Positive pole piece as well as electrochemical device and electronic equipment comprising positive pole piece

Abstract

The invention discloses a positive pole piece as well as an electrochemical device and electronic equipment comprising the same, and relates to the technical field of energy storage. The positive pole piece comprises a positive current collector and a positive active layer arranged on at least one surface of the positive current collector, the positive active layer is sequentially provided with a first coating, a transition layer and a second coating in the direction from the position close to the positive current collector to the position away from the positive current collector, the first coating contains MOF and a positive electrode active material with the particle size of a; the transition layer contains a metal oxide; the second coating contains a positive electrode active material with the particle size of b, and b is less than a. By adding the MOF into the pole piece, low-temperature slow-release electrolyte can be realized, the diffusion conduction path of lithium ions can be optimized by introducing the hierarchical structure, and on the basis, interlayer stripping generated between the first coating and the second coating in the circulation process can be inhibited by introducing the transition layer.

Description

Technical Field

[0001] This application relates to the field of energy storage technology, and more specifically, to positive electrode plates and electrochemical and electronic devices comprising them. Background Technology

[0002] Lithium-ion batteries are among the mainstream commercially available batteries, but their performance at low temperatures is unsatisfactory, severely limiting their use in cold regions or specific applications. This decline in low-temperature performance is mainly due to: increased electrolyte viscosity and even freezing at low temperatures, leading to reduced ion conductivity and significant electrolyte consumption; lattice shrinkage in electrode materials; slowed lithium-ion diffusion rates within the active material; and increased interfacial film impedance, all of which severely affect charge transfer kinetics. These issues result in reduced capacity retention at low temperatures, and with increasing cycle count, the internal resistance and temperature of the cathode also increase, further impacting the electrode's performance at low temperatures. Therefore, developing a cathode material with high capacity retention at low temperatures while maintaining low internal resistance and temperature is of great significance. Summary of the Invention

[0003] The purpose of this application is to solve the problems of reduced capacity retention and increased internal resistance and temperature of existing cathode materials at low temperatures, and to provide a cathode electrode and an electrochemical device and electronic device containing the cathode.

[0004] To achieve the above objectives, this application provides a positive electrode sheet, which includes a positive current collector and a positive active layer disposed on at least one surface of the positive current collector. The positive active layer is provided with a first coating, a transition layer with a thickness of 50-200 nm, and a second coating sequentially from the direction near the positive current collector to the direction away from the positive current collector. The first coating includes a first coating comprising a MOF and a positive active material; the transition layer comprises a metal oxide; and the second coating includes a second coating comprising a positive active material. The average particle size of the positive electrode active material in the first coating is a μm, and the average particle size of the positive electrode active material in the second coating is b μm, where b < a.

[0005] In some embodiments, the areal density of the first coating is σ1g / cm³. 3 The areal density of the second coating is σ²g / cm³. 3 , 1.05≤σ1 / σ2≤1.2.

[0006] In some embodiments, the thickness of the first coating is H1 μm, the thickness of the second coating is H2 μm, and 1.05 ≤ H1 / H2 ≤ 1.2.

[0007] In some implementations, 1.05 ≤ a / b ≤ 3.95.

[0008] In some implementations, 3.2 g / cm 3 ≤σ1≤3.5 g / cm 3 .

[0009] In some implementations, 2.8 g / cm 3 ≤σ2≤3.2 g / cm 3 .

[0010] In some implementations, 42 μm ≤ H1 ≤ 48 μm.

[0011] In some implementations, 38 μm ≤ H2 ≤ 42 μm.

[0012] In some implementations, 2 μm ≤ a ≤ 10 μm.

[0013] In some implementations, 1 μm ≤ b ≤ 3 μm.

[0014] In some implementations, the specific surface area of ​​the MOF is ≥1500 m². 2 / g.

[0015] In some implementations, the MOF includes at least one of ZIF-8 and MIL-101.

[0016] In some implementations, the porosity of the transition layer is 30%-50%.

[0017] In some embodiments, the positive electrode active material in the first coating includes LiNi. 0.6 Co 0.2 Mn 0.2 O2, LiCoO2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.5 Co 0.2 Mn 0.3 At least one of O2.

[0018] In some embodiments, the positive electrode active material in the second coating includes LiFePO4 and Li4Ti5O. 12 LiMn x Fe 1-x At least one of PO4 and LiCoPO4.

[0019] In some embodiments, the mass ratio of the positive electrode active material to the MOF in the first coating is (70-85):(5-15).

[0020] In some embodiments, the second coating also includes carbon fibers.

[0021] In some embodiments, the mass ratio of the positive electrode active material to the carbon fiber in the second coating is (88-93):(1-5).

[0022] In some implementations, the aspect ratio of the carbon fiber is 50-100.

[0023] This application also provides an electrochemical device comprising any of the positive electrode, diaphragm, negative electrode, and electrolyte described herein.

[0024] In some implementations, the electrochemical device can be a lithium-ion secondary battery.

[0025] This application also provides an electronic device that includes the electrochemical device described above.

[0026] Compared with the prior art, the present invention has the following beneficial effects: This application provides a positive electrode with a gradient structure. A MOF material and a positive electrode active material with higher electronic conductivity and larger particle size are introduced into the first coating. A positive electrode active material with higher ionic conductivity and smaller particle size is introduced into the second coating. This optimizes the ion diffusion path. Simultaneously, the MOF can release its adsorbed electrolyte at low temperatures, compensating for electrolyte loss at low temperatures and thus improving the electrochemical performance of the positive electrode at low temperatures. Furthermore, this application introduces a specific transition layer between the two layers to ensure excellent bonding between them. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0028] <Terminology Definition> The term "MOF" refers to Metal Organic Framework, a class of crystalline porous materials with a periodic network structure formed by the self-assembly of inorganic metal centers (metal ions or metal clusters) and bridging organic ligands.

