Positive electrode material and preparation method therefor, and lithium-ion battery

Abstract

A positive electrode material and a preparation method therefor, and a lithium-ion battery, for use in providing a positive electrode material having both high energy density and excellent cycling stability. The positive electrode material comprises: a substrate, and a coating layer covering the surface of the substrate. The crystalline phase structure of the substrate is a layered phase; and the coating layer comprises lithium manganese iron phosphate particles, and a polymer film that fills the gaps among the lithium manganese iron phosphate particles and covers the outermost surface of the lithium manganese iron phosphate particles coated on the surface of the substrate.

Description

Positive electrode material and preparation method thereof, and lithium ion battery

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on December 28, 2023, with application number 202311839496.7 and application name “A positive electrode material, its preparation method, and lithium-ion battery”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present application relates to the technical field of positive electrode materials, and in particular to a positive electrode material and a preparation method thereof, and a lithium-ion battery. Background Art

[0003] With the development of new energy vehicles, higher requirements are placed on the energy density and service life of power batteries, and the performance of positive electrode materials plays a decisive role in the energy density and service life of power batteries.

[0004] The two mainstream materials currently used are layered phase materials (such as ternary cathodes) and olivine-type lithium manganese iron phosphate cathode materials. Layered phase materials, due to their layered structure, are beneficial for lithium ions to diffuse at a higher rate, so they have the advantage of higher energy density. However, the layered phase structure has the problem of insufficient crystal structure stability: on the one hand, it is prone to phase transition, which in turn manifests as particle cracking due to severe lattice strain; on the other hand, it is easily heated during the cycle, resulting in irreversible release of lattice oxygen, so the crystal structure is not stable enough, which in turn manifests as insufficient cycle stability. Olivine-type lithium manganese iron phosphate materials have the advantage of high stability due to the strong PO4 tetrahedral PO covalent bond energy in their olivine structure. However, due to the PO covalent bond in the olivine structure, the electrical conductivity and lithium ion diffusion coefficient of lithium manganese iron phosphate materials are low, so they have the problem of insufficient energy density. Therefore, there is an urgent need for a cathode material with both excellent energy density and cycle stability. Summary of the Invention

[0005] The present application provides a positive electrode material and a preparation method thereof, and a lithium-ion battery, for providing a positive electrode material with excellent energy density and cycle stability.

[0006] In a first aspect, an embodiment of the present application provides a positive electrode material, comprising: a substrate, and a coating layer coated on the surface of the substrate; wherein the crystal phase structure of the substrate is a layered phase,

[0007] The coating layer includes lithium iron manganese phosphate particles and a polymer film filling gaps between the lithium iron manganese phosphate particles and coating the surface of the lithium iron manganese phosphate on the outermost layer of the substrate surface.

[0008] In a possible embodiment, the molecular expression of the substrate is Lig Ni a Co b D c E 1-a-b-c O2, 0.98≤g≤1.02, 0.50≤a≤0.95, 0.025≤b≤0.20, 0.025≤c≤0.30, 0≤1-abc≤0.055, D is selected from Mn and / or Al, and E is selected from at least one of Mg, W, Nb, Zr, Nd, Na and Ti.

[0009] In a possible embodiment, the molecular expression of the lithium manganese iron phosphate particles is Li d Mn e Fe f M 1- e-f PO4, 0.98≤d≤1.02, 0.4≤e≤0.8, 0.2≤f≤0.6, 0≤1-ef≤0.035, M is selected from at least one of Mg, Na, K, Co, Ni, Zn and Ti.

[0010] In one possible embodiment, the polymer film is obtained by heat treating a conductive polymer, a binder, the substrate and the lithium iron manganese phosphate particles together; wherein the median particle size of the substrate is larger than the median particle size of the lithium iron manganese phosphate particles.

[0011] In one possible embodiment, the ratio of the median particle size of the substrate to the median particle size of the lithium manganese iron phosphate particles is greater than or equal to 4; and / or,

[0012] Based on the total mass of the substrate and the lithium iron manganese phosphate particles, the content of the substrate is 40%-70%.

[0013] In one possible implementation manner, the polymer film includes a conductive polymer and a binder.

[0014] In one possible implementation, the conductive polymer includes at least one of 3-hexyl-substituted polythiophene, polyaniline, and polyethylenedioxythiophene; and the binder includes at least one of polyethylene glycol, cellulose acetate, polystyrene, and polyvinylidene fluoride.

[0015] In a possible implementation manner, the surface of the lithium iron manganese phosphate particles contains carbon; wherein the ratio of the mass of the carbon to the mass of the lithium iron manganese phosphate particles is 0.5%-3.5%.

