 L of 85% phosphoric acid solution and react at 85°C. Some gas will be generated during the reaction, and the color of the reactant will gradually change from gray-black to gray-white. When no more gas is generated, the material is sand-milled in a sand mill with 0.3 mm zirconia beads. During the sand-milling process, the color gradually changes to pure white, yielding the first product.

[0101] S2. 3.50 kg of lithium carbonate, 0.52 kg of glucose and 0.020 kg of niobium oxide were gradually added to 10 L of a mixed solution of tartaric acid and malic acid (including 3.3 kg of tartaric acid and 2.8 kg of malic acid), and the mixture was reacted at 80°C. During the reaction, a large amount of gas was generated. The reaction was continued until no gas was generated and a transparent solution was formed to obtain the second product.

[0102] S3. Add the second product to the first product, mix and stir, and continue sand grinding. When the material particle size is ground to an average particle size of the primary particles A1 of about 200 nm, the reaction is completed to obtain the precursor of lithium iron phosphate particles A.

[0103] S4. The precursor of the lithium iron phosphate particles A was spray dried, sintered, and crushed to obtain lithium iron phosphate particles A having a particle size of D10 = 0.31 μm, D50 = 0.72 μm, and D90 = 2.1 μm. The carbon content of the lithium iron phosphate particles A was 1.9% and the specific surface area was 23 m 2 / g, and the content of niobium oxide is 0.005%.

[0104] The spray drying conditions were as follows: air inlet temperature was 280°C, outlet temperature was 110°C;

[0105] The sintering conditions are as follows: sintering temperature is 680°C and sintering time is 15h.

[0106] The crushing conditions are as follows: the crushing method is air flow crushing; the pressure of air flow crushing is 6 MPa; and the frequency of air flow crushing is 80 Hz.

[0107] (2) Preparation of lithium iron phosphate particles B

[0108] S1. Add 5.17 kg of iron powder to 11 L of 85% phosphoric acid solution and react at 85°C. Some gas will be generated during the reaction, and the color of the reactant will gradually change from gray-black to gray-white. When no more gas is generated, the material is sand-milled in a sand mill with 0.3 mm zirconia beads. During the sand-milling process, the color gradually changes to pure white, yielding the first product.

[0109] S2. 3.54 kg of lithium carbonate, 0.44 kg of glucose and 0.25 kg of niobium oxide were gradually added to a 10 L mixed solution of tartaric acid and malic acid (wherein, 3.3 kg of tartaric acid and 2.8 kg of malic acid), and reacted at 80°C. During the reaction, a large amount of gas was generated. The reaction was continued until no gas was generated and a transparent solution was formed to obtain the second product.

[0110] S3. Add the second product to the first product, mix and stir, and continue sand grinding. When the material particle size is ground to an average particle size of the primary particles B1 of about 800 nm, the reaction is completed to obtain the precursor of lithium iron phosphate particles B.

[0111] S4. The precursor of the lithium iron phosphate particles B was spray dried, sintered, and crushed to obtain lithium iron phosphate particles B having a particle size of D10 = 0.45 μm, D50 = 1.34 μm, and D90 = 6.5 μm. The carbon content of the lithium iron phosphate particles B was 1.5%, and the specific surface area was 13 m 2 / g, and the content of niobium oxide is 0.18%.

[0112] The spray drying conditions were as follows: air inlet temperature was 280°C, outlet temperature was 110°C;

[0113] The sintering conditions are as follows: sintering temperature is 720°C and sintering time is 15h.

[0114] The crushing conditions are as follows: the crushing method is air flow crushing; the pressure of air flow crushing is 6 MPa; and the frequency of air flow crushing is 65 Hz.

[0115] (3) Preparation of lithium iron phosphate particles C

[0116] S1. Add 5.17 kg of iron powder to 11 L of 85% phosphoric acid solution and react at 85°C. Some gas will be generated during the reaction, and the color of the reactant will gradually change from gray-black to gray-white. When no more gas is generated, the material is sand-milled in a sand mill with 0.3 mm zirconia beads. During the sand-milling process, the color gradually changes to pure white, yielding the first product.

[0117] S2. Gradually add 3.55 kg of lithium carbonate, 0.40 kg of glucose, and 0.50 kg of niobium oxide to a 10 L mixed solution of tartaric acid and malic acid (3.3 kg of tartaric acid and 2.8 kg of malic acid), and react at 80°C. During this reaction, a large amount of gas will be generated. The reaction is continued until no gas is generated and a transparent solution is formed to obtain the second product.

