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

Abstract

A positive electrode active material and a preparation method therefor, a secondary battery, and an electric device. The positive electrode active material comprises particles containing lithium nickel cobalt manganese oxide. In a range of an extension distance L from the surfaces of the particles to the interiors of the particles along the radial direction of the particles, the mass concentration of Co gradually increases from the interiors of the particles to the surfaces of the particles along the radial direction of the particles, wherein L = 1 µm.

Description

Positive electrode active material and preparation method thereof, secondary battery and electric device

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This disclosure is based on the Chinese patent application with application number 202311460557.9, application date November 3, 2023, and invention name “A positive electrode active material and its preparation method, secondary battery and electrical device”, and claims the priority of the Chinese patent application. The entire content of the Chinese patent application is hereby introduced into the present disclosure as a reference. Technical Field

[0003] The present disclosure relates to the technical field of lithium batteries, and in particular to a positive electrode active material and a preparation method thereof, a secondary battery, and an electrical device. Background Art

[0004] In recent years, the application of lithium-ion batteries has become increasingly widespread. They are widely used in energy storage systems such as hydropower, thermal power, wind power, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, aerospace, and other fields. As lithium-ion batteries have achieved significant development, higher requirements have been placed on their energy density, cycle performance, and safety performance.

[0005] Low-nickel ternary cathode materials have the advantages of high thermal stability and excellent cycle performance. However, as market demand places more demands on the various properties of cathode materials, single-crystal ternary cathode materials are required to have higher stability at high voltages and improved electrical properties.

[0006] Summary of the Invention

[0007] This disclosure addresses the aforementioned challenges and aims to provide a positive electrode active material that enhances the structural stability of the positive electrode active material particles and achieves improved power performance, thereby addressing the issues of poor structural stability, kinetic performance, and cycle performance associated with large particle sizes. This disclosure also provides a method for preparing the positive electrode active material, a secondary battery, and an electrical device.

[0008] In order to achieve the above-mentioned purpose, the first aspect of the present disclosure provides a positive electrode active material, including particles containing lithium nickel cobalt manganese oxide, wherein the mass concentration of Co gradually increases from the inside of the particle to the surface of the particle along the radial direction of the particle within a range of a distance L extending from the surface of the particle to the inside of the particle, where L = 1 μm.

[0009] The content of Co in the positive electrode active material disclosed herein in the area close to the surface of the particle changes in a gradient, which greatly increases the content of Co on the particle surface and in the nearby area, helps to increase the rate of conductive ion transmission in the battery, thereby effectively improving the structural stability of the surface layer of the positive electrode active material particle. In addition, when the voltage is above 4.4 volts, cobalt can contribute to capacity. When the cobalt content on the particle surface is high, it is beneficial to exert the capacity of cobalt at high voltage. Therefore, the secondary battery prepared with the positive electrode active material can obtain improved power performance and cycle performance at high voltage. Therefore, the positive electrode active material disclosed herein can effectively improve the problem of poor power performance caused by the large particle size design of the particles. Moreover, since the particles have an integrated structure, the overall stability of the particles is excellent.

[0010] In addition, the positive electrode active material disclosed herein can achieve the above-mentioned effects while reducing the cobalt content inside the particles because the cobalt element is more concentratedly distributed on the particle surface and the nearby area, thereby reducing the Co content of the positive electrode active material as a whole and reducing the material cost.

[0011] In any embodiment, L is 1.5 μm. In this way, the Co element can be advantageously distributed in a region closer to the particle surface.

[0012] In any embodiment, the Co mass concentration on the surface of the particle is 1.02 to 2 times, optionally 1.1 to 1.8 times, of the Co mass concentration at a distance L=1 μm extending radially toward the interior of the particle. By adjusting the variation of the Co content in the region near the surface of the particle, it is helpful to improve the capacity and electrical performance of a secondary battery prepared with the positive electrode active material. For example, in some embodiments, when the above ratio is larger, the gram capacity of the material is larger and the initial DCR is smaller, so that the secondary battery can have a larger capacity and a smaller initial internal resistance value.

[0013] In any embodiment, the mass concentration of Ni decreases gradually along the radial direction of the particle from the interior of the particle to the surface of the particle. When the Ni content is high, the positive tetravalent nickel on the particle surface is highly oxidizable and easily undergoes side reactions with the electrolyte. Therefore, the lower the nickel content on and near the particle surface, the more favorable the stability of the particle surface.

[0014] In any embodiment, the absolute value of the difference in mass concentration of Li and Mn at any two points in the particle is less than or equal to 20%. Thus, when the difference in mass concentration of Li and Mn at any two points in the particle is less than or equal to 20%, it means that the distribution uniformity of Li and Mn elements in the particle is high.

[0015] In any embodiment, the positive electrode active material is represented by the following formula 1:

[0016] Lix (Ni a Co b Mn c ) d M 1-d O 2-y A y Formula 1

[0017] Wherein, 0.95≤x≤1.3, 0.3≤a≤0.7, 0.01≤b≤0.15, 0.1≤c≤0.5, a+b+c=1, 0.95≤d≤1, 0≤y≤0.1, M comprises at least one selected from Zr, Sr, B, Ti, Mg, Sn, Tb, W, Nb, and Al, and A comprises at least one selected from S, N, F, Cl, Br, and I. Doping with element M effectively binds oxygen atoms, making the positive electrode active material less susceptible to oxygen release during high-temperature cycling and storage, significantly improving the structural stability of the positive electrode active material. Doping with element A further improves the structural stability of the positive electrode active material and reduces side reactions on the particle surface, thereby further improving the battery's cycling performance.

[0018] In any embodiment, the volume distribution particle size Dv50 of the particles is in the range of 3.5 μm ≤ Dv50 ≤ 7.5 μm, and optionally, 4 μm ≤ Dv50 ≤ 7 μm. When the volume distribution particle size of the particles is within the above range, the smaller particle size facilitates electronic conductivity and ion transport, thereby further reducing the impedance of the secondary battery and improving power performance and kinetic performance.

[0019] In any embodiment, the particles further comprise a coating layer. Optionally, the coating layer is an oxide or hydroxide of M', wherein M' comprises at least one selected from Mg, Al, Ca, Ba, Ti, Zr, Zn, Nb, Sb, Ce, La, W, and B. By coating and modifying the surface of the positive electrode active material particles, the particles can be effectively protected, making the particle interface more stable and effectively reducing phase changes in the surface structure. Furthermore, the positive electrode active material surface can be protected from electrolyte corrosion, reducing side reactions, improving the material's resistance to gas generation, reducing battery gas generation, and improving battery cycle performance.

[0020] In any embodiment, the coating layer has a thickness of 0.001-0.5 μm, and optionally 0.01-0.1 μm. By controlling the coating layer thickness within the above range, the electrolyte and the bulk particles can be effectively isolated, side reactions can be reduced, and lithium ions can have a higher diffusion and migration ability in the positive electrode active material.

[0021] The second aspect of the present disclosure further provides a method for preparing the positive electrode active material according to the first aspect of the present disclosure, the preparation method comprising:

[0022] Mixing a lithium source, precursors of Ni, Co and Mn and optionally a compound containing a doping element M and a compound containing a doping element A, and performing a first sintering to obtain initial material particles;

[0023] The starting material particles are mixed with a cobalt-containing compound and subjected to a second sintering to obtain particles containing lithium nickel cobalt manganese oxide.

[0024] The method for preparing a positive electrode active material disclosed herein first prepares initial material particles having a target composition by a first sintering, and then performs a second sintering with a cobalt-containing compound, thereby obtaining a positive electrode active material, wherein the mass concentration of Co gradually decreases from the inside of the particle to the surface of the particle along the radial direction of the particle within a range of L=1 μm. In addition, the particles do not have a clear core-shell structure, and the mass concentration of the Co element (and other elements) in the particles does not have a sudden change in the obvious interface, but is an integrated crystalline particle, especially a single crystal particle.

[0025] In any embodiment, the ratio of the mass of the cobalt-containing compound, calculated as elemental cobalt, to the mass of the particles containing lithium nickel cobalt manganese oxide is in the range of 2000-10000 ppm, and optionally, 4000-7000 ppm. Controlling the amount of the cobalt-containing compound within this range helps achieve an ideal Co mass concentration and a good concentration gradient in the surface layer and adjacent regions of the obtained granular material, thereby achieving the excellent performance of the aforementioned positive electrode active material.

[0026] In any embodiment, the molar ratio of Ni:Co:Mn in the precursor is in the range of (50-70):(6-25):(29-40), and optionally, in the range of (55-65):(6-12):(29-33). By controlling the molar ratio of Ni:Co:Mn in the precursor within the above range, it is helpful to achieve the excellent performance of the aforementioned positive electrode active material.

