Lithium iron phosphate composite material as well as preparation method and application thereof

Abstract

The invention provides a lithium iron phosphate composite material as well as a preparation method and application thereof. The lithium iron phosphate composite material comprises small-particle lithium iron phosphate, large-particle lithium iron phosphate and a super-large-particle lithium supplement agent; the average particle size of the small-particle lithium iron phosphate is less than or equal to 0.3 mu m, the average particle size of the large-particle lithium iron phosphate is 1-2.5 mu m, and the average particle size of the super-large-particle lithium supplementing agent is 3-8 mu m. According to the invention, the high-compaction composite material is obtained by combining oversized particles of the lithium supplement agent with large and small particles of lithium iron phosphate, reasonably designing the three-stage grading relationship of the particles, utilizing the high activity of small particles and the high space utilization rate of large particles, and utilizing the lithium supplement effect and the structure supporting effect of the oversized particle lithium supplement agent; the gram capacity can be effectively improved while the compaction density of the material is improved, so that the capacity performance such as energy density is further improved.

Description

Technical Field

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

[0002] With the continuous advancement of technology and the continuous expansion of application scenarios, the market has higher and higher requirements for the energy density of lithium-ion batteries, whether it is energy storage systems or power batteries (such as batteries used in new energy vehicles). From the perspective of positive electrode materials, obtaining higher-density positive electrode materials is crucial to improving battery energy density.

[0003] Although the compaction density of lithium iron phosphate materials has been greatly improved in recent years, there are still some urgent problems to be solved. That is, high-compacted lithium iron phosphate (powder compaction ≥ 2.65g / cc) can improve the mass of active materials per unit volume (i.e. energy density) due to the generally large particle size of the material. However, the gram capacity of large particles will be limited by the particle size, and the corresponding ideal gram capacity cannot be exerted at high energy density. Therefore, high-compacted lithium iron phosphate is difficult to take into account the demand for high capacity, which will also directly affect the energy density and power density of the battery. Therefore, how to make lithium iron phosphate take into account both high compaction and high electrochemical performance has become the focus of current research.

[0004] Chinese application CN 115275109 A discloses a long-cycle thick lithium iron phosphate electrode, a preparation method thereof, and a lithium-ion battery. The thick lithium iron phosphate electrode includes a current collector and a first coating, a second coating, and a lithium supplement layer arranged on at least one side of the current collector. The first coating includes small particles of lithium iron phosphate, and the second coating includes large particles of lithium iron phosphate. The application uses a double-layer coating structure of large and small particles to improve the liquid phase transmission efficiency and kinetic performance of the thick lithium iron phosphate electrode. The lithium supplement layer can effectively compensate for the loss of irreversible capacity in the thick electrode and improve the cycle performance of the electrode. However, although the above technical solution can improve compaction and improve capacity performance to a certain extent, the effect of improving capacity performance is very limited, and the contact between the coating particles is not close, the stability of the electrode is not high, and layered coating is required. The efficiency of pole piece production is low, and there is still a lot of room for improvement in electrochemical performance. Summary of the invention

[0005] The main purpose of the present invention is to provide a lithium iron phosphate composite material, a preparation method and application thereof, so as to solve the problem that lithium iron phosphate materials in the prior art cannot have both high compaction density and good electrochemical performance.

[0006] To achieve the above object, according to one aspect of the present invention, a lithium iron phosphate composite material is provided. The lithium iron phosphate composite material includes small particle lithium iron phosphate, large particle lithium iron phosphate, and extra-large particle lithium supplement agent; the average particle size of the small particle lithium iron phosphate is ≤0.3 μm, the average particle size of the large particle lithium iron phosphate is 1 - 2.5 μm, and the average particle size of the extra-large particle lithium supplement agent is 3 - 8 μm. The present invention rationally designs the three-level particle size grading relationship, and utilizes the close-packed system formed by two kinds of lithium iron phosphate with different particle sizes and the extra-large particle lithium supplement agent to obtain a composite material with a high tap density. Based on the tight combination of the above lithium supplement agent and the lithium iron phosphate cathode material, the lithium supplement advantage of the lithium supplement agent can be fully exerted, thereby effectively improving the electrochemical performance while increasing the tap density of the material.

[0007] Further, the average particle size of the small particle lithium iron phosphate is ≤0.25 μm, the average particle size of the large particle lithium iron phosphate is 1.5 - 2.5 μm, and the average particle size of the extra-large particle lithium supplement agent is 4 - 6 μm. By controlling the particle sizes of the lithium iron phosphate and the lithium supplement agent within the above preferred ranges, the present invention can form a more coordinated three-level grading of small, large, and extra-large particles, which is beneficial to further increasing the tap density of the material and maintaining good electrochemical performance, enabling the lithium iron phosphate material to better balance high tap density and high capacity.

[0008] Further, the weight ratio of the small particle lithium iron phosphate to the large particle lithium iron phosphate is 1:(7 - 9.5), and the weight ratio of the small particle lithium iron phosphate to the extra-large particle lithium supplement agent is 1:(0.1 - 0.5). This ratio is beneficial to achieving a higher space utilization rate while taking into account the electrical performance.

[0009] Further, the lithium supplement agent includes Li 2 NiO 2 and / or Li 5 FeO 4 ; and / or the lithium iron phosphate composite material further includes carbon, and the weight percentage of carbon in the lithium iron phosphate composite material is 1.0 - 1.6%. The above lithium supplement agent can release extra lithium ions more smoothly during the first charge and discharge of the material, compensating for the lithium lost due to the formation of the SEI film during the first charge and discharge of the lithium iron phosphate. This compensation mechanism can directly improve the first charge specific capacity of the material and stably maintain the capacity performance of the battery in the long term. When the carbon content is within the above range, it not only provides good electronic conduction but also has the least impact on the tap density, and the electrical performance of the lithium iron phosphate composite material is the best.

[0010] According to another aspect of the present invention, there is provided a method for preparing the above-mentioned lithium iron phosphate composite material of the present invention, comprising the following steps: Step S1, mixing a ferric phosphate precursor, a lithium source, an optional carbon source and water, and grinding to obtain a first slurry; dividing the first slurry into two parts, namely the first part of the first slurry and the second part of the first slurry, subjecting the first part of the first slurry to first spray drying to obtain a first dry material; Step S2, adding a coupling agent to the second part of the first slurry to obtain a second slurry; subjecting the second slurry to second spray drying to obtain a second dry material; Step S3, mixing the first dry material, the second dry material and a lithium supplement agent, and performing first sintering and second sintering in sequence to obtain a lithium iron phosphate composite material; wherein, the temperature of the first sintering is lower than the temperature of the second sintering.

[0011] The present invention adopts a process of two-stage pulping, two-stage spray drying, and two-step sintering to achieve precise control of particle size; introduces a lithium supplement additive, combines large particles of the lithium supplement agent with large and small particles of lithium iron phosphate, rationally designs the three-stage particle size distribution relationship, and obtains a high-compactness composite material. By co-sintering the lithium supplement agent with the dry material after spray drying of lithium iron phosphate, the tight combination between the lithium supplement agent and lithium iron phosphate particles is enhanced, thereby improving the tap density of the lithium iron phosphate composite material while effectively improving its electrical properties. The above preparation process is simple, easy to operate, and can be applied to industrial large-scale production.

