A lithium iron phosphate composite and a method for preparing the same

Abstract

The application provides a lithium iron phosphate composite and a preparation method thereof. The preparation method uses a block copolymer with a glass transition temperature matching a spray drying process temperature as a part of a carbon source, which can uniformly adhere to and coat active ingredient particles to form a relatively regular spherical precursor powder in a spray drying process. The lithium iron phosphate composite obtained after calcination of the powder still has a relatively regular spherical appearance, and thus has a high compaction density. The lithium iron phosphate composite as a positive electrode material can improve the rate performance of a battery.

Description

Technical Field

[0001] This invention relates to the field of lithium battery materials technology, specifically a lithium iron phosphate composite and its preparation method. Background Technology

[0002] Lithium iron phosphate (LFP) has an olivine structure and boasts advantages such as abundant raw materials, low cost, good cycle performance, and high specific capacity, making it the preferred cathode material for power lithium-ion batteries. In the manufacturing process of LFP power batteries, the specific capacity of LFP has essentially reached its limit. Therefore, to further improve the battery's specific capacity, increasing the compaction density of the cathode material is a good option. Generally speaking, the higher the compaction density, the higher the battery capacity can be. Therefore, while pursuing specific capacity, improving compaction density can also be considered a direction for material improvement to enhance energy density.

[0003] In the fabrication of lithium iron phosphate (LFP) batteries, controlling the morphology of the precursor is an effective way to improve the compaction density of LFP cathode materials. Currently, there is a method for preparing LFP cathode materials that uses polymer-based carbon sources and small-molecule carbon sources to perform secondary coating of LFP. The mixing, grinding, and drying steps in this method can improve the morphology of the precursor, but this is limited to refining and shaping different particle sizes. The resulting precursor is a dispersed powder, and the polymer-based carbon sources are difficult to form a uniform coating structure on the LFP particles. Therefore, this has little effect on improving the compaction density of the cathode material, which is the sintering product. Summary of the Invention

[0004] The first objective of this invention is to provide a method for preparing a lithium iron phosphate composite that improves the compaction density of the cathode material by increasing the sphericity of the precursor powder.

[0005] A second objective of the present invention is to provide a lithium iron phosphate complex prepared by the aforementioned method.

[0006] To achieve the aforementioned first objective, the present invention provides a method for preparing a lithium iron phosphate complex, characterized by comprising the following steps:

[0007] (1) A block copolymer with a glass transition temperature of 105~145℃ is obtained by polymerizing the monomer composition and the initiator at a preset weight ratio. The monomer composition includes α-methylstyrene, styrene, methacrylate and acrylonitrile monomers.

[0008] (2) The active ingredients, carbon source and deionized water are mixed according to the preset weight ratio to form a precursor slurry. The precursor slurry is ground and spray-dried to obtain spherical precursor powder. The active ingredients include lithium source, iron source and phosphorus source, and the carbon source includes block copolymer.

[0009] (3) The spherical precursor powder was calcined under an inert gas protection state to obtain lithium iron phosphate composite.

[0010] As can be seen from the above scheme, existing lithium iron phosphate precursor powders are usually obtained by spray drying of precursor slurry. The operating parameters of spray drying equipment in related fields, such as the inlet and outlet temperatures, are relatively fixed and can only vary within a small range. Therefore, by controlling the glass transition temperature of the block copolymer used as the precursor carbon source within the operating parameter range of the aforementioned spray drying equipment, the block copolymer can exhibit a certain elasticity during the spray drying process and uniformly adhere to and coat the active ingredient particles under the impact of the fluid, thereby forming a relatively regular spherical precursor powder. The lithium iron phosphate composite obtained after calcination of this spherical precursor powder still retains a relatively regular spherical appearance, thus possessing a high compaction density and helping to improve the energy density of the battery.

[0011] By polymerizing the above monomer composition and initiator according to a preset weight ratio, a block copolymer with a glass transition temperature of 105~145℃ can be obtained. This glass transition temperature range roughly corresponds to the temperature range corresponding to the operating parameters of spray drying equipment in related fields. Therefore, during the spray drying process, the block copolymer can uniformly adhere to the surface of the active ingredient particles, thereby playing a key role in the spheroidization of the precursor. Moreover, the above spheroidization is achieved through conventional spray drying processes, without the need for additional processes such as heating and melting, granulation, etc., making it relatively easier to implement.

