Positive electrode active material as well as preparation method and application thereof

Abstract

The invention discloses a positive electrode active material and a preparation method and application thereof. The positive electrode active material comprises a ternary material, the ternary material comprises a nickel cobalt lithium manganate inner core and a first coating layer coating the nickel cobalt lithium manganate inner core, the chemical formula of the first coating layer is Li (AlxCo1-x) O2-delta F delta, x is larger than or equal to 0.1 and smaller than or equal to 0.6, and delta is larger than or equal to 0.05 and smaller than or equal to 0.2. The positive electrode active material can improve the interface stability, and can improve the interface compatibility of a ternary material and a lithium iron phosphate material when the ternary material and the lithium iron phosphate material are compounded for use.

Description

Technical Field

[0001] This application relates to the field of lithium-ion battery technology, and in particular to a positive electrode active material, its preparation method and application. Background Technology

[0002] With the increasing global demand for clean energy and sustainable development, lithium-ion batteries, due to their high energy density and stable cycle performance, have found increasingly widespread applications in mobile electronic devices, electric vehicles, and large-scale energy storage. As a key component of lithium-ion batteries, the performance of the cathode material directly affects the overall battery performance. Therefore, research on lithium-ion battery cathode materials has significant practical implications. Lithium-ion battery cathode materials need to balance high energy density and high safety.

[0003] Compared to lithium iron phosphate (LFP), ternary materials (NCM) have higher specific capacity, but suffer from poorer ionic conductivity and higher interfacial impedance. Furthermore, theoretically, combining high-specific-capacity ternary materials (NCM) with highly stable lithium iron phosphate (LFP) can achieve complementary performance; however, in practical applications, problems such as poor interfacial compatibility arise, including electrochemical window mismatch, increased interfacial impedance, and differences in ion / electron transport characteristics. Summary of the Invention

[0004] The purpose of this invention is to provide a positive electrode active material, its preparation method, and its application. This positive electrode active material can improve the ionic conductivity of ternary materials, reduce their interfacial impedance, and improve interfacial stability. In particular, when ternary materials are used in combination with lithium iron phosphate materials, it can improve the interfacial compatibility between ternary and lithium iron phosphate materials.

[0005] For the purposes of this invention, the following technical solution is adopted: On one hand, the present invention provides a positive electrode active material, the positive electrode active material comprising a ternary material, the ternary material comprising a lithium nickel cobalt manganese oxide core and a first coating layer coated on the lithium nickel cobalt manganese oxide core, the first coating layer having the chemical formula Li(Al) x Co 1-x )O 2-δ F δ , where 0.1 ≤ x ≤ 0.6, 0.05 ≤ δ ≤ 0.2.

[0006] In an embodiment of the present invention, the positive electrode active material further includes a fast ion conductor layer coated on the surface of the first coating layer.

[0007] Preferably, the material of the fast ion conductor layer is Li1.3Al0.3Ti1.7(PO4)3 (LATP).

[0008] Preferably, the thickness of the first coating layer is 10-20 nm.

[0009] Preferably, the thickness of the fast ion conductor layer is 5-15 nm.

[0010] In an embodiment of the present invention, the positive electrode active material further includes a lithium iron phosphate material, which includes a lithium iron phosphate core and a conductive layer coated on the lithium iron phosphate core, wherein the electronic conductivity of the conductive layer is greater than 1 S / cm.

[0011] In embodiments of the present invention, the conductive layer is made of carbon or poly(3,4-ethylenedioxythiophene-polystyrene sulfonate).

[0012] Preferably, the thickness of the conductive layer is 2-5 nm.

[0013] In an embodiment of the present invention, the surface of the conductive layer is further coated with a lithium iron phosphate buffer layer.

[0014] Preferably, the thickness of the lithium iron phosphate buffer layer is 1-3 nm.

[0015] On the other hand, the present invention also provides a positive electrode sheet, the positive electrode sheet comprising: Positive current collector substrate; and Positive electrode material disposed on the surface of the positive electrode current collector substrate; The positive electrode material includes a conductive agent, a binder, and the positive electrode active material described above; The binder is a polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), and the main link is a phosphorus-containing functional group -PO3H2 (i.e., phosphonic acid group -PO (OH)2).

[0016] Preferably, the weight ratio of the positive electrode active material, conductive agent, and binder is positive electrode active material: conductive agent: binder = 93~97: 2~5: 1~2.

[0017] Furthermore, the present invention also provides a method for preparing the positive electrode sheet as described above, which includes the following steps: S1, Preparation of ternary materials: S1a, a mixture containing aluminum, cobalt, fluorine, and lithium sources is mixed with NCM particles, and evaporated to obtain a precursor; the precursor is then subjected to a first calcination treatment to form a surface coated with Li(Al) x Co 1-x )O 2-δ F δ The ternary material; wherein the molar amounts of the aluminum source, cobalt source, fluorine source, and lithium source satisfy: Al³ + Co²+ :F - Li + =0.01~0.025: 0.01~0.025: 0.004~0.05: 0.002~0.005; S1b, the ternary material obtained in step S1a is mixed with Li1.3Al0.3Ti1.7(PO4)3 sol, and after centrifugation, a ternary material coated with Li1.3Al0.3Ti1.7(PO4)3 gel is obtained; the ternary material coated with Li1.3Al0.3Ti1.7(PO4)3 gel is dried and subjected to a second calcination treatment to obtain a double-layer coated ternary material; S2, Preparation of lithium iron phosphate material: The carbon source is mixed with LFP particles, dried and calcined for the third time to obtain lithium iron phosphate material with a carbon conductive layer on the surface. S3, Preparation of positive electrode sheet: The ternary material obtained in step S1b, the lithium iron phosphate material obtained in step S2, the conductive agent and the binder are mixed in a solvent to obtain a positive electrode slurry; The positive electrode slurry is coated onto the positive electrode current collector substrate, and after drying and pressing, a positive electrode sheet is obtained.

[0018] In an embodiment of the present invention, in step S1a, the first calcination treatment is a two-stage calcination treatment carried out in an oxygen atmosphere; wherein, the first stage of calcination treatment refers to heating to 250-450°C at a rate of 2-5°C / min and holding at that temperature for 1-4 hours; the second stage of calcination treatment refers to heating to 600-800°C at a rate of 2-5°C / min and holding at that temperature for 6-12 hours; The aluminum source is at least one of aluminum nitrate, aluminum chloride, and aluminum sulfate; The cobalt source is at least one of cobalt acetate, cobalt chloride, and cobalt nitrate. The fluorine source is at least one of ammonium fluoride, sodium fluoride, and potassium fluoride; The lithium source is at least one of lithium hydroxide, lithium nitrate, lithium carbonate, lithium dihydrogen phosphate, and lithium chloride. In step S1b, the second calcination treatment specifically involves heating to 550-800°C at a rate of 2-5°C / min under an argon atmosphere and holding at that temperature for 2-8 hours.

[0019] In an embodiment of the present invention, the third calcination in step S2 specifically involves: heating to 400-850°C at a rate of 2-5°C / min under a weakly reducing atmosphere, and holding at that temperature for 4-8 hours; In step S3, the mass ratio of the ternary material to the lithium iron phosphate material is 8:2 to 2:8.

[0020] In an embodiment of the present invention, step S2 further includes: depositing and growing lithium iron phosphate on the surface of the lithium iron phosphate material coated with a carbon conductive layer, followed by annealing treatment, to obtain a lithium iron phosphate material with a carbon conductive layer and a lithium iron phosphate buffer layer coated on the surface.

