Lithium ion battery

Abstract

The invention discloses a lithium ion battery, and relates to the field of batteries. A positive pole piece of the battery cell is provided with a straight area and a corner area, the corner area is coated with a composite coating in a dot matrix mode, the composite coating comprises graphene and ceramic particles with the mass ratio being (0.05-0.2): 1, the Dv50 particle size ratio of the graphene to the ceramic particles is 1: (1.2-2), and the lithium ion battery meets the following relational expression that P is larger than or equal to 0.1 and smaller than or equal to 0.6; p is the ratio of the area of the composite coating to the sum of the areas nearby the corner area. The corner area of the positive pole piece is point-coated with the composite coating, graphene in the coating has excellent conductivity, ceramic particles contain pore structures and can provide more migration channels for lithium ions, the graphene and the ceramic particles are matched by adopting different particle sizes so as to optimize the transmission path of the lithium ions, and multi-level dispersed lithium ion transmission is realized; the lithium precipitation phenomenon of a negative electrode interface is effectively relieved, the dynamic performance of a battery cell is improved, and the cycle life is prolonged.

Description

Technical Field

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

[0002] Lithium-ion batteries, core components of new energy vehicles and energy storage devices, have a direct impact on their safety, lifespan, and energy density. However, in actual use, batteries are prone to lithium deposition during charge and discharge cycles, particularly in the corners of the battery cell. This deposition can lead to capacity degradation, thickness changes, and reduced safety. In severe cases, it can even cause short circuits or thermal runaway.

[0003] During the charge and discharge process of lithium-ion batteries, the speed and path of lithium ion transmission play a critical role in battery performance. Furthermore, due to the high forces and poor electrode contact in the corners of the battery cell, lithium ion transmission is easily blocked, leading to lithium plating. Furthermore, during the cycling of lithium batteries, as lithium ions are inserted and removed, the electrolyte content in the corners decreases, causing lithium plating in the corners due to low electrolyte content in the later stages of the cycle. Lithium plating not only increases the internal resistance of the battery, but also leads to a decrease in battery capacity, shortened battery life, and even potential safety incidents such as short circuits. Therefore, in-depth research is needed to identify solutions to the lithium plating problem in the corners of battery cells. Summary of the Invention

[0004] The present invention provides a lithium-ion battery. By applying a composite coating on the corner area of ​​the positive electrode plate and using graphene and ceramic particles of different particle sizes to cooperate with each other to construct a conductive network, not only can the transmission path and rate of lithium ions be optimized and lithium plating at the negative electrode be reduced, but the conductive network can also reduce the impact of the composite coating on the battery's kinetic performance and improve the battery's high-temperature cycle performance.

[0005] In order to solve the above technical problems, one of the objectives of the present invention is to provide a lithium-ion battery, comprising a positive electrode sheet and a negative electrode sheet, wherein the positive electrode sheet comprises a positive electrode active material layer and a positive electrode current collector, wherein the positive electrode sheet is alternately provided with straight regions and corner regions along its length, and a composite coating is provided on the surface of the positive active material layer in the corner region;

[0006] The composite coating comprises graphene and ceramic particles in a mass ratio of (0.05-0.2):1, wherein the Dv50 particle size ratio of the graphene and ceramic particles is 1:(1.2-2); the composite coating comprises a plurality of coating points arranged in an array, wherein the following relationship is satisfied: 0.1≤P≤0.6;

[0007] Wherein, P is the ratio of the area of ​​the composite coating to the area of ​​the positive electrode plate located between the two ends of the composite coating;

[0008] The position where the straight area and the corner area are connected is defined as the intersection line, and the end of the composite coating is located within 5 mm of the intersection line in the corner area or the end of the composite coating is located within 15 mm of the intersection line in the straight area.

[0009] The present application applies a composite coating on the corner area of ​​the positive electrode sheet. The graphene contained in the composite coating can significantly improve the conductivity of the electrode material, promote the rapid transmission of lithium ions, and effectively inhibit the occurrence of lithium plating. The ceramic particles, due to their unique pore structure and good chemical stability, can provide more migration channels for lithium ions, multi-level dispersed lithium ion transmission, avoid lithium ion accumulation and precipitation at the negative electrode interface, and inhibit excessive evaporation of the electrolyte and side reactions on the electrode surface. At the same time, the composite coating is spot-coated, and the coating area satisfies 0.1≤P≤0.6. Through the synergy of the porous ceramic with a larger particle size and the graphene with a smaller particle size, the porous ceramic with a larger particle size can take into account its adsorption of lithium ions and its support for the electrode structure. The graphene with a smaller particle size can be evenly dispersed in the porous ceramic support to form a conductive network, which can achieve multi-level dispersion of lithium ion transmission in a smaller coating area, effectively alleviating the lithium plating phenomenon at the negative electrode interface. The conductivity of the composite coating and the smaller conductive area can reduce the impact on the kinetic performance of the battery cell and improve the cycle capacity retention rate of the battery cell.

