Positive electrode sheet, winding core and battery

Abstract

The present invention relates to the technical field of lithium-ion batteries, and provides a positive electrode sheet, a coil and a battery. The positive electrode sheet includes a positive electrode current collector, which is divided into an arc area and a flat area; at least one surface of the arc area is provided with a first active material coating, and at least one surface of the flat area is provided with a second active material coating; wherein the first active material coating includes a first active material, and the second active material coating includes a second active material; the volume shrinkage rate of the first active material is greater than the volume shrinkage rate of the second active material, and the volume shrinkage rate of the first active material is 0.1-8%. In the positive electrode sheet, the active material coating with a large volume shrinkage rate is applied to the arc area, so that the battery containing the positive electrode sheet avoids the problem of glue layer clogging of the arc area diaphragm during charging, does not affect the obstruction of lithium ion transmission, and thus effectively solves the problem of black spot lithium precipitation, and improves the cycle performance and safety performance of the lithium ion battery.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and in particular to a positive electrode sheet, a winding core containing the positive electrode sheet, and a battery containing the positive electrode sheet or the battery core. Background Art

[0002] Lithium-ion batteries are becoming increasingly popular in smart digital products, new energy vehicles, and other fields due to their compact size, light weight, and excellent energy storage capacity. Lithium-ion batteries include wound cells, which consist of a rolled positive electrode sheet, a negative electrode sheet, and a separator, wound on a winding machine to form an electrode assembly. In wound cells, the cell can be divided into an unbent flat area and a curved arc area.

[0003] During the charging process of a wound battery cell, the electrode will expand, and internal stress will be generated inside the battery cell due to structural limitations. The internal stress in the planar area is low because it can expand outward in the direction of the battery cell thickness. However, due to the limitations of the arc structure, the internal stress caused by charging expansion in the arc area is greater and more concentrated than in the planar area. This phenomenon will cause the internal structure of the wound core to be squeezed, and the diaphragm in the arc area of the battery cell is prone to clogging with the glue layer, which will lead to obstruction of lithium ion transmission and the occurrence of black spot lithium precipitation.

[0004] Therefore, it is of great significance to develop a battery electrode that can solve the problem of black spot lithium plating. Summary of the Invention

[0005] The present invention aims to overcome the aforementioned problems of the prior art by providing a positive electrode sheet, a winding core containing the positive electrode sheet, and a battery containing the positive electrode sheet or the battery core. The positive electrode sheet is provided with distinct functional zones (arc-shaped zones and flat zones), and the arc-shaped zones are coated with an active material with a high volume shrinkage rate. This prevents pore clogging of the separator in the arc-shaped zones during charging, thus preventing the obstruction of lithium ion transport. This effectively solves the problem of black speckle lithium deposition and improves the cycling performance and safety of lithium-ion batteries.

[0006] The inventors of the present invention discovered that by coating an active material with a large volume shrinkage rate on the arc area of the positive electrode current collector and coating a conventional active material (such as lithium cobalt oxide active material) on the flat area, the active material in the arc area will shrink in volume when the battery is charged and delithiation occurs. The volume shrinkage of the arc area can offset part of the volume expansion caused by lithium insertion in the negative electrode sheet, thereby reducing the stress in the arc area during charging, preventing the diaphragm in the arc area from being blocked, and avoiding the risk of black spots and lithium precipitation in the arc area.

[0007] In order to achieve the above-mentioned objectives, the first aspect of the present invention provides a positive electrode sheet, which includes a positive electrode collector, and the positive electrode collector is divided into an arc area and a flat area; at least one surface of the arc area is provided with a first active material coating, and at least one surface of the flat area is provided with a second active material coating; wherein, the first active material coating includes a first active material, and the second active material coating includes a second active material; the volume shrinkage rate of the first active material is greater than the volume shrinkage rate of the second active material, and the volume shrinkage rate of the first active material is 0.1-8%.

