Method for evaluating particle dispersion of positive electrode material and application thereof

Abstract

The invention relates to a particle dispersion evaluation method of a positive electrode material and application thereof, and the particle dispersion evaluation method comprises the following steps: adding the positive electrode material into a solvent, and carrying out dispersion treatment to obtain a to-be-detected dispersion liquid; the to-be-measured dispersion liquid is placed in a laser particle size analyzer, the D50 particle size and the D10 particle size of the positive electrode material in the to-be-measured dispersion liquid are measured n times in sequence, the particle dispersion Y of the positive electrode material is calculated according to the D50 particle size and the D10 particle size, and n is larger than or equal to 2; wherein the calculation mode of the particle dispersion Y of the positive electrode material comprises the steps that the maximum value in the D50 particle size measured for n times is recorded as X1, the minimum value in the D10 particle size measured for n times is recorded as X2, Y = X1 / X2, and the larger the dispersion Y is, the more the particles of the positive electrode material are agglomerated; the dispersion evaluation method can solve the problem that the particle dispersion degree of the positive electrode material cannot be quantified.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and in particular to a method for evaluating the particle dispersion of a positive electrode material and an application thereof. Background Art

[0002] The electrochemical performance of secondary batteries mainly depends on the electrode materials. The positive electrode materials play a vital role in determining the capacity, battery voltage, energy density, safety and cycle life of secondary batteries. Currently, the commonly used positive electrode materials are LiFePO4, LiFe x Mn 1-x PO4, NCM ternary materials, LiMn2O4, LiNi 0.5 Mn 1.5 O4, etc. The size distribution and dispersion state of the positive electrode material particles directly affect the kinetic properties of the electrode and the overall performance of the secondary battery, because in the preparation process of the secondary battery, the discreteness of the positive electrode material particles is the key factor determining the ion diffusion rate, electrode compaction density and cycle stability.

[0003] In the prior art, most methods for evaluating the degree of discreteness of positive electrode material particles are direct observation and giving evaluation results of the degree of discreteness, mostly relying on scanning electron microscopy (SEM) to observe the agglomeration state of the particles. However, the method of observing particles through SEM and giving the degree of discreteness has the defect of strong subjectivity; because after SEM observation, the degree of discreteness of particles is judged solely by SEM images, and the manual interpretation error is large, and the results are mostly given as "excellent, medium, poor", and the degree of discreteness cannot be quantified. Therefore, how to provide a method that can quantify the degree of discreteness of positive electrode material particles has become the key. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for evaluating the particle dispersion of a positive electrode material and its application, so as to solve the problem in the prior art that the particle dispersion of a positive electrode material cannot be quantified.

[0005] To achieve the purpose of the present invention, the present invention provides the following technical solutions:

[0006] In a first aspect, the present invention provides a method for evaluating the particle discreteness of a positive electrode material, the method comprising: adding a positive electrode material to a solvent, and obtaining a dispersion to be tested after dispersion treatment; placing the dispersion to be tested in a laser particle size analyzer, measuring the D50 particle size and the D10 particle size of the positive electrode material in the dispersion to be tested n times in succession, and calculating the particle discreteness Y of the positive electrode material using the D50 particle size and the D10 particle size, where n≥2; wherein the calculation method of the particle discreteness Y of the positive electrode material comprises: recording the maximum value of the D50 particle size measured n times as X1, and recording the minimum value of the D10 particle size measured n times as X2, Y=X1 / X2, wherein the larger the discreteness Y, the more agglomerated the particles.

[0007] In some embodiments, the calculation method of the particle discreteness Y of the positive electrode material includes: taking the maximum value of the D50 particle size measured from the 2nd to the nth time as X1; taking the minimum value of the D10 particle size measured from the 2nd to the nth time as X2; wherein n≥4.

[0008] In some embodiments, the positive electrode material is added to a solvent and dispersed to obtain the dispersion to be tested, comprising: adding the positive electrode material to a first solvent and dispersing the dispersion to obtain a first dispersion; freezing and thawing the first dispersion in sequence to obtain a second dispersion; and adding the second dispersion to a second solvent to obtain the dispersion to be tested.

[0009] In some embodiments, the first dispersion is sequentially subjected to freezing and thawing treatments to obtain a second dispersion, comprising: placing the first dispersion in a low-temperature environment for freezing to obtain a frozen material, wherein the temperature of the low-temperature environment is -209°C to -190°C; placing the frozen material in a thawing environment for thawing to obtain the second dispersion, wherein the temperature of the thawing environment is 20°C to 40°C.

[0010] In some embodiments, the frozen material is placed in a thawing environment for thawing to obtain the second dispersion, comprising: placing the frozen material in a first thawing environment for primary thawing, wherein the temperature of the first thawing environment is 20°C to 30°C; placing the material after the primary thawing in a second thawing environment for secondary thawing to obtain the second dispersion, wherein the temperature of the second thawing environment is 30°C to 40°C.

[0011] In some embodiments, the low-temperature environment is a liquid nitrogen environment, and the freezing time is ≤10s.

[0012] In some embodiments, the primary thawing is water bath thawing for 5 to 15 seconds.

[0013] In some embodiments, the secondary thawing is water bath thawing for 5s to 15s.

[0014] In some embodiments, the mass ratio of the positive electrode material to the first solvent is 1:(15-30).

[0015] In some embodiments, the volume ratio of the second dispersion to the second solvent is 1:(150-200).

[0016] In a second aspect, the particle dispersion evaluation method provided by the present invention is applied in the preparation process of positive electrode materials.