[0021] Preferably, the atomic layer deposition is performed in an inert atmosphere at 50-300°C for 30-60 minutes.

[0022] Preferably, the post-annealing treatment specifically involves holding the temperature at 200-500°C for 1-3 hours in an inert atmosphere.

[0023] In another aspect, the present invention also provides a lithium-ion battery comprising a positive electrode sheet as described above; and / or a positive electrode sheet prepared according to the preparation method described above.

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows: The positive electrode active material of the present invention comprises a ternary material, wherein the ternary material comprises a lithium nickel cobalt manganese oxide core and a first coating layer coating the lithium nickel cobalt manganese oxide core. The first coating layer is Li. x The CoO2-type spinel-like phase layer, possessing three-dimensional lithium-ion channels and maintaining lattice matching with the lithium nickel cobalt manganese oxide core, enhances the ionic conductivity of the ternary material, reduces interfacial impedance, and improves its interfacial stability. Especially when the ternary material is used in combination with lithium iron phosphate, it improves their interfacial compatibility. The ternary material can be further coated with a fast-ion conductor layer, which provides a rapid lithium-ion transport path; this further reduces the NCM interfacial impedance. Particularly during high-rate charge and discharge, the ternary material, through its two-layer coating, achieves multifunctional integration of "lattice stability, ion conduction, and electron transport."

[0025] Furthermore, the positive electrode active material of the present invention may further include lithium iron phosphate material, wherein the lithium iron phosphate material comprises a lithium iron phosphate core and a conductive layer coated on the lithium iron phosphate core, and the conductive layer has an electronic conductivity greater than 1 S / cm; the conductive layer has high electronic conductivity (>1 S / cm) and maintains lithium-ion permeability, and the conductive layer can solve the kinetic limitation problem. When the lithium iron phosphate material and the ternary material described in the present invention are mixed as positive electrode active materials, the interfacial compatibility between the two is better, the electrochemical window is more matched, the interfacial impedance is reduced, and the difference in ion / electron transport characteristics is also improved.

[0026] Furthermore, the positive electrode sheet of the present invention includes a positive electrode active material containing ternary materials and lithium iron phosphate materials, which uses PVDF-HFP with phosphorus-containing functional groups -PO3H2 as the main linker as a binder. The phosphorus-containing functional groups of PVDF-HFP are bonded to the Al³⁺ on the surface of the ternary material. + Coordination bonds are formed, and at the same time, the HFP segments generate van der Waals forces with the conductive layer on the surface of lithium iron phosphate (such as a carbon conductive layer formed by carbon materials), which makes the interfacial compatibility between ternary materials and lithium iron phosphate materials better. Detailed Implementation

[0027] To better understand and implement this application, the technical solutions of this application will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are only some of the embodiments of this application, and not all of them.

[0028] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein in the specification of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application.

[0029] Unless otherwise stated, all numerical values ​​for the amounts of expressed components, reaction conditions, etc., used in the specification and claims are to be understood as being modified by the term "about". Therefore, unless otherwise indicated, the numerical parameters set forth herein are approximate values ​​that can be varied to obtain the desired performance.

[0030] For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range or individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0031] The word “and / or” as used in this article refers to one or all of the elements mentioned.

[0032] The terms "include" and "contain" as used in this article cover both cases where only the mentioned elements exist and cases where other unmentioned elements exist in addition to the mentioned elements.

[0033] The purpose of this invention is to provide a positive electrode active material, its preparation method, and its application. The following is a detailed description of this application.

[0034] Positive electrode active material This invention provides a positive electrode active material, comprising a ternary material, wherein the ternary material includes a lithium nickel cobalt manganese oxide core and a first coating layer coated on the lithium nickel cobalt manganese oxide core, the first coating layer having the chemical formula Li(Al) x Co 1-x )O2-δ F δ , where 0.1 ≤ x ≤ 0.6, 0.05 ≤ δ ≤ 0.2.

[0035] The material of the "lithium nickel cobalt manganese oxide core" is not limited to unmodified lithium nickel cobalt manganese oxide (NCM), but also includes modified lithium nickel cobalt manganese oxide (NCM), such as doped modified lithium nickel cobalt manganese oxide, as long as it can achieve the purpose of the present invention; preferably, the material of the "lithium nickel cobalt manganese oxide core" is unmodified lithium nickel cobalt manganese oxide (NCM).

[0036] "Coating" is not limited to direct coating, but also includes indirect coating; that is, the first coating layer can be directly coated on the lithium nickel cobalt manganese oxide core, and there can be one or more other structures between the first coating layer and the lithium nickel cobalt manganese oxide core; preferably, the first coating layer is directly coated on the lithium nickel cobalt manganese oxide core, that is, there are no other structures between the first coating layer and the lithium nickel cobalt manganese oxide core.

[0037] "Coating on the lithium nickel cobalt manganese oxide core" can mean coating the entire surface of the lithium nickel cobalt manganese oxide core or coating a portion of the surface of the lithium nickel cobalt manganese oxide core; preferably, "coating on the lithium nickel cobalt manganese oxide core" means coating the entire surface of the lithium nickel cobalt manganese oxide core, that is, the first coating layer completely covers the surface of the lithium nickel cobalt manganese oxide core.

[0038] In the above technical solution, the first coating layer on the surface of the ternary material is Li. x The CoO2-type spinel-like phase layer possesses three-dimensional lithium-ion channels and maintains lattice matching with the lithium nickel cobalt manganese oxide core, thereby improving ionic conductivity, reducing interfacial impedance, and enhancing the interfacial stability of ternary materials. In particular, when ternary materials are used in combination with lithium iron phosphate materials, they can improve the interfacial compatibility between the two.

[0039] In Li(Al) x Co 1-x )O 2-δ F δ In this case, 0.1 ≤ x ≤ 0.6, 0.05 ≤ δ ≤ 0.2; for example, x can be 0.2, 0.25, 0.3, 0.35, 0.4, 0.5, 0.6, etc., and δ can be 0.05, 0.1, 0.15, 0.18, 0.2, etc. Since Li(Al) x Co 1-x )O 2-δ F δ The presence of Co ensures good chemical compatibility and structural similarity between the first coating layer and the lithium nickel cobalt manganese oxide (NCM) core (which also contains Co), which is the basis for achieving lattice matching; due to the presence of Al³⁺... + Al³+ The ionic radius of Co³ + Unlike other materials, its introduction can adjust the lattice parameters, making it closer to the surface structure of the NCM matrix, further promoting matching; at the same time, the strong Al-O bond energy stabilizes the crystal structure. Due to the addition of F... - F - Its electronegativity is much higher than that of O² - F - With metal ions (Al³) + Co³ + The bond formed is more than O² - The bonds formed with metal ions are stronger and more stable; therefore, F - Partially replaces O² - This can further enhance structural stability (suppress oxygen loss) and adjust electronic conductivity; F - The addition of [a specific ingredient] improves the thermal and chemical stability of the material, enhances battery safety, and slows down capacity decay caused by transition metal dissolution and phase transition during cycling.

[0040] "Lattice matching" here specifically refers to the ability of the crystal structure of the cladding layer and the crystal structure of the NCM core to be coherently connected at the interface, with the periodicity of the atomic arrangement and the lattice constant being very close; maintaining lattice matching can reduce interfacial impedance and enhance the stability of the material.

