Lithium iron phosphate composite material, preparation method therefor, and application thereof

Abstract

The present application provides a lithium iron phosphate composite material, a preparation method therefor, and an application thereof. The lithium iron phosphate composite material comprises small-particle lithium iron phosphate, large-particle lithium iron phosphate, and an ultra-large-particle pre-lithiation additive; an average particle size of the small-particle lithium iron phosphate is less than or equal to 0.3 μm, an average particle size of the large-particle lithium iron phosphate is 1 to 2.5 μm, and an average particle size of the ultra-large-particle pre-lithiation additive is 3 to 8 μm. The present application uses ultra-large particles of a pre-lithiation additive in combination with large and small particles of lithium iron phosphate to reasonably design a trimodal particle size distribution relationship, utilizing high activity of the small particles and high space utilization of the large particles, while also leveraging a pre-lithiation effect and structural support function of the ultra-large particle pre-lithiation additive to obtain a composite material with a high compacted density, which is capable of effectively improving specific capacity while simultaneously increasing the compacted density of the material, thereby further improving capacity performance such as energy density.

Description

Lithium iron phosphate composite materials, their preparation methods and applications

[0001] This application claims priority to Chinese patent application 2025103784381, filed on March 27, 2025. The entire contents of the aforementioned Chinese patent application are incorporated herein by reference. Technical Field

[0002] This application relates to the field of lithium-ion battery technology, and more specifically, to a lithium iron phosphate composite material, its preparation method, and its application. Background Technology

[0003] With continuous technological advancements and expanding application scenarios, the market demands increasingly higher energy density for lithium-ion batteries, whether in energy storage systems or power batteries (such as those used in new energy vehicles). From the perspective of cathode materials, obtaining higher-compact cathode materials is crucial for improving battery energy density.

[0004] Although the compaction density of lithium iron phosphate (LFP) materials has improved significantly in recent years, some problems still need to be solved. High-compact LFP (powder compaction ≥ 2.65 g / cc) generally has larger particle sizes. While this increases the mass of active material per unit volume (i.e., energy density), the specific capacity of large particles is limited by their size. At high energy densities, they cannot achieve the ideal specific capacity. Therefore, high-compact LFP struggles to meet the demand for high capacity, which directly affects the energy density and power density of the battery. Thus, how to achieve both high compaction and high electrochemical performance in LFP has become a key research focus.

[0005] Chinese patent application CN 115275109 A discloses a long-cycle lithium iron phosphate thick electrode, its preparation method, 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 replenishment layer disposed on at least one side of the current collector. The first coating includes small-particle lithium iron phosphate, and the second coating includes large-particle lithium iron phosphate. This application uses a double-layer coating structure with both large and small particles, which can improve the liquid phase transport efficiency and kinetic performance of the thick lithium iron phosphate electrode. The lithium replenishment layer can effectively compensate for irreversible capacity loss in the thick electrode and improve the electrode's cycle performance. However, although the above technical solution can improve compaction and capacity performance to some extent, the improvement effect on capacity performance is very limited. Moreover, the contact between the coating particles is not tight, the electrode stability is not high, layered coating is required, the electrode fabrication efficiency is low, and there is still considerable room for improvement in electrochemical performance. Summary of the Invention

[0006] The main objective of this application is to provide a lithium iron phosphate composite material, its preparation method, and its application, so as to solve the problem that lithium iron phosphate materials in the prior art cannot simultaneously achieve high compaction density and good electrochemical performance.

[0007] To achieve the above objectives, according to one aspect of this application, a lithium iron phosphate composite material is provided, comprising small-particle lithium iron phosphate, large-particle lithium iron phosphate, and ultra-large-particle lithium supplementer; 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 supplementer is 3–8 μm. This application rationally designs a three-level particle gradation relationship, utilizing a close-packed system formed by two different particle sizes of lithium iron phosphate and ultra-large-particle lithium supplementer to obtain a high compaction density composite material. Based on the close bonding between the lithium supplementer and the lithium iron phosphate cathode material, the lithium supplementation advantage of the lithium supplementer can be fully utilized, thereby effectively improving the electrochemical performance while increasing the material compaction density.

[0008] Furthermore, the average particle size of small lithium iron phosphate particles is ≤0.25 μm, the average particle size of large lithium iron phosphate particles is 1.5–2.5 μm, and the average particle size of ultra-large lithium supplementary agents is 4–6 μm. By controlling the particle size of lithium iron phosphate and lithium supplementary agents within the above-mentioned preferred ranges, this application can form a more coordinated three-level distribution of small, large, and ultra-large particles, which is beneficial to further improve the compaction density of the material and maintain good electrochemical performance, enabling lithium iron phosphate materials to better balance high compaction and high capacity.

[0009] Furthermore, 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 ultra-large-particle lithium supplement is 1:(0.1–0.5). This ratio is beneficial for achieving higher space utilization while also taking into account electrical performance.

[0010] Furthermore, the lithium replenishing agent includes Li₂NiO₂ and / or Li₅FeO₄; and / or the lithium iron phosphate composite material also includes carbon, with carbon accounting for 1.0–1.6% of the weight of the lithium iron phosphate composite material. The aforementioned lithium replenishing agent allows for more efficient release of additional lithium ions during the first charge and discharge of the material, compensating for the lithium loss due to SEI film formation during the first charge and discharge of the lithium iron phosphate. This compensation mechanism directly improves the initial charge specific capacity of the material and stabilizes the battery's capacity performance over the long term. When the carbon content is within the aforementioned range, it provides good electronic conductivity while minimizing the impact on compaction density, resulting in the optimal electrical performance of the lithium iron phosphate composite material.

[0011] According to another aspect of this application, a method for preparing the lithium iron phosphate composite material described above is provided, comprising the following steps: Step S1, mixing an iron phosphate precursor, a lithium source, an optional carbon source, and water, and grinding them to obtain a first slurry; dividing the first slurry into two parts, namely a first part of the first slurry and a second part of the first slurry, and subjecting the first part of the first slurry 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; subjecting the second slurry to a 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 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.

[0012] This application employs a two-stage pulping and spray-drying process, combined with a two-step sintering process, to achieve precise control over particle size. A lithium-supplementing additive is introduced, utilizing its large particles to combine with the varying sizes of lithium iron phosphate particles, and a rationally designed three-level particle gradation relationship is achieved to obtain a high-compaction composite material. By co-sintering the lithium-supplementing additive with the spray-dried lithium iron phosphate, the tightness of the lithium-supplementing additive and lithium iron phosphate particles is enhanced, thereby increasing the compaction density of the lithium iron phosphate composite material and effectively improving its electrical properties. The above preparation process is simple, easy to operate, and applicable to large-scale industrial production.

