Lithium iron phosphate composite material, preparation method thereof and lithium ion battery

Abstract

The application relates to the technical field of lithium ion batteries, in particular to a lithium iron phosphate composite material, a preparation method thereof and a lithium ion battery. The lithium iron phosphate composite material comprises a lithium iron phosphate base material and a carbon coating layer; the lithium iron phosphate base material contains Fe2P; the lithium iron phosphate base material comprises secondary particles formed by agglomeration of a plurality of primary particles; the average particle size of the primary particles is 50nm-150nm; and the preparation method comprises three-stage sintering, that is, heat preservation under different temperature ranges and different reducing gas concentrations in sequence. The application can effectively and controllably produce Fe2P and keep small primary particles; the carbon coating layer is arranged on the surface of the lithium iron phosphate base material to form a dense three-dimensional conductive network, which can effectively prevent the electrolyte in the battery from reacting with Fe2P to generate a side reaction, and the cycle and storage performance of the lithium iron phosphate composite material is improved.

Description

Technical Field

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

[0002] Lithium iron phosphate (LiFePO4), as a cathode material for lithium-ion batteries, has advantages such as high safety, long cycle life, wide availability of raw materials and low cost, and is one of the mainstream cathode materials in the fields of power batteries and energy storage.

[0003] During the synthesis of lithium iron phosphate, sintering at high temperatures generates secondary phases such as Fe2P and Li3PO4. The reaction principle is as follows: 6LiFePO4 + 8C → 2Fe2P + 2Li3PO4 + 2FeP + 8CO2 → 3Fe2P + 2Li3PO4 + P↑. Among these impurity phases, Fe2P exhibits higher conductivity compared to LiFePO4, while Li3PO4 has extremely low conductivity. Therefore, effectively controlling the formation of the Fe2P phase can increase the rate performance of lithium iron phosphate.

[0004] However, current technologies for producing Fe2P during the synthesis of lithium iron phosphate involve increasing the sintering temperature. Temperatures of 800°C and above are favorable for Fe2P formation. While increasing the sintering temperature can increase the amount of Fe2P generated, it also intensifies the growth of primary particles, leading to coarse particles that increase the lithium-ion transport distance, thus limiting the improvement in the rate performance of lithium iron phosphate. Furthermore, the reduction reaction at high temperatures is intense and difficult to control, making the amount of Fe2P generated sensitive to temperature fluctuations. Over-reduction can easily produce harmful impurities such as FeP, resulting in poor product consistency. Additionally, high-temperature sintering increases energy consumption and costs.

[0005] Furthermore, although Fe2P itself has high electrical conductivity, it is prone to side reactions and decomposition after contact with the electrolyte in the battery chemistry system, which affects the battery cycle life and storage performance.

[0006] In view of this, the present invention is hereby proposed. Summary of the Invention

[0007] The primary objective of this invention is to provide a lithium iron phosphate composite material. By introducing a reducing gas during the sintering process of the lithium iron phosphate composite material, Fe2P can be effectively and controllably produced at a lower sintering temperature while maintaining smaller primary particles, thereby improving rate performance and achieving good material consistency with low energy consumption and cost. Simultaneously, by employing a three-stage sintering process and controlling the temperature and reducing gas concentration of each stage, Fe2P can be generated both inside and on the surface of the lithium iron phosphate matrix material. The internal Fe2P functions to shorten the electron transport distance within the primary particles, constructing a gradient conductive network of "internal conductive bridges + surface conductive shells." This achieves synergistic optimization of the lithium iron phosphate matrix material in terms of electronic conductivity, lithium-ion diffusion kinetics, and structural integrity, further improving rate performance and cycle stability. Furthermore, a carbon coating layer is applied to the surface of the lithium iron phosphate matrix material, forming a dense three-dimensional conductive network that effectively prevents side reactions between the electrolyte and Fe2P in the battery, enhancing the cycle and storage performance of the lithium iron phosphate composite material. This invention solves the problems in the existing technology for synthesizing lithium iron phosphate, such as the problem of large primary particles, poor product consistency, high energy consumption, and high cost caused by increasing the sintering temperature during Fe2P production, and the problem of reduced battery cycle life and storage performance caused by the reaction of Fe2P with the electrolyte.

[0008] The second objective of this invention is to provide a method for preparing lithium iron phosphate composite materials.

[0009] A third objective of this invention is to provide a lithium-ion battery.

[0010] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: This invention first provides a lithium iron phosphate composite material, comprising a lithium iron phosphate matrix material and a carbon coating layer disposed on the surface of the lithium iron phosphate matrix material; the lithium iron phosphate matrix material contains Fe2P; the lithium iron phosphate matrix material includes secondary particles formed by the agglomeration of multiple primary particles; the average particle size of the primary particles is 50nm~150nm; the method for forming the lithium iron phosphate composite material includes a three-stage sintering process, the three-stage sintering process including: holding at a low temperature of 600℃~700℃, a medium temperature of 700℃~720℃, and a high temperature of 720℃~740℃ in a mixed gas containing inert gas and reducing gas; and the volume fraction of the reducing gas in the mixed gas used in the low temperature stage, the medium temperature stage, and the high temperature stage increases sequentially, to 1%~2%, 2%~4%, and 4%~8%, respectively.

