Positive electrode active material, method for manufacturing the same, positive electrode sheet, and lithium ion secondary battery

Abstract

This invention discloses a positive electrode active material and its preparation method, a positive electrode sheet, and a lithium-ion secondary battery. The positive electrode active material includes bulk particles and an M-containing coating on the outer surface of the bulk particles. 1 The elemental oxide coating layer comprises nickel-containing lithium composite oxide particles; the bulk particles are uniformly doped with M. 2 Element; the surface layer of the bulk particles is doped with M 3 The outer doped layer of the element; M 1 Elements and M 3 Each element is independently selected from one or more of Mg, Al, Ca, Ce, Ti, Zr, Zn, Y, and B, M 2 The elements include one or more of Si, Ti, Cr, Mo, V, Ge, Se, Zr, Nb, Ru, Rh, Pd, Sb, Te, Ce, and W. Using the positive electrode active material provided by this invention enables lithium-ion secondary batteries to simultaneously achieve high energy density, high-temperature cycle performance, and high-temperature storage performance.

Description

[0001] This application is a divisional application based on the invention with application number 201910825110.4, application date September 2, 2019, applicant CATL, and invention title "Positive electrode active material and preparation method thereof, positive electrode sheet, lithium-ion secondary battery". Technical Field

[0002] This invention belongs to the field of secondary battery technology, specifically relating to a positive electrode active material and its preparation method, a positive electrode sheet, and a lithium-ion secondary battery. Background Technology

[0003] Lithium-ion rechargeable batteries are a type of rechargeable battery that primarily relies on the movement of lithium ions between the positive and negative electrodes to function. They are currently a widely used clean energy source. The positive electrode active material, as a crucial component of lithium-ion rechargeable batteries, provides the lithium ions that move back and forth between the positive and negative electrodes during the battery's charging and discharging process; therefore, the positive electrode active material is vital to the battery's performance.

[0004] Nickel-containing lithium composite oxides have a high theoretical capacity, and lithium-ion secondary batteries using nickel-containing lithium composite oxides as positive electrode active materials can be expected to achieve high energy density. However, the high-temperature cycling performance of this type of lithium-ion secondary battery is poor in practical applications. Summary of the Invention

[0005] This invention provides a positive electrode active material and its preparation method, a positive electrode sheet, and a lithium-ion secondary battery, aiming to enable the lithium-ion secondary battery to simultaneously achieve high energy density and high-temperature cycle performance.

[0006] A first aspect of this invention provides a positive electrode active material, comprising bulk particles and an M-containing compound coated on the outer surface of the bulk particles. 1 The elemental oxide coating layer comprises nickel-containing lithium composite oxide particles; the bulk particles are uniformly doped with M. 2 Element; the surface layer of the bulk particles is doped with M 3 The outer doped layer of the element; M 1 Elements and M 3 All have the same element and are L elements, where L elements are one or more of Mg, Al, Ca, Ce, Ti, Zr, Zn, Y, and B, and M 2 The elements include one or more of Si, Ti, Cr, Mo, V, Ge, Se, Zr, Nb, Ru, Rh, Pd, Sb, Te, Ce, and W.

[0007] A second aspect of the present invention provides a positive electrode sheet, which includes a positive current collector and a positive active material layer disposed on the positive current collector, wherein the positive active material layer includes a positive active material according to the first aspect of the present invention.

[0008] A third aspect of the present invention provides a lithium-ion secondary battery, which includes a positive electrode sheet according to a second aspect of the present invention.

[0009] A fourth aspect of this invention provides a method for preparing a positive electrode active material, the method comprising:

[0010] Provide a mixture, which includes a nickel-containing transition metal source, a lithium source, and M. 2 Precursors of elements;

[0011] The mixture was sintered to obtain a uniformly doped material containing M. 2 The matrix particles of the element;

[0012] matrix particles and M 3 The precursors of the elements are mixed and sintered to make M 3 Elements are doped onto the surface layer of the matrix particles to form an outer doped layer, thus obtaining the bulk particles;

[0013] Combine bulk particles with M 1 The precursors of the elements are mixed and sintered to form M-containing compounds on the outer surface of the bulk particles. 1 An element oxide coating layer is used to obtain a positive electrode active material;

[0014] Among them, M 1 Elements and M 3 Each element is independently selected from one or more of Mg, Al, Ca, Ce, Ti, Zr, Zn, Y, and B, M 2 The elements include one or more of Si, Ti, Cr, Mo, V, Ge, Se, Zr, Nb, Ru, Rh, Pd, Sb, Te, Ce, and W.

[0015] Compared with the prior art, the embodiments of the present invention have at least the following beneficial effects:

[0016] The positive electrode active material provided in this invention includes a nickel-containing lithium composite oxide. This positive electrode active material has high specific capacity characteristics, and its use enables lithium-ion secondary batteries to have high energy density. The bulk particles are uniformly doped with M. 2 The element can significantly improve the structural stability and high-temperature cycling stability of the positive electrode active material. Meanwhile, the surface layer of the bulk particles is doped with M. 3 The outer doped layer of the element, and the M-containing layer on the outer surface of the bulk particle. 1Element oxide coating, M 1 Elements and M 3 The high lattice matching between the elements and the surface of the bulk particles provides excellent protection for them. Therefore, this invention improves the cycle life of the positive electrode active material, reduces battery gas production, and thus significantly improves the high-temperature cycle performance and high-temperature storage performance of lithium-ion secondary batteries. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the coating and doping of a positive electrode active material according to an embodiment of the present invention.

[0018] Figure 2 M is the bulk particles in Examples 1-28 and Comparative Examples 1-9. 2 Schematic diagram of the test location for element doping mass concentration deviation. Detailed Implementation

[0019] To make the inventive objectives, technical solutions, and beneficial effects of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the embodiments described in this specification are merely illustrative of the invention and are not intended to limit the invention.

[0020] For simplicity, this paper only explicitly discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form an undefined range; and any lower limit can be combined with other lower limits to form an undefined range, just as any upper limit can be combined with any other upper limit to form an undefined range. Furthermore, although not explicitly stated, every point or individual value between the endpoints of a range is included within that range. Therefore, each point or individual value can be used as its own lower or upper limit, combined with any other point or individual value, or combined with other lower or upper limits to form an undefined range.

[0021] In this description, it should be noted that, unless otherwise stated, "above" and "below" include the stated number, and "multiple" in "one or more" means two or more.

[0022] The above description of the invention is not intended to describe every disclosed embodiment or implementation of the invention. Instead, the following description provides more specific examples of exemplary embodiments. Throughout this application, guidance is provided through a series of embodiments that can be used in various combinations. In each example, the examples are listed only as representative groups and should not be construed as exhaustive.

[0023] Positive electrode active material

[0024] One embodiment of the present invention provides a positive electrode active material, please refer to... Figure 1The positive electrode active material includes bulk particles and M-containing materials coated on the outer surface of the bulk particles. 1 The elemental oxide coating layer comprises nickel-containing lithium composite oxide particles; the bulk particles are uniformly doped with M. 2 Element; the surface layer of the bulk particles is doped with M 3 The outer doped layer of the element; M 1 Elements and M 3 Each element is independently selected from one or more of Mg, Al, Ca, Ce, Ti, Zr, Zn, Y, and B, M 2 The elements include one or more of Si, Ti, Cr, Mo, V, Ge, Se, Zr, Nb, Ru, Rh, Pd, Sb, Te, Ce, and W.

[0025] The bulk phase of the aforementioned bulk particle refers to the entire bulk particle; the surface layer of the bulk particle is the region extending from the outer surface of the bulk particle to a predetermined depth in the direction of the core.

[0026] The positive electrode active material in this invention includes a nickel-containing lithium composite oxide. This positive electrode active material has a high specific capacity characteristic, and using this positive electrode active material enables lithium-ion secondary batteries to have a high energy density. Preferably, the molar content of nickel in the nickel-containing lithium composite oxide accounts for 50% to 95% of the total molar content of transition metal site elements, such as 50%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, etc. Batteries using high-nickel-content positive electrode active materials have higher energy densities.

[0027] The nickel-containing lithium composite oxide has a layered structure with lithium sites, transition metal sites, and oxygen sites. The aforementioned transition metal site elements refer to the elements located at the transition metal sites.

[0028] Selected from the above specific types of M 2 Elements are uniformly doped into the bulk phase of the bulk particles, M 2 The element can effectively bind oxygen atoms, making it less likely for the positive electrode active material to release oxygen during high-temperature cycling, and inhibiting the irreversible structural phase transition of the material. This ensures that the material structure remains in a layered phase state with strong electrochemical activity, significantly improving the structural stability and high-temperature cycling stability of the positive electrode active material, thereby improving the cycle performance and safety performance of lithium-ion secondary batteries, especially improving the high-temperature cycling performance of the battery.

