Secondary battery and method for manufacturing the same

Abstract

A secondary battery and a method of manufacturing the same. The secondary battery includes a positive electrode sheet including a current collector, an active material layer, and an insulating layer. The active material layer is disposed on at least one surface of the current collector. The insulating layer is disposed on the surface of the current collector and is adjacent to the active material layer. The content of nitrogen or sulfur in the insulating layer is greater than or equal to 10 ppm, the porosity of the insulating layer is 0.6 to 0.9, and the ratio of the thickness of the insulating layer to the thickness of the active material layer is 0.8 to 1. The secondary battery of the present application has a small width of the thinned area at the edge of the active material layer.

Description

Technical Field

[0001] This application relates to the field of energy storage devices, and in particular to a secondary battery and a method for preparing the secondary battery. Background Technology

[0002] With the increasing demand for high-energy-density batteries, a larger coating weight of active material is typically required on the electrodes to increase battery capacity. This larger coating weight results in a thicker active material layer, creating a noticeable thickness gradient between the insulating layer (used for burr prevention) and the active material layer at the electrode edges. During the coating of the insulating and active coatings, the insulating coating slurry has a lower solids content than the active coating, causing the insulating coating to dry faster than the active coating. This results in the active coating edges collapsing outwards due to surface energy and gravity, creating thinned zones. These thinned zones contain less active material, leading to a loss of mass energy density. Summary of the Invention

[0003] One object of this application is to provide a secondary battery that can improve the problem of thinning at the edge of the active material layer, and a method for preparing the secondary battery.

[0004] This application provides a secondary battery, including a positive electrode sheet, which comprises a current collector, an active material layer, and an insulating layer. The active material layer is disposed on at least one surface of the current collector. The insulating layer is disposed on the surface of the current collector and adjacent to the active material layer. The nitrogen or sulfur content in the insulating layer is greater than or equal to 10 ppm. The porosity of the insulating layer is 0.6 to 0.9, and the ratio of the thickness of the insulating layer to the thickness of the active material layer is 0.8 to 1.

[0005] In the secondary battery provided in this application, the insulating layer is formed by drying an insulating coating doped with a foaming agent. During the drying process, the foaming agent foams, supporting the thickness of the insulating coating. An appropriate amount of foaming agent needs to be added so that when the porosity of the insulating layer is 0.6 to 0.9, the ratio of the thickness of the insulating layer to the thickness of the active material layer reaches 0.8 to 1. At this time, after foaming, the nitrogen or sulfur content in the insulating layer is greater than or equal to 10 ppm, which reduces the thickness difference between the insulating layer and the active material layer and improves the problem of thinning caused by the collapse of the edge of the active coating outward due to surface energy and gravity.

[0006] According to some embodiments of this application, the porosity of the insulating layer is 0.75~0.9, which helps to reduce the width of the thinned area at the edge of the active material layer.

[0007] According to some embodiments of this application, the ratio of the thickness of the insulating layer to the thickness of the active material layer is 0.85 to 1, which helps to reduce the width of the thinned area at the edge of the active material layer.

[0008] According to some embodiments of this application, the active material layer includes an adjacent first region and a second region. The thickness of the active material layer in the first region is less than the thickness of the active material layer in the second region. The width of the first region is 0~0.3mm, which is beneficial to improving the mass energy density.

[0009] According to some embodiments of this application, the width of the first region is 0~0.15mm, which is beneficial to improving the mass energy density.

[0010] According to some embodiments of this application, the insulating layer includes nitrogen or sulfur with a mass content of 10 to 5000 ppm, preferably including nitrogen or sulfur with a mass content of 10 to 200 ppm.

[0011] According to some embodiments of this application, the insulating layer has multiple pores with a size of 0.03~0.08 mm, which helps to reduce the width of the thinned area at the edge of the active material layer.

[0012] A second aspect of this application provides a method for preparing a secondary battery, comprising the following steps:

[0013] An active slurry and an insulating slurry are applied to the surface of the current collector to form an adjacent active coating and an insulating coating, wherein the insulating slurry includes a foaming agent;

[0014] Drying the active coating and insulating coating forms an active material layer and an insulating layer, resulting in a positive electrode sheet;

[0015] A secondary battery is made by stacking or winding the positive electrode, separator, and negative electrode.

[0016] According to some embodiments of this application, the foaming agent includes at least one of diphenylsulfonyl hydrazide ether, azodicarbonamide, or ammonium bicarbonate.