[0016] In a second aspect, embodiments of the present application provide a method for preparing the positive electrode material described in the first aspect and any possible embodiment, comprising:

[0017] In an inert atmosphere, heat-treating a mixture comprising a polymer, a substrate, and lithium iron manganese phosphate particles to obtain the positive electrode material; wherein the heat-treating temperature is greater than or equal to the melting temperature of the polymer and less than the decomposition temperature of the polymer;

[0018] The crystal phase structure of the substrate is a layered phase, and the median particle size of the substrate is larger than the median particle size of the lithium manganese iron phosphate particles.

[0019] In one possible embodiment, heat treating a mixture containing a polymer, a substrate, and lithium iron manganese phosphate particles in an inert atmosphere to obtain the positive electrode material comprises:

[0020] mixing the substrate and the lithium iron manganese phosphate particles to obtain a first mixture;

[0021] mixing the first mixture with the polymer;

[0022] The mixture of the first mixture and the polymer is heat-treated in an inert atmosphere to obtain the positive electrode material.

[0023] In a possible implementation manner, based on the mass percentage of the total mass of the lithium iron manganese phosphate particles and the substrate, the content of the lithium iron manganese phosphate particles is 30%-60%, and the content of the substrate is 40%-70%.

[0024] In one possible implementation, the conductive polymer includes at least one of 3-hexyl-substituted polythiophene, polyaniline, and polyethylenedioxythiophene; and the binder includes at least one of polyethylene glycol, cellulose acetate, polystyrene, and polyvinylidene fluoride.

[0025] In a possible implementation manner, in the first mixture, the lithium manganese iron phosphate material accumulates on the surface of the substrate to form a coating layer.

[0026] In a possible implementation manner, the polymer includes a conductive polymer and a binder; wherein the mass ratio of the conductive polymer to the binder is (0.3-0.5):(0.5-0.7).

[0027] In one possible embodiment, heat treating a mixture containing a polymer, a substrate, and lithium iron manganese phosphate particles in an inert atmosphere to obtain the positive electrode material comprises:

[0028] The mixture is sintered at 280-310° C. for 3-5 hours to obtain the positive electrode material.

[0029] In a third aspect, an embodiment of the present application provides a lithium-ion battery, comprising:

[0030] The positive electrode material described in the first aspect and any possible embodiment.

[0031] One or more technical solutions provided in the embodiments of this application have at least the following beneficial effects:

[0032] In the positive electrode material provided in the embodiments of the present application, an integrally formed polymer film located between the lithium iron manganese phosphate particles and on the surface of the outermost layer of lithium iron manganese phosphate particles allows the lithium iron manganese phosphate particles to be firmly coated on the substrate surface, without falling off with the increase in the number of charge-discharge cycles. Under the action of this polymer film, the positive electrode material can achieve a coating shedding rate of as low as 0.6% after 5 hours of mechanical vibration. This enables the positive electrode material, based on its inherently stable core-shell structure, to effectively improve energy density and cycle stability during the charge-discharge process through the synergistic effect of the lithium iron manganese phosphate particles and the layered phase substrate.

[0033] Secondly, the conductive polymer in the polymer film can further improve the electrochemical properties of the positive electrode material, such as the discharge capacity and rate performance, by improving the conductivity.

[0034] Finally, in the preparation method provided in the embodiment of the present application, by heat treating the mixture containing the polymer, the substrate, and the lithium iron manganese phosphate particles, the polymer can make use of the characteristics of molten flow to enable the polymer attached to the surface of the outermost layer of lithium iron manganese phosphate particles to completely or incompletely cover the outermost layer of lithium iron manganese phosphate particles in the molten state. Since it has a certain fluidity, a part of the molten polymer will enter the gaps between the lithium iron manganese phosphate particles. As a result, the cooled polymer film is integrally formed and located in the gaps between the lithium iron manganese phosphate particles and on the surface of the outermost layer of lithium iron manganese phosphate particles, thereby playing the role of stabilizing the coating layer.

[0035] Furthermore, since the decomposition temperature of the polymer is usually around 350°C or lower, heat treating the mixture at a temperature lower than the decomposition temperature of the polymer can effectively avoid the problem of excessive sintering of the substrate and lithium manganese iron phosphate particles in the mixture.

[0036] Other features and advantages of the present application will be described in the following description and, in part, will become apparent from the description or may be learned through practice of the present application. The objectives and other advantages of the present application may be achieved and obtained through the structures particularly pointed out in the written description, claims, and drawings. It should be understood that the above general description and the detailed description that follow are merely exemplary and explanatory and do not limit the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without any creative work.