[0013] Further, in 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 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 is ≤350 nm; and / or the first part of the first slurry accounts for 10–13% of the weight percentage of the first slurry; 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 percentage of the lithium iron phosphate composite material.

[0014] Within the aforementioned ranges, the molar ratios of iron to phosphorus and lithium to phosphorus in the lithium iron phosphate precursor result in a more suitable stoichiometry for the synthesized lithium iron phosphate, contributing to improved electrochemical performance, particularly capacity performance. A solid content within the aforementioned range in the first slurry further promotes particle formation during subsequent drying and sintering processes, while improving slurry flowability and dispersibility. When the average particle size of the slurry and the proportion of slurry used to prepare the small-particle-size first dry material are within the aforementioned ranges, it is more conducive to ensuring high dispersibility and a higher specific surface area for some material particles, thereby improving lithium-ion diffusion efficiency and the overall density of the material.

[0015] Further, in step S2, the coupling agent includes one or more of trimethyl borate, triethanolamine borate, and N-methyliminodiacetic acid borate; and / or the amount of coupling agent added is 5-20% of the weight of the first slurry in the second part. The above-mentioned coupling agent can also play a coupling role on the surface of material particles, providing the necessary adhesion for particle growth, while not forming a structure that is too dense to affect lithium ion diffusion.

[0016] Further, in step S3, the lithium replenishing agent includes Li2NiO2 and / or Li5FeO4, with an average particle size of 3–8 μm; and / or the weight ratio of the first dry material to the lithium replenishing agent is 1:(0.08–0.45); and / or the first sintering temperature is 400–600 °C, the time is 3–5 h, and the heating rate is 10–20 °C / min; and / or the second sintering temperature 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 amount of lithium replenishing agent to the above ranges can provide a more sufficient lithium replenishment effect while reducing the potential decrease in electrochemical performance due to the introduction of excessive lithium replenishing agent. The relatively fast heating rate of the first sintering, being a short-time low-temperature sintering, can simultaneously meet the requirements of decomposition and the growth process of small-particle lithium iron phosphate crystals. The second sintering heating rate is relatively slow, which is a long-term high-temperature sintering process that can repair crystal defects. 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 particle size distribution.

[0017] According to another aspect of this application, a positive electrode sheet is provided, comprising a current collector and a positive electrode slurry loaded on at least one surface of the current collector. The positive electrode slurry comprises a positive electrode active material, a conductive agent, and a binder. The positive electrode active material comprises the lithium iron phosphate composite material described above in this application. It exhibits significantly improved compaction density and capacity performance.

[0018] According to another aspect of this application, a lithium-ion battery is provided, including the positive electrode sheet described above. It exhibits significantly improved electrochemical performance.

[0019] By applying the technical solution of this application, the higher specific surface area of ​​small-particle lithium iron phosphate helps to improve the rate performance of the battery; while large-particle lithium iron phosphate ensures that the material has high compaction density and conductivity. The ultra-large-particle lithium replenisher can effectively replenish lithium ions during the preparation, sintering, and SEI film formation processes, reducing lithium loss and thus improving the battery's capacity performance. This application utilizes the combination of ultra-large-particle lithium replenisher and large-particle lithium iron phosphate, rationally designing a three-level particle gradation relationship. It leverages the high activity of small particles and the high space utilization of large particles, while utilizing the lithium replenishment effect and structural support effect of the ultra-large-particle lithium replenisher to obtain a high-compaction composite material. This effectively increases the specific capacity while improving the material's compaction density, thereby further improving capacity performance such as energy density. Attached Figure Description

[0020] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings:

[0021] Figure 1 shows a SEM image of the lithium iron phosphate composite material according to Example 1 of this application. Detailed Implementation

[0022] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0023] As described in the background section of this application, existing lithium iron phosphate materials suffer from the problem of not being able to simultaneously achieve high compaction density and good electrochemical performance, especially capacity performance. To address this issue, in a typical embodiment of this application, a lithium iron phosphate composite material is provided. This composite material comprises small-particle lithium iron phosphate, large-particle lithium iron phosphate, and ultra-large-particle lithium supplementer; 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 supplementer is 3–8 μm.

[0024] Small-particle lithium iron phosphate (LFP) with the aforementioned particle size has a higher specific surface area, providing more active sites and promoting rapid lithium-ion diffusion, thereby improving the electrochemical performance of the material. Smaller particles also reduce the lithium-ion diffusion path, enhancing the material's cycle stability. Large-particle LFP promotes compact packing, reducing inter-particle voids and increasing the overall compaction density of the material, without excessively hindering lithium-ion diffusion efficiency due to excessive size. Introducing ultra-large lithium-replenishing particles helps compensate for lithium loss due to lithium-ion volatilization during LFP preparation, and also compensates for lithium loss due to lithium intercalation / deintercalation during cycling, thus improving the material's capacity and cycle life. Furthermore, because their particle size is much larger than that of LFP particles, ultra-large lithium-replenishing particles can also act as a skeleton in the composite material, contributing to increased compaction density. Larger particle sizes increase the characteristic ion diffusion length, leading to a decrease in electrical performance.

[0025] This application rationally designs a three-level particle gradation relationship and utilizes a close-packed system formed by two different particle sizes of lithium iron phosphate and ultra-large particle lithium supplementer to obtain a high compaction density composite material with a theoretical space utilization rate of up to 85%, which has a positive effect on improving compaction density. Based on the close combination of the aforementioned lithium supplementer and lithium iron phosphate cathode material, the lithium supplementer's lithium supplementation advantage can be fully utilized, thereby effectively improving electrochemical performance while increasing the material's compaction density.

[0026] In a preferred embodiment, the average particle size of small lithium iron phosphate particles is ≤0.25 μm, the average particle size of large lithium iron phosphate particles is 1.5–2.5 μm, and the average particle size of ultra-large lithium supplementary agent is 4–6 μm. The small lithium iron phosphate particles with the aforementioned particle sizes have a higher specific surface area, which helps to further improve the rate performance of the battery; the large lithium iron phosphate particles allow the material to have higher compaction density and conductivity, while the ultra-large lithium supplementary agent can more effectively replenish lithium ions during the preparation, sintering, and SEI film formation processes, reducing lithium loss and thus improving the battery's capacity performance. By controlling the particle sizes of lithium iron phosphate and the lithium supplementary agent within the aforementioned preferred range, this application can form a more coordinated three-level distribution of small, large, and ultra-large particles, which is beneficial for further improving the compaction density of the material and maintaining good electrochemical performance, enabling the lithium iron phosphate material to better balance high compaction and high capacity.