[0017] According to some embodiments of this application, the insulating slurry also includes inorganic particles and a binder, and the mass of the foaming agent accounts for 1% to 3.5% of the total mass of the inorganic particles, binder and foaming agent, so that the width of the thinned area of ​​the dried active material layer 12 can be controlled to be 0 to 0.3 mm.

[0018] According to some embodiments of this application, the drying temperature is 70°C to 110°C. Within this temperature range, the foaming agent can foam sufficiently. Attached Figure Description

[0019] The above and / or additional aspects and advantages of this application will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0020] Figure 1 This is a cross-sectional schematic diagram of a positive electrode sheet provided in an embodiment of this application.

[0021] Explanation of main component symbols

[0022] Detailed Implementation

[0023] The technical solutions in the embodiments of this application are described clearly and in detail below. Obviously, the described embodiments are only some, not all, of the embodiments of this application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the specification of this application is for the purpose of describing particular embodiments only and is not intended to limit this application.

[0024] The embodiments of this application will be described in detail below. However, this application may be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, these exemplary embodiments are provided to thereby convey this application thoroughly and in detail to those skilled in the art.

[0025] Additionally, for brevity and clarity, the dimensions or thicknesses of various components and layers may be enlarged in the accompanying drawings. Throughout the text, the same numerical values ​​refer to the same elements. As used herein, the terms "and / or" and "and / or" include any and all combinations of one or more of the associated enumerated items. Furthermore, it should be understood that when element A is referred to as "connecting" element B, element A may be directly connected to element B, or there may be an intermediate element C and element A and element B may be indirectly connected to each other.

[0026] Furthermore, when describing the implementation of this application, the word "may" refers to "one or more implementations of this application".

[0027] The technical terms used herein are for the purpose of describing particular embodiments and are not intended to limit this application. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. It should be further understood that the term "comprising," as used in this specification, means the presence of the described features, values, steps, operations, elements, and / or components, but does not exclude the presence or addition of one or more other features, values, steps, operations, elements, components, and / or combinations thereof.

[0028] Spatial terms, such as "above," may be used herein for convenience in describing the relationship between one element or feature and another element (or feature) or feature (or feature) illustrated in the figures. It should be understood that, in addition to the directions depicted in the figures, spatial terms are intended to include different orientations of the device or apparatus during use or operation. For example, if the device in the figure is flipped, an element described as "above" or "on" other elements or features would be oriented "below" or "under" other elements or features. Therefore, the exemplary term "above" can include both above and below orientations. It should be understood that although the terms first, second, third, etc., may be used herein to describe various elements, components, regions, layers, and / or portions, these elements, components, regions, layers, and / or portions should not be limited by these terms. These terms are used to distinguish one element, component, region, layer, or portion from another element, component, region, layer, or portion. Therefore, a first element, component, region, layer, or portion discussed below may be referred to as a second element, component, region, layer, or portion without departing from the teachings of the exemplary embodiments.

[0029] One embodiment of this application provides a secondary battery, including a casing and a positive electrode, a negative electrode, a separator, and an electrolyte housed in the casing, wherein a separator is disposed between the positive electrode and the negative electrode.

[0030] Please see Figure 1 The positive electrode 10 includes a current collector 11, an active material layer 12, and an insulating layer 13. The current collector 11 includes a first surface 11a and a second surface 11b disposed opposite to each other. The active material layer 12 is disposed on the first surface 11a. The insulating layer 13 is disposed on the first surface 11a and adjacent to the active material layer 12. The active material layer 12 may also be disposed on the first surface 11a and the second surface 11b, which is not limited in this application. The insulating layer 13 has a plurality of pores 13a, and the porosity of the insulating layer 13 is 0.6 to 0.9. Along a first direction X perpendicular to the first surface 11a (the thickness direction of the positive electrode 10), the ratio of the thickness H1 of the insulating layer 13 to the thickness H2 of the active material layer 12 is 0.8 to 1. In one embodiment, the thickness H1 of the insulating layer 13 refers to the maximum thickness of the insulating layer 13, and the thickness H2 of the active material layer 12 refers to the thickness of the middle region of the active material layer 12, that is, the thickness of the second region 122.

