Cylindrical battery

Abstract

The invention provides a cylindrical battery, which comprises a negative plate, the negative plate comprises a negative current collector and a negative active layer covering at least one side of the negative current collector, the negative active layer comprises graphite and a silicon carbon material, the sphericity of the silicon carbon material is Q, and 0.8 < = Q < = 1. According to the cylindrical battery provided by the invention, the energy density of the battery can be improved while the expansion rate of the negative electrode is reduced, the stress borne by the negative electrode is relieved and the dynamic performance of the battery is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of batteries and relates to a cylindrical battery. Background Art

[0002] As an important part of sustainable energy, battery technology has greatly promoted the sustainable development of society and the convenience and intelligence of human life. According to market research, range anxiety and charging problems greatly affect the use experience of new energy vehicles, indicating that there is an urgent need for targeted technological innovation in power batteries. Therefore, lithium-ion power batteries that take into account high energy density and fast charging performance have become the focus of research.

[0003] According to the different structures of the battery casing, lithium-ion power batteries can be divided into soft-pack batteries, square batteries, and cylindrical batteries. Due to the advantages of low cost and high energy density of cylindrical batteries, their market share in new energy vehicles is gradually increasing, and they are also gradually attracting the attention of major manufacturers. However, at present, the kinetic performance of the negative electrode sheet of cylindrical batteries is not good. Therefore, it is necessary to improve the material system of the battery to enhance its performance. Summary of the Invention

[0004] The present invention provides a cylindrical battery. The negative electrode active layer of the cylindrical battery includes graphite and silicon-carbon material with a certain sphericity. The cylindrical battery can improve the energy density while fully exerting the kinetic performance of the negative electrode sheet.

[0005] On the one hand, the present invention provides a cylindrical battery, including a negative electrode sheet. The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer covering at least one side of the negative electrode current collector. The negative electrode active layer includes graphite and silicon-carbon material, and the sphericity of the silicon-carbon material is Q, where 0.8 ≤ Q ≤ 1.

[0006] For the cylindrical battery as described above, based on the total mass of the graphite and the silicon-carbon material, the mass fraction of the silicon-carbon material is 0.1% - 20%.

[0007] For the cylindrical battery as described above, the Dv50 particle size of the graphite is A μm, the Dv50 particle size of the silicon-carbon material is B μm, and A and B satisfy 1.4 ≤ A / B ≤ 3.2.

[0008] For the cylindrical battery as described above, the Dv50 particle size of the silicon-carbon material is 5 μm - 9 μm;

[0009] and / or, the specific surface area of the silicon-carbon material is 2 m 2 / g - 3 m 2 / g.

[0010] For the cylindrical battery as described above, the graphitization degree of the graphite is 92% - 94%;

[0011] And / or, the Dv50 particle size of the graphite is 13 μm to 16 μm;

[0012] And / or, the specific surface area of the graphite is 1.2 m 2 / g to 1.7 m 2 / g.

[0013] For the cylindrical battery as described above, the negative electrode active layer further includes carbon nanotubes;

[0014] Preferably, based on the mass of the negative electrode active layer, the mass ratio of the carbon nanotubes is 0.1% to 1%.

[0015] For the cylindrical battery as described above, the carbon nanotubes are single-walled carbon nanotubes;

[0016] Preferably, the diameter of the single-walled carbon nanotubes is < 3 nm, the length is 1800 nm to 7200 nm, and the aspect ratio is 1200 to 3000.

[0017] For the cylindrical battery as described above, it further includes a positive electrode sheet, the positive electrode sheet includes a positive electrode current collector and a positive electrode active layer covering at least one side of the positive electrode current collector, the positive electrode active layer includes a positive electrode active material having a layered structure, and the chemical formula of the positive electrode active material is Li a Ni x Co y Mn z A k O2, where 0.9 ≤ a ≤ 1.1, 0.8 ≤ x ≤ 0.95, 0 < y ≤ 0.2, 0 < z ≤ 0.2, 0 ≤ k ≤ 0.05, and A includes at least one of Al, Mg, Ti, Y, B, P, Zr.

[0018] For the cylindrical battery as described above, the positive electrode active material includes single crystal particles and polycrystalline particles;

[0019] Preferably, based on the mass of the positive electrode active material, the mass ratio of the polycrystalline particles is 60% - 90%;

[0020] More preferably, based on the total mass of the graphite and the silicon-carbon material, the mass ratio of the silicon-carbon material is D, and C and D satisfy: 4.6 ≤ C / D ≤ 800.

