Preparation method of lithium iron phosphate composite material by adopting two-firing process

Abstract

The invention provides a preparation method of a lithium iron phosphate composite material by adopting a two-firing process, and relates to the technical field of lithium ion batteries. The preparation method of the lithium iron phosphate composite material comprises the following steps: step 1, preparing a mixture containing iron phosphate, a lithium source, a first carbon source and water into first slurry, and carrying out first sintering on the first slurry to prepare a carbon-coated lithium iron phosphate preform; step 2, mixing the carbon-coated lithium iron phosphate preform prepared in the step 1, a surfactant and water to prepare slurry, standing for layering, separating to obtain upper-layer slurry which is second slurry and lower-layer slurry which is third slurry, mixing the second slurry with a second carbon source and water, and then carrying out second sintering; and mixing the third slurry with a third carbon source and water, then carrying out third sintering, and respectively or mixing products obtained after the second sintering and the third sintering as the lithium iron phosphate composite material. According to the invention, the particles with different particle sizes are separated, and a secondary burning process of different carbon supplementing schemes of different primary burning particles is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and particularly to a preparation method of a lithium iron phosphate composite material adopting a two-step sintering process. Background Art

[0002] Lithium iron phosphate batteries exhibit good cycle stability and long cycle life during charge and discharge processes. However, lithium iron phosphate also has the disadvantages of poor conductivity and low lithium ion diffusion rate. Currently, in order to improve the conductivity of lithium iron phosphate materials, most of the lithium iron phosphate materials used in current lithium-ion batteries have undergone in-situ coating treatment with amorphous carbon. However, while pursuing a dense, uniform, and highly graphitized carbon coating layer, when the dosage and combination of the in-situ coating carbon source are inappropriate, it is easy to provide too much spatial position during the sintering process, resulting in insufficient growth of particles and inability to form good particle combinations, leading to a decrease in powder compaction and a reduction in the battery energy density.

[0003] To solve the improvement of carbon coating effect and compaction density during the sintering process, a two-step sintering process is often used for improvement in the industrial production of lithium iron phosphate. However, whether it is the method of re-slurrying the iron lithium after the first sintering for in-situ carbon supplementation and then re-granulating for the second sintering; or the method of non-in-situ carbon coating of the iron lithium after the first sintering by supplementing carbon and metal oxides, etc., it is impossible to avoid the carbon source for carbon supplementation during the second sintering from acting on the large and small iron lithium particles formed after the first sintering at the same time, reducing the effect of the second sintering on carbon coating and improving the combination of large and small particles after the second sintering. In addition, there is also a secondary sintering method that directly completes in-situ carbon coating under high temperature conditions by introducing a gaseous carbon source under the action of separating large and small particles in a fluidized bed after the first sintering, so that the particles after the first sintering of different sizes are coated with different carbon sources in-situ, thereby maximizing the advantages of the two-step sintering process for carbon coating and the combination of large and small particles. However, the fluidized bed reaction device is not a common iron lithium production process, and the addition of equipment and the high requirements of other carbon sources for fire and explosion prevention in the production environment are not conducive to industrialization and cost control of production. Summary of the Invention

[0004] In view of the above analysis, the present invention aims to provide a preparation method of a lithium iron phosphate composite material adopting a two-step sintering process to solve at least one of the following existing problems: improving the battery energy density, reducing the production cost, and differentiating specific products to meet the market demand after industrialization.

[0005] The object of the present invention is mainly achieved through the following technical solutions:

[0006] In a first aspect, the present invention provides a preparation method of a lithium iron phosphate composite material adopting a two-step sintering process, including the following steps:

[0007] Step 1: Prepare a first slurry from a mixture containing iron phosphate, a lithium source, a first carbon source, and water, and perform a first sintering on the first slurry to obtain a carbon-coated lithium iron phosphate preform;

[0008] Step 2: Mix the carbon-coated lithium iron phosphate preform obtained in Step 1, a surfactant, and water to make a slurry. After standing and separating into layers, the upper slurry obtained by separation is the second slurry, and the lower slurry is the third slurry. Mix the second slurry with a second carbon source and water, and then perform a second sintering; mix the third slurry with a third carbon source and water, and then perform a third sintering. The products after the second sintering and the third sintering are used separately or mixed as the lithium iron phosphate composite material;

[0009] Wherein, in Step 2, the surfactant includes a first surfactant and a second surfactant. The first surfactant includes at least one of fatty alcohol polyoxyethylene ether and alkylphenol polyoxyethylene ether; the second surfactant includes polyoxyethylene-polyoxypropylene block polyether; the second carbon source includes at least one of polyethylene glycol, polyvinyl alcohol, polyacrylic acid, and phenolic resin; the third carbon source includes at least one of glucose, sucrose, and citric acid.

[0010] Preferably, the mass ratio of the first surfactant to the second surfactant is (0.5 - 1.5):(1.5 - 2.5).

[0011] Preferably, the number average molecular weight of the first surfactant is 1000 - 3000, preferably 1500 - 2000.

[0012] Preferably, the HLB value of the first surfactant is 11 - 18, preferably 12 - 14.

[0013] Preferably, the number average molecular weight of the second surfactant is 8000 - 15000.

[0014] Preferably, the mass ratio of the mass x of the first carbon source, the total mass y of the non-ionic surfactant, and the total mass (1 - x - y) of the second carbon source and the third carbon source is x:y:(1 - x - y), where x ≥ 0.4, y ≥ 0.20. Preferably, 0.7 ≥ x ≥ 0.4, 0.5 ≥ y ≥ 0.20, and 0.35 ≥ (1 - x - y) ≥ 0.1.

