Secondary battery and electronic device

Abstract

The invention provides a secondary battery and an electronic device, the secondary battery comprises an electrode assembly, the electrode assembly comprises a diaphragm, the diaphragm comprises a base film, a first coating and a second coating, the base film comprises a first edge area, a first main body area, a central area, a second main body area and a second edge area, based on the width of the base film, the width proportion of the first edge area is WA1, and the width proportion of the second main body area is WA2. The width ratio of the first main body region is WB1, the width ratio of the central region is WA2, the width ratio of the second main body region is WB2, the width ratio of the second edge region is WA3, WA1 is smaller than WB1, WA3 is smaller than WB2, and WA1 + WB1 + WA2 + WB2 + WA3 = 1; first coatings are arranged on the surfaces of the first marginal area, the central area and the second marginal area, second coatings are arranged on the surfaces of the first main body area and the second main body area, the liquid retention rate of the first coatings is VA, the liquid retention rate of the second coatings is VB, and VA is larger than VB. According to the arrangement, the cycle performance of the secondary battery can be improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electrochemistry, in particular to a secondary battery and an electronic device. BACKGROUND

[0002] Since the commercial application of lithium ion batteries, they have stood out from various energy storage technologies due to their advantages in energy density, voltage platform, cycle life, no memory effect, green environmental protection, self-discharge and stability, and have been widely used in various consumer electronic products, electric vehicles, energy storage power stations and other applications.

[0003] For lithium ion batteries in the later stage of the cycle, the middle and both ends of the electrode width direction cannot be timely supplemented or supplemented due to insufficient electrolyte, causing poor electrode soaking, forming purple spots, and further lithium precipitation after cycling, etc., which leads to the risk of lithium ion battery capacity diving or short circuit; the single coating and single thickness separator at the present stage usually adopts the technical scheme of increasing the liquid retention amount of the lithium ion battery, but the increase of the liquid retention amount affects the packaging quality of the lithium ion battery, and also increases the fire risk caused by electrolyte leakage when the lithium ion battery abnormally breaks, which cannot effectively improve the above problems. SUMMARY

[0004] The purpose of the present application is to provide a secondary battery to reduce the formation of electrode purple or black spots, improve the liquid supplementing capacity of the secondary battery during the cycle process, thereby enhancing the anti-lithium precipitation performance of the secondary battery and improving the cycle performance of the secondary battery.

[0005] It should be noted that the present application is explained by taking lithium ion batteries as an example in the summary of the present application, but the secondary battery of the present application is not limited to lithium ion batteries. The specific technical solutions are as follows:

[0006] The first aspect of the present application provides a secondary battery, comprising an electrode assembly in a roll structure, the electrode assembly comprising an electrode sheet and a separator, the electrode sheet comprising a current collector and a material layer provided on at least one surface of the current collector; the material layer comprises a main body area and an edge area, the area with a thickness fluctuation difference of ≤1 μm from the center position of the material layer in the thickness direction of the material layer being the main body area, and the edge area being located on either side of the material layer in the width direction; the separator comprises a base film and a first coating and a second coating provided on at least one surface of the base film, and along the width direction after the base film is unfolded, the base film comprises a first edge area, a first main body area, a center area, a second main body area and a second edge area in turn, and based on the width of the base film, the width ratio of the first edge area is W A1 , the width ratio of the first main body area is W B1 , the width ratio of the center area is W A2 , the width ratio of the second main body area is W B2 , and the width ratio of the second edge area is W A3 , and WA1 <W B1 W A3 <W B2 W A1 +W B1 +W A2 +W B2 +W A3 =1; A first coating is provided on the surfaces of the first edge region, the center region, and the second edge region, and a second coating is provided on the surfaces of the first main body region and the second main body region. The liquid retention rate of the first coating is V. A The liquid retention rate of the second coating is V B V A >V B By adjusting the liquid retention rate V of the first coating A The liquid retention rate V is greater than that of the second coating. B The first coating has a stronger electrolyte retention capacity than the second coating, which is beneficial to improving the electrolyte replenishment capacity during the cycle of the secondary battery. The first coating is set in the first edge region and the second edge region on both sides of the base film and the central region in the middle of the base film. Correspondingly, after the separator and the electrode are stacked, the first coating corresponds to the middle and two ends of the electrode. Since the first coating retains more electrolyte, the thickness of the first coating after formation is increased, which can better fill the gaps that appear at both ends of the electrode width direction due to the edge region. Therefore, when the separator meets the above characteristics, the first coating with a higher electrolyte retention rate corresponds to the middle and two ends of the electrode, which can better solve the problem of insufficient electrolyte wetting in the edge region of the middle and two ends of the electrode in the later stage of the cycle, reduce the formation of purple spots or black spots, improve the electrolyte replenishment capacity during the cycle of the secondary battery, thereby improving the lithium plating resistance performance and cycle performance of the secondary battery.

[0007] In one or more embodiments of the present application, the first coating layer comprises a first material, the first material comprises at least one of polystyrene, polyphenylacrylate, polyvinyl alcohol, polyamide, polyurethane or ethyl cellulose, the mass percentage of the first material is M1 based on the mass of the first coating layer, 30%≤M1≤60%. The second coating layer comprises a second material, the second material comprises at least one of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyacrylic acid or polyimide, the mass percentage of the second material is M2 based on the mass of the second coating layer, 30%≤M2≤60%. By regulating the secondary battery to meet the above characteristics, the first coating layer and the second coating layer are selected with appropriate first material and second material and appropriate mass percentage, which is beneficial to make the first coating layer and the second coating layer have appropriate liquid retention rate, and is beneficial to better fill the gap at both ends of the width direction due to the edge area, and better reduce the risk of uneven compression of the pole piece due to the over-thickness of the high-liquid-retention-rate separator. Specifically, the liquid retention capacity of the first material is stronger than that of the second material, and the liquid retention rate of the first coating layer is greater than that of the second coating layer, which is beneficial to better solve the problem of insufficient electrolyte infiltration at the middle and both ends of the pole piece in the later stage of the cycle, reduce the formation of purple or black spots, improve the liquid supplementing capacity of the secondary battery in the cycle process, thereby improving the anti-lithium precipitation performance of the secondary battery and improving the cycle performance of the secondary battery.

[0008] In one or more embodiments of the present application, 5%≤W A1 ≤15%. By regulating the secondary battery to meet the above characteristics, the first edge area has a suitable width, and the first coating layer on the first edge area has a suitable content, which is beneficial to improve the liquid supplementing capacity of the secondary battery in the cycle, reduce the generation of purple or black spots of the pole piece, thereby improve the anti-lithium precipitation performance of the secondary battery and improve the cycle performance of the secondary battery.

[0009] In one or more embodiments of the present application, 20%≤W B1 ≤35%. By regulating the secondary battery to meet the above characteristics, the first main body area has a suitable width, and the second coating layer on the first main body area has a suitable content, which is beneficial to better adjust the compression rate of the whole separator, and at the same time can better solve the problem of insufficient electrolyte infiltration at the edge area at both ends of the pole piece in the later stage of the cycle, thereby improving the cycle performance of the secondary battery.

[0010] In one or more embodiments of the present application, 20%≤W A2 ≤45%. By regulating the secondary battery to meet the above characteristics, the center area has a suitable width, and the first coating layer on the center area has a suitable content, which is beneficial to improve the liquid supplementing capacity of the secondary battery in the cycle, reduce the generation of purple or black spots of the pole piece, thereby improve the anti-lithium precipitation performance of the secondary battery and improve the cycle performance of the secondary battery.

[0011] In one or more embodiments of the present application, 20%≤WB2 ≤ 35%. By regulating the secondary battery to meet the above characteristics, the second main body area has a suitable width, and the second coating has a suitable content, which is conducive to better adjusting the overall compression rate of the separator, and can better solve the problem of insufficient electrolyte infiltration at the edge area of the two ends of the electrode tab in the later stage of the cycle, thereby improving the cycle performance of the secondary battery.

[0012] In one or more embodiments of the present application, 5%≤ W A3 ≤ 15%. By regulating the secondary battery to meet the above characteristics, the second edge area has a suitable width, and the first coating on the second edge area has a suitable content, which is conducive to improving the liquid supplementing capacity of the secondary battery in the cycle, reducing the generation of purple or black spots of the electrode tab, thereby improving the anti-lithium precipitation performance of the secondary battery and improving the cycle performance of the secondary battery.

[0013] In one or more embodiments of the present application, the average thickness of the first coating corresponding to the edge area located in the first edge area or the second edge area along the thickness direction of the electrode assembly is H A1 μm, and the average thickness of the second coating located in the first main body area and the second main body area is H B μm, H A1 ≥ H B . By regulating the secondary battery to meet the above characteristics, the coating on each area of the secondary battery base film after formation has a suitable thickness, which is conducive to better solving the problem of insufficient electrolyte infiltration at the edge area of the middle and both ends of the electrode tab in the later stage of the cycle, reducing the formation of purple or black spots, improving the liquid supplementing capacity of the secondary battery in the cycle, and at the same time, the center area has a suitable thickness, which is conducive to reducing the risk of affecting the packaging quality due to the over-thickness of the center area causing the over-thickness of the electrode assembly, thereby improving the anti-lithium precipitation performance of the secondary battery and improving the cycle performance of the secondary battery.