[0027] 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 ultra-large-particle lithium supplementer is 1:(0.1-0.5). This ratio facilitates higher space utilization while maintaining good electrical performance. The above ratio provides more sufficient small-particle lithium iron phosphate to utilize its high specific surface area to improve the material's specific capacity and cycle stability. Setting the weight ratio of large-particle lithium iron phosphate within the above range allows the composite material to have most of its space occupied by large particles, thereby achieving high compaction density. Limiting the weight ratio of ultra-large-particle lithium supplementer within the above range provides a more sufficient lithium supplementation effect while reducing the potential decline in electrochemical performance due to excessive lithium supplementer introduction. This application, by precisely controlling the weight ratio of three different particle sizes, further facilitates a balance between high compaction density and good capacity performance in the material.

[0028] In a preferred embodiment, the lithium replenishing agent includes Li₂NiO₂ and / or Li₅FeO₄; and / or the lithium iron phosphate composite material further includes carbon, with carbon accounting for 1.0–1.6% of the weight of the lithium iron phosphate composite material. The aforementioned lithium replenishing agent allows for smoother release of additional lithium ions during the first charge and discharge of the material, compensating for the lithium loss due to SEI film formation during the first charge and discharge of the lithium iron phosphate. This compensation mechanism directly improves the initial charge specific capacity of the material and stabilizes the battery's capacity performance over the long term. Carbon, as a conductive agent, effectively improves the electronic conductivity of the material and reduces internal resistance during electrochemical reactions, thereby improving the specific capacity and cycle stability of the material. However, excessively high carbon content increases the bulkiness of the material and reduces compaction density, while excessively low carbon content has a smaller effect on improving electronic conductivity and affects electrochemical performance. When the carbon content is within the above range, it provides good electronic conductivity while having minimal impact on compaction density, resulting in the optimal electrical performance of the lithium iron phosphate composite material.

[0029] In another typical embodiment of this application, a method for preparing the lithium iron phosphate composite material described above is also provided, comprising the following steps: Step S1, mixing an iron phosphate precursor, a lithium source, an optional carbon source, and water, and grinding them to obtain a first slurry; dividing the first slurry into two parts, namely a first part of the first slurry and a second part of the first slurry, and subjecting the first part of the first slurry 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; subjecting the second slurry to a 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 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.

[0030] Specifically, the iron phosphate precursor, lithium source, optional carbon source, and water are first mixed and ground to uniformly disperse the raw materials and reduce the particle size, resulting in a first slurry. A portion of this slurry undergoes a first spray drying process to obtain a small-particle-size first dry material. During this process, the precursor particle size is controlled through ultrafine grinding, ensuring that the carbon source fully coats the surface of the reactants, forming a stable reactant-carbon source system. This system inhibits particle growth during subsequent sintering. A coupling agent is added to another portion of the first slurry to obtain a second slurry. The second slurry undergoes a second spray drying process to obtain a second dry material. During this process, a stable reactant-carbon source-coupling agent system is formed. The coupling agent effectively couples and aggregates the small particles in the slurry, which is beneficial for melt growth during subsequent sintering. Furthermore, during low-temperature sintering, the agent decomposes to form boron oxides, which act as a sintering aid and further promote particle aggregation and growth.

[0031] Finally, the first dry material, the second dry material, and the lithium supplement agent are mixed and then subjected to low-temperature sintering and high-temperature sintering in sequence. During the low-temperature sintering process, the carbon source in the reactant-carbon source system of the first dry material decomposes at low temperature and fully coats the surface of the reactants, inhibiting the particle growth during the phase formation process of lithium iron phosphate. The carbon source and coupling agent in the second dry material also decompose fully. The coupling agent decomposes at low temperature to form boron oxide, 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, resulting in a lithium iron phosphate composite material with a three-level particle gradation.

[0032] Spray drying is a solvent evaporation process. Conventional drying processes often lead to carbon source segregation and uneven distribution. Spray drying effectively avoids this segregation problem. Therefore, in the initial stage of lithium iron phosphate material preparation, after aqueous grinding, spray drying is an essential step to obtain a stable reactant system with uniform carbon source distribution. Furthermore, the material changes during this process are physical changes and have no significant impact on particle size. The temperature and other parameters of spray drying are related to the capacity and spray volume of the spray drying equipment and can be adjusted according to actual production conditions, as long as the drying purpose is achieved. These are concepts understood by those skilled in the art and will not be elaborated upon further here.

[0033] This application involves mixing a slurry containing all raw materials before sintering. Firstly, slurry grinding improves particle dispersibility, and high dispersibility is more conducive to achieving the target gradation relationship and increasing compaction density. Secondly, sintering after grinding enhances the tightness of contact between particles and between particles and the optional carbon layer. Compared with physical layering coating, sintering after mixing the slurry has a better interfacial contact effect, which is beneficial to improving the stability of the composite material. Thirdly, the high-compact composite material of this application can be directly coated with slurry, reducing the layering coating steps, thus improving the electrode manufacturing efficiency.

[0034] In summary, this application employs a two-stage pulping and spray-drying process, combined with a two-step sintering process, to achieve precise control over particle size. By introducing a lithium-supplementing additive, and utilizing the ultra-large particles of the lithium-iron phosphate to combine with the varying particle sizes, a rational three-level particle gradation relationship is designed to obtain a high-compaction composite material. Through co-sintering of the lithium-supplementing additive with the spray-dried lithium-iron phosphate, the tight bonding between the lithium-supplementing additive and the lithium-iron phosphate particles is enhanced, thereby increasing the compaction density of the lithium-iron phosphate composite material and effectively improving its electrical properties. The above preparation process is simple, easy to operate, and applicable to large-scale industrial production.

[0035] In a preferred embodiment, in 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 includes one or more of lithium carbonate, lithium hydroxide, and lithium hydroxide monohydrate, wherein 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 ≤350 nm; and / or the first portion of the first slurry accounts for 10–13% of the weight percentage of the first slurry; 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, wherein the carbon in the carbon source accounts for 1.0–1.6% of the weight percentage of the lithium iron phosphate composite material.