[0027] In any embodiment, the first sintering is carried out at 600° C. to 1000° C. for 5 to 25 hours, or alternatively, at 650° C. to 950° C. for 8 to 20 hours. After the first sintering, the sintered material is cooled to room temperature and then ground for 5 hours to obtain initial material particles.

[0028] In any embodiment, the second sintering is performed in an oxygen-rich atmosphere. Optionally, the oxygen-rich atmosphere has a volume ratio of air to oxygen between (0:10) and (8:2). Performing the second sintering in an oxygen-rich atmosphere helps suppress Li / Ni mixing caused by low cobalt content during the sintering process, thereby extending the life of the material and improving the cycle life of the secondary battery.

[0029] In any embodiment, the second sintering is performed at 650°C-800°C for 5-7 hours. This second sintering process allows the cobalt-containing compound coated on the surface of the initial material particles to become integrated with the initial material particles. Furthermore, the Co content decreases radially from the surface to a certain depth within the particles, thereby contributing to the production of a positive electrode active material with high structural stability and excellent power performance.

[0030] In any embodiment, the preparation method further comprises:

[0031] The particles containing lithium nickel cobalt manganese oxide are mixed with an M' source and subjected to a third sintering step to form a coating layer on the surface of the particles; optionally, the M' comprises at least one selected from Mg, Al, Ca, Ba, Ti, Zr, Zn, Nb, Sb, Ce, La, W, and B. By adjusting the amount of the compound of the M' source to be added within the above range, the resulting coating layer can have the above-mentioned suitable thickness, thereby effectively isolating the particles from the electrolyte, reducing positive electrode side reactions, and ensuring that the positive electrode active material has good lithium ion diffusion and migration capabilities.

[0032] The third aspect of the present disclosure further provides a secondary battery, comprising a positive electrode plate, wherein the positive electrode plate comprises at least the positive electrode active material of the first aspect of the present disclosure, or the positive electrode plate comprises at least the positive electrode active material prepared by the method of the second aspect of the present disclosure.

[0033] A fourth aspect of the present disclosure further provides an electrical device, comprising the secondary battery according to the third aspect of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] FIG1 is a schematic cross-sectional view of a lithium nickel cobalt manganese oxide particle 100 of a positive electrode active material according to an embodiment of the present disclosure.

[0035] FIG2 is an X-ray photoelectron spectroscopy (XPS) spectrum of the positive electrode active materials prepared in Example 1, Example 5, and Example 6 of the present disclosure.

[0036] FIG3 is a schematic diagram of a battery cell according to an embodiment of the present disclosure.

[0037] FIG. 4 is an exploded view of the battery cell according to the embodiment of the present disclosure shown in FIG. 3 .

[0038] FIG5 is a schematic diagram of a battery module according to an embodiment of the present disclosure.

[0039] FIG6 is a schematic diagram of a battery pack according to an embodiment of the present disclosure.

[0040] FIG. 7 is an exploded view of the battery pack shown in FIG. 6 according to an embodiment of the present disclosure.

[0041] FIG8 is a schematic diagram of an electric device using a secondary battery according to an embodiment of the present disclosure as a power source.

[0042] Description of reference numerals:

[0043] 1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 battery cell; 51 housing; 52 electrode assembly; 53 top cover assembly DETAILED DESCRIPTION

[0044] Below, the embodiments of the positive electrode active material and its preparation method, secondary battery and electric device disclosed in the present invention are described in detail with appropriate reference to the drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present disclosure and are not intended to limit the subject matter described in the claims.

[0045] " scope " disclosed in the present disclosure is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and selected lower limit and upper limit define the boundary of special scope.The scope that this mode limits can be to include end value or not include end value, and can be combined arbitrarily, and promptly any lower limit can form a scope with any upper limit combination.For example, if the scope of 60-120 and 80-110 is listed for specific parameter, it is understood that the scope of 60-110 and 80-120 is also expected.In addition, if the minimum range value 1 and 2 listed, and if the maximum range value 3,4 and 5 listed, then the following scope can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5.In the present disclosure, unless otherwise specified, numerical range " ab " represents the abbreviation of any real number combination between a and b, and wherein a and b are all 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.

[0046] Unless otherwise specified, all embodiments and optional embodiments of the present disclosure can be combined with each other to form new technical solutions.

[0047] Unless otherwise specified, all technical features and optional technical features disclosed herein can be combined with each other to form a new technical solution.

[0048] Unless otherwise specified, all steps of the present disclosure may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), which means that step (c) may be added to the method in any order, for example, 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.

[0049] Unless otherwise specified, the terms used in the present disclosure have the common meanings that are generally understood by those skilled in the art.

[0050] Unless otherwise specified, the numerical values ​​of the parameters mentioned in the present disclosure can be measured using various test methods commonly used in the art, for example, they can be measured according to the test methods given in the present disclosure.

[0051] Unless otherwise specified, in the present disclosure, the term "active ions" refers to ions that can be intercalated and extracted back and forth between the positive electrode and the negative electrode of a secondary battery, including but not limited to lithium ions.

[0052] Nickel-cobalt-manganese ternary positive electrode active materials have excellent energy density and cost advantages over lithium cobalt oxide materials. The three elements nickel, cobalt and manganese in the material can be mixed in any proportion. Among them, Mn mainly exists in the form of +4 valence, which can improve the structural stability of the material and enhance the safety of the battery, but does not participate in the electrochemical process. Ni mainly exists in the form of +2 valence. During the lithium de-insertion process, Ni 2+ It will be oxidized to +3 and +4; Co mainly exists in the form of +3. In the process of lithium de-insertion, Co 3+It will be oxidized to +4 valence, and both can contribute to capacity. Since nickel reserves are large and the price is relatively low, increasing the nickel content in the material to increase the gram capacity of the material is the mainstream trend. However, divalent nickel and lithium ions are prone to mixing. As the number of charge and discharge cycles increases, the mixing intensifies and eventually leads to unstable material structure, affecting the cycle life of the secondary battery. In addition, the +4 valence nickel on the surface of the material is prone to side reactions with the electrolyte, which also leads to a decrease in the stability of the material. Cobalt is beneficial to stabilizing the layered structure of the material, preventing the mixing of nickel and lithium, and is beneficial to the cycle performance and rate performance of the battery. However, the price of cobalt is relatively high, and an increase in its content will increase the cost of the material.

[0053] Ternary cathode active materials with large particle sizes have been designed to reduce the specific surface area of ​​the material, reduce side reactions with the electrolyte, and improve the electrical performance of the material at high voltage. However, this results in a decrease in the power performance of the material at high voltage.

[0054] In addition, a ternary positive electrode active material with a high nickel content inside the material and a low nickel content near the surface of the material has been designed, in order to reduce the activity of the material surface and improve the stability of the material. For example, a scheme has been disclosed in which a core-shell structure precursor with different target element contents is prepared by a two-step co-precipitation method, and then sintered. This method can obtain a material with a gradual change in the target element content in the shell, but because the element content of the core and shell parts changes suddenly, the interface between the core and shell is obvious, and the stability of the material is not ideal. For another example, it has been disclosed that the concentration of the raw material solution is adjusted during the preparation of the precursor, and a nickel-cobalt-manganese precursor with a gradual change in the target element content is prepared by a two-step co-precipitation method, and then mixed with a lithium source and sintered to obtain a positive electrode active material in which the nickel content gradually decreases from the center to the surface of the particle, and the cobalt and manganese contents gradually increase from the center to the surface of the particle. However, the material obtained by this method has an unsatisfactory elemental gradient distribution within the particles because elements tend to migrate from high-concentration areas to low-concentration areas during the sintering process. In particular, the concentrations of elements near the particle surface (e.g., approximately 1 micron inward from the particle surface) tend to be constant. Consequently, it is difficult to achieve the desired target element content on the particle surface.

[0055] Based on this, the present disclosure provides a positive electrode active material and a preparation method thereof, a secondary battery, and an electrical device, which are described in detail below with reference to the accompanying drawings.

[0056] positive electrode active material

[0057] The present disclosure proposes a positive electrode active material, which includes particles containing lithium nickel cobalt manganese oxide. Within a range of a distance L extending from the surface of the particle along the radial direction of the particle to the interior of the particle, the mass concentration of Co gradually increases from the interior of the particle to the surface of the particle along the radial direction of the particle, where L = 1 μm.

[0058] Referring to FIG. 1 , a schematic cross-sectional view through the geometric center of a particle 100 comprising lithium nickel cobalt manganese oxide, a positive electrode active material of the present invention, is shown. As shown in FIG. 1 , within a range extending inwardly from the surface of the particle 100 along a radial direction x of the particle to a certain distance (or depth) L, the mass concentration of the Co element 101 gradually increases from the particle interior to the particle surface along the radial direction x of the particle.

[0059] It should be understood that, although a range is defined within the particle by a dotted line in FIG1 , the particle does not have an obvious core-shell structure, but is an integrated crystal particle, especially a single crystal particle.