[0029] Furthermore, M 3 Elements are doped into the surface layer of the bulk particles to form an outer doped layer, and the outer surface of the bulk particles has M-containing elements. 1 The coating layer of the element oxide is selected from the specific type of M mentioned above. 1 Elements and M 3The element has a high lattice matching degree with the surface of the bulk particles, does not damage the structure of the bulk particles, and provides good protection for them. Containing M 1 The oxide coating layer can isolate the bulk particles from contact with the electrolyte, containing M 3 The external doping layer can reduce the activity of side reactions on the surface of the bulk particles. Under the dual protection of the coating layer and the external doping layer, the surface of the positive electrode active material can be effectively protected from electrolyte corrosion, reducing side reactions, improving the high-temperature cycle performance of the battery, and effectively suppressing gas generation during high-temperature storage, thus improving the high-temperature storage performance of the battery.

[0030] In some preferred embodiments, when the positive electrode active material is in a 78% delithiation state, M 2 The element has a valence of +3 or higher, preferably one or more of +4, +5, +6, +7, and +8.

[0031] In this article, "78% delithiation state" refers to the state of the battery during charging when the molar content of lithium extracted from the positive electrode active material accounts for 78% of the theoretical lithium content. In practical use, secondary batteries are generally set to a "fully charged state" and a corresponding "charging cutoff voltage" to ensure safe operation. "Fully charged state" refers to the secondary battery's state of charge (SOC) being 100%. In other words, a secondary battery containing the aforementioned positive electrode active material is charged to the charging cutoff voltage within the reversible charge-discharge range. This "fully charged state" or "charging cutoff voltage" may vary depending on the positive electrode active material or safety requirements. For secondary batteries made with nickel-containing lithium composite oxide positive electrode active materials, the delithiation state of the positive electrode active material in the "fully charged state" is generally around "78% delithiation state" to ensure normal operation.

[0032] In this paper, we combine the correspondence between the "delithiation state" and the charging voltage to study the positive electrode active material in the "78% delithiation state". Specifically, a series of batteries using this positive electrode active material were charged at a 0.1C rate to 2.8V, 2.9V, 3.0V, 3.1V, 3.2V, 3.3V, ..., 4.0V, 4.1V, 4.2V, 4.3V, 4.4V, 4.5V, 4.6V, and 4.7V (i.e., charging voltage intervals of 0.1V). The positive electrode was then removed, the electrolyte was washed away, and the positive electrode active material was digested. The mass concentrations of Li, transition metals, and O in the positive electrode active material were measured using inductively coupled plasma-optical emission spectrometers (ICP-OES). The stoichiometric ratios of each element in the positive electrode active material at that charging voltage were calculated, and the chemical formula of the positive electrode active material at that charging voltage was obtained. This led to the determination of the charging voltage corresponding to the "78% delithiation state".

[0033] By charging the battery containing the cathode active material to be tested to the voltage corresponding to the "78% delithiation state," the cathode active material in the "78% delithiation state" can be disassembled for further research. M in the "78% delithiation state" cathode active material... 2 The valence of an element can be determined by X-ray photoelectron spectroscopy (XPS). More precise determinations can be made by synchrotron radiation photoelectron spectroscopy (SRPES).

[0034] At 78% delithiation state, M in the positive electrode active material 2 When the element has a higher valence state, M... 2 The element can better retain oxygen atoms in their original lattice positions, preventing oxygen release from the cathode active material during heating and high-temperature cycling after lithium delithiation, suppressing irreversible structural phase transitions, and thus further improving the structural stability and high-temperature cycling stability of the cathode active material. Furthermore, this M... 2 The element can provide more electrons to the positive electrode active material, which can more stabilize the structure of the positive electrode active material, reduce the surface activity of the positive electrode active material, and reduce the amount of electrolyte decomposition gas generated during high-temperature cycling and high-temperature storage. Therefore, the high-temperature cycling performance and high-temperature storage performance of the battery are improved. In addition, M 2 The electrons contributed by the elements can also support the release of more lithium ions from the positive electrode active material, thereby further improving the energy density of the battery.

[0035] It is understandable that, in the positive electrode active materials before and after delithiation, M 2The valence state of an element can remain unchanged, M 2 The elements do not participate in redox reactions during battery charging. These M elements... 2 Elements can stabilize the layered crystal structure of positive electrode active materials.

[0036] M in positive electrode active material 2 Elements can also participate in redox reactions during battery charging, M 2 The element has more than two stable valence states, and is in a lower valence state in the positive electrode active material before delithiation, while during battery charging, M... 2 Elements contribute electrons to the positive electrode active material and their valence state increases. During battery charging, M... 2 The electrons contributed by the element cause charge compensation within the material, increasing the number of lithium ions that can be extracted from the positive electrode active material, thereby improving the battery's capacity performance and energy density. Simultaneously, the increased valence state of M... 2 Elements can strengthen the binding of oxygen atoms, improve the structural stability of positive electrode active materials, reduce the surface activity of positive electrode active materials, and improve the high-temperature cycle performance and high-temperature storage performance of batteries.

[0037] In some embodiments, in the "78% delithiation state" positive electrode active material, M 2 An element can have more than two distinct valence states, with M in its highest valence state. 2 The valence of the element is one or more of +4, +5, +6, +7, and +8. M has a higher valence state and exhibits variable valence states. 2 Elements can contribute more electrons to the positive electrode active material, which can further stabilize the material structure and reduce side reactions on the material surface, thereby further improving the high-temperature cycle performance and high-temperature storage performance of the battery.

[0038] Furthermore, when the positive electrode active material is in a 78% delithiation state, M 2 An element has more than two distinct valence states, where the lower valence state is M. 2 The element can further contribute electrons to support the release of more lithium ions at the positive electrode, thereby further improving the energy density of the battery.

[0039] In some preferred embodiments, M at any point in the bulk particles of the positive electrode active material 2 The mass concentration deviation of the element is less than 30%, more preferably less than 20%.

[0040] In this paper, M is located at any point in the bulk particle. 2 The mass concentration of the element is M within the minimum volume at that point. 2The mass concentration of an element relative to all elements can be obtained by elemental analysis using EDX (Energy Dispersive X-ray Spectroscopy) or EDS (Energy Dispersive Spectrometer) combined with single-point scanning using TEM (Transmission Electron Microscope) or SEM (Scanning Electron Microscope), or other similar methods. Specifically, when using EDX or EDS elemental analysis combined with single-point scanning using TEM or SEM, the mass concentration (M) at different sites within the bulk particle, expressed in μg / g, is... 2 The mass concentrations of the elements are denoted as η1, η2, η3, ..., η n n is a positive integer greater than or equal to 15.

[0041] M in bulk particles 2 The average mass concentration of the element is M within a single bulk particle. 2 The mass concentration of an element relative to all elements can be obtained by EDX or EDS elemental analysis combined with TEM or SEM surface scanning to determine the elemental concentration distribution, or by other similar methods. When testing the elemental concentration distribution using EDX or EDS elemental analysis combined with TEM or SEM surface scanning, the test surface includes all points from the single-point tests described above. M in the bulk particles 2 The average mass concentration of an element is denoted as The unit is μg / g.

[0042] M at any point in the bulk particle 2 The mass concentration deviation σ of the element is calculated according to the following formula (1):

[0043]

[0044] M at any point in the bulk particle 2 The mass concentration deviation of the element is less than 30%, preferably less than 20%, which means that M 2 The element is relatively uniformly distributed within the bulk particles. 2 Uniform elemental doping ensures consistent properties throughout the particle, thus allowing M to perform better. 2 The element improves the structural stability of the positive electrode active material, effectively preventing particle cracking. Furthermore, lithium ions in M... 2In particles with uniform element doping, the migration and diffusion capabilities of different regions are at the same level, and the deformation resistance is similar throughout the particle, resulting in a uniform distribution of internal stress. This improves the structural stability of the cathode active material and makes it less prone to breakage. Consequently, the capacity utilization and high-temperature cycling performance of the cathode active material are further enhanced, thereby improving the capacity performance, energy density, and high-temperature cycling performance of lithium-ion secondary batteries.

[0045] M at any point in the bulk particle 2 The smaller the mass concentration deviation of the element, the higher the concentration of M in the bulk particles. 2 The more uniform the distribution of doping elements, the better the capacity utilization and high-temperature cycling performance of the positive electrode active material.

[0046] In some preferred embodiments, M in the positive electrode active material 2 The element content ω and the M in the bulk particles 2 average mass concentration of elements The deviation is ε, where ε < 50%. Preferably, ε ≤ 30%. More preferably, ε ≤ 20%.

