Lithium iron phosphate positive electrode material, preparation method thereof, positive plate and battery

Abstract

The invention relates to the technical field of batteries, in particular to a lithium iron phosphate positive electrode material, a preparation method thereof, a positive plate and a battery. The lithium iron phosphate positive electrode material provided by the invention comprises a doped lithium iron phosphate inner core, and doped elements in the doped lithium iron phosphate inner core comprise one or more of alkaline earth metal, transition metal or non-metallic elements; the first coating layer is coated on at least part of the surface of the doped lithium iron phosphate inner core, and the first coating layer comprises a niobium phosphide coating layer; the second coating layer is coated on at least part of the surface of the first coating layer, and the second coating layer comprises a carbon coating layer. According to the invention, the problem that the compaction density and the low-temperature performance of the lithium iron phosphate positive electrode material are difficult to consider at present can be relieved, and the purpose of considering the compaction density and the low-temperature performance of the high-compaction lithium iron phosphate material can be achieved.

Description

Technical Field

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

[0002] Lithium iron phosphate (LFP) has become a mature commercial cathode material due to its excellent thermal stability, low cost, good electrochemical stability, and its green, economical, and efficient characteristics, finding applications in new energy vehicle power batteries and energy storage power stations. However, LFP's inherent defects lead to poor electronic conductivity and low ion diffusion rate, negatively impacting its rate performance and low-temperature performance. Its relatively low operating voltage, compaction density, and theoretical specific capacity limit its energy density, significantly restricting further applications. Since the voltage window and specific capacity of LFP are fixed, improving its compaction density while maintaining electrochemical performance is crucial for meeting the requirements of power batteries and further enhancing its energy density. Related technologies for modifying high-compact LFP mainly include particle size distribution, elemental doping, surface carbon coating, and material compositing.

[0003] Currently, the mainstream process route for lithium iron phosphate (LFP) synthesis is the iron phosphate route. High-compact LFP produced via the iron phosphate route improves compaction density by controlling the particle size distribution through the precursor, milling particle size, and sintering process. However, the presence of large particles leads to poor low-temperature rate performance. Further improvements to high-compact LFP can be achieved by increasing the carbon coating or reducing the content of large particles. While this can improve low-temperature performance to some extent, it also affects compaction density. Furthermore, higher carbon content results in higher residual carbon content, and even small amounts of residual carbon can directly contact the electrolyte, increasing side reactions and impacting battery cycle performance. Summary of the Invention

[0004] In view of this, the present invention aims to at least partially solve one of the technical problems in the related art. To this end, the present invention provides a lithium iron phosphate cathode material and its preparation method, a cathode sheet, and a battery, which can alleviate the current problem of simultaneously improving the compaction density and low-temperature performance of lithium iron phosphate cathode materials, achieving the goal of balancing the compaction density and low-temperature performance of high-compact lithium iron phosphate materials.

[0005] To solve the above-mentioned technical problems, this application is implemented as follows: According to one aspect of this application, an embodiment of this application provides a lithium iron phosphate cathode material, the lithium iron phosphate cathode material comprising: A doped lithium iron phosphate core, wherein the doping element in the doped lithium iron phosphate core includes one or more of alkaline earth metals, transition metals, or non-metallic elements; A first coating layer covers at least a portion of the surface of the doped lithium iron phosphate core, the first coating layer comprising a niobium phosphide coating layer; A second coating layer covers at least a portion of the surface of the first coating layer, the second coating layer comprising a carbon coating layer.

[0006] In some embodiments, the doped lithium iron phosphate core has the general chemical formula LiM. x Fe （1-x） PO4; where M is a doping element, including at least one of Mg, Nb, Ti, V, Zr, Mo or B; and x ranges from 0.005 to 0.02.

[0007] In some of these implementations, M includes at least Nb.

[0008] In some of these embodiments, the lithium iron phosphate cathode material satisfies at least one of the following features (1) to (3): (1) in the lithium iron phosphate cathode material, the thickness of the niobium phosphide coating layer is 1 nm to 5 nm; (2) in the lithium iron phosphate cathode material, the thickness of the carbon coating layer is 1 nm to 6 nm; (3) in the lithium iron phosphate cathode material, the mass content of carbon element is 1.0% to 1.6%.

[0009] In some embodiments, the lithium iron phosphate cathode material satisfies at least one of the following features (1) to (4): (1) the lithium iron phosphate cathode material comprises large lithium iron phosphate particles and small lithium iron phosphate particles, wherein the particle size of the large lithium iron phosphate particles is larger than the particle size of the small lithium iron phosphate particles; (2) the particle size D of the lithium iron phosphate cathode material is... v 50 is 0.8μm~1.5μm; (3) the powder compaction density of the lithium iron phosphate cathode material is 2.5~2.7g / cm³. 3 (4) The powder resistivity of the lithium iron phosphate cathode material is In some embodiments, the average particle size of the lithium iron phosphate particles is 0.8 μm to 2 μm.

[0010] In some embodiments, the average particle size of the lithium iron phosphate particles is 0.3 μm to 0.5 μm.

[0011] In some embodiments, the volume percentage of large lithium iron phosphate particles in the lithium iron phosphate cathode material is 55% to 70%.

[0012] In some embodiments, the volume percentage of the lithium iron phosphate particles in the lithium iron phosphate cathode material is 30% to 45%.

[0013] According to another aspect of this application, embodiments of this application provide a method for preparing a lithium iron phosphate cathode material, comprising the following steps: Iron phosphate, lithium source, first carbon source and dopant source are added to solvent for the first grinding, followed by the first drying and the first sintering to obtain a doped lithium iron phosphate core; the dopant source includes one or more of the following: transition metal compound, alkaline earth metal compound or non-metal element compound. The doped lithium iron phosphate core, the second carbon source, the phosphorus source and the niobium source are added to a solvent for a second grinding, followed by a second drying and a second sintering to obtain the lithium iron phosphate cathode material. The lithium iron phosphate cathode material includes a doped lithium iron phosphate core, a first coating layer covering at least a portion of the surface of the doped lithium iron phosphate core, and a second coating layer covering at least a portion of the surface of the first coating layer. The first coating layer includes a niobium phosphide coating layer, and the second coating layer includes a carbon coating layer.

[0014] In some of these embodiments, the preparation method satisfies at least one of the following features (1) to (9): (1) The iron-to-phosphorus ratio of the iron phosphate is 96.5% to 98%; (2) The lithium source includes at least one of lithium carbonate, lithium nitrate, lithium hydroxide, lithium oxalate, or lithium acetate; (3) The first carbon source includes at least one of glucose, sucrose, starch, polyvinyl alcohol, polyacryl alcohol, polyethylene glycol, carbon black, carbon fiber tube, graphite, or graphene; (4) The transition metal compound includes at least one of Nb-containing compounds, Ti-containing compounds, V-containing compounds, Zr-containing compounds, or Mo-containing compounds; (5) The alkaline earth gold-containing compound... The compounds include Mg-containing compounds; the non-metallic element-containing compounds include B-containing compounds; (6) the solvent includes water and / or alcohols; (7) the second carbon source includes at least one of glucose, sucrose, starch, polyvinyl alcohol, polyacryl alcohol, polyethylene glycol, carbon black, carbon fiber tube, graphite or graphene; (8) the phosphorus source includes at least one of iron phosphate, tetrabutylphosphine chloride, phosphoric acid or phosphine; (9) the niobium source includes at least one of Nb2O5, NbO2, NbO, NbCl5 or NbCl4.