[0041] Preferably, x = 0.5, δ = 0.2; correspondingly, the chemical formula of the first coating layer is Li(Al0.5Co0.5)O. 1.8 F 0.2 Use F at a ratio of 20%. - Partially replaces O² - (That is, δ = 0.2), which can greatly "anchor" oxygen in the crystal lattice to enhance structural stability, thereby maximally suppressing oxygen loss (oxygen evolution) under high voltage. Setting the Al ratio to 50% (that is, x = 0.5) means that half of the metal sites in the cladding crystal framework are occupied by Al, which greatly enhances the structural stability of the cladding itself; this strengthened framework can better withstand the lattice stress during cycling, especially under high voltage, effectively suppressing the propagation of microcracks caused by volume changes in the NCM core to the surface, and acting as a physical barrier to reduce direct contact and side reactions between the electrolyte and the NCM core.

[0042] In embodiments of the present invention, the positive electrode active material further includes a fast ion conductor layer coated on the surface of the first coating layer. The fast ion conductor layer provides a fast lithium-ion transport path; it further reduces the NCM interface impedance, especially during high-rate charge and discharge. When the first coating layer and the fast ion conductor layer are coated on the NCM core, a multifunctional integration of "lattice stability-ion conduction-electron transport" can be achieved.

[0043] In embodiments of the present invention, the material of the fast ion conductor layer includes Li1.3Al0.3Ti1.7(PO4)3 (LATP) and Li7La3Zr2O. 12 (LLZO), Li 3x La2 / 3-x TiO3 (LLTO), Li 1+x Al x Ge 2-x (PO4)3 (LAGP), Li 10 GeP2S 12 At least one of (LGPS) and Li2S-P2S5 (LPS). These fast ion conductor layer materials have high ionic conductivity (generally up to 10). -4 S / cm~10 - It has a density on the order of S / cm and contains continuous lithium-ion migration channels, providing a fast lithium-ion transport path.

[0044] Preferably, the fast ion conductor layer is made of Li1.3Al0.3Ti1.7(PO4)3 (LATP). LATP not only has high ionic conductivity, but also suppresses side reactions between NCM and electrolyte, reducing transition metal dissolution and interface degradation. LATP is more stable in air, has good compatibility with high-voltage cathode materials such as NCM, and has a wide operating voltage window. At the same time, the interface between LATP and NCM has excellent adhesion, and it is easy to form a dense and continuous nanoscale coating layer through methods such as sol-gel, sputtering, and ALD, effectively isolating the electrolyte from direct contact with NCM, suppressing side reactions, and easily depositing as a thin layer.

[0045] Preferably, the thickness of the first coating layer is 10-20 nm; for example, the thickness of the first coating layer is 10 nm, 11 nm, 12 nm, 15 nm, 16 nm, 18 nm, 20 nm, etc. During charging and discharging, the NCM core (i.e., lithium nickel cobalt manganese oxide core) undergoes reversible lattice volume changes (expansion / contraction); the coating layer with a thickness of 10-20 nm has both good flexibility and sufficient mechanical strength. It can elastically deform to a certain extent with the "breathing" of the NCM core, and is not prone to cracking or peeling off from the surface due to cyclic stress, thereby ensuring the structural integrity and durable protection capability of the coating layer during long-term cycling; an excessively thick coating layer may be more brittle due to excessive rigidity.

[0046] Preferably, the particle size D50v of the lithium nickel cobalt manganese oxide core is 5~8 μm; for example, the particle size of the lithium nickel cobalt manganese oxide core is 5 μm, 5.5 μm, 6 μm, 6.3 μm, 6.8 μm, 7 μm, 7.5 μm, 8 μm, etc. If the particle size is too large (>8 μm), the internal stress generated by lithium ion intercalation and deintercalation during cycling will be more concentrated, increasing the risk of internal cracking of the particles; at the same time, the diffusion path of lithium ions in the solid phase is too long, which will impair rate performance.

[0047] Preferably, the mass of the first coating layer is 2 wt% to 5 wt% of the mass of the lithium nickel cobalt manganese oxide core; for example, the mass of the first coating layer is 2 wt%, 2.5 wt%, 3 wt%, 3.5 wt%, 4 wt%, 4.5 wt%, 5 wt%, etc., of the mass of the lithium nickel cobalt manganese oxide core. Since the specific capacity of the first coating layer itself (especially those containing Co and Al) is lower than that of the NCM core (i.e., the lithium nickel cobalt manganese oxide core), its mass percentage should not be too high, otherwise it will directly reduce the proportion of reversible lithium storage components in the positive electrode active material.

[0048] Preferably, the thickness of the fast ion conductor layer is 5-15 nm; exemplaryly, the thickness of the fast ion conductor layer is 5 nm, 7 nm, 8 nm, 10 nm, 13 nm, 14 nm, 15 nm, etc. A LATP layer with a thickness of 5-15 nm exhibits excellent flexibility and adhesion; during charge and discharge, the fast ion conductor layer within this thickness range can elastically adapt to the volume changes of the NCM core together with the first coating layer, without easily experiencing brittle cracking or peeling. This nanoscale mechanical adaptability ensures the structural integrity and long-lasting protection of the entire coating system through hundreds or even thousands of cycles. Simultaneously, it results in an extremely short absolute diffusion distance for lithium ions to traverse the layer, reducing the introduced additional interfacial impedance to a negligible level; this ensures that the protective layer provides the strongest isolation without compromising the rate performance and power characteristics of the battery.

[0049] Preferably, the mass of the fast ion conductor layer is 0.5 wt% to 1.5 wt% of the mass of the lithium nickel cobalt manganese oxide core; for example, the mass of the fast ion conductor layer is 0.5 wt%, 0.7 wt%, 0.9 wt%, 1.0 wt%, 1.2 wt%, 1.5 wt%, etc., of the mass of the lithium nickel cobalt manganese oxide core. When the mass percentage of the fast ion conductor layer is within this range, not only can the rate performance and fast charging capability be guaranteed, but the integrity of the fast ion conductor layer is also ensured; when the mass percentage is less than 0.5 wt%, it is insufficient to form a completely covered, defect-free ultrathin film; when the mass percentage is greater than 1.5 wt%, it may have a negative impact on the rate performance and fast charging capability of the battery.

[0050] In embodiments of the present invention, the positive electrode active material further includes lithium iron phosphate material, which includes a lithium iron phosphate core and a conductive layer coated on the lithium iron phosphate core, wherein the electronic conductivity of the conductive layer is greater than 1 S / cm.

[0051] The material of the "lithium iron phosphate core" is not limited to unmodified lithium iron phosphate (LFP), but also includes modified lithium iron phosphate (LFP), such as doped modified lithium iron phosphate, as long as it can achieve the purpose of this invention; preferably, the material of the "lithium iron phosphate core" is unmodified lithium iron phosphate.

[0052] "Coating" is not limited to direct coating, but also includes indirect coating; that is, the conductive layer can be directly coated on the lithium iron phosphate core, and there can also be one or more other structures between the conductive layer and the lithium iron phosphate core.

[0053] "Coating on the lithium iron phosphate core" can mean coating the entire surface of the lithium iron phosphate core or coating a portion of the surface of the lithium iron phosphate core; preferably, "coating on the lithium iron phosphate core" means coating the entire surface of the lithium iron phosphate core, that is, the conductive layer completely covers the surface of the lithium iron phosphate core.

[0054] The conductive layer has high electronic conductivity (>1 S / cm) and maintains lithium-ion permeability; the high electronic conductivity can build a continuous electron transport network and reduce electronic contact resistance; at the same time, the lithium-ion permeability ensures that lithium ions can pass smoothly through the conductive layer, realizing efficient cross-interface migration for insertion / extraction.

