A lithium ion battery positive electrode sheet, a preparation method thereof and a lithium ion battery

Abstract

This invention relates to a lithium-ion battery positive electrode sheet, its preparation method, and a lithium-ion battery. The preparation method of the positive electrode sheet includes the following steps: immersing a lithium-ion battery positive electrode sheet blank in an organic solution of a silane coupling agent; after reaction, washing and drying to obtain a modified lithium-ion battery positive electrode sheet blank; then preparing a COF layer on the surface of the positive active material layer of the modified lithium-ion battery positive electrode sheet blank using a vapor phase epitaxy method to obtain a lithium-ion battery positive electrode sheet blank with a COF layer; and then subjecting the COF-loaded lithium-ion battery positive electrode sheet blank to vapor phase sulfonation modification to obtain the lithium-ion battery positive electrode sheet. This invention can solve the problems of transition metal dissolution, capacity decay, and limited rate performance caused by side reactions at the positive electrode / electrolyte interface under high voltage (>4.5 V).

Description

Technical Field

[0001] The present invention relates to a positive electrode sheet of a lithium-ion battery, a preparation method thereof, and a lithium-ion battery, and particularly relates to a high-voltage lithium-ion battery positive electrode sheet based on a gas-phase epitaxial growth COF molecular sieve interface, a preparation method thereof, and a lithium-ion battery, belonging to the field of lithium-ion batteries. Background Art

[0002] In existing high-nickel ternary cathode materials (such as LiNi x Co y Mn 1-x-y O2, where 0 < x < 1, 0 < y < 1, and x + y < 1), there are interface failure problems under high-voltage conditions. With the increasing demand for energy density, high-nickel ternary cathode materials have become the mainstream choice due to their high specific capacity. However, during voltage cycling above 4.5 V, side reactions at the cathode / electrolyte interface can cause the dissolution of transition metal ions (such as Ni 2+ 、Co 3+ ), crystal structure deterioration, and capacity attenuation, severely restricting the cycle life and rate performance of the battery. As a quaternary cathode material, lithium nickel cobalt manganese aluminate (NCMA) suppresses the H2→H3 phase transition of high-nickel materials (such as Ni content ≥ 80%) through Al doping, reducing the probability of microcrack generation. Its typical composition is LiNi 0.89 Co 0.05 Mn 0.05 Al 0.01 O2, and the capacity retention rate can be increased by 12% compared to NCM811 after 100 cycles at 4.4V.

[0003] Currently, the surface modification technology of cathode materials mainly suppresses side reactions at the interface through a coating layer. For example, although a polymer coating (such as PVDF) can physically isolate the electrolyte, its pore size distribution is wide (usually > 5 nm), making it difficult to accurately screen out desolvated lithium ions (hydrated radius about 0.38 nm) and multivalent metal ions (such as Ni 2+ with a hydrated radius of about 0.69 nm), and its mechanical stability is insufficient to effectively resist the volume expansion of the electrode under high voltage. On this basis, the developed inorganic coating materials (such as Al2O3, Li3PO4) can improve the interface stability, but will significantly increase the interface impedance (> 50 Ω·cm 2 ), resulting in a decline in rate performance (5C capacity is usually < 120 mAh / g), and the high-temperature sintering process (> 400°C) is likely to damage the structure of the cathode body.

[0004] In recent years, metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) have been applied to cathode coatings due to their tunable pore characteristics. For example, Chinese invention patent application CN117229525A discloses a coating material for lithium-ion battery cathodes, its preparation method, and its application. Its Ni / Co-MOF / COF hybrid technology requires liquid-phase synthesis combined with high-temperature sintering to form the coating layer. However, this technology still has significant limitations: solvent residues (such as DMF and DMSO) in the liquid-phase process easily clog the pores, reducing the lithium-ion transport number; the high-temperature sintering process may cause the COF framework to collapse, leading to deterioration of pore size uniformity; simultaneously, this approach does not address the core contradictions of high lithium-ion desolvation energy barriers and insufficient dynamic sieving of multivalent ions.

[0005] With the development of solid-state battery technology, the cathode / solid electrolyte interface faces challenges such as high interfacial impedance and lithium dendrite penetration. The difference in thermal expansion coefficients between traditional solid-state electrolytes (such as LLZO and LATP) and high-nickel cathodes (ΔCTE > 5 × 10⁻⁶) is also a significant issue. -6 / K) leads to interface stripping during cycling, and rigid interfaces cannot suppress lithium dendrite penetration. Existing solutions often use polymer buffer layers (such as PEO), but their ionic conductivity is low (<10). -4 Its voltage window is narrow (<4.0V) and it is difficult to adapt to high-voltage positive electrodes.

[0006] Therefore, there is an urgent need to develop a cathode interface engineering technology that combines sub-nanometer pore size precision, high interface stability, and industrial feasibility, so as to ensure efficient lithium-ion transport, achieve dynamic blocking of multivalent metal ions, and avoid the risks of liquid phase corrosion and high-temperature damage. Summary of the Invention

[0007] In view of the shortcomings of the prior art, one of the objectives of this invention is to provide a lithium-ion battery positive electrode sheet with superior electrochemical performance (such as cycle performance) and its preparation method; another objective of this invention is to provide a lithium-ion battery.