[0008] The second aspect of the present invention provides a winding core, which is a winding structure formed by a first diaphragm, a first electrode sheet, a second diaphragm and a second electrode sheet stacked in sequence, and the first electrode sheet or the second electrode sheet is the positive electrode sheet described in the first aspect of the present invention; wherein, along the direction of stretching of the winding core, the width of the arc area is not greater than the bending part of the winding structure, and the width of the flat area is not less than the unbent part of the winding structure.

[0009] A third aspect of the present invention provides a battery comprising at least one of the positive electrode sheet described in the first aspect of the present invention and the winding core described in the second aspect of the present invention.

[0010] The present invention adopts the above technical solution to achieve the following beneficial effects:

[0011] (1) The volume shrinkage of the arc region of the positive electrode provided by the present invention can offset the volume expansion of the negative electrode caused by lithium insertion, thereby reducing the stress in the arc region during charging, preventing the diaphragm in the arc region from being blocked, and avoiding the risk of black spots and lithium precipitation in the arc region;

[0012] (2) The battery provided by the present invention has higher battery cycle performance and safety performance.

[0013] The endpoints of the ranges and any values disclosed herein are not limited to the precise ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 Shown is a schematic diagram of the winding core structure in an embodiment of the present invention.

[0015] Figure 2 Shown is a schematic diagram of the winding core structure in an embodiment of the present invention.

[0016] Figure 3Shown is a schematic diagram of the top view of the positive electrode sheet of the present invention in the width direction (the two ends are not shown).

[0017] Figure 4 Shown is a schematic diagram of the cross-sectional structure of the positive electrode sheet of the present invention in the width direction (the two ends are not shown).

[0018] Description of Reference Numerals

[0019] 101: arc area of positive electrode; 102: flat area of positive electrode; 103: negative electrode. DETAILED DESCRIPTION

[0020] The following is a detailed description of the specific embodiments of the present invention. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention and are not intended to limit the present invention.

[0021] Unless otherwise defined, all scientific and technical terms used in the present invention have the same meanings as commonly understood by one of ordinary skill in the art to which the present invention relates.

[0022] A first aspect of the present invention provides a positive electrode sheet, comprising a positive electrode current collector, the positive electrode current collector being divided into an arc region and a flat region; at least one surface of the arc region being provided with a first active material coating, and at least one surface of the flat region being provided with a second active material coating;

[0023] The first active material coating includes a first active material, and the second active material coating includes a second active material; the volume shrinkage rate of the first active material is greater than that of the second active material, and the volume shrinkage rate of the first active material is 0.1-8%.

[0024] When the positive and negative electrode sheets and the separator are wound to form a core structure, different areas of the positive electrode sheet will present two states: a bent state and an unbent state. Correspondingly, the positive electrode current collector will have a bent area and an unbent area. In the present invention, the "arc area" can be a portion of the bent area of the positive electrode current collector, and this portion includes the inflection point of the bend. The "arc area" can also be the entire bent area of the positive electrode current collector. The "flat area" can be the entire unbent area and a portion of the bent area of the positive electrode current collector, and this portion of the bent area does not include the inflection point of the bend. The "flat area" can also be the entire unbent area of the positive electrode current collector. The inflection point of the bend is the intersection of the centerline of the core and the bent area.

[0025] The inventors of the present invention have found that in currently commonly used battery systems, traditional binary metal lithium salts are mainly used as the cathode active material. For example, lithium cobaltate commonly used, but when lithium cobaltate is charged and lithium is deintercalated, its volume will expand, and when lithium is intercalated during discharge, its volume will contract. Therefore, during the charging process of a wound battery cell, both the positive and negative electrode sheets will expand in volume, which makes it easy for black spots and lithium deposition to appear in the arc area of the battery. Based on this, the inventors coated a multi-component cathode material with a large volume shrinkage rate on the arc area of the positive electrode sheet. During battery charging, the volume of this multi-component cathode material will contract, so it can offset part of the volume expansion caused by lithium intercalation in the negative electrode sheet, thereby reducing the stress during charging in the arc area, preventing the diaphragm in the arc area from being blocked by holes, and avoiding the risk of black spots and lithium deposition in the arc area.

