Positive electrode material, method for preparing the same, and secondary battery

Abstract

This application relates to a positive electrode material, a preparation method thereof, and a secondary battery. The positive electrode material is a nickel-cobalt oxide-based lithium composite oxide. In the XRD pattern of the positive electrode material, the characteristic peak of the (104) crystal plane includes a (104)-Kα1 diffraction peak and a (104)-Kα2 diffraction peak after peak separation. The separation degree between the (104)-Kα1 diffraction peak and the (104)-Kα2 diffraction peak is α, where α is the separation degree between the (104)-Kα1 diffraction peak and the (104)-Kα2 diffraction peak, and α is 0.7≦α≦2.0. The positive electrode material has an appropriate particle size, good particle strength, and a sufficiently large number of internal defects, which are advantageous for reducing polarization phenomena in the positive electrode material and allow secondary batteries based on the positive electrode material to have relatively excellent cycle stability and rate characteristics.

Description

[Technical Field]

[0001] The present application relates to the technical field of positive electrode materials, in particular to a positive electrode material and a preparation method thereof, and a secondary battery. [Background technology]

[0002] Lithium-ion batteries have advantages such as high energy density, excellent safety, long cycle life, and environmental friendliness, which has led to their widespread use in laptops, mobile phones, digital products, etc. The development of high-capacity and high-voltage positive electrode materials will help improve the energy density of lithium-ion batteries and meet the demands of a wider range of markets.

[0003] It is generally believed that increasing the nickel content in ternary positive electrode materials can improve the capacity of the positive electrode material. However, increasing the nickel content intensifies the degree of side reactions between the material and the electrolyte, and as the charge / discharge cycle progresses, the degree of grain expansion or contraction and internal stress of the positive electrode material become more severe, causing cracking and pulverization of the positive electrode material particles, which affects the cycle life and safety of the battery. Furthermore, conventional positive electrode materials have relatively few internal defects, so their relatively large particle size is unfavorable to the diffusion of lithium ions during the charge / discharge process, causing serious electrochemical polarization and concentration polarization phenomena, which affect the electrical properties of the battery. Summary of the Invention [Problem to be solved by the invention]

[0004] It is necessary to provide a positive electrode material that solves at least one of the above problems. There is also a need to provide secondary batteries having the above cathode materials. [Means for solving the problem]

[0005] In a first aspect, the present application provides a positive electrode material, the positive electrode material being a lithium nickel-cobalt oxide-based composite oxide. In the XRD pattern of the positive electrode material, the characteristic peak of the (104) crystal plane includes a (104)-Kα1 diffraction peak and a (104)-Kα2 diffraction peak after peak separation, and the degree of separation between the (104)-Kα1 diffraction peak and the (104)-Kα2 diffraction peak is α, where α is the degree of separation between the (104)-Kα1 diffraction peak and the (104)-Kα2 diffraction peak, and α is 0.7≦α≦2.0.

[0006] In a second aspect, the present application provides a method for preparing a positive electrode material, the method comprising: mixing nickel-cobalt hydroxide and a lithium source to obtain a mixture; and performing primary sintering and grinding of the mixture to obtain a positive electrode material. The primary sintering includes N stepwise heating stages and M isothermal stages, which are performed sequentially, where N is 3 or more and M is 1 or more. The stepwise heating stages include n heating substages, where n is 2 or more, and the heating rate of the nth heating substage is faster than the heating rate of the first heating substage of the same stepwise heating stage, and the heating rate of the nth heating substage is a non-negative value.

[0007] In a third aspect, the present application provides a secondary battery comprising the positive electrode material according to the first aspect above or the positive electrode material prepared by the preparation method according to the second aspect above.

[0008] In the present application, the separation α of the two diffraction peaks of the positive electrode material within a predetermined range is advantageous for controlling the particle size of the positive electrode material, shortening the lithium ion diffusion distance, and reducing the risk of deterioration in the capacity or rate characteristics of the secondary battery. It also enables the positive electrode material to maintain relatively good particle strength, thereby improving the ability to prevent cracking and pulverization of the positive electrode material under high rolling pressure or high voltage window conditions and improving the cycle performance of the positive electrode material. On the other hand, the separation α of the two diffraction peaks of the positive electrode material within a predetermined range is also advantageous for increasing the internal defects of the positive electrode material particles, enriching the lithium ion diffusion paths, and accelerating the lithium ion diffusion rate, thereby further shortening the degree of lithium absorption and desorption between the interior and surface layers of the positive electrode material particles and improving the polarization phenomenon between the surface and interior of the positive electrode material particles. Therefore, the present application controls the particle size of the positive electrode material by controlling the separation α of the two diffraction peaks within a predetermined range, thereby improving the particle strength of the positive electrode material and increasing the internal defects of the material, thereby providing the positive electrode material with relatively excellent cycle stability and rate characteristics.

[0009] In the present application, the method for preparing a positive electrode material includes the stepwise heating stage and constant temperature stage in the primary sintering process. In the stepwise heating stage, a relatively slow heating rate in the first sub-stage is advantageous for maintaining a stable particle growth rate and achieving more uniform particle size, while a relatively fast heating rate in the last sub-stage is advantageous for increasing the particle growth rate and increasing internal defects. Meanwhile, in the constant temperature stage, some of the internal defects of the particles can be repaired as the single-crystal particles grow, and the grain boundary stress inside the particles can be gradually released, which is advantageous for improving the ability to prevent particle cracking and pulverization. Therefore, the method for preparing a positive electrode material according to the present application can control the particle size, particle strength, and internal defects of the positive electrode material, thereby improving the cycle stability and rate performance of the positive electrode material. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a structural schematic diagram of a secondary battery according to the present application during discharge. [Figure 2] FIG. 2 is a structural schematic diagram of the secondary battery according to the present application during charging. [Figure 3] 1 is a temperature graph of primary sintering in a method for preparing a positive electrode material according to Example 1 of the present application. [Figure 4] FIG. 2 is a schematic diagram of peak separation of the characteristic peaks of the (104) crystal plane of the positive electrode material according to Example 1 of the present application. DETAILED DESCRIPTION OF THE INVENTION

[0011] In order to better understand the technical solution of the present application, the present application will be described in more detail below. It should be clarified that the following examples are only some of the examples of the present application, not all of the examples. The following examples are merely examples of the present application, and do not represent or limit the scope of protection of the present application, which is governed by the claims. Based on the examples of the present application, all other examples that a person skilled in the art can derive without creative work fall within the scope of protection of the present application.

[0012] Furthermore, the terms "first" and "second" are for descriptive purposes only and cannot be understood to indicate or imply relative importance or the number of technical features indicated. Thus, a feature qualified by "first" or "second" may explicitly or implicitly include one or more of such features.

[0013] To facilitate understanding of the present invention, certain terms are defined appropriately in this application. Scientific and technical terms used in the present invention have the meanings that are commonly understood by those skilled in the art unless otherwise defined in the present invention.

[0014] The present application provides a positive electrode material, which is a lithium nickel-cobalt oxide-based composite oxide. In the X-ray diffraction (XRD) pattern of the positive electrode material, the characteristic peak of the (104) crystal plane includes a (104)-Kα1 diffraction peak and a (104)-Kα2 diffraction peak after peak separation. The degree of separation between the (104)-Kα1 diffraction peak and the (104)-Kα2 diffraction peak is α, where α is 0.7≦α≦2.0. For example, α may be 0.7, 0.75, 0.8, 0.82, 0.85, 0.88, 0.90, 0.95, 0.98, 1.0, 1.05, 1.08, 1.1, 1.15, 1.2, 1.3, 1.4, 1.58, 1.6, 1.8, 2.0, or any value within a range consisting of any two of the above values.

[0015] In the present application, the separation α of the two diffraction peaks of the positive electrode material within a predetermined range is advantageous for controlling the particle size of the positive electrode material, shortening the lithium ion diffusion distance, increasing the lithium ion diffusion coefficient, and improving the electrochemical polarization and concentration polarization of the positive electrode material. This can improve the phenomenon of the secondary battery's increasing charge internal resistance (DCR) at a high state of charge (SOC) and increasing discharge internal resistance at a low state of charge (SOC). A relatively low internal resistance is beneficial for the secondary battery to maintain a relatively high capacity. Therefore, in the above solution, setting the separation α within a predetermined range can reduce the risk of deterioration in the capacity or rate characteristics of the secondary battery. Furthermore, the positive electrode material maintains relatively good particle strength, thereby improving the positive electrode material's ability to prevent cracking and pulverization under high rolling pressure or high voltage window conditions and improving the cycle performance of the positive electrode material. On the other hand, ensuring that the separation α of the two diffraction peaks of the positive electrode material is within a predetermined range is also beneficial for increasing the internal defects of the positive electrode material particles, enriching the diffusion paths for lithium ions, and accelerating the diffusion rate of lithium ions, thereby making the degree of lithium absorption and desorption in the interior and surface layers of the positive electrode material particles closer, thereby improving the polarization phenomenon between the surface and interior of the positive electrode material particles. Therefore, the positive electrode material of the present application has relatively excellent cycle stability and rate characteristics by controlling the particle size, improving the particle strength, and increasing the internal defects of the material.

