A lithium iron phosphate cathode material, its preparation method, and a lithium battery

Abstract

This invention relates to the field of lithium-ion battery materials technology, providing a lithium iron phosphate cathode material, its preparation method, and a lithium battery. The preparation method of the lithium iron phosphate cathode material includes the following steps: a first iron source solution and a first phosphorus source solution undergo a first co-precipitation reaction in the presence of polymer monomers to obtain iron phosphate seed crystals; a second iron source solution and a second phosphorus source solution undergo a second co-precipitation reaction in the presence of polymer monomers and iron phosphate seed crystals to obtain iron phosphate dihydrate; iron phosphate dihydrate, a lithium source, and a carbon source are mixed, spray-dried, and sintered in a rotary kiln; different sintering temperatures are set to prepare carbon-coated lithium iron phosphate with large and small particle sizes respectively; and the carbon-coated lithium iron phosphate with different particle sizes is mixed in a mass ratio to obtain the lithium iron phosphate cathode material. This lithium iron phosphate cathode material has a good spherical morphology, and therefore, after gradation, it possesses high compaction density and electrical properties.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery materials, and more specifically, to a lithium iron phosphate cathode material, its preparation method, and a lithium battery. Background Technology

[0002] With societal development, the theoretical driving range of new energy vehicles has been continuously improving, but the actual driving range still falls short of the theoretical value. Coupled with long charging times, these issues have caused considerable inconvenience in practical applications. To address these shortcomings, current solutions primarily focus on improving the energy density and fast-charging performance of lithium batteries. Lithium iron phosphate batteries, due to their excellent safety performance, long cycle life, and low cost, have already captured more than half of the power lithium battery market share.

[0003] Despite the rapid development of lithium iron phosphate batteries, they still face technical bottlenecks in fast-charging performance. This is due to structural defects in lithium iron phosphate (LiFePO4) that reduce its electronic conductivity and Li... + The low diffusion rate means that lithium ions cannot escape quickly enough during high-rate discharge, leading to increased electrochemical polarization and higher internal resistance in lithium iron phosphate batteries, which affects their rate performance and makes them unsuitable for fast charging. Secondly, the irregular morphology and varying sizes of lithium iron phosphate result in a low compaction density, further impacting the energy density of lithium iron phosphate batteries.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] The purpose of this invention is to provide a lithium iron phosphate cathode material, its preparation method, and a lithium battery, aiming to improve the compaction density of the lithium iron phosphate cathode material and thus improve the energy density of the corresponding lithium battery.

[0006] This invention is implemented as follows:

[0007] In a first aspect, the present invention provides a method for preparing a lithium iron phosphate cathode material, comprising the following steps:

[0008] S1. The solution of the first iron source and the solution of the first phosphorus source are subjected to a first coprecipitation reaction in the presence of polymer monomers to obtain iron phosphate seed crystals.

[0009] S2. The solution of the second iron source and the solution of the second phosphorus source are subjected to a second coprecipitation reaction in the presence of the polymer monomer and the iron phosphate seed crystals to obtain iron phosphate dihydrate.

[0010] S3. The iron phosphate dihydrate, lithium source, and carbon source are mixed and then spray-dried to obtain a spray material;

[0011] S4. The sprayed material is placed in a rotary kiln for sintering to obtain carbon-coated lithium iron phosphate; wherein, different sintering temperatures are set to prepare first carbon-coated lithium iron phosphate and second carbon-coated lithium iron phosphate respectively.

[0012] S5. The first carbon-coated lithium iron phosphate and the second carbon-coated lithium iron phosphate are mixed in a certain mass ratio to obtain lithium iron phosphate cathode material.

[0013] In an optional embodiment, the concentrations of the first iron source solution and the first phosphorus source solution are 1 mol / L to 2 mol / L, and the concentrations of the second iron source solution and the second phosphorus source solution are 0.1 mol / L to 0.5 mol / L.

[0014] In an optional embodiment, the amount of iron phosphate seed crystals added is 10% to 25% of the total mass of the second iron source and the second phosphorus source.

[0015] In an optional embodiment, the polymer monomer satisfies at least one of the following (1) to (2):

[0016] (1): The polymer monomer includes at least one of pyrrole, aniline and thiophene;

[0017] (2): In steps S1 and S2, the amount of polymer monomer used per gram of iron source is 0.01g to 0.2g.

[0018] In an optional embodiment, the first coprecipitation reaction and / or the second coprecipitation reaction satisfy at least one of the following (3) to (6):

[0019] (3): The coprecipitation reaction includes the addition of a morphology control agent, wherein the morphology control agent includes at least one of polyethylene glycol, polyvinylpyrrolidone and hexadecyltrimethylammonium bromide, and the mass of the morphology control agent is 0.01% to 5% of the mass of the first iron source or the second iron source;

[0020] (4): The coprecipitation reaction includes the addition of an oxidant and / or a pH adjuster, wherein the oxidant includes hydrogen peroxide and the pH adjuster includes ammonia.

