Positive electrode sheet and lithium ion secondary battery

Abstract

This application discloses a cathode material and a lithium-ion secondary battery. The lithium nickel cobalt manganese oxide material in the cathode material includes single-crystal and polycrystalline forms, with a higher Ni content in the single-crystal form than in the polycrystalline form. The single-crystal form includes a first matrix containing relatively uniformly distributed Zr elements. The polycrystalline form includes a second matrix containing primary particles, which includes M1 (Sb, Ta, and / or Y) elements, enriched on the surface of the primary particles. The ratio of Zr content in the single-crystal form to M1 content in the polycrystalline form is 0.5-28. Introducing Zr into the single-crystal form enhances the bulk structure and kinetic performance, while introducing M1 into the polycrystalline form strengthens the surface lattice, stabilizes grain boundaries, improves the interface, and accelerates lithium-ion conduction. Simultaneously controlling the Ni content in the single-crystal and polycrystalline forms, as well as the Zr / M1 content ratio, satisfies the above relationships, allowing for a perfect match of the advantages of single-crystal and polycrystalline forms to meet the demands of next-generation high-energy-density, long-cycle-life power batteries.

Description

Technical Field

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

[0002] High-nickel nickel-cobalt-manganese ternary cathode materials (with Ni content of ≥0.8% in the transition metals) have become one of the mainstream choices for power batteries due to their high discharge specific capacity and operating voltage. Currently, the industry uses ternary single-crystal materials and ternary polycrystalline materials, but the two exhibit significant complementary characteristics and inherent shortcomings in performance.

[0003] Nickel-cobalt-manganese ternary polycrystalline materials are polycrystalline secondary particle materials formed by the agglomeration of primary particles. This morphology is beneficial for improving the processing performance and compaction density of the electrode. However, in practical applications, especially during deep charge-discharge (high lithium depletion state) and long-term cycling, nickel-cobalt-manganese ternary polycrystalline materials face serious capacity decay and safety challenges. Ternary single-crystal materials, on the other hand, exhibit excellent long-cycle structural stability and thermal stability, but they also have significant drawbacks: First, the size of single-crystal particles is usually large (several micrometers), resulting in a longer diffusion path for lithium ions in the solid phase, and their intrinsic rate performance and low-temperature performance are often inferior to those of polycrystalline materials. Second, the hard single-crystal particles are difficult to deform during electrode rolling, making it difficult to improve the electrode compaction density and limiting the volumetric energy density of the battery. Third, the synthesis process of single-crystal materials usually requires higher temperatures and longer sintering times, resulting in significantly higher production costs than polycrystalline materials.

[0004] Due to the limitations of high-nickel ternary cathode materials with a single morphology / grain aggregation state, the field has begun to explore the physical mixing of single-crystal and polycrystalline materials in order to integrate the advantages of both. However, existing simple mixing schemes mostly remain at the conceptual level, lacking systematic design and principle guidance, and the actual results are often unsatisfactory. Summary of the Invention

[0005] In view of this, this application provides a composite cathode system that, through the design of mixing ternary single-crystal materials and ternary polycrystalline materials, can not only give full play to the ultra-long life characteristics of ternary single-crystal materials, but also inherit the excellent kinetics and processability of ternary polycrystalline materials, and can effectively synergize with high-capacity anode designs to meet the needs of next-generation high-energy-density and long-cycle-life power batteries.

[0006] To achieve the above objectives, this application adopts the following technical solution.

[0007] According to an embodiment of this application, in a first aspect, a positive electrode sheet is provided, the positive electrode sheet comprising a positive current collector and a positive active layer located on at least one side surface of the positive current collector, the positive active layer comprising a positive active material; the positive active material comprising lithium nickel cobalt manganese oxide material, the lithium nickel cobalt manganese oxide material comprising single crystal particles and polycrystalline particles, wherein the molar fraction of Ni element in the single crystal particles is n1% based on the molar number of transition metal elements, and the molar fraction of Ni element in the polycrystalline particles is n2% based on the molar number of transition metal elements, satisfying: n1>n2, 99≥n1≥80.5, 98≥n2≥80; The single-crystal particle comprises a first substrate and a first coating layer, the first coating layer being located on at least a portion of the surface of the first substrate; the first substrate comprises at least Zr and Al elements, and the Zr element mass content is a1 ppm based on the mass of the single-crystal particle; the relative standard deviation of the Zr element mass content at 5 locations on the CP-SEM cross-section of the first substrate is R1, satisfying R1≤40%; The polycrystalline particle includes a second matrix and a second coating layer, the second coating layer being located on at least a portion of the surface of the second matrix; the second matrix includes at least element M1, the element M1 including at least one of Sb, Ta, and Y, and the mass content of element M1 is a2 ppm based on the mass of the polycrystalline particle; satisfying: 0.5≤a1 / a2≤28; The second matrix includes 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 the first region, and a region at a depth greater than D / 5 nm from the surface of the primary particle is designated as the second region. Based on the mass of the polycrystalline particles, the mass content of element M1 in the first region is greater than the mass content of element M1 in the second region.

[0008] In some alternative implementations, at least one of the following conditions is met: (1) 800≤a1≤6000; (2) 200≤a2≤2000; (3) 100≤D≤800; (4) The thickness of the first coating layer is h1 nm and the thickness of the second coating layer is h2 nm, satisfying: h1 > h2.

[0009] Furthermore, in some implementations, 10 ≤ h1 ≤ 100 is satisfied.

[0010] Furthermore, in some implementations, 5≤h2≤50 is satisfied.

[0011] In some alternative implementations, at least one of the following conditions is met: (1) The first substrate further includes element G, which includes at least one of Nb and Y; the mass content of element G is 5ppm-3000ppm based on the mass of the single crystal particle; and / or, along the direction from the surface of the first substrate to its interior, a first substrate region within a depth of 35nm is a third region containing element G, and a region from a depth of 35nm from the surface of the first substrate to the center of the first substrate is a fourth region containing element G, and the mass content of element G in the third region is greater than the mass content of element G in the fourth region; (2) Based on the mass of the single crystal particles, the mass content of Co in the first matrix is ​​greater than the mass content of Mn in the first matrix; (3) Based on the surface area of ​​the first substrate, the coverage rate of the first coating layer is 20%-95%; (4) The first coating layer includes at least Co and Al elements. Along the direction from the surface of the first substrate to its interior, the first substrate includes a fifth region and a sixth region. The fifth region is located on the surface of the sixth region. The first coating layer is located on a portion of the surface of the fifth region. The fifth region contains a first spinel phase. Based on the mass of the single crystal grain, the mass content of Co in the fifth region is greater than the mass content of Co in the sixth region, and the mass content of Al in the fifth region is greater than the mass content of Al in the sixth region.

[0012] Furthermore, in some embodiments, the thickness of the fifth region is 10nm-500nm.

[0013] In some optional embodiments, along the direction from the surface to the interior of the first substrate, the region where the G element is at a depth less than or equal to L1 nm in the first substrate is designated as the ninth region, and the region where the G element is at a depth greater than L1 nm in the first substrate is designated as the tenth region, where 2 ≤ L1 ≤ 35; the mass content of the G element in the ninth region is greater than the mass content of the G element in the tenth region. This can be understood as the G element being enriched in the ninth region of the first substrate.

[0014] In some optional embodiments, the first coating layer further includes an M2 element, which includes at least one of Ti, W, and B elements. The mass content of the M2 element is denoted as m ppm based on the mass of the single crystal particle, and satisfies the condition: 0 < m ≤ 6000.

[0015] Furthermore, in some optional embodiments, based on the mass of the single crystal particles, the mass content of Zr is 1000ppm-3000ppm, the mass content of Al is 100ppm-2000ppm, the mass content of W is 300ppm-3000ppm, and the mass content of B is 100ppm-1500ppm.

[0016] In some optional embodiments, the second coating layer includes at least Co; the grain boundaries of the polycrystalline particles contain Co and M1, and the mass content of Co in the grain boundaries is greater than the mass content of M1 in the grain boundaries, based on the mass of the polycrystalline particles.

[0017] In some optional embodiments, the second coating layer includes at least Co element, the particle size of the second substrate is L μm, 5≤L≤18; along the direction from the surface of the second substrate to its interior, the region with a depth of L / 5 μm from the surface of the second substrate to its interior is designated as the seventh region, and the region with a depth greater than L / 5 μm from the surface of the second substrate is designated as the eighth region; based on the mass of the polycrystalline particles, the mass content of Co element in the seventh region is greater than the mass content of Co element in the eighth region.

[0018] Furthermore, in some optional embodiments, the grain boundaries of the polycrystalline particles contain a cobalt-rich layered structure.

[0019] Furthermore, in some alternative embodiments, the seventh region contains a cobalt-rich layered structure.

[0020] Furthermore, in some optional embodiments, 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.

[0021] In some alternative embodiments, the mass content of Co in the second matrix is ​​greater than the mass content of Mn in the second matrix, based on the mass of the polycrystalline particles.

[0022] In some alternative implementations, the first region contains a second spinel phase.

[0023] In some alternative embodiments, the grain boundaries of the polycrystalline grains contain a third spinel phase.

[0024] In some alternative embodiments, the second coating layer further includes at least one element selected from Al, Ti, W, and B.

[0025] In some alternative embodiments, the second matrix further includes a Q element, which includes at least one of Zr, Al, Mo, Ti, and Sr.

[0026] In some optional embodiments, the primary particle includes a first surface and a second surface, the first surface being in contact with the second coating layer and the second surface not being in contact with the second coating layer; based on the mass of the polycrystalline particle, the mass content of Co element on the first surface is less than the mass content of Co element on the second surface.

[0027] In some alternative implementations, at least one of the following conditions is met: (1) The second coating layer includes Co, Al and B elements. Based on the mass of the second 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%. (2) The second matrix includes Sr element; based on the mass of the polycrystalline particles, the mass content of Sr element in the grain boundary of the polycrystalline particles is h3%, and the mass content of Sr element in the primary particles is h4%, satisfying h3>h4; (3) The second matrix includes Sr and M1 elements; based on the mass of the polycrystalline particles, the mass content of Sr is denoted as s1 ppm and the mass content of M1 is denoted as s2 ppm, satisfying: 260≤s1≤2500, 400≤s2≤2000, 0.15≤s1 / s2≤6; (4) The M1 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 second matrix is ​​denoted as R2, which satisfies R2 < 40%. The relative standard deviation of the Zr element mass content at five locations on the CP-SEM cross-section of the second matrix is ​​denoted as R3, which satisfies R3 < 40%. Based on the mass of the polycrystalline particles, the mass content of Sb is 100ppm-1600ppm, the mass content of Sr is 260ppm-2500ppm, 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.

[0028] In some alternative implementations, at least one of the following conditions is met: (1) The particle size Dv50 of the single crystal particle is denoted as D1 μm, and the particle size Dv50 of the polycrystalline particle is denoted as D2 μm, satisfying: 0.1≤D1 / D2≤1.4; (2) Based on the mass of the positive electrode active layer, the mass content of the single crystal particles is e1%, and the mass content of the polycrystalline particles is e2%, satisfying: e1≥e2; (3) The porosity of the positive electrode active layer is 20%-50%; (4) The compaction density of the positive electrode active layer is 3 g / cm³. 3 -3.5g / cm 3 .

[0029] Furthermore, in some implementations, the following condition is satisfied: 1≤D1≤8.