[0017] In a third aspect, the present invention provides a positive electrode material. After the positive electrode material is subjected to the particle dispersion evaluation method described in any one of the embodiments of the first aspect, the particle dispersion Y of the positive electrode material satisfies the following: 2.5≤Y≤8.

[0018] In a fourth aspect, the present invention provides a secondary battery, comprising the positive electrode sheet as described in the fourth aspect.

[0019] The particle dispersion evaluation method of the positive electrode material provided by the present invention directly disperses the positive electrode material and then tests it, calculates the dispersion of the positive electrode material based on the test results, and obtains the quantitative change result of the dispersion degree of the positive electrode material particles. In the quantified dispersion value, the larger the value of the dispersion Y, the more agglomerated the positive electrode material particles are; the smaller the value of the dispersion Y, the looser the positive electrode material particles are; the quantified dispersion degree result can be used to evaluate the dispersion degree of different positive electrode materials and accurately distinguish positive electrode materials with similar dispersion degrees, so that the dispersion degree evaluation of the positive electrode materials is more accurate; the dispersion degree evaluation method is simple in steps, short in evaluation time, accurate in evaluation results, and saves a lot of manpower and material resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0021] Figure 1 is a flow chart of a method for evaluating particle dispersion of a positive electrode material according to an embodiment;

[0022] Figure 2 is a flowchart of step S100 in one embodiment;

[0023] Figure 3 is a flowchart of step S120 in one embodiment;

[0024] Figure 4 is a flowchart of step S122 in one embodiment;

[0025] Figure 5 is a flow chart of a method for preparing a positive electrode material according to an embodiment;

[0026] Figure 6 1 is a scanning electron microscope (SEM) image of the positive electrode materials prepared in Example 1 to Example 4. DETAILED DESCRIPTION

[0027] The following will be combined with the accompanying drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0028] Unless otherwise defined, all technical and scientific terms used herein have the same meanings as commonly understood by those skilled in the art to which this invention pertains. The terms used in this specification are for the purpose of describing specific embodiments only and are not intended to limit the present invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0029] It should be noted that the "ranges" disclosed herein are defined in the form of lower limits and upper limits. A given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundaries of the particular range. Ranges defined in this manner can be inclusive or exclusive of the end values ​​and can be combined arbitrarily, i.e., any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60 to 120 and 80 to 110 are listed for a particular parameter, it is understood that ranges of 60 to 110 and 80 to 120 are also contemplated. In addition, if the minimum range values ​​listed are 1 and 2, and if the maximum range values ​​listed are 3, 4, and 5, the following ranges are all contemplated: 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4, and 2 to 5. In the present invention, unless otherwise specified, the numerical range "a to b" represents an abbreviation for any combination of real numbers between a and b, where a and b are both real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0030] All steps of the present invention may be performed sequentially or randomly, but are preferably performed sequentially. For example, a statement that a method includes steps (a) and (b) indicates that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, a statement that a method may also include step (c) indicates that step (c) may be added to the method in any order, e.g., the method may include steps (a), (b), and (c), or may include steps (a), (c), and (b), or may include steps (c), (a), and (b), etc.

[0031] The following embodiments of the present invention are described in detail with reference to the accompanying drawings. In the absence of conflict, the following embodiments and features in the embodiments may be combined with each other.

[0032] The present invention provides a method for evaluating the particle discreteness of a positive electrode material. The method is used to evaluate the particle discreteness of a positive electrode material and provide a quantitative result of the particle discreteness of the positive electrode material to be tested. The method can quantify the particle discreteness of the positive electrode material and, based on the numerical value of the discreteness, determine whether the particles of the positive electrode material are agglomerated or discrete.

[0033] In some embodiments, the method for evaluating the particle dispersion of the positive electrode material can be found in Figure 1 , specifically including the following steps:

[0034] Step S100: adding the positive electrode material into a solvent and performing a dispersion treatment to obtain a dispersion liquid to be tested.

[0035] Step S200: placing the dispersion to be tested in a laser particle size analyzer, measuring the D50 particle size and D10 particle size of the positive electrode material in the dispersion to be tested n times, and calculating the particle dispersion Y of the positive electrode material using the D50 particle size and D10 particle size, where n≥2.

[0036] Among them, the calculation method of the particle discreteness Y of the positive electrode material includes: recording the maximum value of the D50 particle size measured n times as X1, and recording the minimum value of the D10 particle size measured n times as X2, Y=X1 / X2, wherein the larger the discreteness Y, the more agglomerated the particles of the positive electrode material.

[0037] The particle dispersion evaluation method of the positive electrode material provided by the present invention directly disperses the positive electrode material and then tests it, calculates the dispersion of the positive electrode material based on the test results, and obtains the quantitative change result of the dispersion degree of the positive electrode material particles. In the quantified dispersion value, the larger the value of the dispersion Y, the more agglomerated the particles; the smaller the value of the dispersion Y, the more discrete the positive electrode material particles; the quantified dispersion degree result can be used to evaluate the dispersion degree of different positive electrode materials and accurately distinguish positive electrode materials with similar dispersion degrees, so that the dispersion degree evaluation of the positive electrode materials is more accurate; the dispersion degree evaluation method is simple in steps, short in evaluation time, accurate in evaluation results, and saves a lot of manpower and material resources.

[0038] In addition, in the present invention, the formula Y=X1 / X2 is used to evaluate the particle discreteness. The largest D50 particle size replaces the agglomerate diameter, and the smallest D10 particle size replaces the primary particle diameter. The division approximates the number of primary particles in the radial direction of the agglomerate. The larger this value is, the more primary particles there are in the agglomerate, which means that the material is more agglomerated.

