Positive pole piece, secondary battery and electric equipment

Abstract

The invention provides a positive pole piece, a secondary battery and electric equipment. The positive pole piece comprises a positive pole current collector and a positive pole active layer arranged on at least one surface of the positive pole current collector, the positive pole active layer is sequentially provided with a first coating, a middle conductive layer and a second coating in the direction from the position close to the positive pole current collector to the position away from the positive pole current collector, and the first coating contains O3-phase lithium cobalt oxide; the middle conductive layer comprises a conductive agent and a binder; and the second coating contains O2-phase lithium cobalt oxide. The intermediate conductive layer containing the conductive agent and the binder is added to the O3-phase lithium cobalt oxide layer and the O2-phase lithium cobalt oxide layer, so that the high ion conductivity of the O2-phase lithium cobalt oxide layer on the surface layer can be further extended to the O3-phase lithium cobalt oxide layer at the bottom, and the rate capability of the prepared secondary battery is further improved; and the advantages of high capacity of intermediate O3-phase lithium cobalt oxide and high voltage resistance of O2-phase lithium cobalt oxide can be fully exerted.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a positive electrode sheet, a secondary battery and an electrical device. Background Art

[0002] Lithium-ion batteries have been widely used in terminal products (such as smart phones, digital cameras, laptops and electric vehicles) due to their advantages such as high energy density, high operating voltage, long service life, low self-discharge rate and environmental friendliness, which also puts higher requirements on their capacity, life, fast charging and safety.

[0003] For example, in order to improve the charging efficiency, it is necessary to increase the charging voltage and use a strong current so that the battery can receive a large amount of electrical energy in a shorter time. In the highly delithiated state or high voltage, the O-O bond of O2 phase lithium cobalt oxide is longer, and the dimer structure formed is more stable, making it more difficult for O to separate from the skeleton, thus showing better stability; in addition, the lithium oxygen layer and cobalt oxygen layer of O2 phase lithium cobalt oxide have strong repulsion, and the O-Li-O layer spacing is large, which is conducive to the escape of lithium ions. Therefore, O2 phase lithium cobalt oxide has high voltage stability and Li + Although O2-phase lithium cobalt oxide cathode materials have become an important research direction, they still have many problems in practical use, especially for O2-phase lithium cobalt oxide prepared by ion exchange method. The particles have serious "slip" phenomenon after rolling, and the adhesion between the particles and the current collector is poor, which even causes the active material area to fall off, deteriorating the rate performance and cycle performance of the battery.

[0004] Although the problem of adhesion between O2 phase lithium cobalt oxide particles and current collector can be solved to a certain extent by compounding and homogenizing O3 phase lithium cobalt oxide and O2 phase lithium cobalt oxide and coating, as the voltage increases, especially when the voltage increases to above 4.55V, traditional O3 phase lithium cobalt oxide will undergo more H1-3 irreversible phase changes, accompanied by oxygen escape, particle breakage and other problems, resulting in rapid performance deterioration.

[0005] Therefore, it is necessary to provide a lithium ion battery that can be used in a long-term cycle at a high voltage. Summary of the invention

[0006] The purpose of this application is to overcome the above-mentioned problems in the prior art and improve the electrochemical cycle performance of lithium-ion batteries such as cycle stability, rate performance, and capacity at high voltage.

[0007] To achieve the above-mentioned purpose, the first aspect of the present application provides a positive electrode plate, which includes a positive electrode collector and a positive electrode active layer arranged on at least one surface of the positive electrode collector, and the positive electrode active layer is provided with a first coating layer, an intermediate conductive layer, and a second coating layer in sequence from close to the positive electrode collector to far away from the positive electrode collector, wherein the first coating layer contains O3 phase lithium cobalt oxide; the intermediate conductive layer contains a conductive agent and a binder; and the second coating layer contains O2 phase lithium cobalt oxide.

[0008] As an embodiment of the present application, the positive electrode sheet satisfies: 3.0≤σ1 / σ2≤5.4, where σ1 represents the surface density of the first coating layer, in units of mg / cm 2 ; σ2 represents the surface density of the second coating, in mg / cm 2 .

[0009] As an embodiment of the present application, the surface density of the first coating layer is σ1=11-15 mg / cm 2 .

[0010] As an embodiment of the present application, the surface density of the second coating layer is σ2=2.7-3.8 mg / cm 2 .

[0011] As an embodiment of the present application, the first coating layer and the second coating layer further include a conductive agent and a binder.

[0012] Based on the total mass of the first coating, the mass content of the binder in the first coating is a%; based on the total mass of the second coating, the mass content of the binder in the second coating is b%; based on the total mass of the intermediate conductive layer, the mass content of the binder in the intermediate conductive layer is c%.

[0013] As an embodiment of the present application, the positive electrode sheet satisfies: a <c。

[0014] As an embodiment of the present application, the positive electrode sheet satisfies: b <c。

[0015] As an embodiment of the present application, based on the total mass of the first coating layer, the first coating layer includes the following components in percentage by mass: 95-99% of O3 phase lithium cobalt oxide, 0.5-3% of a conductive agent, and 0.5-3% of a binder.