[0021] For the cylindrical battery as described above, the positive electrode active layer further includes carbon nanotubes;

[0022] Preferably, based on the mass of the positive electrode active layer, the mass ratio of the carbon nanotubes is 0.5% to 1.5%, and the aspect ratio of the carbon nanotubes is 1800 - 2400.

[0023] The cylindrical battery as described above, the single-sided areal density of the negative electrode sheet is 6 mg / cm 2 ~8 mg / cm 2 and the tap density is 1.4 g / cm 3 ~1.8 g / cm 3 .

[0024] The cylindrical battery as described above, the negative electrode active layer includes at least one groove;

[0025] Preferably, the minimum distance between the groove and the edge of the negative electrode sheet is L, and L is 10 mm to 20 mm.

[0026] The cylindrical battery as described above, the depth of the groove is 3 μm to 40 μm, the width is 50 μm to 500 μm, and the distance between adjacent grooves is 400 μm to 5000 μm.

[0027] The cylindrical battery as described above, the negative electrode sheet sequentially includes a first region, a second region, and a first region along the width extension direction;

[0028] The width of the first region is 0 to 20 mm;

[0029] And / or, the tap density of the first region is E, the tap density of the second region is F, and E and F satisfy: E > F;

[0030] Preferably, E is 1.65 g / cm 3 ~1.8 g / cm 3 , F is 1.4 g / cm 3 ~1.65 g / cm 3 .

[0031] The cylindrical battery provided by the present invention can give full play to the kinetic performance of the negative electrode sheet. When the negative electrode active layer includes graphite and a silicon-carbon material with a certain sphericity, graphite serves as a conductive framework, forming a good conductive network with the silicon-carbon material to ensure the rapid transmission of electrons in the electrode. Moreover, the silicon-carbon material with a certain sphericity can embed lithium ions faster during the charging process, and its spherical structure can shorten the distance for lithium ions to be transmitted from the particle surface to the particle center, thereby making its kinetic performance better. The silicon particles are uniformly dispersed in the pores of the porous carbon framework, which can alleviate the volume effect of silicon during the lithium insertion and extraction process and ensure the structural stability of the silicon-carbon composite material. When applied to a cylindrical battery, even if there is a large tensile limit between the electrode sheet and the separator, it can still ensure the lithium storage capacity of the negative electrode material, extend the cycle life of the battery, reduce the swelling rate of the negative electrode during the charging process, and further slow down the stress on the negative electrode, which is beneficial to improving the kinetic performance of the negative electrode. Description of the Drawings

[0032] Figure 1Scanning electron microscope image of silicon-carbon material doped graphite in a specific embodiment of the present invention;

[0033] Figure 2 Schematic diagram of the division of the negative electrode sheet region in a specific embodiment of the present invention. Specific embodiment

[0034] To make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of them. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0035] Silicon materials have a relatively high theoretical specific capacity. Currently, the energy density of the battery is usually increased by adding silicon-containing materials to the negative electrode active layer. However, during the process of active ion insertion and extraction, silicon materials will undergo significant volume changes, which will lead to an increase in the mechanical stress of the materials. Researchers have found that for cylindrical batteries, in the manufacturing process of cylindrical batteries, the positive electrode, separator and negative electrode are wound into a core and then assembled into the shell. During this process, the electrode sheets of the cylindrical battery are wound into a tight spiral structure. Excessive winding tension will cause the electrode sheets to be compressed during the winding process. This compression limits the volume expansion of silicon during charge and discharge, resulting in the ineffective utilization of the capacity of the negative electrode, greatly limiting the kinetic performance of the battery. At the same time, compared with soft-pack batteries, the shell of a cylindrical battery is a metal shell and is not easily deformed, which will also inhibit the expansion of the internal battery core and is not conducive to the utilization of the capacity of the silicon-doped negative electrode, further limiting the kinetic performance of the battery.

[0036] Therefore, the negative electrode active layer can be modified to absorb the stress caused by volume changes while maintaining the energy density and improving the battery performance.

[0037] One aspect of the present invention provides a cylindrical battery, including a negative electrode sheet, the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer covering at least one side of the negative electrode current collector, the negative electrode active layer includes graphite and silicon-carbon material, and the sphericity of the silicon-carbon material is Q, 0.8 ≤ Q ≤ 1.