[0039] At the same time, by quantifying the degree of discreteness of the positive electrode material, the present invention can be further used to evaluate the preparation method of the positive electrode material, and the measured degree of discreteness can be used to modify the preparation process flow of the positive electrode material; applying the results of the particle discreteness evaluation method to the industrialized positive electrode material preparation process can greatly improve the yield of the positive electrode material and reduce the preparation process cost.

[0040] In a specific embodiment, in step S100, the positive electrode material may include one or more of a lithium-ion positive electrode material and a sodium-ion positive electrode material. Among them, the lithium-ion positive electrode material includes a phosphate-based positive electrode material, a nickelate-based positive electrode material, a manganate-based positive electrode material, and a ternary positive electrode material. Optionally, the positive electrode material may include lithium iron phosphate (LFP) and / or lithium manganese iron phosphate (LMFP).

[0041] In a specific embodiment, in step S100, the solvent includes an inorganic solvent and / or an organic solvent. Methods for dispersing the positive electrode material in the solvent include, but are not limited to, one or more of mechanical stirring dispersion, ultrasonic dispersion, and ball milling dispersion. In other embodiments, dispersing the positive electrode material in the solvent further includes adding a surface modifier, adjusting the pH of the solution, and the like.

[0042] In a specific embodiment, in step S200, the laser particle size analyzer is an instrument that analyzes the particle size distribution of a particle group by measuring the diffraction spectrum of the particle group and performing mathematical transformation. Based on the principle of light scattering, when a laser beam is irradiated onto a particle group, particles of different sizes will cause the light to scatter at different angles, thereby measuring the particle size.

[0043] In a specific embodiment, in step S200, measuring the D50 particle size and the D10 particle size of the positive electrode material in the dispersion to be tested n times means performing n measurements on the same dispersion to be tested, obtaining n groups of D50 particle size measurement results for the positive electrode materials and n groups of D10 particle size measurement results for the positive electrode materials. The maximum value among the n groups of D50 particle size measurement results for the positive electrode materials is recorded as X1, and the minimum value among the n groups of D10 particle size measurement results for the positive electrode materials is recorded as X2.

[0044] In a specific embodiment, in step S200, the operating method of measuring the particle size of the positive electrode material using a laser particle size analyzer may include: starting the laser particle size analyzer and preheating it, cleaning the lens of the laser particle size analyzer; weighing a suitable weight of the sample to be tested and placing it in a container; adding deionized water and a stirrer to the container, placing the container on a magnetic stirrer, sealing it and performing magnetic stirring; then placing the container in an ultrasonic instrument for ultrasonication; opening the test software and setting the parameters; measuring the background, initializing the instrument, clicking start, testing the background, the light energy in the image of detectors 0 to 50, the normal background should show a decreasing trend, and the light intensity of detector No. 1 (the first vertical column to the right of the coordinate axis) does not exceed 200.

[0045] In other embodiments, in step S200, the method for measuring the particle size of the positive electrode material using a laser particle size analyzer may be other methods. It is understood that the method for measuring the particle size of the positive electrode material using a laser particle size analyzer may vary depending on the type of laser particle size analyzer, and is not specifically limited in the present invention.

[0046] In a specific embodiment, the value range of the discreteness Y may be 2.5≤Y≤8. Optionally, the value range of the discreteness Y may be 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, or 8.

[0047] In some embodiments, the calculation method of the particle discreteness Y of the positive electrode material includes: taking the maximum value of the D50 particle size measured from the 2nd to the nth time as X1; taking the minimum value of the D10 particle size measured from the 2nd to the nth time as X2; wherein n≥4.

[0048] In a specific embodiment, the same dispersion to be tested is measured 4 times to obtain 4 groups of D50 particle size measurement results and D10 particle size measurement results of the positive electrode material; from the measurement results starting from the second time (a total of three groups), the maximum D50 particle size is recorded as X1, and the minimum D10 particle size is recorded as X2, and the dispersion is calculated according to the formula Y = X1 / X2.

[0049] During the test, the data from the first test may be unstable, leading to increased errors. Subsequent tests will generally stabilize or increase the test results. If the data is generally stable, it means the test has reached a stable state, and the evaluation results are more accurate. If the data increases, it may be due to small particles reaggregating, which will cause errors in the evaluation results. Therefore, it is best to test four or more times, excluding the unstable data from the first test, and using the data from the last three tests for comparison and calculation to achieve a more accurate evaluation result.

[0050] In a specific embodiment, each set of D50 particle size is an average value of multiple measurements, and each set of D10 particle size is an average value of multiple measurements. Specifically, among the four sets of D50 particle size measurement results of the positive electrode materials, the first set of measurement results is taken as an example; in the first test, a laser particle size analyzer is used to perform m parallel tests on the dispersion to be tested, obtaining m values, where m ≥ 2, and the average of the m values ​​is taken to obtain the first set of measurement results; the measurement method for the remaining three sets of D50 particle size measurement results and the four sets of D10 particle size measurement results can refer to this method.

[0051] In a specific embodiment, each set of D50 particle diameters is the average of three measurements, and each set of D10 particle diameters is the average of three measurements, i.e., m=3 in the above description. In the first set of measurement results a1, the average of three measurements is obtained, i.e., a1=(q1+q2+q3) / 3, where q1, q2, and q3 are the three measurements taken when measuring the first set of measurement results a1.

[0052] In some implementations, please refer to Figure 2 , adding the positive electrode material to the solvent, and obtaining the dispersion to be tested after dispersion treatment, which specifically includes the following steps:

[0053] Step S110 , adding the positive electrode material into a first solvent, and dispersing the material by ultrasonication to obtain a first dispersion.

