Positive electrode active material, and preparation method therefor and use thereof

Abstract

Provided in the present application are a positive electrode active material, and a preparation method therefor and the use thereof. The positive electrode active material comprises an LiMnxFe1-xPO4 inner core and a carbon coating layer covering at least part of the surface of the inner core, wherein 0≤x<1. The compaction density of the positive electrode active material is not lower than 2.28 g / cm3; and the impedance of the positive electrode active material is not higher than 1,500 Ω. The compaction density and impedance of the positive electrode active material provided in the present application are defined, such that the positive electrode active material has a relatively high compaction density and good rate capability; and when the positive electrode active material is applied to a lithium-ion battery, the specific capacity, energy density and rate capability of the lithium-ion battery can be improved.

Description

A positive electrode active material, a preparation method and application thereof

[0001] The present disclosure claims priority to the Chinese patent application No. 202411253406.0, filed on September 9, 2024, and entitled "A positive electrode active material, a preparation method and application thereof", the entire content of which is incorporated herein by reference. TECHNICAL FIELD

[0002] The present application belongs to the technical field of lithium ion batteries, and particularly relates to a positive electrode active material, a preparation method and application thereof. BACKGROUND

[0003] Lithium ion batteries, as a kind of secondary batteries, have high energy density and cycle performance, high specific capacity and low carbon environmental protection, and thus can help solve the increasingly severe energy problem. Therefore, the research on lithium ion batteries has become a hot topic in the fields of physics, chemistry, materials, energy and environment. Since the safety and electrochemical performance of lithium ion batteries are largely determined by the positive electrode active material, the design and preparation of high-performance positive electrode active materials have been widely concerned and researched. Common positive electrode active materials for lithium ion batteries include lithium cobaltate, ternary materials, lithium iron phosphate, lithium manganese iron phosphate and some composite materials, etc.

[0004] The main problem of the current positive electrode active material is the low compaction density. In the prior art, the method for improving the compaction density of the positive electrode active material usually makes the rate performance of the material worse. Therefore, it has high research value to prepare a positive electrode active material with high compaction density and excellent rate performance. SUMMARY

[0005] The main purpose of the present application is to provide a positive electrode active material which has high compaction density and excellent rate performance.

[0006] The present application also provides a preparation method of the positive electrode active material, which can prepare the above-mentioned positive electrode active material and has simple process and low cost.

[0007] The present application also provides a positive electrode sheet. Since the positive electrode sheet comprises the above-mentioned positive electrode active material, the positive electrode sheet used in a lithium ion battery can improve the specific capacity, energy density and rate performance of the lithium ion battery.

[0008] The present application also provides a lithium ion battery. Since the lithium ion battery comprises the above-mentioned positive electrode sheet, the lithium ion battery has high specific capacity, energy density and excellent rate performance.

[0009] In a first aspect, the present application provides a positive electrode active material, comprising LiMn x Fe1-x PO4 core and a carbon coating layer covering at least part of the surface of the core, wherein 0≤x<1;

[0010] The compaction density of the positive electrode active material is not less than 2.28 g / cm 3 ;

[0011] The impedance of the positive electrode active material is not higher than 1500 Ω.

[0012] The positive electrode active material as described above, wherein the core comprises first plate-like nanoparticles and first spherical nanoparticles.

[0013] The difference between the average length of the first plate-like nanoparticles and the average particle size of the first spherical nanoparticles is not more than 50 nm.

[0014] The positive electrode active material as described above, wherein the average length of the first plate-like nanoparticles is 100-200 nm.

[0015] The average particle size of the first spherical nanoparticles is 100-250 nm.

[0016] The positive electrode active material as described above, wherein the thickness of the first plate-like nanoparticles is <20 nm.

[0017] The sphericity of the first spherical nanoparticles is greater than 0.5.

[0018] The positive electrode active material as described above, wherein the standard deviation of the length of the first plate-like nanoparticles is <80 nm.

[0019] The particle size distribution SPAN of the first spherical nanoparticles is 1.0-2.0.

[0020] The positive electrode active material as described above, wherein the carbon coating layer accounts for 1-3% of the mass of the positive electrode active material.

[0021] In a second aspect, the application provides a preparation method of the positive electrode active material as described above, comprising the following steps:

[0022] The second plate-like nanoparticles are added to the second spherical nanoparticles in batches and mixed uniformly to obtain a mixed raw material; and a mixed system comprising the mixed raw material and a carbon source is subjected to sintering treatment at 600-800°C under an inert atmosphere to obtain the positive electrode active material.

[0023] The mass fraction of each batch of added second plate-like nanoparticles in the total amount of second plate-like nanoparticles is 10-20%; and the difference between the average length of the second plate-like nanoparticles and the average particle size of the second spherical nanoparticles is not more than 50 nm.

[0024] The preparation method is as described above, and a mass ratio of the second plate-shaped nanoparticles to the second spherical nanoparticles in the mixed raw materials is 0.25-4:1.

[0025] And / or, the carbon source accounts for 5-15% of the mass of the mixed raw materials.

[0026] In a third aspect, the present application provides a positive electrode sheet, which comprises the positive electrode active material as described above or the positive electrode active material prepared by the preparation method as described above.

