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. The positive electrode material comprises: a core layer, the core layer comprising Li, Fe, Mn and PO4 - ions, and a doping element A; and a shell layer, wherein the outer surface of the core layer is coated with at least the surface of the shell layer, and the shell layer comprises first carbon particles and second carbon particles. The doping element A comprises at least one element of Al, Mg, Ni, Co, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn and Y; the distance between the highest point and the lowest point in a single surface of the positive electrode material does not exceed 1 nm, and the surface roughness of the positive electrode material is 0.8 μm to 1.6 μm. By means of the two-time carbon coating process, the present application greatly reduces manganese dissolution while ensuring high capacity and high compaction of the positive electrode material, thereby ensuring the cyclic discharging efficiency of the positive electrode material.

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 October 16, 2023, with application number 202311329054.8 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 lithium batteries, and in particular to a positive electrode material and a preparation method thereof, and a lithium-ion battery. Background Art

[0003] With the booming development of electronic products, the demand for portable and reusable secondary batteries is increasing day by day. Among the existing secondary battery types, olivine-type cathode materials have great potential for application in power lithium-ion batteries due to their many advantages.

[0004] However, inherent drawbacks of olivine-structured phosphate compounds, such as low electronic conductivity and slow one-dimensional lithium ion diffusion, severely impact the electrochemical performance of lithium manganese iron phosphate materials, hindering their further large-scale application. To improve power density, the manganese content is typically increased, but this increase in manganese content inevitably leads to manganese dissolution during the material's cycling. Currently, carbon coating is commonly used to mitigate this manganese dissolution during material cycling, improving both cycling performance and conductivity.

[0005] However, the existing solid-phase dry carbon coating process cannot uniformly coat the material surface with a layer of carbon material. The uneven and bumpy carbon coating not only fails to improve the manganese dissolution problem, but may even affect the conductivity of the material.

[0006] Application Contents

[0007] This application aims to solve at least one of the above technical problems.

[0008] To this end, the first object of the present application is to provide a positive electrode material.

[0009] The second object of this application is to provide a method for preparing a positive electrode material.

[0010] The third object of this application is to provide a lithium-ion battery.

[0011] In order to achieve the first purpose of the present application, the present application provides a positive electrode material, the positive electrode material includes: a core layer, the core layer includes Li, Fe, Mn, PO4 -ions, a doping element A; a shell layer, at least a surface portion of the shell layer is coated on the outer surface of the core layer, and the shell layer includes first carbon particles and second carbon particles; wherein the doping element A includes at least one element selected from the group consisting of Al, Mg, Ni, Co, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn and Y; the difference between the highest point and the lowest point on a single surface of the positive electrode material does not exceed 1 nm, and the surface roughness of the positive electrode material is 0.8 μm-1.6 μm.

[0012] Compared with the existing technology, the technical effect achieved by adopting this technical solution is: the core layer includes Li, Fe, Mn, PO4 - Ions and doping element A, by doping manganese in lithium iron phosphate to replace part of the Fe element, the preparation of lithium manganese iron phosphate material can increase the voltage and increase the energy density per unit mass. It also has good voltage compatibility with current lithium-ion batteries, reducing the difficulty of mutual replacement;

[0013] Furthermore, the doping element A includes at least one element selected from the group consisting of Al, Mg, Ni, Co, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y. By doping the lithium iron phosphate with the element A, not only the elements in the core layer are coordinated, but also the positive electrode material is modified to improve the performance of the positive electrode material. The doping element A can cause Li or M site defects in the manganese iron phosphate lattice, generate vacancies in the material lattice, or change the interatomic bond length, which is beneficial to the movement of Li ions in the lattice and can improve the electrochemical performance.

[0014] Shell layer, at least the surface portion of the shell layer is coated on the outer surface of the core layer, and the shell layer is composed of C elements; as the proportion of manganese increases, manganese dissolution is inevitable during the circulation process of the material, coating the surface with carbon particles can reduce manganese dissolution; however, it is currently not possible to uniformly coat the surface of the material with a layer of carbon material. The uneven and bumpy carbon coating not only fails to improve the manganese dissolution problem, but may even affect the conductivity of the material. The difference in distance between the highest point and the lowest point on a single surface of the positive electrode material of the present application does not exceed 1 nm, and the surface roughness of the positive electrode material is 0.8 μm-1.6 μm. By adjusting the degree of surface roughness, the shell layer on the surface of the positive electrode material is made smooth, thereby improving the manganese dissolution problem and thereby improving the conductivity of the material.

[0015] In one technical solution of the present application, the average diameter of the first carbon particles is 1 μm-5 μm; and / or the average diameter of the second carbon particles is 0.5 μm-1 μm.

