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

Abstract

The application provides a lithium iron phosphate material, a preparation method thereof, a positive plate and a lithium ion battery. The lithium iron phosphate material comprises first lithium iron phosphate particles and second lithium iron phosphate particles; the first lithium iron phosphate particles comprise a first lithium iron phosphate inner core and a first composite coating layer coated on the outer surface of the first lithium iron phosphate inner core; the first lithium iron phosphate inner core contains a doping element, and the doping element is boron and / or fluorine; the second lithium iron phosphate particles comprise a second lithium iron phosphate inner core and a second composite coating layer coated on the outer surface of the second lithium iron phosphate inner core; the first composite coating layer and the second composite coating layer each independently comprise a first coating layer and a second coating layer, the first coating layer comprises a fast ion conductor material, and the second coating layer is a carbon coating layer. The above lithium iron phosphate material can balance high tap density and high rate performance.

Description

Technical Field

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

[0002] Currently, with the continued rapid growth in demand for high-energy-density, low-cost lithium-ion batteries in the power battery and energy storage markets, lithium iron phosphate (LiFePO4) materials have attracted much attention due to their excellent thermal stability, long cycle life, and low cost. However, in order to obtain higher volumetric energy density, the compaction density of lithium iron phosphate materials has become a key bottleneck restricting the improvement of battery performance. The compaction density of lithium iron phosphate materials prepared by traditional processes is generally between 2.2 and 2.4 g / cm³. 3 This makes it difficult to meet the demand for high-density cathode plates in high-energy-density batteries.

[0003] To overcome this limitation, the industry generally adopts the "two-stage sintering method" (i.e., a two-step sintering process) to control particle size distribution. By preparing precursor particles of different sizes, structural optimization is achieved in the second sintering process, with large particles dominating the volume filling and small particles filling the gaps, thereby significantly improving the packing density of the cathode material. However, the existing two-stage sintering technology still has the following technical problems: 1) It is difficult to coordinate and control the temperature and time windows of the two sintering processes. The core particles formed after the first sintering of the precursor and the fine particles added in the second sintering process are prone to uncontrollable agglomeration or abnormal growth, making it difficult to accurately control the gradation relationship and limiting the improvement of compaction density; 2) The increased particle size of high-compact lithium iron phosphate materials leads to the obstruction of lithium-ion diffusion, resulting in the degradation of rate performance; 3) Traditional carbon coating layers are prone to graphitization or uneven distribution during high-temperature sintering, resulting in insufficient electronic conductivity and interface stability, further exacerbating the electrochemical kinetic lag.

[0004] Furthermore, existing technologies often use single metal oxides such as alumina and titanium dioxide as doping or coating components, but their diffusion ability at grain boundaries is weak, making it difficult to form a continuous fast ion-conducting network and thus failing to effectively improve lithium-ion transport efficiency. Meanwhile, traditional carbon sources such as glucose and pitch tend to form dense, disordered carbon layers during pyrolysis, resulting in poor pore structure, which hinders electrolyte penetration and interfacial ion exchange, limiting the electrochemical activity of the material under high-compact conditions. Therefore, there is an urgent need for a novel high-compact lithium iron phosphate material and its preparation method that can precisely control particle size distribution, suppress excessive grain growth, and simultaneously construct an efficient ion and electron transport network. Summary of the Invention

[0005] The main objective of this invention is to provide a lithium iron phosphate material and its preparation method, a positive electrode sheet, and a lithium-ion battery, so as to solve the problem that lithium iron phosphate materials in the prior art are difficult to achieve both high compaction density and high rate performance.

[0006] To achieve the above objectives, according to one aspect of the present invention, a lithium iron phosphate material is provided, comprising first lithium iron phosphate particles and second lithium iron phosphate particles, wherein the mass ratio of the first lithium iron phosphate particles to the second lithium iron phosphate particles is (1~4):1; the first lithium iron phosphate particle comprises a first lithium iron phosphate core and a first composite coating layer covering the outer surface of the first lithium iron phosphate core; the first lithium iron phosphate core contains a dopant element, wherein the dopant element is boron and / or fluorine; the second lithium iron phosphate particle comprises a second lithium iron phosphate core and a second composite coating layer covering the outer surface of the second lithium iron phosphate core; the first composite coating layer and the second composite coating layer each independently comprise a first coating layer and a second coating layer, wherein the first coating layer comprises a fast ion conductor material and the second coating layer is a carbon coating layer; the D50 particle size of the first lithium iron phosphate particle is 1.00~1.50 μm larger than the D50 particle size of the second lithium iron phosphate particle; the powder compaction density of the lithium iron phosphate material is 2.7~2.8 g / cm³. 3 .

[0007] The lithium iron phosphate material of this application uses first and second lithium iron phosphate particles with a D50 particle size difference within the aforementioned range, and controls their mass ratio within the aforementioned range to form a high compaction density gradation relationship, thereby improving the compaction density and rate performance of the lithium iron phosphate material. Introducing boron (B) and / or ferrous (F) elements into the lithium iron phosphate core can effectively broaden the lithium-ion diffusion channels and enhance the stability of the crystal structure under high compaction conditions. Coating the outer surface of the lithium iron phosphate core with a fast ion conductor material can significantly improve the interfacial lithium-ion transport rate, while the carbon coating layer can optimize the electronic conductivity of the lithium iron phosphate material, thereby improving mass transfer efficiency and enhancing electrolyte wetting effect.

[0008] Furthermore, the D50 particle size of the first lithium iron phosphate particle is 1.2~1.9μm, and the D50 particle size of the second lithium iron phosphate particle is 200~400nm; and / or, the mass content of the doped element in the core of the first lithium iron phosphate is 100~3000μg / g.

[0009] Preferably, the D50 particle size of the first and second lithium iron phosphate particles is within the above-mentioned range, which helps to form a high compaction density gradation relationship, thereby improving the compaction density and rate performance of the lithium iron phosphate material. Preferably, the mass content of doping elements in the first lithium iron phosphate core is within the above-mentioned range, which helps to better broaden the lithium-ion diffusion channels and enhance the stability of the crystal structure under high compaction conditions.

[0010] Furthermore, the thickness of the first composite coating layer and the second composite coating layer are each independently 1~65nm; and / or, the mass percentage of the first coating layer in the first lithium iron phosphate particle and the second lithium iron phosphate particle is each independently 0.01~1.00%; the thickness of the first coating layer is 5~50nm; and / or, the mass percentage of the second coating layer in the first lithium iron phosphate particle and the second lithium iron phosphate particle is 1.0~1.65%, and the thickness of the second coating layer is 1~30nm; and / or, the fast ion conductor material is lithium titanium aluminum phosphate.

[0011] Preferably controlling the thickness of the composite coating layer within the aforementioned range helps to synergistically optimize particle packing efficiency and surface coating uniformity, thereby improving the compaction density, ion transport efficiency, and conductivity of the lithium iron phosphate material. Preferably controlling the mass ratio and thickness of the first and second coating layers within the aforementioned range helps to enhance their synergistic effect, thereby further improving the compaction density, ion transport efficiency, and conductivity of the lithium iron phosphate material.

[0012] According to another aspect of the present invention, a method for preparing the above-mentioned lithium iron phosphate material is provided. The method includes: step S1, mixing a first raw material comprising iron phosphate, a first lithium source, and a first carbon source, and then sequentially performing a first grinding, freeze drying, and a first sintering to obtain a first intermediate; dividing the first intermediate into two parts to obtain a first intermediate A1 and a first intermediate A2; step S2, performing ion implantation on the first intermediate A1 using an ion source to obtain a second intermediate; the ion source includes a B ion source and / or an F ion source; step S3, mixing a second raw material comprising the first intermediate A2, the second intermediate, a second carbon source, a second lithium source, a titanium source, and a fast ion conductor precursor, and then sequentially performing a second grinding and pulse sintering to obtain the lithium iron phosphate material; wherein the second carbon source includes an alkyl phosphate ester; the pulse sintering includes sequentially performing a first pulse sintering and a second pulse sintering, the temperature of the second pulse sintering being lower than the temperature of the first pulse sintering.