[0008] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows:

[0009] A method for preparing a positive electrode sheet for a lithium-ion battery includes the following steps:

[0010] S1. Provides a rough blank for the positive electrode sheet of a lithium-ion battery;

[0011] The lithium-ion battery positive electrode blank comprises a current collector and a positive electrode active material layer stacked sequentially; the positive electrode active material in the positive electrode active material layer includes lithium nickel cobalt manganese oxide (its chemical formula may be LiNi). x Co y Mn 1-x-yO2, where 0 < x < 1, 0 < y < 1, and x + y < 1), lithium nickel cobalt aluminate (its chemical formula can be selected as LiNi x Co y Al z O2, where 0 < x < 1, 0 < y < 1, 0 < z < 1, and x + y + z = 1), lithium nickel cobalt manganese aluminate (its chemical formula can be selected as LiNi x Co y Mn z Al w O2, where 0 < x < 1, 0 < y < 1, 0 < z < 1, 0 < w < 0.1, and x + y + z + w = 1); or one or more of the above;

[0012] S2. Immerse the rough blank of the lithium-ion battery positive electrode plate in an organic solution of a silane coupling agent, react, wash, and dry to obtain a modified rough blank of the lithium-ion battery positive electrode plate;

[0013] S3. Prepare a COF layer on the surface of the positive electrode active material layer of the modified rough blank of the lithium-ion battery positive electrode plate by vapor phase epitaxial growth method (VPE) to obtain a rough blank of the lithium-ion battery positive electrode plate loaded with a COF layer;

[0014] S4. Perform gas phase sulfonation modification on the rough blank of the lithium-ion battery positive electrode plate loaded with a COF layer to obtain a lithium-ion battery positive electrode plate.

[0015] Thus, first immerse the rough blank of the lithium-ion battery positive electrode plate in an organic solution of a silane coupling agent to form amino anchor sites on the surface of the positive electrode active material layer, activate the substrate, and lay a good foundation for subsequent preparation of a COF layer on the positive electrode active material layer; subsequently, prepare a COF layer (organic framework molecular sieve layer) on the surface of the positive electrode active material layer of the modified rough blank of the lithium-ion battery positive electrode plate by vapor phase epitaxial growth method, and a COF layer with a tightly bound interface can be obtained on the surface of the positive electrode active material layer; finally, perform gas phase sulfonation modification on the COF layer to obtain a lithium-ion battery positive electrode plate with excellent electrochemical performance.

[0016] Furthermore, in S1, the chemical formula of lithium nickel cobalt manganese aluminate is LiNi x Co y Mn 1-x-y O2, where 0.6 ≤ x < 1, 0 < y < 1, x + y < 1, preferably, 0.6 ≤ x ≤ 0.9; the chemical formula of lithium nickel cobalt aluminate is LiNi x Co y Al z O2, where 0.6 ≤ x < 1, 0 < y < 1, 0 < z < 1, x + y + z = 1, preferably, 0.6 ≤ x ≤ 0.9; the chemical formula of lithium nickel cobalt manganese aluminate is LiNi x Co y Mnz Al w O₂, 0.6 ≤ x < 1, 0 < y < 1, 0 < z < 1, 0 < w < 0.1, x + y + z + w = 1, preferably, 0.6 ≤ x ≤ 0.9.

[0017] Optionally, the lithium nickel cobalt manganate is a common doped lithium nickel cobalt manganate. Optionally, the lithium nickel cobalt aluminate is a common doped lithium nickel cobalt aluminate. Optionally, the lithium nickel cobalt manganese aluminate is a common doped lithium nickel cobalt manganese aluminate.

[0018] Optionally, the positive electrode active material is one or more of NCM811 and NCM622.

[0019] Further, in S2, the concentration of the silane coupling agent in the organic solution is 1 - 10 vol%, more preferably 2 - 8 vol%, and even more preferably 4 - 6 vol%; the silane coupling agent includes one or both of APTES and AEAPTMS; the solvent in the organic solution includes one or more of toluene, xylene, and cyclohexane.

[0020] Further, in S2, the rough blank of the lithium ion battery positive electrode plate is immersed in the organic solution of the silane coupling agent, and under the condition of continuously introducing an inert gas, after reacting at 70 - 90 °C for 1 - 3 h, it is washed and dried to obtain a modified rough blank of the lithium ion battery positive electrode plate; preferably, the flow rate of the inert gas is controlled to be 5 - 30 sccm, more preferably 10 - 25 sccm, and even more preferably 15 - 20 sccm, and the inert gas includes one or both of nitrogen and argon. Here, the continuous introduction of the inert gas can provide antioxidant protection, remove reaction by-products at the same time, promote the condensation reaction to proceed fully, significantly enhance the adhesion of the subsequently grown COF layer and induce vertical orientation. Preferably, it reacts at 75 - 85 °C for 1.5 - 2.5 h.

[0021] In S2, through the condensation reaction, uniform amino anchoring sites can be formed on the surface of the positive electrode active material layer. The XPS detection results show that the intensity of the N 1s peak increases to 2 - 5 times of the initial value. The introduction of the amino anchoring sites can effectively enhance the adhesion of the subsequently in-situ generated COF layer, and effectively reduce the possibility of the COF layer peeling off due to the volume expansion of the electrode during high-voltage cycling.

[0022] Further, in S3, when performing vapor phase epitaxial growth, the modified rough blank of the lithium ion battery positive electrode plate is placed in a reaction chamber with a vacuum degree ≤ 5 × 10 -2 Pa, and undergoes 40 - 60 cycles of deposition at 160 - 200 °C to obtain a rough blank of the lithium ion battery positive electrode plate loaded with a COF layer;

[0023] During each deposition cycle, the following steps are performed sequentially: First, aldehyde monomers are introduced into the reaction chamber using nitrogen as a carrier gas for 20-40 seconds (more preferably 25-35 seconds) to achieve surface adsorption. Preferably, the gas delivery rate is controlled at 5-15 sccm, more preferably 8-12 sccm. Then, inert gas is introduced into the reaction chamber and purged for 15-25 seconds (more preferably 18-22 seconds) to completely remove reaction byproducts. Preferably, the gas input rate is controlled at 30-60 sccm, more preferably 40-50 sccm. Finally, diamine monomers are introduced into the reaction chamber using nitrogen as a carrier gas for 30-50 seconds (more preferably 35-45 seconds) to carry out a condensation reaction to form a COF layer. Preferably, the gas delivery rate is controlled at 5-15 sccm, more preferably 8-12 sccm.

