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

Abstract

The invention relates to the technical field of lithium ion batteries, in particular to a lithium iron phosphate positive electrode material and a preparation method and application thereof. The invention provides a lithium iron phosphate positive electrode material. The lithium iron phosphate positive electrode material is prepared from a first lithium iron phosphate granular material with a high iron-phosphorus ratio and a high doping element content and a second lithium iron phosphate granular material with a low iron-phosphorus ratio and a low doping element content through grain composition. By controlling the lithium iron phosphate positive electrode material to be metal elements with different iron-phosphorus ratios and different doping amounts, particles with different sizes are obtained to form natural gradation, so that the compaction density of finished product powder is improved, and the compaction density of the obtained lithium iron phosphate positive electrode material is relatively high; and on the basis of improving the compaction density, the specific capacity performance of the lithium iron phosphate finished product is ensured, the overall electrochemical performance of the product is improved, and wide application is facilitated.

Description

Technical Field

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

[0002] In recent years, as a member of the new energy field, lithium-ion batteries have been increasingly used in energy storage, power and other application fields. Lithium iron phosphate, lithium iron manganese phosphate, lithium nickel cobalt manganese oxide, etc., as positive electrode materials of lithium-ion batteries, are an important part affecting their performance, and therefore have attracted more and more attention and attention. Among them, lithium iron phosphate is favored by many battery manufacturers and researchers due to its advantages such as stable structure, good safety, low cost and good electrochemical performance.

[0003] However, the current lithium iron phosphate positive electrode material also has shortcomings, namely, its compaction density is not high. This problem restricts the energy density of the final battery product, resulting in poor electrochemical performance of the final product.

[0004] Based on the problem of low compaction density, the current improved method is basically to prepare large particle products and small particle products of different particle sizes separately, and then mix and compound them to improve the compaction density of the overall material. However, this method has complicated steps and requires multiple preparations of lithium iron phosphate materials, which takes a lot of time and is not conducive to improving efficiency and industrial wide application. Summary of the invention

[0005] The purpose of the present application is to provide a lithium iron phosphate positive electrode material and a preparation method and application thereof, aiming to solve the problems of low compaction density and low capacity of lithium iron phosphate positive electrode materials in the prior art.

[0006] In order to achieve the above application purpose, the technical solution adopted in this application is as follows:

[0007] In a first aspect, the present application provides a lithium iron phosphate positive electrode material, which is obtained by grading a first lithium iron phosphate particle material with a high iron-phosphorus ratio and a high content of doped elements and a second lithium iron phosphate particle material with a low iron-phosphorus ratio and a low content of doped elements.

[0008] In some embodiments, the chemical formula of the first lithium iron phosphate particle material is Li x Fe y M a P z O4@C, and 0.98≤x≤1.02, 0.98≤y≤1.02, 0.98≤z≤1.02, 0.015≤a≤0.04; the chemical formula of the second lithium iron phosphate particle material is Li x’ Fe y’ M a’ P z’O4@C, and 0.98≤x'≤1.02, 0.93≤y'≤0.95, 0.98≤z'≤1.02, 0<a'≤0.01; wherein M is a doping element and is selected from at least one of V, Nb, Ti, Mo, and Co.

[0009] In some embodiments, the lithium iron phosphate positive electrode material includes small particle material, medium particle material and large particle material in a volume ratio of 1:(8-10):(25-35).

[0010] In some embodiments, the lithium iron phosphate positive electrode material includes small particle materials with a particle size of 1 to 300 nm, medium particle materials with a particle size of 301 to 700 nm, and large particle materials with a particle size ≥ 701 nm.

[0011] In some embodiments, the particle morphology of the lithium iron phosphate positive electrode material is spherical or quasi-spherical.

[0012] In some embodiments, the compaction density of the lithium iron phosphate positive electrode material is ≥2.65 g / cm 3 .

[0013] In some embodiments, the discharge specific capacity of the lithium iron phosphate positive electrode material at 25° C. and 1C is ≥140 mAh / g.

[0014] In a second aspect, the present application provides a method for preparing a lithium iron phosphate positive electrode material, comprising the following steps:

[0015] Provide precursor A with high iron-phosphorus ratio and high content of doping elements and precursor B with low iron-phosphorus ratio and low content of doping elements respectively.

[0016] The precursor A and the precursor B are laid in layers in a sagger, and the sagger is subjected to a multi-stage gradient sintering treatment in a mixed atmosphere of a protective gas and a volatile liquid carbon source to obtain a primary sintering product;

[0017] The primary sintered product is crushed and mixed, and then subjected to secondary sintering in a mixed atmosphere of a protective gas and a volatile liquid carbon source, and then cooled to obtain a lithium iron phosphate positive electrode material.

[0018] In some embodiments, the step of performing a multi-stage gradient sintering treatment on the sagger includes: first performing a first temperature rising sintering treatment and a second temperature rising sintering treatment in sequence, and then performing a third temperature falling sintering treatment, a fourth temperature falling sintering treatment and a fifth temperature falling sintering treatment in sequence.

[0019] In some embodiments, the first temperature rising sintering treatment is to heat the temperature from room temperature to a first sintering temperature at a heating rate of 1-10°C / min and keep the temperature for 0.5-2 hours, wherein the first sintering temperature is 160-200°C.

[0020] In some embodiments, the second temperature-raising sintering treatment is to raise the temperature from the first sintering temperature to the second sintering temperature at a heating rate of 1-10°C / min and keep the temperature for 5-10 hours, wherein the second sintering temperature is 550-620°C.

[0021] In some embodiments, the third temperature reduction sintering treatment is to reduce the temperature from the second sintering temperature to the third sintering temperature at a cooling rate of 0.3-5°C / min and keep the temperature for 1-3 hours, wherein the third sintering temperature is 520-590°C.

[0022] In some embodiments, the fourth temperature reduction sintering treatment is to reduce the temperature from the third sintering temperature to the fourth sintering temperature at a cooling rate of 0.3-5°C / min and keep the temperature for 1-3 hours, wherein the fourth sintering temperature is 470-540°C.

[0023] In some embodiments, the fifth temperature reduction sintering treatment is to reduce the temperature from the fourth sintering temperature to the fifth sintering temperature at a cooling rate of 0.3-5°C / min and keep the temperature for 1-3 hours, wherein the fifth sintering temperature is 400-470°C.

[0024] In some embodiments, the secondary sintering includes: heating the temperature from room temperature to 750-790° C. at a heating rate of 1-10° C. / min for the secondary sintering, wherein the secondary sintering time is 5-10 hours.

[0025] In some embodiments, the mass ratio of precursor A to precursor B is (1:9) to (4:6).

[0026] In some embodiments, in the step of laying precursor A and precursor B in layers in a sagger, precursor A is laid flat on the bottom of the sagger, and precursor B is laid flat on top of precursor A.

[0027] In some embodiments, the protective gas includes nitrogen or argon.

[0028] In some embodiments, the volatile liquid carbon source includes at least one of methanol, ethanol, acetone, pentane, n-hexane, cyclohexane, and ethyl acetate.