[0055] In an embodiment of the present invention, the surface of the conductive layer is further coated with a lithium iron phosphate buffer layer. The lithium iron phosphate buffer layer directly reduces the insertion / extraction barrier of lithium ions on the surface of the LFP material, thereby improving rate performance; at the same time, it matches the LFP core lattice, reducing interfacial strain; the combination of the conductive layer and the lithium iron phosphate buffer layer can better solve the kinetic limitation problem.

[0056] In embodiments of the present invention, the conductive layer is made of carbon material or poly(3,4-ethylenedioxythiophene-polystyrene sulfonate) (PEDOT:PSS).

[0057] In embodiments of the present invention, the carbon material includes at least one of graphite, carbon black, hard carbon, soft carbon, mesoporous carbon, graphene, carbon nanotubes, and carbon fibers.

[0058] In embodiments of the present invention, the carbon material is formed by carbonization via a carbon source. Preferably, the carbon source is a biomass-based carbon source. More preferably, the biomass-based carbon source includes at least one of glucose, cellulose, lignin, and hemicellulose. Most preferably, the biomass-based carbon source is glucose.

[0059] Preferably, the thickness of the conductive layer is 2-5 nm; for example, the thickness of the conductive layer is 2 nm, 2.5 nm, 3 nm, 3.8 nm, 4 nm, 4.6 nm, 5 nm, etc.

[0060] Preferably, the particle size D50v of the lithium iron phosphate core is 2~4μm; for example, the particle size of the lithium iron phosphate core is 2μm, 2.2μm, 2.6μm, 3μm, 3.4μm, 3.8μm, 4μm, etc. If the particle size of the lithium iron phosphate core is too large, the lithium ion diffusion path will be too long, affecting its performance; but if the particle size is too small, it will bring problems such as excessive specific surface area, more side reactions, and low tap density.

[0061] Preferably, the mass of the conductive layer is 0.4 wt% to 1.2 wt% of the mass of the lithium iron phosphate core. For example, the mass of the conductive layer is 0.4 wt%, 0.6 wt%, 0.7 wt%, 0.9 wt%, 0.95 wt%, 1.0 wt%, 1.1 wt%, 1.2 wt%, etc., of the lithium iron phosphate core. If the mass percentage of the conductive layer is too small, the electronic conductivity is poor, resulting in poor rate capability and cycle life of the battery; if the mass percentage of the conductive layer is too large, ion transport is hindered, leading to a loss of energy density.

[0062] In an embodiment of the present invention, FeP4O is further disposed between the lithium iron phosphate core and the conductive layer. 11 Transition layer. FeP4O 11 Its crystal structure has good compatibility with LFP, and it can serve as a lattice buffer layer to reduce interfacial stress and defects. At the same time, it provides a continuous path for electron transport, reduces interfacial electron transfer resistance, and reduces polarization during charging and discharging.

[0063] Preferably, the lithium iron phosphate buffer layer is a lithium iron phosphate (LiFePO4) buffer layer with lithium vacancies. The lithium iron phosphate (LiFePO4) buffer layer with lithium vacancies can further reduce the lithium-ion intercalation barrier.

[0064] Preferably, the LiFePO4 buffer layer is grown by atomic layer deposition (ALD). Intrinsic LFP surfaces contain lattice defects or impurity phases, resulting in high interfacial resistance when lithium ions enter the solid phase from the electrolyte. The ALD-grown LiFePO4 layer has a high lithium vacancy concentration, providing sites for incoming lithium ions, significantly reducing the activation energy required for insertion, and facilitating a smoother interfacial charge transfer process. The buffer layer and the core have the same crystal structure and similar lattice parameters, forming a lithium ion transport channel from the electrolyte to the interior of the active particles. This eliminates the ion blocking effect commonly found at heterogeneous interfaces (such as direct carbon / electrolyte interfaces or amorphous interfaces).

[0065] Preferably, the thickness of the lithium iron phosphate buffer layer is 1-3 nm. When the thickness of the lithium iron phosphate buffer layer is within this range, it not only ensures that the conductive layer is completely and uniformly covered without any exposed points, thus completely eliminating direct contact between the carbon layer and the electrolyte and fulfilling its high-voltage protection function, but also guarantees compaction density and saves costs. Preferably, the weight ratio of ternary material to lithium iron phosphate material is 2:8 to 8:2. For example, when the weight ratio of ternary material to lithium iron phosphate material is 2:8 or 3:7, the system is dominated by highly stable lithium iron phosphate material, resulting in extremely low risk of thermal runaway and extremely long cycle life, which can significantly reduce the amount of expensive NCM (especially high-nickel) materials used. For example, when the weight ratio of ternary material to lithium iron phosphate material is 4:6, 5:5, or 6:4, the system has a relatively balanced region between ternary material and lithium iron phosphate material, achieving the best comprehensive balance between energy density, power, life, safety, and cost. For example, when the weight ratio of ternary material to lithium iron phosphate material is 7:3 or 8:2, the system is dominated by high-energy ternary material, pursuing the highest volumetric and gravimetric energy density. Lithium iron phosphate material acts as a "stabilizer," effectively buffering the volume changes of ternary material, suppressing phase transitions, providing additional thermal stability protection, and significantly improving the safety and cycle life of the pure NCM system.

[0066] Positive electrode sheet On the other hand, the present invention also provides a positive electrode sheet, the positive electrode sheet comprising: Positive current collector substrate; and Positive electrode material disposed on the surface of the positive electrode current collector substrate; The positive electrode material includes a conductive agent, a binder, and a positive electrode active material. The positive electrode active material includes ternary materials and lithium iron phosphate materials as described above. The binder is polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), and the main linker has phosphorus-containing functional groups —PO3H2.

[0067] When phosphorus-containing functional groups (—PO3H2) are branched onto the main link of PVDF-HFP, these functional groups can interact with Al³⁺ on the surface of the ternary material. + Coordinate bonds are formed, P─O→Al³ + Lewis acid-base interactions (bond energy 60–120 kJ mol) - ¹, which is 1–2 orders of magnitude stronger than van der Waals forces, can break Al³ on the surface of NCM lattice. + The HFP chain segments "pin" the lithium iron phosphate (LFP) material, preventing particle slippage and reducing lattice oxygen precipitation. Simultaneously, van der Waals forces between the HFP segments and the surface conductive layer of the LFP material (such as a carbon conductive layer formed by carbon materials) create an interfacial bridge, enabling high rate capability and high capacity. This cathode material achieves interfacial stability compatibility between ternary and LFP materials while ensuring efficient lithium-ion transport.

[0068] In an embodiment of the present invention, the weight ratio of the positive electrode active material, the conductive agent, and the binder is positive electrode active material: conductive agent: binder = 93~97: 2~5: 1~2.

[0069] In embodiments of the present invention, the positive current collector substrate is selected from at least one of aluminum foil, titanium (Ti) foil, titanium alloy foil, tantalum (Ta) foil, niobium (Nb) foil, and nickel (Ni) foil.

[0070] In an embodiment of the present invention, the polyvinylidene fluoride-hexafluoropropylene copolymer with phosphorus-containing functional group —PO3H2 as the main link is prepared by the following steps: adding vinylphosphonic acid monomer to PVDF-HFP solution, reacting at 60-100°C for 16-32 hours, and then washing and drying to obtain the copolymer.

[0071] Preferably, the molar ratio of vinylphosphonic acid monomer to PVDF-HFP is 1:10~20.

[0072] Preferably, the concentration of PVDF-HFP in the PVDF-HFP solution is 10~20wt%.