[0012] Further, in step S1, in the ferric phosphate precursor, the molar ratio of iron to phosphorus is (0.95 - 0.98):1; and / or the lithium source includes one or more of lithium carbonate, lithium hydroxide, and lithium hydroxide monohydrate, and the molar ratio of lithium in the lithium source to phosphorus in the ferric phosphate precursor is (1 - 1.4):1; and / or the solid content of the first slurry is 20 - 40%, and the average particle size of the first slurry ≤ 350 nm; and / or the weight percentage of the first part of the first slurry in the first slurry is 10 - 13%; and / or the carbon source includes one or more of glucose, sucrose, starch, and polyethylene glycol with a weight average molecular weight of 2000 - 8000, and the carbon in the carbon source accounts for 1.0 - 1.6% of the weight of the lithium iron phosphate composite material.

[0013] The molar ratio of iron to phosphorus and the molar ratio of lithium to phosphorus in the ferric phosphate precursor within the above ranges can make the synthesized lithium iron phosphate have a more appropriate stoichiometric ratio, which helps to improve the electrochemical performance of lithium iron phosphate, especially the capacity performance. When the solid content of the first slurry is within the above range, it can further promote the formation of material particles during subsequent drying and sintering on the premise of improving the fluidity and dispersibility of the slurry. When the average particle size of the slurry and the proportion of the slurry used to prepare the first dry material with small particle size are within the above ranges, it is more conducive to making some material particles have high dispersibility and a relatively high specific surface area, thereby improving the diffusion efficiency of lithium ions and the overall density of the material.

[0014] Further, in step S2, the coupling agent includes one or more of trimethyl borate, triethanolamine borate, and N-methyliminodiacetic acid borate; and / or the addition amount of the coupling agent is 5-20% of the weight of the first slurry in the second part. The above coupling agent can also play a coupling role on the surface of the material particles, providing the necessary bonding force for particle growth, and at the same time not forming a structure that is too tight to affect lithium ion diffusion.

[0015] Further, in step S3, the lithium supplement agent includes Li 2 NiO 2 and / or Li 5 FeO 4 , the average particle size of the lithium supplement agent is 3-8 μm; and / or the weight ratio of the first dry material to the lithium supplement agent is 1:(0.08-0.45); and / or the temperature of the first sintering is 400-600 °C, the time is 3-5 h, and the heating rate is 10-20 °C / min; and / or the temperature of the second sintering is 700-900 °C, the time is 8-10 h, and the heating rate is 1-3 °C / min. Limiting the type, particle size, and addition amount of the lithium supplement agent within the above ranges can provide a more sufficient lithium supplement effect while reducing the decline in electrochemical performance that may be caused by the introduction of excessive lithium supplement agent. The heating rate of the first sintering is relatively fast, which is short-time low-temperature sintering, and can meet the decomposition requirements and the requirements for the growth process of small particle crystals of lithium iron phosphate at the same time. The heating rate of the second sintering is relatively slow, which is long-time high-temperature sintering, can repair crystal defects, and at the same time, the decomposition products of the coupling agent have a fluxing effect at high temperatures, which can induce the formation of large particles and improve the particle size distribution.

[0016] According to another aspect of the present invention, a positive electrode sheet is provided, which includes a current collector and a positive electrode slurry loaded on at least one surface of the current collector. The positive electrode slurry includes a positive electrode active material, a conductive agent, and a binder, and the positive electrode active material includes the lithium iron phosphate composite material of the present invention above. It has significantly improved compaction density and capacity performance.

[0017] According to another aspect of the present invention, a lithium ion battery is provided, which includes the positive electrode sheet of the present invention above. It has significantly improved electrochemical performance.

[0018] Applying the technical solution of the present invention, the specific surface area of the small-particle lithium iron phosphate is relatively high, which helps to improve the rate performance of the battery; the large-particle lithium iron phosphate ensures that the material has a relatively high tap density and conductivity, while the ultra-large-particle lithium supplementing agent can effectively supplement lithium ions during the preparation sintering and SEI film formation processes, reducing lithium loss, thereby improving the capacity performance of the battery. The present invention combines the ultra-large particles of the lithium supplementing agent with the large and small particles of lithium iron phosphate, rationally designs the three-level particle size grading relationship, utilizes the high activity of the small particles and the high space utilization rate of the large particles, and at the same time utilizes the lithium supplementing effect and the structure supporting effect of the ultra-large-particle lithium supplementing agent to obtain a high-tap composite material, which can effectively improve the specific capacity per gram while increasing the tap density of the material, thereby further improving the capacity performance such as the energy density. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The accompanying drawings forming a part of this application are used to provide a further understanding of the present invention. The schematic embodiments and descriptions thereof of the present invention are used to explain the present invention and do not constitute an improper limitation to the present invention. In the drawings:

[0020] Figure 1 The SEM photograph of the lithium iron phosphate composite material according to Embodiment 1 of the present invention is shown. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0021] It should be noted that, without conflict, the embodiments in this application and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the drawings and in conjunction with the embodiments.

[0022] As described in the background art of the present invention, in the prior art, there is a problem that the lithium iron phosphate material cannot take into account both high tap density and good electrochemical performance, especially the capacity performance. To solve the above problems, in a typical embodiment of the present invention, a lithium iron phosphate composite material is provided. The lithium iron phosphate composite material includes small-particle lithium iron phosphate, large-particle lithium iron phosphate and ultra-large-particle lithium supplementing agent; the average particle size of the small-particle lithium iron phosphate ≤ 0.3 μm, the average particle size of the large-particle lithium iron phosphate is 1 - 2.5 μm, and the average particle size of the ultra-large-particle lithium supplementing agent is 3 - 8 μm.

[0023] The small-sized lithium iron phosphate particles with the above particle size have a relatively high specific surface area, which can provide more active sites, promote the rapid diffusion of lithium ions, and thus improve the electrochemical performance of the material. The small-sized particles can also reduce the diffusion path of lithium ions and enhance the cycle stability of the material. The large-sized lithium iron phosphate particles can promote the close packing of the particles, reduce the voids between the particles, and thus increase the overall tap density of the material, without overly affecting the diffusion efficiency of lithium ions due to their excessively large size. Introducing the ultra-large particle lithium supplement agent is beneficial to compensating for the lithium loss caused by the volatilization of lithium ions during the preparation of lithium iron phosphate, and at the same time compensating for the lithium loss caused by the lithium deintercalation during the cycling process of lithium iron phosphate, thereby improving the capacity and cycle life of the material. At the same time, due to its particle size being much larger than that of lithium iron phosphate particles, the ultra-large lithium supplement agent particles can also play a role similar to a skeleton in the composite material, helping to increase the tap density of the material. A larger particle size will increase the characteristic ion diffusion length of the particles, resulting in a decline in electrical performance.

[0024] The present invention rationally designs the particle three-level grading relationship, and uses the close packing system formed by two kinds of lithium iron phosphate with different particle sizes and the ultra-large particle lithium supplement agent to obtain a composite material with a high tap density. The theoretical space utilization rate can reach 85%, which has a positive effect on improving the tap density. Based on the tight combination of the above lithium supplement agent and the lithium iron phosphate cathode material, the lithium supplement advantage of the lithium supplement agent can be fully exerted, thereby effectively improving the electrochemical performance while increasing the tap density of the material.