[0011] Furthermore, the heat preservation time of the low-temperature section is 0.5h to 1h.

[0012] Furthermore, the heat preservation time in the medium temperature section is 1 hour to 3 hours.

[0013] Furthermore, the heat preservation time of the high-temperature section is 2h to 5h.

[0014] Furthermore, the heating rate of the low-temperature section is 1℃ / min to 3℃ / min.

[0015] Furthermore, the heating rate in the intermediate temperature range is 5℃ / min to 10℃ / min.

[0016] Furthermore, the heating rate of the high-temperature section is 5℃ / min to 10℃ / min.

[0017] Furthermore, the inert gas in the mixed gas includes at least one of argon and nitrogen.

[0018] Furthermore, the reducing gas in the mixed gas includes at least one of ammonia, hydrogen, and carbon monoxide.

[0019] This invention also provides a method for preparing lithium iron phosphate composite materials, comprising the following steps: mixing lithium source, iron source, first carbon source and water, followed by a first grinding, and then performing a first spray drying to obtain a precursor; the precursor is subjected to three-stage sintering in a mixed gas containing inert gas and reducing gas to obtain a sintered material; the three-stage sintering includes: sequentially holding at a low temperature of 600℃~700℃, a medium temperature of 700℃~720℃, and a high temperature of 720℃~740℃, wherein the volume fraction of the reducing gas in the mixed gas used in the low temperature stage, the medium temperature stage and the high temperature stage increases sequentially to 1%~2%, 2%~4%, and 4%~8%, respectively; mixing the sintered material, water and a second carbon source, followed by a second grinding and a second spray drying to obtain a mixture; and calcining the mixture in an inert atmosphere to obtain the lithium iron phosphate composite material.

[0020] Furthermore, the molar ratio of lithium in the lithium source to iron in the iron source is 1.02~1.06:1.

[0021] Furthermore, the first carbon source includes at least one of glucose, sucrose, and citric acid.

[0022] Furthermore, the mass of the first carbon source is 5% to 10% of the mass of the iron source.

[0023] Furthermore, the second carbon source includes at least one of polyethylene glycol, polyvinyl alcohol, and phenolic resin.

[0024] Furthermore, the mass of the second carbon source is 1% to 5% of the mass of the sinter.

[0025] Furthermore, the particle size D50 of the solid particles in the first grinding-to-obtain mixed slurry is 200nm~400nm.

[0026] Furthermore, the particle size D50 of the solid particles in the resulting mixed slurry after the second grinding is 100nm~200nm.

[0027] Furthermore, the inert atmosphere used for calcination includes an argon atmosphere or a nitrogen atmosphere.

[0028] Furthermore, the calcination specifically includes: heating to 700℃~740℃ at a heating rate of 1℃ / min~10℃ / min and holding at that temperature for 4h~10h.

[0029] The present invention also provides a lithium-ion battery, including a positive electrode sheet, wherein the positive active layer of the positive electrode sheet contains a lithium iron phosphate composite material or a lithium iron phosphate composite material prepared by a method for preparing lithium iron phosphate composite materials.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) By introducing a reducing gas during the sintering process of preparing lithium iron phosphate composite material, the present invention can effectively and controllably produce Fe2P at a lower sintering temperature while maintaining smaller primary particles, thereby improving the conductivity and rate performance of lithium iron phosphate composite material, and obtaining material with good consistency, low energy consumption and low cost.

[0031] (2) By adopting a three-stage sintering process and controlling the temperature and reducing gas concentration of each stage, Fe2P is generated inside and on the surface of the primary particles of lithium iron phosphate matrix material. The Fe2P present inside can shorten the electron transport distance inside a single primary particle and significantly reduce the electron transport resistance inside a single primary particle. A gradient conductive network of "internal conductive bridge + surface conductive shell" is constructed, which can realize the synergistic optimization of the lithium iron phosphate matrix material in terms of electronic conductivity, lithium ion diffusion kinetics and structural integrity, thereby further improving the rate performance and also improving the cycle stability.

[0032] (3) By setting a carbon coating layer on the surface of the lithium iron phosphate matrix material, the present invention forms a denser three-dimensional conductive network, which can effectively improve the rate performance of the lithium iron phosphate composite material and effectively block the side reaction between the electrolyte and Fe2P in the battery, thereby improving the cycle performance and storage performance of the lithium iron phosphate composite material. Attached Figure Description

[0033] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0034] Figure 1 SEM image of the lithium iron phosphate composite material prepared in Example 1 of this invention; Figure 2 XRD comparison images of lithium iron phosphate composite materials prepared in Example 1, Comparative Example 1 and Comparative Example 2 provided for the present invention; Figure 3 Comparison of the 45°C and 5C / 5C cycle performance of small soft-pack full batteries assembled from the lithium iron phosphate composite materials of Example 1, Comparative Example 1 and Comparative Example 2 provided by the present invention. Figure 4 Comparison chart of capacity recovery rate of small pouch cells assembled from lithium iron phosphate composite materials of Examples 1, 1, and 2 provided by the present invention after 60 days of storage at 60°C. Detailed Implementation

[0035] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings and specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0036] Unless otherwise specified, in this invention, terms such as "first aspect," "second aspect," "third aspect," and "fourth aspect" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor should they be construed as implicitly indicating the importance or quantity of the indicated technical features. Moreover, terms such as "first," "second," "third," and "fourth" serve only as a non-exhaustive enumeration and should be understood not to constitute a closed limitation on quantity.