[0031] The active material layer 12 is formed by drying the active coating, and the insulating layer 13 is formed by drying the insulating coating. The pores 13a in the insulating layer 13 can be formed by foaming with a foaming agent. The insulating coating doped with a foaming agent foams during the drying process, supporting the thickness of the insulating coating. This results in the ratio of the thickness H1 of the insulating layer 13 to the thickness H2 of the active material layer 12 reaching 0.8 to 1 when the porosity of the insulating layer 13 is 0.6 to 0.9. This reduces the thickness difference between the insulating layer 13 and the active material layer 12, and improves the problem of thinning zones caused by the outer collapse of the active coating edge due to surface energy and gravity.

[0032] In one embodiment, the porosity of the insulating layer 13 is 0.75 to 0.9, which helps to reduce the width of the thinned area at the edge of the active material layer.

[0033] In one embodiment, along the first direction X, the ratio of the thickness H1 of the insulating layer 13 to the thickness H2 of the active material layer 12 is 0.85~1, which helps to reduce the width of the thinned area at the edge of the active material layer.

[0034] In one embodiment, the size of the hole 13a is 0.03~0.08mm, which helps to reduce the width of the thinned area at the edge of the active material layer.

[0035] In one embodiment, the active material layer 12 includes a first region 121 and a second region 122 arranged and connected along a second direction Y, with the first region 121 adjacent to the insulating layer 13. The second direction Y is perpendicular to the first direction X. In this application, the second direction Y refers to the width direction of the positive electrode 10. In the second direction Y, the first region 121 may be located on one side or opposite sides of the second region 122, and this application is not limited thereto. In the first direction X, the thickness of the active material layer 12 located in the first region 121 is less than the thickness of the active material layer 12 located in the second region 122. In one embodiment, the thickness of the active material layer 12 located in the first region 121 being less than the thickness of the active material layer 12 located in the second region 122 means that the thickness of the active material layer 12 located at any point in the first region 121 is less than the thickness of the active material layer 12 located in the second region 122.

[0036] In the second direction Y, the width W of the first region 121 is 0~0.3mm. The first region 121 of the active material layer 12 is a thinned region. During the foaming process of the foaming agent, the insulating coating squeezes the edge of the active coating, reducing the width of the thinned region located at the edge of the active coating, so that the width W of the thinned region reaches 0~0.3mm. The smaller the width W of the thinned region, the narrower the portion of the negative electrode active material layer of the negative electrode sheet that extends beyond the active material layer 12 of the positive electrode sheet 10 can be designed. This reduces the amount of negative electrode active material layer used, lightens the weight of the secondary battery, and increases the mass energy density. In one embodiment, the width W of the first region 121 is 0~0.15mm, further improving the mass energy density.

[0037] The insulating layer 13 comprises inorganic particles and a binder. The inorganic particles include at least one selected from boehmite, alumina, zirconium oxide, boron oxide, or hexagonal boron nitride. The binder includes at least one selected from polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and hexafluoropropylene, polyacrylate, polyacrylic acid, polyacrylate, or sodium carboxymethyl cellulose. The foaming agent retains nitrogen or sulfur elements after foaming. The nitrogen or sulfur content is 10 ppm to 5000 ppm by mass, which allows the insulating layer 13 to have the required porosity and the ratio of the thickness of the insulating layer 13 to the thickness of the active material layer 12 to meet the requirements. Preferably, the nitrogen or sulfur content is 10 to 200 ppm by mass, which helps to reduce the width of the first region 121.

[0038] The active material layer 12 may include a positive electrode active material, a conductive agent, and a binder. The positive electrode active material may be any known positive electrode active material capable of reversible intercalation and deintercalation of active ions, and this application is not limited thereto. For example, the positive electrode active material may include at least one of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium iron phosphate, or lithium cobalt phosphate. The conductive agent may be any conductive agent known in the art. For example, the conductive agent may include at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, or graphene. The binder may be any binder known in the art. For example, the binder may include at least one of styrene-butadiene rubber (SBR), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), or sodium carboxymethyl cellulose (CMC-Na).

[0039] The current collector 11 can be any positive current collector known in the art, such as aluminum foil, aluminum alloy foil, or composite current collector.

[0040] The negative electrode sheet includes a negative current collector and a negative active material layer disposed on the surface of the negative current collector. The negative current collector can be any negative current collector known in the art, such as copper foil, copper alloy foil, or composite current collector. The negative active material layer can be any negative active material known in the art, for example, it can include at least one of graphite, hard carbon, soft carbon, silicon, silicon carbide, or silicon oxide. The negative active material may also include a conductive agent and a binder. The conductive agent may include at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, or graphene. The binder may include at least one of styrene-butadiene rubber (SBR), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), or sodium carboxymethyl cellulose (CMC-Na).