[0016] In this field, the separation degree α of the (104)-Kα1 diffraction peak and the (104)-Kα2 diffraction peak after peak separation is related to the peak position Q and the full width at half maximum FWHM of the (104)-Kα1 and (104)-Kα2.

[0017]

number

[0018] Therefore, if the separation α of the two diffraction peaks is too small (for example, less than 0.7), or if the peak positions are essentially constant, it means that the half-width of the two diffraction peaks is too large. According to the Scherrer equation of XRD mechanism, if the particle size of the positive electrode material is too small, it also means that the number of primary particles making up the secondary particles increases, i.e., the number of grain boundaries increases. This phenomenon causes a decrease in particle strength, making the secondary particles more susceptible to destruction and pulverization during electrode sheet rolling and particle charging / discharging, ultimately affecting material properties such as recyclability. This phenomenon also increases the number of internal voids and pore structures within the positive electrode material particles, reducing the material's tap density. Furthermore, a relatively small primary particle size also results in a relatively short lattice order, which makes internal defects such as dislocations and stacking faults more likely to be improved by atomic rearrangement during high-temperature solid-state reactions and expelled from the crystal grains. At the same time, the relatively small primary particle size is generally due to the relatively low temperature of the high-temperature solid-state reactions. At low temperatures, the formation of crystal grains is relatively slow, and it is difficult to incorporate internal defects. Therefore, if the primary particle size of the material is relatively small, the number of internal defects in the crystal grains also decreases, reducing the diffusion paths for lithium ions, and increasing the difference in the degree of lithium absorption and desorption between the surface and interior of the positive electrode material particles, which exacerbates the polarization phenomenon of the positive electrode material and shortens the cycle life of the positive electrode material.

[0019] Conversely, if the separation α between the two diffraction peaks is too large, for example, greater than 2.0, the particle size of the positive electrode material becomes too large, lengthening the diffusion path of lithium ions and hindering the movement of lithium ions within the particles. This exacerbates the concentration polarization and electrochemical polarization phenomena of the positive electrode material, resulting in a corresponding deterioration in the capacity and rate characteristics of the secondary battery. Therefore, by controlling the separation α between the two diffraction peaks within the above range, the positive electrode material can have an appropriate particle size, relatively good particle strength, and a relatively large number of internal defects, thereby achieving relatively excellent cycle characteristics, capacity, and rate characteristics.

[0020] In some embodiments, the lithium nickel cobalt oxide-based composite oxide includes lithium nickel cobalt manganese oxide or lithium nickel cobalt aluminum oxide.

[0021] In some embodiments, the tap density of the cathode material is expressed as Tg / cm 3 , the median diameter D of the positive electrode material 50 If Pμm, then 1≦T-(1.04α-0.25α 2 +0.004P 2 -0.02P)≦1.5.

[0022] In correlation techniques, the cycle stability and rate characteristics of positive electrode materials are generally determined by the tap density T and particle diameter D. 50 However, the influence of the degree of separation α on the electrochemical properties of the cathode material is not taken into consideration, making it difficult to balance the electrical property indicators. For example, to obtain a relatively high energy density, the tap density must be relatively high and the particle diameter D 50 If a cathode material with a relatively low tap density is selected, the rate performance of the cathode material will decrease. 50 If a positive electrode material with a relatively large σ is selected, the energy density and cycle stability of the positive electrode material will decrease.

[0023] This application proposes controlling the degree of separation α within a preset range, and then calculates the tap density T and particle diameter D of the positive electrode material. 50 We further propose three relationships for the degree of separation α and the tap density. If the three relationships are within the above ranges, a relatively high energy density can be obtained by using a relatively high tap density and a 50 When a cathode material with a relatively small D is selected, the degree of separation α can be appropriately increased within the preset range (not exceeding 2.0) to increase the number of internal defects in the cathode material particles, increasing the number of lithium ion diffusion paths, thereby compensating for or improving the rate performance of the cathode material. On the other hand, in order to obtain a relatively high rate performance, a relatively low tap density and D 50When a positive electrode material with a relatively large tap density T and particle diameter D are selected, the degree of separation α can be appropriately controlled to decrease within the preset range (not to fall below 0.7), so that the particle size of the positive electrode material decreases but the particle strength remains relatively good, reducing the risk of cracking or powdering of the positive electrode material at high rolling pressure or high voltage window, thereby compensating for or improving the cycle stability of the positive electrode material. Therefore, in the above solution, the tap density T and particle diameter D of the positive electrode material are 50 By controlling the three relationships of the degree of separation α and the degree of separation α within the above-mentioned predetermined ranges, the energy density, particle strength, and degree of defects of the material can be balanced, resulting in the positive electrode material having relatively excellent cycle stability and rate characteristics.

[0024] In some examples, the gram capacity of the positive electrode material is C mAh / g, and the molar ratio of Ni element to all metal elements other than Li element in the positive electrode material is n Ni Then, 125≦C-(100n Ni -α 2 P)≦135. By controlling the positive electrode material so that the above relationship is satisfied, the particle strength of the positive electrode material can be improved, the polarization phenomenon of the positive electrode material can be reduced, and the positive electrode material can be provided with relatively good gram capacity and cycle stability.

[0025] In correlation technology, the positive electrode material products with different gram capacities are generally characterized by the Ni element content n Ni The resolution α and particle diameter D 50 The influence of Ni on other electrochemical properties of the cathode material has not been fully recognized. For example, to obtain a relatively high energy density, Ni content is relatively high and D 50 When a positive electrode material having a relatively small Ni content is selected, the cycle characteristics and high-temperature gas generation characteristics of the positive electrode material are disadvantageously reduced. Ni , separation degree α, particle size D 50By establishing the above relationship and setting the above predetermined range, various electrochemical properties are balanced. According to the solution of the present application, if a relatively high Ni content is selected to obtain a multi-element material with a relatively high energy density, the separation degree α is set to control the value of the relationship within the predetermined range. 2 and particle diameter D 50 It is necessary to increase the product of the resolution α and particle diameter D 50 (These two can be increased simultaneously or individually.) An increase in the degree of separation α (not exceeding 2.0) means a decrease in the half-width, i.e., an increase in the primary particle size of the material, a decrease in the number of grain boundaries, a decrease in the stress caused by the expansion / contraction of the crystal grains of the multi-component material during charge / discharge cycling, an increase in particle strength, and an assurance of the cycling characteristics of the material. Particle diameter D 50 As the separation degree α and particle diameter D increase, the specific surface area of ​​the material decreases significantly, the degree of side reaction with the electrolyte decreases, and the cycle stability improves, thereby helping to overcome the shortcomings of the related art. 50 One can increase while the other can decrease. 2 It is only necessary to ensure that the P value increases. It is also clear that the degree of separation α has a squared relationship, so while ensuring an improvement in gram capacity, it is even more important to ensure the effects on cycle and storage characteristics. Therefore, in the above solution, the gram capacity C and particle diameter D of the positive electrode material are 50 By controlling the three relationships of the degree of separation α and the degree of separation α within the above-mentioned predetermined ranges, it is possible to achieve a balance between the gram capacity, primary particle size, particle strength, specific surface area, etc. of the material, resulting in a positive electrode material that combines relatively excellent gram capacity and cycle stability.

[0026] In some embodiments, the cycle life of the positive electrode material is 1800≦L−(2245α−5000n) where L is the number of cycles. Ni +100P)≦2400. By controlling the positive electrode material so that the above relationship is satisfied, the particle strength of the positive electrode material can be improved, the polarization phenomenon of the positive electrode material can be reduced, and the positive electrode material can be endowed with relatively excellent cycle stability and gram capacity.

