Positive electrode material and preparation method therefor, secondary battery, and electric device

Abstract

The present application provides a positive electrode material and a preparation method therefor, a secondary battery, and an electric device. By optimizing the particle size of the positive electrode material to make the particle size and morphology distribution thereof meet the following condition: z=2.35+0.258*D150-0.121*D250-0.639*D3mo+0.3125*S+0.0142*n, where 2.3≤z≤2.79, the grading of large and small particles is facilitated, pores between the particles are filled, and the compaction density can be significantly improved, thereby solving the problems of relatively low energy density and relatively poor rate capability of batteries due to relatively low compaction density of electrode sheets caused by a relatively large number of pores between particles in existing lithium-iron-phosphate positive electrode materials.

Description

A positive electrode material, a preparation method thereof, a secondary battery, and an electric device

[0001] The present application claims priority to the Chinese patent application No. 202411003026.1, filed on July 25, 2024, and entitled "A positive electrode material, a preparation method thereof, a secondary battery, and an electric device", the content of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0002] The present application relates to the technical field of battery manufacturing, in particular to a positive electrode material, a preparation method thereof, a secondary battery, and an electric device. BACKGROUND

[0003] Currently, due to the advantages of high cycle stability, good safety performance, and high charge-discharge efficiency when used as a positive electrode material of a lithium ion battery, lithium iron phosphate has been widely applied in the fields of power batteries and energy storage.

[0004] However, when the existing lithium iron phosphate is used as a positive electrode material of a lithium ion battery, the compaction density of the pole piece is small, not only more electrolyte needs to be filled into the pores, resulting in a decrease in energy density, but also the conductive network of the pole piece is not tightly constructed, and the pole piece film resistance is high, resulting in poor rate performance of the battery. SUMMARY

[0005] The present application aims to provide a positive electrode material, a secondary battery, and an electric device to improve the problem of too many pores between existing lithium iron phosphate positive electrode material particles, resulting in a small compaction density of the pole piece, a small energy density of the battery, and poor rate performance.

[0006] To solve the above problems, the present application is realized through the following technical solutions:

[0007] The present application provides a positive electrode material, wherein,

[0008] The positive electrode material comprises a first lithium iron phosphate, a second lithium iron phosphate, and a third lithium iron phosphate.

[0009] The particle size and morphology distribution of the positive electrode material satisfy: z = 2.35 + 0.258 * D 1 50 - 0.121 * D 2 50 - 0.639 * D 3 mo + 0.3125 * S + 0.0142 * n, 2.3 ≤ z ≤ 2.79.

[0010] Wherein, D 1 50 refers to the D50 of the first lithium iron phosphate, D 2 50 refers to the D50 of the second lithium iron phosphate, and D 3mo refers to the particle size corresponding to the maximum volume fraction of the third lithium iron phosphate, S refers to the sphericity of the positive electrode material, and n refers to the mass ratio of the second lithium iron phosphate to the third lithium iron phosphate; 2 μm≤D 1 50≤4 μm, and 0.8 μm≤D 2 50<2 μm, and 0.35 μm≤D 3 mo≤0.6 μm, 0.6≤S≤1.0, and 1≤n≤4.

[0011] Further, in the positive electrode material, 2.6≤z≤2.79.

[0012] Further, in the positive electrode material, the sphericity of the first lithium iron phosphate is S1, the sphericity of the second lithium iron phosphate is S2, and the sphericity of the third lithium iron phosphate is S3, and the S1, S2, and S3 satisfy: 0.6≤S1≤1.0, and / or, 0.6≤S2≤1.0, and / or, 0.6≤S3≤1.0.

[0013] Further, in the positive electrode material, the mass ratio of the first lithium iron phosphate, the second lithium iron phosphate, and the third lithium iron phosphate is (4-5):4:(1-2).

[0014] Further, in the positive electrode material, at least one of the first lithium iron phosphate, the second lithium iron phosphate, and the third lithium iron phosphate has a carbon coating layer.

[0015] Further, in the positive electrode material, the thickness of the carbon coating layer is 2 nm-5 nm.

[0016] The application also provides a preparation method of the positive electrode material, comprising:

[0017] obtaining lithium iron phosphate, wherein the sphericity of the lithium iron phosphate is 0.6-1.0;

[0018] grading the lithium iron phosphate to obtain first lithium iron phosphate, second lithium iron phosphate, and third lithium iron phosphate, wherein the D 1 50, the D 2 50, the D 3 mo satisfies: 2 μm≤D 1 50≤4 μm, and 0.8 μm≤D 2 50<2 μm, and 0.35 μm≤D 3 mo≤0.6 μm;

[0019] The first lithium iron phosphate, the second lithium iron phosphate, and the third lithium iron phosphate are mixed to obtain the positive electrode material, a mass ratio n of the second lithium iron phosphate to the third lithium iron phosphate satisfies: 1≤n≤4, and a sphericity S of the positive electrode material satisfies: 0.6≤S≤1.0; the positive electrode material satisfies:

[0020] z = 2.35 + 0.258 * D 1 50 - 0.121 * D 2 50 - 0.639 * D 3 mo + 0.3125 * S + 0.0142 * n, 2.3≤z≤2.79.