[0054] Step S120 , freezing and thawing the first dispersion in sequence to obtain a second dispersion.

[0055] Step S130: adding the second dispersion liquid into the second solvent to obtain a dispersion liquid to be tested.

[0056] In a specific embodiment, in step S110, the first solvent is deionized water, the positive electrode material is added to the deionized water, and ultrasonic dispersion treatment is performed in an ultrasonic device to obtain a fully dispersed first dispersion.

[0057] In a specific embodiment, in step S120, the fully dispersed first dispersion is placed in a low-temperature environment for freezing, and then the frozen first dispersion is placed in a room temperature or high-temperature environment for thawing. The purpose of the freezing treatment is to strengthen the independence of the positive electrode material particles in the first dispersion, thereby preventing the positive electrode material particles from agglomerating during the measurement process and affecting the evaluation accuracy.

[0058] It should be noted that after freezing the first dispersion, the first dispersion becomes solid, and the deionized water forms ice crystals. These ice crystals grow and squeeze into the gaps between the positive electrode material particles. The ice crystals formed by the deionized water mechanically break up the hard agglomerates, dispersing the positive electrode material particles in the first dispersion. Furthermore, the thermal shock during the thawing process weakens the electrostatic adsorption between the positive electrode material particles, preventing them from self-agglomerating again.

[0059] In a specific embodiment, in step S130, the second solvent is deionized water, and the second dispersion is added to the deionized water, so that the proportion of the positive electrode material in the prepared dispersion to be tested can meet the standard of the laser particle size analyzer.

[0060] In some implementations, please refer to Figure 3 , freezing and thawing the first dispersion in sequence to obtain a second dispersion, which specifically comprises the following steps:

[0061] Step S121 , placing the first dispersion in a low-temperature environment for freezing to obtain a frozen material, wherein the temperature of the low-temperature environment is -209° C. to -190° C.

[0062] Step S122: placing the frozen material in a thawing environment for thawing to obtain a second dispersion, wherein the temperature of the thawing environment is 20° C. to 40° C.

[0063] In a specific embodiment, in step S121, the first dispersion is placed in liquid nitrogen for freezing to obtain a frozen material. The liquid nitrogen is immersed in the sample tube containing the first dispersion to a depth of greater than or equal to 3 cm, and the freezing treatment lasts for greater than or equal to 10 seconds to ensure sufficient freezing.

[0064] In a specific embodiment, in step S121, the temperature of the cryogenic environment can be -209°C, -208°C, -206°C, -204°C, -202°C, -200°C, -198°C, -196°C, -194°C, -192°C, or -190°C. The advantages of using liquid nitrogen for freezing treatment are that the temperature of liquid nitrogen at standard atmospheric pressure falls within the aforementioned temperature range; liquid nitrogen is easily available, inexpensive, and pollution-free; liquid nitrogen can achieve rapid freezing, ensuring that deionized water quickly forms ice crystals to break down hard aggregates; and controlling the temperature of the cryogenic environment within the aforementioned range ensures that the first dispersion is fully frozen and solidified.

[0065] In a specific embodiment, in step S121, the immersion depth of the sample tube can be 3 cm, 4 cm, 5 cm, 6 cm, 7 cm, 8 cm, 9 cm, or 10 cm. The freezing treatment duration is ≤ 10 seconds; alternatively, the freezing treatment duration can be 1 second, 2 seconds, 3 seconds, 4 seconds, 5 seconds, 6 seconds, 7 seconds, 8 seconds, 9 seconds, or 10 seconds. By controlling the immersion depth of the sample tube and the freezing treatment duration within the above ranges, the first dispersion can be fully frozen and solidified, and material damage caused by prolonged exposure of the first dispersion to a low temperature environment can be avoided.

[0066] In a specific embodiment, in step S122, the frozen material is placed in a warm water bath for thawing to obtain a second dispersion. During the thawing process in the warm water bath, the warm water bath is also shaken, and the thawing time is greater than or equal to 10 seconds to ensure sufficient thawing.

[0067] In a specific embodiment, in step S122, the temperature of the thawing environment can be 20° C., 22° C., 24° C., 26° C., 28° C., 30° C., 32° C., 34° C., 36° C., 38° C., or 40° C. By controlling the temperature of the thawing environment within the above range, the risks that may occur during the thawing process can be reduced.

[0068] In a specific embodiment, in step S122, the thawing process duration can be 5 seconds, 6 seconds, 7 seconds, 8 seconds, 9 seconds, 10 seconds, 11 seconds, 12 seconds, 13 seconds, 14 seconds, 15 seconds, 16 seconds, 17 seconds, 18 seconds, 19 seconds, or 20 seconds. By controlling the thawing process duration within the above range, the second dispersion can be fully thawed to prevent ice crystals from affecting the evaluation results.

[0069] In some implementations, please refer to Figure 4 , placing the frozen material in a thawing environment for thawing to obtain a second dispersion, specifically comprising the following steps:

[0070] Step S1221: placing the frozen material in a first thawing environment for thawing, wherein the temperature of the first thawing environment is 20°C to 30°C.

[0071] Step S1222: placing the once thawed material in a second thawing environment for secondary thawing to obtain a second dispersion. The temperature of the second thawing environment is 30° C. to 40° C.

[0072] In a specific embodiment, in step S1221, the temperature of the first thawing environment can be 20° C., 21° C., 22° C., 23° C., 24° C., 25° C., 26° C., 27° C., 28° C., 29° C., or 30° C. By controlling the temperature of the first thawing environment within the above range, the dangers during the thawing process can be reduced and larger ice crystal structures can be pre-destroyed.