[0036] Within the aforementioned ranges, the molar ratios of iron to phosphorus and lithium to phosphorus in the lithium iron phosphate precursor allow for a more appropriate stoichiometry in the synthesized lithium iron phosphate, resulting in a more stable crystal structure. This improves the structural integrity of the lithium iron phosphate while achieving a more complete lithiation process through lithium excess, reducing the formation of metastable products due to Li deficiency. This contributes to improving the electrochemical performance of lithium iron phosphate, particularly its capacity performance. Excessive solid content in the first slurry may lead to excessive viscosity, affecting dispersibility and uniformity; conversely, insufficient solid content may result in inadequate particle formation, affecting the final compaction density. Therefore, this application limits the solid content to the aforementioned range, which, while improving slurry flowability and dispersibility, further promotes particle formation during subsequent drying and sintering processes.

[0037] Furthermore, when the average particle size of the slurry is within the aforementioned range, and the proportion of slurry used to prepare the small-particle-size first dry material is also within this range, it is more conducive to ensuring that some material particles have high dispersibility and a high specific surface area, thereby improving the diffusion efficiency of lithium ions. Simultaneously, small particles are easily dispersed with large and ultra-large particles, further increasing the overall density of the material. A carbon content within the aforementioned range allows the composite material to possess good electronic conductivity while having minimal impact on the compaction density of the material.

[0038] It should be noted that the average particle size of the first slurry is obtained from the test under the slurry system. Due to the influence of agglomeration and solvation effects during the test, the test result is generally larger than the actual particle size. The particle size of small lithium iron phosphate particles in the lithium iron phosphate composite material is the particle size of the sintered finished product, which can be measured in SEM and then calculated.

[0039] It should be noted that controlling the weight percentage of the first slurry in the first part to be 10-13% allows for a control of the weight ratio of small-particle lithium iron phosphate to large-particle lithium iron phosphate at 1:(7-9.5), and the weight ratio of small-particle lithium iron phosphate to ultra-large-particle lithium supplementer at 1:(0.1-0.5). The weight ratio of particles of each size is controlled by the slurry weight. Considering the loss of carbon content during sintering, the dry material burn-off rate is approximately 80%. Therefore, the actual weight ratio of small-particle lithium iron phosphate to lithium supplementer is 80% of the dry material ratio. The weight of large-particle lithium iron phosphate is slightly increased due to the influence of the boron co-solvent quality.

[0040] A suitable coupling agent can not only improve the compaction density of the material, but also enhance the contact tightness between particles and improve electrochemical performance by adjusting the surface properties of the particles. To further promote particle adhesion and thus facilitate the formation of larger lithium iron phosphate particles during subsequent sintering, in a preferred embodiment, in step S2, the coupling agent includes borate esters, specifically one or more of trimethyl borate, triethanolamine borate, and N-methyliminodiacetic acid borate; and / or the amount of coupling agent added is 5-20% of the weight of the first slurry in the second part. The above-mentioned coupling agent can also play a coupling role on the surface of the material particles, providing the necessary adhesion for particle growth without forming an overly dense structure that would hinder lithium-ion diffusion. The melting point of the borate ester can also play a role in aiding sintering during the sintering process.

[0041] Insufficient coupling agent will hinder the formation of sufficient bonding force, potentially leading to particle dispersion and low compaction density. Conversely, excessive coupling agent will result in overly strong coupling on the particle surface, potentially impeding lithium-ion diffusion and affecting capacity performance. Setting the coupling agent dosage within the aforementioned range helps balance the bonding strength between particles and the electrochemical activity of the material, promoting particle adhesion, forming a more ideal compaction structure, and maintaining good electrochemical performance.

[0042] In a preferred embodiment, in step S3, the lithium replenishing agent includes Li2NiO2 and / or Li5FeO4, the average particle size of the lithium replenishing agent is 3-8 μm, preferably, the average particle size of the lithium replenishing agent is 4-6 μm; and / or the weight ratio of the first dry material to the lithium replenishing agent is 1:(0.08-0.45).

[0043] Limiting the type, particle size, and amount of lithium replenishing agent to the above range can provide a more sufficient lithium replenishment effect while reducing the potential decline in electrochemical performance caused by the introduction of excessive lithium replenishing agent. It should be noted that lithium replenishing agents are generally prepared by high-temperature solid-state sintering, which forms a crystalline product. During further sintering, the particles will not grow significantly, thus ensuring the consistency of the lithium replenishing agent particle size before and after sintering.

[0044] 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 hours; 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 hours, 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 hours, and the heating rate is 1–3°C / min. The relatively fast heating rate of the first sintering allows it to reach the carbon source reaction temperature as quickly as possible, promoting the simultaneous and complete decomposition of the coupling agent and the carbon source. Short-time low-temperature sintering can simultaneously meet the requirements of decomposition and the growth process of small-particle lithium iron phosphate crystals. The second sintering heating rate is relatively slow to control the secondary growth process of lithium iron phosphate crystals formed after low-temperature sintering. This is beneficial for obtaining lithium iron phosphate crystals with different particle sizes. Furthermore, long-term high-temperature sintering can promote the graphitization process of the carbon layer and repair crystal defects, resulting in composite materials with better crystallinity. At the same time, the decomposition products of coupling agents (such as boron oxides produced by the decomposition of boric acid coupling agents) have a fluxing effect at high temperatures, which can induce the formation of large particles and improve particle size distribution.

[0045] In another typical embodiment of this application, a positive electrode sheet is also provided, comprising a current collector and a positive electrode slurry loaded on at least one surface of the current collector. The positive electrode slurry comprises a positive electrode active material, a conductive agent, and a binder. The positive electrode active material comprises the lithium iron phosphate composite material described above in this application. Due to the use of the lithium iron phosphate composite material with a three-level particle gradation relationship of this application, a high compaction density composite material is obtained by utilizing a close-packed system formed by two lithium iron phosphate particles of different sizes and ultra-large particle lithium supplementer. Based on the close combination of the lithium supplementer and the lithium iron phosphate positive electrode material, the lithium supplementation advantage of the lithium supplementer can be fully utilized. Therefore, the positive electrode sheet has significantly improved compaction density and capacity performance.

[0046] In another typical embodiment of this application, a lithium-ion battery is also provided, including the positive electrode sheet described above, which has significantly improved electrochemical performance.

[0047] Typical, but not limiting, average particle sizes of small lithium iron phosphate particles are 0.1 μm, 0.12 μm, 0.15 μm, 0.18 μm, 0.2 μm, 0.22 μm, 0.25 μm, 0.28 μm, 0.3 μm, or any two of these values; average particle sizes of large lithium iron phosphate particles are 1 μm, 1.2 μm, 1.5 μm, 1.8 μm, 2 μm, 2.2 μm, 2.5 μm, or any two of these values; and average particle sizes of ultra-large lithium supplements are 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, or any two of these values.