[0041] The separator can be any separator known in the art. For example, the separator can be a film made of one or more materials selected from polyethylene, polypropylene, nonwoven fabric, and polyfiber.

[0042] The electrolyte can be any electrolyte known in the art. For example, the electrolyte is selected from solutions containing one or more carbonate organic esters such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate as solvents, and containing one or more lithium salts selected from LiPF6, LiBF4, LiBOB, LiAsF6, Li(CF3SO2)2N, LiCF3SO3, and LiClO4 as solutes.

[0043] The housing can be any housing known in the art. For example, the housing can be a packaging bag sealed with an encapsulating film, such as aluminum-plastic film or steel-plastic film; or, the housing can be a metal housing, such as a steel housing or an aluminum housing.

[0044] An embodiment of this application also provides a method for preparing the above-mentioned secondary battery, comprising the following steps: coating an active slurry and an insulating slurry on the surface of a current collector 11 to form an adjacent active coating and an insulating coating, wherein the insulating slurry includes a foaming agent; drying the active coating and the insulating coating to form an active material layer 12 and an insulating layer 13, thereby obtaining a positive electrode 10; and stacking or winding the positive electrode 10, the separator, and the negative electrode to prepare a secondary battery.

[0045] During drying, the foaming agent in the insulating coating foams, supporting the thickness of the insulating coating and reducing the thickness difference between the dried insulating layer 13 and the active material layer 12. It also squeezes the edge of the active coating to prevent the edge of the active coating from collapsing outward due to surface energy and gravity, thereby reducing the width of the thinned area (first region 121) of the active material layer 12.

[0046] In one embodiment, the blowing agent includes at least one of diphenylsulfonyl hydrazide ether, azodicarbonamide, or ammonium bicarbonate. After the blowing agent foams, a small amount of nitrogen or sulfur remains in the insulation layer 13.

[0047] In one embodiment, the drying temperature is 70°C to 110°C. Within this temperature range, the foaming agent can foam sufficiently.

[0048] In one embodiment, the insulating slurry includes inorganic particles, a binder, and a foaming agent. The mass of the foaming agent accounts for 1% to 3.5% of the total mass of the inorganic particles, binder, and foaming agent, so that the width of the thinned area of ​​the dried active material layer 12 can be controlled within 0 to 0.3 mm.

[0049] One embodiment of this application also provides an electronic device that includes the secondary battery of this application. The electronic device can be any electrical device that uses an electrochemical device, such as a mobile phone, portable device, laptop computer, electric vehicle, electric car, ship, spacecraft, electric toy, power tool, etc.

[0050] The following specific embodiments and comparative examples are provided to better illustrate this application.

[0051] Example 1

[0052] Lithium cobalt oxide, conductive carbon black, and polyvinylidene fluoride (PVDF) were mixed and then dispersed in N-methylpyrrolidone (NMP) to obtain an active slurry with a solid content of 72%. Boehmite, PVDF, and ammonium bicarbonate were mixed and then dispersed in NMP to obtain an insulating slurry with a solid content of 35%, wherein the mass percentage of the foaming agent was 1% (i.e., the mass percentage of ammonium bicarbonate was 1% of the total mass of boehmite, PVDF, and ammonium bicarbonate). The active and insulating slurries were coated onto aluminum foil to form an active coating and an insulating coating, which were adjacent to each other and had the same thickness. The active and insulating coatings were dried at 95°C to obtain an active material layer and an insulating layer. After cold pressing, die-cutting, and slitting, the final product was obtained as shown below. Figure 1 The positive electrode shown.

[0053] The negative electrode sheet is obtained by mixing graphite, polyvinylidene fluoride, and conductive carbon black and coating them onto copper foil, followed by cold pressing, die cutting, and slitting.

[0054] The positive electrode, polyethylene separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes, and then wound to obtain an electrode assembly. The electrode assembly is placed in an aluminum-plastic film, and after liquid injection and formation, a lithium-ion battery is obtained.

[0055] Examples 2-7 and Comparative Example 2

[0056] Except for the mass ratio of the foaming agent, everything else is the same as in Example 1. The mass ratios of the foaming agent in Examples 2-6 are shown in Table 1.

[0057] Examples 8-13

[0058] In Examples 8-13, the total mass percentage of the foaming agent is the same as in Example 3, which is 2%. When the foaming agent includes two types, the mass ratio of the two types is 1:1; when the foaming agent includes three types, the mass ratio of the three types is 1:1:1. The rest is the same as in Example 3. The types of foaming agents in Examples 8-13 are shown in Table 2.