[0021] (5): The pH of the coprecipitation reaction is 1.5~3.0, and the temperature of the coprecipitation reaction is 75℃~95℃;

[0022] (6): After the coprecipitation reaction is completed, the process also includes aging and solid-liquid separation, and drying the separated solid product; wherein the aging time is 0.5h~6h, the drying temperature is 100℃~130℃, and the drying time is 2h~5h.

[0023] In an optional implementation, the sintering in step S4 includes sintering for 3 to 8 hours, followed by introducing a reducing gas and continuing sintering for another 1 to 5 hours.

[0024] The reducing gas includes at least one of acetylene and methane, and the gas flow rate is 0.01 L / min to 0.15 L / min.

[0025] In an optional embodiment, the preparation method satisfies at least one of the following (7) to (9):

[0026] (7): The sintering temperature corresponding to the first carbon-coated lithium iron phosphate is 730℃~770℃;

[0027] (8): The sintering temperature corresponding to the second carbon-coated lithium iron phosphate is 800℃~830℃;

[0028] (9): The first carbon-coated lithium iron phosphate and the second carbon-coated lithium iron phosphate are mixed at a mass ratio of (2~3):1.

[0029] In an optional implementation, the Dv50 of the first carbon-coated lithium iron phosphate is 0.3 μm to 0.6 μm, and the Dv50 of the second carbon-coated lithium iron phosphate is 1.0 μm to 1.4 μm.

[0030] In a second aspect, the present invention provides a lithium iron phosphate cathode material prepared by the preparation method described in the first aspect.

[0031] Thirdly, the present invention provides a lithium battery comprising the lithium iron phosphate cathode material as provided in the second aspect.

[0032] The present invention has the following beneficial effects:

[0033] (1) This invention prepares spherical iron phosphate particles by combining a crystal form control method and a seed crystal method. The crystal form control method involves adding polymer monomers during the co-precipitation process, where the polymer monomers are in the Fe... 3+ Polymerization is initiated by [a specific method], and the polymerization process occurs simultaneously with that of iron phosphate, thereby suppressing the disordered growth of iron phosphate crystal faces and controlling the growth of iron phosphate into a spherical morphology. The seed crystal method involves first co-precipitating small iron phosphate particles under high-concentration raw materials, and then using these as seed crystals for further co-precipitation under low-concentration raw materials to obtain large iron phosphate particles. Using both methods in combination yields iron phosphate particles with a stable spherical morphology. These iron phosphate particles are further calcined with a lithium source to generate spherical lithium iron phosphate. Spherical lithium iron phosphate reduces porosity and bridging during the stacking process, thereby reducing impedance in the electron conduction path, lowering interface resistance, and improving the rate performance of the battery.

[0034] (2) This invention utilizes rotary kiln vapor deposition combined with different sintering temperatures to prepare spherical carbon-coated lithium iron phosphate with small and large particle sizes. The particles are mixed by mass to form a gradation. The spherical carbon-coated lithium iron phosphate makes the gradation effect better, thereby giving the finished lithium iron phosphate cathode material a higher compaction density and electrical performance. Attached Figure Description

[0035] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0036] Figure 1 The image shows the X-ray diffraction pattern of the lithium iron phosphate cathode material prepared in Example 1.

[0037] Figure 2 This is a scanning electron microscope image of the ferric phosphate dihydrate prepared in Example 1;

[0038] Figure 3 The image shows a scanning electron microscope image of the lithium iron phosphate cathode material prepared in Example 1.

[0039] Figure 4 Transmission electron microscope image of the lithium iron phosphate cathode material prepared in Example 1;

[0040] Figure 5 The image shows a scanning electron microscope image of the lithium iron phosphate cathode material prepared for Comparative Example 1. Detailed Implementation

[0041] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below. However, there may be instances where unnecessary detailed descriptions are omitted. For example, detailed descriptions of well-known matters and repeated descriptions of actually identical structures may be omitted. This is to avoid making the following description unnecessarily lengthy and to facilitate understanding by those skilled in the art.

[0042] Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand the invention and are not intended to limit the subject matter of the claims. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0043] In the description of the embodiments of this application, the terms "first," "second," etc., are only used to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. The term "and / or" is only a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone.

[0044] For the sake of brevity, this invention specifically discloses several numerical ranges, which can be combined to form corresponding embodiments. The endpoints and values ​​of the ranges disclosed herein are not limited to precise ranges or values; these ranges or values ​​should be understood to include values ​​close to them. For numerical ranges, endpoint values ​​of various ranges, endpoint values ​​of various ranges and individual point values, and individual point values ​​can be combined to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0045] Unless otherwise stated, the terminology used in this invention has the common meaning as generally understood by those skilled in the art.

[0046] The following is a detailed description of a lithium iron phosphate cathode material, its preparation method, and a lithium battery provided by the present invention.

[0047] This invention provides a method for preparing lithium iron phosphate cathode material, which includes the following steps:

[0048] S1. The solutions of the first iron source and the first phosphorus source are subjected to a first coprecipitation reaction in the presence of polymer monomers to obtain iron phosphate seed crystals.

[0049] S2. The solutions of the second iron source and the second phosphorus source are subjected to a second coprecipitation reaction in the presence of polymer monomers and iron phosphate seed crystals to obtain iron phosphate dihydrate.