[0014] In one or more embodiments of the present application, the electrode tab is a positive electrode tab, the thickness of the main body area of the positive electrode tab is D1 μm, and 0≤ (H A1 -H B ) / D1≤0.1. When the surface of the base film provided with the first coating and the second coating faces the positive electrode tab, by regulating the secondary battery to meet the above characteristics, the width of the first edge area or the second edge area and the thickness difference between the first main body area and the second main body area are more suitable, which is conducive to better adapting to the thickness of the corresponding positive electrode tab main body area, and the first coating can fill the gap of the edge area at both ends of the positive electrode tab in the width direction after absorbing electrolyte swelling, which is conducive to improving the liquid supplementing capacity of the secondary battery in the cycle, reducing the generation of purple or black spots of the positive electrode tab, thereby improving the anti-lithium precipitation performance of the secondary battery and improving the cycle performance of the secondary battery.

[0015] In one or more embodiments of the present application, the material layer is a positive electrode material layer, the width of the positive electrode material layer is less than the width of the base film, and the edge region width of the positive electrode material layer is less than the width of the first edge region or the second edge region. Along the width direction of the electrode assembly after unfolding, the tab is usually connected on either side of the material layer width direction of the electrode piece. By regulating the secondary battery to meet the above characteristics, the width of the base film is greater than the width of the positive electrode material layer, the width of the first coating layer on the first edge region and the second edge region on both sides of the base film is greater than the width of the edge region of the positive electrode material layer on the positive electrode piece, and the first coating layer covers the tab, which is conducive to reducing the risk of positive and negative contact short circuit caused by tab burr piercing the separator. At the same time, the first coating layer can better fill the gap at both ends of the positive electrode piece in the width direction due to the edge region, better solve the problem of insufficient electrolyte infiltration in the middle and both ends of the electrode piece at the later stage of the cycle, reduce the formation of purple or black spots, improve the liquid supplementing capacity of the secondary battery during the cycle process, thereby improving the anti-lithium precipitation performance of the secondary battery and improving the cycle performance of the secondary battery.

[0016] In one or more embodiments of the present application, based on the width of the positive electrode material layer, the edge region width ratio of the positive electrode material layer is W X1 , 0≤W X1 ≤10%. By regulating the secondary battery to meet the above characteristics, the edge region of the positive electrode material layer of the positive electrode piece has a suitable width, which is conducive to relieving the increased risk of lithium precipitation due to the over-wide edge region and reducing the formation of purple or black spots, thereby improving the anti-lithium precipitation performance of the secondary battery.

[0017] In one or more embodiments of the present application, the electrode piece is a negative electrode piece, the thickness of the main body region of the negative electrode piece is D2 μm, and 0≤(H A1 -H B ) / D2≤0.1. When the electrode piece facing the surface of the base film provided with the first coating layer and the second coating layer is a negative electrode piece, by regulating the secondary battery to meet the above characteristics, the width of the first edge region or the second edge region is more suitable for the thickness difference of the first main body region and the second main body region, which is conducive to better adapting to the thickness of the main body region of the corresponding negative electrode piece. The first coating layer can fill the gap of the edge region at both ends of the electrode piece in the width direction after absorbing electrolyte swelling, which is conducive to improving the liquid supplementing capacity of the secondary battery during the cycle, reducing the generation of purple or black spots of the negative electrode piece, thereby improving the anti-lithium precipitation performance of the secondary battery and improving the cycle performance of the secondary battery.

[0018] In one or more embodiments of the present application, the material layer is a negative electrode material layer, the width of the negative electrode material layer is less than the width of the base film, and the edge region width of the negative electrode material layer is less than the width of the first edge region or the second edge region. Along the width direction of the electrode assembly after being unfolded, the tab is usually connected on either side of the material layer width direction of the electrode piece. By regulating the secondary battery to meet the above characteristics, the width of the base film is greater than the width of the negative electrode material layer, the first coating layer on the first edge region and the second edge region on both sides of the base film has a width greater than the width of the edge region of the negative electrode material layer on the negative electrode piece, and the first coating layer covers the tab, which is conducive to reducing the risk of positive and negative electrode contact short circuit caused by tab burr piercing the separator. At the same time, the first coating layer can better fill the gap at both ends of the negative electrode piece in the width direction due to the edge region, better solve the problem of insufficient electrolyte infiltration at the middle and both ends of the electrode piece in the later stage of the cycle, reduce the formation of purple or black spots, improve the liquid supplementing capacity of the secondary battery in the cycle process, thereby improving the anti-lithium precipitation performance of the secondary battery and improving the cycle performance of the secondary battery.

[0019] In one or more embodiments of the present application, based on the width of the negative electrode material layer, the edge region width of the negative electrode material layer accounts for W X2 , 0≤W X2 ≤10%. By regulating the secondary battery to meet the above characteristics, the edge region of the negative electrode material layer of the negative electrode piece has a suitable width, which is conducive to relieving the increased risk of lithium precipitation due to the over-wide edge region and reducing the formation of purple or black spots, thereby improving the anti-lithium precipitation performance of the secondary battery.

[0020] In one or more embodiments of the present application, the electrode piece includes a positive electrode piece and a negative electrode piece. Along the thickness direction of the separator, the first coating layer and the second coating layer are arranged on one surface of the base film, and the surface of the base film provided with the first coating layer and the second coating layer faces the negative electrode piece. During the cycle of the lithium ion battery, lithium ions mainly accumulate on the surface of the negative electrode piece compared with the positive electrode piece, and therefore lithium precipitation mainly occurs on the surface of the negative electrode piece. Arranging the surface of the base film coated with the first coating layer and the second coating layer to face the negative electrode piece is conducive to better relieving the lithium precipitation of the negative electrode piece. The first coating layer can fill the gap of the edge region at both ends of the electrode piece in the width direction after absorbing electrolyte swelling, which is conducive to improving the liquid supplementing capacity of the secondary battery in the cycle, reducing the generation of purple or black spots of the negative electrode piece, thereby improving the anti-lithium precipitation performance of the secondary battery and improving the cycle performance of the secondary battery.

[0021] The second aspect of the present application provides an electronic device comprising the secondary battery in any of the preceding embodiments. Therefore, the electronic device provided by the present application has good use performance.

[0022] The beneficial effects of the present application are:

[0023] This application provides a secondary battery, including a wound electrode assembly. The electrode assembly includes an electrode sheet and a separator. The electrode sheet includes a current collector and a material layer disposed on at least one surface of the current collector. The material layer includes a main region and an edge region. Along the thickness direction of the material layer, the region with a thickness fluctuation difference ≤1μm from the center position of the material layer is the main region. The edge region is disposed at any end in the width direction of the material layer. The separator includes a base film and a first coating and a second coating disposed on two surfaces of the base film. The base film sequentially includes a first edge region, a first main region, a center region, a second main region, and a second edge region in the width direction. Based on the width of the base film, the width ratio of the first edge region is W. A1 The width of the first main area accounts for W. B1 The width of the central area accounts for W. A2 The width of the second main area accounts for W. B2 The width of the second edge region accounts for W. A3 W A1 <W B W A1 +W B1 +W A2 +W B2 +W A3 =1; A first coating is provided on the surfaces of the first edge region, the center region, and the second edge region, and a second coating is provided on the surfaces of the first main body region and the second main body region. The liquid retention rate of the first coating is V. A The liquid retention rate of the second coating is V B V A >V B By adjusting the secondary battery to meet the above characteristics, it is possible to effectively improve the electrolyte retention between the electrodes, improve the electrolyte wettability of the electrodes in the later stages of cycling, reduce the appearance of purple spots on the electrodes, improve the lithium plating resistance of the secondary battery, and improve the cycle performance of the secondary battery.

[0024] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other embodiments can be obtained based on these accompanying drawings.

[0026] Figure 1 This is a schematic diagram of the winding structure formed by the electrode assembly in some embodiments of this application;

[0027] Figure 2 This is a schematic diagram of the thickness direction of the unfolded electrode assembly in some embodiments of this application;

[0028] Reference numerals: Electrode assembly 001; Positive electrode 10; Positive current collector 11; Positive electrode material layer 12; Negative electrode 20; Negative current collector 21; Negative electrode material layer 22; Separator 30; First edge region 31; First main body region 32; Central region 33; Second main body region 34; Second edge region 35. Detailed Implementation

[0029] The technical solutions of this application will be clearly and completely described below with reference to the embodiments and accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0030] It should be noted that, in the specific embodiments of this application, a lithium-ion battery is used as an example of a secondary battery to explain this application; however, the secondary battery in this application is not limited to lithium-ion batteries. The specific technical solution is as follows:

[0031] This application provides a secondary battery, including a wound electrode assembly. The electrode assembly includes an electrode sheet and a separator. The electrode sheet includes a current collector and a material layer disposed on at least one surface of the current collector. The material layer includes a main region and an edge region. Along the thickness direction of the material layer, the region with a thickness fluctuation difference ≤1μm from the center position of the material layer is the main region, and the edge region is located on either side of the width direction of the material layer. The separator includes a base film and a first coating and a second coating disposed on at least one surface of the base film. Along the width direction of the separator after it is unfolded, the base film sequentially includes a first edge region, a first main region, a center region, a second main region, and a second edge region. Based on the width of the base film, the width ratio of the first edge region is W. A1 The width of the first main area accounts for W. B1 The width of the central area accounts for W. A2 The width of the second main area accounts for W. B2 The width of the second edge region accounts for W. A3 W A1 <W B1 W A3 <W B2 W A1 +W B1 +W A2 +W B2 +W A3 =1; A first coating is provided on the surfaces of the first edge region, the center region, and the second edge region, and a second coating is provided on the surfaces of the first main body region and the second main body region. The liquid retention rate of the first coating is V. A The liquid retention rate of the second coating is V B V A >VB .