[0015] In some embodiments, the preparation method satisfies at least one of the following features (1) to (8): (1) the molar ratio of the iron phosphate, lithium source, first carbon source and dopant source is 1:(1.03-1.04):(0.04-0.15):(0.002-0.01); (2) the molar ratio of the doped lithium iron phosphate core, second carbon source, phosphorus source and niobium source is 1:(0.04-0.2):(0.001-0.015):(0.001-0.015); (3) the first grinding method includes sand milling, wherein the particle size D of the sand milling is... v 50 is 0.3μm~0.5μm; (4) The first drying method includes spray drying; and / or, the second drying method includes spray drying; (5) The conditions for the first sintering include: heating to 200~400℃ at a heating rate of 1~10℃ / min, holding for 1~5h, then heating to 720~820℃ at a heating rate of 1~10℃ / min, and holding for 6~15h; (6) The carbon content of the doped lithium iron phosphate core obtained after the first sintering is 0.05%~0.8% (7) The second grinding method includes sand milling, which yields a first slurry and a second slurry with different particle sizes; the particle size Dv50 of the first slurry is 0.25μm to 0.45μm; the particle size Dv50 of the second slurry is 0.8μm to 1.2μm; (8) The conditions for the second sintering include: heating to 200 to 400℃ at a heating rate of 1 to 10℃ / min, holding for 1 to 5 hours, then heating to 720 to 820℃ at a heating rate of 1 to 10℃ / min, and holding for 6 to 15 hours.

[0016] In some of these embodiments, the mass ratio of the first slurry to the second slurry is (20-30):(70-80).

[0017] According to another aspect of this application, an embodiment of this application provides a positive electrode sheet, which includes: the aforementioned lithium iron phosphate positive electrode material, and / or the lithium iron phosphate positive electrode material obtained by the aforementioned preparation method.

[0018] In some embodiments, the compaction density of the positive electrode sheet is 2.6–2.8 g / cm³. 3 .

[0019] In some embodiments, the positive electrode includes a current collector and an active layer disposed on at least one surface of the current collector, the active layer including the lithium iron phosphate positive electrode material, a conductive agent, a binder, and a dispersant.

[0020] In some of these embodiments, the mass ratio of the lithium iron phosphate cathode material, conductive agent, binder and dispersant is (96-98):(0-1):(1-4):(0-1).

[0021] According to another aspect of this application, an embodiment of this application provides a battery including the aforementioned positive electrode.

[0022] Implementing the technical solution of the present invention has at least the following beneficial effects: In this embodiment, the provided lithium iron phosphate cathode material has a core-shell structure. The core is a doped lithium iron phosphate core, where the doping element includes one or more of alkaline earth metals, transition metals, or non-metallic elements. The outer shell or shell layer includes a first coating layer and a second coating layer. The first coating layer includes a niobium phosphide coating layer, and the second coating layer includes a carbon coating layer. Therefore, the doped lithium iron phosphate core can provide abundant valence electrons, thereby affecting the electronic environment of the transition metal. This can increase the number of d-orbital electrons in the system, causing the valence band and conduction band to shift towards the Fermi level, thus reducing the band gap and improving electronic conductivity. Furthermore, high-valence doped atoms easily form more vacancy defects in the crystal lattice, widening the lithium-ion transport channels and improving ion diffusion. The niobium phosphide coating prevents small particles from fusing into excessively large primary particles during sintering, inhibiting further abnormal growth of large particles that could negatively impact specific capacity, low-temperature performance, and rate capability. Furthermore, niobium phosphide, even in its ultra-thin form, exhibits high conductivity, significantly improving the kinetics of large lithium iron phosphate particles and allowing them to achieve higher specific capacity. This results in a balance between compaction density and low-temperature rate capability in high-compact lithium iron phosphate materials. Simultaneously, the combined effect of the niobium phosphide and carbon coatings significantly reduces the resistivity of lithium iron phosphate powder. In high-compact lithium iron phosphate applications, this reduces or eliminates the need for conductive agents, further increasing the content of active material in the electrode and the compaction density, achieving a simultaneous improvement in both volumetric and gravimetric energy density.

[0023] Therefore, this lithium iron phosphate cathode material with a double-layer coating can achieve good cycle performance and low-temperature performance, and has good application prospects.

[0024] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0025] Figure 1 The image shown is a SEM image of the lithium iron phosphate cathode material provided in an embodiment of the present invention.

[0026] Figure 2 The figures shown are discharge curves of full cells made from lithium iron phosphate cathode materials provided in Example 1 and Comparative Example 1 of the present invention at different temperatures. Detailed Implementation

[0027] The present application will be further described below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the present application.

[0028] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges or individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0029] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions. Unless otherwise specified, all technical features and optional technical features of this application can be combined to form new technical solutions.

[0030] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0031] Unless otherwise specified, the terms "comprising" and "including" as used in this application 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.

[0032] In related technologies, common improvement methods for increasing the compaction density of lithium iron phosphate powder mainly include: raw material processing, adjusting the sintering regime, and particle size distribution. The compaction density of lithium iron phosphate powder is primarily achieved through the sintering and gradation steps in the process flow. However, the presence of large particles leads to poor low-temperature rate performance. Further improvements to high-compact lithium iron phosphate can be achieved by increasing the carbon coating or reducing the content of large particles. While this can improve low-temperature performance to some extent, it also affects the compaction density. Furthermore, higher carbon content results in higher residual carbon content, and a small amount of residual carbon can directly contact the electrolyte, increasing side reactions and affecting battery cycle performance. In other words, existing methods for increasing the compaction density of lithium iron phosphate materials may affect the low-temperature rate performance, while methods for improving low-temperature performance may in turn affect the compaction density, making it difficult to simultaneously consider or balance the relationship between the compaction density and low-temperature performance of lithium iron phosphate.

[0033] Based on the above research, it is necessary to fully consider the characteristics of existing lithium iron phosphate materials and provide a lithium iron phosphate material with high powder compaction density. This material should improve the compaction density while minimizing or avoiding the impact on the material's low-temperature performance, effectively balancing the relationship between compaction density and low-temperature performance. This would achieve a balance between compaction density and low-temperature performance in high-compact lithium iron phosphate materials, thus alleviating the aforementioned problems. Therefore, this application provides a lithium iron phosphate cathode material, its preparation method, a cathode sheet, and a battery. The following is a detailed description of this application.

[0034] [Lithium iron phosphate cathode material] In some embodiments, a lithium iron phosphate cathode material is provided, the lithium iron phosphate cathode material comprising: a core, a first coating layer covering at least a portion of the surface of the core, and a second coating layer covering at least a portion of the surface of the first coating layer; The core is a doped lithium iron phosphate core, and the doping elements in the doped lithium iron phosphate core include one or more of alkaline earth metals, transition metals or non-metallic elements. A first coating layer is applied to at least a portion of the surface of a doped lithium iron phosphate core, the first coating layer comprising a niobium phosphide coating layer (NbP coating layer). A second coating layer covers at least a portion of the surface of the first coating layer, the second coating layer including a carbon coating layer (C coating layer).

[0035] It should be noted that, in this application, the term "coating" is not limited to direct coating, but also includes indirect coating. For example, when a first coating layer coats a doped lithium iron phosphate core, it may mean that there are no other structures between the first coating layer and the outer surface of the doped lithium iron phosphate core, or it may mean that there are one or more other structures between the first coating layer and the outer surface of the doped lithium iron phosphate core. Preferably, there are no other structures between the first coating layer and the outer surface of the doped lithium iron phosphate core. Similarly, when a second coating layer coats a first coating layer, it may mean that there are no other structures between the second coating layer and the outer surface of the first coating layer, or it may mean that there are one or more other structures between the second coating layer and the outer surface of the first coating layer.

[0036] The first coating layer is formed or coated on at least a portion of the surface of the doped lithium iron phosphate core, which can protect or improve the lithium iron phosphate cathode material. It can be used to improve the conductivity and dynamic performance of the material, allowing the material to achieve a higher specific capacity, and achieving a balance between the compaction density and low-temperature rate performance of the high-compact lithium iron phosphate material. The first coating layer being formed on at least a portion of the surface of the doped lithium iron phosphate core means that the first coating layer can completely encapsulate the doped lithium iron phosphate core within the first coating layer, or the first coating layer can only coat a portion of the outer surface of the doped lithium iron phosphate core; that is, the first coating layer can completely coat the doped lithium iron phosphate core, or it can coat a portion of the surface of the doped lithium iron phosphate core, preferably completely coating it.