[0024] The aldehyde monomers include one or more of 1,3,5-tricarboxyloylphloroglucinol, 2,4,6-tricarboxymethyltriazine, 2,5-dihydroxyterephthalaldehyde, 2,5-dimethoxyterephthalaldehyde, 2,5-dichloroterephthalaldehyde, and 2,3,5,6-tetrafluoroterephthalaldehyde; the diamine monomers include one or more of p-phenylenediamine, benzidine, m-phenylenediamine, and o-phenylenediamine; and the inert gas is nitrogen and / or argon. The thickness of the COF layer grown in each deposition cycle is limited, but the thickness of the COF layer can be precisely controlled through multiple cycles. Therefore, controlling the appropriate reaction temperature helps suppress pore distortion caused by molecular thermal vibration, resulting in more regular pores and ensuring the optimal performance of the positive electrode. Performing vapor-phase epitaxial growth in a high-vacuum environment helps reduce vapor-phase collisions, ensuring orderly pore growth. Inert gas purging effectively and promptly removes byproducts (such as H2O), preventing pore hydrolysis and collapse. Selecting suitable monomers helps control the pore size of the COF layer, achieving target pore sizes (e.g., 1.7-1.9 nm), and facilitating the synergistic optimization of pore size sieving and ion selectivity.

[0025] Preferably, the single-cycle growth rate (preferably 0.3-0.5 nm / cycle) is monitored in real time by a quartz crystal microbalance during the deposition process, and in-situ ellipsometry (QCM) is used to ensure film thickness uniformity (CV value <10%).

[0026] Optionally, in S3, 45-55 cycles of deposition are performed at 170-190℃, and preferably, 48-52 cycles of deposition are performed at 175-185℃.

[0027] Further, in S4, the lithium-ion battery positive electrode blank with the COF layer is placed in a sulfonating agent atmosphere and subjected to gas-phase sulfonation modification to obtain a lithium-ion battery positive electrode; wherein the sulfonating agent is SO3 gas, chlorosulfonic acid vapor or SO3-dioxane complex, and the molar ratio of SO3 to dioxane in the SO3-dioxane complex is 1:1-3, and more specifically 1:1.5-2.5.

[0028] Furthermore, the lithium-ion battery positive electrode blank with the COF layer is placed in a sulfonating agent atmosphere of 0.05-0.15MPa (preferably 0.08-0.12MPa) and subjected to gas phase sulfonation modification at 140-160℃ (preferably 145-155℃) for 0.5-2h (preferably 0.8-1.5h, more preferably 0.9-1.2h).

[0029] Based on the difference in diffusion-reaction kinetics, after gas-phase sulfonation modification, a gradient distribution structure of sulfonic acid groups is spontaneously formed in the COF layer (the density of sulfonic acid groups on the surface of the COF layer reaches 1.8-2.2 mmol / g, and the density of sulfonic acid groups on the bottom surface (i.e., at the interface between the COF layer and the positive electrode active material layer) reaches 0.3-0.7 mmol / g), which can be verified by TOF-SIMS or X-ray photoelectron spectroscopy.

[0030] Optionally, in S4, the pore size of the COF layer in the lithium-ion positive electrode is 1.7-1.9 nm, further 1.75-1.85 nm, further 1.76-1.84 nm, further 1.77-1.82 nm, and further 1.78-1.8 nm.

[0031] Furthermore, the thickness of the COF layer is 14-24 nm, preferably 16-22 nm, and more preferably 18-20 nm.

[0032] Preferably, the pores of the COF layer are perpendicular to the surface of the positive electrode active material layer.

[0033] This invention utilizes vapor-phase epitaxy to prepare a COF layer on the surface of a positive electrode active material layer, achieving atomic-level in-situ precision film formation. Specifically, it constructs a COF molecular sieve layer with controllable thickness and vertically oriented pores on the surface of the positive electrode active material layer, facilitating sub-nanometer-scale ion sieving. Simultaneously, vapor-phase epitaxy avoids solvent damage and prevents pore blockage that may occur with solvent residues in liquid-phase methods. Furthermore, activating the substrate before growing the COF layer effectively improves interfacial compatibility, enabling the epitaxial growth to form a covalently bonded interface. This helps suppress interfacial delamination under high voltage, ensuring the normal performance of the electrode and extending the battery's service life.

[0034] By performing vapor-phase sulfonation modification on the COF layer to form a gradient sulfonic acid layer and controlling the effective pore size of the COF layer (1.8±0.1 nm), the synergistic effect of these two methods helps to further improve ion selectivity, increasing the lithium-ion transference number to over 0.85 (a significant improvement compared to the PVDF coating scheme), and also enhancing Ni²⁺... + The permeation rate is suppressed to below 5 ppm / cycle; simultaneously, the mechanical modulus of the COF layer, not less than 15 GPa, effectively suppresses electrode expansion. Combined with the zero solvent residue characteristic of the all-gas phase process, the assembled liquid lithium-ion battery can maintain a capacity retention of ≥90% after 500 cycles at a high voltage of 4.5V. The constructed COF layer has a channel structure that is perpendicular to the positive electrode active material layer, which helps to significantly reduce the lithium-ion transport barrier, achieving a 5C rate discharge capacity of over 160 mAh / g and a 5C / 0.1C capacity ratio of ≥85%. Furthermore, the positive electrode of this invention is adaptable to solid-state battery systems. The 15-25 GPa mechanical modulus of the COF layer can be well matched with the stiffness of solid electrolytes (such as lithium lanthanum zirconate (LLZO)), reducing the interface impedance to 28 Ω·cm. 2 The critical current density is then increased to 2.0 mA / cm². 2 The capacity retention rate is ≥89.5% after 300 cycles at 4.5V. This invention is applicable to solving the problems of transition metal dissolution, interfacial side reactions, and rate performance degradation under high voltage (≥4.5V) conditions. In this invention, the vapor deposition rate of S3 can reach 5 nm / min (10 times higher than the liquid phase method), the film thickness uniformity CV <8%, and no high-temperature sintering step is required, providing a positive electrode interface solution for high-voltage lithium-ion batteries that combines high performance and industrial feasibility. In addition, after obtaining the positive electrode blank using existing processes, this invention only requires further processing of the positive electrode blank, without changing the existing positive electrode manufacturing process. Only steps S2-4 need to be added, which can be more conveniently promoted and applied.