[0032] The winding direction of the electrode assembly is defined as the W direction. For example, as shown... Figure 1 As shown, the electrode assembly 001 includes a positive electrode 10, a negative electrode 20, and a separator 30. The positive electrode 10 includes a positive current collector 11 and a positive electrode material layer 12 disposed on both surfaces of the positive current collector 11. The negative electrode 20 includes a negative current collector 21 and a negative electrode material layer 22 disposed on both surfaces of the negative current collector 21. After unfolding the electrode assembly 001, the thickness direction is defined as Z, and the width direction as Y. A coordinate system is established along the Y and Z directions, for example, as shown... Figure 2 As shown, the diaphragm 30 in the electrode assembly 001 also includes a first edge region 31, a first main body region 32, a central region 33, a second main body region 34, and a second edge region 35.

[0033] With the widespread adoption of wound lithium-ion batteries, improving their cycle performance has become increasingly important. However, due to their structural characteristics, wound lithium-ion batteries typically connect tabs along the width of the electrode assembly, usually on either side of the material layer width. At the tab connection, an edge region exists in the material layer. The thickness of this edge region is less than the material layer thickness, resulting in gaps on one side of the edge region at both ends of the wound lithium-ion battery electrode. In later stages of cycling, the middle and edge regions at both ends of the electrode width cannot be adequately wetted by the electrolyte, leading to poor electrode wetting and the formation of purple or black spots. Further cycling can cause lithium plating, increasing the risk of a significant drop in battery capacity and short circuits. The inventors discovered that by designing the separator, a first edge region, a first main region, a central region, a second main region, and a second edge region are planned on the base film. A first coating with high liquid retention is applied to the surfaces of the first edge region, the central region, and the second edge region, while a second coating with lower liquid retention is applied to the surfaces of the first main region and the second main region. During the formation of a secondary battery, due to the low liquid retention rate V of the first coating... A The liquid retention rate V is greater than that of the second coating. BThe first coating has a stronger electrolyte retention capacity than the second coating, which is beneficial to improving the electrolyte replenishment capacity during the cycle of the secondary battery. The first coating is set in the first edge region and the second edge region on both sides of the base film and the central region in the middle of the base film. Correspondingly, after the separator and the electrode are stacked, the first coating corresponds to the middle and two ends of the electrode. Since the first coating retains more electrolyte, the thickness of the first coating after formation is increased, which can better fill the gaps that appear at both ends of the electrode width direction due to the edge region. Therefore, when the separator meets the above characteristics, the first coating with a higher electrolyte retention rate corresponds to the middle and two ends of the electrode, which can better solve the problem of insufficient electrolyte wetting in the edge region of the middle and two ends of the electrode in the later stage of the cycle, reduce the formation of purple spots or black spots, improve the electrolyte replenishment capacity during the cycle of the secondary battery, thereby improving the lithium plating resistance performance and cycle performance of the secondary battery.

[0034] In one or more embodiments of this application, 7.5% ≤ V A ≤15%. For example, V A The value can be 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, 10.5%, 11%, 11.5%, 12%, 12.5%, 13%, 13.5%, 14%, 14.5%, 15%, or a range consisting of any two of the above values. A The value range can be 7.5% to 15%, 8% to 14.5%, 8.5% to 14%, 9% to 13.5%, 8.5% to 13%, and all of these ranges and sub-ranges. This is achieved by adjusting V... A When the value is within the above range, the first coating has a suitable liquid retention rate, which is beneficial to filling the gaps at both ends of the width direction due to the edge area. It is also beneficial to solve the problem of insufficient electrolyte wetting in the middle and edge areas of the electrode in the later stage of the cycle, reduce the formation of purple spots or black spots, improve the liquid replenishment capacity during the cycle of the secondary battery, thereby improving the lithium plating resistance and cycle performance of the secondary battery.

[0035] In one or more embodiments of this application, 2.9% ≤ V B ≤5.8%. For example, V B The value can be 2.9%, 3.1%, 3.3%, 3.5%, 3.7%, 3.9%, 4.1%, 4.3%, 4.5%, 4.7%, 4.9%, 5.1%, 5.3%, 5.5%, 5.8%, or a range consisting of any two of the above values. B The value range can be 2.9% to 5.8%, 3.1% to 5.5%, 3.3% to 5.3%, 3.5% to 5.1%, 3.7% to 4.9%, and all ranges and sub-ranges thereof. This can be achieved by adjusting V. BWhen the value is within the above range, the second coating has a suitable electrolyte retention rate, which creates a height difference between the first coating and the second coating. This allows the first coating, located on the first edge region and the second edge region, to better fill the gaps at both ends of the electrode caused by the edge region. This helps to better solve the problem of insufficient electrolyte wetting at the edge regions at both ends of the electrode in the later stages of cycling, thereby improving the cycle performance of the secondary battery.

[0036] In one or more embodiments of this application, the first coating includes a first material, which includes at least one selected from polystyrene, polystyrene-propylene, polyvinyl alcohol, polyamide, polyurethane, or ethyl cellulose. Based on the mass of the first coating, the mass percentage of the first material is M1, where 30% ≤ M1 ≤ 60%. For example, the value of M1 can be 30%, 33%, 36%, 39%, 42%, 45%, 48%, 51%, 54%, 57%, 60%, or a range consisting of any two of the above values. The range of M1 can be 30% to 60%, 33% to 57%, 36% to 54%, 39% to 51%, 42% to 48%, and all such ranges and sub-ranges. The second coating includes a second material, which includes at least one selected from polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyacrylic acid, or polyimide. Based on the mass of the second coating, the mass percentage of the second material is M2, where 30% ≤ M2 ≤ 60%. For example, the value of M2 can be 30%, 33%, 36%, 39%, 42%, 45%, 48%, 51%, 54%, 57%, 60%, or any range of two of the above values. The value range of M2 can be 30% to 60%, 33% to 57%, 36% to 54%, 39% to 51%, 42% to 48%, and all ranges and sub-ranges thereof. By adjusting the secondary battery to meet the above characteristics, the first coating and the second coating are made of suitable first and second materials with suitable mass percentages. The first coating and the second coating have suitable liquid retention rates, which is beneficial for better filling the gaps at both ends of the width direction caused by the edge region, and better reducing the risk of uneven pressure on the electrode due to excessively thick separator with high liquid retention rate. Specifically, the first material has a stronger liquid retention capacity than the second material, and the first coating has a higher liquid retention rate than the second coating. This helps to better solve the problem of insufficient electrolyte wetting in the middle and edge areas of the electrode in the later stages of cycling, reduce the formation of purple spots or black spots, improve the liquid replenishment capacity during the cycle of the secondary battery, thereby improving the lithium plating resistance and cycle performance of the secondary battery.

[0037] In one or more embodiments of this application, the first coating further includes inorganic fillers and additives. The inorganic fillers may include at least one of alumina, silica, or zirconium oxide; the additives may include at least one of tricresyl phosphate, triphenyl phosphate, tributyl acetylacetonate, sodium dodecyl sulfate, perfluorooctanoic acid, or polysiloxane derivatives. The mass percentage of the inorganic fillers in the first coating may be 39% to 69%, and the mass percentage of the additives may be 1% to 10%. By controlling the composition of the first coating within the above ranges, it is beneficial to improve the mechanical strength of the first coating and the kinetic performance of the secondary battery, and to improve the lithium-ion transport efficiency, thereby giving the first coating good performance.

[0038] In one or more embodiments of this application, the second coating further includes inorganic fillers and additives. The inorganic fillers may include at least one of alumina, silica, or zirconium oxide; the additives may include at least one of tricresyl phosphate, triphenyl phosphate, tributyl acetylacetonate, sodium dodecyl sulfate, perfluorooctanoic acid, or polysiloxane derivatives. The mass percentage of the inorganic fillers in the second coating may be 39% to 69%, and the mass percentage of the additives may be 1% to 10%. By controlling the composition of the second coating within the above ranges, it is beneficial to improve the mechanical strength of the first coating and the kinetic performance of the secondary battery, and to improve the lithium-ion transport efficiency, thereby giving the first coating good performance.

[0039] In one or more embodiments of this application, 5% ≤ W A1 ≤15%. For example, W A1 The value can be 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, or a range consisting of any two of the above values, W. A1 The value range can be 5% to 15%, 6% to 14%, 7% to 13%, 8% to 12%, 9% to 11%, and all of these ranges and sub-ranges. By adjusting the secondary battery to meet the above characteristics, the first edge region has a suitable width, which is beneficial to improving the electrolyte replenishment capacity of the corresponding areas at both ends of the electrode during secondary battery cycling. This effectively improves the problem that the edge regions at both ends of the electrode width direction cannot be timely wetted by electrolyte or that the degree of electrolyte wetting is insufficient in the later stages of cycling, reducing the generation of purple or black spots on the electrode. At the same time, it is beneficial to alleviate the situation where the secondary battery thickness is too large due to the first edge region being too wide, and effectively improves the surface floating liquid problem, thereby improving the lithium plating resistance performance and cycle performance of the secondary battery.

[0040] In one or more embodiments of this application, 20% ≤ W B1 ≤35%. For example, W B1The value can be 20%, 21.5%, 23%, 24.5%, 26%, 27.5%, 29%, 30.5%, 32%, 33.5%, 35%, or a range consisting of any two of the above values, W. B1 The value range can be 20% to 35%, 21.5% to 33.5%, 23% to 32%, 24.5% to 30.5%, 26% to 29%, and all of these ranges and sub-ranges. By adjusting the secondary battery to meet the above characteristics, the first main body region has a suitable width, which works in conjunction with the first edge region and the second edge region to create a height difference between the first coating and the second coating. This allows the first coating, located on the first edge region and the second edge region, to better fill the gaps at both ends of the electrode due to the edge region. This is beneficial for better adjusting the overall compression ratio of the separator and can also effectively solve the problem of insufficient electrolyte wetting at the edge regions at both ends of the electrode in the later stages of cycling, thereby improving the cycle performance of the secondary battery.