[0029] The term “electronic conductivity” refers to the ability of a solid conductor (such as metals and some semiconductors) to conduct electrons, and its unit is Siemens per meter (S / m).

[0030] The term "lithium-ion diffusion coefficient" refers to the diffusion coefficient of lithium ions in electrode materials. It can be determined according to Fick's second law, for example, by any of the following methods: cyclic voltammetry, electrochemical impedance spectroscopy, etc.

[0031] The term "length-to-diameter ratio" refers to the ratio of the length (L) of an object to its diameter (D), and is dimensionless.

[0032] The embodiments of this application may omit unnecessary detailed descriptions. For example, detailed descriptions of well-known matters and repetitive descriptions of actually identical structures may be omitted. This is to avoid making the following description unnecessarily lengthy and to facilitate understanding by those skilled in the art.

[0033] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0034] In this application, a list of items connected by the term "at least one of" can mean any combination of the listed items. For example, if items A and B are listed, then the phrase "at least one of A and B" means only A; only B; or A and B. In another instance, if items A, B, and C are listed, then the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C.

[0035] In the following description, all figures disclosed in this application are approximate values, regardless of whether the terms "about" or "approximately" are used in conjunction. They may vary by 1%, 2%, 5%, or sometimes 10% to 20%. Whenever a range of values ​​with a lower limit (RL) and an upper limit (RU) is disclosed, any values ​​falling within that range are specifically disclosed. Specifically, the following values ​​within this range are specifically disclosed: R = RL + k * (RU - RL), where k is a variable with a 1% increment from 1% to 100%, i.e., k is 1%, 2%, 3%, 4%, 5%, ..., 50%, 51%, 52%, ..., 95%, 96%, 97%, 98%, 99%, or 100%. Furthermore, any range of values ​​defined by the two R values ​​as defined above are also specifically disclosed.

[0036] In this application, numerical ranges are involved. Unless otherwise specified, the numerical ranges mentioned above are considered continuous and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Any lower limit can be combined with any upper limit to form a range not explicitly stated; and any lower limit can be combined with other lower limits to form a range not explicitly stated, just as any upper limit can be combined with any other upper limit to form a range not explicitly stated. Furthermore, each individually disclosed point or single value can itself serve as a lower or upper limit and be combined with any other point or single value or with other lower or upper limits to form a range not explicitly stated.

[0037] Throughout this specification, references to "implementation," "partial implementation," "one implementation," "some implementations," "another implementation," "specific implementation," or "partial implementation" mean that at least one implementation or embodiment in this application includes the specific features, structures, materials, or characteristics described in that implementation or embodiment.

[0038] I. Positive electrode sheet This application provides a positive electrode sheet, including a positive current collector and a positive active layer disposed on at least one surface of the positive current collector. The positive active layer is provided with a first coating, a transition layer with a thickness of 50-200 nm, and a second coating sequentially from the direction close to the positive current collector to the direction away from the positive current collector. The first coating includes a first coating layer comprising a MOF and a positive active material. The transition layer comprises a metal oxide. The second coating comprises a positive active material. The average particle size of the positive electrode active material in the first coating is a μm, and the average particle size of the positive electrode active material in the second coating is b μm, where b < a.

[0039] The positive electrode sheet provided in this application has a multi-layer structure, wherein the first coating (bottom layer) near the surface of the positive current collector contains MOF material. The porous structure of MOFs can adsorb electrolyte and promote ion transport. At low temperatures, MOFs can release pre-adsorbed electrolyte through pore shrinkage, dynamically compensating for insufficient electrolyte caused by low-temperature solidification. Simultaneously, its metal nodes (such as Zn...) 2+ It can react with Li in the electrolyte. +A cooperative migration pathway is formed, reducing internal resistance at low temperatures. Therefore, the first coating can improve the electron conduction rate. However, during the charging and discharging process of the positive electrode, electron transfer and lithium-ion insertion-extraction mutually promote each other, determining the overall reaction rate. Therefore, simply increasing the electron conduction rate has limited effect on improving battery performance. Based on this, this application also introduces a second coating (upper layer) far from the surface of the positive electrode current collector. The lithium-ion diffusion and conduction pathway is optimized by the hierarchical structure and the difference in the average particle size of the positive electrode active material in the two layers (the average particle size b of the second coating < the average particle size a of the first coating). At low temperatures, the first and second coatings can also form a complementary conductive network, thereby further improving the electrochemical performance of the positive electrode. It should be noted that in order to suppress interlayer delamination between the first and second coatings during cycling and improve the mechanical stability of the positive electrode, a transition layer with a thickness of 50-200 nm is also introduced into the first and second coatings. The reason for using metal oxides to construct the transition layer is primarily due to their mechanical stability and chemical compatibility: nanoscale metal oxides can suppress volume deformation of the cathode material during charging and discharging through high hardness, reducing the risk of interlayer delamination; simultaneously, their surface has strong inertness, preventing reaction with the electrolyte or active materials and avoiding the formation of undesirable byproducts. Furthermore, compared to non-metal oxides, metal oxides typically have superior dielectric constants, which can both isolate electronic short circuits and allow Li... + Through pore transport, the requirements for conductivity and insulation are balanced.

[0040] In some embodiments, the areal density of the first coating is σ1g / cm³. 3 The areal density of the second coating is σ²g / cm³. 3 , 1.05≤σ1 / σ2≤1.2.