[0021] Optionally, in the preparation method, the obtaining of the lithium iron phosphate comprises:

[0022] The iron phosphate and the lithium source are mixed to obtain a lithium iron phosphate precursor, and the lithium iron phosphate precursor is ground, and a particle size D50 of the ground lithium iron phosphate precursor satisfies: 400nm≤D50≤800nm.

[0023] The ground lithium iron phosphate precursor is dried, sintered, and crushed to obtain the lithium iron phosphate, and a sphericity of the lithium iron phosphate is 0.6-1.0.

[0024] Optionally, in the preparation method, the lithium source comprises one or more of Li2CO3 and LiOH.

[0025] Optionally, in the preparation method, the mixing of the iron phosphate and the lithium source to obtain the lithium iron phosphate precursor further comprises: adding a carbon source for mixing, and the carbon source comprises one or more of glucose, starch, and phenolic resin.

[0026] Optionally, in the preparation method, a molar ratio of the iron phosphate, the carbon source, and the lithium source is 1:(0.15-0.2):(1-1.04).

[0027] The application further provides a secondary battery comprising a positive electrode sheet, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer arranged on the positive electrode current collector, and the positive electrode active material layer comprises the positive electrode material or the positive electrode material prepared by the preparation method.

[0028] The application further provides an electric device comprising the secondary battery, and the secondary battery serves as a power supply of the electric device.

[0029] Compared with the prior art, the application has the following advantages:

[0030] In the embodiments of the present application, the positive electrode material comprises a first lithium iron phosphate, a second lithium iron phosphate, and a third lithium iron phosphate, and satisfies: z = 2.35 + 0.258 * D 1 50 - 0.121 * D 2 50 - 0.639 * D 3 mo + 0.3125 * S + 0.0142 * n, 2.3 ≤ z ≤ 2.79; wherein, D 1 50 refers to D50 of the first lithium iron phosphate, D 2 50 refers to D50 of the second lithium iron phosphate, D 3 mo refers to the particle size corresponding to the maximum volume fraction of the third lithium iron phosphate, S refers to the sphericity of the positive electrode material, and n refers to the mass ratio of the second lithium iron phosphate to the third lithium iron phosphate; 2 μm ≤ D 1 50 ≤ 4 μm, 0.8 μm ≤ D 2 50 < 2 μm, 0.35 μm ≤ D 3 mo ≤ 0.6 μm, 0.6 ≤ S ≤ 1.0, and 1 ≤ n ≤ 4. By optimizing the particle size, the particle size distribution of the positive electrode material is limited to satisfy: z = 2.35 + 0.258 * D 1 50 - 0.121 * D 2 50 - 0.639 * D 3 mo + 0.3125 * S + 0.0142 * n, 2.3 ≤ z ≤ 2.79, which is beneficial to the size grading of particles, filling the pores between particles, and significantly improving the compaction density, thereby solving the problem that the existing lithium iron phosphate positive electrode material has many pores between particles, resulting in a small compaction density of the electrode sheet, a small battery energy density, and poor rate performance.

[0031] It should be understood that the foregoing general description and the following detailed description are only exemplary and explanatory, and cannot limit the present application. BRIEF DESCRIPTION OF DRAWINGS

[0032] FIG. 1 is a structural schematic diagram of a positive electrode material according to an embodiment of the present application;

[0033] FIG. 2 is a structural schematic diagram of a two-stage turbine type classifier according to an embodiment of the present application;

[0034] FIG. 3 is a FIB-SEM test result schematic diagram of a positive electrode material in Embodiment 1 of the present application;

[0035] FIG. 4 is a FIB-SEM test result schematic diagram of a positive electrode material in Embodiment 2 of the present application. DETAILED DESCRIPTION

[0036] In order to make the above objectives, characteristics and advantages of the present application more apparent, further specific embodiments will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0037] The applicant of the present application finds that, when existing lithium iron phosphate is used as a positive electrode material of a lithium ion battery, the compaction density of the electrode sheet is small, more electrolyte needs to be filled into the pores, resulting in a reduction in energy density, and the conductive network of the electrode sheet is not tightly constructed, the sheet resistance of the electrode sheet is high, and the rate performance of the battery is poor.

[0038] In order to solve the above problems, the present application provides a positive electrode material, wherein, as shown in Figure 1, the positive electrode material comprises first lithium iron phosphate 11, second lithium iron phosphate 12 and third lithium iron phosphate 13, and the particle size and morphology distribution of the above positive electrode material satisfies: z = 2.35 + 0.258 * D 1 50-0.121*D 2 50-0.639*D 3 mo+0.3125*S+0.0142*n, and 2.3≤z≤2.79;

[0039] Wherein, D 1 50 refers to the D50 of the first lithium iron phosphate, D 2 50 refers to the D50 of the second lithium iron phosphate, D 3 mo refers to the particle size corresponding to the maximum volume fraction of the third lithium iron phosphate, S refers to the sphericity of the positive electrode material, and n refers to the mass ratio of the second lithium iron phosphate to the third lithium iron phosphate; 2μm≤D 1 50≤4μm, 0.8μm≤D 2 50<2μm, 0.35μm≤D 3 mo≤0.6μm, 0.6≤S≤1.0, 1≤n≤4.