[0038] FIG1 is a schematic flow chart of a method for preparing a positive electrode material provided in an embodiment of the present application;

[0039] FIG2 is a SEM image of the positive electrode material in Example 3 provided in the examples of the present application;

[0040] FIG3 is a SEM image of the positive electrode material in Comparative Example 1 provided in an embodiment of the present application. DETAILED DESCRIPTION

[0041] In response to the current problem of lack of positive electrode materials with excellent energy density and cycle stability, the embodiment of the present application provides a positive electrode material with a layered phase material coated with a lithium iron manganese phosphate material as a coating layer, so that the core-shell structured positive electrode material has the advantage of high energy density based on the layered phase material. And because of the polymer film in the coating layer, the lithium iron manganese phosphate particles in the coating layer can be stably coated on the surface of the layered phase material during the charging and discharging process without falling off. Thus, the structural stability advantage of the lithium iron manganese phosphate in the coating layer can be utilized to block most of the heat from being transferred to the inner layer of the layered phase material, so as to effectively alleviate the problem of the inner layer of the layered phase material being heated and causing the oxygen atoms to escape and causing the crystal phase structure to gradually collapse, thereby effectively improving the cycle stability of the layered phase material; and utilizing the structural stability of the lithium iron manganese phosphate itself, the positive electrode material is able to exhibit outstanding cycle stability. Thus, a positive electrode material with excellent energy density and cycle stability is provided.

[0042] In the examples of this application, the subscripts in the molecular expressions are atomic indices, which are used to indicate the relative molar content of the corresponding atoms in the molecule. x B y C z In O4, x, y, and z are all atomic indices, that is, x, y, and z represent the relative molar content of element A, element B, and element C in the molecule respectively.

[0043] The following is a detailed description of a positive electrode material, a preparation method thereof, and a lithium-ion battery provided in the embodiments of the present application. It should be noted that the embodiments described below are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0044] The present application provides a positive electrode material, comprising: a substrate and a coating layer coated on a surface of the substrate. The crystal phase structure of the substrate is a layered phase.

[0045] The coating layer includes lithium iron manganese phosphate particles and a polymer film filling the gaps between the lithium iron manganese phosphate particles and coating the outermost surface of the lithium iron manganese phosphate particles on the surface of the substrate. The polymer film on the outer side of the lithium iron manganese phosphate particles.

[0046] The median particle size of the substrate is larger than the median particle size of the lithium manganese iron phosphate particles. On the surface of the substrate, the lithium manganese iron phosphate can be arranged in a single layer, in overlapping layers, or in staggered layers.

[0047] The polymer film distributed on the surface and gaps of the outermost layer of lithium iron manganese phosphate particles is integrally formed. The polymer film can be obtained by heat-treating the corresponding polymer, substrate, and the lithium iron manganese phosphate particles. In other words, the polymer film can be obtained by heat-treating a mixture containing the polymer, substrate, and the lithium iron manganese phosphate particles.

[0048] The median particle size of the substrate is larger than that of the lithium iron phosphate particles. The integral formation of the polymer film occurs when the small lithium iron phosphate particles, after mixing with the substrate, naturally adsorb to the substrate surface, forming a relatively loose initial coating. The resulting polymer, when heated to or above the melting temperature, naturally flows along the lithium iron phosphate particles on the substrate surface, gradually distributing itself among the lithium iron phosphate particles and entering the gaps between them. Once the polymer cools from its molten state, the integrally formed polymer film is obtained.

[0049] The substrate may be a single crystal material, and its morphology is primary particles; or it may be a polycrystalline material, and its morphology is secondary particles composed of primary particles.

[0050] Furthermore, the ratio of the median particle size of the substrate to the median particle size of the lithium manganese iron oxide particles is greater than or equal to 4. Optionally, the ratio of the median particle size of the substrate to the median particle size of the lithium manganese iron oxide particles is (4-7):1.

[0051] In some embodiments, the polymer may be composed of a conductive polymer to further improve the charge and discharge capacity and rate performance of lithium manganese iron phosphate.

[0052] In some embodiments, the polymer film may include a binder and a conductive polymer to further enhance the stability and conductivity of the core-shell structure of the positive electrode material, thereby preventing the polymer film from falling off during battery charging and discharging, and thereby preventing the lithium manganese iron phosphate particles in the coating layer from falling off.

[0053] The conductive polymer includes at least one of 3-hexyl-substituted polythiophene, polyaniline and polyethylene dioxythiophene. The binder includes at least one of polyethylene glycol, cellulose acetate, polystyrene and polyvinylidene fluoride.

[0054] The substrate is a layered phase material containing Li. In some embodiments, the substrate can be a ternary or quaternary material, and its molecular expression can be Li g Ni a Co b D c E 1-a-b-c O2, 0.98≤g≤1.02, 0.50≤a≤0.95, 0.025≤b≤0.20, 0.025≤c≤0.30, 0≤1-abc≤0.055, D is selected from Mn and / or Al, and E is selected from at least one of Mg, W, Nb, Zr, Nd, Na and Ti.

[0055] Among them, when a is 0.5-0.7, it is a low-nickel material; when a is 0.7-0.8, it is a medium-nickel material; when a is 0.8-0.95, it is a high-nickel material.