[0037] Unless otherwise specified, the terms "comprising" and "including" as used in this invention can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0038] Unless otherwise specified, in this invention, "one or more" or "at least one" refers to any one, any two, or any two or more of the listed items. "Several" refers to any two or more.

[0039] In a first aspect, the present invention provides a lithium iron phosphate composite material, comprising a lithium iron phosphate matrix material and a carbon coating layer disposed on the surface of the lithium iron phosphate matrix material.

[0040] The lithium iron phosphate matrix material contains Fe2P.

[0041] The lithium iron phosphate matrix material comprises secondary particles formed by the agglomeration of multiple primary particles.

[0042] Tests revealed that the primary particles contained Fe2P both inside and on their surface, and the Fe2P content increased from the core to the surface.

[0043] The average particle size of the primary particles is 50nm~150nm, for example 50nm, 60nm, 80nm, 100nm, 120nm, 130nm or 150nm.

[0044] The method for forming the lithium iron phosphate composite material includes a three-stage sintering process, which includes: holding the material at a low temperature range of 600℃~700℃ (e.g., 600℃, 620℃, 630℃, 650℃, 680℃ or 700℃), a medium temperature range of 700℃~720℃ (e.g., 700℃, 710℃ or 720℃), and a high temperature range of 720℃~740℃ (e.g., 720℃, 730℃ or 740℃) under a mixed gas containing inert gas and reducing gas.

[0045] The volume fraction of the reducing gas in the mixed gas used in the low-temperature section, the medium-temperature section, and the high-temperature section increases sequentially. Specifically, the volume fraction of the reducing gas in the mixed gas used in the low-temperature section is 1% to 2%, for example, 1%, 1.3%, 1.5%, 1.8%, or 2%; the volume fraction of the reducing gas in the mixed gas used in the medium-temperature section is 2% to 4%, for example, 2%, 2.3%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.7%, or 4%; and the volume fraction of the reducing gas in the mixed gas used in the high-temperature section is 4% to 8%, for example, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, or 8%.

[0046] This invention introduces a reducing gas during the sintering process of preparing lithium iron phosphate composite materials, enabling the effective and controllable production of Fe2P at a lower sintering temperature while maintaining smaller primary particles. This improves the conductivity and rate performance of the lithium iron phosphate composite materials, and also results in materials with good consistency, low energy consumption, and low cost.

[0047] Because LiFePO4 has extremely low intrinsic electronic conductivity, even though the Fe2P on the surface provides good interparticle conductivity, electron transport within a single particle still relies on the assistance of the internal conductive phase. To address this, this invention employs a three-stage sintering process and controls the temperature and reducing gas concentration in each stage to generate Fe2P both inside and on the surface of the primary particles of the lithium iron phosphate matrix material. The presence of Fe2P inside shortens the electron transport distance within a single primary particle, significantly reducing the electron transport resistance within that particle. If Fe2P is only present on the surface and absent inside, electrons entering the particle surface must still pass through the low-conductivity LiFePO4 bulk phase to reach the vicinity of the internal active lithium ions, resulting in the internal active material not being fully utilized. This invention, through a gradient distribution throughout the entire particle, achieves efficient and coordinated electron transport both inside and outside the particle. Meanwhile, the three-stage sintering process enables the Fe2P concentration to gradually increase from the core to the surface of the primary particles of the lithium iron phosphate matrix material, constructing a gradient conductive network of "internal conductive bridge + surface conductive shell". This allows for synergistic optimization of the lithium iron phosphate matrix material in terms of electronic conductivity, lithium-ion diffusion kinetics, and structural integrity, thereby further improving rate performance and also enhancing cycle stability.

[0048] In addition, by setting a carbon coating layer on the surface of the lithium iron phosphate matrix material, the present invention forms a denser three-dimensional conductive network, which effectively improves the rate performance of the lithium iron phosphate composite material and effectively blocks the side reaction between the electrolyte and Fe2P in the battery, thereby improving the cycle performance and storage performance of the lithium iron phosphate composite material.

[0049] In some specific implementations, the heat preservation time of the low-temperature section is 0.5h to 1h.

[0050] In some specific implementations, the heat preservation time of the medium temperature section is 1h to 3h, for example, 1h, 2h or 3h.

[0051] In some specific implementations, the heat preservation time of the high-temperature section is 2h to 5h, for example, 2h, 3h, 4h or 5h.

