A lithium iron phosphate material, a preparation method thereof and an application thereof

Abstract

The present invention provides a lithium iron phosphate material, a preparation method thereof and an application. The preparation method comprises the following steps: adding iron powder and a reducing agent to phosphoric acid to carry out a first reaction to obtain a mixed solution; then adding an iron oxide and a lithium source to the mixed solution to carry out a second reaction to obtain a lithium iron phosphate precursor; and performing a sintering treatment on the lithium iron phosphate precursor to obtain a lithium iron phosphate material. Among them, the mixed solution comprises ferrous dihydrogen phosphate and phosphoric acid, and the lithium iron phosphate precursor comprises lithium dihydrogen phosphate, iron hydrogen phosphate and an iron oxide. The iron hydrogen phosphate and the iron oxide have different morphologies, realizing the mixing of large and small particles and forming a more compact packing structure, so that the lithium iron phosphate material has both a high tap density and a high rate performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery materials, and in particular relates to a lithium iron phosphate material and a preparation method and application thereof. Background Art

[0002] Lithium iron phosphate materials with olivine crystal structure have the advantages of high gram capacity, long cycle life, good safety performance, low price, non-toxic and pollution-free, etc., and are widely used in 3C digital, two-wheeled electric bicycles, electric vehicles, energy storage, batteries and other fields. However, the compaction density of existing lithium iron phosphate materials is low, which limits its application in the battery field.

[0003] At present, the compaction density of lithium iron phosphate materials is mainly improved by increasing the size of the primary particles of lithium iron phosphate or mixing large and small primary particles. However, increasing the size of the primary particles of lithium iron phosphate will lengthen the lithium ion deintercalation path, resulting in a decrease in the rate performance of the lithium iron phosphate material, and thus a decrease in the rate performance of the battery. The mixing of large and small primary particles is generally achieved by mixing particles of different sizes or mixing iron phosphate precursors of different particle sizes through a sand milling process to form a tighter stacking structure and increase the compaction density of the lithium iron phosphate material. However, this method increases the complexity of the production process and the control points of the production process, resulting in large fluctuations between batches and a decrease in the rate performance of the lithium iron phosphate material.

[0004] Therefore, how to provide a preparation method for lithium iron phosphate material having both high compaction density and high rate performance is a technical problem to be solved urgently in this field. Summary of the invention

[0005] The present invention provides a method for preparing a lithium iron phosphate material. The lithium iron phosphate material prepared by the preparation method has both high compaction density and high rate performance. Meanwhile, the preparation method has a simple process and can realize efficient industrial production.

[0006] The present invention also provides a lithium iron phosphate material, which has both high compaction density and high rate performance.

[0007] The present invention also provides a positive electrode sheet, which has both high compaction density and high rate performance.

[0008] The present invention also provides a battery having high rate performance and excellent cycle performance.

[0009] In a first aspect, the present invention provides a method for preparing a lithium iron phosphate material, comprising the following steps:

[0010] (1) adding iron powder and a reducing agent to phosphoric acid to perform a first reaction to obtain a mixed solution;

[0011] (2) Add iron oxide and lithium source to the mixed solution and conduct a second reaction to obtain a lithium iron phosphate precursor;

[0012] (3) Sinter the lithium iron phosphate precursor to obtain the lithium iron phosphate material;

[0013] The mixed solution includes iron dihydrogen phosphate and phosphoric acid;

[0014] The lithium iron phosphate precursor includes lithium dihydrogen phosphate, iron hydrogen phosphate and iron oxide.

[0015] The preparation method as described above, wherein the molar ratio of the phosphoric acid to the iron powder is z:1, where 2 < z ≤ 6; and / or,

[0016] The molar ratio of the reducing agent to the iron powder is (0.008 - 0.013):1.

[0017] The preparation method as described above, wherein the average primary particle size of the iron oxide is 0.05 μm - 0.5 μm; and / or,

[0018] The iron oxide includes iron(III) oxide and / or iron(II,III) oxide;

[0019] When the iron oxide includes iron(III) oxide, the molar ratio of iron(III) oxide to iron powder is x1:y1, where 0.1 ≤ y1 ≤ 0.5 and 0.25 ≤ x1 ≤ 0.45;

[0020] When the iron oxide includes iron(II,III) oxide, the molar ratio of iron(II,III) oxide to iron powder is x2:y2, where 0.1 ≤ y2 ≤ 0.5 and 0.17 ≤ x2 ≤ 0.30.

[0021] The preparation method as described above, wherein the process of adding iron powder and reducing agent to phosphoric acid to conduct the first reaction to obtain a mixed solution includes: adding iron powder and reducing agent to a phosphoric acid solution to conduct the first reaction to obtain a mixed solution;

[0022] The mass concentration of the phosphoric acid solution is 30 wt% - 80 wt%; and / or,

[0023] The reducing agent includes one or more of ascorbic acid, citric acid, glucose; and / or,

[0024] The lithium source includes one or more of lithium carbonate, lithium hydroxide, lithium oxalate, lithium acetate.

[0025] The preparation method as described above, wherein the temperature of the first reaction is 70 - 90 °C and the time of the first reaction is 6 - 12 h; and / or,

[0026] The temperature of the second reaction is 20 - 30°C, and the time of the second reaction is 60 min - 90 min; and / or,

[0027] The process of the sintering treatment includes a first sintering treatment and a second sintering treatment, and the temperature of the first sintering treatment is higher than that of the second sintering treatment;

[0028] In the first sintering treatment, the sintering temperature is 790°C - 830°C, and the sintering time is 1 - 5 h;

[0029] In the second sintering treatment, the sintering temperature is 730°C - 790°C, and the sintering time is 8 - 10 h.