[0015] Preferably, based on the total weight of the lithium iron phosphate composite material, the total carbon content of the lithium iron phosphate composite material is 1 wt.% - 2 wt.%, preferably 1.3 wt.% - 1.9 wt.%.

[0016] Preferably, the molar ratio of Fe:P:Li in the iron phosphate and the lithium source is 1:(1.01 - 1.03):(1.015 - 1.05).

[0017] Preferably, in step 1, the lithium source is lithium carbonate.

[0018] Preferably, in step 1, the first sintering temperature is 730 - 850 °C and the time is 1 - 20 h.

[0019] Preferably, step 1 further includes a step of sanding the mixture. After sanding, a first slurry is obtained. The D50 of the particles after sanding is 0.3 - 0.8 μm, preferably 0.4 - 0.5 μm.

[0020] Preferably, the first carbon source includes at least one of glucose, sucrose, and citric acid.

[0021] Preferably, in step 2, the separation conditions of the second slurry and the third slurry include: the standing time is 5 - 60 min and the temperature is 25 - 60 °C.

[0022] Preferably, in step 2, the conditions of the second sintering and the third sintering are the same or different and each independently includes: the temperature is 600 - 830 °C and the time is 1 - 20 h.

[0023] Preferably, in step 2, a conductive agent is respectively added during the mixing process of the second slurry and the second carbon source and during the mixing process of the third slurry and the third carbon source. The conductive agent includes one or more of carbon nanotubes, graphene, carbon nanotubes, and conductive carbon black.

[0024] Preferably, the total amount of the conductive agent is 0.1% - 0.75% of the mass of iron phosphate.

[0025] Preferably, in step 2, a metal dopant is further added during the mixing process of the third slurry with the third carbon source, the conductive agent, and water.

[0026] Preferably, the metal dopant includes at least one of compounds of vanadium, titanium, niobium, molybdenum, and boron; more preferably, the amount of the metal dopant is 0.1% - 1.2% of the weight of iron phosphate.

[0027] In a second aspect, the present invention provides a lithium iron phosphate composite material prepared by the preparation method.

[0028] In a third aspect, the present invention provides a lithium-ion battery, including a positive electrode material, and the positive electrode material includes the lithium iron phosphate composite material.

[0029] Compared with the prior art, the present invention can at least achieve one of the following beneficial effects:

[0030] A) The preparation method of the lithium iron phosphate composite material provided by the present invention, by adding a non-ionic surfactant before the second sintering, separating particles of different particle sizes, and then adding a carbon supplementation scheme and re-granulating for the second sintering, easily realizes the second sintering process with different carbon supplementation schemes for different particles after the first sintering.

[0031] B) The preparation method of the lithium iron phosphate composite material provided by the present invention realizes the product layout of high-compactness lithium iron phosphate composite materials with different particle gradings by adopting carbon supplementation or metal doping schemes to different degrees respectively according to specific product requirements during the process of re-pulping primary particles after separation. Description of the Drawings

[0032] Figure 1 It is the particle size distribution diagram of Example 1 and Comparative Example 1 provided by the present invention;

[0033] Figure 2 It is the cycle performance diagram of the 42Ah square battery test of Example 1 and Comparative Example 1 provided by the present invention. Detailed Embodiments

[0034] Next, the preferred embodiments of the present invention will be specifically described in conjunction with the drawings, where the drawings form a part of the present invention and are used together with the embodiments of the present invention to explain the principles of the present invention.

[0035] To solve the problem that it is difficult to make carbon sources / metal oxides of different types and dosages act on the lithium iron phosphate after primary sintering with a preliminary particle size distribution during the second sintering process, in this solution, during the process of re-pulping iron lithium after primary sintering, by adding a quantitative specific surfactant, under the dispersion effect provided by the surfactant, the primary sintered iron lithium in different particle ranges can be effectively separated within a short time. By re-performing different carbon supplementation / metal doping schemes on the separated slurry and then re-granulating, different degrees of carbon coating and doping schemes can be obtained for the primary particle iron lithium of different sizes that have been formed during the second sintering process, so as to maximize the advantages of the second sintering process.

[0036] Hereinafter, the present invention will be described in detail.

[0037] In the first aspect, the present invention provides a preparation method of a high-compactness lithium iron phosphate composite material using a two-step sintering process, and the preparation method includes the following steps:

[0038] Step 1: Obtain a mixture of iron phosphate, a lithium source, a first carbon source (carbon source I) and water, prepare a first slurry (slurry I), and perform a first sintering on the first slurry to obtain a carbon-coated lithium iron phosphate preform;

[0039] Step 2: Mix the carbon-coated lithium iron phosphate preform prepared in Step 1, surfactant and water to make a slurry. After standing and stratifying, the upper slurry separated is the second slurry (slurry IA), and the lower slurry is the third slurry (slurry IB). Mix the second slurry with a second carbon source and water, and then perform a second sintering. Mix the third slurry with a third carbon source, and then perform a third sintering. The products after the second sintering and the third sintering are used separately or mixed as the lithium iron phosphate composite material;

[0040] Among them, in Step 2, the surfactant includes a first surfactant and a second surfactant. The first surfactant is selected from at least one of fatty alcohol polyoxyethylene ether (AEO) and alkylphenol polyoxyethylene ether (APEO); the second surfactant is selected from polyoxyethylene-polyoxypropylene block polyether (EO / PO); the second carbon source is one or more of polyethylene glycol, polyvinyl alcohol, polyacrylic acid, phenolic resin. Preferably, the second carbon source is polyethylene glycol (the number average molecular weight can be 2000-6000); the third carbon source is one or more of glucose, sucrose, citric acid. Preferably, the third carbon source is glucose.

