A positive electrode active material, a positive electrode sheet, and a lithium-ion secondary battery

Abstract

This application discloses a positive electrode active material, a positive electrode sheet, and a lithium-ion secondary battery. The chemical formula of the positive electrode active material is Li. f Ni x Co y Mn z A e O2, A includes at least element M, which includes at least one of Sb, Ta, and Y; the positive electrode active material includes polycrystalline particles and a coating layer containing at least element Co, the grain boundaries of the polycrystalline particles include elements Co and M, and the content of element Co is greater than the content of element M; the polycrystalline particles include primary particles with a particle size of D nm, a region with a depth of D / 5 nm along the direction from the surface of the primary particle to its interior is denoted as the first region, and the remaining regions are denoted as the second region, the content of element M in the first region is greater than the content of element M in the second region. The positive electrode active material of this application can achieve a balance between high capacity and grain boundary enhancement and interface stability, thereby meeting the application requirements of next-generation high-energy-density, long-cycle-life lithium-ion secondary batteries.

Description

Technical Field

[0001] This application relates to the field of battery technology, specifically to a positive electrode active material, a positive electrode sheet, and a lithium-ion secondary battery. Background Technology

[0002] High-nickel ternary materials (with Ni content of more than or equal to 0.8% in transition metals) have high discharge specific capacity and operating voltage. Furthermore, using high-nickel ternary materials with polycrystalline morphology can also improve the processing performance and compaction density of the electrodes.

[0003] However, high-nickel ternary polycrystalline materials face serious capacity decay and safety challenges in practical applications, especially during deep charge-discharge (high delithiation state) and long-term cycling. To address this, existing technologies typically modify high-nickel ternary polycrystalline materials through bulk doping and surface coating, but these methods still cannot effectively alleviate the problem of mechanical failure of polycrystalline particles, and existing improvement methods often come at the cost of sacrificing some specific capacity. Summary of the Invention

[0004] In view of this, this application aims to retain the high capacity of high-nickel ternary polycrystalline materials, strengthen grain boundaries and suppress the initiation and propagation of microcracks by controlling the composition and structure of the materials, and construct an interface structure that can adapt to volume changes, is stable and conductive, thereby fundamentally improving the mechanical integrity and interface stability of high-nickel ternary polycrystalline materials to meet the application requirements of next-generation high-energy-density and long-cycle-life lithium-ion secondary batteries.

[0005] To achieve the above objectives, this application adopts the following technical solution.

[0006] According to embodiments of this application, in a first aspect, a positive electrode active material is provided, the chemical formula of which is Li f Ni x Co y Mn z A e O2, 0.8≤f<1.3, 0.8≤x<1, 0.02≤y≤0.2, 0.01≤z≤0.14, 0.06≤e≤0.1, A includes at least one element M, wherein the element M includes at least one of Sb, Ta, and Y; The positive electrode active material includes polycrystalline particles and a coating layer located on at least a portion of the surface of the polycrystalline particles, the coating layer including at least Co element; The grain boundaries of the polycrystalline particles contain Co and M elements. Based on the mass of the positive electrode active material, the mass content of Co in the grain boundaries is greater than the mass content of M in the grain boundaries. The polycrystalline particles include primary particles with a particle size of D nm, where 50 ≤ D ≤ 800. Along the direction from the surface of the primary particle to its interior, a region extending from the surface of the primary particle to a depth of D / 5 nm is designated as a first region, and a region at a depth greater than D / 5 nm from the surface of the primary particle is designated as a second region. Based on the mass of the positive electrode active material, the mass content of element M in the first region is greater than the mass content of element M in the second region.

[0007] Furthermore, in some alternative implementations, at least one of the following conditions is met: (1) 100≤D≤800; (2) The first region contains a spinel phase; (3) The grain boundaries of the polycrystalline grains contain spinel phase; (4) The A also includes the Q element, which includes at least one of Zr, Al, Mo, Ti and Sr.

[0008] Furthermore, in some optional embodiments, A includes Sr and M elements. Based on the mass of the positive electrode active material, the mass content of Sr element in the grain boundaries of the polycrystalline particles is h1%, and the mass content of Sr element in the primary particles is h2%, satisfying: h1 > h2.

[0009] Furthermore, in some optional embodiments, based on the mass of the positive electrode active material, the mass content of the Sr element is denoted as s1 ppm, and the mass content of the M element is denoted as s2 ppm, satisfying: 260≤s1≤2500, 400≤s2≤2000, and 0.15≤s1 / s2≤6.

[0010] In some optional embodiments, the M element includes Sb and Y elements, and the Q element includes Sr, Zr and Al elements, wherein: the relative standard deviation of the mass content of Al element at at least 5 locations on the cross-section of the polycrystalline particle is denoted as R1, and R1 < 40%; the relative standard deviation of the mass content of Zr element at at least 5 locations on the cross-section of the polycrystalline particle is denoted as R2, and R2 < 40%.

[0011] Furthermore, in some optional embodiments, based on the mass of the positive electrode active material, the mass content of Sb is 100ppm-1600ppm, the mass content of Y is 300ppm-1200ppm, the mass content of Zr is 1500ppm-3500ppm, and the mass content of Al is 600ppm-2500ppm.

[0012] In some optional embodiments, the particle size of the polycrystalline particles is L μm, 5≤L≤18; along the direction from the surface of the polycrystalline particles to their interior, the region with a depth of L / 5 μm from the surface of the polycrystalline particles is designated as the third region, and the region with a depth greater than L / 5 μm from the surface of the polycrystalline particles is designated as the fourth region; based on the mass of the positive electrode active material, the mass content of Co element in the third region is greater than the mass content of Co element in the fourth region.

[0013] Furthermore, in some optional embodiments, a cobalt-rich layered structure exists in the third region and / or the grain boundaries of the polycrystalline particles, wherein the molar content of Co in the cobalt-rich layered structure is greater than or equal to 60% based on the sum of the molar numbers of all transition metal elements in the cobalt-rich layered structure.

[0014] In some alternative implementations, at least one of the following conditions is met: (1) Based on the mass of the positive electrode active material, the mass content of Co in the polycrystalline particles is greater than the mass content of Mn in the polycrystalline particles; (2) The primary particle includes a first surface and a second surface, the first surface is in contact with the coating layer, and the second surface is not in contact with the coating layer; based on the mass of the positive electrode active material, the mass content of Co element on the first surface is less than the mass content of Co element on the second surface; (3) The thickness of the coating layer is 5nm-50nm; (4) The coating layer also includes at least one element selected from Al, Ti, W and B.

[0015] Furthermore, in some optional embodiments, the coating layer includes Co, Al, and B elements, wherein, based on the mass of the coating layer, the mass content of Co element is 50%-98%, the mass content of Al element is 1%-15%, and the mass content of B element is 0.2%-10%.

[0016] In some alternative implementations, at least one of the following conditions is met: (1) The particle size Dv50 of the positive electrode active material is 5μm-18μm; (2) In the X-ray diffraction pattern of the positive electrode active material, there are diffraction peaks at positions of 18.5° - 19.0°, 44.5° - 45.1°, and 37.8° - 38.7°; (3) In the differential scanning calorimetry of the positive electrode active material, the starting temperature of the exothermic peak is T≥ 190℃.

[0017] According to an embodiment of this application, in a second aspect, a positive electrode sheet is provided, including a positive electrode active layer, the positive electrode active layer including a positive electrode active material; the positive electrode active material includes the positive electrode active material described in the first aspect of this application.

[0018] Furthermore, in some optional embodiments, the positive electrode active material further includes lithium nickel cobalt manganese oxide single crystal particles. Based on the mass of the positive electrode active layer, the mass content of the lithium nickel cobalt manganese oxide single crystal particles is denoted as F%, and the mass content of the positive electrode active material is denoted as E%, satisfying: 0 < E / F ≤ 1.

[0019] According to an embodiment of this application, in a third aspect, a lithium-ion secondary battery is also provided, including a negative electrode and the positive electrode described in the second aspect.

[0020] Furthermore, in some optional embodiments, the ratio of the capacity of the negative electrode to the capacity of the positive electrode is denoted as NP, which satisfies: 1.04≤NP≤1.3.

[0021] In some alternative embodiments, the negative electrode sheet includes a negative electrode active material, which includes a silicon-carbon material; and satisfies at least one of the following conditions: (1) The particle size Dv50 of the silicon carbide material is 1μm-25μm; (2) The silicon-carbon material comprises porous carbon and silicon particles located within the porous carbon channels, and the mass content of silicon element is 30%-80% based on the mass of the silicon-carbon material; (3) Based on the mass of the negative electrode active material, the mass content of the silicon-carbon material is 15%~100%.

[0022] In some optional embodiments, the differential capacity curve of the lithium-ion secondary battery has an oxidation peak A in the range of 4.1V-4.2V and a reduction peak B in the range of 3.6V-3.8V; the half-width at half maximum (WHM) of the oxidation peak A is denoted as WA, and the half-width at half maximum (WHM) of the reduction peak B is denoted as WB, satisfying 0.8≤WB / WA≤1.1.

[0023] The technical solution of this application has the following advantages: The positive electrode active material provided in this application has the chemical formula Li. f Ni x Co y Mn z A eO2, 0.8≤f<1.3, 0.8≤x<1, 0.02≤y≤0.2, 0.01≤z≤0.14, 0.06≤e≤0.1, A includes at least element M, wherein element M includes at least one of Sb, Ta, and Y; the positive electrode active material includes polycrystalline particles and a coating layer located on at least a portion of the surface of the polycrystalline particles, wherein the coating layer includes at least element Co; the grain boundaries of the polycrystalline particles contain element Co and element M, and based on the mass of the positive electrode active material, the mass content of element Co in the grain boundaries is greater than the mass content of element M in the grain boundaries; the polycrystalline particles include primary particles, wherein the particle size of the primary particles is D nm, 50≤D≤800; along the direction from the surface of the primary particle to its interior, the region from the surface of the primary particle to its interior at a depth of D / 5 nm is denoted as the first region, and the depth from the surface of the primary particle is greater than D / 5 nm. The region of nm is designated as the second region; based on the mass of the positive electrode active material, the mass content of element M in the first region is greater than the mass content of element M in the second region.