[0047] M in positive electrode active material 2 The element content ω and the M in the bulk particles 2 average mass concentration of elements The deviation is calculated by the following formula (2):

[0048]

[0049] ω is M in ppm for the positive electrode active material. 2 The overall mass concentration of the element, i.e., the amount of M contained in each gram of positive electrode active material, expressed in μg. 2 The mass of the element. Where ω represents the mass of M in the macroscopic cathode active material. 2 The overall content of elements, including M incorporated into the bulk particles of the positive electrode active material. 2 The element, M, is enriched in other phases on the surface of the bulk particles. 2 Elements, and M embedded between the positive electrode active material particles 2 The element ω can be obtained through absorption spectroscopy of the positive electrode active material solution, such as ICP (Inductively Coupled Plasma Emission Spectrometer) and XAFS (X-ray absorption fine structure spectroscopy).

[0050] M in positive electrode active material 2 The content of element ω and M in bulk particles2 average mass concentration of elements If the deviation ε is within the above range, it means that M 2 The elements can be smoothly incorporated into the bulk particles. The content of doped elements distributed in other phases on the surface of the bulk particles and doped elements embedded in the gaps of the positive electrode active material is small. The macroscopic and microscopic consistency of the positive electrode active material is good, the structure is uniform, and the particle stability is high, which is conducive to the positive electrode active material having high capacity utilization and high temperature cycling performance.

[0051] M in positive electrode active material 2 The preferred content of element ω is 500ppm to 5000ppm, such as 500ppm, 800ppm, 1000ppm, 1200ppm, 1500ppm, 1700ppm, 2000ppm, 2500ppm, 3000ppm, 4000ppm, 5000ppm, etc. M in the positive electrode active material 2 When the element content ω is within the above range, it can better improve the high-temperature cycle performance and high-temperature storage performance of the battery, and effectively leverage M. 2 The charge compensation effect of elements in positive electrode active materials.

[0052] M in positive electrode active material 2 When the element content ω is within the above range, it also provides a good carrier for the insertion and extraction of lithium ions in the positive electrode active material, which is conducive to the insertion and extraction of lithium ions. This gives the positive electrode active material a higher initial capacity and cycle capacity retention rate, thereby improving the energy density and high-temperature cycle performance of the battery.

[0053] M in positive electrode active material 1 The content of element α is preferably 100ppm to 2000ppm, such as 100ppm, 300ppm, 500ppm, 600ppm, 800ppm, 900ppm, 1000ppm, 1200ppm, 1500ppm, 1700ppm, 2000ppm, etc.

[0054] M in positive electrode active material 3 The content of element β is preferably 400ppm to 3000ppm, such as 400ppm, 700ppm, 1000ppm, 1300ppm, 1500ppm, 1800ppm, 2000ppm, 2200ppm, 2500ppm, 2700ppm, 3000ppm, etc.

[0055] M in positive electrode active material 1 Elements and M 3Within the aforementioned range, the element content can improve the stability of the positive electrode active material, reduce side reactions of the electrolyte on the material surface, and enhance the battery's high-temperature cycle performance and high-temperature storage performance. Furthermore, since the bulk particles only have a small amount of surface coating and surface doping, the positive electrode active material possesses a higher lithium-ion diffusion and migration capability, thereby contributing to higher rate performance, capacity performance, and cycle performance of the battery.

[0056] The ppm (parts per million) mentioned above represents the mass percentage of a specific element in the cathode active material, representing a fraction of the total mass of the cathode active material. α and β can be obtained through absorption spectroscopy of the cathode active material solution, such as ICP (Inductively Coupled Plasma Emission Spectrometer) or XAFS (X-ray Absorption Fine Structure Spectroscopy).

[0057] Preferably, the thickness of the coating layer is 1 nm to 200 nm, such as 50 nm to 160 nm. A coating layer thickness within this range can isolate the electrolyte from the bulk particles, reduce side reactions, and enable lithium ions to have higher diffusion and migration capabilities in the positive electrode active material. This, in turn, helps the battery to have higher capacity performance, high-temperature cycle performance, and high-temperature storage performance.

[0058] The thickness of the coating layer can be determined using methods known in the art. As an example, a cross-section of the positive electrode active material particles can be prepared using a cross-section polisher (such as the JEOL IB-09010CP argon ion cross-section polisher), passing through the core of the positive electrode active material particles. Then, elemental analysis using EDX or EDS combined with TEM or SEM (such as the Oxford Instruments X-Max EDS combined with the ZEISS Sigma-02-33 SEM) is performed to obtain an elemental distribution map of the cross-section. The thickness of the coating layer is then determined based on the elemental distribution of the cross-section. More precisely, the thickness of the coating layer can be measured at multiple (more than 3, such as 8, 10, 12, etc.) different locations on the cross-section, and the average value is recorded as the coating layer thickness.

[0059] Preferably, the thickness of the outer doped layer is 10% to 30% of the particle size of the bulk particles, such as 15% to 25%. A thickness within this range is beneficial for improving the high-temperature cycle performance and high-temperature storage performance of the battery, and also for maximizing the capacity of the positive electrode active material, resulting in a battery with higher energy density.

[0060] The thickness of the outer doped layer can be determined using methods known in the art. For example, it can be determined using methods for measuring the thickness of the coating layer. As an example, a cross-section of the positive electrode active material particles or bulk particles can be prepared using a cross-section polisher, with the cross-section passing through the particle's core; then, elemental analysis using EDX or EDS combined with TEM or SEM surface scanning can be used to obtain an elemental distribution map of the cross-section; the thickness of the outer doped layer can be obtained based on the elemental distribution of the cross-section. More precisely, the thickness values ​​of the outer doped layer at multiple (more than 3, such as 8, 10, 12, etc.) different locations on the cross-section can be measured, and the average value can be recorded as the thickness of the outer doped layer.

[0061] Similarly, the particle size of the bulk particles can be obtained using the method described above. If the bulk particles are not ideally spherical, the diameters of multiple (more than 3, such as 8, 10, 12, etc.) different orientations of the bulk particles can be tested, and the average value can be recorded as the particle size of the bulk particles.

[0062] In some preferred embodiments, M in the bulk particles 3 The element has a mass concentration gradient that decreases from the outer surface of the bulk particle towards the core. M 3 The mass concentration of elements in the bulk particles tends to decrease from the outer surface to the core of the particles, which can improve the lithium-ion conductivity of the positive electrode active material particles and improve the capacity and cycle performance of the material.

[0063] Furthermore, M in the outer doped layer 3 The mass concentration of the element is less than that of M in the coating layer 1 The mass concentration of the element. That is, the coating layer has a relatively high concentration of M. 1 The element, M, has a relatively low concentration in the outer doped layer. 3 The modified elements are mainly present on the surface of the positive electrode active material, which helps to form a surface protection for the positive electrode active material, improve the high-temperature cycle performance and high-temperature storage performance of the battery, and at the same time enable the battery to have higher capacity utilization and energy density.

[0064] It is understandable that M in the coating layer 1 M in elements and outer doped layers 3 Elements can be different or the same. For example, M in the wrapper layer. 1 M in elements and outer doped layers 3 All components are identical and contain only nitrogen (L), which can be one or more of Mg, Al, Ca, Ce, Ti, Zr, Zn, Y, and B. Furthermore, the concentration of L decreases from the outer surface of the positive electrode active material particles towards the core. This facilitates surface protection of the positive electrode active material, improves the battery's high-temperature cycle performance and high-temperature storage performance, and simultaneously enhances the battery's capacity and energy density.

[0065] In some embodiments, M in the positive electrode active material 1 Elements and M 3 The sum of the element contents and the average particle size D of the positive electrode active material v The 50-to-1000 ppm / μm ratio is preferably 25 ppm / μm to 1000 ppm / μm, and more preferably 200 ppm / μm to 700 ppm / μm. This facilitates the formation of surface protection for the positive electrode active material, improves the high-temperature cycle performance and high-temperature storage performance of the battery, enhances the capacity utilization of the positive electrode active material, and increases the energy density of the battery.

[0066] In some embodiments, the positive electrode active material includes secondary particles formed by the aggregation of primary particles. In these embodiments, the "bulk particles" mentioned above include secondary particles.

[0067] Optionally, the positive electrode active material provided in the embodiments of the present invention has one or more of the morphology of spheres and quasi-spheres.

[0068] The average particle size D of the positive electrode active material v The preferred size of 50 is 3μm to 20μm, more preferably 5μm to 11μm, and even more preferably 6μm to 8μm.

[0069] The average particle size of the positive electrode active material is preferably below 20 μm, more preferably below 11 μm, and even more preferably below 8 μm. This results in shorter migration paths for lithium ions and electrons within the material, improving the transport and diffusion performance of lithium ions and electrons in the positive electrode active material, thereby enhancing the battery's cycle performance and rate capability. The average particle size D of the positive electrode active material... v The particle size of 50 is preferably 3 μm or more, more preferably 5 μm or more, and even more preferably 6 μm or more. This is beneficial for reducing side reactions of the electrolyte on the surface of the positive electrode active material and reducing the agglomeration between particles of the positive electrode active material, thereby improving the cycle performance of the positive electrode active material.

