Lithium iron phosphate positive electrode material and preparation method and application thereof

Abstract

The invention relates to the technical field of lithium ion batteries, in particular to a lithium iron phosphate positive electrode material and a preparation method and application thereof. The lithium iron phosphate positive electrode material provided by the invention comprises a lithium iron phosphate core and a carbon coating layer, in primary particles of the lithium iron phosphate positive electrode material, the quantity ratio of particles with the particle size d being greater than or equal to 600nm is greater than or equal to 40%, and the quantity ratio of particles with the particle size d being less than or equal to 200nm is less than or equal to 20%; the total variance of the carbon coating layer meets the following conditions: # imgabs0 #, n is more than or equal to 10, and s2 is more than or equal to 0.01 and less than or equal to 0.5. According to the lithium iron phosphate positive electrode material, through dual regulation and control of particle size distribution and carbon layer uniformity, the energy density, the cycle life and the overall safety are effectively balanced; and an innovative solution is provided for the development of a high-power power battery.

Description

Technical Field

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

[0002] With the global energy structure transformation, lithium-ion batteries, as the core power source for plug-in hybrid vehicles and pure electric vehicles, have achieved large-scale commercial application. However, the new energy vehicle market has placed higher requirements on battery performance, especially in the three core indicators of energy density, cycle life, and safety performance. High energy density can extend driving range, long cycle life can reduce life cycle costs, and excellent safety is the prerequisite for ensuring user use. In this context, positive electrode materials, as the key determinant of battery performance, have become the focus of technological breakthroughs. Among them, lithium iron phosphate (LFP) is highly favored due to its high theoretical specific capacity, strong thermal stability, low cost, and environmental friendliness, and is regarded as an ideal choice for high-safety power batteries.

[0003] Although lithium iron phosphate has outstanding performance in terms of safety and cost, its intrinsic conductivity is extremely low, close to the level of an insulator, which seriously restricts the rate performance and low-temperature performance of the battery. Low conductivity leads to the obstruction of the transmission of lithium ions and electrons within the material, causing a polarization effect, which in turn reduces the actual capacity and charge and discharge efficiency of the battery. In order to break through this bottleneck, carbon coating technology has been widely adopted. By constructing a conductive carbon layer on the surface of LFP particles, the electronic conductivity of the material can be significantly improved. At the same time, the pore structure in the carbon layer can also promote the diffusion of lithium ions and indirectly improve the ion transfer kinetics. However, the process parameters and effects of carbon coating directly determine the comprehensive performance of the LFP positive electrode. The technical difficulty lies in how to achieve precise control of the thickness, uniformity and structural stability of the carbon layer under complex synthesis conditions.

[0004] The current mainstream carbon coating technology mostly uses solid organic matter (such as glucose and sucrose) as the carbon source, but the problem of uneven carbon distribution is prone to occur during the high-temperature carbonization process. If the carbon source is excessive, the carbon layer is too thick or free "floating carbon" is formed, it will not only increase the interfacial impedance and hinder the migration of lithium ions, but also reduce the compaction density of the material, affecting the volume energy density of the battery; if the carbon source is insufficient, it will lead to incomplete coating, part of the LFP crystal surface is directly exposed, causing the local electron conduction path to break, and aggravating the irreversibility of the electrochemical reaction. In addition, insufficient uniformity of the carbon layer will also cause differences in contact resistance between particles, accelerating the capacity decay during the battery cycle. Although the academic community has tried to optimize the carbon layer structure through new processes such as liquid phase coating and vapor deposition, the industrialization level still faces challenges such as high equipment cost and large process complexity. How to balance the uniformity, conductivity and economy of carbon coating has become a technical barrier that needs to be overcome for the large-scale application of lithium iron phosphate materials. Summary of the Invention

[0005] The purpose of this application is to provide a lithium iron phosphate positive electrode material and its preparation method and application, aiming to solve the problems in the prior art of excessively high proportion of fine particles in the lithium iron phosphate positive electrode material and poor uniformity of the carbon coating layer.

[0006] To achieve the above application objectives, the technical solutions adopted in this application are as follows:

[0007] In a first aspect, the present application provides a lithium iron phosphate positive electrode material, the lithium iron phosphate positive electrode material comprising a lithium iron phosphate core and a carbon coating layer, wherein, among the primary particles of the lithium iron phosphate positive electrode material, the number of particles with a particle size d ≥ 600 nm accounts for ≥ 40%, and the number of particles with a particle size d ≤ 200 nm accounts for ≤ 20%; the overall variance of the carbon coating satisfies: And n ≥ 10, 0.01 ≤ s 2 ≤0.5.

[0008] In some embodiments, the integrated area ratio of the D peak to the G peak in the Raman spectrum of the carbon coating layer is R, and the value range of R is 0.8≤R≤4.5.

[0009] In some embodiments, the total mass of the lithium iron phosphate positive electrode material is 100%, and the carbon content is 0.5 wt % to 5.1 wt %.

[0010] In some embodiments, the average thickness of the carbon coating layer is 1 nm ≤ d A ≤10nm.

[0011] In some embodiments, the value range of R is 1.5≤R≤3.2.

[0012] In some embodiments, among the primary particles of the lithium iron phosphate positive electrode material, the number of particles with a particle size d≥600 nm accounts for 40% to 70%.

[0013] In some embodiments, among the primary particles of the lithium iron phosphate positive electrode material, the number of particles with a particle size d≤200 nm accounts for 3% to 20%.

[0014] In some embodiments, the doping element of the lithium iron phosphate positive electrode material is selected from at least one of Ti, V, Mg, and Nb, and the content of the doping element is 3000 ppm to 10000 ppm.

[0015] In some embodiments, the lithium iron phosphate cathode material has a spherical or quasi-spherical morphology.

[0016] In some embodiments, the powder compaction density of the lithium iron phosphate positive electrode material is 2.55 g / cm 3 ~2.75g / cm 3 .