[0029] In a third aspect, the present application provides a positive electrode sheet, comprising the above-mentioned lithium iron phosphate positive electrode material or the lithium iron phosphate positive electrode material prepared by the above-mentioned method for preparing the lithium iron phosphate positive electrode material.

[0030] In a fourth aspect, the present application provides a lithium-ion battery, comprising the above-mentioned positive electrode sheet.

[0031] The lithium iron phosphate positive electrode material provided in the first aspect of the present application is obtained by particle grading of a first lithium iron phosphate granular material with a high iron-phosphorus ratio and a high content of doped elements and a second lithium iron phosphate granular material with a low iron-phosphorus ratio and a low content of doped elements. By controlling the lithium iron phosphate positive electrode material to have different iron-phosphorus ratios and different doping amounts of metal elements, particles of different sizes are obtained to form a natural grading, thereby improving the compaction density of the finished powder, so that the compaction density of the obtained lithium iron phosphate positive electrode material is higher; and on the basis of improving the compaction density, the specific capacity performance of the lithium iron phosphate finished product is ensured, the overall electrochemical performance of the product is improved, and it is conducive to wide use.

[0032] The second aspect of the present application provides a method for preparing a lithium iron phosphate positive electrode material, which provides a precursor A with a high iron-phosphorus ratio and a high content of doped elements and a precursor B with a low iron-phosphorus ratio and a low content of doped elements, respectively. On the basis of different iron-phosphorus ratios and metal element doping, the inconsistency of heat transfer in the sagger and the CVD carbon coating technology are further used to carry out sintering, prepare high-capacity small particles in the bottom layer, and prepare medium and large particles in the middle and upper layers, so as to achieve natural gradation in the sagger at one time, thereby improving the compaction density of the finished powder; at the same time, through multi-stage gradient sintering treatment and secondary sintering process, the lattice structure stress in the particles is reduced, the crystallinity of the particles is improved, and the particle growth process is controlled, so as to ensure the specific capacity performance of the lithium iron phosphate finished product on the basis of improving compaction. The preparation method is efficient and easy to implement, and the obtained natural gradation lithium iron phosphate powder has good uniformity between batches, and while improving the compaction density, it ensures the due specific capacity.

[0033] The positive electrode sheet provided in the third aspect of the present application includes the above-mentioned lithium iron phosphate positive electrode material or the lithium iron phosphate positive electrode material prepared by the above-mentioned method for preparing the lithium iron phosphate positive electrode material; since the provided lithium iron phosphate positive electrode material has a higher compaction density and specific capacity, the obtained positive electrode sheet has a higher specific capacity and cycle performance.

[0034] The lithium-ion battery provided in the fourth aspect of the present application includes the above-mentioned positive electrode sheet, and the above-mentioned positive electrode sheet has a high specific capacity and cycle performance. Therefore, the obtained lithium-ion battery has excellent electrochemical properties and is suitable for wide application. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0036] Figure 1This is a SEM image of the lithium iron phosphate positive electrode material of Example 1 of the present application.

[0037] Figure 2 This is a SEM image of the lithium iron phosphate positive electrode material of Comparative Example 1 of the present application.

[0038] Figure 3 This is a SEM image of the lithium iron phosphate positive electrode material of Comparative Example 2 of the present application. DETAILED DESCRIPTION

[0039] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present application more clearly understood, the present application is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0040] In this application, the term "and / or" describes the association relationship of associated objects, indicating that there may be three relationships. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone. A and B can be singular or plural. The character " / " generally indicates that the associated objects are in an "or" relationship.

[0041] In this application, "at least one" means one or more, and "plurality" means two or more. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single items or plural items. For example, "at least one of a, b, or c", or "at least one of a, b, and c" can all mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, c can be single or multiple, respectively.

[0042] It should be understood that in the various embodiments of the present application, the size of the serial numbers of the above-mentioned processes does not mean the order of execution, some or all of the steps can be executed in parallel or sequentially, and the execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0043] The terms used in the embodiments of the present application are only for the purpose of describing specific embodiments, and are not intended to limit the present application. The singular forms "a", "an" and "the" used in the embodiments of the present application and the appended claims are also intended to include plural forms, unless the context clearly indicates other meanings.

[0044] The weight of the relevant components mentioned in the embodiment description of the present application can not only refer to the specific content of each component, but also represent the proportional relationship between the weights of the components. Therefore, as long as the content of the relevant components is proportionally enlarged or reduced according to the embodiment description of the present application, it is within the scope disclosed in the embodiment description of the present application. Specifically, the mass in the embodiment description of the present application can be a mass unit known in the chemical industry such as μg, mg, g, kg, etc.

[0045] The terms "first" and "second" are used only for descriptive purposes to distinguish objects such as substances from each other, and should not be understood as indicating or implying relative importance or implicitly indicating the number of indicated technical features. For example, without departing from the scope of the embodiments of the present application, the first XX may also be referred to as the second XX, and similarly, the second XX may also be referred to as the first XX. Thus, features defined as "first" and "second" may explicitly or implicitly include one or more of the features.

[0046] In a first aspect, an embodiment of the present application provides a lithium iron phosphate positive electrode material, which is obtained by particle grading from a first lithium iron phosphate granular material with a high iron-to-phosphorus ratio and a high content of doped elements and a second lithium iron phosphate granular material with a low iron-to-phosphorus ratio and a low content of doped elements.

[0047] The lithium iron phosphate positive electrode material provided in the first aspect of the embodiment of the present application is obtained by grading a first lithium iron phosphate granular material with a high iron-phosphorus ratio and a high content of doped elements and a second lithium iron phosphate granular material with a low iron-phosphorus ratio and a low content of doped elements. By controlling the lithium iron phosphate positive electrode material to have different iron-phosphorus ratios and different doping amounts of metal elements, particles of different sizes are obtained to form a natural grading, thereby improving the compaction density of the finished powder, so that the compaction density of the obtained lithium iron phosphate positive electrode material is higher; and on the basis of improving the compaction density, the specific capacity performance of the lithium iron phosphate finished product is ensured, the overall electrochemical performance of the product is improved, and it is conducive to wide use.

[0048] In some embodiments, the first lithium iron phosphate particles have the properties of high iron-phosphorus ratio and high content of doping elements. Based on the properties of high iron-phosphorus ratio and high content of doping elements, it is beneficial to combine specific preparation conditions during the preparation process to ensure the growth of particles with good crystallinity, high capacity, good morphology and good dispersion by limiting particle growth.

[0049] In some embodiments, the chemical formula of the first lithium iron phosphate particle material is Li x Fe y M a P zO4@C, and 0.98≤x≤1.02, 0.98≤y≤1.02, 0.98≤z≤1.02, 0.015≤a≤0.04; wherein M is a doping element and is selected from at least one of V, Nb, Ti, Mo, and Co.

[0050] In some embodiments, the second lithium iron phosphate particles have the properties of low iron-phosphorus ratio and low content of doping elements. Based on the properties of low iron-phosphorus ratio and low content of doping elements, it is beneficial to combine specific preparation conditions during the preparation process to grow the second lithium iron phosphate particles that can achieve natural gradation with the first lithium iron phosphate particles.