[0027] In a fourth aspect, the present application provides a lithium ion battery, which comprises the positive electrode sheet as described above.

[0028] The positive electrode active material provided by the present application has high compaction density and excellent rate performance, and when applied to a lithium ion battery, can improve the specific capacity, energy density and rate performance of the lithium ion battery. BRIEF DESCRIPTION OF DRAWINGS

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the related art, the drawings needed to be used in the description of the embodiments of the present application or the related art are briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and for those skilled in the art, other drawings can be obtained without creative labor on the basis of these drawings.

[0030] FIG. 1 is an SEM diagram of the positive electrode active material prepared in Example 1 of the present application. DETAILED DESCRIPTION

[0031] In order to make the purpose, technical solutions and advantages of the present application more clear, the technical solutions in the embodiments of the present application will be described clearly and completely below in combination with the embodiments of the present application. Obviously, the described embodiments are only some embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0032] In a first aspect, the present application provides a positive electrode active material, which comprises a LiMn x Fe 1-x PO4 core and a carbon coating layer covering at least part of the surface of the core, wherein 0≤x<1;

[0033] The compaction density of the positive electrode active material is not less than 2.28 g / cm 3 ;

[0034] The impedance of the positive electrode active material is not higher than 1500 Ω.

[0035] It can be understood that the tap density of the positive electrode active material mainly affects the specific capacity and energy density of the battery, and the impedance mainly affects the rate performance of the battery.

[0036] In an embodiment, the tap density of the positive electrode active material of the present application is not less than 2.28 g / cm 3 It can be understood that the tap density of the positive electrode active material should be within a suitable range, so that the battery has higher energy density and better rate performance. In the present embodiment, the positive electrode active material has a suitable tap density, so that the distance between the particles of the material is suitable, the porosity is appropriate, the liquid retention is high, the structure is relatively stable, the lithium ion insertion and extraction are facilitated, and good electronic conductivity can be provided, so that side reactions are not easily generated, capacity loss and polarization caused by looseness are avoided, the capacity performance of the material is better, and the rate performance is improved. Therefore, the suitable tap density of the positive electrode active material can increase the energy density of the battery, and is beneficial to improving the rate performance of the battery.

[0037] In addition, the impedance of the positive electrode active material in the present application is not higher than 1500 Ω, which is within a suitable range. Since the impedance of the positive electrode active material is mainly affected by the electronic conductivity and the lithium ion diffusion rate, the suitable impedance of the positive electrode active material means that the electronic conductivity is high and the lithium ion diffusion rate is fast. The electronic conductivity is high, and the electron migration ability in the positive electrode active material is strong. In the charging and discharging process of the battery, high electronic conductivity can promote the rapid transmission of electrons, thereby improving the charging and discharging rate of the battery. This makes the battery have good performance under the condition of rate charging and discharging, and can provide a large amount of electric energy for the equipment in a short time. The lithium ion diffusion rate is fast, and the lithium ion can rapidly migrate in the positive electrode active material during the charging and discharging process of the battery, which helps to improve the charging and discharging efficiency and rate performance of the battery. Therefore, the suitable impedance of the positive electrode active material makes the rate performance of the positive electrode active material better.

[0038] The positive electrode active material provided by the present application has high tap density and excellent rate performance by limiting the tap density and impedance, and can improve the specific capacity, energy density and rate performance of the lithium ion battery when applied to the lithium ion battery.

[0039] In some embodiments of the present application, the core comprises first plate-like nanoparticles and first spherical nanoparticles.

[0040] The difference between the average length of the first plate-like nanoparticles and the average particle size of the first spherical nanoparticles is not more than 50 nm.

[0041] The positive electrode active material core in the present application comprises first plate-shaped nanoparticles (primary particles) and first spherical nanoparticles (primary particles), and the difference between the average length of the first plate-shaped nanoparticles and the average particle size of the first spherical nanoparticles is not greater than 50 nm. The first plate-shaped nanoparticles and the first spherical nanoparticles have a gap-filling effect and are tightly packed into spherical particles (secondary particles), and the structure is compact, and the compaction density is greater than that of traditional positive electrode active material spherical particles. Moreover, the difference between the average length of the first plate-shaped nanoparticles and the average particle size of the first spherical nanoparticles is small, which can avoid the polarization caused by too large particle size difference, thereby improving the specific capacity, energy density and rate performance of the positive electrode active material, especially improving the rate performance and thus improving the charge and discharge efficiency.

[0042] The present application does not limit the size relationship between the average length of the first plate-shaped nanoparticles and the average particle size of the first spherical nanoparticles, as long as the difference between the average length of the first plate-shaped nanoparticles and the average particle size of the first spherical nanoparticles is not greater than 50 nm.

[0043] Further, the average length of the first plate-shaped nanoparticles is 100-200 nm, and the average particle size of the first spherical nanoparticles is 100-250 nm, which can realize that the difference between the average length of the first plate-shaped nanoparticles and the average particle size of the first spherical nanoparticles is not greater than 50 nm, and further improve the compaction density and rate performance of the positive electrode active material. When applied to a lithium ion battery, the specific capacity, energy density and rate performance of the lithium ion battery can be improved.