[0030] Furthermore, in some implementations, the following condition is satisfied: 5≤D2≤18.

[0031] Furthermore, in some implementations, the following condition is satisfied: 1≤e1 / e2≤9.

[0032] According to an embodiment of this application, in a second aspect, a lithium-ion secondary battery is provided, including a negative electrode, an electrolyte, and the positive electrode described in the first aspect of this application.

[0033] Furthermore, in some optional embodiments, the negative electrode sheet includes a negative electrode active material, which includes a silicon-carbon material; satisfying 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%; (4) The ratio of the capacity of the negative electrode to the capacity of the positive electrode is denoted as NP, which satisfies: 1.06≤NP≤1.35.

[0034] In some alternative embodiments, the electrolyte comprises a nitrile compound, and the nitrile compound content is 0.2%-3% by mass based on the mass of the electrolyte.

[0035] Furthermore, in some optional embodiments, the nitrile compound includes at least one selected from butadionitrile, glutaronitrile, adiponitrile, heptadionitrile, octadionitrile, sebaconitrile, 1,3,6-hexanetrionitrile, glyceroltrionitrile, and 1,2-bis(2-cyanoethoxy)ethane.

[0036] 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: 0.72≤WB / WA≤1.15.

[0037] The technical solution of this application has the following advantages: The positive electrode sheet provided in this application includes a positive current collector and a positive active layer located on at least one side of the surface of the positive current collector. The positive active layer includes a positive active material. The positive active material includes lithium nickel cobalt manganese oxide material, which includes single-crystal particles and polycrystalline particles. The molar fraction of Ni element in the single-crystal particles, based on the molar number of transition metal elements, is n1%, and the molar fraction of Ni element in the polycrystalline particles, based on the molar number of transition metal elements, is n2%, satisfying: n1 > n2, 99 ≥ n1 ≥ 80.5, 98 ≥ n2 ≥ 80. The single-crystal particles include a first substrate and a first coating layer, the first coating layer being located on at least a portion of the surface of the first substrate. The first substrate includes at least Zr and Al elements, and the mass content of Zr element, based on the mass of the single-crystal particles, is a1. The relative standard deviation of the Zr element mass content at five locations on the CP-SEM cross-section of the first substrate is R1, satisfying R1≤40%; the polycrystalline particles include a second substrate and a second coating layer, the second coating layer being located on at least a portion of the surface of the second substrate; the second substrate includes at least one M1 element, the M1 element including at least one of Sb, Ta, and Y, and the mass content of the M1 element based on the mass of the polycrystalline particles is a2 ppm; satisfying: 0.5≤a1 / a2≤28; the second substrate includes primary particles, the primary particles having a particle size of D nm, 50≤D≤800; along the direction from the surface of the primary particles to their interior, the region from the surface of the primary particles to a depth of D / 5 nm is denoted as the first region, and the region at a depth greater than D / 5 nm from the surface of the primary particles is denoted as the second region; based on the mass of the polycrystalline particles, the mass content of the M1 element in the first region is greater than the mass content of the M1 element in the second region.

[0038] This application introduces an appropriate amount of Zr element into the first matrix of single-crystal particles, which can enhance the bulk structure and kinetic properties of the single-crystal particles. Simultaneously, it introduces at least one M1 element from Sb, Ta, and Y into the second matrix of polycrystalline particles and enriches it in the surface layer and grain boundaries of the second matrix, thereby constructing a continuous chemical protection network for the polycrystalline particles, which strengthens their surface lattice, stabilizes grain boundaries, improves interfaces, and accelerates lithium-ion conduction. Furthermore, this application adjusts the molar fraction n1% of Ni element in the single-crystal particles and the molar fraction n1% of Ni element in the polycrystalline particles... The value of fraction n2% is set such that n1 > n2, and the ratio a1 / a2 of the mass content of Zr element in the first matrix of the monocrystalline particle a1ppm to the mass content of M1 element in the second matrix of the polycrystalline particle a2ppm is controlled between 0.5 and 28. This makes the "bulk phase stability" capability of the monocrystalline particle match the "interface protection and grain boundary strengthening" capability of the polycrystalline particle. In this way, the ultra-long lifespan characteristics of the monocrystalline particle can be utilized, while the excellent kinetics and processability of the polycrystalline particle can be inherited, thereby meeting the needs of the next generation of high energy density and long cycle life power batteries.

[0039] 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

[0040] 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.

[0041] Figure 1 This is a schematic diagram of the structure of a single crystal particle in one embodiment of this application; Figure 2 This is a SEM cross-sectional image of a single-crystal particle in one embodiment of this application; Figure 3 According to Figure 2 The Zr element content distribution map obtained by performing EDS line scanning on the line segments in the image; Figure 4 This is a schematic diagram of the structure of polycrystalline particles in one embodiment of this application; Figure 5 yes Figure 4 A magnified view of a portion of the image.

[0042] The reference numerals in the attached figures are explained as follows: 11-First substrate; 12-First coating layer; 13-Thickness of fifth region; L1-Depth of ninth region; h1-Thickness of first coating layer; 21-Primary particle; 22-Coating layer; 23-Polycrystalline particle; 31-First surface; 32-Second surface. Detailed Implementation

[0043] 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.

[0044] It should be noted in the description of this application that the terms "first," "second," "third," "fourth," "fifth," "sixth," "seventh," "eighth," "ninth," and "tenth" 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.

[0045] This research found that battery systems formed by physically mixing ternary single-crystal and ternary polycrystalline materials in related technologies lack systematic optimization design of parameters such as mixing ratio and particle size matching, making it difficult to find the optimal balance between cycle life, rate performance, compaction density, and cost. Furthermore, to ensure high energy density, silicon-based materials are often used for the anode. However, high-capacity silicon-based anodes have low initial coulombic efficiency and large cycle volume expansion. This requires the cathode to not only have slow degradation but also act as a reliable "lithium reservoir" to compensate for active lithium loss, thus placing unprecedentedly high demands on the overall stability of the cathode active material. However, related technologies have also failed to achieve a complete synergistic design of the composite cathode and high-capacity anode within the battery system.

[0046] To alleviate the aforementioned technical problems, this application proposes the following solutions.

[0047] According to a first aspect, this application provides a positive electrode sheet, the positive electrode sheet comprising a positive current collector and a positive active layer located on at least one side surface of the positive current collector, the positive active layer comprising a positive active material; the positive active material comprising lithium nickel cobalt manganese oxide material, the lithium nickel cobalt manganese oxide material comprising single crystal particles and polycrystalline particles, wherein the molar fraction of Ni element in the single crystal particles is n1% based on the molar number of transition metal elements, and the molar fraction of Ni element in the polycrystalline particles is n2% based on the molar number of transition metal elements, satisfying: n1>n2, 99≥n1≥80.5, 98≥n2≥80; The single-crystal particle comprises a first substrate and a first coating layer, the first coating layer being located on at least a portion of the surface of the first substrate; the first substrate comprises at least Zr and Al elements, and the Zr element mass content is a1 ppm based on the mass of the single-crystal particle; the relative standard deviation of the Zr element mass content at 5 locations on the CP-SEM cross-section of the first substrate is R1, satisfying R1≤40%; The polycrystalline particle includes a second matrix and a second coating layer, the second coating layer being located on at least a portion of the surface of the second matrix; the second matrix includes at least element M1, the element M1 including at least one of Sb, Ta, and Y, and the mass content of element M1 is a2 ppm based on the mass of the polycrystalline particle; satisfying: 0.5≤a1 / a2≤28; The second matrix includes 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 the first region, and a region at a depth greater than D / 5 nm from the surface of the primary particle is designated as the second region. Based on the mass of the polycrystalline particles, the mass content of element M1 in the first region is greater than the mass content of element M1 in the second region.

[0048] First, it's important to clarify that 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 RSD value, the smaller the difference between the measured values, indicating a more uniform data distribution. "CP-SEM cross-section" refers to a flat cross-section obtained by argon ion beam processing using a cross-section polisher, followed by observation using a scanning electron microscope.

[0049] In this application, the term "single-crystal particle" refers to a single particle in lithium nickel cobalt manganese oxide powder with a complete crystal structure, exhibiting a single crystallographic orientation under an electron microscope, and its particle boundary being the crystal surface. The term "polycrystalline particle" refers to a secondary agglomerate formed by multiple primary particles (i.e., original nano- or submicron-sized crystal particles that cannot be further mechanically divided) arranged randomly and densely packed in space with different crystallographic orientations, interconnected by grain boundary interfaces; for example, it can be a micro- or nano-scale secondary agglomerate formed by the spontaneous assembly of these primary particles under thermodynamic or chemical driving through grain boundary diffusion, necking growth, and pore evolution, possessing a high-density grain boundary network and hierarchical pore structure, and whose overall mechanical / transport properties are dominated by grain boundary characteristics; it can also be a secondary agglomerate formed by primary particles bonded through grain boundaries, exhibiting lattice distortion and local strain fields, and capable of buffering volume changes and delaying crack propagation during electrochemical cycling.

[0050] This application introduces Zr element into the first matrix of single crystal particles and controls the Zr element mass content to a1ppm, thereby making the Zr element tend to be uniformly distributed in the first matrix (R1≤40%), and playing at least the following roles: (1) forming high bond energy Zr-O bonds, stabilizing lattice oxygen, reducing oxygen release, strengthening the layered skeleton of single crystal particles, suppressing the shrinkage / expansion of the transition metal layer during charging and discharging, reducing lattice strain, and reducing interface reconstruction reactions; (2) utilizing the high-valence Zr 4+ Modulate local charge and stress field to reduce Ni 2+ Migration to the Li layer inhibits cation mixing and maintains the integrity of the R-3m layered structure of the single crystal particles; (3) It causes micro-expansion of the c-axis of the single crystal particles, widening the Li layer. + Diffusion channels are beneficial to improving ionic conductivity and rate performance; (4) Pinning the lattice inhibits the initiation and propagation of microcracks caused by anisotropic volume changes, ensuring the structural stability and cycle life of single crystal particles.

[0051] Meanwhile, by controlling the particle size D of the primary particles to be between 50nm and 800nm, this application can ensure a balance between ion diffusion rate, volumetric strain, grain boundary activity, and interface stability, thereby contributing to the synergistic improvement of battery rate performance, cycle life, and capacity utilization. In addition, by introducing at least one M1 element from Sb, Ta, and Y into the second matrix of the polycrystalline particles and controlling the mass content of the M1 element to a2ppm, this application can enable it to perform at least the following functions: (1) preferentially occupy transition metal sites, form a high-bond-energy chemical framework with oxygen atoms, firmly lock lattice oxygen, suppress oxygen evolution and irreversible phase transition under high pressure / high temperature, and simultaneously improve Ni 2+ The energy barrier for migration to the lithium layer suppresses cation mixing and ensures Li + The channel is smooth, and the c-axis spacing of the lithium layer is appropriately increased to reduce Li + (2) M1 element is enriched in the surface layer of the second matrix (i.e., the first region), which can form a chemically stable passivation layer, effectively isolating the contact between the electrolyte and the polycrystalline particles and suppressing the interface side reaction; and when the polycrystalline particles generate microcracks due to stress, the surface layer of the second matrix pre-enriched with M1 element can make the newly exposed crack interface still have anti-corrosion ability, prevent the electrolyte from directly contacting the fresh active surface, and delay the degradation of the polycrystalline particle structure; (3) Maintaining a low content of M1 element in the second matrix (i.e., the second region) can ensure that the layered structure of the polycrystalline particle bulk phase is not affected and maintain the high capacity of the polycrystalline particles; (4) M1 element can diffuse along the grain boundary of the polycrystalline particles and penetrate into the grain boundary. Through the "pinning" effect of M1 element at the grain boundary, it can guide the primary particles to be arranged radially along the (003) crystal plane and construct a continuous Li + Conductive network, shorten Li +The diffusion path, while uniformly dispersing the volumetric strain, prevents crack initiation and propagation, and maintains the mechanical integrity of polycrystalline particles; (5) reacts with Li in situ to generate spinel phase, which coats the surface of the second matrix, reduces the interface / grain boundary impedance, and at the same time blocks electrolyte erosion and reduces interface side reactions.