[0024] This application introduces an appropriate amount of element M into polycrystalline particles and enriches it on the surface and grain boundaries of the polycrystalline particles, thereby constructing a continuous chemical protection network for the polycrystalline particles. This significantly strengthens the surface lattice, stabilizes grain boundaries, improves the interface, and accelerates lithium-ion conduction, effectively preventing mechanical failure caused by grain boundary stress concentration in polycrystalline materials. This, in turn, helps to simultaneously improve the cycle life, rate performance, and capacity of the battery. Furthermore, this application also provides a Co-containing coating layer on at least part of the surface of the polycrystalline particles. Utilizing its dual effects of physical isolation and chemical passivation, it can stabilize the interface layer, reduce interface side reactions, and lower impedance. By allowing the Co element in the coating layer to penetrate into the grain boundaries of the polycrystalline particles, it can effectively inhibit grain boundary cracking and further strengthen the grain boundary structure. At the same time, it also controls the content of Co element in the grain boundaries to be greater than that of element M, so as to achieve a balance between high specific capacity and grain boundary stability. This fundamentally improves the mechanical integrity and interface stability of high-nickel ternary polycrystalline materials to meet the application requirements of next-generation high-energy-density, long-cycle-life lithium-ion secondary batteries.

[0025] Additional aspects and advantages of the embodiments of this application will be described and shown in part in the following description, or illustrated by practice of the embodiments of this application. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the specific embodiments of this application, the drawings used in the description of the specific embodiments will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0027] Figure 1 This is a scanning electron microscope image of the positive electrode active material in one embodiment of this application; Figure 2 This is a schematic diagram of the structure of the positive electrode active material in one embodiment of this application; Figure 3 yes Figure 2 A magnified view of a portion of the image; Figure 4 This is an X-ray diffraction pattern of the positive electrode active material in one embodiment of this application.

[0028] The reference numerals in the attached figures are explained as follows: 11-Primary particle; 12-Coating layer; 13-Polycrystalline particle; 21-First surface; 22-Second surface. Detailed Implementation

[0029] The following embodiments are provided to better understand this application and are not limited to the preferred embodiments described herein. They do not constitute a limitation on the content and scope of protection of this application. Any product that is the same as or similar to this application, derived by anyone under the guidance of this application or by combining features of this application with other prior art, falls within the scope of protection of this application.

[0030] It should be noted in the description of this application that the terms "first," "second," "third," and "fourth" are used for descriptive purposes only and should not be construed as indicating or implying relative importance. Furthermore, the technical features involved in the different embodiments of this application described below may be combined with each other as long as they do not conflict with each other.

[0031] The fundamental reason why high-nickel ternary polycrystalline materials are prone to capacity decay and safety issues under deep charge-discharge or long-term cycling lies in their inherent polycrystalline structure defects. Specifically, when charged to high voltage, the crystal lattice of high-nickel ternary polycrystalline materials undergoes a transformation from the H2 phase to the H3 phase, accompanied by significant c-axis contraction. Due to the anisotropy of crystallographic orientation within the material, this volume change generates enormous non-uniform stress between particles with different orientations. This stress concentrates at grain boundaries, leading to microcracks. As cycling progresses, these microcracks continuously expand and penetrate, eventually causing particle fracture or even pulverization. Furthermore, particle fracture exposes a large number of new highly active surfaces, exacerbating the interaction between the electrolyte and highly oxidized active materials (such as Ni). 4+ Side reactions between these components lead to the continuous decomposition of the electrolyte and the release of transition metal ions (such as Mn). 2+ Co 2+The dissolution and unstable thickening of the interfacial solid electrolyte phase (CEI film) generate a large amount of gas, increasing the risk of battery swelling and thermal runaway. At the same time, particle breakage can also destroy the established conductive network inside the electrode, causing some active materials to lose electrical connection with the current collector and fail, increasing the polarization resistance of the battery and further accelerating capacity decay.

[0032] This study found that existing modification methods, such as bulk element doping, have limited direct strengthening effects on grain boundary strength. Furthermore, surface coatings are difficult to penetrate into the grain interior to effectively protect the grain boundaries, and are prone to detachment during repeated expansion and contraction of the particles due to mismatch with the matrix's mechanical properties. In other words, the aforementioned modification measures can only achieve "external repair" or "local enhancement," failing to fundamentally resolve the core contradiction of particle mechanical failure caused by grain boundary stress concentration. More importantly, these modification methods often come at the cost of sacrificing some specific capacity or increasing process complexity.

[0033] In order to effectively alleviate the above problems, this application proposes the following solutions.

[0034] According to the first aspect, this application provides a positive electrode active material, the chemical formula of which is Li f Ni x Co y Mn z A e O2, 0.8≤f<1.3, 0.8≤x<1, 0.02≤y≤0.2, 0.01≤z≤0.14, 0.06≤e≤0.1, A includes at least element M, wherein element M includes at least one of Sb, Ta, and Y; the positive electrode active material includes polycrystalline particles and a coating layer located on at least a portion of the surface of the polycrystalline particles, wherein the coating layer includes at least element Co; the grain boundaries of the polycrystalline particles contain element Co and element M, and based on the mass of the positive electrode active material, the mass content of element Co in the grain boundaries is greater than the mass content of element M in the grain boundaries; the polycrystalline particles include primary particles, wherein the particle size of the primary particles is D nm, 50≤D≤800; along the direction from the surface of the primary particle to its interior, the region from the surface of the primary particle to its interior at a depth of D / 5 nm is denoted as the first region, and the depth from the surface of the primary particle is greater than D / 5 nm. The region of nm is designated as the second region; based on the mass of the positive electrode active material, the mass content of element M in the first region is greater than the mass content of element M in the second region.

[0035] In this application, the term "polycrystalline particle" refers to a secondary aggregate (i.e., secondary particle) formed by multiple primary particles arranged randomly and densely packed in space with different crystallographic orientations and interconnected through grain boundary interfaces. Therefore, the polycrystalline particles in this application contain grain boundaries formed by the boundaries of primary particles. This grain boundary network runs through the entire interior of the polycrystalline particle and is the main channel for element diffusion, migration, and interfacial reactions, which has a significant impact on the structural stability and electrochemical performance of the polycrystalline particle.

[0036] This application, by controlling the particle size D of the primary particles to be between 50nm and 800nm, can ensure a balance between ion diffusion rate, volumetric strain, grain boundary activity, and interface stability, thereby contributing to a synergistic improvement in battery rate performance, cycle life, and capacity. Simultaneously, this application introduces at least one M element from Sb, Ta, and Y into the polycrystalline particles. The M element preferentially occupies transition metal sites, forming a high-bond-energy chemical framework with oxygen atoms, firmly locking lattice oxygen, suppressing oxygen evolution and irreversible phase transitions under high pressure / high temperature, and also improving Ni... 2+ The energy barrier for migration to the lithium layer suppresses cation mixing and ensures Li + The channel is smooth, and the doping of element M can also moderately increase the c-axis spacing of the lithium layer and reduce the Li + Diffusion resistance.

[0037] Furthermore, the aforementioned element M can be enriched on the surface of the polycrystalline particles (i.e., the first region). This gradient distribution allows for the formation of a chemically stable passivation layer on the surface of the polycrystalline particles, effectively isolating the electrolyte from contact with the active material and suppressing interfacial side reactions. More importantly, when microcracks develop in the polycrystalline particles due to stress during long-term cycling, the newly exposed crack interface still possesses corrosion resistance because the element M has been pre-enriched on the particle surface. This prevents direct contact between the electrolyte and the fresh active surface, delaying material structural degradation. In addition, the lower content of element M inside the polycrystalline particles (i.e., the second region) ensures that the layered structure of the bulk active material remains unaffected, maintaining the material's high capacity performance.

[0038] At the same time, by "pinning" the M element at the grain boundaries, it is possible to guide the radial alignment of primary grains along the (003) crystal plane, thus constructing a continuous Li... + Conductive network, shorten Li + It can diffuse along a path and uniformly distribute volumetric strain, preventing crack initiation and propagation and maintaining the mechanical integrity of polycrystalline particles. Furthermore, the M element can react in situ with Li to form a spinel phase, which coats the surface and grain boundaries of polycrystalline particles, reducing interfacial / grain boundary impedance, while also blocking electrolyte erosion and reducing interfacial side reactions.

[0039] Building upon this, this application further provides a coating layer containing Co on at least a portion of the surface of the polycrystalline particles, such as... Figure 2 As shown, this coating layer not only provides physical shielding, preventing direct contact between the active material and the electrolyte and reducing interfacial side reactions, but also, due to the significantly higher conductivity of cobalt oxide compared to the bulk nickel-cobalt-manganese polycrystalline particles, allows the Co-containing coating layer to construct continuous surface conductive pathways, reducing contact impedance and interfacial charge transfer impedance between active materials. Furthermore, during charging, a thin and uniform rock salt phase preferentially forms on the Co-rich surface of the active material, stabilizing the interfacial layer and mitigating bulk structural distortion. More importantly, the Co element in the coating layer diffuses along the grain boundaries of the polycrystalline particles and penetrates into them, creating a Co-concentration effect within the grain boundaries. 3+ Enrichment can reduce grain boundary energy, enhance the bonding force between primary particles, inhibit grain boundary cracking and particle breakage, and improve the mechanical stability of polycrystalline particles. Furthermore, this application also controls the content of Co element in the grain boundary of polycrystalline particles to be greater than the content of M element, thereby ensuring that the grain boundary is dominated by cobalt-rich layered phase, supplemented by an appropriate amount of spinel phase, thereby enhancing grain boundary stability while ensuring high specific capacity, inhibiting grain boundary slip and crack propagation caused by volume strain during cycling, and synergistically improving the structural integrity, cycle life and rate performance of the active material.