[0035] This invention relates to a high-voltage lithium-ion battery cathode electrode based on a COF molecular sieve interface grown by vapor phase epitaxy and its preparation method. This invention addresses problems such as transition metal dissolution, capacity decay, and limited rate performance caused by side reactions at the cathode / electrolyte interface under high voltage (>4.5 V). Traditional polymer coatings (such as PVDF) suffer from uncontrollable pore size and are prone to electrode corrosion due to liquid-phase modification, making it difficult to balance ion sieving and interface stability. This invention achieves synergistic optimization of sub-nanometer-scale ion selective transport and interface strengthening through innovative interface structure design and vapor-phase processing.

[0036] This invention is also applicable to the preparation of cathode plates of doped positive electrode active materials such as lithium nickel cobalt manganese aluminum oxide (NCMA). The sulfonated COF layer is expected to synergistically improve the performance of the cathode plate with the doped atoms.

[0037] Based on the same inventive concept, the present invention also provides: a positive electrode sheet for a lithium-ion battery, which is prepared by the preparation method described above.

[0038] Based on the same inventive concept, the present invention also provides: a lithium-ion battery, including the lithium-ion battery positive electrode sheet as described above.

[0039] The technical solution adopted in this invention can be adapted to both liquid battery and solid battery systems.

[0040] When the positive electrode sheet of the present invention is applied to a solid-state battery system, the solid electrolyte can be selected from sulfide systems (such as Li6PS5Cl), oxide systems (such as lithium lanthanum zirconate (LLZO)), or polymer-ceramic composite electrolytes (such as composite electrolytes of PVDF-HFP polymer and LLZTO). The mechanical modulus of the COF layer is about 15-25 GPa, which has plastic deformation capability and can buffer the stress concentration of the rigid interface of the solid electrolyte (such as the mechanical modulus of LLZO is about 150 GPa). Combined with the sulfonic acid groups that are distributed in a gradient in the COF layer, a single-channel lithium-ion transport path is constructed, which can effectively block the growth of lithium dendrites caused by electron leakage. In addition, after the three-layer structure of positive electrode sheet-solid electrolyte is stacked (positive active material layer + COF layer + solid electrolyte), the interface can be integrated by hot pressing at 80-120℃ and 10-30 MPa.

[0041] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0042] This invention utilizes a combination of vapor-phase epitaxial growth and gradient sulfonation processes to construct a COF molecular sieve layer with precise pore size and mechanical stability at the interface of the positive electrode active material layer, achieving multiple technological breakthroughs:

[0043] (1) Synergistic optimization of ion transport and sieving performance: Sulfonic acid groups are distributed in a gradient within the COF layer, and the pore size of the COF layer is precisely controlled (1.8±0.1 nm), thereby increasing the lithium ion transference number to above 0.85, while simultaneously increasing the Ni 2+ The permeation of transition metal ions was suppressed to below 5 ppm / cycle.

[0044] (2) The high voltage cycling stability is significantly improved. The mechanical modulus of the COF layer of not less than 15 GPa can effectively suppress the expansion of the electrode (positive electrode active material layer). Combined with the zero solvent residue characteristics of the gas phase process, the capacity retention rate after 500 cycles at a high voltage of 4.5V is ≥90% (≤76% for unmodified positive electrode).

[0045] (3) Breakthrough in rate performance: The vertically penetrating channel structure in the COF layer can significantly reduce the lithium-ion transport energy barrier, and the 5C rate discharge capacity can reach more than 160 mAh / g, and the 5C discharge capacity is maintained at ≥85% relative to the 0.1C discharge capacity.

[0046] (4) It has significant advantages in industrial application. The vapor phase epitaxial growth rate is increased to 5 nm / min (a significant improvement compared to the liquid phase method), and the film thickness uniformity (CV<8%) and process stability are significantly improved. At the same time, the high-temperature sintering step can be avoided, which reduces the overall cost and is fully adapted to the needs of large-scale production. In addition, the vapor phase epitaxial growth step and the vapor phase sulfonation modification step are both carried out after the preparation of the positive electrode blank, without the need to adjust the existing preparation process of the positive electrode blank, which is conducive to the promotion and application.

[0047] (5) Solid-state battery compatibility: By matching the mechanical modulus of the COF layer (15-25 GPa) with the stiffness of the solid electrolyte, interface peeling can be effectively suppressed; the construction of gradient sulfonic acid channels can realize single-ion conduction of lithium ions and block the growth of lithium dendrites caused by electron leakage; at the same time, the all-gas phase process avoids solvent contact, completely avoids the risk of corrosion of the solid electrolyte by the liquid phase coating method, and can also avoid adverse effects on the positive electrode active material layer. Attached Figure Description

[0048] Figure 1 This is a schematic diagram of the construction process of the COF molecular sieve interface structure in Example 1.

[0049] Figure 2 This is a graph showing the correlation between sulfonation process parameters and performance. Figure 2 a is a graph showing the relationship between surface sulfonic acid density and time; Figure 2 b is a graph showing the relationship between Ni permeation rate and time; Figure 2 c is a graph showing the relationship between aperture change and time.

[0050] Figure 3 This is a comparison chart of film thickness uniformity for Example 1, Comparative Example 1, and Comparative Example 7.

[0051] Figure 4 The cyclic performance curves are for Examples 1, 5, Comparative Examples 1, 3, and 7.

[0052] Figure 5 This is the rate performance curve for Example 1.

[0053] Figure 6 A bar chart comparing lithium-ion transference numbers.

[0054] Figure 7 This is a schematic diagram of a VPE deposition apparatus.

[0055] Figure 8SEM comparison images of the electrode after cycling (where, Figure 8 a is a SEM image of the positive electrode sheet after 500 cycles in Example 1. Figure 8 b is the SEM image of the positive electrode after 300 cycles in Comparative Example 6.