[0016] As an embodiment of the present application, based on the total mass of the second coating layer, the second coating layer includes the following components in percentage by mass: 95-99% of O2-phase lithium cobalt oxide, 0.5-3% of a conductive agent, and 0.5-3% of a binder.

[0017] As an embodiment of the present application, based on the total mass of the intermediate conductive layer, the intermediate conductive layer includes the following components in percentage by mass: 90-97% of a binder, and 3-10% of a conductive agent.

[0018] As an embodiment of the present application, the conductive agent in the first coating layer, the second coating layer, and the intermediate conductive layer independently includes at least one of conductive carbon black, graphite, carbon fiber, carbon nanotubes, and graphene.

[0019] As an embodiment of the present application, the binder in the first coating layer, the second coating layer, and the intermediate conductive layer independently includes at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), styrene-butadiene rubber (SBR) emulsion, and polyimide (PI).

[0020] As an implementation scheme of the present application, the positive electrode plate satisfies: H1:H2:H3=(12.8~17.4):(3.2~4.3):1; wherein H1 represents the thickness of the first coating, in μm; H2 represents the thickness of the second coating, in μm; H3 represents the thickness of the intermediate conductive layer, in μm.

[0021] As an embodiment of the present application, the thickness of the first coating layer H1=25.7-34.7 μm.

[0022] As an embodiment of the present application, the thickness of the second coating layer H2=6.4-8.7 μm.

[0023] As an embodiment of the present application, the thickness H3 of the intermediate conductive layer is 1-3 μm.

[0024] As an embodiment of the present application, the compaction density of the positive electrode sheet is 3.78 to 4.34 g / cm 3 .

[0025] As an embodiment of the present application, the D of the O3 phase lithium cobalt oxide V 50 is 14.0~17.0μm, D V 50 represents the particle size corresponding to when the cumulative volume of the O3 phase lithium cobalt oxide particles reaches 50%.

[0026] As an embodiment of the present application, the D of the O2 phase lithium cobalt oxide V 50' is 5.5~9.0μm, D V 50' represents the particle size corresponding to when the cumulative volume of the O2 phase lithium cobalt oxide particles reaches 50%.

[0027] As an embodiment of the present application, in the intermediate conductive layer, the particle size distribution of the conductive agent satisfies: 0.44≤D V 50” / (DV 90”-D V 10”)≤0.93, where D V 50" indicates the particle size corresponding to when the cumulative volume of the particles of the conductive agent reaches 50%, in μm; D V 90" indicates the particle size corresponding to when the cumulative volume of the particles of the conductive agent reaches 90%, in μm; D V 10" indicates the particle size corresponding to when the cumulative volume of the particles of the conductive agent reaches 10%, in μm.

[0028] A second aspect of the present application provides a secondary battery, wherein the secondary battery comprises the positive electrode plate, a separator, a negative electrode plate and an electrolyte as described in the first aspect of the present application.

[0029] According to a third aspect of the present application, there is provided an electrical device, wherein the electrical device comprises the secondary battery according to the second aspect of the present application.

[0030] Compared with the prior art, the beneficial effects of this application are:

[0031] The present application adds an intermediate conductive layer containing a conductive agent and a binder to the O3 phase lithium cobalt oxide layer and the O2 phase lithium cobalt oxide layer, which can effectively prevent the above two coatings from generating gaps due to different elastic moduli. At the same time, the added conductive agent forms a specific conductive network structure between the O3 phase lithium cobalt oxide layer and the O2 phase lithium cobalt oxide layer, which is beneficial to further extend the high ion conductivity of the surface O2 phase lithium cobalt oxide layer to the bottom O3 phase lithium cobalt oxide layer, further improving the rate performance of the prepared secondary battery, and can give full play to the advantages of high energy density, high capacity of the intermediate O3 phase lithium cobalt oxide and high voltage resistance of the O2 phase lithium cobalt oxide. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a schematic diagram of the structure of the positive electrode plate of Example 1 of the present application. In the figure, 1 represents the second coating (O2 phase lithium cobalt oxide layer), 2 represents the first coating (O3 phase lithium cobalt oxide layer), 3 represents the intermediate conductive layer, and 4 represents the current collector aluminum foil. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application are described clearly and completely below. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present application.

[0034] In the present application, the technical features described in an open manner include closed technical solutions composed of the listed features, and also include open technical solutions containing the listed features.

[0035] In this application, when it comes to numerical ranges, unless otherwise specified, the above numerical ranges are deemed to be continuous and include the minimum and maximum values ​​of the range, as well as each value between such minimum and maximum values. Further, when a range refers to an integer, each integer between the minimum and maximum values ​​of the range is included. In addition, when multiple ranges are provided to describe features or characteristics, the ranges can be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges included therein.

[0036] The reagents or instruments used in this application without indicating the manufacturer are all conventional products that can be obtained through commercial purchase.