[0040] In summary, this application introduces an appropriate amount of element M into polycrystalline particles and enriches it on the surface and grain boundaries of the polycrystalline particles, thereby constructing a continuous chemical protection network for the polycrystalline particles. This significantly strengthens the surface lattice, stabilizes grain boundaries, improves the interface, and accelerates lithium-ion conduction, effectively preventing mechanical failure of polycrystalline materials caused by grain boundary stress concentration. This, in turn, helps to simultaneously improve the cycle life, rate performance, and capacity of the battery. Furthermore, this application also provides a Co-containing coating layer on at least a portion of the surface of the polycrystalline particles. Utilizing its dual effects of physical isolation and chemical passivation, it can stabilize the interface layer, reduce interface side reactions, and lower impedance. By allowing the Co element in the coating layer to penetrate into the grain boundaries of the polycrystalline particles, it can effectively inhibit grain boundary cracking and further strengthen the grain boundary structure. At the same time, it also controls the content of Co element in the grain boundaries to be greater than that of element M, so as to achieve a balance between high specific capacity and grain boundary stability. This fundamentally improves the mechanical integrity and interface stability of high-nickel ternary polycrystalline materials to meet the application requirements of next-generation high-energy-density, long-cycle-life lithium-ion secondary batteries.

[0041] In this application, the coating layer covers more than 50% of the polycrystalline particles. The coating layer coverage rate refers to the percentage of the surface area of ​​the polycrystalline particles covered by the coating layer, which can be obtained by transmission electron microscopy combined with energy dispersive spectroscopy (TEM-EDS). For example, the coating layer coverage rate can be 50%, 60%, 75%, 80%, 90%, 92%, 94%, 96%, 98%, 100%, or a value within any two of the above ranges.

[0042] Furthermore, the coating layer has a coating rate of over 90% on the polycrystalline particles, for example, it can be 90%, 92%, 94%, 96%, 98%, 100%, or a value within the range of any two of the above values.

[0043] In some implementations, further controlling the particle size of the primary particles to be between 100nm and 800nm ​​can better ensure the balance between ion diffusion rate, volumetric strain, grain boundary activity and interface stability, thereby synergistically improving the rate performance, cycle life and capacity utilization of the battery.

[0044] This study found that if the particle size of the primary particles is too small, the contact area between the active material and the electrolyte increases dramatically, exacerbating interfacial side reactions. It also leads to more voids and lower compaction density when secondary particles are stacked, which is not conducive to the development of high-energy-density batteries. Conversely, if the particle size of the primary particles is too large, there are fewer grain boundaries, and the stress concentration at the grain boundaries is obvious, making it easy for grain boundary cracking and secondary particle breakage to occur. In addition, large particle size also leads to longer diffusion paths, increased lithium-ion migration resistance, and severe concentration polarization under high current.

[0045] It should be noted that the particle size of primary particles can be obtained by scanning electron microscopy (SEM). For example, the particle size of primary particles can be 50nm, 100nm, 150nm, 200nm, 250nm, 300nm, 350nm, 400nm, 450nm, 500nm, 550nm, 600nm, 650nm, 700nm, 750nm, 800nm, etc., or values ​​within any two of the above ranges.

[0046] When the total content of element M in the positive electrode active material is the same, if the mass content of element M in the first region is close to that in the second region, it will either affect the surface layer's resistance to electrolyte corrosion or affect the integrity of the layered structure of the active material, making it difficult to maintain the balance of various electrochemical properties of the active material. The relationship between the mass content of element M in the first region and the mass content of element M in the second region can be obtained by energy-dispersive X-ray spectroscopy (EDS) line scan analysis.

[0047] It should be noted that the depth of the first region is closely related to the particle size of the primary particles and the enrichment degree of element M. In the EDS line scan, the boundary between the surface and the bulk phase can be determined based on the fact that the element M content in the surface of the primary particles is greater than that in the bulk phase. The distance between this boundary and the surface of the primary particles is the depth of the first region. The "D / 5 nm" in "the region from the surface of the primary particles to its interior at a depth of D / 5 nm" is actually the maximum value of the depth of the first region.

[0048] When the mass content of Co in the grain boundaries of polycrystalline particles is less than that of M, the effect of inhibiting grain boundary cracking is limited, making it difficult to suppress the mechanical failure of polycrystalline particles, and it also affects the specific capacity of the active material. The relationship between the mass contents of Co and M in the grain boundaries of polycrystalline particles can be obtained by conventional methods in the art, such as energy-dispersive X-ray spectroscopy (EDS) line scanning analysis.

[0049] For example, see Figure 1 , Figure 1 This is a scanning electron microscope image of the positive electrode active material in one embodiment of this application. Figure 1 As can be seen from the data, the positive electrode active material is a polycrystalline particle. The polycrystalline particle is formed by the agglomeration of primary particles. Its surface is relatively smooth, the particles are dense, and there are no obvious cracks.

[0050] In some embodiments, A further includes an element Q, which includes at least one of Zr, Al, Mo, Ti, and Sr. These elements can stabilize lattice oxygen, suppress cation mixing, or segregate along grain boundaries to enhance grain boundary bonding and prevent crack initiation, thereby improving the mechanical integrity of polycrystalline particles and thus improving the cycle stability of the battery.

[0051] Furthermore, in some embodiments, A includes Sr elements and M elements, since Sr 2+ With a larger radius and limited solid solubility in the crystal lattice, it is easy to diffuse and agglomerate towards the grain boundaries during high-temperature sintering. This results in the mass content of Sr elements located in the grain boundaries being greater than that in the primary particles. In this way, the grain boundary bonding force can be strengthened, the initiation and propagation of microcracks can be suppressed, the mechanical integrity of polycrystalline particles can be improved, and the cycle stability of the battery can be improved.

[0052] When the total Sr content in the positive electrode active material is the same, if the mass content h1% of Sr in the grain boundaries of polycrystalline particles is close to equal to the mass content h2% of Sr in the primary particles, either the grain boundary bonding force cannot be effectively strengthened, leading to easy initiation and propagation of microcracks, or there is an excessive amount of Sr. 2+ Intrusion into the crystal lattice causes localized distortion and hinders lithium-ion diffusion, making it difficult to maintain the mechanical integrity of the positive electrode active material and the cycle stability of the battery. The relationship between h1 and h2 can be obtained through energy-dispersive X-ray spectroscopy (EDS) line scan analysis.

[0053] Furthermore, in some embodiments, based on the mass of the positive electrode active material, by controlling the Sr element mass content s1 between 260ppm and 2500ppm, it can be ensured that the Sr element forms an effective "grain boundary modification layer" at the grain boundaries, strengthening the "skeleton connection points" inside the secondary particles and inhibiting the initiation of microcracks. If s1 is too small, the grain boundary strengthening effect is insufficient, and it cannot effectively inhibit the generation and propagation of microcracks; if s1 is too large, an excessive amount of Sr with a larger ionic radius will result in... 2+ It can distort the local crystal structure of materials, hinder lithium-ion diffusion, degrade rate performance, and reduce capacity.

[0054] Meanwhile, controlling the mass content (s2) of element M between 400 ppm and 2000 ppm is crucial. Due to its high charge and strong electronegativity, element M tends to migrate to the surface of polycrystalline particles in the later stages of synthesis, forming a dense "surface passivation layer." This passivation layer can lock in lattice oxygen, inhibit electrolyte erosion, and some element M can penetrate along grain boundaries, forming a continuous chemical protection network. If s2 is too small, the passivation layer will be discontinuous, making it difficult to effectively inhibit electrolyte erosion; if s2 is too large, excessive high-charge element M may cause local charge imbalance, inducing lattice defects, and also sacrificing specific capacity due to excessive passivation layer.

[0055] Based on this, by controlling the ratio s1 / s2 between the mass content of Sr element s1 and the mass content of M element s2 to be between 0.15 and 6, a continuous protective network framework from the grain boundaries of polycrystalline particles to the surface can be formed. This allows Sr and M elements to work together to resist the volume change stress caused by lithium-ion insertion / extraction and the chemical erosion of the electrolyte. The internal grain boundaries are strengthened by Sr, which enhances their mechanical integrity, while the external interfaces are passivated by M, which improves their chemical stability. Together, they block the main failure path of polycrystalline materials from both physical and chemical dimensions, thereby doubling the capacity retention rate and significantly improving the cycle stability and thermal safety of the battery.

[0056] If s1 / s2 is too small, meaning there is a relatively large amount of M element, the mechanical buffer will be insufficient, and the grain boundaries will still be prone to stress cracking. The passivation layer formed by M element will fail due to particle fracture. If s1 / s2 is too large, meaning there is a relatively large amount of Sr element, the chemical anchoring will be insufficient, the surface oxygen stability will be inadequate, and the material may still experience oxygen loss under high voltage, leading to structural collapse. The mechanical support of Sr element will also lose its meaning.