[0037] The second coating layer is formed or covers at least a portion of the surface of the first coating layer. The second coating layer works synergistically with the first coating layer to reduce the resistivity of lithium iron phosphate powder. This reduces the need for conductive agents during the use of lithium iron phosphate cathode materials, thereby increasing the content and compaction density of lithium iron phosphate cathode material in the cathode sheet, improving volumetric and gravimetric energy density, and ultimately enhancing the electrochemical performance of the cathode material. The second coating layer forming at least a portion of the surface of the first coating layer means that the second coating layer can completely encapsulate the first coating layer within itself, or it can be that the second coating layer only covers a portion of the outer surface of the first coating layer; that is, the second coating layer can completely cover the first coating layer, or it can cover a portion of the surface of the first coating layer, preferably completely covering it.

[0038] In a preferred embodiment of this application, the doping element in the doped lithium iron phosphate core includes at least a transition metal, especially a transition metal with a high valence state, and more preferably, the doping element includes at least niobium (Nb).

[0039] To address the challenges of controlling large particle size and poor low-temperature performance in high-compaction lithium iron phosphate (LFP) materials, this invention addresses these issues by controlling the raw materials and processes of the LFP cathode material. Transition metal elements, such as niobium, are introduced into the LFP material. Niobium bulk doping can generate lattice defects, reduce band gaps, and broaden lithium-ion transport channels. It can also form niobium phosphide on the LFP surface. Niobium phosphide exhibits excellent conductivity even in ultra-thin states, reducing the negative impact on LFP material compaction and inhibiting particle fusion to form large particles. It also limits particle growth and prevents the formation of abnormally large particles, thus achieving a balance between compaction density and low-temperature performance in high-compaction LFP materials.

[0040] Specifically, the provided cathode material has a core-shell structure. The provided lithium iron phosphate cathode material has a core-shell structure, with its core being a doped lithium iron phosphate core. The outer shell or shell layer includes a first coating layer and a second coating layer. The first coating layer includes a niobium phosphide coating layer, and the second coating layer includes a carbon coating layer. Thus, the doped lithium iron phosphate core, such as the high-valence Nb... 5+ Doping provides abundant valence electrons, thus affecting the electronic environment of transition metals. It can increase the number of d-orbital electrons in the system, causing the valence and conduction bands to shift towards the Fermi level, thereby reducing the band gap and improving electronic conductivity. Furthermore, high-valence doped atoms tend to form more vacancy defects in the crystal lattice, widening lithium-ion transport channels and improving ion diffusion. Niobium phosphide coating can prevent small particles from fusing into excessively large primary particles during sintering, inhibiting further abnormal growth of large particles that affects specific capacity, low-temperature performance, and rate performance. On the other hand, niobium phosphide also possesses high conductivity even in ultra-thin cases, significantly improving the kinetic performance of large lithium iron phosphate particles, allowing them to achieve higher specific capacity. This achieves a balance between compaction density and low-temperature rate performance in high-compact lithium iron phosphate materials. Meanwhile, the carbon coating also possesses high conductivity, which improves kinetic performance. The combined effect of the niobium phosphide coating and the carbon coating significantly reduces the resistivity of lithium iron phosphate powder. In high-compaction lithium iron phosphate applications, this reduces or eliminates the need for conductive agents, further increasing the content of active material and compaction density of the electrode, achieving a simultaneous increase in both volumetric and gravimetric energy density. Furthermore, this multi-layered structure ensures interfacial stability, thereby enabling long-cycle performance of the cathode material.

[0041] Through in-depth research, the inventors have discovered that, in addition to meeting the above design conditions, the performance of the lithium iron phosphate cathode material can be further improved if it can optionally meet one or more of the following conditions.

[0042] In some embodiments, the general chemical formula of the doped lithium iron phosphate core is LiM x Fe （1-x）PO4; where M is the dopant element, and the value of x ranges from 0.005 to 0.02; for example, x can be 0.005, 0.008, 0.01, 0.012, 0.015, 0.02, etc.

[0043] In the above general chemical formula, M is a dopant element, that is, M can include one or more of alkaline earth metals, transition metals or non-metallic elements; as an example, the alkaline earth metals include, but are not limited to, Mg, the transition metals include, but are not limited to, one or more of Nb, Ti, V, Zr and Mo, and the non-metallic elements include, but are not limited to, B.

[0044] In some embodiments, M includes, but is not limited to, at least one of Mg, Nb, Ti, V, Zr, Mo, or B. Preferably, M includes at least Nb.

[0045] In this embodiment, the dopant element M preferably includes at least Nb, and further may include one or more of Mg, Ti, V, Zr, Mo, or B. As an example, M may be selected from Nb; or M may be selected from Nb and Zr; or M may be selected from Nb, Mg, and B; or M may be selected from Nb and B; or M may be selected from Nb and Ti; or M may be selected from Nb, Mo, and B, etc.

[0046] Optionally, the doped lithium iron phosphate core has a doping amount of Nb of 0.05% to 0.2%.

[0047] Therefore, by introducing niobium into the core of doped lithium iron phosphate, the bulk doping of niobium can generate lattice defects, reduce band gaps, and widen lithium-ion transport channels. It can also form niobium phosphide on the surface of lithium iron phosphate, which is more conducive to achieving both compaction density and low-temperature performance of high-compact lithium iron phosphate materials.

[0048] In some embodiments, the thickness of the first coating layer, namely the niobium phosphide coating layer, in the lithium iron phosphate cathode material is 1 nm to 5 nm. As an example, the thickness of the niobium phosphide coating layer can be any one of 1 nm, 2 nm, 3 nm, 4 nm, and 5 nm, or a range between any two.

[0049] In this embodiment, the niobium phosphide coating layer needs to be relatively thin, such as within the range of 1 nm to 5 nm. This is because niobium phosphide is a material that exhibits excellent conductivity even in an ultra-thin state. By controlling its thickness within the ultra-thin range of 1 nm to 5 nm, the negative impact on the compaction of lithium iron phosphate material can be reduced, the fusion of particles to form large particles can be inhibited, particle growth can be limited to prevent the formation of abnormally large particles, and the conductivity of the material can be improved, thereby enhancing the kinetic performance. If the thickness is greater than 5 nm, it may have a negative impact on the compaction of lithium iron phosphate material. Therefore, the thickness of the niobium phosphide coating layer needs to be set reasonably.

[0050] In some embodiments, the thickness of the second coating layer, i.e., the carbon coating layer, in the lithium iron phosphate cathode material is 1 nm to 6 nm. As an example, the thickness of the carbon coating layer can be any one of 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, and 6 nm, or a range between any two.

[0051] This application, by controlling the thickness of the second carbon coating layer within a suitable range, can improve the compaction density and conductivity of the cathode material while ensuring its high conductivity, thus enhancing its electrochemical performance. If the thickness of the second coating layer is too low, such as less than 1 nm, it will not effectively exert its modification effect and will not effectively improve conductivity, resulting in minimal improvement in cycle performance. If the thickness of the second coating layer is too high, such as greater than 6 nm, it will increase costs, and capacity, rate capability, and other performance characteristics may also be affected. Therefore, the thickness of the second coating layer needs to be set appropriately.

[0052] In some embodiments, the second coating layer in the lithium iron phosphate cathode material contains carbon elements, with a carbon element mass content of 1.0% to 1.6%. As an example, the carbon element mass content can be any one of 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, or 1.6%, or a range between any two.

[0053] Similarly, by adjusting the carbon content within a suitable range, it is possible to improve the compaction density and conductivity of the cathode material while ensuring that the cathode material maintains a high conductivity, which is more conducive to improving the electrochemical performance of the cathode material.

[0054] In some embodiments, the lithium iron phosphate cathode material includes large lithium iron phosphate particles and small lithium iron phosphate particles, wherein the particle size of the large lithium iron phosphate particles is larger than that of the small lithium iron phosphate particles.

[0055] In this embodiment, the lithium iron phosphate cathode material includes large lithium iron phosphate particles and small lithium iron phosphate particles. Those skilled in the art will understand that the particle size of large and small lithium iron phosphate particles is relative. That is, in this embodiment, the particle size of the large lithium iron phosphate particles is larger than that of the small lithium iron phosphate particles.