[0030] In the preparation method as described above, wherein the process of adding the iron oxide and the lithium source to the mixed solution and performing the second reaction to obtain the lithium iron phosphate precursor includes: first adding the iron oxide to the mixed solution, stirring to form a suspension, and then adding the lithium source to the suspension to perform the second reaction to obtain the lithium iron phosphate precursor; and / or,

[0031] The process of sintering the lithium iron phosphate precursor to obtain the lithium iron phosphate material further includes: mixing the lithium iron phosphate precursor obtained in step (2) with a carbon source and an additive, and performing a grinding treatment to obtain a grinding slurry, and then sintering the grinding slurry to obtain the lithium iron phosphate material.

[0032] In the preparation method as described above, wherein the mass ratio of the carbon source to the lithium iron phosphate precursor is (0.1 - 0.15):1; and / or,

[0033] The mass ratio of the additive to the lithium iron phosphate precursor is (0.001 - 0.008):1; and / or,

[0034] The carbon source includes one or more of glucose, sucrose, dextrin, citric acid, fructose, polyethylene glycol, polyvinyl alcohol, and polypropylene glycol; and / or,

[0035] The additive includes a compound containing one or more elements of Al, Mg, Ni, Co, Ti, Cu, Ca, Nb, Cr, Zn, La, Sb, Te, Sr, W, In, and Y; and / or,

[0036] The volume average particle size Dv50 of the grinding slurry is 0.2 μm - 0.6 μm.

[0037] In a second aspect, the present invention provides a lithium iron phosphate material obtained by the preparation method of the first aspect.

[0038] In a third aspect, the present invention provides a positive electrode sheet including the lithium iron phosphate material of the second aspect.

[0039] In a fourth aspect, the present invention provides a battery, comprising the positive electrode sheet described in the third aspect.

[0040] The lithium iron phosphate material prepared by the above method of the present invention has a high tap density and high rate performance, thereby improving the rate performance and cycle performance of the battery. Among them, the lithium iron phosphate precursor includes lithium dihydrogen phosphate, iron hydrogen phosphate and iron oxide. The iron hydrogen phosphate and iron oxide have different morphologies, realizing the mixing of large and small particles, forming a more compact packing structure, and endowing the lithium iron phosphate material with both high tap density and high rate performance. Description of the Drawings

[0041] Figure 1 SEM image of the lithium iron phosphate precursor of Example 1 of the present invention;

[0042] Figure 2 SEM image of the lithium iron phosphate material of Example 1 of the present invention;

[0043] Figure 3 Charge and discharge test chart of the coin cell assembled with the lithium iron phosphate material of Example 1 of the present invention at rates of 0.5C, 1C and 4C in the voltage range of 2.0V to 3.75V. Detailed Embodiments

[0044] To enable those skilled in the art to better understand the solution of the present invention, the present invention will be further described in detail below. The following specific embodiments are only used to describe the principles and features of the present invention, and the examples given are only used to explain the present invention, not to limit the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0045] In a first aspect, the present invention provides a method for preparing a lithium iron phosphate material, comprising the following steps:

[0046] (1) Adding iron powder and a reducing agent to phosphoric acid, and performing a first reaction to obtain a mixed solution;

[0047] (2) Adding iron oxide and a lithium source to the mixed solution, and performing a second reaction to obtain a lithium iron phosphate precursor;

[0048] (3) Performing a sintering treatment on the lithium iron phosphate precursor to obtain a lithium iron phosphate material;

[0049] The mixed solution includes iron dihydrogen phosphate and phosphoric acid;

[0050] The lithium iron phosphate precursor includes lithium dihydrogen phosphate, iron hydrogen phosphate and iron oxide.

[0051] In the present invention, first, iron powder, a reducing agent, and phosphoric acid are subjected to a first reaction to generate a mixed solution (dark green solution) including iron dihydrogen phosphate and phosphoric acid. The reducing agent can prevent the oxidation of iron, improve the reaction efficiency, and ensure the production of high-purity iron dihydrogen phosphate. Then, iron oxide and a lithium source are added to the mixed solution, and a second reaction occurs between the iron dihydrogen phosphate and the lithium source to obtain a lithium iron phosphate precursor containing lithium dihydrogen phosphate, iron hydrogen phosphate, and iron oxide. The iron hydrogen phosphate and the iron oxide are lithium iron phosphate precursors with different morphologies, achieving the mixing of large and small particles, forming a more compact packing structure, and improving the tap density and rate performance of the material. Subsequently, the lithium iron phosphate precursor is sintered to form a lithium iron phosphate material.

[0052] In some embodiments, in step (1), the process of adding iron powder and a reducing agent to phosphoric acid for the first reaction to obtain a mixed solution includes: slowly adding the iron powder and the reducing agent to phosphoric acid for the first reaction to obtain a mixed solution.

[0053] In some embodiments, in step (2), the iron oxide and the lithium source are added to the mixed solution, and a second reaction is carried out under stirring conditions to obtain a lithium iron phosphate precursor.