[0026] To better solve the problem of black spots and lithium deposition in the battery, one or more of the technical features can be further optimized.

[0027] Exemplarily, the positive electrode current collector is a substance with conductivity that does not cause adverse chemical changes in the secondary battery, including but not limited to aluminum, aluminum alloy, nickel, nickel alloy, titanium, and titanium alloy.

[0028] In one example, both opposite surfaces of the arc area are coated with a first active material coating, and both opposite surfaces of the flat area are coated with a second active material coating.

[0029] In one example, the first active material is a multi-component cathode material, and the volume shrinkage rate of the multi-component material is 0.1 - 8%.

[0030] Exemplarily, the volume shrinkage rate of the multi-component material can be 0.1%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8% or the range between any two point values.

[0031] Preferably, the volume shrinkage rate of the first active material is 2 - 7%, and more preferably 3 - 6%.

[0032] Under the condition of meeting the above shrinkage rate range, the specific selection of the multi-component cathode material is not limited and can be selected within the art according to needs.

[0033] In one example, the multi-component cathode material is selected from at least one of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminate, lithium nickel cobalt manganese aluminate, and lithium nickel titanium magnesium oxide. Among them, the chemical formula of lithium nickel cobalt manganese oxide is Li z Ni x Co y Mn 1-x-y O2, where 0.95 ≤ z ≤ 1.05, x > 0, y > 0, 0 < x + y < 1; the chemical formula of lithium nickel cobalt aluminate is Li z Ni x Co y Al1-x-y O2, wherein 0.95≤z≤1.05, x>0, y>0, 0.8≤x+y<1; the chemical formula of the lithium nickel cobalt manganese aluminum oxide is Li z Ni x Co y Mn w Al 1-x-y-w O2, wherein 0.95≤z≤1.05, x>0, y>0, w>0, 0.8≤x+y+w<1; the chemical formula of the lithium nickel titanium magnesium oxide is LiNi x Ti y Mg z O2, where x>0, y>0, z>0, x+y+z=1.

[0034] In one embodiment, the chemical formula of the lithium nickel cobalt manganese oxide is LiNi x Co y Mn 1-x-y O2, of which 0.4<x≤0.8,y> 0, 0 <x+y<1。

[0035] In one embodiment, the chemical formula of the lithium nickel cobalt aluminum oxide is LiNi x Co y Al 1-x-y O2, of which 0.4<x≤0.8,y> 0, 0.8≤x+y<1.

[0036] In one embodiment, the chemical formula of the lithium nickel cobalt manganese aluminum oxide is LiNi x Co y Mn w Al 1-x-y-w O2, of which 0.4<x≤0.8,y> 0, w>0, 0.8≤x+y+w<1.

[0037] In one embodiment, the chemical formula of lithium nickel magnesium titanate is LiNi x Ti y Mg z O2, of which 0.4<x≤0.8,y> 0, z>0, x+y+z=1.

[0038] In one embodiment, the multi-element cathode material is selected from LiNi 0.8 Co 0.1 Mn 0.1 O2、LiNi 0.7 Co 0.2 Mn 0.1 O2、LiNi 0.6 Co 0.2 Mn 0.2 O2、LiNi 0.5 Co 0.2 Mn 0.3O2、LiNi 0.6 Co 0.2 Al 0.2 O2、LiNi 0.5 Co 0.2 Al 0.3 O2、LiNi 0.6 Co 0.2 Mn 0.1 Al 0.1 O2 and LiNi 0.6 Ti 0.2 Mg 0.2 At least one of O2.

[0039] The inventors of the present invention have found through extensive research that although there are many multi-element positive electrode materials, LiNi 0.8 Co 0.1 Mn 0.1 O2 and LiNi 0.8 Co 0.15 Al 0.05 When O2 is used together and the mass ratio of the two meets 1: (0.2-0.6), the volume shrinkage rate can be as high as 7-8%, and it has the effect of improving the capacity retention rate of the battery and reducing the cycle expansion rate of the battery cell.