[0118] S3. Add the second product to the first product, mix and stir, and continue sand grinding. When the material particle size is ground to an average particle size of the primary particles C1 of about 1500 nm, the reaction is completed to obtain the precursor of lithium iron phosphate particles C.

[0119] S4. The precursor of the lithium iron phosphate particles C was spray dried, sintered, and crushed to obtain lithium iron phosphate particles C with a particle size of D10 = 0.58 μm, D50 = 1.57 μm, and D90 = 7.8 μm. The carbon content of the lithium iron phosphate particles C was 1.1% and the specific surface area was 10 m 2 / g, and the content of niobium oxide is 0.35%.

[0120] The spray drying conditions were as follows: air inlet temperature was 280°C, outlet temperature was 110°C;

[0121] The sintering conditions are as follows: sintering temperature is 750°C and sintering time is 20h.

[0122] The crushing conditions are as follows: the crushing method is air flow crushing; the pressure of air flow crushing is 6 MPa; and the frequency of air flow crushing is 60 Hz.

[0123] (4) Lithium iron phosphate material

[0124] The lithium iron phosphate particles A, lithium iron phosphate particles B and lithium iron phosphate particles C prepared in the above steps are mixed in a mass ratio of 5:2:3 by a mechanical mixing device to obtain the lithium iron phosphate material.

[0125] The lithium iron phosphate material prepared in this embodiment has a high compacted density, with the powder compacted density reaching 2.58 g / cm3. Furthermore, a synergistic effect can be achieved, significantly improving the product's electrical properties, processing performance, and other comprehensive properties.

[0126] Example 2

[0127] The difference from Example 1 is that the mass ratio of lithium iron phosphate particles A, lithium iron phosphate particles B and lithium iron phosphate particles C is 4:2:4.

[0128] Example 3

[0129] The difference from Example 1 is that the mass ratio of lithium iron phosphate particles A, lithium iron phosphate particles B and lithium iron phosphate particles C is 7:1:2.

[0130] Example 4

[0131] The difference from Example 1 is that the average particle size of the primary particles A1 is 50 nm, the average particle size of the primary particles B1 is 800 nm, and the average particle size of the primary particles C1 is 1500 nm.

[0132] Example 5

[0133] The difference from Example 1 is that the average particle size of the primary particles A1 is 300 nm, the average particle size of the primary particles B1 is 600 nm, and the average particle size of the primary particles C1 is 1200 nm.

[0134] Comparative Example 1

[0135] The only difference from Example 1 is that the mass ratio of lithium iron phosphate particles A, lithium iron phosphate particles B, and lithium iron phosphate particles C is 12:2:3.

[0136] Comparative Example 2

[0137] The only difference from Example 1 is that the mass ratio of lithium iron phosphate particles A, lithium iron phosphate particles B, and lithium iron phosphate particles C is 1:2:3.

[0138] Comparative Example 3

[0139] The only difference from Example 1 is that the mass ratio of lithium iron phosphate particles A, lithium iron phosphate particles B, and lithium iron phosphate particles C is 5:10:3.

[0140] Comparative Example 4

[0141] The only difference from Example 1 is that the mass ratio of lithium iron phosphate particles A, lithium iron phosphate particles B, and lithium iron phosphate particles C is 5:0.5:3.

[0142] Comparative Example 5

[0143] The only difference from Example 1 is that the mass ratio of lithium iron phosphate particles A, lithium iron phosphate particles B, and lithium iron phosphate particles C is 5:2:9.

[0144] Comparative Example 6

[0145] The only difference from Example 1 is that the mass ratio of lithium iron phosphate particles A, lithium iron phosphate particles B, and lithium iron phosphate particles C is 5:2:1.

[0146] Comparative Example 7

[0147] The difference from Example 1 is that the average particle diameter of the primary particles A1 is 400 nm, the average particle diameter of the primary particles B1 is 800 nm, and the average particle diameter of the primary particles B1 is 1500 nm.

[0148] Comparative Example 8

[0149] The difference from Example 1 is that the average particle size of the primary particles A1 is 300 nm, the average particle size of the primary particles B1 is 1200 nm, and the average particle size of the primary particles C1 is 1500 nm.