[0044] In some embodiments of the present application, the thickness of the first plate-shaped nanoparticles is <20 nm.

[0045] The sphericity of the first spherical nanoparticles is greater than 0.5.

[0046] In the present application, the thickness of the first plate-shaped nanoparticles is controlled to be <20 nm, and the thickness value is low, and the sphericity of the first spherical nanoparticles is greater than 0.5, and the sphericity is high, which can further realize that the first plate-shaped nanoparticles and the first spherical nanoparticles are packed into a positive electrode active material with a compact structure through a gap-filling effect, thereby improving the compaction density and rate performance thereof.

[0047] In some embodiments of the present application, the standard deviation of the length of the first plate-shaped nanoparticles is <80 nm.

[0048] The particle size distribution SPAN of the first spherical nanoparticles is 1.0-2.0.

[0049] The standard deviation of the length of the first plate-like nanoparticles is defined in the present application to be < 80 nm; the particle size distribution SPAN of the first spherical nanoparticles is 1.0-2.0, which indicates that the lengths or particle sizes of the first plate-like nanoparticles and the first spherical nanoparticles are relatively uniform, so that when the two are stacked into the positive electrode active material through the interstitial effect, the polarization of the positive electrode active material can be reduced, thereby further improving the compaction density and rate capability of the positive electrode active material.

[0050] The SPAN of the positive electrode active material is calculated according to the formula (D90-D10) / D50. The present application does not limit the specific values of D90, D50 and D10 of the positive electrode active material, as long as the particle size distribution SPAN of the positive electrode active material is within the above range.

[0051] In some embodiments of the present application, the carbon coating layer accounts for 1-3% of the mass of the positive electrode active material.

[0052] It can be understood that the content of the carbon coating layer will affect the compaction density and electrical conductivity of the positive electrode active material to some extent, thereby affecting the energy density and rate capability of the positive electrode active material.

[0053] In one embodiment, the carbon coating layer accounts for 1-3% of the mass of the positive electrode active material, which is within a suitable range. Suitable carbon coating of the positive electrode active material can inhibit the growth of particles and reduce the diffusion distance of lithium ions, and on the other hand, carbon has excellent electrical conductivity, which is beneficial to the transmission of electrons and improves the electronic conductivity of the material, thereby improving the rate capability of the battery. In addition, the carbon coating layer can reduce the voids between the positive electrode active material and increase the bonding force between the material particles, thereby improving the compaction density and rate capability of the positive electrode active material.

[0054] In a second aspect, the present application provides a preparation method of the positive electrode active material as described above, comprising the following steps:

[0055] The second plate-like nanoparticles are added to the second spherical nanoparticles in batches and mixed uniformly to obtain a mixed raw material; and a mixed system comprising the mixed raw material and a carbon source is subjected to sintering treatment at 600-800°C under an inert atmosphere to obtain the positive electrode active material.

[0056] In the present application, the mass fraction of each batch of added second plate-like nanoparticles in the total amount of second plate-like nanoparticles is 10-20%; and the difference between the average length of the second plate-like nanoparticles and the average particle size of the second spherical nanoparticles is not greater than 50 nm.

[0057] In the present application, the second plate-like nanoparticles are added to the second spherical nanoparticles in batches and mixed uniformly, wherein the mass fraction of the second plate-like nanoparticles in each batch accounts for 10-20% of the total amount of the second plate-like nanoparticles, and the difference between the average length of the second plate-like nanoparticles and the average particle size of the second spherical nanoparticles is not greater than 50 nm. The interstitial effect of the second plate-like nanoparticles and the second spherical nanoparticles can be effectively realized, the compaction density of the final positive electrode active material is improved, the impedance is reduced, and then the mixed raw materials are mixed with a carbon source. After sintering at 600-800°C in an inert atmosphere, the positive electrode active material of the first aspect described above can be prepared. The carbon source can be at least one of glucose, polyethylene glycol (PEG), sucrose, citric acid, polyvinylpyrrolidone (PVP), acetylene black, graphite, carbon black, carbon nanotubes, and graphene. The inert atmosphere can be a nitrogen atmosphere. The heating rate of the sintering process can be 3°C / min.

[0058] The present application does not limit the source of the second plate-like nanoparticles and the second spherical nanoparticles, which can be purchased or prepared, for example, the second plate-like nanoparticles and the second spherical nanoparticles can be prepared by hydrothermal method, controlling the hydrothermal process temperature and time, and the feeding sequence.

[0059] Specifically, when preparing the second plate-like nanoparticles, the iron source and / or manganese source and the lithium source are first dissolved in an aqueous glycol solution, ultrasonic dispersion and stirring are uniform, and the iron source and / or manganese source solution and the lithium source solution are obtained. The phosphorus source is added to the iron source and / or manganese source solution, stirred uniformly, then the lithium source solution is slowly added, stirred thoroughly, then transferred into a high-pressure kettle, and reacted at 150-250°C for 3-20h to obtain second plate-like nanoparticles with uniform morphology. That is, the feeding sequence is iron source and / or manganese source, phosphorus source, and lithium source, and the molar ratio of iron and / or manganese, phosphorus, and lithium in the iron source and / or manganese source, phosphorus source, and lithium source can be 1:0.9-1.1:1-1.2. The addition sequence in the hydrothermal method is M (M is Mn, Fe source or mixture thereof)-PO4 3- -Li helps to generate Li-M-PO4 3- gel-like precursor, and then nucleate at a lower temperature to generate LiMn x Fe 1-x PO4, wherein 0≤x<1, LiMn x Fe 1-x PO4 particles will grow by a combination of ion diffusion onto existing nuclei and irreversible aggregation of nuclei. This is followed by promoting the growth of crystal faces, and due to the different growth rates of crystal faces, the final LiMn x Fe 1-x PO4 particles will have good crystal morphology and specific face orientation, and the material tends to generate plate-like material.