[0060] It should be understood that the oval shape of the particle 100 shown in FIG. 1 is merely exemplary and is not intended to limit the particle shape of the positive electrode active material of the present disclosure.

[0061] The positive electrode active material of the embodiment of the present invention includes lithium nickel cobalt manganese oxide, which is a nickel cobalt manganese ternary positive electrode active material. This material has a high gram capacity, and the use of this positive electrode active material can make the lithium ion secondary battery have a higher capacity. The positive electrode active material disclosed herein has a gradient change in the area close to the surface of the particle, which greatly increases the Co content on the surface of the particle and in the nearby area, helps to increase the rate of conductive ion transmission in the battery, thereby effectively improving the structural stability of the surface layer of the positive electrode active material particle. In addition, when the voltage is above 4.4 volts, cobalt can contribute to the capacity. When the cobalt content on the surface of the particle is high, it is beneficial to exert the capacity of cobalt at high voltage. Therefore, the secondary battery prepared with this positive electrode active material can obtain improved power performance and cycle performance at high voltage. Therefore, the positive electrode active material disclosed herein can effectively improve the problem of poor power performance caused by the large particle size design of the particles. Moreover, because the particles have an integrated structure, the overall stability of the particles is excellent.

[0062] In addition, the positive electrode active material disclosed herein can achieve the above-mentioned effects while reducing the cobalt content inside the particles because the cobalt element is more concentratedly distributed on the particle surface and the nearby area, thereby reducing the Co content of the positive electrode active material as a whole and reducing the material cost.

[0063] In some embodiments, L is 1.5 μm. That is, within a range of 1.5 μm extending radially from the particle surface toward the particle interior, the mass concentration of Co gradually increases radially from the particle interior to the particle surface. In these embodiments, the Co element is advantageously distributed in an area closer to the particle surface. In some embodiments, the mass concentration of Co at the particle surface is 1.02 to 2 times, and optionally 1.1 to 1.8 times, the mass concentration of Co at a distance L = 1 μm extending radially toward the particle interior.

[0064] Here, the Co mass concentration on the particle surface is the ratio of the Co mass concentration at a distance L = 1 μm extending from the radial direction of the particle to the interior of the particle. It can be the two end points of the above range, or any value between the two end points, for example, it can be: 1.02, 1.05, 1.08, 1.0, 1.05, 1.1, 1.15, 1.2, 1.25, 1.3, 1.35, 1.4, 1.45, 1.5, 1.55, 1.6, 1.65, 1.7, 1.75, 1.8, etc.

[0065] In the present disclosure, by adjusting the variation range of the Co content in the region near the particle surface, it is helpful to improve the capacity and electrical performance of the secondary battery prepared with the positive electrode active material. For example, in some embodiments, when the above ratio is larger, the gram capacity of the material is larger and the initial DCR is smaller, so that the secondary battery can have a larger capacity and a smaller initial internal resistance value. However, when the ratio is too large, the excess Co on the surface of the material will be dissolved during the charge and discharge cycle of the secondary battery, causing the structure of the material to be destroyed, which in turn affects the cycle life of the secondary battery.

[0066] Further reference is made to Figure 2, which shows the X-ray photoelectron spectroscopy (XPS) of the positive electrode active materials of some examples of the present disclosure. From the peak height corresponding to the Co element at a binding energy of 777.8 eV in Figure 2, it can be seen that the peak height values ​​of the surface Co of Sample 1 (corresponding to Example 1 in the subsequent Examples section), Sample 2 (corresponding to Example 5 in the subsequent Examples section), and Sample 3 (corresponding to Example 6 in the subsequent Examples section) are essentially the same, indicating that the surface layers of each sample have roughly equivalent Co content.

[0067] Further comparison of the peak heights corresponding to the Co element at 777.8 eV at different depths from the surface of the particles in Sample 1 in Figure 2 (corresponding to Example 1 in the subsequent Examples section) shows that in Sample 1, the Co content shows a clear downward trend at multiple depths from the particle surface to the particle interior. This trend is more pronounced within the range of 1000 nm from the particle surface to within the surface, and the downward trend of the Co content slows down within the range of 1000 nm to 2000 nm within the particle surface.

[0068] In the present disclosure, the central region of the particle of the material refers to the region extending from the particle surface along the radial direction of the particle to the geometric center of the particle at a distance L, wherein L = 1 μm, optionally, L = 1.5 μm.

[0069] In some embodiments, the mass concentration of Ni gradually decreases from the interior of the particle to the surface of the particle along the radial direction of the particle.

[0070] Understandably, a higher Ni content increases the gram capacity of the ternary material, which is more beneficial for improving the initial discharge gram capacity of the electrochemical energy storage device. However, when the Ni content is high, the tetravalent nickel on the particle surface is highly oxidizable and prone to side reactions with the electrolyte. Therefore, a lower Ni content on and near the particle surface improves particle surface stability.

[0071] In addition, Ni 2+ With Li + It is easy to mix up. The reason is that Ni 2+ With Li + The radius is similar. In the process of lithium insertion and extraction, Ni 2+ Easier migration to Li + The vacancies formed by the insertion and removal of Li lead to the precipitation of Li. During the repeated charge and discharge process, Ni 2+ With Li + The mixing ratio of the ternary material is increasing, and the layered structure of the ternary material may collapse, which makes Li + Intercalation and deintercalation within the layered structure of ternary materials become increasingly difficult, ultimately leading to deterioration in the cycling performance of secondary batteries. The presence of Co effectively stabilizes the layered structure of ternary materials and inhibits lithium-nickel intercalation. Therefore, adjusting the nickel content in and near the particle surface of ternary cathode active materials to a relatively low level and the cobalt content to a relatively high level is beneficial for improving the material's stability and enhancing the cycling performance of secondary batteries.

[0072] The present disclosure does not particularly limit the overall nickel content in the positive electrode active material, and the gradient change rate. According to some embodiments, the positive electrode active material may contain any suitable nickel content, in particular a low nickel content, or a medium nickel content. The gradient change rate of the nickel content in the positive electrode active material may be that the nickel content in a certain area inside the particle is relatively uniform, and in the area on and near the surface of the particle, the nickel content gradually decreases from the inside of the particle to the surface. For example, it may be an area extending from the surface of the particle along the radial direction of the particle to the inside within a range of L, where L = 1.5 μm, 1 μm or 0.5 μm. In some embodiments, the nickel content may be gradually decreasing from the center of the particle to the surface of the particle.

[0073] Since in the embodiment of the present disclosure, the mass concentration of Co gradually increases from the inside of the particle to the surface of the particle along the radial direction of the particle, while the mass concentration of Ni gradually decreases, the present disclosure, under the premise of a low-cost metal ratio with unchanged overall Ni / Co content in the material, reduces the Ni content of the surface layer and increases the Co content of the surface layer by changing the concentrations of cobalt and nickel at different positions in the particle in a gradient manner, thereby providing a positive electrode active material with high structural stability and good power performance at high voltage.

[0074] Likewise, in the embodiment of the present disclosure, since there is no obvious core-shell structure, there is no sudden change in the concentration of any element from the interior of the particle to the surface of the particle, which helps to obtain a positive electrode active material with better performance stability.

[0075] In some embodiments, the absolute value of the difference in mass concentration of Li and Mn at any two points in the particles is less than or equal to 20%.

[0076] In the present disclosure, the mass concentration of Li and Mn elements at any two points in the particle is the mass concentration of Li and Mn elements in all elements within the extremely small volume of the two points. In the present disclosure, the difference in mass concentration of Li and Mn at any two points in the particle is less than or equal to 20%, which means that the distribution uniformity of Li and Mn elements in the particle is relatively high. Optionally, the difference in mass concentration of Li and Mn at any two points in the particle is less than or equal to 15%, less than or equal to 10%, or even less than or equal to 5%. The uniform distribution of Li and Mn elements can better exert the performance improvement effect of Li and Mn elements on the positive electrode active material. In addition, lithium ions located in different regions inside the particle can also obtain more consistent migration and diffusion capabilities within the uniform structure of the Mn element.

[0077] Without being bound by existing theories, it is speculated that the relatively uniform distribution of Li and Mn elements in the particles may be due to the fact that their migration ability is stronger than that of Ni and Co elements during the sintering process.

[0078] In some embodiments, the positive electrode active material is represented by the following formula 1:

[0079] Li x (Ni a Co b Mn c ) d M 1-d O 2-y A y Formula 1

[0080] Among them, 0.95≤x≤1.3, 0.3≤a≤0.7, 0.01≤b≤0.15, 0.1≤c≤0.5, a+b+c=1, 0.95≤d≤1, 0≤y≤0.1, M includes at least one selected from Zr, Sr, B, Ti, Mg, Sn, Tb, W, Nb and Al, and A includes at least one selected from S, N, F, Cl, Br and I.