[0040] The first active material has a greater volume shrinkage than the second active material. It is understood that this condition can also be met when the volume shrinkage of the second active material is negative (i.e., when it exhibits volume expansion). Therefore, the present invention encompasses the case where the first active material exhibits volume shrinkage and the second active material exhibits volume expansion.

[0041] In one example, the second active material is a lithium-containing inorganic salt.

[0042] Illustratively, the lithium-containing inorganic salt may be selected from at least one of lithium cobaltate, lithium manganate, lithium nickelate, lithium iron phosphate, lithium tantalate, lithium zirconate, lithium titanium phosphate, lithium lanthanum titanate, and lithium aluminum titanium phosphate.

[0043] In one example, the lithium-containing inorganic salt is lithium cobaltate.

[0044] In the present invention, when lithium cobalt oxide is used as the positive electrode material, the inventors discovered that coating the arc region with a multi-element positive electrode material with a large volume shrinkage rate, while still coating the flat region with lithium cobalt oxide, can effectively solve the problem of black speckle lithium deposition in lithium cobalt oxide batteries. However, if the arc region and the flat region are not distinguished, and a certain amount of multi-element positive electrode material is mixed into the lithium cobalt oxide system, the volume of the lithium cobalt oxide will expand when delithiation occurs during charging, while the volume of the ternary positive electrode material will shrink when delithiation occurs. In this case, the shrinkage of the ternary positive electrode material can only offset the volume expansion caused by the lithium cobalt oxide, but cannot offset the volume expansion caused by the negative electrode. The problem of high internal stress in the arc region still exists, and the problem of black speckle lithium deposition in the arc region will still occur.

[0045] In one example, based on the total mass of the first active material coating layer, the content of the first active material is 90-98%.

[0046] Illustratively, the content of the first active material may be 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97% and 98%.

[0047] In one example, based on the total mass of the second active material coating layer, the content of the second active material is 90-98%.

[0048] Illustratively, the content of the second active material may be 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97% and 98%.

[0049] In one example, the first active material coating layer and the second active material coating layer further independently include a positive electrode conductor and a positive electrode binder.

[0050] The positive electrode conductor and the positive electrode binder can be selected according to needs in the art and are not specifically limited.

[0051] Illustratively, the positive electrode conductive agent is selected from at least one of conductive graphite, ultrafine graphite, acetylene black, conductive carbon black SP, superconducting carbon black, carbon nanotubes, and conductive carbon fibers.

[0052] Illustratively, the positive electrode binder is at least one selected from polyvinylidene fluoride, polytetrafluoroethylene, sodium carboxymethyl cellulose, styrene-butadiene rubber, polyurethane, polyvinyl alcohol, polyvinylidene fluoride, and a copolymer of vinylidene fluoride and fluorinated olefin.

[0053] The second aspect of the present invention provides a winding core, which is a winding structure formed by a first diaphragm, a first electrode sheet, a second diaphragm and a second electrode sheet stacked in sequence, and the first electrode sheet or the second electrode sheet is the positive electrode sheet described in the first aspect of the present invention; wherein, along the direction of stretching of the winding core, the width of the arc area is not greater than the bending part of the winding structure, and the width of the flat area is not less than the unbent part of the winding structure.

[0054] like Figure 1 , Figure 3 and Figure 4 As shown, the winding core is a wound structure formed by sequentially stacking a first separator, a negative electrode sheet, a second separator, and a positive electrode sheet. Along the direction of core stretching, the width of the positive electrode sheet arc area 101 is the width of the winding structure's curved portion, while the width of the positive electrode sheet flat area 102 is the width of the uncurved portion of the winding structure.

[0055] In the present invention, the width direction of the separator is the same as the direction in which the winding core is stretched.