[0054] In some embodiments, in step (3), the process of sintering the lithium iron phosphate precursor further includes: sequentially performing spray drying treatment, sintering treatment, and pulverization drying treatment on the lithium iron phosphate precursor to obtain a lithium iron phosphate material. Among them, the spray drying treatment can rapidly evaporate the liquid in the lithium iron phosphate precursor to obtain lithium iron phosphate precursor particles with a uniform particle size distribution, and the pulverization drying treatment can further reduce the particle size of the lithium iron phosphate material.

[0055] The spray drying treatment and the pulverization drying treatment of the present invention are both conventional treatments in the art and are not particularly limited.

[0056] Specifically, the sintering treatment can be carried out in a roller hearth kiln.

[0057] In some embodiments, the molar ratio of phosphoric acid (here phosphoric acid refers to the phosphoric acid solute) to iron powder is z:1, where 2 < z ≤ 6, so that the phosphoric acid can fully react with the iron powder and generate iron dihydrogen phosphate.

[0058] Exemplarily, the molar ratio z:1 of phosphoric acid to iron powder can be 2.02:1, 2.5:1, 3:1, 3.5:1, 4:1, 4.5:1, 5:1, 5.5:1, or 6:1, etc.

[0059] In some embodiments, the molar ratio of the reducing agent to the iron powder is (0.008 - 0.013):1, which can better prevent the oxidation of iron and promote the reaction between the iron powder and the phosphoric acid to generate high-purity iron dihydrogen phosphate.

[0060] Exemplarily, the molar ratio of the reducing agent to the iron powder can be 0.008:1, 0.009:1, 0.01:1, 0.011:1, 0.012:1, 0.013:1, etc.

[0061] According to the technical solution of the present invention, the average primary particle size of the iron oxide is 0.05 μm - 0.5 μm, which helps the iron oxide to be evenly distributed in the mixed solution to form a stable suspension, and has a relatively high specific surface area, which helps to shorten the diffusion path of lithium ions and improve the rate performance of the lithium iron phosphate material.

[0062] Exemplarily, the average primary particle size of the iron oxide is 0.05 μm, 0.1 μm, 0.15 μm, 0.2 μm, 0.25 μm, 0.3 μm, 0.35 μm, 0.4 μm, 0.45 μm or 0.5 μm, that is, nano iron oxide.

[0063] In the present invention, the iron oxide includes iron(III) oxide and / or iron(II,III) oxide, that is, the iron oxide may only include iron(III) oxide, may only include iron(II,III) oxide, or may also be a combination of iron(III) oxide and iron(II,III) oxide.

[0064] In some embodiments, when the iron oxide includes iron(III) oxide, the molar ratio of iron(III) oxide to the iron powder is x1:y1, where 0.1 ≤ y1 ≤ 0.5 and 0.25 ≤ x1 ≤ 0.45.

[0065] Exemplarily, the molar ratio of iron(III) oxide to the iron powder is x1:y1. y1 can be 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, etc., and x1 can be 0.25, 0.3, 0.35, 0.4, 0.45, etc.

[0066] In some embodiments, when the iron oxide includes iron(II,III) oxide, the molar ratio of iron(II,III) oxide to the iron powder is x2:y2, where 0.1 ≤ y2 ≤ 0.5 and 0.17 ≤ x2 ≤ 0.30.

[0067] Exemplarily, the molar ratio of iron(II,III) oxide to the iron powder is x2:y2. y2 can be 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, etc., and x2 can be 0.17, 0.19, 0.21, 0.23, 0.25, 0.27, 0.29, 0.3, etc.

[0068] In some embodiments, the process of adding iron powder and a reducing agent to phosphoric acid to carry out a first reaction to obtain a mixed solution includes: adding iron powder and a reducing agent to a phosphoric acid solution to carry out a first reaction to obtain a mixed solution.

[0069] In some embodiments, the mass concentration of the phosphoric acid solution is 30wt%-80wt%, which helps to ensure the completeness of the first reaction, reduce the generation of by-products, and improve the efficiency of the first reaction.

[0070] Exemplarily, the mass concentration of the phosphoric acid solution can be 30wt%, 40wt%, 50wt%, 60wt%, 70wt% or 80wt% etc.

[0071] In some embodiments, the reducing agent includes one or more of ascorbic acid, citric acid, and glucose, which enables the lithium iron phosphate material to have better electrochemical performance.

[0072] In some embodiments, the lithium source includes one or more of lithium carbonate, lithium hydroxide, lithium oxalate, and lithium acetate.

[0073] In the present invention, the temperature of the first reaction is 70-90°C, and the time of the first reaction is 6-12h, which can improve the efficiency of the first reaction while saving energy consumption and time costs.

[0074] Exemplarily, the temperature of the first reaction can be 70°C, 75°C, 80°C, 85°C or 90°C etc.

[0075] Exemplarily, the time of the first reaction can be 6h, 7h, 8h, 9h, 10h, 11h or 12h etc.

[0076] In some embodiments, the temperature of the second reaction is 20-30°C, and the time of the second reaction is 60min-90min, which can improve the efficiency of the second reaction while saving energy consumption and time costs, and reduce unnecessary side reactions.

[0077] Exemplarily, the temperature of the second reaction can be 21°C, 23°C, 25°C, 27°C or 30°C etc.

[0078] Exemplarily, the time of the second reaction can be 60min, 65min, 70min, 75min, 80min, 85min or 90min etc.