[0051] In some embodiments, the chemical formula of the second lithium iron phosphate particle material is Li x’ Fe y’ M a’ P z’ O4@C, and 0.98≤x'≤1.02, 0.93≤y'≤0.95, 0.98≤z'≤1.02, 0<a'≤0.01; wherein M is a doping element and is selected from at least one of V, Nb, Ti, Mo, and Co.

[0052] It is particularly noted that, for the first lithium iron phosphate granule material and the second lithium iron phosphate granule material, M in the two materials must be the same doping element. If different doping elements are used, other effects such as side reactions may occur during the mixed material sintering process.

[0053] In some embodiments, the lithium iron phosphate cathode material includes small particle materials, medium particle materials, and large particle materials in a volume ratio of 1: (8-10): (25-35). By controlling the compounding of materials with different particle sizes, it is beneficial to improve the overall compaction density of the lithium iron phosphate cathode material.

[0054] In some embodiments, the lithium iron phosphate positive electrode material includes small particle materials with a particle size of 1 to 300 nm, medium particle materials with a particle size of 301 to 700 nm, and large particle materials with a particle size of ≥701 nm.

[0055] In some specific embodiments, in the lithium iron phosphate positive electrode material, the particle size of the small particle material includes but is not limited to typical but non-limiting values ​​such as 1nm, 10nm, 30nm, 50nm, 70nm, 100nm, 120nm, 150nm, 170nm, 200nm, 220nm, 250nm, 270nm, and 300nm.

[0056] In some specific embodiments, in the lithium iron phosphate positive electrode material, the particle size of the medium particle material includes but is not limited to typical but non-limiting values ​​such as 301nm, 350nm, 400nm, 450nm, 500nm, 550nm, 600nm, 650nm, and 700nm.

[0057] In some embodiments, the first lithium iron phosphate particles and the second lithium iron phosphate particles are provided in a spherical shape. Controlling the morphology of the two lithium iron phosphate particles to be spherical is beneficial to improving the grading effect of the two during the grading process, thereby improving the overall compaction density of the lithium iron phosphate particles. In some embodiments, the compaction density of the lithium iron phosphate positive electrode material is ≥2.65g / cm 3 .

[0058] In some embodiments, the discharge specific capacity of the lithium iron phosphate positive electrode material at 25° C. and 1C is ≥140 mAh / g.

[0059] A second aspect of the present application provides a method for preparing a lithium iron phosphate positive electrode material, comprising the following steps:

[0060] S01. Provide a precursor A with a high iron-phosphorus ratio and a high content of doping elements and a precursor B with a low iron-phosphorus ratio and a low content of doping elements, respectively.

[0061] S02. The precursor A and the precursor B are layered and laid in a sagger, and a multi-stage gradient sintering treatment is performed on the sagger in a mixed atmosphere of a protective gas and a volatile liquid carbon source to obtain a primary sintering product;

[0062] S03. The primary sintered product is crushed and mixed, and then secondary sintered in a mixed atmosphere of a protective gas and a volatile liquid carbon source, and then cooled to obtain a lithium iron phosphate positive electrode material.

[0063] The second aspect of the present application embodiment provides a method for preparing a lithium iron phosphate positive electrode material, which provides a precursor A with a high iron-phosphorus ratio and a high content of doped elements and a precursor B with a low iron-phosphorus ratio and a low content of doped elements, respectively. On the basis of different iron-phosphorus ratios and metal element doping, the inconsistency of heat transfer in the sagger and the CVD carbon coating technology are further used to carry out sintering, prepare high-capacity small particles in the bottom layer, and prepare medium and large particles in the middle and upper layers, so as to achieve natural gradation in the sagger at one time, thereby improving the compaction density of the finished powder; at the same time, through multi-stage gradient sintering treatment and secondary sintering process, the lattice structure stress in the particles is reduced, the crystallinity of the particles is improved, and the particle growth process is controlled, and the specific capacity performance of the lithium iron phosphate finished product is ensured on the basis of improving compaction. The preparation method is efficient and easy to implement, and the obtained natural gradation lithium iron phosphate powder has good uniformity between batches, and while improving the compaction density, it ensures that the specific capacity is properly exerted.

[0064] In step S01, a precursor A having a high iron-phosphorus ratio and a high content of doping elements and a precursor B having a low iron-phosphorus ratio and a low content of doping elements are provided respectively.

[0065] The provided precursor can be prepared by using a sol-gel method, a liquid phase method, a solid phase method, etc. commonly used in the art, and a specific method can be selected according to specific needs.

[0066] In some specific embodiments, the preparation method of precursor A with a high iron-phosphorus ratio and a high content of doping elements includes the following steps: in molar ratio Li:Fe:P:M = (0.98~1.02): (0.98-1.2): (0.98-1.2): (0.015~0.04), weigh raw materials such as Li source, Fe source, P source, doping metal source, and carbon material, wherein the amount of carbon material added accounts for 4wt%~10wt% of the total mass of the raw materials; add the above materials into deionized water and mix, stirring for 30 minutes to ensure that the above raw materials are completely dissolved / dispersed, and obtain solution A after completion; under stirring conditions, heat and dry solution A to obtain a solid mixture, and then crush it to obtain precursor A.

[0067] The heating and drying temperature is 80-130° C., and the stirring rate is sufficient to ensure that the solution is stirred evenly.

[0068] In some specific embodiments, the preparation method of precursor B with low iron-phosphorus ratio and low content of doping elements includes the following steps: in molar ratio Li:Fe:P:M = (0.98~1.02): (0.93~0.95): (0.98~1.2): (0~0.01) weigh raw materials such as Li source, Fe source, P source, doping metal source, and carbon material, wherein the added amount of carbon material accounts for 2wt%~5wt% of the total mass of the raw materials; add the above materials into deionized water and mix, stir for 30 minutes to ensure that the above raw materials are completely dissolved / dispersed, and obtain B solution after completion; under stirring conditions, heat and dry the B solution to obtain a solid mixture, and then crush it to obtain precursor B.

[0069] The heating and drying temperature is 80-130° C., and the stirring rate is sufficient to ensure that the solution is stirred evenly.

[0070] In some embodiments, the Li source includes, but is not limited to, lithium carbonate, lithium hydroxide, lithium chloride, lithium phosphate, lithium oxalate, lithium nitrate and other water-soluble Li salts.

[0071] In some embodiments, the Fe source includes but is not limited to soluble Fe salts such as ferric nitrate, ferric nitrite, and ferric chloride. The amount of Fe is adjusted here to control the iron-phosphorus ratio of the precursor, thereby controlling the particle growth state during subsequent sintering and the final product capacity performance.

[0072] In some embodiments, the P source includes but is not limited to phosphoric acid, diammonium phosphate, diammonium hydrogen phosphate, and the like.

[0073] In some embodiments, the doping metal source includes a transition metal salt of the V element and a transition metal salt of the Nb element. The use of the above-mentioned metal doping is intended to broaden the Li diffusion path in the crystal, ensure the dynamics of the lithium iron phosphate electrochemical particles, and at the same time affect the particle growth state during the subsequent sintering process, thereby achieving a grading effect.