[0070] In addition, the average particle size D of the positive electrode active material v Within the above range, 50 also helps to make the positive electrode active material have a higher compaction density, thereby improving the energy density of the battery.

[0071] The specific surface area of ​​the positive electrode active material is preferably 0.2 m². 2 / g~1.5m 2 / g, more preferably 0.3m 2 / g~1m 2 / g. The specific surface area of ​​the positive electrode active material is within the above range, which ensures that the positive electrode active material has a high active specific surface area, and at the same time helps to reduce the side reactions of the electrolyte on the surface of the positive electrode active material, thereby improving the capacity utilization and cycle life of the positive electrode active material.

[0072] The tap density of the positive electrode active material is preferably 2.3 g / cm³. 3 ~2.8g / cm 3 A tap density of the positive electrode active material within the above-mentioned range is beneficial for achieving a higher energy density in the battery.

[0073] In some embodiments, the nickel-containing lithium composite oxide is a compound represented by Formula 1, and the positive electrode active material has M-containing compounds on the outer surface of bulk particles comprising the compound represented by Formula 1. 1 Element oxide coating layer.

[0074] Li 1+α [Ni x Co y Mn z M 2 b M 3 d ]O 2-p X p Chemical Formula 1

[0075] In chemical formula 1, M 2 This refers to doping substitution at one or more of the nickel, cobalt, and manganese sites in the bulk phase of the bulk particles; M 3 The process involves doping and substituting one or more of the nickel, cobalt, and manganese sites on the surface layer of the bulk particles; X can substitute for oxygen sites in the bulk phase or surface layer of the bulk particles, or substitute for at least a portion of the oxygen elements in the coating layer. X is preferably selected from one or more of F, N, P, and S; and the following conditions apply: 0.5 ≤ x < 1, 0 ≤ y < 0.3, 0 ≤ z < 0.3, -0.2 < a < 0.2, 0 < b < 0.2, 0 < d < 0.2, 0 ≤ p < 0.2, x + y + z + b + d = 1. Batteries using this high-nickel ternary material can achieve a combination of high energy density, high-temperature cycle performance, and high-temperature storage performance.

[0076] Furthermore, 0.6≤x≤0.9, such as 0.7≤x≤0.8; 0<y<0.3; 0<z<0.3.

[0077] In this paper, the average particle size D of the positive electrode active material v 50 is a well-known concept in the art, also known as the median particle size, representing the particle size corresponding to 50% of the volume distribution of the positive electrode active material particles. The average particle size D of the positive electrode active material... v50 can be measured using instruments and methods known in the art, such as a laser particle size analyzer, like the Mastersizer 3000 laser particle size analyzer from Malvern Instruments Ltd., UK.

[0078] The specific surface area of ​​the positive electrode active material is a well-known concept in the art and can be measured using instruments and methods known in the art. For example, it can be tested using the nitrogen adsorption specific surface area analysis method and calculated using the BET (Brunauer Emmett Teller) method. The nitrogen adsorption specific surface area analysis can be performed using a NOVA2000e specific surface area and pore size analyzer from CANTA Corporation. As a specific example, the test method is as follows: Take 8.000g to 15.000g of positive electrode active material from a weighed empty sample tube. Stir the positive electrode active material evenly and weigh it. Place the sample tube in the NOVA2000e degassing station for degassing. Weigh the total mass of the degassed positive electrode active material and the sample tube. Subtract the mass of the empty sample tube from the total mass to calculate the mass G of the degassed positive electrode active material. The sample tube was placed in NOVA 2000e to measure the amount of nitrogen adsorbed on the surface of the positive electrode active material under different relative pressures. The amount of monolayer adsorption was obtained based on the Brownnor-Etter-Teller multilayer adsorption theory and its formula, and then the total surface area A of the positive electrode active material was calculated. The specific surface area of ​​the positive electrode active material was obtained by calculating A / G.

[0079] The tap density of the positive electrode active material can be determined using instruments and methods known in the art, such as a tap density meter, like the FZS4-4B type tap density meter.

[0080] The following describes a method for preparing a positive electrode active material. This method can be used to prepare any of the aforementioned positive electrode active materials. The preparation method includes:

[0081] S10 provides a mixture, which includes a nickel-containing transition metal source, a lithium source, and M. 2 Precursors of elements.

[0082] The nickel-containing transition metal source is, for example, one or more of oxides, hydroxides and carbonates containing Ni and optionally Co and / or Mn, such as hydroxides containing Ni, Co and Mn.

[0083] Nickel-containing transition metal sources can be obtained by methods known in the art, such as co-precipitation, gelation or solid-phase methods.

[0084] As an example of preparing hydroxides containing Ni, Co, and Mn, a Ni source, a Co source, and a Mn source are dispersed in a solvent to obtain a mixed solution. Using a continuous co-current reaction, the mixed solution, a strong alkali solution, and a complexing agent solution are simultaneously pumped into a stirred reactor. The pH of the reaction solution is controlled at 10–13, and the temperature inside the reactor is maintained at 25°C–90°C. An inert gas is used for protection during the reaction. After the reaction is complete, the hydroxide is aged, filtered, washed, and vacuum dried to obtain the hydroxide containing Ni, Co, and Mn.

[0085] The Ni source can be a soluble nickel salt, such as one or more of nickel sulfate, nickel nitrate, nickel chloride, nickel oxalate, and nickel acetate, or more specifically, one or more of nickel sulfate and nickel nitrate, or more specifically, nickel sulfate; the Co source can be a soluble cobalt salt, such as one or more of cobalt sulfate, cobalt nitrate, cobalt chloride, cobalt oxalate, and cobalt acetate, or more specifically, one or more of cobalt sulfate and cobalt nitrate, or more specifically, cobalt sulfate; the Mn source can be a soluble manganese salt, such as one or more of manganese sulfate, manganese nitrate, manganese chloride, manganese oxalate, and manganese acetate, or more specifically, one or more of manganese sulfate and manganese nitrate, or more specifically, manganese sulfate.

[0086] The strong base can be one or more of LiOH, NaOH, and KOH, such as NaOH. The complexing agent can be one or more of ammonia, ammonium sulfate, ammonium nitrate, ammonium chloride, ammonium citrate, and disodium ethylenediaminetetraacetate (EDTA), such as ammonia.

[0087] There are no particular restrictions on the solvents for mixed solutions, strong base solutions, and complexing agent solutions. For example, the solvents for mixed solutions, strong base solutions, and complexing agent solutions can be one or more of deionized water, methanol, ethanol, acetone, isopropanol, and n-hexanol, such as deionized water.

[0088] The inert gas introduced during the reaction is one or more of nitrogen, argon, and helium.

[0089] The lithium source mentioned above 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). Further, the lithium source is one or more of lithium carbonate, lithium hydroxide, and lithium nitrate; even further, the lithium source is lithium carbonate.

[0090] M 2 The precursor of an element can be M 2 One or more of the following can be selected: oxides of the element, nitrate compounds, carbonate compounds, hydroxides, and acetic acid compounds, depending on the actual needs.

[0091] In step S10, a ball mill or high-speed mixer can be used to mix the materials to obtain a uniformly mixed material. For example, a nickel-containing transition metal source, a lithium source, and M can be used. 2 The precursor of the element is added to a high-speed mixer for mixing, and the mixing time is 0.5h to 2h.

[0092] S20, the mixture is sintered to obtain uniformly doped with M. 2 The matrix particles of the element.

[0093] In step S20, the mixture can be added to an atmosphere sintering furnace for sintering. The sintering atmosphere is an oxygen-containing atmosphere, such as an air atmosphere or an oxygen atmosphere; the oxygen concentration in the sintering atmosphere is, for example, 70% or more, further 80% or more, and even more 85% or more. The sintering temperature is, for example, 600℃ to 1000℃, further 600℃ to 900℃, and even more further 650℃ to 850℃, which is beneficial for M... 2 The elements exhibit high doping uniformity. The sintering time can be adjusted according to actual conditions, for example, 5h to 25h, or even 5h to 15h.

[0094] It should be noted that, during the preparation of positive electrode active materials, there are several theoretically feasible methods to control the M content in lithium-nickel composite oxides. 2 The distribution of elements and the valence states at "78% delithiation state", such as M 2 The valence state of the element precursor itself, different M 2 The proportion of elemental valence state precursors, the oxidizing nature of the sintering atmosphere during doping, the number of sintering cycles, mixing uniformity, sintering temperature, and sintering time are all considered. This application document lists methods for controlling the type of doping precursor, sintering time, and temperature in step S20 to obtain a series of cathode active materials. These cathode active materials exhibit high energy density, thermal stability, and high-temperature cycling stability. Preferably, after further control of M... 2 Homogeneous elemental doping, M in a 78% delithiation state 2 Positive electrode active materials with characteristic elemental valence states have better performance.

