Secondary battery

Abstract

The invention discloses a secondary battery, and relates to the field of batteries. Comprising a positive pole piece and a negative pole piece, the positive pole piece is provided with a straight area and a corner area, the surface of a positive active material layer of the corner area is provided with a conductive coating, and the secondary battery meets the following relational expressions: C / P < = 6300, and P < = 0.1 < = P < = 0.5; c is the unit volume capacity of the negative pole piece; p is the ratio of the area of the conductive coating to the sum of the areas near the corner region. The conductive coating forming the stripe is coated in the gap of the corner area of the positive pole piece, so that the conductivity and the heat conduction performance of the pole piece are improved, meanwhile, the conveying distance of part of lithium ions is increased, lithium ion transmission is dispersed in a multi-level mode, lithium ions are prevented from being accumulated and separated out in a negative pole structure, concentration polarization and interface impedance are reduced, and the service life of the lithium ion battery is prolonged. And gaps between the adjacent stripe bands can also store electrolyte, so that the lithium precipitation effect of the battery is reduced, relatively high cycle capacity is maintained, and the cycle life of the battery is prolonged.

Description

Technical Field

[0001] The present invention relates to the field of secondary batteries, and in particular to a secondary battery. Background Art

[0002] With the rapid development of new energy vehicles and energy storage devices, lithium-ion batteries have become the mainstream choice due to their high energy density, long cycle life, and excellent safety. However, in actual use, lithium-ion batteries are prone to lithium deposition during charge and discharge cycles, especially in the corners of the battery cell. Lithium deposition not only causes battery capacity degradation but can also lead to internal short circuits and thermal runaway, seriously affecting battery safety and service life.

[0003] The corners of wound lithium-ion battery cells are prone to lithium plating due to their special structure, low temperature, and poor contact. Under high-rate charge and discharge conditions, the negative electrode material at the corners is more prone to overpotential accumulation, leading to Li dendrite growth and electrolyte decomposition, which in turn causes lithium plating. Therefore, lithium plating at the corners of battery cells is a common technical difficulty in the battery industry and a problem in high-rate charging of wound structures. It is necessary to focus on studying the factors affecting lithium plating in the corners of battery cells and solutions to promote the development of high-energy, high-rate wound batteries. Summary of the Invention

[0004] The present invention provides a secondary battery. By applying a conductive coating on the corner area of ​​the positive electrode plate and defining the relationship between the conductive coating and the unit volume capacity of the negative electrode, the transmission of lithium ions can be dispersed at multiple levels, thereby alleviating the problem of lithium plating in the corner area of ​​the negative electrode, while ensuring the dynamic performance of the secondary battery and extending the service life of the battery.

[0005] In order to solve the above technical problems, the present invention aims to provide a secondary battery, comprising a positive electrode sheet and a negative electrode sheet, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer, wherein the positive electrode current collector is alternately provided with a straight region and a corner region along its length, wherein the position where the straight region and the corner region are connected is defined as an intersection line, and a conductive coating is provided on the surface of the positive electrode active material in the corner region, wherein an end of the conductive coating extends to a position within 2 mm of the corner region from the intersection line or an end of the conductive coating extends to a position within 15 mm of the straight region from the intersection line;

[0006] The conductive coating is composed of a plurality of stripes arranged along the length direction of the positive electrode sheet, and the secondary battery satisfies the following relationship: C / P≤6300, 0.1≤P≤0.5;

[0007] Where C is the unit volume capacity of the negative electrode, in mAh / cm 3; P is the ratio of the area of ​​the conductive coating to the area of ​​the region of the positive electrode sheet located between the two ends of the conductive coating.

[0008] In order to alleviate the lithium deposition problem in the corner area of ​​the negative electrode, the present application applies a conductive coating to the corner area of ​​the positive electrode. The conductive particles have excellent conductivity, which can significantly improve the conductivity and cycle stability of the electrode material. At the same time, the conductive coating is a plurality of stripes, which can disperse the transmission of lithium ions, increase the transport distance of some lithium ions, and make the lithium ions of the positive electrode enter the negative electrode structure in an orderly manner, avoiding the accumulation of lithium ions in the negative electrode structure and precipitation. At the same time, the gaps between the stripes can store electrolyte, increase the electrolyte content in the corner area, and effectively slow down the lithium deposition phenomenon. The ratio P of the conductive coating area to the area near the corner area and the unit volume capacity of the negative electrode are controlled to meet the relationship C / P≤6300, which can disperse the lithium ion transmission distance at multiple levels, reduce concentration polarization and interface impedance, and control the P value to meet 0.1≤P≤0.5. It can avoid that the coating area of ​​the conductive coating is too large and affects the dynamic performance of the battery, so that the battery can maintain a higher cycle capacity.