[0052] Based on this, this application adjusts the molar fraction of Ni in single-crystal particles (n1%) to be greater than that in polycrystalline particles (n2%). This allows single-crystal particles, with their more stable lattice structure, to act as a "high-energy framework" without structural collapse at high delithiation depths, ensuring the long-cycle stability of the battery. At the same time, maintaining a relatively low Ni molar fraction in polycrystalline particles can alleviate stress concentration at grain boundaries caused by the huge volume changes of grains with different orientations, reduce grain boundary microcracks, and improve the structural stability of polycrystalline particles. This, in turn, ensures the structural integrity and interface stability of the composite cathode system under high-voltage cycling.

[0053] Furthermore, this application further regulates the ratio a1 / a2 of the mass content of Zr element a1 in the first matrix of the monocrystalline particles and the mass content of M1 element a2 in the second matrix of the polycrystalline particles to be between 0.5 and 28. This allows the "bulk phase stability" capability of the monocrystalline particles to match the "interface protection and grain boundary strengthening" capability of the polycrystalline particles, achieving synergy in their aging rates and preventing premature failure of one of them from becoming a performance bottleneck. This is beneficial for balancing the high energy density, high rate performance, and long cycle life of the battery.

[0054] In summary, this application introduces an appropriate amount of Zr element into the first matrix of single-crystal particles, which can enhance the bulk structure and kinetic properties of the single-crystal particles. Simultaneously, it introduces at least one M1 element from Sb, Ta, and Y into the second matrix of polycrystalline particles and enriches it in the surface layer and grain boundaries of the second matrix, thereby constructing a continuous chemical protection network for the polycrystalline particles, which strengthens their surface lattice, stabilizes grain boundaries, improves interfaces, and accelerates lithium-ion conduction. Furthermore, this application adjusts the molar fraction n1% of Ni element in the single-crystal particles and the molar fraction n1% of Ni element in the polycrystalline particles. The mole fraction n2% of the element is set such that n1 > n2, and the ratio a1 / a2 of the mass content of Zr element in the first matrix of the single crystal particle and the mass content of M1 element in the second matrix of the polycrystalline particle is controlled between 0.5 and 28. This makes the "bulk phase stability" capability of the single crystal particle match the "interface protection and grain boundary strengthening" capability of the polycrystalline particle. In this way, the ultra-long lifespan characteristics of the single crystal particle can be utilized, while the excellent kinetics and processability of the polycrystalline particle can be inherited, thereby meeting the needs of the next generation of high energy density and long cycle life power batteries.

[0055] In this application, as Figure 4As shown, the second coating layer covers more than 50% of the polycrystalline particles. The coating rate refers to the percentage of the surface area of ​​the polycrystalline particles covered by the second coating layer, relative to the total surface area of ​​the polycrystalline particles. This percentage can be obtained using conventional methods in the art, such as transmission electron microscopy combined with energy dispersive spectroscopy (TEM). For example, the coating rate can be 50%, 60%, 75%, 80%, 90%, 92%, 94%, 96%, 98%, 100%, or a value within any two of the above ranges.

[0056] Furthermore, the second coating layer has a coating rate of over 90% for the polycrystalline particles, such as 90%, 92%, 94%, 96%, 98%, 100%, or values ​​within any two of the above ranges.

[0057] This study found that if the molar fraction of Ni in single-crystal particles (n1%) is similar to that in polycrystalline particles (n2%), the skeletal stability of single-crystal particles may be insufficient, making it difficult to maintain the structure without collapsing at high delithiation depths. Meanwhile, the relatively high molar fraction of Ni in polycrystalline particles will exacerbate grain boundary stress concentration and microcrack propagation, thereby weakening the overall cycle stability and structural integrity of the composite cathode.

[0058] If the molar fraction of Ni in single-crystal particles is too low, their discharge specific capacity will be significantly reduced, making it difficult to meet the design requirements for high energy density. If the molar fraction of Ni in polycrystalline particles is too low, their rate performance and compaction density advantages cannot be fully utilized, affecting the overall kinetics and volumetric energy density of the composite cathode.

[0059] It should be noted that the molar fraction of Ni in single-crystal and polycrystalline particles can be obtained by conventional methods in the art, such as inductively coupled plasma optical emission spectrometry (ICP-OES). For example, the molar fraction of Ni in single-crystal particles can be 80.5%, 82.0%, 83.5%, 85.0%, 86.5%, 88.0%, 89.5%, 91.0%, 92.5%, 94.0%, 95.5%, 97.0%, 98.0%, 98.5%, 99.0%, etc., or values ​​within any two of the above ranges; the molar fraction of Ni in polycrystalline particles can be 80%, 81%, 82%, 84%, 85%, 86%, 88%, 89%, 90%, 92%, 94%, 95%, 96%, 97%, 98%, etc., or values ​​within any two of the above ranges.

[0060] If Zr is unevenly distributed in the first matrix (i.e., R1 > 40%), microcracks and cation mixing are likely to occur in the Zr-deficient areas of the first matrix, while lattice distortion is likely to occur in the Zr-rich areas, and lithium-ion transport channels are blocked. Overall, this results in unstable bulk structure of the active material, reduced cycle life and rate performance.

[0061] The relative standard deviation R1 of the Zr element mass content in single crystal particles can be obtained by conventional methods in the art, such as cross-section polishing-scanning electron microscopy combined with energy dispersive spectroscopy. For example, the value of R1 can be 3%, 6%, 9%, 12%, 15%, 18%, 21%, 24%, 27%, 30%, 33%, 36%, 39%, 40%, etc., or a value within the range of any two of the above values.

[0062] For example, see Figure 2 , Figure 2 This is a SEM cross-sectional image of a single-crystal particle in one embodiment of this application. The image shows the EDS line scan path, which starts from one end of the particle cross-section, traverses the entire interior of the particle, and extends to the other end of the particle cross-section. Zr element mass content data is collected along this scan path, resulting in the image shown below. Figure 3 The Zr element content distribution curve shown is from Figure 3 As can be seen, the mass content of Zr within the particles exhibits relatively small overall fluctuations, with no significant local enrichment or deficiency regions. This indicates that Zr is relatively uniformly distributed within the first matrix (RSD ≤ 40%). Therefore, Zr... 4+ It can effectively pin the lattice and suppress crack initiation and propagation at various locations in the first substrate, which is beneficial to stabilizing the first substrate structure and extending the cycle life of the battery.

[0063] If the a1 / a2 ratio is too small, the Zr doping in the single crystal particles is relatively insufficient, weakening their bulk stability and making it difficult to maintain lattice integrity at high delithiation depths. This can easily lead to premature structural degradation or microcracks, becoming a bottleneck for electrode failure. If the a1 / a2 ratio is too large, the M1 element content in the polycrystalline particles is relatively insufficient, preventing them from fully exerting their interface protection and grain boundary strengthening effects. Cracks are easily generated at the grain boundaries, triggering side reactions and causing premature failure of the polycrystalline particles. This disrupts the synergistic aging of single and polycrystalline particles, making it impossible to achieve both high energy density and long cycle life.

[0064] The mass content of Zr and M1 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 value of a1 / a2 can be 0.5, 1, 1.5, 2, 3, 4, 5, 6, 8, 10, 12, 15, 18, 22, 28, etc., or a value within the range of any two of the above values.

[0065] When the total M1 content in polycrystalline particles is the same, if the mass content of M1 in the first region is close to that in the second region, it will either affect the surface layer of the polycrystalline particles' resistance to electrolyte corrosion or affect the integrity of the layered structure of the second matrix, making it difficult to maintain the balance of various electrochemical properties of the active material. The relationship between the mass content of M1 in the first region and the mass content of M1 in the second region can be obtained by energy-dispersive X-ray spectroscopy (EDS) line scan analysis.

[0066] 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 the M1 element. In the EDS line scan, the boundary between the surface and the bulk phase can be determined based on the fact that the M1 element 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.

[0067] 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.

[0068] This study found that if the primary particle size is too small, the contact area between the polycrystalline particles 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 primary particle size 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.

[0069] It should be noted that the particle size of primary particles can be obtained by conventional methods in the art, such as scanning electron microscopy (SEM). For example, the particle size of primary particles can be 50 nm, 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, 500 nm, 550 nm, 600 nm, 650 nm, 700 nm, 750 nm, 800 nm, or a value within any two of the above values.

[0070] Furthermore, in some embodiments, by controlling the mass content of Zr in the single crystal particles to be between 800 ppm and 6000 ppm, Zr can be... 4+ Fully distributed within the first matrix of single-crystal particles, Zr effectively anchors the crystal lattice, suppresses microcracks, and stabilizes the layered structure, thereby enhancing the cycle stability and thermal safety of the cathode active material. If the Zr content is too low, sufficient Zr-O bond anchoring effect cannot be formed, resulting in insignificant suppression of lattice strain and crack initiation. Conversely, if the Zr content is too high, it may introduce excessive inert phase or cause local lattice distortion, thereby increasing interfacial impedance and reducing capacity utilization and rate performance.

[0071] Furthermore, in some embodiments, by controlling the mass content of M1 element in the second matrix to be between 200ppm and 2000ppm, M1 element can preferentially occupy transition metal sites and accumulate on the surface and grain boundaries of the second matrix, effectively locking lattice oxygen, inhibiting grain boundary cracking, and forming an interfacial passivation layer, thereby improving the corrosion resistance and structural integrity of the positive electrode active material. If the content of M1 element is too low, the grain boundary strengthening and surface protection effects are insufficient, and it cannot effectively inhibit microcrack propagation and interfacial side reactions; if the content of M1 element is too high, it may over-occupy active sites, reduce reversible capacity, increase material cost, and even cause local phase transitions due to excessive doping, which is detrimental to electrochemical performance.

[0072] For example, the mass content of Zr element can be 800ppm, 1000ppm, 1200ppm, 1400ppm, 1600ppm, 1800ppm, 2000ppm, 2400ppm, 2800ppm, 3200ppm, 3600ppm, 4000ppm, 4500ppm, 5000ppm, 6000ppm, etc., or values ​​within the range of any two of the above values; the mass content of M1 element can be 200ppm, 300ppm, 400ppm, 500ppm, 600ppm, 700ppm, 800ppm, 900ppm, 1000ppm, 1100ppm, 1200ppm, 1400ppm, 1600ppm, 1800ppm, 2000ppm, etc., or values ​​within the range of any two of the above values.