[0079] In some embodiments, the process of the sintering treatment includes a first sintering treatment and a second sintering treatment. The temperature of the first sintering treatment is higher than that of the second sintering treatment. In the first sintering treatment, the sintering temperature is 790°C-830°C, and the sintering time is 1-5h. In the second sintering treatment, the sintering temperature is 730°C-790°C, and the sintering time is 8-10h, which can improve the crystallinity of the lithium iron phosphate material, promote the growth of grains and the completion of phase transformation.

[0080] Exemplarily, in the first sintering process, the sintering temperature can be 790°C, 800°C, 810°C, 820°C, 830°C, etc., and the sintering time can be 1 h, 2 h, 3 h, 4 h, 5 h, etc.

[0081] Exemplarily, in the second sintering process, the sintering temperature can be 730°C, 740°C, 750°C, 760°C, 770°C, 780°C, 790°C, etc., and the sintering time can be 8 h, 9 h, 10 h, etc.

[0082] In some embodiments, the sintering process is carried out in an inert atmosphere, which can effectively prevent oxidation and maintain the chemical stability of the lithium iron phosphate material. Among them, the inert atmosphere includes one of nitrogen, argon, and neon.

[0083] In some embodiments, the process of adding iron oxide and lithium source to the mixed solution and carrying out the second reaction to obtain the lithium iron phosphate precursor includes: adding iron oxide to the mixed solution, stirring to form a suspension, then adding the lithium source to the suspension, and carrying out the second reaction to obtain the lithium iron phosphate precursor.

[0084] In some embodiments, the process of sintering the lithium iron phosphate precursor to obtain the lithium iron phosphate material further includes: mixing the lithium iron phosphate precursor obtained in step (2) with a carbon source and an additive, and carrying out a grinding process to obtain a grinding slurry, and then sequentially carrying out a drying process and a sintering process on the grinding slurry to obtain the lithium iron phosphate material. The carbon source and the additive can improve the conductivity and electrochemical performance of the lithium iron phosphate material.

[0085] By adjusting the mass ratio of the carbon source to the lithium iron phosphate precursor (referring to the solid component in the lithium iron phosphate precursor), and the mass ratio of the additive to the lithium iron phosphate precursor (referring to the solid component in the lithium iron phosphate precursor), the conductivity and electrochemical performance of the lithium iron phosphate material are further improved. In some embodiments, the mass ratio of the carbon source to the lithium iron phosphate precursor is (0.1 - 0.15):1, and the mass ratio of the additive to the lithium iron phosphate precursor is (0.001 - 0.008):1.

[0086] Exemplarily, the mass ratio of the carbon source to the lithium iron phosphate precursor can be 0.1:1, 0.11:1, 0.12:1, 0.13:1, 0.14:1, 0.15:1, etc.

[0087] Exemplarily, the mass ratio of the additive to the lithium iron phosphate precursor can be 0.001:1, 0.002:1, 0.003:1, 0.004:1, 0.005:1, 0.006:1, 0.007:1, 0.008:1, etc.

[0088] In some embodiments, the carbon source includes one or more of glucose, sucrose, dextrin, citric acid, fructose, polyethylene glycol, polyvinyl alcohol, and polypropylene glycol, enabling the lithium iron phosphate material to have better electrochemical performance.

[0089] In some embodiments, the additive includes a compound containing one or more elements of Al, Mg, Ni, Co, Ti, Cu, Ca, Nb, Cr, Zn, La, Sb, Te, Sr, W, In, and Y. The additive includes oxides containing the above elements. The oxides may include aluminum oxide, magnesium oxide, titanium oxide, zinc oxide, calcium oxide, tungsten oxide, cobalt oxide, etc. The additive can enhance the electronic conductivity of the lithium iron phosphate material and improve the rate performance of the lithium iron phosphate material; at the same time, it improves the transmission path of lithium ions, reduces the ion diffusion resistance, and improves the specific capacity and charge-discharge efficiency of the lithium iron phosphate material.

[0090] In the present invention, the volume average particle size (here referring to the volume average particle size of solid particles in the grinding slurry) Dv50 of the grinding slurry is 0.2 μm - 0.6 μm, which can further improve the tap density and rate performance of the lithium iron phosphate material.

[0091] Exemplarily, the volume average particle size Dv50 of the grinding slurry can be 0.2 μm, 0.25 μm, 0.3 μm, 0.35 μm, 0.4 μm, 0.45 μm, 0.5 μm, 0.55 μm, or 0.6 μm, etc.

[0092] In a second aspect, the present invention provides a lithium iron phosphate material obtained by the preparation method described in the first aspect. This lithium iron phosphate material has both high tap density and high rate performance.

[0093] In a third aspect, the present invention provides a positive electrode sheet including the lithium iron phosphate material described in the second aspect. This positive electrode sheet has both high tap density and high rate performance.

[0094] Specifically, the positive electrode sheet includes a positive electrode current collector and a positive electrode active layer located on at least one surface of the positive electrode current collector. Specifically, the positive electrode active layer can be provided on one surface in the thickness direction of the positive electrode current collector, or positive electrode active layers can be respectively provided on opposite surfaces in the thickness direction of the positive electrode current collector.