[0056] Optionally, the average particle size of the large lithium iron phosphate particles is 0.8 μm to 2 μm; as an example, the average particle size of the large lithium iron phosphate particles can be any one of 0.8 μm, 0.9 μm, 1 μm, 1.2 μm, 1.5 μm, 1.6 μm, 1.8 μm, and 2 μm, or a range between any two. By limiting the average particle size of the large lithium iron phosphate particles, it is possible to avoid the problem of large lithium iron phosphate particles cracking and breaking during long-term cycling, or undergoing side reactions with the electrolyte in the battery, thereby accelerating the cycle degradation of the battery.

[0057] Optionally, the average particle size of the lithium iron phosphate particles is 0.3 μm to 0.5 μm; as an example, the average particle size of the lithium iron phosphate particles can be any one of 0.3 μm, 0.35 μm, 0.4 μm, 0.45 μm, and 0.5 μm, or a range between any two. By limiting the average particle size of the lithium iron phosphate particles, the compaction density of the lithium iron phosphate cathode material can be increased, thereby allowing for better fusion of large and small lithium iron phosphate particles and improving the specific capacity.

[0058] The fusion of large and small lithium iron phosphate (LFP) particles can improve the cracking and breakage of large LFP particles during cycling, thereby improving the cycle performance of the battery. Thus, by fusing large and small LFP particles, the LFP cathode material can maintain good porosity even at high compaction densities, resulting in higher energy density and superior kinetic or cycle performance.

[0059] In this application, the large lithium iron phosphate particles and the small lithium iron phosphate particles may be the same in composition, and the main difference is in the particle size range or morphology.

[0060] Optionally, in the lithium iron phosphate cathode material, the volume percentage of large lithium iron phosphate particles is 55% to 70%; as an example, the volume percentage of large lithium iron phosphate particles can be any one of 55%, 60%, 65%, 70% or any range between two.

[0061] Optionally, in the lithium iron phosphate cathode material, the volume percentage of lithium iron phosphate particles is 30% to 45%; as an example, the volume percentage of lithium iron phosphate particles can be any one of 30%, 35%, 40%, and 45%, or any range between two.

[0062] By controlling the volume ratio of large and small lithium iron phosphate particles within the aforementioned range, it is possible to fill the gaps between large particles with small particle size, forming a multi-stage filling, thereby achieving high compaction density, or achieving a high compaction density to ensure volumetric energy density, and ensuring the rate capability or low-temperature performance of high-compact lithium iron phosphate cathode materials.

[0063] In some embodiments, the overall particle size D of the lithium iron phosphate cathode material v 50 is 0.8μm~1.5μm; as an example, the particle size D of lithium iron phosphate cathode material v 50 can be any one or any two of the following values: 0.8μm, 0.9μm, 1.0μm, 1.1μm, 1.2μm, 1.3μm, 1.4μm, and 1.5μm.

[0064] The particle size D of lithium iron phosphate cathode material v The value 50 has a meaning known in the art and can be tested using methods known in the art. For example, a laser particle size analyzer can be used for testing.

[0065] By adjusting the particle size D of the lithium iron phosphate cathode material v By controlling the concentration of lithium ions within the above range, the compaction density of the cathode material can be further improved while ensuring a high lithium ion transport rate.

[0066] In some embodiments, the powder compaction density of the lithium iron phosphate cathode material is 2.5–2.7 g / cm³. 3 As an example, the powder compaction density of lithium iron phosphate cathode material can be 2.5 g / cm³. 3 2.55g / cm 3 2.6g / cm 3 2.65g / cm 3 2.7g / cm 3 The value of any one of the points or the range between any two.

[0067] Powder compaction density is a well-known concept in the art and can be tested using methods known in the art. During the compression process of powder under external force, as the powder moves and deforms, larger voids are filled, the contact area between particles increases, generating attractive forces between atoms and enhancing the mechanical wedging effect between particles, thereby forming a compact with a certain strength. The density of this compact is the powder compaction density.

[0068] By limiting the range of powder compaction density of lithium iron phosphate cathode materials, it is possible to better balance the requirements of energy density and low-temperature fast charging, thereby achieving optimal application results.

[0069] In some embodiments, the powder resistivity of the lithium iron phosphate cathode material is... As an example, the powder resistivity of lithium iron phosphate cathode material can be... , , , , , , , The value of any one of the points or the range between any two.

[0070] By limiting the powder resistivity of lithium iron phosphate cathode materials, the effectiveness of carbon coating and particle contact of lithium iron phosphate materials is ensured, thereby improving electronic conductivity, rate performance, and interfacial reaction kinetics, resulting in better power characteristics, energy density, and cycle stability.

[0071] Therefore, based on the above settings, compared with existing lithium iron phosphate materials, the lithium iron phosphate cathode material provided by the present invention can achieve both high compaction density and low-temperature performance of high compaction lithium iron phosphate materials, improve the conductivity and kinetic performance of the material, and achieve the effect of simultaneously improving volumetric and gravimetric energy density.

[0072] [Preparation method of lithium iron phosphate cathode material] In some embodiments, this application provides a method for preparing a lithium iron phosphate cathode material, the method comprising the following steps: The iron phosphate, lithium source, first carbon source and dopant source are added to a solvent for the first grinding, followed by the first drying and the first sintering to obtain the doped lithium iron phosphate core; the dopant source includes one or more of the following: transition metal compounds, alkaline earth metal compounds or non-metallic element compounds. The doped lithium iron phosphate core, the second carbon source, the phosphorus source and the niobium source are added to a solvent for a second grinding, followed by a second drying and a second sintering to obtain the lithium iron phosphate cathode material. The lithium iron phosphate cathode material includes a doped lithium iron phosphate core, a first coating layer covering at least a portion of the surface of the doped lithium iron phosphate core, and a second coating layer covering at least a portion of the surface of the first coating layer. The first coating layer includes a niobium phosphide coating layer, and the second coating layer includes a carbon coating layer.

[0073] The method for preparing lithium iron phosphate cathode material provided by the present invention can first use a one-time sintering process to prepare a doped lithium iron phosphate core, then use a two-time sintering process to form a first coating layer on the surface of the doped lithium iron phosphate core, and then form a second coating layer on the surface of the first coating layer to obtain the lithium iron phosphate cathode material.

[0074] The preparation method provided in this invention is simple, easy to operate, highly feasible, and easy to industrialize. This method can be used to prepare lithium iron phosphate cathode materials, which improves the compaction density of the material and also improves the low-temperature performance to a certain extent.

[0075] It should be understood that all the features and advantages described above regarding "lithium iron phosphate cathode material" also apply to the "preparation method of lithium iron phosphate cathode material", and will not be repeated here.

[0076] In some specific embodiments, the preparation method of the lithium iron phosphate cathode material specifically includes the following steps (a) to (b): (a) Preparation of doped lithium iron phosphate core.

[0077] In step (a), iron phosphate, lithium source, first carbon source and dopant source are dispersed in solvent, ground for the first time, dried for the first time, such as spray drying, to obtain spray balls, and then sintered for the first time to obtain doped lithium iron phosphate core.

[0078] Optionally, in step (a), ferric phosphate may comprise one or more types, such as single-type or multi-type ferric phosphate. As an example, ferric phosphate can be synthesized via different synthetic routes, such as the ammonium method, sodium method, and iron method. The morphology of ferric phosphate obtained from different routes will differ; for example, ferric phosphate synthesized via the ammonium and sodium methods is generally coral-like, while ferric phosphate synthesized via the iron method is generally lamellar. Therefore, the ferric phosphate may comprise single-morphology or multi-morphology ferric phosphate.

[0079] Optionally, the iron-to-phosphorus ratio of ferric phosphate is 96.5% to 98%; as an example, the iron-to-phosphorus ratio of ferric phosphate can be 96.5%, 97%, 97.5%, 98%, etc.

[0080] The aforementioned ferric phosphate can be obtained commercially or prepared by methods known in the art. This application does not limit the source of ferric phosphate.

[0081] In some embodiments, in step (a), the lithium source includes, but is not limited to, at least one of lithium carbonate, lithium nitrate, lithium hydroxide, lithium oxalate, lithium acetate, lithium phosphate, or lithium dihydrogen phosphate. Preferably, the lithium source is selected from one or more of lithium carbonate, lithium hydroxide, lithium nitrate, or lithium acetate.