[0013] In step S1 of the preparation method of this application, precursor particles with small nucleation sites are formed through first grinding. Combined with freeze-drying technology, a loose network structure is formed between the particles, inhibiting particle growth during secondary sintering (pulse sintering). Ion implantation of the first intermediate provides a sintering aid, promoting particle growth during secondary sintering. The first intermediate is divided into two parts: one part is implanted with boron or fluorine ions to serve as a structure-regulating component, while the other part remains in its original state. These two parts are mixed in step S3 and synergistically interact with each other during pulse sintering, achieving a gradient distribution of lithium iron phosphate core particle size and controllable formation of the surface coating layer. The boron or fluorine elements introduced by ion implantation form local defects in the crystal lattice, promoting appropriate grain growth during sintering and inducing lattice expansion, thereby improving mass transfer efficiency. By introducing a titanium source and a fast-ion conductor precursor, combined with the pyrolysis of a second carbon source, the fast-ion conductor precursor serves as a reaction anchor during pulse sintering. This results in a double coating effect of carbon and fast-ion conductor with a loose porous structure, thereby improving mass transfer efficiency and enhancing electrolyte wetting. This application designs a two-step pulse sintering method: high-temperature pulses induce rapid phase formation of the fast-ion conductor precursor, while low-temperature pulses repair lattice defects, further improving the ionic conductivity of lithium iron phosphate materials and thus enhancing the electrochemical performance of the battery.

[0014] Further, in step S1, the molar ratio of the first lithium source (based on lithium element) and the iron phosphate (based on phosphorus element) is (1.005~1.1):1; and / or, the specific surface area of ​​the iron phosphate is 8~10 m². 2 / g, the molar ratio of Fe to P in iron phosphate is (0.97~0.99):1; the first lithium source and the second lithium source are each independently selected from any one or more of lithium carbonate, lithium hydroxide and lithium hydroxide monohydrate; the first carbon source is selected from one or more of glucose, sucrose and polyethylene glycol; and / or, the first raw material also includes a solvent, the solvent being water; and / or, the first grinding is ultrafine sand milling, the D50 particle size of the mixture after the first grinding is 150~250nm, and the solid content of the first raw material is 30~60%; and / or, the freeze-drying temperature is -40~-80℃, and the freeze-drying time is 12~48h; and / or, the atmosphere for the first sintering is nitrogen and / or argon, the first sintering temperature is 450~550℃, and the first sintering time is 3~10h; the mass content of carbon in the first intermediate is 0.01~1%; and / or, the mass ratio of the first intermediate A1 to the first intermediate A2 is (1~4):1.

[0015] Preferably controlling the types and molar ratios of lithium source and iron phosphate, as well as the type of the first carbon source, within the aforementioned ranges helps in the formation of the first intermediate. Preferably adding a solvent helps control the solid content of the first raw material within the aforementioned range, thereby improving its dispersibility and uniformity during ultrafine milling, ensuring the D50 particle size of the first milled mixture remains stable within the aforementioned range, and further enabling a loose network structure between particles under freeze-drying conditions, mitigating further particle growth during pulse sintering. Preferably, the temperature, time, and atmosphere of the first sintering within the aforementioned ranges help improve the phase purity of the first intermediate. Preferably, the mass content of carbon in the first intermediate within the aforementioned ranges helps control particle growth to obtain an intermediate with the target particle size, and forms a conductive network between particles, improving particle dispersibility and thus enhancing the rate performance of the material. Preferably, the mass ratio of the first intermediate A1 to the first intermediate A2 is within the aforementioned range. The first intermediate A1 easily grows into a large particle size after ion implantation and sintering, while the first intermediate A2 retains a small particle size morphology. This mass ratio helps form a densely packed state with higher space utilization, thereby increasing the compaction density of the material.

[0016] Further, in step S2, the mass ratio of B ions and / or F ions in the ion source to the mass of the first intermediate A1 is 0.05~0.1:100; and / or, the ion source is any one or more of boron trifluoride, diborane, fluorine, carbon tetrafluoride, and hydrogen fluoride; and / or, the ion implantation energy is 1~5keV, and the ion implantation depth is 5~30nm.

[0017] Preferably, the mass ratio of B ions and / or F ions in the preferred ion source to the mass of the first intermediate A1 is within the above-mentioned range. This facilitates the doping of B or F ions, thereby optimizing the lithium-ion migration channels at grain boundaries and improving the ion diffusion efficiency of the lithium iron phosphate material. Preferably, using the above-mentioned ion source for ion implantation helps improve the purity of the dopant elements and reduce the formation of impurity phases. Preferably, the ion implantation energy and depth are within the above-mentioned range. This facilitates better incorporation of B or F ions into the surface and near-surface lattice regions of the first intermediate A1, thus avoiding excessive lattice distortion or phase transition, while also optimizing the lithium-ion migration channels at grain boundaries, further improving the ion diffusion efficiency of the lithium iron phosphate material.

[0018] Further, in step S3, the alkyl phosphate ester is selected from any one or more of tributyl phosphate ester, dodecyl phosphate ester, and tetradecyl phosphate ester; the titanium source is titanium oxide, and the D50 particle size of the titanium source is 50~200 nm; the fast ion conductor precursor is aluminum phosphate molecular sieve; the molar ratio of Al to P in the aluminum phosphate molecular sieve is 0.8~1:1, and the pore volume of the aluminum phosphate molecular sieve is 0.01~0.5 cm³. 3 / g, the average pore size of aluminum phosphate molecular sieve is 0.1~20nm; the fast ion conductor precursor and the second carbon source are based on the total amount of P element, the second lithium source is based on the amount of lithium element, and the titanium source is based on the amount of titanium element. The ratio of the total molar amount of the fast ion conductor precursor and the second carbon source, the molar amount of the second lithium source and the molar amount of the titanium source is 3:1.0~1.3:1.5~2.0; the mass ratio of the first intermediate A2 and the second intermediate is 1:(1~4).

[0019] The preferred second carbon source is an alkyl phosphate ester, whose phosphate groups can be anchored on the surface of the lithium iron phosphate intermediate, facilitating ultrathin and uniform coating. Simultaneously, its thermal decomposition releases phosphorus, which helps suppress interfacial side reactions of LFP locally, thereby achieving secondary coating of the LFP particle surface and comprehensively improving rate performance, cycle performance, low-temperature performance, and interfacial stability. The preferred aluminum phosphate molecular sieve has a micro-mesoporous structure; controlling its pore volume and average pore size within the aforementioned range helps improve electrolyte wettability and optimize interfacial lithium-ion transport. Controlling the type of titanium source and D50 particle size within the aforementioned range helps it disperse at grain boundaries, reducing abnormal grain growth during pulse sintering and enhancing structural stability. The preferred ratio of the total molar amount of the fast ion conductor precursor and the second carbon source, the molar amount of the second lithium source, and the molar amount of the titanium source within the aforementioned range helps to better form a double coating layer containing both a fast ion conductor and a carbon coating layer on the outer surface of the lithium iron phosphate, thereby further improving mass transfer efficiency and electrolyte wettability. The optimal mass ratio of the first intermediate A2 and the second intermediate helps to form lithium iron phosphate particles of different sizes, thereby improving the compaction density and rate performance of the material.

[0020] Further, in step S3, the second grinding is dry grinding, and the D50 particle size of the mixture after the second grinding is 200~300nm; and / or, the temperature of the first pulse sintering is 800~1000℃, and the single pulse time of the first pulse sintering is 100~300s; the temperature of the second pulse sintering is 500~700℃, and the single pulse time of the second pulse sintering is 50~100s; the voltage of the pulse sintering is 10~80V, the current of the pulse sintering is 0.5~80A, and the atmosphere of the pulse sintering is nitrogen and / or argon.

[0021] The above-mentioned second grinding process brings the D50 particle size of the mixture after the second grinding to the above range, which helps to maintain the original gradation structure of the first intermediate A1 (large particles) and the second intermediate A2 (small particles), optimizes the particle packing density and pore structure, and thus facilitates the formation of interconnected micropores during the high-pressure compaction process, promoting electrolyte penetration.

