Phosphate-based positive electrode material, preparation method thereof and lithium ion battery

Abstract

The application provides a phosphate-based positive electrode material, a preparation method thereof and a lithium ion battery, and relates to the technical field of battery materials. The preparation method of the positive electrode material comprises the following steps: mixing first raw materials containing a first phosphorus source, a lithium source, a metal M source and a carbon source, performing first sintering, and obtaining first crystalline bodies; mixing the first crystalline bodies with second raw materials containing a second phosphorus source, the lithium source, the metal M source, the carbon source and a doping precursor, performing second sintering, and obtaining second crystalline bodies; mixing the second crystalline bodies with the second raw materials, performing third sintering, and obtaining third crystalline bodies; mixing the third crystalline bodies with the second raw materials, performing fourth sintering, and obtaining the phosphate-based positive electrode material. With the increase of the sintering times, the sintering temperature is increased in a gradient, which is beneficial to realize layered crystallization. The second phosphorus source is excessively added, so that the P element and the doping element form a ligand, the self-agglomeration of the doping element in the cycle process is effectively inhibited, and the cycle stability is improved.

Description

Technical Field

[0001] This application relates to the field of battery materials technology, and in particular to a phosphate-based cathode material, its preparation method, and a lithium-ion battery. Background Technology

[0002] The main problems with current lithium manganese iron phosphate (LFP) cathode materials include manganese plateau decay, poor cycle performance, and poor high-temperature storage. The key factors causing these problems lie in the stability of the crystal structure, lithium-ion diffusion resistance, and electron conductivity. Current methods to improve these issues mainly include carbon coating, elemental doping, particle morphology control, and particle size control. Among these, elemental doping is the most common and effective method. By doping different sites, the one-dimensional transport channels of lithium ions can be broadened, or the tetrahedral covalent bonds of PO in phosphate-based materials can be broken, thereby improving electron conductivity.

[0003] For practical applications of lithium-ion batteries, the main concerns are high-temperature storage, cycle performance, and safety. However, as is well known, the cycle performance of phosphate-based cathode materials is primarily related to intrinsic factors such as crystal structure, main element loss, and structural collapse. Although the crystal volume of phosphate-based cathode materials does not change significantly, their structure changes rapidly, leading to intra-lattice aggregation of doped metals, which severely affects cycle performance.

[0004] Currently, the industry lacks effective methods to suppress or resolve this problem. Therefore, a new method is needed to improve the cycle performance of phosphate-based cathode materials. Summary of the Invention

[0005] The purpose of this application is to provide a phosphate-based cathode material, its preparation method, and a lithium-ion battery. By employing a novel doping modification method, the doping modification of the dopant element can be achieved, and the self-doping of the main element and the coordination and protection of the dopant element can be utilized. This largely suppresses the self-aggregation of dopant elements in the crystal lattice during cycling, thereby effectively improving the cycle stability of lithium-ion batteries made from phosphate-based cathode materials.

[0006] To achieve the above objectives, the technical solution of this application is as follows:

[0007] This application provides a method for preparing a phosphate-based cathode material, including:

[0008] A first raw material comprising a first phosphorus source, a lithium source, a metal M source, and a carbon source is mixed and subjected to a first sintering to obtain a first crystalline body.

[0009] The first crystal is mixed with a second raw material containing a second phosphorus source, the lithium source, the metal M source, the carbon source and the precursor of the doped element, and then subjected to a second sintering to obtain a second crystal.

[0010] The second crystal is mixed with the second raw material and then subjected to a third sintering process to obtain the third crystal.

[0011] The third crystal is mixed with the second raw material and subjected to a fourth sintering to obtain the phosphate-based cathode material;

[0012] As the number of sintering cycles increases, the sintering temperature increases in a gradient.

[0013] In the second raw material, the second phosphorus source is added in excess.

[0014] Preferably, in the second raw material, the molar ratio of P element in the second phosphorus source to Li element in the lithium source is (1.001~1.03):1;

[0015] And / or, in the second raw material, the molar ratio of the dopant element in the doped element precursor to the Li element in the lithium source is (0.001~0.05):1;

[0016] And / or, in the second raw material, the second phosphorus source includes an inorganic phosphorus source and an organic phosphorus source, wherein the molar ratio of P element in the organic phosphorus source to dopant element in the dopant element precursor is (2.2~5.0):1;

[0017] And / or, in the first raw material, the mass of the carbon source is 8%-15% of the mass of the first crystal;

[0018] And / or, in the second raw material, the mass of the carbon source is 8% to 15% of the mass of the crystals obtained by sintering the second raw material itself.

[0019] Preferably, when the metal source M is an iron source and a manganese source, the molar ratio of Li:Mn:Fe:P in the first raw material is (1~1.05):x:y:1, where x+y=1, 0 <x<1,0<y<1;

[0020] And / or, when the metal source M is an iron source and a manganese source, the molar ratio of Li:Mn:Fe:doping element:P in the second raw material is 1:x:y:z:(1.001~1.03), where x+y+z=1, z=0.001~0.05, 0 <x<1-z,0<y<1-z;

[0021] And / or, the molar ratio of Li element in the second raw material to Li element in the first crystal is (0.05~0.3):1.

[0022] Preferably, the temperature of the first sintering is 350℃~450℃;

[0023] And / or, the second sintering temperature is 500℃~600℃;

[0024] And / or, the temperature of the third sintering is 650℃~730℃;

[0025] And / or, the temperature of the fourth sintering is 750℃~800℃;

[0026] And / or, the first sintering, the second sintering, the third sintering and the fourth sintering are all carried out in an inert gas atmosphere;

[0027] And / or, in the first sintering, the second sintering, the third sintering and the fourth sintering processes, the heating rate in the heating stage and the cooling rate in the cooling stage are each independently 0.5℃ / min to 1.5℃ / min;

[0028] And / or, the sintering time of the first sintering, the second sintering, the third sintering and the fourth sintering is each independently 5h to 8h;

[0029] And / or, with the increase of the number of sintering times, the sintering temperature of the later sintering is 50℃~150℃ higher than that of the previous sintering.

[0030] Preferably, before the first sintering, the method further includes: mixing the first raw material with a solvent to obtain a mixture; drying and crushing the mixture to obtain a first precursor;

[0031] And / or, before the second sintering, the method further includes: dispersing the first crystal in a solvent to form a first slurry; adding the second raw material to the first slurry, stirring, and then ball milling and drying to obtain a second precursor;

[0032] And / or, before the third sintering, the method further includes: dispersing the second crystal in a solvent to form a second slurry; adding the second raw material to the second slurry, stirring, then ball milling, and drying to obtain a third precursor;

[0033] And / or, before the fourth sintering, the process further includes: dispersing the third crystal in a solvent to form a third slurry; adding the second raw material to the third slurry, stirring, then ball milling, and drying to obtain the fourth precursor;

[0034] And / or, in the second sintering process, the third sintering process, and the fourth sintering process, the amount of the second raw material added is the same in each sintering process, or decreases arithmetically with the increase of the number of sintering times;

[0035] And / or, after the fourth sintering, the method further includes: mixing the fourth crystal obtained by the fourth sintering with the second raw material, performing a fifth sintering to obtain a fifth crystal; and repeating the mixing and sintering operation n times to obtain the phosphate-based cathode material, where n = 0, 1, 2, 3, 4, ..., n, and n is an integer.