[0017] In a second aspect, the present application provides a method for preparing a lithium iron phosphate positive electrode material, comprising the following steps:

[0018] Providing a lithium iron phosphate precursor containing doping elements;

[0019] A carbon coating layer is prepared on the surface of a lithium iron phosphate precursor using plasma deposition technology to obtain a lithium iron phosphate positive electrode material.

[0020] In some embodiments, the step of preparing a carbon coating on the surface of a lithium iron phosphate precursor using plasma deposition technology includes: placing the lithium iron phosphate precursor in a rotating microwave plasma generator, passing an inert gas through it and then evacuating it, passing a gaseous carbon source in a pulsed form and performing plasma treatment, so that carbon atoms are uniformly deposited on the surface of the lithium iron phosphate precursor to form a carbon coating.

[0021] In some embodiments, the reaction chamber rotation speed of the rotary microwave plasma generator is 10 rpm to 100 rpm.

[0022] In some embodiments, the inert gas is N2 or Ar.

[0023] In some embodiments, the purge gas flow rate is 10 sccm to 150 sccm, and the purge time is 1 min to 10 min.

[0024] In some embodiments, the gaseous carbon source is at least one of methane, ethane, propane, acetylene, and propyne, and the flow rate of the gaseous carbon source is 10 sccm to 50 sccm.

[0025] In some embodiments, the plasma treatment power is 400W to 800W, and the treatment time is 10 minutes to 600 minutes.

[0026] In some embodiments, a method for preparing a lithium iron phosphate precursor containing doping elements comprises the following steps: thoroughly mixing, grinding, and drying a lithium source, an iron phosphate precursor, and a dopant, and sintering at a high temperature in a reducing atmosphere to obtain a lithium iron phosphate precursor.

[0027] In some embodiments, the molar ratio of the lithium source to the iron phosphate precursor is (0.98-1.02):1.

[0028] In some embodiments, the dopant comprises at least one of Ti, V, Mg, and Nb, accounting for 3000 ppm to 10000 ppm of the mass of the finished product.

[0029] In some embodiments, the iron-phosphorus atomic ratio of the lithium iron phosphate precursor is (0.955-0.970):1, and the specific surface area is 4m 2 / g~10m 2 / g.

[0030] In some embodiments, the high temperature sintering temperature is 300° C. to 850° C., and the sintering time is 8 hours to 36 hours.

[0031] In some embodiments, the reducing atmosphere includes any one of Ar / H2 mixed gas, N2 / H2 mixed gas, Ar / CO mixed gas, and N2 / CO mixed gas, wherein the volume proportion of H2 or CO in each mixed gas is 5% to 20%.

[0032] In some embodiments, milling comprises ball milling or sand milling.

[0033] In some embodiments, drying comprises vacuum drying or spray drying.

[0034] In a third aspect, the present application provides a lithium-ion battery, comprising the above-mentioned lithium iron phosphate positive electrode material or the lithium iron phosphate positive electrode material prepared by the above-mentioned preparation method of the lithium iron phosphate positive electrode material.

[0035] The lithium iron phosphate positive electrode material provided in the first aspect of the present application reduces the risk of interface side reactions of fine particles by controlling the proportion of particles with a particle size of ≥600nm in the primary particles to ≥40%. At the same time, the close packing of large particles increases the compaction density, thereby improving the volume energy density of the battery, and can enhance the structural stability of the material and extend the cycle life of the battery; further, the overall variance of the carbon coating layer is controlled to meet 0.01≤s 2 ≤0.5 and n≥10 indicates that the carbon layer thickness of the particles is within the nanometer scale and exhibits a low dispersion distribution, achieving highly uniform carbon layer coverage. This uniform coverage prevents localized excessive thickness, thinness, or missing carbon layers, thereby ensuring good conductivity while also facilitating electrolyte infiltration and lithium ion insertion and extraction. By dually regulating primary particle size distribution and carbon layer uniformity, this lithium iron phosphate cathode material effectively balances energy density, cycle life, and overall safety, providing an innovative solution for the development of high-power power batteries.

[0036] The second aspect of this application provides a method for preparing a lithium iron phosphate positive electrode material. This method uses a lithium iron phosphate precursor containing doping elements to react, effectively improving the intrinsic electronic conductivity and lithium ion diffusion rate of the lithium iron phosphate through lattice doping during the material synthesis process. At the same time, combined with plasma deposition technology, an ultra-thin and uniform carbon layer is formed on the surface of the lithium iron phosphate precursor. Its coverage uniformity is significantly better than that of traditional solid-phase carbonization methods, achieving nano-scale uniform carbon coating. This process can complete carbon coating at a lower temperature, avoiding the problems of particle coarsening or doping element segregation caused by high-temperature sintering, and ensuring precise control of the material particle size distribution. In addition, the process technology is simple, can significantly reduce energy consumption and production costs, and provides a reliable technical path for the large-scale preparation of high-performance lithium iron phosphate batteries.

[0037] The lithium-ion battery provided in the third aspect of the present application significantly improves the energy density, cycle life and overall safety of the battery by optimizing the particle size distribution and the uniformity design of the carbon coating layer, which is conducive to the application of the provided lithium-ion battery in various fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the embodiments or descriptions of the prior art. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0039] Figure 1 This is a flow chart of an embodiment of the present application;

[0040] Figure 2 This is a scanning electron microscope (SEM) image of the positive electrode material prepared in Example A1 of the present application;

[0041] Figure 3 This is a transmission electron microscope (TEM) image of the positive electrode material prepared in Example A1 of the present application;

[0042] Figure 4 This is the Raman spectrum of the positive electrode material prepared in Example A1 of the present application;

[0043] Figure 5 This is the charge and discharge curve of the positive electrode material prepared in Application Example B1. DETAILED DESCRIPTION

[0044] In order to make the technical problems, technical solutions and beneficial effects to be solved by this application more clearly understood, the present application is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit this application.