[0073] In a specific embodiment, in step S1221, the primary thawing is performed in a water bath for 5 to 15 seconds. Optionally, the duration of the primary thawing process can be 5 seconds, 6 seconds, 7 seconds, 8 seconds, 9 seconds, 10 seconds, 11 seconds, 12 seconds, 13 seconds, 14 seconds, or 15 seconds. By controlling the duration of the thawing process within the above range, it can be ensured that the primary thawing process is sufficient, does not cause danger, and can achieve the effect of initial thawing.

[0074] In a specific embodiment, in step S1222, the temperature of the second thawing environment can be 30° C., 31° C., 32° C., 33° C., 34° C., 35° C., 36° C., 37° C., 38° C., 39° C., or 40° C. By controlling the temperature of the second thawing environment within the above range, ice crystals in the deionized water can be fully liquefied to form the second solution, and the impact of ice crystal particles on the evaluation results can be avoided.

[0075] In a specific embodiment, in step S1222, the secondary thawing is performed in a water bath for 5 to 15 seconds. Optionally, the secondary thawing duration can be 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15 seconds. By controlling the thawing duration within this range, sufficient secondary thawing can be ensured, preventing ice crystals from affecting the evaluation results.

[0076] In some embodiments, the mass ratio of the positive electrode material to the first solvent is 1:(15-30). By controlling the mass ratio of the positive electrode material to the first solvent within the above range, it is possible to ensure that the positive electrode material can be fully dispersed, and the first solvent can create a larger space to cope with the freezing process, so that the ice crystals of the frozen first solvent can mechanically destroy hard agglomerates.

[0077] In specific embodiments, the mass ratio of the positive electrode material to the first solvent can be 1:15, 1:16, 1:18, 1:20, 1:22, 1:24, 1:26, 1:28, or 1:30.

[0078] In some embodiments, the volume ratio of the second dispersion to the second solvent is 1:(150-200). By controlling the volume ratio of the second dispersion to the second solvent within the above range, it can be ensured that the proportion of the positive electrode material in the prepared dispersion to be tested meets the standards of the laser particle size analyzer.

[0079] In a specific embodiment, the volume ratio of the second dispersion to the second solvent may be 1:150, 1:160, 1:170, 1:180, 1:190, or 1:200.

[0080] In some embodiments, the particle dispersion evaluation method of the positive electrode material provided by the present invention can be applied in the positive electrode material preparation process to evaluate the particle dispersion of the prepared positive electrode material to determine the processability of the prepared positive electrode material.

[0081] In some embodiments, the present invention provides a method for preparing a positive electrode material, please refer to Figure 5 , specifically including the following steps:

[0082] In step S300 , the cathode material precursor is subjected to a first calcination treatment and a carbon source is introduced, and then cooled to obtain a calcined material.

[0083] In step S400 , the first-calcined material is subjected to a second calcination treatment and a carbon source is introduced, and then cooled to obtain a positive electrode material.

[0084] In a specific embodiment, in step S300, the positive electrode material precursor includes one or more of a lithium-ion positive electrode material precursor and a sodium-ion positive electrode material precursor. The lithium-ion positive electrode material precursor includes a phosphate-based positive electrode material precursor, a nickelate-based positive electrode material precursor, a manganate-based positive electrode material precursor, and a ternary positive electrode material precursor. Alternatively, the positive electrode material precursor may include a lithium iron phosphate precursor and / or a lithium iron manganese phosphate precursor.

[0085] In a specific embodiment, in step S300, the heating rate of the first calcination process may be 5°C / min to 30°C / min. Optionally, the heating rate of the first calcination process may be 5°C / min, 10°C / min, 15°C / min, 20°C / min, 25°C / min, or 30°C / min.

[0086] In a specific embodiment, in step S300 , the temperature of the first calcination treatment may be 500° C. to 800° C. Optionally, the temperature of the first calcination treatment may be 500° C., 550° C., 600° C., 650° C., 700° C., 760° C., or 800° C.

[0087] In a specific embodiment, in step S300, the holding time of the first calcination treatment can be 10 hours to 24 hours. Optionally, the holding time of the first calcination treatment can be 10 hours, 12 hours, 14 hours, 16 hours, 18 hours, 20 hours, 22 hours, or 24 hours.

[0088] In a specific embodiment, in step S300, the carbon source introduced includes an oxygen-containing organic compound and water vapor, and the oxygen-containing organic compound and water vapor are used together to perform CVD carbon coating on the positive electrode material at high temperature. Optionally, the oxygen-containing organic compound includes one or more of alcohols, ketones, aldehydes, phenols, ethers, esters, and carboxylic acids.

[0089] In a specific embodiment, in step S300, the mass of the oxygen-containing organic matter introduced is 1% to 5% of the mass of the positive electrode material precursor. Optionally, the mass of the oxygen-containing organic matter introduced is 1%, 2%, 3%, 4%, or 5% of the mass of the positive electrode material precursor.

[0090] In a specific embodiment, in step S400, the heating rate of the second calcination process may be 2°C / min to 20°C / min. Optionally, the heating rate of the second calcination process may be 2°C / min, 3°C / min, 5°C / min, 10°C / min, 15°C / min, or 20°C / min.

[0091] In a specific embodiment, in step S400 , the temperature of the second calcination treatment may be 500° C. to 800° C. Optionally, the temperature of the second calcination treatment may be 500° C., 550° C., 600° C., 650° C., 700° C., 760° C., or 800° C.

[0092] In a specific embodiment, in step S400, the holding time of the second calcination treatment can be 6 hours to 18 hours. Optionally, the holding time of the second calcination treatment can be 6 hours, 8 hours, 10 hours, 12 hours, 14 hours, 16 hours, or 18 hours.