[0081] It should be noted that in the present disclosure, the chemical formula of the lithium nickel manganese oxide material is the chemical formula of the material used in the battery preparation process. In the positive electrode sheets, battery cells, and electrical equipment, due to the processes of formation and circulation, those skilled in the art will understand that the elements in the above chemical formula may be lost. For example, in the positive electrode sheets, battery cells, and electrical equipment, due to the processes of circulation, the oxygen element in the positive electrode active material is lost, so the measured oxygen element content in the positive electrode active material will decrease. In addition, due to the processes of formation and circulation, lithium ions will be consumed in the positive electrode sheets, battery cells, and electrical equipment, so the measured lithium element content in the positive electrode active material will decrease. At the same time, if the positive electrode sheets and negative electrode sheets are supplemented with lithium, after the processes of formation and circulation, the measured lithium element content in the positive electrode active material will increase.

[0082] In some embodiments, the amount of Li in Formula 1 is represented by a value of x. Optionally, the value of x can be 0.95, 0.98, 1.0, 1.1, 1.2, or 1.3.

[0083] In the above formula 1, the amounts of Ni, Co, and Mn are represented by the values ​​of a, b, and c, respectively. Optionally, 0.5≤a≤0.65, 0.05≤b≤0.12, 0.95≤d≤0.99, and the values ​​of a may be 0.3, 0.4, 0.5, 0.6, or 0.7; the values ​​of b may be 0.04, 0.06, 0.08, 0.10, 0.11, 0.12, 0.13, 0.14, or 0.15; and the values ​​of c may be 0.1, 0.2, 0.3, 0.4, or 0.5. As described above, the mass concentration of the cobalt element in the region near the surface of the particles of the positive electrode active material varies in a gradient, resulting in a higher Co content on the particle surface and a relatively lower Co content inside the particles. Therefore, the cathode active material of the present disclosure can achieve improved structural stability and power performance at high voltage with reduced Co content (0.01≤b≤0.15, optionally 0.05≤b≤0.12).

[0084] In addition, the mass concentration of nickel in the area near the surface of the particles of the positive active material also changes in a gradient, resulting in a lower Ni content on the particle surface and a relatively higher Ni content inside the particles. Therefore, the positive active material of the present disclosure can be suitable for designs with low to medium nickel content (0.3≤a≤0.7, optionally 0.5≤a≤0.65) while improving the stability of the particle surface, thereby having an ideal gram capacity.

[0085] In the above formula 1, M is a doping element. In the present disclosure, the positive electrode active material may not contain the doping element M. A preferred embodiment is that the positive electrode active material contains the doping element M. By doping with the M element, oxygen atoms can be effectively bound, making the positive electrode active material less likely to release oxygen during high-temperature cycling and high-temperature storage, significantly improving the structural stability of the positive electrode active material. In some embodiments, the M element can be at least one of Zr, Sr, W, Na, Mg, F, P, etc., or a combination thereof.

[0086] The doping amount of the M element is expressed as the value of (1-d). Optionally, 0.95≤d≤0.99. The value of d can be 0.95, 0.96, 0.97, 0.98, or 0.99.

[0087] In the above formula 1, A is also a doping element. In the present disclosure, the positive electrode active material may or may not contain the doping element A. Doping with the doping element A can further improve the structural stability of the positive electrode active material, reduce side reactions on the particle surface, and thus further improve the cycle performance of the battery.

[0088] The doping amount of element A is represented by the value y. Optionally, 0.02≤y≤0.1. The value of y can be 0.02, 0.05, 0.07, 0.09, or 0.1.

[0089] In the present disclosure, the concentration of an element in the positive electrode active material can be obtained by EDX (Energy Dispersive X-Ray Spectroscopy) or EDS elemental analysis combined with TEM (Transmission Electron Microscope) or SEM (Scanning Electron Microscope) single-point scanning test element concentration distribution or other similar methods. When EDX or EDS elemental analysis is combined with TEM or SEM single-point scanning test, taking Li as an example, the mass concentration of Li element in μg / g at different sites in the particle is recorded as η1, η2, η3, ..., η n , n is a positive integer greater than or equal to 15.

[0090] The average mass concentration of Li in a particle is the mass concentration of Li as a percentage of all elements within a single particle. This can be determined by EDX or EDS elemental analysis combined with TEM or SEM surface scanning to measure elemental concentration distribution, or other similar methods. When using EDX or EDS elemental analysis combined with TEM or SEM surface scanning to measure elemental concentration distribution, the test surface includes all points in the aforementioned single-point test. The average mass concentration of Li in a particle is denoted as η, and its unit is ppm (i.e., parts per million, μg / g).

[0091] Similarly, the average mass concentrations of Ni, Co, Mn, M element, and A element in the particles can be measured and calculated using the above method.

[0092] The mass concentration deviation σ of the Li element at any point in the particle is calculated according to the following formula 2:

[0093] It can be understood that the mass concentration deviation of the Mn element between any two points in the particle can also be obtained using the above-mentioned test and calculation methods.

[0094] Similarly, the average mass concentration of Co or Ni in a certain region of a particle, or the mass concentration at a certain point, can be obtained using the above-described test and calculation methods. The Co mass concentration at the particle surface can thus be calculated as a multiple of the Co mass concentration at a point extending radially inward from the particle at a distance L = 1 μm.

[0095] In some embodiments, the volume distribution particle size Dv50 of the particles is in the range of 3.5 μm ≤ Dv50 ≤ 7.5 μm, optionally, 4 μm ≤ Dv50 ≤ 7 μm.

[0096] In this disclosure, the volume distribution particle size Dv50 of the positive electrode active material particles is generally known in the art and represents the particle size corresponding to 50% of the cumulative volume distribution percentage of the material. It can be measured using instruments and methods known in the art. For example, it can be measured using a laser particle size analyzer in accordance with GB / T 19077-2016. The testing instrument can be a Mastersizer 3000 laser particle size analyzer from Malvern Instruments Ltd. in the UK.

[0097] Here, the value of the volume distribution particle size range of the particles can be the end point value, or any value between the end points, for example, it can be: 3.5μm, 3.7μm, 3.9μm, 4μm, 4.1μm, 4.2μm, 4.3μm, 4.5μm, 4.7μm, 4.9μm, 5μm, 5.1μm, 5.3μm, 5.5μm, 5.7μm, 5.9μm, 6μm, 6.1μm, 6.3μm, 6.5μm, 6.7μm, 7μm, 7.2μm, 7.3μm, 7.5μm, etc.

[0098] When the volume distribution particle size of the particles is within the above range, the smaller the particle size, the better the electronic conductivity and ion transport, thereby further reducing the impedance of the secondary battery and improving the power performance and kinetic performance.

[0099] In some embodiments, the positive electrode active material further comprises a coating layer. Optionally, the coating layer is an oxide or hydroxide of M', wherein M' comprises at least one selected from Mg, Al, Ca, Ba, Ti, Zr, Zn, Nb, Sb, Ce, La, W, and B. Optionally, M' comprises at least one selected from Al, Ti, Zr, Nb, and W.

[0100] In this disclosure, by coating and modifying the surface of the positive electrode active material particles, the particles are effectively protected, making the particle interface more stable and effectively reducing phase changes in the surface structure. Furthermore, the presence of the coating layer effectively protects the positive electrode active material surface from electrolyte corrosion, reduces side reactions, improves the material's resistance to gas generation, reduces battery gas production, and improves the battery's cycle performance.

[0101] In some embodiments, the coating layer has a thickness of 0.001-0.5 μm, optionally 0.01-0.1 μm.

[0102] Here, the value of the coating thickness can be the end point value or any value between the end points, for example, it can be: 0.005μm, 0.01μm, 0.015μm, 0.02μm, 0.025μm, 0.03μm, 0.035μm, 0.04μm, 0.045μm, 0.05μm, 0.055μm, 0.06μm, 0.065μm, 0.07μm, 0.075μm, 0.080μm, 0.085μm, 0.09μm, 0.095μm, 0.1μm, 0.2μm, 0.3μm, 0.4μm, 0.5μm.

[0103] In the present disclosure, by controlling the thickness of the coating layer within the above range, the contact between the electrolyte and the bulk particles can be effectively isolated, side reactions can be reduced, and lithium ions can have a higher diffusion and migration ability in the positive electrode active material.

[0104] The present disclosure also provides a method for preparing the above-mentioned positive electrode active material, the preparation method comprising:

[0105] S1: mixing a lithium salt, hydroxides of Ni, Co and Mn as precursors, and an optional compound containing a doping element M, and performing a first sintering to obtain initial material particles;

[0106] S2: mixing the starting material particles with a cobalt-containing compound and performing a second sintering to obtain particles containing lithium nickel cobalt manganese oxide.