[0016] Compared with the existing technology, the technical effect achieved by adopting this technical solution is: the average diameter of the first carbon particles is 1μm-5μm, and the average diameter of the second carbon particles is 0.5μm-1μm; the inter-particle pores of carbon particles with different particle sizes are used to coat the positive electrode material twice, and the gap difference of the first carbon coating layer is utilized to introduce the second carbon particles to achieve uniform and flat coating, thereby obtaining a positive electrode material with high carbon coating rate and flat coating.

[0017] In one technical solution of the present application, the composition of the positive electrode material is as shown in formula (I): Li 1+a Fe 1-x-y Mn x A y (PO 4-b )·C b Formula (I); wherein the value ranges of a, x, y, and b are as follows: -0.1≤a≤0.4, 0.5≤x≤0.7, 0.005≤y≤0.05, 0<b≤0.3.

[0018] Compared with the existing technology, the technical effect achieved by adopting this technical solution is: - As the matrix of the core layer, the prepared positive electrode material has better conductivity, and its theoretical capacity and conductivity are high; the synthesis process is relatively simple, and large-scale production and research promotion can be achieved. By adjusting the ratio of different molecules, Li ions need to be deintercalated in the positive electrode material; manganese is added to replace part of the Fe element to prepare lithium iron manganese phosphate material, which can increase voltage, increase unit mass energy density, stabilize the material structure, prevent drastic changes in volume, and increase battery life. However, if only manganese is added, the crystal structure will still produce strain after multiple cycles, and the battery life is still not optimized; therefore, the crystal structure is further stabilized by doping element A; and carbon particles are coated on the outer surface of the core layer to prevent manganese dissolution due to increased manganese content, further improving the various performance of the positive electrode material; and by controlling the coating ratio of doped element A and element C, the coating effect and conductive performance are balanced, so that the prepared material is not only evenly coated but also has excellent conductivity.

[0019] In one technical solution of the present application, the specific surface area of ​​the positive electrode material is 10m 2 / g-25m 2 / g; and / or the diameter of the core layer is 200nm-400nm; and / or the thickness of the shell layer is 1nm-5nm.

[0020] Compared with the existing technology, the technical effect achieved by adopting this technical solution is: when the specific surface area of ​​the positive electrode material is 10m 2 / g-25m 2 / g, the cycle discharge efficiency of the positive electrode material is the highest;

[0021] The thickness of the shell is 1nm-5nm, and the diameter of the core is 200nm-400nm. By adjusting the thickness of the core shell, it is convenient for Li + During charging, the electrons are released from the positive electrode and embedded in the negative electrode through the electrolyte. At the same time, the compensation charge of the electrons is supplied to the negative electrode through the external circuit to ensure the balance of charge between the positive and negative electrodes. The opposite is true during discharge. + It is deintercalated from the negative electrode and embedded into the positive electrode through the electrolyte; at the same time, it is more conducive to the penetration of the doping element A.

[0022] In one technical solution of the present application, the brightness of the positive electrode material is 0-25; and / or the chroma of the positive electrode material is 0-3.6.

[0023] Compared with the existing technology, the technical effect achieved by adopting this technical solution is: the lightness of the positive electrode material is 0-25, and the chroma of the positive electrode material is 0-3.6. The analysis results can effectively judge the overall situation of the carbon coating of the prepared lithium manganese iron phosphate positive electrode material. The carbon layer with appropriate thickness and uniform surface can not only improve the conductivity of the material, but also prevent particle contact and inhibit particle growth, making it easy to obtain nano-scale positive electrode materials.

[0024] In one technical solution of the present application, the positive electrode material has a 0.1C first discharge capacity of not less than 160 mAh / g in the range of 2.0V-4.3V; and / or the volumetric energy density of the positive electrode material is not less than 80 mAh / cm 3 ; and / or the 200-cycle retention rate of the positive electrode material reaches 95.62%.

[0025] Compared with the existing technology, the technical effects achieved by adopting this technical solution are as follows: the first discharge gram capacity refers to the amount of electricity that the battery can release in the first charge and discharge cycle. The first discharge gram capacity is one of the important indicators of battery quality because it directly affects the service life and performance of the battery. The positive electrode material provided in this application has a 0.1C first discharge gram capacity of not less than 160mAh / g in the range of 2.0V-4.3V, and the 200-week cycle retention rate of the positive electrode material reaches 95.62%; the volume energy density refers to the ratio between the energy possessed by a certain energy source and its volume, which can reflect the volume consumed by the battery during use and is one of the important indicators of battery quality. The volume specific energy density of the positive electrode material provided in this application is not less than 80mAh / cm 3 , thus having a higher cycle capacity retention rate and a lower DC internal resistance growth rate, thereby proving that the positive electrode material of the present application has excellent performance, a longer service life, and is in line with economic benefits.