[0056] Figure 9 This is a schematic diagram of the solid-state battery interface structure in Example 5.

[0057] Figure 10 This is a TEM image of the COF layer in Comparative Example 7. Detailed Implementation

[0058] The present invention will be described in detail below with reference to embodiments. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0059] All examples and comparative examples used a uniform method to prepare the electrode sheets (NCM811 electrode sheets, NCA electrode sheets), the specific process of which is as follows:

[0060] First, the high-voltage lithium-ion battery cathode material (from Xinxiang Tianli Lithium Energy, specifically NCM811 model TLD806, chemical composition LiNi) was used. 0.8 Co0.1Mn 0.1 O2; NCA model is TLP910A, chemical composition LiNi 0.8 Co 0.15 Al 0.05 O2, conductive carbon black (Super P), and binder (PVDF) were mixed in a mass ratio of 96:2:2, and solvent (N-methylpyrrolidone) was added. The mixture was then vacuum ball-milled for 4 hours to form a homogeneous slurry with a solid content of 70 wt%. The slurry was then uniformly coated onto a 20 μm thick aluminum foil current collector (with a controlled areal density of 20 mg / cm²) and dried at 120°C for 2 hours to completely remove the solvent. Finally, the mixture was densified by rolling under 10 MPa pressure and cut into circular electrode sheets with a diameter of 14 mm to ensure that the active material loading was precisely controlled at 4.5 mg / cm².

[0061] The performance testing and characterization instruments and methods used in the relevant embodiments and comparative examples are as follows:

[0062] (1) Physical characterization:

[0063] Film thickness uniformity: In-situ ellipsometer (Sentech SE850), CV value = standard deviation / mean × 100%.

[0064] Pore ​​size distribution: N2 adsorption-desorption (Micromeritics ASAP 2460), NLDFT model fitting.

[0065] Mechanical modulus testing: A nanoindenter (Keysent G200) with a Berkovich probe was used, with a loading rate of 0.5 mN / s and a Poisson's ratio of 0.3.

[0066] Surface morphology: Scanning electron microscopy (SEM, Hitachi SU8010) combined with energy dispersive spectroscopy (EDS).

[0067] (2) Electrochemical testing:

[0068] Liquid battery assembly: negative electrode: lithium sheet; electrolyte: 1 M LiPF6 in EC / DMC (1:1 vol%); testing equipment: Landian CT3001A.

[0069] Solid-state battery assembly: negative electrode: lithium metal; solid electrolyte: Li6PS5Cl / LLZO; equipment: argon glove box (H2O < 0.1 ppm).

[0070] Cycle performance: 4.5 V cutoff, 1C constant current charge and discharge, record capacity retention rate for 500 cycles (liquid battery) or 300 cycles (solid battery); Rate performance: 0.1C to 5C stepped discharge, calculate 5C capacity retention rate.

[0071] Ion transport number: Bruce-Vincent method (polarization voltage 10 mV).

[0072] Interfacial impedance: AC impedance spectroscopy (Chenhua CH760e electrochemical workstation), frequency 1 MHz~0.1 Hz, amplitude 10mV.

[0073] Critical current density (CCD): Step current method (0.2 mA / cm²) 2 (Step, 10 min / step); a voltage drop >50% indicates failure.

[0074] Low temperature performance: After standing in a -40℃ constant temperature chamber (MSK-TE906-150L) for 24 hours, a 0.1C discharge test was performed.

[0075] Example 1

[0076] In this embodiment, the fabrication process of the positive electrode sheet is as follows:

[0077] (1) Substrate activation: The NCM811 electrode sheet was immersed in a 5 vol% APTES / toluene solution and reacted at 80°C for 2 hours under a continuous nitrogen flow of 15 sccm. It was then washed three times with toluene and dried under vacuum at 120°C to obtain a modified lithium-ion battery positive electrode blank. XPS characterization showed that the N 1s peak intensity increased to 3.2 times the initial value, confirming the successful construction of the amino anchor site.

[0078] (2) Vapor phase epitaxial growth: under a vacuum of 5×10 -3 A reaction chamber with a pressure of Pa and a temperature of 180℃ (see schematic diagram for details) Figure 7 To enhance deposition efficiency and consistency, the positive active material layer of a modified lithium-ion battery positive electrode blank was deposited in 50 cycles on its surface to obtain an electrode with a loaded COF layer. Each cycle consisted of the following steps: First, Tp (1,3,5-tricarboxymethyl phloroglucinol) monomer was transported and adsorbed for 30 seconds using 10 sccm nitrogen carrier gas; then, nitrogen was purged for 20 seconds at 50 sccm to remove byproducts; finally, Pa (p-phenylenediamine) was transported with 10 sccm nitrogen carrier gas for a 40-second condensation reaction. The reaction process is described in [reference needed]. Figure 1 During this period, the growth rate of a single cycle (0.4 nm / cycle) was monitored in real time using a quartz crystal microbalance, and the film thickness was measured to be 21 ± 1 nm (CV = 6%) using an in-situ ellipsometer.

[0079] (3) Gas-phase sulfonation: The electrode with the COF layer was placed in a SO3 atmosphere of 0.1 MPa and reacted at 150 °C for 1 hour to obtain the positive electrode of the lithium-ion battery. The sulfur (S 2p) element density decreased from 2.1 mmol / g on the surface of the positive electrode of the lithium-ion battery to 0.6 mmol / g on the substrate (positive electrode active material layer) by X-ray photoelectron spectroscopy layer etching (argon ion sputtering).

[0080] In this embodiment, the TpPa-COF system was selected. Tp and Pa condense to form a β-ketoenamine structure. The theoretical pore size of the β-ketoenamine structure is approximately 1.82 nm (which can be obtained through DFT calculation). Nitrogen adsorption-desorption tests showed that the specific surface area of ​​the obtained COF layer was 1680 m². 2 / g indicates that the pores in the COF layer have high permeability.