[0012] Furthermore, α-methylstyrene and styrene can introduce highly graphitized benzene ring-derived carbon into the carbon coating layer of lithium iron phosphate composites, which is beneficial for reducing interfacial resistance and improving electron transport efficiency. Acrylonitrile monomers can be doped with nitrogen atoms in the aforementioned carbon coating layer to improve its conductivity and also help improve the battery's capacity retention at high rates. The ester groups in methacrylates have strong polarity and can form hydrogen bonds with the surface of polar active components, thereby improving the adhesion of block copolymers to active components. The homopolymer of α-methylstyrene has a high glass transition temperature, and its introduction as a monomer facilitates the control of the glass transition temperature of block copolymers to the desired range.

[0013] A further option is that, by weight, 45-65 parts of α-methylstyrene, 15-30 parts of styrene, 15-35 parts of methacrylate and 15-30 parts of acrylonitrile monomers are used in step (1) for polymerization reaction, and the weight of the initiator accounts for 0.5-1.5% of the weight of the monomer composition.

[0014] As can be seen from the above scheme, monomers and initiators that meet the above ratio can fully polymerize under suitable reaction conditions to form block copolymers with glass transition temperatures reaching the expected range.

[0015] A further option is that the methacrylate is methyl methacrylate and / or ethyl methacrylate, and the acrylonitrile monomer is acrylonitrile and / or methacrylonitrile.

[0016] As can be seen from the above scheme, methyl methacrylate and ethyl methacrylate are the same type of organic compounds, while acrylonitrile and methacrylonitrile are derivatives. The physicochemical properties of various material options of the same type of monomers are similar. Therefore, block copolymers with glass transition temperatures that meet the expectations can be obtained by using different material options for various monomers.

[0017] A further option is that the polymerization reaction in step (1) is carried out in a solution system, the polymerization temperature is 60~80℃, and the reaction time is 6~10h.

[0018] As can be seen from the above scheme, in a solution system using organic solvents, combined with suitable initiators and reaction conditions, the aforementioned monomer composition can be polymerized to form a block copolymer with a glass transition temperature within the expected range.

[0019] A further option is that the polymerization reaction in step (1) is carried out in an emulsion system, the polymerization temperature is 70~90℃, and the reaction time is 8~12h.

[0020] As can be seen from the above scheme, selecting appropriate surfactants and initiators in the emulsion system can also ensure the full progress of the polymerization reaction.

[0021] A further approach is to mix 43-53 parts of active ingredient, 6-7 parts of carbon source and 40-50 parts of deionized water in step (2) to form a precursor slurry.

[0022] As can be seen from the above scheme, the ratio of active ingredients and carbon source in the above precursor slurry is relatively well matched, and the slurry contains an appropriate amount of deionized water, which can ensure that the active ingredients and carbon source are fully dispersed and mixed, and can also avoid excessive heat energy consumption during the spray drying process.

[0023] A further option is to include polyvinyl alcohol and / or glucose as carbon sources, with block copolymers accounting for 40-80% of the total mass of the carbon source.

[0024] As can be seen from the above schemes, in addition to block copolymers, there are a variety of conventional carbon source options. Glucose, as a biomass carbon source, can form a conductive carbon skeleton when decomposed at high temperature. Polyvinyl alcohol can form a nanoscale carbon layer after high-temperature pyrolysis. Both can improve the conductivity of lithium iron phosphate composite materials. The latter can also reduce the volume change of the material during charging and discharging through the buffering effect of the carbon layer.

[0025] A further option is that the grinding process in step (2) includes:

[0026] After coarse grinding of the precursor slurry in a wet ball mill for 20-40 minutes, it is transferred to a sand mill for fine grinding until the particle size reaches 400-600 nm.

[0027] As can be seen from the above scheme, the precursor slurry can be fully refined and dispersed by the above two grinding processes, so that the particle size of the spherical precursor powder obtained by spray drying reaches the preset range.