[0048] Typical, but not limiting, weight ratios of small-particle lithium iron phosphate to large-particle lithium iron phosphate are 1:7, 1:7.5, 1:8, 1:8.5, 1:9, 1:9.5, or any two of these values.

[0049] Typical, but not limiting, weight ratios of small-particle lithium iron phosphate to ultra-large-particle lithium supplements are 1:0.1, 1:0.2, 1:0.3, 1:0.4, 1:0.5, or any two of these values.

[0050] Typical, but not limited, lithium iron phosphate composites also include carbon, with carbon accounting for 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6% by weight of the lithium iron phosphate composite, or any combination of two such values.

[0051] Typical, but not limiting, amounts of coupling agent added are 5%, 8%, 10%, 12%, 15%, 18%, 20% of the weight of the first slurry in the second part, or any combination of two of these values.

[0052] Typical, but not limiting, the first sintering temperature is 400℃, 450℃, 500℃, 550℃, 600℃ or any two of these values; the time is 3h, 3.5h, 4h, 4.5h, 5h or any two of these values; and the heating rate is 10℃ / min, 12℃ / min, 15℃ / min, 18℃ / min, 20℃ / min or any two of these values.

[0053] Typical, but not limiting, the second sintering temperature is 700℃, 750℃, 800℃, 850℃, 900℃ or any two of these values; the time is 8h, 8.5h, 9h, 9.5h, 10h or any two of these values; and the heating rate is 1℃ / min, 1.5℃ / min, 2℃ / min, 2.5℃ / min, 3℃ / min or any two of these values.

[0054] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0055] Example 1

[0056] Step S1: The iron phosphate precursor and lithium carbonate are mixed in a Li:Fe:P molar ratio of 1.2:0.968:1 and then ultra-finely ground. Glucose is added so that the carbon content of the lithium iron phosphate composite material is 1.2% by weight, resulting in a first slurry with a solid content of 35% and an average particle size of 300 nm (the average particle size of the slurry is the D50 value obtained by measuring the slurry using a Malvern 3000 laser particle size analyzer). 10 wt.% of the first slurry is then spray-dried to obtain the first dry material.

[0057] Step S2: Add 10 wt% trimethyl borate to the remaining first slurry, mix well to obtain the second slurry, and then spray dry to obtain the second dry material.

[0058] Step S3: The first dry material, the second dry material, and the lithium supplement Li5FeO4 are added to a high-speed mixer for uniform mixing. The average particle size of the lithium supplement is 4.5 μm, and the weight ratio of the lithium supplement to the first dry material is 1:0.2. Then, the first sintering and the second sintering are carried out in sequence. First, the temperature is raised to 450°C at a heating rate of 20°C / min and sintered for 5 hours. Then, the temperature is raised to 800°C at a heating rate of 1°C / min and sintered for 8 hours to obtain the lithium iron phosphate composite material.

[0059] Figure 1 shows the SEM image of the lithium iron phosphate composite material, which clearly shows a three-level particle size distribution.

[0060] Example 2

[0061] Step S1: The iron phosphate precursor and lithium carbonate are mixed in a Li:Fe:P molar ratio of 1.4:0.970:1 and then ultra-finely ground. Glucose is added so that the carbon content of the lithium iron phosphate composite material is 1.3% by weight, and a first slurry with a solid content of 30% and an average particle size of 330 nm is obtained. 13 wt.% of the first slurry is taken and spray-dried to obtain the first dry material.

[0062] In step S2, 13 wt% of triethanolamine borate is added to the remaining first slurry, mixed evenly to obtain the second slurry, and then spray-dried to obtain the second dry material.

[0063] Step S3: The first dry material, the second dry material, and the lithium supplement Li5FeO4 are added to a high-speed mixer for uniform mixing. The average particle size of the lithium supplement is 6μm, and the weight ratio of the lithium supplement to the first dry material is 1:0.1. Then, the first sintering and the second sintering are carried out in sequence. First, the temperature is raised to 500℃ at a heating rate of 10℃ / min and sintered for 3h. Then, the temperature is raised to 810℃ at a heating rate of 2℃ / min and sintered for 10h to obtain the lithium iron phosphate composite material.

[0064] Example 3

[0065] Step S1: The iron phosphate precursor and lithium carbonate are mixed in a Li:Fe:P molar ratio of 1.4:0.970:1 and then ultra-finely ground. Polyethylene glycol with a weight average molecular weight of 2000 is added so that the carbon content of the lithium iron phosphate composite material is 1.3% by weight, and a first slurry with a solid content of 40% and an average particle size of 250 nm is obtained. 10 wt.% of the first slurry is taken and spray-dried to obtain the first dry material.

[0066] In step S2, 8 wt% of triethanolamine borate is added to the remaining first slurry, mixed evenly to obtain the second slurry, and then spray-dried to obtain the second dry material.

[0067] Step S3: The first dry material, the second dry material, and the lithium supplement Li5FeO4 are added to a high-speed mixer for uniform mixing. The average particle size of the lithium supplement is 5 μm, and the weight ratio of the lithium supplement to the first dry material is 1:0.15. Then, the first sintering and the second sintering are carried out in sequence. First, the temperature is raised to 500℃ at a heating rate of 10℃ / min and sintered for 5 hours. Then, the temperature is raised to 770℃ at a heating rate of 3℃ / min and sintered for 10 hours to obtain the lithium iron phosphate composite material.

[0068] Example 4

[0069] Step S1: The iron phosphate precursor and lithium carbonate are mixed in a Li:Fe:P molar ratio of 1.1:0.975:1 and then ultra-finely ground. Polyethylene glycol with a weight average molecular weight of 6000 is added so that the carbon content of the lithium iron phosphate composite material is 1.5% by weight, and a first slurry with a solid content of 40% and an average particle size of 230 nm is obtained. 13 wt.% of the first slurry is taken and spray-dried to obtain the first dry material.

[0070] In step S2, 20 wt% of N-methylimino diacetate borate is added to the remaining first slurry, and the mixture is stirred evenly to obtain the second slurry. Then, the mixture is spray-dried to obtain the second dry material.