[0095] Specifically, the positive electrode active layer includes lithium iron phosphate material, a conductive agent, and a binder. In the positive electrode active layer, the mass percentage content of the positive electrode active material can be 70% to 99%, such as 70%, 75%, 80%, 85%, 90%, 93%, 95%, 97%, 99%, or the range composed of any two of them. The mass fraction of the conductive agent can be 0.5% to 15%, such as 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 8%, 10%, 13%, 15%, or the range composed of any two of them. The mass fraction of the binder can be 0.5% to 15%, such as 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 8%, 10%, 13%, 15%, or the range composed of any two of them.

[0096] In the embodiments of the present invention, the conductive agent in the positive electrode active layer can be a conventional conductive material in the art. For example, the conductive agent in the positive electrode active layer can include one or more of conductive carbon black, conductive graphite, carbon nanotubes (CNT), carbon fibers, graphene, acetylene black, and Ketjen black.

[0097] In the embodiments of the present invention, the binder in the positive electrode active layer can be a conventional binder material in the art. For example, the binder in the positive electrode active layer can include one or more of polyvinylidene fluoride (PVDF), polyvinylidene difluoride, polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, etc.

[0098] In the embodiments of the present invention, a conventional positive electrode current collector in the art can be used. For example, the positive electrode current collector includes aluminum foil.

[0099] In the embodiments of the present invention, the positive electrode sheet can be prepared by a conventional method in the art. For example, it can be prepared by a coating method. Specifically, components such as the positive electrode active material, the conductive agent, and the binder for forming the positive electrode active layer can be dispersed in a solvent. The solvent includes, for example, N-methylpyrrolidone (NMP), to prepare a positive electrode slurry, and then it is coated on the surface of the positive electrode current collector. After processes such as drying, the positive electrode sheet is obtained. Among them, the processes involved such as coating and drying are conventional operations for preparing the positive electrode sheet by the coating method, and no special limitation is made thereto.

[0100] Fourthly, the present invention provides a battery, including the positive electrode sheet described in the third aspect. This battery has high rate performance and excellent cycling performance.

[0101] Generally, a battery includes an electrolyte, a battery cell, and a shell that encapsulates the battery cell. The electrolyte is injected into the battery cell in the shell. The battery cell includes a positive electrode sheet, a negative electrode sheet, and a separator between the positive electrode sheet and the negative electrode sheet. The battery cell can be a laminated battery cell, that is, the battery cell is formed by staggered stacking of positive electrode sheets, separators, and negative electrode sheets; or the battery cell can also be a wound battery cell, that is, the battery cell is formed by stacking positive electrode sheets, separators, and negative electrode sheets and then winding them.

[0102] The electrolyte of the embodiment of the present invention can be a conventional electrolyte in the art. For example, the electrolyte is a non-aqueous electrolyte, which can specifically include an organic solvent and an electrolyte salt. The organic solvent includes, for example, one or more of ethylene carbonate (EC), diethyl carbonate (DEC) and propylene carbonate (PC). The electrolyte salt can include a lithium salt. The lithium salt includes, for example, lithium cobalt oxide, etc., but is not limited thereto.

[0103] In the embodiment of the present invention, the battery cell may be packaged with conventional shell materials in the art, and the shell may include, for example, soft packaging materials such as aluminum-plastic film, but is not limited thereto.

[0104] The embodiments of the present invention can assemble components such as positive electrode sheets, separators and negative electrode sheets into a battery by conventional methods in the art. For example, the positive electrode sheets, separators and negative electrode sheets can be stacked in an alternating manner to produce a stacked battery cell (or wound into a wound battery cell); the battery cell is then placed in a shell (outer packaging), and after conventional processes such as liquid injection (i.e., injecting electrolyte) and packaging, the battery is obtained.

[0105] Hereinafter, the present invention will be further described in detail through specific examples.

[0106] Example 1

[0107] The lithium iron phosphate material of this embodiment is prepared by a method comprising the following steps:

[0108] (1) adding iron powder and ascorbic acid to 50 wt % phosphoric acid, and performing a first reaction at 80° C. for 6 h to obtain a mixed solution of ferric dihydrogen phosphate and phosphoric acid, wherein the molar ratio of phosphoric acid to iron powder is 3:1, and the molar ratio of ascorbic acid to iron powder is 0.01:1;

[0109] (2) adding ferroferric oxide to the mixed solution obtained in step (1), stirring and dispersing the mixture to form a suspension, then slowly adding lithium hydroxide monohydrate to the suspension, and performing a second reaction at 25° C. for 60 minutes to obtain a lithium iron phosphate precursor comprising lithium dihydrogen phosphate, ferric hydrogen phosphate and ferroferric oxide, wherein the molar ratio of ferroferric oxide to iron powder is 0.33:1, and the average primary particle size of the ferroferric oxide is 0.2 μm;

[0110] (3) Grind the lithium iron phosphate precursor, glucose, and titanium dioxide obtained in step (2) according to a mass ratio of 1:0.13:0.006 to obtain a ground slurry, and the volume average particle size Dv50 of the ground slurry is 0.35 µm;

[0111] (4) Subject the ground slurry obtained in step (3) to spray drying, sintering, and pulverization drying treatments in sequence to obtain a lithium iron phosphate material. Among them, the sintering process includes a first sintering treatment and a second sintering treatment under nitrogen protection. The temperature of the first sintering treatment is 810 °C, the time of the first sintering treatment is 2 h, the temperature of the second sintering treatment is 780 °C, and the time of the second sintering treatment is 8 h.