[0041] By controlling the areal density ratio, the mass distribution of the first coating (bottom layer) and the second coating (top layer) can be adjusted, optimizing the charge transport dynamics and mechanical stability of the electrode. Specifically, the first coating, with a slightly higher areal density σ1, ensures rapid electron conduction near the current collector, while the second coating, with a slightly lower σ2, reduces lithium-ion diffusion resistance. Simultaneously, the gradient of areal density on both sides of the transition layer (σ1 slightly higher than σ2) alleviates volumetric stress during charging and discharging, suppressing interlayer delamination.

[0042] In this application, the areal density of the coating can be determined by, but is not limited to, the following methods: This invention does not limit the method for detecting the areal density of the first coating and the areal density of the second coating. Those skilled in the art can detect their areal densities using conventional techniques. For example, the areal density of the first coating and the areal density of the second coating can be detected by the following method: placing an electrode sheet on a die-cutting machine, weighing the obtained sheet, and deducting the weight of the foil to obtain the areal density of the coating.

[0043] In this application, the areal density of the coating can be adjusted in, but is not limited to, the following ways: Adjust the proportion of positive electrode active material in the coating material; reduce the doctor blade / roller speed or reduce the coating die gap when coating is applied.

[0044] In some implementations, σ1 / σ2 can be 1.05, 1.06, 1.07, 1.08, 1.09, 1.1, 1.11, 1.12, 1.13, 1.14, 1.15, 1.16, 1.17, 1.18, 1.19, 1.2, or fall within the range of any two of the above values.

[0045] In some embodiments, the thickness of the first coating is H1 μm, the thickness of the second coating is H2 μm, and 1.05 ≤ H1 / H2 ≤ 1.2.

[0046] An appropriate thickness gradient can accommodate the electrolyte slow-release function of MOF (the bottom layer needs more pores for electrolyte storage). Therefore, it is necessary to ensure that the first coating (bottom layer) is slightly thicker than the second coating (top layer) to avoid uneven electrolyte distribution caused by an excessively thick top layer. At the same time, an appropriate thickness gradient can optimize the ion-electron conduction path and provide a buffer for the volume expansion of the cathode material, preventing interlayer delamination during charging and discharging.

[0047] In this application, the thickness of the coating can be determined by, but is not limited to, the following methods: The positive electrode sheet of the empty secondary battery was removed, and the thickness of each layer was then observed and measured using a cross-sectional polished scanning electron microscope (CP-SEM).

[0048] In this application, the thickness of the coating can be adjusted in, but is not limited to, the following ways: Adjust the proportion of positive electrode active material in the coating material, or adjust the process parameters when coating, such as the doctor blade gap, coating speed, or roller pressure (such as reducing the gap or speed to thicken a single layer of coating), or perform multiple coatings.

[0049] In some implementations, H1 / H2 can be 1.05, 1.06, 1.07, 1.08, 1.09, 1.1, 1.11, 1.12, 1.13, 1.14, 1.15, 1.16, 1.17, 1.18, 1.19, 1.2, or fall within the range of any two of the above values.

[0050] In some implementations, 1.05 ≤ a / b ≤ 3.95.

[0051] In this application, the two layers of active material with a certain gradient in particle size can achieve better electrochemical performance. This is mainly because: a certain gradient in particle size distribution can optimize ion-electron transport. The large-particle bottom layer (a) reduces the tortuosity of the electron transport path (high electronic conductivity materials such as NCM / LCO), and the small-particle surface layer (b) shortens the lithium-ion diffusion distance (such as LiFePO4), thereby improving rate performance. At the same time, this multi-layered particle size distribution structure can optimize electrolyte wetting and alleviate the volume expansion stress of the cathode material during charging and discharging.

[0052] In this application, the average particle size of the positive electrode active material can be determined by, but is not limited to, the following methods: The positive electrode sample was prepared using an argon ion cross-section polisher (JEOL IB-09010CP) to ensure a smooth and undamaged surface. Elemental surface scanning of the cross-section was performed using SEM (ZEISS Sigma-02-33) and EDS (Oxford X-Max). High-resolution images of the cross-section were acquired using TEM, focusing on 8-12 different regions. Particle size was statistically analyzed using image analysis software (NanoMeasurer), and the average particle size was calculated.

[0053] In this application, the average particle size of the positive electrode active material can be controlled by, but is not limited to, mechanical ball milling.

[0054] In some implementations, a / b can be 1.05, 1.1, 1.15, 1.2, 1.25, 1.3, 1.35, 1.4, 1.45, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.05, 2.2, 2.5, 3, 3.5, 3.6, 3.9, 3.95, or within a range of any two of the above values.

[0055] In some implementations, 3.2 g / cm 3 ≤σ1≤3.5 g / cm 3 For example, σ1 can be 3.2 g / cm³. 3 3.25 g / cm 3 3.3 g / cm 3 3.35 g / cm3 3.4 g / cm 3 3.45 g / cm 3 3.5 g / cm 3 Or it falls within the range of any two of the above values.

[0056] In some implementations, 2.8 g / cm 3 ≤σ2≤3.2 g / cm 3 For example, σ² can be 2.8 g / cm³. 3 2.85 g / cm 3 2.9 g / cm 3 2.95 g / cm 3 3 g / cm 3 3.05 g / cm 3 3.15 g / cm 3 3.2 g / cm 3 Or it falls within the range of any two of the above values.

[0057] In some implementations, 42 μm ≤ H1 ≤ 48 μm. For example, H1 can be 42 μm, 42.5 μm, 43 μm, 43.5 μm, 44 μm, 44.5 μm, 45 μm, 45.5 μm, 46 μm, 46.5 μm, 47 μm, 47.5 μm, 48 μm, or within any two of the above values.