[0056] In some embodiments, the substrate may be at least one of lithium nickelate with a molecular expression of LiNiO2, lithium cobaltate with a molecular expression of LiCoO2, and lithium manganate with a molecular expression of LiMnO2.

[0057] The molecular expression of the above-mentioned lithium manganese iron phosphate particles is Li d Mn e Fe f M 1-e-f PO4, 0.98≤d≤1.02, 0.4≤e≤0.8, 0.2≤f≤0.6, 0≤1-ef≤0.035, M is selected from at least one of Mg, Na, K, Co, Ni, Zn and Ti.

[0058] Furthermore, the lithium iron manganese phosphate particles are carbon-coated lithium iron manganese phosphate particles, and the surface of the lithium iron manganese phosphate particles contains carbon.

[0059] The ratio of the mass of carbon to the mass of lithium manganese iron phosphate is 0.5%-3.5%.

[0060] Based on the same inventive concept, the present embodiment provides a method for preparing the above-mentioned positive electrode material, which includes the following implementation steps:

[0061] In an inert atmosphere, a mixture containing a polymer, a substrate and lithium manganese iron phosphate particles is heat-treated to obtain a positive electrode material.

[0062] The heat treatment temperature is greater than or equal to the melting temperature of the polymer and less than the decomposition temperature of the polymer. The heat treatment time can be 3-5 hours.

[0063] The substrate has a layered crystal structure, and a median particle size of the substrate is larger than that of the lithium manganese iron phosphate particles. In some embodiments, the median particle size of the substrate is less than 25 μm.

[0064] Specifically, the polymer may include a conductive polymer to further improve the charge and discharge capacity and rate performance of lithium manganese iron phosphate.

[0065] The polymer may also include a binder and a conductive polymer to further improve the stability and conductivity of the core-shell structure of the positive electrode material, prevent the polymer film from falling off during battery charging and discharging, and thus prevent the lithium manganese iron phosphate particles in the coating layer from falling off.

[0066] Wherein, the mass ratio between the conductive polymer and the binder is (0.3-0.5): (0.5-0.7).

[0067] When the polymer is a mixture of conductive polymers, or a mixture of a conductive polymer and a binder, the heat treatment temperature is set based on the highest melting temperature of each polymer and the lowest decomposition temperature of each polymer. That is, the heat treatment temperature is greater than or equal to the highest melting temperature and less than the lowest decomposition temperature.

[0068] Since the melting temperature of polymers is usually less than 280°C and the decomposition temperature is greater than 300°C, in some embodiments, the temperature condition of the heat treatment may be 280-300°C.

[0069] In some embodiments, the conductive polymer includes at least one of 3-hexyl-substituted polythiophene, polyaniline, and polyethylenedioxythiophene.

[0070] The binder includes at least one of polyethylene glycol, cellulose acetate, polystyrene, and polyvinylidene fluoride.

[0071] In some embodiments, a ratio of a median particle size of the substrate to a median particle size of the lithium manganese iron phosphate particles is greater than or equal to 4.

[0072] Preferably, the ratio of the aforementioned median particle sizes can be (4-7):1. The median particle size of the substrate can be, for example, 8-25 μm, and the size of the lithium manganese iron phosphate particles can be set accordingly based on the aforementioned relative relationship of the median particle sizes. Alternatively, the lithium manganese iron phosphate particles can be, for example, 1.5-5 μm, and the size of the substrate can be set accordingly based on the aforementioned relative relationship of the median particle sizes.

[0073] In some embodiments, the substrate may be a molecular formula of Li gNi a Co b D c E 1-a-b-c Ternary or quaternary material of O2, 0.98≤g≤1.02, 0.50≤a≤0.95, 0.025≤b≤0.20, 0.025≤c≤0.30, 0≤1-abc≤0.055, D is selected from Mn and / or Al, and E is selected from at least one of Mg, W, Nb, Zr, Nd, Na and Ti.

[0074] In some embodiments, the substrate may be at least one of lithium nickelate with a molecular expression of LiNiO2, lithium cobaltate with a molecular expression of LiCoO2, and lithium manganate with a molecular expression of LiMnO2.

[0075] The molecular expression of the above-mentioned lithium manganese iron phosphate particles can be Li d Mn e Fe f M 1-e-f PO4, 0.98≤d≤1.02, 0.4≤e≤0.8, 0.2≤f≤0.6, 0≤1-ef≤0.035, M is selected from at least one of Mg, Na, K, Co, Ni, Zn and Ti.

[0076] Furthermore, the positive electrode material can be obtained by directly mixing the polymer, the substrate and the lithium iron manganese phosphate particles together and then sintering them.