[0036] More preferably, the solvent includes at least one selected from pure water, ethanol, and acetone;

[0037] And / or, the solid content in the first slurry, the second slurry and the third slurry is each independently 50% to 60%;

[0038] And / or, the arithmetic decrease is 5% to 25% of the amount of the second raw material added during the second sintering process.

[0039] Preferably, the first phosphorus source is an inorganic phosphorus source, which includes at least one of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, lithium dihydrogen phosphate, and dilithium hydrogen phosphate.

[0040] And / or, the organic phosphorus source in the second phosphorus source includes at least one of tricyclopentylphosphine, dodecylphosphonic acid, and di-tert-butyl phosphonate;

[0041] And / or, the lithium source includes at least one of lithium oxide, lithium carbonate, lithium dihydrogen phosphate, lithium hydroxide, lithium acetate, and lithium nitrate;

[0042] And / or, when the metal M source is an iron source, the iron source includes at least one of ferric chloride, ferric nitrate, ferric oxide, and ferric sulfate;

[0043] And / or, when the metal M source is a manganese source, the manganese source includes at least one of manganese dioxide, manganese nitrate, manganese sulfate, and manganese phosphate;

[0044] And / or, the carbon source includes at least one of PEG, glucose, sucrose, EG, and Ketjen black;

[0045] And / or, the doped element precursor includes at least one of the following: an oxide containing a doped element, a soluble salt containing a doped element;

[0046] And / or, the doping element in the doped element precursor includes at least one of Ti, Mg, V, and Nb.

[0047] Preferably, the phosphate-based cathode material includes any one of lithium iron phosphate, lithium manganese iron phosphate, lithium manganese phosphate, lithium vanadium phosphate, and lithium manganese iron titanium phosphate.

[0048] This application also provides a phosphate-based cathode material, which is prepared using the above-described method for preparing phosphate-based cathode materials.

[0049] This application also provides a lithium-ion battery, including the above-mentioned phosphate-based cathode material.

[0050] The beneficial effects of this application are:

[0051] In the preparation method of the phosphate-based cathode material of this application, no doping is performed during the initial sintering stage, followed by multiple doping and sintering processes. This improves the conductivity of the phosphate-based material itself. Furthermore, by controlling the temperature of each sintering stage, highly crystalline crystals are formed within the particles, achieving layered crystallization. Additionally, during doping and sintering, this application utilizes an excess phosphorus source to coordinate with the dopant element, forming uniformly dispersed ligands. This is equivalent to using the self-doping of phosphorus to form a protective layer around the dopant element, thus preventing the agglomeration of the dopant element during cycling. This application utilizes the difference in crystallinity caused by layered crystallization to promote the diffusion kinetics of lithium ions in the cathode material, reducing the diffusion resistance of lithium ions. Simultaneously, layered crystallization allows the excess phosphorus source added in each layer to fully coordinate with the dopant element to form organic ligands, and makes the ligands more uniformly dispersed in the crystal, further suppressing the self-agglomeration of the dopant element during cycling.

[0052] In the phosphate-based cathode material of this application, the ionic and electronic conductivity of the cathode material is improved by doping modification with doping elements. Furthermore, the self-doping of P element forms a protective layer around the doping element to prevent the doping element from self-aggregating during cycling. In addition, layered crystallization is used to reduce vacancy defects caused by doping elements, improve the stability of the crystal structure, ensure that the excess phosphorus source coordinates with the doping element, and ensure the dispersion of the doping element in the crystal, thereby improving the cycle stability of the lithium-ion battery. Attached Figure Description

[0053] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of this application and should not be regarded as a limitation on the scope of this application.

[0054] Figure 1 This is a schematic diagram of the preparation process of lithium manganese iron phosphate cathode material in Example 1 and Comparative Example 1;

[0055] Figure 2FIB-TEM images of cross-sections of lithium manganese iron phosphate cathode materials prepared in Example 1 and Comparative Example 1, as well as distribution maps of Ti and P elements;

[0056] Figure 3 Capacity cycle performance of lithium-ion batteries prepared in Example 1 and Comparative Example 1 after 100 charge-discharge cycles. Detailed Implementation

[0057] As used in this article:

[0058] "Prepared from" is synonymous with "comprising". The terms "comprising", "including", "having", "containing", or any other variations thereof as used herein are intended to cover non-exclusive inclusion. For example, a composition, step, method, article, or apparatus that includes the listed elements is not necessarily limited to those elements, but may include other elements not expressly listed or elements inherent to such a composition, step, method, article, or apparatus. The conjunction "composed of" excludes any unnamed elements, steps, or components.

[0059] When a quantity, concentration, or other value or parameter is expressed as a range, a preferred range, or a range defined by a series of upper and lower preferred values, this should be understood as specifically disclosing all ranges formed by any pair of any upper or preferred value with any lower or preferred value, regardless of whether the range is disclosed individually. For example, when the range “1–5” is disclosed, the described range should be interpreted as including ranges “1–4”, “1–3”, “1–2”, “1–2 and 4–5”, “1–3 and 5”, etc. When numerical ranges are described herein, unless otherwise stated, the range is intended to include its endpoints and all integers and fractions within that range.

[0060] In these embodiments, unless otherwise specified, the portions and percentages are all by weight.

[0061] "Parts by mass" refers to the basic unit of measurement that expresses the mass ratio of multiple components. One part can represent any unit mass, such as 1g or 2.689g. If we say that component A has "a" parts by mass and component B has "b" parts by mass, it means the ratio of the mass of component A to the mass of component B is a:b. Alternatively, it can mean that the mass of component A is aK and the mass of component B is bK (K is any number representing a multiplier). It is important to understand that, unlike the number of parts by mass, the sum of the mass parts of all components is not limited to 100 parts.

[0062] "And / or" is used to indicate that one or both of the described situations may occur, for example, A and / or B includes (A and B) and (A or B).

[0063] This application provides a method for preparing a phosphate-based cathode material, including:

[0064] S1. Mix the first raw material containing the first phosphorus source, lithium source, metal M source and carbon source, and perform the first sintering to obtain the first crystal;

[0065] S2. The first crystal is mixed with a second raw material including a second phosphorus source, the lithium source, the metal M source, the carbon source and the doping element precursor, and then subjected to a second sintering to obtain a second crystal.