[0028] A further option is that the roasting process in step (3) includes:

[0029] Heat the sample at a rate of 5-10℃ / min to 300-350℃ and hold for 1-4 hours, then heat it to 650-800℃ and hold for 6-18 hours.

[0030] As can be seen from the above scheme, the first roasting treatment stage mainly involves the carbonization process of the carbon source material coating the active material particles, while the second roasting treatment stage at a higher temperature mainly involves the solid-phase reaction between the lithium source, iron source and phosphorus source to generate the lithium iron phosphate olivine structure. At the same time, the graphitized carbon of the block copolymer in the carbon coating layer combines with the amorphous carbon formed by other carbon source materials to form a multi-level carbon network structure, which ensures conductivity and avoids the carbon layer being too thick to hinder lithium ion diffusion.

[0031] To achieve the second objective mentioned above, the present invention provides a lithium iron phosphate complex, which is characterized in that the lithium iron phosphate complex is prepared by the aforementioned method for preparing lithium iron phosphate complex.

[0032] As can be seen from the above scheme, during the spray drying stage of the lithium iron phosphate composite preparation process, the block copolymer can uniformly adhere to and coat the active ingredient particles to form a regular spherical precursor powder. The powder has a high degree of sphericity, and the lithium iron phosphate composite obtained after subsequent calcination still retains a relatively regular spherical appearance, thus having a high compaction density, which helps to improve the energy density of related batteries.

[0033] The preparation method of the present invention uses a block copolymer with a glass transition temperature matching the spray drying process temperature as a carbon source. During the spray drying process, it can uniformly adhere to and coat the active ingredient particles to form a relatively regular spherical precursor powder. After the powder is calcined, the resulting lithium iron phosphate composite still retains a relatively regular spherical appearance, thus having a high compaction density. Using it as a positive electrode material can improve the rate performance of the battery. Attached Figure Description

[0034] Figure 1 This is a 1000x electron microscope scan image of the spherical precursor powder prepared in Example 1.

[0035] Figure 2This is a 5000x electron microscope scan image of the spherical precursor powder prepared in Example 1. Detailed Implementation

[0036] This invention provides a method for preparing a lithium iron phosphate complex, comprising the following steps:

[0037] (1) A block copolymer with a glass transition temperature of 105~145℃ is obtained by polymerizing the monomer composition and the initiator at a preset weight ratio. The monomer composition includes α-methylstyrene, styrene, methacrylate and acrylonitrile monomers.

[0038] (2) The active ingredients, carbon source and deionized water are mixed according to the preset weight ratio to form a precursor slurry. The precursor slurry is ground and spray-dried to obtain spherical precursor powder. The active ingredients include lithium source, iron source and phosphorus source, and the carbon source includes block copolymer.

[0039] (3) The spherical precursor powder was calcined under an inert gas protection state to obtain lithium iron phosphate composite.

[0040] Preferably, in step (1), 45-65 parts of α-methylstyrene, 15-30 parts of styrene, 15-35 parts of methacrylate and 15-30 parts of acrylonitrile monomers are used for polymerization reaction, and the weight of the initiator accounts for 0.5-1.5% of the weight of the monomer composition.

[0041] Preferably, the methacrylate is methyl methacrylate and / or ethyl methacrylate, and the acrylonitrile monomer is acrylonitrile and / or methacrylonitrile.

[0042] Preferably, the polymerization reaction in step (1) is carried out in a solution system, the temperature of the polymerization reaction is 60~80℃, and the reaction time is 6~10h.

[0043] Preferably, the polymerization reaction in step (1) is carried out in an emulsion system, the polymerization temperature is 70~90℃, and the reaction time is 8~12h.

[0044] Preferably, in step (2), 43-53 parts of active ingredient, 6-7 parts of carbon source and 40-50 parts of deionized water are mixed to form a precursor slurry.

[0045] Preferably, the carbon source also includes polyvinyl alcohol and / or glucose, and the block copolymer accounts for 40-80% of the total mass of the carbon source.