[0010] In addition, the tail end of the composite coating is located between 10 mm from the intersection line in the corner area and 20 mm from the intersection line in the straight area. This can prevent the coating area from being too large to affect lithium ion transmission, resulting in a decrease in the capacity density of the battery cell. At the same time, it can avoid the coating area being too small to effectively disperse the transmission of lithium ions, thereby improving the lithium plating problem.

[0011] As a preferred solution, the lithium-ion battery satisfies the following relationship: C / P≤5220;

[0012] Where C is the unit volume capacity of the negative electrode, in mAh / cm 3 ; P is the ratio of the area of ​​the composite coating to the area of ​​the positive electrode plate located between the two ends of the composite coating.

[0013] The coating area P of the composite coating in this application and the unit volume capacity C of the negative electrode satisfy C / P≤5220, and different negative electrode capacities can be used to optimize the transmission rate and transmission distance of lithium ions to ensure that multi-level dispersed lithium ions can enter the negative electrode structure in an orderly manner, reduce concentration polarization and interface impedance, further improve the negative electrode interface, especially the lithium plating problem in the corner area, while ensuring the kinetic performance of the battery cell, and further improving the cycle capacity and cycle life.

[0014] As a preferred solution, the mass ratio of the graphene to the ceramic particles is in the range of any one or any two of 0.05:1, 0.1:1, 0.15:1, and 0.2:1.

[0015] The graphene in the composite coating of the present application has excellent electrical and thermal conductivity, can effectively disperse heat in the corner area, avoid the risk of local overheating, and at the same time improve the transmission rate of lithium ions. Combined with the ceramic particles, due to their unique pore structure and good chemical stability, they can provide more migration channels for lithium ions and optimize the transmission path of lithium ions. If there is too much graphene in the composite coating, it will not be able to effectively disperse the transmission of lithium ions in multiple levels, and if there is too little, it will not have the heat dissipation and conductivity functions. Controlling the two within the above-mentioned ratio range can optimize the transmission path and rate of lithium ions, while reducing the impact on the dynamic performance of the battery cell and improving the cycle life of the battery cell.

[0016] As a preferred embodiment, the ceramic particles are at least one of silicon dioxide, aluminum oxide, boehmite, titanium oxide, zirconium oxide, silicon carbide, titanium carbide, silicon nitride, aluminum nitride, zirconium diboride and titanium diboride.

[0017] As a preferred solution, the average pore size of the ceramic particles is 10-300 nm.

[0018] As a preferred solution, the average pore size of the ceramic particles is 50-200 nm.

[0019] As a preferred embodiment, the composite coating comprises graphene, ceramic particles and a binder in a mass ratio of (0.05-0.2):1:(0.004-0.03).

[0020] As a preferred embodiment, the graphene has a Dv50 particle size of 100-1000 nm.

[0021] As a preferred solution, the Dv50 particle size of the ceramic particles is 200-1500 nm.

[0022] As a preferred solution, the Dv50 particle size of the graphene is 400-800 nm.

[0023] As a preferred solution, the Dv50 particle size of the ceramic particles is 500-1000 nm.

[0024] Controlling the particle size of porous ceramics and graphene within the above range not only takes into account their adsorption capacity for lithium ions, enhances their absorption capacity for electrolytes, and improves the migration efficiency of lithium ions, but also supports the electrode structure, inhibits volume expansion, and reduces the electrode fragmentation rate. If the graphene particle size is too large, it is easy to reduce the contact area between the graphene and the active material, which is not conducive to the formation of a stable conductive network. If the particle size of porous ceramics is too large, it will lead to a decrease in specific surface area, affecting the overall adsorption of lithium ions by the composite coating, making it impossible to effectively increase the transmission distance and effectively achieve multi-level dispersed lithium ion transmission. If the particle size of graphene and porous ceramics is too small, they are prone to agglomeration, which in turn blocks the diffusion channels of lithium ions and affects the kinetic performance of lithium ions.

[0025] As a preferred solution, the composite coating includes a main coating area and two finishing coating areas, and the coating point thickness of the finishing coating area decreases in a direction away from the main coating area.

[0026] As a preferred embodiment, the end of the finishing coating area close to the main coating area is the starting end, the thickness of the first column of coating points in the finishing coating area close to the starting end is X, the thickness of the coating points in the first column is equal to the thickness of the coating points in the main coating area, the thickness of the dots in the nth column is Y, and X and Y satisfy the following relationship: Y = X-(n-1)dX, 0.15≤d≤0.55.

[0027] The composite coating on the tail end of the positive electrode plate of the present application is applied in a step-by-step decreasing manner to avoid large step thickness differences in the uncoated area after winding, otherwise it is easy to cause stress concentration, affecting the cycle of the battery cell, and the expansion of the battery cell after the cycle is easy to cause breakage in the stress concentration area.