[0150] Comparative Example 9

[0151] The difference from Example 1 is that the average particle size of the primary particles A1 is 300 nm, the average particle size of the primary particles B1 is 800 nm, and the average particle size of the primary particles C1 is 1600 nm.

[0152] Effect Example 1

[0153] The composition of lithium iron phosphate material was characterized, and the test results are shown in Table 2:

[0154] (1) Carbon content: The carbon content of the lithium iron phosphate materials prepared in the examples and comparative examples was tested using a carbon-sulfur analyzer (manufacturer: Keguo Instruments, model: HCS-500).

[0155] (2) Iron content: The lithium iron phosphate materials prepared in the examples and comparative examples were tested using redox titration, with reference to the national standard GBT 33822-2017.

[0156] (3) Phosphorus content: The phosphorus content in the lithium iron phosphate materials prepared in the examples and comparative examples was tested using the quinoline molybdate weight method, with reference to the national standard GBT 33822-2017.

[0157] (4) Dopant content: The dopant content in the lithium iron phosphate materials prepared in the examples and comparative examples was tested using inductively coupled atomic emission spectroscopy (ICP-OES) (manufacturer: Thermo Fisher, model: ICPAPRO).

[0158] (5) pH test method: The lithium iron phosphate material prepared in the embodiment and the comparative example was mixed with deionized water at a mass ratio of 1:9 and stirred for 30 minutes. The supernatant was taken for testing using a pH meter (manufacturer: Mettler, model: SD50).

[0159] (6) Powder resistivity test method: The powder resistivity of the lithium iron phosphate materials prepared in the examples and comparative examples was tested using a powder resistivity tester (manufacturer: Suzhou Jingge Electronics Co., Ltd. Model: ST2742B)

[0160] (7) Particle size: Equipment model: Malvern 3000

[0161] (8) Specific surface area: Equipment model: TB400

[0162] Table 2

[0163] Effect Example 2

[0164] The electrical properties of lithium iron phosphate materials were characterized, and the test results are shown in Table 3:

[0165] The specific preparation method of button battery is as follows:

[0166] 1) Positive electrode production

[0167] The lithium iron phosphate material, conductive carbon black, and polyvinylidene fluoride from Example 1 and Comparative Examples 1-6 were dissolved in an NMP solution at a mass ratio of 90:5:5 and stirred in a vacuum mixer for 3 hours. The solid content of the slurry was controlled to 50% to prepare a positive electrode slurry. The positive electrode slurry was evenly coated on aluminum foil and then dried in a vacuum drying oven at 120°C for 12 hours. After drying, the slurry was punched into 12 mm diameter discs to serve as positive electrode sheets.

[0168] 2) Production of button batteries

[0169] A lithium metal sheet was used as the negative electrode, a Celgard 2400 microporous membrane as the separator, and a 1.0 mol / L LiPF6 solution as the electrolyte. The solvent was a 1:1:1 volume ratio of ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). The positive electrode sheet, negative electrode sheet, separator, and electrolyte were assembled into a CR2016 button cell in an argon-filled glove box.

[0170] The electrical performance of the button cell produced using the above method was tested. During the test, the cutoff voltage during the charging process was 3.75V, and the cutoff voltage during the discharging process was 2.0V.

[0171] Charge at a constant current of 0.1C to a voltage of 3.75V, then switch to constant voltage charging to a current of 0.02C, and discharge at a constant current of 0.1C to a voltage of 2.0V. Take the 0.1C charging specific capacity as the test result.

[0172] Charge at a constant current of 1C to a voltage of 3.75V, then switch to constant voltage charging to a current of 0.02C, and discharge at a constant current of 1C to a voltage of 2.0V. Take the 1C charging specific capacity as the test result.

[0173] Table 3

[0174] According to the results in Table 2 and Table 3, it can be seen that the lithium iron phosphate material of the present invention has both good compaction density and discharge specific capacity, and also has a lower powder resistivity. Research and development found that the powder compaction density is mainly affected by the particle size, and is also affected by other factors such as carbon content, the shape of the primary particles, and whether the surface is smooth. The present invention has found through research that when the particle size is compounded according to a certain size and distribution, the powder compaction density can be significantly improved. Large particle sizes often have lower powder compaction density and poor electrical properties, so adding a certain amount of lithium or dopant can improve these shortcomings. The added lithium will cause the finished product to easily absorb water. By compounding according to a certain ratio, the defect of the mixture being easily absorbed by water can be overcome.