[0058] In some implementations, 38 μm ≤ H2 ≤ 42 μm. For example, H2 can be 38 μm, 38.5 μm, 39 μm, 39.5 μm, 40 μm, 40.5 μm, 41 μm, 41.5 μm, 42 μm, or within any two of the above values.

[0059] In some implementations, 2 μm ≤ a ≤ 10 μm. For example, a can be 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm, 8.5 μm, 9 μm, 9.5 μm, 10 μm, or within any two of the above values.

[0060] In some implementations, 1 μm ≤ b ≤ 3 μm. For example, b can be 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, or within any two of the above values.

[0061] In some implementations, the MOF includes at least one of ZIF-8 and MIL-101.

[0062] In some embodiments, the metal oxide of the transition layer includes at least one of Al2O3, ZrO2, and TiO2.

[0063] In some embodiments, the porosity of the transition layer is 30%-50%. For example, the porosity of the transition layer may be 30%, 31%, 32%, 35%, 37%, 40%, 42%, 45%, 50%, or within any two of the above values.

[0064] In some implementations, the porosity of the transition layer can be determined by, but is not limited to, the following methods: The secondary battery was disassembled in an empty state. The obtained positive electrode sheet was first rinsed with deionized water, then quickly rinsed with 3.5M sulfuric acid, then washed with deionized water and dried. The second coating was scraped off, and the transition layer was scraped off. The scraped powder was soaked in N-methylpyrrolidone solution for 2 hours, then filtered to obtain solid particles. The solid particles were dried at 60°C for 24 hours and then tested using a specific surface area analyzer (McTristar II 3020) at a nitrogen atmosphere and a test temperature of -196°C. The pore volume q cm⁻¹ of the test sample was measured. 3 / g, pore volume refers to the total volume of fine pores (usually micropores and mesopores) in a unit mass of transition layer material. The true density (wg / cm³) of the solid particles was then tested using a true density meter (Micromeritics / ACCUPYS II 1340, USA) to confirm the true density. 3 Then the porosity P = q / [q + (1 / w)].

[0065] In some embodiments, the transition layer also includes conductive carbon black.

[0066] In some embodiments, the conductive carbon black has an average particle size of 20-50 nm.

[0067] In some embodiments, the mass ratio of conductive carbon black to metal oxide in the transition layer is (1-3):1.

[0068] In some embodiments, the transition layer can be prepared in, but is not limited to, the following ways: Mix all components thoroughly, ball mill them, disperse them in a solvent, and then spray them to obtain the final product.

[0069] In some implementations, the porosity of the transition layer can be controlled by adjusting the particle size of each component in the transition layer (which can be adjusted by the ball milling speed, time, etc.).

[0070] In some embodiments, the components in the transition layer form a core-shell structure, wherein the core layer is conductive carbon black and the shell layer is a metal oxide.

[0071] In some embodiments, the shell thickness in the transition layer of the core-shell structure is 5-10 nm.

[0072] In some embodiments, the transition layer forming the core-shell structure of the components can be prepared in, but is not limited to, the following ways: The metal oxide precursor is deposited in vapor phase on the surface of conductive carbon black at a temperature of 140-180℃ to obtain the product.

[0073] In the above embodiments, the porosity of the transition layer can be controlled by adjusting the average particle size of the conductive carbon black (which can be controlled by ball milling) and by controlling the thickness of the deposited shell (which can be controlled by the deposition time).

[0074] In some embodiments, the shell thickness in the transition layer forming the core-shell structure can be adjusted by changing the amount of metal oxide precursor added, the time of vapor deposition, and the temperature.

[0075] In some embodiments, the metal oxide precursor includes at least one of trimethylaluminum, zirconium tetrachloride, and titanium tetrachloride.

[0076] In some embodiments, the positive electrode active material in the first coating includes LiNi. 0.6 Co 0.2 Mn 0.2 O2, LiCoO2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.5 Co 0.2 Mn 0.3 At least one of O2.

[0077] The aforementioned positive electrode active material has been experimentally verified by the inventors to have good electronic conductivity in the system of this application, and can make full use of the gradient structure of this application to optimize the electronic conduction path and improve the low-temperature electrochemical performance of the positive electrode.

[0078] In some embodiments, the first coating also includes at least one of a conductive agent and a binder.

[0079] In some embodiments, the conductive agent in the first coating includes at least one of carbon, carbon black, graphite, expanded graphite, graphene, graphene nanosheets, superconducting carbon, acetylene black, conductive carbon black, Ketjen black, carbon dots, carbon fibers, carbon nanofibers, graphitized carbon sheets, carbon nanotubes, activated carbon, and mesoporous carbon. The positive electrode conductive agent in this application is not limited to the above materials, but also includes other materials that can be used as positive electrode conductive agents in batteries.

[0080] In some embodiments, the binder in the first coating includes at least one of polyvinylidene fluoride (PVDF), poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, fluorinated acrylate resin, polyacrylic acid, polyacrylonitrile, polyimide, polyurethane, polyvinyl butyral, polyvinylpyrrolidone (PVP), acrylic acid-acrylonitrile-acrylamide copolymer, and acrylic acid-acrylonitrile-acrylate copolymer. The positive electrode binder of this application is not limited to the above materials, but also includes other materials that can be used as battery positive electrode binders.

[0081] In some embodiments, the mass ratio of the positive electrode active material to the MOF in the first coating is (70-85):(5-15).

[0082] In some embodiments, based on the total mass of the first coating, the first coating comprises the following components by mass percentage: 70%-85% positive electrode active material, 5%-15% MOF, 5%-20% conductive agent, and 2%-7% binder.

[0083] In some embodiments, the components in the first coating form a core-shell structure, wherein the core layer is a positive electrode active material and the shell layer includes a MOF, a conductive agent, and a binder.