[0025] In a preferred embodiment, the average particle size of the small-sized lithium iron phosphate is ≤0.25 μm, the average particle size of the large-sized lithium iron phosphate is 1.5 - 2.5 μm, and the average particle size of the ultra-large particle lithium supplement agent is 4 - 6 μm. The small-sized lithium iron phosphate particles with the above particle size have a higher specific surface area, which helps to further improve the rate performance of the battery; the large-sized lithium iron phosphate particles can endow the material with a higher tap density and conductivity, while the ultra-large particle lithium supplement agent can more effectively supplement lithium ions during the preparation sintering and SEI film formation processes, reduce lithium loss, and thus improve the capacity performance of the battery. By controlling the particle sizes of the lithium iron phosphate and the lithium supplement agent within the above preferred ranges, the present invention can form a more coordinated three-level grading of small, large, and ultra-large particles, which is beneficial to further increasing the tap density of the material and maintaining good electrochemical performance, enabling the lithium iron phosphate material to better balance high tap density and high capacity.

[0026] For similar reasons, in a preferred embodiment, the weight ratio of small particle lithium iron phosphate to large particle lithium iron phosphate is 1:(7 - 9.5), and the weight ratio of small particle lithium iron phosphate to super-large particle lithium supplementing agent is 1:(0.1 - 0.5). This ratio is conducive to achieving higher space utilization while taking into account the electrical properties. The above ratio can provide a more sufficient amount of small particle lithium iron phosphate to utilize its high specific surface area characteristics to improve the specific capacity and cycle stability of the material. Setting the weight ratio of large particle lithium iron phosphate within the above range is beneficial for most of the space of the composite material to be occupied by large particles, thereby achieving a high tap density. Limiting the weight ratio of the super-large particle lithium supplementing agent within the above range can provide a more sufficient lithium supplementing effect while reducing the possible decline in electrochemical performance caused by the introduction of excessive lithium supplementing agent. By precisely controlling the weight ratios of the three kinds of particles with different particle sizes, the present invention is more conducive to achieving the balance between high tap density and good capacity performance of the material.

[0027] In a preferred embodiment, the lithium supplementing agent includes Li 2 NiO 2 and / or Li 5 FeO 4 ; and / or the lithium iron phosphate composite material further includes carbon, and the weight percentage of carbon in the lithium iron phosphate composite material is 1.0 - 1.6%. The above lithium supplementing agent can release additional lithium ions more smoothly during the first charge and discharge of the material, compensating for the lithium lost due to the formation of the SEI film during the first charge and discharge of lithium iron phosphate. This compensation mechanism can directly improve the first charge specific capacity of the material and stably maintain the capacity performance of the battery in the long term. As a conductive agent, carbon can effectively improve the electronic conductivity of the material, reduce the internal resistance during the electrochemical reaction process, thereby increasing the specific capacity and cycle stability of the material. However, too high a carbon content will increase the fluffiness of the material and reduce the tap density, and when it is too low, the improvement effect on the electronic conductivity is small, affecting the electrochemical performance. When the carbon content is within the above range, it not only provides good electronic conduction but also has the least impact on the tap density, and the electrical performance of the lithium iron phosphate composite material is the best.

[0028] In another typical embodiment of the present invention, a method for preparing the above lithium iron phosphate composite material of the present invention is further provided, including the following steps: Step S1, mixing a ferric phosphate precursor, a lithium source, an optional carbon source, and water, and grinding to obtain a first slurry; dividing the first slurry into two parts, namely the first part of the first slurry and the second part of the first slurry, and performing first spray drying on the first part of the first slurry to obtain a first dry material; Step S2, adding a coupling agent to the second part of the first slurry to obtain a second slurry; performing second spray drying on the second slurry to obtain a second dry material; Step S3, mixing the first dry material, the second dry material, and the lithium supplementing agent, and performing first sintering and second sintering in sequence to obtain a lithium iron phosphate composite material; wherein, the temperature of the first sintering is lower than the temperature of the second sintering.

[0029] Specifically, first mix the iron phosphate precursor, lithium source, optional carbon source and water, and perform grinding to make the raw materials evenly dispersed and reduce the particle size, obtaining a first slurry. Part of the first slurry is subjected to first spray drying to obtain a first dry material with a small particle size. During this process, the particle size of the precursor is controlled by ultrafine grinding, so that the carbon source fully wraps the surface of the reactants, forming a stable reactant-carbon source system, thereby playing an inhibitory role in particle growth during the subsequent sintering process. A coupling agent is added to another part of the first slurry to obtain a second slurry; the second slurry is subjected to second spray drying to obtain a second dry material. During this process, a stable reactant-carbon source-coupling agent system is formed. The coupling agent can fully couple and aggregate the small particles in the slurry, which is beneficial to melting and growth during the subsequent sintering process, and decomposes to form boron oxides during low-temperature sintering, playing a sintering aid role and further promoting particle aggregation and growth.

[0030] Finally, mix the first dry material, the second dry material and the lithium supplement agent, and perform low-temperature sintering and high-temperature sintering in sequence. During the low-temperature sintering process, the carbon source in the reactant-carbon source system in the first dry material will decompose at low temperature and fully wrap the surface of the reactants, inhibiting particle growth during the lithium iron phosphate phase formation process; the carbon source and coupling agent in the second dry material will also fully decompose. Among them, the coupling agent decomposes at low temperature to form boron oxides, which can promote the melting of particles to form large particles in the subsequent high-temperature stage. At the same time, the carbon source decomposes, which can promote the contact between the lithium supplement agent and the lithium iron phosphate particles, obtaining a lithium iron phosphate composite material with a three-level particle size distribution.

[0031] Among them, spray drying is a solvent evaporation process. The ordinary drying process usually leads to carbon source segregation and uneven distribution. Spray drying can effectively avoid the segregation problem. Therefore, at the initial stage of preparing the lithium iron phosphate material, after aqueous grinding, spray drying is an essential step to obtain a stable reactant system with uniform carbon source distribution. And the change of materials in this process is a physical change, which has no obvious influence on the particle size of the materials. The parameters such as the temperature of spray drying are related to the capacity of the spray drying equipment, the spray volume, etc., and can be adjusted in combination with the actual production as long as the drying purpose can be achieved. These are understandable to those skilled in the art and will not be elaborated here.

[0032] In the present invention, the slurries containing all raw materials are mixed and then sintered. First, slurry grinding can improve the particle dispersibility, and high dispersibility is more conducive to achieving the target grading relationship and improving the compaction density; second, sintering after grinding can enhance the contact tightness between particles and the contact tightness between particles and the optional carbon layer. Compared with physical layer-by-layer coating, sintering after mixing slurries has a better interface contact effect, which is beneficial to improving the stability of the composite material; third, the high-compaction composite material of the present invention can be directly mixed and coated, reducing the steps of layer-by-layer coating, thus improving the production efficiency of the electrode sheet.

[0033] In summary, the present invention adopts a process of two-stage pulping, two-stage spray drying, and two-step sintering to achieve precise control of particle size; a lithium supplement additive is introduced, and the super-large particles of the lithium supplement are combined with the large and small particles of lithium iron phosphate, and the three-stage particle size grading relationship is reasonably designed to obtain a high-compact composite material. By co-sintering the lithium supplement and the dried material after spray drying of lithium iron phosphate, the tight combination between the lithium supplement and the lithium iron phosphate particles is enhanced, thereby improving the compaction density of the lithium iron phosphate composite material and effectively improving its electrical properties while. The above preparation process is simple and easy to operate, and can be applied to industrial mass production.