[0026] In one technical solution of the present application, the preparation method comprises the following steps:

[0027] S100, mixing a Li source, a Mn source, an Fe source, a P source, and a dopant containing a doping element A, sequentially performing a preheating treatment, a crushing treatment, and a drying treatment, and performing a sintering treatment under a reducing atmosphere to obtain a first positive electrode material;

[0028] S200: performing a secondary sintering treatment on the first cathode material and the first carbon particles under a reducing atmosphere to obtain a second cathode material;

[0029] S300: adding a second positive electrode material, second carbon particles, and a binder in sequence for premixing, and then performing three sintering processes under a reducing atmosphere to obtain the positive electrode material;

[0030] Wherein, the average diameter of the first carbon particles is 1 μm-5 μm;

[0031] The average diameter of the second carbon particles is 0.5 μm to 1 μm.

[0032] Compared with the existing technology, the technical effect achieved by adopting this technical solution is as follows: step S100 is sintered once to obtain the first positive electrode material, namely the core layer, and the inside of the core layer is adjusted synergistically by adding dopant A to improve the performance of the positive electrode material; step S200 is sintered twice, and element C is coated on the surface of the core layer by the second sintering. However, due to the uneven and bumpy surface of the first coating, not only the manganese dissolution problem is not improved, but the conductivity of the material is even affected; therefore, the average diameter of the first carbon particles is 1μm-5μm, and the average diameter of the second carbon particles is 0.5μm-1μm; the inter-particle pores of carbon particles with different particle sizes are used to coat the positive electrode material twice, and the gap difference of the first carbon coating layer is utilized to introduce the second carbon particles to achieve uniform and flat coating, thereby obtaining a positive electrode material with high carbon coating rate and flat coating; thus, after two carbon coating processes, while ensuring the high capacity and high compaction of the positive electrode material, manganese dissolution is greatly reduced, thereby ensuring the cyclic discharge efficiency of the positive electrode material.

[0033] In one technical solution of the present application, in S100, the molar ratio of the Li source, Mn source + Fe source, P source, and dopant is (1.01-1.04): (0.98-1): 1: (0.05-0.1); and / or in S200, the molar ratio of the first positive electrode material to the first carbon particles is 100: (0.5-1); and / or in S300, the molar ratio of the second positive electrode material to the second carbon particles and the binder is 100: (0.5-0.8): (0.2-0.5).

[0034] Compared with the existing technology, the technical effect achieved by adopting this technical solution is as follows: by adjusting the molar ratio of the Li source, Mn source, Fe source, P source and dopant, the performance of the positive electrode material is improved; the carbon content of the three-stage sintering is less than that of the two-stage sintering; since the surface of the positive electrode material has been completely coated by the first sintering, but the surface is uneven, the secondary coating is performed by the three-stage sintering, and the surface is more completely coated, and finally an olivine-shaped positive electrode material with uniform and flat carbon coating is prepared.

[0035] In one technical solution of the present application, the pulverization treatment is sequentially performed by ball milling and sand milling; wherein, when the median particle size of the mixture treated by ball milling does not exceed 1 μm, the treatment is switched to sand milling; the diameter of the medium treated by ball milling does not exceed 0.8 μm; and / or the diameter of the medium treated by sand milling does not exceed 0.3 μm.

[0036] Compared with the existing technology, the technical effect achieved by adopting this technical solution is as follows: crushing treatment is carried out after preheating treatment, ball milling treatment is first carried out, and the diameter of the medium for ball milling treatment does not exceed 0.8μm, so that the median particle size of the mixed material does not exceed 1μm, and then it is turned to sand milling treatment, and the diameter of the medium for sand milling treatment does not exceed 0.3μm. By first using ball milling to quickly crush the mixed material and then performing sand milling, it is suitable for industrial production and reduces costs.

[0037] In one technical solution of the present application, the following further comprises: in S100, the median particle size of the particles after the crushing treatment does not exceed 6 μm; and / or in S100, the median particle size of the particles after the drying treatment does not exceed 4 μm; and / or in S200, the average particle size of the first carbon particles is 1 μm-5 μm; and / or in S200, the powder compaction density of the first carbon particles at a pressure of 2T is 3.00 g / cm 3 -3.20g / cm 3 ; and / or in S300, the binder includes any one of polyvinylidene fluoride, polyamide, polyimide, polyacrylic acid, polyvinyl alcohol, and styrene-butadiene rubber, or a combination of two or more thereof.