[0112] Example 2

[0113] The difference between the preparation method of the lithium iron phosphate material provided in this example and that of Example 1 is that in step (2), the average primary particle size of the iron trioxide is 0.3 µm, and in step (4), the sintering process includes a first sintering treatment and a second sintering treatment under nitrogen protection. The temperature of the first sintering treatment is 815 °C, the time of the first sintering treatment is 2 h, the temperature of the second sintering treatment is 760 °C, and the time of the second sintering treatment is 8 h.

[0114] Example 3

[0115] The difference between the preparation method of the lithium iron phosphate material provided in this example and that of Example 1 is that in step (2), the average primary particle size of the iron trioxide is 0.4 µm, and in step (3), the volume average particle size Dv50 of the ground slurry is 0.30 µm.

[0116] Example 4

[0117] The lithium iron phosphate material of this example is prepared by a method including the following steps:

[0118] (1) Add iron powder and ascorbic acid to 60 wt% phosphoric acid, and carry out a first reaction at 90 °C for 7 h to obtain a mixed solution of ferrous dihydrogen phosphate and phosphoric acid. The molar ratio of phosphoric acid to iron powder is 2.02:1, and the molar ratio of ascorbic acid to iron powder is 0.012:1;

[0119] (2) Add iron oxide to the mixed solution obtained in step (1) and stir to disperse to form a suspension, then slowly add lithium hydroxide monohydrate to the suspension, and carry out a second reaction at 25 °C for 80 min to obtain a lithium iron phosphate precursor including lithium dihydrogen phosphate, iron hydrogen phosphate, and iron oxide. The molar ratio of iron oxide to iron powder is 0.5:1, and the average primary particle size of the iron oxide is 0.35 µm;

[0120] (3) Grind the lithium iron phosphate precursor, carbon source, and vanadium pentoxide obtained in step (2) according to a mass ratio of 1:0.12:0.003 to obtain a ground slurry; wherein, the carbon source includes glucose and polyethylene glycol (the weight-average molecular weight of polyethylene glycol is 6000 - 8000), the mass ratio of glucose to polyethylene glycol is 2:1, and the volume-average particle size Dv50 of the ground slurry is 0.4 µm;

[0121] (4) Perform spray drying, sintering, and pulverization drying on the ground slurry obtained in step (3) in sequence to obtain a lithium iron phosphate material. Among them, the sintering process includes a first sintering treatment and a second sintering treatment under nitrogen protection. The temperature of the first sintering treatment is 815 °C, the time of the first sintering treatment is 1 h, the temperature of the second sintering treatment is 770 °C, and the time of the second sintering treatment is 9 h.

[0122] Example 5

[0123] The difference between the preparation method of the lithium iron phosphate material provided in this example and that of Example 4 is that in step (1), the temperature of the first reaction is 80 °C, in step (2), the average primary particle size of iron oxide is 0.6 µm, and in step (3), the volume-average particle size Dv50 of the ground slurry is 0.7 µm.

[0124] Example 6

[0125] The difference between the preparation method of the lithium iron phosphate material provided in this example and that of Example 1 is that in step (2), lithium hydroxide monohydrate is replaced with lithium carbonate, and in step (4), titanium dioxide is replaced with magnesium oxide, and the mass ratio of lithium iron phosphate precursor, glucose, and magnesium oxide is 1:0.15:0.001.

[0126] Example 7

[0127] The difference between the preparation method of the lithium iron phosphate material provided in this example and that of Example 1 is that in step (2), the average primary particle size of magnetite is 0.05 µm, lithium hydroxide monohydrate is replaced with lithium acetate, in step (3), the volume-average particle size Dv50 of the ground slurry is 0.2 µm, and in step (4), glucose is replaced with citric acid, and titanium dioxide is replaced with aluminum oxide.

[0128] Example 8

[0129] The difference between the preparation method of the lithium iron phosphate material provided in this example and that of Example 1 is that in step (2), the average primary particle size of magnetite is 0.6 µm, and in step (3), the volume-average particle size Dv50 of the ground slurry is 0.7 µm.

[0130] Example 9

[0131] The preparation method of the lithium iron phosphate material provided in this example is basically the same as that in Example 1, except that in step (1), the first reaction is carried out at 60 °C.

[0132] Example 10

[0133] The preparation method of the lithium iron phosphate material provided in this example is basically the same as that in Example 1, except that in step (4), the sintering treatment conditions are: holding at 780 °C for 8 h under nitrogen protection.

[0134] Example 11

[0135] The preparation method of the lithium iron phosphate material provided in this example is basically the same as that in Example 1, except that in step (2), the molar ratio of the reducing agent to iron powder is 0.016:1.

[0136] Comparative Example 1

[0137] The preparation method of the lithium iron phosphate material provided in this comparative example is basically the same as that in Example 1, except that the molar ratio of iron powder to phosphoric acid is 1:1 and iron tetroxide is not used.

[0138] Comparative Example 2

[0139] The preparation method of the lithium iron phosphate material provided in this comparative example is basically the same as that in Example 1, except that iron powder is not used and the molar ratio of iron tetroxide to phosphoric acid is 1:3.