[0095] In some embodiments, the sintered product from step S20 can be crushed and sieved to obtain a positive electrode active material with optimized particle size distribution and specific surface area. There are no particular limitations on the crushing method; it can be selected according to actual needs, such as using a particle crusher.

[0096] S30, combining matrix particles with M 3 The precursors of the elements are mixed and sintered to make M 3Elements are doped onto the surface layer of the matrix particles to form an outer doped layer, thus obtaining the bulk particles.

[0097] M 3 The precursor of an element can be M 3 One or more of the following: chlorides, sulfates, nitrates, oxides, hydroxides, fluorides, carbonates, phosphates, dihydrogen phosphates, and organic compounds of the element, but not limited thereto.

[0098] In step S30, a ball mill or a high-speed mixer can be used to mix the materials. For example, the matrix material is mixed with M... 3 The precursor of the element is added to a high-speed mixer for mixing, and the mixing time can be 0.5h to 2h.

[0099] The mixed materials are added to an atmosphere sintering furnace for sintering. The sintering atmosphere is an oxygen-containing atmosphere, such as an air atmosphere or an oxygen atmosphere; the sintering temperature is, for example, 400℃~750℃, and the sintering time can be 3h~25h, such as 5h~10h. During the sintering process, M 3 Elements diffuse from the outer surface of the matrix particles into the bulk phase to a predetermined depth, forming an outer doped layer. Due to M... 3 Element doping is performed after complete lithiation, which is beneficial for M 3 Elements should be present on the surface of the bulk particles as much as possible, and M should be made 3 The concentration of the element gradually decreases from the outer surface of the particle to the core.

[0100] S40, combining the bulk particles with M 1 The precursors of the elements are mixed and sintered to form M-containing compounds on the outer surface of the bulk particles. 1 An element oxide coating layer is used to obtain a positive electrode active material.

[0101] M 1 The precursor of an element can be M 1 One or more of the following: chlorides, sulfates, nitrates, oxides, hydroxides, fluorides, carbonates, phosphates, dihydrogen phosphates, and organic compounds of the element, but not limited thereto.

[0102] In step S40, a ball mill or high-speed mixer can be used to mix the materials. For example, mixing bulk particles with M... 1 The precursor of the element is added to a high-speed mixer for mixing, and the mixing time is 0.5h to 2h.

[0103] The mixed materials are added to an atmosphere sintering furnace for sintering. The sintering atmosphere is an oxygen-containing atmosphere, such as an air atmosphere or an oxygen atmosphere; the sintering temperature is, for example, 100℃~500℃; the sintering time can be 3h~25h, such as 5h~10h. Due to the relatively low sintering temperature, M1 The oxides of elements do not easily diffuse into the interior of the bulk particles, but instead form a coating layer on the outer surface of the bulk particles. M 1 The oxides of the elements are matched with the surface of the bulk particles, resulting in a tight bond between the coating layer and the bulk particles. The coating layer does not damage the structure of the bulk particles, thus achieving reliable protection for the bulk particles.

[0104] In some embodiments, the mixture in step S10 may contain a precursor of element X to dope the bulk phase of the bulk particles with element X; the mixture in step S30 may contain a precursor of element X to dope the surface layer of the bulk particles with element X, further wherein the concentration of element X gradually decreases from the outer surface of the bulk particles towards the core; the mixture in step S40 may contain a precursor of element X to dope the coating layer with element X. There are no specific limitations on the type of precursor containing element X, and those skilled in the art can select it according to actual needs.

[0105] Positive electrode sheet

[0106] Another aspect of the present invention provides a positive electrode sheet, which adopts the positive electrode active material of the first aspect of the present invention.

[0107] Because of the use of the positive electrode active material of the first aspect of the present invention, the positive electrode sheet of the present invention enables the lithium-ion secondary battery to simultaneously achieve good high-temperature cycle performance, high-temperature storage performance and high energy density.

[0108] The positive electrode sheet may include a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. For example, the positive current collector includes two opposing surfaces in its thickness direction, and the positive active material layer is stacked on either or both of the two surfaces of the positive current collector.

[0109] The positive electrode active material layer includes any one or more positive electrode active materials according to one aspect of the embodiments of the present invention.

[0110] In addition, the positive electrode active material layer may also include a conductive agent and a binder. This invention does not impose specific limitations on the types of conductive agents and binders in the positive electrode active material layer; they can be selected according to requirements.

[0111] As an example, the conductive agent may be one or more of graphite, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers; the binder may be one or more of styrene-butadiene rubber (SBR), water-based acrylic resin, sodium carboxymethyl cellulose (CMC), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl butyral (PVB), ethylene-vinyl acetate copolymer (EVA), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, fluorinated acrylic resin, and polyvinyl alcohol (PVA).

[0112] The positive current collector can be made of metal foil or porous metal plate with good conductivity and mechanical properties, such as aluminum foil.

[0113] The positive electrode sheet can be prepared according to conventional methods in the art. For example, the positive electrode active material, conductive agent and binder are dispersed in a solvent, which can be N-methylpyrrolidone (NMP), to form a uniform positive electrode slurry. The positive electrode slurry is coated on the positive electrode current collector, and after drying, rolling and other processes, the positive electrode sheet is obtained.

[0114] Lithium-ion secondary batteries

[0115] In another aspect, the present invention provides a lithium-ion secondary battery, which includes a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode is the positive electrode of another aspect of the present invention.

[0116] By employing the positive electrode active material of one aspect of the present invention, the lithium-ion secondary battery can simultaneously achieve good high-temperature cycle performance, high-temperature storage performance, and high energy density.

[0117] The aforementioned negative electrode can be a lithium metal sheet.

[0118] The negative electrode sheet may also include a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector. For example, the negative electrode current collector includes two opposing surfaces in its thickness direction, and the negative electrode active material layer is stacked on either or both of the two surfaces of the negative electrode current collector.

[0119] The negative electrode active material layer includes a negative electrode active material. This invention does not specifically limit the type of negative electrode active material; it can be selected according to actual needs. As an example, the negative electrode active material can be natural graphite, artificial graphite, mesophase microcarbon spheres (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composites, or SiO₂. m(0 < m < 2, e.g., m = 1), Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel-structured lithium titanate Li4Ti5O 12 One or more of Li-Al alloys and metallic lithium.

[0120] The negative electrode active material layer may also include a conductive agent and a binder. This invention does not impose specific limitations on the types of conductive agents and binders in the negative electrode active material layer; they can be selected according to actual needs. As an example, the conductive agent may be one or more of graphite, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers; the binder may be one or more of styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl butyral (PVB), water-based acrylic resin, and sodium carboxymethyl cellulose (CMC).

[0121] The negative electrode active material layer may also optionally include a thickener, such as sodium carboxymethyl cellulose (CMC).

[0122] The negative electrode current collector can be made of metal foil or porous metal plate with good electrical conductivity and mechanical properties, such as copper foil.

[0123] The negative electrode sheet can be prepared according to conventional methods in the art. For example, the negative electrode active material, conductive agent, binder and thickener are dispersed in a solvent, which can be N-methylpyrrolidone (NMP) or deionized water, to form a uniform negative electrode slurry. The negative electrode slurry is coated on the negative electrode current collector, and after drying, rolling and other processes, the negative electrode sheet is obtained.

[0124] In the lithium-ion secondary battery of this invention, the electrolyte can be a solid electrolyte, such as a polymer electrolyte or an inorganic solid electrolyte, but is not limited thereto. The electrolyte can also be a liquid electrolyte. The liquid electrolyte can consist of a solvent and a lithium salt dissolved in the solvent.

[0125] The solvent can be a non-aqueous organic solvent, such as one or more of ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), and ethyl butyrate (EB).

[0126] The lithium salt can be one or more of LiPF6 (lithium hexafluorophosphate), LiBF4 (lithium tetrafluoroborate), LiClO4 (lithium perchlorate), LiAsF6 (lithium hexafluoroarsenate), LiFSI (lithium bis(fluorosulfonyl)imide), LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluorooxalate borate), LiBOB (lithium bis(oxalate borate), LiPO2F2 (lithium difluorophosphate), LiDFOP (lithium difluorooxalate phosphate), and LiTFOP (lithium tetrafluorooxalate phosphate), for example, one or more of LiPF6 (lithium hexafluorophosphate), LiBF4 (lithium tetrafluoroborate), LiBOB (lithium bis(oxalate borate), LiDFOB (lithium difluorooxalate borate), LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), and LiFSI (lithium bis(fluorosulfonyl)imide).