[0081] See Figure 2The pore size of the COF layer in Example 1 is about 1.78 nm. Based on Example 1, further experiments were conducted under other sulfonation time conditions (i.e., only the reaction time in step (3) was changed). It can be seen that controlling the appropriate reaction time helps to control the effective pore size of the COF layer and the density of sulfonic acid groups on the surface of the COF layer, thereby reducing the Ni permeability.

[0082] Example 2

[0083] Example 1 was repeated, except that the APTES concentration in step (1) was adjusted to 2 vol%. XPS showed that the N 1s peak intensity only increased to 1.8 times the initial value.

[0084] Example 3

[0085] Repeat Example 1, except that in step (2), the number of cycles is 40 (otherwise the same as in Example 1), and the film thickness is 16±2 nm (QCM data 0.4 nm / cycle).

[0086] Example 4

[0087] Repeat Example 1, except that in step (1), NCA electrode plates are used instead of NCM811 electrode plates, and the APTES concentration is 3 vol%. The rest is the same as in Example 1.

[0088] Example 5

[0089] First, the lithium-ion battery positive electrode sheet prepared in Example 1 was used. Then, sulfide-germanium sulfide solid electrolyte (Li6PS5Cl) powder was cold-pressed into a 200 μm thick electrolyte layer; see [link to previous section]. Figure 9 After the positive electrode active material layer, COF layer and electrolyte layer are stacked in sequence, they are hot-pressed at 120℃ and 20 MPa for 10 minutes to achieve interface fusion; finally, coin cell solid-state batteries are assembled with lithium metal as the negative electrode to complete the full battery integration.

[0090] Comparative Example 1

[0091] Repeat Example 1, except that step (1) is omitted and VPE deposition is performed directly (otherwise the same as in Example 1).

[0092] Comparative Example 2

[0093] Repeat Example 1, except that in step (2), the number of cycles is 70 (otherwise the same as in Example 1), and the film thickness is 28±3 nm (CV=15%).

[0094] Comparative Example 3

[0095] Repeat Example 1, except that the gas phase sulfonation step (i.e., step (3)) is omitted.

[0096] Comparative Example 4

[0097] Repeat Example 1, except that the gas phase sulfonation step (i.e., step (3)) is replaced with liquid phase sulfonation, that is, the electrode loaded with COF layer is immersed in a dichloroethane solution of chlorosulfonic acid with a concentration of 1 M and reacted at 25°C for 1 hour (other aspects are the same as in Example 1).

[0098] Comparative Example 5

[0099] Repeat Example 1, except that in step (2), the nitrogen purging step is omitted (otherwise the same as Example 1).

[0100] Comparative Example 6

[0101] Example 5 is repeated, except that NCM811 electrode sheets (i.e., electrodes that are not activated by the substrate, not loaded with a COF layer, and not modified by sulfonation) are used instead of the lithium-ion battery positive electrode sheet prepared in Example 1. Otherwise, the same applies to Example 5.

[0102] See Figure 8 The positive electrode sheet of this invention remains intact after 500 cycles; while the conventional NCM811 electrode sheet shows significant damage to the active material particles after 300 cycles. This demonstrates that the lithium-ion battery positive electrode sheet of this invention exhibits higher structural stability during service.

[0103] Comparative Example 7

[0104] Repeat Example 1, except that the aldehyde monomer is replaced with 2,6-naphthyldicarboxaldehyde (NDA) and the diamine monomer is replaced with 4,4'-diaminodiphenylmethane (DDM).

[0105] The results analysis shows that:

[0106] The combination of NDA and DDM induces significant pore structure collapse. Nitrogen adsorption-desorption tests show that the specific surface area of ​​the resulting COF layer is only 182 m². 2 / g, compared to 1680 m in Example 1 2 The density per gram ( / g) decreases by nearly an order of magnitude, and over 85% of the effective channel size is less than 0.8 nm, failing to meet the size requirements for lithium-ion transport. See also... Figure 10 Based on transmission electron microscopy (TEM) observations, it can be inferred that the naphthalene ring plane of NDA was excessively densely packed between layers due to strong π-π stacking, resulting in irreversible collapse of the pore network.

[0107] Comparative Example 8

[0108] Repeat Example 1, except that in step (2), the temperature inside the reaction chamber is controlled to be 150°C.

[0109] The lithium-ion transference number dropped to 0.32, and the resulting liquid battery had a 5C rate discharge capacity of 63.6 mAh / g, which was significantly lower than that of Example 1. The possible reason is that the low temperature environment of 150°C resulted in insufficient monomer condensation reaction, and the defect rate of the β-ketoenamine structure in the COF framework was significantly increased compared with Example 1. This caused pore distortion and made it impossible to form a COF molecular sieve layer with regular ion channels, resulting in a decrease in performance.

[0110] Comparative Example 9

[0111] Repeat Example 1, except that in step (2), the temperature inside the reaction chamber is controlled to be 200°C.

[0112] The results showed that the performance of the obtained lithium-ion battery positive electrode was also significantly reduced. The possible reason is that the reaction temperature was too high. The high temperature environment caused the crosslinking density to rise abnormally, and the stress accumulation in the COF layer formed micron-sized cracks, making it difficult to obtain a structurally complete COF layer. During cycling, the electrolyte seeped into the positive electrode interface along the cracks, triggering side reactions such as the dissolution of transition metals. After 200 cycles, the DC internal resistance (DCIR) increased significantly compared with Example 1.

[0113] See Figure 3 The COF layer of the electrode sheet prepared by the method of the present invention has excellent film thickness uniformity, while the film thickness uniformity is poor when the substrate is not activated or when other monomers are used to prepare the COF layer.

[0114] See Figure 4 The liquid battery assembled with the lithium-ion battery positive electrode sheet prepared using the method of this invention retains a capacity retention rate of up to 92.5% after 500 cycles, while the solid battery assembled with it retains a capacity retention rate of up to 90% after 300 cycles. See also Figure 5 Liquid batteries assembled from lithium-ion battery positive electrode sheets prepared using the method of this invention exhibit high discharge capacity under rate conditions of 0.1~5C.