[0037] In a first aspect of the present application, a positive electrode plate is provided, which includes a positive electrode collector and a positive electrode active layer arranged on at least one surface of the positive electrode collector, wherein the positive electrode active layer is provided with a first coating layer, an intermediate conductive layer, and a second coating layer in sequence from close to the positive electrode collector to far away from the positive electrode collector, wherein the first coating layer contains O3 phase lithium cobalt oxide; the intermediate conductive layer contains a conductive agent and a binder; and the second coating layer contains O2 phase lithium cobalt oxide.

[0038] This application can avoid the defect of poor adhesion between O2 phase lithium cobalt oxide and current collector by setting the first coating (O3 phase lithium cobalt oxide layer) on the surface of current collector, and setting the second coating (O2 phase lithium cobalt oxide layer) on the surface of positive electrode plate, which can accelerate the conduction speed of Li and further enhance the low temperature and rate performance. At the same time, O2 phase lithium cobalt oxide has a low voltage discharge voltage platform, and coating it on the surface is more conducive to the performance of gram capacity and increase the capacity of battery. However, due to the different elastic modulus of O2 phase lithium cobalt oxide and O3 phase lithium cobalt oxide, gaps are prone to appear during the cycle, resulting in the destruction of the "conductive network" and deterioration of cycle performance.

[0039] Therefore, the inventor of the present application further adds an intermediate conductive layer containing a conductive agent and a binder to the O3 phase lithium cobalt oxide layer and the O2 phase lithium cobalt oxide layer. The presence of the binder can effectively prevent the above two coatings from generating gaps due to different elastic moduli. At the same time, the added conductive agent forms a specific conductive network structure between the O3 phase lithium cobalt oxide layer and the O2 phase lithium cobalt oxide layer, which is conducive to further extending the high ion conductivity of the surface O2 phase lithium cobalt oxide layer to the bottom O3 phase lithium cobalt oxide layer, further improving the rate performance of the prepared secondary battery. In addition, the addition of the intermediate conductive layer can further improve the protection of the O3 phase lithium cobalt oxide layer under high voltage, reduce the irreversible transformation of the O3 phase under high voltage, and improve the cycle stability of the prepared secondary battery.

[0040] In some embodiments, the positive electrode sheet satisfies: 3.0≤σ1 / σ2≤5.4, where σ1 represents the surface density of the first coating layer, in mg / cm 2 ; σ2 represents the surface density of the second coating, in mg / cm 2 .

[0041] The lattice of O2 phase lithium cobalt oxide belongs to the P63mc space group, with a close-packed oxygen atom stacking mode of ABCBAB, which helps to improve the stability of the material under high pressure. The relatively high density of O2 phase lithium cobalt oxide layer can improve the volume energy density of the secondary battery prepared by it, making it have a higher capacity; at the same time, the high stacking density O2 phase lithium cobalt oxide layer has a smaller average pore size and a narrower pore size distribution, which can effectively protect the inner layer of O3 phase lithium cobalt oxide from phase transformation under high voltage, thereby improving the cycle stability.

[0042] The crystal structure of O3 phase lithium cobalt oxide belongs to the R3m space group and has an ABCABC stacking pattern of tightly stacked oxygen atoms. Therefore, it has a higher energy density. During the charging process, O3 phase lithium cobalt oxide undergoes a structural transformation from O3 to O1 phase, accompanied by layer slip, resulting in drastic changes in unit cell parameters and changes in the volume of material particles, which may lead to structural and capacity degradation.

[0043] Therefore, the value of σ1 / σ2 can be any value among 3.0, 3.4, 4.0, 4.6, 5.4, or an interval range consisting of any two values.

[0044] In some embodiments, the surface density of the first coating is σ1=11-15 mg / cm 2 For example, the surface density of the first coating (O3 phase lithium cobalt oxide layer) may be 11.04 mg / cm 2 、12.98mg / cm 2 、14.93mg / cm 2Any value in , or an interval consisting of any two values.

[0045] In some embodiments, the surface density of the second coating is σ2=2.7-3.8 mg / cm 2 For example, the surface density of the second coating (O2 phase lithium cobalt oxide layer) may be 2.76 mg / cm 2 、3.25mg / cm 2 、3.73mg / cm 2 Any value in , or an interval consisting of any two values.

[0046] The provision of the above-mentioned O2 phase lithium cobalt oxide layer and O3 phase lithium cobalt oxide layer with appropriate density is beneficial to simultaneously improving the energy density and cycle stability under high pressure of the prepared battery.

[0047] In some embodiments, the first coating layer and the second coating layer further include a conductive agent and a binder. In this application, the types of the conductive agent and the binder are not limited, and the conductive agent and the binder commonly used in secondary batteries can be used in this application to prepare the slurry of the positive electrode active layer.

[0048] In some embodiments, the conductive agents in the first coating layer, the second coating layer, and the intermediate conductive layer independently include, but are not limited to, at least one of conductive carbon black, graphite, carbon fiber, carbon nanotubes, and graphene.

[0049] In some embodiments, the binder in the first coating layer, the second coating layer, and the intermediate conductive layer independently includes, but is not limited to, at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), styrene-butadiene rubber (SBR) emulsion, and polyimide (PI).