[0028] As a preferred solution, the tail ends of the finishing coating areas on both sides of the positive electrode plate are staggered by 2-15 mm.

[0029] As a preferred solution, the tail ends of the finishing coating areas on both sides of the positive electrode plate are staggered by 5-10 mm.

[0030] As a preferred solution, the lithium-ion battery satisfies the following relationship: 1500≤C / P≤3700.

[0031] 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, 0.5, 0.55, and 0.6.

[0032] The composite coating of the positive electrode plate of the present application contains graphene and ceramic particles. The electrical conductivity of graphene and the porous characteristics of ceramic particles can be used to improve the electrical conductivity and thermal conductivity of the electrode material. The transmission rate and path of lithium ions can be optimized in conjunction with the unit volume capacity of the negative electrode, reducing the impact of the composite coating on lithium ion transmission, thereby minimizing lithium plating and improving the kinetic performance of the battery cell.

[0033] As a preferred solution, the unit volume capacity C value of the negative electrode plate is 500-1600.

[0034] As a preferred solution, the coating point thickness of the main coating area is 2-10 μm.

[0035] As a preferred solution, the coating point thickness of the main coating area is in the range of any one or any two of 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, and 10μm.

[0036] As a preferred solution, the diameter of the coating dots is 0.2-1.5 mm.

[0037] As a preferred solution, the spacing between the coating points is 0.2-3 mm.

[0038] 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.

[0039] 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.

[0040] 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).

[0041] 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).

[0042] As a preferred solution, the lithium-ion battery further includes a separator.

[0043] As a preferred solution, the separator is at least one of polyethylene, polypropylene, polyethylene terephthalate, polyimide, aramid, and polybutylene terephthalate.

[0044] As a preferred embodiment, the lithium-ion battery further comprises an electrolyte, wherein the electrolyte comprises a lithium salt and a solvent in a mass ratio of (5-10):(90-95).

[0045] As a preferred embodiment, the solvent is at least one of ethylene carbonate, diethyl carbonate, propylene carbonate, propyl propionate and vinylene carbonate.

[0046] Compared with the prior art, the present invention has the following beneficial effects:

[0047] 1. The present application has a composite coating on the corner area of ​​the positive electrode sheet, and the coating area satisfies 0.1≤P≤0.6, which can avoid the excessive coating area of ​​the composite coating affecting the transmission of lithium ions. The graphene in the composite coating can significantly improve the conductivity of the electrode material and promote the rapid transmission of lithium ions. The ceramic particles can provide more migration channels for lithium ions due to their unique pore structure. The combination of smaller-sized graphene and larger-sized ceramic particles can realize the multi-level dispersion of lithium ion transmission. The smaller-sized graphene can be evenly dispersed in the porous ceramic support to form a conductive network, which can realize multi-level dispersion of lithium ion transmission in a smaller coating area, effectively alleviating lithium precipitation at the negative electrode interface, while improving the battery's kinetic performance, and increasing the cycle capacity and cycle life.

[0048] 2. The coating area P of the composite coating in this application and the unit volume capacity C of the negative electrode satisfy C / P≤5220. Different negative electrode capacities can be used to optimize the transmission rate and transmission distance of lithium ions to ensure that multi-level dispersed lithium ions can enter the negative electrode structure in an orderly manner, further improve the lithium plating problem at the negative electrode interface, especially in the corner area, while ensuring the kinetic performance of the battery cell.

[0049] 3. The composite coating on the tail end of the positive electrode of the present application is applied in a step-by-step decreasing manner, and the tail ends of the composite coating are staggered to avoid large step thickness differences in the uncoated area after winding, thereby avoiding stress concentration, ensuring the battery cycle, and reducing the breakage rate of the electrode. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 This is a schematic diagram of the array arrangement of a composite coating on a positive electrode sheet in a lithium-ion battery according to an embodiment of the present invention;

[0051] Figure 2 This is a schematic diagram of the position of the composite coating on the positive electrode sheet of a lithium-ion battery after winding according to an embodiment of the present invention. DETAILED DESCRIPTION

[0052] 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.

[0053] 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.

[0054] 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.

[0055] As used herein:

[0056] "Prepared from" is synonymous with "comprising." As used herein, the terms "comprising," "including," "having," "containing," or any other variations thereof, are intended to cover a non-exclusive inclusion. For example, a composition, process, method, article, or apparatus that comprises the listed elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such composition, process, method, article, or apparatus.

[0057] The conjunction "consisting of" excludes any unspecified element, step, or component. If used in a claim, this phrase renders the claim closed, excluding materials other than those described, except for conventional impurities associated therewith. When the phrase "consisting of" appears in a clause of the body of a claim, rather than immediately following the subject matter, it limits only the elements described in that clause; other elements are not excluded from the claim as a whole.

[0058] 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.

[0059] 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.