[0041] In one or more embodiments of this application, 20% ≤ W A2 ≤45%. For example, W A2 The value can be 20%, 22.5%, 25%, 27.5%, 30%, 32.5%, 35%, 37.5%, 40%, 42.5%, 45%, or a range of any two of the above values, W. A2 The value range can be 20% to 45%, 22.5% to 42.5%, 25% to 40%, 27.5% to 37.5%, 30% to 35%, and all ranges and sub-ranges thereof. By adjusting the secondary battery to meet the above characteristics, the central region has a suitable width, which is beneficial to improve the electrolyte replenishment capacity of the corresponding area in the middle of the electrode during secondary battery cycling. It effectively improves the problem that the central region of the electrode in the width direction cannot be timely wetted by electrolyte or the degree of electrolyte wetting is insufficient in the later stage of cycling, reduces the generation of purple spots or black spots on the electrode, and at the same time, it helps to alleviate the situation that the secondary battery thickness is too large due to the excessive width of the central region, and effectively improves the surface floating liquid problem, thereby improving the lithium plating resistance performance and cycle performance of the secondary battery.

[0042] In one or more embodiments of this application, 20% ≤ W B2 ≤35%. For example, W B2 The value can be 20%, 21.5%, 23%, 24.5%, 26%, 27.5%, 29%, 30.5%, 32%, 33.5%, 35%, or a range consisting of any two of the above values, W. B2The value range can be 20% to 35%, 21.5% to 33.5%, 23% to 32%, 24.5% to 30.5%, 26% to 29%, and all of these ranges and sub-ranges. By adjusting the secondary battery to meet the above characteristics, the second main body region has a suitable width, which works in conjunction with the first edge region and the second edge region to create a height difference between the first coating and the second coating. This allows the first coating, located on the first edge region and the second edge region, to better fill the gaps at both ends of the electrode due to the edge region. This is beneficial for better adjusting the overall compression ratio of the separator and can also effectively solve the problem of insufficient electrolyte wetting at the edge regions at both ends of the electrode in the later stages of cycling, thereby improving the cycle performance of the secondary battery.

[0043] In one or more embodiments of this application, 5% ≤ W A3 ≤15%. For example, W A3 The value can be 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, or a range consisting of any two of the above values, W. A3 The value range can be 5% to 15%, 6% to 14%, 7% to 13%, 8% to 12%, 9% to 11%, and all of these ranges and sub-ranges. By adjusting the secondary battery to meet the above characteristics, the second edge region has a suitable width, which is beneficial to improving the electrolyte replenishment capacity during secondary battery cycling. It effectively addresses the problem that the edge regions at both ends of the electrode width direction cannot be timely wetted by electrolyte or that the degree of electrolyte wetting is insufficient in the later stages of cycling, reducing the generation of purple or black spots on the electrode. At the same time, it helps to alleviate the situation where the secondary battery thickness is too large due to an excessively wide second edge region, and effectively improves the surface floating liquid problem, thereby improving the lithium plating resistance and cycle performance of the secondary battery.

[0044] In one or more embodiments of this application, along the thickness direction of the electrode assembly, the average thickness of the first coating located in the first edge region or the second edge region corresponding to the edge region is H. A1 µm, the average thickness of the second coating located in the first and second main regions is H B µm, H A1 ≥H B Specifically, such as Figure 2 As shown, along the thickness direction Z, the thickness of the first edge region 31, which is opposite to the edge region of the positive electrode material layer of the positive electrode sheet 10, is H. A1 The thickness of the second edge region 35, which is opposite to the edge region of the negative electrode material layer of the negative electrode sheet 20, is H. A1 The thickness of the first main body region 32 and the second main body region 34 is H. B The thickness of the central region 33 is H. A2The first and second coatings have different electrolyte retention rates and amounts. Specifically, the first coating has a higher electrolyte retention rate and absorbs more electrolyte than the second coating. During formation, the first coating, after absorbing electrolyte and swelling, can fill the gaps at both ends of the electrode width direction. The edge regions can be located at either end of the electrode width direction, corresponding to either the first or second edge region of the base film. By adjusting the secondary battery to meet the above characteristics, the coatings on each region of the secondary battery base film after formation have appropriate thicknesses, reducing the formation of purple or black spots and improving the electrolyte replenishment capacity during battery cycling. Simultaneously, the appropriate thickness in the central region helps reduce the risk of excessive electrode assembly thickness affecting packaging quality due to an overly thick central region, thereby improving the secondary battery's lithium plating resistance and cycle performance.

[0045] In this application, in the electrode assembly 001 prepared after cold pressing, the average thickness of the first coating located in the central region is H. A2 µm, the distance between the positive electrode 10 and the negative electrode 20 and the separator is equal, and the distance is equal to the thickness H of the first main body region 32 and the second main body region 34. B The first coating, located in the central region 33, fills the gap between the electrode and the diaphragm after absorbing liquid and swelling. Therefore, the thickness H of the first coating in the central region after cold pressing is... A2 Not greater than H B H A2 With H B They tend to be completely equal.

[0046] In one or more embodiments of this application, the electrode is a positive electrode, and the thickness of the main body region of the positive electrode is D1µm, 0≤(H A1 -H B ) / D1≤0.1. For example, (H A1 -H B The value of ) / D1 can be 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, or a range consisting of any two of the above values. (H) A1 -H BThe value range of ) / D1 can be 0 to 0.1, 0.01 to 0.09, 0.02 to 0.08, 0.03 to 0.07, 0.04 to 0.06, and all of these ranges and sub-ranges. When the electrode with the base film surface having the first coating and the second coating facing is the positive electrode, by adjusting the secondary battery to meet the above characteristics, the width of the first edge region or the second edge region is more suitable than the thickness difference between the first main region and the second main region, which is beneficial to better adapt to the thickness of the corresponding positive electrode body region. After the first coating absorbs the electrolyte and swells, it can fill the gaps in the edge regions at both ends of the electrode width direction, which is beneficial to improve the electrolyte replenishment capacity during the cycle of the secondary battery, reduce the generation of purple spots or black spots on the positive electrode, thereby improving the lithium plating resistance performance and cycle performance of the secondary battery.

[0047] In one or more embodiments of this application, 8≤H A1 ≤60. For example, H A1 The value can be 8, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, or a range of any two of the above values. A1 The value range can be 8 to 60, 10 to 55, 15 to 50, 20 to 45, 25 to 40, and all of these ranges and sub-ranges. This can be achieved by adjusting H... A1 Within the above range, the first coating located in the first or second edge region corresponding to the electrode edge region has a suitable thickness, which is beneficial to better fill the gaps at both ends of the electrode caused by the edge region, reduce the formation of purple spots or black spots, improve the liquid replenishment capacity during the cycle of the secondary battery, and at the same time, the suitable thickness can better reduce the risk of the secondary battery being too thick due to the first coating being too thick, thereby improving the lithium plating resistance of the secondary battery and improving the cycle performance of the secondary battery.

[0048] In one or more embodiments of this application, 0 < H B ≤20. For example, H B The value can be 1, 2, 4, 8, 10, 12, 14, 16, 18, 20, or a range consisting of any two of the above values. B The value range can be 1 to 20, 2 to 18, 4 to 16, 6 to 14, 8 to 12, and all of these ranges and subranges. This is achieved by adjusting H... B The value is within the above range, and the second coating located in the first main body area and the second main body area has a suitable thickness, which is beneficial to ensuring the thickness requirement of the separator itself in the secondary battery of this application, thereby benefiting the secondary battery to have better performance.

[0049] In one or more embodiments of this application, 90 ≤ D1 ≤ 400. For example, the value of D1 can be 90, 100, 120, 140, 160, 180, 200, 250, 300, 350, 400, or a range consisting of any two of the above values. The value range of D1 can be 90 to 400, 100 to 350, 120 to 300, 140 to 250, 160 to 200, and all such ranges and sub-ranges. By adjusting the value of D1 within the above range, the positive electrode sheet has a suitable thickness, which is beneficial to the thickness requirement of the positive electrode sheet itself in the secondary battery of this application, thereby contributing to better performance of the secondary battery.

[0050] In this application, it will be understood that (H) A1 -H B When H / D1=0, the material layer only has the main region and no edge region. Therefore, along the thickness direction of the electrode assembly, H A1 The average thickness of the first coating located in the first edge region or the second edge region is H. A1 µm, H A2 The average thickness of the first coating located in the central area is H. A2 µm, H B The average thickness of the second coating located in the first and second main regions is H. B µm.

[0051] In one or more embodiments of this application, the material layer is a positive electrode material layer, the width of which is smaller than the width of the base film, and the width of the edge region of the positive electrode material layer is smaller than the width of the first edge region or the second edge region. Along the width direction of the unfolded electrode assembly, tabs are typically connected to either side of the material layer width direction of the electrode sheet. By adjusting the secondary battery to meet the above characteristics, the width of the base film is greater than the width of the positive electrode material layer. The width of the first coating disposed on the first and second edge regions on both sides of the base film is greater than the width of the edge region of the positive electrode material layer on the positive electrode sheet. The first coating covers the tabs, which helps reduce the risk of short circuits caused by tab burrs piercing the separator. Simultaneously, the first coating can better fill the gaps at both ends of the positive electrode sheet due to the edge region, better solving the problem of insufficient electrolyte wetting at the middle and both edge regions of the electrode sheet in the later stages of cycling, reducing the formation of purple or black spots, and improving the electrolyte replenishment capacity during the secondary battery cycle, thereby improving the lithium plating resistance and cycle performance of the secondary battery.