[0175] The difference between Comparative Examples 1 and 2 and Example 1 is that the amount of lithium iron phosphate particles A used is not within the scope of the present invention. It can be seen that, under the same conditions, if the proportion of lithium iron phosphate particles A is too large or too small, the technical effects of the present invention cannot be achieved. If the proportion of lithium iron phosphate particles A is too large, although the electrical performance is better, the compaction density is reduced; if the proportion is too small, both the compaction density and electrical performance are reduced. Smaller particles have a larger specific surface area, and increasing the proportion of lithium iron phosphate particles A is beneficial to increasing the specific surface area.

[0176] The difference between Comparative Examples 3 and 4 and Example 1 is that the amount of lithium iron phosphate particles B is not within the scope of the present invention. It can be seen that, under the same conditions, the technical effects of the present invention cannot be achieved if the proportion of lithium iron phosphate particles B is too large or too small. If the proportion of lithium iron phosphate particles B is too large, the compaction density and electrical performance will be reduced; if the proportion is too small, although the electrical performance is better, the compaction density will be reduced.

[0177] The difference between Comparative Examples 5 and 6 and Example 1 is that the amount of lithium iron phosphate particles C used is not within the scope of the present invention. It can be seen that, under the same conditions, the technical effects of the present invention cannot be achieved if the proportion of lithium iron phosphate particles C is too large or too small. If the proportion of lithium iron phosphate particles C is too large, the compaction density and electrical performance are reduced; if the proportion is too small, although the electrical performance is good, the compaction density is reduced.

[0178] Comparative Examples 3-6 all show that when the electrical properties are good, the powder compaction density is obviously too small. When the electrical properties are too small, the powder compaction density is improved, but not significantly, and the overall performance is poor.

[0179] Compared with Example 1, the difference between Comparative Examples 7-9 is that the particle sizes of the three particles are too large and the particle size compounding is inappropriate, resulting in a significant decrease in electrical properties and powder compaction density, and the larger the particle size, the worse the electrical properties.

[0180] Examples 1-3 used different mass ratios to prepare lithium iron phosphate materials, all of which achieved good electrical properties and compaction density. The 0.1C discharge capacity was 159.4 mAh / g or above, the 1C discharge capacity was 144.3 mAh / g or above, and the compaction density was also 2.53 g / cm 3 and above.

[0181] Examples 1, 4, and 5 used a compounding of particles of different particle sizes, and also achieved good electrical properties and compaction density.

[0182] FIG1 is a SEM electron microscope image of the lithium iron phosphate material prepared in Example 1, and FIG2 is a TEM image of the lithium iron phosphate material prepared in Example 1. Both images show that the lithium iron phosphate material contains particles of different sizes.

[0183] Figure 3 is a particle size distribution diagram of the lithium iron phosphate material prepared in Example 1. The particle size distribution of the particles in Example 1 shows that the particle size of the lithium iron phosphate material exhibits a multi-peak pattern, with a significantly higher proportion of small particles, which is conducive to providing good electrical performance. At the same time, the distribution of the three peaks satisfies a certain particle size relationship, enabling good grading and resulting in a high compaction density of the obtained material.

[0184] Although the above describes specific embodiments of the present invention, it should be understood by those skilled in the art that these are merely illustrative and that various changes or modifications may be made to these embodiments without departing from the principles and essence of the present invention. Therefore, the scope of protection of the present invention is defined by the appended claims.

Claims

1. A lithium iron phosphate material, characterized in that, It includes lithium iron phosphate particles A, lithium iron phosphate particles B and lithium iron phosphate particles C; among them, The lithium iron phosphate particles A are formed by agglomeration of primary particles A1, and the average particle size of the primary particles A1 is 50nm - 300nm; The lithium iron phosphate particles B are formed by agglomeration of primary particles B1, the average particle size of the primary particles B1 is 600nm - 1000nm, and in the lithium iron phosphate particles B, the content of the dopant is 0.05 - 0.2%; The lithium iron phosphate particles C are formed by agglomeration of primary particles C1, the average particle size of the primary particles C1 is 1200nm - 1500nm; in the lithium iron phosphate particles C, the content of the dopant is 0.2 - 0.5%; Among them, the dopant is at least one of titanium compounds, vanadium compounds, strontium compounds and niobium compounds; the content of the dopant is the atomic ratio of the dopant to the lithium iron phosphate particles B and the lithium iron phosphate particles C respectively; The mass ratio of the lithium iron phosphate particles A, the lithium iron phosphate particles B and the lithium iron phosphate particles C is (4 - 7):(1 - 2):(2 - 4).