[0107] The method for preparing the positive electrode active material disclosed herein first prepares initial material particles having a target composition by a first sintering, and then performs a second sintering with a cobalt-containing compound, thereby obtaining a positive electrode active material having the above-mentioned characteristics, wherein the mass concentration of Co gradually decreases from the inside of the particle to the surface of the particle along the radial direction of the particle within a range of L=1 μm. In addition, the particles do not have a clear core-shell structure, and the mass concentration of the Co element (and other elements) in the particles does not have a sudden change in the obvious interface, but is an integrated crystalline particle, especially a single crystal particle.

[0108] During the second sintering process, the cobalt-containing compound is coated on the surface of the initial material particles. As a result, the Co content in the initial material particles is relatively low relative to the surface. Co ions diffuse from the particle surface to the bulk phase. From the particle surface to a certain depth inside the particle, the Co content gradually decreases without a sudden change. This results in the positive electrode active material.

[0109] In S1, a lithium source, precursors of Ni, Co and Mn and optionally a compound containing a doping element M and a compound containing a doping element A are mixed and first sintered to obtain initial material particles.

[0110] As an example of preparing particles containing the starting materials, a lithium salt, Ni, Co, and Mn hydroxides as precursors, and, if applicable, a compound containing the M element and / or a compound containing the A element are mixed in, for example, a ball mill or a high-speed mixer. The mixing time can be, for example, 0.5 to 3 hours.

[0111] The present disclosure does not particularly limit the lithium source used in the first sintering. For example, the lithium source can be one or more of lithium oxide (Li2O), lithium phosphate (Li3PO4), lithium dihydrogen phosphate (LiH2PO4), lithium acetate (CH3COOLi), lithium hydroxide (LiOH), lithium carbonate (Li2CO3), and lithium nitrate (LiNO3). Furthermore, the lithium source is one or more of lithium carbonate, lithium hydroxide, and lithium nitrate; and even more preferably, the lithium source is lithium carbonate.

[0112] The present disclosure also does not particularly limit the precursors of Ni, Co, and Mn used in the first sintering. For example, the precursor is one or more of an oxide, hydroxide, and carbonate containing Ni, Co, and Mn, such as a hydroxide containing Ni, Co, and Mn. The precursor can be prepared by any method. For example, it can be prepared by a coprecipitation method, a gel method, or a solid phase method, but is not limited thereto.

[0113] In some embodiments, the precursor is prepared by a co-precipitation method. The following uses the co-precipitation method as an example to illustrate the preparation method of the hydroxide precursors of Ni, Co and Mn.

[0114] A Ni source, a Co source and a Mn source are dispersed in a solvent to obtain a mixed solution; the mixed solution, a strong base solution and a complexing agent solution are simultaneously pumped into a stirred reactor in a continuous parallel flow reaction mode, the pH value of the reaction solution is controlled to be 10-13, the temperature in the reactor is 25°C-90°C, and an inert gas is passed during the reaction process; after the reaction is completed, the hydroxide containing Ni, Co and Mn is obtained through aging, filtration, washing and vacuum drying.

[0115] The Ni source is a soluble nickel salt. For example, the Ni source is one or more of nickel sulfate, nickel nitrate, nickel chloride, nickel oxalate, and nickel acetate. Alternatively, the Ni source is one or more of nickel sulfate and nickel nitrate, and further is nickel sulfate.

[0116] The Co source is a soluble cobalt salt, such as one or more of cobalt sulfate, cobalt nitrate, cobalt chloride, cobalt oxalate and cobalt acetate. Alternatively, the Co source is one or more of cobalt sulfate and cobalt nitrate, further cobalt sulfate.

[0117] The Mn source is a soluble manganese salt, such as one or more of manganese sulfate, manganese nitrate, manganese chloride, manganese oxalate, and manganese acetate. Alternatively, the Mn source is one or more of manganese sulfate and manganese nitrate, further manganese sulfate.

[0118] The strong base is one or more of LiOH, NaOH and KOH.

[0119] The complexing agent is one or more of ammonia water, ammonium sulfate, ammonium nitrate, ammonium chloride, ammonium citrate and disodium ethylenediaminetetraacetate (EDTA), such as ammonia water.

[0120] There is no particular restriction on the solvents of the mixed solution, the strong base solution and the complexing agent solution. For example, the solvents of the mixed solution, the strong base solution and the complexing agent solution are each independently one or more of deionized water, methanol, ethanol, acetone, isopropanol and n-hexanol, such as deionized water.

[0121] The inert gas introduced during the reaction is, for example, one or more of nitrogen, argon, and helium.

[0122] In some embodiments, the molar ratio of Ni:Co:Mn in the precursor is in the range of (50-70):(6-25):(29-40), optionally, in the range of (55-65):(6-12):(29-33).

[0123] By controlling the molar ratio of Ni:Co:Mn in the precursor to be within the above range, it is helpful to achieve the excellent performance of the above-mentioned positive electrode active material.

[0124] In some embodiments, the positive electrode active material includes a doping element M. In such embodiments, a compound containing the doping element M is added during the first sintering. The doping element M is as defined above. The compound containing the doping element M is one or more of an oxide, nitrate, carbonate, bicarbonate, sulfate, chloride, hydroxide, and acetate of the element M. Those skilled in the art can determine a suitable compound containing the element M based on the selected element M.

[0125] In some embodiments, the positive electrode active material further comprises a doping element A. In such embodiments, a compound comprising the doping element A is added during the first sintering. The doping element A is as defined above. The compound comprising the doping element A is one or more of an ammonium salt and a lithium salt of the element A, for example, one or more of NH4F, NH4Cl, NH4Br, (NH4)2S, LiF, LiCl, LiBr, and Li2S.

[0126] The lithium source, precursors of Ni, Co and Mn, and optionally a compound containing a doping element M and a compound containing a doping element A, may be mixed according to the stoichiometric ratio of each element shown in Formula 1 above and subjected to a first sintering.

[0127] In some embodiments, the first sintering is carried out at a temperature of 600° C. to 1000° C. for 5 to 25 hours, and optionally at a temperature of 650° C. to 950° C. for 8 to 20 hours.

[0128] The first sintering temperature may be an end point value or any value between the end points, for example, 600°C, 620°C, 650°C, 670°C, 700°C, 720°C, 750°C, 780°C, 800°C, 820°C, 850°C, 880°C, 900°C, 920°C, 950°C, 980°C, 1000°C, etc. The holding time at the first sintering temperature may be an end point value or any value between the end points, for example, 5 hours, 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, 13 hours, 14 hours, 15 hours, 16 hours, 17 hours, 18 hours, 19 hours, 20 hours, 22 hours, 25 hours, etc.

[0129] After the first sintering, the sintered material was cooled to room temperature and then ground for 5 hours to obtain initial material particles.

[0130] The process conditions of the first sintering are substantially the same as those of the conventional ternary cathode active material preparation process.

[0131] In S2 , the starting material particles are mixed with a cobalt-containing compound and subjected to a second sintering to obtain particles containing lithium nickel cobalt manganese oxide.

[0132] For example, the obtained starting material particles can be mixed with the cobalt-containing compound in, for example, a high-speed mixer.

[0133] The cobalt-containing compound can be any one or more of the above-mentioned cobalt sources, which will not be described in detail here.

[0134] In some embodiments, the ratio of the amount of the cobalt-containing compound to the mass of the particles containing lithium nickel cobalt manganese oxide is in the range of 2000-10000 ppm, optionally, 4000-7000 ppm. The above ratio range refers to the range of the amount of the cobalt-containing compound added when mixed with the initial material particles. Specifically, the amount of the cobalt-containing compound added in the above range is calculated based on the mass (in μg) of the cobalt-containing compound required to be added relative to 1 g of the particles containing lithium nickel cobalt manganese oxide finally obtained.

[0135] Here, the amount of the cobalt-containing compound, calculated as the ratio of the mass of the cobalt element to the mass of the particles containing lithium nickel cobalt manganese oxide, can be either end points or any value between the end points, for example, 2200ppm, 2500ppm, 2800ppm, 3000ppm, 3200ppm, 3500ppm, 3800ppm, 4000ppm, 4200ppm, 4500ppm, 4800ppm, 5000ppm, 5500ppm, 6000ppm, 6500ppm, 7000ppm, 7200ppm, 7500ppm, 8000ppm, 8200ppm, 8500ppm, 9000ppm, 9500ppm, 9800ppm, 10000ppm, etc.

[0136] By controlling the addition amount of the cobalt-containing compound within the above range, the surface layer and the vicinity of the obtained granular material can have an ideal Co mass concentration and a good concentration gradient, thereby achieving the excellent performance of the aforementioned positive electrode active material.

[0137] In some embodiments, the second sintering is carried out at 650° C.-800° C. for 5 h-7 h.

[0138] For example, the second sintering temperature may be an end point value or any value between the end points, for example, 650°C, 660°C, 670°C, 680°C, 690°C, 700°C, 710°C, 720°C, 730°C, 740°C, 750°C, 760°C, 770°C, 780°C, 790°C, 800°C, etc. The holding time of the second sintering temperature (also referred to as the second sintering time) may be an end point value or any value between the end points, for example, 5h, 5.5h, 6h, 6.5h, 7h, etc.