[0052] In some specific embodiments, the heating rate of the low-temperature section is 1℃ / min to 3℃ / min, for example, 1℃ / min, 1.5℃ / min, 2℃ / min, 2.5℃ / min or 3℃ / min.

[0053] In some specific embodiments, the heating rate in the intermediate temperature range is 5℃ / min to 10℃ / min, for example, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min or 10℃ / min.

[0054] In some specific embodiments, the heating rate of the high-temperature section is 5℃ / min to 10℃ / min, for example, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min or 10℃ / min.

[0055] This invention divides the three-stage sintering process into three temperature zones: a low-temperature zone, a medium-temperature zone, and a high-temperature zone. It also controls the concentration of reducing gas, the holding time, and the heating rate in each zone. By utilizing the reaction kinetics limitation in the low-temperature zone and the difference in diffusion rate in the high-temperature zone, the gradient distribution of the Fe2P phase from the interior to the surface of the primary lithium iron phosphate particles can be controlled, ultimately obtaining a gradient distribution structure with low internal content and high surface content.

[0056] In some specific embodiments, the inert gas in the mixed gas includes at least one of argon and nitrogen.

[0057] In some specific embodiments, the reducing gas in the mixed gas includes at least one of ammonia, hydrogen, and carbon monoxide.

[0058] Secondly, the present invention provides a method for preparing lithium iron phosphate composite materials, comprising the following steps: (a) The lithium source, iron source, first carbon source and water are mixed and then subjected to first grinding, followed by first spray drying to obtain the precursor.

[0059] Preferably, the water in step (a) is deionized water.

[0060] Preferably, the solid content of the mixed slurry obtained after mixing the lithium source, iron source, first carbon source and water in step (a) is 30% to 45%, wherein the solid content refers to the percentage of the total mass of all solid substances in the mixed slurry to the total mass of the mixed slurry.

[0061] Preferably, the lithium source in step (a) includes a lithium-containing compound, such as lithium carbonate, but is not limited thereto; the iron source in step (a) includes an iron-containing compound, such as iron phosphate, but is not limited thereto.

[0062] (b) The precursor is sintered in three stages under a mixed gas containing inert gas and reducing gas, and after cooling, a sintered material containing Fe2P on the surface and inside of the primary particles is obtained.

[0063] The three-stage sintering process includes sequentially holding the temperature in a low-temperature section of 600℃~700℃, a medium-temperature section of 700℃~720℃, and a high-temperature section of 720℃~740℃. Specifically, the temperature is first held in the low-temperature section of 600~700℃, then in the medium-temperature section of 700~720℃, and finally in the high-temperature section of 720~740℃.

[0064] Furthermore, the volume fraction of the reducing gas in the mixed gas used in the low-temperature section, the medium-temperature section, and the high-temperature section increases sequentially. Specifically, the volume fraction of the reducing gas in the mixed gas used in the low-temperature section is 1% to 2%, the volume fraction of the reducing gas in the mixed gas used in the medium-temperature section is 2% to 4%, and the volume fraction of the reducing gas in the mixed gas used in the high-temperature section is 4% to 8%.

[0065] In the three-stage sintering process, the first carbon source plays a reduction role, reducing the trivalent iron in the precursor to divalent iron required to generate lithium iron phosphate; on the other hand, the first carbon source is used to form a conductive carbon network, i.e., a carbon coating layer.

[0066] Therefore, the surface of the sintered material also has a carbon coating layer.

[0067] (c) The sintering material, water and the second carbon source are mixed and then subjected to a second grinding and a second spray drying to obtain a mixture.

[0068] Preferably, the water in step (c) is deionized water.

[0069] The second carbon source is mainly used to form a carbon coating layer on the surface of lithium iron phosphate matrix material.

[0070] The present invention performs two carbon coatings, which can form a more complete, continuous and dense conductive network.

[0071] (d) The mixture is calcined under an inert atmosphere and then cooled to obtain a lithium iron phosphate composite material with secondary carbon coating.

[0072] This invention introduces a reducing gas during the sintering process of lithium iron phosphate composite materials, enabling efficient and controllable production of Fe2P at lower sintering temperatures while maintaining smaller primary particle sizes. This solves the problem of existing technologies being unable to simultaneously achieve Fe2P production and the formation of smaller primary particles. Consequently, the conductivity and rate performance of the lithium iron phosphate composite materials are improved, and the resulting material exhibits good consistency with low energy consumption and cost.

[0073] This invention utilizes the differences in reduction reaction kinetics of lithium iron phosphate precursors at different temperature ranges during heating. By progressively increasing the sintering temperature and simultaneously increasing the concentration of reducing gas in a stepwise manner, a very small number of Fe2P nuclei are generated inside the particles during the low-temperature, low-concentration stage. As the temperature and reduction concentration increase stepwise, due to the limitation of the solid-phase diffusion rate, a large amount of subsequent reduction reactions are forced to occur in the particle surface region. This results in a gradient distribution of Fe2P content that gradually increases from the inside to the surface of the primary particles. This gradient structure fully utilizes the high conductivity of surface Fe2P to improve rate performance while significantly reducing the electron transport resistance inside individual primary particles, maintaining the stability of the internal main structure, and laying the structural foundation for subsequent secondary carbon coating densification and isolation of electrolyte side reactions.