[0052] In one or more embodiments of this application, the edge region width ratio of the positive electrode material layer is W, based on the width of the positive electrode material layer. X1 , 0≤W X1 ≤10%. For example, W X1The value can be 0, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a range of any two of the above values, W. X1 The value range can be 0 to 10%, 1% to 9%, 2% to 8%, 3% to 7%, 4% to 6%, and all of these ranges and sub-ranges. By adjusting the secondary battery to meet the above characteristics, the edge region of the positive electrode material layer of the positive electrode sheet has a suitable width, which helps to alleviate the increased risk of lithium plating caused by an excessively wide edge region, reduces the formation of purple spots or black spots, and thus improves the lithium plating resistance of the secondary battery.

[0053] In one or more embodiments of this application, the electrode is a negative electrode, and the thickness of the main body region of the negative electrode is D2μm, 0≤(H A1 -H B ) / D2≤0.1. For example (H A1 -H B The value of ) / D2 can be 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, or a range of any two of the above values. (H) A1 -H B The value range of ) / D2 can be 0 to 0.1, 0.01 to 0.09, 0.02 to 0.08, 0.03 to 0.07, 0.04 to 0.06, and all of these ranges and sub-ranges. When the electrode facing the base film surface with the first coating and the second coating is the negative electrode, by adjusting the secondary battery to meet the above characteristics, the width of the first edge region or the second edge region is more suitable than the thickness difference between the first main region and the second main region, which is beneficial to better adapt to the thickness of the corresponding negative electrode body region. After the first coating absorbs the electrolyte and swells, it can fill the gaps at both ends of the edge region in the width direction of the electrode, which is beneficial to improve the electrolyte replenishment capacity during the cycle of the secondary battery, reduce the generation of purple spots or black spots on the negative electrode, thereby improving the lithium plating resistance performance and cycle performance of the secondary battery.

[0054] In one or more embodiments of this application, 60 ≤ D2 ≤ 270. For example, the value of D2 can be 60, 80, 100, 120, 140, 160, 180, 200, 220, 240, 270, or a range consisting of any two of the above values. The value range of D2 can be 60 to 270, 80 to 240, 100 to 220, 120 to 200, 140 to 180, and all such ranges and sub-ranges. By adjusting the value of D2 within the above range, the negative electrode sheet has a suitable thickness, which is beneficial to the thickness requirement of the negative electrode sheet itself in the secondary battery of this application, thereby contributing to better performance of the secondary battery.

[0055] In one or more embodiments of this application, the material layer is a negative electrode material layer, the width of which is smaller than the width of the base film, and the width of the edge region of the negative electrode material layer is smaller than the width of the first edge region or the second edge region. Along the width direction of the unfolded electrode assembly, tabs are typically connected to either side of the material layer width direction of the electrode sheet. By adjusting the secondary battery to meet the above characteristics, the width of the base film is greater than the width of the negative electrode material layer. The width of the first coating disposed on the first and second edge regions on both sides of the base film is greater than the width of the edge region of the negative electrode material layer on the negative electrode sheet. The first coating covers the tabs, which helps reduce the risk of short circuits caused by tab burrs piercing the separator. Simultaneously, the first coating can better fill the gaps at both ends of the negative electrode sheet due to the edge region, better solving the problem of insufficient electrolyte wetting at the middle and both edge regions of the electrode sheet in the later stages of cycling, reducing the formation of purple or black spots, improving the electrolyte replenishment capacity during the secondary battery cycle, thereby improving the lithium plating resistance and cycle performance of the secondary battery.

[0056] In one or more embodiments of this application, the edge region width ratio of the negative electrode material layer is W, based on the width of the negative electrode material layer. X2 , 0≤W X2 ≤10%. For example, W X2 The value can be 0, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a range of any two of the above values, W. X2 The value range can be 0 to 10%, 1% to 9%, 2% to 8%, 3% to 7%, 4% to 6%, and all of these ranges and sub-ranges. By adjusting the secondary battery to meet the above characteristics, the edge region of the negative electrode material layer has a suitable width, which helps to alleviate the increased risk of lithium plating caused by an excessively wide edge region, reduces the formation of purple spots or black spots, and thus improves the lithium plating resistance of the secondary battery.

[0057] In one or more embodiments of this application, the electrode includes a positive electrode and a negative electrode. Along the thickness direction of the separator, a first coating and a second coating are disposed on one surface of the base film, with the base film surface having the first and second coatings facing the negative electrode. During the cycling process of a lithium-ion battery, lithium ions are mainly concentrated on the surface of the negative electrode compared to the positive electrode; therefore, lithium plating mainly occurs on the surface of the negative electrode. Having the side of the base film coated with the first and second coatings facing the negative electrode helps to better alleviate lithium plating on the negative electrode. After the first coating absorbs the electrolyte and swells, it can fill the gaps at both ends of the electrode's width, which helps to improve the electrolyte replenishment capacity during battery cycling, reduces the generation of purple or black spots on the negative electrode, thereby improving the lithium plating resistance and cycle performance of the secondary battery.

[0058] In this application, the liquid retention rate of the first coating can be controlled by adjusting the type and / or mass percentage of the first material. For example, under the same conditions, as M1 increases, V... A Increase; M1 decreases, V A Decrease.

[0059] In this application, the liquid retention rate of the second coating can be controlled by adjusting the type and / or mass percentage of the second material. For example, under the same conditions, as M2 increases, V... B Increase; M2 decreases, V B Decrease.

[0060] In this application, H can be controlled by adjusting the liquid retention rate of the first coating. A1 The value of V. For example, all other things being equal, V A Increase, H A1 Increase; V A Decrease, H A1 Decrease.

[0061] In this application, H can be adjusted by controlling the coating weight of the first coating. A1 The value of H. For example, under the same conditions, if the coating weight of the first coating increases, H A1 Increase; the coating weight of the first coating layer decreases, H A1 Decrease.

[0062] In this application, H can be adjusted by controlling the coating weight of the first coating. A2 The value of H. For example, under the same conditions, if the coating weight of the first coating increases, H A2 Increase; the coating weight of the first coating layer decreases, H A2 Decrease.

[0063] In this application, H can be controlled by adjusting the coating weight of the second coating. B The value of H. For example, under the same conditions, if the coating weight of the second coating increases, H B Increase; the coating weight of the second coating decreases, H B Decrease.

[0064] In this application, the value of D1 can be controlled by adjusting the coating weight or compaction density of the cathode material layer. For example, under the same conditions, increasing the coating weight of the cathode material layer increases D1; ​​decreasing the coating weight of the cathode material layer decreases D1; ​​increasing the compaction density of the cathode material layer decreases D1; ​​and decreasing the compaction density of the cathode material layer increases D1.

[0065] In this application, the value of D2 can be controlled by adjusting the coating weight or compaction density of the negative electrode material layer. For example, under the same conditions, increasing the coating weight of the negative electrode material layer increases D2; decreasing the coating weight of the negative electrode material layer decreases D2; increasing the compaction density of the negative electrode material layer decreases D2; and decreasing the compaction density of the negative electrode material layer increases D2.

[0066] This application does not impose any particular restrictions on the preparation method of the diaphragm, as long as the purpose of this application can be achieved. For example, the diaphragm can be prepared by the following method: (1) Mix the first material, inorganic filler and additive in proportion, add solvent, stir evenly to obtain the first coating slurry; mix the second material, inorganic filler and additive in proportion, add solvent, stir evenly to obtain the second coating slurry. (2) Take the base film, and determine the first edge area, the first main body area, the center area, the second main body area and the second edge area in sequence along the width direction of the base film. Apply the first coating slurry to the center area, the first edge area and the second edge area, apply the second coating slurry to the first main body area and the second main body area, and dry to obtain the finished diaphragm.

[0067] In this application, "a material layer disposed on at least one surface of the current collector" means that the material layer can be disposed on one surface of the current collector along its own thickness direction, or on two surfaces of the current collector along its own thickness direction. It should be noted that the "surface" here can be the entire area of ​​the current collector surface or a part of the current collector surface. This application has no particular limitation, as long as the purpose of this application can be achieved.

[0068] This application does not impose any particular restrictions on the positive electrode current collector, as long as it can achieve the purpose of this application. For example, it may include aluminum foil, aluminum alloy foil, or composite current collector (such as aluminum-carbon composite current collector).

[0069] The positive electrode material layer includes a positive electrode active material. This application does not impose any particular restrictions on the positive electrode active material, as long as it can achieve the purpose of this application. For example, the positive electrode active material may include, but is not limited to, lithium nickel cobalt manganese oxide (e.g., NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide (LiCoO2), lithium manganese oxide, lithium manganese iron phosphate, or lithium titanate.

[0070] The positive electrode material layer may also include a conductive agent and a binder. This application does not impose any particular restrictions on the types of conductive agents and binders, as long as they achieve the purpose of this application. For example, the binder may include at least one of polyacrylate, polyimide, polyamide, polyamide-imide, polyvinylidene fluoride, polystyrene-butadiene copolymer (styrene-butadiene rubber), sodium alginate, polyvinyl alcohol, polytetrafluoroethylene, polyacrylonitrile, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, sodium carboxymethyl cellulose, or potassium carboxymethyl cellulose. The conductive agent may include at least one of conductive carbon black, carbon nanotubes (CNTs), carbon fibers, flake graphite, or graphene. The conductive carbon black may be at least one of acetylene black, Super P, or Ketjen black. The carbon nanotubes may be at least one of single-walled carbon nanotubes or multi-walled carbon nanotubes. The carbon fibers may be at least one of vapor-grown carbon fibers (VGCF) or carbon nanofibers. This application does not impose any particular restrictions on the mass ratio of the positive electrode active material, conductive agent, and binder in the positive electrode material layer. Those skilled in the art can select according to actual needs, as long as the purpose of this application is achieved.