[0038] Compared with the existing technology, the technical effects achieved by adopting this technical solution are as follows: by continuously adjusting the particle size, the median particle size of the particles after crushing treatment does not exceed 6μm; the median particle size of the particles after drying treatment does not exceed 4μm, and the particle size of the primary carbon coating is larger than the particle size of the secondary carbon coating, thereby controlling the thickness of the positive electrode material to remain in the optimal range; and since the powder compaction density of the first carbon particles under 2T pressure is 3.00g / cm 3 -3.20g / cm 3, so that the positive electrode material has a good normal distribution of particles, which can increase the discharge capacity of the battery, reduce internal resistance, reduce polarization loss, extend the cycle life of the battery, and improve the utilization rate of lithium-ion batteries; when the compaction density is too large or too small, it is not conducive to the embedding and extraction of lithium ions.

[0039] In one technical solution of the present application, in S100, the temperature of the preheating treatment is 50°C-100°C; and / or in S100, the time of the preheating treatment is 0.1h-1h; and / or in S100, the temperature of the primary sintering treatment is 500°C-800°C; and / or in S100, the time of the primary sintering treatment is 8h-14h; and / or in S200, the temperature of the secondary sintering treatment is 500°C-800°C; and / or in S200, the time of the secondary sintering treatment is 6h-12h; and / or in S300, the time of the premixing treatment is 1h-1.5h; and / or in S300, the temperature of the tertiary sintering treatment is 500°C-800°C; and / or in S300, the time of the tertiary sintering treatment is 6h-12h.

[0040] Compared with the existing technology, the technical effect achieved by adopting this technical solution is as follows: by controlling the reaction temperature, time and heating rate, the construction of the core layer and shell layer materials is realized; specifically, through the staged sintering process, combined with specific process parameters, the first positive electrode material is sintered, firstly, a preheating treatment is performed, in the preheating stage, the temperature is low, and the mixture of Li source, Mn source, Fe source, P source and dopant is thermally decomposed into a precursor, and the sintering time is controlled to be 0.1h-1h; then a first sintering is performed to fully decompose the raw materials and combine to form a nickel-cobalt-manganese-lithium compound, and the sintering temperature is increased to 500-800℃ at a rate of 1℃ / min-4℃ / min for 8h-14h; further, a second stage of temperature rise sintering is performed to coat the first positive electrode material with carbon elements, and the second sintering time is 6h-12h; finally, the second positive electrode material is sintered to modify the morphology, and the time and temperature are the same as the second sintering to make it uniformly coated.

[0041] The present application also provides a lithium-ion battery comprising the positive electrode material of any of the above technical solutions, thereby having all the beneficial effects of any of the above technical solutions, which will not be described in detail here.

[0042] By adopting the technical solution of this application, the following technical effects can be achieved:

[0043] (1) The present application coats the positive electrode material twice through the inter-particle pores of carbon particles of different particle sizes. By utilizing the gap difference of the first carbon coating layer, the second coating is introduced so that the carbon elements of the small particles can enter the surface gaps of the positive electrode material and fill the gaps, thereby achieving uniform and flat coating of the positive electrode material and obtaining a positive electrode material with a high carbon coating rate. This not only ensures the high capacity of the positive electrode material, but also greatly reduces the dissolution of manganese on the basis of high compaction, while ensuring the cycle discharge efficiency of the positive electrode material.

[0044] (2) The carbon-coated positive electrode material and the preparation method thereof of the present application have a simple process, and the prepared carbon-coated positive electrode material can maintain a high capacitance and has good discharge performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] To more clearly illustrate the technical solutions of the embodiments of the present application, the following briefly introduces the drawings to be used in describing the embodiments. Obviously, the drawings described below are only some embodiments of the present application, and those skilled in the art can derive other drawings based on these drawings without inventive effort.

[0046] FIG1 is a SEM image of a second cathode material after secondary sintering provided in an embodiment of the present application;

[0047] FIG2 is an SEM image of a cathode material after triple sintering provided in an embodiment of the present application;

[0048] FIG3 is a SEM image of a positive electrode material provided in a comparative example of the present application;

[0049] FIG4 is a schematic diagram of a triple sintering and coating process after a secondary sintering process provided in an embodiment of the present application. DETAILED DESCRIPTION

[0050] To make the above-mentioned purposes, features, and advantages of this application more obvious and easy to understand, the technical solutions in the embodiments of this application are clearly and completely described. Obviously, the described embodiments are only part of the embodiments of this application, not all of them. Based on the embodiments of this application, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of this application.