[0046] Preferably, the grinding process in step (2) includes:

[0047] After coarse grinding of the precursor slurry in a wet ball mill for 20-40 minutes, it is transferred to a sand mill for fine grinding until the particle size reaches 400-600 nm.

[0048] Preferably, the roasting process in step (3) includes:

[0049] Heat the sample at a rate of 5-10℃ / min to 300-350℃ and hold for 1-4 hours, then heat it to 650-800℃ and hold for 6-18 hours.

[0050] The present invention also provides a lithium iron phosphate complex, which is prepared by the aforementioned method for preparing lithium iron phosphate complex.

[0051] Examples and comparative examples of lithium iron phosphate composites and their preparation methods

[0052] The lithium iron phosphate complex of Example 1 was prepared according to the following steps:

[0053] (1) After evacuating the glass reactor, nitrogen was repeatedly introduced three times to form an inert gas protective environment. Then, 1000g of α-methylstyrene, 200g of styrene, 400g of methyl methacrylate and 300g of acrylonitrile were added as reaction substrates, and 3150g of toluene was added as solvent. After stirring to dissolve and mixing, the mixture was heated to 80°C under circulating water bath conditions and kept warm. 10.5g of benzoyl peroxide was dissolved in 200g of styrene as an initiator solution and continuously and slowly injected into the reactor for polymerization. During the reaction, the stirring paddle was kept running at a speed of 200r / min and the reaction time was controlled at 8h. After the reaction system was cooled to room temperature, the solvent was removed by vacuum distillation to obtain the block copolymer.

[0054] (2) Weigh 400g of lithium carbonate as lithium source, 1600g of iron phosphate as phosphorus source and iron source, 120g of block copolymer, 60g of glucose and 60g of polyvinyl alcohol as carbon source and mix with 2kg of deionized water to form a precursor slurry. Put the precursor slurry into a wet ball mill for coarse grinding for 20~40min and then transfer it to a sand mill for fine grinding until the particle size reaches 400~600nm. After spray drying, obtain spherical precursor powder.

[0055] (3) The spherical precursor powder was heated to 300°C at a rate of 5~10°C / min under nitrogen protection and held for 1h as the first stage of calcination treatment. Then, it was heated to 700°C and held for 6h as the second stage of calcination treatment. The calcination product of this stage was used as the lithium iron phosphate composite.

[0056] The preparation process of the lithium iron phosphate composite in Example 2 is roughly the same as that in Example 1, except that: in step (1), 1200g of α-methylstyrene, 100g of styrene, 400g of methyl methacrylate, 100g of ethyl methacrylate and 500g of methacrylonitrile are used as reaction substrates, the amount of toluene solvent is 3750g, the initiator solution is obtained by dissolving 25g of benzoyl peroxide in 200g of styrene, the reaction temperature of the polymerization reaction is 70℃, and the reaction time is 6h; in step (2), 344g of lithium carbonate is used as lithium source, 1376g of iron phosphate is used as phosphorus source and iron source, 168g of block copolymer, 56g of glucose and 56g of polyvinyl alcohol are used as carbon source and 1600g of deionized water to prepare precursor slurry; in step (3), the first stage of calcination treatment is kept at 330℃ for 2h, and the second stage is kept at 750℃ for 8h.

[0057] The preparation process of the lithium iron phosphate composite in Example 3 is roughly the same as that in Example 1, except that: in step (1), 1300g of α-methylstyrene, 160g of styrene, 300g of methyl methacrylate and 400g of acrylonitrile are used as reaction substrates, the amount of toluene solvent is 3540g, the initiator solution is obtained by dissolving 35.4g of benzoyl peroxide in 200g of styrene, the reaction temperature of the polymerization reaction is 60℃, and the reaction time is 10h; in step (2), 368g of lithium carbonate is used as lithium source, 1472g of iron phosphate is used as phosphorus source and iron source, 182g of block copolymer, 39g of glucose and 39g of polyvinyl alcohol are used as carbon source and 1800g of deionized water to prepare precursor slurry; in step (3), the first stage of calcination treatment is kept at 350℃ for 3h, and the second stage is kept at 650℃ for 10h.