[0073] Preferably, in the PVDF-HFP solution, the solvent is N-methylpyrrolidone (NMP).

[0074] Preferably, drying refers to drying at 60-80°C for 32-48 hours.

[0075] This invention does not impose any particular restrictions on the type of conductive agent, as long as it can achieve the purpose of this application; preferably, the conductive agent is conductive carbon black.

[0076] Preparation method of positive electrode sheet Furthermore, the present invention also provides a method for preparing the positive electrode sheet as described above, which includes the following steps: S1, Preparation of ternary materials: S1a, a mixture containing aluminum, cobalt, fluorine, and lithium sources is mixed with NCM particles, and evaporated to obtain a precursor; the precursor is then subjected to a first calcination treatment to form a surface coated with Li(Al) x Co 1-x )O 2-δ F δ The ternary material; wherein the molar amounts of the aluminum source, cobalt source, fluorine source, and lithium source satisfy: Al³ + Co² + :F - Li + =0.01~0.025: 0.01~0.025: 0.004~0.05: 0.002~0.005; S1b, the ternary material obtained in step S1a is mixed with Li1.3Al0.3Ti1.7(PO4)3 sol, and after centrifugation, a ternary material coated with Li1.3Al0.3Ti1.7(PO4)3 gel is obtained; the ternary material coated with Li1.3Al0.3Ti1.7(PO4)3 gel is dried and subjected to a second calcination treatment to obtain a double-layer coated ternary material; S2, Preparation of lithium iron phosphate materials: The carbon source is mixed with LFP particles, and after drying and a third calcination treatment, lithium iron phosphate material with a carbon conductive layer on the surface is obtained. S3, Preparation of positive electrode sheet: The ternary material obtained in step S1b, the lithium iron phosphate material obtained in step S2b, the conductive agent and the binder are mixed in a solvent to obtain a positive electrode slurry; The positive electrode slurry is coated onto the positive electrode current collector substrate, and after drying and pressing, a positive electrode sheet is obtained.

[0077] By generating Li(Al) coating on the surface of NCM core through in-situ reaction x Co 1-x )O 2-δ F δ Ternary materials (rather than mechanically added Li(Al)) x Co 1-x )O 2-δ F δ This allows the first coating layer to "adapt" to the lattice of the NCM core, thereby achieving better lattice matching.

[0078] In an embodiment of the present invention, in step S1a, the first calcination treatment is a two-stage calcination treatment performed in an oxygen atmosphere; wherein, the first stage of calcination treatment refers to heating to 250-450°C at a rate of 2-5°C / min and holding at that temperature for 1-4 hours; the second stage of calcination treatment refers to heating to 600-800°C at a rate of 2-5°C / min and holding at that temperature for 6-12 hours. The first stage of calcination treatment can completely decompose the aluminum source and cobalt source (especially aluminum nitrate and cobalt acetate); the second stage of calcination treatment can promote the decomposition of Al³⁺. + / Co² + / F - It undergoes a solid-state reaction with the NCM surface to form stable, lattice-matched Li(Al) x Co 1-x )O 2-δ F δThe first stage of calcination ensures the "clean and uniform" decomposition of the precursor. After decomposition at a relatively low temperature, the generated metal oxides can spread evenly on the NCM surface in a more "peaceful" manner, forming a thin and dense precursor film. This provides an ideal and uniform raw material layer for the second stage of solid-phase reaction. The second stage of calcination promotes ion diffusion and the formation of a stable phase, which is also a favorable condition for repairing cation mixing defects that may exist in the synthesis of NCM materials and improving their crystallinity. In an oxygen atmosphere, it also helps to keep transition metals (especially cobalt and nickel) in a high valence state and stabilize the material structure.

[0079] Preferably, the aluminum source is at least one of aluminum nitrate, aluminum chloride, and aluminum sulfate.

[0080] Preferably, the cobalt source is at least one of cobalt acetate, cobalt chloride, and cobalt nitrate.

[0081] Preferably, the fluorine source is at least one of ammonium fluoride, sodium fluoride, and potassium fluoride.

[0082] Preferably, the lithium source is at least one of lithium hydroxide, lithium nitrate, lithium carbonate, lithium dihydrogen phosphate, and lithium chloride.

[0083] Preferably, the molar ratio of lithium source to NCM particles satisfies: Li + : NCM monomer = 0.002~0.005: 1.

[0084] Preferably, in step S1b, the second calcination treatment specifically involves heating to 550-800°C at a rate of 2-5°C / min under an argon atmosphere and holding at that temperature for 2-8 hours.

[0085] In an embodiment of the present invention, in step S1b, the Li1.3Al0.3Ti1.7(PO4)3 sol is prepared by the following steps: lithium source, aluminum source, and phosphorus source are mixed in a solvent to obtain a mixture; under a strong stirring in an ice-water bath at 2000-3000 rpm, a titanium source is slowly added dropwise to the mixture, and the mixture is stirred at 1000-2000 rpm for 8-16 hours to obtain the Li1.3Al0.3Ti1.7(PO4)3 sol. A transparent and stable LATP sol can be obtained through the above steps.

[0086] Preferably, the lithium source includes at least one of lithium nitrate (LiNO3), lithium chloride (LiCl), and lithium carbonate (Li2CO3).

[0087] Preferably, the aluminum source includes at least one of aluminum isopropoxide (Al(C3H7O)3), aluminum chloride (AlCl3), and aluminum nitrate (Al(NO3)3).

[0088] Preferably, the titanium source includes at least one of tetrabutyl titanate (Ti(O C4H9)4), titanium chloride (TiCl4), and bis(acetylacetonyl)diisopropyl titanate (Ti(acac)2(OiPr)2).

[0089] Preferably, the phosphorus source includes at least one of triethyl phosphate ((C2H5O)3PO), ammonium dihydrogen phosphate (NH4H2PO4), sodium dihydrogen phosphate (NaH2PO4), and potassium dihydrogen phosphate (KH2PO4).

[0090] Preferably, the molar amounts of lithium source, aluminum source, titanium source, and phosphorus source satisfy the following: Li + Al³ + Ti 4+ P = (1.3~1.8):(0.3~0.8):(1.7~2.2):(3~3.5), more preferably 1.3:0.3:1.7:3.

[0091] Preferably, the solvent is a mixture of aqueous ethanol and ethylene glycol methyl ether. More preferably, the volume ratio of aqueous ethanol to ethylene glycol methyl ether is 1:1.

[0092] Preferably, in step S2, the third calcination specifically involves heating to 400-850°C at a rate of 2-5°C / min under a weakly reducing atmosphere and holding at that temperature for 4-8 hours. Further, the weakly reducing atmosphere is composed of a mixture of argon and hydrogen in a volume ratio of 95:5.

[0093] Preferably, in step S2, the carbon source is glucose, and the mass ratio of the carbon source to LFP particles is 3-7:100.

[0094] Preferably, in step S2, FeP4O is further formed between the carbon conductive layer and the LFP particles. 11 Transition layer. Carbon source (such as glucose (C6H4O)). 12 O6 decomposes during heating, producing reducing gases. These gases, combined with the external weakly reducing atmosphere (95% Ar / 5% H2), create a locally strong reducing microenvironment around the LFP particles. Within this reducing microenvironment, the Fe³⁺ layer on the outermost surface of the LFP (LiFePO₄) particles... + Partially reduced to Fe² + Simultaneously, a small amount of lithium is released and phosphorus-oxygen tetrahedra (PO4³⁺) are formed. - Local reconstruction of the structure, phosphorus (P) released from the LFP lattice combines with oxygen and accumulates on the surface; the surface-enriched Fe²⁺ + It reacts with phosphorus oxides (such as P2O5 or PO4 units) at the interface, and epitaxially grows to form a thin and continuous layer of FeP4O. 11 Compounds.