[0045] In this application, the term "and / or" describes the relationship between associated objects, indicating that three possible relationships exist. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone. A and B can be singular or plural. The character " / " generally indicates that the associated objects are in an "or" relationship.

[0046] In this application, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single items or plural items. For example, "at least one of a, b, or c", or "at least one of a, b, and c" can all mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple.

[0047] It should be understood that in the various embodiments of the present application, the size of the serial numbers of the above-mentioned processes does not mean the order of execution. Some or all of the steps can be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0048] The terms used in the embodiments of the present application are only for the purpose of describing specific embodiments and are not intended to limit the present application. The singular forms "a", "an" and "the" used in the embodiments of the present application and the appended claims are also intended to include plural forms unless the context clearly indicates otherwise.

[0049] The weights of the relevant components mentioned in the examples of this application may not only refer to the specific content of each component, but also represent the weight ratio between the components. Therefore, as long as the content of the relevant components is proportionally enlarged or reduced according to the examples of this application, it is within the scope disclosed in the examples of this application. Specifically, the mass in the examples of this application may be μg, mg, g, kg, etc., which are mass units known in the chemical industry.

[0050] The terms "first" and "second" are used solely for descriptive purposes to distinguish objects, such as substances, from one another and should not be construed as indicating or implying relative importance or implicitly specifying the quantity of the technical features being referred to. For example, a first XX could also be referred to as a second XX, and similarly, a second XX could also be referred to as a first XX, without departing from the scope of the embodiments of this application. Thus, features defined as "first" or "second" may explicitly or implicitly include one or more of such features.

[0051] In a first aspect, an embodiment of the present application provides a lithium iron phosphate positive electrode material, the lithium iron phosphate positive electrode material comprising a lithium iron phosphate core and a carbon coating layer, wherein, among the primary particles of the lithium iron phosphate positive electrode material, the number of particles with a particle size d ≥ 600 nm accounts for ≥ 40%, and the number of particles with a particle size d ≤ 200 nm accounts for ≤ 20%; and the overall variance of the carbon coating layer satisfies: And n ≥ 10, 0.01 ≤ s 2 ≤0.5.

[0052] The lithium iron phosphate positive electrode material provided in the first aspect of the embodiment of the present application reduces the risk of interface side reactions of fine particles by controlling the proportion of particles with a particle size of ≥600nm in the primary particles to ≥40%. At the same time, the close packing of large particles increases the compaction density, thereby improving the volume energy density of the battery, and can enhance the structural stability of the material and extend the cycle life of the battery; further, the overall variance of the carbon coating layer is controlled to meet 0.01≤s 2 ≤0.5 and n≥10 indicates that the carbon layer thickness of the particles is within the nanometer scale and exhibits a low dispersion distribution, achieving highly uniform carbon layer coverage. This uniform coverage not only prevents localized excessive thickness, thinness, or missing carbon layers, thereby ensuring good conductivity, but also facilitates electrolyte infiltration and the insertion and extraction of lithium ions. By dually regulating primary particle size distribution and carbon layer uniformity, this lithium iron phosphate cathode material effectively balances energy density, cycle life, and overall safety, providing an innovative solution for the development of high-power power batteries.

[0053] In some embodiments, among the primary particles of lithium iron phosphate positive electrode materials, the number of particles with a particle size of d ≥ 600 nm accounts for ≥ 40%, and the number of particles with a particle size of d ≤ 200 nm accounts for ≤ 20%. By controlling the proportion of particles with a particle size of ≥ 600 nm in the primary particles to be ≥ 40%, the risk of interfacial side reactions of fine particles (≤ 200 nm accounts for ≤ 20%) is reduced, which ensures that the particle size is large and uniform. The close packing of large particles increases the electrode compaction density, thereby improving the volume energy density of the battery. In addition, the size distribution dominated by large particles can alleviate the volume expansion stress during the charge and discharge process, enhance the structural stability of the material, and extend the cycle life.

[0054] In some embodiments, the primary particles of the lithium iron phosphate cathode material have a particle size of d ≥ 600 nm, accounting for 40% to 70% of the total. In some embodiments, the primary particles of the lithium iron phosphate cathode material have a particle size of d ≤ 200 nm, accounting for 3% to 20% of the total. Simultaneously controlling the percentages of particles with a particle size of d ≥ 600 nm and particles with a particle size of d ≤ 200 nm facilitates small particles filling the gaps between larger particles, ensuring that the resulting lithium iron phosphate cathode material achieves high density and high capacity.

[0055] In some embodiments, the population variance of the carbon coating satisfies: And n ≥ 10, 0.01 ≤ s 2 ≤0.5. Control the overall variance of the carbon coating layer to meet 0.01≤s 2≤0.5 and n≥10, indicating that the thickness of the carbon layer is within the nanometer scale and presents a low discreteness distribution, achieving highly uniform coverage of the carbon layer. This uniform coverage can not only prevent the local carbon layer from being too thick, which can reduce the ion migration resistance caused by "floating carbon", but also prevent the local carbon layer from being too thin or missing, which is conducive to eliminating the electron conduction bottleneck caused by exposed crystal surfaces, thereby simultaneously optimizing electronic conductivity and lithium ion diffusion kinetics, and significantly improving the rate performance and low-temperature performance of the battery. In some specific embodiments, the overall variance s 2 Typical but non-limiting values ​​include, but are not limited to, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, etc.

[0056] In some embodiments, the integrated area ratio of the D peak and the G peak in the Raman spectrum of the carbon coating layer is R, and the value range of R is 0.8≤R≤4.5. The D peak in the Raman spectrum reflects the sp 3 Hybrid disordered structure or defect, the G peak in the Raman spectrum corresponds to sp 2 Hybrid graphitized ordered structure. By controlling the R value within the range of 0.8 to 4.5, an appropriate amount of graphitized ordered domains can be retained to provide high electronic conductivity, while moderate defects can increase active sites and promote lithium ion interfacial transport. In some specific embodiments, the value range of R includes, but is not limited to, typical but non-limiting values ​​such as 0.8, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, and 4.5.