[0060] Example 1

[0061] A lithium-ion 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 aluminum foil. The positive electrode sheet is alternately provided with straight regions and corner regions along the length of the winding, and the surfaces of the positive electrode active material layers on both sides of the corner regions are dotted with a composite coating. The composite coating comprises a main coating region and two tail coating regions. Along the length direction of the positive electrode sheet, the two ends of the main coating region are respectively connected to the two tail coating regions.

[0062] The intersection line is defined as the location on the positive electrode sheet where the straight region and the corner region meet. The end of the finishing coating region away from the main coating region is located 5 mm from the intersection line in the straight region. Assuming that when the end of the finishing coating region away from the main coating region is located at the intersection line, the distance between the end and the intersection line is 0, and when the end of the finishing coating region away from the main coating region is located in the corner region, the distance between the end and the intersection line is a negative value, and when the end of the finishing coating region away from the main coating region is located in the straight region, the distance between the end and the intersection line is a positive value. Therefore, the distance H between the end of the finishing coating region away from the main coating region and the intersection line in Example 1 can be defined as 5 mm.

[0063] Composite coatings include several Figure 1 The coating points are arranged vertically and horizontally, and the diameter of a single coating point is 1mm. Lithium-ion batteries satisfy the following relationship: C / P = 2363, P = 0.4; where C is the unit volume capacity of the negative electrode sheet, in mAh / cm 3 ; P is the ratio of the composite coating area to the sum of the areas of the main coating area and the finishing coating area of ​​the positive electrode sheet.

[0064] The coating thickness of the main coating area is consistent and is 6μm. Figure 2As shown, the thickness of the coating points in the finishing coating area decreases as it moves away from the main coating area. The end of the finishing coating area close to the main coating area is the starting end. The thickness of the first column of coating points in the finishing coating area close to the starting end is X. The thickness of the coating points in the first column is equal to the thickness of the coating points in the main coating area. The thickness of the coating points in the nth column is Y. X and Y satisfy the following relationship: Y = X - (n - 1) dX, d = 0.3.

[0065] The positive electrode active material layer comprises lithium cobalt oxide, acetylene black, and polyvinylidene fluoride in a mass ratio of 98:1.2:0.8. The negative electrode sheet comprises a negative electrode active material layer and copper foil. The negative electrode active material layer comprises negative electrode active material, acetylene black, styrene-butadiene rubber, and sodium carboxymethyl cellulose in a mass ratio of 98:0.6:0.7:0.7. The negative electrode active material comprises silicon carbon and graphite in a mass ratio of 15:85. The separator is a polyethylene porous film with an alumina ceramic coating on both sides. The electrolyte comprises LiPF6 and an organic solvent in a mass ratio of 8:92. The organic solvent comprises ethylene carbonate, diethyl carbonate, propylene carbonate, propyl propionate, and vinylene carbonate in a mass ratio of 25:25:15:31:4.

[0066] The unit volume capacity C value of the negative electrode plate is calculated according to the following formula: unit volume capacity of the negative electrode plate = negative electrode compaction density × negative electrode plate gram capacity. The unit volume capacity C value of the negative electrode plate is 945mAh / cm 3 The compacted density of the negative electrode active material is 1.75 g / cm 3 The gram capacity of the negative electrode active material is 540mAh / g.

[0067] The composite coating also includes a particulate material and a binder. The particulate material includes conductive particles and ceramic particles in a mass ratio of 0.1:1. The conductive particles are graphene and the ceramic particles are silica. The ceramic particles have an average pore size of 185 nm and a Dv50 particle size of 850 nm. The Dv50 particle size of the conductive particles is 600 nm. The Dv50 particle size ratio (F) of the conductive particles to the ceramic particles is 1:1.42.

[0068] The method for preparing a lithium-ion battery in the above embodiment 1 comprises the following steps:

[0069] (1) Adding granular material and polyvinylidene fluoride in a mass ratio of 98:2 to N-methylpyrrolidone solvent, wherein the granular material includes conductive particles and ceramic particles in a mass ratio of 0.1:1, to prepare a composite coating slurry, wherein the mass fraction of N-methylpyrrolidone in the composite coating slurry is 28%; lithium cobalt oxide, acetylene black, and polyvinylidene fluoride are fully stirred and mixed in an N-methylpyrrolidone solvent system in a mass ratio of 98:1.2:0.8 to prepare a positive electrode slurry with a solid content of 72%, which is then coated on aluminum foil, and after drying and rolling, a positive electrode active material layer is formed, and the composite coating slurry is spot-coated on the surface of the positive electrode active material layer on both sides of the positive electrode sheet in the main coating area and the finishing coating area, and after drying and stripping, a positive electrode sheet is obtained;

[0070] (2) The negative electrode active material, acetylene black, styrene-butadiene rubber, and sodium carboxymethyl cellulose were thoroughly stirred and mixed in deionized water at a mass ratio of 98:0.6:0.7:0.7. The negative electrode active material included silicon carbon and graphite at a mass ratio of 15:85 to prepare a negative electrode slurry with a solid content of 40%. The slurry was then coated on a copper foil at 105°C in a vacuum, and dried, rolled, and slit to obtain a negative electrode sheet.