[0095] In an embodiment of the present invention, step S2 further includes: depositing and growing lithium iron phosphate on the surface of a lithium iron phosphate material coated with a carbon conductive layer, followed by annealing treatment, to obtain a lithium iron phosphate material coated with a carbon conductive layer and a lithium iron phosphate buffer layer.

[0096] Preferably, atomic layer deposition is performed in an inert atmosphere at 50-300°C for 30-60 minutes.

[0097] Preferably, the post-annealing treatment specifically involves holding the temperature at 200-500°C for 1-3 hours in an inert atmosphere. Further, the inert atmosphere includes at least one of nitrogen, argon, and helium.

[0098] Preferably, in step S3, the mass ratio of ternary material to lithium iron phosphate material is 8:2 to 2:8.

[0099] Lithium-ion batteries In another aspect, the present invention also provides a lithium-ion battery comprising a positive electrode sheet as described above; and / or a positive electrode sheet prepared according to the preparation method described above.

[0100] In embodiments of the present invention, the lithium-ion battery further includes a negative electrode, an electrolyte, and a separator. The separator is disposed between the positive and negative electrodes, and the electrolyte fills the pores of the separator and wets both the positive and negative electrodes. During battery charging and discharging, active ions repeatedly insert and extract between the positive and negative electrodes; the electrolyte acts as a conductor between the positive and negative electrodes; and the separator, disposed between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through. The present invention does not impose any particular limitations on the negative electrode, electrolyte, or separator, as long as the objective of this application is achieved.

[0101] The present invention will be further illustrated below with reference to the embodiments: Example 1: The method for preparing a lithium-ion battery in this embodiment includes the following steps: 1. Preparation of positive electrode sheet (1) Preparation process of NCM modified materials: First layer: Li(Al0.5Co0.5)O 1.8 F 0.2Inner layer (15 nm): Weigh 200 ml of a mixed solvent of anhydrous ethanol and deionized water (volume ratio 4:1). Dissolve aluminum nitrate Al(NO3)3·9H2O, cobalt acetate Co(CH3COO)2·4H2O (molar amount 0.015 each) and NH4F (molar amount 0.03) in the mixed solvent and stir magnetically for 30 minutes until completely dissolved. Add lithium hydroxide (LiOH) or lithium nitrate (LiNO3) (molar amount 0.003) to the above solution to provide lithium for the reaction, and stir magnetically for 30 minutes. Weigh 100 g of NCM powder and slowly add it to the above solution, stirring continuously in a 60°C water bath to ensure the solution uniformly adheres to the particle surface. Remove the solvent using a rotary evaporator at 60°C and -0.1 MPa to obtain the precursor mixed powder. The precursor powder was subjected to a two-stage annealing process in a tube furnace under an oxygen atmosphere (flow rate: 100 sccm): Stage 1: Heating to 300°C at 2°C / min and holding for 2 hours to completely decompose nitrates and acetates; Stage 2: Heating to 700°C at 5°C / min and holding for 10 hours to promote Al³⁺ annealing. + / Co² + / F - It undergoes a solid-state reaction with the NCM surface to form a stable, lattice-matched Li(Al0.5Co0.5)O 1.8 F 0.2 The layers were allowed to cool naturally to room temperature, then removed, ground, and sieved to form an NCM core-Li(Al0.5Co0.5)O. 1.8 F 0.2 Core-shell materials, that is, materials with Li(Al0.5Co0.5)O coated on the surface. 1.8 F 0.2 Ternary materials.

[0102] Second layer: Li1.3Al0.3Ti1.7(PO4)3 (LATP) fast ion conductor intermediate layer (10nm): Lithium nitrate LiNO3, aluminum isopropoxide Al(C3H7O)3, tetrabutyl titanate Ti(OC4H9)4, and triethyl phosphate (C2H5O)3PO (molar ratio 1.3:0.3:1.7:3) were dissolved in 200ml of a mixture of water, ethanol, and ethylene glycol methyl ether (volume ratio 1:1). The titanium source was slowly added dropwise to the mixture under ice-water bath and vigorous stirring to avoid rapid hydrolysis of the titanium source. Stirring was continued for 12 hours to form a transparent and stable LATP sol. The NCM powder coated with the first layer (i.e., with Li(Al0.5Co0.5)O) was then added. 1.8 F 0.2The ternary material was added to a sol, ultrasonically dispersed for 30 minutes, and then mechanically stirred for 6 hours to ensure uniform dispersion and adsorption onto the particle surface. The gel-coated powder was obtained by centrifugation and dried in a vacuum oven at 60°C for 12 hours. The dried powder was then heated to 650°C at a rate of 3°C / min in a tube furnace under argon atmosphere and held for 6 hours. After natural cooling, the powder was removed and ground to obtain a double-layer-coated ternary material (i.e., NCM-modified material). (2) Preparation process of LFP modified materials: Step 1: Constructing a carbon-rich conductive layer and FeP4O 11 Phase (3nm): Glucose was completely dissolved in deionized water. LFP powder was added, and the mixture was stirred and sonicated at 60°C to form a homogeneous slurry (glucose to LFP mass ratio of 5:100). The slurry was dried using a spray dryer to obtain the precursor composite powder. The precursor powder was placed in a tube furnace and heat-treated in an argon / hydrogen mixed atmosphere (Ar / H2 = 95:5, flow rate: 200 sccm). The temperature was increased to 650°C at 5°C / min and held for 6 hours. After the treatment, the mixture was cooled to room temperature under a protective atmosphere to obtain a surface with a carbon conductive layer and FeP4O. 11 LFP@C composite material of phase.

[0103] Step 2: Constructing the Lithium Iron Phosphate Buffer Layer (2nm): The LFP@C composite material was placed in an ALD fluidized bed reactor, and the reactor temperature was set to 200°C. A stable inert carrier gas (N2 / Ar) was introduced into the bottom of the reactor. The gas flow rate caused the powder below to be blown up by the gas, and it tumbled violently and uniformly throughout the reaction chamber, achieving a "fluidized state". Deposition was carried out for 40 minutes. After deposition, it was post-annealed at 300°C for two hours under an inert atmosphere to obtain lithium iron phosphate material (i.e., LFP modified material) with a carbon conductive layer and a lithium iron phosphate buffer layer on the surface.

[0104] (3) Fabrication process of interface bridging design: In a nitrogen-filled three-necked flask, PVDF-HFP was dissolved in NMP (15 wt%), heated to 80°C, and stirred. Vinylphosphonic acid monomer (molar ratio to PVDF-HFP: 1:15) was added. The reaction was carried out at 80°C for 20 hours to graft the phosphorus-containing monomer onto the PVDF-HFP backbone. After the reaction was complete, the solution was cooled to room temperature, and the precipitate was repeatedly washed with deionized water to remove unreacted monomers and homopolymers. The final product was dried in a vacuum oven at 60°C for 36 hours to obtain PVDF-HFP-g-functional binder (i.e., PVDF-HFP with phosphorus-containing functional groups (—PO3H2) grafted onto the backbone).

[0105] (4) Sheet preparation: The positive electrode active material (including LFP modified material and NCM modified material), conductive agent (conductive carbon black) and binder (PVDF-HFP-g-functional binder) are mixed at a mass ratio of 96 (the mass ratio of LFP modified material to ternary modified material is 7:3):2:2 to obtain a mixed material. The mixed material is thoroughly stirred at high speed in NMP to obtain a positive electrode slurry. The positive electrode slurry is coated on conventional aluminum foil, and after drying and pressing, a positive electrode sheet is obtained.