[0084] In some embodiments, the first coating forming the core-shell structure of the components can be prepared in, but is not limited to, the following manner: MOF, conductive agent and binder are dispersed in solvent and mixed to obtain slurry. Then, positive electrode active material is mixed with slurry to obtain the final product.

[0085] In some embodiments, the positive electrode active material in the second coating includes LiFePO4 and Li4Ti5O. 12 LiMn x Fe 1-x At least one of PO4 and LiCoPO4.

[0086] The aforementioned positive electrode active material has been experimentally verified by the inventors to exhibit good ionic conductivity (ionic conductivity is related to the particle size of the material, etc.) in the system of this application, and can fully utilize the gradient structure of this application to achieve synchronous optimization of the ion conduction path.

[0087] In some embodiments, the second coating also includes carbon fibers.

[0088] Adding carbon fiber to the second coating can utilize its high thermal conductivity to evenly distribute heat on the electrode and alleviate localized polarization at low temperatures.

[0089] In some embodiments, the mass ratio of the positive electrode active material to the carbon fiber in the second coating is (88-93):(1-5).

[0090] In some embodiments, the second coating also includes at least one of a conductive agent and a binder.

[0091] In some embodiments, the conductive agent in the second coating includes at least one of carbon, carbon black, graphite, expanded graphite, graphene, graphene nanosheets, superconducting carbon, acetylene black, conductive carbon black, Ketjen black, carbon dots, graphitized carbon sheets, carbon nanotubes, carbon nanotubes, activated carbon, and mesoporous carbon. The positive electrode conductive agent in this application is not limited to the above materials, but also includes other materials that can be used as positive electrode conductive agents in batteries.

[0092] In some embodiments, the binder in the second coating includes at least one of polyvinylidene fluoride (PVDF), poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, fluorinated acrylate resin, polyacrylic acid, polyacrylonitrile, polyimide, polyurethane, polyvinyl butyral, polyvinylpyrrolidone (PVP), acrylic acid-acrylonitrile-acrylamide copolymer, and acrylic acid-acrylonitrile-acrylate copolymer. The positive electrode binder of this application is not limited to the above materials, but also includes other materials that can be used as battery positive electrode binders.

[0093] In some embodiments, based on the total mass of the second coating, the second coating comprises the following components by mass percentage: 88%-93% positive electrode active material, 1%-5% carbon fiber, 1%-5% conductive agent, and 1%-5% binder.

[0094] In some implementations, the aspect ratio of the carbon fiber is 50-100.

[0095] In some embodiments, the carbon fibers have a length of 250-1000 μm and a diameter of 5-10 μm.

[0096] In some embodiments, the positive current collector is a metal foil or a composite current collector. In some embodiments, the metal foil is aluminum foil. The composite current collector may include a metal foil substrate and a conductive layer disposed on at least one side of the metal foil substrate. In some embodiments, the conductive layer may include at least one of carbon, carbon black, graphite, expanded graphite, graphene, graphene nanosheets, carbon fibers, carbon nanofibers, graphitized carbon sheets, carbon nanotubes, activated carbon, and mesoporous carbon.

[0097] II. Electrochemical Device This application also provides an electrochemical device comprising any of the positive electrode, diaphragm, negative electrode, and electrolyte described herein.

[0098] In some implementations, the electrochemical device can be a lithium-ion secondary battery.

[0099] 1. Negative electrode In some embodiments, the negative electrode may include a negative electrode current collector and a negative electrode active material layer disposed on at least one side of the negative electrode current collector.

[0100] In some embodiments, the negative current collector is a metal foil or a composite current collector. In some embodiments, the metal foil is a copper foil. The composite current collector may include a metal foil substrate and a conductive layer disposed on at least one side of the metal foil substrate.

[0101] In some embodiments, the conductive layer may include at least one of carbon, carbon black, graphite, expanded graphite, graphene, graphene nanosheets, carbon fiber, carbon nanofiber, graphitized carbon sheet, carbon tube, carbon nanotube, activated carbon, and mesoporous carbon.

[0102] In some embodiments, the negative electrode active material may include natural graphite particles, synthetic graphite particles, hard carbon, soft carbon, mesophase carbon microspheres (MCMB), Sn, SnO2, SnO, Li4Ti5O 12 (LTO), Si materials, silicon-carbon (Si-C) composites The material is selected from at least one of silicon-nitrogen (Si-N) composite materials and silicon-oxygen (Si-O) composite materials. The negative electrode active material of this application is not limited to the above-mentioned materials, but also includes other materials that can be used as negative electrode active materials for batteries.

[0103] In some embodiments, the negative electrode binder may include at least one of polyacrylic acid, polymethacrylic acid, polyacrylate, polymethacrylate, polyacrylamide, styrene-butadiene rubber, acrylic styrene-butadiene rubber, acrylic acid-acrylonitrile-acrylamide copolymer, acrylic acid-acrylonitrile-acrylate copolymer, acrylonitrile-butadiene rubber, nitrile rubber, acrylonitrile-styrene-butadiene copolymer, acryloyl rubber, butyl rubber, fluororubber, polytetrafluoroethylene, polyvinyl alcohol, polyvinyl acetate, polyepoxychloropropane, polyphosphazene, polyacrylonitrile, polystyrene, latex, acrylic resin, phenolic resin, epoxy resin, carboxymethyl cellulose, hydroxypropyl cellulose, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl cellulose, carboxymethyl chitosan, polyester, polyamide, polyether, polyimide, polycarboxylic acid ester, polycarboxylic acid, polyurethane, alginate, fluorinated polymer, chlorinated polymer, polyvinylidene fluoride, and poly(vinylidene fluoride)-hexafluoropropylene. The negative electrode binder of this application is not limited to the above-mentioned materials, but also includes other materials that can be used as battery negative electrode binders.