[0074] In some embodiments, the carbon source includes, but is not limited to, glucose, sucrose, citric acid, tartaric acid, oxalic acid, ascorbic acid, PVP, PEG, PVA, starch and other water-soluble carbon sources.

[0075] In step S02, the precursor A and the precursor B are layered in a sagger, and the sagger is subjected to a multi-stage gradient sintering treatment in a mixed atmosphere of a protective gas and a volatile liquid carbon source to obtain a primary sintering product.

[0076] In some embodiments, the mass ratio of precursor A to precursor B is (1:9) to (4:6). If there is too much precursor A, the proportion of medium and small particles will be too high, which may damage the overall particle grading system and thus reduce compaction; if there is too much precursor B, the proportion of large particles may be too high, which will also damage the particle grading system, slightly reduce compaction, and the capacity will be greatly affected.

[0077] In some specific embodiments, the mass ratio of precursor A to precursor B includes, but is not limited to, typical but non-limiting values ​​such as 1:9, 2:8, 3:7, 4:6, etc.

[0078] In some embodiments, in the step of laying precursor A and precursor B in layers in the sagger, precursor A is laid flat on the bottom of the sagger, and precursor B is laid flat on the top of precursor A. Due to the inconsistency of thermal conductivity of the sagger, the high iron-phosphorus ratio and high-doping precursor are laid at the bottom for sintering, mainly because the heat transfer at the bottom is better, and at the same time, the high iron-phosphorus ratio and high-doping properties of the precursor are used to limit the growth of particles, and small particles with good crystallinity, high capacity, good morphology and discreteness are generated; the low iron-phosphorus ratio material occupies the main body and is placed in the middle and upper layers, and medium and large particles are formed from the middle layer upwards. When sintering is completed, natural grading is achieved at one time, and a small amount of doping and liquid carbon source CVD carbon coating are used to prevent the particles from growing too large or agglomerating, while ensuring that the "large particles" occupying the grading have a certain capacity.

[0079] In some embodiments, the protective gas includes nitrogen or argon.

[0080] In some embodiments, the volatile liquid carbon source includes at least one of methanol, ethanol, acetone, pentane, n-hexane, cyclohexane, and ethyl acetate.

[0081] In some embodiments, the step of performing a multi-stage gradient sintering treatment on the sagger includes: first performing a first temperature rising sintering treatment and a second temperature rising sintering treatment in sequence, and then performing a third temperature falling sintering treatment, a fourth temperature falling sintering treatment and a fifth temperature falling sintering treatment in sequence.

[0082] In some embodiments, the first temperature rise sintering treatment is to heat the temperature from room temperature to the first sintering temperature at a heating rate of 1-10° C. / min and keep the temperature for 0.5-2 hours, wherein the first sintering temperature is 160-200° C. The first temperature rise sintering treatment is intended to ensure the removal of excess volatiles in the precursor to facilitate subsequent particle crystal growth.

[0083] In some embodiments, the second temperature-raising sintering treatment is to raise the temperature from the first sintering temperature to the second sintering temperature at a heating rate of 1-10° C. / min and keep the temperature for 5-10 hours, wherein the second sintering temperature is 550-620° C. The second temperature-raising sintering treatment is intended to provide sufficient temperature and energy for particle nucleation and growth.

[0084] In some embodiments, the third temperature reduction sintering treatment is to reduce the temperature from the second sintering temperature to the third sintering temperature at a cooling rate of 0.3-5°C / min and keep the temperature for 1-3 hours, wherein the third sintering temperature is 520-590°C.

[0085] In some embodiments, the fourth temperature reduction sintering treatment is to reduce the temperature from the third sintering temperature to the fourth sintering temperature at a cooling rate of 0.3-5°C / min and keep the temperature for 1-3 hours, wherein the fourth sintering temperature is 470-540°C.

[0086] In some embodiments, the fifth temperature reduction sintering treatment is to reduce the temperature from the fourth sintering temperature to the fifth sintering temperature at a cooling rate of 0.3-5°C / min and keep the temperature for 1-3 hours, wherein the fifth sintering temperature is 400-470°C.

[0087] The third, fourth and fifth sintering stages are intended to provide a gradient sintering cooling environment after the nucleation and initial growth of lithium iron phosphate are completed, to ensure the slow and controllable growth of the particles, to avoid excessive growth of the particles, and to avoid high lattice stress caused by sudden cooling, thereby obtaining lithium iron phosphate materials with a good lattice structure.

[0088] In step S03, the primary sintered product is crushed and mixed, and then subjected to secondary sintering in a mixed atmosphere of a protective gas and a volatile liquid carbon source, and then cooled to obtain a lithium iron phosphate positive electrode material.

[0089] In some embodiments, the secondary sintering includes: heating from room temperature to 750-790°C at a heating rate of 1-10°C / min for secondary sintering, wherein the secondary sintering time is 5-10 hours. The main purpose of the secondary sintering is to make the particles grow further, with better morphology such as sphericity and discreteness, so as to complete the final grading, and improve the crystallinity of the lithium iron phosphate particles to ensure the capacity of the finished lithium iron phosphate material.

[0090] A third aspect of an embodiment of the present application provides a positive electrode sheet, comprising the above-mentioned lithium iron phosphate positive electrode material or the lithium iron phosphate positive electrode material prepared by the above-mentioned method for preparing the lithium iron phosphate positive electrode material.

[0091] The positive electrode sheet provided in the third aspect of the embodiment of the present application includes the above-mentioned lithium iron phosphate positive electrode material or the lithium iron phosphate positive electrode material prepared by the above-mentioned method for preparing the lithium iron phosphate positive electrode material; since the provided lithium iron phosphate positive electrode material has a higher compaction density and specific capacity, the obtained positive electrode sheet has a higher specific capacity and cycle performance.

[0092] A fourth aspect of an embodiment of the present application provides a lithium-ion battery, comprising the above-mentioned positive electrode sheet.

[0093] The lithium-ion battery provided in the fourth aspect of the embodiment of the present application includes the above-mentioned positive electrode sheet, and the above-mentioned positive electrode sheet has a high specific capacity and cycle performance. Therefore, the obtained lithium-ion battery has excellent electrochemical properties and is suitable for wide application.

[0094] The following describes the invention in conjunction with specific embodiments.