[0041] It should be noted that the first sintering in Step 1 can be understood as a primary sintering, and the second sintering and the third sintering in Step 2 can be understood as secondary sinterings. It should be noted that the first, second, third, etc. terms in the present invention are used to facilitate understanding and distinguish the same terms, and are not a limitation on the order of the method steps.

[0042] In the present invention, after re-pulping the lithium iron phosphate after the first firing, a composite surfactant is added to the slurry. Under the dispersion action of the surfactant, the separation of the lithium iron with different primary sintering particle size ranges in the slurry can be completed. By performing different carbon supplementation schemes on the primary sintered lithium iron with different separated particle size ranges, the primary particles after re-granulation can complete the secondary sintering under the in-situ carbon coating of different schemes, giving full play to the advantages of the secondary sintering. And by adjusting the range of the separated primary lithium iron particle size, it is convenient to perform specific product differentiation of the lithium iron after the second firing in terms of specific capacity / compaction and other properties to meet the market demand after industrialization.

[0043] The non-ionic surfactant used in the present invention provides the main effect for the separation effect. The non-ionic surfactant used is a compound. ① One is fatty alcohol polyoxyethylene ether (AEO) or alkylphenol polyoxyethylene ether (APEO), which provides emulsifying characteristics for the slurry, and effectively captures medium and large particles under the emulsifying action, facilitating the rapid sedimentation of medium and large particles. ② The other is polyoxyethylene polyoxypropylene block polyether (EO / PO), which provides wetting and dispersing effects for the slurry. The block polyether surfactant aggregates on the surface of small particles and gradually forms a critical micelle state, which can effectively prevent the sedimentation of small particles, including the situation where the sedimentation of large particles drives the sedimentation of small particles. By adjusting the dosage and ratio of the two surfactants, the large and small particles in a specific particle size range can be effectively and rapidly separated.

[0044] It should be noted that in the preparation method provided by the present invention, when no surfactant is used in step 2, the second slurry and the third slurry cannot be obtained by static layering separation. When a single type of surfactant is used, the layering effect is poor, and the separation of large and small particles is incomplete, thereby affecting the subsequent carbon supplementation effect and resulting in poor performance of the lithium iron phosphate composite material.

[0045] In the specific embodiment of the present invention, the mass ratio of the first surfactant to the second surfactant is (0.5 - 1.5):(1.5 - 2.5). For example, the values in the range of 0.5 - 1.5 can be 0.5, 0.7, 1, 1.2, 1.5, etc., and the values in the range of 1.5 - 2.5 can be 1.5, 1.7, 2, 2.2, 2.5, etc. It should be noted that the mass ratio of the first surfactant to the second surfactant will affect the yields of the first slurry and the second slurry. When the mass ratio of the two is lower or higher than the range defined by the present invention, the separation effect required by the present invention cannot be obtained, thereby resulting in poor performance of the lithium iron phosphate composite material.

[0046] In the specific embodiment of the present invention, the number average molecular weight of the first surfactant is between 1000 and 3000, preferably 1500 - 2000, and the HLB value is between 11 and 18, such as 11, 12, 13, 14, 15, 16, 17, 18, etc., preferably 12 - 14. By adjusting the dosage, molecular weight, and HLB value of the surfactant, different degrees of emulsifying power can be provided for the slurry, which is manifested as the specific particle size level of the medium particles in the medium and large particles, and the particles larger than this particle size level can be effectively sedimented. At the same time, due to the low molecular weight of the surfactant, it has the characteristics of low carbon residue content and small steric hindrance provided, which is conducive to the re-melting and growth of medium and large particles during the second sintering process, forming lithium iron phosphate products with high crystallinity and regular particle morphology.

[0047] In a specific embodiment of the present invention, the number-average molecular weight of the second surfactant is between 8,000 and 15,000, and its wetting and dispersing properties can effectively prevent the sedimentation of small particles, including the situation where the sedimentation of large particles drives the sedimentation of small particles. At the same time, since the surfactant mainly remains in the small particle slurry and has a relatively high molecular weight, it has the characteristics of a high carbon residue content and a large steric hindrance provided, which is beneficial to preventing the re-melting and growth of small particles during the second calcination process, thereby forming a high-crystallinity and regularly shaped finished product of lithium iron phosphate with a highly graphitized, dense and evenly distributed carbon coating layer.

[0048] In the present invention, the carbon source used in the carbon coating in the preparation method is a composite carbon source, including a first carbon source, a surfactant, a second carbon source, and a third carbon source. Specifically, the mass ratio of the total mass x of the first carbon source, the total amount y of the non-ionic surfactant, and the total amount (1 - x - y) of the second carbon source and the third carbon source is x:y:(1 - x - y), where x ≥ 0.4, y ≥ 0.2. For example, x can be 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, etc., y can be 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, etc., and (1 - x - y) can be 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, etc. Preferably, 0.7 ≥ x ≥ 0.4, 0.5 ≥ y ≥ 0.2, and 0.35 ≥ (1 - x - y) ≥ 0.1.

[0049] In a specific embodiment of the present invention, based on the total weight of the finally obtained lithium iron phosphate composite material, the total carbon content is 1 wt.% - 2 wt.%, that is, the coated carbon accounts for 1 wt.% - 2 wt.% of the total mass of the nanoscale lithium iron phosphate composite material. Preferably, the coated carbon accounts for 1.3 wt.% - 1.9 wt.% of the total mass of the nanoscale lithium iron phosphate composite material.