[0009] As a preferred embodiment, the conductive coating includes a corner coating area and two tailing areas respectively connected to both ends of the corner coating area, and the thickness of the stripes in the tailing areas decreases in the direction away from the corner coating area.

[0010] As a preferred embodiment, the end of the trailing area close to the corner coating area is the starting end, the thickness of the first column of stripes in the trailing area close to the starting end is X, the thickness of the stripes in the first column is equal to the thickness of the stripes in the corner coating area, the thickness of the stripes in the nth column is Y, and X and Y satisfy the following relationship: Y=X-(n-1)dX, 0.1≤d≤0.4.

[0011] The conductive coating on the positive electrode sheet of this application is provided with a trailing zone near the corners and flat areas to avoid the formation of large thickness steps with the uncoated area, preventing the accumulation of stress concentration caused by the battery cell after winding, which affects the lithium ion transport dynamics. The thickness of the stripes in the trailing zone is controlled to gradually decrease, which can effectively reduce stress concentration in the transition area of ​​the coating, avoid battery cycle breakage, and improve cycle life.

[0012] As a preferred solution, the tail ends of the trailing areas on both sides of the positive electrode sheet are staggered by 2-15 mm. Staggering the tail ends of the trailing areas of the positive electrode sheet can further reduce the stacking thickness of the transition area, effectively reduce the stress concentration in the transition area of ​​the coating, avoid battery cycle breakage, and further improve the cycle life.

[0013] As a preferred embodiment, the P value is any one or any two of 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, and 0.5.

[0014] As a preferred solution, the secondary battery satisfies the following relationship: 1500≤C / P≤4100.

[0015] As a preferred solution, the secondary battery satisfies the following relationship: 2000≤C / P≤3500.

[0016] The present application controls the ratio P of the area near the conductive coating and the corner area to satisfy the above-mentioned relationship with the unit volume capacity of the negative electrode plate, which can effectively disperse lithium ion transmission at multiple levels to match the unit volume capacity of the negative electrode plate, thereby further improving the lithium plating effect. The lithium plating effect increases with the decrease of the C / P ratio, avoiding excessively large P values. By balancing the relationship between the C value and the P value, the lithium plating effect can be reduced while improving the cycle capacity retention rate of the battery cell and improving the kinetic performance.

[0017] As a preferred solution, the unit volume capacity C value of the negative electrode plate is 500-1600.

[0018] As a preferred embodiment, the conductive coating includes conductive particles, and the conductive particles include at least one of graphene, carbon nanotubes, and conductive carbon black, preferably graphene, which has good thermal conductivity, can effectively disperse heat in the corner area, reduce the risk of local overheating, and improve the battery cycle life.

[0019] As a preferred solution, the conductive coating comprises conductive particles and a binder in a mass ratio of (98-99.7):(0.3-2.0).

[0020] As a preferred solution, the Dv50 particle size of the conductive particles is 50-2000 nm.

[0021] The present application controls the particle size of the conductive particles in the conductive coating within the above range. When the particle size is too large, it is easy to cause agglomeration and an imperfect conductive network. At the same time, the contact area with the active material is reduced, which is not conducive to the formation of a stable conductive network and leads to a decrease in electron transmission efficiency. When the particle size is too small, the particles are more likely to aggregate together to form larger clusters, which not only affects the uniformity of the material, but may also block the diffusion channel of lithium ions.

[0022] As a preferred solution, the thickness of the conductive coating in the corner coating area is 2-8 μm.

[0023] As a preferred solution, the thickness of the conductive coating in the corner coating area is within the range of any one or any two of 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, and 8 μm.

[0024] The present application controls the thickness of the conductive coating in the corner coating area to meet the above range, which can avoid the conductive coating thickness being too large and affecting the lithium ion transmission, resulting in insufficient dynamic performance; at the same time, it can avoid the conductive coating thickness being too small, which cannot effectively increase the lithium ion transmission distance and cannot disperse the lithium ion transmission at multiple levels, thereby causing the lithium precipitation phenomenon to not be improved.