[0040] Wherein, D50 is the particle size corresponding to the cumulative volume percentage of 50% of the lithium iron phosphate material in the Malvern particle size tester; and Dmo is the particle size corresponding to the maximum volume fraction of the lithium iron phosphate material in the Malvern particle size tester.

[0041] In the embodiment of the present application, the sphericity S of the positive electrode material is the average sphericity of the positive electrode material particles.

[0042] In the embodiment of the present application, by optimizing the particle size and morphology, the particle size distribution of the lithium iron phosphate in the positive electrode material satisfies: z = 2.35 + 0.258 * D 1 50-0.121*D 2 50-0.639*D 3mo+0.3125*S+0.0142*n, 2.3≤z≤2.79, which is conducive to the size particle grading, filling the pores between the particles, can significantly improve the compaction density, and make the conductive network of the pole piece compact, which can effectively improve the energy density and rate performance of the battery, thereby solving the problem that the existing lithium iron phosphate positive electrode material has more pores between the particles, the compaction density of the pole piece is small, and the energy density and rate performance of the battery are poor.

[0043] Optionally, in some embodiments, 2.6≤z≤2.79 is met, which can make the size particle grading of the lithium iron phosphate in the positive electrode material more reasonable, can tightly fill the pores between the particles, can further improve the compaction density, and can make the conductive network of the pole piece more compact, thereby more effectively improving the energy density and rate performance of the battery.

[0044] Optionally, in an embodiment, the shape of each of the lithium iron phosphates with the above particle size is spherical, which not only effectively ensures the size particle grading, but also makes the positive electrode material have more stable results and more suitable specific surface area, and further improves the play of the electrochemical performance of the battery.

[0045] Optionally, in a specific embodiment, the sphericity of the first lithium iron phosphate is S1, the sphericity of the second lithium iron phosphate is S2, and the sphericity of the third lithium iron phosphate is S3, S1, S2, and S3 satisfy: 0.6≤S1≤1.0, 0.6≤S2≤1.0, and 0.6≤S3≤1.0; the sphericity S1, S2, and S3 of each lithium iron phosphate satisfy 0.6≤S1≤1.0, and / or 0.6≤S2≤1.0, and / or 0.6≤S3≤1.0, for example, one of 0.6, 0.7, 0.8, 0.9, and 1.0 or a range value of any two thereof. The sphericity of the lithium iron phosphate in the range can better meet the accumulation of the mixed positive electrode material, and can further improve the structural stability of the positive electrode material.

[0046] Optionally, in an embodiment, the mass ratio of the first lithium iron phosphate, the second lithium iron phosphate, and the third lithium iron phosphate in the above positive electrode material is 4-5:4:1-2, which can be but is not limited to 4:4:1, 4:4:2, 4.5:4:1, 4.5:4:1.5, 4.5:4:2, 5:4:1, and 5:4:2, which can effectively balance the capacity density and structural stability of the material.

[0047] In the above positive electrode material, at least one of the first lithium iron phosphate, the second lithium iron phosphate, and the third lithium iron phosphate has a carbon coating layer, the carbon coating is on the surface of the lithium iron phosphate, which can promote the transmission of electric charge on the surface of the particles, improve the electrical conductivity, and also can prevent Fe 2+ from being oxidized to Fe 3+ as a reducing agent, thereby ensuring the purity of the finished material.

[0048] Optionally, in one specific embodiment, the thickness of the carbon coating is 2nm-5nm, for example, one of 2nm, 3nm, 4nm, 5nm or a range value of any two of them, which is moderate and can effectively balance the conductivity and energy density of the material.

[0049] The application also provides a preparation method of the above positive electrode material, comprising steps 201-203:

[0050] Step 201, obtaining lithium iron phosphate, the sphericity of which is 0.6-1.0.

[0051] In one embodiment of the application, the lithium iron phosphate with the sphericity S satisfying 0.6≤S≤1.0 can be obtained by mixing and grinding iron phosphate and lithium source, granulating, sintering and airflow crushing. The lithium iron phosphate material with the sphericity satisfying the condition can also be prepared by other conventional methods for obtaining lithium iron phosphate.

[0052] Step 202, grading the lithium iron phosphate to obtain first lithium iron phosphate, second lithium iron phosphate and third lithium iron phosphate, wherein the D 1 50, 2 50, 3 mo satisfies: 2μm≤D 1 50≤4μm, 0.8μm≤D 2 50<2μm, 0.35μm≤D 3 mo≤0.6μm.

[0053] In this step, the first lithium iron phosphate with D 1 50 at 2μm-4μm, the second lithium iron phosphate with D 2 50 at 0.8μm-2μm and the third lithium iron phosphate with D 3 mo at 0.35μm-0.6μm are obtained by adjusting the grading rotor speed of the turbine airflow classifier.

[0054] In actual application, the first lithium iron phosphate, the second lithium iron phosphate and the third lithium iron phosphate are obtained by using two-stage turbine classifiers in series, and the lithium iron phosphate samples with different particle size distributions and narrow distribution width can be obtained by grading through the two-stage turbine classifiers in series.