[0027] In correlation technology, cathode material products with different cycle life are generally classified as D 50 The degree of separation α can be obtained by controlling the size and Ni content, but the effect of the degree of separation α on the electrochemical properties of the positive electrode material, such as the cycle life, has not been fully recognized. For example, to obtain a relatively long cycle life, the Ni content is relatively low and D 50 If a positive electrode material having a relatively large Ni content is selected, the capacity and rate characteristics of the positive electrode material will be reduced. Ni , separation degree α, particle size D 50 By relating the above, establishing the above relationship, and setting the above predetermined range, various electrochemical properties are balanced. Similarly, according to the solution of the present application, in order to obtain a relatively long cycle life, the Ni content is relatively low and D 50 If a positive electrode material with a relatively large value is selected, the value of the degree of separation α must be reduced to control the value of the relationship within a preset range. A reduction in the degree of separation α (not below 0.7) means an increase in the half-width, i.e., a reduction in the primary particles of the material, and a shortening of the diffusion path of lithium ions. At the same time, the defects in the material particles are not too few, which ensures the capacity and rate characteristics of the material and helps overcome the shortcomings of related technologies. Therefore, in the above solution, the cycle life L of the positive electrode material, the Ni content n Ni , particle diameter D 50 and the degree of separation α within the above-mentioned predetermined range, it is possible to achieve a balance between the internal defects of the material, the size of the primary particles, etc., and as a result, the positive electrode material is endowed with relatively excellent gram capacity, rate, and cycle stability.

[0028] In some embodiments, the tap density of the cathode material is expressed as Tg / cm 3For example, T may be 1.7≦T≦2.5. For example, T may be 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, or any value within a range consisting of any two of the above values. The tap density of a positive electrode material is one indicator of the material's energy density. If the tap density of a positive electrode material is too high, the positive electrode sheet will be too tight, which will hinder the electrolyte from wetting the positive electrode sheet, inhibiting lithium ion absorption and reducing the battery's rate performance. If the tap density of a positive electrode material is too low, the material's energy density will decrease. Controlling the tap density of a positive electrode material within the above range is advantageous for the positive electrode material to combine high energy density with excellent rate performance.

[0029] In some embodiments, the median diameter D of the positive electrode material 50 When P μm is used, 3≦P≦16. For example, P may be 3, 4, 5, 8, 10, 12, 15, 16, or any value within a range consisting of any two of the above values. Median diameter D 50 indicates the particle size of the material particles at the point where the cumulative particle size distribution percentage reaches 50% by volume. 50 When the D of the positive electrode material is relatively small, the particle size is relatively small, the tap density is relatively low, and the specific surface area is large, which makes it easy for side reactions between the particle surface and the electrolyte to occur seriously, resulting in reduced safety and cycle life. 50 When D is relatively large, the particle size becomes relatively large, the internal stress of the particles increases, and at the same time, the electrochemical polarization and concentration polarization of the lithium ions inside and outside the particles become severe, resulting in a decrease in the capacity and rate characteristics of the positive electrode material. 50 Controlling the value of the positive electrode material within the above range is advantageous for maintaining relatively excellent green density, gram capacity, and cycle life of the positive electrode material.

[0030] In some embodiments, the gram capacity of the positive electrode material, C mAh / g, satisfies 140≦C≦230. For example, C may be 140, 160, 170, 180, 190, 200, 210, 220, 230, or any value within a range consisting of any two of the above values. The gram capacity of the positive electrode material is the discharge capacity of a full battery made with the positive electrode material at 0.33 C / 0.33 C @ 3.0 V to 4.3 V at 25°C. Controlling the gram capacity within the above range is advantageous for the positive electrode material to maintain relatively excellent energy density and cycle life. Preferably, 170≦C≦220.

[0031] In some embodiments, the molar ratio of Ni element to all metal elements other than Li element in the positive electrode material is n Ni Then, 0.33≦n Ni ≦1. For example, n Ni n may be 0.33, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.8, 0.9, 1, or any value in the range consisting of any two of the above numbers. Ni Controlling n within the above range is advantageous in controlling the gram capacity of the positive electrode material within an appropriate range and improving the energy density of the lithium ion battery. Ni This is advantageous in that it can suppress an increase in the cobalt content due to a decrease in the cobalt content, thereby reducing production costs and improving the cost-effectiveness of the unit energy density.

[0032] In some embodiments, when the number of cycle life cycles of the positive electrode material is L, 300 ≤ L ≤ 6000. For example, L can be 300, 500, 600, 800, 1100, 1200, 1500, 1600, 1700, 1800, 2100, 2200, 2400, 2500, 2900, 3000, 3500, 3900, 4000, 4500, 6000, or any value within the range composed of any two of the above numerical values. Note that when the full battery made of the positive electrode material is repeatedly charged and discharged under the conditions of 1C / 1C@3.0V to 4.3V and 25°C until the capacity retention rate reaches 80%, the recorded number of cycles is the cycle life cycle number L. Controlling L within the above range is advantageous for the positive electrode material to have relatively excellent energy density and cycle life. L is the measured value of the cycle life of the corresponding lithium-ion battery, n Ni is greatly affected by, and is also closely related to both the particle size D 50 and the degree of separation α. When the particle size D 50 and the degree of separation α fluctuate and the L value deviates from the above range, the cycle life of the battery becomes too low, affecting practicality. To balance the energy density and practicality of the positive electrode material, preferably, 1000 ≤ L ≤ 4000.

[0033] In some embodiments, the chemical general formula of the positive electrode material is Li a Ni x Co y M1 z M2 k O2, where M1 includes one or two of Mn and Al, M2 includes one or more of Ni, Co, Mn, Na, K, Mg, Ca, Sr, Al, Ti, Y, Zr, W, Nb, Ce, La, and Dy, 0.9 < a ≤ 1.1, 0.33 ≤ x ≤ 1, 0 ≤ y ≤ 0.33, 0 < z < 0.33, 0 ≤ k < 0.1, and x + y + z + k = 1. Note that based on the chemical general formula of the above positive electrode material, the molar ratio of the Ni element in all metal elements other than the Li element is n Ni = N Ni / N Ni + N Co + N M1 + N M2 Here, NNi is the molar mass of Ni element, and N Co is the molar mass of Co element, N M1 is the molar mass of the M1 element, and N M2 is the molar mass of the M2 element. For example, a may be 0.9, 0.95, 0.98, 1.0, 1.01, 1.02, 1.03, 1.05, 1.1, or any value within the range consisting of any two of the above numbers. x may be 0.33, 0.45, 0.5, 0.55, 0.6, 0.7, 0.8, 0.9, 1, or any value within the range consisting of any two of the above numbers. y may be 0, 0.02, 0.05, 0.07, 0.08, 0.1, 0.15, 0.2, 0.25, 0.28, 0.30, 0.33, or any value within the range consisting of any two of the above numbers. z may be 0.01, 0.06, 0.1, 0.15, 0.2, 0.25, 0.3, or any value within the range consisting of any two of the above numbers. k may be 0, 0.001, 0.005, 0.01, 0.02, 0.05, 0.099, or any value within a range consisting of any two of the above values. The content of each element in the positive electrode material can be measured using known instruments such as ICP and ICP-MS that perform qualitative and / or quantitative analysis of each element.

[0034] When M2 includes any of the above elements, these elements are doped into the surface lattice of the positive electrode material, changing the lattice constant of the positive electrode material or the valence state of the elements in the material itself, reducing the mixed arrangement of cations, improving the electronic conductivity and ionic conductivity of the material, improving the stability of the material structure, and reducing the risk of structure collapse, which is advantageous for improving the cycle stability of the positive electrode material.

[0035] In some examples, scanning electron microscope images obtained by analyzing a cathode material using a scanning electron microscope reveal that the cathode material appears to be a single-crystalline cathode material. Single-crystalline cathode materials have a more stable structure, a more uniform distribution of bulk components, and superior particle strength than polycrystalline cathode materials. This is advantageous for providing better cycling stability and safety to lithium-ion batteries. It can also reduce particle cracking during the electrode sheet pressing process, increasing the green density and volumetric energy density of the electrode sheet. The difference between single-crystalline cathode materials and polycrystalline cathode materials (i.e., polycrystalline secondary particles) is that the smallest particle in polycrystalline secondary particles is a secondary particle formed by the aggregation of primary particles. In the case of single-crystalline cathode materials, the smallest particle is typically a single primary particle at the micron level. In addition to EBSD testing, characterization methods such as scanning electron microscopy (SEM) can also be used to determine whether a resulting cathode product is a single-crystalline material. For example, in the case of single-crystal cathode materials, the shape of the single-crystal particles can be characterized by SEM, which shows that the appearance of single-crystal particles generally takes the form of regular or irregular polyhedrons without significant particle aggregation. The orientation of single-crystal cathode materials can also be characterized by EBSD, which shows that the orientation within the grains is uniform, and that grains with the same orientation are single crystals. It is important to note that the "single-crystal cathode materials" known to those skilled in the art are not "single crystals" in the strict sense. In crystallography, an ideal single crystal is a crystal with exactly the same alignment and orientation. However, ideal single crystals are extremely rare due to impurities, distortion, and crystal defects, and are difficult to produce in a laboratory. Therefore, most single-crystal cathode materials known in the art are actually "single-crystal-like" cathode materials, which merely exhibit large grain sizes similar to those of single crystals, but are different from polycrystals, which are composed of many small primary particles.