[0025] As a preferred solution, the stripe widths of the corner coating area and the trailing area are independent and range from 0.1 to 1 mm.

[0026] As a preferred embodiment, the stripe widths of the corner coating area and the trailing area are independent of each other and are in the range of any one or any two of 0.1mm, 0.2mm, 0.3mm, 0.4mm, 0.5mm, 0.6mm, 0.7mm, 0.8mm, 0.9mm, and 1mm.

[0027] As a preferred solution, the spacing between adjacent stripes in the corner coating area and the trailing area is independent and ranges from 0.1 to 2 mm.

[0028] As a preferred embodiment, the spacing between adjacent stripe bands in the corner coating area and the trailing area is independent of each other and is in the range of any one or any two of 0.1mm, 0.2mm, 0.3mm, 0.4mm, 0.5mm, 0.6mm, 0.7mm, 0.8mm, 0.9mm, 1mm, 1.1mm, 1.2mm, 1.3mm, 1.4mm, 1.5mm, 1.6mm, 1.7mm, 1.8mm, 1.9mm, and 2mm.

[0029] Since lithium deposition is prone to occur in the corner area of ​​the negative electrode interface, the present application controls the spacing between adjacent stripes on the positive electrode sheet to meet the above range. The gap between adjacent stripes can effectively store electrolyte, increase the liquid retention capacity of the battery cell, and further reduce the risk of lithium deposition in the corner area. If the spacing is too small or too large, the electrolyte may not be effectively stored.

[0030] As a preferred solution, the negative electrode plate includes a negative electrode active material layer and a negative electrode current collector, and the negative electrode active material layer includes a negative electrode active material.

[0031] As a preferred embodiment, the negative electrode active material is at least one of a carbon-based compound, a silicon-based compound, a titanium-based compound, a tin-based alloy, and a transition metal nitride.

[0032] As a preferred embodiment, the positive electrode active material layer comprises a positive electrode active material, a positive electrode binder and a positive electrode conductor in a mass ratio of (92-99):(0.5-4):(0.5-2).

[0033] As a preferred embodiment, the negative electrode active material layer includes a negative electrode active material, a negative electrode binder, a negative electrode conductor and a plasticizer in a mass ratio of (92-99): (0.5-5): (0.5-1.5): (0-1).

[0034] As a preferred solution, the secondary battery further includes a separator, and the separator is at least one of polyethylene, polypropylene, polyethylene terephthalate, polyimide, aramid, and polybutylene terephthalate.

[0035] As a preferred embodiment, the secondary battery further comprises an electrolyte, wherein the electrolyte comprises a lithium salt and a solvent in a mass ratio of (5-10):(90-95), and the solvent is at least one of ethylene carbonate, diethyl carbonate, propylene carbonate, propyl propionate and vinylene carbonate.

[0036] As a preferred embodiment, the organic solvent is N-methylpyrrolidone.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] 1. The present application applies a conductive coating with stripes in the gaps of the corner areas of the positive electrode sheet, which not only significantly improves the conductivity and cycle stability of the electrode material, but also increases the transport distance of some lithium ions. The ratio P of the conductive coating area to the area near the corner area and the unit volume capacity of the negative electrode sheet are controlled to satisfy the relationship C / P≤6300, which can disperse lithium ions for transport at multiple levels, avoid the accumulation and precipitation of lithium ions in the negative electrode structure, reduce concentration polarization and interface impedance, and control the P value to satisfy 0.1≤P≤0.5, so as to avoid the conductive coating area from being too large and affecting the dynamic performance of the battery, reduce the lithium precipitation effect of the battery while maintaining a higher cycle capacity.

[0039] 2. The conductive coating on the positive electrode sheet of the present application sets a tail area near the corner area and the flat area, which can avoid the formation of steps with a large thickness difference from the uncoated area, prevent the superposition and increase after the battery cell is wound, resulting in cyclic stress concentration, thereby affecting the lithium ion transmission dynamics. The thickness of the tail area decreases step by step, effectively reducing the stress concentration in the transition area of ​​the coating, avoiding battery cycle breakage, and improving the cycle life.