[0050] Based on the total mass of the first coating, the mass content of the binder in the first coating is recorded as a%; based on the total mass of the second coating, the mass content of the binder in the second coating is recorded as b%; based on the total mass of the intermediate conductive layer, the mass content of the binder in the intermediate conductive layer is recorded as c%.

[0051] In some embodiments, the positive electrode sheet satisfies: a <c。

[0052] In some embodiments, the positive electrode sheet satisfies: b <c。

[0053] In this application, the presence of sufficient binder in the middle conductive layer can effectively make up for the gaps between the first coating and the second coating due to different elastic moduli. Adding a conductive agent in the middle layer can effectively transfer the high conductivity of the surface O2 phase lithium cobalt oxide to the inner O3 phase lithium cobalt oxide layer, significantly improving the conductivity of the entire electrode and effectively improving the high voltage fast charging performance of the battery.

[0054] In some embodiments, based on the total mass of the first coating layer, the first coating layer includes the following components in percentage by mass: 95-99% of O3 phase lithium cobalt oxide, 0.5-3% of a conductive agent, and 0.5-3% of a binder.

[0055] In some embodiments, based on the total mass of the second coating layer, the second coating layer includes the following components in percentage by mass: 95-99% of O2-phase lithium cobalt oxide, 0.5-3% of a conductive agent, and 0.5-3% of a binder.

[0056] In some embodiments, based on the total mass of the intermediate conductive layer, the intermediate conductive layer includes the following components in percentage by mass: 90-97% of a binder, and 3-10% of a conductive agent.

[0057] In some embodiments, the positive electrode plate satisfies: H1:H2:H3=(12.8~17.4):(3.2~4.3):1; wherein H1 represents the thickness of the first coating, in μm; H2 represents the thickness of the second coating, in μm; H3 represents the thickness of the intermediate conductive layer, in μm.

[0058] Providing an intermediate layer of moderate thickness in the first and second coatings can ensure excellent interface bonding performance between the first and second coatings while having high ionic conductivity, thereby improving the fast charging performance of the prepared secondary battery and meeting the demand for thin and light use of electronic products.

[0059] As an embodiment of the present application, the thickness H1 of the first coating layer is 25.7-34.7 μm. For example, the thickness H1 of the first coating layer can be any value of 25.7 μm, 29.5 μm, 30.2 μm, 30.9 μm, 32.5 μm, 34.2 μm, 34.7 μm, or an interval consisting of any two values.

[0060] As an embodiment of the present application, the thickness H2 of the second coating layer is 6.4-8.7 μm. For example, the thickness H2 of the first coating layer can be any value of 6.4 μm, 7.4 μm, 7.5 μm, 7.7 μm, 8.1 μm, 8.5 μm, 8.7 μm, or an interval consisting of any two values.

[0061] As an embodiment of the present application, the thickness H3 of the intermediate conductive layer is 1-3 μm. Exemplarily, the thickness H3 of the first coating layer can be any value of 1 μm, 2 μm, 3 μm, or an interval consisting of any two values.

[0062] In some embodiments, the particle size distribution of the conductive agent in the intermediate conductive layer satisfies: 0.44≤D V 50” / (D V 90”-D V 10”)≤0.93, where D V 50" indicates the particle size corresponding to when the cumulative volume of the particles of the conductive agent reaches 50%, in μm; D V 90" indicates the particle size corresponding to when the cumulative volume of the particles of the conductive agent reaches 90%, in μm; D V 10" indicates the particle size corresponding to the cumulative volume of the conductive agent particles reaching 10%, in μm. The particle size distribution of the conductive agent meets this range, and can form a better dispersion in the binder matrix of the intermediate conductive layer. A stable conductive network can be formed between the conductive agents of different sizes, and the conductive permeability can be reduced. Therefore, at a lower addition amount of the conductive agent, it is beneficial for ions to be quickly conducted from the O2 phase lithium cobalt oxide layer to the inner O3 phase lithium cobalt oxide layer. The D V 50” / (D V 90”-D V 10”) can be any value among 0.44, 0.54, 0.79, 0.93, or an interval consisting of any two values.

[0063] In some embodiments, the particle size D of the conductive agent in the intermediate conductive layer is V 50" = 0.35-0.65 μm, specifically, it can be any value among 0.35 μm, 0.40 μm, 0.55 μm, 0.65 μm, or an interval consisting of any two values.

[0064] In some embodiments, the particle size D of the conductive agent in the intermediate conductive layer is V 90" = 0.80-1.05 μm, specifically, it can be any value among 0.80 μm, 0.95 μm, 1.05 μm, or an interval consisting of any two values.

[0065] In some embodiments, the particle size D of the conductive agent in the intermediate conductive layer is V 10"=0.05-0.35 μm, specifically, it can be any value among 0.05 μm, 0.15 μm, 0.25 μm, 0.35 μm, or an interval consisting of any two values.