[0022] This application employs a segmented pulse sintering process, preferably with the first pulse sintering temperature within the aforementioned range. This facilitates the rapid phase formation of fast ion conductor precursors under high-temperature pulses. Subsequently, the temperature is lowered to the aforementioned range for a second pulse, which helps to better repair lattice defects and alleviate abnormal grain growth. This allows for the simultaneous optimization of ion diffusion channels and synergistic improvement of lattice integrity under high real density, thereby enhancing the rate performance and cycle stability of lithium iron phosphate materials.

[0023] According to another aspect of the present invention, a positive electrode sheet is provided, comprising a current collector and a positive electrode active layer disposed on at least one side surface of the current collector, the positive electrode active layer comprising a positive electrode active material, which is the lithium iron phosphate material described above or prepared by the preparation method described above.

[0024] Using the aforementioned lithium iron phosphate material in the positive electrode can improve the density and structural integrity of the electrode, reduce the interface resistance, and enhance the electrolyte wetting ability. This allows for improved rate performance and cycle stability while maintaining high capacity retention, making it suitable for the high-compactness and low-impedance requirements of high-energy-density lithium-ion batteries.

[0025] According to another aspect of the present invention, a lithium-ion battery is provided, comprising a positive electrode, a negative electrode, and a separator, wherein the positive electrode is the aforementioned positive electrode.

[0026] The lithium-ion battery using the above-mentioned positive electrode has high energy density, high rate performance and good cycle stability.

[0027] By applying the technical solution of this invention, the introduction of boron (B) and / or ferric (F) elements into the lithium iron phosphate (LFP) core can effectively broaden the lithium-ion diffusion channels and enhance the stability of the crystal structure under high compaction conditions. Coating the LFP core with a fast ion-conducting material significantly improves the interfacial lithium-ion transport rate, while the carbon coating optimizes the electronic conductivity of the LFP material, thereby improving mass transfer efficiency and enhancing electrolyte wetting. The synergistic effect of these three elements significantly increases the powder compaction density of the LFP material to the aforementioned range, while maintaining high ion diffusion efficiency and structural integrity. This fundamentally overcomes the technical bottleneck of ion transport obstruction and lattice distortion instability caused by compaction in traditional high-compaction LFP materials, thereby improving the energy density, rate performance, and cycle life of the LFP material, and ultimately enhancing the overall performance of lithium-ion batteries. Attached Figure Description

[0028] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0029] Figure 1The HRTEM image of the lithium iron phosphate material in Example 1 of this application is shown;

[0030] Figure 2 The SEM image of the lithium iron phosphate material in Example 1 of this application is shown.

[0031] The above figures include the following reference numerals:

[0032] 1. First coating layer; 2. Second coating layer. Detailed Implementation

[0033] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0034] As analyzed in the background section of this application, existing lithium iron phosphate materials have the problem of difficulty in simultaneously achieving high compaction density and high rate performance. In order to solve the above problems, this application provides a lithium iron phosphate material, its preparation method, a positive electrode sheet, and a lithium-ion battery.

[0035] In a typical embodiment of this application, a lithium iron phosphate material is provided, comprising first lithium iron phosphate particles and second lithium iron phosphate particles, wherein the mass ratio of the first lithium iron phosphate particles to the second lithium iron phosphate particles is (1~4):1; the first lithium iron phosphate particle comprises a first lithium iron phosphate core and a first composite coating layer covering the outer surface of the first lithium iron phosphate core; the first lithium iron phosphate core contains a dopant element, wherein the dopant element is boron and / or fluorine; the second lithium iron phosphate particle comprises a second lithium iron phosphate core and a second composite coating layer covering the outer surface of the second lithium iron phosphate core; the first composite coating layer and the second composite coating layer each independently comprise a first coating layer and a second coating layer, wherein the first coating layer comprises a fast ion conductor material and the second coating layer is a carbon coating layer; the D50 particle size of the first lithium iron phosphate particle is 1.00~1.50 μm larger than the D50 particle size of the second lithium iron phosphate particle; the powder compaction density of the lithium iron phosphate material is 2.7~2.8 g / cm³. 3 .

[0036] The lithium iron phosphate material of this application uses first and second lithium iron phosphate particles with a D50 particle size difference within the aforementioned range, and controls their mass ratio within the aforementioned range to form a high compaction density gradation relationship, thereby improving the compaction density and rate performance of the lithium iron phosphate material. Introducing boron (B) and / or ferric (F) elements into the lithium iron phosphate core effectively widens the lithium-ion diffusion channels and enhances the stability of the crystal structure under high compaction conditions. Coating the outer surface of the lithium iron phosphate core with a fast ion conductor material significantly improves the interfacial lithium-ion transport rate, while the carbon coating layer optimizes the electronic conductivity of the lithium iron phosphate material, thereby improving mass transfer efficiency and enhancing electrolyte wetting. The synergistic effect of these three factors significantly improves the powder compaction density of the lithium iron phosphate material to the aforementioned range, while maintaining high ion diffusion efficiency and structural integrity. This fundamentally overcomes the technical bottleneck of ion transport obstruction and lattice distortion instability caused by compaction in traditional high-compact lithium iron phosphate materials, thereby improving the energy density, rate performance, and cycle life of the lithium iron phosphate material, and ultimately improving the overall performance of lithium-ion batteries.

[0037] D50 is the median particle size, which is the particle size value corresponding to the cumulative distribution reaching 50% in the particle size distribution. That is, 50% of the particles have a diameter smaller than this value, and 50% have a diameter larger than this value.

[0038] In one embodiment of this application, the D50 particle size of the first lithium iron phosphate particle is 1.2~1.9μm, the D50 particle size of the second lithium iron phosphate particle is 200~400nm; and / or, the mass content of doped elements in the first lithium iron phosphate core is 100-~3000μg / g.

[0039] Preferably, the D50 particle size of the first and second lithium iron phosphate particles is within the above range, which helps to form a high compaction density gradation relationship, thereby improving the compaction density and rate performance of lithium iron phosphate materials.

[0040] The optimal mass content of doping elements in the first lithium iron phosphate core within the aforementioned range helps to better broaden the lithium-ion diffusion channels and enhance the stability of the crystal structure under high-pressure compaction conditions. When the doping elements in the first lithium iron phosphate core are a combination of boron and fluorine, a molar ratio of boron to fluorine of 1:1 to 3 is preferred, which helps to form a low-melting-point Li-BOF system, thereby improving the mass transfer rate, accelerating grain growth, and enhancing densification. Simultaneously, F... - Replace O 2- B 3+ Replace P 5 + This helps to synergistically increase the concentration of electron carriers, suppress distortion, and reduce phase transition stress, thereby achieving a simultaneous improvement in compaction density and rate performance.

[0041] In one embodiment of this application, the thickness of the first composite coating layer and the second composite coating layer are each independently 1~65nm; and / or, the mass percentage of the first coating layer in the first lithium iron phosphate particle and the second lithium iron phosphate particle is each independently 0.01~1.00%; the thickness of the first coating layer is 5~50nm; and / or, the mass percentage of the second coating layer in the first lithium iron phosphate particle and the second lithium iron phosphate particle is 1.0~1.65%, and the thickness of the second coating layer is 1~30nm; and / or, the fast ion conductor material is lithium titanium aluminum phosphate.

[0042] Excessive thickness of the composite coating layer increases ion transport resistance, while insufficient thickness leads to inadequate structural support. This application preferably controls the thickness of the composite coating layer within the aforementioned range, which helps to synergistically optimize particle packing efficiency and surface coating uniformity, thereby improving the compaction density, ion transport efficiency, and conductivity of the lithium iron phosphate material. Preferably, controlling the mass ratio and thickness of the first and second coating layers within the aforementioned range helps to enhance their synergistic effect, further improving the compaction density, ion transport efficiency, and conductivity of the lithium iron phosphate material. Preferably, the type of fast ion conductor material is within the aforementioned range, which helps to further improve the lithium-ion transport rate. Furthermore, the porous structure of the carbon coating layer also has a mechanical buffering function, helping to alleviate stress concentration during particle compaction, reducing lithium iron phosphate core cracking and coating peeling, thereby achieving high energy density while further improving the cycle stability and structural integrity of the lithium iron phosphate material.