[0050] In some embodiments, the concentrations of the first iron source solution and the first phosphorus source solution are 1 mol / L to 2 mol / L, for example, any one of 1 mol / L, 1.2 mol / L, 1.5 mol / L, 1.8 mol / L, 2.0 mol / L, or any range between two of them; the concentrations of the second iron source solution and the second phosphorus source solution are 0.1 mol / L to 0.5 mol / L, for example, any one of 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, or any range between two of them.

[0051] In some embodiments, the amount of iron phosphate seed crystals added is 10% to 25% of the total mass of the second iron source and the second phosphorus source, for example, it can be any one of 10%, 15%, 20%, 25% or a range between any two.

[0052] The particle size of the material was controlled by adjusting the concentrations of the iron and phosphorus source solutions, resulting in the preparation of small-sized iron phosphate seed crystals and large-sized iron phosphate dihydrate. In the first coprecipitation reaction, high concentrations of the first iron and phosphorus source solutions were used, leading to a higher raw material solution concentration and a crystal nucleation rate exceeding the crystal growth rate, thus primarily producing small-sized iron phosphate seed crystals. In the second coprecipitation reaction, low concentrations of the second iron and phosphorus source solutions were used, resulting in a lower raw material solution concentration and a lower seed crystal nucleation rate than the crystal growth rate. This favored continued growth on the iron phosphate seed crystals already present in the reaction system, thus producing large-sized iron phosphate dihydrate.

[0053] In some embodiments, the polymer monomer includes at least one of pyrrole, aniline and thiophene; the amount of polymer monomer used per gram of iron source is 0.01g to 0.2g, for example, it can be any one of 0.01g, 0.05g, 0.1g, 0.15g, 0.2g or a range between any two.

[0054] During the coprecipitation reaction, polymer monomers are introduced, and the Fe in the iron source solution... 3+ As an initiator, the polymer monomers are polymerized in situ. The in-situ polymerization and the growth of iron phosphate occur simultaneously, thus suppressing the disordered growth of iron phosphate crystal faces and controlling the growth of iron phosphate into a spherical morphology. Secondly, the long-chain polymer formed by the monomer reaction polymerization can serve as a carbon source, thereby reducing the amount of external carbon source used in the subsequent preparation of lithium iron phosphate.

[0055] In the coprecipitation reaction, polymer monomers are introduced to regulate the concentration of the raw material solution, so that the small-sized iron phosphate seed crystals have a spherical morphology, and then the large-sized iron phosphate dihydrate is prepared from these seed crystals, which also have a spherical morphology.

[0056] In some embodiments, the first coprecipitation reaction and / or the second coprecipitation reaction include the addition of a morphology control agent. The morphology control agent includes at least one selected from polyethylene glycol, polyvinylpyrrolidone, and hexadecyltrimethylammonium bromide. The mass of the morphology control agent is 0.01% to 5% of the mass of the first or second iron source, for example, it can be any one of 0.01%, 0.1%, 1%, 3%, 5%, or a range between any two. The morphology control agent can regulate the primary particle morphology of ferric phosphate, thereby obtaining ferric phosphate with a uniform morphology.

[0057] In some embodiments, the first coprecipitation reaction and / or the second coprecipitation reaction include the addition of an oxidizing agent and / or a pH adjuster, wherein the oxidizing agent includes hydrogen peroxide and the pH adjuster includes ammonia. When the iron source is Fe... 2+ When an oxidizing agent is added, Fe 2+ Converted to Fe 3+ It participates in coprecipitation reactions.

[0058] In some embodiments, the pH of the first coprecipitation reaction and / or the second coprecipitation reaction is 1.5 to 3.0, for example, it can be any one of 1.5, 2.0, 2.5, 3.0 or a range between any two; the temperature of the first coprecipitation reaction and / or the second coprecipitation reaction is 75°C to 95°C, for example, it can be any one of 75°C, 80°C, 85°C, 90°C, 95°C or a range between any two.

[0059] In some embodiments, after the first coprecipitation reaction and / or the second coprecipitation are completed, aging and solid-liquid separation are further performed, and the separated solid product is dried; wherein, the aging time is 0.5h~6h, the drying temperature is 100℃~130℃, and the drying time is 2h~5h. The completion of the first coprecipitation reaction and / or the second coprecipitation means that the ion concentration in the coprecipitation reaction system no longer changes.

[0060] In summary, by introducing polymerizable monomers and controlling the concentration of the raw material solution during the coprecipitation reaction, small-particle-size spherical iron phosphate seed crystals are first prepared. Then, the small-particle-size spherical iron phosphate seed crystals are used to promote crystallization, further generating large-particle-size spherical iron phosphate dihydrate.

[0061] In the coprecipitation reaction system, ammonia water not only acts as a pH adjuster, but its NH4+... + It will complex on the surface of iron phosphate, and during the subsequent sintering process to prepare lithium iron phosphate, NH4 + It transforms into NH3, which creates pores, making the material porous, accelerating lithium-ion transport, and improving electrical performance. Unlike other researchers who calcine ferric phosphate dihydrate to obtain anhydrous ferric phosphate, this invention does not calcine ferric phosphate dihydrate because NH4+ is produced during calcination. + The conversion to NH3 will destroy the spherical morphology of the iron phosphate material, which is not conducive to the subsequent preparation of spherical lithium iron phosphate.