[0055] The two-stage turbine classifier structure is shown in FIG. 2, which includes a first turbine classifier 21 and a second turbine classifier 22 connected in series. The first-stage coarse powder classified by the first turbine classifier is collected as the first lithium iron phosphate, the second-stage fine powder classified by the second turbine classifier is collected as the second lithium iron phosphate, and the fine powder in the bag collector at the end of the second turbine classifier is collected as the third lithium iron phosphate.

[0056] In step 203, the first lithium iron phosphate, the second lithium iron phosphate, and the third lithium iron phosphate are mixed to obtain the positive electrode material. The mass ratio n of the second lithium iron phosphate to the third lithium iron phosphate satisfies 1≤n≤4, and the sphericity S of the positive electrode material satisfies 0.6≤S≤1.0. The positive electrode material satisfies z=2.35+0.258*D. 1 50-0.121*D 2 50-0.639*D 3 mo+0.3125*S+0.0142*n, and 2.3≤z≤2.79, to obtain the positive electrode material.

[0057] In step 203, the first lithium iron phosphate, the second lithium iron phosphate, and the third lithium iron phosphate are mixed to obtain the positive electrode material. The mass ratio n of the second lithium iron phosphate to the third lithium iron phosphate satisfies 1≤n≤4, and the sphericity S of the positive electrode material satisfies 0.6≤S≤1.0. The positive electrode material satisfies z=2.35+0.258*D. 1 50-0.121*D 2 50-0.639*D 3 mo+0.3125*S+0.0142*n, and 2.3≤z≤2.79, to obtain the positive electrode material.

[0058] In the embodiment, the product lithium iron phosphate is classified to obtain samples with different sizes, and the samples with different sizes are matched according to the formula z to obtain higher electrode sheet compaction.

[0059] Optionally, in an embodiment, the step 201 includes steps 211-212.

[0060] In step 211, the iron phosphate and the lithium source are mixed to obtain the lithium iron phosphate precursor, and the particle size D50 of the lithium iron phosphate precursor after grinding satisfies 400nm≤D50≤800nm.

[0061] Optionally, in a specific embodiment, mixing the iron phosphate and the lithium source to obtain the lithium iron phosphate precursor further includes adding a carbon source, and the carbon source includes one or more of glucose, starch, and phenolic resin.

[0062] In the embodiment, the carbon source can coat carbon on the lithium iron phosphate sample in the high-temperature synthesis process, promote the transmission of electric charge on the surface of the particles, improve the conductivity, and also hinder the further growth of lithium iron phosphate grains, so that lithium iron phosphate crystals with moderate particle size are synthesized. Meanwhile, the carbon can also prevent Fe 2+ from being oxidized to Fe 3+ , so as to ensure the purity of the finished material. The carbon source can be glucose, starch, phenolic resin, etc., and the lithium source can be Li2CO3, LiOH, etc. The carbon source can form a carbon coating layer with a thickness of 2-5 nm on the surface of lithium iron phosphate.

[0063] In the embodiment, the iron phosphate, the carbon source, and the lithium source are mixed and ground by a sand mill, so that the lithium iron phosphate precursor with a particle size D50 of 400-800 nm is obtained.

[0064] The iron phosphate, the carbon source, and the lithium source are mixed at a mass ratio of 1:0.15-0.2:1-1.04, so that the reaction can be ensured to be sufficient and the lithium can be prevented from being wasted. Meanwhile, the carbon can form a coating layer with a thickness of 2-5 nm on the surface of lithium iron phosphate, and the thickness is moderate, so that the conductivity and the energy density of the material can be effectively balanced.

[0065] Optionally, the grinding can be performed by the sand mill at a rotation speed of 470-490 r / min, so that the particle size D50 of the ground particles is 400-800 nm.

[0066] In step 212, the ground lithium iron phosphate precursor is dried, sintered, and crushed to obtain the lithium iron phosphate, and the sphericity of the lithium iron phosphate is 0.6-1.0.

[0067] In the above step 212, the lithium iron phosphate precursor is dried and sintered, so that ferrous phosphate and lithium are combined to form lithium iron phosphate, and then the lithium iron phosphate is airflow crushed, so that the sphericity S of the lithium iron phosphate satisfies 0.6≤S≤1.0.

[0068] Optionally, the drying process can be performed by spray drying and granulation, the spray moisture is controlled to be less than or equal to 2%, the spray inlet temperature is 230-250°C, and the spray outlet temperature is 85-95°C, so that the particles with different particle sizes are uniformly distributed, the subsequent sintering reaction is facilitated, and the comprehensive performance of the material is improved.

[0069] Optionally, the dried powder obtained by spraying is placed in a sintering furnace with a protective gas atmosphere such as nitrogen, and sintered at a sintering temperature of 765-775°C and an oxygen content in the sintering furnace less than or equal to 10 ppm, so that the iron phosphate and the lithium source are reacted to form lithium iron phosphate.

[0070] Optionally, the sintered block material is subjected to airflow crushing, and the airflow pressure is 0.15-0.25 MPa, so that the sphericity of the lithium iron phosphate is 0.6-1.0.