[0066] In some embodiments, the O3 phase lithium cobalt oxide has a D V 50 is 14.0~17.0μm, D V50 represents the particle size corresponding to when the cumulative volume of the O3 phase lithium cobalt oxide particles reaches 50%, which can be any value among 14.0 μm, 14.5 μm, 15.5 μm, 17.0 μm, or an interval consisting of any two values.

[0067] In some embodiments, the O2 phase lithium cobalt oxide has a D V 50' is 5.5~9.0μm, D V 50' represents the particle size corresponding to when the cumulative volume of the O2 phase lithium cobalt oxide particles reaches 50%, which can be any value among 5.5 μm, 6.5 μm, 8.0 μm, 9.0 μm, or an interval consisting of any two values.

[0068] For the positive electrode active material lithium cobalt oxide, a smaller particle size can effectively improve the wettability with the electrolyte and improve the ion conductivity of the battery. However, the smaller the particle size, the larger the specific surface area and the more surface active sites. As a result, it may have side reactions with the components in the electrolyte due to its higher activity, thereby reducing the kinetic cycle performance of the battery. Therefore, lithium cobalt oxide can have both high ion conductivity and cycle stability within the above-mentioned suitable particle size range.

[0069] In addition, lithium cobalt oxide can also be doped with elements. The matrix formula of doped lithium cobalt oxide is: LiCo 1-x Q x O 2- y Z y , 0<x≤0.05, 0<z≤0.05; wherein Q and Z represent doping elements, the Q element can be at least one of Mg, Al, Ti, La, Y, Zr, Ni, Mn, Ce, Sc, W, Nb and V; the Z element can be at least one of S, F and Cl to improve the conductivity of lithium cobalt oxide.

[0070] In some embodiments, the compaction density of the positive electrode sheet is 3.96 to 4.36 g / cm 3 The positive electrode sheet has good energy density and cycle stability within the above compaction density range.

[0071] In a second aspect of the present application, a secondary battery is provided, the secondary battery comprising the positive electrode sheet, the separator, the negative electrode sheet and the electrolyte as described in the first aspect of the present application. It should be noted that the electrolyte may be a solid electrolyte or an electrolyte solution, and the electrolyte solution contains an ionizable electrolyte.

[0072] In some embodiments, the negative electrode plate includes a negative electrode current collector and a negative electrode active layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode active layer includes a negative electrode active material. The present application does not limit the type of negative electrode active material, and the commonly used negative electrode active materials in the art can be used to prepare a secondary battery in the present application. The negative electrode active material includes but is not limited to at least one of a silicon negative electrode, a carbon material, and a silicon-carbon negative electrode material.

[0073] In the present application, there is no limitation on the types of positive electrode current collector, negative electrode current collector, separator and electrolyte, which can be selected according to the requirements. Common current collector materials, separators and electrolytes in the art can be used in the present application.

[0074] In some embodiments, the negative electrode current collector may preferably be made of copper foil or carbon-coated copper foil.

[0075] The positive electrode current collector may be made of metal materials such as aluminum, stainless steel, nickel plating, titanium, tantalum, etc.; carbon materials such as carbon cloth, carbon paper, etc.; a composite material formed by a polymer and a metal layer. In some embodiments, the positive electrode current collector is preferably made of aluminum foil.

[0076] In some embodiments, the type of solvent used to form the positive electrode slurry and / or the negative electrode slurry is not limited, as long as it is a solvent that can dissolve or disperse the positive electrode active material, the negative electrode active material, the conductive agent, the binder, and the dispersant.

[0077] In the secondary battery described in the present application, the type of the separator is not particularly limited and can be selected according to actual needs. The separator can be a polypropylene film, a polyethylene film, a polyvinylidene fluoride, a spandex film, an aramid film, or a multilayer composite film modified by a coating.

[0078] In the secondary battery described in the present application, the type of electrolyte is not particularly limited and can be selected according to actual needs.

[0079] In some embodiments, the preparation of a secondary battery includes: stacking the positive electrode sheet, the separator, and the negative electrode sheet in order, placing the separator between the positive and negative electrode sheets to play an isolating role, and then winding them into a square bare cell, loading them into a battery casing, and then baking them at 65 to 95°C to remove water, injecting electrolyte, sealing, and obtaining a secondary battery after standing, hot and cold pressing, formation, clamping, capacity division and other processes.

[0080] In some embodiments, the secondary battery may include an outer package, which may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the secondary battery may also be a soft package, such as a bag-type soft package, and the material of the soft package may be plastic, such as one or more of polypropylene, polybutylene terephthalate, polybutylene succinate, etc., or an aluminum-plastic film, such as an aluminum-plastic film formed by a composite of a PA layer, an aluminum layer, and a PP layer. There is no particular limitation on the shape of the secondary battery, which may be cylindrical, square, or any other shape.