[0104] In some embodiments, the negative electrode conductive agent may include at least one of carbon, carbon black, graphite, expanded graphite, graphene, graphene nanosheets, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon fibers, carbon nanofibers, graphitized carbon sheets, carbon nanotubes, carbon nanotubes, activated carbon, and mesoporous carbon. The negative electrode conductive agent of this application is not limited to the above-mentioned materials, but also includes other materials that can be used as battery negative electrode conductive agents.

[0105] 2. Electrolytes In some embodiments, the electrolyte may include at least one of a gel electrolyte, a solid electrolyte, and a liquid electrolyte.

[0106] In some embodiments, the liquid electrolyte may include a non-aqueous solvent and a lithium salt.

[0107] In some embodiments, the lithium salt may include at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, LiSiF6, LiBOB, and lithium difluoroborate.

[0108] In some embodiments, the non-aqueous solvent may be at least one of carbonate compounds, carboxylic acid ester compounds, and ether compounds.

[0109] In some embodiments, the carbonate compound may include at least one of chain carbonate compounds, cyclic carbonate compounds, and fluorocarbonate compounds.

[0110] In some embodiments, the chain carbonate compound may include diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), and combinations thereof.

[0111] In some embodiments, the cyclic carbonate compound may include ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), vinyl ethylene carbonate (VEC), and combinations thereof.

[0112] In some embodiments, the fluorocarbonate compound may include at least one of fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, and trifluoromethylethylene carbonate.

[0113] In some embodiments, the carboxylic acid ester compound may include at least one of methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanoic acid lactone, valerate lactone, mevalonate lactone, caprolactone, and methyl formate.

[0114] In some embodiments, the ether compound may include dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, and combinations thereof.

[0115] In some embodiments, the non-aqueous solvent may also include at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, trioctyl phosphate, and phosphate esters.

[0116] III. Electronic Equipment This application also provides an electronic device that includes the electrochemical device described above.

[0117] The electronic device described in this application is not particularly limited and can be any electronic device known in the prior art. The electrochemical device described in this application is also not particularly limited in its use and can be used in any electronic device known in the prior art. According to some embodiments of this application, the electronic device includes, but is not limited to, mobile phones, smartphones, laptops, tablets, wearable devices, smartwatches, smart bracelets, smart glasses, power banks, televisions, game consoles, game controllers, digital cameras, smart speakers, headphones, keyboards, mice, monitors, drones, audio equipment, home appliances, toys, power tools, automobiles, motorcycles, electric bicycles, bicycles, robots, robot dogs, industrial robots, and androids.

[0118] IV. Examples It should be noted that, in the specific embodiments of this application, lithium-ion batteries are used as an example of electrochemical devices to explain this application, but the electrochemical devices of this application are not limited to lithium-ion batteries.

[0119] Unless otherwise specified, all reagents, materials, and instruments used in the following examples and comparative examples are commercially available. Furthermore, unless otherwise specified, "parts" and "%" refer to mass measurements.

[0120] In the following examples and comparative examples, the use of some reagents and materials is as follows: MOF-1: ZIF-8, particle size 150-250nm, BET specific surface area ≥1500m² 2 / g.

[0121] MOF-2: MIL-101, particle size 100-300μm, BET specific surface area ≥2500m² 2 / g.

[0122] Carbon fiber-1: Aspect ratio 80 (diameter 7μm, length 560μm).

[0123] Carbon fiber-2: Aspect ratio 50 (diameter 8μm, length 400μm).

[0124] Carbon fiber-3: Aspect ratio 100 (5μm diameter, 500μm length).

[0125] Carbon fiber-4: Aspect ratio 60 (diameter 6μm, length 360μm).

[0126] Carbon fiber-5: Aspect ratio 40 (diameter 10μm, length 400μm).

[0127] Carbon fiber-6: Aspect ratio 120 (diameter 5μm, length 600μm).

[0128] Example 1 This embodiment provides a positive electrode sheet, including a positive current collector and a positive active layer disposed on one surface of the positive current collector. The positive active layer is provided with a first coating layer, a transition layer and a second coating layer sequentially from the direction close to the positive current collector to the direction away from the positive current collector. The first coating comprises a first coating layer, which, based on its total mass, comprises the following components by mass percentage: 75.67% LiNi. 0.6 Co 0.2 Mn 0.2 O2, 7.35% MOF-1, 11.06% conductive agent, 5.89% binder; The transition layer comprises Al2O3 and conductive carbon black, and has a core-shell structure, wherein the core layer is conductive carbon black with a particle size of 30 nm, the shell layer is Al2O3 with a thickness of 7.5 nm, the porosity of the transition layer is 40%, and the thickness is 100 nm. Based on the total mass of the second coating, the second coating comprises the following components in mass percentage: 91% LiFePO4, 3% carbon fiber-1, 3% conductive agent, and 3% binder. The average particle size of the positive electrode active material in the first coating is a=6 μm, and the average particle size of the positive electrode active material in the second coating is b=2 μm; The areal density of the first coating is σ1 = 3.4 g / cm³. 3 The areal density of the second coating is σ² = 3 g / cm³. 3 ; The thickness of the first coating is H1=45 μm, and the thickness of the second coating is H2=40 μm.

[0129] This embodiment also provides a lithium-ion battery (electrochemical device) including the above-mentioned positive electrode, negative electrode and electrolyte, wherein the negative electrode includes graphite as the negative electrode active material and the electrolyte is LiPF6.