[0036] In a second aspect, the present application provides a method for preparing a cathode material, the method comprising: mixing a precursor and a lithium source to obtain a mixture; and performing primary sintering and grinding of the mixture to obtain a cathode material. The primary sintering comprises N stepwise heating stages and M isothermal stages, which are performed in sequence, where N is 3 or more and M is 1 or more, and each stepwise heating stage comprises n heating substages, where n is 2 or more and, if n>1, the heating rate of the nth heating substage is faster than the heating rate of the first heating substage of the same stepwise heating stage, and the heating rate of the nth heating substage is a non-negative value.

[0037] In this application, research has found that by setting multiple stepwise heating stages and setting a difference between the heating rate of the first sub-stage and the heating rate of the final sub-stage, the separation degree α of the positive electrode material can be well controlled within a predetermined range. It can be understood that when the heating rate is relatively fast, the particle growth rate changes relatively quickly, the particle size becomes non-uniform, and the number of internal defects in the particles increases. When the heating rate is relatively slow, the particles tend to grow stably, the particle size becomes uniform, and some of the internal defects of the particles are repaired as the single crystal grows. By alternating between high and low heating rates, the internal stress of the crystal grains and the number of defects can be balanced, so that the positive electrode material will not have insufficient particle strength or easy cracking due to excessive internal stress, and will not have insufficient internal defects, resulting in Li + There is no reduction in diffusion paths or deterioration of polarization.

[0038] Therefore, in the present application, the method for preparing a positive electrode material includes the stepwise heating stage and the constant temperature stage in the primary sintering process. In the stepwise heating stage, a relatively slow heating rate in the first sub-stage is advantageous for maintaining a stable particle growth rate and achieving more uniform particle size, while a relatively fast heating rate in the last sub-stage is advantageous for increasing the particle growth rate and increasing internal defects in the particles. Meanwhile, in the constant temperature stage, some of the internal defects in the particles can be repaired as the single-crystal particles grow, and the grain boundary stress inside the particles is gradually released, which is advantageous for improving the ability to prevent particle cracking and pulverization. Therefore, the method for preparing a positive electrode material according to the present application can control the particle size, particle strength, and internal defects of the positive electrode material, thereby improving the cycle stability and rate performance of the positive electrode material.

[0039] In some embodiments, the maximum temperature of the stepwise temperature ramp is between 700° C. and 1000° C. For example, the maximum temperature of the stepwise temperature ramp can be 700° C., 800° C., 850° C., 900° C., 950° C., 980° C., 1000° C., or any value within a range consisting of any two of the foregoing values.

[0040] In some embodiments, the time for the stepwise temperature ramp-up is between 6 hours and 14 hours, for example, the time for the stepwise temperature ramp-up may be 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 12 hours, 14 hours, or any value within a range consisting of any two of the above values.

[0041] In some embodiments, the sintering temperature of the isothermal stage is (1150-500n Ni )℃~(1150-280n Ni ) °C, and n Ni represents the molar ratio of Ni element to all metal elements other than Li element in the positive electrode material, and 0.33≦n Ni Based on the general chemical formula of the positive electrode material, the molar ratio of Ni to all metal elements other than Li is n Ni =N Ni / NNi +N Co +N M1 +N M2 where N Ni is the molar mass of Ni element, and N Co is the molar mass of Co element, and N M1 is the molar mass of the M1 element, and N M2 is the molar mass of the M2 element.

[0042] The isothermal phase may be a period in which the temperature remains constant or may consist of multiple temperature steps that alternate between temperature steps. In some embodiments, the isothermal phase may include alternating temperature reduction and temperature increase sub-steps, and the temperature of the temperature reduction sub-step of a isothermal phase is lower than the temperature of the adjacent temperature increase sub-step.

[0043] In some embodiments, the mixture further comprises a dopant, the dopant element comprising one or more of Ni, Co, Mn, Na, K, Mg, Ca, Sr, Al, Ti, Y, Zr, W, Nb, Ce, La, and Dy. For example, the dopant may be a salt or oxide of any of the above elements.

[0044] In some embodiments, the above preparation method further includes, after pulverization, mixing the product of the primary sintering with a coating agent and performing secondary sintering to obtain a positive electrode material. The coating agent includes one or more elements of Ni, Co, Mn, Na, K, Mg, Ca, Sr, Al, Ti, Y, Zr, W, Nb, Ce, La, and Dy. For example, the coating agent may be a salt or oxide of any of the above elements. Alternatively, preferably, the coating agent includes a Co compound or a W compound.

[0045] The preparation method of the present application will be described in detail below with reference to examples. In some embodiments, the general chemical formula of the cathode material precursor is: a1 Co b1 M1 c1 O dwhere 0.33≦a1≦1, 0≦b1≦0.33, 0≦c1<0.33, 4 / 3≦d≦3 / 2, and a1+b1+c1=1, and M1 includes one or both of Mn and Al.

[0046] In some embodiments, the general chemical formula of the cathode material precursor is: a2 Co b2 M1 c2 (OH)2, where 0.33≦a2≦1, 0≦b2≦0.33, 0≦c2<0.33, and a2+b2+c2=1, and M1 contains one or both of Mn and Al.

[0047] In some embodiments, the median diameter of the positive electrode material precursor is 3 μm to 18 μm, for example, 3 μm, 4 μm, 5 μm, 7 μm, 8 μm, 10 μm, 12 μm, 15 μm, 18 μm, or any value within a range consisting of any two of the above values.

[0048] In some embodiments, the lithium source comprises at least one of lithium hydroxide, lithium carbonate, lithium oxide, lithium nitrate, lithium acetate, lithium sulfate, and lithium oxalate.

[0049] In some embodiments, the amounts of the lithium source and the positive electrode material precursor added satisfy the ratio of the molar amount of Li to the total molar amount of all metals in the positive electrode material precursor of (0.9-1.1):1. For example, the ratio may be 0.9:1, 0.95:1, 0.98:1, 1:1, 1.02:1, 1.05:1, 1.1:1, or any value within a range consisting of any two of the above values. Within this range, the degree of mixed arrangement of Li / Ni cations can be reduced, and excessive lithium remaining on the surface of the fired product can be prevented from affecting processability and safety.

[0050] In some embodiments, the amount of dopant added is controlled so that the ratio of the molar amount of M2 to the total molar amount of transition metals in the prepared positive electrode material is (0-0.1): 1. For example, the ratio may be 0.01:1, 0.02:1, 0.05:1, 0.07:1, 0.08:1, 0.09:1, 0.1:1, or any value within a range consisting of any two of the above values.

[0051] In some embodiments, the mixing includes solid-state mixing, and the solid-state mixing method may be dry grinding or ball milling, etc., and is not limited thereto, as long as the components are mixed uniformly.

[0052] In some embodiments, the mixing device may be one or more of a ball mill, a three-dimensional mixer, a high speed mixer, a coater, and a VC mixer.

[0053] In some embodiments, the primary sintering is performed in an oxygen-containing atmosphere, and the volume content of oxygen gas in the oxygen-containing atmosphere is 20% or more. For example, the volume content of oxygen gas in the oxygen-containing atmosphere may be 20%, 30%, 40%, 50%, 60%, 70%, 80%, 100%, or any value within a range consisting of any two of the above values. To ensure sufficient reaction, the volume content of oxygen gas in the oxygen-containing atmosphere is preferably 90% or more.

[0054] In some embodiments, the comminuting method comprises one or more of a double roll, a plowshare mixer, a plowshare crusher, a jet mill, and a mechanical mill.

[0055] In some embodiments, the mass ratio of the coating material to the primary sintered product is (0-0.1):1. For example, the mass ratio may be 0:1, 0.002:1, 0.005:1, 0.01:1, 0.03:1, 0.05:1, 0.1:1, or any value within a range consisting of any two of the foregoing values.