[0071] (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 lithium-ion battery.

[0072] Examples 2-5

[0073] A lithium-ion battery is different from Example 1 in that the Dv50 particle sizes of the conductive particles and the ceramic particles are different, so as to change the Dv50 particle size ratio F of the conductive particles and the ceramic particles, as shown in Table 2.

[0074] Examples 6-7

[0075] A lithium-ion battery, which is different from Example 1 in that the diameters of the coating points are different and the thicknesses of the coating points in the main coating area are different, as shown in Table 2.

[0076] Examples 8-10

[0077] A lithium-ion battery is different from Example 1 in that the distance H between the end of the finishing coating area away from the main coating area and the intersection line is different, as shown in Table 2.

[0078] Examples 11-12

[0079] A lithium-ion battery, which differs from Example 1 in that d is different in the formula Y=X-(n-1)dX, where d=0 indicates that the coating point thickness in the tail coating area is consistent and equal to the coating point thickness in the corner coating area, as shown in Table 2.

[0080] Examples 13-14

[0081] A lithium-ion battery, which is different from Example 1 in that the composition of the ceramic particles is different, is as shown in Table 2.

[0082] Examples 15-16

[0083] A lithium-ion battery, which is different from Example 1 in that the mass ratio Q of the conductive particles to the ceramic particles is different, as shown in Table 2.

[0084] Examples 17-22

[0085] A lithium-ion battery differs from Example 1 in that the mass ratio of silicon, carbon, and graphite in the negative electrode active material is varied to achieve a change in the gram capacity of the negative electrode active material. The negative electrode unit volume capacity, the negative electrode active material compaction density, and the negative electrode active material gram capacity are varied 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 varied, and the ratio P of the composite coating area to the sum of the areas of the main coating area and the finishing coating area of ​​the positive electrode sheet is varied to achieve a change in the C / P ratio, as specifically shown in Table 2. The negative electrode active material of Example 17 contains only graphite and no silicon or carbon.

[0086] Examples 23-24

[0087] A lithium-ion battery differs from Example 1 in that the ratio P of the composite coating area to the sum of the areas of the main coating area and the finishing coating area of ​​the positive electrode plate is different to achieve a change in the C / P ratio, as shown in Table 2.

[0088] Example 25

[0089] A lithium-ion battery, which differs from Example 1 in that the tail ends of the double-sided finishing coating areas of the positive electrode plate are staggered by 10 mm from each other, the tail end of the finishing coating area located on the inner arc surface of the corner area after winding is located at the intersection line, and the tail end of the finishing coating area located on the outer arc surface of the corner area after winding is located in the straight area and is 10 mm away from the intersection line.

[0090] Comparative Examples 1-3

[0091] A lithium-ion battery, which differs from Example 1 in that the ratio P of the composite coating area to the sum of the areas of the main coating area and the finishing coating area of ​​the positive electrode plate is 0-1, where P=0 indicates that the composite coating is not applied to the positive electrode plate, and P=1 indicates that the composite coating is fully applied, as shown in Table 2.

[0092] Comparative Example 4

[0093] A lithium-ion 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 composite coating area to the sum of the areas of the main coating area and the finishing coating area of ​​the positive electrode sheet is different to achieve a change in the C / P ratio, as specifically shown in Table 2.

[0094] Comparative Examples 5-6

[0095] A lithium-ion battery is different from Example 1 in that the distance H between the end of the finishing coating area away from the main coating area and the intersection line is different, as shown in Table 2.

[0096] Comparative Examples 7-8

[0097] A lithium-ion battery is different from Example 1 in that the composition of the conductive particles is different, as shown in Table 2.

[0098] Comparative Examples 9-10

[0099] A lithium-ion battery, which is different from Example 1 in that the mass ratio Q of the conductive particles to the ceramic particles is different, as shown in Table 2.

[0100] Comparative Examples 11-12

[0101] A lithium-ion battery differs from Example 1 in that the average pore size of the ceramic particles is different, and the Dv50 particle size of the conductive particles and the ceramic particles is different, so as to change the Dv50 particle size ratio F of the conductive particles and the ceramic particles, as shown in Table 2.

[0102] Comparative Example 13

[0103] A lithium-ion battery differs from Example 1 in that the particulate material in the composite coating is silicon dioxide and no graphene is added. The ratio P of the composite coating area to the sum of the areas of the main coating area and the finishing coating area of ​​the positive electrode sheet is different to achieve a change in the C / P ratio, as shown in Table 2.

[0104] Comparative Example 14

[0105] A lithium-ion battery, which differs from Example 1 in that the particulate material in the composite coating is graphene and no silicon dioxide is added, the negative electrode unit volume capacity C is different, and the ratio P of the composite coating area to the sum of the areas of the main coating area and the finishing coating area of ​​the positive electrode sheet is different to achieve a change in the C / P ratio, as shown in Table 2.