[0062] S3. After mixing iron phosphate dihydrate, lithium source and carbon source, the mixture is spray-dried to obtain spray material.

[0063] The mixing process employs techniques including sand milling.

[0064] In some embodiments, the carbon source includes at least one of sucrose, glucose, cyclodextrin, cellulose, tannic acid, polyethylene glycol, vitamin C, and bitumen.

[0065] In some embodiments, the molar ratio of Fe to Li in the spray material is 1:(1~1.05), and the mass of the carbon source accounts for 4%~12% of the total mass of iron phosphate dihydrate and lithium source.

[0066] In some embodiments, the inlet air temperature of the spray dryer is 200°C to 230°C, and the outlet air temperature is 90°C to 110°C.

[0067] S4. The sprayed material is placed in a rotary kiln for sintering to obtain carbon-coated lithium iron phosphate; wherein, different sintering temperatures are set to prepare first carbon-coated lithium iron phosphate and second carbon-coated lithium iron phosphate respectively.

[0068] In some embodiments, the sintering includes sintering for 3 to 8 hours, followed by introducing a reducing gas and continuing sintering for 1 to 5 hours. The reducing gas includes at least one of acetylene and methane, and the gas flow rate is 0.01 L / min to 0.15 L / min, for example, it can be any one or a range between any two of 0.01 L / min, 0.03 L / min, 0.05 L / min, 0.07 L / min, 0.09 L / min, 0.11 L / min, 0.13 L / min, and 0.15 L / min.

[0069] A rotary kiln with uniform heating is used to sinter the sprayed lithium iron phosphate, ensuring even heating during growth and resulting in lithium iron phosphate particles with uniform size. Simultaneously, a reducing gas is introduced during the later stages of sintering to perform vapor deposition, producing lithium iron phosphate with a carbon coating. Vapor deposition increases the graphitization degree of the carbon coating, thereby improving the conductivity and rate performance of the carbon-coated lithium iron phosphate.

[0070] In some embodiments, the sintering temperature for the first carbon-coated lithium iron phosphate is 730°C to 770°C, for example, it can be any one or a range between 730°C, 740°C, 750°C, 760°C, and 770°C. The sintering temperature for the second carbon-coated lithium iron phosphate is 800°C to 830°C, for example, it can be any one or a range between 800°C, 810°C, 820°C, and 830°C.

[0071] The particle size of carbon-coated lithium iron phosphate can be controlled by adjusting the sintering temperature. When the sintering temperature is too low, such as 730℃~770℃, crystal growth is slow, so the particle size of the first carbon-coated lithium iron phosphate is relatively small, ranging from 0.3μm to 0.6μm. When the sintering temperature is too high, such as 800℃~830℃, crystal growth is rapid, so the particle size of the second carbon-coated lithium iron phosphate is relatively large, ranging from 1.0μm to 1.4μm.

[0072] Iron phosphate, as a precursor to lithium iron phosphate, directly influences the morphology of lithium iron phosphate; spherical iron phosphate often results in spherical lithium iron phosphate. Therefore, when the spherical iron phosphate obtained in S2 is used as a precursor in this invention, the carbon-coated lithium iron phosphate prepared has a spherical morphology. The spherical morphology of the carbon-coated lithium iron phosphate increases the contact area between adjacent particles, reducing porosity and bridging during the stacking process, thereby reducing impedance in the electron conduction path, lowering the interface resistance, and improving the electrical performance of the battery.

[0073] S5. Mix the first carbon-coated lithium iron phosphate and the second carbon-coated lithium iron phosphate at a mass ratio of (2~3):1 to obtain the lithium iron phosphate cathode material.

[0074] In some embodiments, the Dv50 of the first carbon-coated lithium iron phosphate is 0.3 μm to 0.6 μm, for example, it can be any one of 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm or any two of these values; the Dv50 of the second carbon-coated lithium iron phosphate is 1.0 μm to 1.4 μm, for example, it can be any one of 1.0 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm or any two of these values.

[0075] The first carbon-coated lithium iron phosphate and the second carbon-coated lithium iron phosphate are mixed in a certain mass ratio to form a gradation. The small-diameter first carbon-coated lithium iron phosphate fills the gaps between the large-diameter second carbon-coated lithium iron phosphate, thereby improving the overall compaction density of the material.

[0076] In summary, the preparation of spherical carbon-coated lithium iron phosphate and the use of carbon-coated lithium iron phosphate particles of varying sizes to form a graded structure, together enable the prepared lithium iron phosphate cathode material to possess high compaction density and high rate performance.

[0077] The features and performance of the present invention will be further described in detail below with reference to the accompanying drawings, embodiments and comparative examples.