[0056] In some embodiments, the secondary sintering is performed in an oxygen-containing atmosphere, and the volume content of oxygen gas in the oxygen-containing atmosphere is 20% or more. For example, the volume content of oxygen gas in the oxygen-containing atmosphere may be 20%, 30%, 40%, 50%, 60%, 70%, 80%, 100%, or any value within a range consisting of any two of the foregoing values.

[0057] In some embodiments, the secondary sintering temperature is between 300° C. and 900° C. For example, the secondary sintering temperature may be 300° C., 380° C., 450° C., 550° C., 600° C., 700° C., 750° C., 800° C., 840° C., 900° C., or any value within a range consisting of any two of the foregoing values.

[0058] In some embodiments, the incubation time for the secondary sintering is 6 hours to 24 hours, for example, 6 hours, 8 hours, 10 hours, 12 hours, 15 hours, 18 hours, 24 hours, or any value within a range consisting of any two of the above values.

[0059] In some embodiments, the preparation method also includes cooling, shaping, and sieving the secondary sintered product, where shaping includes one or more of crushing, grinding, ball milling, and jet milling.

[0060] In some embodiments, the mesh of the sieve used for sieving is 300 mesh to 400 mesh.

[0061] In a third aspect, the present application provides a secondary battery comprising the above-described positive electrode material or a positive electrode material prepared by the above-described preparation method.

[0062] The secondary battery includes a case, an electrode assembly, and an electrolyte. The electrode assembly and the electrolyte are disposed in the case. The case may be a sealed packaging bag with a packaging film (e.g., aluminum-plastic film), and the secondary battery is, for example, a soft-pack battery. In other embodiments, the secondary battery may be a steel-cased battery, an aluminum-cased battery, or the like.

[0063] 1 and 2, the electrode assembly of the secondary battery includes a positive electrode sheet 110, a negative electrode sheet 120, and a separator 130, with the separator 130 being disposed between the positive electrode sheet 110 and the negative electrode sheet 120. The electrode assembly may have a layered structure in which the positive electrode sheet 110, the separator 130, the negative electrode sheet 120, and the separator 130 are alternately stacked in this order. In some other embodiments, the electrode assembly may have a wound structure in which the positive electrode sheet 110, the separator 130, the negative electrode sheet 120, and the separator 130 are stacked in this order and then wound.

[0064] Positive electrode sheet The positive electrode sheet 110 includes a positive electrode current collector 111 and a positive electrode active material layer 112 provided on at least one surface of the positive electrode current collector 111. The positive electrode current collector 111 may be made of aluminum foil, nickel foil, or any composite current collector disclosed in the prior art, including, but not limited to, the aforementioned current collector made by combining a conductive foil with a polymer substrate. The positive electrode active material layer 112 includes the above-described positive electrode material. The positive electrode active material layer 112 further includes a binder that can adhere the positive electrode active material particles to facilitate the formation of a film layer and can also enhance the bonding strength between the positive electrode active material layer and the positive electrode current collector 111. In some embodiments, the binder includes at least one of, but is not limited to, polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, poly(1,1-vinylidene difluoride), polyethylene, polypropylene, styrene butadiene rubber, acrylated styrene butadiene rubber, epoxy resin, or nylon.

[0065] The positive electrode active material layer 112 may further include a conductive material, and the conductive material includes, but is not limited to, a carbon-based material, a conductive polymer, or any combination thereof. In some embodiments, the carbon-based material includes, but is not limited to, natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, or any combination thereof. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0066] Negative electrode sheet The negative electrode sheet 120 includes a negative electrode current collector 121 and a negative electrode active material layer 122 provided on at least one surface of the negative electrode current collector 121. The negative electrode current collector 121 may be made of at least one of a copper foil, a nickel foil, a stainless steel foil, a titanium foil, or a carbon-based current collector, or may be a composite current collector disclosed in any of the prior arts. For example, it may be a current collector in which the aforementioned conductive foil and a polymer substrate are bonded, but is not limited thereto. The negative electrode active material layer 122 includes a negative electrode material, and the negative electrode material includes, but is not limited to, at least one of graphite, mesophase microcarbon beads (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, SiOx (0.5 < x < 1.6), Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, lithium titanate having a spinel structure, lithiated TiO2-Li4Ti5O 12 , Li-Al alloy, and metallic lithium.

[0067] The negative electrode active material layer 122 further includes a binder that can adhere the negative electrode active material particles to facilitate the formation of a film layer and can also enhance the binding strength between the negative electrode active material layer and the negative electrode current collector 121. In some embodiments, the binder includes, but is not limited to, polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, poly(1,1-vinylidene difluoride), polyethylene, polypropylene, styrene butadiene rubber, acrylated styrene butadiene rubber, epoxy resin, or nylon.

[0068] The negative electrode active material layer 122 may further include a conductive material, including, but not limited to, a carbon-based material, a metal-based material, a conductive polymer, or any combination thereof. In some embodiments, the carbon-based material may include, but is not limited to, natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, or a combination thereof. In some embodiments, the metal-based material may include, but is not limited to, a metal powder or metal fiber, such as, but not limited to, copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0069] Separator Separator 130 includes a membrane layer having a porous structure, and its material includes, but is not limited to, at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, and aramid. For example, separator 130 may be a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene composite porous membrane.

[0070] electrolyte The electrolyte serves to conduct ions between the positive electrode sheet 110 and the negative electrode sheet 120. The electrolyte may be in one or more of a gel, solid, and liquid state. In some embodiments, an electrolyte solution is used as the electrolyte. The electrolyte solution serves to conduct active ions between the positive electrode sheet 110 and the negative electrode sheet 120. In some embodiments, the electrolyte solution includes a lithium salt and an organic solvent. Lithium salts include lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), lithium perchlorate (LiClO4), lithium tetraphenylborate (LiB(C6H5)4), lithium methanesulfonate (LiCH3SO3), lithium bisfluorosulfonylimide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiCF3SO3), and lithium bis(trifluoromethanesulfonyl)imide (LiN(SO2C The organic solvent may be one or more selected from, but not limited to, lithium fluoride (LiF3), tris(trifluoromethanesulfonyl)methyllithium (LiC(SO2CF3)3), lithium bis(oxalatoborate) (LiBOB), and lithium difluorophosphate (LiPO2F2). For example, LiPF6 is selected as the lithium salt because it provides high ionic conductivity and can improve cycle characteristics. The organic solvent may be a carbonate compound, a carboxylic acid ester compound, an ether compound, a nitrile compound, another organic solvent, or a combination thereof.Examples of carbonate compounds include diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), ethyl methyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinyl ethylene carbonate (VEC), fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, The fluoroethylene carbonate may be, but is not limited to, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, trifluoromethylethylene carbonate, or a combination thereof.

[0071] Referring to FIG. 1, when the battery is in a discharged state, lithium ions 140 are released from the lattice of the negative electrode material, pass through the separator 130 via the electrolyte or electrolytic solution, and are absorbed into the lattice of the positive electrode material. Electrons are generated on the negative electrode side and move to the positive electrode side via an external circuit. The reverse flow of electrons forms an electric current, which can supply power to an electrical device. Referring to FIG. 2, when the battery is charged by adding an external circuit, lithium ions 140 are released from the lattice of the positive electrode material, pass through the separator 130 via the electrolyte or electrolytic solution, move to the negative electrode side, and are embedded in the lattice of the negative electrode material. As the lithium ions 140 shuttle between the positive and negative electrodes, the battery can achieve multiple discharge and charge cycles.

[0072] The present application will be described in detail below through specific examples and comparative examples. Those skilled in the art should understand that the preparation methods described in this application are merely examples, and any other suitable preparation methods are also within the scope of this application.

[0073] Example 1 The preparation method of the positive electrode material includes the following steps: S1:D 50 Precursor Ni with a diameter of 4.0 μm 0.6 Co 0.1 Mn 0.3 (OH)2, Li2CO3, and ZrO2 were mixed uniformly in a ratio of 1:1.05:0.0025 to obtain a mixture. S2: The mixture obtained in S1 was subjected to primary sintering in a dry air atmosphere according to the following temperature settings: The temperature settings include temperatures run in sequence as follows: Stepwise heating stage 1: 0℃→0.2h, 50℃→0.5h, 250℃; Stepwise heating stage 2: 250℃→0.5h, 270℃→0.5h, 400℃; Stepwise heating stage 3: 400℃→0.5h, 420℃→0.5h, 550℃; Stepwise heating stage 4: 550℃→0.5h, 570℃→0.5h, 700℃; Stepwise heating stage 5: 700℃→0.5h, 720℃→0.5h, 850℃; Stepwise heating stage 6: 850℃→0.5h, 830℃→0.8h, 920℃; Stepwise heating stage 7: 920 °C → 1 h, 900 °C → 1 h, 970 °C; and Constant temperature stage: 970℃→1h, 950℃→1h, 970℃→1h, 950℃→1h, 970℃→1h, 950℃→1h, 970℃→1h, 950℃→1h, 970℃→1h, 950℃→1h, 970℃. S3: The material sintered in S2 was crushed and sieved to obtain the substrate material. S4: The substrate material obtained in S3, Al2O3, and Co(OH)2 were uniformly mixed in a weight ratio of 1:0.002:0.04, fired at 850°C for 8 hours, cooled, crushed, sieved, and demagnetized to obtain the positive electrode material.