[0059] Examples 14-18

[0060] Except for the drying temperature, everything else is the same as in Example 3. The drying temperatures for Examples 14-18 are shown in Table 3.

[0061] Comparative Example 1

[0062] Except that the insulating grout does not contain a foaming agent, it is the same as in Example 1.

[0063] The nitrogen or sulfur content of the insulating layer, the porosity of the insulating layer, the size of the pores, the thickness H1 of the insulating layer, the thickness H2 of the active material layer, and the width W of the first region were measured in Examples 1-19 and Comparative Example 1.

[0064] Measurements of the insulation layer thickness, the active material layer thickness, and the width of the first region:

[0065] Take the positive electrode sheet and cut it horizontally with a slitting knife. Analyze the cross-section with a high-power microscope (SEM) to see a white insulating layer and a black active material layer.

[0066] The thickness H1 of the insulation layer: The distance from the highest point of the insulation layer to the current collector is measured using the "dot-line mode". The above steps are repeated three times, and the average value is the thickness H1 of the insulation layer.

[0067] Active material layer thickness H2: Along the width direction of the electrode, move laterally 25-35mm inward from the edge of the black active material layer, take 3 points to measure the distance from the surface of the active material layer to the current collector, and then calculate the average value, which is the thickness H2 of the active material layer;

[0068] The width W of the first region: Along the width direction of the electrode, the point where the edge of the black active material layer intersects with the white insulating layer is the starting point of the first region. Moving laterally from the starting point towards the inner side of the membrane region, until the distance from the surface of the active material layer to the current collector satisfies x / H2 = 0.98 (x is the distance from the surface of the black active material to the current collector, and H2 is the thickness of the active material layer), this is the ending point of the first region. Draw parallel lines perpendicular to the bottom surface from the starting point and ending point of the first region (i.e., the thinned area). Measure the distance between the two parallel lines using the "parallel line mode" as the width of the first region. Repeat the above steps three times and calculate the average value to obtain the width of the first region. Nitrogen or sulfur content measurement:

[0069] Use a blade to scrape off approximately 1-20 mg of the insulating layer powder from the positive electrode sheet; use a nitrogen and sulfur analyzer with argon as the carrier gas, place the powder sample in the sample cell, and select the appropriate nitrogen or sulfur measurement mode to measure the nitrogen or sulfur content.

[0070] Porosity measurement of insulating layer:

[0071] The insulating layer is peeled off from the positive current collector to obtain a suitable amount of block sample (so that the volume of the precipitate after crushing and stirring in distilled water can reach 3-5 mL). The block sample is crushed and placed in distilled water, stirred thoroughly, and allowed to precipitate. After it is completely submerged, the volume of the precipitate is measured, which is the volume V of the insulating layer sample. The sample is then filtered and dried (baked at 100-150°C for 12 hours) and placed in the sample cell of a mercury porosimeter. The mercury porosimeter pressure is tested at 10 MPa to obtain the pore volume v. The porosity of the sample is v / V. The above steps are repeated 3 times, and the porosity of the three samples is calculated and averaged to obtain the porosity of the insulating layer.

[0072] Hole size measurement:

[0073] The insulation layer was vertically cut to obtain a cross-section, which was then observed using a high-powered microscope. A region with a width of 30 μm (thickness direction) × 200 μm (horizontal direction) in the center of the cross-section was selected as the statistical region. The diameter of all holes within the statistical region was measured, and the average value was calculated as the size of the hole.

[0074] Mass energy density test:

[0075] The lithium-ion battery was charged at a constant current rate of 0.2C to 4.45V, and then charged at a constant voltage rate to 0.02C to complete the full charge. Next, it was discharged at a constant current rate of 0.2C until the voltage dropped to 3.0V. The total capacity discharged during the discharge process was recorded as C (in Ah) and the voltage plateau of the lithium-ion battery was recorded as U (in V). The actual weight M of the lithium-ion battery was measured, and the mass energy density was calculated as C * U / M.

[0076] The data and measurement results of Examples 1-7 and Comparative Examples 1-2 are shown in Table 1.

[0077] Table 1

[0078]

[0079] In this context, " / " represents nothing.

[0080] As shown in Table 1, by adding a foaming agent, this application significantly increases the porosity, pore size, and thickness ratio H1 / H2 of the insulating layer, and significantly reduces the width of the first region (thinning region) of the active material layer, thereby improving the mass energy density.