[0050] In a specific embodiment of the present invention, in step 1, the iron phosphate has a high iron-to-phosphorus ratio, the iron-to-phosphorus ratio is 0.980 - 0.987, and the specific surface area is 10 - 15 m 2 g -1 . Preferably, the iron-to-phosphorus ratio is 0.983 - 0.985, and the specific surface area is 12 - 13 m 2 g -1 .

[0051] In a specific embodiment of the present invention, in step 1, the lithium source is battery-grade lithium carbonate.

[0052] In a specific embodiment of the present invention, in step 1, the molar ratio of Fe:P:Li in the iron phosphate and the lithium source is 1:(1.01 - 1.03):(1.015 - 1.05).

[0053] In a specific embodiment of the present invention, in step 1, the first carbon source I is one or more of glucose, sucrose, and citric acid. Preferably, the first carbon source is glucose.

[0054] In a specific embodiment of the present invention, in step 1, the solid content of the first slurry is 30 wt.% to 40 wt.%.

[0055] In a specific embodiment of the present invention, in step 1, after mixing the iron source, lithium source, first carbon source, and water, a sanding step is further included. After sanding, the first slurry is prepared. The median particle size (D50) of the particle material after sanding is 0.3 to 0.8 μm. Preferably, D50 is 0.4 to 0.5 μm. The sanding medium used is zirconium balls with a diameter of 0.2 to 0.3 mm.

[0056] In a specific embodiment of the present invention, in step 1, the first sintering is carried out under the protection of a nitrogen atmosphere. The sintering temperature is 730 - 850 °C, and the time is 1 - 20 h. Preferably, in step 1, before sintering the first slurry, a spray drying step is further included, and after sintering, an air flow pulverization and sieving step is further included.

[0057] In a specific embodiment of the present invention, in step 2, the carbon-coated lithium iron phosphate preform, surfactant, and water are mixed to make a slurry. The solid content of this slurry is 40 wt.% to 80 wt.%.

[0058] In a specific embodiment of the present invention, in step 2, the separation conditions of the second slurry IA and the third slurry IB are such that after the carbon-coated lithium iron phosphate preform, surfactant, and water are mixed evenly, they are left standing for 5 - 60 min, and the slurry temperature is 25 - 60 °C; the second slurry IA and the third slurry IB are obtained by separating the supernatant in the area occupied by the outflow of the second slurry IA.

[0059] In a specific embodiment of the present invention, in step 2, a conductive agent can be added respectively during the mixing process of the second slurry and the second carbon source and during the mixing process of the third slurry and the third carbon source. The conductive agent is one or more of carbon nanotubes, graphene, carbon nanotubes, and conductive carbon black. Preferably, the conductive agent is carbon nanotubes, graphene, or a mixture thereof. Further, depending on the different conductive agents or their combinations used, the total amount of the conductive agent used is 0.1% to 0.75% of the mass of the iron phosphate used.

[0060] In a specific embodiment of the present invention, in step 2, during the mixing process of the third slurry IB with the third carbon source, conductive agent and water, a metal dopant is further added. Preferably, the metal dopant is one or more compounds (preferably oxides) of elements such as vanadium, titanium, niobium, molybdenum, boron, etc., and the doping elements are one or more of vanadium, titanium, niobium, molybdenum, boron. The amount of the metal dopant is 0.1% - 1.2% of the weight of iron phosphate. Preferably, the metal dopant is one or more of titanium oxide, vanadium oxide, molybdenum oxide, niobium oxide.

[0061] It should be noted that the mass ratio of the second carbon source mixed with the second slurry IA to the third carbon source mixed with the third slurry IB is the same as the mass ratio of the solid contents of the second slurry IA and the third slurry IB. For example, if the mass ratio of the solid contents of the second slurry IA and the third slurry IB is 1:3, then the mass of the second carbon source mixed with the second slurry IA: the mass of the third carbon source mixed with the third slurry IB = 1:3. The mass ratio of the conductive agents respectively mixed with the second slurry IA and the third slurry IB is similar to the above situation and will not be elaborated here.

[0062] In a specific embodiment of the present invention, in step 2, the conditions of the second sintering and the third sintering are the same or different, and each is independently carried out under the protection of a nitrogen atmosphere, the sintering temperature is 600 - 830 °C, and the constant temperature is 1 - 20 h. Preferably, the second slurry and the third slurry further include a spray drying step before sintering, and a crushing and sieving step after sintering.

[0063] The preparation method of the lithium iron phosphate composite material provided by the present invention uses a solid-phase method to prepare a phosphate-based cathode material, and adapts the secondary sintering process by adjusting the separation effect of the primary particles of lithium iron phosphate after the first sintering. By adding a non-ionic surfactant, etc., to improve the dispersion effect of the slurry, so that the large and small particles in different height ranges of the slurry maintain a specific particle size range distribution. By adjusting the height of the discharge hole in the container, the primary particles of different particle sizes can be easily separated, and then a customized carbon supplementation / doping, etc. scheme is adopted, and re-granulation is carried out for secondary sintering, and a secondary sintering process with different carbon supplementation / doping schemes for different primary sintering particles can be realized.

[0064] In a specific embodiment of the present invention, the specific steps of the preparation method provided by the present invention are as follows:

[0065] Step 1: Grind the iron source, lithium source, carbon source I and deionized water together to a suitable particle size to obtain slurry I. After spray drying the slurry I, sinter it at 730 - 850 °C under the protection of a nitrogen atmosphere for 1 - 20 h, and after discharging the furnace, carry out air flow crushing to prepare a high-graphitized and high-crystallinity carbon-coated lithium iron phosphate pre-product.