[0081] The third aspect of the present application provides an electric device, which includes the secondary battery described in the second aspect of the present application. The electric device can be an application device such as a vehicle, a mobile phone, a portable device, a laptop computer, a ship, a spacecraft, an electric toy, and an electric tool. The vehicle can be a new energy vehicle, and the new energy vehicle can be a pure electric vehicle, a hybrid vehicle, or an extended-range vehicle, etc.; the spacecraft includes an airplane, a rocket, a space shuttle, and a spacecraft, etc.; the electric toy includes a fixed or mobile electric toy, such as a game console, an electric car toy, an electric ship toy, and an electric airplane toy, etc.; the electric tool includes a metal cutting electric tool, a grinding electric tool, an assembly electric tool, and a railway electric tool, such as an electric drill, an electric grinder, an electric wrench, an electric screwdriver, an electric hammer, an impact drill, a concrete vibrator, and an electric planer, etc. The embodiment of the present application does not impose any special restrictions on the above-mentioned device.

[0082] The following are specific embodiments of the present application, and the technical scheme of the present application is further described in conjunction with the embodiments, but the present application is not limited to these embodiments. The reagents, methods and equipment used in the present application, unless otherwise specified, are conventional reagents, methods and equipment in the art.

[0083] Example 1

[0084] Provide a positive electrode sheet, according to Figure 1 As shown in the structural schematic diagram, the first coating layer 2, the intermediate conductive layer 3, and the second coating layer 1 are sequentially coated on the surface of the positive electrode current collector (aluminum foil is selected), and after drying, the positive electrode sheet can be obtained. The specific preparation method includes the following steps:

[0085] (1) First coating (O3 phase lithium cobalt oxide layer):

[0086] O3 phase lithium cobalt oxide (LiCoO2), conductive agent carbon black, and PVDF were mixed and dissolved in a solvent N-methylpyrrolidone (NMP) at a mass ratio of 98:1:1 to prepare a slurry, which was then coated on one side of the surface of an aluminum foil, dried in an oven at 100°C to constant weight, and subjected to an XRD test. The test results showed the diffraction peaks (003), (101), (102), (104), (105), (107), (108), (110), and (113) that are unique to O3 phase lithium cobalt oxide.

[0087] (2) Intermediate conductive layer:

[0088] The conductive agent carbon black and the binder PVDF are mixed and dissolved in a solvent N-methylpyrrolidone (NMP) in a mass ratio of 5:95 to prepare a slurry, and then applied to the surface of the first coating formed by coating in step (1);

[0089] (3) Second coating (O2 phase lithium cobalt oxide layer):

[0090] O2 phase lithium cobalt oxide (LiCoO2), conductive agent carbon black, and PVDF are mixed and dissolved in a solvent N-methylpyrrolidone (NMP) in a mass ratio of 98:1:1 to prepare a slurry, which is then coated on the surface of the intermediate conductive layer formed by coating in step (2), and dried in an oven at 100° C. to constant weight and rolled to obtain the positive electrode sheet. XRD test is performed again, and the test results show diffraction peaks (002), (100), (101), (102), (103), (104), (106), (105), (110), and (112) that are unique to O2 phase lithium cobalt oxide;

[0091] The structural parameters of the relevant raw materials in each coating and the relevant parameters of the obtained positive electrode sheet are shown in Table 1.

[0092] Embodiments 2 to 22

[0093] A series of positive electrode sheets are provided, which are prepared according to the method of Example 1. By adjusting the raw materials and related preparation processes, positive electrode sheets with different parameter structures described in Table 1 can be obtained.

[0094] Table 1

[0095]

[0096]

[0097] Embodiment 23

[0098] This embodiment provides a positive electrode sheet, which is prepared by referring to the method of the embodiment. The difference from embodiment 1 is that the lithium cobalt oxide in the O2 phase and the O3 phase is doped with elements, and the general structural formula is: LiCo0.95 Mg 0.05 O 1.98 S 0.02 , other parameters are the same as the positive electrode sheet of Example 1.

[0099] Comparative Example 1

[0100] This comparative example provides a positive electrode plate, which is prepared by referring to the method of the embodiment. The difference from Example 1 is that the second coating (lithium cobalt oxide layer containing O2 phase) is directly coated on the surface of the positive electrode current collector aluminum foil, and the coating thickness = the first layer thickness + the intermediate conductive layer thickness + the second coating thickness.

[0101] Comparative Example 2

[0102] This comparative example provides a positive electrode plate, which is prepared by referring to the method of the embodiment. The difference from Example 1 is that only the first coating (O3 phase lithium cobalt oxide layer) is directly coated on the surface of the positive electrode current collector aluminum foil, and the coating thickness = the first layer thickness + the intermediate conductive layer thickness + the second coating thickness.

[0103] Comparative Example 3

[0104] This comparative example provides a positive electrode plate, which is prepared by referring to the method of the embodiment. The difference from embodiment 1 is that the second coating is directly coated on the surface of the first coating without coating the intermediate conductive layer.

[0105] Comparative Example 4

[0106] This comparative example provides a positive electrode plate, which is prepared by referring to the method of the embodiment. The difference from Example 1 is that the first coating slurry and the second coating slurry are mixed into a positive electrode active slurry, which is then directly coated on one side of the surface of the positive electrode collector aluminum foil, and the positive electrode plate is obtained after drying and rolling. The other parameters remain the same as those of the plate in Example 1.