[0040] 3. The gaps between adjacent stripes of the conductive coating of the present application can store electrolyte, increase the electrolyte content in the corner area, and effectively slow down the lithium desorption phenomenon. When graphene is used as the conductive particles, it has good thermal conductivity and can effectively disperse the heat in the corner area, reduce the risk of local overheating, and improve the battery cycle life. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 : A schematic diagram showing a conductive coating arranged in an array on a positive electrode plate to form a plurality of stripes in an embodiment of the present invention;

[0042] Figure 2 : Schematic diagram of the position of the conductive coating on the positive electrode sheet after winding in an embodiment of the present invention. DETAILED DESCRIPTION

[0043] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0044] It should be understood that the terms described herein are intended only to describe particular embodiments and are not intended to limit the present invention. In addition, for numerical ranges herein, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. The intermediate value within any stated value or stated range, and each smaller range between any other stated value or intermediate value within the stated range, is also encompassed within the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded within the scope.

[0045] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of any conflict with any incorporated document, the contents of this specification shall prevail.

[0046] In the description of the present invention, it should be understood that the terms "up", "down", "left", "right", "top", "bottom", etc., indicating directions or positional relationships, are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and therefore cannot be understood as a limitation on the present invention.

[0047] To further illustrate the present invention, the present invention is described in detail below with reference to the following examples, but they should not be construed as limiting the scope of the present invention. Unless otherwise specified, the sources of the raw materials used in the following examples and comparative examples of the present application are commercially available, and the same raw materials were used in parallel experiments.

[0048] Example 1

[0049] A secondary battery comprises a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. The positive electrode sheet, separator, and negative electrode sheet are stacked and wound in sequence to form a battery cell, and the outermost circle of the battery cell ends with a hollow foil section of the positive electrode sheet. The positive electrode sheet comprises aluminum foil and positive electrode active material layers located on both sides of the positive electrode sheet. The positive electrode active material layers comprise lithium cobalt oxide positive electrode active material, acetylene black (SP) conductive agent, and polyvinylidene fluoride (PVDF) binder in a mass ratio of 98:1.2:0.8. The positive electrode sheet has straight areas and corner areas alternately arranged along its winding direction. The surfaces of the positive electrode active material layers on both sides of the corner area are coated with a conductive coating.

[0050] The conductive coating includes a corner coating area and two tail areas. In the length direction of the positive electrode sheet, the two tail areas are respectively connected to the two ends of the corner coating in the length direction of the positive electrode sheet. The connection position of the corner area and the straight area of ​​the positive electrode sheet is defined as the intersection line, and the end of the tail area away from the corner coating area is set as the tail end. In this embodiment, the tail end of the tail area is located at a position 10 mm away from the intersection line in the straight area. Assuming that when the tail end of the tail area is located at the intersection line, the distance between the tail end and the intersection line is 0 mm, when the tail end of the tail area is located in the straight area, the distance between the tail end and the intersection line is a positive value, and when the tail end of the tail area is located in the corner area, the distance between the tail end and the intersection line is a negative value, then the distance between the tail end of the tail area and the intersection line in Example 1 is 10 mm.

[0051] The conductive coating includes conductive particles and a binder. The conductive particles are graphene, and the Dv50 particle size of the conductive particles is 1000nm. The conductive coating consists of a number of stripes arranged in an array along the length of the positive electrode sheet, such as Figure 1 As shown, the spacing between adjacent stripes is 1 mm, and the thickness of the stripes located in the corner coating area is 6 μm.

[0052] like Figure 2 As shown, the thickness of the stripes in the tailing area decreases as it moves away from the corner coating area. The end of the tailing area close to the corner coating area is the starting end. The thickness of the first column of stripes in the tailing area close to the starting end is X. The thickness of the first column of stripes is equal to the thickness of the stripes in the corner coating area. The thickness of the nth column of stripes is Y. X and Y satisfy the following relationship: Y = X-(n-1)dX, d = 0.2.

[0053] The secondary battery satisfies the following relationship: C / P = 3060, P = 0.3; where C is the unit volume capacity of the negative electrode, in mAh / cm 3 ; P is the ratio of the conductive coating area to the sum of the areas of the corner coating area and the tail area of ​​the positive electrode.

[0054] The negative electrode sheet includes a copper foil and a negative electrode active material layer. The negative electrode active material layer includes a negative electrode active material, an acetylene black conductive agent, a styrene-butadiene rubber binder and a sodium carboxymethyl cellulose thickener in a mass ratio of 98:0.6:0.7:0.7. The negative electrode active material includes silicon carbon and graphite in a mass ratio of 15:85.