[0078] Example 1

[0079] This embodiment provides a lithium iron phosphate cathode material, the preparation method of which includes:

[0080] S1. Add 1.0 mol / L ferrous sulfate solution and 1.0 mol / L phosphoric acid solution to a reaction vessel, stir well, add polyethylene glycol, hydrogen peroxide and pyrrole, and add ammonia to adjust the pH of the system in the reaction vessel to 2.0. Carry out the first coprecipitation reaction at 90℃. After the first coprecipitation reaction is completed, age for 1 hour, separate the solid and liquid, and dry the separated solid product at 110℃ for 4 hours to obtain ferric phosphate seed crystals.

[0081] The molar ratio of ferrous sulfate:phosphoric acid:hydrogen peroxide is 1:1:1, the mass of polyethylene glycol is 1% of the mass of ferrous sulfate, and the amount of pyrrole used per gram of ferrous sulfate is 0.05g.

[0082] S2. Add 0.5 mol / L ferrous sulfate solution and 0.5 mol / L phosphoric acid solution to the reaction vessel, stir well, add ferric phosphate seed crystals, polyethylene glycol, hydrogen peroxide and pyrrole, and add ammonia to adjust the pH of the system in the reaction vessel to 2.0. Carry out the second coprecipitation reaction at 95℃. After the second coprecipitation reaction is completed, age for 4 hours, separate the solid and liquid, and dry the separated solid product at 130℃ for 5 hours to obtain ferric phosphate dihydrate.

[0083] The molar ratio of ferrous sulfate:phosphoric acid:hydrogen peroxide is 1:1:1; the mass of polyethylene glycol is 0.05% of the mass of ferrous sulfate; the amount of pyrrole corresponding to each gram of ferrous sulfate is 0.05g; and the amount of ferric phosphate seed crystals added is 20% of the total mass of ferrous sulfate and phosphoric acid in S2.

[0084] S3. After grinding and mixing ferric phosphate dihydrate, lithium carbonate and glucose evenly, the mixture is spray-dried to obtain the spray material.

[0085] The molar ratio of ferric phosphate dihydrate to lithium carbonate is 1:1.04; the mass of glucose accounts for 8% of the total mass of ferric phosphate dihydrate and lithium carbonate; the inlet air temperature of the spray dryer is 220℃ and the outlet air temperature is 95℃.

[0086] S4. After sintering the sprayed material in a rotary kiln for 5 hours under a nitrogen atmosphere, acetylene gas is introduced and sintering continues for 3 hours to obtain carbon-coated lithium iron phosphate. The sintering temperatures are set to 750℃ to prepare the first type of carbon-coated lithium iron phosphate and 820℃ to prepare the second type.

[0087] The gas flow rate is 0.06 L / min.

[0088] S5. First carbon-coated lithium iron phosphate and second carbon-coated lithium iron phosphate are mixed at a mass ratio of 2.3:1 to obtain lithium iron phosphate cathode material.

[0089] Appendix Figure 1 The X-ray diffraction pattern of the lithium iron phosphate cathode material obtained in this embodiment shows that its peak position and intensity correspond one-to-one with the standard card, proving that the prepared material is lithium iron phosphate.

[0090] Appendix Figure 2 The image shown is a scanning electron microscope image of ferric phosphate dihydrate obtained in this embodiment. It can be seen that the morphology of ferric phosphate is a spherical particle formed by primary particle agglomeration.

[0091] Appendix Figure 3The image shown is a scanning electron microscope image of the lithium iron phosphate cathode material obtained in this embodiment. It can be seen that the lithium iron phosphate cathode material has a spherical morphology, and the particles vary in size, with small particles being the majority.

[0092] Appendix Figure 4 The image shown is a transmission electron microscope image of the lithium iron phosphate cathode material obtained in this embodiment. It can be seen that a carbon layer is uniformly coated on the surface of lithium iron phosphate, with a thickness of about 1 nm to 2 nm. This uniform carbon layer is beneficial to improving the conductivity of the material, thereby enhancing the rate performance of the material.

[0093] Example 2

[0094] This embodiment provides a lithium iron phosphate cathode material, the preparation method of which includes:

[0095] S1. Add 1.5 mol / L ferrous sulfate solution and 1.5 mol / L phosphoric acid solution to a reaction vessel, stir well, add polyethylene glycol, hydrogen peroxide and thiophene, and add ammonia to adjust the pH of the system in the reaction vessel to 1.5. Carry out the first coprecipitation reaction at 75℃. After the first coprecipitation reaction is completed, age for 6 hours, separate the solid and liquid, and dry the separated solid product at 100℃ for 2 hours to obtain ferric phosphate seed crystals.

[0096] The molar ratio of ferrous sulfate:phosphoric acid:hydrogen peroxide is 1:1:1, the mass of polyethylene glycol is 5% of the mass of ferrous sulfate, and the amount of thiophene used per gram of ferrous sulfate is 0.01g.

[0097] S2. Add 0.1 mol / L ferrous sulfate solution and 0.1 mol / L phosphoric acid solution to the reaction vessel, stir well, add ferric phosphate seed crystals, polyethylene glycol, hydrogen peroxide and thiophene, and add ammonia to adjust the pH of the system in the reaction vessel to 1.5. Carry out the second coprecipitation reaction at 90℃. After the second coprecipitation reaction is completed, age for 4 hours, separate the solid and liquid, and dry the separated solid product at 130℃ for 5 hours to obtain ferric phosphate dihydrate.