[0074] 3 shows a graph of the temperature change of the primary sintering set in the method for preparing a positive electrode material according to Example 1 of the present application. As can be understood by referring to FIG. 3, the above-mentioned "Stepwise temperature increase stage 1: 0°C → 0.2 h, 50°C → 0.5 h, 250°C" means that in the current stepwise temperature increase stage, the temperature is increased from 0°C to 50°C within 0.2 h, i.e., the temperature increase rate is about 250°C / h, and then the temperature is increased from 50°C to 250°C within 0.5 h, i.e., the temperature increase rate is about 400°C / h. The above "Stepwise heating stage 6: 850°C → 0.5 hours, 830°C → 0.8 hours, 920°C" means that in the current stepwise heating stage, the temperature is lowered from 850°C to 830°C within 0.5 hours, i.e., the heating rate is a negative value, and then the temperature is raised from 830°C to 920°C within 0.8 hours, i.e., the heating rate is a non-negative value. By analogy, the specific heating or cooling methods of temperature setting described in this application can be understood.

[0075] Example 2 The following points are different from Example 1. In S2, the temperature setting for the primary sintering is as follows: Stepwise temperature increase stage 6 was adjusted to 850°C → 1 hour, 830°C → 0.8 hours, and 920°C. Stepwise temperature increase stage 7 was adjusted to 920°C → 0.7h, 900°C → 1h, 970°C, The constant temperature stage is adjusted to 970℃ → 8h, 970℃.

[0076] Example 3 Compared with Example 1, the following points are different: In S2, the temperature setting for the primary sintering includes temperatures that are executed in sequence as follows: Stepwise heating stage 1: 0℃→0.2h, 50℃→0.5h, 250℃; Stepwise heating stage 2: 250℃→0.5h, 270℃→0.3h, 400℃; Stepwise heating stage 3: 400℃→0.6h, 420℃→0.4h, 550℃; Stepwise heating stage 4: 550℃→0.5h, 570℃→0.4h, 700℃; Stepwise heating stage 5: 700℃→0.5h, 720℃→0.3h, 850℃; Stepwise heating stage 6: 850 °C → 0.5 h, 870 °C → 0.5 h, 970 °C; and Constant temperature stage: 970℃→1h, 950℃→1h, 970℃→1h, 950℃→1h, 970℃→1h, 950℃→1h, 970℃→1h, 950℃→1h, 970℃→1h, 950℃→1h, 970℃.

[0077] Example 4 The following points are different from Example 1. In S2, the temperature setting for the primary sintering is as follows: Stepwise temperature increase stage 6 was adjusted to 850°C → 1 hour, 830°C → 0.8 hours, and 920°C. The stepwise temperature increase stage 7 was adjusted to 920°C → 1 hour, 900°C → 1 hour, and 950°C. The isothermal stage is adjusted as follows: 950℃→1h, 930℃→1h, 950℃→1h, 930℃→1h, 950℃→1h, 930℃→1h, 950℃→1h, 930℃→1h, 950℃→1h, 930℃→1h, 950℃.

[0078] Example 5 Compared with Example 4, the following points are different: In S1, the precursor D 50 is adjusted to 10 μm.

[0079] Example 6 Compared with Example 4, the following points are different: In S1, the precursor D 50 is adjusted to 16 μm.

[0080] Example 7 Compared with Example 1, the following points are different: In S1, the precursor is D 50 Ni with a thickness of 4.0 μm 0.5 Co 0.2 Mn 0.3 (OH)2, and in S2, the temperature setting for the primary sintering was Stepwise temperature increase stage 6 was adjusted to 850°C → 1 hour, 830°C → 0.8 hours, and 920°C. Stepwise temperature increase stage 7 was adjusted to 920°C → 0.7h, 900°C → 1h, 990°C, The isothermal stage is adjusted as follows: 990℃→1h, 970℃→1h, 990℃→1h, 970℃→1h, 990℃→1h, 970℃→1h, 990℃→1h, 970℃→1h, 990℃→1h, 990℃.

[0081] Example 8 Compared with Example 1, the following points are different: In S1, the precursor is D 50 Ni with a thickness of 4.0 μm 0.7 Co 0.1 Mn 0.2 (OH)2 and Li2CO3 were changed to LiOH·H2O. In S2, the temperature setting for the primary sintering was Stepwise temperature increase stage 6 was adjusted to 850°C → 1 hour, 830°C → 0.8 hours, and 920°C. Stepwise temperature increase stage 7 was adjusted to 920°C → 0.7h, 900°C → 1h, 930°C, The isothermal stage is adjusted as follows: 930℃→1h, 910℃→1h, 930℃→1h, 910℃→1h, 930℃→1h, 910℃→1h, 930℃→1h, 910℃→1h, 930℃→1h, 930℃.

[0082] Example 9 Compared with Example 1, the following points are different: In S1, the precursor is D 50 Ni with a thickness of 4.0 μm 0.8 Co 0.1 Mn 0.1 (OH)2 and Li2CO3 were changed to LiOH·H2O. In S2, the temperature setting for the primary sintering was Stepwise temperature increase stage 6 was adjusted to 850°C → 1 hour, 830°C → 0.8 hours, and 860°C. Stepwise temperature increase stage 7 was adjusted to 860°C → 0.7h, 840°C → 1h, 870°C, The isothermal stage is adjusted as follows: 870℃→1h, 890℃→1h, 870℃→1h, 890℃→1h, 870℃→1h, 890℃→1h, 870℃→1h, 890℃→1h, 870℃→1h, 890℃→1h, 870℃→1h, 890℃.

[0083] Example 10 Compared with Example 1, the following points are different: In S1, the precursor is D 50 Ni with a thickness of 4.0 μm0.9 Co 0.05 Mn 0.05 (OH)2 and Li2CO3 were changed to LiOH·H2O. In S2, in the temperature setting of the primary sintering, stepwise temperature increase stage 6 and stepwise temperature increase stage 7 were omitted, and the constant temperature stages were adjusted to 850°C → 1 h, 830°C → 1 h, 850°C → 1 h, 830°C → 1 h, 850°C → 1 h, 830°C → 1 h, 850°C → 1 h, 830°C → 1 h, 850°C → 1 h, 830°C → 1 h, 850°C → 1 h, 830°C.

[0084] Example 11 Compared with Example 1, the following points are different: In S1, the precursor is D 50 Ni with a thickness of 4.0 μm 0.9 Co 0.05 Mn 0.05 (OH)2 and Li2CO3 were changed to LiOH·H2O. In S2, in the temperature setting of the primary sintering, stepwise temperature increase stage 6 and stepwise temperature increase stage 7 were omitted, and the constant temperature stages were adjusted to 830°C → 1 h, 850°C → 1 h, 830°C → 1 h, 850°C → 1 h, 830°C → 1 h, 850°C → 1 h, 830°C → 1 h, 850°C → 1 h, 830°C → 1 h, 850°C → 1 h, 830°C → 1 h, 850°C.

[0085] Comparative Example 1 Compared with Example 1, the following points were different: In S2, the temperature setting for the primary sintering was adjusted to 0°C → 8 h, 950°C → 8 h, 950°C, i.e., the primary sintering was performed in a constant temperature rise stage and a constant temperature stage.

[0086] Comparative Example 2 Compared with Example 1, the following points were different: In S2, the temperature setting for the primary sintering was adjusted to 0°C → 2 h, 960°C → 8 h, 960°C, i.e., the primary sintering was performed in a constant temperature rise stage and a constant temperature stage.

[0087] Comparative Example 3 The following points were different from Example 1: In S2, the temperature settings for primary sintering were adjusted as follows: 0°C → 4 hours, 850°C → 3 hours, 850°C → 1 hour, 950°C → 8 hours, 950°C.