[0034] In a preferred embodiment, in step S1, in the iron phosphate precursor, the molar ratio of iron to phosphorus is (0.95 - 0.98):1; and / or the lithium source includes one or more of lithium carbonate, lithium hydroxide, and lithium hydroxide monohydrate, and the molar ratio of lithium in the lithium source to phosphorus in the iron phosphate precursor is (1 - 1.4):1; and / or the solid content of the first slurry is 20 - 40%, and the average particle size of the first slurry ≤ 350 nm; and / or the weight percentage of the first part of the first slurry in the first slurry is 10 - 13%; and / or the carbon source includes one or more of glucose, sucrose, starch, and polyethylene glycol with a weight average molecular weight of 2000 - 8000, and the carbon in the carbon source accounts for 1.0 - 1.6% of the weight of the lithium iron phosphate composite material.

[0035] The molar ratio of iron to phosphorus and the molar ratio of lithium to phosphorus in the iron phosphate lithium precursor within the above ranges can make the synthesized lithium iron phosphate have a more appropriate stoichiometric ratio, thereby forming a more stable crystal structure. While improving the structural integrity of lithium iron phosphate, a more complete lithiation process is achieved through the excess of lithium, reducing the generation of metastable products with insufficient Li, which helps to improve the electrochemical performance of lithium iron phosphate, especially the capacity performance. If the solid content of the first slurry is too high, it may lead to too high viscosity of the slurry, affecting the dispersibility and uniformity; if it is too low, it may cause insufficient formation of particles, affecting the final compaction density. Therefore, the present invention limits the solid content within the above range, which can further promote the formation of material particles during subsequent drying and sintering on the premise of improving the fluidity and dispersibility of the slurry.

[0036] In addition, when the average particle size of the slurry is within the above range and the proportion of the slurry used to prepare the first dry material with small particle size is within the above range, it is more conducive to making some material particles have high dispersibility and a relatively high specific surface area, thereby improving the diffusion efficiency of lithium ions. At the same time, small particles are easy to be dispersed and distributed with large and super-large particles to further improve the overall density of the material. The carbon content of the carbon source within the above range can make the composite material have good electronic conductivity and have a small impact on the compaction density of the material.

[0037] It should be noted that the average particle size of the first slurry is measured under the slurry system. Due to the influence of agglomeration and solvation effects during the measurement, the measurement result is generally larger than the actual particle size. The particle size of the small particle lithium iron phosphate in the lithium iron phosphate composite material is the sintered finished product particle size, which can be obtained by measurement and calculation in SEM.

[0038] It should be noted that by controlling the weight percentage of the first part of the first slurry in the first slurry to be 10-13%, the weight ratio of small particle lithium iron phosphate to large particle lithium iron phosphate can be controlled to be 1:(7-9.5), and the weight ratio of small particle lithium iron phosphate to the oversize lithium supplementing agent can be controlled to be 1:(0.1-0.5). The weight ratio of particles of each particle size is controlled by the weight of the slurry. Considering the loss of sintered carbon content at the same time, the dry material burnout rate is about 80%. Therefore, the actual weight ratio of small particle lithium iron phosphate to the lithium supplementing agent is 80% of the dry material ratio. The weight of the large particle lithium iron phosphate is slightly increased due to the influence of the mass of the cosolvent boron.

[0039] A suitable coupling agent can not only improve the tap density of the material, but also enhance the electrochemical performance by adjusting the surface properties of the particles and increasing the contact tightness between the particles. For the purpose of further promoting the bonding between the particles, which is beneficial to the formation of larger-sized lithium iron phosphate particles during the subsequent sintering process, in a preferred embodiment, in step S2, the coupling agent includes borate esters, specifically including one or more of trimethyl borate, triethanolamine borate, and N-methyliminodiacetic acid borate; and / or the addition amount of the coupling agent is 5-20% of the weight of the second part of the first slurry. The above coupling agent can also play a coupling role on the surface of the material particles, providing the necessary bonding force for particle growth, and at the same time not forming a structure that is too tight to affect the diffusion of lithium ions. The melting point property of the borate ester can also play a role in assisting sintering during the sintering process.

[0040] When the addition amount of the coupling agent is too small, it is not conducive to the formation of sufficient bonding force, which may lead to particle dispersion and low tap density; while when the addition amount of the coupling agent is too large, the coupling effect on the particle surface is too strong, which may hinder the diffusion of lithium ions and affect the capacity performance. Setting the addition amount of the coupling agent within the above range is beneficial to balancing the bonding strength between the particles and the electrochemical activity of the material, promoting the bonding between the particles, forming an ideal tap structure, and maintaining good electrochemical performance.

[0041] In a preferred embodiment, in step S3, the lithium supplementing agent includes Li 2 NiO 2 and / or Li 5 FeO 4 , the average particle size of the lithium supplementing agent is 3-8 μm, preferably, the average particle size of the lithium supplementing agent is 4-6 μm; and / or the weight ratio of the first dry material to the lithium supplementing agent is 1:(0.08-0.45).

[0042] Limiting the type, particle size, and addition amount of the lithium supplement agent within the above ranges can provide a more sufficient lithium supplement effect while reducing the decline in electrochemical performance that may be caused by the introduction of excessive lithium supplement agent. It should be noted that the preparation of the lithium supplement agent generally uses high-temperature solid-phase sintering, and a crystalline product has been formed. When sintered again, the particles will basically not grow significantly, so the consistency of the particle size of the lithium supplement agent before and after sintering can be ensured.

[0043] Preferably, the temperature of the first sintering is lower than that of the second sintering, with a difference of 270 - 400 °C, the time of the first sintering is shorter than that of the second sintering, with a difference of 3 - 7 h, and the heating rate of the first sintering is greater than that of the second sintering, with a difference of 7 - 19 °C / min. More preferably, the temperature of the first sintering is 400 - 600 °C, the time is 3 - 5 h, and the heating rate is 10 - 20 °C / min; and / or the temperature of the second sintering is 700 - 900 °C, the time is 8 - 10 h, and the heating rate is 1 - 3 °C / min. The relatively fast heating rate of the first sintering can reach the reaction temperature of the carbon source as soon as possible, promoting the simultaneous and sufficient decomposition of the coupling agent and the carbon source. Short-time low-temperature sintering can meet the decomposition requirements and the requirements for the growth of small particle crystals of lithium iron phosphate simultaneously. The relatively slow heating rate of the second sintering can control the secondary growth process of the lithium iron phosphate crystals formed after low-temperature sintering, which is beneficial to obtaining lithium iron phosphate crystals with different particle sizes. And long-time high-temperature sintering can promote the graphitization process of the carbon layer, repair crystal defects at the same time, obtain a composite material with better crystallinity. At the same time, the decomposition products of the coupling agent (such as boron oxides produced by the decomposition of boric acid coupling agent) have a fluxing effect at high temperatures, which can induce the formation of large particles and improve the particle size distribution.