[0066] Step 2: Repulping the lithium iron phosphate preform, non-ionic surfactant, and deionized water. After separation, a slurry ⅠA of small particles and a slurry ⅠB of large particles are obtained. Mix the slurry ⅠA, carbon source Ⅱ, conductive agent, and deionized water to prepare slurry ⅡA; similarly, obtain slurry ⅡB (the solid contents of slurry ⅡA and slurry ⅡB are 30wt.% - 50wt.%). After spray-drying slurry ⅡA and slurry ⅡB respectively, sinter them at 600 - 830°C under a nitrogen atmosphere protection for 1 - 20h. After taking out of the furnace, through crushing, sieving, and mixing, a finished lithium iron phosphate composite material is prepared.

[0067] In the second aspect, the present invention provides a lithium iron phosphate composite material prepared by the above preparation method.

[0068] In the third aspect, the present invention provides a lithium-ion battery using the above lithium iron phosphate composite material as a cathode material.

[0069] The following specifically describes the preferred embodiments of the present invention to explain the principle of the present invention, not for limiting the scope of the present invention.

[0070] Unless otherwise specified, the following reagents are all commercially available products.

[0071] Example 1

[0072] Step 1: Prepare a carbon-coated lithium iron phosphate preform:

[0073] Prepare iron phosphate with an iron-to-phosphorus ratio of 0.984, and add lithium carbonate according to a Li / Fe molar ratio of 1.028:1. The carbon source is glucose, and the addition amount of glucose accounts for 50% of the total carbon source mass.

[0074] Pour the above required materials into a ball mill, add pure water, with a solid content of 30%, carry out ball milling, and control the ball milling particle size at D50 of 0.4 - 0.45 microns to obtain mixed slurry Ⅰ.

[0075] Carry out spray drying on the above mixed slurry Ⅰ, with a spray pressure of 4MPa, an inlet temperature of the spray dryer of 260°C, and an outlet temperature of 110°C. After the temperature is qualified, carry out spray granulation. The precursor obtained after spray drying is sintered under nitrogen. The environmental requirement of the sintering furnace is that the oxygen content in nitrogen is less than 20ppm, isolated from the outside air. The loading thickness of the precursor in the sagger is 3cm. The total sintering time is 15 hours. Specifically, heat up the precursor to 780°C in 6 hours, then keep it at this temperature for 9 hours, and then cool it. Discharge when the temperature drops below 50°C. Finally, carry out sieving and air flow crushing on the sintered material in a constant temperature and humidity chamber. The temperature of the constant temperature and humidity chamber is 20°C, the relative humidity is 4%, and the sieving mesh number is 400 to obtain a carbon-coated lithium iron phosphate preform.

[0076] Step 2: Preparation of lithium iron phosphate composite material:

[0077] Pour the above carbon-coated lithium iron phosphate preform and non-ionic surfactant into a container, add pure water, with a solid content of 60%, let it stand at room temperature of 25 °C for 30 min, and draw out the supernatant to obtain slurry IA, and the remaining slurry is slurry IB. Among them, the dosage of the non-ionic surfactant accounts for 30% of the total carbon source dosage. The non-ionic surfactant is fatty alcohol polyoxyethylene ether (AEO) and polyoxyethylene-polyoxypropylene block polyether (EO / PO), and the mass ratio is 1:2. The molecular mass of AEO is 2000, and the HLB value is 13.2; the molecular mass of EO / PO is 10500.

[0078] Redisperse slurry IA, the second carbon source, the conductive agent, and pure water to make a new slurry with a solid content of 39% to obtain slurry IIA. Among them, the second carbon source is polyethylene glycol (molecular weight 3000), and the dosage accounts for 5% of the total carbon source.

[0079] Redisperse slurry IB, the third carbon source, the metal dopant, the conductive agent, and pure water to make a new slurry with a solid content of 39% to obtain slurry IIB. Among them, the third carbon source is glucose, and the dosage accounts for 15% of the total carbon source. The metal dopant is titanium dioxide, and the dosage is 0.3% of the weight of iron phosphate.

[0080] The conductive agent is graphene, and the total dosage is 1.5% of the mass of iron phosphate. Among them, the amount mixed with slurry IA is 0.375% of the mass of iron phosphate, and the amount mixed with slurry IB is 1.125% of the mass of iron phosphate.

[0081] Perform spray drying on the above slurry IIA / slurry IIB respectively. The spray pressure is 4 MPa, the inlet temperature of the spray dryer is 260 °C, and the outlet temperature is 110 °C. After the temperature is qualified, perform spray granulation. The precursor obtained after spray drying is sintered under nitrogen. The environmental requirement of the sintering furnace is that the oxygen content in nitrogen is less than 20 ppm, and it is isolated from the outside air. The loading thickness of the precursor in the sagger is 3 cm. The total sintering time is 15 hours. Specifically, heat the precursor to 700 °C in 6 hours, then keep it at this temperature for 9 hours, then cool it, and discharge it when the temperature drops below 50 °C. Finally, screen and batch the sintered material in a constant temperature and humidity chamber. The temperature of the constant temperature and humidity chamber is 20 °C, the relative humidity is 4%, and the screening mesh size is 400 to obtain a lithium iron phosphate composite material sample (calculated based on the total weight of the finally prepared lithium iron phosphate composite material, the total carbon content is 1.3 wt.%).