[0140] Comparative Example 3

[0141] The difference between the preparation method of the lithium iron phosphate material in this comparative example and that in Example 1 lies in changing the feeding order of iron tetroxide. The specific steps are as follows:

[0142] (1) Add iron powder, iron tetroxide and ascorbic acid to 50 wt% phosphoric acid, and carry out the first reaction at 80 °C for 6 h to obtain a mixed solution of iron dihydrogen phosphate, iron oxide and phosphoric acid. The molar ratio of phosphoric acid to iron powder is 3:1, the molar ratio of ascorbic acid to iron powder is 0.01:1, the molar ratio of iron tetroxide to iron powder is 0.33:1, and the average primary particle size of iron tetroxide is 0.2 μm;

[0143] (2) Slowly add lithium hydroxide monohydrate to the mixed solution, and carry out the second reaction at 25 °C for 60 min to obtain a lithium iron phosphate precursor including lithium dihydrogen phosphate, iron hydrogen phosphate and iron tetroxide;

[0144] (3) Grind the lithium iron phosphate precursor, glucose and titanium dioxide obtained in step (2) according to a mass ratio of 1:0.13:0.006 to obtain a ground slurry, and the volume average particle size Dv50 of the ground slurry is 0.35 µm;

[0145] (4) The ground slurry obtained in step (3) is subjected to spray drying, sintering, and pulverization drying in sequence to obtain a lithium iron phosphate material. The sintering process includes a first sintering treatment and a second sintering treatment under nitrogen protection. The temperature of the first sintering treatment is 810 °C, the time of the first sintering treatment is 2 h, the temperature of the second sintering treatment is 780 °C, and the time of the second sintering treatment is 8 h.

[0146] Application examples and application comparative examples

[0147] Polyvinylidene fluoride (PVDF) is dissolved in N-methyl-pyrrolidone to prepare a PVDF solution with a concentration of 0.2 g / L. The lithium iron phosphate materials, conductive carbon black, and PVDF solution prepared in the above-mentioned examples and comparative examples are weighed according to the mass ratio of 92:5:3, mixed and stirred into a slurry mixture. The slurry mixture is evenly coated on an aluminum foil, dried at 80 °C for 30 min, cut into circular positive electrode sheets, and dried in a vacuum oven at 120 °C for 8 h. In an argon atmosphere glove box with a water content < 1 ppm and an oxygen content < 1 ppm, the above-mentioned positive electrode sheets (surface density is 70 g / m 2 ), negative Li metal circular sheets, and 1 mol / L lithium cobaltate electrolyte (the solvent is ethylene carbonate) are assembled into a coin cell using a 2025-type coin cell case.

[0148] Performance test:

[0149] (1) Apparent density: Take a positive electrode sheet sample (specifically, a cutter can be used to cut the positive electrode sheet to obtain a positive electrode sheet sample with appropriate dimensions), and test the total mass m1 of the positive electrode sheet sample, the total thickness T1 of the positive electrode sheet sample (T1 = the total thickness of the positive electrode active layer + the thickness of the positive electrode current collector. When positive electrode active layers are provided on both the front and back surfaces of the positive electrode current collector, the total thickness of the positive electrode active layer = the thickness of the positive electrode active layer on one surface of the positive electrode current collector + the thickness of the positive electrode active layer on the other surface of the positive electrode current collector), and the single-sided surface area S in the thickness direction of the positive electrode sheet sample; then scrape off the positive electrode active layer on the positive electrode sheet sample, and test the mass m2 of the obtained positive electrode current collector and the thickness T2 of the positive electrode current collector. Then the total thickness of the positive electrode active layer = T1 - T2, the surface density of the positive electrode active layer = (m1 - m2) / S, and the apparent density of the positive electrode active layer = the surface density of the positive electrode active layer / the total thickness of the positive electrode active layer = (m1 - m2) / (S×(T1 - T2)).

[0150] (2) 0.5C (1C = 170 mAh / g) rate performance test: Under normal temperature (25 °C) conditions, charge the coin cells of the application example and the comparative example at a constant current of 0.5C to 3.75V, then charge at a constant voltage of 3.75V until the current reaches 0.02C. Let the coin cells stand for 5 min to stabilize the chemical state; then discharge at a constant current of 0.5C to 2.0V. Repeat the above charge-discharge process 2 times, and record the average value of the discharge capacities obtained in the 2 times. The test results are shown in Table 1;

[0151] (3) 1C rate performance test: Under normal temperature (25 °C) conditions, charge the battery at a constant current of 0.5C to 3.75V, then charge at a constant voltage of 3.75V until the current reaches 0.02C, and let it stand for 5 min; then discharge at 0.5C to 2.0V for 2 cycles; then charge the battery at a constant current of 1C to 3.75V, then charge at a constant voltage of 3.75V until the current reaches 0.02C, and let it stand for 5 min; then discharge at 1C to 2.0V for 2 cycles, and record the average value of the 1C capacities obtained in the 2 cycles. The test results are shown in Table 1;

[0152] (4) 4C rate performance test: Under normal temperature (25 °C) conditions, charge the battery at a constant current of 0.5C to 3.75V, then charge at a constant voltage of 3.75V until the current reaches 0.02C, and let it stand for 5 min; then discharge at 0.5C to 2.0V for 2 cycles; then charge the battery at a constant current of 4C to 3.75V, then charge at a constant voltage of 3.75V until the current reaches 0.02C, and let it stand for 5 min; then discharge at 4C to 2.0V for 2 cycles, and record the average value of the 4C capacities obtained in the 2 cycles. The test results are shown in Table 1.