[0073] In some implementations, by controlling the thickness of the first coating layer to be greater than that of the second coating layer, single-crystal particles can obtain a thicker surface protective layer, effectively isolating the electrolyte, suppressing surface side reactions and oxygen loss under high voltage, and ensuring their long-cycle stability as a high-energy framework. Simultaneously, polycrystalline particles can obtain a thinner coating layer, facilitating the penetration of Co elements along grain boundaries, constructing a three-dimensional conductive network, and strengthening grain boundary bonding. This maintains good power characteristics while delaying grain boundary failure, achieving system-level optimization of the overall electrode structure with a "stable framework and smooth interface." In particular, when the thickness of the first coating layer is between 10nm and 100nm, a good balance can be further achieved between interface protection and electronic conduction. If the thickness of the first coating layer is too small, the protective effect is insufficient; if the thickness of the first coating layer is too large, it easily increases interface impedance and reduces energy density. Alternatively, when the thickness of the second coating layer is between 5nm and 50nm, a grain boundary conductive network can be constructed more effectively and the interfacial impedance can be reduced. If the thickness of the second coating layer is too small, it is difficult to form a continuous protective layer. If the thickness of the second coating layer is too large, it may hinder lithium-ion diffusion and reduce the utilization rate of active materials. If the thickness of the first coating layer is smaller than the thickness of the second coating layer, the surface protection capability of the single crystal particles is insufficient, and side reactions and structural degradation are likely to occur under high voltage. At the same time, if the coating layer of polycrystalline particles is too thick, it will hinder lithium-ion diffusion and increase interfacial impedance, which is not conducive to the overall performance balance of the electrode.

[0074] It should be noted that the thicknesses of the first and second coating layers can be obtained by conventional methods in the art, such as transmission electron microscopy combined with energy dispersive spectroscopy (TEM-EDS). For example, the thickness of the first coating layer can be 10 nm, 16 nm, 22 nm, 28 nm, 34 nm, 40 nm, 46 nm, 52 nm, 58 nm, 64 nm, 70 nm, 76 nm, 82 nm, 88 nm, 100 nm, or a value within any two of the above values; the thickness of the second 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, 50 nm, or a value within any two of the above values.

[0075] In some embodiments, the first matrix further includes element G, which includes at least one of elements Nb and Y, and the mass content of element G in the single crystal particles is controlled between 5 ppm and 3000 ppm. Thus, the appropriate amount of element G doping in the first matrix can also achieve the following effects: For Y element doping, since Y 3+ The radius is much larger than that of Li + The radius of the transition metal ion is similar, and it preferentially occupies Li sites, acting as a "pillar" to expand the lithium layer, thus widening the interlayer spacing. + The diffusion channels are smoother; moreover, strong YO bonds can anchor the crystal lattice, suppress oxygen evolution, fix the transition metal layer, hinder cation mixing, further stabilize the bulk structure, and accelerate the lithium-ion transport rate, making it particularly suitable for high-nickel ternary materials.

[0076] For Nb element doping, due to Nb 5+ It readily accumulates on the surface of the first substrate and reacts with residual lithium on the surface of the first substrate to form LiNbO3 or Li-Nb-O compounds with a fast ion conductor structure, significantly reducing interfacial impedance and improving rate performance; simultaneously, Nb 5+ During high-temperature sintering, it can suppress the formation of "sintering necks" between particles, improve the dispersion of single-crystal particles, and reduce agglomeration; in addition, when microcracks are generated in the particles during cycling, Nb 5+ It will preferentially migrate to the crack and accumulate, forming an in-situ repair layer, inhibiting the intrusion of electrolyte along the crack, delaying side reactions and structural degradation, thereby further improving the cycle life and high-temperature stability of the battery.

[0077] It should be noted that the mass content of element G in the first matrix can be determined 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 element G can be 5 ppm, 200 ppm, 400 ppm, 600 ppm, 800 ppm, 1000 ppm, 1200 ppm, 1400 ppm, 1600 ppm, 1800 ppm, 2000 ppm, 2200 ppm, 2400 ppm, 2600 ppm, 2800 ppm, 3000 ppm, or values ​​within any range of two of the above values.

[0078] In some embodiments, along the direction from the surface to the interior of the first substrate, the region in which the depth of the G element in the first substrate is less than or equal to L1 nm is designated as the ninth region, and the region in which the depth of the G element in the first substrate is greater than L1 nm is designated as the tenth region, where 2 ≤ L1 ≤ 35; the mass content of the G element in the ninth region is greater than the mass content of the G element in the tenth region.

[0079] By controlling the depth L1 of the ninth region in the first substrate within the range of 2nm-35nm, the enrichment depth of G element can be ensured to be appropriate. This allows G element to fully exert its "pillar effect" in stabilizing the crystal structure and reducing interfacial impedance with almost no sacrifice in discharge capacity, thereby further improving structural stability and rate performance. If L1 is too small, it means that the content of G element is too low, making it difficult to exert the above effects. Conversely, if L1 is too large, it means that the content of G element is too high, which will lead to a decrease in the orderliness of the layered structure, an increase in impurities, and blockage of lithium-ion transport channels, thus affecting the battery's discharge capacity, cycle life, and rate performance.

[0080] It should be noted that the depth L1 of the ninth region in the first matrix can be obtained by conventional methods in the art, such as transmission electron microscopy combined with energy dispersive X-ray spectroscopy (TEM-EDS). For example, the value of L1 nm can be 2 nm, 4 nm, 7 nm, 9 nm, 12 nm, 14 nm, 17 nm, 19 nm, 22 nm, 24 nm, 27 nm, 29 nm, 32 nm, 34 nm, 35 nm, or a value within any two of the above ranges. The mass content and magnitude relationship of element G in the ninth and tenth regions of the first matrix can be obtained by energy dispersive X-ray spectroscopy (EDS) line scan analysis.

[0081] As an example, in one specific embodiment, along the direction from the surface of the first substrate to its interior, a first substrate region extending from the surface of the first substrate to a depth of 35 nm is designated as a third region, which contains the G element. A fourth region is designated as the region extending from a depth of 35 nm from the surface of the first substrate to the center of the first substrate, which also contains the G element. The mass content of the G element in the third region is greater than that in the fourth region. The mass content and relative magnitude of the G element in the third and fourth regions of the first substrate can be obtained through energy-dispersive X-ray spectroscopy (EDS) line scanning analysis.

[0082] In some embodiments, the X-ray diffraction pattern of the single crystal particles shows a diffraction peak of the first spinel phase and a diffraction peak of the (003) crystal plane at a position of 17°-20°. When the content of the first spinel phase is low and its interplanar spacing (such as the (111) crystal plane) is similar to that of the (003) crystal plane of the layered phase, the diffraction peaks of the two phases will partially overlap or even completely coincide, resulting in an asymmetric broad peak or single peak in the XRD pattern. In addition, the presence of slight cation mixing or lattice distortion in the material will also exacerbate the degree of peak overlap.

[0083] It should be noted that the diffraction peaks in the X-ray diffraction pattern of single crystal particles can be obtained by conventional methods in the field, such as by X-ray diffraction analysis.

[0084] In some embodiments, based on the mass of the single crystal particles, by controlling the mass content of Co in the first matrix to be greater than the mass content of Mn, Co can be utilized. 3+ / Co 4+ The energy level structure widens the electron transition channels and reduces the band gap, thereby significantly improving the electronic conductivity inside the positive electrode 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 inside the first matrix will widen, and the electronic conductivity will decrease, thus hindering electron transport within the first matrix and reducing the rate performance of the battery.

[0085] It should be noted that the mass content and size relationship of Co and Mn elements can be obtained by conventional methods in this field, such as inductively coupled plasma optical emission spectrometry (ICP-OES) or inductively coupled plasma mass spectrometry (ICP-MS).

[0086] In some embodiments, based on the surface area of ​​the first substrate, the coverage rate of the first coating layer is controlled within the range of 20%-95%, such as... Figure 1As shown, while ensuring the first coating layer effectively isolates the electrolyte and reduces side reactions, it retains a sufficient amount of uncoated area to maintain electronic conductivity and elastic deformation capability. This prevents the coating rate from being too low, resulting in a discontinuous first coating layer, poor isolation effect, difficulty in sufficiently reducing the residual alkali on the surface of the single crystal particles, and still severe interfacial side reactions, thus offering limited improvement to cycle stability. Alternatively, it prevents the coating rate from being too high, approaching complete coating, which would block electronic conductivity, increase interfacial impedance, and also limit the local elastic deformation capability of the first substrate, causing the single crystal particles to be more prone to cracking during charging and discharging due to the inability to effectively release C-axis tensile strain.

[0087] It should be noted that the coverage rate of the first coating layer refers to the percentage of the area of ​​the first coating layer covering the surface of the first substrate relative to the total surface area of ​​the first substrate. This percentage can be obtained using conventional methods in the art, such as transmission electron microscopy combined with energy dispersive spectroscopy (TEM-EDS). For example, the coverage rate of the first coating layer can be 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or a value within any range of two of the aforementioned values.

[0088] In some embodiments, this application introduces Co and Al elements into the first coating layer. During the preparation of the single crystal particles, Al and Co elements migrate to the surface region of the first substrate, where they react in situ with Li, Zr, O, and other elements on the surface of the first substrate to form a Li-Zr-Co-Al-O first spinel phase (i.e., the fifth region). This results in a higher content of Co and Al elements in the fifth region compared to the sixth region. This not only consumes residual alkaline impurities such as Li₂CO₃ and LiOH on the surface of the first substrate, reducing the residual alkali content on the surface of the single crystal particles and lowering the interfacial impedance, but also... Compared to the layered structure of the first matrix phase and the Zr-free Li-Co-Al-O first spinel structure, the first spinel phase structure in the present invention has at least the following advantages: (1) The bonding is more stable, the resistance to electrolyte corrosion is stronger, the dissolution of transition metals can be significantly suppressed, the capacity loss and interfacial side reactions can be reduced, and the oxygen release, heat generation and gas generation can be suppressed, thus improving the thermal safety performance of the battery; (2) The structure is more rigid, the volume expansion rate is smaller, the surface lattice can be constrained, the irreversible phase transition can be suppressed, the particle cracking can be reduced, and the capacity decay can be delayed; (3) The surface lattice can be reconstructed, the cation mixing can be reduced, and the spinel phase structure has three-dimensional Li + Diffusion channels can accelerate lithium-ion insertion / extraction and improve rate performance.

[0089] It should be noted that the relative mass content of Co in regions 5 and 6 can be obtained by conventional methods in the art, such as energy-dispersive X-ray spectroscopy (EDS) line scan analysis. Similarly, the relative mass content of Al in regions 5 and 6 can be obtained by conventional methods in the art, such as energy-dispersive X-ray spectroscopy (EDS) line scan analysis.

[0090] In some embodiments, the thickness of the fifth region is further controlled to be 10nm-500nm. This ensures that the first substrate contains a sufficient amount of spinel phase, which helps to consume residual alkali on the surface, promote surface lattice reconstruction, improve structural rigidity, and enhance resistance to electrolyte corrosion, thereby further improving the cycle life and rate performance of the battery. If the thickness of the fifth region is too small, it is difficult to achieve the above effects; conversely, if the thickness of the fifth region is too large, the spinel phase content will be excessive, which can easily cause lattice distortion and anisotropic strain, superimposed on Li + The volumetric stress caused by deintercalation leads to the initiation of cracks in the matrix, accelerates failure, increases charge transfer impedance, increases polarization, deteriorates rate performance, continuously shifts the discharge voltage plateau, and significantly reduces energy density.