2. The lithium iron phosphate material according to claim 1, wherein The mass ratio of the lithium iron phosphate particles A, the lithium iron phosphate particles B and the lithium iron phosphate particles C is (4 - 6.5):(1.5 - 2):(2 - 3.5), such as 5:2:3; And / or, the D10 particle size of the lithium iron phosphate material is ≥0.35μm, such as 0.35μm, 0.36μm, 0.40μm, 0.45μm or 0.51μm; the D50 particle size is 0.5 - 1.5μm, such as 1.09μm, 1.12μm, 1.17μm, 1.23μm or 1.36μm; the D99 particle size is ≤25μm, such as 5.98μm, 5.65μm, 5.37μm, 5.08 or 4.97μm; And / or, the specific surface area of the lithium iron phosphate material is 10-20 m 2 / g, such as 12.74 m 2 / g, 10.80 m 2 / g, 13.92 m 2 / g, 13.41 m 2 / g or 11.78 m 2 / g; And / or, the niobium compound is niobium oxide.

3. The lithium iron phosphate material according to claim 1 or 2, characterized in that, The element content of the lithium iron phosphate material satisfies one or more of the following conditions a - d: a. The carbon content of the lithium iron phosphate material is 1.0% - 2.0%, preferably 1.5% - 2.0%, such as 1.57%, 1.51%, 1.70%, 1.61% or 1.51%, and the percentage is the mass percentage of carbon element in the total mass of the lithium iron phosphate material; b. The iron content of the lithium iron phosphate material is 33% - 35%, such as 33.54%, 33.43%, 33.52%, 33.47% or 33.64%, and the percentage is the mass percentage of iron element in the total mass of the lithium iron phosphate material; c. The phosphorus content of the lithium iron phosphate material is 18% - 20%, such as 19.84%, 19.87% or 19.72%, and the percentage is the mass percentage of phosphorus element in the total mass of the lithium iron phosphate material; d. The content of the doped element in the lithium iron phosphate material is 0.005 - 0.03%, such as 0.085%, 0.0104%, 0.0057%, 0.0085% or 0.0090%, and the percentage is the atomic percentage of the dopant in the lithium iron phosphate material.

4. The lithium iron phosphate material according to any one of claims 1-3, characterized in that, The lithium iron phosphate particle A satisfies one or more of the following conditions e - h: e. The carbon content in the lithium iron phosphate particle A is 1.6% - 2.0%, for example 1.9%, and the percentage is the mass percentage of carbon element in the total mass of the lithium iron phosphate particle A; f. The specific surface area of the lithium iron phosphate particles A is 15 m 2 / g - 25 m 2 / g, such as 17 m 2 / g, 20 m 2 / g or 23 m 2 / g; g. The average particle size of the primary particle A1 is 50 - 260 nm, for example 200 nm; h. The content of the dopant in the lithium iron phosphate particle A is 0 - 0.005%, for example 0% or 0.005%; And / or, the lithium iron phosphate particle B satisfies one or more of the following conditions i - l: i. The carbon content in the lithium iron phosphate particle B is 1.3% - 1.6%, for example 1.5%, and the percentage is the mass percentage of carbon element in the total mass of the lithium iron phosphate particle B; j. The specific surface area of the lithium iron phosphate particles B is 10 m 2 / g - 15 m 2 / g, for example 11 m 2 / g, 13 m 2 / g or 15 m 2 / g; k. The average particle size of the primary particle B1 is 700 - 1000 nm, for example 800 nm or 900 nm; l. The dopant content in the lithium iron phosphate particle B is 0.1% - 0.2%, for example 0.18%; And / or, the lithium iron phosphate particle C satisfies one or more of the following conditions m - p: m. The carbon content in the lithium iron phosphate particle C is 1.0% - 1.3%, for example 1.1%, and the percentage is the mass percentage of carbon element in the total mass of the lithium iron phosphate particle C; n. The specific surface area of the lithium iron phosphate particles C is 8 m 2 / g - 12 m 2 / g, for example 8 m 2 / g, 10 m 2 / g or 12 m 2 / g; o. The average particle size of the primary particle C1 is 1300 - 1500 nm, for example 1500 nm; p. The dopant content in the lithium iron phosphate particle C is 0.3% - 0.5%, for example 0.35%.