[0074] Furthermore, the present invention performs two carbon coatings, which can form a carbon coating layer with a continuous and dense three-dimensional conductive network on the surface of the lithium iron phosphate matrix material. This can effectively prevent side reactions between the electrolyte and Fe2P in the battery, thereby improving the cycle performance and storage performance of the lithium iron phosphate composite material.

[0075] In step (b), the heat preservation time of the low-temperature section is 0.5h to 1h. The heat preservation time of the medium-temperature section is 1h to 3h, for example, 1h, 2h or 3h. The heat preservation time of the high-temperature section is 2h to 5h, for example, 2h, 3h, 4h or 5h.

[0076] In step (b), the heating rate of the low-temperature section is 1℃ / min to 3℃ / min, for example, 1℃ / min, 1.5℃ / min, 2℃ / min, 2.5℃ / min, or 3℃ / min. The heating rate of the medium-temperature section is 5℃ / min to 10℃ / min, for example, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, or 10℃ / min. The heating rate of the high-temperature section is 5℃ / min to 10℃ / min, for example, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, or 10℃ / min.

[0077] In step (b), the inert gas includes at least one of argon and nitrogen. The reducing gas includes at least one of ammonia, hydrogen, and carbon monoxide.

[0078] In some specific implementations, in step (a), the molar ratio of lithium in the lithium source to iron in the iron source is 1.02 to 1.06:1, for example, 1.02:1, 1.03:1, 1.04:1, 1.05:1 or 1.06:1.

[0079] In some specific embodiments, in step (a), the first carbon source includes at least one of glucose, sucrose, and citric acid. In step (c), the second carbon source includes at least one of polyethylene glycol, polyvinyl alcohol, and phenolic resin. This invention, by using different carbon sources for two-stage carbon coating, can form a carbon coating layer with a continuous and dense three-dimensional conductive network on the surface of the lithium iron phosphate matrix material. This effectively blocks side reactions between the electrolyte and Fe2P in the battery, suppresses Fe2P-induced catalytic side reactions and iron ion dissolution at high temperatures, and controls the primary particle size to a small range of 50nm to 150nm, significantly shortening the diffusion path of lithium ions in the solid phase. Furthermore, the gradient concentration distribution of Fe2P from the inside out ensures efficient and coordinated electron transport within and on the surface of the particles. These three factors synergistically enable rapid and reversible intercalation and deintercalation of lithium ions after high-temperature storage, while effectively suppressing interfacial side reactions, thereby achieving a high degree of capacity recovery after high-temperature storage.

[0080] In some specific implementations, in step (a), the mass of the first carbon source is 5% to 10% of the mass of the iron source, for example, 5%, 6%, 7%, 8%, 9% or 10%.

[0081] In some specific embodiments, in step (c), the mass of the second carbon source is 1% to 5% of the mass of the sintering material, for example, 1%, 2%, 3%, 4% or 5%.

[0082] In some specific embodiments, in step (a), the particle size D50 of the solid particles in the resulting mixed slurry after the first grinding is 200 nm to 400 nm, for example, 200 nm, 250 nm, 300 nm, 350 nm, or 400 nm. Controlling the degree of the first grinding can, on the one hand, ensure that the components are mixed uniformly and provide sufficient reaction interfaces to facilitate the formation of the Fe2P gradient structure and grain growth in the subsequent three-stage sintering process; on the other hand, it can further reduce the probability of excessively high sintering activity and excessive primary particle growth caused by excessively fine particles.

[0083] In some specific embodiments, in step (c), the particle size D50 of the solid particles in the second grinding to obtain the mixed slurry is 100nm~200nm, for example 100nm, 120nm, 130nm, 150nm, 160nm, 180nm or 200nm.

[0084] In some specific embodiments, in step (d), the inert atmosphere for calcination includes an argon atmosphere or a nitrogen atmosphere.

[0085] In some specific embodiments, step (d) specifically includes: heating to 700℃~740℃ at a heating rate of 1℃ / min~10℃ / min and holding at that temperature for 4h~10h. The heating rate can be, for example, 1℃ / min, 3℃ / min, 5℃ / min, 7℃ / min, or 10℃ / min; the calcination temperature can be, for example, 700℃, 710℃, 720℃, 730℃, or 740℃; and the holding time can be, for example, 4h, 5h, 6h, 7h, or 10h.

[0086] Thirdly, the present invention provides a lithium-ion battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte; the positive electrode comprises a positive current collector and a positive active layer disposed on the surface of the positive current collector; the positive active layer comprises a lithium iron phosphate composite material as described above or a lithium iron phosphate composite material prepared according to the preparation method of the lithium iron phosphate composite material described above.

[0087] This lithium-ion battery exhibits excellent electrochemical performance, especially high rate capability, long cycle life, and good storage performance, making it a promising candidate for a wide range of applications.