[0043] The preferred thickness ratio of the first coating layer and the second coating layer is (1~5):1, which helps to improve the synergistic effect of the two, thereby further improving the compaction density, ion transport efficiency and conductivity of lithium iron phosphate material.

[0044] In another typical embodiment of this application, a method for preparing the above-mentioned lithium iron phosphate material is provided. The method includes: step S1, mixing a first raw material comprising iron phosphate, a first lithium source, and a first carbon source, and then sequentially performing a first grinding, freeze drying, and a first sintering to obtain a first intermediate; dividing the first intermediate into two parts to obtain a first intermediate A1 and a first intermediate A2; step S2, performing ion implantation on the first intermediate A1 using an ion source to obtain a second intermediate; the ion source includes a B ion source and / or an F ion source; step S3, mixing a second raw material comprising the first intermediate A2, the second intermediate, a second carbon source, a second lithium source, a titanium source, and a fast ion conductor precursor, and then sequentially performing a second grinding and pulse sintering to obtain the lithium iron phosphate material; wherein the second carbon source includes an alkyl phosphate ester; the pulse sintering includes sequentially performing a first pulse sintering and a second pulse sintering, the temperature of the second pulse sintering being lower than the temperature of the first pulse sintering.

[0045] The preparation method of this application improves the compaction density of lithium iron phosphate material by controlling the particle size distribution. In step S1, the first grinding process forms precursor particles with smaller nucleation sites, and the freeze-drying technology creates a loose network structure between the particles, inhibiting particle growth during secondary sintering (pulse sintering). Ion implantation of the first intermediate provides a sintering aid, promoting particle growth during secondary sintering. The first intermediate is divided into two parts: one part is implanted with boron or fluorine ions to act as a structure-regulating component, while the other part remains in its original state. The two parts are mixed in step S3 and work synergistically with the pulse sintering process to achieve a gradient distribution of lithium iron phosphate core particle size and controllable formation of the surface coating layer. The boron or fluorine elements introduced by ion implantation form local defects in the crystal lattice, promoting appropriate grain growth during sintering and inducing lattice expansion, thereby improving mass transfer efficiency. By introducing a titanium source and a fast-ion conductor precursor, combined with the pyrolysis of a second carbon source, the fast-ion conductor precursor serves as a reaction anchor during pulse sintering. This results in a double coating effect of carbon and fast-ion conductor with a loose porous structure, thereby improving mass transfer efficiency and enhancing electrolyte wetting. This application designs a two-step pulse sintering method: a high-temperature pulse induces rapid phase formation of the fast-ion conductor precursor, and a low-temperature pulse repairs lattice defects, further improving the ionic conductivity of the lithium iron phosphate material and thus enhancing the electrochemical performance of the battery. The preparation method of this application yields lithium iron phosphate materials with two different particle sizes, thereby maintaining high powder compaction density while further improving electrochemical transport efficiency, achieving a balance between high energy density and good rate performance.

[0046] In one embodiment of this application, in step S1 above, the first lithium source, calculated by its lithium element, and the iron phosphate, calculated by its phosphorus element, have a molar ratio of (1.005~1.1):1; and / or, the specific surface area of ​​the iron phosphate is 8~10 m². 2 / g, the molar ratio of Fe to P in iron phosphate is (0.97~0.99):1; the first lithium source and the second lithium source are each independently selected from any one or more of lithium carbonate, lithium hydroxide and lithium hydroxide monohydrate; the first carbon source is selected from one or more of glucose, sucrose and polyethylene glycol; and / or, the first raw material also includes a solvent, the solvent being water; and / or, the first grinding is ultrafine sand milling, the D50 particle size of the mixture after the first grinding is 150~250nm, and the solid content of the first raw material is 30~60%; and / or, the freeze-drying temperature is -40~-80℃, and the freeze-drying time is 12~48h; and / or, the atmosphere for the first sintering is nitrogen and / or argon, the first sintering temperature is 450~550℃, and the first sintering time is 3~10h; the mass content of carbon in the first intermediate is 0.01~1%; and / or, the mass ratio of the first intermediate A1 to the first intermediate A2 is (1~4):1.

[0047] Preferably controlling the types and molar ratios of lithium source and iron phosphate, as well as the type of the first carbon source, within the aforementioned ranges helps in the formation of the first intermediate. Preferably adding a solvent helps control the solid content of the first raw material within the aforementioned range, thereby improving its dispersibility and uniformity during ultrafine milling, ensuring the D50 particle size of the first milled mixture remains stable within the aforementioned range, and further enabling a loose network structure between particles under freeze-drying conditions, thus mitigating further particle growth during pulse sintering. Preferably, the temperature, time, and atmosphere of the first sintering within the aforementioned ranges help improve the phase purity of the first intermediate. Preferably, the mass content of carbon in the first intermediate within the aforementioned ranges helps control particle growth to obtain an intermediate with the target particle size, and forms a conductive network between particles, improving particle dispersibility and thus enhancing the rate performance of the material.

[0048] The preferred mass ratio of the first intermediate A1 to the first intermediate A2 is within the above range. After ion implantation, the first intermediate A1 is easy to grow into a large particle size by sintering, while the first intermediate A2 still maintains a small particle size morphology. This mass ratio helps to form a densely packed state with higher space utilization, thereby improving the compaction density of the material.

[0049] In one embodiment of this application, in step S2 above, the mass ratio of B ions and / or F ions in the ion source to the mass of the first intermediate A1 is 0.05~0.1:100; and / or, the ion source is any one or more of boron trifluoride, diborane, fluorine, carbon tetrafluoride, and hydrogen fluoride; and / or, the ion implantation energy is 1~5keV, and the ion implantation depth is 5~30nm.

[0050] Preferably, the mass ratio of B ions and / or F ions in the preferred ion source to the mass of the first intermediate A1 is within the above-mentioned range. This facilitates the doping of B or F ions, thereby optimizing the lithium-ion migration channels at grain boundaries and improving the ion diffusion efficiency of the lithium iron phosphate material. Preferably, using the above-mentioned ion source for ion implantation helps improve the purity of the dopant elements and reduce the formation of impurity phases. Preferably, the ion implantation energy and depth are within the above-mentioned range. This facilitates better incorporation of B or F ions into the surface and near-surface lattice regions of the first intermediate A1, thus avoiding excessive lattice distortion or phase transition, while also optimizing the lithium-ion migration channels at grain boundaries, further improving the ion diffusion efficiency of the lithium iron phosphate material.

[0051] In one embodiment of this application, in step S3 above, the alkyl phosphate is selected from any one or more of tributyl phosphate, dodecyl phosphate, and tetradecyl phosphate; the titanium source is titanium oxide, and the D50 particle size of the titanium source is 50~200 nm; the fast ion conductor precursor is aluminum phosphate molecular sieve; the molar ratio of Al to P in the aluminum phosphate molecular sieve is 0.8~1:1, and the pore volume of the aluminum phosphate molecular sieve is 0.01~0.5 cm³. 3 / g, the average pore size of aluminum phosphate molecular sieve is 0.1~20nm; the fast ion conductor precursor and the second carbon source are based on the total amount of P element, the second lithium source is based on the amount of lithium element, and the titanium source is based on the amount of titanium element. The ratio of the total molar amount of the fast ion conductor precursor and the second carbon source, the molar amount of the second lithium source and the molar amount of the titanium source is 3:1.0~1.3:1.5~2.0; the mass ratio of the first intermediate A2 and the second intermediate is 1:(1~4).