[0057] In some embodiments, the value range of R is 1.5≤R≤3.2. Further controlling the value range of R to 1.5≤R≤3.2 is beneficial to improving the degree of graphitization of the carbon layer, improving the conductivity, and thus improving the dynamic properties of the material. In some specific embodiments, the value range of R includes but is not limited to typical but non-limiting values ​​such as 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, and 3.2.

[0058] In some embodiments, the total mass of the lithium iron phosphate positive electrode material is 100%, and the carbon content is 0.5wt% to 5.1wt%. If the carbon content is too high, the thickness of the obtained carbon coating layer is too thick, which is not conducive to the deintercalation of lithium ions; if the carbon content is too low, it will cause uneven coating of the carbon coating layer, or even the situation where the entire lithium iron phosphate precursor cannot be completely coated, and the overall properties of the positive electrode material cannot be improved. By controlling the carbon content, the thickness of the carbon coating layer can be correspondingly controlled. In some specific embodiments, the total mass of the lithium iron phosphate positive electrode material is 100%, and the carbon content includes but is not limited to typical but non-limiting values ​​such as 0.5wt%, 1wt%, 1.5wt%, 2wt%, 2.5wt%, 3wt%, 3.5wt%, 4wt%, 4.5wt%, 5wt%, 5.1wt%.

[0059] In some embodiments, the average thickness of the carbon coating layer is 1 nm ≤ d A ≤10nm. The lithium iron phosphate positive electrode material can achieve high electronic conductivity and low ion migration resistance by precisely controlling the average thickness of the carbon coating layer in the range of 1nm to 10nm. When the average thickness of the carbon coating layer is less than 1nm, the carbon layer cannot form a complete coating layer, cannot form a continuous conductive network, and thus cannot effectively improve the electronic conductivity; when the average thickness of the carbon coating layer is greater than 10nm, the carbon coating layer is too thick, resulting in a long diffusion path for lithium ions in the carbon layer, resulting in increased lithium ion migration resistance. Therefore, controlling the average thickness of the carbon coating layer to 1nm to -10nm is conducive to achieving the best balance between electronic conduction, ion transport, interface stability and process cost, and provides key technical support for the development of high energy density and long life power batteries.

[0060] In some specific embodiments, the average thickness of the carbon coating layer includes, but is not limited to, typical but non-limiting values ​​such as 1 nm, 1.5 nm, 2 nm, 2.5 nm, 3 nm, 3.5 nm, 4 nm, 4.5 nm, 5 nm, 5.5 nm, 6 nm, 6.5 nm, 7 nm, 7.5 nm, 8 nm, 8.5 nm, 9 nm, 9.5 nm, and 10 nm.

[0061] In some embodiments, the doping element of the lithium iron phosphate positive electrode material, wherein the doping element is selected from at least one of Ti, V, Mg, and Nb, wherein the content of the doping element is 3000ppm to 10000ppm. Providing doping elements can effectively improve the intrinsic electronic conductivity and lithium ion diffusion rate of lithium iron phosphate. The doping elements can also inhibit lattice distortion during charging and discharging, alleviate volume expansion, thereby improving the structural stability of the material and the cycle life of the battery. In some specific embodiments, the content of the doping element includes but is not limited to typical but non-limiting values ​​such as 3000ppm, 3500ppm, 4000ppm, 4500ppm, 5000ppm, 5500ppm, 6000ppm, 6500ppm, 7000ppm, 7500ppm, 8000ppm, 8500ppm, 9000ppm, 9500ppm, 10000ppm.

[0062] In some embodiments, the lithium iron phosphate cathode material has a spherical or quasi-spherical morphology.

[0063] In some embodiments, the powder compaction density of the lithium iron phosphate positive electrode material is 2.55 g / cm 3 ~2.75g / cm 3 In some specific embodiments, the powder compaction density of the lithium iron phosphate positive electrode material includes but is not limited to 2.55 g / cm 3 , 2.60g / cm 3 , 2.65g / cm 3 , 2.70g / cm 3 , 2.75g / cm 3 These are typical but not limiting values.

[0064] A second aspect of the present invention provides a method for preparing a lithium iron phosphate positive electrode material, comprising the following steps:

[0065] S01 provides a lithium iron phosphate precursor containing a doping element;

[0066] S02. Prepare a carbon coating layer on the surface of a lithium iron phosphate precursor using plasma deposition technology to obtain a lithium iron phosphate positive electrode material.

[0067] The second aspect of the present application provides a method for preparing a lithium iron phosphate positive electrode material. In this preparation method, a lithium iron phosphate precursor containing a doping element is used for reaction. During the material synthesis process, lattice doping is used to effectively improve the intrinsic electronic conductivity and lithium ion diffusion rate of the lithium iron phosphate. At the same time, combined with plasma deposition technology, an ultra-thin and uniform carbon layer is formed on the surface of the lithium iron phosphate precursor. Its coverage uniformity is significantly better than that of traditional solid-phase carbonization methods, achieving nano-level uniform carbon coating. This process can complete carbon coating at a lower temperature, avoiding the problems of particle coarsening or doping element segregation caused by high-temperature sintering, and ensuring precise control of the material particle size distribution. In addition, the process technology is simple, can significantly reduce energy consumption and production costs, and provides a reliable technical path for the large-scale preparation of high-performance lithium iron phosphate batteries.

[0068] In step S01 , a lithium iron phosphate precursor containing a doping element is provided.

[0069] In some embodiments, a method for preparing a lithium iron phosphate precursor containing doping elements comprises the following steps: thoroughly mixing, grinding, and drying a lithium source, an iron phosphate precursor, and a dopant, and sintering at a high temperature in a reducing atmosphere to obtain a lithium iron phosphate precursor.