[0115] See Figure 6 The lithium-ion battery positive electrode prepared by the method of the present invention has a higher lithium-ion transference number.

[0116] The performance test results of each embodiment and comparative example are shown in Table 1.

[0117]

[0118] Note: In Table 1, the capacity retention rate refers to the capacity retention rate after 500 cycles.

[0119] As shown in Table 1, activating the substrate before vapor phase epitaxial growth helps obtain a COF layer with a smaller coefficient of variation and results in a better electrochemical performance of the positive electrode. The possible reasons are as follows: substrate activation enhances the adhesion of the in-situ generated COF layer, constructing chemical anchoring points on the surface of the positive electrode active material layer, preventing the COF layer from peeling off due to electrode volume expansion during high-voltage cycling, thus improving cycle performance; the anchored amino groups regulate the epitaxial growth orientation, and the -NH2 groups react directionally with the COF monomer (the aldehyde group of Tp), inducing the COF lattice to grow in a direction perpendicular to the positive electrode active material layer, ensuring vertical channel connectivity; furthermore, substrate activation helps eliminate interfacial impedance, replacing traditional physical coating methods, forming a covalent bond interface, and reducing Li... + Transport barriers help increase ion transport numbers.

[0120] The chemical mechanism of step (1) in Example 1 is explained as follows:

[0121] The -OH groups on the surface of the positive electrode active material layer of the NCM811 electrode react with the ethoxy groups of APTES to form Si-OM bonds, exposing the terminal -NH2 groups. This exposed -NH2 groups are then anchored to the surface of the positive electrode active material layer, thus activating the substrate. The relevant reaction formulas are as follows:

[0122] Step 1 (hydrolysis): (EtO)3Si-(CH2)3-NH2 + 3H2O → (HO)3Si-(CH2)3-NH2 + 3EtOH;

[0123] Step 2 (condensation): NCM811-OH+(HO)3Si-(CH2)3-NH2→NCM811-O-Si-(CH2)3-NH2+H2O.

[0124] As shown in Table 1, the CV value of the positive electrode obtained by first activating the substrate and then performing vapor phase epitaxial growth is significantly lower. This may be because the anchored -NH2 groups during substrate activation have a template effect: the Tp aldehyde groups preferentially condense with the anchoring sites, inducing the COF lattice to grow perpendicularly along the crystal plane, which helps to obtain a COF layer with more regular channels and more uniform film thickness. The chemical reaction equations occurring during vapor phase epitaxial growth are as follows:

[0125] Tp(-CHO)3+Pa(-NH2)2→TpPa-COF(-C=N-)+3H2O.

[0126] As shown in Examples 1, 3, and 4, vapor-phase sulfonation modification helps to more effectively improve electrochemical performance such as cycle performance and reduce the permeability of transition metal ions such as nickel ions. This may be because introducing sulfonic acid groups into the pores of the COF layer allows for precise control of ion selectivity, enabling the positive electrode to preferentially adsorb Li through electrostatic interactions. + And it repels multivalent metal ions (Ni 2+ Co 3+ (etc.) to achieve the "ion sieving" function; the use of gas phase sulfonation modification method to replace the traditional liquid phase method (such as common chlorosulfonic acid solution) can eliminate the risk of solvent corrosion to COF film and electrode substrate, avoid interface corrosion, and ensure the integrity and stability of positive electrode active material layer, current collector, etc.; in addition, the introduction of sulfonic acid groups in COF layer can enhance the compatibility of COF layer with electrolyte, suppress side reactions under high voltage, improve interface stability, and thus improve battery cycle life.

[0127] The present invention further explains the specific reaction mechanism and action mechanism of the gas-phase sulfonation modification process in Example 1 as follows:

[0128] (1) Chemical bonding process in gas-phase sulfonation modification

[0129] SO3 gas → permeates the COF layer pores → reacts with imine bonds (-C=N-) → forms sulfonamide groups (-CN-SO3H).

[0130] The reaction formula is as follows:

[0131] COF-C=N+SO3→COF-CN-SO3H.

[0132] (2) Ion transport regulation mechanism

[0133] Li + Selective transport: The negatively charged channels of -SO3H attract Li + Simultaneously, Ni is blocked through size sieving and electrostatic repulsion. 2+ Equal-sized multivalent ions.

[0134] Desolvation promotion: Sulfonic acid groups compete with EC solvent (the main component of the electrolyte) for coordination of Li. + weaken Li + -EC binding energy, reducing the desolvation energy barrier.

[0135] In Example 1, the sulfur (S 2p) element density decreased from 2.1 mmol / g on the surface of the lithium-ion battery positive electrode to 0.6 mmol / g on the substrate (positive electrode active material layer). A gradient-decreasing sulfonic acid group structure was constructed within the COF layer, which simultaneously optimized interfacial ion transport kinetics and chemical stability: the higher sulfonic acid group density near the solid electrolyte side provides a fast lithium-ion channel, while the lower density near the positive electrode active material layer helps to effectively suppress the oxidative corrosion of the positive electrode material by sulfonic acid groups under high voltage. Simultaneously, the gradient change buffers interfacial stress during charge and discharge. The present invention attempts to explain the relevant reasons as follows:

[0136] (1) Diffusion-reaction competition model

[0137] According to Fick's diffusion law, the concentration of gas in the COF layer pores decreases exponentially with depth, resulting in a higher concentration of sulfonic acid groups in the surface COF layer than in the bottom layer, forming a structure with a gradient of sulfonic acid groups.

[0138] (2) Synergistic effect of temperature field

[0139] Substrate thermal conductivity effect: The thermal conductivity of the NCM811 electrode sheet is greater than that of the COF layer, resulting in a lower substrate interface temperature and a higher COF layer surface temperature. Furthermore, the reaction rate constant follows the Arrhenius equation, causing the gas sulfonation reaction on the COF layer surface to be more complete than that on the substrate layer, forming a structure with gradient sulfonic acid groups.