[0071] Optionally, the positive electrode may further include a conductive layer located between the positive current collector and the positive electrode material layer. The composition of the conductive layer is not particularly limited and can be any conductive layer commonly used in the art. The conductive layer includes a conductive agent and a binder. This application does not impose any particular limitation on the conductive agent and binder in the conductive layer; for example, it can be at least one of the aforementioned conductive agents and binders.

[0072] This application does not impose any particular restrictions on the negative electrode current collector, as long as it can achieve the purpose of this application. For example, it may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or composite current collector. For example, the composite current collector may be lithium copper composite current collector, carbon copper composite current collector, nickel copper composite current collector, titanium copper composite current collector, etc.

[0073] The negative electrode material layer includes a negative electrode active material. This application does not impose any particular limitation on the negative electrode active material, as long as it can achieve the purpose of this application. For example, the negative electrode active material may include, but is not limited to, natural graphite, artificial graphite, mesophase micro carbon spheres, hard carbon, soft carbon, silicon, silicon-carbon composites, Li-Sn alloys, Li-Sn-O alloys, Sn, SnO, SnO2, and spinel-structured lithiated TiO2-Li4Ti5O. 12 Or at least one of Li-Al alloys.

[0074] The negative electrode material layer may also include a conductive agent and a binder. This application does not impose any particular restrictions on the types of conductive agents and binders, as long as they achieve the purpose of this application. For example, it may be at least one of the aforementioned conductive agents and binders. This application does not impose any particular restrictions on the mass ratio of the negative electrode active material, conductive agent, and binder in the negative electrode material layer. Those skilled in the art can select according to actual needs, as long as the purpose of this application is achieved.

[0075] Optionally, the negative electrode sheet may further include a conductive layer located between the negative electrode current collector and the negative electrode material layer. This application does not impose any particular limitation on the composition of the conductive layer, which can be a conductive layer commonly used in the art. For example, the conductive layer may include a conductive agent and a binder. This application does not impose any particular limitation on the conductive agent and binder in the conductive layer; for example, it can be at least one of the aforementioned conductive agents and binders.

[0076] This application does not impose any particular limitation on the type of base membrane, as long as it can achieve the purpose of this application. For example, the base membrane can be a nonwoven fabric or composite membrane with a porous structure, and the material of the base membrane can include at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Optionally, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane can be used.

[0077] In this application, the secondary battery also includes an electrolyte, which includes lithium salts and non-aqueous solvents.

[0078] This application does not impose any particular limitation on the lithium salt, as long as it achieves the purpose of this application. For example, the lithium salt may include, but is not limited to, at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalato)borate (LiBOB), or lithium difluoroborate. This application also does not impose any particular limitation on the content of the lithium salt in the electrolyte, as long as it achieves the purpose of this application.

[0079] This application does not impose any particular restrictions on non-aqueous solvents, as long as they can achieve the purpose of this application. For example, non-aqueous solvents may include, but are not limited to, at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents.

[0080] The aforementioned carbonate compounds may include, but are not limited to, at least one of chain carbonate compounds, cyclic carbonate compounds, or fluorocarbonate compounds. The aforementioned chain carbonate compounds may include, but are not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), or methyl ethyl carbonate (MEC). The aforementioned cyclic carbonates may include, but are not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), or vinyl ethylene carbonate (VEC). Fluorocarbonate compounds may include, but are not limited to, at least one of fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, or trifluoromethylethylene carbonate. The aforementioned carboxylic acid ester compounds may include, but are not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valproic acid lactone, or caprolactone. The aforementioned ether compounds may include, but are not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. Other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate. This application does not impose any particular limitation on the content of non-aqueous solvents in the electrolyte, as long as the purpose of this application is achieved.

[0081] The secondary battery of this application also includes a packaging bag for containing the positive electrode, separator, negative electrode, and electrolyte, as well as other components known in the art for secondary batteries. This application does not limit the aforementioned other components. This application does not have any particular limitation on the packaging bag; it can be any packaging bag known in the art, as long as it can achieve the purpose of this application.

[0082] This application does not impose any particular limitation on the type of secondary battery, which may include any device in which an electrochemical reaction occurs. In this application, the secondary battery may include, but is not limited to: lithium metal secondary batteries, lithium-ion secondary batteries (lithium-ion batteries), lithium polymer secondary batteries, or lithium-ion polymer secondary batteries (lithium-ion polymer batteries), etc.

[0083] The preparation process of the secondary battery described in this application is well known to those skilled in the art, and this application does not impose any particular limitations. For example, it may include, but is not limited to, the following steps: stacking a separator, a negative electrode sheet, a separator, and a positive electrode sheet in sequence, and performing operations such as winding and folding as needed to obtain a wound electrode assembly; placing the electrode assembly in a packaging bag; injecting electrolyte into the packaging bag and sealing it to obtain the secondary battery. Furthermore, overcurrent protection components, conductive plates, etc., may be placed in the packaging bag as needed to prevent pressure rise and overcharging / discharging inside the secondary battery.

[0084] A second aspect of this application provides an electronic device that includes the secondary battery found in any of the foregoing embodiments. Therefore, the electronic device provided by this application has good performance.

[0085] This application does not specifically limit the type of electronic device; it can be any electronic device known in the prior art. In one or more embodiments of this application, the electronic device may include, but is not limited to, laptops, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors, etc.

[0086] Example

[0087] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0088] Test methods and apparatuses:

[0089] Electrode assembly sampling:

[0090] At 25°C, the lithium-ion battery was charged at a constant current of 0.3C to 3.6V, then charged at a constant voltage of 3.6V until the current cutoff was 0.05C. After standing for 15 minutes, it was discharged at a constant current of 0.3C to 2.0V. Subsequently, the discharged lithium-ion battery was disassembled under an argon atmosphere, the electrode assembly was removed, and the electrode assembly was soaked in dimethyl carbonate solvent (DMC) for 2 hours and dried at 60°C for 1 hour to obtain the electrode assembly. The discharge cutoff voltage of the lithium-ion battery in the embodiments and comparative examples of this application is 2.5V. It can be understood that when the voltage range marked on the battery packaging is 2.5V to 3.6V, the charging cutoff voltage is 3.6V and the discharging cutoff voltage is 2.5V.

[0091] Unless otherwise specified, the following test methods shall be performed using the electrode assembly obtained in the above manner.

[0092] W A1 W B1 W A2 W B2 W A3 D1, D2, W1, W X1 W2, W X2 H A1 H A2 H B test

[0093] After disassembling the electrode assembly, a diaphragm was obtained. The diaphragm was then immersed in DMC solvent for 6 hours and removed. The first and second coatings on the diaphragm exhibited a thickness difference after liquid absorption. Scanning electron microscopy (SEM) was used to distinguish the first edge region, second edge region, first main body region, second main body region, and central region. After measuring the width of each region and the width W0 of the base film, W could be calculated. A1 W B1 W A2 W B2 W A3 The value of .

[0094] The electrode assembly was photographed using a scanning electron microscope (SEM) along its thickness direction. Five thickness values ​​were measured along the thickness direction of the unfolded electrode assembly, specifically in the main body regions of the positive and negative electrodes. The average values ​​were then used to obtain the thicknesses D1 and D2 of the main body regions of the positive and negative electrodes, respectively. The width of the material layers of the electrodes was measured along the width direction of the unfolded electrode assembly, yielding the widths W1 and W2 of the positive and negative material layers, respectively. Simultaneously, thickness curves of the electrodes could be plotted based on the SEM images. According to these curves, the starting point of the edge region was determined when the thickness fluctuation difference from the center of the material layer was >1 μm. The width of the electrode edge region could then be measured and calculated. X1 and WX2 Along the thickness direction of the electrode assembly, based on the SEM images, five thickness values ​​were measured at different locations in the central region, the first main body region, and the second main body region. The average value was then used to obtain H. A2 and H B Based on the SEM image, a thickness curve of the first or second edge region opposite to the electrode edge region can be plotted. The average thickness H of the first or second edge region can then be calculated from this thickness curve. A1 .

[0095] Liquid retention rate of the first coating and liquid retention rate of the second coating were tested.

[0096] After disassembling the electrode assembly, a diaphragm was obtained. The diaphragm was immersed in DMC solvent for 6 hours and then removed. The first and second coatings on the diaphragm showed a thickness difference after liquid absorption. Scanning electron microscopy (SEM) was used to distinguish the first edge region, second edge region, first main body region, second main body region, and central region. Samples with an area of ​​1540.25 mm² were taken from the first edge region, second edge region, first main body region, second main body region, and central region, respectively. 2 Five samples of each type were used. After simply wiping off any residual solvent from the surface with lint-free paper, each sample was weighed to obtain sample weight M1. The samples were then placed in a vacuum oven at 120°C for 12 hours to dry under vacuum. After drying, each sample was removed and weighed to obtain the type of dried sample M2. The liquid retention rate V of the first coating was calculated by taking the average value of the five samples in each group. A Liquid retention rate V of the second coating B Liquid retention rate = (M1 - M2) / M2.