[0093] In a specific embodiment, in step S400, the carbon source introduced includes an oxygen-containing organic compound and water vapor, and the oxygen-containing organic compound and water vapor are used together to perform CVD carbon coating on the positive electrode material at high temperature. Optionally, the oxygen-containing organic compound includes one or more of alcohols, ketones, aldehydes, phenols, ethers, esters, and carboxylic acids.

[0094] In a specific embodiment, in step S400, the mass of the oxygen-containing organic matter introduced is 10% to 20% of the mass of the calcined material. Optionally, the mass of the oxygen-containing organic matter introduced is 10%, 12%, 14%, 16%, 18%, or 20% of the mass of the calcined material.

[0095] In a specific embodiment, in steps S300 and S400, the sintering furnace used includes a high-temperature sintering furnace with a protective atmosphere, such as a tubular furnace, a roller kiln, or a rotary kiln. The inert atmosphere includes one or more of nitrogen and argon.

[0096] The invention also provides a positive electrode material.

[0097] In some embodiments, the positive electrode material is prepared by the positive electrode material preparation method provided in the above embodiment, and after the positive electrode material is subjected to the particle dispersion evaluation method, the particle dispersion Y of the positive electrode material satisfies: 2.5≤Y≤8.

[0098] In some embodiments, the positive electrode material has a core-shell structure, comprising a core of a positive electrode active material and a carbon coating, wherein the carbon coating is coated on the outer surface of the core of the positive electrode active material. The core of the positive electrode active material comprises one or more of the various positive electrode materials provided in the above embodiments.

[0099] The present invention also provides a positive electrode plate.

[0100] In some embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode film layer located on at least one surface of the positive electrode current collector. The positive electrode film layer includes the positive electrode material obtained by the above-described preparation method or the above-described positive electrode material. Optionally, the positive electrode current collector has two opposing surfaces in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposing surfaces of the positive electrode current collector.

[0101] In some embodiments, the positive electrode film layer may optionally include a positive electrode conductive agent. The present invention does not particularly limit the type of positive electrode conductive agent. By way of example, the positive electrode conductive agent may include at least one of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene oxide, and carbon nanofibers. In some embodiments, the mass percentage of the positive electrode conductive agent based on the total mass of the positive electrode film layer is ≤5%.

[0102] In some embodiments, the positive electrode film layer may further optionally include a positive electrode binder. The present invention has no particular restrictions on the type of positive electrode binder. As an example, the positive electrode binder may 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. In some embodiments, the mass percentage of the positive electrode binder is ≤5% based on the total mass of the positive electrode film layer.

[0103] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. As an example of a metal foil, aluminum foil may be used. The composite current collector may include a polymer material base layer and a metal material layer formed on at least one surface of the polymer material base layer. As an example, the metal material may include at least one of aluminum, an aluminum alloy, nickel, a nickel alloy, titanium, a titanium alloy, silver, and a silver alloy. As an example, the polymer material base layer may include at least one of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0104] In some embodiments, the positive electrode film layer is typically formed by coating a positive electrode slurry onto a positive electrode current collector, drying, and cold pressing. The positive electrode slurry is typically formed by dispersing the positive electrode material, an optional conductive agent, an optional binder, and any other components in a solvent and stirring them uniformly. The solvent may be, but is not limited to, N-methylpyrrolidone (NMP).

[0105] The present invention also provides a secondary battery.

[0106] In some embodiments, the secondary battery comprises the cathode material obtained by the above-mentioned preparation method or the above-mentioned cathode material. The secondary battery of the present invention comprises the cathode material of the embodiment of the present invention or the cathode material prepared by the method of the embodiment of the present invention.

[0107] The technical solution of the present invention is described in detail below through specific embodiments.

[0108] Example 1

[0109] This embodiment provides a method for preparing a positive electrode material, which specifically includes the following steps:

[0110] (1) Lithium carbonate, ammonium dihydrogen phosphate, and ferric nitrate are weighed in a molar ratio of 1:1:1; the weighed materials are added to citric acid for reaction to obtain a precursor A; the precursor A is crushed and placed in a nitrogen atmosphere for drying at a drying temperature of 120°C to obtain a precursor B.

[0111] (2) Precursor B is placed in a sintering furnace for the first calcination treatment, wherein the heating rate of the first calcination treatment is 15°C / min, and the temperature is raised to 650°C; a carbon source is introduced during the heating process, and the carbon source is a mixture of oxygen-containing organic matter and water vapor (water vapor is doped in the oxygen-containing organic matter), and the mass of the carbon source is 5% of the mass of precursor B; the insulation time of the first calcination treatment is 15h; after the calcination is completed, the temperature is quickly cooled to room temperature, and the cooling rate is 20°C / min; the calcined product is then crushed and sieved to obtain lithium iron phosphate A.

[0112] (3) Lithium iron phosphate A is placed in a sintering furnace for a second calcination treatment, wherein the heating rate of the second calcination treatment is 5°C / min, and the temperature is raised to 700°C; a carbon source is introduced during the heating process, and the carbon source is a mixture of oxygen-containing organic matter and water vapor (water vapor is doped in the oxygen-containing organic matter), and the mass of the carbon source is 10% of the mass of lithium iron phosphate A; the insulation time of the second calcination treatment is 8 hours; after the calcination is completed, it is naturally cooled to room temperature; then the calcined product is crushed and sieved to obtain a positive electrode material of lithium iron phosphate.

[0113] This embodiment also provides a method for evaluating the particle dispersion of a positive electrode material, which specifically includes the following steps:

[0114] (4) An appropriate amount of the positive electrode material prepared in step (3) was added to a 316L stainless steel sample tube filled with 2 ml of deionized water and ultrasonically dispersed to obtain a first dispersion.