[0057] It should be noted that the mass content of Sr and M elements can be obtained by conventional methods in the art, such as inductively coupled plasma optical emission spectrometry (ICP-OES) or inductively coupled plasma mass spectrometry (ICP-MS). For example, the mass content of Sr can be 260 ppm, 420 ppm, 580 ppm, 740 ppm, 900 ppm, 1060 ppm, 1220 ppm, 1380 ppm, 1540 ppm, 1700 ppm, 1860 ppm, 2020 ppm, 2180 ppm, 2340 ppm, 2500 ppm, or values ​​within any two of the above ranges; the mass content of M can be 400 ppm, 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm, etc. 0ppm, 1000ppm, 1100ppm, 1200ppm, 1300ppm, 1400ppm, 1500ppm, 1700ppm, 1900ppm, 2000ppm, etc., or values ​​within the range of any two of the above values; the value of s1 / s2 can be, for example, 0.15, 0.30, 0.50, 0.80, 1.00, 1.50, 2.00, 2.50, 3.00, 3.50, 4.00, 4.50, 5.00, 5.50, 6.00, etc., or values ​​within the range of any two of the above values.

[0058] The relative standard deviation (RSD) is a statistical indicator used to measure the dispersion of data. It is defined as the ratio of the standard deviation to the mean (expressed as a percentage). The smaller the value, the smaller the difference between the measurements, that is, the more uniform the data distribution.

[0059] In some embodiments, the M element includes Sb and Y elements, and the Q element includes Sr, Zr, and Al elements. These elements can form a gradient-stable chemical protection network from the bulk phase of the polycrystalline particles to the grain boundaries and then to the surface layer, thereby further improving the structural integrity of the cathode active material and the cycle stability of the battery. Simultaneously, at five locations on the CP-SEM cross-section of the polycrystalline particles, this application ensures that the relative standard deviation of the mass content of Al and Zr elements is less than 40%, thus ensuring uniform distribution of Al and Zr elements in the bulk phase of the polycrystalline particles. This fully leverages their functions of stabilizing the crystal structure, suppressing harmful H2-H3 phase transitions, and enhancing the strength of transition metal-oxygen bonds, thereby improving the cycle stability and structural integrity of the material. If the relative standard deviation of the mass content of Al and Zr elements is greater than 40%, the distribution of Al and Zr elements in the bulk phase of the material will be uneven. Insufficient doping in local areas will lead to decreased crystal stability, while excessive doping in local areas may cause crystal distortion or capacity loss, thereby deteriorating the cycle performance of the battery.

[0060] It should be noted that "CP-SEM cross section" refers to a flat cross section obtained by argon ion beam processing using a cross section polisher, which is then observed using a scanning electron microscope.

[0061] The relative standard deviation (RSD) of the mass content of Al and Zr can be obtained by cross-sectional polishing-scanning electron microscopy combined with energy dispersive spectroscopy (EDS). Specifically, argon ion beam polishing (CP) is used to process the cross-section of polycrystalline particles with argon ion beams to obtain a flat, stress-free observation surface. The obtained flat cross-section is then observed using a scanning electron microscope, and five different locations are selected on this cross-section. The mass content of Al or Zr at each location is measured using an EDS. The ratio (expressed as a percentage) of the standard deviation of these contents to the average value is then calculated, which is the RSD of the mass content of Al or Zr. For example, the RSD of the mass content of Al or Zr can be 2.5%, 5%, 7.5%, 10%, 12.5%, 15%, 17.5%, 20%, 22.5%, 25%, 27.5%, 30%, 32.5%, 35%, etc., or values ​​within any two of the above ranges.

[0062] Furthermore, in some embodiments, based on the mass of the positive electrode active material, the Sb element content is 100ppm-1600ppm, which allows the Sb element to form a dense "surface passivation layer" on the surface of the polycrystalline particles, locking in lattice oxygen, inhibiting electrolyte erosion, and penetrating along grain boundaries to form a continuous chemical protection network; further, controlling the Y element content between 300ppm-1200ppm allows the Y element to form a stable Y-based compound, further enhancing the stability of the interface structure; controlling the Zr element content between 1500ppm-3500ppm allows the Zr element to be more uniformly distributed in the bulk phase of the polycrystalline particles, so as to more fully stabilize the lattice structure and inhibit the harmful H2-H3 phase transition; controlling the Al element content between 600ppm-2500ppm allows the Al element to be uniformly distributed in the bulk phase as well, further enhancing the bonding strength between the transition metal and oxygen. By further controlling the mass content of the above elements within a suitable range, the effects of grain boundary strengthening, surface passivation, and bulk phase stabilization can be more fully utilized, thereby improving the mechanical stability of polycrystalline particles while maintaining high capacity, and thus significantly enhancing the cycle performance of the battery.

[0063] It should be noted that the mass content of Sb, Y, Zr, and Al elements can be obtained by inductively coupled plasma optical emission spectrometry (ICP-OES) or inductively coupled plasma mass spectrometry (ICP-MS).

[0064] For example, the mass content of Sb can be, for example, 100ppm, 200ppm, 300ppm, 400ppm, 500ppm, 600ppm, 700ppm, 800ppm, 900ppm, 1000ppm, 1100ppm, 1200ppm, 1300ppm, 1400ppm, 1500ppm, 1600ppm, etc., or values ​​within the range of any two of the above values; the mass content of Y can be, for example, 300ppm, 360ppm, 420ppm, 480ppm, 540ppm, 600ppm, 660ppm, 720ppm, 780ppm, 840ppm, 900ppm, 960ppm, 1020ppm, 1080ppm, 1200ppm, etc., or values ​​within the range of any two of the above values; Zr element The mass content of the element can be, for example, 1500ppm, 1650ppm, 1800ppm, 1950ppm, 2100ppm, 2250ppm, 2400ppm, 2550ppm, 2700ppm, 2850ppm, 3000ppm, 3150ppm, 3300ppm, 3450ppm, 3500ppm, etc., or values ​​within the range of any two of the above values; the mass content of the Al element can be, for example, 600ppm, 740ppm, 880ppm, 1020ppm, 1160ppm, 1300ppm, 1440ppm, 1580ppm, 1720ppm, 1860ppm, 2000ppm, 2140ppm, 2280ppm, 2420ppm, 2500ppm, etc., or values ​​within the range of any two of the above values.

[0065] In some implementations, by controlling the particle size L of the polycrystalline particles to be between 5 μm and 18 μm, a reasonable distribution of the grain boundary network between primary particles can be ensured. This is beneficial for the rapid diffusion of lithium ions within the secondary particles and can effectively disperse the volume change stress generated during charging and discharging, suppressing the initiation and propagation of microcracks, thereby further improving the cycle stability and structural integrity of the cathode active material. If the particle size of the polycrystalline particles is too small, the specific surface area will be too large and the number of grain boundaries will be too high, which will aggravate side reactions, reduce compaction density, and be detrimental to energy density improvement. If the particle size of the polycrystalline particles is too large, the lithium ion diffusion path will be longer, the rate performance will decrease, and the local stress generated by the anisotropic volume change between primary particles will be more concentrated, making it easier for microcracks to propagate and particles to break, thereby accelerating capacity decay.

[0066] It should be noted that the particle size of the polycrystalline particles can be obtained by scanning electron microscopy (SEM). For example, the particle size of the polycrystalline particles can be 5 μm, 7.6 μm, 8.4 μm, 9.2 μm, 10.0 μm, 10.8 μm, 11.6 μm, 12.4 μm, 13.2 μm, 14.0 μm, 14.8 μm, 15.6 μm, 16.4 μm, 17.2 μm, 18.0 μm, or values ​​within any two of the above ranges.

[0067] In some embodiments, the region extending from the surface of the polycrystalline particle to a depth of L / 5 μm is designated as the third region, and the region extending at a depth greater than L / 5 μm from the surface of the polycrystalline particle is designated as the fourth region. Since the Co element in the coating layer diffuses and permeates along the grain boundaries into the interior of the polycrystalline particle, and accumulates at the grain boundaries and near the surface, the mass content of Co element in the third region can be greater than that in the fourth region. This reduces the grain boundary energy, enhances the bonding force between primary particles, inhibits grain boundary cracking and particle breakage, thereby improving the mechanical stability of the polycrystalline particle and enhancing the cycle life of the battery. The relative mass contents of Co element in the third and fourth regions can be obtained through energy-dispersive X-ray spectroscopy (EDS) line scan analysis.

[0068] Furthermore, in some embodiments, a cobalt-rich layered structure exists in the third region and / or the grain boundaries of the polycrystalline particles. During the preparation of the positive electrode active material, the Co element in the coating layer will migrate to the third region and grain boundaries of the polycrystalline particles. At the same time, the Co element in the bulk phase of the polycrystalline particles may also undergo re-diffusion under high-temperature sintering conditions. Therefore, based on the sum of the molar numbers of all transition metal elements in the cobalt-rich layered structure, the molar content of the Co element is greater than or equal to 60%. The Co element that migrates to the third region and grain boundaries reacts with the excess lithium (usually from excess lithium source or surface residual lithium) present in the polycrystalline particles to generate a cobalt-rich phase with a layered crystal structure in situ. The crystal structure of this cobalt-rich phase is highly similar to that of LiCoO2 and can be regarded as a LiCoO2-like layered phase.

[0069] This cobalt-rich layered structure exhibits excellent electronic conductivity and lithium-ion transport capabilities, enabling the formation of a highly efficient "electron / ion fast transport network" in the third region and at grain boundaries. This effectively reduces interfacial impedance and improves the rate performance and cycle stability of the battery. However, if the molar content of Co in the cobalt-rich layered structure is less than 60%, the conductivity of the cobalt-rich layered phase decreases significantly, making it difficult to form continuous fast transport channels. This leads to increased impedance for lithium ions to enter and exit the particle, resulting in deteriorated rate performance. Simultaneously, the obstructed electron transport at grain boundaries exacerbates local stress concentration, which is detrimental to maintaining the structural stability of the material.

[0070] It should be noted that the molar content of Co in the cobalt-rich phase can be qualitatively determined by energy-dispersive X-ray spectroscopy (EDS).