[0056] The inventors of the present invention have discovered that in the winding structure of the winding core, the inflection point of the bend area is the part subject to the greatest internal stress and is more prone to black spot lithium deposition. Therefore, coating the inflection point with a multi-element positive electrode material can effectively suppress black spot lithium deposition. To further suppress black spot lithium deposition, the multi-element positive electrode material can also be coated on the portion of the bend area including the inflection point, or on the entire bend area.

[0057] In one example, Figure 2 As shown, the positive electrode arc area 101 is part of the diaphragm bending area and includes the bending inflection point (for example, the bending part in the black dotted box), and the flat area is the entire unbent area and partial bent area of the diaphragm and does not include the bending inflection point.

[0058] In the present invention, the inflection point of the bend is the intersection of the center line of the winding core and the bending area.

[0059] In one example, Figure 1 As shown, the positive electrode sheet arc area 101 is the entire curved area of the positive electrode sheet, and the flat area is the entire uncurved area of the positive electrode sheet.

[0060] In one embodiment, the width L of the arc area is 0.01 mm. mm, wherein d is the thickness of the core.

[0061] In the present invention, the core curved region is configured as a semicircular structure with the core thickness d as its diameter. The arc length of the semicircle is equal to the maximum width of the arc region. However, the actual shape of the arc region is not limited to a standard semicircle. For example, the core structure is similar to the curved portion formed by winding multiple layers of cloth, which is not a standard semicircle.

[0062] In one example, the minimum width of the arc region of 0.01 mm refers to the minimum width of the inflection point of the bending region of the diaphragm.

[0063] In one embodiment, the maximum width of the arc region is mm refers to the arc length of a semicircle with the core thickness d as the diameter.

[0064] Exemplarily, the width of the arc area may be 0.1πd, 0.2πd, 0.3πd, 0.4πd, or 0.5πd.

[0065] The structures of the battery except the winding core can be manufactured according to the methods in the art, and can achieve the effect of suppressing black spot lithium precipitation.

[0066] A third aspect of the present invention provides a battery comprising at least one of the positive electrode sheet described in the first aspect of the present invention and the winding core described in the second aspect of the present invention.

[0067] In one embodiment, the battery includes, in addition to the positive electrode sheet described in the first aspect of the present invention, a negative electrode sheet, a non-aqueous electrolyte, and a separator.

[0068] In one example, the battery includes the winding core according to the second aspect of the present invention.

[0069] In one embodiment, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer coated on one side or both sides of the negative electrode current collector.

[0070] Illustratively, the negative electrode current collector is a material that has conductivity and does not cause adverse chemical changes in the secondary battery, and can be selected from copper, stainless steel, aluminum, nickel, titanium, carbon cloth, or a composite of these materials.

[0071] The negative electrode active material layer includes a negative electrode active material, a negative electrode conductive agent, and a negative electrode binder. The negative electrode active material is not particularly limited, and any common negative electrode active material in the art can be used, for example, at least one of graphite, natural graphite, soft carbon, hard carbon, pure silicon, silicon oxide, and silicon carbon.

[0072] In one example, the negative electrode conductive agent is independently selected from at least one of conductive graphite, ultrafine graphite, acetylene black, conductive carbon black SP, superconducting carbon black, carbon nanotubes, and conductive carbon fibers.

[0073] In one example, the negative electrode binder is independently selected from at least one of polyvinylidene fluoride, polytetrafluoroethylene, sodium carboxymethyl cellulose, styrene-butadiene rubber, polyurethane, polyvinyl alcohol, polyvinylidene fluoride, and a copolymer of vinylidene fluoride and fluorinated olefin.

[0074] The non-aqueous electrolyte can be any commonly used electrolyte in the art and is not specifically limited here.

[0075] In the present invention, when numbers are used to distinguish terms, such as "first active material", "second active material", etc., the numbers in this expression only serve to distinguish and do not indicate the order of precedence. Unless otherwise specified, the size of the numbers does not have any limiting effect on the technical solution.

[0076] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0077] Unless otherwise specified, the materials and reagents used in the following examples can be obtained from commercial sources.