[0071] In the embodiments of the present application, the particle size distribution of the lithium iron phosphate positive electrode material is limited to satisfy z=2.35+0.258*D 1 50-0.121*D 2 50-0.639*D 3 mo+0.3125*S+0.0142*n, and 2.3≤z≤2.79, so that the size particle grading and the pores between the particles are filled, and the compaction density of the material is improved, so that the lithium iron phosphate positive electrode material can fully play the characteristics of high cycle stability, good safety performance and high charge-discharge efficiency.

[0072] In the actual preparation process, the particle size distribution of the positive electrode material can be adjusted by changing the ball milling parameters to satisfy the above relationship. Specifically, the particle size of the positive electrode material can be adjusted by adjusting the ball milling time and the rotation speed, thereby affecting the z value.

[0073] The embodiments of the present application also provide a secondary battery including a positive electrode sheet, wherein the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector, and the positive electrode active material layer includes the positive electrode material as described above.

[0074] In some embodiments, the areal density of the positive electrode sheet is 360-440 mg / cm 2 . 2 The areal density of the positive electrode sheet is within the above range, which can effectively balance the rate performance and energy density of the battery and slow down the polarization.

[0075] Optionally, in an embodiment, the positive electrode sheet further includes a binder and a conductive agent, and the binder can include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene-fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer and fluorine-containing acrylic resin.

[0076] Optionally, in an embodiment, the conductive agent can be acetylene black, carbon fiber, carbon nanotube, ketjen black, artificial graphite, natural graphite, hard carbon, soft carbon, carbon black, graphene, etc.

[0077] Optionally, in an embodiment, during the homogenization of the cathode material, the cathode material accounts for 95wt%-98wt%, the binder accounts for 0.5wt%-3wt%, and the conductive agent accounts for 0.5wt%-3wt%, the proportion of each component is within the above range, which can effectively balance the rate performance and energy density of the battery. For example, the cathode material accounts for 96.8wt%, the binder accounts for 2.42wt%, and the conductive agent accounts for 0.78wt%.

[0078] In some embodiments, the cathode sheet is prepared as follows: the components for preparing the cathode sheet, such as the cathode active material including the above-mentioned cathode material, the binder, and any other components, are dispersed in a solvent such as N-methyl pyrrolidone to form a cathode slurry; the cathode slurry is coated on the cathode current collector; after drying, cold pressing, and other processes, the cathode sheet is obtained.

[0079] The cathode sheet provided in the embodiments of the present application has a compaction density of 2.3g / cm 3 ~2.79g / cm 3 , not only can balance the rate performance and energy density, but also can significantly reduce the strip breaking situation.

[0080] The secondary battery provided in the embodiments of the present application further includes a negative electrode sheet, a separator, and an electrolyte.

[0081] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, and the negative electrode active material layer can include at least one of artificial graphite, natural graphite, hard carbon, soft carbon, and carbon black.

[0082] The electrolyte plays a role in conducting ions between the cathode sheet and the negative electrode sheet, and the electrolyte can be in a liquid state, a gel state, or a full solid state. In some embodiments, the electrolyte uses an electrolyte solution, and the electrolyte solution includes an electrolyte salt and a solvent, and the electrolyte salt is a lithium salt.

[0083] The embodiments of the present application further provide a power-using device, which includes the above-mentioned secondary battery as a power supply for the power-using device.

[0084] For the above-mentioned secondary battery embodiments and power-using device embodiments, the cathode sheet includes a cathode active material layer, the cathode active material layer includes the above-mentioned cathode material, and the same technical effects can be achieved, and to avoid repetition, the relevant parts are described in the part of the cathode material embodiments.

[0085] In order to make the purposes, technical solutions, and beneficial effects of the present application clearer, the present application will be further described below in conjunction with embodiments. It should be understood that these embodiments are only used to illustrate the present application and do not limit the scope of the present application.

[0086] The application will be described in detail below by way of examples.

[0087] Performance test method

[0088] (1) Sphericity test:

[0089] A focused ion beam scanning electron microscope (FIB-SEM) of Crossbeam 550 was used for the test. Specifically, the LFP pole piece after rolling was cut into a sample of 20*20 μm, and the cutting and photographing were performed at an interval of 50 nm. The 3D model was established by using the automatic identification of gray value of the law of large numbers, and the minimum particle size was set to 50 nm. The sphericity was calculated according to the aspect ratio, wherein the sphericity of the positive electrode material was the average value of the sphericity of the tested positive electrode material particles.

[0090] (2) Carbon coating thickness test: The thickness of the carbon coating of lithium iron phosphate was detected by a projection electron microscope.

[0091] (3) Compaction density test:

[0092] The pole piece after rolling was punched into a circular piece with a radius of 20 mm, and the thickness of the circular piece was tested using a micrometer to calculate the compaction of the pole piece.

[0093] (4) First charge specific capacity test:

[0094] The pole piece after rolling was punched into a small circular piece, and a button cell was assembled in a glove box. The glove box environment was: 25°C, water <0.1 ppm, oxygen <1 ppm.