[0060] In the preparation of the second spherical nanoparticles, the iron source and / or manganese source and the lithium source can be dissolved in an aqueous ethylene glycol solution respectively, and then uniformly dispersed and stirred by ultrasonic to obtain an iron source and / or manganese source solution and a lithium source solution. The phosphorus source is added to the lithium source solution, and then the iron source and / or manganese source solution is slowly added after uniform stirring. The mixture is then transferred into an autoclave tank and reacted at 150-250°C for 3-20h to obtain second spherical nanoparticles with uniform morphology. That is, the order of adding the raw materials is lithium source, phosphorus source, iron source and / or manganese source, and the molar ratio of iron and / or manganese, phosphorus and lithium in the iron source and / or manganese source, phosphorus source and lithium source can be 1:0.9-1.1:2-4. Li and PO4 3- If the order of adding the raw materials is changed, a large amount of Li3PO4 and LiH2PO4 will precipitate before the solvothermal reaction, some PO4 3– will be consumed, and the PO4 2+ reacted with Fe 3– will be reduced. LiMn x Fe 1-x PO4 nucleation only occurs when the temperature reaches a certain point and the diffusion ion concentration reaches saturation. At this time, a large number of crystal nuclei are produced by instantaneous nucleation, thereby reducing the amount of material available for subsequent growth. The product of this rapid nucleation process has a small particle size and a random orientation, and this randomness promotes the material to become a spherical crystal material.

[0061] The preparation method provided by the present application can prepare the positive electrode active material of the first aspect described above, and the positive electrode active material is applied to a lithium ion battery, which can improve the specific capacity, energy density and rate performance of the lithium ion battery.

[0062] In some embodiments of the present application, the mass ratio of the second plate-like nanoparticles and the second spherical nanoparticles in the mixed raw materials is 0.25-4:1.

[0063] And / or, the carbon source accounts for 5-15% of the mass of the mixed raw materials.

[0064] The present application controls the mass ratio of the second plate-like nanoparticles and the second spherical nanoparticles in the mixed raw materials, and the mass ratio of the carbon source in the mixed raw materials in a suitable range, so that the mass ratio of the first plate-like nanoparticles and the first spherical nanoparticles in the positive electrode active material is in a suitable range, and the mass ratio of the carbon coating layer is in a suitable range, thereby improving the compaction density and impedance of the positive electrode active material, and further improving the specific capacity, energy density and rate performance of the lithium ion battery.

[0065] In a third aspect, the present application provides a positive electrode sheet comprising the positive electrode active material described above or prepared by the preparation method described above.

[0066] The positive electrode sheet of the present application can be prepared by using conventional techniques in the art. Specifically, the positive electrode active material, the conductive agent, and the binder can be uniformly dispersed in a solvent to obtain a positive electrode active layer slurry, and then the positive electrode active layer slurry can be coated on at least one functional surface of the positive electrode current collector, and after drying, the positive electrode sheet of the present application can be obtained.

[0067] The present application does not make special limitations on the specific types of conductive agents and binders. The components such as conductive agents and binders can be selected from conventional materials in the art. For example, the conductive agent can be selected from one or more of conductive carbon black, carbon nanotubes, conductive graphite, and graphene, and the binder can be selected from one or more of polyvinylidene fluoride (PVDF), acrylic modified PVDF, polyacrylate polymer, polyimide, butadiene styrene rubber, and styrene butadiene rubber.

[0068] The present application does not make special limitations on the coating method. Any one of gravure coating, extrusion coating, spraying, and screen printing can be used to coat the positive electrode active layer slurry.

[0069] Since the positive electrode sheet provided by the present application includes the positive electrode active material described above, the positive electrode sheet used in a lithium ion battery can improve the specific capacity, energy density, and rate performance of the lithium ion battery.

[0070] In a fourth aspect, the present application provides a lithium ion battery comprising the positive electrode sheet described above.

[0071] The lithium ion battery of the present application includes, in addition to the positive electrode sheet, a separator, a negative electrode sheet, and an electrolyte. The composition of the negative electrode sheet can refer to conventional negative electrode sheets in the art, and the separator can also use conventional separators in the art, such as PP film and PE film.

[0072] The lithium ion battery of the present application can be prepared by using conventional methods in the art. Specifically, the positive electrode sheet, the separator, and the negative electrode sheet can be sequentially stacked and placed, and then an electric core can be obtained by lamination or winding process, and then the above-mentioned lithium ion battery can be obtained by subsequent processes such as baking, liquid injection, formation, and packaging.