[0051] Olivine-type cathode materials, with their numerous advantages, hold great potential for use in power lithium-ion batteries. However, inherent drawbacks of olivine-structured phosphate compounds, such as low electronic conductivity and slow one-dimensional lithium ion diffusion, severely impact the electrochemical performance of lithium manganese iron phosphate materials, hindering their further large-scale application. To improve capacity density, the manganese content is typically increased, but this increase in manganese content inevitably leads to manganese dissolution during the material's cycling.

[0052] At present, carbon coating is usually used to alleviate the problem of manganese dissolution during the material recycling process, which can not only improve the recycling performance, but also improve the conductivity of the material. However, the existing solid-phase dry carbon coating process cannot uniformly coat a layer of carbon material on the surface of the material. The uneven and bumpy carbon coating not only fails to improve the manganese dissolution problem, but may even affect the conductivity of the material.

[0053] Therefore, the embodiment of the present application provides a method for preparing a positive electrode material, as well as the prepared positive electrode material and a battery prepared using the positive electrode material; through two carbon coating processes, while ensuring the high capacity and high compaction of the positive electrode material, manganese dissolution is greatly reduced, thereby ensuring the cyclic discharge efficiency of the positive electrode material.

[0054] Specifically, a Li source, a Mn source, an Fe source, a P source, and a dopant containing a doping element A are mixed, preheated, crushed, and dried in sequence, and sintered once in a reducing atmosphere to obtain a first positive electrode material;

[0055] Preferably, the Li source includes one or more of lithium dihydrogen phosphate, lithium carbonate, and lithium hydroxide; the Mn source includes one or more of manganese carbonate and manganese tetraoxide; the Fe source includes one or more of ferric hydroxide, ferric phosphate, and ferroferric oxide; the P source includes one or more of ferric phosphate and magnesium phosphate; the dopant includes one or more of magnesium oxide and magnesium carbonate; the material is mixed with deionized water, preheated, and thermally decomposed into a precursor;

[0056] Preferably, the molar ratio of Li source, Mn source + Fe source, P source, and dopant is (1.01-1.04): (0.98-1): 1: (0.05-0.1), and the preferred molar ratio of Mn source: Fe source is 1: (0.66-2). The preheating treatment can be carried out in a stirred tank with a speed of 500 rpm-1500 rpm, a temperature of 120° C.-180° C., and a time of 0.1 h-1 h; the reducing atmosphere is preferably nitrogen or argon;

[0057] Further, the precursor is pulverized, and the pulverization is followed by ball milling and sand milling. A high-energy ball mill can be used for the ball milling, and the diameter of the ball milling medium does not exceed 0.8 μm. The ball milling time in the high-energy ball mill is 1 h to 3 h, and the ball milling speed is 1000 rpm to 3000 rpm.

[0058] Preferably, when the median particle size of the mixed material after ball milling is no more than 1 μm, the mixed material is subjected to sand milling. A two-stage crushing process is adopted, which is suitable for industrial production and reduces costs. Preferably, a high-energy sand mill can be used for sand milling, and the diameter of the sand milling medium does not exceed 0.3 μm. The sand milling time in the high-energy sand mill is 2 hours to 5 hours, and the ball milling speed is 1000 rpm to 3000 rpm, so that the median particle size of the mixed particles does not exceed 6 μm, which is convenient for subsequent steps.

[0059] Furthermore, a centrifugal spray dryer can be used for drying, with an air inlet temperature of 200°C-500°C, an air outlet temperature of 100°C-300°C, an atomizing disk speed of 15000rpm-30000rmp, and a feed rate of 40L·h-1, so that the median particle size of the particles does not exceed 4μm; a primary sintering treatment is performed under a reducing atmosphere to obtain the first positive electrode material, so that the primary sintering is more uniform and the doping elements better penetrate the core layer; the primary sintering is heated to 500°C-650°C at a rate of 1°C / min-4°C / min for 8h-12h;

[0060] Specifically, the first cathode material and the first carbon particles are subjected to a secondary sintering treatment in a reducing atmosphere to obtain a second cathode material, and the element C is coated on the surface of the core layer by secondary sintering. The first carbon content is 0.9% of the cathode material. The secondary sintering is heated to 600°C-700°C at a rate of 1°C / min-4°C / min, and the secondary sintering time is 6h-10h;