[0077] Alternatively, in order to ensure that the polymer film after heat treatment is mainly located on the surface of the outermost layer of lithium iron manganese phosphate particles on the substrate surface, in some embodiments, the substrate and lithium iron manganese phosphate particles can be mixed to obtain a first mixture, and then the polymer is mixed in and sintered. The following is a detailed description, please refer to Figure 1:

[0078] Step 101: Mix a substrate and lithium iron manganese phosphate particles to obtain a first mixture.

[0079] Specifically, the substrate and the lithium iron manganese phosphate particles can be mixed in a high-speed mixer at a rotation speed of 1000-1600 r / min for 0.5-2 hours to obtain a first mixture, in which the lithium iron manganese phosphate particles adhere to the surface of the substrate due to physical adsorption.

[0080] In the first mixture, the black substrate and the gray lithium iron manganese phosphate particles can be mixed until they are relatively evenly distributed in the mixing setting without agglomeration.

[0081] Step 102: Mix the first mixture and the polymer.

[0082] Similarly, the mixture can be mixed in a high-speed mixer at a rotation speed of 1000-1600 r / min for 0.5-1.5 hours to obtain a second mixture. In the mixture of the first mixture and the polymer, the polymer is attached to the surface of the lithium manganese iron phosphate particles on the substrate surface.

[0083] Step 103: heat-treating the second mixture in an inert atmosphere to obtain the positive electrode material.

[0084] Specifically, the inert atmosphere can be nitrogen and / or argon, and the temperature condition is greater than or equal to the melting temperature of the polymer and less than the decomposition temperature of the polymer; for example, it can be 280-310° C., and the heat treatment time can be 3-5 hours.

[0085] Furthermore, to avoid the low conductivity of lithium iron manganese phosphate affecting the electrochemical performance of the final cathode material, in some embodiments, the lithium iron manganese phosphate material is lithium iron manganese phosphate particles containing carbon on their surface. The aforementioned lithium iron manganese phosphate mass β is the total mass of the lithium iron manganese phosphate particles and the carbon, where the ratio of the mass of the carbon to the total mass is 0.005-0.035.

[0086] When the surface of the lithium manganese iron phosphate contains carbon, the ratio α / (β+θ) of the mass α of the polymer to the sum of the mass β of the carbon-containing lithium manganese iron phosphate particles and the mass θ of the substrate is 0.5%-7%.

[0087] The ratio θ / (θ+β) of the mass θ of the substrate to the sum of the mass θ of the substrate and the mass β of the lithium iron manganese phosphate particles is 40%-70%. That is, the content of the substrate is 40%-70% based on the total mass of the substrate and the lithium iron manganese phosphate particles.

[0088] It should be noted that since the carbon content on the surface of lithium manganese iron phosphate is trace, when the surface of lithium manganese iron phosphate does not contain carbon, the ratio of the mass of the polymer to the sum of the masses of lithium manganese iron phosphate and the substrate, as well as the ratio of the mass of lithium manganese iron phosphate to the sum of the masses of lithium manganese iron phosphate and the substrate can still refer to the above ratios.

[0089] The following is a detailed description of the preparation method of lithium manganese iron phosphate particles containing carbon on the surface:

[0090] First, a mixture containing a lithium source, a manganese source, an iron source, a phosphorus source, a carbon source, and a doping metal material is added to water to obtain a slurry with a solid content of about 35%. The carbon source can be selected from at least one of glucose, sucrose, polyethylene glycol, starch, and polystyrene.

[0091] Then, the slurry is coarsely ground and sand-milled in succession, and then dried to obtain a lithium iron manganese phosphate precursor. Specifically, the aforementioned slurry can be coarsely ground in a coarse grinder with a rotation speed of 1100-1600r / min for 0.5-2h to obtain an intermediate slurry with a median particle size of less than 14μm. The intermediate slurry is continued to be sand-milled in a sand mill with a rotation speed of 1200-1700 for 1-3 hours to obtain a target slurry with a median particle size of 0.2-7μm. Finally, the target slurry is dried for 5-9 hours by a spray dryer with an air pressure of 0.1-0.6MPa, an inlet temperature of 150-260℃, and an outlet temperature of 80-120℃ to obtain a lithium iron manganese phosphate precursor.

[0092] Next, the lithium manganese iron phosphate is sintered in an inert atmosphere to obtain carbon-coated lithium manganese iron phosphate particles. Specifically, this can be achieved through a secondary sintering process, wherein the primary sintering temperature is 280-370°C for 4-7 hours, and the secondary sintering temperature is 560-670°C for 7-12 hours.

[0093] Based on the same inventive concept, the present invention also provides a secondary battery comprising the aforementioned positive electrode material. Due to the advantages of the positive electrode material, both excellent energy density and cycle stability, the service life and user experience of the secondary battery can be significantly improved.

[0094] The following is a detailed description through examples and comparative examples.