[0088] The embodiments of the present invention will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0089] Example 1 The preparation method of the lithium iron phosphate composite material provided in this embodiment includes the following steps: (1) Weigh lithium carbonate and iron phosphate in a molar ratio of Li to Fe of 1.04:1, and weigh glucose accounting for 8% of the mass of iron phosphate. Mix the raw materials and add them to a dispersion tank containing deionized water to prepare a mixed slurry with a solid content of 40%. Stir for 30 minutes to mix evenly. Then, use a sand mill to grind the above mixed slurry for the first time. After grinding until the particle size D50 of the solid particles in the obtained mixed slurry is 200 nm, transfer it to a spray dryer for the first spray drying to obtain the precursor.

[0090] (2) The precursor obtained in step (1) is placed in a box furnace and a mixture of nitrogen and hydrogen is introduced into the box furnace for three-stage sintering: first, the temperature is raised to 600℃ at a heating rate of 1℃ / min and held for 1h, which is the low temperature stage; then, the temperature is raised to 700℃ at a heating rate of 5℃ / min and held for 3h, which is the medium temperature stage; then, the temperature is raised to 720℃ at a heating rate of 7℃ / min and held for 5h, which is the high temperature stage; and the volume fraction of hydrogen in the mixed gas used in the low temperature stage, medium temperature stage and high temperature stage increases sequentially to 1%, 2.5% and 5%, respectively; then, it is naturally cooled to room temperature to obtain the sintered material.

[0091] (3) Add the sintered material obtained in step (2) to deionized water and add polyethylene glycol to it, wherein the mass of polyethylene glycol accounts for 4% of the mass of the sintered material, to prepare a mixed slurry with a solid content of 30%, and stir for 30 minutes to mix evenly; then transfer the evenly mixed slurry into a sand mill for a second grinding, and after the second grinding until the particle size D50 of the solid particles in the obtained mixed slurry is 150nm, transfer it into a spray dryer for a second spray drying to obtain a mixed material.

[0092] (4) The mixture obtained in step (3) is placed in a box furnace and nitrogen is introduced. The temperature is raised to 720°C at a heating rate of 5°C / min for calcination. After holding at the temperature for 10 hours, it is naturally cooled to room temperature to obtain lithium iron phosphate composite material.

[0093] The lithium iron phosphate composite material prepared in this embodiment includes a lithium iron phosphate matrix material and a carbon coating layer disposed on the surface of the lithium iron phosphate matrix material; the lithium iron phosphate matrix material contains Fe2P; the lithium iron phosphate matrix material includes secondary particles formed by the agglomeration of multiple primary particles; the average particle size of the primary particles is 92 nm.

[0094] Example 2 The difference between the preparation method of lithium iron phosphate composite material provided in this embodiment and that in embodiment 1 is as follows: In step (2), the three-stage sintering is as follows: first, the temperature is raised to 680℃ at a heating rate of 3℃ / min and held for 0.5h, which is the low temperature stage; then, the temperature is raised to 720℃ at a heating rate of 8℃ / min and held for 1h, which is the medium temperature stage; then, the temperature is raised to 740℃ at a heating rate of 10℃ / min and held for 3h, which is the high temperature stage; wherein the volume fraction of hydrogen in the mixed gas used in the low temperature stage, the medium temperature stage and the high temperature stage is the same as in embodiment 1.

[0095] Example 3 The difference between the preparation method of lithium iron phosphate composite material provided in this embodiment and that in embodiment 1 is that in the three-stage sintering of step (2), the volume fraction of hydrogen in the mixed gas used in the low temperature stage, the medium temperature stage and the high temperature stage is 2%, 4% and 6%, respectively.

[0096] Example 4 The difference between the preparation method of lithium iron phosphate composite material provided in this embodiment and that in embodiment 1 is that glucose is replaced with an equal mass of citric acid in step (1), and polyethylene glycol is replaced with an equal mass of polyvinyl alcohol in step (3).

[0097] Example 5 The difference between the preparation method of the lithium iron phosphate composite material provided in this embodiment and that in embodiment 1 is that the mass of glucose in step (1) is adjusted to account for 5% of the mass of iron phosphate.

[0098] Example 6 The difference between the preparation method of lithium iron phosphate composite material provided in this embodiment and that in embodiment 1 is that the particle size D50 of the solid particles in the first grinding to the obtained mixed slurry is 400 nm.

[0099] Comparative Example 1 The difference between the preparation method of the lithium iron phosphate composite material provided in this comparative example and that in Example 1 is that the three-stage sintering in step (2) is carried out in a nitrogen atmosphere throughout.

[0100] Comparative Example 2 The lithium iron phosphate composite material provided in this comparative example is the sintered material obtained in step (2) of Example 1, i.e., without steps (3) and (4).

[0101] Comparative Example 3 The difference between the preparation method of the lithium iron phosphate composite material provided in this comparative example and Example 1 is that: in step (2), instead of three-stage sintering, one-stage sintering is used: the temperature is raised to 800°C at a heating rate of 5°C / min and held for 9 hours, and the volume fraction of hydrogen in the mixed gas used for one-stage sintering is 5%.