[0130] The preparation method of the above-mentioned lithium-ion battery (electrochemical device) includes the following steps: 1. Preparation of positive electrode sheet (1) First coating The binder was dissolved in the solvent and stirred at 60°C for 2 hours until completely dissolved. The conductive agent was added and ultrasonically dispersed at 300W for 30 min. Then, MOF-1 was added and ball-milled at 400 rpm with a grinding bead diameter of 3 mm for 4 h to obtain a slurry with a viscosity of 3000±200 mPa·s. The mass ratio of the conductive agent, binder, MOF-1 and solvent in the slurry was 15:8:10:67. The conductive agent included carbon nanotubes and graphene in a mass ratio of 2:8. The binder included PVDF and PAN in a mass ratio of 4:1. The solvent included N-methylpyrrolidone. The slurry prepared above is mixed with LiNi 0.6 Co 0.2 Mn 0.2 O2 was mixed with a solid content of 70% and ball-milled in a planetary ball mill for 2 hours (30 rpm revolution and 15 rpm rotation) to obtain the first coating material. The first coating material was coated on a 10 μm aluminum foil with a wet film thickness of 150 μm, pre-dried in an oven at 80℃ for 10 minutes, and then vacuum-dried at 120℃ for 4 hours. The coating was then rolled to obtain the first coating.

[0131] (2) Transition layer Trimethylaluminum was vapor-deposited onto the surface of conductive carbon black at 150°C to obtain the transition layer material. The vapor deposition time was 45 min. The transition layer material was dispersed in ethanol and sprayed onto the surface of the first coating layer according to the set thickness. It was then dried at 80°C to obtain the transition layer.

[0132] (3) Second coating LiFePO4, conductive agent, and binder were added to N-methylpyrrolidone at a mass ratio of 94:3:3 to obtain a second coating slurry. After addition, the solid content in the solvent was 65%. 2 wt% of carbon fiber-1 of the second coating slurry was added, stirred and dispersed, and then coated on the transition layer to a wet film thickness of 100 μm. After drying at 80 °C, it was rolled to a thickness of 40 μm to obtain the second coating.

[0133] 2. Preparation of the negative electrode The preparation of graphite negative electrode sheets includes the following steps: graphite, carbon nanotubes and binder PVDF are mixed in solvent NMP at a mass ratio of 94%:3%:3% to form a uniform slurry. The slurry is then uniformly coated onto copper foil current collector by a coating machine. After drying at 100℃, the slurry is compacted (roller density ~1.7 g / cm³) and finally slit to obtain the electrode sheet.

[0134] 3. Preparation of electrolytes The electrolyte is prepared by dissolving LiPF6 in a solvent in a 1:1 volume ratio of ethylene carbonate and dimethyl carbonate to form a 1 mol / L solution.

[0135] 4. Preparation of lithium-ion batteries (electrochemical devices) The positive electrode (with its second coating close to the negative electrode), separator, and negative electrode prepared above are stacked in sequence and wound to obtain an electrode assembly; the electrode assembly is placed in an aluminum-plastic film packaging bag, dried, and then injected with electrolyte. After vacuum sealing, standing, formation, degassing, and edge trimming, a lithium-ion secondary battery is obtained.

[0136] Examples 2-21 and Comparative Examples 1-4 A series of positive electrode sheets are provided and prepared according to the method of Example 1. By adjusting the raw materials and related preparation processes, positive electrode sheets with different parameter structures as shown in Table 1 can be obtained.

[0137] The average particle size of the positive electrode active material is adjusted by ball milling, and the areal density and thickness of the coating are adjusted by coating parameters (scraper gap, coating speed and roller pressure, etc.).

[0138] Table 1. Examples 22-24 The difference between Examples 22-24 and Example 1 is that the amount and type of MOF added in the first coating are different, as shown in Table 2. All other aspects are the same as in Example 1.

[0139] Table 2. Comparative Example 6 The difference between Comparative Example 6 and Example 1 is that MOF-1 is not added to the first coating, while the rest are the same as Example 1.

[0140] Comparative Example 7 The difference between Comparative Example 7 and Example 1 is that no transition layer is introduced; otherwise, they are the same as Example 1.

[0141] Comparative Example 8 The difference between Comparative Example 8 and Example 1 is that the transition layer includes a non-metallic oxide SiO2, while the rest are the same as in Example 1.

[0142] The only difference between this comparative example and Example 1 is that: When preparing the transition layer, the trimethylaluminum precursor was replaced with tetraethyl orthosilicate.

[0143] V. Testing Methods 1. -20℃ Capacity Retention Rate Test Lithium-ion batteries were fabricated using the cathode materials described in the embodiments and comparative examples of this application, and tested using the method described in this application. The lithium-ion batteries were repeatedly charged and discharged to calculate their capacity retention rate. First, the lithium-ion batteries were placed in an environment of -20°C for the first charge and discharge cycle. Constant current charging was performed at a charging current of 1C until the upper limit voltage reached 4.35V. Then, constant current discharging was performed at a discharging current of 1C until the final voltage reached 2.5V. This constituted the first charge-discharge cycle, and the discharge capacity of the first cycle was recorded. Then, the above method was repeated for 100 charge-discharge cycles, and the discharge capacity of the 100th cycle was recorded to calculate the capacity retention rate.

[0144] 2. Internal resistance growth test Test method: AC impedance spectroscopy (EIS) combined with DC internal resistance (DCR) was used for testing at 25℃ and -20℃ respectively.

[0145] EIS test: frequency range 0.1Hz-100kHz, amplitude 10mV, ohmic internal resistance (RΩ) obtained through the real axis intercept in the high frequency region, and charge transfer resistance (Rct) fitted by a semicircle in the mid frequency region. DCR test: 1C pulse charge and discharge (10s), calculate the total internal resistance based on ΔV / ΔI; Cyclic comparison: Test once every 50 cycles, calculate the internal resistance growth rate (%) = (R) n -R0) / R0×100%.