[0038] Among them, the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer covering one or both sides of the negative electrode current collector. The negative electrode active layer includes graphite and silicon-carbon material. As Figure 1 shown, it is a scanning electron microscope picture of the doping of silicon-carbon material and graphite in a specific embodiment.

[0039] In the present invention, the sphericity Q of the silicon-carbon material is Q = da / d1, where d1 is the equivalent diameter of the perimeter of the silicon-carbon material and da is the equivalent diameter of the surface area of the silicon-carbon material. A high-resolution image of the silicon-carbon material can be obtained using a scanning electron microscope or an optical microscope. Subsequently, the contour of the silicon-carbon material is analyzed using image processing software to calculate the perimeter of the silicon-carbon material, and the equivalent diameter d1 of the perimeter can be inversely derived through the circumference formula.

[0040] In the present invention, the equivalent diameter da of the surface area of the silicon-carbon material can be calculated in the following manner: The specific surface area of the particles is measured using the nitrogen adsorption method, and the equivalent diameter of the surface area is calculated from the specific surface area.

[0041] The present invention does not limit the specific type of the silicon-carbon material, and common silicon-carbon materials in the art can be selected, such as coated silicon-carbon, supported silicon-carbon, and dispersed silicon-carbon.

[0042] The negative electrode current collector of the present invention can be selected from common types, such as at least one of copper foil, nickel foam, and copper foam.

[0043] It can be understood that, in order to improve the structural stability of the negative electrode active layer, the negative electrode active layer may further include a binder, and the binder can be selected from at least one of carboxymethyl cellulose, styrene-butadiene rubber, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide, polyvinyl alcohol, and sodium polyacrylate.

[0044] The cylindrical battery provided by the present invention can maintain a high energy density while fully exerting the kinetic performance of the negative electrode sheet. On the one hand, when the negative electrode active layer includes graphite, a silicon-carbon material, and a conductive agent (such as carbon nanotubes), graphite serves as a conductive skeleton, silicon-carbon particles are embedded in the gaps between its particles, and then cross-linked and interpenetrated between the particles through carbon nanotubes, thereby forming a good conductive network between the active substances to ensure the rapid transmission of electrons in the active layer; on the other hand, the silicon-carbon material with a certain sphericity has lower orientation, can embed lithium ions faster during the charging process, and its spherical structure can shorten the distance for lithium ions to be transmitted from the particle surface to the particle center, thereby reducing the diffusion impedance of lithium ions and making its kinetic performance better. In addition, in the silicon-carbon material, silicon particles are uniformly dispersed in the pores of the porous carbon skeleton, which can alleviate the volume effect of silicon during the lithium insertion and extraction process, reduce the expansion rate of the negative electrode during the charging process, and further slow down the stress on the negative electrode, improve the structural stability of the silicon-carbon composite material, ensure the lithium insertion ability of the negative electrode material, and extend the cycle life of the battery.

[0045] Since the negative electrode active layer includes graphite and silicon-carbon materials, the proportional relationship between graphite and silicon-carbon materials has an important impact on battery performance. In a specific embodiment, based on the total mass of graphite and silicon-carbon materials, the mass fraction of silicon-carbon materials is 0.1% to 20%.

[0046] Specifically, the mass fraction of silicon-carbon materials includes, but is not limited to, 0.1%, 1%, 2%, 4%, 6%, 8%, 10%, 12%, 14%, 16%, 18%, 20%, or the range formed by any two of them.

[0047] When, based on the total mass of graphite and silicon-carbon materials, the mass fraction of silicon-carbon materials is within the above range, neither will the energy density be too low due to too little addition of silicon-carbon materials, nor will the kinetic performance of the battery negative electrode and the interface stability decrease due to too much addition of silicon-carbon materials, thereby affecting the fast charging performance and cycle performance of the battery.

[0048] In a specific embodiment, the Dv50 particle size of graphite is A μm, and the Dv50 particle size of the silicon-carbon material is B μm, and A and B satisfy 1.4 ≤ A / B ≤ 3.2.

[0049] Dv50 represents the particle size distribution of powder materials or granular materials. Specifically, Dv50 means that 50% of the particle volume in the cumulative distribution is less than or equal to this value, and it is also called the median volume diameter or volume median diameter. By using equipment such as a laser particle size analyzer, the Dv50 value can be measured and calculated.