[0082] It should be noted that the lithium source includes lithium-containing salts, and is not limited to the substances listed above. It can be used to provide lithium elements and does not limit the purpose of this invention. Other similar lithium salts can also be used in this invention.

[0083] In some embodiments, in step (a), the first carbon source includes, but is not limited to, at least one of glucose, sucrose, starch, polyvinyl alcohol, polyacrylol, polyethylene glycol, carbon black, carbon fiber tube, graphite, or graphene. The first carbon source is preferably one or more of glucose, sucrose, starch, polyethylene glycol, carbon black, carbon fiber tube, graphite, or graphene.

[0084] In this embodiment, a small amount of the first carbon source is added in step (a), which mainly serves as a reducing agent. Furthermore, the method in this embodiment employs a secondary sintering process. In the primary sintering process, iron phosphate, a lithium source, a carbon source, and a dopant source are added. The iron phosphate, lithium source, and carbon source react to obtain lithium iron phosphate, and bulk doping is completed by adding the dopant source.

[0085] In some embodiments, in step (a), the doping source includes one or more of transition metal compounds, alkaline earth metal compounds, or non-metallic element compounds; wherein, the transition metal compounds include at least one of Nb compounds, Ti compounds, V compounds, Zr compounds, or Mo compounds; the alkaline earth metal compounds include Mg compounds; and the non-metallic element compounds include B compounds.

[0086] As an example, the doping source includes one or more of Nb-containing compounds, Ti-containing compounds, V-containing compounds, Zr-containing compounds, Mo-containing compounds, Mg-containing compounds, or B-containing compounds. Preferably, the doping source includes at least an Nb-containing compound; furthermore, the doping source may also include one or more of Ti-containing compounds, V-containing compounds, Zr-containing compounds, Mo-containing compounds, Mg-containing compounds, or B-containing compounds.

[0087] Optionally, the Nb-containing compound includes at least one of Nb₂O₅, NbO₂, NbO, NbCl₅, or NbCl₄.

[0088] In this embodiment, the Nb-containing compound can be an oxide of Nb or a chloride of Nb, etc. Similarly, Ti-containing compounds, V-containing compounds, Zr-containing compounds, Mo-containing compounds, Mg-containing compounds, or B-containing compounds can be their oxides or chlorides, etc.

[0089] In some embodiments, in step (a), the solvent includes water and / or alcohols; the alcohols may be, for example, conventional alcohol solvents such as methanol and ethanol. Preferably, the solvent in step (a) is water.

[0090] In some embodiments, in step (a), the molar ratio of iron phosphate, lithium source, first carbon source and dopant source is 1:(1.03~1.04):(0.04~0.15):(0.002~0.01); for example, it can be 1:1.03:0.04:0.002, 1:1.03:0.06:0.005, 1:1.03:0.1:0.008, 1:1.03:0.15:0.01, 1:1.04:0.04:0.002, 1:1.04:0.015:0.01, etc.

[0091] By controlling the molar ratio of iron phosphate, lithium source, first carbon source and dopant source within the range of 1:(1.03~1.04):(0.04~0.15):(0.002~0.01), it is beneficial to obtain the doped lithium iron phosphate core with the required doping amount, meet the doping requirements, and improve the doping effect.

[0092] In some embodiments, in step (a), the first grinding method includes sand milling, where the particle size D is... v 50 is 0.3μm to 0.5μm.

[0093] In this embodiment, the raw materials are ground and mixed using a sand mill, and the particle size D of the resulting slurry is... v 50 represents a particle size of 0.3μm to 0.5μm, for example, 0.3μm, 0.4μm, 0.5μm, etc. Thus, by controlling the particle size obtained after the first grinding within this range, the particle size of the final product, lithium iron phosphate cathode material, can be effectively controlled. If it exceeds this range, the particle size of the final product, lithium iron phosphate cathode material, will increase, and the particles will also grow during the sintering process, thereby affecting the specific capacity, low-temperature performance, and cycle performance of the cathode material.

[0094] In some embodiments, the first drying method in step (a) includes spray drying; In some embodiments, the conditions for the first sintering in step (a) include: heating to 200–400°C at a heating rate of 1–10°C / min, holding at that temperature for 1–5 hours, and then heating to 720–820°C at the same heating rate, such as 1–10°C / min, and holding at that temperature for 6–15 hours. For example, the heating rate can be 1°C / min, 2°C / min, 3°C / min, 5°C / min, 6°C / min, 8°C / min, 10°C / min, etc.; the initial heating temperature can be 200°C, 250°C, 300°C, 350°C, 400°C, etc., and the holding time can be 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, etc.; the subsequent heating temperature can be 720°C, 750°C, 760°C, 780°C, 800°C, 820°C, etc., and the holding time can be 6 hours, 8 hours, 10 hours, 12 hours, 15 hours, etc.

[0095] In the synthesis of lithium iron phosphate cathode materials, the first sintering conditions have a certain impact on the final product. For example, if the sintering temperature is too high, it will damage the crystal structure of lithium iron phosphate, increase side reactions, and reduce the uniformity of the particles. By limiting the sintering conditions for preparing doped lithium iron phosphate cores, the particle size of the doped lithium iron phosphate cores can be well controlled, which is beneficial to improving the performance of the final lithium iron phosphate cathode material.

[0096] In some embodiments, in step (a), the carbon content of the doped lithium iron phosphate core obtained after the first sintering is 0.05% to 0.8%; for example, it can be 0.05%, 0.08%, 0.1%, 0.2%, 0.5%, 0.8%, etc.

[0097] (b) Preparation of lithium iron phosphate cathode material.

[0098] In step (b), the doped lithium iron phosphate core, second carbon source, phosphorus source and niobium source obtained in step (a) are uniformly mixed in an appropriate solvent such as water, and then ground for a second time to obtain slurries with different particle sizes, such as first slurry and second slurry with different particle sizes. The first slurry and the second slurry are mixed in a suitable ratio, and then dried for a second time, such as spray drying, and then sintered for a second time and pulverized to obtain lithium iron phosphate cathode material.

[0099] In some embodiments, in step (b), the second carbon source includes, but is not limited to, at least one of glucose, sucrose, starch, polyvinyl alcohol, polyacrylol, polyethylene glycol, carbon black, carbon fiber tube, graphite, or graphene. The second carbon source is preferably one or more of glucose, sucrose, starch, polyethylene glycol, carbon black, carbon fiber tube, graphite, or graphene.

[0100] In this embodiment, the first carbon source added in step (a) and the second carbon source added in step (b) can be of the same type or different types, but preferably the same type.

[0101] In some embodiments, in step (b), the phosphorus source includes, but is not limited to, at least one of iron phosphate, tetrabutylphosphine chloride, phosphoric acid, or phosphine.

[0102] In some embodiments, in step (b), the niobium source, i.e., the niobium-containing compound, includes, but is not limited to, at least one of Nb2O5, NbO2, NbO, NbCl5, or NbCl4.

[0103] In step (b), by adding a phosphorus source and a niobium source, the phosphorus source and the niobium source can react to obtain niobium phosphide, which in turn forms a first coating layer on the surface of the doped lithium iron phosphate core, namely the niobium phosphide coating layer.

[0104] In some embodiments, in step (b), the molar ratio of the doped lithium iron phosphate core, the second carbon source, the phosphorus source, and the niobium source is 1:(0.04~0.2):(0.001~0.015):(0.001~0.015); for example, it can be 1:0.04:0.001:0.001, 1:0.04:0.008:0.008, 1:0.06:0.008:0.01, 1:0.1:0.008:0.006, 1:0.2:0.015:0.015, etc.

[0105] In some embodiments, in step (b), the second grinding method includes sand milling, which yields a first slurry and a second slurry with different particle sizes; the particle size D of the first slurry... v 50 is 0.25μm to 0.45μm, for example, it can be 0.25μm, 0.3μm, 0.35μm, 0.4μm, 0.45μm, etc.; the particle size D of the second slurry v 50 is 0.8μm to 1.2μm, for example, it can be 0.8μm, 0.9μm, 1.0μm, 1.2μm, etc. Preferably, the mass ratio of the first slurry to the second slurry is (20 to 30): (70 to 80).