[0052] The preferred second carbon source is an alkyl phosphate ester, whose phosphate groups can be anchored on the surface of the lithium iron phosphate intermediate, facilitating ultrathin and uniform coating. Simultaneously, its thermal decomposition releases phosphorus, which helps suppress interfacial side reactions of LFP locally, thereby achieving secondary coating of the LFP particle surface and comprehensively improving rate performance, cycle performance, low-temperature performance, and interfacial stability. The preferred aluminum phosphate molecular sieve has a micro-mesoporous structure; controlling its pore volume and average pore size within the aforementioned range helps improve electrolyte wettability and optimize interfacial lithium-ion transport. Controlling the type of titanium source and D50 particle size within the aforementioned range helps it disperse at grain boundaries, reducing abnormal grain growth during pulse sintering and enhancing structural stability. The preferred ratio of the total molar amount of the fast ion conductor precursor and the second carbon source, the molar amount of the second lithium source, and the molar amount of the titanium source within the aforementioned range helps to better form a double coating layer containing both a fast ion conductor and a carbon coating layer on the outer surface of the lithium iron phosphate, thereby further improving mass transfer efficiency and electrolyte wettability. The optimal mass ratio of the first intermediate A2 and the second intermediate helps to form lithium iron phosphate particles of different sizes, thereby improving the compaction density and rate performance of the material.

[0053] In one embodiment of this application, in step S3 above, the second grinding is dry grinding, and the D50 particle size of the mixture after the second grinding is 200~300nm; and / or, the temperature of the first pulse sintering is 800~1000℃, the single pulse time of the first pulse sintering is 100~300s; the temperature of the second pulse sintering is 500~700℃, the single pulse time of the second pulse sintering is 50~100s; the voltage of the pulse sintering is 10~80V, the current of the pulse sintering is 0.5~80A, and the atmosphere of the pulse sintering is nitrogen and / or argon.

[0054] The above-mentioned second grinding process brings the D50 particle size of the mixture after the second grinding to the above range, which helps to maintain the original gradation structure of the first intermediate A1 (large particles) and the second intermediate A2 (small particles), optimizes the particle packing density and pore structure, and thus facilitates the formation of interconnected micropores during the high-pressure compaction process, promoting electrolyte penetration.

[0055] This application employs a segmented pulse sintering process, preferably with the first pulse sintering temperature within the aforementioned range. This facilitates the rapid phase formation of fast ion conductor precursors under high-temperature pulses. Subsequently, the temperature is lowered to the aforementioned range for a second pulse, which helps to better repair lattice defects and alleviate abnormal grain growth. This allows for the simultaneous optimization of ion diffusion channels and synergistic improvement of lattice integrity under high real density, thereby enhancing the rate performance and cycle stability of lithium iron phosphate materials.

[0056] The preferred second pulse sintering temperature is 200-300°C lower than the first pulse sintering temperature, and the second pulse sintering time is 100-200 seconds shorter than the first pulse sintering time. This helps to further improve the ionic conductivity of lithium iron phosphate materials, reduce lattice defects, and thus further improve the compaction density, rate performance and cycle stability of lithium iron phosphate materials.

[0057] In another typical embodiment of this application, a positive electrode sheet is provided, including a current collector and a positive electrode active layer disposed on at least one side surface of the current collector. The positive electrode active layer includes a positive electrode active material, which is the lithium iron phosphate material described above or prepared by the preparation method described above.

[0058] Using the aforementioned lithium iron phosphate material in the positive electrode can improve the density and structural integrity of the electrode, reduce the interface resistance, and enhance the electrolyte wetting ability. This allows for improved rate performance and cycle stability while maintaining high capacity retention, making it suitable for the high-compactness and low-impedance requirements of high-energy-density lithium-ion batteries.

[0059] In another typical embodiment of this application, a lithium-ion battery is provided, including a positive electrode, a negative electrode, and a separator, wherein the positive electrode is the aforementioned positive electrode.

[0060] The lithium-ion battery using the above-mentioned positive electrode has high energy density, high rate performance and good cycle stability.

[0061] The beneficial effects of this application will be further illustrated below with reference to the embodiments.

[0062] Example 1

[0063] Step S1, with a specific surface area of ​​8.3 m² 2A mixture of ferric phosphate (Fe to P molar ratio of 0.980:1), lithium carbonate (first lithium source), glucose (first carbon source), and water was prepared to obtain a first mixture with a solid content of 50%. The molar ratio of lithium carbonate to ferric phosphate was 1.01:1, calculated based on lithium in lithium carbonate and phosphorus in ferric phosphate. The mixture was then subjected to a first grinding process (ultrafine grinding) to obtain a first ground mixture with a D50 particle size of 150 nm. The mixture was further freeze-dried at -80°C for 24 h, followed by a first sintering process at 500°C for 3 h under a nitrogen atmosphere to obtain a first intermediate with a carbon content of 0.8%. The first intermediate was divided into two parts, resulting in first intermediate A1 and first intermediate A2, with a mass ratio of 7:3.

[0064] In step S2, the first intermediate A1 is implanted with B ions using the ion source diborane to obtain the second intermediate; wherein the mass ratio of the implanted ions to the first intermediate A1 is 0.05:100, the ion implantation energy is 1.0 keV, and the ion implantation depth is 10 nm.

[0065] Step S3 involves mixing the first intermediate A2 and the second intermediate (mass ratio 1:3), the second carbon source tributyl phosphate, the titanium source titanium oxide (D50 particle size 50 nm), the fast ion conductor precursor aluminum phosphate molecular sieve, and the second lithium source lithium carbonate to obtain a second mixture. The aluminum phosphate molecular sieve has a 1:1 molar ratio of aluminum to phosphorus and a pore volume of 0.05 cm³. 3 / g, with an average pore size of 1.2nm. The fast ion conductor precursor and the second carbon source are calculated based on the total amount of P, the second lithium source based on the amount of lithium, and the titanium source based on the amount of titanium. The molar ratio of the total molar amount of the fast ion conductor precursor and the second carbon source, the molar amount of the second lithium source, and the molar amount of the titanium source is 3:1.05:1.7. The second mixture is subjected to a second dry grinding process, resulting in a D50 particle size of 200nm. Pulse sintering is then performed under a nitrogen atmosphere. The pulse sintering process is as follows: the temperature is raised to 800℃ for the first pulse sintering, with a single pulse duration of 200s, followed by cooling to 600℃ for the second pulse sintering, with a single pulse duration of 50s, yielding lithium iron phosphate material.

[0066] The lithium iron phosphate material consists of two types of particles: first lithium iron phosphate particles and second lithium iron phosphate particles. The first lithium iron phosphate particle comprises a first lithium iron phosphate core and a first composite coating layer covering the outer surface of the first lithium iron phosphate core. The first lithium iron phosphate core contains boron dopant at a doping level of 700 μg / g. The second lithium iron phosphate particle comprises a second lithium iron phosphate core and a second composite coating layer covering the outer surface of the second lithium iron phosphate core. Both the first and second composite coating layers contain a first coating layer and a second coating layer. The first coating layer comprises a fast ion conductor material (lithium titanium aluminum phosphate), and the second coating layer is a carbon coating layer. The first lithium iron phosphate particle has a D50 particle size of 1.5 μm, the second lithium iron phosphate particle has a D50 particle size of 250 nm, the mass ratio of first lithium iron phosphate particles to second lithium iron phosphate particles is 3:1, the thickness of the first coating layer is 15 nm, and the mass content of the first coating layer is 0.65%. The thickness of the second coating layer is 5 nm, and the mass content of the second coating layer is 1.36%. The total thickness of both the first and second composite coating layers is 25 nm.

[0067] Example 2

[0068] The difference from Example 1 is that in step S1, the molar ratio of lithium carbonate to iron phosphate is 1.1:1.

[0069] In step S3, the molar ratio of aluminum to phosphorus in the aluminum phosphate molecular sieve is 0.8:1, and the pore volume is 0.08 cm³. 3 / g, with an average pore size of 2nm. The pulse sintering process is as follows: the temperature is raised to 1000℃ for the first pulse sintering, with a single pulse time of 200s, and then the temperature is lowered to 600℃ for the second pulse sintering, with a single pulse time of 100s, to obtain lithium iron phosphate material.

[0070] In the lithium iron phosphate material, the first lithium iron phosphate particle has a D50 size of 1.3 μm, the second lithium iron phosphate particle has a D50 size of 220 nm, the mass ratio of the first lithium iron phosphate particle to the second lithium iron phosphate particle is 3:1, the thickness of the first coating layer is 12 nm, and the mass content of the first coating layer is 0.5%. The thickness of the second coating layer is 10 nm, and the mass content of the second coating layer is 1.58%.