[0106] 2. Preparation of negative electrode sheet: The negative electrode active material (graphite), conductive agent (conductive carbon black) and binder (CMC) are mixed in a mass ratio of 90:3:7 to obtain a mixed material. The mixed material is thoroughly stirred at high speed in deionized water to obtain a negative electrode slurry. The negative electrode slurry is coated on conventional copper foil, and after drying and pressing, a negative electrode sheet is obtained.

[0107] 3. Separating membrane: PP porous film is used as the separating membrane.

[0108] 4. Electrolyte: Ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1. Then, fully dried lithium salt LiPF6 is dissolved in the mixed organic solvent at a ratio of 1 mol / L to prepare the electrolyte. 5. Preparation of Lithium-ion Batteries: After weighing, the electrode sheets were directly used as the negative electrodes of lithium-ion batteries. 2032 coin cells were assembled in an Ar-filled glove box. The assembly sequence from top to bottom was: negative electrode shell, gasket, spacer, lithium sheet, separator, self-supporting electrode sheet, and positive electrode shell. The resulting coin cells were further tested on a charge-discharge tester. The cycle stability of the synthesized composite negative electrode material was tested for 300 cycles at a current density of 100 mA / g.

[0109] Example 2-3 The temperatures of the tube furnace in the second layer of the NCM modified material preparation process were set to 550℃ and 800℃ respectively, and the remaining steps were the same as in Example 1, resulting in Example 2 and Example 3.

[0110] Examples 4-5 In the first step of the preparation process of LFP modified materials, the mass ratio of glucose to LFP was set to 3:100 and 7:100, respectively. The remaining steps were the same as in Example 1, resulting in Examples 4 and 5. In these examples, the carbon-rich conductive layer and FeP4O... 11 The phase thicknesses are 5 nm and 8 nm, respectively.

[0111] Examples 6-7 The deposition time in the second step of the LFP modified material preparation process was set to 30 min and 60 min respectively, and the remaining steps were the same as in Example 1, resulting in Example 6 and Example 7.

[0112] Examples 8-9 In the preparation process of the interface bridging design, the molar ratio of vinylphosphonic acid monomer to PVDF-HFP was set to 1:10 and 1:20, respectively, and the remaining steps were the same as in Example 1, resulting in Examples 8 and 9.

[0113] Examples 10-11 The mass ratios of the positive electrode active material (including LFP modified material and NCM modified material), conductive agent (conductive carbon black), and binder (PVDF-HFP-g-functional binder) in the film preparation were set to 93 (the mass ratio of LFP modified material to ternary modified material is 7:3):5:2) and 97 (the mass ratio of LFP modified material to ternary modified material is 7:3):2:1), respectively. The remaining steps were the same as in Example 1, resulting in Examples 10 and 11.

[0114] Examples 12-13 The mass ratio of LFP modified material to ternary modified material in the preparation was set to 8:2 and 2:8, respectively. The remaining steps were the same as in Example 1, resulting in Examples 12 and 13.

[0115] Example 14 The second layer of the preparation process of NCM modified material is omitted, and the remaining steps are the same as in Example 1.

[0116] Example 15 The second step in the preparation process of LFP modified materials is omitted, and the remaining steps are the same as in Example 1.

[0117] Example 16 The LFP material was not modified, that is, the LFP modified material preparation process in step (2) was not included; and in step (4) of the film preparation, the LFP modified material was replaced with the unmodified LFP material; the remaining steps were the same as in Example 1.

[0118] Example 17 The interface bridging design was not included, that is, the preparation process of the interface bridging design in step (3) was not included, and in step (4) the adhesive was replaced with PVDF-HFP; the remaining steps were the same as in Example 1.

[0119] Comparative Example 1 The NCM was not modified, that is, the preparation process of NCM modified material in step (1) was not included, and in step (4) of the film preparation, the NCM modified material was replaced with unmodified NCM material; the remaining steps were the same as in Example 1.

[0120] Comparative Example 2 No modification was performed on NCM and LFP, that is, the preparation process of NCM modified material in step (1) and LFP modified material in step (2) were not included. In step (4) of film preparation, NCM modified material was replaced with unmodified NCM material and LFP modified material was replaced with unmodified LFP material; the remaining steps were the same as in Example 1.

[0121] Comparative Example 3 No NCM and LFP were modified, and no interface bridging design was performed. That is, the preparation process of NCM modified material in step (1), the preparation process of LFP modified material in step (2), and the preparation process of interface bridging design in step (3) were not included. In step (4) of film preparation, the NCM modified material was replaced with unmodified NCM material, and the binder was replaced with PVDF-HFP. The remaining steps were the same as in Example 1.

[0122] Test example: The lithium-ion batteries of each embodiment and comparative example were tested according to the following methods, and the test results are shown in Table 1: (1) Internal resistance test: The internal resistance of different groups of batteries is obtained by using a battery internal resistance meter.

[0123] (2) Battery cycle performance test: At 25℃, the battery was charged at a constant current of 0.33C to a voltage of 3.85V, and then charged at a constant voltage to a current of 0.05C. After standing for 5 minutes, the battery was discharged at a constant current of 0.33C to 2.5V, and the discharge capacity at this time was recorded, which is the discharge capacity C0 of the first cycle. The cycle test was performed according to this procedure, and the capacity at the 300th cycle was recorded. It is denoted as C, and the capacity retention rate after 300 cycles is obtained by using the formula C / C0*100%.

[0124] (3) EIS test: The battery was tested using an electrochemical workstation. Test frequency: 0.01HZ-100KHZ; voltage perturbation: 5mV, and the charge transfer resistance value Rct was obtained.

[0125] (4) Peel force test between active material coating and current collector: core equipment – ​​tensile testing machine. Sample preparation: the sample width is 25mm and the length is at least 200mm. At least 3 strips are tested for each material (i.e., the negative electrode sheets prepared in each embodiment and comparative example). One end of the sample is attached to the standard test plate (stainless steel plate) and rolled with a roller to ensure adhesion. The other end of the sample needs to be folded and fixed on the fixture to ensure that the peel line is in the position of minimum force.

[0126] Table 1 Performance test results of each embodiment and comparative example

[0127] As can be seen from the test results of Examples 1-17 and Comparative Examples 1-3 in Table 1, compared with Comparative Examples 1-3, the modified NCM and LFP in Examples 1-15, using the modified NCM and LFP as positive electrode active materials and combining them with PVDF-HFP with phosphorus-containing functional groups (-PO3H2) as a binder, resulted in a lower internal resistance and improved capacity retention in the prepared lithium-ion batteries. The main reasons are: firstly, the good interfacial compatibility between the modified NCM and LFP reduces the interfacial impedance, improves interfacial stability, and significantly reduces charge transfer impedance; secondly, the phosphorus-containing functional groups (-PO3H2) and the Al³⁺ on the surface of the modified NCM... + Coordination bonds are formed, and van der Waals forces are generated between the HFP segments and the carbon layer on the modified LFP surface. Phosphorus groups interact with the Al³⁺ on the NCM surface. + Coordination bonds are formed, and van der Waals forces are generated between the HFP chain segments and the carbon layer on the LFP surface to form an interface bridge, which can achieve a combination of high rate and high capacity, and can also reduce internal resistance and improve capacity retention.