[0127] The electrolyte may also optionally contain other additives, such as vinylene carbonate (VC), ethylene ethylene carbonate (VEC), fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), trifluoromethyl ethylene carbonate (TFPC), succinic anionyl (SN), adiponitrile (ADN), glutaronitrile (GLN), hexanetrionitrile (HTN), 1,3-propanesulfonyl lactone (1,3-PS), vinyl sulfate (DTD), methylene disulfonate (MMDS), and 1-propene-1,3-sulfonyl lactone. One or more of the following, but not limited to: (PST), 4-methyl ethylene sulfate (PCS), 4-ethyl ethylene sulfate (PES), 4-propyl ethylene sulfate (PEGLST), propylene sulfate (TS), 1,4-butane sulpholactone (1,4-BS), ethylene sulfite (DTO), dimethyl sulfite (DMS), diethyl sulfite (DES), sulfonate cyclic quaternary ammonium salts, tris(trimethylsilane) phosphate (TMSP), and tris(trimethylsilane) borate (TMSB).

[0128] The lithium-ion secondary battery of this invention does not have any particular limitation on the separator. Any well-known porous structure separator with electrochemical and mechanical stability can be selected, such as one or more single-layer or multi-layer films including glass fiber, non-woven fabric, polyethylene (PE), polypropylene (PP) and polyvinylidene fluoride (PVDF).

[0129] A battery cell is obtained by alternately stacking positive and negative electrode sheets and placing a separator between them. Alternatively, the battery cell can be obtained by winding the electrodes. The battery cell is then placed in a casing, electrolyte is injected, and the casing is sealed to obtain a lithium-ion secondary battery.

[0130] Example

[0131] The following examples describe the disclosure of this invention in more detail. These examples are merely illustrative, as various modifications and variations will be apparent to those skilled in the art within the scope of this disclosure. Unless otherwise stated, all parts, percentages, and ratios reported in the following examples are based on weight, and all reagents used in the examples are commercially available or synthesized by conventional methods and can be used directly without further processing. The instruments used in the examples are also commercially available.

[0132] Example 1

[0133] Preparation of positive electrode active materials

[0134] (1) The positive electrode active material precursor [Ni 0.8 Co 0.1 Mn 0.1 Lithium hydroxide (LiOH), antimony trioxide (Sb₂O₃), and antimony pentoxide (Sb₂O₅) were added to a high-speed mixer and mixed for 1 hour to obtain a mixture. The molar ratio of the positive electrode active material precursor to lithium hydroxide (Li / Me) was 1.05, where Me represents the total molar amount of Ni, Co, and Mn in the positive electrode active material precursor. Sb₂O₃ accounted for 50% of the total molar amount of Sb₂O₃ and Sb₂O₅, and the addition of Sb₂O₃ and Sb₂O₅ resulted in an Sb content of 3120 ppm in the positive electrode active material. The mixture was then sintered in an atmosphere sintering furnace at 830℃ in an oxygen-containing atmosphere with an O₂ concentration of 90% for 15 hours. After crushing and sieving, the matrix particles were obtained, in which Sb was uniformly doped into the bulk structure of the matrix particles.

[0135] (2) The matrix particles and alumina (Al2O3) were added to a high-speed mixer and mixed for 1 hour. The amount of Al2O3 added was such that the Al content in the outer doped layer of the matrix particles was 2210 ppm, which is the content in the positive electrode active material. The mixed material was placed in an atmosphere sintering furnace and sintered at a temperature of 700°C and an oxygen-containing atmosphere with an O2 concentration of 90% for 15 hours. This allowed Al to be doped onto the surface layer of the matrix particles to form an outer doped layer, resulting in the matrix particles. The thickness of the outer doped layer was 21% of the particle size of the matrix particles. The Al element in the outer doped layer had a concentration gradient that gradually decreased from the outer surface of the matrix particles to the core.

[0136] (3) The bulk particles and alumina (Al2O3) were added to a high-speed mixer and mixed for 1 hour. The amount of Al2O3 added resulted in an Al content of 1207 ppm in the coating layer, which is the content in the positive electrode active material. The mixed material was then placed in an atmosphere sintering furnace for sintering at a temperature of 450°C and an oxygen-containing atmosphere with an O2 concentration of 90% for 14 hours. An Al2O3 coating layer was formed on the outer surface of the bulk particles, resulting in the positive electrode active material with a coating layer thickness of 98 nm.

[0137] Preparation of electrolyte

[0138] EC, DEC, and DMC are mixed in a volume ratio of 1:1:1 to obtain a solvent. Lithium salt LiPF6 is then dissolved in the solvent to obtain an electrolyte, wherein the concentration of LiPF6 is 1 mol / L.

[0139] Preparation of button cells

[0140] The above-prepared positive electrode active material, conductive carbon black and binder PVDF are dispersed in the solvent N-methylpyrrolidone (NMP) at a weight ratio of 90:5:5 and mixed evenly to obtain a positive electrode slurry; the positive electrode slurry is evenly coated on the positive electrode current collector aluminum foil, and after drying and cold pressing, a positive electrode sheet is obtained.

[0141] In the coin cell, the positive electrode, separator and lithium metal sheet are stacked in sequence and injected with the electrolyte to assemble a coin cell.

[0142] Preparation of full cells

[0143] The above-prepared positive electrode active material, conductive agent acetylene black, and binder PVDF are dispersed in solvent NMP at a weight ratio of 94:3:3 and mixed evenly to obtain a positive electrode slurry. The positive electrode slurry is evenly coated on the positive electrode current collector aluminum foil, and after drying and cold pressing, a positive electrode sheet is obtained.

[0144] The negative electrode active materials, artificial graphite, hard carbon, conductive agent acetylene black, binder styrene-butadiene rubber (SBR), and thickener sodium carboxymethyl cellulose (CMC) are dispersed in deionized water at a weight ratio of 90:5:2:2:1 and mixed evenly to obtain a negative electrode slurry. The negative electrode slurry is then evenly coated onto the negative electrode current collector aluminum foil, dried, and cold-pressed to obtain the negative electrode sheet.

[0145] A porous polyethylene (PE) polymer film is used as the separator. The positive electrode, separator, and negative electrode are stacked in sequence to obtain a bare cell. The bare cell is placed in an outer package, injected with the above-mentioned electrolyte, and sealed. After formation and other processes, a full cell is obtained.

[0146] Examples 2-28 and Comparative Examples 1-9

[0147] Unlike Example 1, the relevant parameters in the preparation steps of the positive electrode active material were changed to obtain a positive electrode active material with predetermined parameter characteristics, as detailed in Tables 1-1 and 1-2.

[0148] In all of them, the positive electrode active material precursors in Examples 2-26 and Comparative Examples 1-4 were [Ni 0.8 Co 0.1 Mn 0.1 [OH]2; the positive electrode active material precursors in Examples 27 and Comparative Examples 5-8 were all [Ni] 0.6 Co 0.2 Mn 0.2 [OH]2; The positive electrode active material precursors in Example 28 and Comparative Example 9 were both [Ni] 0.5 Co 0.2 Mn 0.3 (OH)2;

[0149] The doping element precursors in Examples 3 and 24-26 were WO2 and WO3; the doping element precursor in Example 4 was SiO and SiO2; the doping element precursors in Examples 5 and 19-21 were NbO2 and Nb2O5; the doping element precursors in Examples 6 and 22-23 were V2O3 and V2O4; the doping element precursor in Example 7 was TeO2 and TeO3; the doping element precursor in Example 8 was MoO2 and MoO3; and the doping element precursor in Example 9 was Sb2O3, Sb2O5, WO2, and WO3, with the contents of the four precursors being basically the same.

[0150] The remaining doping elements M that differ from those in Example 1 3 and the covering element M 1 The precursors are selected from CaO, TiO2, B2O3, MgO, and ZrO2;

[0151] In Example 20, step (1) the sintering temperature was 720℃ and the sintering time was 8h; step (2) the sintering temperature was 600℃ and the sintering time was 10h; step (3) the sintering temperature was 380℃ and the sintering time was 11h.

[0152] In Example 21, step (1) the sintering temperature was 650℃ and the sintering time was 4h; step (2) the sintering temperature was 570℃ and the sintering time was 6h; step (3) the sintering temperature was 260℃ and the sintering time was 8h.

[0153] In Example 22, step (1) the sintering temperature was 710℃ and the sintering time was 7h; step (2) the sintering temperature was 520℃ and the sintering time was 9h; step (3) the sintering temperature was 210℃ and the sintering time was 6h.

[0154] In Example 23, step (1) the sintering temperature is 600℃ and the sintering time is 4h; step (2) the sintering temperature is 440℃ and the sintering time is 3h; step (3) the sintering temperature is 120℃ and the sintering time is 3h.

[0155] In Example 27 and Comparative Examples 5-8, step (1) was sintering at 800°C for 14 hours; step (2) was sintering at 700°C for 13 hours; and step (3) was sintering at 450°C for 11 hours.

[0156] In Example 28 and Comparative Example 9, step (1) was sintering at 780°C for 13 hours; step (2) was sintering at 700°C for 12 hours; and step (3) was sintering at 450°C for 10 hours.