[0070] In some embodiments, the molar ratio of the lithium source to the iron phosphate precursor is (0.98-1.02):1.

[0071] In some embodiments, the dopant comprises at least one of Ti, V, Mg, and Nb, accounting for 3,000 ppm to 10,000 ppm of the finished product mass. Adding the dopant can broaden the lithium ion transport channels of the material and improve the kinetic properties of the material.

[0072] In some embodiments, the iron-phosphorus atomic ratio of the lithium iron phosphate precursor is (0.955-0.970):1, and the specific surface area is 4m 2 / g~10m 2 Limiting the iron-phosphorus atomic ratio and specific surface area is primarily to ensure the formation of uniform large particles, reducing the risk of interfacial side reactions of fine particles. This ensures that the particle size is large and uniform, and the close packing of large particles increases the electrode compaction density, thereby improving the volume energy density of the battery.

[0073] In some embodiments, milling comprises ball milling or sand milling.

[0074] In some embodiments, drying comprises vacuum drying or spray drying.

[0075] Furthermore, the lithium iron phosphate precursor is obtained by high-temperature sintering in a reducing atmosphere. During the preparation of the lithium iron phosphate precursor, a reducing atmosphere is provided by a reducing gas rather than a traditional organic carbon source as a reducing agent. This carbon-free process, without the constraints of a carbon layer, allows the lithium iron phosphate precursor particles to grow more easily and more uniformly, facilitating improved powder compaction.

[0076] In some embodiments, the reducing atmosphere includes any one of an Ar / H2 mixed gas, an N2 / H2 mixed gas, an Ar / CO mixed gas, and an N2 / CO mixed gas, wherein the volume proportion of H2 or CO in each mixed gas is 5% to 20%. The reducing atmosphere provided is a mixed gas, and the use of the mixed gas is to improve safety while ensuring a sufficient reducing atmosphere.

[0077] In some embodiments, the high temperature sintering temperature is 300°C to 850°C, and the time is 8 hours to 36 hours. In some specific embodiments, the high temperature sintering temperature includes, but is not limited to, typical but non-limiting values ​​such as 300°C, 350°C, 400°C, 450°C, 500°C, 550°C, 600°C, 650°C, 700°C, 750°C, 800°C, and 850°C. In some specific embodiments, the high temperature sintering time includes, but is not limited to, typical but non-limiting values ​​such as 8 hours, 10 hours, 15 hours, 20 hours, 25 hours, 30 hours, and 36 hours.

[0078] In step S02, a carbon coating layer is prepared on the surface of the lithium iron phosphate precursor by using plasma deposition technology to obtain a lithium iron phosphate positive electrode material.

[0079] In some embodiments, the step of preparing a carbon coating on the surface of a lithium iron phosphate precursor using plasma deposition technology includes placing the lithium iron phosphate precursor in a rotating microwave plasma generator, purging it with an inert gas, and then evacuating the system. A gaseous carbon source is introduced in a pulsed manner and plasma treated to uniformly deposit carbon atoms on the surface of the lithium iron phosphate precursor to form a carbon coating. Placing the lithium iron phosphate precursor in a rotating reaction chamber and introducing a carbon source for plasma treatment makes it easier to precisely control the thickness of the carbon layer and better control the uniformity of the carbon layer.

[0080] In some embodiments, the reaction chamber speed of the rotary microwave plasma generator is 10 rpm to 100 rpm. In some specific embodiments, the reaction chamber speed of the rotary microwave plasma generator includes, but is not limited to, typical but non-limiting values ​​such as 10 rpm, 20 rpm, 30 rpm, 40 rpm, 50 rpm, 60 rpm, 70 rpm, 80 rpm, 90 rpm, and 100 rpm.

[0081] In some embodiments, the inert gas is N2 or Ar.

[0082] In some embodiments, the purge gas flow rate is 10 sccm to 150 sccm, and the purge time is 1 min to 10 min. Controlling the gas flow rate and purge time of the inert gas purge, and controlling the flow rate and purge rate of the inert gas is mainly to ensure that the oxygen in the cavity is emptied to avoid oxidation of the material during the reaction. In some specific embodiments, the purge gas flow rate includes but is not limited to typical but non-limiting values ​​such as 10 sccm, 20 sccm, 30 sccm, 40 sccm, 50 sccm, 60 sccm, 70 sccm, 80 sccm, 90 sccm, 100 sccm, 110 sccm, 120 sccm, 130 sccm, 140 sccm, 150 sccm. If the flow rate and rate are too large, it is easy to lift the powder material, which is then carried away by the gas, causing material loss, and it is easy to clog the pipeline. In some specific embodiments, the purge time includes, but is not limited to, typical but non-limiting values ​​such as 1 min, 2 min, 3 min, 4 min, 5 min, 6 min, 7 min, 8 min, 9 min, and 10 min.

[0083] In some embodiments, the gaseous carbon source is at least one of methane, ethane, propane, acetylene, and propyne, and the flow rate of the gaseous carbon source is 10 sccm to 50 sccm.

[0084] In some embodiments, the plasma treatment power is 400W to 800W, and the treatment time is 10min to 600min. In some specific embodiments, the plasma treatment power includes but is not limited to typical but non-limiting values ​​such as 400W, 450W, 500W, 550W, 600W, 650W, 700W, 750W, and 800W. If the plasma treatment power is too small, the reaction will not be thorough, or the carbon coating will not be complete; if the plasma treatment power is too high, it will easily damage the material itself and cause lattice defects. In some specific embodiments, the treatment time includes but is not limited to typical but non-limiting values ​​such as 10min, 50min, 100min, 150min, 200min, 250min, 300min, 350min, 400min, 450min, 500min, 550min, and 600min.

[0085] A third aspect of an embodiment of the present application provides a lithium-ion battery, comprising the above-mentioned lithium iron phosphate positive electrode material or the lithium iron phosphate positive electrode material prepared by the above-mentioned method for preparing the lithium iron phosphate positive electrode material.