[0140] Ion transference number (t) of the positive electrode plates in Examples 1-4 + A value ≥ 0.85 indicates that the pores in the COF layer of the obtained positive electrode sheet affect the Li... + The transport exhibits high selectivity (other ions are sieved). The rate performance (5C) of the liquid batteries assembled with the positive electrode sheets of Examples 1-4 all reach over 152 mAh / g, indicating that the pore size of the COF layer of the obtained positive electrode sheet is well-suited for rapid desolvation of Li. + The transmission.

[0141] The performance test results of the solid-state batteries in Example 5 and Comparative Example 6 are shown in Table 2.

[0142]

[0143] As shown in Table 2, the solid-state battery assembled using the positive electrode sheet of the present invention has significantly better performance in terms of cycle performance, lithium-ion transference number, interface impedance, CCD, and low-temperature discharge capacity compared with the solid-state battery assembled using the traditional positive electrode sheet.

[0144] Table 3 shows a table of Chinese and English translations of the technical terms used in this invention.

[0145]

[0146] The above embodiments should be understood as being used only to illustrate the present invention more clearly, and not to limit the scope of the present invention. After reading the present invention, any modifications of the present invention in various equivalent forms by those skilled in the art fall within the scope defined by the appended claims.

Claims

1. A method for preparing a positive electrode sheet for a lithium-ion battery, characterized in that, Includes the following steps: S1. Provides a rough blank for the positive electrode sheet of a lithium-ion battery; The lithium-ion battery positive electrode blank includes a current collector and a positive active material layer stacked sequentially; the positive active material in the positive active material layer includes one or more of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and lithium nickel cobalt manganese aluminum oxide. S2. The lithium-ion battery positive electrode blank is immersed in an organic solution of silane coupling agent. After reaction, it is washed and dried to obtain the modified lithium-ion battery positive electrode blank. S3. A COF layer is prepared on the surface of the positive active material layer of the modified lithium-ion battery positive electrode blank using aldehyde monomers and diamine monomers as raw materials by vapor phase epitaxy to obtain a lithium-ion battery positive electrode blank with a COF layer; S4. The lithium-ion battery positive electrode blank with a COF layer is subjected to vapor phase sulfonation modification to obtain a lithium-ion battery positive electrode. The aldehyde monomer includes one or more of 1,3,5-tricarboxyloylphloroglucinol, 2,4,6-tricarboxylotriazine, 2,5-dihydroxyterephthalaldehyde, 2,5-dimethoxyterephthalaldehyde, 2,5-dichloroterephthalaldehyde, and 2,3,5,6-tetrafluoroterephthalaldehyde; the diamine monomer includes one or more of p-phenylenediamine, benzidine, m-phenylenediamine, and o-phenylenediamine.

2. The preparation method according to claim 1, characterized in that, In S1, the chemical formula of lithium nickel cobalt manganese oxide is LiNi x Co y Mn 1-x-y O2, where 0.6 ≤ x < 1, 0 < y < 1, and x + y < 1; the chemical formula of lithium nickel cobalt aluminate is LiNi x Co y Al z O2, where 0.6 ≤ x < 1, 0 < y < 1, 0 < z < 1, and x + y + z = 1; the chemical formula of lithium nickel cobalt manganese aluminate is LiNi x Co y Mn z Al w O2, where 0.6 ≤ x < 1, 0 < y < 1, 0 < z < 1, 0 < w < 0.1, and x + y + z + w = 1.

3. The preparation method according to claim 1, characterized in that, In S2, the concentration of the silane coupling agent in the organic solution is 1-10 vol%. The silane coupling agent includes one or two of APTES and AEAPTMS. The solvent in the organic solution includes one or more of toluene, xylene, and cyclohexane.

4. The preparation method according to claim 1, characterized in that, In step S2, the lithium-ion battery positive electrode blank is immersed in an organic solution of silane coupling agent and reacted at 70-90°C for 1-3 hours under continuous inert atmosphere. After washing and drying, the modified lithium-ion battery positive electrode blank is obtained. The flow rate of the inert atmosphere is controlled at 5-30 sccm, and the inert atmosphere includes one or both of nitrogen and argon.

5. The preparation method according to claim 1, characterized in that, In step S3, during vapor phase epitaxial growth, the modified lithium-ion battery positive electrode blank is placed under a vacuum of ≤5×10⁻⁶. -2 In the reaction chamber of Pa, 40-60 cycles of deposition are performed at 160-200℃ to obtain a rough blank of lithium-ion battery positive electrode loaded with COF layer. During each deposition cycle, the following steps are performed sequentially: First, nitrogen is used as a carrier gas to deliver aldehyde monomers into the reaction chamber for 20-40 seconds; then, an inert atmosphere is introduced into the reaction chamber and purged for 15-25 seconds; finally, nitrogen is used as a carrier gas to deliver diamine monomers into the reaction chamber for 30-50 seconds. The inert atmosphere is nitrogen and / or argon.

6. The preparation method according to any one of claims 1-5, characterized in that, In step S4, the lithium-ion battery positive electrode blank with the COF layer is placed in a sulfonating agent atmosphere and subjected to gas-phase sulfonation modification to obtain the lithium-ion battery positive electrode; wherein the sulfonating agent is SO3 gas, chlorosulfonic acid vapor or SO3-dioxane complex, and the molar ratio of SO3 to dioxane in the SO3-dioxane complex is 1:1-3.

7. The preparation method according to claim 6, characterized in that, The lithium-ion battery positive electrode blank with the COF layer is placed in a sulfonating agent atmosphere of 0.05-0.15 MPa and subjected to gas phase sulfonation modification at 140-160 °C for 0.5-2 h.

8. The preparation method according to claim 7, characterized in that, The thickness of the COF layer is 14-24 nm.

9. A positive electrode sheet for a lithium-ion battery, characterized in that, It is prepared by the preparation method according to any one of claims 1-8.

10. A lithium-ion battery, characterized in that, Including the lithium-ion battery positive electrode sheet as described in claim 9.

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