[0071] In some embodiments, based on the mass of the positive electrode active material, by controlling the mass content of Co in the polycrystalline particles to be greater than the mass content of Mn in the polycrystalline particles, Co can be utilized. 3+ / Co 4+ The energy level structure broadens the electron transition channels and reduces the band gap, thereby significantly improving the electronic conductivity within the ternary cathode active material. This allows electrons to be rapidly transported to the surface to participate in the reaction, laying the foundation for improved rate performance and capacity. If the mass content of Co is lower than that of Mn, the electronic band gap within the matrix widens, reducing electronic conductivity and hindering electron transport within the matrix, thus decreasing the battery's rate performance.

[0072] It should be noted that the mass content of Co and Mn elements can be obtained by conventional methods in the field, such as inductively coupled plasma optical emission spectrometry (ICP-OES) or inductively coupled plasma mass spectrometry (ICP-MS).

[0073] In some implementations, such as Figure 3 As shown, the primary particle includes a first surface (red line) and a second surface (blue line). The first surface is in contact with the coating layer, while the second surface is not in contact with the coating layer. Based on the mass of the positive electrode active material, the mass content of Co on the first surface is less than that on the second surface. Thus, the lower Co content on the first surface allows for a relatively higher content of coating elements such as Al, Ti, W, and B, which is beneficial for forming a highly stable and dense chemical protective shell, effectively addressing the issues of chemical compatibility with the electrolyte and oxygen loss. Simultaneously, Co is enriched on the second surface, particularly at the grain boundaries of the polycrystalline particles, which helps to construct a continuous and efficient "electron / ion fast transport network" within the polycrystalline particles, enhancing grain boundary conductivity and mechanical strength, and suppressing the generation and propagation of microcracks. This "high Co internally, low Co externally" design balances the electronic conduction and mechanical integrity within the polycrystalline particles with the interfacial chemical stability of the outer surface, thereby synergistically improving the rate performance and cycle stability of the battery.

[0074] If the mass content of Co on the first surface is greater than that on the second surface, Co will be excessively enriched on the outer surface of the polycrystalline particles, while the Co content inside the polycrystalline particles will be insufficient. This will lead to discontinuity in the internal conductive network and a decrease in grain boundary strength. At the same time, the lack of suitable elemental combinations on the outer surface will make it difficult to effectively suppress electrolyte side reactions and oxygen release, thereby accelerating the failure of the positive electrode active material. The relationship between the mass contents of Co on the first and second surfaces can be determined by conventional methods in the art, such as energy-dispersive X-ray spectroscopy (EDS) line scanning analysis.

[0075] In some implementations, by controlling the thickness of the coating layer between 5nm and 50nm, the cross-interface transport efficiency of lithium ions and electrons can be ensured while effectively isolating the electrolyte from contact with the active material. If the coating layer is too thin, it is difficult to form a continuous and stable protective layer, and it is impossible to suppress side reactions and transition metal dissolution; if the coating layer is too thick, it will significantly increase the interfacial impedance, hinder lithium ion diffusion, and lead to a decrease in the rate performance of the battery.

[0076] It should be noted that the thickness of the coating layer can be measured using conventional methods in the art, such as transmission electron microscopy combined with energy dispersive spectroscopy (TEM-EDS). For example, the thickness of the coating layer can be 5 nm, 8 nm, 11 nm, 14 nm, 17 nm, 20 nm, 23 nm, 26 nm, 29 nm, 32 nm, 35 nm, 38 nm, 41 nm, 44 nm, 47 nm, 50 nm, or a value within any two of the above ranges.

[0077] In some embodiments, the coating layer further includes at least one element selected from Al, Ti, W, and B. These elements can synergistically work with Co to further enhance the density, chemical stability, or ionic conductivity of the coating layer, thereby more effectively suppressing electrolyte side reactions, reducing interfacial impedance, and improving the cycle stability and rate performance of the battery.

[0078] Furthermore, in some embodiments, the coating layer includes Co, Al, and B elements. Based on the mass of the coating layer, by controlling the mass content of Co element between 50% and 98%, sufficient electronic conductivity of the coating layer can be ensured; controlling the mass content of Al element between 1% and 15% can enhance the structural compactness and chemical stability of the coating layer; and controlling the mass content of B element between 0.2% and 10% can promote the formation of a fast ion conductor interface layer and reduce interface impedance.

[0079] For example, the mass content of Co can be 50%, 53%, 57%, 60%, 64%, 67%, 71%, 74%, 78%, 81%, 85%, 88%, 92%, 95%, 98%, or values ​​within any two of the above ranges; the mass content of Al can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, or values ​​within any two of the above ranges; the mass content of B can be 0.2%, 0.9%, 1.6%, 2.3%, 3.0%, 3.7%, 4.4%, 5.1%, 5.8%, 6.5%, 7.2%, 7.9%, 8.6%, 9.3%, 10.0%, or values ​​within any two of the above ranges.

[0080] In some implementations, by controlling the particle size Dv50 of the positive electrode active material between 5 μm and 18 μm, it is possible to ensure that the material has good processing performance, suitable compaction density, and a short lithium-ion diffusion path, thereby balancing the energy density and rate performance of the battery. If the particle size Dv50 is too small (less than 5 μm), the specific surface area of ​​the material is too large, which will aggravate the side reactions with the electrolyte, and the compaction density will be too low; if the particle size Dv50 is too large (greater than 18 μm), the lithium-ion solid-phase diffusion distance increases, the rate performance decreases, and scratches are easily generated during electrode coating, affecting processing consistency.

[0081] It should be noted that the particle size Dv50 of the positive electrode active material refers to the particle size corresponding to a cumulative volume distribution percentage of 50%, that is, the volume content of particles smaller than or equal to this size accounts for 50% of the total particle volume. The Dv50 test method can be obtained by conventional methods in this field, such as referring to the standard GB / T19077-2016 / ISO 13320:2009, and using a laser particle size analyzer (Malvern Master Size 3000) for testing.

[0082] For example, the particle size Dv50 of the positive electrode active material can be, for example, 5 μm, 7.5 μm, 8.0 μm, 9.0 μm, 10.0 μm, 11.0 μm, 11.5 μm, 12.0 μm, 13.0 μm, 14.0 μm, 15.0 μm, 15.5 μm, 16.0 μm, 17.0 μm, 18.0 μm, etc., or a value within the range of any two of the above values.

[0083] In some embodiments, the X-ray diffraction pattern of the positive electrode active material, such as Figure 4As shown, diffraction peaks are present at positions of 18.5°-19.0°, 37.8°-38.7°, and 44.5°-45.1°. Among them, the peak in the range of 18.5°-19.0° corresponds to the (003) crystal plane, the peak in the range of 37.8°-38.7° corresponds to the (101) crystal plane, and the peak in the range of 44.5°-45.1° corresponds to the (104) crystal plane. The presence of these peaks indicates that the material has a well-crystallized α-NaFeO2 type layered structure (space group R-3m), which is beneficial to the reversible insertion and extraction of lithium ions and the structural stability of the material.

[0084] It should be noted that the diffraction peaks in the X-ray diffraction pattern of the positive electrode active material can be obtained by X-ray diffraction analysis.

[0085] In the differential scanning calorimetry (DSC) of positive electrode active materials, the onset temperature of the exothermic peak can indicate the thermal stability of the positive electrode active material as it undergoes structural collapse and oxygen release from the layered structure to the spinel or rock salt phase. The higher the onset temperature, the better the thermal stability of the material.

[0086] In this application, the differential scanning calorimetry (DSC) of the positive electrode active material shows that the starting temperature of the exothermic peak is T≥190℃. This confirms that the positive electrode active material in this application has excellent resistance to thermal runaway under high temperature or high voltage, which can significantly improve the thermal safety performance of the battery.

[0087] It should be noted that the exothermic peak in the differential scanning calorimetry (DSC) of the positive electrode active material can be obtained by DSC.

[0088] In this application, the positive electrode active material can be prepared by the following method, including the following steps: The first step involves preparing a nickel-cobalt-manganese hydroxide precursor using a co-precipitation method. At least one of the following sources—antimony, tantalum, yttrium, zirconium, aluminum, molybdenum, titanium, and strontium—is loaded onto the precursor surface. This precursor is then mixed with a lithium source at a specific molar ratio and sintered under an oxygen atmosphere to obtain polycrystalline particles. The antimony source can be an oxide, chloride, antimonylic acid, or fluoride; the tantalum source can be an oxide or chloride; the yttrium source can be an oxide, chloride, nitrate, hydroxide, fluoride, or phosphate; the zirconium source can be an oxide, hydroxide, chloride, fluoride, sulfate, or nitrate; the aluminum source can be an oxide, hydroxide, chloride, nitrate, or sulfate; the molybdenum source can be an oxide, chloride, molybdenum nitrate, or ammonium molybdate; the titanium source can be an oxide, chloride, hydroxide, or sulfuric acid; and the strontium source can be an oxide, carbonate, hydroxide, chloride, or nitrate.

[0089] The second step involves preparing a mixed solution of polycrystalline particles and a cobalt source (optionally, an aluminum source, tungsten source, boron source, and / or titanium source). This solution is then sprayed onto the surface of the polycrystalline particles, and after sintering, the positive electrode active material is obtained. The cobalt source can be an oxide, chloride, nitrate, hydroxide, phosphate, or sulfate; the aluminum source can be an oxide, hydroxide, chloride, nitrate, or sulfate; the tungsten source can be an oxide, tungstate, or tungstic acid; the boron source can be an oxide or boric acid; the titanium source can be an oxide, chloride, hydroxide, or sulfate; or the titanium source can be an oxide, chloride, hydroxide, or sulfuric acid.

[0090] Based on the amount of each raw material used in the above preparation method, the chemical formula of the positive electrode active material can be calculated.