[0044] In another typical embodiment of the present invention, a positive electrode sheet is also provided, which includes a current collector and a positive electrode paste loaded on at least one surface of the current collector. The positive electrode paste includes a positive electrode active material, a conductive agent, and a binder. The positive electrode active material includes the lithium iron phosphate composite material of the present invention. Due to the use of the lithium iron phosphate composite material with a three-level particle size distribution relationship of the present invention, a closely packed system formed by two different particle sizes of lithium iron phosphate and super-large particle lithium supplement agent is utilized to obtain a composite material with a high tap density. And based on the close combination of the above lithium supplement agent and the lithium iron phosphate positive electrode material, the lithium supplement advantage of the lithium supplement agent can be fully exerted. Therefore, the positive electrode sheet has significantly improved tap density and capacity performance.

[0045] In another typical embodiment of the present invention, a lithium-ion battery is also provided, which includes the positive electrode sheet of the present invention above, and it has significantly improved electrochemical performance.

[0046] Typically but not limited to, the average particle size of the small particle lithium iron phosphate is 0.1μm, 0.12μm, 0.15μm, 0.18μm, 0.2μm, 0.22μm, 0.25μm, 0.28μm, 0.3μm or a range value composed of any two of these values, the average particle size of the large particle lithium iron phosphate is 1μm, 1.2μm, 1.5μm, 1.8μm, 2μm, 2.2μm, 2.5μm or a range value composed of any two of these values, and the average particle size of the ultra-large particle lithium supplement agent is 3μm, 4μm, 5μm, 6μm, 7μm, 8μm or a range value composed of any two of these values.

[0047] Typically but not limited to, the weight ratio of the small particle lithium iron phosphate to the large particle lithium iron phosphate is 1:7, 1:7.5, 1:8, 1:8.5, 1:9, 1:9.5 or a range value composed of any two of these values.

[0048] Typically but not limited to, the weight ratio of the small particle lithium iron phosphate to the ultra-large particle lithium supplement agent is 1:0.1, 1:0.2, 1:0.3, 1:0.4, 1:0.5 or a range value composed of any two of these values.

[0049] Typically but not limited to, the lithium iron phosphate composite material also includes carbon, and the weight percentage of carbon in the lithium iron phosphate composite material is 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6% or a range value composed of any two of these values.

[0050] Typically but not limited to, the addition amount of the coupling agent is 5%, 8%, 10%, 12%, 15%, 18%, 20% of the weight of the first slurry in the second part or a range value composed of any two of these values.

[0051] Typically but not limited to, the temperature of the first sintering is 400℃, 450℃, 500℃, 550℃, 600℃ or a range value composed of any two of these values, the time is 3h, 3.5h, 4h, 4.5h, 5h or a range value composed of any two of these values, and the heating rate is 10℃ / min, 12℃ / min, 15℃ / min, 18℃ / min, 20℃ / min or a range value composed of any two of these values.

[0052] Typically but not limited to, the temperature of the second sintering is 700℃, 750℃, 800℃, 850℃, 900℃ or a range value composed of any two of these values, the time is 8h, 8.5h, 9h, 9.5h, 10h or a range value composed of any two of these values, and the heating rate is 1℃ / min, 1.5℃ / min, 2℃ / min, 2.5℃ / min, 3℃ / min or a range value composed of any two of these values.

[0053] The present application will be further described in detail below in conjunction with specific embodiments, and these embodiments should not be construed as limiting the scope claimed by the present application.

[0054] Example 1

[0055] Step S1: Mix the iron phosphate precursor and lithium carbonate in a ratio of Li:Fe:P molar ratio of 1.2:0.968:1, and then perform ultra-fine grinding. Add glucose so that the carbon content accounts for 1.2% of the weight of the lithium iron phosphate composite material to obtain a first slurry with a solid content of 35% and an average particle size of 300 nm (here, it is the average particle size of the slurry, that is, the D50 value obtained by testing the slurry with a Malvern 3000 laser particle size analyzer); Take 10 wt.% of the first slurry for spray drying to obtain a first dry material.

[0056] Step S2: Add 10 wt% of trimethyl borate to the remaining first slurry, mix evenly to obtain a second slurry, and then perform spray drying to obtain a second dry material.

[0057] Step S3: Put the first dry material, the second dry material and the lithium supplement agent Li 5 FeO 4 into a high-speed mixer for uniform mixing. The average particle size of the lithium supplement agent is 4.5 μm, and the weight ratio of the lithium supplement agent to the first dry material is 1:0.2. Then, perform the first sintering and the second sintering in sequence. First, heat up to 450 °C at a heating rate of 20 °C / min and sinter for 5 h, and then heat up to 800 °C at a heating rate of 1 °C / min and sinter for 8 h to obtain the lithium iron phosphate composite material.

[0058] The SEM photograph of the lithium iron phosphate composite material is shown in Figure 1 , and obvious three-level particle grading can be seen from the figure.

[0059] Example 2

[0060] Step S1: Mix the iron phosphate precursor and lithium carbonate in a ratio of Li:Fe:P molar ratio of 1.4:0.970:1, and then perform ultra-fine grinding. Add glucose so that the carbon content accounts for 1.3% of the weight of the lithium iron phosphate composite material to obtain a first slurry with a solid content of 30% and an average particle size of 330 nm; Take 13 wt.% of the first slurry for spray drying to obtain a first dry material.

[0061] Step S2: Add 13 wt% of triethanolamine borate to the remaining first slurry, mix evenly to obtain a second slurry, and then perform spray drying to obtain a second dry material.

[0062] Step S3: Put the first dry material, the second dry material and the lithium supplement agent Li 5 FeO 4Add them to a high-speed mixer for uniform mixing. The average particle size of the lithium supplement agent is 6 μm, and the weight ratio of the lithium supplement agent to the first dry material is 1:0.1. Then, perform the first sintering and the second sintering in sequence. First, heat up to 500 °C at a heating rate of 10 °C / min and sinter for 3 h. Then, heat up to 810 °C at a heating rate of 2 °C / min and sinter for 10 h to obtain the lithium iron phosphate composite material.

[0063] Example 3

[0064] Step S1: Mix the iron phosphate precursor and lithium carbonate in a molar ratio of Li:Fe:P of 1.4:0.970:1, and then perform ultrafine grinding. Add polyethylene glycol with a weight-average molecular weight of 2000 so that the carbon accounts for 1.3% by weight of the lithium iron phosphate composite material to obtain the first slurry, with a solid content of 40% and an average particle size of 250 nm. Pipette 10 wt.% of the first slurry for spray drying to obtain the first dry material.

[0065] Step S2: Add 8 wt% of triethanolamine borate to the remaining first slurry, mix evenly to obtain the second slurry, and then perform spray drying to obtain the second dry material.

[0066] Step S3: Add the first dry material, the second dry material, and the lithium supplement agent Li 5 FeO 4 Add them to a high-speed mixer for uniform mixing. The average particle size of the lithium supplement agent is 5 μm, and the weight ratio of the lithium supplement agent to the first dry material is 1:0.15. Then, perform the first sintering and the second sintering in sequence. First, heat up to 500 °C at a heating rate of 10 °C / min and sinter for 5 h. Then, heat up to 770 °C at a heating rate of 3 °C / min and sinter for 10 h to obtain the lithium iron phosphate composite material.

[0067] Example 4

[0068] Step S1: Mix the iron phosphate precursor and lithium carbonate in a molar ratio of Li:Fe:P of 1.1:0.975:1, and then perform ultrafine grinding. Add polyethylene glycol with a weight-average molecular weight of 6000 so that the carbon accounts for 1.5% by weight of the lithium iron phosphate composite material to obtain the first slurry, with a solid content of 40% and an average particle size of 230 nm. Pipette 13 wt.% of the first slurry for spray drying to obtain the first dry material.