[0128] As shown in Examples 1, 16, and Comparative Example 2, modification of NCM reduces the internal resistance and improves the capacity retention of lithium-ion batteries. This is mainly because the ternary material is coated with Li(Al0.5Co0.5)O2. 1.8 F 0.2 , which is Li x The CoO2-type spinel-like phase layer has three-dimensional lithium-ion channels and maintains lattice matching with the lithium nickel cobalt manganese oxide core, which improves the ionic conductivity of the ternary material, reduces the interfacial impedance, and enhances the interfacial stability of the ternary material.

[0129] Example 1 not only modified NCM but also LFP. Compared with Example 16, the internal resistance of the lithium-ion battery in Example 1 was further reduced and the capacity retention rate was further improved. This is mainly because: a carbon conductive layer is provided on the surface of LFP. The conductive layer has high electronic conductivity and maintains lithium-ion permeability. The setting of the conductive layer can solve the kinetic limitation problem. A lithium iron phosphate buffer layer is further provided on the surface of the carbon conductive layer. The lithium iron phosphate buffer layer directly reduces the insertion / extraction barrier of lithium ions on the surface of LFP material and improves rate performance. At the same time, it matches the LFP core lattice and reduces interfacial strain.

[0130] Comparing Examples 1, 14, and 15, it can be seen that the Rct value of Example 1 is smaller. This is mainly because Example 1 adds a fast ion conductor layer compared to Example 14, and adds a lithium iron phosphate buffer layer compared to Example 15. The fast ion conductor layer and the lithium iron phosphate buffer layer can improve the electron / ion transport effect of the cathode material, effectively coordinate the kinetic differences between NCM and LFP, and reduce the additional interface impedance caused by transport mismatch.

[0131] Comparing Examples 1 and 17, it can be seen that compared to ordinary PVDF-HFP, when using PVDF-HFP with phosphorus-containing functional groups (-PO3H2) as the binder, the peel force between the active material coating and the current collector (copper foil) is significantly improved. This indicates that using PVDF-HFP with phosphorus-containing functional groups (-PO3H2) as the binder can significantly improve the bonding strength between the active material coating containing modified NCM and modified LFP on the positive electrode and the current collector. Simultaneously, using PVDF-HFP with phosphorus-containing functional groups (-PO3H2) as the binder can also reduce internal resistance and improve capacity retention.

[0132] The technical means disclosed in this application are not limited to those disclosed in the above embodiments, but also include technical solutions composed of any combination of the above technical features. It should be noted that those skilled in the art can make several improvements and modifications without departing from the principles of this application, and these improvements and modifications are also considered to be within the scope of protection of this application.

Claims

1. A positive electrode active material, characterized in that: The positive electrode active material includes a ternary material, which comprises a lithium nickel cobalt manganese oxide core and a first coating layer coated on the lithium nickel cobalt manganese oxide core. The first coating layer has the chemical formula Li(Al) x Co 1-x )O 2-δ F δ， in, 0.1 ≤ x ≤ 0.6, 0.05 ≤ δ ≤ 0.

2.

2. The positive electrode active material as described in claim 1, characterized in that: The positive electrode active material also includes a fast ion conductor layer coated on the surface of the first coating layer.

3. The positive electrode active material as described in claim 2, characterized in that: At least one of the following conditions must be met: (1) The material of the fast ion conductor layer is Li1.3Al0.3Ti1.7(PO4)3; (2) The thickness of the first coating layer is 10-20 nm; (3) The thickness of the fast ion conductor layer is 5-15 nm.

4. The positive electrode active material as described in claim 1, characterized in that: The positive electrode active material also includes lithium iron phosphate material, which includes a lithium iron phosphate core and a conductive layer coated on the lithium iron phosphate core, wherein the electronic conductivity of the conductive layer is greater than 1 S / cm.

5. The positive electrode active material as described in claim 4, characterized in that: The conductive layer is made of carbon or poly(3,4-ethylenedioxythiophene-polystyrene sulfonate); and / or... The thickness of the conductive layer is 2-5 nm.

6. A positive electrode plate, characterized in that: The positive electrode sheet includes: Positive current collector substrate; and Positive electrode material disposed on the surface of the positive electrode current collector substrate; The positive electrode material includes a conductive agent, a binder, and the positive electrode active material as described in claim 4 or 5; The adhesive is a polyvinylidene fluoride-hexafluoropropylene copolymer, and the main link is a phosphorus-containing functional group -PO3H2; The weight ratio of the positive electrode active material, conductive agent, and binder is positive electrode active material: conductive agent: binder = 93~97: 2~5: 1~2.

7. A method for preparing a positive electrode sheet as described in claim 6, characterized in that, Includes the following steps: S1, Preparation of ternary materials: S1a, a mixture containing aluminum, cobalt, fluorine, and lithium sources is mixed with NCM particles, and evaporated to obtain a precursor; the precursor is then subjected to a first calcination treatment to form a surface coated with Li(Al) x Co 1-x )O 2-δ F δ The ternary material; wherein the molar amounts of the aluminum source, cobalt source, fluorine source, and lithium source satisfy: Al³ + Co² + :F - Li + =0.01~0.025: 0.01~0.025: 0.004~0.05: 0.002~0.005; S1b, the ternary material obtained in step S1a is mixed with Li1.3Al0.3Ti1.7(PO4)3 sol, and after centrifugation, a ternary material coated with Li1.3Al0.3Ti1.7(PO4)3 gel is obtained; the ternary material coated with Li1.3Al0.3Ti1.7(PO4)3 gel is dried and subjected to a second calcination treatment to obtain a double-layer coated ternary material; S2, Preparation of lithium iron phosphate material: The carbon source is mixed with LFP particles, dried and calcined for the third time to obtain lithium iron phosphate material with a carbon conductive layer on the surface. S3, Preparation of positive electrode sheet: The ternary material obtained in step S1b, the lithium iron phosphate material obtained in step S2, the conductive agent and the binder are mixed in a solvent to obtain a positive electrode slurry; The positive electrode slurry is coated onto the positive electrode current collector substrate, and after drying and pressing, a positive electrode sheet is obtained.

8. The preparation method according to claim 7, characterized in that: In step S1a, the first calcination treatment is a two-stage calcination treatment carried out in an oxygen atmosphere; wherein, the first stage of calcination treatment refers to heating to 250-450°C at a rate of 2-5°C / min and holding at that temperature for 1-4 hours; the second stage of calcination treatment refers to heating to 600-800°C at a rate of 2-5°C / min and holding at that temperature for 6-12 hours. The aluminum source is at least one of aluminum nitrate, aluminum chloride, and aluminum sulfate; The cobalt source is at least one of cobalt acetate, cobalt chloride, and cobalt nitrate. The fluorine source is at least one of ammonium fluoride, sodium fluoride, and potassium fluoride; The lithium source is at least one of lithium hydroxide, lithium nitrate, lithium carbonate, lithium dihydrogen phosphate, and lithium chloride. In step S1b, the second calcination treatment specifically involves heating to 550-800°C at a rate of 2-5°C / min under an argon atmosphere and holding at that temperature for 2-8 hours.

9. The preparation method according to claim 7, characterized in that: In step S2, the third calcination specifically involves: heating to 400-850°C at a rate of 2-5°C / min under a weakly reducing atmosphere, and holding at that temperature for 4-8 hours. In step S3, the mass ratio of the ternary material to the lithium iron phosphate material is 8:2 to 2:

8.

10. A lithium-ion battery, characterized in that, Includes the positive electrode sheet as described in claim 6; and / or, the positive electrode sheet prepared by the preparation method according to any one of claims 7 to 9.

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