[0061] Furthermore, since the surface of the primary coating is uneven and bumpy, it not only fails to improve the manganese dissolution problem, but also affects the conductivity of the material. Therefore, the second positive electrode material, the second carbon particles, and the binder are added in sequence and mixed for 1h-1.5h, so that the secondary small carbon particles fully enter the gap, and then the secondary coating, i.e., the third firing, is performed. The second carbon content is 0.6% of the positive electrode material. The third firing is heated to 650℃-750℃ at a rate of 1℃ / min-4℃ / min, and the second firing time is 6h-10h; the second positive electrode material is sintered to modify the morphology, and the time and temperature are the same as the second firing to ensure uniform coating;

[0062] The test method of lightness and chroma is as follows: L represents lightness, a and b represent chroma, and the color difference value △E=[(△L) 2 +△a) 2 +(△b) 2 ] 1 / 2 ; Use a spectrophotometer to measure the colorimetric value of the sample. Place the material close to the colorimeter probe, with the long axis of the lens perpendicular to the surface of the sample being measured. Flash the probe light source three times continuously and then release it, and record the value.

[0063] [First embodiment]

[0064] S100, mixing Li:Mn:Fe:P according to the molar ratio of 1.04:0.6:0.4:1, adding deionized water and mixing uniformly in a stirring tank; preheating at a heating temperature of 120°C, a rotation speed of 1000 rpm, and a time of 0.5 h, crushing, drying in a centrifugal spray dryer at an air inlet temperature of 220°C, an air outlet temperature of 100°C, an atomizing disk rotation speed of 30000 rpm, and a feed rate of 40 L / h, and performing a primary sintering treatment at a temperature of 650°C for 10 h in a nitrogen atmosphere to obtain a first positive electrode material;

[0065] Among them, the pulverization process is first carried out in a high-energy ball mill, the ball milling medium in the high-energy ball mill is zirconium beads, the diameter of the zirconium beads is 0.8 μm, the speed is 1500 rpm, and the time is 2 hours of ball milling; when the median particle size D50 of the particles reaches 1 μm, it is transferred to a high-energy sand mill, and the zirconium beads are 0.3 μm in diameter and the sand milling time is 3 hours;

[0066] S200: The first positive electrode material and the first carbon particles are mixed in a molar ratio of 100:0.9, and the mixture is heated at a temperature of

[0067] The second cathode material was obtained by secondary sintering at 680°C for 8 hours, as shown in FIG1 ;

[0068] The average diameter of the first carbon particles is in the range of 1 μm to 5 μm, and the powder compaction density of the first carbon particles is 3.00 g / cm 3 -3.20g / cm 3 ;

[0069] S300: adding the second cathode material, the second carbon particles, and PVDF in sequence, and mixing them at a molar ratio of 100:0.6:0.3 for 1 hour, and performing a sintering treatment at 700° C. for 8 hours under a nitrogen atmosphere to obtain the cathode material, as shown in FIG2 ;

[0070] wherein the average diameter of the second carbon particles is in the range of 0.5 μm-1 μm;

[0071] [Second embodiment]

[0072] This embodiment provides a lithium ion battery and a preparation method thereof. The preparation method is similar to that of Example 1, with the difference being the selection and molar ratio of the components, as shown in Table 1.

[0073] Table 1

[0074] [Third embodiment]

[0075] This embodiment provides a lithium-ion battery and a preparation method thereof. The preparation method is similar to that of Example 1, except that the doping elements and binders of the components are selected differently, as shown in Table 2.

[0076] Table 2

[0077] [Fourth embodiment]

[0078] This embodiment provides a lithium-ion battery and a preparation method thereof. The preparation method is similar to that of Example 1, except that the process parameters are different, as shown in Table 3.

[0079] Table 3

[0080]

First comparison

[0081] This embodiment provides a lithium ion battery and a preparation method thereof, wherein the preparation method is similar to that of Example 1, except that in S200, a first positive electrode material and first carbon particles are mixed in a molar ratio of 100:1.8; wherein the average diameter of the first carbon particles is in the range of 1.2 μm-5.2 μm, and the powder compaction density of the first carbon particles is 3.50 g / cm 3 -3.80g / cm 3 ;

[0082] [Second comparison]

[0083] This embodiment provides a lithium-ion battery and a preparation method thereof, wherein the preparation method is similar to that of Example 1, except that in S200 the first cathode material is mixed with other metal sources in a molar ratio of 100:0.2, as shown in FIG3 .

[0084]

Third comparison

[0085] This embodiment provides a lithium ion battery and a preparation method thereof, wherein the preparation method is similar to that of Example 1, except that in S200 the average diameter of the second carbon particles is in the range of 1.1 μm to 1.5 μm.

[0086]

Fourth comparison

[0087] This embodiment provides a lithium-ion battery and a method for preparing the same. The preparation method is similar to that in Example 1, except that no binder is added in S300.