[0086] The lithium-ion battery provided in the third aspect of the embodiment of the present application significantly improves the energy density, cycle life and overall safety of the battery by optimizing the particle size distribution and the uniformity design of the carbon coating layer, which is conducive to the application of the provided lithium-ion battery in various fields.

[0087] The following describes the details in conjunction with specific embodiments.

[0088] Example A1

[0089] S1: Weigh 100kg of ferric phosphate (iron-phosphorus ratio of 0.967, specific surface area of ​​7.55m 2 / g), 25.3kg lithium carbonate and 0.6kg titanium dioxide are mixed and mixed, stirred evenly and then sand-milled to control the particle size D of the sand-milled slurry. 50 The particle size of the spray-dried material was 0.50 μm, and then spray-dried; 500 g of the spray-dried material was placed in a tubular furnace and calcined at a high temperature under the condition of passing an Ar / H2 mixed gas, wherein the volume proportion of H2 was 10%, the sintering temperature was 780 ° C, and the sintering time was 24 h to obtain a lithium iron phosphate precursor;

[0090] S2: The product in S1 is placed in the reaction chamber of a rotary microwave-assisted plasma generator, the rotation speed of the cavity is set to 20 rpm, high-purity Ar is introduced for purging, the gas flow rate is 20 sccm, and the purging time is 10 minutes, then vacuum is applied, and methane gas is introduced in a pulsed form with a flow rate of 10 sccm. The power of the plasma generator is set to 700 W, and the plasma treatment time is 60 minutes. After the end, high-purity Ar is introduced for purging again, the gas flow rate is 15 sccm, and the purging time is 10 minutes. The product is uniformly carbon-coated lithium iron phosphate powder.

[0091] Example A2

[0092] The operation steps of this embodiment are basically the same as those of Example A1, except that in step S2, the plasma treatment time is adjusted to 120 minutes.

[0093] Example A3

[0094] The operating steps of this embodiment are basically the same as those of embodiment A1, except that the calcination temperature is changed to 850° C. and the sintering time is 30 h.

[0095] Example A4

[0096] The operation steps of this embodiment are basically the same as those of Example A1, except that in step S1, the dopant is adjusted to 0.7 kg of vanadium pentoxide.

[0097] Example A5

[0098] The operation steps of this embodiment are basically the same as those of embodiment A1, except that in step S2, the gaseous carbon source is adjusted to ethane and the gas flow rate is adjusted to 10 sccm.

[0099] Comparative Example A1

[0100] This comparative example was carried out with reference to step S1 in Example A1, and the obtained product was a lithium iron phosphate positive electrode material.

[0101] Comparative Example A2

[0102] This comparative example was carried out with reference to step S1 in Example A1, except that 9 kg of glucose and 3 kg of polyethylene glycol were additionally added, and other steps and parameters remained unchanged to obtain a carbon-coated lithium iron phosphate positive electrode material.

[0103] Comparative Example A3

[0104] In addition to adjusting the iron-phosphorus atomic ratio of the iron phosphate precursor to 0.975:1 and the specific surface area to 10.5 m 2 / g.

[0105] Examples B1 to B5 and Comparative Examples B1 to B3

[0106] The powder materials prepared in Examples A1 to A5 and Comparative Examples A1 to A3 were prepared into button batteries, respectively, and the specific steps were as follows:

[0107] (1) Preparation of slurry: The powder materials prepared in Examples A1 to A5 and Comparative Examples A1 to A3 were provided as positive electrode active materials, superconducting carbon black (SP), and a binder, polyvinylidene fluoride (PVDF), were simultaneously added to an agate ball mill, and then a solvent, N-methylpyrrolidone (NMP), was added and ball milled to prepare a slurry;

[0108] (2) Slurry coating: adjust the scale of the scraper of the coating machine, evenly apply the milled slurry on the aluminum foil, and place the coated electrode in a vacuum drying oven for drying;

[0109] (3) Rolling and punching: Place the aluminum foil coated with slurry flatly in the middle of the roller and roll the electrode; place the front of the rolled electrode close to the punched area and punch the electrode in sequence; place the punched electrode in a vacuum drying oven and dry it;

[0110] (4) Assemble button batteries in a glove box in the order of negative electrode shell, spring, steel sheet, lithium sheet, diaphragm, positive electrode sheet and positive electrode shell. In the process, inject electrolyte, and then use a sealing machine to seal the button battery. The electrochemical performance of the button battery is tested.

[0111] Property Test

[0112] (1) Various property tests were performed on the lithium iron phosphate positive electrode materials of Examples A1 to A5 and Comparative Examples A1 to A3.

[0113] (2) Taking the lithium iron phosphate positive electrode material obtained in Example A1 as an example, the lithium iron phosphate positive electrode material obtained in Example A1 was subjected to scanning electron microscopy analysis, transmission electron microscopy analysis, and Raman spectroscopy analysis.

[0114] (3) Various property tests were performed on the button batteries of Examples B1 to B5 and Comparative Examples B1 to B3.

[0115] Result Analysis

[0116] (1) The properties of the lithium iron phosphate positive electrode materials of Examples A1 to A5 and Comparative Examples A1 to A3 were tested. The test results are shown in Table 1. In Examples A1 to A5, the proportion of large particles is ≥40% (40.5% to 46.5%), and the corresponding compaction density is ≥2.588 g / cm 3 (Maximum 2.702g / cm 3 ); Comparative Example A2 has a large particle size of only 25.7%, and the compaction density is significantly reduced to 2.443g / cm 3 It can be concluded that a large particle ratio of ≥40% can optimize the particle packing density and improve the volume energy density. The small particle ratio of Examples A1 to A5 is ≤18.8%, while that of Comparative Example A3 is as high as 33.1%, resulting in the lowest compaction density (2.388 g / cm 3 ), it can be concluded that a high proportion of small particles will increase the interfacial porosity and hinder the close arrangement of particles.