[0097] Purple spot test

[0098] After placing the lithium-ion battery in a 45℃ environment for 60 minutes, charge it to 3.6V at a constant current of 1C, then charge it to the cutoff current of 0.05C at a constant voltage of 3.6V, let it stand for 5 minutes, and then discharge it to 2.5V at a constant current of 1C, and let it stand for 5 minutes. This process is considered one cycle. Repeat the above cycle with the lithium-ion battery. After the 100th cycle, disassemble the lithium-ion battery to obtain the positive and negative electrode sheets. Soak the positive and negative electrode sheets in dimethyl carbonate solvent for 2 hours and dry them at 60℃ for 1 hour to obtain the positive and negative electrode sheets. Use a scanning electron microscope (SEM) to photograph the positive and negative electrode sheets and observe whether purple spots appear on the surface of the electrode sheets.

[0099] Lithium plating test

[0100] After placing the lithium-ion battery in a 15°C environment for 60 minutes, charge it to 3.6V at a constant current of 1C, then charge it to 0.05C at a constant voltage of 3.6V until the cutoff current is reached. Let it rest for 5 minutes, then discharge it to 2.5V at a constant current of 1C and let it rest for 5 minutes. This process is considered one cycle. Repeat the above cycle for the lithium-ion battery. After the 10th cycle, disassemble the lithium-ion battery to obtain the negative electrode. Observe the lithium deposition state on the surface of the negative electrode material layer. The non-lithium-deposited area on the surface of the negative electrode material layer is golden yellow, and the lithium-deposited area is grayish-white. Measure the lithium deposition area on the surface of the negative electrode material layer in the flat area and the bent area respectively. For each example and comparative example, the lithium deposition state of the negative electrode material layer surface in 10 groups of lithium-ion batteries is statistically analyzed, and the average value is calculated to obtain the percentage of lithium deposition area to evaluate the lithium deposition state of the negative electrode material layer.

[0101] The criteria for judging the lithium plating state on the surface of the negative electrode material layer are as follows: lithium plating area less than or equal to 1% is no lithium plating, lithium plating area greater than 1% and less than or equal to 3% is slight lithium plating, lithium plating area greater than 3% and less than or equal to 5% is moderate lithium plating, and lithium plating area greater than 5% is severe lithium plating.

[0102] Cyclic capacity retention test:

[0103] At 45℃, the lithium-ion battery was charged at a constant current of 0.5C to 3.6V, then charged at a constant voltage of 3.6V to 0.05C, allowed to rest for 5 minutes, and then discharged at a constant current of 1C to 2.5V, allowed to rest for 5 minutes. This constitutes one charge-discharge cycle, and the discharge capacity of each cycle was recorded. The lithium-ion battery was subjected to 1000 cycles under the above conditions, and the discharge capacity was measured after each cycle. Cycle capacity retention (%) = (Capacity after 1000 discharge cycles / First discharge cycle capacity) × 100%.

[0104] Example 1

[0105] <Preparation of the positive electrode>

[0106] Lithium iron phosphate (LiFePO4, Dv50=12μm), the positive electrode active material, polyvinylidene fluoride (PVDF), the positive electrode binder, and Super P, were added to N-methylpyrrolidone (NMP) at a solid mass ratio of 95:2.5:2.5 and mixed evenly under vacuum stirring to obtain a positive electrode material slurry with a solid content of 60wt%. The positive electrode material slurry was then uniformly coated onto one surface of a 13μm thick aluminum foil for the positive electrode current collector and dried at 80℃ for 1 hour to obtain a single-sided coated positive electrode sheet with a single-sided coating weight of 300mg / 1540.25mm. 2Then, the above steps are repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of positive electrode material. After cold pressing, cutting, and welding of tabs, a positive electrode sheet with a size of 1600mm×64mm is obtained. The thickness of the single-sided positive electrode material layer is 43.5μm, and the thickness of the main body area of ​​the prepared positive electrode sheet is 100μm.

[0107] <Preparation of Negative Electrode Sheets>

[0108] Artificial graphite (negative electrode active material), styrene-butadiene rubber (binder), and acetylene black (conductive agent) were mixed in a mass ratio of 97.4:1.4:1.2. Deionized water was added as a solvent to prepare a slurry with a solid content of 45 wt%. The slurry was stirred evenly in a vacuum mixer to obtain the negative electrode slurry. The negative electrode slurry was uniformly coated onto one surface of a 6 μm thick copper foil current collector and dried at 120°C for 1 hour to obtain a negative electrode sheet with a single-sided negative electrode material layer. The coating weight of the negative electrode material layer was 230 mg / 1540.25 mm. 2 The above steps are then repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided coating of negative electrode material. After drying at 120℃, it is cold-pressed, then cut and welded with tabs to obtain a negative electrode sheet with dimensions of 1742mm×67mm for later use. The thickness of the single-sided negative electrode material layer is 57μm, and the thickness of the main body area of ​​the prepared negative electrode sheet is 120μm.

[0109] <Preparation of the diaphragm>

[0110] The first material, polystyrene, inorganic alumina particles, and tricresyl phosphate additive were mixed in a mass ratio of 45:50:5, and NMP solvent was added. After vacuum stirring, the first coating slurry was obtained.

[0111] The second material, polyvinylidene fluoride, inorganic alumina particles, and tricresyl phosphate additive were mixed in a mass ratio of 45:50:5, and NMP solvent was added. After vacuum stirring, the second coating slurry was obtained.

[0112] A 9μm thick polyethylene base membrane was used as the base membrane for the separator. Along the width of the base membrane, a first edge region, a first main region, a central region, a second main region, and a second edge region were defined. The proportions of the first edge region were 10%, the first main region 25%, the central region 30%, the second main region 25%, and the second edge region 10%. A first coating slurry was applied to the central region with a coating weight of 120mg / 1540.25mm. 2 The first coating slurry applied to the first and second edge regions has a coating weight of 130 mg / 1540.25 mm. 2A second coating slurry was applied to the first and second main areas, with a coating weight of 125 mg / 1540.25 mm. 2 After drying, a diaphragm is obtained.

[0113] <Preparation of Electrolyte>

[0114] In an environment with a water content of less than 10 ppm, dimethyl carbonate, diethyl carbonate, and ethylene carbonate were mixed in a mass ratio of 1:1:1 to obtain an organic solvent. Then, the electrolyte salt LiPF6 was added to the organic solvent and mixed thoroughly to obtain the electrolyte. Based on the mass of the electrolyte, the electrolyte salt comprised 12.5% ​​by mass, with the remainder being the organic solvent.

[0115] <Preparation of Lithium-ion Batteries>

[0116] The prepared positive electrode, separator, and negative electrode are stacked in the order of separator, negative electrode, separator, and positive electrode, with the separator positioned between the positive and negative electrodes to provide insulation. The electrode assembly is then wound to obtain the electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag and dehydrated at 80°C. The electrolyte prepared above is then injected, and the battery undergoes vacuum sealing, settling, formation, degassing, and edge trimming to obtain a lithium-ion battery. The upper limit of the formation voltage is 3.6V, the formation temperature is 70°C, and the formation settling time is 2 hours.

[0117] Examples 2 to 9

[0118] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Example 1. Specifically, when the mass percentage of the first material in the first coating changes, the mass percentage of the additive tricresyl phosphate in the first coating remains unchanged, while the mass percentage of the inorganic filler alumina changes accordingly; when the mass percentage of the second material in the second coating changes, the mass percentage of the additive tricresyl phosphate in the second coating remains unchanged, while the mass percentage of the inorganic filler alumina changes accordingly.

[0119] Examples 10 to 18

[0120] Except for adjusting the relevant preparation parameters according to Table 1, everything else is the same as in Example 1.

[0121] Example 19

[0122] Except in the <diaphragm preparation> step, the coating weight of the first coating slurry in the first and second edge regions is adjusted to 135 mg / 1540.25 mm. 2 Except for the above, the rest is the same as in Example 1.

[0123] Example 20

[0124] Except in the <diaphragm preparation> step, the coating weight of the first coating slurry in the first and second edge regions is adjusted to 140 mg / 1540.25 mm. 2 Except for the above, the rest is the same as in Example 1.

[0125] Examples 21 to 23

[0126] Except for adjusting the relevant preparation parameters according to Table 1, everything else is the same as in Example 1.

[0127] Comparative Example 1

[0128] Except for the use of a 9 μm thick porous polyethylene film (provided by Celgard) as the diaphragm in the <Preparation of the Diaphragm> section, the rest is the same as in Example 1.

[0129] Comparative Example 2

[0130] Except for applying a second coating to the first edge region, second edge region, and center region, and applying a first coating to the first body region and second body region in the <Preparation of the Separator>, the rest is the same as in Example 1.

[0131] Table 1

[0132] Note: In Table 1, " / " indicates that the corresponding preparation parameter or substance does not exist; M1 represents the mass percentage of the first material; M2 represents the mass percentage of the second material; V A V represents the liquid retention rate of the first coating. B Indicates the liquid retention rate of the second coating; W A1 Indicates the width percentage of the first edge region; W B1 Indicates the width percentage of the first main area; W A2 Indicates the width percentage of the central area; W B2 Indicates the width percentage of the second main area; W A3 Indicates the width percentage of the second edge region; H A1 H represents the average thickness of the first coating located in the first or second edge region, corresponding to the edge region; A2 H represents the average thickness of the first coating located in the central area; B D1 represents the average thickness of the second coating located in the first or second main body region; D2 represents the thickness of the positive electrode main body region; D2 represents the thickness of the negative electrode main body region; W X1 W represents the width percentage of the edge region of the cathode material layer. X2 This indicates the width percentage of the edge region of the negative electrode material layer.