[0095] Example 1

[0096] S1. Lithium carbonate, manganese carbonate, iron phosphate, manganese dihydrogen phosphate, glucose, polyethylene glycol, and magnesium phosphate are mixed. The mixing ratio is set according to the molar ratio of Mn:Fe:Mg of 0.6:0.38:0.02, and the carbon coating amount accounts for 2% of the total mass. Water is then added to adjust the solid content of the slurry to 35%. After the slurry is evenly mixed, it is added to a coarse grinder with a speed of 1500 r / min (the speed is not less than 1100 r / min) and coarsely ground for 1 hour. Then, it is transferred to a sand mill with a speed set at 1500 r / min (the speed is not less than 1200 r / min) and sand-milled for 2 hours to obtain the target slurry.

[0097] S2. The target slurry is slowly transported to a spray drying tower for drying. The inlet temperature is controlled to be 200°C, the outlet temperature is set to be 100°C, the compressed air pressure is 0.4MPa, and the drying time is 7h. The powder particles obtained by spray drying are placed in a tube furnace and sintered once in a nitrogen atmosphere at a controlled temperature of 280°C for 4h; then a secondary sintering is performed at a controlled temperature of 560°C for 12h. After sintering, the particles are naturally cooled to room temperature to obtain carbon-coated lithium manganese iron phosphate particles LiMn with a median particle size of 3.2μm. 0.6 Fe 0.38 Mg 0.02 PO4.

[0098] S3, the nickel ternary material substrate LiNi with a median particle size of 13 μm 0.7 Co 0.1 Mn 0.2 O2 and lithium manganese iron phosphate particles with a median particle size of 3.2 μm were added into a high-speed mixer at a rotation speed of 1000 r / min in a mass ratio of 3:7 and mixed for 2 hours to obtain a first mixture.

[0099] S4. Add the polyaniline and polyvinylidene fluoride mixture to the first mixture and continue mixing for 1.5 hours to obtain a second mixture. The mass of the polyaniline and polyvinylidene fluoride mixture added is 0.5% of the mass of the first mixture. The mass ratio of polyaniline to polyvinylidene fluoride is 0.3:0.7.

[0100] S5. Place the second mixture in a tube furnace, and heat treat it at 310° C. for 3 hours under a nitrogen atmosphere, and then cool it naturally to room temperature to obtain a ternary composite material containing a lithium manganese iron phosphate coating layer.

[0101] Example 2

[0102] The difference from Example 1 is that the substrate mixed with the lithium manganese iron phosphate particles in step S3 is the medium nickel ternary material LiNi 0.7 Co 0.095 Mn 0.2 W 0.005 O2.

[0103] The remaining steps and parameters are the same as those in Example 1.

[0104] Example 3

[0105] The difference from Example 1 is that in step S3, the low nickel ternary material substrate LiNi with a median particle size of 24 μm is 0.5 Co 0.2 Mn 0.3O2 and lithium manganese iron phosphate particles with a median particle size of 3.2 μm were added into a high-speed mixer at a rotation speed of 1000 r / min in a mass ratio of 6:4 and mixed.

[0106] The remaining steps and parameters are the same as those in Example 1. The SEM image of the positive electrode material is shown in Figure 2. As shown in Figure 2, the lithium manganese iron phosphate is evenly coated on the surface of the secondary particles without obvious agglomeration.

[0107] Example 4

[0108] The difference from Example 1 is that the speed of the sand mill in S1 is 1700 r / min and the sand milling is 3 h; the median particle size of the lithium manganese iron phosphate particles in S2 is 1.5 μm;

[0109] In step S3, the low nickel ternary material substrate LiNi with a median particle size of 9 μm is 0.5 Co 0.2 Mn 0.3 O2 is mixed with lithium manganese iron phosphate particles with a median particle size of 1.5 μm;

[0110] In step S4, the mass of the mixture of 3-hexyl-substituted polythiophene and polyvinylidene fluoride added is 7% of the mass of the first mixture, wherein the mass ratio between 3-hexyl-substituted polythiophene and polyvinylidene fluoride is 1:1.

[0111] The remaining steps and parameters are the same as those in Example 1.

[0112] Example 5

[0113] The difference from Example 1 is that the speed of the sand mill in S1 is 1700 r / min and the sand milling is 2 h; the median particle size of the lithium manganese iron phosphate particles in S2 is 2.9 μm;

[0114] In step S3, the high nickel ternary material substrate LiNi with a median particle size of 20 μm is 0.8 Co 0.1 Mn 0.1 O2 and lithium manganese iron phosphate particles with a median particle size of 2.9 μm;

[0115] In S5, the heat treatment temperature is 280° C. and the time is 5 h.

[0116] The remaining steps and parameters are the same as those in Example 1.

[0117] Example 6

[0118] The difference from Example 1 is that in step S4, 3-hexyl-substituted polythiophene and polyethylene glycol are added to the first mixture, wherein the mass ratio of 3-hexyl-substituted polythiophene to polyethylene glycol is 0.45:0.55;

[0119] In step S5, the heat treatment temperature is 280° C. and the heat treatment time is 5 hours.