[0071] Step S3: The first dry material, the second dry material, and the lithium supplement Li2NiO2 are added to a high-speed mixer for uniform mixing. The average particle size of the lithium supplement is 6μm, and the weight ratio of the lithium supplement to the first dry material is 1:0.1. Then, the first sintering and the second sintering are carried out in sequence. First, the temperature is raised to 500℃ at a heating rate of 10℃ / min and sintered for 3h. Then, the temperature is raised to 810℃ at a heating rate of 2℃ / min and sintered for 10h to obtain the lithium iron phosphate composite material.

[0072] Example 5

[0073] Step S1: The iron phosphate precursor and lithium carbonate are mixed in a Li:Fe:P molar ratio of 1.2:0.968:1 and then ultra-finely ground. Glucose is added so that the carbon content of the lithium iron phosphate composite material is 1.5% by weight, and a first slurry is obtained with a solid content of 38% and an average particle size of 200 nm. 10 wt.% of the first slurry is taken and spray-dried to obtain the first dry material.

[0074] Step S2: Add 10 wt% N-methylimino diacetate borate to the remaining first slurry, mix well to obtain the second slurry, and then spray dry to obtain the second dry material.

[0075] Step S3: The first dry material, the second dry material, and the lithium supplement Li2NiO2 are added to a high-speed mixer for uniform mixing. The average particle size of the lithium supplement is 5.6 μm, and the weight ratio of the lithium supplement to the first dry material is 1:0.2. Then, the first sintering and the second sintering are carried out in sequence. First, the temperature is raised to 400℃ at a heating rate of 15℃ / min and sintered for 5 hours. Then, the temperature is raised to 800℃ at a heating rate of 1℃ / min and sintered for 10 hours to obtain the lithium iron phosphate composite material.

[0076] Example 6

[0077] Step S1: The iron phosphate precursor and lithium carbonate are mixed in a Li:Fe:P molar ratio of 1.2:0.97:1 and then ultra-finely ground. Glucose is added so that the carbon content of the lithium iron phosphate composite material is 1.3% by weight, and a first slurry is obtained with a solid content of 30% and an average particle size of 280 nm. 13 wt.% of the first slurry is taken and spray-dried to obtain the first dry material.

[0078] In step S2, 12 wt% trimethyl borate is added to the remaining first slurry, mixed evenly to obtain the second slurry, and then spray-dried to obtain the second dry material.

[0079] Step S3: The first dry material, the second dry material, and the lithium supplement Li2NiO2 are added to a high-speed mixer for uniform mixing. The average particle size of the lithium supplement is 6μm, and the weight ratio of the first dry material to the lithium supplement is 1:0.3. Then, the first sintering and the second sintering are carried out in sequence. First, the temperature is raised to 500℃ at a heating rate of 15℃ / min and sintered for 4h. Then, the temperature is raised to 800℃ at a heating rate of 2℃ / min and sintered for 9h to obtain the lithium iron phosphate composite material.

[0080] Example 7

[0081] Step S1: The iron phosphate precursor and lithium hydroxide are mixed in a Li:Fe:P molar ratio of 1:0.95:1 and then ultra-finely ground. Sucrose is added so that the carbon content of the lithium iron phosphate composite material is 1.0% by weight, and a first slurry is obtained with a solid content of 40% and an average particle size of 250 nm. 10 wt.% of the first slurry is taken and spray-dried to obtain the first dry material.

[0082] Step S2: Add 10 wt% trimethyl borate to the remaining first slurry, mix well to obtain the second slurry, and then spray dry to obtain the second dry material.

[0083] Step S3: The first dry material, the second dry material, and the lithium supplement Li5FeO4 are added to a high-speed mixer for uniform mixing. The average particle size of the lithium supplement is 4.5 μm, and the weight ratio of the lithium supplement to the first dry material is 1:0.2. Then, the first sintering and the second sintering are carried out in sequence. First, the temperature is raised to 450°C at a heating rate of 20°C / min and sintered for 5 hours. Then, the temperature is raised to 800°C at a heating rate of 1°C / min and sintered for 8 hours to obtain the lithium iron phosphate composite material.

[0084] Example 8

[0085] Step S1: The iron phosphate precursor and lithium hydroxide monohydrate are mixed in a Li:Fe:P molar ratio of 1.4:0.98:1 and then ultra-finely ground. Starch is added so that the carbon content of the lithium iron phosphate composite material is 1.6% by weight, and a first slurry with a solid content of 20% and an average particle size of 280 nm is obtained. 13 wt.% of the first slurry is taken and spray-dried to obtain the first dry material.

[0086] Step S2: Add 10 wt% trimethyl borate to the remaining first slurry, mix well to obtain the second slurry, and then spray dry to obtain the second dry material.

[0087] Step S3: The first dry material, the second dry material, and the lithium supplement Li5FeO4 are added to a high-speed mixer for uniform mixing. The average particle size of the lithium supplement is 4.5 μm, and the weight ratio of the lithium supplement to the first dry material is 1:0.2. Then, the first sintering and the second sintering are carried out in sequence. First, the temperature is raised to 450°C at a heating rate of 20°C / min and sintered for 5 hours. Then, the temperature is raised to 800°C at a heating rate of 1°C / min and sintered for 8 hours to obtain the lithium iron phosphate composite material.

[0088] Example 9

[0089] Step S1: The iron phosphate precursor and lithium carbonate are mixed in a Li:Fe:P molar ratio of 1.2:0.968:1 and then ultra-finely ground. Glucose is added so that the carbon content of the lithium iron phosphate composite material is 1.2% by weight, and a first slurry is obtained with a solid content of 35% and an average particle size of 300 nm. 10 wt.% of the first slurry is taken and spray-dried to obtain the first dry material.

[0090] Step S2: Add 5 wt% trimethyl borate to the remaining first slurry, mix well to obtain the second slurry, and then spray dry to obtain the second dry material.

[0091] Step S3: The first dry material, the second dry material, and the lithium supplement Li5FeO4 are added to a high-speed mixer for uniform mixing. The average particle size of the lithium supplement is 4.5 μm, and the weight ratio of the lithium supplement to the first dry material is 1:0.2. Then, the first sintering and the second sintering are carried out in sequence. First, the temperature is raised to 450°C at a heating rate of 20°C / min and sintered for 5 hours. Then, the temperature is raised to 800°C at a heating rate of 1°C / min and sintered for 8 hours to obtain the lithium iron phosphate composite material.