[0082] Comparative Example 1

[0083] Step 1: Preparation of carbon-coated lithium iron phosphate preform:

[0084] Prepare iron phosphate with an iron to phosphorus ratio of 0.984, and add lithium carbonate according to a Li / Fe molar ratio of 1.028:1. The carbon source is glucose, and the addition amount of glucose accounts for 50% of the total carbon source mass.

[0085] Pour the above-mentioned required materials into a ball mill, add pure water, with a solid content of 30%, and carry out ball milling. The ball milling particle size is controlled with a D50 of 0.4 - 0.45 microns to obtain mixed slurry I.

[0086] Carry out spray drying on the above-mentioned mixed slurry I. The spray pressure is 4 MPa, the inlet temperature of the spray dryer is 260 °C, and the outlet temperature is 110 °C. After the temperature is qualified, carry out spray granulation. The precursor obtained after spray drying is sintered under nitrogen. The environmental requirement of the sintering furnace is that the oxygen content in nitrogen is less than 20 ppm, isolated from the outside air. The loading thickness of the precursor in the sagger is 3 cm. The total sintering time is 15 hours. Specifically, heat up the precursor to 780 °C in 6 hours, then hold at this temperature for 9 hours, then cool, and discharge when the temperature drops below 50 °C. Finally, screen and carry out air flow pulverization on the sintered material in a constant temperature and humidity room. The temperature of the constant temperature and humidity room is 20 °C, the relative humidity is 4%, and the screening mesh number is 400 to obtain a carbon-coated lithium iron phosphate preform.

[0087] Step 2: Prepare lithium iron phosphate composite material:

[0088] Pour the above-mentioned lithium iron phosphate preform, non-ionic surfactant, metal dopant, carbon source II, and conductive agent into a container, add pure water, with a solid content of 39%, to obtain slurry II. The dosage of the non-ionic surfactant accounts for 30% of the total carbon source dosage. The non-ionic surfactant is fatty alcohol polyoxyethylene ether (AEO) and polyoxyethylene polyoxypropylene block polyether (EO / PO), with a mass ratio of 1:2. The molecular mass of AEO is 2000, and the HLB value is 13.2; the molecular mass of EO / PO is 10500. The metal dopant is titanium dioxide, with a dosage of 0.3% of the weight of iron phosphate. The conductive agent is graphene, with a dosage of 1.5% of the mass of iron phosphate. Carbon source II is a composite of glucose and polyethylene glycol, with a mass ratio of 3:1, and the dosage of carbon source II accounts for 20% of the total carbon source mass.

[0089] The above-mentioned slurry II is spray-dried. The spray pressure is 4 MPa, the inlet temperature of the spray dryer is 260 °C, and the outlet temperature is 110 °C. After the temperature is qualified, spray granulation is carried out. The precursor obtained after spray drying is sintered under nitrogen. The environmental requirement of the sintering furnace is that the oxygen content in nitrogen is less than 20 ppm, and it is isolated from the outside air. The charging thickness of the precursor in the sagger is 3 cm. The total sintering time is 15 hours. Specifically, the precursor is heated to 700 °C in 6 hours, then held at this temperature for 9 hours, then cooled, and discharged when the temperature drops below 50 °C. Finally, the sintered material is sieved in a constant temperature and humidity chamber. The temperature of the constant temperature and humidity chamber is 20 °C, the relative humidity is 4%, and the sieve mesh number is 400, obtaining a lithium iron phosphate sample (calculated based on the total weight of the finally prepared lithium iron phosphate sample, the total carbon content is 1.3 wt.%).

[0090] Example 2

[0091] Step 1: Prepare a carbon-coated lithium iron phosphate preform:

[0092] Prepare iron phosphate with an iron-to-phosphorus ratio of 0.984, and add lithium carbonate according to a Li / Fe molar ratio of 1.028:1. The carbon source is glucose, and the addition amount of glucose accounts for 50% of the total carbon source mass.

[0093] Pour the above-mentioned required materials into a ball mill, add pure water, and the solid content is 30%. Carry out ball milling, and control the ball milling particle size at D50 of 0.4 - 0.45 microns to obtain a mixed slurry I.

[0094] The above-mentioned mixed slurry I is spray-dried. The spray pressure is 4 MPa, the inlet temperature of the spray dryer is 260 °C, and the outlet temperature is 110 °C. After the temperature is qualified, spray granulation is carried out. The precursor obtained after spray drying is sintered under nitrogen. The environmental requirement of the sintering furnace is that the oxygen content in nitrogen is less than 20 ppm, and it is isolated from the outside air. The charging thickness of the precursor in the sagger is 3 cm. The total sintering time is 15 hours. Specifically, the precursor is heated to 780 °C in 6 hours, then held at this temperature for 9 hours, then cooled, and discharged when the temperature drops below 50 °C. Finally, the sintered material is sieved and air-flow pulverized in a constant temperature and humidity chamber. The temperature of the constant temperature and humidity chamber is 20 °C, the relative humidity is 4%, and the sieve mesh number is 400, obtaining a carbon-coated lithium iron phosphate preform.

[0095] Step 2: Prepare a lithium iron phosphate composite material:

[0096] Pour the above-mentioned lithium iron phosphate preform and non-ionic surfactant into a container, add pure water, with a solid content of 60%, let it stand at room temperature of 25°C for 30 min, draw out the supernatant to obtain slurry IA, and the remaining slurry is slurry IB. Among them, the dosage of the non-ionic surfactant accounts for 30% of the total carbon source dosage. The non-ionic surfactant is fatty alcohol polyoxyethylene ether (AEO) and polyoxyethylene-polyoxypropylene block polyether (EO / PO), and the mass ratio is 1:2. The molecular mass of AEO is 2000, and the HLB value is 13.2; the molecular mass of EO / PO is 10500.