[0107] The positive electrode sheets prepared in the above examples and comparative examples were prepared into secondary batteries, and then the dynamic performance of the secondary batteries was tested.

[0108] The preparation of the secondary battery includes the following steps:

[0109] Positive electrode preparation

[0110] See Examples 1-23 and Comparative Examples 1-4.

[0111] Negative electrode preparation

[0112] The negative electrode active material (artificial graphite), conductive agent carbon black, carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were mixed in a mass ratio of 94.3:1.5:1.2:3, and deionized water was added as a solvent. The mixture was stirred in a stirrer until the system was uniform to obtain a negative electrode slurry. The negative electrode slurry was evenly coated on the negative electrode current collector copper foil, dried in an oven, and rolled and cut to obtain a negative electrode sheet (the compaction density of the sheet was 1.60 g / cm 3 ).

[0113] Electrolyte preparation

[0114] Ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1 to obtain a mixed organic solvent, and then LiPF6 is dissolved in the mixed organic solvent. After mixing evenly, an electrolyte with a lithium ion concentration of 1 mol / L is obtained.

[0115] Assembly of secondary batteries

[0116] The prepared positive electrode sheet, separator (polypropylene separator, Celgard 2300, purchased from Celgard, USA), and negative electrode sheet are stacked in order so that the separator is between the positive and negative electrode sheets, and then wound into a battery cell and loaded into a soft package shell. After top and side sealing, liquid injection (electrolyte), formation, sorting and other processes, a lithium-ion secondary battery is obtained.

[0117] The performance test of secondary batteries is as follows:

[0118] (1) Gram capacity test (mAh / g):

[0119] At room temperature (25±5°C), the prepared secondary battery was charged to 4.55V at a constant current of 0.5C, with a cut-off current of 0.02C, and then discharged to 3.0V at a constant current of 0.2C at room temperature. The obtained room temperature discharge capacity was recorded as Cap, and the total mass of O3 phase lithium cobalt oxide and O2 phase lithium cobalt oxide in the positive electrode sheet was recorded as m, and the gram capacity = Cap / m;

[0120] (2) Rate performance test:

[0121] At room temperature (25±5℃), the secondary battery is charged to 4.55V at 0.5C constant current and constant voltage, the cut-off current is 0.02C, and then discharged to 3.0V at 0.2C constant current. The room temperature discharge capacity is recorded as Cap1;

[0122] At room temperature, the soft-pack battery cell is charged to 4.55V at 0.5C constant current and constant voltage, with a cut-off current of 0.02C, and then discharged to 3.0V at 3.0C constant current. The room temperature discharge capacity is recorded as Cap2.

[0123] The rate performance is the ratio of Cap2 / Cap1;

[0124] (3) High temperature and high pressure cycle performance:

[0125] Place the secondary battery in an environment of 45°C, charge it to 4.55V at 3C constant current and constant voltage, cut off the current at 0.05C, and discharge it to 3.0V at 0.7C constant current. This step of charge and discharge is recorded as one cycle. Repeat this cycle until the capacity retention rate of the battery is less than 80%. Stop the test and record the cycle at this time.

[0126] The test results are shown in Table 2.

[0127] Table 2

[0128]

[0129]

[0130] From the above results, we can see that:

[0131] By adding an intermediate conductive layer containing a conductive agent and a binder between the O3 phase lithium cobalt oxide layer and the O2 phase lithium cobalt oxide layer, it is possible to effectively prevent the above two coatings from generating gaps due to different elastic moduli. At the same time, the added conductive agent forms a specific conductive network structure between the O3 phase lithium cobalt oxide layer and the O2 phase lithium cobalt oxide layer, which is beneficial to further extend the high ion conductivity of the surface O2 phase lithium cobalt oxide layer to the bottom O3 phase lithium cobalt oxide layer, further improving the rate performance of the prepared secondary battery, and giving full play to the advantages of the high capacity of the intermediate O3 phase lithium cobalt oxide and the high voltage resistance of the O2 phase lithium cobalt oxide.

[0132] The initial capacity of the secondary batteries prepared using the positive electrode sheets of the embodiments of the present application is above 191.2 mAh / g, the rate performance at 3C is above 85%, and the number of cycles at 4.55 V high voltage is above 500.

[0133] In Comparative Example 1, an O2-phase lithium cobalt oxide layer is directly coated on the surface of the positive electrode current collector. During the cycle, the current collector slips, resulting in a significant decrease in the number of cycles.

[0134] In Comparative Example 2, an O3 phase lithium cobalt oxide layer is directly coated on the surface of the positive electrode current collector, and both the gram capacity and the cycle are poor.

[0135] In Comparative Example 3, the O3 phase lithium cobalt oxide layer and the O2 phase lithium cobalt oxide layer are directly combined. During the cycle, due to the different elastic moduli of the two materials, a layer gap appears, which further affects the cycle life of the secondary battery.