[0091] It should be noted that in the first step of polycrystalline particle preparation, the particle size of the primary particles can be controlled by adjusting the sintering temperature and time, which in turn will change the particle size of the polycrystalline particles and the particle size Dv50 of the positive electrode active material. By controlling the amount of M element (such as antimony source or yttrium source) added, the mass content and distribution of M element in the positive electrode active material can be controlled. By controlling the molar ratio of cobalt and manganese elements in the co-precipitation step, the mass content of cobalt and manganese elements in the polycrystalline particles can be controlled.

[0092] In the second coating step, the mass content and distribution of Co in the positive electrode active material can be controlled by adjusting the amount of cobalt source added, the sintering temperature, and the sintering time. The mass content of elements such as Al, W, Ti, and B in the positive electrode active material and the thickness of the coating layer can be controlled by adjusting the amount of aluminum, tungsten, titanium, and boron sources added, the sintering temperature, and the sintering time.

[0093] In the first step of polycrystalline particle preparation, by controlling the amount of strontium source added, the mass content of Sr element in the cathode active material, the mass content of Sr element in the grain boundaries of polycrystalline particles and the interior of primary particles in the cathode active material can be regulated.

[0094] By controlling the amount of tantalum source added, the mass content of Ta element in the positive electrode active material can be adjusted.

[0095] By controlling the amount of zirconium source added, the mass content of Zr in the cathode active material can be adjusted. In combination with the adjustment of sintering temperature, the relative standard deviation of the mass content of Zr in the cathode active material can be controlled.

[0096] By controlling the amount of aluminum source added, the mass content of Al in the positive electrode active material can be adjusted. In combination with the adjustment of sintering temperature, the relative standard deviation of the mass content of Al in the positive electrode active material can be controlled.

[0097] By controlling the amount of yttrium source added, the mass content of Y element in the positive electrode active material can be adjusted.

[0098] According to a second aspect of this application, a positive electrode sheet is provided, including a positive electrode active layer, the positive electrode active layer including a positive electrode active material; the positive electrode active material includes the positive electrode active material described in the first aspect of this application.

[0099] Furthermore, in some embodiments, the positive electrode active material further includes lithium nickel cobalt manganese oxide single crystal particles. Based on the mass of the positive electrode active layer, the mass content of the lithium nickel cobalt manganese oxide single crystal particles is denoted as F%, and the mass content of the positive electrode active material is denoted as E%, satisfying: 0 < E / F ≤ 1. Thus, when polycrystalline particles and lithium nickel cobalt manganese oxide single crystal particles are used in combination, the polycrystalline particles, due to their faster lithium-ion diffusion kinetics, bear more current during cycling, thereby reducing the effective current density and polarization on the surface of the lithium nickel cobalt manganese oxide single crystal particles and suppressing the release of lattice oxygen from the single crystal under high voltage. At the same time, the polycrystalline particles continuously absorb the volume change stress of the electrode during charging and discharging through internal grain boundary sliding and microscale fragmentation, protecting the lithium nickel cobalt manganese oxide single crystal particles from cracking and maintaining their interface stability. If E / F > 1, the mass content of polycrystalline particles exceeds that of lithium nickel cobalt manganese oxide monocrystalline particles. This will result in an excessively large total area of ​​highly active grain boundaries continuously exposed by polycrystalline particles during cycling, leading to uncontrolled side reactions and total gas production. This will accelerate battery capacity decay, shorten cycle life, and may cause battery swelling due to increased gas production, thus reducing battery safety performance.

[0100] For example, the value of E / F can be 0.14, 0.21, 0.29, 0.36, 0.43, 0.50, 0.57, 0.64, 0.71, 0.79, 0.86, 0.93, 1.00, or a value within the range of any two of the above values.

[0101] According to a third aspect of this application, a lithium-ion secondary battery is also provided, comprising a negative electrode and a positive electrode as described in the second aspect of this application.

[0102] Furthermore, in some embodiments, the ratio of the capacity of the negative electrode to the capacity of the positive electrode is denoted as NP. By controlling NP between 1.04 and 1.3, the capacity of the positive electrode can be relatively excessive compared to the negative electrode, thereby using a portion of the positive electrode capacity as a "buffer" to compensate for the low initial coulombic efficiency of the negative electrode and the continuous consumption of active lithium caused by the volume expansion or rupture of silicon-carbon materials during cycling, thus ensuring a long cycle life of the battery.

[0103] If the NP is too small, the compensation will be insufficient, the loss of negative electrode lithium cannot be effectively replenished, and the battery capacity will decay rapidly. If the NP is too large, the positive electrode capacity utilization rate will be low, thus sacrificing the battery's energy density.

[0104] It should be noted that the NP test method includes: assembling the positive and negative electrodes into half-cells at 25°C, and measuring their first-cycle discharge capacity at a 0.1C charge-discharge rate. NP = first-cycle discharge capacity of the negative electrode / first-cycle discharge capacity of the positive electrode. For example, the value of NP can be 1.04, 1.05, 1.06, 1.08, 1.10, 1.12, 1.14, 1.16, 1.18, 1.20, 1.22, 1.24, 1.26, 1.28, 1.30, or a value within any two of the above ranges.

[0105] In some embodiments, the negative electrode sheet includes a negative electrode active material, which includes silicon-carbon material. By controlling the particle size Dv50 of the silicon-carbon material between 1μm and 25μm, it can be ensured that the silicon-carbon particles have a suitable size, which is beneficial for uniform mixing with negative electrode materials such as graphite, and also provides sufficient buffer space for the volume expansion of silicon. If the particle size Dv50 of the silicon-carbon material is too small, it will result in an excessively large specific surface area, excessive growth of the SEI film, and consumption of a large amount of active lithium, thereby aggravating the irreversible capacity loss of the battery. If the particle size Dv50 of the silicon-carbon material is too large, it will cause stress concentration inside the silicon-carbon particles, making them prone to cracking and pulverization, thereby reducing the cycle stability of the battery.

[0106] It should be noted that the particle size Dv50 of silicon carbide materials refers to the particle size corresponding to a cumulative volume distribution percentage of 50%, that is, the volume content of particles smaller than or equal to this size accounts for 50% of the total particle volume. The Dv50 test method can refer to the standard GB / T19077-2016 / ISO 13320:2009, and is tested using a laser particle size analyzer (Malvern Master Size3000).

[0107] For example, the particle size Dv50 of silicon carbide material can be 1μm, 2μm, 3μm, 5μm, 7μm, 9μm, 11μm, 13μm, 15μm, 17μm, 19μm, 21μm, 23μm, 24μm, 25μm, etc., or a value within the range of any two of the above values.

[0108] In some embodiments, the silicon-carbon material comprises porous carbon and silicon particles located within the pores of the porous carbon. Based on the mass of the silicon-carbon material, by controlling the silicon content to be between 30% and 80%, the silicon-carbon material can be ensured to possess both high specific capacity and good cycle stability. This prevents insufficient silicon content from leading to inadequate specific capacity improvement and difficulty in guaranteeing high energy density of the battery; or prevents excessive silicon content from causing drastic volume expansion and easy particle pulverization, thereby drastically reducing the cycle life of the battery.

[0109] It should be noted that the mass content of silicon can be obtained by conventional methods in the art, such as inductively coupled plasma optical emission spectrometry (ICP-OES) or energy dispersive spectroscopy (EDS). For example, the mass content of silicon can be 30%, 34%, 38%, 42%, 46%, 50%, 54%, 58%, 62%, 66%, 70%, 72%, 74%, 78%, 80%, or values ​​within any two of the above ranges.

[0110] In some embodiments, the silicon-carbon material content is between 15% and 100% based on the mass of the negative electrode active material, which can significantly improve the specific capacity of the negative electrode and thus improve the mass energy density of the lithium-ion secondary battery.

[0111] It should be noted that the mass content of silicon-carbon materials can be obtained through conventional testing methods in the art, such as peak separation using thermogravimetric analysis (TGA) combined with differential weight loss curves (DTG), or through ICP testing. For example, the mass content of silicon-carbon materials can be 15%, 21%, 27%, 33%, 39%, 45%, 51%, 57%, 63%, 69%, 75%, 81%, 87%, 93%, 100%, or values ​​within any range of two of the above values.

[0112] In some embodiments, the negative electrode active material further includes graphite material. Based on the mass of the negative electrode active material, by controlling the mass content of the graphite material between 0-85%, the overall specific capacity, initial coulombic efficiency, and cycle stability of the negative electrode can be adjusted. This prevents the negative electrode from having an excessively low specific capacity due to too low a mass content of graphite material, resulting in poor matching with silicon-carbon and a low initial efficiency; or the negative electrode from having an excessively high mass content of graphite material, resulting in insufficient specific capacity and difficulty in ensuring high energy density of the battery.

[0113] It should be noted that the mass content of graphite material can be obtained by conventional methods in the art, such as peak separation using a thermogravimetric analyzer (TGA) combined with differential weight loss curves (DTG). For example, the mass content of graphite material can be 0%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 80%, 85%, etc., or values ​​within any range of two of the above values.

[0114] The differential capacity curve (dQ / dV-V curve) is obtained by taking the first derivative of the battery's charge / discharge capacity (Q) with respect to the voltage (V). It can intuitively reflect the electrochemical reactions, phase transitions, and polarization behaviors that occur during the charging and discharging process. In this curve, the peaks that appear during charging are called oxidation peaks, representing the phase transitions or reaction steps when lithium ions are extracted; the peaks that appear during discharging are called reduction peaks, representing the corresponding steps when lithium ions are inserted. The intensity of the peak (i.e., the peak height) is related to the severity of the reaction. The more severe the reaction, the greater the peak intensity. The half-width at half-maximum (HWHM) of the peak (i.e., the width at half the peak height) reflects the uniformity of the reaction process and the kinetic resistance. The narrower the HWHM, the more concentrated the reaction and the smaller the resistance.