[0117] The thickness of the carbon layer (5.47nm-7.35nm) in the examples is positively correlated with the carbon content (1.01wt%-1.47wt%) (e.g. A2: dA=7.35nm, carbon content 1.44wt%). However, in comparative example A1, the carbon layer is too thin (0.05nm) and the carbon content is only 0.18wt%, resulting in coating failure. 2 All of them are ≤0.0923 (A5 is the highest), indicating that the thickness of the carbon layer is evenly distributed; while the s 2 =0.896, indicating severe nonuniformity in carbon layer thickness, leading to increased interfacial impedance. The R values ​​for the examples range from 2.813 to 3.561 (A3 is slightly higher due to R=3.561), indicating that the carbon layer has both moderately graphitized ordered domains (G peak) and defects (D peak), resulting in synergistic electronic conductivity and ion transport efficiency. Comparative Examples A1 (R=9.369 (excessive defects)) and A2 (R=5.796 (high defects)) both result in poor conductivity.

[0118] Table 1

[0119]

[0120] (2) Taking the lithium iron phosphate positive electrode material obtained in Example A1 as an example, the lithium iron phosphate positive electrode material obtained in Example A1 was subjected to scanning electron microscopy analysis, transmission electron microscopy analysis, and Raman spectroscopy analysis.

[0121] The SEM image of the lithium iron phosphate product prepared in Example A1 is as follows: Figure 2 As shown in the figure, it can be seen that the morphology of lithium iron phosphate particles is spherical, and the primary particle size d A The number of particles ≥600 nm accounts for 41.3%, and the number of primary particle size d≤200 nm accounts for 17.6%.

[0122] The TEM image of lithium iron phosphate prepared in Example A1 is as follows: Figure 3 As shown, the Ti doping amount of lithium iron phosphate is 3776ppm and the carbon content is 1.01%. Figure 3 It can be seen that in the TEM image, the carbon layer thickness of 10 areas randomly measured is marked as d1~d 10 , where d1 = 5.39nm, d2 = 5.51nm, d3 = 5.65nm, d4 = 5.75nm, d5 = 5.21nm, d6 = 5.17nm, d7 = 5.31nm, d8 = 5.71nm, d9 = 5.12nm, d 10 =5.85nm, calculate the average carbon layer thickness d A is 5.47 nm, and the overall variance of the carbon layer thickness s 2 It is 0.0696, indicating that the carbon layer coating is very uniform.

[0123] The Raman spectrum of lithium iron phosphate prepared in Example A1 is as follows: Figure 4 As shown in the figure, there are two obvious characteristic peaks: D peak and G peak. Among them, D peak is located at 1350cm -1 Nearby, it represents graphite-like lattice defects, edge disordered arrangement and low-symmetry carbon structure, also known as structural disorder peak; G peak is located at 1580cm -1 Nearby, it is the sp inherent in natural graphite 2 The stretching vibration peak of the hybrid is also called the structural order peak; the area ratio of the D peak and the G peak in the Raman spectrum is R (A D / A G ) is used to quantitatively describe the degree of graphitization of the carbon layer. The lower R is, the higher the degree of graphitization is. Here, R = 2.937.

[0124] (3) Various property tests were performed on the button batteries of Examples B1 to B5 and Comparative Examples B1 to B3.

[0125] Figure 5The charge-discharge curves for the cathode material prepared in Example B1 are shown in Table 2. Specifically, the 0.1C capacities of Examples B1-B5 are all close to 160 mAh / g to 161 mAh / g (with actual utilization rates of approximately 94% to 95%), demonstrating the material's high intrinsic activity and lithium-ion deintercalation efficiency. Comparative Example B1, due to carbon coating failure, has a capacity of only 75.5 mAh / g (with a utilization rate of 44%), demonstrating the critical role of the carbon layer in electron conduction.

[0126] The 5C capacities of Examples B1 to B5 are all ≥120 mAh / g (maximum 125.7 mAh / g), which is due to the following advantages: uniform carbon coating (s 2 ≤0.1): reduce interface impedance and improve electron conduction rate; moderate R value (2.8~3.5): balance conductivity and lithium ion diffusion efficiency; large particle-dominated particle size distribution (d≥600nm accounted for ≥40%): reduce inter-particle contact resistance; comparative example B2 has too high a proportion of small particles (26.9%) and uneven carbon layer (s 2 =0.896), the 5C capacity dropped to 118.9 mAh / g.

[0127] The cycle retention rates of Examples B1 to B5 are all ≥94.1% (maximum 96.5%), and the key supporting factors include: Ti / V doping amount close (3500ppm~4000ppm): suppressing lattice distortion and alleviating volume expansion; carbon layer integrity (thickness 5-7nm, coverage ≥95%): reducing electrolyte side reactions (such as Fe dissolution); low variance carbon layer (s 2 ≤0.1): avoid local stress concentration leading to carbon layer peeling. However, the cycle retention rate of comparative example B1 is only 10.5% due to ineffective coating (carbon layer 0.05nm); the cycle retention rate of comparative example B2 is only 10.5% due to uneven carbon layer (s 2 =0.896) and the particle size distribution was unbalanced, and the retention rate dropped to 92.5%.

[0128] The capacity decay rate of Examples B1 to B5 (3.5% to 5.9% after 500 cycles) is positively correlated with the uniformity of the carbon layer: B1 (s 2 =0.0696) retention rate is 96.5%, better than B5(s 2 =0.0923) is 94.1%; B3 (R = 3.561) has slightly more carbon layer defects, so the retention rate is slightly lower than B1 (95.6% vs 96.5%).