[0091] It should be noted that the thickness of the fifth region can be measured using conventional methods in the art, such as transmission electron microscopy combined with energy dispersive spectroscopy (TEM). For example, the thickness of the fifth region can be 10 nm, 35 nm, 70 nm, 105 nm, 140 nm, 175 nm, 210 nm, 245 nm, 280 nm, 315 nm, 350 nm, 385 nm, 420 nm, 455 nm, 490 nm, 500 nm, or a value within any range of two of the above values.

[0092] In some embodiments, the first coating layer further includes an M2 element, which includes at least one of Ti, W, and B. These elements do not migrate into the first matrix during the calcination of the single crystal particles, but are mainly retained in the first coating layer. This can further improve the structural stability of the first coating layer and improve the transport dynamics of lithium ions at the interface, thereby enhancing the cycle stability of the positive electrode active material.

[0093] Based on the mass of the single crystal particles, by controlling the mass content m ppm of the M2 element to satisfy: 0 < m ≤ 6000, it can be ensured that the M2 element is uniformly distributed in the first coating layer and plays a synergistic modifying role without causing negative effects. If the content of the M2 element is too small, it will not be able to effectively enhance the structural strength and ion conductivity of the first coating layer; if the content of the M2 element is too large, the excess M2 element may form an inert impurity phase, hindering lithium ion migration and even reducing the specific capacity of the material.

[0094] It should be noted that the mass content of element M2 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 element M2 can be 50 ppm, 500 ppm, 1000 ppm, 1500 ppm, 2000 ppm, 2500 ppm, 3000 ppm, 3500 ppm, 4000 ppm, 4500 ppm, 5000 ppm, 5500 ppm, 6000 ppm, or a value within any range of two of the above values.

[0095] Furthermore, in some embodiments, based on the mass of the single crystal particles, by controlling the mass content of Zr element between 1000ppm and 3000ppm, the mass content of Al element between 100ppm and 2000ppm, the mass content of W element between 300ppm and 3000ppm, and the mass content of B element between 100ppm and 1500ppm, it is possible to ensure the stability of the bulk structure and interface modification while preventing excessive doping from causing capacity loss or increased impedance of the battery, and also preventing insufficient doping from causing insufficient modification effect, thereby improving the cycle performance and rate performance of the battery.

[0096] It should be noted that the mass content of Zr, Al, W, and B elements can be obtained by conventional methods in the art. For example, the mass content of Zr can be 1000 ppm, 1150 ppm, 1300 ppm, 1450 ppm, 1600 ppm, 1750 ppm, 1900 ppm, 2050 ppm, 2200 ppm, 2350 ppm, 2500 ppm, 2650 ppm, 2800 ppm, 2950 ppm, 3000 ppm, etc., or values ​​within any two of the above ranges; the mass content of Al can be 100 ppm, 200 ppm, 300 ppm, 400 ppm, 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm, 1000 ppm, 1200 ppm, 1400 ppm, 1600 ppm, 1800 ppm, 2000 ppm, etc., or values ​​within any two of the above ranges. The values ​​within the range; the mass content of element W can be, for example, 300ppm, 500ppm, 700ppm, 900ppm, 1100ppm, 1300ppm, 1500ppm, 1700ppm, 1900ppm, 2100ppm, 2300ppm, 2500ppm, 2700ppm, 2900ppm, 3000ppm, etc., or values ​​within the range of any two of the above values; the mass content of element B can be, for example, 100ppm, 200ppm, 300ppm, 400ppm, 500ppm, 600ppm, 700ppm, 800ppm, 900ppm, 1000ppm, 1100ppm, 1200ppm, 1300ppm, 1400ppm, 1500ppm, etc., or values ​​within the range of any two of the above values.

[0097] In this application, the chemical formula of the single crystal particles can be Li. f Ni x Co y Mn z Z e O2, 0.8≤f≤1.3, 0.8≤x<1, 0.02≤y≤0.2, 0.01≤z≤0.14, 0.001≤e≤0.22, Z includes at least Zr and Al elements, Z may also include G elements and / or M2 elements, the G element includes at least one of Nb and Y elements, and the M2 element includes at least one of Ti, W and B elements.

[0098] In this application, single-crystal particles can be prepared using the following method, including the following steps: The first step involves preparing a nickel-cobalt-manganese hydroxide precursor using a co-precipitation method. A zirconium source is loaded onto the surface of the precursor. In some embodiments, an aluminum source, a niobium source, and / or a yttrium source may also be loaded onto the surface of the precursor. The precursor is then mixed with a lithium source at a specific molar ratio and sintered under an oxygen atmosphere to obtain the first matrix material. 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 niobium source can be an oxide, fluoride, or chloride; and the yttrium source can be an oxide, chloride, nitrate, fluoride, hydroxide, or sulfate.

[0099] The second step involves preparing a mixed solution of the first matrix material with an aluminum source and a cobalt source (optionally, a tungsten source, a boron source, and / or a titanium source). This solution is then sprayed onto the surface of the matrix material. After sintering at 500℃~1200℃, single-crystal particles containing the first coating layer are obtained. 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 cobalt source can be an oxide, chloride, nitrate, hydroxide, phosphate, or sulfate; and the titanium source can be an oxide, chloride, hydroxide, or sulfate.

[0100] Based on the amount of each raw material used in the above preparation method, the chemical formula of the single crystal particles can be calculated.

[0101] It should be noted that in the preparation of the first matrix material in the first step, the orientation of the atomic arrangement between grains can be controlled by controlling the sintering temperature, thereby obtaining a single-crystal matrix or a polycrystalline matrix; the mass content of Zr in the single-crystal particles can be controlled by controlling the amount of zirconium source added, and the relative standard deviation of the mass content of Zr in the single-crystal particles can be controlled by adjusting the sintering temperature; the mass content of Al, Nb, Y and other elements in the single-crystal particles and the depth of the ninth region can be controlled by controlling the amount of aluminum source, niobium source and / or yttrium source added.

[0102] In the second coating step, the mass content of Al, W, and B elements in the single crystal particles can be controlled by adjusting the amount of aluminum, tungsten, and boron sources added; the mass content of Co in the single crystal particles can be controlled by adjusting the amount of cobalt source added; the thickness and coverage of the first coating layer can be controlled by adjusting the amount of aluminum, cobalt, and at least one of tungsten, titanium, and boron sources added, as well as the sintering temperature; and the thickness of the fifth region can be controlled by adjusting the amount of aluminum and cobalt sources added.

[0103] It is understandable that the Co and Al elements in the first coating material migrate to the surface region of the first substrate during sintering, resulting in a higher Co and Al content in the fifth region of the first substrate compared to the sixth region. Zr, however, is introduced during the preparation of the first substrate; the first coating layer does not contain Zr. Figure 2 and Figure 3 It can be seen that the Zr element is relatively uniformly distributed throughout the entire first matrix.

[0104] In some embodiments, this application further provides a second coating layer containing Co on the surface of the second substrate. 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, because the conductivity of cobalt oxide is much higher than that of the second substrate, the Co-containing second coating layer can construct a continuous surface conductive pathway, reducing the contact impedance between active materials and the interfacial charge transfer impedance. Furthermore, during charging, a thin and uniform rock salt phase preferentially forms on the Co-rich surface of the second substrate, stabilizing the interfacial layer and mitigating bulk structure distortion. More importantly, the Co element in the second coating layer also diffuses along the grain boundaries of the second substrate and penetrates into the grain boundaries, where Co... 3+ Enrichment of the cobalt phase can reduce grain boundary energy, enhance the bonding force between primary particles, suppress 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 M1 element, thereby ensuring that the grain boundary is dominated by cobalt-rich layered phase, supplemented by an appropriate amount of inert spinel phase. This enhances grain boundary stability while ensuring high specific capacity, suppresses grain boundary slip and crack propagation caused by volume strain during cycling, and synergistically improves the structural integrity, cycle life and rate performance of the active material.

[0105] It should be noted that the mass content and magnitude relationship of Co and M1 elements within the grain boundaries of the second matrix can be obtained by conventional methods in the art, such as energy dispersive X-ray spectroscopy (EDS) line scan analysis.

[0106] In some implementations, by controlling the particle size L of the second matrix to be between 5 μm and 18 μm, a reasonable distribution of the grain boundary network between the 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 second matrix 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 second matrix 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 the primary particles will be more concentrated, making it easier for microcracks to propagate and particles to break, thereby accelerating capacity decay.

[0107] It should be noted that the particle size of the second matrix can be obtained by conventional methods in the art, such as scanning electron microscopy (SEM). For example, the particle size of the second matrix 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.

[0108] Furthermore, along the direction from the surface of the second substrate to its interior, the region with a depth of L / 5 μm from the surface of the second substrate is designated as the seventh region, and the region with a depth greater than L / 5 μm from the surface of the second substrate is designated as the eighth region. Since the Co element in the second coating layer diffuses and penetrates into the interior of the second substrate along the grain boundaries and is enriched in the grain boundaries and near-surface regions, the mass content of the Co element in the seventh region can be greater than that in the eighth 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 polycrystalline particles and thus improving the cycle life of the battery.

[0109] It should be noted that the mass content and magnitude relationship of Co in regions 7 and 8 can be obtained by conventional methods in the field, such as energy dispersive X-ray spectroscopy (EDS) line scan analysis.

[0110] Furthermore, in some embodiments, during the preparation of polycrystalline particles, the Co element in the second coating layer migrates to the seventh region and grain boundaries of the second matrix. The Co element that migrates to the seventh region and grain boundaries reacts with excess lithium present in the polycrystalline particles (usually from excess lithium source or residual lithium on the surface) 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, i.e., a cobalt-rich layered structure.

[0111] Furthermore, in some embodiments, the Co element in the bulk phase of the second matrix may also undergo re-diffusion under high-temperature sintering conditions, which would result in the Co element molar content in the cobalt-rich layered structure being 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.

[0112] 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" at the seventh region and grain boundaries. This effectively reduces interfacial impedance and improves the battery's rate performance and cycle stability. 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 entering and exiting the particles, resulting in deteriorated rate performance. Simultaneously, impeded electron transport at grain boundaries exacerbates local stress concentration, which is detrimental to maintaining the material's structural stability.

[0113] It should be noted that the molar content of Co can be obtained by conventional methods in the field, such as qualitative determination by energy-dispersive X-ray spectroscopy (EDS).

[0114] In some embodiments, based on the mass of the polycrystalline particles, by controlling the mass content of Co in the second matrix to be greater than the mass content of Mn in the second matrix, Co can be utilized. 3+ / Co 4+ The energy level structure widens the electron transition channels and reduces the band gap, thereby significantly improving the electronic conductivity inside the positive electrode 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 inside the second matrix will widen, and the electronic conductivity will decrease, thus hindering electron transport within the second matrix and reducing the rate performance of the battery.

[0115] It should be noted that the mass content of Co and Mn elements in the second matrix 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).

[0116] In some embodiments, the second 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 second coating layer, thereby more effectively suppressing electrolyte side reactions, reducing interfacial impedance, and improving the cycle stability and rate performance of the battery.

[0117] In some embodiments, the second matrix further includes a Q element, 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.