[0066] S3. Mix the second crystal with the second raw material and perform a third sintering to obtain the third crystal;

[0067] S4. The third crystal is mixed with the second raw material and subjected to a fourth sintering to obtain the phosphate-based cathode material.

[0068] Among them, as the number of sintering times increases, the sintering temperature in S1, S2, S3 and S4 increases in a gradient; and in the second raw materials of S2, S3 and S4, the second phosphorus source is added in excess.

[0069] It should be noted that, in the preparation of phosphate-based cathode materials, the molar ratio between Li (Li) and P (P) in the lithium source should theoretically be 1:1. For example, the molar ratio of Li to P in lithium iron phosphate cathode materials is 1:1. Therefore, the excessive addition of the second phosphorus source in this application can be indicated by the fact that the molar number of P in the second phosphorus source is higher than the molar number of Li in the second lithium source.

[0070] In one embodiment of this application, the molar ratio of phosphorus (P) in the second phosphorus source to lithium (Li) in the second raw material is (1.001–1.03):1, for example, it can be 1.001:1, 1.005:1, 1.01:1, 1.015:1, 1.02:1, 1.025:1, 1.03:1, or any value between (1.001–1.03):1. This means that the molar content of P in the second phosphorus source is 0.1 mol%–3 mol% higher than the molar content of Li in the second raw material lithium source.

[0071] In one embodiment of this application, the second phosphorus source in the second raw material includes an inorganic phosphorus source and an organic phosphorus source; wherein the molar ratio of the P element in the organic phosphorus source to the dopant element in the dopant precursor is (2.2 to 5.0):1, for example, it can be 2.2:1, 2.5:1, 3:1, 3.5:1, 4:1, 4.5:1, 5:1 or any value between (2.2 to 5.0):1.

[0072] In one embodiment of this application, the inorganic phosphorus source includes at least one of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, lithium dihydrogen phosphate, and dilithium hydrogen phosphate; the organic phosphorus source includes at least one of tricyclopentylphosphine, dodecylphosphonic acid, and di-tert-butyl phosphonate.

[0073] It should be noted that in the prepared phosphate-based cathode material, phosphorus atoms mainly exist in the form of PO tetrahedra. Dopants will enter lithium sites or iron and manganese sites, and the dopants will be surrounded by a relatively large number of phosphorus-oxygen tetrahedra. When the second phosphorus source includes a mixture of organic and inorganic phosphorus sources, the organic phosphorus will form organic ligands with the added dopants, surrounding the dopants. This is equivalent to forming a protective layer around the dopants, which effectively protects the dopants from entering the crystal lattice and ensuring uniform dispersion, further preventing self-aggregation between dopants.

[0074] This application achieves self-doping coordination of the main element by using an excess of organic phosphorus source, which can greatly suppress the aggregation of dopants in the lattice during cycling and improve the cycling performance of phosphate-based cathode materials.

[0075] In one embodiment of this application, the first sintering temperature is 350°C to 450°C, for example, it can be 350°C, 370°C, 380°C, 400°C, 420°C, 450°C, or any value between 350°C and 450°C.

[0076] In one embodiment of this application, the second sintering temperature is 500°C to 600°C, for example, it can be 500°C, 520°C, 540°C, 560°C, 580°C, 600°C or any value between 500°C and 600°C.

[0077] In one embodiment of this application, the third sintering temperature is 650°C to 730°C, for example, it can be 650°C, 680°C, 700°C, 710°C, 720°C, 730°C or any value between 650°C and 730°C.

[0078] In one embodiment of this application, the fourth sintering temperature is 750°C to 800°C, for example, it can be 750°C, 760°C, 770°C, 780°C, 790°C, 800°C or any value between 750°C and 800°C.

[0079] In one embodiment of this application, the first sintering, the second sintering, the third sintering, and the fourth sintering are each independently heated at a heating rate of 0.5℃ / min to 1.5℃ / min, for example, 0.5℃ / min, 0.6℃ / min, 0.8℃ / min, 1.0℃ / min, 1.2℃ / min, 1.5℃ / min, or any value between 0.5℃ / min and 1.5℃ / min, and then held at that temperature for 5h to 8h, for example, 5h, 6h, 7h, 8h, or any value between 5h and 8h; and then each is independently cooled at a cooling rate of 0.5℃ / min to 1.5℃ / min.

[0080] In one embodiment of this application, as the number of sintering cycles increases, the sintering temperature of the later sintering cycle is increased by 50°C to 150°C compared to the previous sintering temperature. For example, it can be 50°C, 60°C, 80°C, 100°C, 120°C, 150°C, or any value between 50°C and 150°C.

[0081] This application first performs low-temperature sintering, then gradually increases the sintering temperature to a high-temperature sintering, with each heating and cooling rate being relatively slow, resulting in a long sintering process. The purpose of this is to allow a highly crystalline core material to fully form inside the particles. Subsequently, a second raw material containing doped elements and an excess of phosphorus source is continuously added for sintering, which can self-dopat to form a layered crystalline cathode material product.

[0082] The purpose of layered crystallization is twofold: first, to allow the excess organophosphorus source to coordinate with the dopant elements to form uniformly dispersed ligands, which are more dispersed within the bulk phase of the crystal; second, different sintering temperatures will result in differences in crystallinity between layers, especially as the sintering temperature increases, which will promote the formation of poor diffusion kinetics of lithium ions, leading to faster insertion and extraction of lithium ions.

[0083] In one embodiment of this application, the first phosphorus source in the first raw material is an inorganic phosphorus source. Since no doping element is added to the first raw material, it is not necessary to add an excessive amount of organic phosphorus source to form organic ligands with the doping element when preparing the first crystal.

[0084] In one embodiment of this application, the lithium source in the first raw material and the second raw material includes at least one of lithium oxide, lithium carbonate, lithium dihydrogen phosphate, lithium hydroxide, lithium acetate, and lithium nitrate; the carbon source includes at least one of PEG, glucose, sucrose, EG, and Ketjen black.

[0085] In one embodiment of this application, the phosphate-based cathode material includes at least one of lithium iron phosphate, lithium manganese iron phosphate, lithium manganese phosphate, lithium vanadium phosphate, and lithium manganese iron titanium phosphate.

[0086] When the phosphate-based positive electrode material is a lithium iron phosphate positive electrode material, the metal M source is an iron source; when the phosphate-based positive electrode material is a lithium manganese phosphate positive electrode material, the metal M source is a manganese source; when the phosphate-based positive electrode material is a lithium manganese iron phosphate positive electrode material, the metal M source is an iron source and a manganese source; when the phosphate-based positive electrode material is a lithium vanadium phosphate positive electrode material, the metal M source is a vanadium source.

[0087] Wherein, the iron source comprises at least one of ferric chloride, ferric nitrate, ferric oxide and ferric sulfate; the manganese source comprises at least one of manganese dioxide, manganese nitrate, manganese sulfate and manganese phosphate.