[0073] Since the lithium ion battery provided by the present application includes the positive electrode sheet described above, the lithium ion battery has high specific capacity, high energy density, and excellent rate performance.

[0074] In the following, the technical solutions of the present application are further illustrated by specific examples.

[0075] Example 1

[0076] The preparation method of the positive electrode active material of the present embodiment includes the following steps:

[0077] 1) Preparation of the second plate-like nanoparticles: 0.03 mol of MnSO4, 0.02 mol of FeSO4 and 0.06 mol of LiOH H2O were respectively dissolved in 100 mL of an aqueous ethylene glycol solution (mass ratio of ethylene glycol to water was 3:1), and stirred uniformly under ultrasonic dispersion to obtain a manganese source and iron source solution and a lithium source solution. 0.05 mol of H3PO4 was added to the manganese source and iron source solution and stirred uniformly, and then the lithium source solution was slowly added. After being stirred uniformly, the mixture was transferred into an autoclave tank and reacted at 180°C for 15 h to obtain second plate-like nanoparticles with uniform morphology, wherein the average length of the second plate-like nanoparticles was 100 nm.

[0078] 2) Preparation of the second spherical nanoparticles: 0.03 mol of MnSO4, 0.02 mol of FeSO4 and 0.15 mol of LiOH H2O were respectively dissolved in 100 mL of an aqueous ethylene glycol solution (mass ratio of ethylene glycol to water was 3:1), and stirred uniformly under ultrasonic dispersion to obtain a manganese source and iron source solution and a lithium source solution. 0.05 mol of H3PO4 was added to the lithium source solution and stirred uniformly, and then the manganese source and iron source solution was slowly added. After being stirred uniformly, the mixture was transferred into an autoclave tank and reacted at 180°C for 15 h to obtain second spherical nanoparticles with uniform morphology, wherein the average particle size of the second spherical nanoparticles was 130 nm.

[0079] 3) The second plate-like nanoparticles were added to the second spherical nanoparticles in five batches to obtain a mixed raw material, wherein the second plate-like nanoparticles in each batch accounted for 20% of the total amount of the second plate-like nanoparticles, and the mass ratio of the second plate-like nanoparticles to the second spherical nanoparticles in the mixed raw material was 1:1. The mixed raw material was mixed with glucose and PEG at a mass ratio of 10:0.9:0.1, ground for 10 min, and then sintered at 620°C for 5 h under N2 atmosphere at a heating rate of 3°C / min to obtain a positive electrode active material with a LiMn 0.6 Fe 0.4 PO4 core. The carbon coating layer accounted for 2.12% of the mass of the positive electrode active material, which was denoted as the carbon content. The average length of the first plate-like nanoparticles in the positive electrode active material was 120 nm, the average particle size of the first spherical nanoparticles was 150 nm, and the difference between the average length of the first plate-like nanoparticles and the average particle size of the first spherical nanoparticles was 30 nm.

[0080] Example 2

[0081] Example 2 and the preparation method of the positive electrode active material of Example 1 were basically the same, except that the mass ratio of the second plate-like nanoparticles to the second spherical nanoparticles in the mixed raw material of step 3) was 4:1, and finally a positive electrode active material with a LiMn 0.6 Fe 0.4A positive electrode active material of LiFePO4.

[0082] Example 3

[0083] The preparation method of the positive electrode active material of Example 3 is basically the same as that of Example 1, except that the mass ratio of the second plate-shaped nanoparticles to the second spherical nanoparticles in the mixed raw materials of step 3) is 1:4, and finally a positive electrode active material of LiMn 0.6 Fe 0.4 PO4.

[0084] Example 4

[0085] The preparation method of the positive electrode active material of Example 4 is basically the same as that of Example 1, except that the mass ratio of the second plate-shaped nanoparticles to the second spherical nanoparticles in the mixed raw materials of step 3) is 3:7, and finally a positive electrode active material of LiMn 0.6 Fe 0.4 PO4.

[0086] Example 5

[0087] The preparation method of the positive electrode active material of Example 5 is basically the same as that of Example 1, except that the mixed raw materials are mixed with glucose in a mass ratio of 10:1 in step 3), and finally a positive electrode active material of LiMn 0.6 Fe 0.4 PO4.

[0088] Example 6

[0089] The preparation method of the positive electrode active material of Example 6 is basically the same as that of Example 1, except that the amount of LiOH H2O added in step 1) is 0.055 mol, and the second plate-shaped nanoparticles with an average length of 170 nm are prepared; the amount of LiOH H2O added in step 2) is 0.12 mol, and the hydrothermal time is extended to 20 h, and the second spherical nanoparticles with an average particle size of 200 nm are prepared, and finally a positive electrode active material of LiMn 0.6 Fe 0.4 PO4.

[0090] Example 7

[0091] The preparation method of the positive electrode active material of Example 7 is basically the same as that of Example 1, except that the amount of LiOH H2O added in step 1) is 0.06 mol, and the second plate-shaped nanoparticles with an average length of 90 nm are prepared; the amount of LiOH H2O added in step 2) is 0.18 mol, and the second spherical nanoparticles with an average particle size of 110 nm are prepared, and finally a positive electrode active material of LiMn 0.6 Fe 0.4A positive electrode active material of LiFePO4.