[0055] The unit volume capacity C value of the negative electrode plate is calculated according to the following formula: The unit volume capacity C value of the negative electrode plate = the compaction density of the negative electrode active material × the gram capacity of the negative electrode active material. The gram capacity of the negative electrode active material is 540mAh / g; the compaction density of the negative electrode active material is 1.7g / cm 3 The unit volume capacity C value of the negative electrode is 918 mAh / cm 3 .

[0056] The separator is a polyethylene porous film coated on both sides with alumina ceramic. The electrolyte consists of a lithium salt, LiPF6, and a non-aqueous organic solvent in a mass ratio of 8:92. The non-aqueous organic solvent consists of ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), propyl propionate (PP), and vinylene carbonate (VC) in a mass ratio of 25:25:15:31:4.

[0057] The method for preparing a secondary battery in the above embodiment 1 comprises the following steps:

[0058] (1) adding conductive particles and polyvinylidene fluoride (PVDF) in a mass ratio of 98:2 to N-methylpyrrolidone and stirring uniformly to prepare a conductive coating slurry, wherein the mass fraction of NMP in the conductive coating slurry is 27%, and adding lithium cobalt oxide positive electrode active material, acetylene black (SP) conductive agent, and polyvinylidene fluoride (PVDF) binder in a mass ratio of 98:1.2:0.8 to N-methylpyrrolidone solvent system and stirring uniformly to prepare a positive electrode slurry with a solid content of 73%, which is then coated on aluminum foil, dried and rolled to form a positive electrode active material layer, and the conductive coating slurry is applied to the surface of the positive electrode active material layer on both sides of the corner coating area and the trailing area of ​​the positive electrode sheet, and dried and slit to obtain a positive electrode sheet;

[0059] (2) The negative electrode active material, conductive agent, styrene-butadiene rubber binder, and sodium carboxymethyl cellulose thickener were fully stirred and mixed in deionized water at a mass ratio of 98:0.6:0.7:0.7 to prepare a negative electrode slurry with a solid content of 42%. The negative electrode active material included silicon carbon and graphite at a mass ratio of 15:85. The slurry was coated on a copper foil at 105°C in a vacuum, and after drying, rolling, and slitting, a negative electrode sheet was obtained;

[0060] (3) Stack the positive electrode sheet, separator and negative electrode sheet in order, so that the separator is placed between the positive electrode sheet and the negative electrode sheet to play a role of safety isolation, and wind them to obtain an electrode assembly, place the electrode assembly in a packaging shell, inject electrolyte, and encapsulate and form it to obtain a secondary battery.

[0061] Example 2-3

[0062] A secondary battery is different from Example 1 in that the particle size of graphene in the conductive coating is different, as shown in Table 2.

[0063] Examples 4-5

[0064] A secondary battery is different from Example 1 in that the thickness of the stripe bands in the corner coating area is different, as shown in Table 2.

[0065] Examples 6-8

[0066] A secondary battery, which is different from Example 1 in that the d value in the formula Y=X-(n-1)dX is different, wherein d=0 indicates that the thickness of the stripe bands in the tail area is consistent and the same as the thickness of the stripe bands in the corner coating area, as shown in Table 2.

[0067] Examples 9-10

[0068] A secondary battery is different from Example 1 in that the distance between the tail end of the trailing region and the intersection line is different, as shown in Table 2.

[0069] Examples 11-12

[0070] A secondary battery is different from Example 1 in that the spacing between adjacent stripes is different, as shown in Table 2.

[0071] Example 13

[0072] A secondary battery is different from the embodiment 1 in that the width of the stripe is 0.5 mm, and the spacing between the stripes is calculated using the P value.

[0073] Examples 14-18

[0074] A secondary battery differs from Example 1 in that the mass ratio of silicon-carbon to graphite in the negative electrode active material is different to achieve a change in the gram capacity of the negative electrode active material, and the negative electrode unit volume capacity, the negative electrode active material compaction density, and the negative electrode active material gram capacity are different to achieve a change in the negative electrode unit volume capacity C, as specifically shown in Table 1; the negative electrode unit volume capacity C is different, and the ratio P of the conductive coating area to the sum of the areas of the corner coating area and the tail area of ​​the positive electrode sheet is different to achieve a change in the C / P ratio, as specifically shown in Table 2.

[0075] Examples 19-20

[0076] A secondary battery differs from Example 1 in that the ratio P of the total area of ​​the corner coating area and the tail area of ​​the positive electrode plate is different to achieve a change in the C / P ratio, as shown in Table 2.