[0092] Example 10

[0093] Step S1: The iron phosphate precursor and lithium carbonate are mixed in a Li:Fe:P molar ratio of 1.2:0.968:1 and then ultra-finely ground. Glucose is added so that the carbon content of the lithium iron phosphate composite material is 1.2% by weight, and a first slurry is obtained with a solid content of 35% and an average particle size of 300 nm. 10 wt.% of the first slurry is taken and spray-dried to obtain the first dry material.

[0094] In step S2, 20 wt% trimethyl borate is added to the remaining first slurry, mixed evenly to obtain the second slurry, and then spray-dried to obtain the second dry material.

[0095] Step S3: The first dry material, the second dry material, and the lithium supplement Li5FeO4 are added to a high-speed mixer for uniform mixing. The average particle size of the lithium supplement is 4.5 μm, and the weight ratio of the lithium supplement to the first dry material is 1:0.2. Then, the first sintering and the second sintering are carried out in sequence. First, the temperature is raised to 450°C at a heating rate of 20°C / min and sintered for 5 hours. Then, the temperature is raised to 800°C at a heating rate of 1°C / min and sintered for 8 hours to obtain the lithium iron phosphate composite material.

[0096] Example 11

[0097] Step S1: The iron phosphate precursor and lithium carbonate are mixed in a Li:Fe:P molar ratio of 1.2:0.968:1 and then ultra-finely ground. Glucose is added so that the carbon content of the lithium iron phosphate composite material is 1.2% by weight, and a first slurry is obtained with a solid content of 35% and an average particle size of 300 nm. 10 wt.% of the first slurry is taken and spray-dried to obtain the first dry material.

[0098] Step S2: Add 10 wt% trimethyl borate to the remaining first slurry, mix well to obtain the second slurry, and then spray dry to obtain the second dry material.

[0099] Step S3: The first dry material, the second dry material, and the lithium supplement Li5FeO4 are added to a high-speed mixer for uniform mixing. The average particle size of the lithium supplement is 3 μm, and the weight ratio of the first dry material to the lithium supplement is 1:0.45. Then, the first sintering and the second sintering are carried out in sequence. First, the temperature is raised to 400℃ at a heating rate of 10℃ / min and sintered for 5 hours. Then, the temperature is raised to 700℃ at a heating rate of 1℃ / min and sintered for 10 hours to obtain the lithium iron phosphate composite material.

[0100] Example 12

[0101] Step S1: The iron phosphate precursor and lithium carbonate are mixed in a Li:Fe:P molar ratio of 1.2:0.968:1 and then ultra-finely ground. Glucose is added so that the carbon content of the lithium iron phosphate composite material is 1.2% by weight, and a first slurry is obtained with a solid content of 35% and an average particle size of 300 nm. 10 wt.% of the first slurry is taken and spray-dried to obtain the first dry material.

[0102] Step S2: Add 10 wt% trimethyl borate to the remaining first slurry, mix well to obtain the second slurry, and then spray dry to obtain the second dry material.

[0103] Step S3: The first dry material, the second dry material, and the lithium supplement Li5FeO4 are added to a high-speed mixer for uniform mixing. The average particle size of the lithium supplement is 8 μm, and the weight ratio of the first dry material to the lithium supplement is 1:0.08. Then, the first sintering and the second sintering are carried out in sequence. First, the temperature is raised to 600℃ at a heating rate of 20℃ / min and sintered for 3 hours. Then, the temperature is raised to 900℃ at a heating rate of 3℃ / min and sintered for 8 hours to obtain the lithium iron phosphate composite material.

[0104] Comparative Example 1

[0105] Step S1: The iron phosphate precursor and lithium carbonate are mixed in a Li:Fe:P molar ratio of 1.2:0.968:1 and then ultra-finely ground. Glucose is added so that the carbon content of the lithium iron phosphate composite material is 1.2% by weight, and a first slurry is obtained with a solid content of 35% and an average particle size of 300 nm. 10 wt.% of the first slurry is taken and spray-dried to obtain the first dry material.

[0106] Step S2: Add 10 wt% trimethyl borate to the remaining first slurry, mix well to obtain the second slurry, and then spray dry to obtain the second dry material.

[0107] Step S3: The first dry material and the second dry material are added to a high-speed mixer for uniform mixing, and then the first sintering and the second sintering are carried out in sequence. First, the temperature is raised to 450°C at a heating rate of 20°C / min and sintered for 5 hours. Then, the temperature is raised to 800°C at a heating rate of 1°C / min and sintered for 8 hours to obtain lithium iron phosphate material.

[0108] Comparative Example 2

[0109] Step S1: The iron phosphate precursor and lithium carbonate are mixed in a Li:Fe:P molar ratio of 1.2:0.968:1 and then ultra-finely ground. Glucose is added so that the carbon content of the lithium iron phosphate composite material is 1.2% by weight, and a first slurry is obtained with a solid content of 35% and an average particle size of 300 nm. 10 wt.% of the first slurry is taken and spray-dried to obtain the first dry material.

[0110] In step S2, the first dry material and the lithium supplement Li5FeO4 are added to a high-speed mixer for uniform mixing. The average particle size of the lithium supplement is 4.5 μm, and the weight ratio of the lithium supplement to the first dry material is 1:0.2. Then, the first sintering and the second sintering are carried out in sequence. First, the temperature is raised to 450°C at a heating rate of 20°C / min and sintered for 5 hours. Then, the temperature is raised to 800°C at a heating rate of 1°C / min and sintered for 8 hours to obtain lithium iron phosphate material.

[0111] Comparative Example 3

[0112] Step S1: The iron phosphate precursor and lithium carbonate are mixed in a Li:Fe:P molar ratio of 1.2:0.968:1 and then ultra-finely ground. Glucose is added so that the carbon content of the lithium iron phosphate composite material is 1.2% by weight to obtain the first slurry. 10 wt% of trimethyl borate is added to the slurry and mixed evenly to obtain the second slurry. Then, the mixture is spray-dried to obtain the second dry material.

[0113] In step S2, the first dry material, the second dry material, and the lithium supplement Li5FeO4 are added to a high-speed mixer for uniform mixing. The average particle size of the lithium supplement is 4.5 μm, and the weight ratio of the lithium supplement to the first dry material is 1:0.2. Then, the first sintering and the second sintering are carried out in sequence. First, the temperature is raised to 450°C at a heating rate of 20°C / min and sintered for 5 hours. Then, the temperature is raised to 800°C at a heating rate of 1°C / min and sintered for 8 hours to obtain lithium iron phosphate material.