[0088] In one embodiment of the present application, the doping precursor in the second raw material comprises an oxide or soluble salt containing a doping element.

[0089] Further preferably, the doping element comprises at least one of Ti, Mg, V and Nb, and the doping precursor comprises at least one of TiO₂, TiCl₄, MgO, MgCl₂, V₂O₅ and Nb₂O₅.

[0090] In one embodiment of the present application, in the second raw material, the molar ratio of the doping element in the doping element precursor to the Li element in the lithium source is (0.001~0.05):1, for example, it can be 0.001:1, 0.005:1, 0.01:1, 0.02:1, 0.03:1, 0.04:1, 0.05:1, or any value between (0.001~0.05):1.

[0091] In one embodiment of the present application, the mass of the carbon source in the first raw material is 8% to 15% of the mass of the first crystal.

[0092] In one embodiment of the present application, the mass of the carbon source in the second raw material is 8% to 15% of the mass of the crystal obtained by sintering the second raw material itself.

[0093] By limiting the carbon source, it can be ensured that the surface of the sintered crystal has a carbon coating layer, thereby maintaining the stability of the crystal structure and improving the electron conduction capability.

[0094] In one embodiment of the present application, when the metal M source is an iron source and a manganese source, the molar ratio of Li element: Mn element: Fe element: P element in the first raw material is (1~1.05):x:y:1, wherein x+y=1, 0<x<1, 0<y<1, for example, it can be 1:0.3:0.7:1, 1:0.5:0.5:1, 1:0.6:0.4:1, 1:0.8:0.2:1, 1.05:0.7:0.3:1 or 1.03:0.55:0.45:1, more preferably 1:0.6:0.4:1.

[0095] In one embodiment of the present application, when the metal M source is an iron source and a manganese source, the molar ratio of Li element: Mn element: Fe element: doping element: P element in the second raw material is 1:x:y:z:(1.001 to 1.03), wherein x+y+z=1, z=0.001 to 0.05, 0<x<1-z, 0<y<1-z, for example, it can be 1:0.3:0.7:0.001:1.001, 1:0.5:0.5:0.01:1.005, 1:0.6:0.4:0.02:1.01, 1:0.8:0.2:0.03:1.02 or 1:0.7:0.3:0.04:1.03, more preferably 1:0.6:0.4:(0.001 to 0.05):(1.001 to 1.03).

[0096] In one embodiment of the present application, the molar ratio of the Li element in the second raw material to the Li element in the first crystal is (0.05 to 0.3):1. Through the molar ratio of Li elements, it is shown that in the second, third or fourth sintering process, after the second raw material added in each sintering is sintered, the molar content of the obtained doped phosphate material is 5% to 30% of the molar content of the first crystal.

[0097] In one embodiment of the present application, in the second sintering process, the third sintering process and the fourth sintering process, the addition amount of the second raw material is the same in each sintering process, or decreases arithmetically with the increase of the number of sintering times.

[0098] Further preferably, the arithmetically decreasing reduction amount is 5% to 25% of the addition amount of the second raw material in the second sintering process.

[0099] It can be understood that the addition amount of the second raw material is the same in each sintering process, that is, in the second sintering and each subsequent sintering process, the same amount of the second raw material is added each time to mix with the sintered crystal. Alternatively, the addition amount of the second raw material decreases arithmetically with the increase of the number of sintering times, and it is calculated according to the proportion that the arithmetically decreasing reduction amount each time is 5% to 25% of the addition amount of the second raw material in the second sintering process.

[0100] For example, if the second raw material lithium source is 0.5 mol during the second sintering process, then when adding the second raw material to the second crystal obtained by sintering for the third sintering, the amount of lithium source in the added second raw material can be reduced by 0.025 mol to 0.125 mol, that is, the amount of lithium source added is 0.375 mol to 0.475 mol. Subsequently, when adding the second raw material to the third crystal obtained by sintering for the fourth sintering, the amount of lithium source in the added second raw material can be further reduced by 0.025 mol to 0.125 mol based on the amount of lithium source in the second raw material during the third sintering, that is, the amount of lithium source added during the third sintering is 0.4 mol, which is 0.1 mol less than in the second sintering. Therefore, the amount of lithium source added during the fourth sintering is 0.3 mol, which is further reduced by 0.1 mol. Similarly, because there is a fixed molar ratio between the phosphorus source, metal M source, dopant precursor, etc. in the second raw material and the lithium source, as the lithium source decreases, the other raw materials will also decrease accordingly, thus making the overall amount of the second raw material used in each sintering process show an arithmetic progression.

[0101] In one embodiment of this application, before the first sintering is performed in step S1, the method further includes: mixing the first raw material with a solvent to obtain a mixture; and drying and crushing the mixture to obtain a first precursor.

[0102] Specifically, the solvent includes at least one of pure water, ethanol, and acetone. When drying the mixture, the solvent is completely evaporated by heating. The resulting solid substance is then ground and crushed, which is the first precursor.

[0103] In one embodiment of this application, before the second sintering in S2, the process further includes: dispersing the first crystal in a solvent to form a first slurry; adding a second raw material to the first slurry, stirring for 1 to 3 hours, and then ball milling and drying to obtain a second precursor.

[0104] The solvent used for dispersion also includes at least one of pure water, ethanol, and acetone, and the solid content in the first slurry is 50% to 60%, for example, it can be 50%, 52%, 55%, 58%, 60%, or any value between 50% and 60%.

[0105] In one embodiment of this application, before the third sintering in S3 and before the fourth sintering in S4, the same process steps as before the second sintering need to be repeated. For example, the crystal obtained by sintering is first made into a slurry, then a second raw material is added to the slurry, and after stirring and reacting for 1 to 3 hours, it is ball-milled and dried to obtain the precursor before sintering.

[0106] In one embodiment of this application, the first sintering, the second sintering, the third sintering, and the fourth sintering are all carried out in an inert gas atmosphere. The inert gas includes at least one of nitrogen, argon, helium, and neon.

[0107] In one embodiment of this application, after the fourth sintering in S4, the process further includes: mixing the fourth crystal obtained from the fourth sintering with the second raw material, performing a fifth sintering to obtain a fifth crystal; and repeating the similar mixing and sintering operation n times to obtain a phosphate-based cathode material, where n = 0, 1, 2, 3, 4, ..., n, and n is an integer.

[0108] Similarly, as the number of sintering cycles increases, the sintering temperature of each subsequent sintering cycle increases in a gradient compared to the previous sintering temperature.

[0109] It is understandable that the preparation method of the phosphate-based cathode material in this application is not limited to four sintering and four crystallization processes; five sintering and five crystallization processes, or even more sintering processes, can be performed to obtain a crystalline material with more layers. However, it should be noted that the overall sintering temperature transitions from low to high, meaning that the temperature of the later sintering process is higher than the temperature of the previous sintering process.