[0153] Table 1

[0154]

[0155] As Figure 1 shown, in Example 1, the lithium iron phosphate precursor achieved a mixed doping of large and small particles, forming a dense packing structure. As Figure 2 shown, the lithium iron phosphate material obtained by sintering the lithium iron phosphate precursor in Example 1 also achieved a mixed doping of large and small particles and had a denser packing structure.

[0156] As Figure 3 shown, the coin cell assembled with the lithium iron phosphate material in Example 1 had a high specific capacity in the voltage range of 2.0V to 3.75V and a small attenuation, indicating that the lithium iron phosphate material had excellent rate performance.

[0157] As can be seen from the analysis of Table 1, by comparing Examples 1-11 with Comparative Examples 1-3, it can be seen that the lithium iron phosphate materials in Examples 1-11 have a higher tap density, and the coin cells containing the lithium iron phosphate materials in Examples 1-11 have a higher discharge specific capacity. This indicates that the iron hydrogen phosphate and iron oxides in the lithium iron phosphate precursor prepared by the preparation method of the present invention have different morphologies, realizing the mixed doping of large and small particles and forming a more compact packing structure, thereby enabling the lithium iron phosphate material to have both a high tap density and a high rate performance.

[0158] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a lithium iron phosphate material, characterized in that: The following steps are involved: (1) adding iron powder and a reducing agent to phosphoric acid to perform a first reaction to obtain a mixed solution; (2) adding iron oxide and a lithium source to the mixed solution to carry out a second reaction to obtain a lithium iron phosphate precursor; (3) sintering the lithium iron phosphate precursor to obtain the lithium iron phosphate material; The mixed solution includes ferrous dihydrogen phosphate and phosphoric acid; The lithium iron phosphate precursor includes lithium dihydrogen phosphate, iron hydrogen phosphate and iron oxide; The molar ratio of the phosphoric acid to the iron powder is z:1, wherein 2<z≤6; The iron oxide includes ferric oxide and / or ferrosinite; When the iron oxide comprises ferric oxide, the molar ratio of ferric oxide to iron powder is x1:y1, wherein 0.1≤y1≤0.5, 0.25≤x1≤0.45; When the iron oxide includes ferroferric oxide, the molar ratio of ferroferric oxide to iron powder is x2:y2, wherein 0.1≤y2≤0.5, 0.17≤x2≤0.

30.

2. The preparation method according to claim 1, characterized in that: The molar ratio of the reducing agent to the iron powder is (0.008-0.013):

1.

3. The preparation method according to claim 1, characterized in that: The average primary particle size of the iron oxide is 0.05 μm-0.5 μm.

4. The preparation method according to claim 1, characterized in that: The process of adding iron powder and a reducing agent to phosphoric acid to perform a first reaction to obtain a mixed solution comprises: adding iron powder and a reducing agent to a phosphoric acid solution to perform a first reaction to obtain a mixed solution; The mass concentration of the phosphoric acid solution is 30wt%-80wt%; and / or, The reducing agent includes one or more of ascorbic acid, citric acid, and glucose; and / or, The lithium source includes one or more of lithium carbonate, lithium hydroxide, lithium oxalate and lithium acetate.

5. The preparation method according to claim 1, characterized in that: The temperature of the first reaction is 70-90° C., and the time of the first reaction is 6-12 hours; and / or, The temperature of the second reaction is 20-30° C., and the time of the second reaction is 60 min-90 min; and / or, The sintering process includes a first sintering process and a second sintering process, and the temperature of the first sintering process is greater than the temperature of the second sintering process; In the first sintering process, the sintering temperature is 790°C-830°C, and the sintering time is 1-5h; In the second sintering treatment, the sintering temperature is 730° C.-790° C., and the sintering time is 8-10 hours.

6. The preparation method according to claim 1, characterized in that: The process of adding the iron oxide and the lithium source to the mixed solution and performing a second reaction to obtain the lithium iron phosphate precursor comprises: firstly adding the iron oxide to the mixed solution, stirring to form a suspension, then adding the lithium source to the suspension, performing a second reaction, and obtaining the lithium iron phosphate precursor; And / or, the process of sintering the lithium iron phosphate precursor to obtain the lithium iron phosphate material also includes: mixing the lithium iron phosphate precursor obtained in step (2) with a carbon source and an additive and grinding them to obtain a grinding slurry, and then drying and sintering the grinding slurry in sequence to obtain the lithium iron phosphate material.

7. The preparation method according to claim 6, characterized in that: The mass ratio of the carbon source to the lithium iron phosphate precursor is (0.1-0.15): 1; and / or, The mass ratio of the additive to the lithium iron phosphate precursor is (0.001-0.008): 1; and / or, The carbon source comprises one or more of glucose, sucrose, dextrin, citric acid, fructose, polyethylene glycol, polyvinyl alcohol, and polypropylene glycol; and / or, The additive includes a compound containing one or more elements of Al, Mg, Ni, Co, Ti, Cu, Ca, Nb, Cr, Zn, La, Sb, Te, Sr, W, In, and Y; and / or, The volume average particle size Dv50 of the grinding slurry is 0.2 μm-0.6 μm.

8. A lithium iron phosphate material, characterized in that: Obtained by the preparation method according to any one of claims 1 to 7.

9. A positive electrode sheet, characterized in that: The invention comprises the lithium iron phosphate material obtained by the preparation method according to any one of claims 1 to 7 or the lithium iron phosphate material according to claim 8.

10. A battery, characterized in that: Including the positive electrode sheet as described in claim 9.

Images