[0088] For the above Examples 1-4 and Comparative Examples 1-4, a person skilled in the art measured the positive electrode materials by the following method, and the specific data are shown in Table 4;

[0089] In the range of 2.0V-4.3V, its 0.1C first discharge capacity: the assembled button battery is tested in a blue light device at a temperature of 25±1℃, a test voltage of 2.0V-4.3V, and a charge and discharge rate of 0.1C / 0.1C. The charge cut-off current is 0.05C.

[0090] 200-cycle retention rate: The full battery adopts Xinwei CT3008-5V3A-A1, cycles at 0℃, cycle voltage 2V-4V, constant voltage cut-off current 20mA, and cycles 200 times;

[0091] Lightness and chroma: L represents lightness, a and b represent chroma, and the color difference value △E=[(△L) 2 +△a) 2 +(△b) 2 ] 1 / 2 ; Use a spectrophotometer to measure the colorimetric value of the sample. Place the material close to the colorimeter probe, with the long axis of the lens perpendicular to the surface of the sample being measured. Flash the probe light source three times continuously and then release it, and record the value.

[0092] The difference in distance between the highest and lowest points on a single surface: displayed on a TEM image.

[0093] Surface roughness: The instrument is a surface roughness tester. The stylus of the tester is guided by a constant speed driver so that it contacts the workpiece surface to be tested vertically and moves horizontally along the surface of the workpiece to be tested. The movement of the stylus can effectively reflect the surface contour. The subtle changes in the stylus movement are converted into electrical signals through the sensor, and the surface roughness value is displayed on the screen after calculation and processing.

[0094] Table 4

[0095] When the positive electrode materials of Examples 1-4 and Comparative Examples 1-4 were made into batteries, the 0.1C first discharge capacity was 154 mAh / g when measured in the range of 2.0V-4.3V. After 200 cycles, the cycle retention rate reached 95.62%.

[0096] It can be seen from Example 1 and Comparative Example 2 that the first positive electrode material is coated with a layer of porous carbon, which can effectively inhibit the growth of the first positive electrode material particles. By adding a new carbon layer, the second layer of carbon can fill the gaps in the first layer. The double carbon layer formed by the inner carbon layer and the outer carbon layer can improve the electrical conductivity of the material, which is beneficial to improving the electrochemical properties of the material.

[0097] This proves that by limiting the particle size of carbon twice and compacting it, it can be evenly coated on the surface of the positive electrode material. As shown in Figure 4, since the first carbon coating will inevitably not be uniform, the second coating has small carbon particles so that they can enter the gaps on the surface of the positive electrode material and fill the gaps, thereby obtaining a positive electrode material with a high carbon coverage rate. Finally, high-speed mixing and polishing are used to make the carbon coating on the surface of the positive electrode material smooth, and finally an olivine-shaped positive electrode material with uniform and flat carbon coating is prepared. After two carbon coating processes, while ensuring the high capacity and high compaction of the positive electrode material, the manganese dissolution is greatly reduced, thereby ensuring the cycle discharge efficiency of the positive electrode material.

[0098] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A positive electrode material, characterized in that: The positive electrode material comprises: The core layer includes Li, Fe, Mn, PO4 - ions, doping element A; a shell layer, wherein at least a surface portion of the shell layer covers the outer surface of the core layer, and the shell layer comprises first carbon particles and second carbon particles; Wherein, the doping element A includes at least one element selected from the group consisting of Al, Mg, Ni, Co, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn and Y; The difference in distance between the highest point and the lowest point on a single surface of the positive electrode material does not exceed 1 nm, and the surface roughness of the positive electrode material is 0.8 μm-1.6 μm.

2. The positive electrode material according to claim 1, wherein The average diameter of the first carbon particles is 1 μm-5 μm.

3. The positive electrode material according to claim 1 or 2, characterized in that The average diameter of the second carbon particles is 0.5 μm-1 μm.

4. The positive electrode material according to claim 1, wherein The composition of the positive electrode material is shown in formula (I): Li 1+a Fe 1-x-y Mn x A y (PO 4-b )·C b Formula (I); The value ranges of a, x, y, and b are as follows: -0.1≤a≤0.4, 0.5≤x≤0.7, 0.005≤y≤0.05, 0<b≤0.

3.

5. The positive electrode material according to claim 1, wherein The specific surface area of ​​the positive electrode material is 10 m 2 / g-25m 2 / g.

6. The positive electrode material according to claim 1 or 5, wherein The diameter of the core layer is 200nm-400nm.

7. The positive electrode material according to any one of claims 1, 5 or 6, wherein The thickness of the shell layer is 1nm-5nm.