5. A method for preparing a lithium iron phosphate material as described in any one of claims 1-4, characterized in that, Mix the lithium iron phosphate particle A, the lithium iron phosphate particle B, and the lithium iron phosphate particle C as described in any one of claims 1 - 4 to obtain the lithium iron phosphate material.

6. The preparation method of the lithium iron phosphate material according to claim 5, wherein, The preparation methods of the lithium iron phosphate particle A, the lithium iron phosphate particle B, and the lithium iron phosphate particle C all include the following steps: S1. React the mixture I of the iron source and phosphoric acid, and obtain a first product after the reaction is completed; React the mixture II of the organic acid, the lithium source, and the carbon source, and obtain a second product after the reaction is completed; The first product The order of preparation of the first product and the second product is not limited; S2. Grind and sinter the mixture of the first product and the second product to obtain the lithium iron phosphate particle A, the lithium iron phosphate particle B, or the lithium iron phosphate particle C.

7. The preparation method of the lithium iron phosphate material according to claim 5 or 6, characterized in that When preparing the lithium iron phosphate particle A, the preparation method satisfies one or more of the following conditions (1) - (7): (1) The molar ratio of iron element in the iron source to phosphorus element in the phosphoric acid is (0.95 - 1.01):1, for example 0.99:1; (2) The molar ratio of lithium element in the lithium source to phosphorus element in the phosphoric acid is (1.00 - 1.05):1, for example 1.02:1; (3) At least one of the mixture I and the mixture II further includes a dopant; the molar ratio of the dopant to phosphorus element in the phosphoric acid is (0 - 0.005):1, for example 0.01:1; the dopant is preferably at least one of titanium compounds, vanadium compounds, strontium compounds, and niobium compounds, for example niobium oxide; (4) The dosage of the organic acid is 70%-170% of the mass of the iron source, such as 118%; (5) The sintering temperature is 650°C-700°C, such as 680°C; (6) The sintering time is 10h-30h, such as 10h; (7) The average particle size of the slurry obtained by the grinding is 50nm-260nm, such as 200nm.

8. The preparation method of the lithium iron phosphate material according to any one of claims 5-7, characterized in that, When preparing the lithium iron phosphate particle B, the preparation method satisfies one or more of the following conditions ①-⑦ ① The molar ratio of iron element in the iron source to phosphorus element in the phosphoric acid is (0.97-1.01):1, such as 0.99:1; ② The molar ratio of lithium element in the lithium source to phosphorus element in the phosphoric acid is (1.00-1.05):1, such as 1.03:1; ③ The molar ratio of the dopant to phosphorus element in the phosphoric acid is (0.01-0.013):1, such as 0.01:1; ④ The dosage of the organic acid is 50%-120% of the mass of the iron source; such as 118%; ⑤ The sintering temperature is 700°C-740°C, such as 700°C; ⑥ The sintering time is 15h-20h, such as 15h; ⑦ The average particle size of the slurry obtained by the grinding is 700nm-1000nm, such as 800nm.

9. The preparation method of the lithium iron phosphate material according to any one of claims 5-8, characterized in that, When preparing the lithium iron phosphate particle C, the preparation method satisfies one or more of the following conditions 1)-7): 1) The molar ratio of iron element in the iron source to phosphorus element in the phosphoric acid is (0.97-1.01):1, such as 0.985:1; 2) The molar ratio of lithium element in the lithium source to phosphorus element in the phosphoric acid is (1.00-1.05):1, such as 1.035:1; 3) The molar ratio of the dopant to phosphorus element in the phosphoric acid is (0.02-0.05):1, such as 0.03:1; 4) The dosage of the organic acid accounts for 70%-120% of the mass of the iron source, such as 118%; 5) The sintering temperature is 740°C-780°C, such as 740°C; 6) The sintering time is 15h-30h, such as 25h; 7) The average particle size of the slurry obtained by the grinding is 1300nm-1500nm, such as 1500nm.

10. The application of the lithium iron phosphate material as claimed in any one of claims 1-4 as a cathode active material in the field of lithium ion batteries.

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