[0097] Redisperse slurry IA, the second carbon source, the conductive agent, and pure water to make a new slurry with a solid content of 39% to obtain slurry IIA. Among them, the second carbon source is polyethylene glycol, and the dosage accounts for 5% of the total carbon source.

[0098] Redisperse slurry IB, the third carbon source, the metal dopant, the conductive agent, and pure water to make a new slurry with a solid content of 39% to obtain slurry IIB. Among them, the third carbon source is glucose, and the dosage accounts for 15% of the total carbon source. The metal dopant is titanium dioxide, and the dosage is 0.3% of the weight of iron phosphate.

[0099] The conductive agent is selected as graphene, and the total dosage is 1.5% of the mass of iron phosphate. Among them, the amount mixed with slurry IA is 0.375% of the mass of iron phosphate, and the amount mixed with slurry IB is 1.125% of the mass of iron phosphate.

[0100] Perform spray drying on the above-mentioned slurry IIA. The spray pressure is 4 MPa, the inlet temperature of the spray dryer is 260°C, and the outlet temperature is 110°C. After the temperature is qualified, perform spray granulation. The precursor obtained after spray drying is sintered under nitrogen. The environmental requirement of the sintering furnace is that the oxygen content in nitrogen is less than 20 ppm, and it is isolated from the outside air. The loading thickness of the precursor in the sagger is 3 cm. The total sintering time is 15 hours. Specifically, heat up the precursor to 700°C in 6 hours, then keep it at this temperature for 9 hours, then cool it, and discharge it when the temperature drops below 50°C. Finally, screen the sintered material in a constant temperature and humidity chamber. The temperature of the constant temperature and humidity chamber is 20°C, the relative humidity is 4%, and the screening mesh number is 400 to obtain the lithium iron phosphate sample prepared from slurry IIA.

[0101] Example 3

[0102] The mixed slurry IIB prepared in Example 2 was spray-dried at a spray pressure of 4 MPa. The inlet temperature of the spray dryer was 260 °C and the outlet temperature was 110 °C. After the temperature was qualified, spray granulation was carried out. The precursor obtained after spray drying was sintered under nitrogen. The environmental requirement of the sintering furnace was that the oxygen content in nitrogen was less than 20 ppm, and it was isolated from the outside air. The charging thickness of the precursor in the sagger was 3 cm. The total sintering time was 15 hours. Specifically, the precursor was heated to 700 °C in 6 hours, then held at this temperature for 9 hours, and then cooled. The temperature was lowered below 50 °C for discharging. Finally, the sintered material was sieved in a constant temperature and humidity chamber. The temperature of the constant temperature and humidity chamber was 20 °C, the relative humidity was 4%, and the sieve mesh number was 400, obtaining the lithium iron phosphate sample prepared from slurry IIB.

[0103] Example 4

[0104] This example is basically the same as Example 1, except that in step 2, the non-ionic surfactant is fatty alcohol polyoxyethylene ether (AEO) and polyoxyethylene polyoxypropylene block polyether (EO / PO), and the mass ratio is 1:2. The molecular mass of AEO is 2000 and the HLB value is 15; the molecular mass of EO / PO is 10500. The dosage of the non-ionic surfactant accounts for 30% of the total carbon source dosage.

[0105] Comparative Example 2

[0106] This comparative example is basically the same as Example 1, except that in step 2, only the non-ionic surfactant fatty alcohol polyoxyethylene ether (AEO) is added. The dosage of the non-ionic surfactant accounts for 30% of the total carbon source dosage. The molecular mass of AEO is 2000 and the HLB value is 13.2.

[0107] Comparative Example 3

[0108] This comparative example is basically the same as Example 1, except that in step 2, only the non-ionic surfactant polyoxyethylene polyoxypropylene block polyether (EO / PO) is added. The molecular mass of EO / PO is 10500. The dosage of the non-ionic surfactant accounts for 30% of the total carbon source dosage.

[0109] Comparative Example 4

[0110] This comparative example is basically the same as Comparative Example 1, except that in step 2, no non-ionic surfactant is added, and the dosage of carbon source II is 50% of the total carbon source mass.

[0111] Test Example

[0112] (1) The Dv50 was tested using a Malvern 3000 particle size analyzer.

[0113] (2) The test result of the powder compaction was under a pressure of 5 tons.

[0114] (3) The test conditions for the specific capacity of the coin cell discharged at 0.1C are: 0.1C CCCV / 0.1C DC 2.0V to 3.75V.

[0115] (4) The test conditions for the cycle test of the 42Ah rectangular battery are: 25°C, 0.5C / 0.5C, 2.5V to 3.65V.

[0116] Table 1 Performance tests of examples and comparative examples

[0117]

[0118] In Table 1, compared with Example 1, the Dv50 of Comparative Example 1 is lower, the compaction is lower, the specific capacity is lower, and the cycle retention rate is lower. The reason is that under the action of the mixed carbon source, the steric hindrance of small particles during the second sintering process is insufficient, and the small particles grow, resulting in a decrease in specific capacity; the steric hindrance of medium and large particles is relatively large, making it difficult for them to grow into large particles, resulting in a decrease in Dv50; the reduction of the particle size distribution of large and small particles leads to a lower powder compaction; under the action of metal doping on small particles, small particles are more easily affected by metal doping due to their active surface energy, resulting in the formation of more defects, so that more lithium vacancies are missing during the cycle process, leading to a decrease in the cycle retention rate. Examples 2 and 3 show that the sample products can be obtained by mixing different ratios, including adjusting the surfactant scheme, carbon supplementation and doping scheme during the sample preparation process to obtain Slurry II with specific requirements, and realizing products with specialized compaction, specific capacity and cycle performance.