[0071] Example 3

[0072] The difference from Example 1 is that in step S1, the molar ratio of lithium carbonate to iron phosphate is 1.08:1, and the mass ratio of the first intermediate A1 to the first intermediate A2 is 2.5:1.

[0073] In step S3, the total molar amounts of the fast ion conductor precursor and the second carbon source (based on the total amount of P), the second lithium source (based on the amount of lithium), and the titanium source (based on the amount of titanium) are calculated. The molar ratios of the total molar amounts of the fast ion conductor precursor and the second carbon source, the second lithium source, and the titanium source are 3:1:1.5. The pulse sintering process is as follows: the temperature is raised to 800℃ for the first pulse sintering, with a single pulse duration of 100s, followed by cooling to 700℃ for the second pulse sintering, with a single pulse duration of 100s, to obtain lithium iron phosphate material.

[0074] In the lithium iron phosphate material, the first lithium iron phosphate particle has a D50 size of 1.4 μm, the second lithium iron phosphate particle has a D50 size of 210 nm, the mass ratio of the first lithium iron phosphate particle to the second lithium iron phosphate particle is 2.5:1, the thickness of the first coating layer is 10 nm, and the mass content of the first coating layer is 0.40%. The thickness of the second coating layer is 5 nm, and the mass content of the second coating layer is 1.36%.

[0075] Example 4

[0076] The difference from Example 1 is that in step S1, the molar ratio of lithium carbonate to iron phosphate is 1.1:1, the D50 particle size of the first milled mixture is 100 nm, and the mass ratio of the first intermediate A1 to the first intermediate A2 is 2.5:1.

[0077] In step S3, the pore volume of the aluminum phosphate molecular sieve is 0.4 cm³. 3 / g, with an average pore size of 10nm. The pulse sintering process is as follows: the temperature is raised to 1000℃ for the first pulse sintering, with a single pulse time of 100s, and then the temperature is lowered to 500℃ for the second pulse sintering, with a single pulse time of 100s, to obtain lithium iron phosphate material.

[0078] In the lithium iron phosphate material, the first lithium iron phosphate particle has a D50 size of 1.3 μm, the second lithium iron phosphate particle has an average D50 size of 220 nm, the mass ratio of the first lithium iron phosphate particles to the second lithium iron phosphate particles is 2.5:1, the first coating layer (fast ion conductor layer) has a thickness of 40 nm and a mass content of 0.91%, and the second coating layer has a thickness of 8 nm and a mass content of 1.44%.

[0079] Example 5

[0080] The difference from Example 1 is that in step S1, the surface area with a specific surface area of ​​8m² is... 2A mixture of ferric phosphate (Fe to P molar ratio of 0.980), lithium hydroxide (first lithium source), polyethylene glycol (first carbon source), and water was prepared to obtain a first mixture with a solid content of 30%. The molar ratio of lithium hydroxide to ferric phosphate was 1.09:1, calculated based on lithium in lithium hydroxide and phosphorus in ferric phosphate. The mixture was then subjected to a first grinding process (ultrafine grinding) to obtain a first ground mixture with a D50 particle size of 200 nm. The mixture was then freeze-dried at -60°C for 48 h, followed by a first sintering process at 450°C for 10 h under a nitrogen atmosphere to obtain a first intermediate with a carbon content of 1%. The first intermediate was divided into two parts, A1 and A2, with a mass ratio of 8:2.

[0081] Step S2: F-ion implantation is performed on the first intermediate A1 using hydrogen fluoride ion source to obtain the second intermediate; wherein, the mass ratio of the implanted ions to the first intermediate A1 is 0.1:100, the ion implantation energy is 5keV, and the ion implantation depth is 22nm.

[0082] Step S3 involves mixing the first intermediate A2 and the second intermediate (mass ratio 1:4), the second carbon source dodecyl phosphate, the titanium source titanium oxide (D50 particle size 100 nm), the fast ion conductor precursor aluminum phosphate molecular sieve, and the second lithium source lithium hydroxide to obtain a second mixture. The aluminum phosphate molecular sieve has a molar ratio of aluminum to phosphorus of 0.9:1 and a pore volume of 0.03 cm³. 3 / g, with an average pore size of 0.5nm. The fast ion conductor precursor and the second carbon source are calculated based on the total amount of P, the second lithium source based on the amount of lithium, and the titanium source based on the amount of titanium. The molar ratio of the total molar amount of the fast ion conductor precursor and the second carbon source, the molar amount of the second lithium source, and the molar amount of the titanium source is 3:1.05:1.9. The second mixture is subjected to a second dry grinding process, resulting in a D50 particle size of 300nm. Pulse sintering is then performed under a nitrogen atmosphere. The pulse sintering process is as follows: the temperature is raised to 900℃ for the first pulse sintering, with a single pulse duration of 300s, followed by cooling to 600℃ for the second pulse sintering, with a single pulse duration of 80s, yielding lithium iron phosphate material.

[0083] The lithium iron phosphate material has a first lithium iron phosphate particle with a D50 size of 1.3 μm and a second lithium iron phosphate particle with a D50 size of 220 nm. The mass ratio of the first lithium iron phosphate particles to the second lithium iron phosphate particles is 4:1. The first lithium iron phosphate core contains fluorine as a dopant at a doping amount of 2500 μg / g. The first coating layer has a thickness of 48 nm and a mass content of 1%. The second coating layer has a thickness of 15 nm and a mass content of 1.62%.

[0084] Example 6

[0085] The difference from Example 1 is that in step S1, the surface area with a specific surface area of ​​8m² is... 2 A mixture of ferric phosphate (Fe to P molar ratio of 0.980), lithium hydroxide (first lithium source), polyethylene glycol (first carbon source), and water was prepared to obtain a first mixture with a solid content of 60%. The molar ratio of lithium hydroxide to ferric phosphate was 1.09:1, calculated based on lithium in lithium hydroxide and phosphorus in ferric phosphate. The mixture was then subjected to a first grinding process (ultrafine grinding) to obtain a first ground mixture with a D50 particle size of 180 nm. The mixture was then freeze-dried at -60°C for 48 h, followed by a first sintering process at 550°C for 3 h under a nitrogen atmosphere to obtain a first intermediate with a carbon content of 0.1%. The first intermediate was divided into two parts, resulting in first intermediate A1 and first intermediate A2, with a mass ratio of 6.5:3.5.

[0086] In step S2, fluorine gas is used as an ion source to implant F ions into the first intermediate A1 to obtain the second intermediate; wherein the mass ratio of the implanted ions to the first intermediate A1 is 0.08:100, the ion implantation energy is 2keV, and the ion implantation depth is 8nm.

[0087] Step S3 involves mixing the first intermediate A2, the second intermediate, the second carbon source dodecyl phosphate, the titanium source titanium dioxide (D50 particle size of 100 nm), the fast ion conductor precursor aluminum phosphate molecular sieve, and the second lithium source lithium hydroxide to obtain a second mixture. The aluminum phosphate molecular sieve has a molar ratio of aluminum to phosphorus of 0.85:1 and a pore volume of 0.5 cm³. 3 / g, with an average pore size of 20nm. The fast ion conductor precursor and the second carbon source are calculated based on the total amount of P, the second lithium source based on the amount of lithium, and the titanium source based on the amount of titanium. The molar ratio of the total molar amount of the fast ion conductor precursor and the second carbon source, the molar amount of the second lithium source, and the molar amount of the titanium source is 3:1.3:1.5. The second mixture is subjected to a second dry grinding process, resulting in a D50 particle size of 250nm. Pulse sintering is then performed under a nitrogen atmosphere. The pulse sintering process is as follows: the temperature is raised to 950℃ for the first pulse sintering, with a single pulse duration of 250s, followed by cooling to 650℃ for the second pulse sintering, with a single pulse duration of 70s, yielding lithium iron phosphate material.