[0069] Step S2: Add 20 wt% of N-methyliminodiacetic acid borate to the remaining first slurry, mix evenly to obtain the second slurry, and then perform spray drying to obtain the second dry material.

[0070] Step S3: Add the first dry material, the second dry material, and the lithium supplement agent Li 2 NiO 2Add them to a high-speed mixer for uniform mixing. The average particle size of the lithium supplement agent is 6 μm, and the weight ratio of the lithium supplement agent to the first dry material is 1:0.1. Then, perform the first sintering and the second sintering in sequence. First, heat up to 500 °C at a heating rate of 10 °C / min and sinter for 3 h. Then, heat up to 810 °C at a heating rate of 2 °C / min and sinter for 10 h to obtain the lithium iron phosphate composite material.

[0071] Example 5

[0072] Step S1: Mix the iron phosphate precursor and lithium carbonate in a molar ratio of Li:Fe:P of 1.2:0.968:1, and then perform ultrafine grinding. Add glucose so that the carbon content accounts for 1.5% of the weight of the lithium iron phosphate composite material to obtain the first slurry, with a solid content of 38% and an average particle size of 200 nm. Take 10 wt.% of the first slurry for spray drying to obtain the first dry material.

[0073] Step S2: Add 10 wt% of N-methyliminodiacetic acid borate to the remaining first slurry, mix evenly to obtain the second slurry, and then perform spray drying to obtain the second dry material.

[0074] Step S3: Add the first dry material, the second dry material, and the lithium supplement agent Li 2 NiO 2 Add them to a high-speed mixer for uniform mixing. The average particle size of the lithium supplement agent is 5.6 μm, and the weight ratio of the lithium supplement agent to the first dry material is 1:0.2. Then, perform the first sintering and the second sintering in sequence. First, heat up to 400 °C at a heating rate of 15 °C / min and sinter for 5 h. Then, heat up to 800 °C at a heating rate of 1 °C / min and sinter for 10 h to obtain the lithium iron phosphate composite material.

[0075] Example 6

[0076] Step S1: Mix the iron phosphate precursor and lithium carbonate in a molar ratio of Li:Fe:P of 1.2:0.97:1, and then perform ultrafine grinding. Add glucose so that the carbon content accounts for 1.3% of the weight of the lithium iron phosphate composite material to obtain the first slurry, with a solid content of 30% and an average particle size of 280 nm. Take 13 wt.% of the first slurry for spray drying to obtain the first dry material.

[0077] Step S2: Add 12 wt% of trimethyl borate to the remaining first slurry, mix evenly to obtain the second slurry, and then perform spray drying to obtain the second dry material.

[0078] Step S3: Add the first dry material, the second dry material, and the lithium supplement agent Li 2 NiO 2Add them to a high-speed mixer for uniform mixing. The average particle size of the lithium supplement agent is 6 μm, and the weight ratio of the first dry material to the lithium supplement agent is 1:0.3. Then, perform the first sintering and the second sintering successively. First, heat up to 500 °C at a heating rate of 15 °C / min and sinter for 4 h. Then, heat up to 800 °C at a heating rate of 2 °C / min and sinter for 9 h to obtain the lithium iron phosphate composite material.

[0079] Example 7

[0080] Step S1: Mix the iron phosphate precursor and lithium hydroxide in a molar ratio of Li:Fe:P of 1:0.95:1, and then perform ultrafine grinding. Add sucrose so that the carbon content accounts for 1.0% by weight of the lithium iron phosphate composite material to obtain the first slurry with a solid content of 40% and an average particle size of 250 nm. Pipette 10 wt.% of the first slurry for spray drying to obtain the first dry material.

[0081] Step S2: Add 10 wt.% of trimethyl borate to the remaining first slurry, mix evenly to obtain the second slurry, and then perform spray drying to obtain the second dry material.

[0082] Step S3: Add the first dry material, the second dry material, and the lithium supplement agent Li 5 FeO 4 Add them to a high-speed mixer for uniform mixing. The average particle size of the lithium supplement agent is 4.5 μm, and the weight ratio of the lithium supplement agent to the first dry material is 1:0.2. Then, perform the first sintering and the second sintering successively. First, heat up to 450 °C at a heating rate of 20 °C / min and sinter for 5 h. Then, heat up to 800 °C at a heating rate of 1 °C / min and sinter for 8 h to obtain the lithium iron phosphate composite material.

[0083] Example 8

[0084] Step S1: Mix the iron phosphate precursor and lithium hydroxide monohydrate in a molar ratio of Li:Fe:P of 1.4:0.98:1, and then perform ultrafine grinding. Add starch so that the carbon content accounts for 1.6% by weight of the lithium iron phosphate composite material to obtain the first slurry with a solid content of 20% and an average particle size of 280 nm. Pipette 13 wt.% of the first slurry for spray drying to obtain the first dry material.

[0085] Step S2: Add 10 wt.% of trimethyl borate to the remaining first slurry, mix evenly to obtain the second slurry, and then perform spray drying to obtain the second dry material.

[0086] Step S3: Add the first dry material, the second dry material, and the lithium supplement agent Li 5 FeO 4Add them to a high-speed mixer for uniform mixing. The average particle size of the lithium supplement agent is 4.5 μm, and the weight ratio of the lithium supplement agent to the first dry material is 1:0.2. Then, perform the first sintering and the second sintering in sequence. First, heat up to 450 °C at a heating rate of 20 °C / min and sinter for 5 h. Then, heat up to 800 °C at a heating rate of 1 °C / min and sinter for 8 h to obtain the lithium iron phosphate composite material.

[0087] Example 9

[0088] Step S1: Mix the iron phosphate precursor and lithium carbonate in a molar ratio of Li:Fe:P of 1.2:0.968:1, and then perform ultrafine grinding. Add glucose so that the carbon content accounts for 1.2% by weight of the lithium iron phosphate composite material to obtain the first slurry, whose solid content is 35% and average particle size is 300 nm. Take 10 wt.% of the first slurry for spray drying to obtain the first dry material.

[0089] Step S2: Add 5 wt% of trimethyl borate to the remaining first slurry, mix evenly to obtain the second slurry, and then perform spray drying to obtain the second dry material.

[0090] Step S3: Add the first dry material, the second dry material, and the lithium supplement agent Li 5 FeO 4 Add them to a high-speed mixer for uniform mixing. The average particle size of the lithium supplement agent is 4.5 μm, and the weight ratio of the lithium supplement agent to the first dry material is 1:0.2. Then, perform the first sintering and the second sintering in sequence. First, heat up to 450 °C at a heating rate of 20 °C / min and sinter for 5 h. Then, heat up to 800 °C at a heating rate of 1 °C / min and sinter for 8 h to obtain the lithium iron phosphate composite material.

[0091] Example 10

[0092] Step S1: Mix the iron phosphate precursor and lithium carbonate in a molar ratio of Li:Fe:P of 1.2:0.968:1, and then perform ultrafine grinding. Add glucose so that the carbon content accounts for 1.2% by weight of the lithium iron phosphate composite material to obtain the first slurry, whose solid content is 35% and average particle size is 300 nm. Take 10 wt.% of the first slurry for spray drying to obtain the first dry material.