[0095] Example 1

[0096] A method for preparing a high energy density lithium iron phosphate positive electrode material comprises the following steps:

[0097] (1) Lithium carbonate, ferric nitrate, ammonium dihydrogen phosphate, vanadium oxalate, and glucose were weighed according to a molar ratio of Li:Fe:P:M:C=1:0.98:1:0.025:0.3, and deionized water equivalent to 200% of the total mass of the above raw materials was added and mixed, and the mixture was stirred for 30 minutes to make the solution uniform. After completion, solution A was obtained;

[0098] (2) Lithium carbonate, ferric nitrate, ammonium dihydrogen phosphate, vanadium oxalate, and glucose were weighed according to a molar ratio of Li:Fe:P:M:C = 1:0.93:1:0.005:0.15, and deionized water equivalent to 200% of the total mass ratio of the above raw materials was added and mixed, and the solution was stirred for 30 minutes to make the solution uniform. After completion, solution B was obtained;

[0099] (3) Under stirring conditions, heating solutions A and B to 90° C. respectively to evaporate the water therein to obtain a solid mixture, and crushing the mixture to obtain precursor A and precursor B;

[0100] (4) Weigh the above precursors in a mass ratio of a:b = 1:9, spread precursor A evenly on the bottom of the sagger, and spread precursor B evenly on the top of precursor A, then send the sagger into a nitrogen and ethanol mixed atmosphere for sintering according to the following procedure: increase the temperature from room temperature to 180°C at a heating rate of 5°C / min, and keep it warm for 1 hour; increase the temperature from 180°C to 580°C at a heating rate of 5°C / min, and keep it warm for 7 hours; decrease the temperature from 580°C to 530°C at a cooling rate of 2°C / min, and keep it warm for 2 hours; decrease the temperature from 530°C to 480°C at a cooling rate of 2°C / min, and keep it warm for 2 hours; decrease the temperature from 480°C to 430°C at a cooling rate of 2°C / min, and keep it warm for 2 hours. After the end, finally cool it to room temperature with the furnace to obtain a primary sintered powder;

[0101] (5) The above-mentioned primary sintered material is crushed and mixed, and then spread into a sagger for sintering again. It is sent into a nitrogen and ethanol mixed atmosphere for CVD secondary sintering. The sintering is carried out according to the following procedure: the temperature is increased from room temperature to 780°C at a heating rate of 5°C / min, and the temperature is kept for 8 hours. After the sintering, it is finally cooled to room temperature to obtain a lithium iron phosphate positive electrode material.

[0102] Example 2

[0103] A method for preparing a high energy density lithium iron phosphate positive electrode material comprises the following steps:

[0104] (1) Lithium carbonate, ferric nitrate, ammonium dihydrogen phosphate, vanadium oxalate, and glucose were weighed according to a molar ratio of Li:Fe:P:M:C=1:0.98:1:0.025:0.3, and deionized water equivalent to 200% of the total mass of the above raw materials was added and mixed, and the mixture was stirred for 30 minutes to make the solution uniform. After completion, solution A was obtained;

[0105] (2) Lithium carbonate, ferric nitrate, ammonium dihydrogen phosphate, vanadium oxalate, and glucose were weighed according to a molar ratio of Li:Fe:P:M:C = 1:0.93:1:0.005:0.15, and deionized water equivalent to 200% of the total mass ratio of the above raw materials was added and mixed, and the solution was stirred for 30 minutes to make the solution uniform. After completion, solution B was obtained;

[0106] (3) Under stirring conditions, heating solutions A and B to 90° C. respectively to evaporate the water therein to obtain a solid mixture, and crushing the mixture to obtain precursor A and precursor B;

[0107] (4) Weigh the above precursors in a mass ratio of a:b = 2:8, spread precursor A evenly on the bottom of the sagger, and spread precursor B evenly on the top of precursor A, then send the sagger into a nitrogen and ethanol mixed atmosphere for sintering according to the following procedure: increase the temperature from room temperature to 180°C at a heating rate of 5°C / min, and keep it warm for 1 hour; increase the temperature from 180°C to 580°C at a heating rate of 5°C / min, and keep it warm for 7 hours; decrease the temperature from 580°C to 530°C at a cooling rate of 2°C / min, and keep it warm for 2 hours; decrease the temperature from 530°C to 480°C at a cooling rate of 2°C / min, and keep it warm for 2 hours; decrease the temperature from 480°C to 430°C at a cooling rate of 2°C / min, and keep it warm for 2 hours. After the end, finally cool it to room temperature with the furnace to obtain a primary sintered powder;

[0108] (5) The above-mentioned primary sintered material is crushed and mixed, and then spread into a sagger for sintering again. It is sent into a nitrogen and ethanol mixed atmosphere for CVD secondary sintering. The sintering is carried out according to the following procedure: the temperature is increased from room temperature to 780°C at a heating rate of 5°C / min, and the temperature is kept for 8 hours. After the sintering, it is finally cooled to room temperature to obtain a lithium iron phosphate positive electrode material.

[0109] Example 3

[0110] A method for preparing a high energy density lithium iron phosphate positive electrode material comprises the following steps:

[0111] (1) Lithium carbonate, ferric nitrate, ammonium dihydrogen phosphate, vanadium oxalate, and glucose were weighed according to a molar ratio of Li:Fe:P:M:C=1:0.98:1:0.025:0.3, and deionized water equivalent to 200% of the total mass of the above raw materials was added and mixed, and the mixture was stirred for 30 minutes to make the solution uniform. After completion, solution A was obtained;

[0112] (2) Lithium carbonate, ferric nitrate, ammonium dihydrogen phosphate, vanadium oxalate, and glucose were weighed according to a molar ratio of Li:Fe:P:M:C = 1:0.93:1:0.005:0.15, and deionized water equivalent to 200% of the total mass ratio of the above raw materials was added and mixed, and the solution was stirred for 30 minutes to make the solution uniform. After completion, solution B was obtained;

[0113] (3) Under stirring conditions, heating solutions A and B to 90° C. respectively to evaporate the water therein to obtain a solid mixture, and crushing the mixture to obtain precursor A and precursor B;

[0114] (4) Weigh the above precursors in a mass ratio of a:b = 1:9, spread precursor A evenly on the bottom of the sagger, and spread precursor B evenly on the top of precursor A, then send the sagger into a nitrogen and ethanol mixed atmosphere for sintering according to the following procedure: increase the temperature from room temperature to 180°C at a heating rate of 5°C / min, and keep it warm for 1 hour; increase the temperature from 180°C to 620°C at a heating rate of 5°C / min, and keep it warm for 7 hours; decrease the temperature from 620°C to 570°C at a cooling rate of 2°C / min, and keep it warm for 2 hours; decrease the temperature from 570°C to 520°C at a cooling rate of 2°C / min, and keep it warm for 2 hours; decrease the temperature from 520°C to 470°C at a cooling rate of 2°C / min, and keep it warm for 2 hours. After the end, finally cool it to room temperature with the furnace to obtain a primary sintered powder;

[0115] (5) The above-mentioned primary sintered material is crushed and mixed, and then spread into a sagger for sintering again. It is sent into a nitrogen and ethanol mixed atmosphere for CVD secondary sintering. The sintering is carried out according to the following procedure: the temperature is increased from room temperature to 780°C at a heating rate of 5°C / min, and the temperature is kept for 8 hours. After the sintering, it is finally cooled to room temperature to obtain a lithium iron phosphate positive electrode material.