[0058] The preparation process of the lithium iron phosphate composite in Example 4 is roughly the same as that in Example 1, except that the polymerization reaction in step (1) is carried out in an emulsion system, using 1040g of α-methylstyrene, 300g of styrene, 360g of ethyl methacrylate, and 340g of methacrylonitrile as reaction substrates. Then, 3360g of deionized water and 23g of sodium dodecyl sulfate are added and mixed at a stirring speed of 200r / min to form an emulsion. The initiator solution is prepared by dissolving 26.9g of benzoyl peroxide in 20 0g of styrene was obtained. The polymerization reaction temperature was 70℃ and the reaction time was 10h. After precipitation with anhydrous ethanol and washing more than 3 times, the block copolymer was obtained by vacuum drying. In step (2), 424g of lithium carbonate was used as the lithium source, 1696g of iron phosphate was used as the phosphorus source and iron source, 224g of block copolymer, 28g of glucose and 28g of polyvinyl alcohol were used as carbon sources and 2000g of deionized water to prepare the precursor slurry. In step (3), the first stage of the calcination treatment was kept at 320℃ for 4h, and the second stage was kept at 800℃ for 15h.

[0059] The preparation process of the lithium iron phosphate composite in Example 5 is roughly the same as that in Example 4, except that: in step (1), 900g of α-methylstyrene, 400g of styrene, 700g of ethyl methacrylate, 280g of acrylonitrile and 280g of methacrylonitrile are used as reaction substrates, and then 4140g of deionized water and 28g of sodium dodecyl sulfate are added and stirred to form an emulsion. The initiator solution is obtained by dissolving 22.1g of benzoyl peroxide in 200g of styrene. The polymerization temperature is 80℃ and the reaction time is 12h. In step (2), 384g of lithium carbonate is used as the lithium source, 1536g of iron phosphate is used as the phosphorus source and iron source, 96g of block copolymer, 72g of glucose and 72g of polyvinyl alcohol are used as carbon sources and 1800g of deionized water to prepare the precursor slurry. In step (3), the first stage of the calcination treatment is kept at 340℃ for 2.5h and the second stage is kept at 770℃ for 12h.

[0060] The preparation process of the lithium iron phosphate composite in Example 6 is roughly the same as that in Example 4, except that: in step (1), 1100g of α-methylstyrene, 240g of styrene, 60g of methyl methacrylate, 600g of ethyl methacrylate and 600g of acrylonitrile are used as reaction substrates, followed by the addition of 4200g of deionized water and 28g of sodium dodecyl sulfate to form an emulsion. The initiator solution is obtained by dissolving 28g of benzoyl peroxide in 200g of styrene. The polymerization temperature is 90℃ and the reaction time is 8h. In step (2), 400g of lithium carbonate is used as the lithium source, 1600g of iron phosphate is used as the phosphorus source and iron source, 156g of block copolymer, 52g of glucose and 52g of polyvinyl alcohol are used as carbon sources and 1600g of deionized water to prepare the precursor slurry. In step (3), the first stage of the calcination treatment is kept at 310℃ for 3.5h and the second stage is kept at 680℃ for 18h.

[0061] Testing revealed that the glass transition temperatures of the block copolymers obtained in Examples 1-6 were all within the range of 105-145℃. The lithium iron phosphate composites prepared in Examples 1-6 were subjected to compaction density testing according to Appendix L of GB / T 24533-2019, "Method for Determination of Powder Compacted Density". Three sets of samples were tested repeatedly, and the average value was taken as the measured compaction density. The test results are shown in Table 1.

[0062] Table 1: Test Results of Sample Compacted Density

[0063]

[0064] As shown in Table 1, the compaction density values ​​of the lithium iron phosphate composites prepared in the six examples are quite similar and all fall within the range of 2.5 g / cm³. 3 Around 100,000, which is considered a relatively high compaction density level.

[0065] The spherical precursor powder obtained in step (2) of Example 1 was observed using a scanning electron microscope, and its appearance was as follows: Figure 1 and Figure 2 As shown. Figure 1 It's a 1000x esports scan image, from Figure 1 As can be seen from the data, the spherical precursor powder prepared in Example 1 has a relatively regular particle shape and a high degree of sphericity. Figure 2 As can be seen from the example, the spherical precursor powder prepared in Example 1 has an outermost structural layer formed by the uniform attachment and coating of block copolymer microparticles.