[0081] Comparative examples 1-7 show that when the mass percentage of the foaming agent is less than or equal to 3.5%, the higher the mass percentage of the foaming agent, the higher the nitrogen content and porosity in the insulation layer, the larger the H1 / H2 ratio, and the smaller the width of the thinned area. However, when the mass percentage of the foaming agent further increases, for example, to 4.0%, the porosity of the insulation layer remains almost unchanged, and therefore the H1 / H2 ratio and the width of the thinned area do not change significantly. Specifically, when the mass percentage of the foaming agent is 1% to 3.5%, the porosity is 0.61 to 0.9, and the H1 / H2 ratio is 0.8 to 0.99.

[0082] The data and measurement results for Examples 3 and 8-13 are shown in Table 2. In the foaming agent category, A refers to diphenylsulfonyl hydrazine ether, B refers to azodicarbonamide, and C refers to ammonium bicarbonate. The foaming agent categories A, B, and C indicate that the foaming agent includes A, B, and C.

[0083] Table 2

[0084]

[0085] As shown in Table 2, when the foaming agent is ammonium bicarbonate or dibenzenesulfonyl hydrazine ether and ammonium bicarbonate, the porosity is greater, the width of the thinned zone is smaller, and the mass energy density is higher.

[0086] The data and measurement results for Examples 3, 14-18 are shown in Table 3.

[0087] Table 3

[0088]

[0089] As shown in Table 3, with the increase of drying temperature, the pore size increases and the width of the thinned area decreases.

[0090] The above-disclosed embodiments are merely preferred embodiments of this application and should not be construed as limiting the scope of this application. Therefore, any equivalent variations made in accordance with this application are still within the scope of this application.

Claims

1. A secondary battery, comprising a positive electrode, wherein, The positive electrode sheet includes: current collector; An active material layer is disposed on at least one surface of the current collector; An insulating layer is disposed on the surface of the current collector and adjacent to the active material layer. The insulating layer is made of an insulating slurry, which includes a foaming agent. The nitrogen or sulfur content in the insulating layer is greater than or equal to 10 ppm. The nitrogen is the residual nitrogen element after foaming of the foaming agent, and the sulfur is the residual sulfur element after foaming of the foaming agent. The porosity of the insulating layer is 0.6 to 0.9, and the ratio of the thickness of the insulating layer to the thickness of the active material layer is 0.8 to 1.

2. The secondary battery as described in claim 1, wherein, The porosity of the insulating layer is 0.75~0.

9.

3. The secondary battery as described in claim 1, wherein, The ratio of the thickness of the insulating layer to the thickness of the active material layer is 0.85 to 1.

4. The secondary battery as described in claim 1, wherein, The active material layer includes a first region and a second region that are adjacent to each other. The thickness of the active material layer located in the first region is less than the thickness of the active material layer located in the second region. The width of the first region is 0~0.3mm.

5. The secondary battery as described in claim 4, wherein, The width of the first region is 0~0.15mm.

6. The secondary battery as described in claim 1, wherein, The insulating layer comprises nitrogen or sulfur at a mass content of 10 to 5000 ppm.

7. The secondary battery as described in claim 1, wherein, The insulating layer comprises nitrogen or sulfur at a mass content of 10 to 200 ppm.

8. The secondary battery as described in claim 1, wherein, The insulating layer has multiple pores, the size of which is 0.03~0.08mm.

9. A method for preparing a secondary battery, used to prepare a secondary battery as described in any one of claims 1 to 8, wherein, Includes the following steps: The active slurry and the insulating slurry are applied to the surface of the current collector to form an adjacent active coating and an insulating coating; Drying the active coating and insulating coating forms an active material layer and an insulating layer, resulting in a positive electrode sheet; A secondary battery is made by stacking or winding the positive electrode, separator, and negative electrode.

10. The method for preparing a secondary battery as described in claim 9, wherein, The foaming agent includes at least one of diphenylsulfonyl hydrazide, azodicarbonamide, or ammonium bicarbonate.

11. The method for preparing a secondary battery as described in claim 9, wherein, The insulating slurry also includes inorganic particles and a binder, wherein the mass of the foaming agent accounts for 1% to 3.5% of the total mass of the inorganic particles, the binder and the foaming agent.

12. The method for preparing a secondary battery as described in claim 9, wherein, The drying temperature is 70℃~110℃.

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