[0110] This application provides a phosphate-based cathode material, which is prepared by the above-described method for preparing phosphate-based cathode materials.

[0111] This application also provides a lithium-ion battery, which includes the aforementioned phosphate-based cathode material.

[0112] The implementation schemes of this application will be described in detail below with reference to specific embodiments. However, those skilled in the art will understand that the following embodiments are only for illustrating this application and should not be regarded as limiting the scope of this application. Unless otherwise specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments used without specified manufacturers are all conventional products that can be purchased commercially.

[0113] Example 1

[0114] This embodiment provides a lithium manganese iron phosphate cathode material (LMFP), and the synthesis process of this cathode material is shown in the schematic diagram below. Figure 1 As shown in (a), the specific preparation method includes:

[0115] (1) Weigh out 1 mol of LiNO3, 0.4 mol of Fe(NO3)3, 1 mol of NH4H2PO4, and 0.6 mol of Mn(NO3)2. Then weigh out 15.72 g of the carbon source glucose (this mass is 1 mol of LiMn). 0.6 Fe 0.4Add 10% (by mass of PO4) to 500 mL of water and mix thoroughly to form a solution. Then heat and stir until the water in the solution evaporates to obtain a solid-phase first precursor material, which is then crushed.

[0116] (2) The crushed first precursor material is placed in a sintering furnace and heated to 400℃ at a heating rate of 0.5℃ / min under a nitrogen atmosphere and kept at a constant temperature for 6 hours. Then it is cooled to room temperature at a rate of 0.5℃ / min, taken out and crushed to obtain the first crystal.

[0117] (3) The first crystal is dispersed in water (the solid content of which is about 56%), and then a second raw material is added to it, wherein the second raw material includes: 0.2 mol of LiNO3, 0.08 mol of Fe(NO3)3, 0.186 mol of NH4H2PO4, 0.016 mol of dodecylphosphonic acid (the total molar amount of phosphorus source is 0.202 mol, and the molar ratio of P element in the total phosphorus source to Li element in 0.2 mol of lithium nitrate is 1.01:1), and 0.12 mol of Mn(NO3)3. 2. 0.004 mol TiCl4 (the molar ratio of Ti in the doped precursor to Li in 0.2 mol lithium nitrate is 0.02:1; the molar ratio of P in 0.016 mol dodecylphosphonic acid to Ti in the doped precursor is 4:1), 3.144 g glucose (the mass of glucose is approximately 10% of the mass of 0.2 mol lithium manganese iron phosphate doped with Ti and P), stirred for 2 h, then ball-milled, and then the water was evaporated to obtain the second precursor.

[0118] (4) The second precursor is placed in a sintering furnace and heated to 530°C at a heating rate of 0.5°C / min under a nitrogen atmosphere. The temperature is then maintained at 530°C for 6 hours. After cooling to room temperature at a rate of 0.5°C / min, the precursor is taken out and crushed to obtain the second crystal.

[0119] (5) Disperse the second crystal in water and add the third raw material, wherein the third raw material is the same as the second raw material, and repeat the contents of step (3) to obtain the third precursor;

[0120] (6) Repeat step (4) with the third precursor, sintering temperature: 660℃, other conditions are the same, to obtain the third crystal;

[0121] (7) Disperse the third crystal in water and add the fourth raw material, which is the same as the second raw material. Repeat step (3) to obtain the fourth precursor.

[0122] (8) Repeat step (4) with the fourth precursor, sintering temperature: 760℃, other conditions are the same, to obtain the final product of the fourth crystal, which is the doped lithium manganese iron phosphate cathode material doped with Ti and P.

[0123] Example 2

[0124] This embodiment provides a lithium manganese phosphate cathode material (LMP), which is prepared in the same way as in Example 1, except that:

[0125] In step (1), instead of adding 0.4 mol of Fe(NO3)3, 1 mol of Mn(NO3)2 is added directly. In steps (3), (5), and (7), instead of adding 0.08 mol of Fe(NO3)3, 0.2 mol of Mn(NO3)2 is added directly. The other raw materials and preparation methods remain unchanged.

[0126] Example 3

[0127] This embodiment provides a lithium iron phosphate cathode material (LFP), which is prepared in the same way as in Example 1, except that:

[0128] In step (1), instead of adding 0.6 mol of Mn(NO3)2, 1 mol of Fe(NO3)3 is added directly. In steps (3), (5), and (7), instead of adding 0.12 mol of Mn(NO3)2, 0.2 mol of Fe(NO3)3 is added directly. The other raw materials and preparation methods remain unchanged.

[0129] Example 4

[0130] This embodiment provides a lithium manganese iron phosphate cathode material (LMFP), which is prepared in the same way as in Example 1, except that:

[0131] In steps (3), (5), and (7), add 0.1842 mol of NH4H2PO4 and 0.016 mol of dodecylphosphonic acid (the total number of moles of phosphorus source is 0.2002 mol, and the molar ratio of P element in the total phosphorus source to Li element in 0.2 mol of lithium nitrate is 1.001:1), while keeping the other raw materials unchanged.

[0132] Example 5

[0133] This embodiment provides a lithium manganese iron phosphate cathode material (LMFP), which is prepared in the same way as in Example 1, except that:

[0134] In steps (3), (5), and (7), 0.19 mol of NH4H2PO4 and 0.016 mol of dodecylphosphonic acid (the total number of moles of phosphorus source is 0.206 mol, and the molar ratio of P element in the total phosphorus source to Li element in 0.2 mol of lithium nitrate is 1.03:1) are added, while the other raw materials remain unchanged.

[0135] Example 6

[0136] This embodiment provides a lithium manganese iron phosphate cathode material (LMFP), which is prepared in the same way as in Example 1, except that:

[0137] In steps (3), (5), and (7), add 0.1932 mol of NH4H2PO4 and 0.0088 mol of dodecylphosphonic acid (the total phosphorus source has a molar ratio of 0.202 mol, the molar ratio of P in the total phosphorus source to Li in 0.2 mol of lithium nitrate is 1.01:1, and the molar ratio of P in 0.0088 mol of organic phosphorus source dodecylphosphonic acid to Ti in 0.004 mol of TiCl4 is 2.2:1), while the remaining raw materials remain unchanged.

[0138] Example 7

[0139] This embodiment provides a lithium manganese iron phosphate cathode material (LMFP), which is prepared in the same way as in Example 1, except that:

[0140] In steps (3), (5), and (7), add 0.182 mol of NH4H2PO4 and 0.02 mol of dodecylphosphonic acid (the total phosphorus source has a molar ratio of 0.202 mol, the molar ratio of P in the total phosphorus source to Li in 0.2 mol of lithium nitrate is 1.01:1, and the molar ratio of P in 0.02 mol of organic phosphorus source dodecylphosphonic acid to Ti in 0.004 mol of TiCl4 is 5:1), while the remaining raw materials remain unchanged.