[0106] Comparative Example 15

[0107] A lithium-ion battery, which differs from Example 1 in that a composite coating is applied to the surface of a positive electrode active material layer on one side of a main coating region and a finishing coating region; in step (1) of the preparation method, a granular material and polyvinylidene fluoride are added to an N-methylpyrrolidone solvent in a mass ratio of 98:2, the granular material comprising conductive particles and ceramic particles in a mass ratio of 0.1:1, to prepare a composite coating slurry, wherein the mass fraction of N-methylpyrrolidone in the composite coating slurry is 28%; lithium cobaltate, acetylene black, and polyvinylidene fluoride are fully stirred and mixed in an N-methylpyrrolidone solvent system in a mass ratio of 98:1.2:0.8 to prepare a positive electrode slurry with a solid content of 72%, which is then coated on an aluminum foil, dried and rolled to form a positive electrode active material layer, and the composite coating slurry is spot-coated on the surface of the positive electrode active material layer on one side of a positive electrode plate located in the main coating region and the finishing coating region, and dried and slit to obtain a positive electrode plate.

[0108] In the lithium-ion 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.

[0109] In the lithium-ion batteries prepared in the above embodiments and comparative examples, the Dv50 particle size of the conductive particles, the composition of the conductive particles, the average pore size of the ceramic particles, the Dv50 particle size of the ceramic particles, the composition of the ceramic particles, the Dv50 particle size ratio F of the conductive particles and the ceramic particles, the mass ratio Q of the conductive particles and the ceramic particles, the coating point diameter, the coating point thickness of the main coating area, d in the formula Y=X-(n-1)dX, the unit volume capacity C of the negative electrode, the ratio P of the composite coating area to the sum of the areas of the main coating area and the finishing coating area of ​​the positive electrode sheet, the C / P ratio, and the distance H between the tail end of the finishing coating area and the intersection line are all shown in Table 2.

[0110] Table 1 - Parameters of negative electrode active materials and unit volume capacity in negative electrode sheets of Examples and Comparative Examples

[0111]

[0112]

[0113] Table 2 - Parameters of the composite coating provided in the battery cells in the examples and comparative examples of the present application

[0114]

[0115]

[0116]

[0117] Performance testing

[0118] 1. Capacity retention rate: The lithium-ion batteries prepared in the above embodiments and comparative examples were subjected to charge and discharge cycle tests at room temperature and 45°C, respectively. 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. The capacity retention rates of 1200 cycles at room temperature and 800 cycles at 45°C were recorded. The lithium plating in the negative electrode material area was observed for 1000 cycles at room temperature and 600 cycles at 45°C. The pole piece breakage rates of 1200 cycles at room temperature and 700 cycles at 45°C were observed and counted. The test results are shown in Table 3 below.

[0119] The level of lithium plating at the negative electrode interface is evaluated in the following manner:

[0120] No lithium deposition: There is no lithium deposition on the surface of the main area of ​​the negative electrode sheet, and there is no lithium deposition on the surface of the corner area of ​​the negative electrode sheet;

[0121] Slight lithium deposition: The lithium deposition area on the surface of the main area of ​​the negative electrode is less than 5%, and the lithium deposition area on the surface of the corner area of ​​the negative electrode is less than 10%, and does not grow to the main area;

[0122] Moderate lithium deposition: lithium deposition area on the surface of the main area of ​​the negative electrode sheet is 5% to 20%, and lithium deposition area on the corner of the negative electrode sheet is 10% to 40%, and does not grow to the main area;

[0123] Severe lithium deposition: The lithium deposition area on the surface of the main area of ​​the negative electrode is greater than 20%, the lithium deposition area at the corner of the negative electrode is greater than 40%, or it grows to the main area.

[0124] Table 3 - Performance test results of battery cells prepared in Examples and Comparative Examples of the present application

[0125]

[0126]

[0127]

[0128] As shown in Table 3, in Example 1 of the present application, a composite coating is applied to the corner area of ​​the positive electrode sheet. The graphene of the composite coating can significantly improve the conductivity of the electrode material and promote the rapid transmission of lithium ions. The ceramic particles can provide more migration channels for lithium ions due to their unique pore structure and good chemical stability, while inhibiting excessive evaporation of the electrolyte and side reactions on the electrode surface. By combining graphene and silicon dioxide of different particle sizes, and controlling the coating area to meet 0.1≤P≤0.6, porous ceramics with larger particle sizes are used to provide adsorption of lithium ions and support for the electrode structure. Graphene with smaller particle sizes can be evenly dispersed in the porous ceramic support to form a conductive network, which can achieve multi-level dispersion of lithium ion transmission in a smaller coating area, effectively alleviating the lithium plating phenomenon at the negative electrode interface, and the conductivity of the composite coating and the smaller conductive area can reduce the impact on the kinetic performance of the battery cell and improve the cycle capacity retention rate of the battery cell. The positive electrode sheet of Comparative Example 1 is not coated with a composite coating, and the transmission of lithium ions cannot be optimized. Lithium ions are easily accumulated and precipitated at the negative electrode interface, causing serious lithium deposition, affecting the cycle life of the battery cell, and causing cycle capacity decay.