[0120] The remaining steps and parameters are the same as those in Example 1.

[0121] Example 7

[0122] The difference from Example 1 is that in step S3, the medium nickel ternary material substrate LiNi 0.7 Co 0.1 Mn 0.2 The mass ratio of O2 to lithium iron manganese phosphate particles is 7:3. The remaining steps and parameters are the same as those in Example 1.

[0123] Example 8

[0124] The difference from Example 1 is that in step S1, lithium carbonate, manganese carbonate, iron phosphate, manganese dihydrogen phosphate, glucose, polyethylene glycol, and nickel oxide are mixed in a mixing ratio of 0.6:0.39:0.01 based on the molar ratio of Mn:Fe:Ni;

[0125] The molecular expression of the lithium manganese iron phosphate cathode material obtained in step S2 is LiMn 0.6 Fe 0.39 Ni 0.01 PO4; in step S3, the medium nickel ternary material substrate LiNi with a median particle size of 13 μm 0.7 Co 0.1 Mn 0.2 O2 mixed lithium manganese iron phosphate particles are LiMn 0.6 Fe 0.39 Ni 0.01 PO4.

[0126] The remaining steps and parameters are the same as those in Example 1.

[0127] Example 9

[0128] The difference from Example 1 is that in step S3, the medium nickel ternary material substrate LiNi with a median particle size of 6 μm is 0.7 Co 0.1 Mn 0.2 O2 was mixed with lithium manganese iron phosphate particles with a median particle size of 3 μm, and the remaining steps and parameters were the same as those in Example 1.

[0129] Comparative Example 1

[0130] The difference from Example 1 is that the secondary sintering temperature in step S2 is 530°C and the sintering time is 12 hours; furthermore, step S4 is omitted. The remaining steps and parameters are the same as in Example 1. See Figure 3 for an SEM image of this positive electrode material. As shown in Figure 3, without step S4, the lithium manganese iron phosphate (LMFP) on the surface of the secondary particles exhibits significant agglomeration, resulting in uneven coating.

[0131] Comparative Example 2

[0132] The remaining steps and parameters are the same as those in Example 1.

[0133] Comparative Example 3

[0134] The speed of the sand mill in S1 is 1200 r / min, and the sand milling is performed for 1.5 h, so that the lithium manganese iron phosphate positive electrode material with a median particle size of 6.1 μm is obtained in step S2;

[0135] In step S3, the medium nickel ternary material substrate LiNi with a median particle size of 6.4 μm is 0.7 Co 0.1 Mn 0.2 O2 is mixed with lithium manganese iron phosphate particles.

[0136] The remaining steps and parameters are the same as those in Example 1.

[0137] Furthermore, the following tests were performed on the positive electrode materials in the above embodiments and comparative examples to test whether the coating layer in the positive electrode material is stable. The test method is first described below:

[0138] Select a sieve with a mesh size that only allows the lithium manganese iron phosphate to pass through the vibrating sieve. The powder obtained after the vibrating sieve is the lithium manganese iron phosphate particles that have fallen off the positive electrode material coating, and the shedding rate is calculated based on this. The test parameters are: a vibration frequency of 150 times per minute and a vibration time of 1 hour. The first shedding rate is determined as m1 / M, where m1 is the mass of the particles falling through the sieve mesh, and M is the mass of the sample before vibrating.

[0139] After continuing to vibrate at the same vibration frequency for 2 hours (a total of 3 hours), the second shedding rate is determined as (m1+m2) / M; wherein m2 is the mass of particles falling through the sieve holes during the continued vibration for 2 hours.

[0140] After vibrating for another 2 hours at the same frequency (a total of 5 hours), determine the third shedding rate (m1 + m2 + m3) / M, where m3 is the mass of particles falling through the sieve during the 2-hour vibration period. See Table 1 below for test parameters.

[0141] Table 1

[0142] As can be seen from Table 1, the coating layer of the positive electrode material in the embodiment exhibits excellent mechanical stability, which enables the lithium manganese iron phosphate particles in the coating layer to be stably coated on the surface of the substrate during the charge-discharge cycle of the positive electrode material.

[0143] In particular, by comparing the shedding rates of Example 1 with that of Comparative Example 2, it can be seen that the polymer film formed by heat treatment can effectively improve the stability of the coating layer.

[0144] Furthermore, a secondary battery was prepared using the positive electrode materials in the above examples and comparative examples. During the preparation, a positive electrode slurry (with a surface density of 13.2 mg / cm 2 ). Then, the compacted density after roller pressing is 2.83g / cm 3 The positive electrode.

[0145] The mass ratio of the positive electrode active material: the conductive agent: the binder in the positive electrode slurry is 8:1:1.