[0116] Example 4

[0117] A method for preparing a high energy density lithium iron phosphate positive electrode material comprises the following steps:

[0118] (1) Lithium carbonate, ferric nitrate, ammonium dihydrogen phosphate, vanadium oxalate, and glucose were weighed according to a molar ratio of Li:Fe:P:M:C=1:0.98:1:0.025:0.3, and deionized water equivalent to 200% of the total mass of the above raw materials was added and mixed, and the mixture was stirred for 30 minutes to make the solution uniform. After completion, solution A was obtained;

[0119] (2) Lithium carbonate, ferric nitrate, ammonium dihydrogen phosphate, vanadium oxalate, and glucose were weighed according to a molar ratio of Li:Fe:P:M:C = 1:0.93:1:0.005:0.15, and deionized water equivalent to 200% of the total mass ratio of the above raw materials was added and mixed, and the solution was stirred for 30 minutes to make the solution uniform. After completion, solution B was obtained;

[0120] (3) Under stirring conditions, heating solutions A and B to 90° C. respectively to evaporate the water therein to obtain a solid mixture, and crushing the mixture to obtain precursor A and precursor B;

[0121] (4) Weigh the above precursors in a mass ratio of a:b = 1:9, spread precursor A evenly on the bottom of the sagger, and spread precursor B evenly on the top of precursor A, then send the sagger into a nitrogen and ethanol mixed atmosphere for sintering according to the following procedure: increase the temperature from room temperature to 160°C at a heating rate of 5°C / min, and keep it warm for 1 hour; increase the temperature from 160°C to 550°C at a heating rate of 5°C / min, and keep it warm for 7 hours; decrease the temperature from 550°C to 520°C at a cooling rate of 2°C / min, and keep it warm for 2 hours; decrease the temperature from 520°C to 470°C at a cooling rate of 2°C / min, and keep it warm for 2 hours; decrease the temperature from 470°C to 420°C at a cooling rate of 2°C / min, and keep it warm for 2 hours. After the end, finally cool it to room temperature with the furnace to obtain a primary sintered powder;

[0122] (5) The above-mentioned primary sintered material is crushed and mixed, and then spread into a sagger for sintering again. It is sent into a nitrogen and ethanol mixed atmosphere for CVD secondary sintering. The sintering is carried out according to the following procedure: the temperature is increased from room temperature to 780°C at a heating rate of 5°C / min, and the temperature is kept for 8 hours. After the sintering, it is finally cooled to room temperature to obtain a lithium iron phosphate positive electrode material.

[0123] Comparative Example 1

[0124] A method for preparing a lithium iron phosphate positive electrode material comprises the following steps:

[0125] (1) Lithium carbonate, ferric nitrate, ammonium dihydrogen phosphate, vanadium oxalate, and glucose were weighed according to a molar ratio of Li:Fe:P:M:C=1:0.98:1:0.025:0.3, and deionized water equivalent to 200% of the total mass of the above raw materials was added and mixed, and the mixture was stirred for 30 minutes to make the solution uniform. After completion, solution A was obtained;

[0126] (2) heating solution A to 90° C. under stirring to evaporate the water therein to obtain a solid mixture, and crushing the mixture to obtain precursor A;

[0127] (3) Weigh the above-mentioned precursor A and spread it evenly into the sagger, then send the sagger into a nitrogen and ethanol mixed atmosphere for sintering according to the following procedure: increase the temperature from room temperature to 180°C at a heating rate of 5°C / min, and keep it at this temperature for 1 hour; increase the temperature from 180°C to 580°C at a heating rate of 5°C / min, and keep it at this temperature for 7 hours; decrease the temperature from 580°C to 530°C at a cooling rate of 2°C / min, and keep it at this temperature for 2 hours; decrease the temperature from 530°C to 480°C at a cooling rate of 2°C / min, and keep it at this temperature for 2 hours; decrease the temperature from 480°C to 430°C at a cooling rate of 2°C / min, and keep it at this temperature for 2 hours. After the temperature is reduced, it is finally cooled to room temperature with the furnace to obtain a primary sintered powder;

[0128] (4) The above-mentioned primary sintered material is crushed and mixed, and then spread into a sagger for sintering again. It is sent into a nitrogen and ethanol mixed atmosphere for CVD secondary sintering. The sintering is carried out according to the following procedure: the temperature is increased from room temperature to 780°C at a heating rate of 5°C / min, and the temperature is kept for 8 hours. After the sintering, it is finally cooled to room temperature to obtain a lithium iron phosphate positive electrode material.

[0129] Comparative Example 2

[0130] A method for preparing a lithium iron phosphate positive electrode material comprises the following steps:

[0131] (1) Lithium carbonate, ferric nitrate, ammonium dihydrogen phosphate, vanadium oxalate, and glucose were weighed according to a molar ratio of Li:Fe:P:M:C=1:0.93:1:0.005:0.15, and deionized water equivalent to 200% of the total mass of the above raw materials was added and mixed, and the mixture was stirred for 30 minutes to make the solution uniform. After completion, solution A was obtained;

[0132] (2) heating solution A to 90° C. under stirring to evaporate the water therein to obtain a solid mixture, and crushing the mixture to obtain precursor A;

[0133] (3) Weigh the above-mentioned precursor A and spread it evenly into the sagger, then send the sagger into a nitrogen and ethanol mixed atmosphere for sintering according to the following procedure: increase the temperature from room temperature to 180°C at a heating rate of 5°C / min, and keep it at this temperature for 1 hour; increase the temperature from 180°C to 580°C at a heating rate of 5°C / min, and keep it at this temperature for 7 hours; decrease the temperature from 580°C to 530°C at a cooling rate of 2°C / min, and keep it at this temperature for 2 hours; decrease the temperature from 530°C to 480°C at a cooling rate of 2°C / min, and keep it at this temperature for 2 hours; decrease the temperature from 480°C to 430°C at a cooling rate of 2°C / min, and keep it at this temperature for 2 hours. After the temperature is reduced, it is finally cooled to room temperature with the furnace to obtain a primary sintered powder;

[0134] (4) The above-mentioned primary sintered material is crushed and mixed, and then spread into a sagger for sintering again. It is sent into a nitrogen and ethanol mixed atmosphere for CVD secondary sintering. The sintering is carried out according to the following procedure: the temperature is increased from room temperature to 780°C at a heating rate of 5°C / min, and the temperature is kept for 8 hours. After the sintering, it is finally cooled to room temperature to obtain a lithium iron phosphate positive electrode material.

[0135] Comparative Example 3

[0136] A method for preparing a high energy density lithium iron phosphate positive electrode material comprises the following steps:

[0137] (1) Lithium carbonate, ferric nitrate, ammonium dihydrogen phosphate, vanadium oxalate, and glucose were weighed according to a molar ratio of Li:Fe:P:M:C=1:0.98:1:0.025:0.3, and deionized water equivalent to 200% of the total mass of the above raw materials was added and mixed, and the mixture was stirred for 30 minutes to make the solution uniform. After completion, solution A was obtained;

[0138] (2) Lithium carbonate, ferric nitrate, ammonium dihydrogen phosphate, vanadium oxalate, and glucose were weighed according to a molar ratio of Li:Fe:P:M:C = 1:0.93:1:0.005:0.15, and deionized water equivalent to 200% of the total mass ratio of the above raw materials was added and mixed, and the solution was stirred for 30 minutes to make the solution uniform. After completion, solution B was obtained;

[0139] (3) Under stirring conditions, heating solutions A and B to 90° C. respectively to evaporate the water therein to obtain a solid mixture, and crushing the mixture to obtain precursor A and precursor B;

[0140] (4) Weigh the above precursors in a mass ratio of a:b = 1:9, spread precursor A evenly on the bottom of the sagger, and spread precursor B evenly on the top of precursor A, then send the sagger into a nitrogen and ethanol mixed atmosphere for sintering according to the following procedure: increase the temperature from room temperature to 180°C at a heating rate of 5°C / min, and keep it warm for 1 hour; increase the temperature from 180°C to 580°C at a heating rate of 5°C / min, and keep it warm for 7 hours; decrease the temperature from 580°C to 530°C at a cooling rate of 2°C / min, and keep it warm for 2 hours; decrease the temperature from 530°C to 480°C at a cooling rate of 2°C / min, and keep it warm for 2 hours; decrease the temperature from 480°C to 430°C at a cooling rate of 2°C / min, and keep it warm for 2 hours. After the end, finally cool it to room temperature in the furnace to obtain a lithium iron phosphate positive electrode material.