[0098] The molar ratio of ferrous sulfate:phosphoric acid:hydrogen peroxide is 1:1:1; the mass of polyethylene glycol is 0.01% of the mass of ferrous sulfate; the amount of thiophene corresponding to each gram of ferrous sulfate is 0.1g; and the amount of ferric phosphate seed crystals added is 10% of the total mass of ferrous sulfate and phosphoric acid in S2.

[0099] S3. After uniformly mixing by sand milling, ferric phosphate dihydrate, lithium carbonate and tannic acid are spray-dried to obtain spray material.

[0100] The molar ratio of iron phosphate dihydrate to lithium carbonate is 1:1; the mass of tannic acid accounts for 12% of the total mass of iron phosphate dihydrate and lithium carbonate; the inlet air temperature of the spray dryer is 220℃ and the outlet air temperature is 90℃.

[0101] S4. Under a nitrogen atmosphere, the sprayed material was sintered in a rotary kiln for 3 hours, followed by acetylene gas and sintering for another 5 hours to obtain carbon-coated lithium iron phosphate. The first carbon-coated lithium iron phosphate was prepared at 730℃, and the second carbon-coated lithium iron phosphate was prepared at 830℃.

[0102] The gas flow rate is 0.01 L / min.

[0103] S5. Mix the first carbon-coated lithium iron phosphate and the second carbon-coated lithium iron phosphate at a mass ratio of 2:1 to obtain the lithium iron phosphate cathode material.

[0104] Example 3

[0105] This embodiment provides a lithium iron phosphate cathode material, the preparation method of which includes:

[0106] S1. Add 2.0 mol / L ferrous sulfate solution and 2.0 mol / L phosphoric acid solution to a reaction vessel, stir well, add polyethylene glycol, hydrogen peroxide and pyrrole, and add ammonia to adjust the pH of the system in the reaction vessel to 3.0. Carry out the first coprecipitation reaction at 80℃. After the first coprecipitation reaction is completed, age for 1 hour, separate the solid and liquid, and dry the separated solid product at 130℃ for 5 hours to obtain ferric phosphate seed crystals.

[0107] The molar ratio of ferrous sulfate:phosphoric acid:hydrogen peroxide is 1:1.01:1.1, the mass of polyethylene glycol is 2% of the mass of ferrous sulfate, and the amount of pyrrole used per gram of ferrous sulfate is 0.01g.

[0108] S2. Add 0.3 mol / L ferrous sulfate solution and 0.3 mol / L phosphoric acid solution to the reaction vessel, stir well, add ferric phosphate seed crystals, hexadecyltrimethylammonium bromide, hydrogen peroxide and pyrrole, and add ammonia to adjust the pH of the system in the reaction vessel to 2.0. Carry out the second coprecipitation reaction at 95℃. After the second coprecipitation reaction is completed, age for 3 hours, separate the solid and liquid, and dry the separated solid product at 100℃ for 5 hours to obtain ferric phosphate dihydrate.

[0109] The molar ratio of ferrous sulfate:phosphoric acid:hydrogen peroxide is 1:1:1.1; the mass of hexadecyltrimethylammonium bromide is 3% of the mass of ferrous sulfate; the amount of pyrrole used per gram of ferrous sulfate is 0.2g; and the amount of ferric phosphate seed crystals added is 25% of the total mass of ferrous sulfate and phosphoric acid in S2.

[0110] S3. After grinding and mixing ferric phosphate dihydrate, lithium carbonate and glucose evenly, the mixture is spray-dried to obtain the spray material.

[0111] The molar ratio of ferric phosphate dihydrate to lithium carbonate is 1:1.05; the mass of glucose accounts for 4% of the total mass of ferric phosphate dihydrate and lithium carbonate; the inlet air temperature of the spray dryer is 220℃ and the outlet air temperature is 108℃.

[0112] S4. After sintering the sprayed material in a rotary kiln for 8 hours under a nitrogen atmosphere, methane gas is introduced and sintering continues for 1 hour to obtain carbon-coated lithium iron phosphate. The sintering temperature is set to 770℃ to prepare the first carbon-coated lithium iron phosphate and 800℃ to prepare the second carbon-coated lithium iron phosphate.

[0113] The gas flow rate is 0.15 L / min.

[0114] S5. Mix the first carbon-coated lithium iron phosphate and the second carbon-coated lithium iron phosphate at a mass ratio of 3:1 to obtain the lithium iron phosphate cathode material.

[0115] Example 4

[0116] This embodiment provides a lithium iron phosphate cathode material, which differs from Example 1 only in that the amount of pyrrole used per gram of ferrous sulfate in S1 is 0.02g.

[0117] Example 5

[0118] This embodiment provides a lithium iron phosphate cathode material, which differs from Example 1 only in that the amount of pyrrole used per gram of ferrous sulfate in S1 is 0.15g.

[0119] Example 6

[0120] This embodiment provides a lithium iron phosphate cathode material, which differs from Example 1 only in that the amount of pyrrole used per gram of ferrous sulfate in S2 is 0.02g.