[0077] Examples 21-22

[0078] A secondary battery differs from Example 1 in that the mass ratio of silicon-carbon to graphite in the negative electrode active material is different to achieve a change in the gram capacity of the negative electrode active material, and the negative electrode unit volume capacity, the negative electrode active material compaction density, and the negative electrode active material gram capacity are different to achieve a change in the negative electrode unit volume capacity C, as specifically shown in Table 1; the negative electrode unit volume capacity C is different, and the ratio P of the conductive coating area to the sum of the areas of the corner coating area and the tail area of ​​the positive electrode sheet is different to achieve a change in the C / P ratio, as specifically shown in Table 2.

[0079] Examples 23-24

[0080] A secondary battery is different from Example 1 in that the conductive particle composition and Dv50 particle size of the conductive coating are different, as shown in Table 2.

[0081] Example 25

[0082] A secondary battery, which differs from Example 1 in that the tail ends of the double-sided tail areas of the positive electrode plate are staggered by 10 mm from each other, the tail end of the tail area located on the inner arc surface of the corner area after winding is located in the straight area and is 5 mm away from the intersection line, and the tail end of the tail area located on the outer arc surface of the corner area after winding is located in the straight area and is 15 mm away from the intersection line.

[0083] Comparative Examples 1-3

[0084] A secondary battery, which is different from Example 1 in that the ratio P of the conductive coating area to the sum of the areas of the corner coating area and the tail area of ​​the positive electrode plate is 0-1, where P=0 indicates that the corner coating area and the tail area are not coated with the conductive coating, and P=1 indicates that the corner coating area and the tail area are fully coated with the conductive coating, as shown in Table 2.

[0085] Comparative Example 4

[0086] A secondary battery differs from Example 1 in that the mass ratio of silicon-carbon to graphite in the negative electrode active material is different to achieve a change in the gram capacity of the negative electrode active material, and the negative electrode unit volume capacity, the negative electrode active material compaction density, and the negative electrode active material gram capacity are different to achieve a change in the negative electrode unit volume capacity C, as specifically shown in Table 1; the negative electrode unit volume capacity C is different, and the ratio P of the conductive coating area to the sum of the areas of the corner coating area and the tail area of ​​the positive electrode sheet is different to achieve a change in the C / P ratio, as specifically shown in Table 2.

[0087] Comparative Example 5

[0088] A secondary battery differs from Example 1 in that the ratio P of the total area of ​​the corner coating area and the tail area of ​​the positive electrode plate is different to achieve a change in the C / P ratio, as shown in Table 2.

[0089] Comparative Examples 6-7

[0090] A secondary battery is different from Example 1 in that the distance between the tail end of the trailing region and the intersection line is different, as shown in Table 2.

[0091] Comparative Example 8

[0092] A secondary battery, which differs from Example 1 in that a porous ceramic coating is applied to the surface of the positive electrode active material layer in the corner coating area and the tail area and located on one side; in step (1), conductive particles and polyvinylidene fluoride (PVDF) are added to N-methyl pyrrolidone in a mass ratio of 98:2 and stirred evenly to prepare a conductive coating slurry, wherein the mass fraction of NMP in the conductive coating slurry is 27%; lithium cobalt oxide positive electrode active material, acetylene black (SP) conductive agent, and polyvinylidene fluoride (PVDF) binder are fully stirred and mixed in an N-methyl pyrrolidone solvent system in a mass ratio of 98:1.2:0.8 to prepare a positive electrode slurry with a solid content of 73%, which is then coated on aluminum foil, dried and rolled, and then the conductive coating slurry is applied to the surface of the positive electrode active material layer on one side of the positive electrode sheet located in the corner coating area and the tail area. After drying and stripping, a positive electrode sheet is obtained.

[0093] In the secondary batteries prepared in the above embodiments and comparative examples, the unit volume capacity of the negative electrode sheet, the compacted density of the negative electrode active material, the gram capacity of the negative electrode active material, and the mass ratio of silicon carbon to graphite in the negative electrode active material are all shown in Table 1.

[0094] In the secondary batteries prepared in the above embodiments and comparative examples, the conductive particle composition, the conductive particle Dv50 particle size, the thickness of the stripe band in the corner coating area, d in the formula Y=X-(n-1)dX, the stripe band spacing, the width of adjacent stripe bands, the negative electrode unit volume capacity C, the ratio P of the conductive coating area to the sum of the areas of the corner coating area and the tail area of ​​the positive electrode plate, the C / P ratio, and the distance between the tail end of the tail area and the intersection line are all shown in Table 2.