[0141] Comparative Example 4

[0142] A method for preparing a high energy density lithium iron phosphate positive electrode material comprises the following steps:

[0143] (1) Lithium carbonate, ferric nitrate, ammonium dihydrogen phosphate, vanadium oxalate, and glucose were weighed according to a molar ratio of Li:Fe:P:M:C=1:0.98:1:0.025:0.3, and deionized water equivalent to 200% of the total mass of the above raw materials was added and mixed, and the mixture was stirred for 30 minutes to make the solution uniform. After completion, solution A was obtained;

[0144] (2) Lithium carbonate, ferric nitrate, ammonium dihydrogen phosphate, vanadium oxalate, and glucose were weighed according to a molar ratio of Li:Fe:P:M:C = 1:0.93:1:0.005:0.15, and deionized water equivalent to 200% of the total mass ratio of the above raw materials was added and mixed, and the solution was stirred for 30 minutes to make the solution uniform. After completion, solution B was obtained;

[0145] (3) Under stirring conditions, heating solutions A and B to 90° C. respectively to evaporate the water therein to obtain a solid mixture, and crushing the mixture to obtain precursor A and precursor B;

[0146] (4) Weigh the above precursors in a mass ratio of a:b = 1:9, spread precursor A evenly on the bottom of the sagger, and spread precursor B evenly on the top of precursor A. Then, send the sagger into a nitrogen and ethanol mixed atmosphere and sinter according to the following procedure: increase the temperature from room temperature to 780°C at a heating rate of 5°C / min, keep the temperature for 8h, and finally cool to room temperature to obtain the lithium iron phosphate positive electrode material.

[0147] Comparative Example 5

[0148] A method for preparing a high energy density lithium iron phosphate positive electrode material comprises the following steps:

[0149] (1) Lithium carbonate, ferric nitrate, ammonium dihydrogen phosphate, vanadium oxalate, and glucose were weighed according to a molar ratio of Li:Fe:P:M:C=1:0.98:1:0.025:0.3, and deionized water equivalent to 200% of the total mass of the above raw materials was added and mixed, and the mixture was stirred for 30 minutes to make the solution uniform. After completion, solution A was obtained;

[0150] (2) Lithium carbonate, ferric nitrate, ammonium dihydrogen phosphate, vanadium oxalate, and glucose were weighed according to a molar ratio of Li:Fe:P:M:C = 1:0.93:1:0.005:0.15, and deionized water equivalent to 200% of the total mass ratio of the above raw materials was added and mixed, and the solution was stirred for 30 minutes to make the solution uniform. After completion, solution B was obtained;

[0151] (3) Under stirring conditions, heating solutions A and B to 90° C. respectively to evaporate the water therein to obtain a solid mixture, and crushing the mixture to obtain precursor A and precursor B;

[0152] (4) Weigh the above precursors in a mass ratio of a:b = 1:9, spread precursor A evenly on the bottom of the sagger, and spread precursor B evenly on the top of precursor A. Then, send the sagger into a nitrogen and ethanol mixed atmosphere and sinter according to the following procedure: increase the temperature from room temperature to 580°C at a heating rate of 5°C / min, and keep warm for 14 hours;

[0153] (5) The above-mentioned primary sintered material is crushed and mixed, and then spread into a sagger for sintering again. It is sent into a nitrogen and ethanol mixed atmosphere for CVD secondary sintering. The sintering is carried out according to the following procedure: the temperature is increased from room temperature to 780°C at a heating rate of 5°C / min, and the temperature is kept for 8 hours. After the sintering, it is finally cooled to room temperature to obtain a lithium iron phosphate positive electrode material.

[0154] Performance testing and result analysis

[0155] (1) Compacted density

[0156] The overall compaction density of the lithium iron phosphate positive electrode materials obtained by testing examples 1 to 4 and comparative examples 1 to 5 is specifically shown in Table 1. It can be seen from Table 1 that the lithium iron phosphate positive electrode material prepared by the preparation method provided by the present invention can maintain both the specific capacity and the compaction density at a high level, while the sample using a single ratio / single gradient / single sintering cannot take into account both the specific capacity and the compaction, and even both of them are at a relatively low level.

[0157] (II) SEM image analysis of lithium iron phosphate positive electrode material

[0158] The SEM images of the lithium iron phosphate positive electrode materials of Example 1, Comparative Example 1 and Comparative Example 2 were analyzed, respectively. Figure 1 (SEM image of the lithium iron phosphate positive electrode material of Example 1), Figure 2 (SEM image of the lithium iron phosphate cathode material of Comparative Example 1) and Figure 3 (SEM image of the lithium iron phosphate positive electrode material of Comparative Example 2), from the SEM image of Example 1 provided, this should be related to the better large and small particle grading brought about by the preparation method of dual-ratio precursor compounding and annealing-like gradient secondary sintering. The SEM of the materials obtained by single-ratio sintering of Comparative Example 1 and Comparative Example 2 are in a state of being overall smaller or overall larger, resulting in poor compaction density / insufficient specific capacity.

[0159] (III) Testing of lithium iron phosphate positive electrode material D of each embodiment and comparative example 10 Particle size, D 50 Particle size, carbon content, and compacted density.

[0160] As shown in Table 1, the lithium iron phosphate positive electrode material D obtained in Examples 1 to 4 10 The particle size is 0.40-0.43 μm. It can be seen that during the compaction process of the lithium iron phosphate material obtained in Examples 1-4, the fine particles can be better filled into the gaps between large particles, which helps to increase the compaction density of the electrode and further improve the energy density of the battery.

[0161] Lithium iron phosphate positive electrode material D obtained in Examples 1 to 4 50 The particle size is 0.90~0.98μm, and the compacted density is ≥2.651g / cm 3 It can be seen that the lithium iron phosphate materials obtained in Examples 1 to 4 have a moderate particle size and a high compaction density. Therefore, the particles can be naturally graded to improve the compaction density of the finished powder, so that the compaction density of the obtained lithium iron phosphate positive electrode material is higher; this is beneficial to ensure the specific capacity performance of the lithium iron phosphate finished product, improve the overall electrochemical performance of the product, and is conducive to its wide use.