[0095] The assembled button cell was discharged to 5 mV at 0.1C, and then rested for 5 minutes. It was discharged to 5 mV at 0.05 mA, and then rested for 5 minutes. It was discharged to 5 mV at 0.01 mA, and then rested for 5 minutes. It was charged to 3.8V at 0.1C, and the battery capacity was measured to calculate the battery specific capacity.

[0096] (5) Energy density test:

[0097] At room temperature 25°C, constant current charge and discharge was carried out at 1 / 3C to 2.0V-3.8V, and the discharge capacity C, the average discharge voltage V, and the battery mass M were recorded. The energy density was calculated as C*V / M.

[0098] (6) Rate performance test:

[0099] At room temperature 25°C, a battery test cabinet was used to discharge at 1 / 3C, 0.5C, 1C, and 2C rates in turn at a voltage of 2.5V-3.8V, and the discharge capacity retention rate was calculated as the rate performance index.

[0100] Example 1

[0101] (1) Preparation of the positive electrode material:

[0102] a. Mix the iron phosphate, carbon source, and lithium source in a mass ratio of 1:0.2:1.04, and then grind them using a sand mill at a speed of 480 r / min to obtain particles with a particle size D50 of 680 nm after sanding; wherein the carbon source is phenolic resin, and the lithium source is LiOH;

[0103] b. Spray dry the material after sanding to control the moisture content to be less than or equal to 2%, with the inlet air temperature being 240°C and the outlet air temperature being 90°C;

[0104] c. Put the dry powder obtained by spraying into a sintering furnace with a protective gas atmosphere such as nitrogen, control the sintering temperature to be 770°C, and the sintering time to be 9h;

[0105] d. Adjust the rotational speed of the first-stage classification rotor cage and the second-stage classification rotor cage of the two-stage turbine classifier in series to be 800 r / min, use compressed air as the working medium, and collect the first-stage coarse powder first lithium iron phosphate after the first-stage classification, the second-stage fine powder second lithium iron phosphate after the second-stage classification, and the microfine powder third lithium iron phosphate in the bag collector; wherein the D50(D 1 50) of the first lithium iron phosphate is 2 microns; the D50(D 2 50) of the second lithium iron phosphate is 1.5 microns; and the Dmo(D 3 mo) of the third lithium iron phosphate in the bag collector is 0.378 microns;

[0106] e. Mix the first lithium iron phosphate, the second lithium iron phosphate, and the third lithium iron phosphate collected by classification in a mass ratio of 4:4:2 using a drum tank mill to obtain a sphericity of 0.73, and obtain the positive electrode material.

[0107] (2) Preparation of the positive electrode sheet

[0108] Disperse the positive electrode material prepared above, PVDF, and conductive carbon black in NMP in a mass ratio of 96.8:2.42:0.78, homogenize, and uniformly coat on the positive electrode current collector. After high-temperature drying, rolling, cutting, and slitting, the positive electrode sheet is prepared, wherein the rolling pressure is controlled to be 40T.

[0109] (3) Preparation of the negative electrode sheet

[0110] Disperse the negative electrode material hard carbon, styrene-butadiene rubber (SBR), conductive carbon black, and sodium carboxymethyl cellulose (CMC) in a mass ratio of 93.3:0.03:0.02:0.015, and uniformly coat on the negative electrode current collector. Control the sheet area density to be 8 mg / cm 2 , and after high-temperature drying, rolling, cutting, and slitting, the negative electrode sheet is prepared.

[0111] (4) Preparation of lithium ion battery

[0112] After winding the positive electrode, the negative electrode and the separator, the lithium ion battery is prepared by packaging and injecting electrolyte (the composition of the electrolyte is LiPF6, EC, EMC, DEC, DMC = 12%:22.5%:52.5%:6.5%:6.5%).

[0113] Example 2

[0114] The difference between Example 2 and Example 1 is that in step (a), the iron phosphate, carbon source and lithium source are mixed in a mass ratio of 1:0.18:1.03.

[0115] In step (c), the sintering temperature is adjusted to 775℃;

[0116] In step (d), the rotating speed of the first-stage classification rotating cage and the second-stage classification rotating cage is adjusted to 600r / min, the D50 of the first lithium iron phosphate is 3 microns, the D50 of the second lithium iron phosphate is 1.97 microns, and the Dmo of the third lithium iron phosphate in the bag collector is 0.6 microns.

[0117] In step (e), the first lithium iron phosphate, the second lithium iron phosphate and the third lithium iron phosphate collected by classification are mixed in a mass ratio of 4:4:2 using a drum tank mill to a sphericity of 0.78.

[0118] Example 3

[0119] The difference between Example 3 and Example 1 is that in step (a), the iron phosphate, carbon source and lithium source are mixed in a mass ratio of 1:0.19:1.03.

[0120] In step (c), the sintering temperature is adjusted to 772℃;

[0121] In step (d), the rotating speed of the first-stage classification rotating cage and the second-stage classification rotating cage is adjusted to 200r / min, the D50 of the first lithium iron phosphate is 2.2 microns, the D50 of the second lithium iron phosphate is 1.8 microns, and the Dmo of the third lithium iron phosphate in the bag collector is 0.52 microns.