[0078] The present invention will be described in detail below with reference to specific embodiments. These embodiments are intended to help you understand the present invention but are not intended to limit it.

[0079] The volume shrinkage of the active materials in the following examples and comparative examples was obtained by XRD testing, and the testing method is as follows:

[0080] XRD was used to test the XRD of the positive electrode sheet before and after charging, and then Rietveld refinement was performed on the XRD data to obtain the lattice parameters of the positive electrode active material. The unit cell volume of the positive electrode active material before and after charging was then calculated. The corresponding volume shrinkage rate = (V(before charging) - V(after charging)) / (Vbefore charging) × 100%.

[0081] Example 1

[0082] 1. Preparation of positive electrode

[0083] (1) Preparation of first active material coating slurry

[0084] Lithium nickel cobalt manganese oxide LiNi 0.6 Co 0.2 Mn 0.2 O2 is the first active material. The positive electrode slurry is prepared according to the ratio of 96% of the first active material, 2.5% of conductive carbon black, and 1.5% of polyvinylidene fluoride. The viscosity of the first active material coating slurry is about 5000mPa.s and the solid content is about 75%.

[0085] Among them, lithium nickel cobalt manganese oxide LiNi 0.8 Co 0.1 Mn 0.1 The shrinkage of O2 is about 6%;

[0086] (2) Preparation of second active material coating slurry

[0087] Lithium cobalt oxide was used as the second active material. A positive electrode slurry was prepared according to a ratio of 96% of the second active material, 2.5% of conductive carbon black, and 1.5% of polyvinylidene fluoride. The slurry had a viscosity of approximately 5000 mPa.s and a solid content of approximately 75%.

[0088] (3) Spray the two slurries onto the two surfaces of the positive electrode current collector through different nozzles of the coating machine to form a first active material coating in the arc area and a second active material coating in the flat area (such as Figure 3 and Figure 4 As shown, the thickness of the two coatings is the same), wherein the length L of the first active material coating on the positive arc area is 5.966 mm.

[0089] 2. Preparation of negative electrode sheet

[0090] A negative electrode slurry was prepared using graphite as the negative electrode active material, with a ratio of 96.8% negative electrode active material, 1.2% conductive carbon black, and 2% polyvinylidene fluoride. The slurry had a viscosity of approximately 4000 mPa·s and a solids content of 45%. The slurry was sieved and then coated onto the negative electrode current collector.

[0091] 3. Preparation of batteries

[0092] The positive and negative electrode sheets obtained above are rolled, die-cut and slit, and then assembled into a winding core (such as Figure 1 ), after passing the short-circuit test, it is packaged with aluminum-plastic film, baked in an oven to remove moisture until the moisture standard required for injection is reached, and then the electrolyte is injected. After aging for 24-48 hours, the first charge is completed by the hot pressing process to obtain the activated battery cell.

[0093] Example 2

[0094] The method is carried out with reference to Example 1, except that:

[0095] 1. Preparation of positive electrode

[0096] (1) Preparation of first active material coating slurry

[0097] Lithium nickel cobalt manganese oxide LiNi 0.8 Co 0.15 Al 0.05 O2 is the first active material, and the positive electrode slurry is prepared according to the ratio of 98% of the first active material, 1% of conductive carbon black, and 1% of polyvinylidene fluoride. The viscosity of the first active material coating slurry is about 6000mPa.s and the solid content is about 70%.

[0098] Among them, lithium nickel cobalt manganese oxide LiNi 0.5 Co 0.2 Mn 0.3 The shrinkage of O2 is about 5%;

[0099] (2) Preparation of second active material coating slurry

[0100] Lithium cobalt oxide is used as the second active material. The positive electrode slurry is prepared according to a certain batching process with a ratio of 98% of the second active material, 1% of conductive carbon black, and 1% of polyvinylidene fluoride. The slurry viscosity is about 3000 mPa.s and the solid content is about 70%.