[0157] Comparative Example 1 and Comparative Example 5 (undoped M) 2 Elements, Comparative Examples 2 and 6, undoped M 3 Elements, uncovered M in Comparative Examples 3 and 7 1 The elements in Comparative Examples 4 and 8-9 were not doped or coated.

[0158] The remaining parameters are shown in Tables 1-1 and 1-2.

[0159] In Tables 1-1 and 1-2, "M at 78% delithiation state" 2 "Valence state" lists the M values ​​of the positive electrode active material when it is in a 78% delithiation state. 2 The lowest and highest oxidation states of an element; σ represents the M at any point in the bulk particle. 2 The mass concentration deviation of elements; the percentage of outer doped layer thickness, i.e., the percentage of the outer doped layer thickness to the bulk particle size; κ represents the mass concentration of M in the positive electrode active material. 1 Elements and M 3 The sum of the element contents and the average particle size D of the positive electrode active material v The ratio of 50 is expressed in ppm / μm; M 1 Element, M 2 Element, M 3 The content of each element refers to its content in the positive electrode active material.

[0160] Test section

[0161] 1) M in the "78% delithiation state" positive electrode active material 2Valence distribution test of elements

[0162] a. Determining the 78% delithiation state

[0163] At 25°C, eight coin cells were charged at a constant current of 1C to the upper limit of the charge / discharge cutoff voltage, then charged at a constant voltage until the current was less than or equal to 0.05mA. After that, they were left to rest for 2 minutes, and then discharged at a constant current of 1C to the lower limit of the charge / discharge cutoff voltage.

[0164] Then, the eight coin cells that had been charged and discharged were charged at a rate of 0.1C to 4.0V, 4.1V, 4.2V, 4.3V, 4.4V, 4.5V, 4.6V, and 4.7V, respectively. Each charged coin cell was disassembled in a drying room, and the positive electrode was removed as a sample. The sample mass was recorded and placed in a digestion vessel. 10 mL of aqua regia was slowly added as a digestion reagent. After assembly, the sample was placed in a CEM-Mars5 microwave digester and digested at a microwave emission frequency of 2450 Hz. The digested sample solution was transferred to a volumetric flask and shaken well. A sample was then placed in a PE 7000DV inductively coupled plasma optical emission spectrometer (ICP-OES) sample introduction system. The mass concentrations of Li, O, Ni, Co, Mn, and doping elements in the positive electrode active material were measured at 0.6 MPa argon pressure and 1300 W RF power. Based on the mass concentrations of each element, the chemical formula at each voltage was calculated, and thus the delithiation state at each voltage was obtained. For example, the chemical formula of the positive electrode active material calculated at 4.3 V is Ui. 0.22 Ni 0.8 Co 0.1 Mn 0.1 If O2 is present, the corresponding delithiation state is (1-0.22)×100%=78% delithiation state, which means the battery voltage corresponding to the 78% delithiation state is 4.3V.

[0165] The coin cells were charged at 25°C and at a rate of 0.1C to the voltage corresponding to 78% delithiation, and samples in the 78% delithiation state were obtained. Then, the following operations were performed:

[0166] b. XPS test element valence state

[0167] ① Disassemble the 78% delithiated cell in the drying room, take out the entire positive electrode sheet and put it into a beaker, pour in an appropriate amount of high-purity anhydrous dimethyl carbonate (DMC), replace the DMC every 8 hours, and clean it 3 times in a row. Then put it into the vacuum settling chamber of the drying room and keep it under vacuum (-0.096MPa) for 12 hours. After drying, scrape and grind the positive electrode sheet with a blade in the drying room, and weigh about 50mg of positive active material powder.

[0168] ② Wipe the surface of an aluminum foil piece approximately 2cm x 2cm clean with acetone. Cut a piece of double-sided tape approximately 1cm x 1cm and attach it to the center of the aluminum foil. Spread the powder sample onto the double-sided tape and use a clean stainless steel sampling spoon to evenly spread the powder over the entire tape. Take another piece of aluminum foil cleaned with acetone, cover the sample, and place the entire piece between two flat stainless steel modules. Then, use a tablet press to compress the sample, applying a pressure of approximately 10MPa and holding for 15 seconds.

[0169] ③ Using an escalab 250Xi X-ray photoelectron spectrometer from Thermo Fisher Scientific, the pressed sample was placed in the sample chamber. A monochromatic Al Kα (hv = 1486.6 eV) excitation source was set, with an X-ray power of 150 W and a focused spot size of 500 μm. The 2p or 3d spectra of the doped elements were acquired and processed using XPSpeak software for peak separation. The M values ​​were then determined. 2 Valence distribution of elements.

[0170] 2) M at any point in the bulk particle 2 Elemental mass concentration deviation test

[0171] Weigh 2g of positive electrode active material powder sample and evenly sprinkle the sample onto the sample stage coated with conductive adhesive. Then gently press to fix the powder. Alternatively, cut a 1cm × 1cm electrode sheet from the battery's positive electrode and attach it to the sample stage as the sample to be tested. Place the sample stage into the vacuum sample chamber and secure it. Use a JEOL IB-09010CP cross-section polisher to prepare the cross-section of the positive electrode active material particles, thus obtaining the cross-section of the bulk particles. Figure 2 As shown; refer to Figure 2 Seventeen sampling points were taken at the cross-section of the particle shown, each with an area of ​​20 nm × 20 nm. The mass concentration of the dopant element at these 17 sites was measured using an X-Max energy dispersive spectrometer (EDS) from Oxford Instruments (UK) combined with a Sigma-02-33 scanning electron microscope (SEM) from ZEISS (Germany). The testing method is as follows: Li, O, Ni, Co, Mn, and the dopant element were selected for detection. The SEM parameters were set to 20 kV accelerating voltage, 60 μm aperture, 8.5 mm working distance, and 2.335 A current. During EDS testing, the test was stopped and data was collected when the spectral area reached more than 250,000 cts (controlled by acquisition time and acquisition rate). The mass concentration of the dopant element at each site was obtained. 2 The mass concentrations of the elements are denoted as η1, η2, η3, ..., ηn. 17 .

[0172] M in bulk particles 2 Method for determining the average mass concentration η of an element: The above-mentioned EDS-SEM testing method is adopted, such as... Figure 2As shown in the dashed box, the test area covers all points scanned by the above-mentioned body particle and does not exceed the cross-section of the body particle.

[0173] Then, M at any point in the bulk particle is calculated according to equation (1) mentioned above. 2 The mass concentration deviation of the element σ.

[0174] 3) M in the positive electrode active material 1 Element, M 2 Element, M 3 Element content

[0175] The M content in the positive electrode active material was measured using a PE 7000DV inductively coupled plasma-emission spectrometer (ICP-OES) from PE Corporation, USA. 1 Element, M 2 Element, M 3 The element content is tested using the following method: Take an electrode containing positive electrode active material, punch it into a round disc with a total mass greater than 0.5g, or take at least 5g of positive electrode active material powder sample. Weigh and record the sample mass, then place it in a digestion vessel. Slowly add 10mL of aqua regia as a digestion reagent, and then place it in a Mars 5 microwave digester from CEM Corporation (USA) for digestion at a microwave emission frequency of 2450Hz. Transfer the digested sample solution to a volumetric flask, shake well, and then place the sample into an ICP-OES injection system. Perform M content analysis on the positive electrode active material using 0.6MPa argon gas pressure and 1300W radio frequency power. 1 Element, M 2 Element, M 3 Element content testing.

[0176] Then, based on the formula (2) mentioned above, the M in the positive electrode active material is calculated. 2 The content of elements and M in the bulk particles 2 Deviation in the average mass concentration of an element.

[0177] 4) Initial specific capacity test of button cells

[0178] At 25°C, the battery is charged at a constant current of 0.1C to the upper limit of the charge / discharge cutoff voltage, then charged at a constant voltage until the current is less than or equal to 0.05mA. After that, it is left to stand for 2 minutes, and then discharged at a constant current of 0.1C to the lower limit of the charge / discharge cutoff voltage. The discharge capacity at this time is the initial specific capacity of the coin cell.

[0179] 5) Initial specific capacity test of the full battery

[0180] At 25°C, charge the battery at a constant current of 1 / 3C to the upper limit of the charge / discharge cutoff voltage, then charge it at a constant voltage until the current is less than or equal to 0.05mA. After that, let it rest for 5 minutes, and then discharge it at a constant current of 1 / 3C to the lower limit of the charge / discharge cutoff voltage. The discharge capacity at this time is the initial specific capacity of the whole battery.