[0093] Step S2: Add 20 wt% of trimethyl borate to the remaining first slurry, mix evenly to obtain the second slurry, and then perform spray drying to obtain the second dry material.

[0094] Step S3: Add the first dry material, the second dry material, and the lithium supplement agent Li 5 FeO 4Add them to a high-speed mixer for uniform mixing. The average particle size of the lithium supplement agent is 4.5 μm, and the weight ratio of the lithium supplement agent to the first dry material is 1:0.2. Then, perform the first sintering and the second sintering in sequence. First, heat up to 450 °C at a heating rate of 20 °C / min and sinter for 5 h. Then, heat up to 800 °C at a heating rate of 1 °C / min and sinter for 8 h to obtain the lithium iron phosphate composite material.

[0095] Example 11

[0096] Step S1: Mix the iron phosphate precursor and lithium carbonate in a molar ratio of Li:Fe:P of 1.2:0.968:1, and then perform ultrafine grinding. Add glucose so that the carbon accounts for 1.2% by weight of the lithium iron phosphate composite material to obtain the first slurry, with a solid content of 35% and an average particle size of 300 nm. Take 10 wt.% of the first slurry for spray drying to obtain the first dry material.

[0097] Step S2: Add 10 wt% of trimethyl borate to the remaining first slurry, mix evenly to obtain the second slurry, and then perform spray drying to obtain the second dry material.

[0098] Step S3: Add the first dry material, the second dry material, and the lithium supplement agent Li 5 FeO 4 Add them to a high-speed mixer for uniform mixing. The average particle size of the lithium supplement agent is 3 μm, and the weight ratio of the first dry material to the lithium supplement agent is 1:0.45. Then, perform the first sintering and the second sintering in sequence. First, heat up to 400 °C at a heating rate of 10 °C / min and sinter for 5 h. Then, heat up to 700 °C at a heating rate of 1 °C / min and sinter for 10 h to obtain the lithium iron phosphate composite material.

[0099] Example 12

[0100] Step S1: Mix the iron phosphate precursor and lithium carbonate in a molar ratio of Li:Fe:P of 1.2:0.968:1, and then perform ultrafine grinding. Add glucose so that the carbon accounts for 1.2% by weight of the lithium iron phosphate composite material to obtain the first slurry, with a solid content of 35% and an average particle size of 300 nm. Take 10 wt.% of the first slurry for spray drying to obtain the first dry material.

[0101] Step S2: Add 10 wt% of trimethyl borate to the remaining first slurry, mix evenly to obtain the second slurry, and then perform spray drying to obtain the second dry material.

[0102] Step S3: Add the first dry material, the second dry material, and the lithium supplement agent Li 5 FeO 4Add them to a high-speed mixer for uniform mixing. The average particle size of the lithium supplement agent is 8 μm, and the weight ratio of the first dry material to the lithium supplement agent is 1:0.08. Then, conduct the first sintering and the second sintering in sequence. First, heat up to 600 °C at a heating rate of 20 °C / min and sinter for 3 h. Then, heat up to 900 °C at a heating rate of 3 °C / min and sinter for 8 h to obtain the lithium iron phosphate composite material.

[0103] Comparative Example 1

[0104] Step S1: Mix the iron phosphate precursor and lithium carbonate in a molar ratio of Li:Fe:P of 1.2:0.968:1, and then conduct ultrafine grinding. Add glucose so that the carbon content accounts for 1.2% by weight of the lithium iron phosphate composite material to obtain the first slurry, whose solid content is 35% and average particle size is 300 nm. Take 10 wt.% of the first slurry for spray drying to obtain the first dry material.

[0105] Step S2: Add 10 wt% of trimethyl borate to the remaining first slurry, mix evenly to obtain the second slurry, and then conduct spray drying to obtain the second dry material.

[0106] Step S3: Add the first dry material and the second dry material to a high-speed mixer for uniform mixing, and then conduct the first sintering and the second sintering in sequence. First, heat up to 450 °C at a heating rate of 20 °C / min and sinter for 5 h. Then, heat up to 800 °C at a heating rate of 1 °C / min and sinter for 8 h to obtain the lithium iron phosphate material.

[0107] Comparative Example 2

[0108] Step S1: Mix the iron phosphate precursor and lithium carbonate in a molar ratio of Li:Fe:P of 1.2:0.968:1, and then conduct ultrafine grinding. Add glucose so that the carbon content accounts for 1.2% by weight of the lithium iron phosphate composite material to obtain the first slurry, whose solid content is 35% and average particle size is 300 nm. Take 10 wt.% of the first slurry for spray drying to obtain the first dry material.

[0109] Step S2: Add the first dry material and the lithium supplement agent Li 5 FeO 4 to a high-speed mixer for uniform mixing. The average particle size of the lithium supplement agent is 4.5 μm, and the weight ratio of the lithium supplement agent to the first dry material is 1:0.2. Then, conduct the first sintering and the second sintering in sequence. First, heat up to 450 °C at a heating rate of 20 °C / min and sinter for 5 h. Then, heat up to 800 °C at a heating rate of 1 °C / min and sinter for 8 h to obtain the lithium iron phosphate material.

[0110] Comparative Example 3

[0111] Step S1: Mix the iron phosphate precursor and lithium carbonate in a ratio of Li:Fe:P molar ratio of 1.2:0.968:1, followed by ultra-fine grinding. Add glucose such that the carbon content accounts for 1.2% by weight of the lithium iron phosphate composite material to obtain the first slurry. Add 10 wt% of trimethyl borate to the slurry, mix evenly to obtain the second slurry, and then perform spray drying to obtain the second dry material.

[0112] Step S2: Add the first dry material, the second dry material, and the lithium supplement agent Li 5 FeO 4 to a high-speed mixer for uniform mixing. The average particle size of the lithium supplement agent is 4.5 μm, and the weight ratio of the lithium supplement agent to the first dry material is 1:0.2. Then, perform the first sintering and the second sintering in sequence. First, heat up to 450 °C at a heating rate of 20 °C / min and sinter for 5 h. Then, heat up to 800 °C at a heating rate of 1 °C / min and sinter for 8 h to obtain the lithium iron phosphate material.

[0113] Comparative Example 4

[0114] Step S1: Mix the iron phosphate precursor and lithium carbonate in a ratio of Li:Fe:P molar ratio of 1.2:0.968:1, followed by ultra-fine grinding. Add glucose such that the carbon content accounts for 1.2% by weight of the lithium iron phosphate composite material to obtain the first slurry, with a solid content of 35% and an average particle size of 300 nm. Pipette 10 wt.% of the first slurry for spray drying to obtain the first dry material.

[0115] Step S2: Add 10 wt% of trimethyl borate to the remaining first slurry, mix evenly to obtain the second slurry, and then perform spray drying to obtain the second dry material.

[0116] Step S3: Add the first dry material, the second dry material, and the lithium supplement agent Li 5 FeO 4 to a high-speed mixer for uniform mixing. The average particle size of the lithium supplement agent is 4.5 μm, and the weight ratio of the lithium supplement agent to the first dry material is 1:0.2. Then, perform one-step sintering, heat up to 800 °C at a heating rate of 1 °C / min, and sinter for 8 h to obtain the lithium iron phosphate material.

[0117] Perform performance tests on the lithium iron phosphate materials prepared in the above examples and comparative examples, and the results are shown in Tables 1 and 2.