[0095] Table 1 - Parameters of negative electrode active materials and unit volume capacity in negative electrode sheets of Examples and Comparative Examples

[0096]

[0097]

[0098] Table 2 - Parameter values ​​of secondary batteries in Examples and Comparative Examples

[0099]

[0100]

[0101]

[0102] Performance testing

[0103] 1. Capacity retention rate: The secondary batteries prepared in the above embodiments and comparative examples were subjected to charge and discharge cycle tests at room temperature. The battery was charged to 4.53V at a constant current of 0.5C, charged to a cut-off current of 0.05C at a constant voltage, and discharged to 3.0V at a constant current of 0.5C. This charge and discharge step was recorded as one cycle. The number of cycles at which the capacity retention rate reached 80% was recorded. The lithium deposition in the negative electrode material area was observed after 1000 cycles at room temperature. The lithium deposition level determination method is shown in Table 3. At the same time, the electrode fragment breakage rate after 1200 cycles was observed and counted. The test results are shown in Table 4 below.

[0104] Table 3 - Determination method of lithium plating level at negative electrode interface

[0105]

[0106]

[0107] Table 4 - Performance test results of secondary batteries prepared in Examples and Comparative Examples of the present application

[0108]

[0109]

[0110] As shown in Table 4, in Example 1 of the present application, in order to alleviate the lithium precipitation phenomenon in the corner area of ​​the negative electrode interface, a conductive coating is intermittently applied to the corner area of ​​the positive electrode sheet, and the negative electrode unit volume capacity and the conductive coating coating area P are controlled to meet C / P≤6300, 0.1≤P≤0.5, which can effectively disperse the lithium ion transmission, increase the transport distance of some lithium ions, and make the lithium ions of the positive electrode enter the negative electrode structure in an orderly manner, avoid the lithium ions from accumulating and precipitating in the negative electrode structure, and at the same time avoid the conductive coating coating area being too large to affect the dynamic performance. The gaps between the striped coatings can store electrolyte, increase the electrolyte content in the corner area, and effectively slow down the lithium precipitation phenomenon. Graphene has excellent electrical conductivity and thermal conductivity, which can effectively disperse the heat in the corner area, reduce the risk of local overheating, and improve the battery cycle capacity retention rate. However, in Comparative Example 1, no conductive coating is applied, the lithium ion transmission distance is short, and it is easy to accumulate at the negative electrode interface to cause serious lithium precipitation, and reduce the battery cycle capacity retention rate and the pole piece fragmentation rate.

[0111] Compared with Example 1, the conductive coating of Comparative Example 2 is fully coated near the corner area of ​​the positive electrode plate. Although it increases the transmission distance of lithium ions, it cannot disperse lithium ion transmission at multiple levels, and affects the kinetic performance of the battery cell, reducing the lithium ion transmission rate, and ultimately greatly reducing the battery's cycle capacity retention rate. In addition, the excessive coating area of ​​the conductive coating increases lithium plating at the negative electrode interface.

[0112] Compared with Examples 1 and 19-20, in which the conductive coating area ratio P is 0.2-0.5, the conductive coating area ratio P of Comparative Examples 3-4 is too large, 0.6, indicating that the conductive coating area is too large, which will affect the lithium ion transmission on the electrode, resulting in the battery's kinetic performance failing to meet the requirements and a reduced cycle capacity retention rate.

[0113] The ratio P of the conductive coating area to the area near the corner area in Examples 1 and 19-20 and the relationship C / P of the unit volume capacity of the negative electrode sheet range from 1530 to 4590, and as the C / P ratio decreases, the lithium plating mitigation effect at the negative electrode corner interface becomes more obvious; the C / P in Comparative Example 5 is 9180, which is significantly beyond the C / P range of Examples 1 and 19-20. The C / P ratio is too large, indicating that the conductive coating cannot effectively disperse lithium ion transport at multiple levels, resulting in serious lithium plating in the negative electrode corner area.