[0101] Example 3

[0102] The method is carried out with reference to Example 1, except that:

[0103] 1. Preparation of positive electrode

[0104] (1) Preparation of first active material coating slurry

[0105] Lithium nickel cobalt manganese oxide LiNi 0.6 Co 0.2 Al 0.2 O2 is the first active material. The positive electrode slurry is prepared according to the ratio of 95% first active material, 2.5% conductive carbon black, and 2.5% polyvinylidene fluoride. The viscosity of the first active material coating slurry is about 7000mPa.s and the solid content is about 80%.

[0106] Among them, lithium nickel cobalt manganese oxide LiNi 0.6 Co 0.2 Al 0.2 The shrinkage of O2 is 4%;

[0107] (2) Preparation of second active material coating slurry

[0108] Lithium cobalt oxide is used as the second active material. The positive electrode slurry is prepared according to a certain batching process with a ratio of 95% of the second active material, 3% of conductive carbon black, and 3% of polyvinylidene fluoride. The slurry viscosity is about 5000 mPa.s and the solid content is about 80%.

[0109] Example 4

[0110] Refer to Example 1, the difference from Example 1 is that: LiNi 0.33 Co 0.33 Mn 0.33 O2 is the first active material, and its shrinkage rate is 1.2%.

[0111] Example 5

[0112] Refer to Example 1, the difference from Example 1 is that: LiNi 0.8 Co 0.1 Mn 0.1 O2 and LiNi 0.8 Co 0.15 Al0.05 O2 is the first active material, the mass ratio of the two satisfies 1:0.5, and the shrinkage rate of the composition is about 7%.

[0113] Example 6

[0114] The process is carried out with reference to Example 1, except that the width L of the arc area is 2.983 mm, including the inflection point of the bending area of the diaphragm.

[0115] Example 7

[0116] The process is carried out with reference to Example 1, except that the width L of the arc area is 1.4915 mm, including the inflection point of the bending area of the diaphragm.

[0117] Example 8

[0118] The process is carried out with reference to Example 1, except that the width L of the arc area is 0.01 mm, including the inflection point of the bending area of the diaphragm.

[0119] Comparative Example 1

[0120] The process was carried out with reference to Example 1, except that the arc area and the flat area were not distinguished, and the positive electrode sheet used lithium cobalt oxide as the active material.

[0121] Comparative Example 2

[0122] The method is carried out with reference to Example 1, except that:

[0123] (1) Preparation of positive electrode active material coating slurry

[0124] Lithium nickel cobalt manganese oxide LiNi 0.6 Co 0.2 Mn 0.2 O2 and lithium cobalt oxide are used as the first active materials. The positive electrode slurry is prepared according to a certain batching process according to the ratio of 2% lithium nickel cobalt manganese oxide, 94% lithium cobalt oxide, 2.5% conductive carbon black, and 1.5% polyvinylidene fluoride. The viscosity of the first active material coating slurry is about 5000 mPa.s and the solid content is 75%;

[0125] (2) The positive electrode current collector does not distinguish between the arc area and the flat area, and the above slurry is sprayed onto the two surfaces of the positive electrode current collector through different nozzles of the coating machine.

[0126] Experimental example

[0127] (1) Normal temperature cycle test

[0128] The battery was placed in a 25°C environment and charged at 3C constant current and constant voltage to 4.45V. Then, it was charged at 4.45V to a cutoff current of 0.05C. The battery was then left to rest for 15 minutes before being discharged at 1C to 3V. The initial capacity was recorded as Q1, and the capacity after 600 cycles was recorded as Q2. The capacity retention rate after room temperature cycling was calculated using the following formula: Capacity retention rate (%) = (Q2 / Q1) 100%.

[0129] (2) Battery expansion rate test

[0130] The initial thickness M1 of the electrode sheet was measured, and the thickness after 600 cycles was measured as M2. The expansion rate of the battery after room temperature cycling was calculated by the following formula: battery expansion rate (%) = [(M2-M1) / M1]×100%.