[0129] Compared with Example 1, the composite coating of the positive electrode plate of Comparative Example 2 is fully coated, and the composite coating area P value of Comparative Example 3-4 is too large, at 0.7. Too much coating area will affect the transmission of lithium ions, resulting in a decrease in the kinetic performance of the battery cell and a decrease in the capacity density. In addition, the composite coating cannot store more electrolyte, the liquid retention capacity of the battery cell is reduced, and the cycle capacity of the battery cell is greatly reduced.

[0130] In Examples 1, 8-10 of the present application, the tail end of the composite coating is controlled at a position between 5 mm from the intersection line in the corner area and 15 mm from the intersection line in the straight area, which can achieve multi-level dispersed lithium ion transmission, and is not easy to affect the dynamic performance of the battery cell, reducing negative electrode lithium plating. However, the coating area of ​​the composite coating in Comparative Example 5 is too small, and it cannot effectively disperse the transmission of lithium ions, resulting in negative electrode lithium plating; the coating area of ​​the composite coating in Comparative Example 6 is too large, which will affect the lithium ion transmission, resulting in a decrease in the capacity density of the battery cell, and the composite coating extends too much into the straight area, resulting in an increase in the thickness of the battery cell. The volume expansion during the cycle process easily increases the pole piece fragmentation rate, and ultimately the cycle capacity retention rate of the battery cell is reduced.

[0131] Compared to Example 1, the graphene in the composite coating of Comparative Example 7 was replaced with conductive carbon black, and that of Comparative Example 8 was replaced with carbon nanotubes. Because the electrical and thermal conductivity of conductive carbon black and carbon nanotubes are inferior to those of graphene, the local heat dissipation effect in the corner area of ​​the battery cell is reduced, affecting the cycle life of the battery cell and significantly reducing the cycle capacity retention rate of the battery cell. Furthermore, the composite coatings of Comparative Examples 7-8 were unable to effectively coordinate with the negative electrode's unit volume capacity to optimize the lithium ion transmission rate and path, resulting in lithium plating at the negative electrode.

[0132] As shown in Table 3, the particle size of graphene and silicon dioxide in the composite coating of Example 1 is controlled at 1:1.42, and the mass ratio is controlled at 0.1:1. The porous ceramic with a larger particle size is used to provide adsorption of lithium ions and support for the electrode structure. The graphene with a smaller particle size can be evenly dispersed in the porous ceramic support to form a conductive network, thereby optimizing the transmission path and rate of lithium ions and effectively alleviating lithium plating at the negative electrode interface. At the same time, the conductivity of the composite coating can also prevent the coating area from affecting the dynamic performance of the battery. In Comparative Example 9, the graphene content of the composite coating is too low, resulting in a decrease in the conductivity of the composite coating, affecting the transmission rate of lithium ions and reducing the cycle capacity retention rate; in Comparative Example 10, the graphene content of the composite coating is too high, and the nano-particle graphene is combined with silica to disperse the transmission of lithium ions, resulting in an aggravation of the lithium plating phenomenon at the negative electrode; in Comparative Example 11, the particle size of silica is significantly larger than that of graphene, resulting in a deterioration in the compatibility of the two particles, and the graphene is easily agglomerated and blocks the transmission channel of lithium ions, resulting in a significant decrease in the cycle capacity; in Comparative Example 12, the particle size of graphene is significantly larger than that of silica, resulting in the inability of the composite coating to optimize the transmission path of lithium ions and the inability to disperse the transmission of lithium ions at multiple levels, resulting in moderate lithium plating.

[0133] The composite coating coating area P of Example 23 of the present application is 0.2, and the negative electrode unit volume capacity is 945 mAh / cm 3 , which is different from Example 23 in that the composite coating of Comparative Example 13 only contains silica ceramic particles and no conductive graphene, resulting in reduced conductivity of the electrode material. At this time, the composite coating cannot effectively cooperate with the higher negative electrode unit volume capacity to disperse lithium ions in multiple levels, resulting in increased lithium plating at the negative electrode. The composite coating coating area P value of Example 20 of this application is 0.6, and the negative electrode unit volume capacity is 1224 mAh / cm 3 The difference from Example 20 is that only graphene is added to the composite coating of Comparative Example 14, and no silica ceramic particles are added. The coating area of ​​the composite coating here is too large, which will affect the transmission of lithium ions, resulting in a decrease in the kinetic performance of the battery cell and a significant decrease in the cycle capacity retention rate of the battery cell.