[0118] In some implementations, such as Figure 5 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 second coating layer, while the second surface is not in contact with the second coating layer. Based on the mass of the polycrystalline particle, 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, the enrichment of Co on the second surface, particularly at the grain boundaries of the polycrystalline particle, helps to construct a continuous and efficient "electron / ion fast transport network" within the polycrystalline particle, enhancing grain boundary conductivity and mechanical strength, and suppressing the generation and propagation of microcracks. This design balances the electronic conduction and mechanical integrity within the polycrystalline particle with the interfacial chemical stability of the outer surface, thereby synergistically improving the rate performance and cycle stability of the battery.

[0119] 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.

[0120] In some embodiments, the second coating layer includes Co, Al, and B elements. Based on the mass of the second coating layer, by controlling the mass content of Co element between 50% and 98%, sufficient electronic conductivity of the second 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 second 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.

[0121] It should be noted that the mass content of Co, Al, and B elements in the second coating layer can be obtained by conventional methods in the art. For example, the mass content of Co element can be 50%, 53%, 57%, 60%, 64%, 67%, 71%, 74%, 78%, 81%, 85%, 88%, 92%, 95%, 98%, etc., or values ​​within any two of the above ranges; the mass content of Al element can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, etc., or values ​​within any two of the above ranges; the mass content of B element 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%, etc., or values ​​within any two of the above ranges.

[0122] In some embodiments, the second matrix includes an Sr element, 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 Sr element mass content in the grain boundaries of polycrystalline particles 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.

[0123] When the total Sr content in the second matrix is ​​the same, if the mass content h3% of Sr located at the grain boundaries is close to equal to the mass content h4% of Sr located in the primary grains, 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+ Entering the crystal lattice causes local 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 h3 and h4 can be determined by conventional methods in the art, such as energy-dispersive X-ray spectroscopy (EDS) line scan analysis.

[0124] In some embodiments, based on the mass of the polycrystalline particles, 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.

[0125] Meanwhile, the mass content (s2) of M1 element is controlled between 400 ppm and 2000 ppm. Due to its high charge and strong electronegativity, M1 element 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 M1 element can penetrate along grain boundaries to form 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 M1 element may cause local charge imbalance, induce lattice defects, and also sacrifice specific capacity due to excessive passivation layer.

[0126] Based on this, by controlling the ratio s1 / s2 between the mass content of Sr element s1 and the mass content of M1 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 M1 elements to synergistically 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 M1, which improves their chemical stability. Together, they block the main failure paths 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.

[0127] If s1 / s2 is too small, that is, there is a relatively large amount of M1 element, the mechanical buffer will be insufficient, and the grain boundary will still be prone to stress cracking. The passivation layer formed by M1 element will fail due to particle fracture. If s1 / s2 is too large, that is, there is a relatively large amount of Sr element, the chemical anchoring will be insufficient, the surface oxygen stability will be insufficient, and the material may still lose oxygen under high voltage, leading to structural collapse. The mechanical support of Sr element will also lose its meaning.

[0128] It should be noted that the mass content of Sr and M1 elements can be obtained by conventional methods in the art. 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 M1 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.

[0129] In some embodiments, the M1 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 second substrate, 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 second substrate. This allows them to fully exert 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 second substrate 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.

[0130] It should be noted that the relative standard deviation of the mass content of Al and Zr can be obtained by conventional methods in the art, such as cross-sectional polishing-scanning electron microscopy combined with energy dispersive spectroscopy. For example, the relative standard deviation of the mass content of Al and Zr can be 2.5%, 5%, 7.5%, 10%, 12.5%, 15%, 17.5%, 20%, 22.5%, 25%, 27.5%, 30%, 32.5%, 35%, or values ​​within any range of two of the above values.

[0131] Furthermore, in some embodiments, based on the mass of the polycrystalline particles, the mass content of Sb is further controlled between 100 ppm and 1600 ppm, so that Sb is mainly enriched in the first region and grain boundaries of the second matrix, thereby constructing a chemically inert passivation layer and more effectively suppressing electrolyte erosion; the mass content of Y is controlled between 300 ppm and 1200 ppm, so that Y forms a stable Y-based compound, further enhancing the stability of the interface structure; the mass content of Sr is controlled between 260 ppm and 2... With a concentration between 500 ppm, Sr preferentially segregates at grain boundaries, strengthening the "skeleton connection points" of grain boundaries and better improving the mechanical strength of polycrystalline particles. Controlling the Zr content between 1500 ppm and 3500 ppm allows Zr to be uniformly distributed in the second matrix, more fully stabilizing the crystal structure and suppressing the harmful H2-H3 phase transition. Controlling the Al content between 600 ppm and 2500 ppm also allows Al to be uniformly distributed in the second matrix, further enhancing the bonding strength between transition metals and oxygen. By further controlling the mass content of these elements within appropriate ranges, the effects of grain boundary strengthening, surface passivation, and bulk stabilization can be more fully utilized, thereby further improving the mechanical stability of polycrystalline particles while maintaining high capacity, and thus significantly improving the cycle performance of the battery.

[0132] For example, the mass content of Sb can be 100ppm, 200ppm, 300ppm, 400ppm, 500ppm, 600ppm, 700ppm, 800ppm, 900ppm, 1000ppm, 1100ppm, 1200ppm, 1300ppm, 1400ppm, 1500ppm, 1600ppm, etc., or values ​​within any two of the above ranges; the mass content of Y can be 300ppm, 360ppm, 420ppm, etc. The values ​​can be ppm, 480ppm, 540ppm, 600ppm, 660ppm, 720ppm, 780ppm, 840ppm, 900ppm, 960ppm, 1020ppm, 1080ppm, 1200ppm, etc., or values ​​within any two of the above ranges; the mass content of Sr can be, for example, 260ppm, 420ppm, 580ppm, 740ppm, 900ppm, 1060ppm, 1220ppm, 1380ppm, 1540ppm, etc. The values ​​are 1700ppm, 1860ppm, 2020ppm, 2180ppm, 2340ppm, 2500ppm, etc., or values ​​within any two of the above ranges; the mass content of Zr can be, for example, 1500ppm, 1650ppm, 1800ppm, 1950ppm, 2100ppm, 2250ppm, 2400ppm, 2550ppm, 2700ppm, 2850ppm, 3000ppm, 3150ppm, 3300ppm. The values ​​may be 3450ppm, 3500ppm, etc., or values ​​within the range of any two of the above values; the mass content of Al element may 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.

[0133] In this application, the chemical formula of the polycrystalline particles can be Li. f Ni x Co y Mn z E 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, E includes at least the M1 element, which includes at least one of Sb, Ta, and Y, and E may also include the Q element, which includes at least one of Zr, Al, Mo, Ti, and Sr, and / or E may also include at least one of Ti, W, and B.

[0134] In this application, polycrystalline particles can be prepared using 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 a second matrix. 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.

[0135] The second step involves preparing a mixed solution of the second substrate 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 second substrate. After sintering, polycrystalline particles containing the second coating layer are 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.

[0136] Based on the amount of each raw material used in the above preparation method, the chemical formula of the polycrystalline particles can be calculated.

[0137] It should be noted that in the first step of preparing the second matrix, 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 second matrix and the particle size Dv50 of the polycrystalline particles. The mass content and distribution of the M1 element in the polycrystalline particles can be controlled by adjusting the amount of M1 element (such as antimony source or yttrium source). The mass content of cobalt and manganese elements in the second matrix can be controlled by adjusting the molar ratio of cobalt and manganese elements in the co-precipitation step.

[0138] In the second coating step, the mass content and distribution of Co in the polycrystalline particles 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 polycrystalline particles 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.

[0139] In the preparation of the second matrix in the first step, the mass content of Sr element in the polycrystalline particles, the mass content of Sr element in the grain boundaries of the polycrystalline particles, and the mass content of Sr element in the primary particles can be controlled by adjusting the amount of Sr source added.

[0140] By controlling the amount of tantalum source added, the mass content of Ta element in polycrystalline particles can be adjusted.

[0141] By controlling the amount of zirconium source added, the mass content of Zr in polycrystalline particles can be adjusted. In combination with the adjustment of sintering temperature, the relative standard deviation of the mass content of Zr in polycrystalline particles can be controlled.

[0142] By controlling the amount of aluminum source added, the mass content of Al in polycrystalline particles can be adjusted. In combination with the adjustment of sintering temperature, the relative standard deviation of the mass content of Al in polycrystalline particles can be controlled.

[0143] By controlling the amount of yttrium source added, the mass content of Y element in polycrystalline particles can be adjusted.

[0144] In some embodiments, the particle size Dv50 of the single-crystal particles is denoted as D1 μm, and the particle size Dv50 of the polycrystalline particles is denoted as D2 μm. By controlling the value of D1 / D2 within the range of 0.1-1.4, the small-diameter single-crystal particles can be tightly packed into the gaps between the large-diameter polycrystalline particles, forming a high-compact packing. Simultaneously, the single crystal acts as a highly stable framework, providing crack resistance, while the polycrystalline particle acts as a fast ion channel, ensuring rate performance. In particular, when the particle size D1 of the single-crystal particles is between 1 μm and 8 μm, the structural stability and processing performance of the single-crystal particles as the main framework can be further guaranteed. Or, especially, when the particle size D2 of the polycrystalline particles is between 5 μm and 18 μm, its grain boundary network can be further utilized to provide fast ion channels, improving rate performance.

[0145] If the value of D1 / D2 is too small, the single crystal particles will be too small, the specific surface area will be too large, and the interfacial side reactions will be aggravated; if the value of D1 / D2 is too large, the particle size will be poorly matched, the particles will not be densely packed, and the volumetric energy density will decrease.

[0146] If D1 is too small, the specific surface area increases, and the interface stability deteriorates; if D1 is too large, the lithium-ion diffusion path becomes longer, and the rate performance decreases. If D2 is too small, the packing density decreases and agglomeration is more likely; if D2 is too large, the lithium-ion solid-phase diffusion distance increases, and the rate performance decreases.

[0147] It should be noted that the particle size Dv50 of single-crystal and polycrystalline particles refers to the average particle size based on volume statistics, that is, the particle size value corresponding to when the cumulative volume distribution of the particles reaches 50%. The particle size D1 of single-crystal particles and the particle size D2 of polycrystalline particles can be obtained by conventional methods in the art, such as by laser particle size analyzer. For example, the value of D1 can be 1.0μm, 1.5μm, 2.0μm, 2.5μm, 3.0μm, 3.5μm, 4.0μm, 4.5μm, 5.0μm, 5.5μm, 6.0μm, 6.5μm, 7.0μm, 7.5μm, 8.0μm, etc., or a value within the range of any two of the above values; the value of D2 can be 5μm, 5.5μm, 6μm, 7μm, 8μm, 9μm, etc. 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, etc., or values ​​within the range of any two of the above values; the value of D1 / D2 can be, for example, 0.1, 0.15, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, etc., or values ​​within the range of any two of the above values.

[0148] In some embodiments, based on the mass of the positive electrode active layer, by controlling the mass content of monocrystalline particles (e1%) to be greater than or equal to the mass content of polycrystalline particles (e2%), and further controlling the ratio of the mass content of monocrystalline particles (e1) to the mass content of polycrystalline particles (e2%) to be between 1 and 9, a combined system can be formed with monocrystalline particles as the main body of a high-stability framework and polycrystalline particles as fast ion conduction channels. Monocrystalline particles provide crack resistance and long-term cycle stability, while polycrystalline particles provide ion diffusion paths and rate performance, thereby enabling the battery to achieve both high energy density, high compaction density, and excellent cycle life. If e1 / e2 is too large, the proportion of monocrystalline particles is too high, and there is a lack of sufficient polycrystalline particles to build a grain boundary conductive network, resulting in a decrease in rate performance and an increase in electrode polarization. If e1 / e2 is too small, the proportion of polycrystalline particles is too high, and since polycrystalline materials themselves are prone to grain boundary cracks and interfacial side reactions, the cycle stability is significantly reduced and gas production is aggravated.