[0118] Test method:

[0119] Average particle size of the slurry: The D50 value of the particle size distribution obtained by testing the slurry with a Malvern 3000 laser particle size analyzer.

[0120] Average particle sizes of large and small particles in dry materials and composite materials: The average particle sizes of lithium iron phosphate and the lithium supplement agent were measured by scanning electron microscopy, and the number of particles within the relevant particle size ranges was calculated under the microscope.

[0121] Element proportion: The weight percentage of carbon in the lithium iron phosphate composite material was measured using a carbon and sulfur analyzer.

[0122] Apparent density: The material was tested using an apparent density tester with a test pressure of 3 t.

[0123] Specific capacity: Lithium iron phosphate (composite) material, PVDF, and Super-p were mixed at a mass ratio of 90:5:5 to prepare a slurry with a solid content of 30%. The slurry was homogenized, then coated, dried, and punched to obtain circular electrodes. Finally, in a glove box, the circular electrodes, separator (polypropylene microporous membrane), electrolyte (1 mol / L LiPF 6 , with the solvent being EC:DMC:EMC = 1:1:1 (v / v / v)), and lithium sheets were assembled into coin cells. Charge-discharge tests were carried out in the voltage range of 2.0 - 4.5 V. First, it was charged at 0.2C to 4.5 V, then discharged at 0.2C to 2.0 V, then charged at 0.2C to 4.0 V, and then discharged at 0.2C to 2.0 V, for two cycles.

[0124] Table 1

[0125]

[0126] Table 2

[0127]

[0128]

[0129] It can be seen that compared with Example 1, in Comparative Example 1, no lithium supplement agent was added, and no lithium supplement effect was provided, resulting in a significant decrease in both the first charge specific capacity and the discharge specific capacity. In Comparative Example 2, a coupling agent was not used to prepare the second dry material, and large particles could not be effectively formed, resulting in the inability to form an effective three-stage grading relationship and a significant decrease in compaction. In Comparative Example 3, the first dry material was not prepared. Therefore, due to the significant decrease in the proportion of small particles in the composite material, a three-stage grading relationship was not formed, resulting in a decrease in compaction. At the same time, due to the aggregation of large particles, the polarization of the electrode was large, and the capacity was difficult to exert. In Comparative Example 4, two-step sintering was not carried out, the controllability of coupling agent decomposition and crystal growth decreased, resulting in the failure to achieve a rational grading relationship, and the uneven distribution of carbon sources led to a decrease in electrical performance.

[0130] As can be seen from the above, compared with the comparative examples, in each embodiment of the present invention, the super-large particles of the lithium supplement are combined with the large and small particles of lithium iron phosphate, and the three-level particle grading relationship is reasonably designed. By utilizing the high activity of the small particles and the high space utilization rate of the large particles, and at the same time using the lithium supplementing effect and the structure supporting effect of the super-large particle lithium supplement, a high-compaction composite material is obtained, which can effectively improve the specific capacity while increasing the compaction density of the material, thereby further improving the capacity performance such as energy density.

[0131] In addition, it can be seen that when each process parameter is within the preferred range of the present invention, the comprehensive effect is better.

[0132] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A lithium iron phosphate composite material, characterized in that: The lithium iron phosphate composite material includes small-particle lithium iron phosphate, large-particle lithium iron phosphate and ultra-large-particle lithium supplement; the average particle size of the small-particle lithium iron phosphate is ≤0.3 μm, the average particle size of the large-particle lithium iron phosphate is 1-2.5 μm, and the average particle size of the ultra-large-particle lithium supplement is 3-8 μm.

2. The lithium iron phosphate composite material according to claim 1, characterized in that: The average particle size of the small-particle lithium iron phosphate is ≤0.25 μm, the average particle size of the large-particle lithium iron phosphate is 1.5-2.5 μm, and the average particle size of the super-large-particle lithium supplement is 4-6 μm.

3. The lithium iron phosphate composite material according to claim 1 or 2, characterized in that: The weight ratio of the small-particle lithium iron phosphate to the large-particle lithium iron phosphate is 1:(7-9.5), and the weight ratio of the small-particle lithium iron phosphate to the super-large-particle lithium supplement is 1:(0.1-0.5).

4. The lithium iron phosphate composite material according to any one of claims 1 to 3, characterized in that: The lithium supplement comprises Li2NiO2 and / or Li5FeO4; and / or The lithium iron phosphate composite material further includes carbon, and the weight percentage of the carbon in the lithium iron phosphate composite material is 1.0-1.6%.

5. The method for preparing the lithium iron phosphate composite material according to any one of claims 1 to 4, characterized in that: The following steps are involved: Step S1, mixing an iron phosphate precursor, a lithium source, an optional carbon source and water, and grinding them to obtain a first slurry; The first slurry is divided into two parts, namely a first part of the first slurry and a second part of the first slurry, and the first part of the first slurry is subjected to a first spray drying to obtain a first dry material; Step S2, adding a coupling agent to the second part of the first slurry to obtain a second slurry; performing a second spray drying on the second slurry to obtain a second dry material; Step S3, mixing the first dry material, the second dry material and the lithium supplement agent, and sequentially performing a first sintering and a second sintering to obtain the lithium iron phosphate composite material; Wherein, the temperature of the first sintering is lower than the temperature of the second sintering.

6. The method for preparing the lithium iron phosphate composite material according to claim 5, characterized in that: In the step S1, the molar ratio of iron to phosphorus in the iron phosphate precursor is (0.95-0.98):1; and / or The lithium source comprises one or more of lithium carbonate, lithium hydroxide and lithium hydroxide monohydrate, and the molar ratio of lithium in the lithium source to phosphorus in the iron phosphate precursor is (1-1.4):1; and / or The solid content of the first slurry is 20-40%, and the average particle size of the first slurry is ≤350nm; and / or The weight percentage of the first part of the first slurry to the first slurry is 10-13%; and / or The carbon source comprises one or more of glucose, sucrose, starch, and polyethylene glycol with a weight average molecular weight of 2000-8000, and the weight percentage of carbon in the carbon source to the lithium iron phosphate composite material is 1.0-1.6%.

7. The method for preparing the lithium iron phosphate composite material according to claim 5 or 6, characterized in that: In the step S2, The coupling agent includes one or more of trimethyl borate, triethanolamine borate and N-methyliminodiacetic acid borate; and / or The added amount of the coupling agent is 5-20% of the weight of the second part of the first slurry.

8. The method for preparing the lithium iron phosphate composite material according to any one of claims 5 to 7, characterized in that: In the step S3, The lithium supplement agent comprises Li2NiO2 and / or Li5FeO4, and the average particle size of the lithium supplement agent is 3 to 8 μm; and / or The weight ratio of the first dry material to the lithium supplement agent is 1:(0.08-0.45); and / or The first sintering temperature is 400-600° C., the time is 3-5 hours, and the heating rate is 10-20° C. / min; and / or The second sintering is carried out at a temperature of 700-900° C., for a time of 8-10 hours, and at a heating rate of 1-3° C. / min.

9. A positive electrode sheet, comprising a current collector and a positive electrode slurry loaded on at least one surface of the current collector, wherein the positive electrode slurry comprises a positive electrode active material, a conductive agent and a binder, characterized in that: The positive electrode active material includes the lithium iron phosphate composite material according to any one of claims 1 to 4.

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

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