[0133] As can be seen from Examples 1 to 23 and Comparative Examples 1 to 2, by setting a first edge region, a first main region, a central region, a second main region, and a second edge region that satisfy the characteristics of this application in the separator, and by controlling the liquid retention rate of the first coating and the second coating within the range of this application, the secondary battery has a lower lithium plating rate and a higher cycle capacity retention rate. Comparative Example 1 uses a conventional separator, and the secondary battery developed purple spots during cycling, severe lithium plating occurred in the later stages of cycling, and it had a poor cycle capacity retention rate. In Comparative Example 2, the liquid retention rate of the first coating was lower than that of the second coating, and the liquid retention capacity of the first edge region and the second edge region of the separator was poor, which could not fill the gaps at both ends of the electrode width direction caused by the edge region. The secondary battery developed purple spots during cycling, severe lithium plating occurred in the later stages of cycling, and it had a poor cycle capacity retention rate. This indicates that embodiments that do not meet the requirements of this application have poor lithium plating resistance and cycle performance, while embodiments of this application, while reducing the possibility of purple spots, have a lower lithium plating rate and a higher cycle capacity retention rate. This shows that embodiments that meet the features of this application can reduce the formation of purple or black spots on the electrode, improve the liquid replenishment capacity during the cycle of the secondary battery, thereby enhancing the lithium plating resistance and improving the cycle performance of the secondary battery.

[0134] As can be seen from Examples 1 to 5, the mass percentage of the first material affects the lithium plating resistance and cycle performance of the secondary battery. When the value of M1 is adjusted within the range of this application, the first coating can have a suitable liquid retention rate. The first coating located in the first edge region or the second edge region corresponding to the edge region can better fill the gaps caused by the electrode edge region, thereby improving the liquid replenishment capacity during the cycle of the secondary battery. While reducing the formation of purple spots or black spots on the electrode, it has a lower lithium plating rate and a higher cycle capacity retention rate, indicating that the embodiments provided in this application are beneficial to enhancing the lithium plating resistance and improving the cycle performance of the secondary battery.

[0135] As can be seen from Examples 1, 6 to 9, the mass percentage of the second material affects the lithium plating resistance and cycle performance of the secondary battery. When the value of M2 is adjusted within the range of this application, the second coating can have a suitable liquid retention rate, thereby improving the liquid replenishment capacity during the cycle of the secondary battery. While reducing the formation of purple or black spots on the electrode, it has a lower lithium plating rate and a higher cycle capacity retention rate, indicating that the embodiments provided in this application are beneficial for enhancing the lithium plating resistance and improving the cycle performance of the secondary battery.

[0136] As can be seen from Examples 1, 10 to 12, the types of the first and second materials affect the lithium plating resistance and cycle performance of the secondary battery. When the types of the first and second materials are controlled within the scope of this application, the first and second coatings can have suitable liquid retention rates, which can better fill the gaps generated by the electrode edge region, thereby improving the liquid replenishment capacity during the cycle of the secondary battery. While reducing the formation of purple or black spots on the electrode, it has a lower lithium plating rate and a higher cycle capacity retention rate, indicating that the embodiments provided in this application are beneficial to enhancing the lithium plating resistance and improving the cycle performance of the secondary battery.

[0137] As can be seen from Examples 1, 13 to 17, the proportion of each region on the separator affects the lithium plating resistance and cycle performance of the secondary battery. When the proportions of the first edge region, the first main region, the central region, the second main region, and the second edge region are adjusted within the scope of this application, each region has a suitable width, which is beneficial to improving the electrolyte replenishment capacity of the corresponding regions at both ends of the electrode during secondary battery cycling. This effectively addresses the problem that the edge regions at both ends of the electrode width direction cannot be timely wetted by the electrolyte or that the electrolyte wetting degree is insufficient in the later stages of cycling. While reducing the formation of purple or black spots on the electrode, it also exhibits a lower lithium plating rate and a higher cycle capacity retention rate, indicating that the embodiments provided in this application are beneficial to enhancing the lithium plating resistance and improving the cycle performance of the secondary battery.

[0138] As can be seen from Examples 1, 18 to 20, (H) A1 -H B ) / D1 and (H A1 -H B The value of (H) / D2 affects the lithium plating resistance and cycle performance of the secondary battery. When (H) / D2 is adjusted... A1 -H B ) / D1 and (H A1 -H B When the value of D2 is within the range of this application, the coating on each region of the secondary battery base film after formation has a suitable thickness, which is beneficial to reduce the formation of purple spots or black spots and improve the liquid replenishment capacity during the secondary battery cycle. Thus, the secondary battery has a lower lithium plating rate and a higher cycle capacity retention rate while reducing the formation of purple spots or black spots on the electrode. At the same time, the central region has a suitable thickness, which is beneficial to reduce the risk of the electrode assembly being too thick due to excessive thickness of the central region, which affects the packaging quality. This indicates that the embodiments provided in this application are beneficial to enhancing the lithium plating resistance of the secondary battery and improving the cycle performance of the secondary battery.

[0139] As can be seen from Examples 1 and 21 to 23, the width ratio of the edge region of the positive electrode material layer and the width ratio of the edge region of the negative electrode material layer affect the lithium plating resistance and cycle performance of the secondary battery. When the width ratios of the edge regions of the positive and negative electrode material layers are controlled within the scope of this application, it is beneficial to alleviate the increased risk of lithium plating caused by excessively wide edge regions, allowing the first coating to better fill the voids generated by the edge regions and improving the electrolyte replenishment capacity during the cycle of the secondary battery. The embodiment of this application, which reduces the formation of purple or black spots on the electrode while exhibiting a low lithium plating rate and a high cycle capacity retention rate, indicates that it is beneficial to enhance the lithium plating resistance and improve the cycle performance of the secondary battery.

[0140] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or article that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, or article.

[0141] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A secondary battery, comprising a wound electrode assembly, the electrode assembly comprising an electrode sheet and a separator, the electrode sheet comprising a current collector and a material layer disposed on at least one surface of the current collector; the material layer comprising a main region and an edge region, wherein the main region is a region with a thickness fluctuation difference ≤1 μm from the center position of the material layer along the thickness direction of the material layer, and the edge region is located on either side of the width direction of the material layer; The diaphragm includes a base film and a first coating and a second coating disposed on at least one surface of the base film. Along the width direction of the unfolded diaphragm, the base film sequentially includes a first edge region, a first main body region, a central region, a second main body region, and a second edge region. Based on the width of the base film, the width of the first edge region accounts for W. A1 The width of the first main body area accounts for W. B1 The width of the central area accounts for W. A2 The width of the second main area accounts for W. B2 The width ratio of the second edge region is W. A3 W A1 <W B1 W A3 <W B2 W A1 +W B1 +W A2 +W B2 +W A3 =1; The first coating is applied to the surfaces of the first edge region, the central region, and the second edge region, and the second coating is applied to the surfaces of the first main body region and the second main body region. The liquid retention rate of the first coating is V. A The liquid retention rate of the second coating is V B V A >V B .

2. The secondary battery according to claim 1, wherein, The first coating comprises a first material, which includes at least one of polystyrene, polystyrene-propylene, polyvinyl alcohol, polyamide, polyurethane, or ethyl cellulose, and the mass percentage of the first material is M1 based on the mass of the first coating, where 30% ≤ M1 ≤ 60%. The second coating includes a second material, which includes at least one of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyacrylic acid or polyimide, and the mass percentage of the second material is M2 based on the mass of the second coating, where 30% ≤ M2 ≤ 60%.

3. The secondary battery according to claim 1, wherein, The secondary battery satisfies at least one of the following characteristics: （1）5%≤W A1 ≤15%； （2）20%≤W B1 ≤35%； （3）20%≤W A2 ≤45%； （4）20%≤W B2 ≤35%； （5）5%≤W A3 ≤15%。 4. The secondary battery according to claim 1, wherein, Along the thickness direction of the electrode assembly, the average thickness of the first coating located in the first edge region or the second edge region corresponding to the edge region is H. A1 μm, the average thickness of the second coating located in the first main body region or the second main body region is H B μm, H A1 ≥H B .

5. The secondary battery according to claim 4, wherein, The electrode is a positive electrode, and the thickness of the main body region of the positive electrode is D1 μm, 0≤(H A1 -H B ) / D1≤0.

1.

6. The secondary battery according to claim 5, wherein, The material layer is a positive electrode material layer, the width of the positive electrode material layer is smaller than the width of the base film, and the width of the edge region of the positive electrode material layer is smaller than the width of the first edge region or the second edge region.

7. The secondary battery according to claim 6, wherein, Based on the width of the positive electrode material layer, the width ratio of the edge region of the positive electrode material layer is W. X1 , 0≤W X1 ≤10%.

8. The secondary battery according to claim 4, wherein, The electrode is a negative electrode, and the thickness of the main body region of the negative electrode is D2 μm, 0≤(H A1 -H B ) / D2≤0.

1.

9. The secondary battery according to claim 8, wherein, The material layer is a negative electrode material layer, the width of the negative electrode material layer is smaller than the width of the base film, and the width of the edge region of the negative electrode material layer is smaller than the width of the first edge region or the second edge region.

10. The secondary battery according to claim 9, wherein, Based on the width of the negative electrode material layer, the width ratio of the edge region of the negative electrode material layer is W. X2 , 0≤W X2 ≤10%.

11. The secondary battery according to claim 1, wherein, The electrode includes a positive electrode and a negative electrode. Along the thickness direction of the separator, the first coating and the second coating are disposed on one surface of the base film, and the surface of the base film with the first coating and the second coating faces the negative electrode.

12. An electronic device comprising a secondary battery according to any one of claims 1 to 11.

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