[0106] By forming a first slurry and a second slurry with different particle sizes in a second grinding process, such as sand milling, it is possible to ultimately obtain large and small lithium iron phosphate particles with different particle sizes. Furthermore, by limiting the abrasive particle size of the first and second slurries during the sand milling process, the particle size of the final product—large and small lithium iron phosphate particles—can be effectively controlled. If the particle size exceeds the specified range, the final product will have a larger particle size, and the particles will also grow larger during sintering, thus affecting the specific capacity, low-temperature performance, and cycle performance of the cathode material.

[0107] In some embodiments, the second drying method in step (b) includes spray drying.

[0108] In some embodiments, the conditions for the second sintering in step (b) include: heating to 200–400°C at a heating rate of 1–10°C / min, holding at that temperature for 1–5 hours, and then heating again to 720–820°C at the same heating rate, such as 1–10°C / min, and holding at that temperature for 6–15 hours. As an example, the heating rate can be 1°C / min, 2°C / min, 3°C / min, 5°C / min, 6°C / min, 8°C / min, 10°C / min, etc.; the initial heating temperature can be 200°C, 250°C, 300°C, 350°C, 400°C, etc., and the holding time can be 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, etc.; the subsequent heating temperature can be 720°C, 750°C, 760°C, 780°C, 800°C, 820°C, etc., and the holding time can be 6 hours, 8 hours, 10 hours, 12 hours, 15 hours, etc.

[0109] In the synthesis of lithium iron phosphate cathode materials, the second sintering conditions have a certain impact on the final product. For example, if the sintering temperature is too high, it will damage the crystal structure of lithium iron phosphate, increase side reactions, and reduce the uniformity of the particles. By limiting the sintering conditions for preparing doped lithium iron phosphate cores, the formation conditions of the first and second coating layers can be well controlled, which is beneficial to improving the performance of the final lithium iron phosphate cathode material.

[0110] The preparation method of the lithium iron phosphate cathode material provided in this application embodiment is simple, feasible, efficient and environmentally friendly, and easy to achieve large-scale production.

[0111] [Positive electrode tablets] This invention also provides a positive electrode sheet, which includes: lithium iron phosphate positive electrode material according to any embodiment of the present invention, and / or lithium iron phosphate positive electrode material obtained by any preparation method according to any embodiment of the present invention.

[0112] The positive electrode includes the lithium iron phosphate positive electrode material provided in the embodiments of this application. Therefore, the positive electrode also has the characteristics of excellent electrochemical performance.

[0113] In some embodiments, the positive electrode includes a current collector and an active layer disposed on at least one surface of the current collector; the active layer includes the aforementioned lithium iron phosphate positive electrode material. Further, the positive electrode active material layer may also include an optional conductive agent, a binder, and an optional dispersant.

[0114] Optionally, the compaction density of the cathode sheet prepared from the lithium iron phosphate cathode material is 2.6–2.8 g / cm³. 3 For example, it can be 2.6 g / cm³. 3 2.7g / cm 3 2.8g / cm 3 wait.

[0115] This application does not impose any particular restrictions on the material of the current collector in the positive electrode sheet, as long as it can achieve the purpose of this application, it can be selected according to actual needs. For example, in some embodiments, the positive electrode current collector may include aluminum foil, aluminum alloy foil, etc. Of course, in other embodiments, composite current collectors (such as aluminum-carbon composite current collectors) may also be used.

[0116] In some embodiments, the mass ratio of lithium iron phosphate cathode material, conductive agent, binder, and dispersant in the active layer is (96-98):(0-1):(1-4):(0-1). In this embodiment, the mass content of lithium iron phosphate cathode material in the active layer is 96%-98%; the mass content of conductive agent is 0%-1%; the mass content of phosphorus binder is 1%-4%; and the mass content of dispersant is 0%-1%.

[0117] It should be noted that the lithium iron phosphate in this application has high conductivity, thus reducing the content of conductive agent in the active layer, or even eliminating the need for conductive agent addition. That is, the mass content of conductive agent in the active layer can be 0, or greater than 0 and less than or equal to 1%. Therefore, this application can reduce or even eliminate the use of conductive agent in the high-compaction lithium iron phosphate cathode material process, further increasing the active material content and compaction density in the active layer, achieving a simultaneous increase in both volumetric and gravimetric energy density.

[0118] In addition, the dispersant in the active layer can be added or not added depending on the actual situation. For example, the mass content of the dispersant in the active layer can be 0, or it can be greater than 0 and less than or equal to 1%.

[0119] This application does not impose any particular limitation on the types of conductive agents, binders, and dispersants in the active layer, as long as they can achieve the purpose of this application. For example, in some embodiments, the binder may include, but is not limited to, one or more of polyacrylate, polyimide, polyvinyl alcohol, polyamide, polyamide-imide, polyvinylidene fluoride, polyacrylonitrile, polytetrafluoroethylene, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, or sodium carboxymethyl cellulose. The conductive agent may include, but is not limited to, at least one of conductive carbon black (such as acetylene black, Ketjen black), carbon nanotubes (CNTs), carbon fibers, graphene, etc. The aforementioned carbon nanotubes may be single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The dispersant may be polyvinylpyrrolidone (PVP), polyacrylic acid (PAA), etc.

[0120] [Battery] This invention also provides a battery comprising: a positive electrode sheet according to any of the above embodiments of this invention.

[0121] In this embodiment of the invention, the battery also includes a negative electrode, a separator, and an electrolyte; and the specific type of battery is not limited. For example, the battery can be any one of a cylindrical lithium battery, a pouch lithium battery, or an aluminum-cased lithium battery.

[0122] The battery includes the lithium iron phosphate cathode material provided in the embodiments of this application. Therefore, the battery, like a lithium-ion secondary battery, also has excellent electrochemical performance, such as long cycle life, high compaction density, and good low-temperature performance.

[0123] Since the battery provided in this embodiment of the invention adopts all the technical solutions of the above embodiments, it has at least all the beneficial effects brought about by the technical solutions of the above embodiments, which will not be repeated here.

[0124] The following describes the implementation methods of this application. The implementation methods described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the implementation methods, they shall be performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents, materials, or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0125] Example 1 The preparation of lithium iron phosphate cathode material includes the following steps: (a) With an Fe / P ratio of 0.970 and a specific surface area of ​​10 m², 2 A mixture of iron phosphate and lithium carbonate with a D50 of 6 μm and a lithium-iron ratio of 1.038 was prepared. Glucose and polyethylene glycol (PEG, molecular weight 6000) were then mixed in a 1:1 mass ratio to form the first carbon source. The molar ratio of iron phosphate to the first carbon source was 1:0.05. The molar ratio of the added dopant source to iron phosphate was 0.005:1. The molar ratio of titanium dioxide to niobium oxide was 3:2. After being mixed evenly in an aqueous solvent, the mixture underwent a first grinding process, i.e., a sand milling process, with a particle size Dv50 of 0.38 μm. After spray drying, the mixture underwent a first sintering process. The first sintering was carried out by increasing the temperature at 2℃ / min to 350℃ and sintering for 2 hours, followed by increasing the temperature at 5℃ / min to 750℃ and sintering for 10 hours. After the first sintering, a doped lithium iron phosphate core (lithium iron phosphate precursor) was obtained.

[0126] (b) The lithium iron phosphate precursor, polyethylene glycol, phosphine, and niobium pentoxide were mixed uniformly in a ratio of 1:0.07:0.0012:0.0012. After a second grinding, i.e., a second sand milling, slurries with different particle sizes were obtained. The first slurry, i.e., the second sand milling slurry, had a small particle size Dv50 of 0.30 μm, and the second slurry, i.e., the large particle size Dv50, had a large particle size of 1.0 μm. The first slurry and the second slurry were graded and mixed uniformly in a ratio of 2:8. After spray drying, a second sintering was performed. The second sintering was carried out by heating to 350°C at a heating rate of 2°C / min and holding for 1 h, and then heating to 780°C at a heating rate of 5°C / min and holding for 6 h. After air jet pulverization, the lithium iron phosphate cathode material was obtained.