[0088] Comparative Example 4 Compared with Example 5, the following differences exist: In S2, the temperature setting for the primary sintering was adjusted to 0°C → 8 h, 950°C → 8 h, 950°C, i.e., the primary sintering was performed in a constant temperature rise stage and a constant temperature stage.

[0089] Comparative Example 5 Compared with Example 6, the following points were different: In S2, the temperature setting for the primary sintering was adjusted to 0°C → 8 h, 950°C → 8 h, 950°C, i.e., the primary sintering was performed in a constant temperature rise stage and a constant temperature stage.

[0090] In the present application, lithium ion batteries were also prepared using the positive electrode materials of Examples 1 to 11 and Comparative Examples 1 to 5, respectively, and the preparation method included the following steps.

[0091] S1: Positive electrode material, polyvinylidene fluoride (PVDF) (binder), acetylene black (conductive agent), and polyaniline (PANI) (conductive liquid) were dissolved in the solvent N-methylpyrrolidone (NMP) in a weight ratio of 96:2:1.5:0.5, and the mixture was thoroughly stirred and homogeneously mixed to obtain a positive electrode slurry. The obtained positive electrode slurry was uniformly applied to a positive electrode current collector with primer slurry, and after drying, cold pressing, and slitting, a positive electrode sheet was obtained. Measurements showed that the surface density of the obtained positive electrode sheet was approximately 350 g / m 2 , the green density is approximately 3.5 g / cm 3 It was.

[0092] S2: Graphite (active material), styrene butadiene rubber (SBR) (binder), and acetylene black (conductive agent) were dissolved in deionized water as a solvent in a weight ratio of 96:2:2 and mixed uniformly to prepare a negative electrode slurry. The resulting negative electrode slurry was applied to copper foil, dried, and then cold-pressed and slit to obtain a negative electrode sheet. Measurements showed that the surface density of the resulting negative electrode sheet was approximately 210 g / m. 2 , the green density is approximately 1.6 g / cm 3 It was.

[0093] S3: Polyethylene film is used as the separator, and the surface of the polyethylene film is coated with polyvinylidene fluoride and aluminum oxide to improve adhesion and heat resistance.

[0094] S4: Ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed in a 1:1:1 volume ratio, and LiPF6 was added and dissolved to obtain a lithium salt concentration of 1.1 mol / L. The electrolyte further contained 2 wt% vinylene carbonate and 1 wt% polysulfone as additives.

[0095] S5: The ratio of the negative electrode capacity to the positive electrode discharge capacity of the battery (N / P) was controlled to 1.16, and the amount of electrolyte injected was 3.2 g / Ah. The positive electrode sheet, separator, negative electrode sheet, and separator were stacked in this order so that the separator was interposed between the positive electrode sheet and the negative electrode sheet to act as an insulator, and then rolled up to form a bare cell. A tab was welded to the bare cell, and the bare cell was placed in an aluminum case and baked at 80°C to remove moisture. The electrolyte was then injected and sealed to form an uncharged battery. The uncharged battery was then subjected to further processes such as standing, hot and cold pressing, chemical formation, shaping, and capacity testing to obtain a lithium-ion battery.

[0096] Testing characteristics 1. Battery capacity test method: At 25°C, the battery was charged to 4.3V in CCCV mode (charging current 0.33C, constant voltage cutoff current 0.01C), allowed to stand for 10 minutes, and then discharged to 3.0V in CC mode (discharging current 0.33C). The discharge capacity (CmAh / g) was calculated from the discharge capacity.

[0097] 2. Battery rate test method: After the battery capacity test is completed, leave it at 25°C for 10 minutes, charge it to 4.3V in CCCV mode (charge current 0.33C, constant voltage cut-off current 0.01C), leave it for 10 minutes, discharge it to 3.0V in CC mode (discharge current 2.0C), and measure the discharge capacity. R Recorded as mAh / g and calculated as rate characteristic R = CR / C×100% was obtained.

[0098] 3. Battery cycle life test method: After formation and capacity grading, the battery was charged to 4.3 V in CCCV mode at 25°C (charging current 1.0 C, constant voltage cutoff current 0.1 C), allowed to stand for 10 minutes, and then discharged to 3.0 V in CC mode (discharging current 1.0 C) to obtain an initial gram capacity C1mAh / g. The battery was then allowed to stand for 10 minutes. The CCCV mode charging and CC mode discharging were then repeated until the discharge gram capacity fell below 0.8*C1mAh / g. The cumulative number of discharges was recorded as the cycle life L.

[0099] 4. Test method for tap density of positive electrode material: GB / T5162-1985 Metal powder - Tap density assay, test results are Tg cm -3 was recorded as.

[0100] 5. Particle diameter D of the positive electrode material 50 Test method: Refer to GB / T19077-2016. Measurements can be easily performed using a laser particle size analyzer such as the Mastersizer 3000 laser particle size analyzer manufactured by Malvern Instruments Ltd., UK. Test results are recorded as Pμm.

[0101] 6. Cathode materials Ni Test method: Refer to GB / T24194-2009. It can be easily measured using an inductively coupled plasma optical emission spectrometer (ICP-OES) such as the Agilent 5110 ICP-OES. The molar mass of Ni element measured by ICP is then converted to N. Ni , the molar mass of Co element is N Co , the molar mass of M1 element is N M1 , M2 is the molar mass of the element N M2 The molar ratio of Ni element to all metal elements other than Li element is nNi=N Ni / N Ni +N Co +N M1 +N M2 is.

[0102] 7. XRD test of cathode material: XRD radiation was used to measure the cathode material, and a characteristic peak corresponding to the (104) crystal plane of the cathode material was obtained. This characteristic peak was then separated to obtain two diffraction peaks, (104)-Kα1 and (104)-Kα2. The peak positions of the (104)-Kα1 and (104)-Kα2 diffraction peaks are Q1 and Q2, respectively, and the half-widths are FWHM1 and FWHM2, respectively. The separation α can be calculated using the following formula:

[0103]

number

[0104] The XRD test conditions were as follows: Cu target, voltage 40 kV, current 40 mA, scan range 43.5°-45°, step size 0.02°, scan rate 2° / min. The peak separation method was as follows: In Origin 8.5 software, select the XRD test data → plot in Line mode → baseline removal (Peaks and baseline in the Analysis window → PeakAnalyzer → Opening Dialog → select Manual in the Recalculate option, select FitPeaks(Pro) in the Goal option → click Next → select Constant in the BaselineMode option, set Constant=Minimum → click Next) → peak separation (uncheck AutoSubtractBaseline and AutoRescale options, and set FixBaselineParameter Check the meters option and click Next → uncheck EnableAutoFind, check the SmoothingWindowSize option in PeakFindingSettings to Auto, select Positive in the Direction option, select LocalMaximum as the Method, set the LocalPoints parameter to 2, select ByNumber as the PeakFiltration Method, change NumberofPeaks to 2 → click Find → click Next) → perform fitting correction (select NoWeightin as the Weight Method, select Max. NumberofIterations as 50 in FitControl, select Tolerance as 0.05 → click Fit) → click Finish. Once completed, you can get the Q1, Q2, FWHM1, and FWHM2 values ​​from the peak separation report.

[0105] Taking Example 1 as an example and referring to FIG. 4, the characteristic peak of the (104) crystal plane of the positive electrode material has an asymmetric structure, and the two diffraction peaks after peak separation have a symmetric structure, that is, the diffraction peak curve follows a normal distribution.

[0106] In this application, T-(1.04α-0.25α 2 +0.004P 2 -0.02P) value E, C-(100n Ni -α 2 P) value of H, L-(2245α-5000n Ni +100P) is defined as F. The results of the above tests or calculations are shown in Table 1.

[0107] Table 1. Test results of the parameters of the positive electrode materials and the lithium ion battery characteristics of Examples 1 to 11 and Comparative Examples 1 to 5 of the present application

[0108] [Table 1]

[0109] Referring to Table 1, in Examples 1 to 11, a cathode material precursor and a lithium source were sintered at specific temperatures. The temperature was then varied through multiple stepwise heating and constant-temperature steps to control the particle size of the resulting cathode material and appropriately increase the internal defects of the particles, resulting in a cathode material with a separation degree α within a predetermined range. These cathode material particles have an interior suitable for lithium ion diffusion and good particle strength, which is advantageous for reducing polarization and improving the ability to prevent cracking or pulverization of the cathode material, thereby maintaining good capacity, rate performance, and cycle stability. Furthermore, the E, H, and F values ​​of these cathode materials all fall within the predetermined ranges, resulting in these cathode materials exhibiting relatively excellent cycle stability and rate performance.