[0066] Electrochemical performance testing

[0067] The comparative examples used in the following tests were prepared in accordance with the method of Example 1 to prepare block copolymers and lithium iron phosphate complexes. The difference was that commercially available polystyrene was used to replace the block copolymer component in the carbon source, and therefore the block copolymer preparation process in step (1) was not involved.

[0068] Using the lithium iron phosphate composites prepared in Example 1 and the comparative example as cathode materials, cathode sheets were fabricated and batteries were assembled according to the same method. The battery assembly process is as follows:

[0069] Using N-methylpyrrolidone as a solvent, lithium iron phosphate, binder (PVDF-hsv-900), and conductive agent (SP) were added to the solvent at a mass ratio of 95:3:2 for homogenization. Then, aluminum foil was used as the current collector for coating on an infrared flatbed coating machine. After the sample was vacuum dried, it was rolled, die-cut, selected, and weighed to obtain the positive electrode sheet. The weighed electrode sheet was transferred to a glove box for coin cell assembly. The coin cell case was model CR2032, and the negative electrode was lithium sheet.

[0070] Using the battery corresponding to Example 1 as Sample 1 and the battery corresponding to the comparative example as Sample 2, electrochemical performance tests were conducted on the two sample batteries. The 0.2C discharge capacity of Sample 1 was 158.1 mAh / g, and the 1C discharge capacity was 147.2 mAh / g. The 0.2C discharge capacity of Sample 2 was 151.3 mAh / g, and the 1C discharge capacity was 133.4 mAh / g. This demonstrates that the carbon coating layer obtained from the block copolymer conversion in the lithium iron phosphate composite of the present invention possesses excellent conductivity, which can significantly improve the rate performance of the battery.

[0071] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0072] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a lithium iron phosphate composite, characterized in that, Includes the following steps: (1) A block copolymer with a glass transition temperature of 105-145°C is obtained by polymerizing a monomer composition and an initiator at a predetermined weight ratio. The monomer composition includes 45-65 parts of α-methylstyrene, 15-30 parts of styrene, 15-35 parts of methacrylate and 15-30 parts of acrylonitrile monomers. The methacrylate is methyl methacrylate and / or ethyl methacrylate, so the acrylonitrile monomers are acrylonitrile and / or methacrylonitrile. The weight of the initiator accounts for 0.5-1.5% of the weight of the monomer composition. (2) The active ingredient, carbon source and deionized water are mixed in a preset weight ratio to form a precursor slurry. The precursor slurry is then ground and spray-dried to obtain spherical precursor powder. The active ingredient includes lithium source, iron source and phosphorus source. The precursor slurry includes 43-53 parts of the active ingredient, 6-7 parts of the carbon source and 40-50 parts of deionized water. The carbon source includes the block copolymer and polyvinyl alcohol and / or glucose. The block copolymer accounts for 40-80% of the total mass of the carbon source. (3) The spherical precursor powder is calcined under an inert gas atmosphere to obtain a lithium iron phosphate composite, wherein the calcination process includes: Heat the sample at a rate of 5-10℃ / min to 300-350℃ and hold for 1-4 hours, then heat it to 650-800℃ and hold for 6-18 hours.

2. The method for preparing the lithium iron phosphate composite as described in claim 1, characterized in that: The polymerization reaction in step (1) is carried out in a solution system at a temperature of 60-80℃ and a reaction time of 6-10h.

3. The method for preparing the lithium iron phosphate composite as described in claim 1, characterized in that: The polymerization reaction in step (1) is carried out in an emulsion system at a temperature of 70-90°C for 8-12 hours.

4. The method for preparing the lithium iron phosphate composite as described in claim 1, characterized in that: The grinding process described in step (2) includes: The precursor slurry is coarsely ground in a wet ball mill for 20-40 minutes and then finely ground in a sand mill until the particle size reaches 400-600 nm.

5. A lithium iron phosphate complex, characterized in that: The lithium iron phosphate composite was prepared by the method described in any one of claims 1 to 4.

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