[0162] (IV) Electrochemical performance

[0163] The lithium iron phosphate positive electrode materials obtained in Examples 1 to 4 and Comparative Examples 1 to 5 were used as positive electrode materials to assemble lithium ion batteries. The discharge specific capacity of each lithium ion battery was tested at 25° C. and 1C.

[0164] As shown in Table 1, under the conditions of 25°C and 1C, the discharge capacity of the lithium ion batteries obtained in Examples 1 to 4 is ≥140 mAh / g, and the discharge capacity of the lithium ion battery obtained in Comparative Example 1 is 147 mAh / g. Although the discharge capacity is very high, the compaction density is only 2.588 g / cm 3 The specific discharge capacities of the lithium-ion batteries obtained in Comparative Examples 2 to 5 are all ≤138 mAh / g, which are relatively low.

[0165] Therefore, the battery assembled from the materials provided in the embodiments of the present application ensures the specific capacity performance of the finished lithium iron phosphate product on the basis of improving the compaction density, improves the overall electrochemical performance of the product, and is conducive to wide use.

[0166] Table 1

[0167]

[0168]

[0169] In summary, the lithium iron phosphate positive electrode material provided in the embodiment of the present application is obtained by particle grading from a first lithium iron phosphate granule material with a high iron-phosphorus ratio and a high content of doped elements and a second lithium iron phosphate granule material with a low iron-phosphorus ratio and a low content of doped elements. By controlling the lithium iron phosphate positive electrode material to have different iron-phosphorus ratios and different doping amounts of metal elements, particles of different sizes are obtained to form a natural grading, thereby improving the compaction density of the finished powder, so that the compaction density of the obtained lithium iron phosphate positive electrode material is higher; and on the basis of improving the compaction density, the specific capacity performance of the lithium iron phosphate finished product is ensured, the overall electrochemical performance of the product is improved, and it is conducive to wide use.

[0170] The above are only preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application should be included in the protection scope of the present application.

Claims

1. A lithium iron phosphate positive electrode material, characterized in that: The lithium iron phosphate positive electrode material is obtained by particle grading from a first lithium iron phosphate particle material with a high iron-phosphorus ratio and a high content of doping elements and a second lithium iron phosphate particle material with a low iron-phosphorus ratio and a low content of doping elements.

2. The lithium iron phosphate positive electrode material according to claim 1, characterized in that: The chemical formula of the first lithium iron phosphate particle material is Li x Fe y M a P z O4@C, and 0.98≤x≤1.02, 0.98≤y≤1.02, 0.98≤z≤1.02, 0.015≤a≤0.04; The chemical formula of the second lithium iron phosphate particle material is Li x’ Fe y’ M a’ P z’ O4@C, and 0.98≤x'≤1.02, 0.93≤y'≤0.95, 0.98≤z'≤1.02, 0<a'≤0.01; Wherein, the M is a doping element and is selected from at least one of V, Nb, Ti, Mo, and Co.

3. The lithium iron phosphate positive electrode material according to claim 1, characterized in that: The lithium iron phosphate positive electrode material comprises small particle material, medium particle material and large particle material in a volume ratio of 1:(8-10):(25-35); and / or, The lithium iron phosphate positive electrode material includes small particle materials with a particle size of 1 to 300 nm, medium particle materials with a particle size of 301 to 700 nm, and large particle materials with a particle size of ≥701 nm; and / or, The particle morphology of the lithium iron phosphate positive electrode material is spherical or quasi-spherical; and / or, The compaction density of the lithium iron phosphate positive electrode material is ≥2.65g / cm 3 and / or, The discharge specific capacity of the lithium iron phosphate positive electrode material under the conditions of 25° C. and 1C is ≥140 mAh / g.

4. A method for preparing a lithium iron phosphate positive electrode material, characterized in that: The steps include: Provide precursor A with high iron-phosphorus ratio and high content of doping elements and precursor B with low iron-phosphorus ratio and low content of doping elements respectively. Laying the precursor A and the precursor B in layers in a sagger, and subjecting the sagger to a multi-stage gradient sintering treatment in a mixed atmosphere of a protective gas and a volatile liquid carbon source to obtain a primary sintering product; The primary sintered product is crushed and mixed, and then subjected to secondary sintering in a mixed atmosphere of a protective gas and a volatile liquid carbon source, and then cooled to obtain a lithium iron phosphate positive electrode material.

5. The method for preparing the lithium iron phosphate positive electrode material according to claim 4, characterized in that: The step of performing multi-stage gradient sintering on the sagger includes: first performing a first temperature rise sintering treatment and a second temperature rise sintering treatment in sequence, and then performing a third temperature drop sintering treatment, a fourth temperature drop sintering treatment and a fifth temperature drop sintering treatment in sequence.

6. The method for preparing the lithium iron phosphate positive electrode material according to claim 5, characterized in that: The first temperature-raising sintering treatment is to raise the temperature from room temperature to the first sintering temperature at a heating rate of 1 to 10° C. / min and keep the temperature for 0.5 to 2 hours, wherein the first sintering temperature is 160 to 200° C.; and / or, The second temperature-raising sintering treatment is to raise the temperature from the first sintering temperature to the second sintering temperature at a heating rate of 1 to 10°C / min and keep the temperature for 5 to 10 hours, wherein the second sintering temperature is 550 to 620°C; and / or, The third temperature reduction sintering treatment is to reduce the temperature from the second sintering temperature to the third sintering temperature at a cooling rate of 0.3-5°C / min and keep the temperature for 1-3 hours, wherein the third sintering temperature is 520-590°C; and / or, The fourth temperature reduction sintering treatment is to reduce the temperature from the third sintering temperature to the fourth sintering temperature at a cooling rate of 0.3-5°C / min and keep the temperature for 1-3 hours, wherein the fourth sintering temperature is 470-540°C; and / or, The fifth temperature reduction sintering treatment is to reduce the temperature from the fourth sintering temperature to the fifth sintering temperature at a cooling rate of 0.3-5°C / min and keep the temperature for 1-3 hours, wherein the fifth sintering temperature is 400-470°C.

7. The method for preparing a lithium iron phosphate positive electrode material according to claim 4, characterized in that: The secondary sintering comprises: heating from room temperature to 750-790° C. at a heating rate of 1-10° C. / min for secondary sintering, wherein the secondary sintering time is 5-10 hours.

8. The method for preparing a lithium iron phosphate positive electrode material according to claim 4, characterized in that: The mass ratio of the precursor A to the precursor B is (1:9) to (4:6); and / or, In the step of laying the precursor A and the precursor B in layers in a sagger, the precursor A is laid flat on the bottom of the sagger, and the precursor B is laid flat on top of the precursor A; and / or, The protective gas comprises nitrogen or argon; and / or, The volatile liquid carbon source includes at least one of methanol, ethanol, acetone, pentane, n-hexane, cyclohexane and ethyl acetate.

9. A positive electrode sheet, characterized in that: The invention comprises the lithium iron phosphate positive electrode material according to any one of claims 1 to 3 or the lithium iron phosphate positive electrode material prepared by the preparation method of the lithium iron phosphate positive electrode material according to any one of claims 4 to 8.

10. A lithium ion battery, characterized in that: Including the positive electrode sheet as described in claim 9.

Images