[0141] Example 8

[0142] This embodiment provides a lithium manganese iron phosphate cathode material (LMFP), which is prepared in the same way as in Example 1, except that:

[0143] The heating rate and cooling rate in steps (2), (4), (6), and (8) are all changed to 1.5℃ / min.

[0144] Example 9

[0145] This embodiment provides a lithium manganese iron phosphate cathode material (LMFP), which is prepared in the same way as in Example 1, except that:

[0146] The heating rate and cooling rate in steps (2), (4), (6), and (8) are all changed to 1℃ / min.

[0147] Example 10

[0148] This embodiment provides a lithium manganese iron phosphate cathode material (LMFP), which is prepared in the same way as in Example 1, except that:

[0149] The sintering temperature in step (2) is 350℃; the sintering temperature in step (4) is 500℃; the sintering temperature in step (6) is 650℃; and the sintering temperature in step (8) is 750℃.

[0150] Example 11

[0151] This embodiment provides a lithium manganese iron phosphate cathode material (LMFP), which is prepared in the same way as in Example 1, except that:

[0152] The sintering temperature in step (2) is 450℃; the sintering temperature in step (4) is 600℃; the sintering temperature in step (6) is 730℃; and the sintering temperature in step (8) is 800℃.

[0153] Example 12

[0154] This embodiment provides a lithium manganese iron phosphate cathode material (LMFP), which is prepared in the same way as in Example 1, except that:

[0155] After obtaining the fourth crystal in step (8), continue to repeat the contents of steps (3) and (4) to obtain the fifth crystal, which is the final cathode material.

[0156] Example 13

[0157] This embodiment provides a lithium manganese iron phosphate cathode material (LMFP), which is prepared in the same way as in Example 1, except that:

[0158] In step (5), when repeating the content of step (3), the amount of the third raw material added changes and the following needs to be added: 0.15 mol of LiNO3, 0.06 mol of Fe(NO3)3, 0.1395 mol of NH4H2PO4, 0.012 mol of dodecylphosphonic acid, 0.09 mol of Mn(NO3)2, 0.003 mol of TiCl4, and 2.358 g of glucose (approximately 10% of the mass of 0.15 mol of lithium manganese iron phosphate doped with Ti and P).

[0159] In step (7), when repeating the content of step (3), the amount of the fourth raw material added also changes, and the following needs to be added respectively: 0.1 mol of LiNO3, 0.04 mol of Fe(NO3)3, 0.093 mol of NH4H2PO4, 0.008 mol of dodecylphosphonic acid, 0.06 mol of Mn(NO3)2, 0.002 mol of TiCl4, and 1.572 g of glucose (approximately 10% of the mass of 0.1 mol of lithium manganese iron phosphate doped with Ti and P).

[0160] Comparative Example 1

[0161] This comparative example provides a lithium manganese iron phosphate cathode material (LMFP), and a schematic diagram of the synthesis process of this cathode material is shown below. Figure 1 As shown in (b), the specific preparation method includes:

[0162] (1) Weigh out 1.6 mol of LiNO3, 0.64 mol of Fe(NO3)3, 1.6 mol of NH4H2PO4, 0.96 mol of Mn(NO3)2, and 0.0032 mol of titanium chloride. Then weigh out 25.18 g of the carbon source glucose (this mass is equivalent to 1.6 mol of LiMn). 0.6 Fe 0.4 Add 10% of the mass of PO4 to 500 mL of water and mix well to form a solution. Then heat and stir until the water in the solution evaporates to obtain a solid precursor material, which is then crushed.

[0163] (2) The crushed solid precursor material was placed in a sintering furnace and heated to 760°C at a heating rate of 0.5°C / min under a nitrogen atmosphere. The temperature was then maintained at 760°C for 6 hours. After cooling to room temperature at a rate of 0.5°C / min, the material was taken out and crushed to obtain Ti-doped lithium manganese iron phosphate cathode material.

[0164] Comparative Example 2

[0165] This comparative example provides a lithium manganese iron phosphate cathode material (LMFP), which is prepared using the same method as in Example 1, except that:

[0166] In steps (3), (5), and (7), only 0.2 mol of NH4H2PO4 is added as the phosphorus source, and dodecylphosphonic acid is not added. The other raw materials remain unchanged.

[0167] Comparative Example 3

[0168] This comparative example provides a lithium manganese iron phosphate cathode material (LMFP), which is prepared using the same method as in Example 1, except that:

[0169] In steps (3), (5), and (7), 0.204 mol of NH4H2PO4 and 0.016 mol of dodecylphosphonic acid (the total number of moles of phosphorus source is 0.22 mol, and the molar ratio of P element in the total phosphorus source to Li element in 0.2 mol of lithium nitrate is 1.1:1) are added, while the other raw materials remain unchanged.

[0170] Comparative Example 4

[0171] This comparative example provides a lithium manganese iron phosphate cathode material (LMFP), which is prepared using the same method as in Example 1, except that:

[0172] In steps (3), (5), and (7), add 0.17 mol of NH4H2PO4 and 0.032 mol of dodecylphosphonic acid (the total phosphorus source has a molar ratio of 0.202 mol, the molar ratio of P in the total phosphorus source to Li in 0.2 mol of lithium nitrate is 1.01:1, and the molar ratio of P in 0.032 mol of organic phosphorus source dodecylphosphonic acid to Ti in 0.004 mol of TiCl4 is 8:1), while the remaining raw materials remain unchanged.

[0173] Comparative Example 5

[0174] This comparative example provides a lithium manganese iron phosphate cathode material (LMFP), which is prepared using the same method as in Example 1, except that:

[0175] In steps (3), (5), and (7), add 0.198 mol of NH4H2PO4 and 0.004 mol of dodecylphosphonic acid (the total phosphorus source has a molar ratio of 0.202 mol, the molar ratio of P in the total phosphorus source to Li in 0.2 mol of lithium nitrate is 1.01:1, and the molar ratio of P in 0.004 mol of organic phosphorus source dodecylphosphonic acid to Ti in 0.004 mol of TiCl4 is 1:1), while the remaining raw materials remain unchanged.

[0176] Comparative Example 6

[0177] This comparative example provides a lithium manganese iron phosphate cathode material (LMFP), which is prepared using the same method as in Example 1, except that:

[0178] The heating rate and cooling rate in steps (2), (4), (6), and (8) are all changed to 5℃ / min.

[0179] Comparative Example 7

[0180] This comparative example provides a lithium manganese iron phosphate cathode material (LMFP), which is prepared using the same method as in Example 1, except that:

[0181] The sintering temperature in step (2) is 500℃; the sintering temperature in step (4) is 650℃; the sintering temperature in step (6) is 800℃; and the sintering temperature in step (8) is 900℃.