[0102] Experimental Example The average particle size of the primary particles of the lithium iron phosphate matrix material in the lithium iron phosphate composite materials prepared in each embodiment and comparative example is shown in Table 1. The average particle size of the primary particles was measured using the Nanomeasure method.

[0103] Furthermore, the lithium iron phosphate composite materials prepared in each embodiment and comparative example were used as positive electrode active materials and assembled into mold half-cells according to the following method for material specific capacity testing: The positive electrode active material, conductive agent SuperP, and binder PVDF were mixed in a mass ratio of 8:1:1, and N-methylpyrrolidone (NMP) was added and stirred evenly to obtain a positive electrode slurry. This positive electrode slurry was coated onto an aluminum foil current collector, then dried, rolled, and cut to obtain a positive electrode sheet. Using a lithium metal sheet as the negative electrode, Celgard 2400 as the separator, and a 1 mol / L LiPF6 solution dissolved in EC / DMC / EMC (volume ratio 1:1:1) as the electrolyte, a mold battery was assembled in an argon-filled glove box. Then, the specific capacity and electrochemical performance of the positive electrode material of each assembled mold battery were tested: After the mold battery was placed in a constant temperature chamber at 25°C for 12 hours, charge and discharge tests were performed using the Blue Battery Test System. Constant current charge and discharge were performed at rates of 0.5C, 10C, and 20C, with a voltage range of 2.0V~3.8V. The discharge specific capacity at each rate was recorded, and the test results are shown in Table 1.

[0104] Furthermore, using the lithium iron phosphate composite materials prepared in each embodiment and comparative example as the positive electrode active material, small soft-pack full batteries were assembled according to the following method for cycle and storage testing: The positive electrode active material, SP, and PVDF were mixed in a mass ratio of 95:2:3, and N-methylpyrrolidone (NMP) was added and stirred until homogeneous to obtain a positive electrode slurry. This positive electrode slurry was coated onto an aluminum foil current collector, then dried, rolled, and cut to obtain a positive electrode sheet. The negative electrode active material (graphite) was mixed with CMC, SP, and SBR in a mass ratio of 96:1.2:1:1.8 to obtain a negative electrode slurry. This negative electrode slurry was coated onto a copper foil current collector, then dried, rolled, and cut to obtain a negative electrode sheet. TC-EGX21 was used as the electrolyte. The cycle performance test method is as follows: The small pouch cell full battery is subjected to constant current charge-discharge cycle test at a current density of 5C / 5C in a 45°C constant temperature chamber, with a voltage range of 2.0V~3.65V, for 200 cycles. The capacity retention rate is recorded, where the capacity retention rate = discharge specific capacity at the 200th cycle / discharge specific capacity at the first cycle × 100%. The high-temperature storage performance test method is as follows: The small pouch cell full battery is charged to full capacity (3.65V) at 1C, and the initial discharge capacity is measured. Then, the fully charged battery is stored in a 60°C constant temperature chamber for 60 days, removed, cooled to room temperature, and discharged at a 1C rate. The remaining discharge capacity is recorded, where the capacity recovery rate = discharge capacity after storage / initial discharge capacity × 100%. The test results are shown in Table 1.

[0105] Table 1. Results of primary particle average size and electrochemical performance tests

[0106] As shown in Table 1, compared with the comparative examples, the lithium iron phosphate composite materials prepared in each embodiment of the present invention possess superior high-rate discharge performance, high-temperature cycle stability, and high-temperature storage performance. This indicates that each embodiment achieves controllable production of Fe2P while maintaining smaller primary particles, and achieves Fe2P generation both inside and on the surface of the lithium iron phosphate matrix material, with the Fe2P content increasing from the core to the surface of the primary particles. Simultaneously, a continuous and dense carbon coating layer is formed on the surface of the lithium iron phosphate matrix material.

[0107] In contrast, Comparative Example 1, due to the lack of reducing gas during the sintering process, could not effectively generate a highly conductive Fe2P phase in the lithium iron phosphate matrix material, resulting in insufficient electronic conductivity and significantly lower discharge specific capacity at 10C and 20C compared to the embodiments.

[0108] Comparative Example 2, due to the lack of secondary carbon coating, had an incomplete and non-dense carbon coating layer, which could not effectively block the side reaction between the electrolyte and Fe2P, resulting in a significant decrease in high-temperature cycling capacity retention and storage capacity recovery rate.

[0109] Comparative Example 3 uses a traditional one-stage high-temperature sintering method at 800℃ to generate Fe2P. Although high temperature is conducive to the formation of Fe2P phase, the intense high-temperature reaction leads to severe coarsening of primary particles. The average particle size of the primary particles far exceeds the range of 50nm~150nm of this invention. The lithium-ion solid-phase diffusion path is significantly increased. At the same time, the high temperature triggers an excessive reduction reaction to generate harmful impurity phases such as FeP. Ultimately, its high-rate performance, cycle stability and storage performance are inferior to those of the other examples.

[0110] Additionally, the SEM image of the lithium iron phosphate composite material prepared in Example 1 is shown below. Figure 1 As shown.