[0136] In Comparative Example 4, O2-phase lithium cobalt oxide and O3-phase lithium cobalt oxide are prepared into a mixed slurry for coating, and the cycle stability of the obtained secondary battery performance is poor.

[0137] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present application rather than to limit the scope of protection of the present application. Although the present application has been described in detail with reference to the preferred embodiments, ordinary technicians in this field should understand that the technical solution of the present application can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present application.

Claims

1. A positive electrode sheet, characterized in that: The positive electrode plate includes a positive electrode collector and a positive electrode active layer arranged on at least one surface of the positive electrode collector, wherein the positive electrode active layer is provided with a first coating layer, an intermediate conductive layer, and a second coating layer in sequence from close to the positive electrode collector to far away from the positive electrode collector, wherein the first coating layer contains O3 phase lithium cobalt oxide; the intermediate conductive layer contains a conductive agent and a binder; and the second coating layer contains O2 phase lithium cobalt oxide.

2. The positive electrode sheet according to claim 1, characterized in that: The surface density of the first coating is denoted as σ1 mg / cm 2 , the surface density of the second coating is denoted as σ2mg / cm 2 , satisfying: 3.0≤σ1 / σ2≤5.

4.

3. The positive electrode sheet according to any one of claims 1 or 2, characterized in that: The surface density of the first coating is σ1=11-15 mg / cm 2 ; The surface density of the second coating σ2 = 2.7 ~ 3.8 mg / cm 2 .

4. The positive electrode sheet according to claim 1, characterized in that: The first coating and the second coating further include a conductive agent and a binder; based on the total mass of the first coating, the mass content of the binder in the first coating is a%; based on the total mass of the second coating, the mass content of the binder in the second coating is b%; based on the total mass of the intermediate conductive layer, the mass content of the binder in the intermediate conductive layer is c%; at least one of the following characteristics is met: (1)a <c; (2)b <c。 5. The positive electrode sheet according to any one of claims 1 or 4, characterized in that: Satisfy at least one of the following characteristics: (1) Based on the total mass of the first coating layer, the first coating layer comprises the following components in percentage by mass: 95-99% of O3 phase lithium cobalt oxide, 0.5-3% of a conductive agent, and 0.5-3% of a binder; (2) Based on the total mass of the second coating layer, the second coating layer comprises the following components in percentage by mass: 95-99% of O2-phase lithium cobalt oxide, 0.5-3% of a conductive agent, and 0.5-3% of a binder; (3) Based on the total mass of the intermediate conductive layer, the intermediate conductive layer includes the following components in percentage by mass: 90-97% of a binder and 3-10% of a conductive agent.

6. The positive electrode sheet according to claim 5, characterized in that: At least one of the following conditions is met: (1) The conductive agent in the first coating layer, the second coating layer, and the intermediate conductive layer independently comprises at least one of carbon black, graphite, carbon fiber, carbon nanotube, and graphene; (2) The binders in the first coating layer, the second coating layer, and the intermediate conductive layer independently include at least one of polyvinylidene fluoride, polytetrafluoroethylene, polyacrylic acid, styrene-butadiene rubber latex, and polyimide.

7. The positive electrode sheet according to claim 1, characterized in that: Satisfies: H1:H2:H3=(12.8~17.4):(3.2~4.3):1; wherein H1 represents the thickness of the first coating, in μm; H2 represents the thickness of the second coating, in μm; H3 represents the thickness of the intermediate conductive layer, in μm.

8. The positive electrode sheet according to any one of claims 1 to 6, characterized in that: Satisfy at least one of the following characteristics: (1) The compaction density of the positive electrode sheet is 3.78 to 4.34 g / cm 3 ; (2) D of the O3 phase lithium cobalt oxide V 50 is 14.0~17.0μm, D V 50 represents the particle size corresponding to when the cumulative volume of the O3 phase lithium cobalt oxide particles reaches 50%; (3) D of the O2 phase lithium cobalt oxide V 50' is 5.5~9.0μm, D V 50' represents the particle size corresponding to when the cumulative volume of the O2 phase lithium cobalt oxide particles reaches 50%; (4) The thickness of the first coating layer H1 = 25.7-34.7 μm; (5) The thickness of the second coating layer H2 = 6.4-8.7 μm; (6) The thickness of the intermediate conductive layer H3 = 1 to 3 μm; (7) In the intermediate conductive layer, the particle size distribution of the conductive agent satisfies: 0.44≤D V 50” / (D V 90”-D V 10”)≤0.93, where D V 50" indicates the particle size corresponding to when the cumulative volume of the particles of the conductive agent reaches 50%, in μm; D V 90" indicates the particle size corresponding to when the cumulative volume of the particles of the conductive agent reaches 90%, in μm; D V 10" indicates the particle size corresponding to when the cumulative volume of the particles of the conductive agent reaches 10%, in μm.

9. A secondary battery, characterized in that: The invention comprises the positive electrode sheet, the separator, the negative electrode sheet and the electrolyte as claimed in any one of claims 1 to 8.

10. An electrical device, characterized in that: The electric device comprises the secondary battery according to claim 9.

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