[0182] Comparative Example 8

[0183] This comparative example provides a lithium manganese iron phosphate cathode material (LMFP), which is prepared using the same method as in Example 1, except that:

[0184] The sintering temperature in step (2) is 250℃; the sintering temperature in step (4) is 400℃; the sintering temperature in step (6) is 600℃; and the sintering temperature in step (8) is 700℃.

[0185] Comparative Example 9

[0186] This comparative example provides a lithium manganese iron phosphate cathode material (LMFP), which is prepared using the same method as in Example 1, except that:

[0187] The sintering temperature for steps (2), (4), (6), and (8) is 600℃.

[0188] Comparative Example 10

[0189] This comparative example provides a lithium manganese iron phosphate cathode material (LMFP), which is prepared using the same method as in Example 1, except that:

[0190] The sintering temperatures for steps (2), (4), (6), and (8) are 800℃, 700℃, 600℃, and 400℃, respectively.

[0191] Comparative Example 11

[0192] This comparative example provides a lithium manganese iron phosphate cathode material (LMFP), which is prepared using the same method as in Example 1, except that:

[0193] The cathode material is prepared directly through step (6) without steps (7) and (8).

[0194] Comparative Example 12

[0195] This comparative example provides a lithium manganese iron phosphate cathode material (LMFP), which is prepared using the same method as in Example 1, except that:

[0196] The cathode material is prepared directly through step (4) without steps (5), (6), (7) and (8).

[0197] The cathode materials prepared in the above embodiments and comparative examples were used to fabricate coin cells, and the electrochemical performance of the cells was tested, specifically as follows:

[0198] The above-mentioned positive electrode material, conductive agent (conductive carbon black), binder (polyvinylidene fluoride) and solvent (N-methylpyrrolidone) were stirred in a mixer for 2 hours at a mass ratio of 93.5:2.5:4:100 to obtain a positive electrode slurry. The positive electrode slurry was coated on aluminum foil, evenly smoothed with a scraper, dried at 130°C, and then rolled to obtain a positive electrode sheet.

[0199] Using the prepared positive electrode sheet as the positive electrode, a lithium metal sheet as the negative electrode, a Celgard 2400 microporous membrane as the separator, and a 1.0 mol / L LiPF6 solution as the electrolyte (the electrolyte being a mixture of ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) in a 1:1:1 volume ratio), coin cells were assembled in a glove box. The electrochemical performance of these coin cells was tested using a LAND electrochemical analyzer. The charging termination voltage was 4.2 V, and the discharge termination voltage was 2.0 V. The test results are shown in Table 1.

[0200] Figure 2 The paper presents FIB-TEM (Focused Ion Beam Transmission Electron Microscopy) images of the cross-sections of the cathode materials of Example 1 and Comparative Example 1. Surface scan analysis of Ti and P elements in the materials was also performed, and distribution maps of Ti and P elements were obtained, respectively.

[0201] from Figure 2 As can be seen, compared with the cross-section of Comparative Example 1, the Ti element in the cathode material prepared in Example 1 is more uniformly distributed within the crystal structure, while the titanium element in Comparative Example 1 shows obvious agglomeration within the crystal structure. These results indicate that the Ti element underwent a coordination reaction with the organophosphorus source, and the self-doped P element is beneficial for improving the dispersion of the Ti element.

[0202] Figure 3 Cycle performance graphs of the batteries prepared in Example 1 and Comparative Example 1 after 100 charge-discharge cycles at 0.1C are provided. Figure 3 It is evident that the battery prepared in Example 1 has better cycle stability than the battery in Comparative Example 1.

[0203] In addition, this application also separately sintered the second raw material in the above embodiments and comparative examples to prepare crystals, and tested the crystals using inductively coupled plasma atomic emission spectrometry (ICP) to determine the content of Li, P, and doped Ti elements. In Comparative Example 1, the cathode material prepared from its raw material was subjected to ICP testing. The specific ICP testing method was as follows: the prepared powder material was dissolved in nitric acid, brought to a constant volume, and then sent to the ICP instrument for detection. The test results are shown in Table 1.

[0204] Table 1 Test results for each embodiment and comparative example

[0205]

[0206]

[0207] As shown in Table 1, the lithium manganese iron phosphate cathode material formed by single-stage doping and crystallization in Comparative Example 1 exhibits poor cycle performance, while the lithium manganese iron phosphate cathode material prepared by multiple crystallization and doping in Example 1 demonstrates better cycle performance. This is mainly because an excess of organophosphorus source was added during the multiple doping process in this application. This excess organophosphorus undergoes self-doping and forms organic ligands with the doped Ti element, effectively forming a protective layer around the Ti element. This prevents self-aggregation between doped elements, thereby improving the battery's cycle performance.

[0208] The electrochemical performance of Comparative Example 2 and Example 1 was compared, showing that the discharge capacity and cycle performance of the doped cathode material were significantly higher than those of the undoped cathode material. That is, doping is beneficial to improving the conductivity of the cathode material, thereby improving the battery capacity and cycle life.

[0209] Comparing Comparative Example 3 with Examples 1, 4, and 5, it was found that during the multiple sintering processes in Comparative Example 3, not only were organic phosphorus sources added in each sintering process, but inorganic phosphorus sources were also added in excess. This resulted in an excessively high P element content in the final cathode material, which in turn affected the conductivity of the material, leading to a significant decrease in both capacity and cycle performance.

[0210] Comparing Comparative Examples 4 and 5 with Examples 1, 6, and 7, it was found that the molar ratio of P to Ti in the organophosphorus source cannot be too large or too small. Only within the range of (2.2–5):1, preferably 4:1, can better performance be obtained. This is because a Ti atom needs to be coordinated with four P atoms to form a more stable structure. Too many P atoms surrounding the Ti atom will affect the charge transport efficiency of the dopant, while too few P atoms will make it difficult to prevent aggregation between adjacent Ti atoms.

[0211] Comparing Comparative Example 6 with Examples 1, 8, and 9, it was found that the excessively high sintering heating rate in Comparative Example 6 resulted in lower capacity and cycle performance of the cathode material. This may be because the rapid heating prevented the formation of a highly crystalline core within the cathode particles, making it difficult to obtain a highly crystalline and structurally stable cathode material even with subsequent multiple doping and sintering processes.

[0212] Comparing Comparative Examples 7, 8, 9, and 10 with Examples 1, 10, and 11 shows that the sintering process of this application is not just about multiple sinterings, but also requires strict temperature control for each sintering, gradient heating, and the formation of a layered crystalline material structure in order to effectively improve the electrochemical performance of the battery.

[0213] Comparing Comparative Examples 11 and 12 with Examples 1 and 12, it was found that although Comparative Examples 11 and 12 also underwent doping and sintering with excess phosphorus source and Ti element after the first crystal was prepared, the performance improvement effect on the cathode material was not as significant as that of Examples 1 and 12, which had more doping times, because the number of doping times was too small.