[0139] The second sintering process integrates the cobalt-containing compound coated on the surface of the initial material particles with the initial material particles. Because the cobalt content in the surface layer of the particles is higher than that in the initial material particles, the cobalt gradually diffuses into the interior of the initial material particles during the second sintering process, forming particles with a high surface Co content and a gradually decreasing Co content in the radial direction from the surface to a certain depth within the particles. This helps to obtain a positive electrode active material with high structural stability and good power performance.

[0140] In order to avoid the situation where the Co content in the surface layer of the particles finally obtained tends to be consistent, the second sintering temperature should not be too high, and the sintering time should not be too long. At the same time, the second sintering temperature should not be too low, and the sintering time should not be too short, so as to prevent the cobalt-containing compound from failing to completely form an integrated single crystal particle with the initial material particles. Within the range of the above-mentioned second sintering temperature and the second sintering time, the final integrated particle containing lithium nickel cobalt manganese oxide can be obtained, and the surface layer of the particle has an ideal Co content, and within a range extending a certain distance from the particle surface along the radial direction of the particle to the particle interior, the mass concentration of Co gradually increases along the radial direction of the particle from the particle interior to the particle surface.

[0141] At the same time, some of the Ni and Mn elements in the initial material particles will diffuse to the surface of the final lithium nickel cobalt manganese oxide particles. Since only cobalt-containing compounds are further added during the second sintering, the Ni content in the surface layer and nearby areas of the final particles will gradually decrease from the interior to the surface. Mn, likely due to its strong elemental migration ability, has a more uniform distribution within the particles.

[0142] In some embodiments, the second sintering is performed in an oxygen-rich atmosphere. Optionally, the oxygen-rich atmosphere has a volume ratio of air to oxygen of between (0:10) and (8:2).

[0143] Here, the volume ratio of air:oxygen can be either end point values ​​or any value between the end point values, for example, it can be: 0:10, 1:9, 2:8, 3:7, 4:6, 5:6, 6:4, 8:2, etc.

[0144] In the present disclosure, the second sintering is carried out in an oxygen-rich sintering atmosphere, which helps to suppress the Li / Ni mixing caused by low cobalt during the sintering process, can extend the life of the material, and help improve the cycle life of the secondary battery.

[0145] In some embodiments, the preparation method further comprises: mixing the particles containing lithium nickel cobalt manganese oxide with an M' source and performing a third sintering to form a coating layer on the surface of the particles. Optionally, the M' comprises at least one selected from Mg, Al, Ca, Ba, Ti, Zr, Zn, Nb, Sb, Ce, La, W, and B.

[0146] The present disclosure does not particularly limit the source of M'. For example, the source of M' may be one or more of, but is not limited to, chlorides, sulfates, nitrates, oxides, hydroxides, fluorides, carbonates, bicarbonates, acetates, phosphates, dihydrogen phosphates, and organic compounds of the aforementioned elements. Those skilled in the art can determine suitable compounds as M' sources based on the selected M element.

[0147] In this embodiment, the amount of the compound used as the M' source added to the final positive electrode active material is 100 ppm to 3000 ppm, calculated as the M' element, such as 100 ppm, 300 ppm, 400 ppm, 700 ppm, 1000 ppm, 1300 ppm, 1500 ppm, 1800 ppm, 2000 ppm, 2200 ppm, 2500 ppm, 2700 ppm, 3000 ppm, etc. When the amount of the compound used as the M' source is within the above range, the final coating layer can have the above-mentioned suitable thickness, thereby effectively isolating the particle body from the electrolyte, reducing positive electrode side reactions, and ensuring that the positive electrode active material has good lithium ion diffusion and migration capabilities.

[0148] In some exemplary embodiments, performing a third sintering operation includes uniformly mixing the lithium nickel cobalt manganese oxide particles obtained in the second sintering process with a compound serving as an M' source, and then sintering at a temperature of 250°C to 350°C for 4 to 6 hours to obtain positive electrode active material particles having an M' oxide coating layer. Due to the low sintering temperature, the M' element is less likely to diffuse into the bulk particles, and the elements in the bulk particles are also less likely to diffuse further.

[0149] The present disclosure also provides a secondary battery and an electric device.

[0150] The secondary battery and the electric device of the present disclosure will be described below with reference to the accompanying drawings as appropriate.

[0151] In one embodiment of the present disclosure, a secondary battery is provided.

[0152] The term "secondary battery" mentioned herein refers to a battery cell, a battery module, or a battery pack. Each of these is described below.

[0153] Typically, a secondary battery cell consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During the battery's charge and discharge process, active ions (i.e., lithium ions) are embedded and released back and forth between the positive and negative electrodes. The electrolyte conducts ions between the positive and negative electrodes. The separator is placed between the positive and negative electrodes, primarily to prevent a short circuit between the positive and negative electrodes while allowing ions to pass through.

[0154] Positive electrode

[0155] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode film layer includes the positive electrode active material disclosed herein.

[0156] As an example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive electrode current collector.

[0157] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0158] The positive electrode active material of the secondary battery of the present disclosure includes at least the above-mentioned positive electrode active material of the present disclosure. In some embodiments, the positive electrode active material may also include a positive electrode active material for lithium ion batteries well known in the art. As an example, the positive electrode active material may also include at least one of the following materials: lithium-containing phosphates with an olivine structure, lithium transition metal oxides and their respective modified compounds. However, the present disclosure is not limited to these materials, and other traditional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Among them, examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also referred to as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2 (also referred to as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2 (also referred to as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2 (also referred to as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2 (also referred to as NCM 811 ), lithium nickel cobalt aluminum oxide (such as LiNi0.85 Co 0.15 Al 0.05 O2) and its modified compounds. Examples of olivine-structured lithium-containing phosphates may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon.

[0159] The battery's charge and discharge processes involve the intercalation and deintercalation of lithium, leading to different molar Li contents at different discharge states. The molar Li contents listed in this disclosure for positive electrode active materials refer to the material's initial state, i.e., before addition. When a positive electrode active material is used in a battery system, its molar Li content will change over the course of charge and discharge cycles.

[0160] In the list of positive electrode active materials in this disclosure, the molar content of O is only a theoretical value. Oxygen release from the lattice will cause the molar content of oxygen to change, and the actual molar content of O will fluctuate.

[0161] In some embodiments, the positive electrode film layer may further optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin.

[0162] In some embodiments, the positive electrode film layer may further include a conductive agent. For example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0163] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0164] Negative electrode

[0165] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer provided on at least one surface of the negative electrode current collector, wherein the negative electrode film layer includes a negative electrode active material.

[0166] As an example, the negative electrode current collector has two surfaces opposite to each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0167] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one surface of the polymer base material. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0168] In some embodiments, the negative electrode active material may be a negative electrode active material for a battery that is well known in the art. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, lithium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxides, and tin alloys. However, the present disclosure is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0169] In some embodiments, the negative electrode film layer may further include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0170] In some embodiments, the negative electrode film layer may further include a conductive agent, which may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0171] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0172] In some embodiments, the negative electrode sheet can be prepared by the following method: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0173] electrolytes

[0174] The electrolyte conducts ions between the positive and negative electrodes. The present disclosure does not specifically limit the type of electrolyte, and the electrolyte can be selected based on needs. For example, the electrolyte can be liquid.

[0175] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0176] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0177] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclopentane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

[0178] In some embodiments, the electrolyte may further include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0179] Isolation film

[0180] In some embodiments, the battery cell further includes a separator. The present disclosure has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

[0181] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0182] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0183] In some embodiments, the battery cell may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0184] In some embodiments, the outer packaging of the battery cell may be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the battery cell may be a soft shell, such as a pouch-type soft shell. The soft shell may be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0185] The present disclosure has no particular limitation on the shape of the battery cell, which may be cylindrical, square, or any other shape. For example, FIG3 shows a battery cell 5 having a square structure as an example.

[0186] In some embodiments, referring to Figure 4, the outer packaging may include a shell 51 and a top cover assembly 53. Among them, the shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the top cover assembly 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can form an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the battery cell 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0187] In some embodiments, battery cells may be assembled into a battery module. The battery module may contain one or more battery cells. The specific number may be selected by those skilled in the art based on the application and capacity of the battery module.

[0188] FIG5 shows an example battery module 4. Referring to FIG5 , in the battery module 4, multiple battery cells 5 may be arranged sequentially along the length of the battery module 4. Of course, they may also be arranged in any other manner. Furthermore, the multiple battery cells 5 may be secured together using fasteners.

[0189] Optionally, the battery module 4 may further include a housing having an accommodation space, and the plurality of battery cells 5 are accommodated in the accommodation space.

[0190] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art based on the application and capacity of the battery pack.