[0119] Comparative Example 2 and Comparative Example 3 only use a single type of surfactant, and Comparative Example 4 does not use a surfactant. The Dv50 of Comparative Example 2, Comparative Example 3 and Comparative Example 4 is higher, the compaction is lower, and the specific capacity is lower. The effect is far less than that of the lithium iron phosphate composite prepared by using two compound surfactants.

[0120] It should be noted that the above-mentioned embodiments are only used to explain the present invention and do not constitute any limitation to the present invention. The present invention has been described by referring to typical embodiments, but it should be understood that the words used therein are descriptive and explanatory words, rather than limiting words. Modifications can be made to the present invention within the scope of the claims of the present invention, and the present invention can be revised without departing from the scope and spirit of the present invention. Although the present invention described therein relates to specific methods, materials and embodiments, it does not mean that the present invention is limited to the specific examples disclosed therein. On the contrary, the present invention can be extended to all other methods and applications with the same functions.

Claims

1. A method for preparing a lithium iron phosphate composite material using a di-sintering process, characterized in that: The following steps are involved: Step 1: preparing a first slurry from a mixture comprising iron phosphate, a lithium source, a first carbon source and water, and performing a first slurry sintering to obtain a carbon-coated lithium iron phosphate preform; Step 2: The carbon-coated lithium iron phosphate preform obtained in step 1, a surfactant and water are mixed to prepare a slurry. After standing and stratification, the upper slurry obtained by separation is the second slurry, and the lower slurry is the third slurry. The second slurry is mixed with a second carbon source and water, and then a second sintering is performed; the third slurry is mixed with a third carbon source and water, and then a third sintering is performed. The products after the second sintering and the third sintering are separately or mixed as the lithium iron phosphate composite material; Wherein, in step 2, the surfactant includes a first surfactant and a second surfactant, the first surfactant includes at least one of fatty alcohol polyoxyethylene ether and alkylphenol polyoxyethylene ether; the second surfactant includes polyoxyethylene polyoxypropylene block polyether; the second carbon source includes at least one of polyethylene glycol, polyvinyl alcohol, polyacrylic acid, and phenolic resin; the third carbon source includes at least one of glucose, sucrose, and citric acid.

2. The preparation method according to claim 1, characterized in that: The mass ratio of the first surfactant to the second surfactant is (0.5-1.5):(1.5-2.5); and / or, the number average molecular weight of the first surfactant is 1000 to 3000, preferably 1500 to 2000, and / or, the HLB value of the first surfactant is 11 to 18, preferably 12 to 14; And / or, the number average molecular weight of the second surfactant is 8,000 to 15,000.

3. The preparation method according to claim 1 or 2, characterized in that: The mass ratio of the mass x of the first carbon source, the total mass y of the surfactant, and the total mass (1-xy) of the second carbon source and the third carbon source is x:y:(1-xy), x≥0.4, y≥0.20, preferably, 0.7≥x≥0.4, 0.5≥y≥0.20, 0.35≥(1-xy)≥0.1; And / or, based on the total weight of the lithium iron phosphate composite material, the total carbon content of the lithium iron phosphate composite material is 1 wt.%-2 wt.%, preferably 1.3 wt.%-1.9 wt.%.

4. The preparation method according to claim 1 or 2, characterized in that: The molar ratio of Fe:P:Li in the iron phosphate and lithium source is 1:(1.01-1.03):(1.015-1.05); And / or, in step 1, the lithium source is lithium carbonate; And / or, in step 1, the first sintering temperature is 730-850° C. and the time is 1-20 h; And / or, step 1 further comprises the step of sand grinding the mixture to obtain a first slurry, and the D50 of the particles after sand grinding is 0.3-0.8 μm, preferably, the D50 is 0.4-0.5 μm.

5. The preparation method according to claim 1 or 2, characterized in that: The first carbon source includes at least one of glucose, sucrose, and citric acid.

6. The preparation method according to claim 1 or 2, characterized in that: In step 2, the separation conditions of the second slurry and the third slurry include: a standing time of 5 to 60 minutes and a temperature of 25 to 60° C.; And / or, in step 2, the conditions of the second sintering and the third sintering are the same or different, and independently include: a temperature of 600-830° C. and a time of 1-20 h.

7. The preparation method according to claim 1 or 2, characterized in that: In step 2, a conductive agent is added during the mixing of the second slurry and the second carbon source and during the mixing of the third slurry and the third carbon source, respectively, wherein the conductive agent comprises at least one of carbon nanotubes, graphene, carbon nanotubes, and conductive carbon black; Preferably, the total amount of the conductive agent is 0.1% to 0.75% of the mass of the iron phosphate.

8. The preparation method according to claim 7, characterized in that: In step 2, a metal dopant is added during the mixing of the third slurry with the third carbon source, the conductive agent and water. Preferably, the metal dopant comprises at least one of compounds of vanadium, titanium, niobium, molybdenum, and boron; More preferably, the amount of the metal dopant is 0.1% to 1.2% by weight of the iron phosphate.

9. A lithium iron phosphate composite material obtained by the preparation method according to any one of claims 1 to 8.

10. A lithium ion battery, characterized in that: It comprises a positive electrode material, wherein the positive electrode material comprises the lithium iron phosphate composite material according to claim 9.

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