[0122] In step (e), the first lithium iron phosphate, the second lithium iron phosphate and the third lithium iron phosphate collected by classification are mixed in a mass ratio of 4:4:2 using a drum tank mill to a sphericity of 0.69.

[0123] Example 4

[0124] Example 4 differs from Example 1 in that in step (e), the first, second, and third lithium iron phosphates collected in the stages are mixed in a mass ratio of 5:4:1 using a tumbler jar mill to a sphericity of 0.73.

[0125] Example 5

[0126] Example 5 differs from Example 1 in that in step (e), the first, second, and third lithium iron phosphates collected in the stages are mixed in a mass ratio of 4.5:4:1.5 using a tumbler jar mill to a sphericity of 0.73.

[0127] Example 6

[0128] Example 6 differs from Example 1 in that in step (e), the first, second, and third lithium iron phosphates collected in the stages are mixed in a mass ratio of 4:4:4 using a tumbler jar mill to a sphericity of 0.73.

[0129] Example 7

[0130] Example 7 differs from Example 1 in that in step (d), the rotational speed of the first-stage classification rotor cage and the second-stage classification rotor cage is adjusted to 700 r / min, and the D50 of the first lithium iron phosphate collected is 1 micron; the D50 of the second lithium iron phosphate collected is 0.8 micron; and the Dmo of the third lithium iron phosphate collected in the bag collector is 0.35 micron.

[0131] Example 8

[0132] Example 8 differs from Example 1 in that in step (d), the rotational speed of the first-stage classification rotor cage and the second-stage classification rotor cage is adjusted to 750 r / min, and the D50 of the first lithium iron phosphate collected is 2 micron; the D50 of the second lithium iron phosphate collected is 1.99 micron; and the Dmo of the third lithium iron phosphate collected in the bag collector is 0.6 micron.

[0133] Example 9

[0134] Example 9 differs from Example 1 in that in step (e), the first, second, and third lithium iron phosphates collected in the stages are mixed using a tumbler jar mill to a sphericity of 0.6.

[0135] Example 10

[0136] Example 10 differs from Example 1 in that in step (e), the first, second, and third lithium iron phosphates collected in the stages are mixed using a tumbler jar mill to a sphericity of 1.

[0137] Comparative Example 1

[0138] The difference between Comparative Example 1 and Example 1 is that in step (d), the rotation speeds of the first-stage classification rotating cage and the second-stage classification rotating cage are adjusted to 500 r / min and 200 r / min, the D50 of the first lithium iron phosphate collected is 2 microns; the D50 of the second lithium iron phosphate is 1.99 microns; and the Dmo of the third lithium iron phosphate collected in the bag collector is 0.59 microns.

[0139] In step (e), the first lithium iron phosphate, the second lithium iron phosphate and the third lithium iron phosphate collected in the classification are mixed in a mass ratio of 4:4:2 using a drum tank mill to a sphericity of 0.69.

[0140] Comparative Example 2

[0141] The difference between Comparative Example 2 and Example 1 is that in step (d), the rotation speeds of the first-stage classification rotating cage and the second-stage classification rotating cage are adjusted to 600 r / min and 670 r / min, the D50 of the first lithium iron phosphate collected is 3 microns; the D50 of the second lithium iron phosphate is 1.97 microns; and the Dmo of the third lithium iron phosphate collected in the bag collector is 0.58 microns.

[0142] In step (e), the first lithium iron phosphate, the second lithium iron phosphate and the third lithium iron phosphate collected in the classification are mixed in a mass ratio of 4:4:2 using a drum tank mill to a sphericity of 0.81.

[0143] The positive electrode materials of Examples 1 and 2 are subjected to FIB-SEM testing, and the test results are shown in FIGS. 3 and 4, respectively.

[0144] The sphericity of the positive electrode materials of each example and the thickness of the carbon coating layer are tested, and the test data are shown in Table 1.

[0145] The batteries prepared in each example are subjected to compaction density testing, first charge capacity testing, energy density testing and rate performance testing, and the test data are shown in Table 2.

[0146] Table 1

[0147] In Table 1, z = 2.35 + 0.258 * D 1 50 - 0.121 * D 2 50 - 0.639 * D 3 mo + 0.3125 * S + 0.0142 * n, n refers to the mass ratio between the second lithium iron phosphate and the third lithium iron phosphate.

[0148] Table 2

[0149] According to the above data, by optimizing the particle size, the particle size distribution of the positive electrode material satisfies z = 2.35 + 0.258 * D 150 - 0.121 * D 2 50 - 0.639 * D 3 Mo+0.3125*S+0.0142*n, 2.3≤z≤2.79, which is beneficial to size grading of particles, filling the pores between particles, and significantly improving the compaction density, thereby solving the problem that the existing lithium iron phosphate positive electrode material has more pores between particles, which results in a smaller compaction density of the electrode sheet, and leads to a smaller battery energy density and poorer rate performance.

[0150] Although the preferred embodiments of the present application have been described, those skilled in the art who have the basic inventive concept can make further changes and modifications to the embodiments. Therefore, the claims are intended to include the preferred embodiments and all changes and modifications falling within the scope of the embodiments of the present application.