[0134] The relationship between the coating area of ​​the composite coating and the unit volume capacity of the negative electrode in Example 1 is C / P≤5220. The composite coating can combine the transmission of multi-level dispersed lithium ions with the unit volume capacity of the negative electrode to effectively alleviate the lithium plating phenomenon at the negative electrode interface. Compared with Example 1, the relationship between the coating area of ​​the composite coating and the unit volume capacity of the negative electrode in Example 23 is C / P, which is higher at 4725, resulting in slight lithium plating after battery cycling. It can be found that as the C / P decreases, the lithium plating improvement effect is better; the relationship between the coating area of ​​the composite coating and the unit volume capacity of the negative electrode in Example 24 is C / P, which is lower at 1575. Due to the large coating area P value, the battery's cycle capacity retention rate decreases. This shows that when the unit volume capacity of the negative electrode is large and the coating area P value of the composite coating is too small, it is difficult to effectively cooperate with the unit volume capacity of the negative electrode to optimize the lithium ion transmission rate and path, which easily leads to excessive accumulation of lithium ions at the negative electrode interface and precipitation.

[0135] 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 lithium-ion battery, characterized in that: The positive electrode sheet comprises a positive electrode plate and a negative electrode plate, wherein the positive electrode plate comprises a positive electrode active material layer and a positive electrode current collector, wherein the positive electrode plate is alternately provided with a straight region and a corner region along its length direction, and a composite coating is provided on the surface of the positive electrode active material layer in the corner region; The composite coating comprises graphene and ceramic particles in a mass ratio of (0.05-0.2):1, wherein the Dv50 particle size ratio of the graphene and ceramic particles is 1:(1.2-2); the composite coating comprises a plurality of coating points arranged in an array, wherein 0.1≤P≤0.6; Wherein, P is the ratio of the area of ​​the composite coating to the area of ​​the positive electrode plate located between the two ends of the composite coating; The position where the straight area and the corner area are connected is defined as the intersection line, and the end of the composite coating is located within 5 mm of the intersection line in the corner area or the end of the composite coating extends to the position within 15 mm of the intersection line in the straight area.

2. The lithium-ion battery according to claim 1, wherein The lithium-ion battery satisfies the following relationship: C / P≤5220; Where C is the unit volume capacity of the negative electrode, in mAh / cm 3 ; P is the ratio of the area of ​​the composite coating to the area of ​​the region of the positive electrode sheet located between the two ends of the composite coating.

3. The lithium-ion battery according to claim 1, wherein The ceramic particles are at least one of silicon dioxide, aluminum oxide, boehmite, titanium oxide, zirconium oxide, silicon carbide, titanium carbide, silicon nitride, aluminum nitride, zirconium diboride and titanium diboride; And / or, the graphene has a Dv50 particle size of 100-1000 nm; and / or, the Dv50 particle size of the ceramic particles is 200-1500 nm, and / or, the average pore size of the ceramic particles is 10-300 nm; And / or, the composite coating comprises graphene, ceramic particles and a binder in a mass ratio of (0.05-0.2):1:(0.004-0.03).

4. The lithium-ion battery according to claim 1, wherein The composite coating includes a main coating area and two finishing coating areas, and the thickness of the coating points in the finishing coating area decreases in a direction away from the main coating area.

5. The lithium-ion battery according to claim 4, wherein The end of the finishing coating area close to the main coating area is the starting end, the thickness of the coating points in the first column of the finishing coating area close to the starting end is X, the thickness of the coating points in the first column is equal to the thickness of the coating points in the main coating area, the thickness of the coating points in the nth column is Y, and X and Y satisfy the following relationship: Y=X-(n-1)dX, 0.15≤d≤0.55; And / or, the tail ends of the finishing coating areas on both sides of the positive electrode plate are staggered by 2-15 mm.

6. The lithium-ion battery according to claim 2, wherein The lithium-ion battery satisfies the following relationship: 1500≤C / P≤3700.

7. The lithium-ion battery according to claim 2, wherein The unit volume capacity C value of the negative electrode plate is 500-1600.

8. The lithium-ion battery according to claim 4, wherein The coating point thickness of the main coating area is 2-10 μm; And / or, the coating dots are round dots, and the diameter of the round dots is 0.2-1.5 mm; And / or, the spacing between the coating points is 0.2-3 mm.

9. The lithium-ion 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.

10. The lithium-ion battery according to claim 9, 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 comprises a negative electrode active material, a negative electrode binder, a negative electrode conductive agent and a plasticizer in a mass ratio of (92-99): (0.5-5): (0.5-1.5): (0-1); And / or, the lithium-ion battery further comprises a separator, wherein the separator is at least one of polyethylene, polypropylene, polyethylene terephthalate, polyimide, aramid, and polybutylene terephthalate; And / or, the lithium-ion battery further comprises an electrolyte, wherein the electrolyte comprises a lithium salt and a solvent in a mass ratio of (5-10):(90-95).

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