[0115] In some embodiments, the differential capacity curve of the lithium-ion secondary battery described in this application has an oxidation peak A in the range of 4.1V-4.2V and a reduction peak B in the range of 3.6V-3.8V. The half-width at half maximum (WHM) of the oxidation peak A is denoted as WA, and the half-width at half maximum (WHM) of the reduction peak B is denoted as WB. By controlling the ratio WB / WA between the half-width at half maximum (WHM) of the oxidation peak A and the half-width at half maximum (WHM) of the reduction peak B to be between 0.8 and 1.1, it can be shown that the resistance experienced by lithium-ion extraction and insertion is highly symmetrical, the material structure has excellent elasticity, can almost completely recover after volume change, and the polycrystalline particles and coating layer are robust and have good lithium-ion conductivity, thereby ensuring that the battery still has good cycle performance at high voltage.

[0116] If WB / WA is too small, it indicates that the resistance in the reduction process is significantly greater than that in the oxidation process. This may be due to the CEI film being too thick or uneven, resulting in slow and dispersed lithium-ion insertion, which leads to a decrease in the cycle performance of the battery at high voltage. If WB / WA is too large, it indicates that the resistance in the oxidation process is relatively greater. This may be related to side reactions or structural degradation on the surface of the positive electrode active material, which is also not conducive to the long-term cycle stability of the battery.

[0117] It should be noted that the testing methods for WB and WA include: performing a constant current charge-discharge test on the battery at a low rate (e.g., 0.1C), recording the voltage V and capacity Q data, with a sampling interval of every 1~10 mV or every 1~5 seconds; differentiating the obtained data to obtain the dQ / dV curve; locating the peaks of the reduction peak B and oxidation peak A on the smoothed dQ / dV curve, determining the baseline, calculating the peak intensity, and then calculating the half-peak width based on the half-peak height position, denoted as WB and WA respectively. For example, the value of WB / WA can be 0.80, 0.82, 0.84, 0.86, 0.88, 0.90, 0.92, 0.94, 0.96, 0.98, 1.00, 1.02, 1.04, 1.06, 1.08, 1.10, or values ​​within any range of two of the above values.

[0118] The present application will be further described in detail below with reference to specific embodiments. These embodiments should not be construed as limiting the scope of protection claimed in this application. Where specific experimental steps or conditions are not specified in the embodiments and comparative examples, they can be performed according to the conventional experimental steps or conditions described in the literature in the art. Reagents or instruments used, unless otherwise specified, are all commercially available conventional reagent products.

[0119] Example 1 This embodiment provides a method for preparing a lithium-ion secondary battery, including the following steps: Step 1: Preparation of positive electrode active material A hydroxide precursor with a nickel-cobalt-manganese molar ratio of 90.5:4.5:5.0 was prepared by co-precipitation. 0.2455 wt% Al₂O₃, 0.3782 wt% ZrO₂, 0.0953 wt% Y₂O₃, 0.2359 wt% SrCO₃, and 0.2274 wt% Sb₂O₃ (all based on precursor mass) were uniformly loaded onto the precursor surface in a liquid phase. The loaded precursor was then mixed with a lithium source at a Li / (Ni+Co+Mn) molar ratio of 1.01–1.06 and sintered at 750–850 °C under an oxygen atmosphere to obtain pentagonal phase-doped polycrystalline particles. Subsequently, 0.8345 wt% Al(NO₃)₃·9H₂O and 2.7240 wt% Sb₂O₃ were added. A mixed solution of wt% Co3O4 (based on the mass of polycrystalline particles) was prepared and sprayed onto the surface of the polycrystalline particles. The mixture was then sintered at 400℃~550℃ to form a primary coating layer. Finally, a solution of 0.5721 wt% H3BO3 (based on the mass of polycrystalline particles) was prepared and sprayed onto the surface again. The mixture was then sintered a second time at 250℃~400℃ to obtain polycrystalline particles with a coating layer (i.e., the positive electrode active material), with the chemical formula Li(Ni). 0.887 Co 0.064 Mn0.029 Sr 0.0019 Sb 0.00074 Y 0.00076 Zr 0.0023 Al 0.0059 B 0.0084 )O2.

[0120] In addition, a hydroxide precursor with a nickel-cobalt-manganese molar ratio of 95:3:2 was prepared by co-precipitation. The precursor was mixed with a lithium source at a Li / (Ni+Co+Mn) molar ratio of 1.02~1.08, and sintered at a high temperature of 850~980℃ under an oxygen atmosphere. After crushing and classification, lithium nickel cobalt-manganese oxide single crystal particles with the chemical formula LiNi were obtained. 0.95 Co 0.03 Mn 0.02 O2.

[0121] Step 2: Preparation of the positive electrode sheet The positive electrode active material (lithium nickel cobalt manganese oxide single crystal particles to the positive electrode active material obtained in the first step at a mass ratio of 60:40), polyvinylidene fluoride, single-walled carbon nanotubes, and multi-walled carbon nanotubes were mixed evenly at a mass ratio of 97.4:1.2:0.4:1. N-methylpyrrolidone was added to prepare a positive electrode slurry with a solid content of 65%. The positive electrode slurry was then uniformly coated onto an aluminum foil with a thickness of 10 μm using a coating machine. After drying, rolling, die-cutting, and sheet forming, a positive electrode sheet was obtained, wherein the compacted density of the positive electrode sheet was 3.35 g / cm³. 3 .

[0122] Step 3: Preparation of the negative electrode Artificial graphite, silicon carbide material, conductive agent, and binder are mixed evenly in a mass ratio of 36.6:54.9:5:3.5. Deionized water is added to prepare a negative electrode slurry with a solid content of 30%. The negative electrode slurry is then uniformly coated onto a high-strength carbon-coated copper foil with a thickness of 6μm. After drying, rolling, die-cutting, and sheet forming, a negative electrode sheet is obtained.

[0123] Step 4: Preparation of the diaphragm A 2 μm thick composite coating is applied to the third surface of a polyethylene substrate with a thickness of 7 μm and a porosity of 40%. The composite coating comprises alumina, melamine cyanurate, methacrylic acid, and sodium polymethyl cellulose. Based on the total mass of the composite coating, the melamine cyanurate content is 92 wt%, the methacrylic acid content is 4 wt%, and the sodium polymethyl cellulose content is 4 wt%. Polyvinylidene fluoride (PVDF) is coated onto the fourth surface of the polyethylene substrate opposite the third surface and the surface of the composite coating, respectively, with a coating amount of 0.4 g / m². 2 After being dried in an oven, a diaphragm is obtained.

[0124] Step 5: Preparation of Electrolyte In a glove box filled with inert gas (argon) and with a moisture content of less than 0.1 ppm and an oxygen content of less than 0.1 ppm, ethylene carbonate, propylene carbonate, diethyl carbonate, and propyl propionate were mixed evenly in a mass ratio of 15:10:10:65. Then, fully dried lithium hexafluorophosphate with a concentration of 1.25 mol / L was added and stirred evenly. Finally, 0.5% succinate and 15 wt% FEC were added according to the total mass of the electrolyte. After the moisture and free acid content were tested and found to be qualified, the desired electrolyte was obtained.

[0125] Step 6: Preparation of lithium-ion secondary batteries The positive electrode sheet obtained in the second step, the negative electrode sheet obtained in the third step, and the separator obtained in the fourth step are wound together by a winding machine to obtain a battery core in which the positive electrode sheet and the negative electrode sheet are separated by the separator. Then, through welding, encapsulation, injection of electrolyte prepared in the fifth step, formation, gas bag cutting, sorting and other processes, a lithium-ion secondary battery is obtained.

[0126] The preparation methods of Examples 2-23 and Comparative Examples 1-5 are basically the same as those of Example 1. The differences are shown in Tables 1-3. In Examples 22-23, the total mass percentage of the negative electrode active material remains unchanged. "This means that the parameter values ​​are the same as in Example 1, but it does not exclude the possibility of reasonably foreseeable errors such as test errors."

[0127] It is understandable that, during the preparation of polycrystalline particles, the mass content of nickel, cobalt, and manganese in the cathode active material can be adjusted by controlling the molar ratio of nickel, cobalt, and manganese in the co-precipitation step; the particle size D of the primary particles, the particle size L of the polycrystalline particles, and the particle size Dv50 of the cathode active material can be simultaneously controlled by adjusting the sintering temperature; the specific types of M-containing substances such as Sb2O3 and Y2O3 and their mass content and distribution in the cathode active material can be adjusted by controlling the types and amounts of Q-containing substances such as Al2O3, ZrO2, and SrCO3; similarly, the specific types of Q-containing substances and their mass content and distribution in the cathode active material can be controlled by adjusting the types and amounts of Q-containing substances such as Al2O3, ZrO2, and SrCO3.

[0128] In the process of preparing polycrystalline particle coating, the mass content and distribution of Co in the positive electrode active material can be controlled by adjusting the amount of Co3O4 added and the sintering temperature. At the same time, by controlling the types and amounts of coating materials such as Al(NO3)3·9H2O and H3BO3, as well as the sintering temperature, the thickness of the coating and the mass content of elements such as Co, Al, B, Ti, and W in the coating can be adjusted, and the particle size Dv50 of the positive electrode active material can also be adjusted simultaneously.

[0129] Table 1

[0130] Table 2

[0131] Table 3

[0132] Test case 1. High-temperature cycling performance test The battery was placed in a constant temperature environment of 45℃ and charged at a constant current rate of 1.8C to 4.3V, then discharged at a constant current rate of 3.0C to 2.0V. This cycle was repeated 300 times. The discharge capacity of each cycle was recorded. The capacity retention rate after 300 cycles was obtained by dividing the discharge capacity of the 300th cycle by the discharge capacity of the first cycle.