[0088] The lithium iron phosphate material has a first lithium iron phosphate particle with a D50 size of 1.4 μm and a second lithium iron phosphate particle with a D50 size of 240 nm. The mass ratio of the first lithium iron phosphate particles to the second lithium iron phosphate particles is 1.85:1. The first lithium iron phosphate core contains fluorine as a dopant at a doping amount of 200 μg / g. The first coating layer has a thickness of 6 nm and a mass content of 0.1%. The second coating layer has a thickness of 5 nm and a mass content of 1.36%.

[0089] Example 7

[0090] The difference from Example 1 is that the D50 particle size of the first lithium iron phosphate particle in the lithium iron phosphate material is 1.54 μm, and the D50 particle size of the second lithium iron phosphate particle is 400 nm, thus obtaining the lithium iron phosphate material.

[0091] Example 8

[0092] The difference from Example 1 is that the total thickness of the first composite coating layer and the second composite coating layer is 50 nm, and the thickness ratio of the first coating layer and the second coating layer is 5:1, thus obtaining the lithium iron phosphate material.

[0093] Example 9

[0094] The difference from Example 1 is that the total thickness of the first composite coating layer and the second composite coating layer is 100 nm, and the thickness ratio of the first coating layer and the second coating layer is 10:1, thus obtaining the lithium iron phosphate material.

[0095] Example 10

[0096] The difference from Example 1 is that the ion implantation energy is 5keV and the ion implantation depth is 30nm, ultimately yielding lithium iron phosphate material.

[0097] Example 11

[0098] The difference from Example 1 is that the ion implantation energy is 10keV and the ion implantation depth is 120nm, ultimately yielding lithium iron phosphate material.

[0099] Example 12

[0100] The difference from Example 1 is that, based on the total amount of P in the fast ion conductor precursor and the second carbon source, based on the amount of lithium in the second lithium source, and based on the amount of titanium in the titanium source, the ratio of the total molar amount of the fast ion conductor precursor and the second carbon source, the molar amount of the second lithium source, and the molar amount of the titanium source is 3:1:2, and finally, lithium iron phosphate material is obtained.

[0101] Example 13

[0102] The difference from Example 1 is that, based on the total amount of P in the fast ion conductor precursor and the second carbon source, based on the amount of lithium in the second lithium source, and based on the amount of titanium in the titanium source, the ratio of the total molar amount of the fast ion conductor precursor and the second carbon source, the molar amount of the second lithium source, and the molar amount of the titanium source is 2:1.0:1.5, and finally, lithium iron phosphate material is obtained.

[0103] Example 14

[0104] The difference from Example 1 is that the pulse sintering process is as follows: the temperature is raised to 800°C for the first pulse sintering, with a single pulse duration of 300s, followed by cooling to 500°C for the second pulse sintering, with a single pulse duration of 100s, to obtain lithium iron phosphate material. Example 15

[0105] The difference from Example 1 is that the pulse sintering process is as follows: the temperature is raised to 1200°C for the first pulse sintering, and the single pulse time is 300s. Then the temperature is lowered to 900°C for the second pulse sintering, and the single pulse time is 50s, to obtain lithium iron phosphate material.

[0106] Example 16

[0107] The difference from Example 1 is that in step S2, boric acid and fluorine gas are used to implant B ions and F ions into the first intermediate A1 to obtain the second intermediate, and finally the lithium iron phosphate material is obtained. The molar ratio of boron to fluorine in the first lithium iron phosphate core is 1:1.

[0108] Example 17

[0109] The difference from Example 1 is that in step S2, boric acid and fluorine gas are used to implant B ions and F ions into the first intermediate A1 to obtain the second intermediate, and finally the lithium iron phosphate material is obtained. The molar ratio of boron to fluorine in the first lithium iron phosphate core is 2:1.

[0110] Comparative Example 1

[0111] The difference from Example 1 is that the first intermediate A1 is not ion implanted. Instead, all of the first intermediate is mixed with the second carbon source tributyl phosphate, the titanium source titanium oxide, and the fast ion conductor precursor aluminum phosphate molecular sieve to finally obtain lithium iron phosphate material.

[0112] Comparative Example 2

[0113] The difference from Example 1 is that the second mixture is subjected to a second grinding and then sintered at 800°C under a nitrogen atmosphere to obtain lithium iron phosphate material.

[0114] Comparative Example 3

[0115] The difference from Example 1 is that phosphoric acid is used instead of aluminum phosphate molecular sieve, a fast ion conductor precursor, to finally obtain lithium iron phosphate material.

[0116] Comparative Example 4

[0117] The difference from Example 1 is that glucose is used instead of tributyl phosphate as the second carbon source, and lithium iron phosphate material is finally obtained.

[0118] Comparative Example 5

[0119] The difference from Example 1 is that the D50 particle size of the first lithium iron phosphate particle is 2.0 μm, the D50 particle size of the second lithium iron phosphate particle is 100 nm, the D50 particle size of the first lithium iron phosphate particle is 1.9 μm larger than that of the second lithium iron phosphate particle, and the mass ratio of the first lithium iron phosphate particle to the second lithium iron phosphate particle is 1:1, thus obtaining the lithium iron phosphate material.

[0120] Comparative Example 6

[0121] The difference from Example 1 is that the temperature of the first pulse sintering is 600°C and the temperature of the second pulse sintering is 800°C, and lithium iron phosphate material is finally obtained.

[0122] Test method:

[0123] Compacted density: The compacted density tester was used for testing, and the test pressure was 3t.

[0124] Electrical performance: The lithium iron phosphate cathode material, PVDF, and Super-P prepared in the examples and comparative examples were mixed with solvent NMP in a weight ratio of 90:5:5 and homogenized to form a slurry with a solid content of 30%. The slurry was coated, dried, and punched to obtain a circular cathode sheet. Finally, the cathode sheet, separator, and lithium sheet were assembled into a coin cell for testing in a glove box. The separator was a polypropylene microporous membrane, and the electrolyte was prepared by mixing 1 mol / L LiPF6 with an organic solvent (ethylene carbonate EC, dimethyl carbonate DMC, and ethyl methyl carbonate EMC in a volume ratio of 1:1:1).

[0125] The button cell battery was charged and discharged within a voltage range of 2.0~4.0V. The test procedure was as follows: charge to 4.0V at 0.2C, then discharge to 2.0V at 0.2C, then charge to 4.0V at 1C, then discharge to 2.0V, and finally charge to 4.0V at 2C, then discharge to 4.0V. The discharge capacity was recorded.

[0126] The test results are shown in Table 1.

[0127] Table 1

[0128]

[0129] As can be seen from the above, the lithium iron phosphate material in Comparative Example 1 lacks boron doping, resulting in limited large particle growth and a significant decrease in compaction density. Comparative Example 2 did not employ pulse sintering, making it difficult to form the surface and interface structure, thus reducing rate performance. Comparative Example 3 did not introduce aluminum phosphate molecular sieves; instead, phosphoric acid was used to equimolarly replace phosphorus (P), preventing the formation of a coating layer through the molecular sieve's pore structure, hindering ion transport, and thus reducing rate performance. Comparative Example 4 did not use a phosphate ester carbon source, resulting in poor carbon coating and reduced controllability of particle growth, leading to decreased rate and compaction density. In Comparative Example 5, the particle size of the mixture after the first grinding was too large, resulting in poor elemental uniformity, reduced reactivity, inability to form a high compaction density gradation, and poor rate performance. In Comparative Example 6, the second sintering temperature was higher than the first sintering temperature, leading to abnormal phase formation of lithium iron phosphate particles, significantly affecting both powder compaction density and rate performance.

[0130] in, Figure 1 This is an HRTEM image of the lithium iron phosphate material in Example 1, from... Figure 1 As can be seen in (a), there is an LTP coating layer (Ti aggregate) with a thickness of 15 nm on the surface of the particles. Figure 1 As can be seen in (b), the surface of LFP particles has a double coating layer with different morphologies. The outer layer is the second coating layer 2 (carbon coating layer) with a thickness of 5 nm, and the inner layer is the first coating layer 1 (fast ion conductor coating layer) with a thickness of 15 nm.