[0110] Examples 1 to 4 further demonstrate the specific effects of the temperature settings on the particle size and internal defects of the positive electrode material. Compared to Example 1, the temperature fluctuations in the constant temperature stage of Example 2 were relatively small, and the growth of the positive electrode material particles during this treatment stage was relatively stable. This resulted in a certain increase in the particle size of the positive electrode material, and an increase in the degree of separation α. ​​This indicates that, within a certain range of the number of primary particles, increasing the size of the primary particles can extend the cycle life of the positive electrode material. Compared to Example 1, the stepwise heating stage of Example 3 had a relatively fast heating rate, which promoted the generation of more internal defects in the positive electrode material particles during this treatment stage, and an increase in the degree of separation α. ​​This indicates that, within a certain range of the number of primary particles, increasing the internal defects in the positive electrode material can further extend the cycle life of the positive electrode material. Compared with Example 1, the temperature of the constant temperature stage in Example 4 is relatively low, and the growth rate of the positive electrode material particles in this treatment stage is relatively slow, so the particle size of the positive electrode material is reduced to a certain extent, and the separation degree α is also reduced. This indicates that, within a certain range of the number of primary particles, reducing the size of the primary particles will shorten the cycle life of the positive electrode material.

[0111] As can be seen from Examples 1 to 4, as the degree of separation α increases, the tap density T of the positive electrode material increases, the gram capacity C decreases, and the cycle life L extends, and the tap density T, gram capacity C, and cycle life L of the positive electrode material all have a good correlation with the degree of separation α. ​​Therefore, controlling the degree of separation α improves the particle strength of the positive electrode material, contributes to reducing the polarization phenomenon of the positive electrode material, and can provide the positive electrode material with relatively excellent cycle stability and rate characteristics.

[0112] Comparing Comparative Examples 1 to 3 with Example 1, Comparative Example 4 with Example 5, and Comparative Example 5 with Example 6, we found that none of these Comparative Examples used the stepwise heating and constant temperature stages described above for sintering. The temperature fluctuations during the sintering process were significantly lower, resulting in significantly fewer internal defects in the positive electrode material particles. The P values ​​of the positive electrode materials obtained in Comparative Examples 1, 3, 4, and 5 were similar to those of the corresponding Examples, but the internal defects in the positive electrode material particles were significantly insufficient, resulting in a too-low separation α. ​​On the other hand, in Comparative Example 2, the low-temperature time was relatively short, and the high-temperature stage was entered before a sufficient number of crystal nuclei were formed. This resulted in the crystal grain size of the final positive electrode material being too large, resulting in a significantly higher P value than in Example 1. Therefore, although the internal defects in the positive electrode material particles were relatively small, the large crystal grain size resulted in a too-high separation α. ​​Therefore, the separation α of the positive electrode materials in Comparative Examples 1 to 5 was outside the range specified in the present application, making it difficult for these positive electrode materials to achieve both excellent cycle stability and rate characteristics.

[0113] The above embodiments are only used to explain the technical solution of the present application and are not intended to limit the present application. Although the present application has been described in detail with reference to the above preferred embodiments, it should be understood that those skilled in the art can make modifications or equivalent substitutions to the technical solution of the present application without departing from the spirit and scope of the technical solution of the present application. [Explanation of symbols]

[0114] 110 Positive electrode sheet 111 Positive electrode current collector 112 Positive electrode active material layer 120 negative electrode sheet 121 Negative electrode current collector 122 Negative electrode active material layer 130 Separator 140 lithium ion

Claims

1. A positive electrode material, the positive electrode material being a lithium nickel cobalt oxide-based composite oxide, In the XRD pattern of the positive electrode material, the characteristic peak of the (104) crystal plane includes a (104)-Kα1 diffraction peak and a (104)-Kα2 diffraction peak after peak separation, and when the degree of separation between the (104)-Kα1 diffraction peak and the (104)-Kα2 diffraction peak is α, the degree of separation satisfies 0.7≦α≦2.

0.

2. The tap density of the positive electrode material is Tg / cm ３ , the median diameter D of the positive electrode material ５０ If Pμm, 1≦T-(1.04α-0.25α ２ +0.004P ２ 2. The positive electrode material of claim 1, wherein the positive electrode material satisfies the condition of −0.02P≦1.

5.

3. The gram capacity of the positive electrode material is CmAh / g, and the median diameter D ５０ is P μm, and the molar ratio of Ni element to all metal elements other than Li element in the positive electrode material is n Ｎｉ Then, 125≦C-(100n Ｎｉ -α ２ 2. The cathode material of claim 1, wherein P)≦135.

4. The cycle life cycle number under the condition that the positive electrode material is charged to 4.3 V at a current of 1.0 C and discharged to 3.0 V at a current of 1.0 C is L, and the median diameter D of the positive electrode material is ５０ is P μm, and the molar ratio of Ni element to all metal elements other than Li element in the positive electrode material is n Ｎｉ Then, 1800≦L-(2245α-5000n Ｎｉ 2. The cathode material of claim 1, wherein the σ is 0.05 or 1.

00.

5. The tap density of the positive electrode material is Tg / cm ３ 2. The positive electrode material according to claim 1, wherein 1.7≦T≦2.

5.

6. 2. The positive electrode material according to claim 1, wherein the gram capacity of the positive electrode material is CmAh / g, and 140≦C≦230.

7. 2. The positive electrode material according to claim 1, wherein the cycle life cycle number under the condition of charging the positive electrode material to 4.3 V at a current of 1.0 C and discharging it to 3.0 V at a current of 1.0 C is L, and the cycle number is 300≦L≦6000.

8. The median diameter D of the positive electrode material ５０ 2. The positive electrode material according to claim 1, wherein P μm satisfies 3≦P≦16.

9. In the positive electrode material, the molar ratio of Ni element to all metal elements other than Li element is n Ｎｉ Then, 0.33≦n Ｎｉ 2. The cathode material of claim 1, wherein R is 0.01 or less.

10. The cathode material according to claim 1 , wherein the cathode material is a single crystal cathode material.

11. The general chemical formula of the positive electrode material is Li ａ Ni ｘ Co ｙ M1 ｚ M2 ｋ O ２ wherein M1 includes one or two of Mn and Al, and M2 includes one or more of Ni, Co, Mn, Na, K, Mg, Ca, Sr, Al, Ti, Y, Zr, W, Nb, Ce, La, and Dy; 0.9<a≦1.1 0.33≦x≦1 0≦y≦0.33 0<z<0.33 0≦k<0.1 2. The cathode material of claim 1, wherein x+y+z+k=1.

12. A method for preparing a positive electrode material, comprising: mixing a precursor and a lithium source to obtain a mixture; and performing primary sintering and pulverization on the mixture to obtain the positive electrode material; The primary sintering includes N stepwise temperature-raising stages and M constant temperature stages that are performed in sequence, where N is 3 or more and M is 1 or more; The method for preparing a positive electrode material, wherein the stepwise heating stage includes n heating sub-stages, n being 2 or greater, and when n is greater than 1, the heating rate of the nth heating sub-stage is faster than the heating rate of the first heating sub-stage of the same stepwise heating stage, and the heating rate of the nth heating sub-stage is a non-negative value.

13. The preparation method comprises: (1) The maximum temperature of the stepwise heating stage is 700°C to 1000°C; (2) The time of the stepwise heating stage is 6 hours to 14 hours; (3) The temperature of the constant temperature stage is (1150-500n Ｎｉ )℃~(1150-280n Ｎｉ ) ° C., where n Ｎｉ represents the molar ratio of Ni element to all metal elements other than Li element in the positive electrode material; and (4) The preparation method according to claim 12, wherein the constant temperature stage includes alternating temperature-lowering sub-stages and temperature-rising sub-stages, and the temperature of the temperature-lowering sub-stage in the constant temperature stage is lower than the temperature of the temperature-rising sub-stage adjacent thereto.

14. After the grinding, 13. The method of claim 12, further comprising the step of mixing the product of the primary sintering with a coating agent to perform secondary sintering, wherein the elements of the coating agent include one or more of Ni, Co, Mn, Na, K, Mg, Ca, Sr, Al, Ti, Y, Zr, W, Nb, Ce, La, and Dy.

15. A secondary battery, A secondary battery comprising the positive electrode material according to any one of claims 1 to 11 or the positive electrode material prepared by the preparation method according to any one of claims 12 to 14.

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