[0115] (5) Immerse the sample tube containing the first dispersion into liquid nitrogen with an immersion depth of ≥3 cm and an immersion time of 10 s to obtain a frozen material.

[0116] (6) The sample tube containing the frozen material was taken out of liquid nitrogen and placed in a warm water bath at 25°C for thawing. The sample tube was shaken during thawing. The thawing time was 10 seconds.

[0117] (7) The sample tube containing the frozen material was placed in a warm water bath at 35°C for secondary thawing. The sample tube was shaken during thawing. The duration of the secondary thawing was 10 seconds to obtain a second dispersion.

[0118] (8) Prepare 500 mL of deionized water and ultrasonically vibrate the deionized water until the opacity of the deionized water in the glass beaker is 0%. Add the second dispersion into the deionized water until the opacity of the liquid in the glass beaker reaches 10%-11%, thereby obtaining the dispersion to be tested.

[0119] (9) The D50 particle size and D10 particle size of four groups of positive electrode materials were measured using a laser particle size analyzer. The D50 particle size of each group was the average value of three measurements, and the D10 particle size of each group was the average value of three measurements. The maximum value of the last three measurements of the four groups of D50 particle size was selected as X1, and the minimum value of the last three measurements of the four groups of D10 particle size was selected as X2. The particle dispersion Y of the positive electrode material was calculated according to the formula Y = X1 / X2.

[0120] Example 2

[0121] This embodiment provides a method for preparing a positive electrode material. The difference between the preparation method provided in this embodiment and that in Example 1 is that in step (2), the temperature of the first calcination treatment is raised to 600° C. This embodiment uses the same positive electrode material particle dispersion evaluation method as in Example 1 to test the prepared positive electrode material.

[0122] Example 3

[0123] This embodiment provides a method for preparing a positive electrode material. The difference between the preparation method provided in this embodiment and that in Example 1 is that in step (2), the temperature of the first calcination treatment is raised to 550° C. This embodiment uses the same positive electrode material particle dispersion evaluation method as in Example 1 to test the prepared positive electrode material.

[0124] Example 4

[0125] This embodiment provides a method for preparing a positive electrode material. The difference between the preparation method provided in this embodiment and that in Example 1 is that in step (2), the temperature of the first calcination treatment is raised to 830° C. This embodiment uses the same positive electrode material particle dispersion evaluation method as in Example 1 to test the prepared positive electrode material.

[0126] Comparative Example 1:

[0127] In this comparative example, the positive electrode material was prepared using the same positive electrode material preparation method as in Example 1. The particle size of the positive electrode material was observed using a scanning electron microscope (SEM), and the degree of dispersion of the positive electrode material was determined manually.

[0128] Comparative Example 2:

[0129] In this comparative example, the positive electrode material was prepared using the same positive electrode material preparation method as in Example 2. The particle size of the positive electrode material was observed using a scanning electron microscope (SEM), and the degree of dispersion of the positive electrode material was determined manually.

[0130] Comparative Example 3:

[0131] In this comparative example, the positive electrode material was prepared using the same positive electrode material preparation method as in Example 3. The particle size of the positive electrode material was observed using a scanning electron microscope (SEM), and the degree of dispersion of the positive electrode material was determined manually.

[0132] Comparative Example 4:

[0133] In this comparative example, the positive electrode material was prepared using the same positive electrode material preparation method as in Example 4. The particle size of the positive electrode material was observed using a scanning electron microscope (SEM), and the degree of dispersion of the positive electrode material was determined manually.

[0134] The comparison results of the discreteness of the positive electrode materials provided by Examples 1 to 4 and Comparative Examples 1 to 4 are shown in Table 1:

[0135] Table 1 Comparison of the discreteness of the positive electrode materials of the embodiment and the comparative example

[0136]

[0137] From the comparison results provided by Examples 1 to 4 and Comparative Examples 1 to 4 in Table 1, it can be seen that the quantitative results of the discreteness of the positive electrode material particles can be obtained by the evaluation method provided by the present invention. Among them, Comparative Examples 1 to 4 use the same positive electrode materials as Examples 1 to 4, and observe and detect the positive electrode materials manually. The larger the value of the discreteness Y provided by the present invention, the more agglomerated the particles are; the smaller the value of the discreteness Y, the more discrete the positive electrode material particles are; the evaluation results of Example 3 and Example 4 are the maximum and minimum values ​​in Example 1 to Example 4, respectively, which are also consistent with the evaluation results of Comparative Examples 3 and 4 (poor and good, respectively).

[0138] Figure 6 The following are scanning electron microscope (SEM) images of the positive electrode materials prepared in Examples 1 to 4; 1) is the SEM image of Example 1, 2) is the SEM image of Example 2, 3) is the SEM image of Example 3, and 4) is the SEM image of Example 4. It is also clear from the figures that the positive electrode material provided by Example 4 has a better degree of dispersion, which is consistent with the result that the positive electrode material provided by Example 4 has the smallest dispersion value Y. At the same time, combined with the dispersion value Y of Examples 1 to 4 in Table 1, it can be confirmed that the positive electrode material provided by Example 3 has a poor degree of dispersion, which is consistent with the result that the positive electrode material provided by Example 3 has the largest dispersion value Y.