[0131] Figure 2 This is a SEM image of the lithium iron phosphate material in Example 1. Figure 2 As can be seen, the lithium iron phosphate material in Example 1 is composed of two types of particles of different sizes, with the large particles having an average particle size of 1.5 μm and the small particles having an average particle size of 250 nm.

[0132] As can be seen from the above description, the embodiments of the present invention achieve the following technical effects:

[0133] This application introduces boron (B) and / or ferric (F) elements into the lithium iron phosphate (LFP) core, which effectively broadens the lithium-ion diffusion channels and enhances the stability of the crystal structure under high compaction conditions. Coating the LFP core with a fast ion-conducting material significantly improves the interfacial lithium-ion transport rate, while the carbon coating optimizes the electronic conductivity of the LFP material, thereby improving mass transfer efficiency and enhancing electrolyte wetting. The synergistic effect of these three elements significantly increases the powder compaction density of the LFP material to the aforementioned range, while maintaining high ion diffusion efficiency and structural integrity. This fundamentally overcomes the technical bottleneck of ion transport obstruction and lattice distortion instability caused by compaction in traditional high-compaction LFP materials, thereby improving the energy density, rate performance, and cycle life of the LFP material, ultimately enhancing the overall performance of lithium-ion batteries.

[0134] The above are merely embodiments of the present invention and are not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A lithium iron phosphate material, characterized in that, The lithium iron phosphate material includes first lithium iron phosphate particles and second lithium iron phosphate particles, and the mass ratio of the first lithium iron phosphate particles to the second lithium iron phosphate particles is (1~4):

1. The first lithium iron phosphate particle includes a first lithium iron phosphate core and a first composite coating layer covering the outer surface of the first lithium iron phosphate core; the first lithium iron phosphate core contains a doping element, wherein the doping element is boron and / or fluorine; the second lithium iron phosphate particle includes a second lithium iron phosphate core and a second composite coating layer covering the outer surface of the second lithium iron phosphate core; the first composite coating layer and the second composite coating layer each independently include a first coating layer and a second coating layer, wherein the first coating layer includes a fast ion conductor material and the second coating layer is a carbon coating layer; The D50 particle size of the first lithium iron phosphate particle is 1.00~1.50 μm larger than that of the second lithium iron phosphate particle; the powder compaction density of the lithium iron phosphate material is 2.7~2.8 g / cm³. 3 .

2. The lithium iron phosphate material according to claim 1, characterized in that, The first lithium iron phosphate particle has a D50 particle size of 1.2~1.9μm, and the second lithium iron phosphate particle has a D50 particle size of 200~400nm; And / or, the mass content of the doping element in the first lithium iron phosphate core is 100~3000μg / g.

3. The lithium iron phosphate material according to claim 1 or 2, characterized in that, The thickness of the first composite coating layer and the second composite coating layer are each independently 1~65nm; And / or, the mass percentage of the first coating layer in the first lithium iron phosphate particle and the second lithium iron phosphate particle is independently 0.01~1.00%; the thickness of the first coating layer is 5~50nm; and / or, the mass percentage of the second coating layer in the first lithium iron phosphate particle and the second lithium iron phosphate particle is 1.0~1.65%, and the thickness of the second coating layer is 1~30nm; and / or, the fast ion conductor material is lithium titanium aluminum phosphate.

4. A method for preparing the lithium iron phosphate material according to any one of claims 1 to 3, characterized in that, The preparation method includes: Step S1: Mix the first raw material, which includes iron phosphate, a first lithium source and a first carbon source, and then perform first grinding, freeze drying and first sintering in sequence to obtain a first intermediate; divide the first intermediate into two parts to obtain first intermediate A1 and first intermediate A2. Step S2: Ion implantation is performed on the first intermediate A1 using an ion source to obtain a second intermediate; the ion source includes a B ion source and / or an F ion source; Step S3: The second raw material, which includes the first intermediate A2, the second intermediate, the second carbon source, the second lithium source, the titanium source and the fast ion conductor precursor, is mixed and then subjected to second grinding and pulse sintering in sequence to obtain the lithium iron phosphate material. The second carbon source includes an alkyl phosphate ester; the pulse sintering includes a first pulse sintering and a second pulse sintering performed sequentially, wherein the temperature of the second pulse sintering is lower than the temperature of the first pulse sintering.

5. The preparation method according to claim 4, characterized in that, In step S1, the first lithium source is calculated based on the lithium element therein, and the iron phosphate is calculated based on the phosphorus element therein. The molar ratio of the first lithium source to the iron phosphate is (1.005~1.1):

1. And / or, the specific surface area of ​​the iron phosphate is 8~10 m². 2 / g, wherein the molar ratio of Fe to P in the iron phosphate is (0.97~0.99):1; the first lithium source and the second lithium source are each independently selected from any one or more of lithium carbonate, lithium hydroxide and lithium hydroxide monohydrate; the first carbon source is selected from one or more of glucose, sucrose and polyethylene glycol; And / or, the first raw material further includes a solvent, wherein the solvent is water; And / or, the first grinding is an ultrafine grinding, the D50 particle size of the mixture after the first grinding is 150~250nm, and the solid content of the first raw material is 30~60%; And / or, the freeze-drying temperature is -40 to -80°C, and the freeze-drying time is 12 to 48 hours; And / or, the atmosphere for the first sintering is nitrogen and / or argon, the temperature for the first sintering is 450~550℃, and the time for the first sintering is 3~10h; the mass content of carbon in the first intermediate is 0.01~1%; And / or, the mass ratio of the first intermediate A1 to the first intermediate A2 is (1~4):

1.

6. The preparation method according to claim 4 or 5, characterized in that, In step S2, the mass ratio of B ions and / or F ions in the ion source to the mass of the first intermediate Al is 0.05~0.1:100; And / or, the ion source is any one or more of boron trifluoride, diborane, fluorine, carbon tetrafluoride, and hydrogen fluoride; And / or, the ion implantation energy is 1~5keV, and the ion implantation depth is 5~30nm.

7. The preparation method according to claim 4 or 5, characterized in that, In step S3, the alkyl phosphate is selected from any one or more of tributyl phosphate, dodecyl phosphate, and tetradecyl phosphate; the titanium source is titanium oxide, and the D50 particle size of the titanium source is 50~200 nm; the fast ion conductor precursor is aluminum phosphate molecular sieve; the molar ratio of Al to P in the aluminum phosphate molecular sieve is 0.8~1:1, and the pore volume of the aluminum phosphate molecular sieve is 0.01~0.5 cm³. 3 / g, wherein the average pore size of the aluminum phosphate molecular sieve is 0.1~20nm; The total amount of P in the fast ion conductor precursor and the second carbon source, the amount of lithium in the second lithium source, and the amount of titanium in the titanium source are all calculated. The ratio of the total molar amount of the fast ion conductor precursor and the second carbon source, the molar amount of the second lithium source, and the molar amount of the titanium source is 3:1.0~1.3:1.5~2.

0. The mass ratio of the first intermediate A2 to the second intermediate is 1:(1~4).

8. The preparation method according to claim 4 or 5, characterized in that, In step S3, the second grinding is dry grinding, and the D50 particle size of the mixture after the second grinding is 200~300nm. And / or, the temperature of the first pulse sintering is 800~1000℃, and the single pulse time of the first pulse sintering is 100~300s; The temperature of the second pulse sintering is 500~700℃, the single pulse time of the second pulse sintering is 50~100s; the voltage of the pulse sintering is 10~80V, the current of the pulse sintering is 0.5~80A, and the atmosphere of the pulse sintering is nitrogen and / or argon.

9. A positive electrode sheet, comprising a current collector and a positive electrode active layer disposed on at least one surface of the current collector, the positive electrode active layer comprising a positive electrode active material, characterized in that, The positive electrode active material is the lithium iron phosphate material according to any one of claims 1 to 3 or prepared by the preparation method according to any one of claims 4 to 8.

10. A lithium-ion battery, comprising a positive electrode, a negative electrode, and a separator, characterized in that, The positive electrode is the positive electrode as described in claim 9.

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