[0214] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application 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 or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

[0215] Furthermore, those skilled in the art will understand that although some embodiments herein include certain features included in other embodiments but not others, combinations of features from different embodiments are intended to be within the scope of this application and form different embodiments. For example, any of the embodiments claimed above can be used in any combination. The information disclosed in this background section is intended only to enhance the understanding of the general background of this application and should not be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

Claims

1. A method for preparing a phosphate-based cathode material, characterized in that, include: A first raw material comprising a first phosphorus source, a lithium source, a metal M source, and a carbon source is mixed and subjected to a first sintering to obtain a first crystal; The first crystal is mixed with a second raw material containing a second phosphorus source, the lithium source, the metal M source, the carbon source and the precursor of the doped element, and then subjected to a second sintering to obtain a second crystal. The second crystal is mixed with the second raw material and then subjected to a third sintering process to obtain the third crystal. The third crystal is mixed with the second raw material and subjected to a fourth sintering to obtain the phosphate-based cathode material; As the number of sintering cycles increases, the sintering temperature increases in a gradient. In the second raw material, the second phosphorus source is added in excess; In the second raw material, the second phosphorus source includes an inorganic phosphorus source and an organic phosphorus source, wherein the molar ratio of the P element in the organic phosphorus source to the dopant element in the dopant precursor is (2.2~5.0):

1.

2. The method for preparing the phosphate-based cathode material as described in claim 1, characterized in that, In the second raw material, the molar ratio of P element in the second phosphorus source to Li element in the lithium source is (1.001~1.03):1; And / or, in the second raw material, the molar ratio of the dopant element in the dopant precursor to the Li element in the lithium source is (0.001~0.05):1; And / or, in the first raw material, the mass of the carbon source is 8% to 15% of the mass of the first crystal; And / or, in the second raw material, the mass of the carbon source is 8% to 15% of the mass of the crystals obtained by sintering the second raw material itself.

3. The method for preparing the phosphate-based cathode material as described in claim 1, characterized in that, When the metal source M is an iron source and a manganese source, the molar ratio of Li:Mn:Fe:P in the first raw material is (1~1.05):x:y:1, where x+y=1, 0 <x<1,0<y<1; And / or, when the metal M source is an iron source and a manganese source, the molar ratio of Li element:Mn element:Fe element:doping element:P element in the second raw material is 1:x:y:z:(1.001~1.03), where x+y+z=1, z=0.001~0.05, 0 <x<1-z,0<y<1-z; And / or, the molar ratio of Li element in the second raw material to Li element in the first crystal is (0.05~0.3):

1.

4. The method for preparing the phosphate-based cathode material as described in claim 1, characterized in that, The first sintering temperature is 350℃~450℃; And / or, the second sintering temperature is 500℃~600℃; And / or, the temperature of the third sintering is 650℃~730℃; And / or, the fourth sintering temperature is 750℃~800℃; And / or, the first sintering, the second sintering, the third sintering and the fourth sintering are all carried out in an inert gas atmosphere; And / or, in the first sintering, the second sintering, the third sintering and the fourth sintering processes, the heating rate in the heating stage and the cooling rate in the cooling stage are each independently 0.5℃ / min~1.5℃ / min; And / or, the sintering time of the first sintering, the second sintering, the third sintering and the fourth sintering is each independently 5h~8h; And / or, with the increase of the number of sintering times, the sintering temperature of the later sintering is 50℃~150℃ higher than that of the previous sintering.

5. The method for preparing the phosphate-based cathode material as described in claim 1, characterized in that, Before the first sintering, the process further includes: mixing the first raw material with a solvent to obtain a mixture; drying and crushing the mixture to obtain a first precursor. And / or, before the second sintering, the method further includes: dispersing the first crystal in a solvent to form a first slurry; adding the second raw material to the first slurry, stirring, and then ball milling and drying to obtain a second precursor; And / or, before the third sintering, the method further includes: dispersing the second crystal in a solvent to form a second slurry; adding the second raw material to the second slurry, stirring, then ball milling, and drying to obtain a third precursor; And / or, before the fourth sintering, the process further includes: dispersing the third crystal in a solvent to form a third slurry; adding the second raw material to the third slurry, stirring, then ball milling, and drying to obtain the fourth precursor; And / or, in the second sintering process, the third sintering process, and the fourth sintering process, the amount of the second raw material added is the same in each sintering process, or decreases arithmetically with the increase of the number of sintering times; And / or, after the fourth sintering, the method further includes: mixing the fourth crystal obtained by the fourth sintering with the second raw material, performing a fifth sintering to obtain a fifth crystal; and repeating the mixing and sintering operation n times to obtain the phosphate-based cathode material, where n = 0, 1, 2, 3, 4, ..., n, and n is an integer.

6. The method for preparing the phosphate-based cathode material as described in claim 5, characterized in that, The solvent includes at least one of pure water, ethanol, and acetone; And / or, the solid content of the first slurry, the second slurry, and the third slurry is each independently 50%~60%; And / or, the arithmetic decrease is 5% to 25% of the amount of the second raw material added during the second sintering process.

7. The method for preparing the phosphate-based cathode material as described in claim 1, characterized in that, The first phosphorus source is an inorganic phosphorus source, which includes at least one of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, lithium dihydrogen phosphate, and dilithium hydrogen phosphate. And / or, the organic phosphorus source in the second phosphorus source includes at least one of tricyclopentylphosphine, dodecylphosphonic acid, and di-tert-butyl phosphonate; And / or, the lithium source includes at least one of lithium oxide, lithium carbonate, lithium dihydrogen phosphate, lithium hydroxide, lithium acetate, and lithium nitrate; And / or, when the metal M source is an iron source, the iron source includes at least one of ferric chloride, ferric nitrate, ferric oxide, and ferric sulfate; And / or, when the metal M source is a manganese source, the manganese source includes at least one of manganese dioxide, manganese nitrate, manganese sulfate, and manganese phosphate; And / or, the carbon source includes at least one of PEG, glucose, sucrose, EG, and Ketjen black; And / or, the doped element precursor includes at least one of the following: an oxide containing a doped element, a soluble salt containing a doped element; And / or, the doping element in the doped element precursor includes at least one of Ti, Mg, V, and Nb.

8. The method for preparing the phosphate-based cathode material according to any one of claims 1-7, characterized in that, The phosphate-based cathode material includes any one of lithium iron phosphate, lithium manganese iron phosphate, lithium manganese phosphate, lithium vanadium phosphate, and lithium manganese iron titanium phosphate.

9. A phosphate-based cathode material, characterized in that, It is prepared using the preparation method of the phosphate-based cathode material according to any one of claims 1-8.

10. A lithium-ion battery, characterized in that, Including the phosphate-based cathode material as described in claim 9.

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