[0127] Example 2 The preparation method of lithium iron phosphate cathode material is similar to that in Example 1, except that: In step (a), the molar ratio of the added dopant source to iron phosphate is 0.004:1, and the molar ratio of titanium dioxide to niobium oxide is 3:1. In step (b), lithium iron phosphate precursor, polyethylene glycol, phosphine, and niobium pentoxide are mixed evenly in a ratio of 1:0.07:0.003:0.003; then a trace amount of citric acid is added. Citric acid acts as a catalyst to promote the uniform growth of niobium phosphide and reduce agglomeration.

[0128] Example 3 The preparation method of lithium iron phosphate cathode material is similar to that in Example 1, except that: In step (a), the molar ratio of the added dopant source to iron phosphate is 0.0035:1, and the molar ratio of titanium dioxide to niobium oxide is 6:1. In step (b), the lithium iron phosphate precursor, polyethylene glycol, phosphine, and niobium pentoxide are mixed in a ratio of 1:0.07:0.005:0.005.

[0129] Example 4 The preparation method of lithium iron phosphate cathode material is similar to that in Example 1, except that: In step (b), the lithium iron phosphate precursor, polyethylene glycol, phosphine, and niobium pentoxide are mixed evenly in a ratio of 1:0.07:0.003:0.003.

[0130] Example 5 The preparation method of lithium iron phosphate cathode material is similar to that in Example 1, except that: In step (a), the molar ratio of the added dopant source to iron phosphate is 0.005:1, and the molar ratio of magnesium oxide to niobium oxide is 3:2.

[0131] Example 6 The preparation method of lithium iron phosphate cathode material is similar to that in Example 1, except that: In step (a), the molar ratio of the added dopant source to iron phosphate is 0.004:1, and the molar ratio of boric acid to niobium oxide is 2:2.

[0132] Example 7 The preparation method of lithium iron phosphate cathode material is similar to that in Example 1, except that: In step (a), the particle size Dv50 of the first sand mill is 0.38 μm, and after spray drying, the first sintering is carried out. The first sintering is carried out by heating to 350℃ at 2℃ / min for 2h, and then heating to 730℃ at 5℃ / min for 10h.

[0133] In step (b), the first slurry, i.e. the secondary grinding slurry, has a small particle size Dv50 of 0.30 μm, and the second slurry, i.e. the large particle size Dv50, has a large particle size of 1.0 μm. The first slurry and the second slurry are mixed evenly in a gradation ratio of 3:7.

[0134] Example 8 The preparation method of lithium iron phosphate cathode material is similar to that in Example 1, except that: In step (a), the molar ratio of iron phosphate to the first carbon source is 1:0.08.

[0135] In step (b), the molar ratio of lithium iron phosphate precursor to polyethylene glycol is 1:0.06.

[0136] Comparative Example 1 The preparation of lithium iron phosphate cathode material includes the following steps: (a) With an Fe / P ratio of 0.970 and a specific surface area of ​​10 m², 2 A mixture of iron phosphate and lithium carbonate with a D50 of 6 μm and a lithium-iron ratio of 1.038 was prepared. Glucose and polyethylene glycol (PEG, molecular weight 6000) were then mixed in a 1:1 mass ratio to form a carbon source. The molar ratio of iron phosphate to the first carbon source was 1:0.05. Titanium dioxide, a dopant source, was added with a molar ratio of 0.003:1 to iron phosphate. The mixture was then thoroughly mixed in an aqueous solvent and milled once to a particle size Dv50 of 0.38 μm. After spray drying, the mixture underwent a first sintering process. The first sintering was performed by increasing the temperature at 2 °C / min to 350 °C and sintering for 2 h, followed by increasing the temperature at 5 °C / min to 750 °C and sintering for 10 h. The resulting lithium iron phosphate precursor was obtained after the first sintering.

[0137] (b) Polyethylene glycol (PEG) (molecular weight 6000) was mixed with lithium iron phosphate precursor and water solvent. The molar ratio of PEG to iron phosphate was 0.07:1. The mixed slurry was subjected to secondary sand milling to obtain slurries with different particle sizes. The first slurry, i.e., the secondary sand milled slurry, had a small particle size Dv50 of 0.30 μm, and the second slurry, i.e., the large particle size Dv50, had a large particle size of 1.0 μm. The first slurry and the second slurry were graded and mixed evenly in a ratio of 2:8. After spray drying, they were subjected to a second sintering. The second sintering was carried out by heating to 350°C at a heating rate of 2°C / min and holding for 1 h, and then heating to 780°C at a heating rate of 5°C / min and holding for 6 h. After air jet milling, lithium iron phosphate cathode material was obtained.

[0138] Performance testing 1. Battery manufacturing The prepared lithium iron phosphate cathode material, binder polyvinylidene fluoride (PVDF), and conductive agent conductive carbon black were mixed in a mass ratio of 97.8:1.7:0.5, and then added to N-methylpyrrolidone solvent (NMP). The mixture was stirred in a drying chamber to form a slurry, thus obtaining the cathode slurry. The cathode slurry was then uniformly coated onto the cathode current collector aluminum foil. After drying and cold pressing, the cathode sheet was obtained. The compacted density of the rolled electrode sheet was 2.65 g / cm³. 3 .

[0139] The negative electrode active material graphite, conductive agent conductive carbon, binder SBR and dispersant CMC are mixed in a mass ratio of 96.5:1.0:1.5:1.0, water is added, and the mixture is stirred in a drying room to form a slurry to obtain the negative electrode slurry. The negative electrode slurry is uniformly coated on the negative electrode current collector copper foil, and after drying, cold pressing and other processes, the negative electrode sheet is obtained.

[0140] The electrolyte was a 1 mol / L LiPF6 solution in EC+DEC+DMC in a volume ratio of 1:1:1.

[0141] The above-mentioned positive electrode sheet, negative electrode sheet, and separator are wound together to obtain a core. The core is then placed into an aluminum-plastic film, vacuum dried, electrolyte injected into the battery case, packaged, left to stand, formed, and tested for capacity to obtain a lithium-ion battery.

[0142] 2. Performance tests were conducted on the lithium iron phosphate cathode materials or batteries prepared in the above embodiments and comparative examples, specifically including: (1) Test of compaction density of lithium iron phosphate cathode material powder: Refer to the standard: GB / T 44330—2024 Determination of compaction density of lithium-ion battery cathode material powder.

[0143] (2) Resistivity test of lithium iron phosphate cathode material powder: Refer to standard: GB / T45324—2025 Determination of resistivity of lithium-ion battery cathode material powder.

[0144] (3) Lithium-ion battery 1C discharge specific capacity test: In the voltage range of 2.5-3.65V, charge and discharge at 1C / 1C for 1 week, record the battery capacity and calculate the specific capacity; Specifically, under normal temperature (25℃) conditions, charge the lithium-ion battery at 1C constant current to 3.65V, then charge at 3.65V constant voltage to a current of 0.05C, and let it rest for 5 minutes; The discharge capacity obtained by discharging at 1C to 2.5V is recorded as the 1C discharge specific capacity C0, and the specific capacity = capacity / weight of active material.

[0145] (3) Low temperature capacity retention test of lithium-ion battery (-20℃ capacity retention): The battery cell whose 1C discharge capacity C0 at room temperature was measured above was then charged to 3.65V at 1C constant current at 25℃, and then charged to 0.05C at a constant voltage of 3.65V. The fully charged battery was then left to stand at -20℃ for 2 hours, and then discharged to 2.0V at 1C constant current at -20℃. The discharge capacity at this time was recorded as C1. The capacity retention rate at -20℃ was calculated as follows: -20℃ capacity retention rate = (-20℃ discharge capacity C1) / (room temperature discharge capacity C0).

[0146] The test results are shown in Table 1.

[0147] Table 1 As can be seen from Table 1, the lithium iron phosphate material without niobium doping and niobium phosphide coating (Comparative Example 1) has a relatively high powder resistivity, resulting in poor specific capacity and low-temperature performance of the prepared cells. This may be related to the poor overall material kinetics. When niobium doping is combined with other doping elements and appropriate niobium phosphide coating, the lithium iron phosphate gradation effect is better, the large particle size is controlled, the powder resistivity is reduced under high powder compaction, and the cells exhibit higher initial specific capacity and higher capacity retention at -20℃.