[0133] 2. Cyclic Thickness Expansion Rate Test The battery was placed in a constant temperature environment of 45℃ and charged at a constant current rate of 1.8C to 4.3V, then discharged at a constant current rate of 3.0C to 2.0V. This cycle was repeated 300 times. The battery thickness was recorded every 100 cycles. The battery thickness after the 300th cycle was divided by the initial battery thickness to obtain the thickness expansion rate after 300 cycles.

[0134] 3. Mass energy density test Charge the battery at a constant current of 0.5C to the upper limit voltage of 4.3V, then charge it at a constant current of 0.05C to the cutoff current, and finally discharge it at a constant current of 0.5C to the lower limit voltage of 2.0V. Record the discharge capacity and average operating voltage. Measure the mass of the battery using a balance and calculate the battery's mass energy density using the following formula: Mass energy density = (Discharge capacity × Average operating voltage) / Battery mass.

[0135] 4. Ratio Performance Test Perform the following operations on the battery at an environment of 25±1℃: 1) Let it sit for 5 minutes; 2) Discharge at a constant current of 0.5C to the lower limit voltage; 3) Let it sit for 1 hour; 4) Charge at a constant current of 0.5C to the upper limit voltage, then charge at a constant voltage to the cutoff current of 0.05C; 5) Let it sit for 30 minutes; 6) Discharge at a constant current of 3C rate to the lower limit voltage.

[0136] Repeat steps 3)-6), changing the discharge rate in step 6) sequentially until all specified rates have been tested. After the test, record the battery's voltage, internal resistance, thickness, and appearance at 0% SOC.

[0137] Capacity retention at 3C rate is calculated using the following formula: 3C capacity retention rate = 3C discharge capacity / 0.5C discharge capacity.

[0138] The test results are shown in Table 4.

[0139] Table 4

[0140] As shown in Tables 1-4, this application introduces an appropriate amount of element M into the polycrystalline particles and enriches it in the surface and grain boundaries of the polycrystalline particles, thereby constructing a continuous chemical protection network for the polycrystalline particles. This significantly strengthens the surface lattice, stabilizes grain boundaries, improves the interface, and accelerates lithium-ion conduction, effectively blocking mechanical failure caused by grain boundary stress concentration in polycrystalline materials. This, in turn, helps to simultaneously improve the cycle life, rate performance, and capacity of the battery. Furthermore, this application also provides a Co-containing coating layer on at least part of the surface of the polycrystalline particles. Utilizing its dual effects of physical isolation and chemical passivation, it can stabilize the interface layer, reduce interface side reactions, and lower impedance. By allowing the Co element in the coating layer to penetrate into the grain boundaries of the polycrystalline particles, it can effectively inhibit grain boundary cracking and further strengthen the grain boundary structure. At the same time, it also controls the content of Co element in the grain boundaries to be greater than that of element M, so as to achieve a balance between high specific capacity and grain boundary stability. This fundamentally improves the mechanical integrity and interface stability of high-nickel ternary polycrystalline materials to meet the application requirements of next-generation high-energy-density, long-cycle-life lithium-ion secondary batteries.

[0141] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this application.

Claims

1. A positive electrode active material, characterized in that, The chemical formula of the positive electrode active material is Li f Ni x Co y Mn z A e O2, 0.8≤f<1.3, 0.8≤x<1, 0.02≤y≤0.2, 0.01≤z≤0.14, 0.06≤e≤0.1, A includes at least one element M, wherein the element M includes at least one of Sb, Ta, and Y; The positive electrode active material includes polycrystalline particles and a coating layer located on at least a portion of the surface of the polycrystalline particles, the coating layer including at least Co element; The grain boundaries of the polycrystalline particles contain Co and M elements. Based on the mass of the positive electrode active material, the mass content of Co in the grain boundaries is greater than the mass content of M in the grain boundaries. The polycrystalline particles include primary particles with a particle size of D nm, where 50 ≤ D ≤ 800. Along the direction from the surface of the primary particle to its interior, a region extending from the surface of the primary particle to a depth of D / 5 nm is designated as a first region, and a region at a depth greater than D / 5 nm from the surface of the primary particle is designated as a second region. Based on the mass of the positive electrode active material, the mass content of element M in the first region is greater than the mass content of element M in the second region.

2. The positive electrode active material according to claim 1, characterized in that, At least one of the following conditions must be met: （1）100≤D≤800； (2) The first region contains a spinel phase; (3) The grain boundaries of the polycrystalline grains contain spinel phase; (4) The A further includes the Q element, which includes at least one of Zr, Al, Mo, Ti, and Sr; Preferably, A includes Sr and M elements. Based on the mass of the positive electrode active material, the mass content of Sr in the grain boundaries of the polycrystalline particles is h1%, and the mass content of Sr in the primary particles is h2%, satisfying: h1 > h2. More preferably, based on the mass of the positive electrode active material, the mass content of the Sr element is denoted as s1 ppm and the mass content of the M element is denoted as s2 ppm, satisfying: 260≤s1≤2500, 400≤s2≤2000, and 0.15≤s1 / s2≤6.

3. The positive electrode active material according to claim 2, characterized in that, The M element includes Sb and Y elements, and the Q element includes Sr, Zr, and Al elements, wherein: The relative standard deviation of the Al element mass content at five locations on the CP-SEM cross-section of the polycrystalline particles is denoted as R1, which satisfies R1 < 40%. The relative standard deviation of the Zr element mass content at five locations on the CP-SEM cross-section of the polycrystalline particles is denoted as R2, which satisfies R2 < 40%. Preferably, based on the mass of the positive electrode active material, the mass content of Sb is 100ppm-1600ppm, the mass content of Y is 300ppm-1200ppm, the mass content of Zr is 1500ppm-3500ppm, and the mass content of Al is 600ppm-2500ppm.

4. The positive electrode active material according to claim 1, characterized in that, The particle size of the polycrystalline particles is L μm, 5≤L≤18; Along the direction from the surface of the polycrystalline particle to its interior, the region extending from the surface of the polycrystalline particle to a depth of L / 5 μm is designated as the third region, and the region extending from the surface of the polycrystalline particle to a depth greater than L / 5 μm is designated as the fourth region; based on the mass of the positive electrode active material, the mass content of Co element in the third region is greater than the mass content of Co element in the fourth region. Preferably, a cobalt-rich layered structure exists in the third region and / or the grain boundaries of the polycrystalline particles, and the molar content of Co in the cobalt-rich layered structure is greater than or equal to 60%, based on the sum of the molar numbers of all transition metal elements in the cobalt-rich layered structure.

5. The positive electrode active material according to any one of claims 1-4, characterized in that, At least one of the following conditions must be met: (1) Based on the mass of the positive electrode active material, the mass content of Co in the polycrystalline particles is greater than the mass content of Mn in the polycrystalline particles; (2) The primary particle includes a first surface and a second surface, the first surface is in contact with the coating layer, and the second surface is not in contact with the coating layer; based on the mass of the positive electrode active material, the mass content of Co element on the first surface is less than the mass content of Co element on the second surface; (3) The thickness of the coating layer is 5nm-50nm; (4) The coating layer further includes at least one element selected from Al, Ti, W, and B; Preferably, the coating layer includes Co, Al and B elements, and based on the mass of the coating layer, the mass content of Co element is 50%-98%, the mass content of Al element is 1%-15%, and the mass content of B element is 0.2%-10%.

6. The positive electrode active material according to claim 5, characterized in that, At least one of the following conditions must be met: (1) The particle size Dv50 of the positive electrode active material is 5μm-18μm; (2) In the X-ray diffraction pattern of the positive electrode active material, there are diffraction peaks at positions of 18.5° - 19.0°, 44.5° - 45.1°, and 37.8° - 38.7°; (3) In the differential scanning calorimetry of the positive electrode active material, the starting temperature of the exothermic peak is T≥190℃.

7. A positive electrode sheet, comprising a positive electrode active layer, wherein the positive electrode active layer comprises a positive electrode active material; characterized in that, The positive electrode active material includes the positive electrode active material according to any one of claims 1-6; Preferably, the positive electrode active material further includes lithium nickel cobalt manganese oxide single crystal particles. Based on the mass of the positive electrode active layer, the mass content of the lithium nickel cobalt manganese oxide single crystal particles is denoted as F%, and the mass content of the positive electrode active material is denoted as E%, satisfying: 0 < E / F ≤ 1.

8. A lithium-ion secondary battery, characterized in that, Includes the negative electrode and the positive electrode as described in claim 7; Preferably, the ratio of the capacity of the negative electrode to the capacity of the positive electrode is denoted as NP, and satisfies: 1.04≤NP≤1.

3.

9. The lithium-ion secondary battery according to claim 8, characterized in that, The negative electrode sheet includes a negative electrode active material, which includes silicon-carbon material; and satisfies at least one of the following conditions: (1) The particle size Dv50 of the silicon carbide material is 1μm-25μm; (2) The silicon-carbon material comprises porous carbon and silicon particles located within the porous carbon channels, and the mass content of silicon element is 30%-80% based on the mass of the silicon-carbon material; (3) Based on the mass of the negative electrode active material, the mass content of the silicon-carbon material is 15%~100%.

10. The lithium-ion secondary battery according to claim 8 or 9, characterized in that, The differential capacity curve of the lithium-ion secondary battery has an oxidation peak A in the range of 4.1V-4.2V and a reduction peak B in the range of 3.6V-3.8V. The half-width at half maximum (WHM) of the oxidation peak A is denoted as WA, and the half-width at half maximum (WHM) of the reduction peak B is denoted as WB, satisfying 0.8 ≤ WB / WA ≤ 1.1.

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