[0092] Example 8

[0093] The preparation method of the positive electrode active material of Example 8 is basically the same as that of Example 1, except that the reaction time in step 1) is shortened to 8h, and the average length of the second plate-like nanoparticles is 100nm; the reaction time in step 2) is shortened to 8h, and the average particle size of the second spherical nanoparticles is 120nm. Finally, the core of the positive electrode active material is LiFePO4. 0.6 Fe 0.4 A positive electrode active material of LiFePO4.

[0094] Example 9

[0095] The preparation method of the positive electrode active material of Example 9 is basically the same as that of Example 1, except that in step 3), the second plate-like nanoparticles are added to the second spherical nanoparticles in 10 batches and mixed uniformly to obtain a mixed raw material, wherein each batch of second plate-like nanoparticles accounts for 10% of the total amount of second plate-like nanoparticles.

[0096] Example 10

[0097] The preparation method of the positive electrode active material of this embodiment includes the following steps:

[0098] 1) Preparation of second plate-like nanoparticles: 0.05mol FeSO4 and 0.06mol LiOH·H2O are respectively dissolved in 100mL aqueous ethylene glycol solution (mass ratio of ethylene glycol to water is 3:1), and ultrasonic dispersion stirring is uniform to obtain iron source solution, lithium source solution. 0.05mol H3PO4 is added to the iron source solution and stirred uniformly, then the lithium source solution is slowly added, and after uniform stirring, it is transferred into a high-pressure kettle and reacted at 180℃ for 15h to obtain second plate-like nanoparticles with uniform morphology, wherein the average length of the second plate-like nanoparticles is 110nm.

[0099] 2) Preparation of second spherical nanoparticles: 0.05mol FeSO4 and 0.15mol LiOH·H2O are respectively dissolved in 100mL aqueous ethylene glycol solution (mass ratio of ethylene glycol to water is 3:1), and ultrasonic dispersion stirring is uniform to obtain iron source solution, lithium source solution. 0.05mol H3PO4 is added to the lithium source solution and stirred uniformly, then the iron source solution is slowly added, and after uniform stirring, it is transferred into a high-pressure kettle and reacted at 180℃ for 15h to obtain second spherical nanoparticles with uniform morphology, wherein the average particle size of the second spherical nanoparticles is 150nm.

[0100] 3) The second plate-like nanoparticles are added into the second spherical nanoparticles in five batches and mixed uniformly to obtain a mixed raw material, wherein the second plate-like nanoparticles in each batch account for 20% of the total amount of the second plate-like nanoparticles, the mass ratio of the second plate-like nanoparticles to the second spherical nanoparticles in the mixed raw material is 1:1, the mixed raw material, glucose and PEG are mixed in a mass ratio of 10:0.9:0.1, grinded for 10 min, and then sintered at 620°C for 5 h under N2 atmosphere at a heating rate of 3°C / min to obtain the positive electrode active material with LiFePO4 as the core. The carbon coating layer accounts for 2.12% of the mass of the positive electrode active material, which is denoted as the carbon content. The average length of the first plate-like nanoparticles in the positive electrode active material is 140 nm, the average particle size of the first spherical nanoparticles is 170 nm, and the difference between the average length of the first plate-like nanoparticles and the average particle size of the first spherical nanoparticles is 30 nm.

[0101] Example 11

[0102] The preparation method of the positive electrode active material of Example 11 is basically the same as that of Example 1, except that the addition amount of LiOH H2O in step 1) is 0.055 mol, and the second plate-like nanoparticles with an average length of 170 nm are prepared; the addition amount of LiOH H2O in step 2) is 0.18 mol, and the second spherical nanoparticles with an average particle size of 110 nm are prepared; and finally the positive electrode active material with LiMn 0.6 Fe 0.4 PO4 as the core is obtained.

[0103] Example 12

[0104] The preparation method of the positive electrode active material of Example 12 is basically the same as that of Example 1, except that the addition amount of LiOH H2O in step 1) is 0.053 mol, and the second plate-like nanoparticles with an average length of 200 nm are prepared; the addition amount of LiOH H2O in step 2) is 0.10 mol, and the hydrothermal time is prolonged to 20 h, and the second spherical nanoparticles with an average particle size of 250 nm are prepared; and finally the positive electrode active material with LiMn 0.6 Fe 0.4 PO4 as the core is obtained.

[0105] Comparative Example 1

[0106] The preparation method of the positive electrode active material of Comparative Example 1 includes the following steps:

[0107] 1) Preparation of second plate-like nanoparticles: 0.03 mol of MnSO4, 0.02 mol of FeSO4 and 0.05 mol of LiOH H2O were respectively dissolved in 100 mL of an aqueous ethylene glycol solution (mass ratio of ethylene glycol to water was 3:1), and uniformly stirred by ultrasonic dispersion to obtain a manganese source and iron source solution and a lithium source solution. 0.05 mol of H3PO4 was added to the manganese source and iron source solution and uniformly stirred, and then the lithium source solution was slowly added. After uniform stirring, the mixture was transferred into an autoclave tank and reacted at 180°C for 15 h to obtain second plate-like nanoparticles with uniform morphology, wherein the average length of the second plate-like nanoparticles was 100 nm.