[0146] The above batteries were tested for initial discharge capacity, 200-cycle discharge capacity, and capacity retention within a voltage window of 2.5-4.25V. See the table below for test parameters.

[0147] Table 2

[0148] As can be seen from Table 2, the capacity retention rate of the embodiment is significantly improved. From the test data of Comparative Example 3, it can be seen that because the size of the lithium manganese iron phosphate and the substrate (ternary material) are similar, no effective coating is formed, so its capacity retention rate shows a huge difference from that of the embodiment.

[0149] Comparing Example 7 with other examples, when the amount of substrate added is less than 0.7, based on the total mass of the substrate (i.e., the ternary material) and the lithium manganese iron phosphate particles, it is beneficial to form a complete and dense coating layer on the surface of the substrate, thereby exhibiting better cycle stability.

[0150] Further comparison of the embodiment with comparative example 2 shows that the density of the polymer film formed by heat treatment and the stability of the relative position of the coating layer on the substrate surface are significantly improved, thereby promoting the cycle stability of the positive electrode material in the embodiment to show obvious advantages.

[0151] Obviously, those skilled in the art may make various changes and modifications to this application without departing from the spirit and scope of this application. Thus, if these modifications and variations of this application fall within the scope of the claims of this application and their equivalents, this application is intended to include these modifications and variations.

Claims

1. A positive electrode material, wherein, Comprising: a substrate, and a coating layer coated on the surface of the substrate; wherein, the crystal phase structure of the substrate is a layered phase, the coating layer includes lithium iron manganese phosphate particles, and a polymer film that fills the gaps between the lithium iron manganese phosphate particles and coats the outermost surface of the substrate on the surface of the lithium iron manganese phosphate.

2. The positive electrode material according to claim 1, wherein, The molecular formula of the substrate is Li g Ni a Co b D c E 1-a-b-c O2, 0.98 ≤ g ≤ 1.02, 0.50 ≤ a ≤ 0.95, 0.025 ≤ b ≤ 0.20, 0.025 ≤ c ≤ 0.30, 0 ≤ 1 - a - b - c ≤ 0.055, D is selected from Mn and / or Al, and E is selected from at least one of Mg, W, Nb, Zr, Nd, Na, and Ti.

3. The cathode material according to claim 1 or 2, wherein, The molecular formula of the lithium iron manganese phosphate particles is Li d Mn e Fe f M 1-e-f PO4, where 0.98 ≤ d ≤ 1.02, 0.4 ≤ e ≤ 0.8, 0.2 ≤ f ≤ 0.6, 0 ≤ 1 - e - f ≤ 0.035, and M is selected from at least one of Mg, Na, K, Co, Ni, Zn, and Ti.

4. The cathode material according to any one of claims 1 to 3, wherein, The polymer film is obtained by co-heat treating a conductive polymer, a binder, the substrate, and the lithium iron manganese phosphate particles; wherein, the median particle size of the substrate is greater than the median particle size of the lithium iron manganese phosphate particles.

5. The cathode material according to any one of claims 1-4, wherein, The ratio of the median particle size of the substrate to the median particle size of the lithium iron manganese phosphate particles is greater than or equal to 4; based on the total mass of the substrate and the lithium iron manganese phosphate particles, the content of the substrate is 40%-70%.

6. The cathode material according to any one of claims 1-5, wherein, The polymer film includes a conductive polymer and a binder.

7. The cathode material according to any one of claims 1-6, wherein, The surface of the lithium iron manganese phosphate particles includes carbon; the mass ratio of carbon to the mass of the lithium iron manganese phosphate particles is 0.5%-3.5%.

8. The positive electrode material according to claim 4, wherein, The conductive polymer includes at least one of 3-hexyl-substituted polythiophene, polyaniline, and polyethylenedioxythiophene; the binder includes at least one of polyethylene glycol, cellulose acetate, polystyrene, and polyvinylidene fluoride.

9. A preparation method for preparing the cathode material according to any one of claims 1-8, wherein, Comprising: in an inert atmosphere, heat treating a mixture containing a polymer, a substrate, and lithium iron manganese phosphate particles to obtain the positive electrode material; wherein, the temperature of the heat treatment is greater than or equal to the melting temperature of the polymer and less than the decomposition temperature of the polymer; the crystal phase structure of the substrate is a layered phase, and the median particle size of the substrate is greater than the median particle size of the lithium iron manganese phosphate particles.

10. The preparation method according to claim 9, wherein, The polymer includes a conductive polymer and a binder; wherein, the mass ratio between the conductive polymer and the binder is (0.3-0.5):(0.5-0.7); and / or, the treatment temperature of the heat treatment is 280-310°C, and the treatment time is 3-5 hours.

11. A lithium-ion battery, wherein, Comprising: the positive electrode material according to any one of claims 1-8.

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