[0146] 3. Peel strength test Test method: Refer to ASTM D903 standard and use a universal testing machine to perform a 180° peel test.

[0147] Sample preparation: Fix the electrode (25mm wide) to the current collector (aluminum foil) with double-sided tape, and clamp the unbonded end to the fixture; Test conditions: tensile speed 50 mm / min, record peel force (N), calculate unit width strength (N / cm); Data requirements: Take the average value of 5 points, and the ideal peel strength is ≥1N / cm.

[0148] 4. Electrode temperature difference test Test method: Infrared thermal imager combined with multi-point thermocouple monitoring.

[0149] Operating condition simulation: During a 1C charge-discharge cycle, the electrode surface is scanned using a FLIR infrared camera (resolution ≤ 0.05℃); Point arrangement: Place type K thermocouples (accuracy ±0.5℃) at the center / edge of the electrode to record the temperature simultaneously; Data Analysis: Calculate the maximum temperature difference ΔT = Tmax -T min The requirement is that ΔT < 5℃ during the cycle.

[0150] The performance test results are shown in Table 3 below: Table 3. As can be seen from Table 3 above, the positive electrode sheet provided by this application has significantly improved low-temperature electrochemical performance. The secondary battery prepared using the positive electrode sheet of the embodiment of this application has an average capacity retention rate of over 72.8% after 100 charge-discharge cycles at -20°C, an internal resistance increase of no more than 32.6 mΩ, an electrode temperature difference of no more than 7.8°C, and an interlayer peel strength between the first coating and the second coating of over 7.6 N / m.

[0151] According to Comparative Examples 1-2 and 5, when the particle size of the positive electrode active material in the first and second coatings does not satisfy the relationship b < a, a good electron-ion conduction path cannot be formed, which affects the electrode performance.

[0152] According to Comparative Examples 3-4, both excessively high and excessively low thickness of the transition layer will affect the peel strength between the first and second coatings, thereby affecting the electrochemical performance of the electrode.

[0153] According to Comparative Example 6, the absence of MOF in the first coating will result in the cathode failing to achieve good low-temperature performance.

[0154] According to Comparative Example 7, without the introduction of a transition layer, the volume expansion of the positive electrode material during battery charging and discharging will lead to interlayer delamination, which will seriously affect the low-temperature electrochemical performance of the battery.

[0155] According to Comparative Example 8, the dielectric constant of the non-metallic oxide transition layer is unsuitable, and it cannot realize the function of the metal oxide transition layer in this application.

[0156] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A positive electrode sheet, characterized in that, The positive electrode includes a positive current collector and a positive active layer disposed on at least one surface of the positive current collector. The positive active layer is provided with a first coating, a transition layer with a thickness of 50-200 nm, and a second coating in sequence from the direction close to the positive current collector to the direction away from the positive current collector. The first coating includes a MOF and a positive active material; the transition layer includes a metal oxide; and the second coating includes a positive active material. The average particle size of the positive electrode active material in the first coating is a μm, and the average particle size of the positive electrode active material in the second coating is b μm, where b < a.

2. The positive electrode sheet as described in claim 1, characterized in that, Satisfy at least one of the following (1)-(3): (1) The areal density of the first coating is σ1g / cm³. 3 The areal density of the second coating is σ²g / cm³. 3 , 1.05≤σ1 / σ2≤1.2; (2) The thickness of the first coating is H1μm, the thickness of the second coating is H2μm, and 1.05≤H1 / H2≤1.2; (3) 1.05≤a / b≤3.

95.

3. The positive electrode sheet as described in claim 2, characterized in that, Satisfy at least one of the following (1)-(6): (1)3.2 g / cm 3 ≤σ1≤3.5 g / cm 3 ; (2)2.8 g / cm 3 ≤σ2≤3.2 g / cm 3 ; (3) 42 μm≤H1≤48 μm; (4)38 μm≤H2≤42 μm; (5) 2 μm ≤ a ≤ 10 μm; (6) 1 μm≤b≤3 μm.

4. The positive electrode sheet as described in claim 1, characterized in that, Satisfy at least one of the following (1)-(2): (1) The MOF includes at least one of ZIF-8 and MIL-101; (2) The porosity of the transition layer is 30%-50%.

5. The positive electrode sheet as described in claim 1, characterized in that, Satisfy at least one of the following (1)-(2): (1) The positive electrode active material in the first coating includes LiNi 0.6 Co 0.2 Mn 0.2 O2, LiCoO2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.5 Co 0.2 Mn 0.3 At least one of O2; (2) The positive electrode active material in the second coating includes LiFePO4 and Li4Ti5O 12 LiMn x Fe 1-x At least one of PO4 and LiCoPO4.

6. The positive electrode sheet as described in claim 1, characterized in that, The second coating also includes carbon fibers.

7. The positive electrode sheet as described in claim 6, characterized in that, Satisfy at least one of the following (1)-(2): (1) The mass ratio of the positive electrode active material to the MOF in the first coating is (70-85):(5-15); (2) The mass ratio of the positive electrode active material to the carbon fiber in the second coating is (88-93):(1-5).

8. The positive electrode sheet as described in claim 6, characterized in that, The aspect ratio of the carbon fiber is 50-100.

9. An electrochemical device, characterized in that, It includes the positive electrode, separator, negative electrode and electrolyte as described in any one of claims 1-8.

10. An electronic device, characterized in that, The electronic device includes the electrochemical device of claim 9.

Images