[0148] The parts of this invention not described in detail are techniques known to those skilled in the art.

[0149] The basic principles of the present invention have been described above with reference to specific embodiments. However, it should be noted that the advantages, benefits, and effects mentioned in the present invention are merely examples and not limitations, and should not be considered as essential features of each embodiment of the present invention. Furthermore, the specific details disclosed above are for illustrative and facilitative purposes only, and are not limitations. These details do not limit the present invention to the necessity of employing the aforementioned specific details.

[0150] It should be noted that the terms "and / or" or " / " used herein are merely descriptions of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. The singular forms "a," "described," and "the" used in the embodiments of the invention and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0151] In the detailed description and claims, a list of items connected by the terms "at least one of," "at least one of," "at least one of," or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, then the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, then the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may contain a single element or multiple elements. Item B may contain a single element or multiple elements. Item C may contain a single element or multiple elements.

[0152] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A lithium iron phosphate cathode material, characterized in that, include: A doped lithium iron phosphate core, wherein the doping element in the doped lithium iron phosphate core includes one or more of alkaline earth metals, transition metals, or non-metallic elements; A first coating layer covers at least a portion of the surface of the doped lithium iron phosphate core, the first coating layer comprising a niobium phosphide coating layer; A second coating layer is applied to at least a portion of the surface of the first coating layer, the second coating layer comprising a carbon coating layer.

2. The lithium iron phosphate cathode material according to claim 1, characterized in that, The chemical formula of the doped lithium iron phosphate core is LiM x Fe （1-x） PO4; Wherein, M is a doping element, and M includes at least one of Mg, Nb, Ti, V, Zr, Mo or B; the value of x ranges from 0.005 to 0.02; Preferably, M includes at least Nb.

3. The lithium iron phosphate cathode material according to claim 1, characterized in that, The lithium iron phosphate cathode material satisfies at least one of the following characteristics (1) to (3): (1) In the lithium iron phosphate cathode material, the thickness of the niobium phosphide coating layer is 1 nm to 5 nm; (2) In the lithium iron phosphate cathode material, the thickness of the carbon coating layer is 1 nm to 6 nm; (3) In the lithium iron phosphate cathode material, the mass content of carbon element is 1.0% to 1.6%.

4. The lithium iron phosphate cathode material according to any one of claims 1 to 3, characterized in that, The lithium iron phosphate cathode material satisfies at least one of the following characteristics (1) to (4): (1) The lithium iron phosphate cathode material includes large lithium iron phosphate particles and small lithium iron phosphate particles, wherein the particle size of the large lithium iron phosphate particles is larger than the particle size of the small lithium iron phosphate particles. Preferably, the average particle size of the lithium iron phosphate particles is 0.8 μm to 2 μm; Preferably, the average particle size of the lithium iron phosphate particles is 0.3 μm to 0.5 μm; Preferably, in the lithium iron phosphate cathode material, the volume percentage of the large lithium iron phosphate particles is 55% to 70%. Preferably, in the lithium iron phosphate cathode material, the volume percentage of the lithium iron phosphate particles is 30% to 45%. (2) The particle size D of the lithium iron phosphate cathode material v 50 is 0.8μm~1.5μm; (3) The compacted density of the lithium iron phosphate cathode material is 2.5–2.7 g / cm³. 3 ; (4) The resistivity of the lithium iron phosphate cathode material is .

5. A method for preparing a lithium iron phosphate cathode material, characterized in that, Includes the following steps: Iron phosphate, lithium source, first carbon source and dopant source are added to solvent for the first grinding, followed by the first drying and the first sintering to obtain a doped lithium iron phosphate core; the dopant source includes one or more of the following: transition metal compound, alkaline earth metal compound or non-metal element compound. The doped lithium iron phosphate core, the second carbon source, the phosphorus source and the niobium source are added to a solvent for a second grinding, followed by a second drying and a second sintering to obtain the lithium iron phosphate cathode material. The lithium iron phosphate cathode material includes a doped lithium iron phosphate core, a first coating layer covering at least a portion of the surface of the doped lithium iron phosphate core, and a second coating layer covering at least a portion of the surface of the first coating layer. The first coating layer includes a niobium phosphide coating layer, and the second coating layer includes a carbon coating layer.

6. The method for preparing the lithium iron phosphate cathode material according to claim 5, characterized in that, The preparation method satisfies at least one of the following features (1) to (9): (1) The iron-to-phosphorus ratio of the iron phosphate is 96.5% to 98%; (2) The lithium source includes at least one of lithium carbonate, lithium nitrate, lithium hydroxide, lithium oxalate or lithium acetate; (3) The first carbon source includes at least one of glucose, sucrose, starch, polyvinyl alcohol, polyacryl alcohol, polyethylene glycol, carbon black, carbon fiber tube, graphite or graphene; (4) The transition metal compound includes at least one of Nb-containing compounds, Ti-containing compounds, V-containing compounds, Zr-containing compounds, or Mo-containing compounds; (5) The alkaline earth metal compounds include Mg-containing compounds; The non-metallic compound includes compounds containing B; (6) The solvent includes water and / or alcohols; (7) The second carbon source includes at least one of glucose, sucrose, starch, polyvinyl alcohol, polyacryl alcohol, polyethylene glycol, carbon black, carbon fiber tube, graphite or graphene; (8) The phosphorus source includes at least one of iron phosphate, tetrabutylphosphine chloride, phosphoric acid, or phosphine; (9) The niobium source includes at least one of Nb2O5, NbO2, NbO, NbCl5 or NbCl4.

7. The method for preparing lithium iron phosphate cathode material according to claim 5 or 6, characterized in that, The preparation method satisfies at least one of the following features (1) to (8): (1) The molar ratio of the iron phosphate, lithium source, first carbon source and dopant source is 1:(1.03~1.04):(0.04~0.15):(0.002~0.01); (2) The molar ratio of the doped lithium iron phosphate core, the second carbon source, the phosphorus source and the niobium source is 1:(0.04~0.2):(0.001~0.015):(0.001~0.015); (3) The first grinding method includes sand milling, wherein the particle size D of the sand milling is... v 50 is 0.3μm~0.5μm; (4) The first drying method includes spray drying; and / or, the second drying method includes spray drying; (5) The conditions for the first sintering include: heating to 200-400°C at a heating rate of 1-10°C / min, holding for 1-5 hours, then heating to 720-820°C at a heating rate of 1-10°C / min, and holding for 6-15 hours. (6) The carbon content of the doped lithium iron phosphate core obtained after the first sintering is 0.05% to 0.8%; (7) The second grinding method includes sand milling, which yields a first slurry and a second slurry with different particle sizes; the particle size Dv50 of the first slurry is 0.25μm to 0.45μm; the particle size Dv50 of the second slurry is 0.8μm to 1.2μm; Preferably, the mass ratio of the first slurry to the second slurry is (20-30):(70-80); (8) The conditions for the second sintering include: heating to 200-400°C at a heating rate of 1-10°C / min, holding for 1-5 hours, then heating to 720-820°C at a heating rate of 1-10°C / min, and holding for 6-15 hours.

8. A positive electrode plate, characterized in that, include: The lithium iron phosphate cathode material according to any one of claims 1 to 4, and / or the lithium iron phosphate cathode material obtained by the preparation method according to any one of claims 5 to 7.

9. The positive electrode sheet according to claim 8, characterized in that, The compaction density of the positive electrode sheet is 2.6–2.8 g / cm³. 3 ; and / or, The positive electrode sheet includes a current collector and an active layer disposed on at least one surface of the current collector, the active layer including the lithium iron phosphate positive electrode material, a conductive agent, a binder, and a dispersant; Preferably, the mass ratio of the lithium iron phosphate cathode material, conductive agent, binder and dispersant is (96-98):(0-1):(1-4):(0-1).

10. A battery, characterized in that, include: The positive electrode sheet according to claim 8 or 9.

Images