[0108] 2) The second plate-like nanoparticles were mixed with glucose and PEG in a mass ratio of 10:0.9:0.1, ground for 10 min, and then sintered at 620°C for 5 h under N2 atmosphere at a heating rate of 3°C / min to obtain a positive electrode active material with a LiMn 0.6 Fe 0.4 PO4 core. The carbon coating layer accounted for 2.22% of the mass of the positive electrode active material, which was recorded as the carbon content. The average length of the first plate-like nanoparticles in the positive electrode active material was 120 nm.

[0109] Comparative Example 2

[0110] The preparation method of the positive electrode active material of Comparative Example 2 included the following steps:

[0111] 1) Preparation of second plate-like nanoparticles: 0.03 mol of MnSO4, 0.02 mol of FeSO4 and 0.05 mol of LiOH H2O were respectively dissolved in 100 mL of an aqueous ethylene glycol solution (mass ratio of ethylene glycol to water was 3:1), and uniformly stirred by ultrasonic dispersion to obtain a manganese source and iron source solution and a lithium source solution. 0.05 mol of H3PO4 was added to the manganese source and iron source solution and uniformly stirred, and then the lithium source solution was slowly added. After uniform stirring, the mixture was transferred into an autoclave tank and reacted at 180°C for 15 h to obtain second plate-like nanoparticles with uniform morphology, wherein the average length of the second plate-like nanoparticles was 100 nm.

[0112] 2) The second plate-like nanoparticles were mixed with glucose and PEG in a mass ratio of 10:0.9:0.1, ground for 10 min, and then sintered at 620°C for 5 h under N2 atmosphere at a heating rate of 3°C / min to obtain a positive electrode active material with a LiMn 0.6 Fe 0.4 PO4 core. The carbon coating layer accounted for 2.22% of the mass of the positive electrode active material, which was recorded as the carbon content. The average length of the first plate-like nanoparticles in the positive electrode active material was 120 nm.

[0113] Test Example:

[0114] 1. Compaction density: The positive electrode active material, conductive carbon black, and binder polyvinylidene fluoride (PVDF) of each example and comparative example were mixed in a mass percentage ratio of 95:3:2 in N-methylpyrrolidone solvent under stirring to obtain a positive electrode slurry. The positive electrode slurry was coated on an aluminum foil, and the surface density was controlled at 300 mg / cm 2 The electrode sheet was rolled to test the compaction density. The results are shown in Table 1.

[0115] 2. Impedance: The positive electrode active material was tested for the electrochemical impedance spectrum (EIS) using a Princeton electrochemical workstation. The test conditions were: an alternating excitation signal amplitude of 10 mV, a frequency range of 10 -2 -10 5 Hz, and a voltage range of 2.0-4.3 V. The results are shown in Table 1.

[0116] 3. Carbon coating layer mass: The mass fraction was confirmed by a carbon-sulfur analyzer and calculated. The results are shown in Table 1.

[0117] 4. Average length of plate-shaped nanoparticles, average thickness of plate-shaped nanoparticles, and average particle size of spherical nanoparticles: The plate-shaped nanoparticles and spherical nanoparticles were confirmed by SEM images, and the length, thickness, and particle size of 50 plate-shaped nanoparticles and spherical nanoparticles in the SEM images were tested, respectively, and the average length and average thickness of the plate-shaped nanoparticles and the average particle size of the spherical nanoparticles were calculated. The results are shown in Table 1.

[0118] 5. SPAN: The particle sizes of one hundred first spherical nanoparticles were counted by SEM to obtain D10, D50, and D90, and SPAN=(D90-D10) / D50.

[0119] 6. Sphericity: The spherical particles in the SEM images were scanned using Metis software to obtain the projection area S and the circumference L, and the sphericity R was calculated according to the formula: R=4πS / L 2 .

[0120] 7. Standard deviation: The lengths of one hundred plate-shaped nanoparticles were counted by SEM, and the length of the i-th nanoparticle was X i , and the average length was X0. The standard deviation was calculated according to the formula

[0121] 8、Specific capacity: at 25°C, under normal pressure (0.1 MPa), the positive electrode active material, conductive carbon black and binder polyvinylidene fluoride (PVDF) of each example and comparative example were mixed in N-methyl pyrrolidone solvent at a mass percentage ratio of 90:5:5, and then stirred sufficiently to obtain a positive electrode slurry. The positive electrode slurry was coated on an aluminum foil, and then dried, cold-pressed to obtain a positive electrode sheet containing a positive electrode active layer with a thickness of 100 μm. Then, the positive electrode sheet was treated by drying and weighing with a film die with a small round piece of 12 mm in diameter. In an Ar-protected glove box, a 2025 button cell shell was used to assemble a button cell with a Li metal round piece as a negative electrode, and a conventional high-voltage lithium cobaltate electrolyte. The button cell was charged at a rate of 0.1C / 1C / 4C to 4.3 V, and then charged at a constant voltage of 4.3 V until the current was equal to 0.05C. At this time, the charging capacity was recorded as the first cycle specific charging capacity. Then, the button cell was rested for 5 min, and then discharged at a rate of 0.1C / 1C / 4C to a voltage of 2.0 V. The discharge capacity at this time was recorded as the first cycle specific discharge capacity of the battery.