[0131] (3) Battery lithium deposition detection

[0132] The battery that has been cycled for 600 cycles is disassembled to observe whether lithium plating occurs.

[0133] Table 1

[0134]

[0135] Note: In Comparative Example 1, ~-3% means that the volume of the lithium cobalt oxide material expanded by about 3% after delithiation.

[0136] As can be seen from Table 1, the experimental data of the embodiment show that providing the first active material coating in the arc area can effectively improve the lithium deposition in the arc area, and the greater the volume shrinkage rate of the first active material in the arc area, the less likely the arc area is to deposit lithium. Therefore, applying the first active material coating can offset part of the expansion volume caused by lithium insertion in the negative electrode, reduce the internal stress in the arc area during overcharge, and avoid the risk of lithium deposition caused by pore blockage in the diaphragm in the arc area due to large internal stress.

[0137] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A positive electrode sheet, characterized in that: The positive electrode sheet includes a positive electrode current collector, which is divided into an arc region and a flat region; at least one surface of the arc region is provided with a first active material coating, and at least one surface of the flat region is provided with a second active material coating; wherein the first active material coating comprises a first active material, and the second active material coating comprises a second active material; The first active material is a multi-element positive electrode material, and the multi-element positive electrode material is selected from at least one of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese aluminum oxide and lithium nickel titanium magnesium oxide; The second active material is lithium cobalt oxide; The volume shrinkage rate of the first active material is greater than that of the second active material, and the volume shrinkage rate of the first active material is 0.1-8%.

2. The positive electrode sheet according to claim 1, wherein: The volume shrinkage rate of the first active material is 5-8%.

3. The positive electrode sheet according to claim 1, wherein: The chemical formula of the lithium nickel cobalt manganese oxide is LiNi x Co y Mn 1-x-y O2, where 0.4 < x ≤ 0.8, y > 0, 0 < x + y < 1; and / or, The chemical formula of the lithium nickel cobalt aluminum oxide is LiNi x Co y Al 1-x-y O2, of which 0.4<x≤0.8,y> 0, 0.8 ≤ x + y < 1; and / or, The chemical formula of the lithium nickel cobalt manganese aluminum oxide is LiNi x Co y Mn w Al 1-x-y-w O2, of which 0.4<x≤0.8,y> 0, w>0, 0.8≤x+y+w<1; and / or, The chemical formula of lithium nickel titanium magnesium oxide is LiNi x Ti y Mg z O2, of which 0.4<x≤0.8,y> 0, z>0, x+y+z=1.

4. The positive electrode sheet according to claim 1, wherein: The multi-element positive electrode material is selected from LiNi 0.6 Co 0.2 Mn 0.2 O2、LiNi 0.5 Co 0.2 Mn 0.3 O2、LiNi 0.6 Co 0.2 Al 0.2 O2、LiNi 0.5 Co 0.2 Al 0.3 O2、LiNi 0.6 Co 0.2 Mn 0.1 Al 0.1 O2 and LiNi 0.6 Ti 0.2 Mg 0.2 At least one of O2.

5. The positive electrode sheet according to claim 1, wherein: Based on the total mass of the first active material coating, the content of the first active material is 90-98%; and / or, Based on the total mass of the second active material coating, the content of the second active material is 90-98%.

6. A winding core, characterized in that: The winding core is a winding structure formed by sequentially stacking a first diaphragm, a first electrode sheet, a second diaphragm, and a second electrode sheet, wherein the first electrode sheet or the second electrode sheet is the positive electrode sheet according to any one of claims 1 to 5; Wherein, along the direction of the winding core stretching, the width of the arc area is not greater than the bent portion of the winding structure, and the width of the flat area is not less than the unbent portion of the winding structure.

7. The winding core according to claim 6, wherein: The width L of the arc area is Wherein, d is the thickness of the core.

8. A battery, characterized in that: The battery comprises at least one of the positive electrode sheet according to any one of claims 1 to 5 and the winding core according to claim 6 or 7.

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