[0191] Figures 6 and 7 illustrate an example battery pack 1. Referring to Figures 6 and 7 , the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box comprises an upper case 2 and a lower case 3. The upper case 2 can be positioned over the lower case 3 to form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0192] In addition, the present disclosure further provides an electrical device, the electrical device including the secondary battery provided in the present disclosure. The secondary battery can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0193] As the electrical device, a battery cell, a battery module or a battery pack can be selected according to its usage requirements.

[0194] Figure 8 shows an example of an electric device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery, a battery pack or battery module can be used.

[0195] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is usually required to be lightweight and thin, and may use a battery cell as a power source.

[0196] Example

[0197] The following examples are provided. The examples described below are illustrative and are intended only to explain the present disclosure and are not to be construed as limiting the present disclosure. Where specific techniques or conditions are not specified in the examples, the methods were performed according to those described in the literature in the art or according to the product specifications. Reagents or instruments used without manufacturer's indication are all commercially available conventional products.

[0198] Example

[0199] Preparation of positive electrode active materials

[0200] Example 1

[0201] Preparation of positive electrode active material precursor: In a continuous stirred tank reactor, nickel sulfate, manganese sulfate, and cobalt sulfate were added to deionized water in a molar ratio of Ni:Co:Mn=55:6:39 (based on each element) to prepare a transition metal salt solution with a total molar concentration of 2 mol / L. 4 mol / L sodium hydroxide was added as a precipitant and 0.4 mol / L ammonia water was added as a complexing agent. The precursor Ni was prepared by coprecipitation reaction at pH 11.3 for 24 hours. 0.55 Co 0.06 Mn 0.39 (OH)2.

[0202] Preparation method of positive electrode active material:

[0203] Step S1: The above-mentioned precursor and lithium carbonate (as a Li-containing compound) are mixed in a high-speed mixer at a molar ratio of 1:1.07, 1000 ppm of Al2O3 as Al element, and 2000 ppm of WO3 as W element. The mixture is then sintered in a kiln at 900°C for 12 hours. After cooling to room temperature, the mixture is mechanically ground by a jet mill for 5 hours to obtain product N1.

[0204] Step S2: Place a mixture of the product N1 obtained in the previous step and 4000ppm (in terms of cobalt element) of Co(OH)2 in a high-speed mixer for mixing, and then place it in a kiln for sintering at 700°C for 6h to obtain product N2. After high-temperature sintering, metal ions diffuse to form a structure with a high Co content on the surface and a low Co content at a certain depth inside.

[0205] Step S3: The product N2 obtained in step S2 is uniformly mixed with 2000 ppm TiO2 and sintered at 300° C. for 5 h to obtain the single crystal positive electrode active material disclosed herein. The Dv50 of the single crystal positive electrode active material is about 4 μm. The stoichiometric formula of the product N1 is shown in Table 1.

[0206] Example 2-26

[0207] The positive electrode active materials of each example were prepared using a method similar to that of Example 1. The material compositions, raw material amounts, and preparation process conditions that were changed are shown in Table 1. Any changes not listed in Table 1 are the same as those of Example 1. The Dv50 values ​​of the positive electrode active materials obtained in these examples were all approximately 4 μm.

[0208] Comparative Example 1

[0209] The preparation method of the positive electrode active material was referred to in Example 1, except that step S2 was omitted. The Dv50 of the obtained positive electrode active materials was about 4 μm.

[0210] Comparative Example 2

[0211] The preparation method of the positive electrode active material was referred to as in Example 1, except that Step S2 and Step S3 were omitted. The Dv50 of the obtained positive electrode active materials was about 4 μm.

[0212] Preparation of lithium-ion batteries

[0213] Preparation of positive electrode sheets: The positive electrode active materials prepared in Examples 1-26 and Comparative Examples 1-2 were dispersed with conductive carbon black SP and binder PVDF in a weight ratio of 98:1:1 in NMP solvent and mixed evenly to obtain positive electrode slurry. The positive electrode slurry was evenly coated on the positive electrode current collector aluminum foil, dried, and cold pressed to obtain positive electrode sheets. The coating amount per unit area was 0.27 g / 1540.25 mm 2 .

[0214] Preparation of negative electrode materials: Graphite (active negative electrode material), sodium carboxymethyl cellulose (thickener), styrene-butadiene rubber (binder), and acetylene black (conductive agent) were mixed in a mass ratio of 97:1:1:1, deionized water was added, and negative electrode slurry was obtained using a vacuum mixer. The negative electrode slurry was evenly coated on copper foil, dried at room temperature, and then transferred to a 120°C oven for 1 hour. The negative electrode sheets were then cold pressed and cut into pieces. The coating weight per unit area was 0.17g / 1540.25mm. 2 .

[0215] Preparation of the electrolyte: The organic solvent is a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC), with the volume ratio of EC, EMC, and DEC being 20:20:60. In an argon atmosphere glove box with a water content of <10 ppm, thoroughly dried lithium salt (lithium hexafluorophosphate) is dissolved in the organic solvent and mixed thoroughly to obtain the electrolyte. The concentration of the lithium salt is 1 mol / L.

[0216] Isolation film: Use 12μm thick polypropylene isolation film.

[0217] Preparation of lithium-ion batteries: The positive electrode sheet, separator, and negative electrode sheet prepared above are stacked in order, with the separator placed between the positive and negative electrode sheets to act as an isolation. After winding into a square bare cell, an aluminum-plastic film is placed in the cell. After baking at 80°C to remove water, the electrolyte prepared above is injected and sealed. After the formation and capacity separation processes, the finished battery is obtained.

[0218] Performance Testing

[0219] The following tests were performed on the positive electrode active materials and batteries obtained in the above examples and comparative examples.

[0220] 1. X-ray photoelectron spectroscopy (XPS)

[0221] The positive electrode active material prepared in Example 1 was subjected to surface and internal analysis using Shimadzu's Axis Supra+ X-ray photoelectron spectroscopy. The specific test results are shown in Figure 2. To determine the internal conditions of the positive electrode active material, the positive electrode active material prepared in Example 1 was cut using argon ion polishing prior to X-ray photoelectron spectroscopy analysis to remove outer layers of material of predetermined thicknesses (500 nm, 1000 nm, and 2000 nm).

[0222] The positive electrode active materials prepared in Example 5 and Example 6 were subjected to surface analysis using Shimadzu's Axis Supra+ X-ray photoelectron spectroscopy. Please refer to FIG2 for specific test results.

[0223] The same method was used to perform surface and internal analysis on the positive electrode active materials prepared in Example 3 and Example 8. Similarly, in order to determine the internal conditions of the positive electrode active materials, before performing the X-ray photoelectron spectroscopy analysis, the positive electrode active materials prepared in Example 3 and Example 8 were cut using argon ion polishing technology to remove a predetermined thickness of outer layer material and obtain the conditions at 1000 nm (1 μm) from the surface.

[0224] By calculating the peak area of ​​the curves obtained in Example 1, Example 3 and Example 8 at 777.8 eV (dividing the peak area at the surface by the peak area at 1000 nm), the ratio of the Co mass concentration on the surface of the positive electrode active material particle to the Co mass concentration at a distance L = 1 μm extending radially toward the interior of the particle is obtained.

[0225] In Example 1, the ratio of the Co mass concentration on the surface of the positive electrode active material particles to the Co mass concentration at a distance L = 1 μm extending radially toward the interior of the particles is 1.5; in Example 3, the ratio of the Co mass concentration on the surface of the positive electrode active material particles to the Co mass concentration at a distance L = 1 μm extending radially toward the interior of the particles is 1.2; in Example 8, the ratio of the Co mass concentration on the surface of the positive electrode active material particles to the Co mass concentration at a distance L = 1 μm extending radially toward the interior of the particles is 1.6.

[0226] 2. Battery capacity in grams

[0227] Place the soft pack battery at a constant temperature of 25°C for 2 hours, then charge it to 4.5V at 1 / 3C at 2.8V~4.5V, then charge it at a constant voltage at 4.5V until the current is ≤0.05mA, place it at rest for 5 minutes, and then discharge it to 2.8V at 1C. Record the capacity C of the battery. 放 The gram capacity of the battery is calculated as follows: Gram capacity = battery capacity C 放(mAh) / mass of positive electrode active material (g).

[0228] 3. DC resistance (DCR)

[0229] Place the lithium-ion batteries prepared above at room temperature until thermal equilibrium is reached. Perform three standard charge and discharge cycles at a current of 1 / 3C with a cut-off voltage of 4.5V, and record the capacity of the battery at the third standard charge and discharge cycle as the standard capacity C. Place at a test temperature of 25°C until thermal equilibrium is reached. Adjust the battery to 50% SOC at a discharge rate of 1 / 3C and discharge at a current of 1C for 30S. Record the battery voltage V2 before discharge termination, the current I, and the battery voltage V1 after the battery voltage stabilizes. Calculate the DC internal resistance DCR according to the formula DCR = (V2-V1) / I.