[0129] Table 2

[0130]

[0131]

[0132] In summary, the lithium iron phosphate positive electrode material provided in the embodiment of the present application reduces the risk of interface side reactions of fine particles by controlling the proportion of particles with a particle size of ≥600nm in the primary particles to ≥40%. At the same time, the close packing of large particles increases the electrode compaction density, thereby improving the volume energy density of the battery, and can enhance the structural stability of the material and extend the cycle life; further, the overall variance of the carbon coating layer is controlled to meet 0.01≤s 2 ≤0.5 and n≥10, indicating highly uniform carbon layer thickness with low dispersion at the nanoscale. This uniform coverage prevents localized excessive carbon layer thickness from being too thick while also preventing excessive thinness or missing carbon layers, thereby ensuring good conductivity while also facilitating electrolyte infiltration and lithium ion insertion and extraction. By dually regulating particle size distribution and carbon layer uniformity, this lithium iron phosphate cathode material effectively balances energy density, cycle life, and overall safety, providing an innovative solution for the development of high-power power batteries.

[0133] The above are only preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, and improvements made within the spirit and principles of the present application should be included in the scope of protection of the present application.

Claims

1. A lithium iron phosphate positive electrode material, characterized in that: The lithium iron phosphate positive electrode material includes a lithium iron phosphate core and a carbon coating layer, wherein, among the primary particles of the lithium iron phosphate positive electrode material, the number of particles with a particle size d≥600nm accounts for ≥40%, and the number of particles with a particle size d≤200nm accounts for ≤20%; the overall variance of the carbon coating layer satisfies: And n ≥ 10, 0.01 ≤ s 2 ≤0.

5.

2. The lithium iron phosphate positive electrode material according to claim 1, characterized in that The integrated area ratio of the D peak and the G peak in the Raman spectrum of the carbon coating layer is R, and the value range of R is 0.8≤R≤4.5; and / or, Taking the total mass of the lithium iron phosphate positive electrode material as 100%, the carbon content is 0.5wt% to 5.1wt%; and / or, The average thickness of the carbon coating layer is 1nm≤d A ≤10nm.

3. The lithium iron phosphate positive electrode material according to claim 2, characterized in that The value range of R is 1.5≤R≤3.2; and / or, Among the primary particles of the lithium iron phosphate positive electrode material, the number of particles with a particle size d≥600 nm accounts for 40% to 70%; and / or, Among the primary particles of the lithium iron phosphate positive electrode material, the number of particles with a particle size d≤200 nm accounts for 3% to 20%.

4. The lithium iron phosphate cathode material according to claim 1, characterized in that The doping element of the lithium iron phosphate positive electrode material, wherein the doping element is selected from at least one of Ti, V, Mg, and Nb, wherein the content of the doping element is 3000ppm to 10000ppm; and / or, The lithium iron phosphate cathode material has a spherical or quasi-spherical morphology; and / or, The powder compaction density of the lithium iron phosphate positive electrode material is 2.55 g / cm 3 ~2.75g / cm 3 .

5. A method for preparing a lithium iron phosphate positive electrode material, characterized in that: The steps include: Providing a lithium iron phosphate precursor containing doping elements; A carbon coating layer is prepared on the surface of the lithium iron phosphate precursor by adopting plasma deposition technology to obtain a lithium iron phosphate positive electrode material.

6. The method for preparing the lithium iron phosphate positive electrode material according to claim 5, characterized in that: The step of preparing a carbon coating layer on the surface of the lithium iron phosphate precursor using plasma deposition technology includes: placing the lithium iron phosphate precursor in a rotating microwave plasma generator, passing an inert gas to purge it and then evacuating it, passing a gaseous carbon source in a pulsed form and performing plasma treatment, so that carbon atoms are uniformly deposited on the surface of the lithium iron phosphate precursor to form a carbon coating layer.

7. The method for preparing the lithium iron phosphate positive electrode material according to claim 6, characterized in that: The reaction chamber rotation speed of the rotary microwave plasma generator is 10 rpm to 100 rpm; and / or, The inert gas is N2 or Ar; and / or, The purge gas flow rate is 10 sccm to 150 sccm, and the purge time is 1 min to 10 min; and / or, The gaseous carbon source is at least one of methane, ethane, propane, acetylene, and propyne, and the flow rate of the gaseous carbon source is 10 sccm to 50 sccm; and / or, The plasma treatment power is 400W to 800W, and the treatment time is 10min to 600min.

8. The method for preparing a lithium iron phosphate cathode material according to claim 5, wherein: The method for preparing the lithium iron phosphate precursor containing doping elements comprises the following steps: fully mixing a lithium source, an iron phosphate precursor and a dopant, grinding and drying them, and sintering them at a high temperature in a reducing atmosphere to obtain the lithium iron phosphate precursor.

9. The method for preparing a lithium iron phosphate cathode material according to claim 8, characterized in that: The molar ratio of the lithium source to the iron phosphate precursor is (0.98-1.02):1; and / or, The dopant is at least one of Ti, V, Mg and Nb, accounting for 3000ppm to 10000ppm of the finished product mass; and / or, The iron-phosphorus atomic ratio of the lithium iron phosphate precursor is (0.955-0.970):1, and the specific surface area is 4m 2 / g~10m 2 / g.

10. The method for preparing a lithium iron phosphate positive electrode material according to claim 8, characterized in that: The high temperature sintering temperature is 300° C. to 850° C. and the time is 8 hours to 36 hours; and / or, The reducing atmosphere includes any one of Ar / H2 mixed gas, N2 / H2 mixed gas, Ar / CO mixed gas, and N2 / CO mixed gas, wherein the volume proportion of H2 or CO in each mixed gas is 5% to 20%; and / or, The grinding comprises ball milling or sand milling; and / or, The drying includes vacuum drying or spray drying.

11. A lithium ion battery, characterized in that: The invention comprises the lithium iron phosphate positive electrode material according to any one of claims 1 to 4 or the lithium iron phosphate positive electrode material prepared by the preparation method of the lithium iron phosphate positive electrode material according to any one of claims 5 to 10.

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