[0149] For example, the value of e1 / e2 can be 1, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 9, etc., or a value within the range of any two of the above values.

[0150] By controlling the particle size and mass content of single-crystal and polycrystalline particles within the aforementioned ranges, the compaction density of the positive electrode active layer can be achieved to be 3 g / cm³. 3 -3.5g / cm 3 This results in a tight packing of small-diameter single crystals filling the gaps between large-diameter polycrystalline particles, which improves the tap density and volumetric energy density of the electrodes while reducing the risk of grain boundary cracking of pure polycrystalline particles under high compaction. Furthermore, the introduction of polycrystalline particles can uniformly distribute the surface current of single crystal particles, alleviate the problem of local excessive delithiation caused by the large intrinsic polarization of single crystals, reduce the risk of active material failure, and thus improve the cycle performance of the cell.

[0151] It should be noted that the compaction density of the positive electrode active layer can be obtained by conventional methods in the art, such as by referring to the national standard GB / T 44330-2024. For example, the compaction density of the positive electrode active layer can be 3.00 g / cm³. 3 3.06 g / cm 3 3.13 g / cm 3 3.19 g / cm 3 3.25g / cm 3 3.31 g / cm 3 3.38g / cm 3 3.44 g / cm 3 3.50g / cm 3 Values ​​equal to or within the range of any two of the above values.

[0152] In some implementations, by controlling the porosity of the positive electrode active layer to 20%-50%, it is possible to ensure that the electrolyte fully wets the active material, providing a good ion transport channel while maintaining a high volumetric energy density and electrode structural integrity. If the porosity is too small, electrolyte wetting becomes difficult, lithium-ion diffusion is hindered, resulting in a decrease in the battery's rate performance; if the porosity is too large, the proportion of active material decreases, the volumetric energy density drops significantly, and the particles inside the electrode become loosely connected, making the conductive network prone to breakage and capacity decay during cycling.

[0153] It should be noted that the porosity of the positive electrode active layer can be obtained by conventional methods in the art, such as mercury porosimetry. For example, the porosity of the positive electrode active layer can be 20%, 22%, 24%, 26%, 28%, 30%, 33%, 35%, 37%, 39%, 41%, 43%, 45%, 47%, 50%, or a value within any two of the above ranges.

[0154] According to a second aspect of this application, a lithium-ion secondary battery is also provided, comprising a negative electrode, an electrolyte, and a positive electrode as described in the first aspect of this application.

[0155] Furthermore, 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.

[0156] 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).

[0157] 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.

[0158] 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.

[0159] It should be noted that the mass content of silicon can be determined 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 range of two of the above values.

[0160] 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.

[0161] 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.

[0162] 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.

[0163] The mass content of graphite material can be, for example, 0%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 80%, 85%, etc., or a value within any two of the above ranges.

[0164] In some implementations, by controlling the ratio NP of the capacity of the negative electrode to the capacity of the positive electrode to be between 1.06 and 1.35, the capacity of the positive electrode can be made to be in excess relative to the negative electrode. This allows a portion of the positive electrode capacity to be used 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, thereby ensuring a long cycle life of the battery.

[0165] 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.

[0166] It should be noted that the value of NP can be obtained by conventional methods in the art. For example, at 25°C, the positive and negative electrodes are assembled into half-cells, and their first-cycle discharge capacity is measured at a charge-discharge rate of 0.1C. 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 1.06, 1.10, 1.14, 1.18, 1.22, 1.26, 1.30, 1.33, 1.35, or a value within any two of the above ranges.

[0167] In some embodiments, the electrolyte includes nitrile compounds. Based on the mass of the electrolyte, by controlling the mass content of the nitrile compounds to be between 0.2% and 3%, the specific coordination between the Co enriched on the surface of the positive electrode material and the cyano groups of the nitrile compounds can be utilized to induce the preferential adsorption of nitrile compounds on the positive electrode surface and the formation of a stable interfacial protective film. This significantly inhibits the oxidative decomposition, transition metal dissolution, and gas generation reaction of the electrolyte under high voltage, reduces the increase in interfacial impedance, and enables the battery to maintain excellent cycle stability and rate performance under high voltage (≥4.3V) and high temperature conditions. If the mass content of the nitrile compounds is too low, it is difficult to form a complete and dense adsorption layer, resulting in insufficient protective effect. If the mass content of the nitrile compounds is too high, it may excessively reduce the ionic conductivity of the electrolyte, affecting the rate performance of the battery.

[0168] For example, the nitrile compounds include at least one selected from succinic acid, glutaronitrile, adiponitrile, heptonitrile, octanoic acid, sebaconitrile, 1,3,6-hexanetrionitrile, glyceroltrionitrile, and 1,2-bis(2-cyanoethoxy)ethane.

[0169] It should be noted that the mass content of nitrile compounds can be obtained by conventional methods in the art, such as gas chromatography (GC) or high-performance liquid chromatography (HPLC). For example, the mass content of nitrile compounds may be 0.20%, 0.55%, 0.90%, 1.25%, 1.60%, 1.95%, 2.30%, 2.65%, 3.00%, or values ​​within any two of the above ranges.

[0170] 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.

[0171] 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 of the half-widths at half maximum (WHM) of the reduction peak B and the oxidation peak A to be between 0.72 and 1.15, it can be indicated that the electrode polarization is small and the redox reversibility is good. This is due to the synergistic design of single crystal and polycrystalline mixing, the Co coating layer, and the effect of elemental doping on reducing interfacial impedance and lattice strain, thereby ensuring that the battery has excellent cycle stability and rate performance at high voltage. If WB / WA is too small, the reduction peak is relatively narrow, which may mean that the electrochemical polarization is increased or the reaction kinetics are hindered. If WB / WA is too large, the reduction peak is relatively wide, reflecting that there is a large polarization inhomogeneity or side reaction interference inside the electrode.

[0172] It should be noted that WB and WA can be obtained through conventional methods in this field. For example, the battery can be subjected to constant current charge-discharge testing at a low rate (e.g., 0.1C), and the voltage V and capacity Q data can be recorded at sampling intervals of 1-10 mV or 1-5 seconds. The obtained data can be differentiated to obtain a dQ / dV curve. The peaks of the reduction peak B and the oxidation peak A can be located on the smoothed dQ / dV curve, the baseline can be determined, the peak intensity can be calculated, and the half-peak width can be calculated based on the half-peak height position, which are denoted as WB and WA, respectively. For example, the value of WB / WA can be 0.72, 0.75, 0.78, 0.81, 0.84, 0.87, 0.90, 0.93, 0.96, 0.99, 1.02, 1.05, 1.08, 1.11, 1.15, etc., or values ​​within any two of the above ranges.

[0173] 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.

[0174] Example E1 This embodiment provides a method for preparing a lithium-ion secondary battery, including the following steps: Step 1: Preparation of positive electrode active material 1) Preparation of single crystal particles First, a hydroxide precursor with a nickel-cobalt-manganese molar ratio of 95:3:2 was prepared by co-precipitation. 0.32 wt% ZrO2 was uniformly loaded onto the precursor surface and then mixed with a lithium source at a Li / (Ni+Co+Mn) molar ratio of 1.02–1.08. The mixture was sintered at 850–980 °C under an oxygen atmosphere, and the first matrix was obtained after crushing and grading. Subsequently, an aluminum source (based on 800 ppm of Al in the first matrix, equivalent to approximately 1510 ppm of Al2O3), a cobalt source (based on 20000 ppm of Co in the first matrix, equivalent to approximately 27400 ppm of Co3O4), and a tungsten source (based on 900 ppm of W in the first matrix, equivalent to approximately 1260 ppm of ammonium metatungstate) were used. A mixed solution of boron (ppm) was prepared and sprayed onto the surface of the first substrate. The substrate was then sintered at 400-600℃ to form a primary coating layer. Finally, a boron source (equivalent to approximately 6290 ppm of H3BO3 raw material based on 1100 ppm of B by mass of the primary coating layer) was sprayed onto the surface of the primary substrate. After low-temperature sintering at 250-400℃, single-crystal particles with the primary coating layer were obtained, with the chemical formula Li(Ni) 0.897 Co 0.069 Mn 0.019 Zr 0.0022 Al 0.003 4W 0.00078 B 0.0089 )O2.

[0175] 2) Preparation of polycrystalline particles 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 a pentagonal phase-doped second matrix. 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 a second substrate. The substrate 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 substrate again. The substrate was then sintered a second time at 250℃~400℃ to obtain polycrystalline particles with a second coating layer. The chemical formula of these particles is Li(NiO2). 0.887 Co0.064 Mn 0.029 Sr 0.0019 Sb 0.00074 Y 0.00076 Zr 0.0023 Al 0.0059 B 0.0084 )O2.

[0176] Step 2: Preparation of the positive electrode sheet The positive electrode active material (mass ratio of single crystal particles to polycrystalline particles of 80:20), polyvinylidene fluoride, single-walled carbon nanotubes, and multi-walled carbon nanotubes prepared in the first step 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 .

[0177] Step 3: Preparation of the negative electrode Artificial graphite, silicon carbide, 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.

[0178] Step 4: Preparation of the diaphragm A 2 μm thick composite coating is applied to the first 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 second surface of the polyethylene substrate opposite the first 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.

[0179] 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.

[0180] 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.

[0181] The preparation methods of Examples S1-S8, Examples T1-T15, Examples E2-E17, and Comparative Examples 1-4 are basically the same as those of Example E1. The differences are shown in Tables 1-5.

[0182] In the negative electrode sheets of Examples E16 and E17, 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."

[0183] It is understandable that, during the preparation of the first matrix, the mass content of nickel, cobalt, and manganese in the single crystal particles can be adjusted by controlling the molar ratio of nickel, cobalt, and manganese in the co-precipitation step; the particle size Dv50 of the single crystal particles can be controlled by adjusting the sintering temperature; at the same time, the mass content and distribution of Zr in the single crystal particles can be adjusted by controlling the amount of ZrO2 added; similarly, the specific types of G-containing substances such as Nb2O3 and Y2O3 and their mass content and distribution in the single crystal particles can be controlled by adjusting the types and amounts of G-containing substances.

[0184] In the process of preparing the first coating layer, the mass content and distribution of Co in the polycrystalline particles can be controlled by adjusting the amount of Co3O4 added and the sintering temperature. By adjusting the types and amounts of coating materials such as Al2O3, ammonium metatungstate, and H3BO3, as well as the sintering temperature, the coating layer thickness, coating rate, and mass content of elements such as Co, Al, B, Ti, and W in the coating layer can be adjusted, and the particle size Dv50 of the single crystal particles can also be adjusted simultaneously.

[0185] In the preparation of the second matrix, the mass content of nickel, cobalt, and manganese in the polycrystalline particles 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 second matrix, and the particle size Dv50 of the polycrystalline particles can be simultaneously controlled by adjusting the sintering temperature; the specific types of M1-containing substances such as Sb2O3 and Y2O3 and their mass content and distribution in the polycrystalline particles 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 polycrystalline particles can be controlled by adjusting the types and amounts of Q-containing substances such as Al2O3, ZrO2, and SrCO3.