[0114] Compared with Example 1, the tail end of the conductive coating of Comparative Example 6 is located in the corner area 15 mm away from the intersection line. At this time, the coating area of ​​the conductive coating is too small to effectively disperse the transmission of lithium ions in multiple levels, and lithium deposition still exists at the negative electrode interface; the tail end of the conductive coating of Comparative Example 7 is located in the straight area 25 mm away from the intersection line. At this time, the coating area of ​​the conductive coating extends too much into the straight area, resulting in a significant increase in the back of the battery cell after winding, and a decrease in the capacity density, which will affect the cycle capacity retention rate of the battery cell and increase the breakage rate of the electrode.

[0115] The specific embodiments described above further illustrate the objectives, technical solutions, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the scope of protection of the present invention. In particular, it should be noted that any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included within the scope of protection of the present invention for those skilled in the art.

Claims

1. A secondary battery, characterized in that: The invention comprises a positive electrode sheet and a negative electrode sheet, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer, wherein the positive electrode current collector is alternately provided with a straight region and a corner region along its length, wherein the position where the straight region and the corner region are connected is defined as an intersection line, and a conductive coating is provided on the surface of the positive electrode active material layer in the corner region, wherein an end of the conductive coating extends to a position within 2 mm of the corner region from the intersection line or an end of the conductive coating extends to a position within 15 mm of the straight region from the intersection line; The conductive coating is composed of a plurality of stripes arranged along the length direction of the positive electrode sheet, and the secondary battery satisfies the following relationship: C / P≤6300, 0.1≤P≤0.5; Where C is the unit volume capacity of the negative electrode, in mAh / cm 3 ; P is the ratio of the area of ​​the conductive coating to the area of ​​the region of the positive electrode sheet located between the two ends of the conductive coating.

2. The secondary battery according to claim 1, wherein: The conductive coating includes a corner coating area and two tailing areas respectively connected to both ends of the corner coating area. The thickness of the stripes in the tailing areas decreases in a direction away from the corner coating area.

3. The secondary battery according to claim 2, wherein The end of the trailing region close to the corner coating region is the starting end, the thickness of the first row of stripes in the trailing region close to the starting end is X, the thickness of the stripes in the first row is equal to the thickness of the stripes in the corner coating region, the thickness of the stripes in the nth row is Y, and X and Y satisfy the following relationship: Y=X-(n-1)dX, 0.1≤d≤0.4; And / or, the tail ends of the trailing regions on both sides of the positive electrode sheet are staggered by 2-15 mm.

4. The secondary battery according to claim 1, wherein The conductive coating satisfies the following relationship: 1500≤C / P≤4100.

5. The secondary battery according to claim 1, wherein The unit volume capacity C value of the negative electrode plate is 500-1600.

6. The secondary battery according to claim 1, wherein The conductive coating includes conductive particles, and the conductive coating includes at least one of graphene, carbon nanotubes, and conductive carbon black; And / or, the conductive coating comprises conductive particles and a binder in a mass ratio of (98-99.7):(0.3-2.0); And / or, the Dv50 particle size of the conductive particles is 50-2000 nm.

7. The secondary battery according to claim 2, wherein The thickness of the conductive coating in the corner coating area is 2-8 μm; And / or, the stripe widths of the corner coating area and the trailing area are independent and range from 0.1 to 1 mm; And / or, the spacing between adjacent stripes in the corner coating area and the trailing area is independent and ranges from 0.1 to 2 mm.

8. The secondary battery according to claim 1, wherein The negative electrode plate includes a negative electrode active material layer and a negative electrode current collector. The negative electrode active material layer includes a negative electrode active material. The negative electrode active material is at least one of a carbon-based compound, a silicon-based compound, a titanium-based compound, a tin-based alloy, and a transition metal nitride.

9. The secondary battery according to claim 8, wherein The positive electrode active material layer comprises a positive electrode active material, a positive electrode binder and a positive electrode conductor in a mass ratio of (92-99): (0.5-4): (0.5-2); And / or, the negative electrode active material layer includes a negative electrode active material, a negative electrode binder, a negative electrode conductor, and a plasticizer in a mass ratio of (92-99):(0.5-5):(0.5-1.5):(0-1).

10. The secondary battery according to claim 1, wherein The secondary battery further includes a separator, wherein the separator is at least one of polyethylene, polypropylene, polyethylene terephthalate, polyimide, aramid, and polybutylene terephthalate; And / or, the secondary battery further comprises an electrolyte, the electrolyte comprising a lithium salt and a solvent in a mass ratio of (5-10):(90-95), the solvent being at least one of ethylene carbonate, diethyl carbonate, propylene carbonate, propyl propionate and vinylene carbonate.

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