[0122] 9、Rate performance: the positive electrode active material was coated into a positive electrode sheet according to the method in the button capacity test. Graphite was used as a negative electrode, and a separator polyethylene and an electrolyte (the electrolyte was LiPF6, and the solvent was EC / DMC) were used to form a full battery. The full battery was charged at a rate of 0.1C to a voltage of 4.3 V, and then charged at a constant voltage of 4.3 V until the current was equal to 0.05C. Then, the full battery was rested for 5 min, and then discharged at a rate of 0.1C to a voltage of 2.5 V. At this time, the capacity was recorded as the discharge capacity at a rate of 0.1C. The full battery was rested for 10 min, and then charged at a rate of 1C to a voltage of 4.3 V, and then charged at a constant voltage of 4.3 V until the current was equal to 0.05C. Then, the full battery was rested for 5 min, and then discharged at a rate of 1C to a voltage of 2.5 V. At this time, the capacity was recorded as the discharge capacity at a rate of 1C. The full battery was rested for 10 min, and then charged at a rate of 4C to a voltage of 4.3 V, and then charged at a constant voltage of 4.3 V until the current was equal to 0.05C. Then, the full battery was rested for 5 min, and then discharged at a rate of 4C to a voltage of 2.5 V. At this time, the capacity was recorded as the discharge capacity at a rate of 4C. The discharge capacity at a rate of 1C was the 1C rate performance compared with the discharge capacity at a rate of 0.1C. The discharge capacity at a rate of 4C was the 4C rate performance compared with the discharge capacity at a rate of 1C.

[0123] FIG. 1 is an SEM image of the positive electrode active material prepared in Example 1.

[0124] As shown in FIG. 1, the positive electrode active material prepared in Example 1 of the present application is a plate-like positive electrode active material with a size close to a spherical positive electrode active material, and the plate-like positive electrode active material and the spherical positive electrode active material are mixed alternately, and there is a certain interstitial effect.

[0125] Table 1

[0126] Table 2

[0127] As shown in Table 1, compared with the comparative example, the positive electrode active material provided in this application, by limiting its compaction density and impedance, enables the positive electrode active material to have both high compaction density and excellent rate performance. When applied to lithium-ion batteries, it can improve the specific capacity and rate performance of lithium-ion batteries.

[0128] As can be seen from the comparison between Example 1 and Comparative Examples 1-2, the positive electrode active material provided in this application, by limiting its compaction density and impedance, enables the positive electrode active material to have both high compaction density and excellent rate performance. When applied to lithium-ion batteries, it can improve the specific capacity and rate performance of lithium-ion batteries.

[0129] 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.

Claims

1. A positive electrode active material, characterized by, comprising LiMn x Fe 1-x PO4 core and a carbon coating layer covering at least part of the surface of the core, wherein 0≤x<1; The compacted density of the positive electrode active material is not less than 2.28 g / cm 3 ; The impedance of the positive electrode active material is not higher than 1500 Ω.

2. The positive electrode active material according to claim 1, characterized by The inner core comprises first plate-like nanoparticles and first spherical nanoparticles. The difference between the average length of the first plate-like nanoparticles and the average particle size of the first spherical nanoparticles is not more than 50 nm.

3. The positive electrode active material according to claim 2, characterized by The average length of the first plate-like nanoparticles is 100-200 nm. The average particle size of the first spherical nanoparticles is 100-250 nm.

4. The positive electrode active material according to claim 2 or 3, characterized by The thickness of the first plate-like nanoparticles is less than 20 nm. The sphericity of the first spherical nanoparticles is greater than 0.

5.

5. The positive electrode active material according to any one of claims 2 to 4, characterized by The standard deviation of the length of the first plate-like nanoparticles is less than 80 nm. The particle size distribution SPAN of the first spherical nanoparticles is 1.0-2.

0.

6. The positive electrode active material according to any one of claims 1 to 5, characterized by The carbon coating layer accounts for 1-3% of the mass of the positive electrode active material.

7. A method for producing the positive electrode active material as claimed in any one of claims 1 to 6, characterized by, The method comprises the following steps: The second plate-like nanoparticles are added to the second spherical nanoparticles in batches and mixed uniformly to obtain a mixed raw material; and a mixed system comprising the mixed raw material and a carbon source is subjected to sintering treatment at 600-800 ℃ under an inert atmosphere to obtain the positive electrode active material. The mass fraction of each batch of added second plate-like nanoparticles in the total amount of second plate-like nanoparticles is 10-20%; and the difference between the average length of the second plate-like nanoparticles and the average particle size of the second spherical nanoparticles is not more than 50 nm.

8. The preparation method according to claim 7, characterized in that, The mass ratio of the second plate-like nanoparticles to the second spherical nanoparticles in the mixed raw material is 0.25-4:

1. And / or, the carbon source accounts for 5-15% of the mass of the mixed raw material.

9. A positive electrode sheet characterized by comprising: The positive electrode active material comprises the positive electrode active material according to any one of claims 1-6 or the positive electrode active material prepared by the preparation method according to claim 7 or 8.

10. A lithium-ion battery, characterized by, The positive electrode sheet comprises the positive electrode active material according to claim 9.

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