[0121] Example 7

[0122] This embodiment provides a lithium iron phosphate cathode material, which differs from Example 1 only in that the amount of pyrrole used per gram of ferrous sulfate in S2 is 0.1g.

[0123] Example 8

[0124] This embodiment provides a lithium iron phosphate cathode material, which differs from Example 1 only in that the amount of pyrrole used per gram of ferrous sulfate in S2 is 0.18g.

[0125] Example 9

[0126] This embodiment provides a lithium iron phosphate cathode material, which differs from Embodiment 1 only in that, in S2, the amount of iron phosphate seed crystals added is 15%.

[0127] Example 10

[0128] This embodiment provides a lithium iron phosphate cathode material, which differs from Embodiment 1 only in that, in S2, the amount of iron phosphate seed crystals added is 23%.

[0129] Example 11

[0130] This embodiment provides a lithium iron phosphate cathode material, which differs from Embodiment 1 only in that polyethylene glycol is not added in S2.

[0131] Comparative Example 1

[0132] This comparative example provides a lithium iron phosphate cathode material, which differs from Example 1 only in that pyrrole is not added in S1.

[0133] Appendix Figure 5 The image shows a scanning electron microscope image of the lithium iron phosphate cathode material obtained in this comparative example. It can be seen that the lithium iron phosphate cathode material has a spherical morphology, with particles of varying sizes. The larger the particles, the more pronounced their edges and corners.

[0134] Comparative Example 2

[0135] This comparative example provides a lithium iron phosphate cathode material, which differs from Example 1 in that pyrrole is not added in S1 and S2.

[0136] Comparative Example 3

[0137] This comparative example provides a lithium iron phosphate cathode material, which differs from Example 1 in that it does not have S1, that is, iron phosphate seed crystals are not added in S2.

[0138] Comparative Example 4

[0139] This comparative example provides a lithium iron phosphate cathode material, which differs from Example 1 in that acetylene gas is not introduced during the rotary kiln sintering process in S4.

[0140] Comparative Example 5

[0141] This comparative example provides a lithium iron phosphate cathode material, which differs from Example 1 in that a roller kiln is used for sintering during the S4 sintering process, and acetylene gas is not introduced.

[0142] Comparative Example 6

[0143] This comparative example provides a lithium iron phosphate cathode material, which differs from Example 1 in that the lithium iron phosphate cathode material in S5 is only lithium iron phosphate coated with first carbon.

[0144] Comparative Example 7

[0145] This comparative example provides a lithium iron phosphate cathode material, which differs from Example 1 in that the lithium iron phosphate cathode material in S5 is only lithium iron phosphate coated with second carbon.

[0146] Experimental Example 1

[0147] The crystal structure of lithium iron phosphate cathode material was tested using an X-ray diffractometer.

[0148] Experimental Example 2

[0149] The microstructure of lithium iron phosphate cathode material was tested using scanning electron microscopy and transmission electron microscopy.

[0150] Experimental Example 3

[0151] The particle size and distribution of lithium iron phosphate cathode material were tested using a laser particle size analyzer. The particle size test results are expressed as Dv50 and are shown in Table 1.

[0152] Test Example 4

[0153] The lithium iron phosphate cathode material was tested using a Sansi longitudinal and transverse compaction density meter, and the test results are shown in Table 1.

[0154] Experimental Example 5

[0155] The lithium iron phosphate cathode material, acetylene black, and polyvinylidene fluoride obtained in the above examples and comparative examples were uniformly mixed in N-methylpyrrolidone at a mass ratio of 90:5:5. This mixture was then coated onto aluminum foil and dried in a vacuum drying oven. After drying, it was pressed into a cathode sheet using a tablet press. A coin cell was assembled according to the following sequence: negative electrode shell - lithium sheet - LiPF6 electrolyte - Celgard 2500 separator - LiPF6 electrolyte - cathode sheet - gasket - spring sheet - cathode shell. The coin cell was then allowed to stand for 10 hours in a 25°C constant temperature chamber. After standing, the electrochemical performance of the coin cell was tested using a LAND battery testing system, with a voltage range of 2.0V~3.75V. The test results are shown in Table 1.

[0156] Table 1

[0157]

[0158] Table 1 shows that, as demonstrated in Examples 1, 4-8, and Comparative Examples 1-2, the presence and amount of pyrrole affect the morphology of iron phosphate, and consequently, the morphology and physicochemical properties of lithium iron phosphate. Iron phosphate dihydrate was prepared without the addition of pyrrole, and the lithium iron phosphate synthesized using it exhibited a non-spherical morphology. This indicates that the presence of the polymer monomer can control the crystal plane growth of iron phosphate to synthesize spherical iron phosphate, and thus spherical lithium iron phosphate. When the amount of pyrrole is too low, the coating carbon content is low, resulting in limited improvement in electronic conductivity and capacity at 5C rate. When the amount of pyrrole is too high, the coating carbon layer is too thick, affecting the compaction performance of the material.