[0111] XRD comparison images of the lithium iron phosphate composite materials prepared in Example 1, Comparative Example 1, and Comparative Example 2 are shown below. Figure 2 As shown.

[0112] The comparison chart of the 45°C, 5C / 5C cycle performance of the small pouch cells assembled from lithium iron phosphate composite materials in Examples 1, 1, and 2 is shown in the figure. Figure 3 As shown.

[0113] The capacity recovery rate comparison chart of the small pouch cells assembled from lithium iron phosphate composite materials in Examples 1, 1, and 2 after 60 days of storage at 60°C is shown in the figure. Figure 4 As shown.

[0114] Although the present invention has been illustrated and described with specific embodiments, it should be understood that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; those skilled in the art should understand that modifications can be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein, without departing from the spirit and scope of the present invention; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention; therefore, this means that all such substitutions and modifications that fall within the scope of the present invention are included in the appended claims.

Claims

1. A lithium iron phosphate composite material, characterized in that, It includes a lithium iron phosphate matrix material and a carbon coating layer disposed on the surface of the lithium iron phosphate matrix material; The lithium iron phosphate matrix material contains Fe2P; The lithium iron phosphate matrix material includes secondary particles formed by the agglomeration of multiple primary particles; The average particle size of the primary particles is 50 nm to 150 nm. The method for forming the lithium iron phosphate composite material includes a three-stage sintering process, wherein the three-stage sintering process includes: holding the mixture in a low-temperature stage (600℃~700℃), a medium-temperature stage (700℃~720℃), and a high-temperature stage (720℃~740℃) in a mixed gas containing an inert gas and a reducing gas; and wherein the volume fraction of the reducing gas in the mixed gas used in the low-temperature stage, the medium-temperature stage, and the high-temperature stage increases sequentially to 1%~2%, 2%~4%, and 4%~8%, respectively.

2. The lithium iron phosphate composite material according to claim 1, characterized in that, The heat preservation time of the low temperature section is 0.5h~1h; The heat preservation time for the medium temperature section is 1 hour to 3 hours; The heat preservation time for the high-temperature section is 2h to 5h.

3. The lithium iron phosphate composite material according to claim 1, characterized in that, The heating rate of the low-temperature section is 1℃ / min to 3℃ / min; The heating rate in the intermediate temperature range is 5℃ / min to 10℃ / min; The heating rate of the high-temperature section is 5℃ / min to 10℃ / min.

4. The lithium iron phosphate composite material according to claim 1, characterized in that, The inert gas in the mixed gas includes at least one of argon and nitrogen; The reducing gas in the mixed gas includes at least one of ammonia, hydrogen, and carbon monoxide.

5. A method for preparing a lithium iron phosphate composite material as described in any one of claims 1 to 4, characterized in that, Includes the following steps: The lithium source, iron source, first carbon source and water are mixed and then subjected to first grinding, followed by first spray drying to obtain the precursor; The precursor is sintered in three stages under a mixed gas containing inert gas and reducing gas to obtain sintered material; The three-stage sintering process includes sequentially holding the gas at a low temperature of 600℃~700℃, a medium temperature of 700℃~720℃, and a high temperature of 720℃~740℃, wherein the volume fraction of the reducing gas in the mixed gas used in the low temperature stage, the medium temperature stage, and the high temperature stage increases sequentially to 1%~2%, 2%~4%, and 4%~8%, respectively. The sintering material, water, and second carbon source are mixed and then subjected to a second grinding and a second spray drying to obtain a mixture. The mixture was calcined under an inert atmosphere to obtain a lithium iron phosphate composite material.

6. The method for preparing the lithium iron phosphate composite material according to claim 5, characterized in that, The molar ratio of lithium in the lithium source to iron in the iron source is 1.02~1.06:

1.

7. The method for preparing the lithium iron phosphate composite material according to claim 5, characterized in that, The first carbon source includes at least one of glucose, sucrose, and citric acid; The mass of the first carbon source is 5% to 10% of the mass of the iron source; The second carbon source includes at least one of polyethylene glycol, polyvinyl alcohol, and phenolic resin; The mass of the second carbon source is 1% to 5% of the mass of the sintering material.

8. The method for preparing the lithium iron phosphate composite material according to claim 5, characterized in that, The particle size D50 of the solid particles in the first grinding-to-obtain mixed slurry is 200nm~400nm; The particle size D50 of the solid particles in the resulting mixed slurry after the second grinding is 100nm~200nm.

9. The method for preparing the lithium iron phosphate composite material according to claim 5, characterized in that, The inert atmosphere used in the calcination includes an argon atmosphere or a nitrogen atmosphere. The calcination specifically includes: heating to 700℃~740℃ at a heating rate of 1℃ / min~10℃ / min and holding at that temperature for 4h~10h.

10. A lithium-ion battery, characterized in that, The positive electrode includes a positive electrode sheet, wherein the positive active layer of the positive electrode sheet contains the lithium iron phosphate composite material as described in any one of claims 1 to 4, or the lithium iron phosphate composite material prepared by the method of preparing the lithium iron phosphate composite material as described in any one of claims 5 to 9.

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