8. The positive electrode material according to claim 1, wherein The brightness of the positive electrode material is 0-25.

9. The positive electrode material according to claim 1 or 8, wherein The chroma of the positive electrode material is 0-3.

6.

10. The positive electrode material according to claim 1, wherein The positive electrode material has a 0.1C first discharge gram capacity of not less than 160 mAh / g within the range of 2.0V-4.3V.

11. The positive electrode material according to claim 1 or 10, wherein The volume specific energy density of the positive electrode material is not less than 80 mAh / cm 3 .

12. The positive electrode material according to any one of claims 1, 10 or 11, wherein The 200-cycle cycle retention rate of the positive electrode material reaches 95.62%.

13. A method for preparing a positive electrode material, wherein: The preparation method comprises the following steps: S100, mixing a Li source, a Mn source, a Fe source, a P source, and a dopant containing a doping element A, sequentially performing a preheating treatment, a pulverizing treatment, and a drying treatment, and performing a sintering treatment once in a reducing atmosphere to obtain a first positive electrode material; S200: performing a secondary sintering treatment on the first positive electrode material and the first carbon particles under a reducing atmosphere to obtain a second positive electrode material; S300: sequentially adding the second positive electrode material, the second carbon particles and the binder for premixing, and then sintering three times in a reducing atmosphere to obtain the positive electrode material; Wherein, the average diameter of the first carbon particles is 1 μm-5 μm; The average diameter of the second carbon particles is 0.5 μm-1 μm.

14. The preparation method according to claim 13, wherein: In S100, the molar ratio of the Li source, the Mn source+Fe source, the P source, and the dopant is (1.01-1.04): (0.98-1): 1: (0.05-0.1).

15. The preparation method according to claim 13 or 14, wherein: In S200 , the molar ratio of the first positive electrode material to the first carbon particles is 100:(0.5-1).

16. The preparation method according to any one of claims 13 to 15, wherein: In S300 , the molar ratio of the second positive electrode material to the second carbon particles and the binder is 100:(0.5-0.8):(0.2-0.5).

17. The preparation method according to claim 13, wherein: In S100: The pulverizing treatment is performed by ball milling and sand milling in sequence; When the median particle size of the mixed material after ball milling does not exceed 1 μm, it is turned to sand milling; The diameter of the ball milling medium does not exceed 0.8 μm.

18. The preparation method according to claim 13 or 17, wherein: In S100: The diameter of the sand-milled media does not exceed 0.3 μm.

19. The preparation method according to claim 13, wherein: In S100, the median particle size of the particles after the crushing process does not exceed 6 μm.

20. The preparation method according to claim 13 or 19, wherein: In S100, the median particle size of the particles after the drying process does not exceed 4 μm.

21. The preparation method according to any one of claims 13, 19 or 20, wherein: In S200 , the average particle size of the first carbon particles is 1 μm-5 μm.

22. The preparation method according to any one of claims 13, 19-21, wherein: In S200, the powder compaction density of the first carbon particles under a pressure of 2T is 3.00 g / cm 3 -3.20g / cm 3 .

23. The preparation method according to any one of claims 13, 19-22, wherein: In S300, the binder includes any one of polyvinylidene fluoride, polyamide, polyimide, polyacrylic acid, polyvinyl alcohol, and styrene-butadiene rubber, or a combination of two or more thereof.

24. The preparation method according to claim 13, characterized in that: In S100, the temperature of the preheating treatment is 120°C-180°C.

25. The preparation method according to claim 13 or 24, wherein: In S100, the preheating treatment time is 0.1h-1h.

26. The preparation method according to any one of claims 13, 24 or 25, wherein: In S100, the temperature of the primary sintering process is 500°C-650°C.

27. The preparation method according to any one of claims 13, 24-26, wherein: In S100, the time of the primary sintering process is 8h-12h.

28. The preparation method according to any one of claims 13, 24-27, wherein: In S200, the temperature of the secondary sintering process is 600°C-700°C.

29. The preparation method according to any one of claims 13, 24-28, wherein: In S200, the secondary sintering process is performed for 6 hours to 10 hours.

30. The preparation method according to any one of claims 13, 24-29, characterized in that: In S300, the premixing treatment time is 1h-1.

5.

31. The preparation method according to any one of claims 13, 24-30, wherein: In S300, the temperature of the three sintering processes is 650°C-750°C.

32. The preparation method according to any one of claims 13, 24-31, wherein: In S300, the three sintering processes are performed for 6 hours to 10 hours.

33. A lithium ion battery, characterized in that: Contains the positive electrode material according to any one of claims 1 to 12.

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