[0181] 6) High-temperature cycle performance test of the full battery

[0182] At 45℃, the battery is charged at a constant current of 1C to the upper limit of the charge / discharge cutoff voltage, then charged at a constant voltage until the current is less than or equal to 0.05mA. After resting for 5 minutes, it is discharged at a constant current of 1C to the lower limit of the charge / discharge cutoff voltage. This constitutes one charge / discharge cycle, and the discharge capacity of this cycle is recorded as the discharge specific capacity D1 of the first cycle. The battery is subjected to 400 charge / discharge cycles using the above method, and the discharge specific capacity D1 of the 400th cycle is recorded. 400 .

[0183] Full cell capacity retention rate (%) after 400 cycles at 45℃ and 1C / 1C = D 400 / D1×100%

[0184] 7) High-temperature storage performance test of the full battery

[0185] At 25℃, the battery was charged at a constant current rate of 1C to the upper limit of the charge / discharge cutoff voltage, and then charged at a constant voltage until the current was less than or equal to 0.05mA. The volume of the battery at this point was measured and recorded as V0. The battery was then placed in an 80℃ constant temperature chamber and its volume was measured and recorded as V1 after 10 days of storage. In this test, the water displacement method was used to measure the battery volume.

[0186] The volume expansion rate of a full battery after storage at 80°C for 10 days is ΔV (%) = (V1 - V0) / V0 × 100%.

[0187] In tests 1), 4) to 7),

[0188] In Examples 1-26 and Comparative Examples 1-4, the charge / discharge cutoff voltage of the button cell was 2.8V-4.25V, and the charge / discharge cutoff voltage of the full cell was 2.8V-4.2V.

[0189] In Examples 27-28 and Comparative Examples 5-9, the charge / discharge cutoff voltage of the button cell was 2.8V-4.35V, and the charge / discharge cutoff voltage of the full cell was 2.8V-4.3V.

[0190] The test results of Examples 1-28 and Comparative Examples 1-9 are shown in Table 2.

[0191]

[0192]

[0193] Table 2

[0194]

[0195] The comparison results of Examples 1-28 and Comparative Examples 1-9 show that by uniformly doping the bulk phase of the nickel-containing lithium composite oxide particles with M... 2 The surface layer of the bulk particles is doped with M. 3 Elements, and elements containing M on the outer surface of the bulk particles. 1 Element oxide coating, and M 1 Element, M 2 Elements and M 3 Each element is selected from a specific type of element. Lithium-ion secondary batteries not only have a high initial specific capacity, but also have high high-temperature cycle performance and high-temperature storage performance.

[0196] As can be seen from the comparison of Examples 19-21, reducing M 2 The mass concentration deviation of an element at any point in the bulk particles of the positive electrode active material is less than 35%, preferably less than 20%, which can improve the initial specific capacity, high-temperature cycle performance and high-temperature storage performance of the battery.

[0197] The results of Examples 6, 22, and 23 show that when ε is small, more dopants can be incorporated into the cathode active material particles, fully leveraging the dopant's role in improving the structural stability of the cathode material. This enhances the thermal stability of the cathode material while improving battery capacity and high-temperature cycle performance. However, when ε is too large, more dopants are distributed in the gaps or on the surface of the cathode active material particles, resulting in poor improvement of the cathode active material and poor thermal stability. While the dopants distributed on the surface provide some coating, isolating the electrolyte and reducing side reactions, the cell capacity and high-temperature cycle performance decrease slightly in this case.

[0198] The results from Examples 1, 10-18 show that M in the positive electrode active material... 1 Element, M 2 Elements and M 3 When the content of elements is within an appropriate range, it can more effectively improve the battery's specific capacity, high-temperature cycle performance, and high-temperature storage performance.

[0199] As can be seen from the results of Examples 3, 24-26, keeping the thickness of the outer doped layer and the coating layer within an appropriate range can enable the battery to have a high specific capacity while improving the battery's high-temperature cycle performance and high-temperature storage performance.

[0200] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and these modifications or substitutions should all be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A positive electrode active material, characterized in that, Includes the main body particles and the M-containing particles coating the outer surface of the main body particles. 1 The elemental oxide coating layer, wherein the bulk particles include a nickel-containing lithium composite oxide; The bulk particles are uniformly doped with M. 2 element; The surface layer of the bulk particles is doped with M. 3 The outer doping layer of the element; The M 1 Elements and M 3 All particles are identical and contain only the element L, which has a decreasing mass concentration gradient from the outer surface of the positive electrode active material particles towards the core. The L element is one or more of Mg, Al, Ca, Ce, Ti, Zr, Zn, Y, and B. 2 The elements include one or more of Si, Ti, Cr, Mo, V, Ge, Se, Zr, Nb, Ru, Rh, Pd, Sb, Te, Ce, and W; The content of L element in the bulk phase of the bulk particles is 100ppm to 2000ppm; The average particle size Dv50 of the positive electrode active material is 5μm to 11μm, the thickness of the coating layer is 50nm to 160nm, and the thickness of the outer doped layer is 15% to 25% of the particle size of the bulk particles.

2. The positive electrode active material according to claim 1, characterized in that, When the positive electrode active material is in a 78% delithiation state, the M 2 The valence of the element is +3 or higher; And / or, when the positive electrode active material is in a 78% delithiation state, the M 2 The element M has two or more distinct valence states, and is in its highest valence state. 2 The valence of an element is one or more of the following: +4, +5, +6, +7, and +8.

3. The positive electrode active material according to claim 2, characterized in that, When the positive electrode active material is in a 78% delithiation state, the M 2 The valence of an element is one or more of the following: +4, +5, +6, +7, and +8.

4. The positive electrode active material according to claim 1, characterized in that, The M at any point in the bulk particle 2 The mass concentration deviation of the element is less than 20%; And / or, the M in the positive electrode active material 2 The content of the element and the M in the bulk particles 2 The deviation ε of the average mass concentration of the element is less than 30%.

5. The positive electrode active material according to claim 4, characterized in that, M in the positive electrode active material 2 The content of the element and the M in the bulk particles 2 The deviation ε of the average mass concentration of the element is less than 20%.

6. The positive electrode active material according to claim 1, characterized in that, In the positive electrode active material, the content of L element in the outer doping layer is 400ppm to 3000ppm, and the content of M... 2 The element content is 500ppm to 5000ppm.

7. The positive electrode active material according to claim 1, characterized in that, M in the positive electrode active material 1 Element and the M 3 The sum of the contents of the elements and the average particle size D of the positive electrode active material v The ratio of 50 is 25ppm / μm to 1000ppm / μm.

8. The positive electrode active material according to any one of claims 1 to 7, characterized in that, The average particle size D of the positive electrode active material v 50 is 6μm to 8μm; And / or, the specific surface area of ​​the positive electrode active material is 0.2 m². 2 / g~1.5m 2 / g; And / or, the tap density of the positive electrode active material is 2.3 g / cm³. 3 ~2.8g / cm 3 .

9. The positive electrode active material according to claim 8, characterized in that, The specific surface area of ​​the positive electrode active material is 0.3 m². 2 / g~1m 2 / g.

10. The positive electrode active material according to claim 1, characterized in that, The nickel-containing lithium composite oxide is a compound represented by Formula 1. Li 1+a [Ni x Co y Mn z M 2 b M 3 d O 2-p X p Formula 1 In Equation 1, X is selected from one or more of F, N, P, and S, where 0.5 ≤ x < 1, 0 ≤ y < 0.3, 0 ≤ z < 0.3, and -0.

2. <a<0.2,0<b<0.2,0<d<0.2,0≤p<0.2,x+y+z+b+d=1。 11. A positive electrode plate, characterized in that, It includes a positive current collector and a positive active material layer disposed on the positive current collector, wherein the positive active material layer includes the positive active material according to any one of claims 1 to 10.

12. A lithium-ion secondary battery, characterized in that, Including the positive electrode sheet according to claim 11.

13. A method for preparing a positive electrode active material, characterized in that, The method includes: A mixture is provided, the mixture comprising a nickel-containing transition metal source, a lithium source, and M. 2 Precursors of elements; The mixture is sintered to obtain a uniformly doped material containing M. 2 The matrix particles of the element; The matrix particles and M 3 The precursors of the elements are mixed and sintered to make M 3 Elements are doped onto the surface layer of the matrix particles to form an outer doped layer, thus obtaining bulk particles; The bulk particles and M 1 The precursors of the elements are mixed and sintered to form an M-containing matrix on the outer surface of the bulk particles. 1 An element oxide coating layer is used to obtain a positive electrode active material; Wherein, the M 1 Elements and M 3 All particles are identical and contain only the element L, which has a decreasing mass concentration gradient from the outer surface of the positive electrode active material particles towards the core. The L element is one or more of Mg, Al, Ca, Ce, Ti, Zr, Zn, Y, and B. 2 The elements include one or more of Si, Ti, Cr, Mo, V, Ge, Se, Zr, Nb, Ru, Rh, Pd, Sb, Te, Ce and W, and the content of the L element in the bulk phase of the bulk particles is 100ppm to 2000ppm.

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