[0139] From the comparison results provided in Table 1 for Examples 1 and 2 and Comparative Examples 1 and 2, it can be seen that, while the scanning electron microscopy (SEM) evaluation results are similar, Comparative Example 1 and Comparative Example 2 cannot more accurately compare the positive electrode material with a higher degree of dispersion. By quantifying the degree of dispersion and obtaining the value of the dispersion Y, the present invention can accurately conclude that the degree of dispersion of the positive electrode material particles in Example 1 is greater than that in Example 2; thus, the results of evaluating the degree of dispersion of the positive electrode material particles are more accurate.

[0140] Combine the discreteness Y calculated in Example 1 and Example 2 Figure 6 As can be seen from 1) and 2) above, when the scanning electron microscope (SEM) evaluation results are similar, it is difficult to compare the particle dispersion of the positive electrode materials in Example 1 or Example 2 using conventional evaluation methods. In the prior art, it is difficult to compare the dispersion of the two using the methods of Comparative Example 1 and Comparative Example 2. The method provided by the present invention can simplify the process of manual visual inspection or rough measurement, and the dispersion of similar particles can be compared through calculation.

[0141] At the same time, from the comparative results provided by Examples 1 to 4 in Table 1, it can be concluded that the preparation method provided by Example 4 can prepare a positive electrode material with the highest degree of discreteness, while the preparation method provided by Example 1 can prepare a positive electrode material with a degree of discreteness better than that of Example 2, so the preparation method provided by Example 1 is more advantageous than that of Example 2. Therefore, the present invention can be further used to evaluate the preparation method by quantifying the degree of discreteness of the positive electrode material, and the preparation process flow of the positive electrode material can be modified using the measured degree of discreteness. Applying the results of this particle discreteness evaluation method to the industrialized positive electrode material preparation process can greatly improve the yield rate of the positive electrode material and reduce the cost of the preparation process.

[0142] In the description of the embodiments of the present invention, it should be noted that the orientation or positional relationship of terms such as "center", "up", "down", "left", "right", "vertical", "horizontal", "inside" and "outside" is based on the orientation or positional relationship described in the accompanying drawings, which is only for the convenience of describing the present invention and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention.

[0143] The above disclosure is only a preferred embodiment of the present invention, and certainly cannot be used to limit the scope of the rights of the present invention. Ordinary technicians in this field can understand that all or part of the processes of the above embodiment and equivalent changes made in accordance with the claims of the present invention are still within the scope of the present invention.

Claims

1. A method for evaluating particle dispersion of a positive electrode material, characterized in that: The particle dispersion evaluation method includes: The positive electrode material is added to the solvent and dispersed to obtain a dispersion to be tested; Placing the dispersion to be tested in a laser particle size analyzer, measuring the D50 particle size and the D10 particle size of the positive electrode material in the dispersion to be tested n times, and calculating the particle dispersion Y of the positive electrode material using the D50 particle size and the D10 particle size, where n≥2; The calculation method of the particle dispersion Y of the positive electrode material includes: The maximum value of the D50 particle size measured n times is recorded as X1, and the minimum value of the D10 particle size measured n times is recorded as X2, Y=X1 / X2, wherein the larger the dispersion Y is, the more agglomerated the particles of the positive electrode material are.

2. The particle dispersion evaluation method according to claim 1, characterized in that: The calculation method of the particle dispersion Y of the positive electrode material includes: The maximum value of the D50 particle size measured from the second to the nth time is recorded as X1; The minimum value of the D10 particle size measured from the second to the nth time is recorded as X2; Among them, n≥4.

3. The particle dispersion evaluation method according to claim 1, characterized in that: The positive electrode material is added to a solvent and dispersed to obtain the dispersion to be tested, comprising: Adding the positive electrode material to a first solvent and dispersing the material through ultrasonic wave to obtain a first dispersion; freezing and thawing the first dispersion in sequence to obtain a second dispersion; The second dispersion liquid is added into the second solvent to obtain the dispersion liquid to be tested.

4. The particle dispersion evaluation method according to claim 3, characterized in that: The first dispersion liquid is sequentially subjected to freezing and thawing treatments to obtain a second dispersion liquid, comprising: placing the first dispersion in a low-temperature environment for freezing to obtain a frozen material, wherein the temperature of the low-temperature environment is -209°C to -190°C; The frozen material is placed in a thawing environment for thawing to obtain the second dispersion, wherein the temperature of the thawing environment is 20° C. to 40° C.

5. The particle dispersion evaluation method according to claim 4, characterized in that: Placing the frozen material in a thawing environment for thawing to obtain the second dispersion, comprising: Placing the frozen material in a first thawing environment for thawing, wherein the temperature of the first thawing environment is 20° C. to 30° C.; The material after the primary thawing is placed in a second thawing environment for secondary thawing to obtain the second dispersion, wherein the temperature of the second thawing environment is 30° C. to 40° C.

6. The particle dispersion evaluation method according to claim 5, characterized in that: The low temperature environment is a liquid nitrogen environment, and the freezing time is ≤10s; And / or, the primary thawing is performed by thawing in a water bath for 5 to 15 seconds; And / or, the secondary thawing is water bath thawing, the time is 5s to 15s.

7. The particle dispersion evaluation method according to claim 3, characterized in that: The mass ratio of the positive electrode material to the first solvent is 1:(15-30); and / or The volume ratio of the second dispersion liquid to the second solvent is 1:(150-200).

8. The particle dispersion evaluation method according to any one of claims 1 to 7, characterized in that: Application of the particle dispersion evaluation method in the preparation process of positive electrode materials.

9. A positive electrode material, characterized in that After the positive electrode material is subjected to the particle dispersion evaluation method according to any one of claims 1 to 7, the particle dispersion Y of the positive electrode material satisfies the following: 2.5≤Y≤8.

10. A secondary battery, characterized in that: The secondary battery includes the positive electrode material according to claim 9.

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