[0159] As shown in Examples 1, 9-10, and Comparative Example 3, when no seed crystals are added during the preparation of iron phosphate dihydrate, the nucleation rate of the seed crystals is slow in the low-concentration raw material environment, resulting in larger particles and morphological changes that lead to poor spherical morphology. Consequently, the spherical morphology of the subsequent lithium iron phosphate cathode material is also poor, resulting in lower compaction density and electrochemical performance. The amount of seed crystals added affects the particle size of the material. When the amount added is too small, the added seed crystals are insufficient to consume the supersaturation in the mother liquor, and the number of growth sites provided by the seed crystals is limited, leading to excessively high uncontrollable factors in the crystallization growth process and uneven particle size distribution. When the amount added is too large, the particle size of the material is too small, the contact area with the electrolyte is increased, and the electrical performance of the lithium iron phosphate cathode material is affected.

[0160] As can be seen from Example 1 and Comparative Examples 4-5, lithium iron phosphate cathode materials that do not generate a high graphitized carbon coating layer through vapor deposition have lower rate performance.

[0161] As shown in Example 1 and Comparative Examples 6-7, gradation can improve the compaction density and electrochemical performance of lithium iron phosphate cathode materials. When lithium iron phosphate cathode materials contain only large particles or only small particles, their compaction density is limited. Only by filling the gaps between large particles with small particles in a certain proportion can the space for compaction improvement be maximized.

[0162] The above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing a lithium iron phosphate cathode material, characterized in that, Includes the following steps: S1. The solution of the first iron source and the solution of the first phosphorus source are subjected to a first coprecipitation reaction in the presence of polymer monomers to obtain iron phosphate seed crystals. S2. The solution of the second iron source and the solution of the second phosphorus source are subjected to a second coprecipitation reaction in the presence of the polymer monomer and the iron phosphate seed crystals to obtain iron phosphate dihydrate. S3. The iron phosphate dihydrate, lithium source, and carbon source are mixed and then spray-dried to obtain a spray material; S4. The sprayed material is placed in a rotary kiln for sintering to obtain carbon-coated lithium iron phosphate; wherein, different sintering temperatures are set to prepare first carbon-coated lithium iron phosphate and second carbon-coated lithium iron phosphate respectively. S5. The first carbon-coated lithium iron phosphate and the second carbon-coated lithium iron phosphate are mixed in a certain mass ratio to obtain lithium iron phosphate cathode material.

2. The preparation method according to claim 1, characterized in that, The concentrations of the first iron source solution and the first phosphorus source solution are 1 mol / L to 2 mol / L, and the concentrations of the second iron source solution and the second phosphorus source solution are 0.1 mol / L to 0.5 mol / L.

3. The preparation method according to claim 1, characterized in that, The amount of iron phosphate seed crystals added is 10% to 25% of the total mass of the second iron source and the second phosphorus source.

4. The preparation method according to claim 1, characterized in that, The polymer monomer satisfies at least one of the following (1) to (2): (1): The polymer monomer includes at least one of pyrrole, aniline and thiophene; (2): In steps S1 and S2, the amount of polymer monomer used per gram of iron source is 0.01g to 0.2g.

5. The preparation method according to claim 1, characterized in that, The first coprecipitation reaction and / or the second coprecipitation reaction satisfy at least one of the following (3) to (6): (3): The coprecipitation reaction includes the addition of a morphology control agent, wherein the morphology control agent includes at least one of polyethylene glycol, polyvinylpyrrolidone and hexadecyltrimethylammonium bromide, and the mass of the morphology control agent is 0.01% to 5% of the mass of the first iron source or the second iron source; (4): The coprecipitation reaction includes the addition of an oxidant and / or a pH adjuster, wherein the oxidant includes hydrogen peroxide and the pH adjuster includes ammonia. (5): The pH of the coprecipitation reaction is 1.5~3.0, and the temperature of the coprecipitation reaction is 75℃~95℃; (6): After the coprecipitation reaction is completed, the process also includes aging and solid-liquid separation, and drying the separated solid product; wherein the aging time is 0.5h~6h, the drying temperature is 100℃~130℃, and the drying time is 2h~5h.

6. The preparation method according to claim 1, characterized in that, The sintering described in step S4 includes sintering for 3 to 8 hours, followed by introducing a reducing gas and continuing sintering for another 1 to 5 hours. The reducing gas includes at least one of acetylene and methane, and the gas flow rate is 0.01 L / min to 0.15 L / min.

7. The preparation method according to claim 1, characterized in that, Satisfy at least one of the following (7) to (9): (7): The sintering temperature corresponding to the first carbon-coated lithium iron phosphate is 730℃~770℃; (8): The sintering temperature corresponding to the second carbon-coated lithium iron phosphate is 800℃~830℃; (9): The first carbon-coated lithium iron phosphate and the second carbon-coated lithium iron phosphate are mixed at a mass ratio of (2~3):

1.

8. The preparation method according to claim 1, characterized in that, The first carbon-coated lithium iron phosphate has a Dv50 of 0.3 μm to 0.6 μm, and the second carbon-coated lithium iron phosphate has a Dv50 of 1.0 μm to 1.4 μm.

9. A lithium iron phosphate cathode material, characterized in that, It is prepared according to any one of claims 1 to 8.

10. A lithium battery, characterized in that, Including the lithium iron phosphate cathode material as described in claim 9.

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