[0186] In the process of preparing the second coating layer, the mass content and distribution of Co in the polycrystalline particles can be controlled by adjusting the amount of Co3O4 added and the sintering temperature. At the same time, by adjusting 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 layer and the mass content of elements such as Co, Al, B, Ti, and W in the coating layer can be adjusted, and the particle size Dv50 of the polycrystalline particles can also be adjusted simultaneously.

[0187] Table 1 Chemical composition and structure of single crystal particles

[0188] Table 2 Chemical composition and structure of polycrystalline particles

[0189] Table 3 Chemical composition and structure of polycrystalline particles

[0190] Table 4 Composition and Structure of Polycrystalline Particles & Positive Electrode Sheets

[0191] Table 5 Composition / Performance of Positive Electrode and Battery

[0192] Test case 1. Cyclic 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.

[0193] 2. Battery 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 battery thickness expansion rate after 300 cycles.

[0194] 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.

[0195] 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 rate of 2C to the lower limit voltage.

[0196] 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.

[0197] Capacity retention at 2C rate is calculated using the following formula: 2C capacity retention rate = 2C discharge capacity / 0.5C discharge capacity.

[0198] The test results are shown in Tables 6-8.

[0199] Table 6

[0200] Table 7

[0201] Table 8

[0202] As shown in Tables 1-8, this application enhances the bulk structure and kinetic properties of single-crystal particles by introducing an appropriate amount of Zr into the first matrix of the polycrystalline particles. Simultaneously, by introducing at least one M1 element from Sb, Ta, and Y into the second matrix of the polycrystalline particles and enriching it in the surface layer and grain boundaries of the second matrix, a continuous chemical protection network is constructed for the polycrystalline particles, thereby strengthening their surface lattice, stabilizing grain boundaries, improving interfaces, and accelerating lithium-ion conduction. Furthermore, this application adjusts the molar fraction n1% of Ni in the single-crystal particles and the polycrystalline particles... The molar fraction of Ni in the particles, n2%, is set such that n1 > n2. Furthermore, the ratio a1 / a2 of the mass content of Zr in the first matrix of the monocrystalline particles and the mass content of M1 in the second matrix of the polycrystalline particles is controlled between 0.5 and 28. This ensures that the "bulk phase stability" capability of the monocrystalline particles matches the "interface protection and grain boundary strengthening" capability of the polycrystalline particles. In this way, the ultra-long lifespan characteristics of monocrystalline particles can be utilized, while the excellent kinetics and processability of polycrystalline particles can be inherited, thereby meeting the needs of the next generation of high energy density and long cycle life power batteries.

[0203] 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 sheet, the positive electrode sheet comprising a positive current collector and a positive active layer located on at least one surface of the positive current collector, the positive active layer comprising a positive active material; characterized in that, The positive electrode active material includes lithium nickel cobalt manganese oxide material, which includes single crystal particles and polycrystalline particles. The molar fraction of Ni element in the single crystal particles is n1% based on the molar number of transition metal elements, and the molar fraction of Ni element in the polycrystalline particles is n2% based on the molar number of transition metal elements, satisfying: n1>n2, 99≥n1≥80.5, 98≥n2≥80. The single-crystal particle comprises a first substrate and a first coating layer, the first coating layer being located on at least a portion of the surface of the first substrate; the first substrate comprises at least Zr and Al elements, and the Zr element mass content is a1 ppm based on the mass of the single-crystal particle; the relative standard deviation of the Zr element mass content at 5 locations on the CP-SEM cross-section of the first substrate is R1, satisfying R1≤40%; The polycrystalline particle includes a second matrix and a second coating layer, the second coating layer being located on at least a portion of the surface of the second matrix; the second matrix includes at least element M1, the element M1 including at least one of Sb, Ta, and Y, and the mass content of element M1 is a2 ppm based on the mass of the polycrystalline particle; satisfying: 0.5≤a1 / a2≤28; The second matrix includes 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 the first region, and a region at a depth greater than D / 5 nm from the surface of the primary particle is designated as the second region. Based on the mass of the polycrystalline particles, the mass content of element M1 in the first region is greater than the mass content of element M1 in the second region.

2. The positive electrode sheet according to claim 1, characterized in that, At least one of the following conditions must be met: （1）800≤a1≤6000； （2）200≤a2≤2000； （3）100≤D≤800； (4) The thickness of the first coating layer is h1 nm and the thickness of the second coating layer is h2 nm, satisfying: h1 > h2; Preferably, 10 ≤ h1 ≤ 100; Preferably, 5 ≤ h2 ≤ 50.

3. The positive electrode sheet according to claim 1, characterized in that, At least one of the following conditions must be met: (1) The first substrate further includes element G, which includes at least one of Nb and Y; the mass content of element G is 5ppm-3000ppm based on the mass of the single crystal particle; and / or, along the direction from the surface of the first substrate to its interior, a first substrate region within a depth of 35nm is a third region containing element G, and a region from a depth of 35nm from the surface of the first substrate to the center of the first substrate is a fourth region containing element G, and the mass content of element G in the third region is greater than the mass content of element G in the fourth region; (2) Based on the mass of the single crystal particles, the mass content of Co in the first matrix is ​​greater than the mass content of Mn in the first matrix; (3) Based on the surface area of ​​the first substrate, the coverage rate of the first coating layer is 20%-95%; (4) The first coating layer includes at least Co and Al elements. Along the direction from the surface of the first substrate to its interior, the first substrate includes a fifth region and a sixth region. The fifth region is located on the surface of the sixth region. The first coating layer is located on a portion of the surface of the fifth region. The fifth region contains a first spinel phase. Based on the mass of the single crystal grain, the mass content of Co in the fifth region is greater than the mass content of Co in the sixth region, and the mass content of Al in the fifth region is greater than the mass content of Al in the sixth region. Preferably, the thickness of the fifth region is 10 nm-500 nm.

4. The positive electrode sheet according to claim 3, characterized in that, The first coating layer also includes the M2 element, which includes at least one of Ti, W, and B elements. Based on the mass of the single crystal particle, the mass content of the M2 element is denoted as m ppm, which satisfies the following condition: 0 < m ≤ 6000. Preferably, based on the mass of the single crystal particles, the mass content of Zr is 1000ppm-3000ppm, the mass content of Al is 100ppm-2000ppm, the mass content of W is 300ppm-3000ppm, and the mass content of B is 100ppm-1500ppm.

5. The positive electrode sheet according to claim 1, characterized in that, The second coating layer includes at least Co. The grain boundaries of the polycrystalline particles contain Co and M1 elements. Based on the mass of the polycrystalline particles, the mass content of Co in the grain boundaries is greater than the mass content of M1 in the grain boundaries. And / or, the particle size of the second matrix is ​​L μm, 5≤L≤18; along the direction from the surface of the second matrix to its interior, the region with a depth of L / 5 μm from the surface of the second matrix to its interior is designated as the seventh region, and the region with a depth greater than L / 5 μm from the surface of the second matrix is ​​designated as the eighth region; based on the mass of the polycrystalline particles, the mass content of Co element in the seventh region is greater than the mass content of Co element in the eighth region; Preferably, the grain boundaries of the polycrystalline particles contain a cobalt-rich layered structure; Preferably, the seventh region contains a cobalt-rich layered structure; More preferably, based on the sum of the molar numbers of all transition metal elements in the cobalt-rich layered structure, the molar content of Co in the cobalt-rich layered structure is greater than or equal to 60%.

6. The positive electrode sheet according to claim 1 or 5, characterized in that, Based on the mass of the polycrystalline particles, the mass content of Co in the second matrix is ​​greater than the mass content of Mn in the second matrix; And / or, the first region contains a second spinel phase; And / or, the grain boundaries of the polycrystalline grains contain a third spinel phase; And / or, the second coating layer further includes at least one element selected from Al, Ti, W, and B; And / or, the second matrix further includes a Q element, which includes at least one of Zr, Al, Mo, Ti, and Sr; And / or, the primary particle includes a first surface and a second surface, the first surface being in contact with the second coating layer, and the second surface not being in contact with the second coating layer; based on the mass of the polycrystalline particle, the mass content of Co element on the first surface is less than the mass content of Co element on the second surface.

7. The positive electrode sheet according to claim 6, characterized in that, At least one of the following conditions must be met: (1) The second coating layer includes Co, Al and B elements. Based on the mass of the second 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%. (2) The second matrix includes Sr element; based on the mass of the polycrystalline particles, the mass content of Sr element in the grain boundary of the polycrystalline particles is h3%, and the mass content of Sr element in the primary particles is h4%, satisfying h3>h4; (3) The second matrix includes Sr and M1 elements; based on the mass of the polycrystalline particles, the mass content of Sr is denoted as s1 ppm and the mass content of M1 is denoted as s2 ppm, satisfying: 260≤s1≤2500, 400≤s2≤2000, 0.15≤s1 / s2≤6; (4) The M1 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 second matrix is ​​denoted as R2, which satisfies R2 < 40%. The relative standard deviation of the Zr element mass content at five locations on the CP-SEM cross-section of the second matrix is ​​denoted as R3, which satisfies R3 < 40%. Based on the mass of the polycrystalline particles, the mass content of Sb is 100ppm-1600ppm, the mass content of Sr is 260ppm-2500ppm, 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.

8. The positive electrode sheet according to claim 1, 2, 3, 4, 5 or 7, characterized in that, At least one of the following conditions must be met: (1) The particle size Dv50 of the single crystal particle is denoted as D1 μm, and the particle size Dv50 of the polycrystalline particle is denoted as D2 μm, satisfying: 0.1≤D1 / D2≤1.4; Preferably, 1 ≤ D1 ≤ 8; Preferably, 5 ≤ D2 ≤ 18; (2) Based on the mass of the positive electrode active layer, the mass content of the single crystal particles is e1% and the mass content of the polycrystalline particles is e2%, satisfying: e1≥e2, preferably, satisfying: 1≤e1 / e2≤9; (3) The porosity of the positive electrode active layer is 20%-50%; (4) The compaction density of the positive electrode active layer is 3 g / cm³. 3 -3.5g / cm 3 .

9. A lithium-ion secondary battery, characterized in that, Includes a negative electrode, an electrolyte, and a positive electrode as described in any one of claims 1-8; Preferably, the negative electrode sheet comprises a negative electrode active material, and the negative electrode active material comprises silicon-carbon material; satisfying 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%; (4) The ratio of the capacity of the negative electrode to the capacity of the positive electrode is denoted as NP, which satisfies: 1.06≤NP≤1.

35.

10. The lithium-ion secondary battery according to claim 9, characterized in that, The electrolyte includes nitrile compounds, and the mass content of the nitrile compounds is 0.2%-3% based on the mass of the electrolyte. Preferably, the nitrile compound includes at least one selected from butadionitrile, glutaronitrile, adiponitrile, heptanonitrile, octanilide, semolinatrionitrile, 1,3,6-hexanetrionitrile, glyceroltrionitrile, and 1,2-bis(2-cyanoethoxy)ethane; And / or, 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 (WWHM) of the oxidation peak A is denoted as WA, and the half width at half maximum (WWHM) of the reduction peak B is denoted as WB, satisfying: 0.72≤WB / WA≤1.15.

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