[0114] Comparative Example 4

[0115] Step S1: The iron phosphate precursor and lithium carbonate are mixed in a Li:Fe:P molar ratio of 1.2:0.968:1 and then ultra-finely ground. Glucose is added so that the carbon content of the lithium iron phosphate composite material is 1.2% by weight, and a first slurry is obtained with a solid content of 35% and an average particle size of 300 nm. 10 wt.% of the first slurry is taken and spray-dried to obtain the first dry material.

[0116] Step S2: Add 10 wt% trimethyl borate to the remaining first slurry, mix well to obtain the second slurry, and then spray dry to obtain the second dry material.

[0117] In step S3, the first dry material, the second dry material, and the lithium supplement Li5FeO4 are added to a high-speed mixer for uniform mixing. The average particle size of the lithium supplement is 4.5μm, and the weight ratio of the lithium supplement to the first dry material is 1:0.2. Then, a sintering step is performed, with the temperature increased to 800℃ at a heating rate of 1℃ / min and sintered for 8h to obtain lithium iron phosphate material.

[0118] The lithium iron phosphate materials prepared in the above examples and comparative examples were subjected to performance tests, and the results are shown in Tables 1 and 2.

[0119] Test method:

[0120] Average particle size of slurry: The particle size distribution D50 value of the slurry obtained by testing with a Malvern 3000 laser particle size analyzer.

[0121] Average particle size of dry materials and composite materials: The average particle size of lithium iron phosphate and lithium supplementer was tested by scanning electron microscopy, and the number of particles that fall within the relevant particle size range was calculated under the microscope.

[0122] Elemental percentage: The percentage of carbon in the weight of the lithium iron phosphate composite material is determined by a carbon-sulfur analyzer.

[0123] Compacted density: The material was tested using a compacted density tester at a pressure of 3t.

[0124] Specific capacity: Lithium iron phosphate (composite) material, PVDF, and Super-p are mixed at a mass ratio of 90:5:5 to form a slurry with a solid content of 30%. The slurry is 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 LiPF6, solvent EC:DMC:EMC = 1:1:1 (v / v / v)) and lithium sheet are assembled into a coin cell. Charge and discharge tests are performed in the voltage range of 2.0 to 4.5V. First, charge to 4.5V at 0.2C, then discharge to 2.0V at 0.2C, then charge to 4.0V at 0.2C, and then discharge to 2.0V at 0.2C, repeating the cycle twice.

[0125] Table 1

[0126] Table 2

[0127] As can be seen, compared with Example 1, Comparative Example 1 did not add a lithium replenishing agent, thus failing to provide a lithium replenishment effect, resulting in a significant decrease in both the initial charge specific capacity and the discharge specific capacity. Comparative Example 2 did not use a coupling agent to prepare the second dry material, preventing the effective formation of large particles and thus failing to establish an effective tertiary gradation relationship, leading to a significant decrease in compaction. Comparative Example 3 did not prepare the first dry material; therefore, due to the significantly reduced proportion of small particles in the composite material, a tertiary gradation relationship was not formed, resulting in decreased compaction. Simultaneously, the aggregation of large particles led to significant electrode polarization, hindering capacity utilization. Comparative Example 4 did not undergo two-step sintering, reducing the controllability of coupling agent decomposition and crystal growth, resulting in an unrealized gradation relationship and uneven carbon source distribution, leading to reduced electrical performance.

[0128] As can be seen from the above, compared with the comparative example, the embodiments of this application utilize the ultra-large particles of lithium supplementer combined with the small and large particles of lithium iron phosphate, and rationally design the three-level gradation relationship of particles. By utilizing the high activity of small particles and the high space utilization of large particles, and at the same time utilizing the lithium supplementation effect and structural support effect of ultra-large particle lithium supplementer, a high-compaction composite material is obtained. This can effectively improve the specific capacity while increasing the compaction density of the material, thereby further improving the capacity performance such as energy density.

[0129] Furthermore, it can be seen that the overall effect is better when all process parameters are within the preferred range of this application.

[0130] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

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 supplementer; the average particle size of the small-particle lithium iron phosphate is ≤0.3μm, the average particle size of the large-particle lithium iron phosphate is 1-2.5μm, and the average particle size of the ultra-large-particle lithium supplementer is 3-8μm.

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

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 ultra-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 includes Li2NiO2 and / or Li5FeO4; and / or The lithium iron phosphate composite material also includes carbon, and the carbon accounts for 1.0 to 1.6% of the weight of the lithium iron phosphate composite material.

5. The method for preparing the lithium iron phosphate composite material according to any one of claims 1 to 4, characterized in that, Includes the following steps: Step S1: Mix the iron phosphate precursor, lithium source, optional carbon source and water, and grind them to obtain the first slurry; The first slurry is divided into two parts, namely a first part first slurry and a second part first slurry. The first part first slurry is subjected to a first spray drying to obtain a first dry material. Step S2: Add a coupling agent to the first slurry in the second part to obtain a second slurry; perform a second spray drying on the second slurry to obtain a second dry material; Step S3: Mix the first dry material, the second dry material and the lithium supplement, and perform the first sintering and the second sintering in sequence to obtain the lithium iron phosphate composite material; 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 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 includes one or more of lithium carbonate, lithium hydroxide, and lithium hydroxide monohydrate, wherein the molar ratio of lithium in the lithium source to phosphorus in the iron phosphate precursor is (1-1.4):1; and / or The first slurry has a solid content of 20-40% and an average particle size of ≤350nm; and / or The first portion of the first slurry accounts for 10-13% of the weight percentage of the first slurry; and / or The carbon source includes one or more of glucose, sucrose, starch, and polyethylene glycol with a weight average molecular weight of 2000 to 8000, and the carbon in the carbon source accounts for 1.0 to 1.6% of the weight of the lithium iron phosphate composite material.

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

8. The method for preparing the lithium iron phosphate composite material according to any one of claims 5 to 7, characterized in that, In step S3 The lithium supplement agent comprises Li₂NiO₂ and / or Li₅FeO₄, and 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 is 1:(0.08~0.45); and / or The first sintering temperature is 400–600℃, the time is 3–5 h, and the heating rate is 10–20℃ / min; and / or The second sintering temperature is 700–900℃, the time is 8–10 h, and the heating rate is 1–3℃ / min; and / or The temperature of the first sintering is lower than that of the second sintering, with a difference of 270–400 °C; and / or The first sintering time is less than the second sintering time, with a difference of 3 to 7 hours; and / or The heating rate of the first sintering is greater than that of the second sintering, with a difference of 7 to 19 °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, the positive electrode slurry comprising 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, Includes the positive electrode sheet as described in claim 9.

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