[0151] The above provides a kind of positive electrode material and its preparation method, secondary battery and electric equipment provided by the present application, which are described in detail in the present application.The principles and implementation modes of the present application are described by applying specific examples.The above description of the embodiments is only used to help understand the method of the present application and its core idea;Meanwhile, for those skilled in the art, according to the idea of the present application, the specific implementation mode and application range will be changed;In conclusion, the content of the specification should not be understood as a limitation of the present application.

Claims

1. A positive electrode material, characterized by, The positive electrode material includes a first lithium iron phosphate, a second lithium iron phosphate, and a third lithium iron phosphate, and the particle size and morphology distribution of the positive electrode material satisfy: z = 2.35 + 0.258 * D 1 50 - 0.121 * D 2 50 - 0.639 * D 3 mo + 0.3125 * S + 0.0142 * n, 2.3 <= z <= 2.79; D 1 50 refers to D50 of the first lithium iron phosphate, D 2 50 refers to D50 of the second lithium iron phosphate, D 3 mo refers to the particle size corresponding to the maximum volume fraction of the third lithium iron phosphate, S refers to the sphericity of the positive electrode material, and n refers to the mass ratio of the second lithium iron phosphate to the third lithium iron phosphate; 2 mu m <= D 1 50 <= 4 mu m, and 0.8 mu m <= D 2 50 < 2 mu m, and 0.35 mu m <= D 3 mo <= 0.6 mu m, 0.6 <= S <= 1.0, and 1 <= n <= 4.

2. The positive electrode material of claim 1, wherein, 2.6≤z≤2.79。 3. The positive electrode material of claim 1, wherein, The sphericity of the first lithium iron phosphate is S1, the sphericity of the second lithium iron phosphate is S2, and the sphericity of the third lithium iron phosphate is S3, and the S1, S2, and S3 satisfy: 0.6≤S1≤1.0, and / or, 0.6≤S2≤1.0, and / or, 0.6≤S3≤1.

0.

4. The positive electrode material of claim 1, wherein, In the positive electrode material, the mass ratio of the first lithium iron phosphate, the second lithium iron phosphate, and the third lithium iron phosphate is (4-5):4:(1-2).

5. The cathode material of claim 1, wherein, At least one of the first lithium iron phosphate, the second lithium iron phosphate, and the third lithium iron phosphate has a carbon coating layer.

6. The positive electrode material according to claim 5, characterized in that, The thickness of the carbon coating layer is 2-5 nm.

7. A method for producing the positive electrode material according to any one of claims 1 to 6, characterized by, Comprising: Obtaining lithium iron phosphate, the sphericity of which is 0.6-1.0; grading the lithium iron phosphate to obtain a first lithium iron phosphate, a second lithium iron phosphate and a third lithium iron phosphate, wherein the D 1 50、the D 2 50、the D 3 mo satisfies: 2 μm≤D 1 50≤4 μm, 0.8 μm≤D 2 50<2 μm, 0.35 μm≤D 3 mo≤0.6 μm; The first lithium iron phosphate, the second lithium iron phosphate, the third lithium iron phosphate are mixed to obtain the positive electrode material, the mass ratio n of the second lithium iron phosphate to the third lithium iron phosphate satisfies: 1≤n≤4, the sphericity S of the positive electrode material satisfies: 0.6≤S≤1.0; the positive electrode material satisfies: z=2.35+0.258*D 1 50-0.121*D 2 50-0.639*D 3 mo+0.3125*S+0.0142*n, 2.3≤z≤2.

79.

8. The method of claim 7, wherein the method further comprises a step of calcining the mixture at a temperature of 700-900°C for 5-20 hours. Obtaining lithium iron phosphate comprises: Mixing iron phosphate and a lithium source to obtain a lithium iron phosphate precursor, and grinding the lithium iron phosphate precursor, wherein the particle size D50 of the ground lithium iron phosphate precursor satisfies: 400 nm≤D50≤800 nm. Drying, sintering, and crushing the ground lithium iron phosphate precursor to obtain the lithium iron phosphate, the sphericity of which is 0.6-1.

0.

9. The method of claim 8, wherein the method further comprises a step of calcining the mixture at a temperature of 700-900°C for 5-20 hours. The lithium source comprises one or more of Li2CO3 and LiOH.

10. The method of claim 8, wherein the method further comprises a step of calcining the mixture at a temperature of 700-900°C for 5-20 hours. Mixing iron phosphate and a lithium source to obtain a lithium iron phosphate precursor further comprises adding a carbon source, wherein the carbon source comprises one or more of glucose, starch, and phenolic resin.

11. The method for preparing the cathode material according to claim 10, characterized in that, The molar ratio of the iron phosphate, the carbon source, and the lithium source is 1:(0.15-0.2):(1-1.04).

12. A secondary battery characterized by comprising: Comprising a positive electrode tab, the positive electrode tab comprising a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector, the positive electrode active material layer comprising the positive electrode material of any one of claims 1-6 or the positive electrode material prepared by the preparation method of any one of claims 7-11.

13. An electrical device, characterized by Comprising the secondary battery of claim 12, the secondary battery serving as a power supply for the electric device.

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