[0230] 4. Cycle performance test

[0231] The battery was charged and discharged at 25°C. The battery was charged at a constant current and constant voltage (CCCV) of 1 / 3C to 4.5V, left to rest for 10 minutes, and then discharged at a constant current of 1C to 2.8V. The final result is expressed as the number of cycles required for the capacity to decay to 80% at a 0.33C / 1C cycle.

[0232] The test results of items 2-4 of the lithium ions prepared by the above examples and comparative examples are shown in Table 1 below:

[0233] As can be seen from Table 1 above, compared to the positive electrode active materials of Comparative Example 1 (with a coating layer) and Comparative Example 2 (without a coating layer) that did not undergo a second sintering step, Examples 1-26, especially Example 1, all achieved significant improvements in the power performance (reduction in DC resistance (DCR)) and cycle performance of the secondary batteries. Furthermore, the specific capacity of the positive electrode active materials was also improved.

[0234] Specifically, in Examples 7-12, the positive electrode active materials obtained by adding different amounts of Co(OH)2 in the second sintering process all contribute to increasing the battery's gram capacity, reducing impedance, and improving cycle performance. Preferably, the cobalt-containing compound is added in an amount of 4000-7000 ppm, calculated as Co element, to achieve the best balance of various properties. If the addition amount is too low, the improvement in kinetic performance is limited, while if the addition amount is too high, on the one hand, it increases cost, and on the other hand, too high an addition amount is prone to the phenomenon of material structure destruction due to Co dissolution during the cycle, resulting in deterioration of material performance, which is not conducive to improving cycle performance.

[0235] Different second sintering temperatures were used in Examples 13-16. From Examples 13-14 and comparison with Example 1, it can be seen that using a temperature of approximately 650-800°C for the second sintering helps improve the specific capacity of the material and the cycle performance of the secondary battery. If the second sintering temperature is too low, the sintering effect is poor, and the Co element is enriched in the surface layer of the material, failing to form an ideal gradient, which is not conducive to improving the kinetics of the secondary battery. If the sintering temperature is too high, the Co element in the surface layer of the material diffuses into the bulk phase, resulting in insufficient Co content in the surface layer, which is not conducive to improving the cycle performance.

[0236] In Examples 17-20, different second sintering times were used. As can be seen from Examples 17-18, combined with Example 1, a second sintering time of 6-7 hours is more advantageous for improving both the power performance and cycle performance of the secondary battery. However, if the second sintering time is too short, the Co element will not migrate sufficiently and will be concentrated on the surface of the material. If the second sintering time is too long, the Co content in the surface of the material will be insufficient, and therefore, neither of these times can significantly improve the dynamic performance of the secondary battery.

[0237] In Examples 1-4, the ratios of Ni, Co, and Mn in the precursor are changed. As the ratio of Co increases, the secondary battery can have a significantly reduced DC impedance, indicating that the power performance of the secondary battery is significantly improved. In addition, the cycle performance is also improved.

[0238] In Examples 5 and 6, the power performance and cycle performance of the secondary battery were affected to a certain extent by reducing the addition amount of the doping elements Al and W compared with Example 1.

[0239] In addition, the oxygen content during the second sintering step was adjusted in Examples 24-26. The introduction of oxygen can reduce the mixing of lithium and nickel in the material, helping to achieve a more stable bulk structure and improve cycling performance. When the oxygen content was increased (Examples 24 and 25), the cycling performance of the secondary battery was significantly improved.

[0240] It should be noted that the present disclosure is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and within the scope of the technical solution of the present disclosure, embodiments having substantially the same structure as the technical concept and exerting the same effects are all included in the technical scope of the present disclosure. In addition, within the scope of the present disclosure, various modifications that can be imagined by those skilled in the art to the embodiments, and other methods constructed by combining some of the constituent elements of the embodiments are also included in the scope of the present disclosure.

Claims

1. A positive electrode active material comprising particles containing lithium nickel cobalt manganese oxide, Within a range of a distance L extending from the particle surface along the radial direction of the particle to the particle interior, the mass concentration of Co gradually increases along the radial direction of the particle from the particle interior to the particle surface, where L=1 μm.

2. The positive electrode active material according to claim 1, wherein The L is 1.5 μm.

3. The positive electrode active material according to claim 1, wherein The Co mass concentration on the surface of the particle is 1.02 to 2 times, and optionally 1.1 to 1.8 times, the Co mass concentration at a distance L=1 μm extending from the particle radially to the inside of the particle.

4. The positive electrode active material according to any one of claims 1 to 3, wherein The mass concentration of Ni gradually decreases from the inside of the particle to the surface of the particle along the radial direction of the particle.

5. The positive electrode active material according to any one of claims 1 to 4, wherein The absolute value of the difference in mass concentration of Li and Mn at any two points in the particles is less than or equal to 20%.

6. The positive electrode active material according to any one of claims 1 to 5, wherein The positive electrode active material is represented by the following formula 1: Li x (Ni a Co b Mn c ) d M 1-d O 2-y A y Formula 1 Among them, 0.95≤x≤1.3, 0.3≤a≤0.7, 0.01≤b≤0.15, 0.1≤c≤0.5, a+b+c=1, 0.95≤d≤1, 0≤y≤0.1, M includes at least one selected from Zr, Sr, B, Ti, Mg, Sn, Tb, W, Nb and Al, and A includes at least one selected from S, N, F, Cl, Br and I.

7. The positive electrode active material according to any one of claims 1 to 6, wherein The volume distribution particle size Dv50 of the particles is in the range of 3.5 μm≤Dv50≤7.5 μm, optionally, 4 μm≤Dv50≤7 μm.

8. The positive electrode active material according to any one of claims 1 to 7, wherein The particles also have a coating layer. Optionally, the coating layer is an oxide or hydroxide of M', wherein M' includes at least one selected from Mg, Al, Ca, Ba, Ti, Zr, Zn, Nb, Sb, Ce, La, W and B.

9. The positive electrode active material according to claim 8, wherein The coating layer has a thickness of 0.001-0.5 μm, and optionally 0.01-0.1 μm.

10. A method for preparing the positive electrode active material according to any one of claims 1 to 9, the method comprising: Mixing a lithium source, precursors of Ni, Co and Mn and optionally a compound containing a doping element M and a compound containing a doping element A, and performing a first sintering to obtain initial material particles; The starting material particles are mixed with a cobalt-containing compound and subjected to a second sintering to obtain particles containing lithium nickel cobalt manganese oxide.

11. The preparation method according to claim 10, wherein: The lithium source is selected from one or more of lithium oxide, lithium phosphate, lithium dihydrogen phosphate, lithium acetate, lithium hydroxide, lithium carbonate and lithium nitrate. Optionally, the lithium source is one or more of lithium carbonate, lithium hydroxide and lithium nitrate.

12. The preparation method according to claim 10 or 11, wherein: The precursor is selected from one or more of oxides, hydroxides and carbonates containing Ni, Co and Mn. Optionally, the precursor is a hydroxide containing Ni, Co and Mn.

13. The preparation method according to any one of claims 10 to 12, wherein: The ratio of the mass of the cobalt-containing compound in terms of cobalt element to the mass of the particles containing lithium nickel cobalt manganese oxide is in the range of 2000-10000 ppm, and optionally, in the range of 4000-7000 ppm.

14. The preparation method according to any one of claims 10 to 13, wherein The molar ratio of Ni:Co:Mn in the precursor is in the range of (50-70):(6-25):(29-40), optionally, in the range of (55-65):(6-12):(29-33).

15. The preparation method according to any one of claims 10 to 14, wherein The first sintering is carried out at 600° C.-1000° C. for 5 h to 25 h, and optionally at 650° C.-950° C. for 8 h to 20 h.

16. The preparation method according to any one of claims 10 to 15, wherein The second sintering is performed in an oxygen-rich atmosphere. Optionally, the oxygen-rich atmosphere has a volume ratio of air to oxygen of between (0:10) and (8:2).

17. The preparation method according to any one of claims 10 to 16, wherein The second sintering is carried out at 650° C.-800° C. for 5 h-7 h.

18. The preparation method according to any one of claims 10 to 17, wherein The preparation method further comprises: The particles containing lithium nickel cobalt manganese oxide are mixed with an M' source and subjected to a third sintering to form a coating layer on the surface of the particles; optionally, the M' includes at least one selected from Mg, Al, Ca, Ba, Ti, Zr, Zn, Nb, Sb, Ce, La, W and B.

19. A secondary battery, comprising a positive electrode plate, wherein the positive electrode plate comprises at least the positive electrode active material according to any one of claims 1 to 9, or the positive electrode plate comprises at least the positive electrode active material prepared by the preparation method according to any one of claims 10 to 18.

20. An electrical device comprising the secondary battery according to claim 19.

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