[0050] Specifically, A / B includes, but is not limited to, 1.4, 1.6, 1.8, 2.0, 2.2, 2.2, 2.4, 2.6, 2.8, 3.0, 3.2, or the range formed by any two of them.

[0051] When the particle sizes of the silicon-carbon material and graphite meet the above conditions, the mutual matching of large and small particles between graphite particles and silicon-carbon particles can be realized, forming a good co-insertion environment, that is, the silicon-carbon material can effectively fill the voids between graphite particles, thereby improving the space utilization rate and the tap density. At the same time, during charging, the voids between graphite can provide a buffer space for the expansion of the silicon-carbon material, thereby reducing the overall expansion of the negative electrode during the charge-discharge cycle, being beneficial to maintaining the integrity of the negative electrode interface, and further improving the battery cycle life.

[0052] Furthermore, in a specific embodiment, the Dv50 of the silicon-carbon material is 5 μm to 9 μm; specifically, the Dv50 of the silicon-carbon material includes, but is not limited to, 5 μm, 5.5 μm, 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm, 8.5 μm, 9 μm, or the range formed by any two of them.

[0053] During the charge and discharge process, silicon-carbon materials will experience significant volume changes, which may lead to electrode pulverization and a decrease in cycle life. The above particle size helps to alleviate the impact of these volume changes on the electrode structure, reducing the pulverization and shedding of electrode materials.

[0054] In another specific embodiment, the specific surface area of the silicon-carbon material is 2 m 2 / g to 3 m 2 / g; specifically, the specific surface area of the silicon-carbon material includes but is not limited to 2 m 2 / g, 2.2 m 2 / g, 2.4 m 2 / g, 2.6 m 2 / g, 2.8 m 2 / g, 3 m 2 / g or the range composed of any two of them.

[0055] When the specific surface area of the silicon-carbon material is within the above range, it can provide sufficient active sites for the insertion and extraction of metal ions, improving the charge and discharge efficiency of the battery. Moreover, the specific surface area within the above range helps to reduce the impact of volume changes of the electrode material during the charge and discharge process on the structure, improving the mechanical stability of the electrode and extending the cycle life of the battery.

[0056] As for graphite, the degree of graphitization has a certain impact on the conductivity, structural stability, etc. of the battery. In a specific embodiment, the graphitization degree of the graphite is 92% - 94%.

[0057] Specifically, the degree of graphitization refers to the proportion of carbon atoms in the graphite material arranged in an ideal graphite crystal structure. Common methods in the art can be used to measure and calculate the degree of graphitization of graphite, such as X-ray diffraction, Raman spectroscopy, transmission electron microscopy, thermogravimetric analysis, etc.

[0058] Specifically, the graphitization degree of the graphite includes but is not limited to 92%, 92.5%, 93%, 93.5%, 94% or the range composed of any two of them.

[0059] When the graphitization degree of the graphite is within the above range, it means that the structure of the graphite is relatively ordered and close to the ideal graphite crystal structure. This not only enables electrons to move more easily between the graphite layers, improving the overall conductivity of the battery, but also can maintain the structural integrity during multiple cycles, reducing the pulverization or shedding of materials caused by volume changes, and improving the cycle stability of the battery.

[0060] In another specific embodiment, the Dv50 particle size of the graphite is 13 μm to 16 μm; specifically, the Dv50 particle size of the graphite includes but is not limited to 13 μm, 13.5 μm, 14 μm, 14.5 μm, 15 μm, 15.5 μm, 16 μm, or the range composed of any two of them.

[0061] When the Dv50 of the graphite is within the above range, it will neither be unfavorable to the diffusion of lithium ions due to too large particle size, hindering the insertion of lithium ions; nor will it be unable to form a good particle inter-matching structure with silicon-carbon particles due to too low particle size. Moreover, the moderate particle size range helps to form a good conductive network, improve the overall conductivity of the electrode, and enhance the rate performance of the battery.

[0062] In another specific embodiment, the specific surface area of the graphite is 1.3 m 2 / g to 1.6 m 2 / g; specifically, the specific surface area of the graphite is 1.3 m 2 / g, 1.35 m 2 / g, 1.4 m 2 / g, 1.45 m 2 / g, 1.5 m 2 / g, 1.55 m 2 / g, 1.6 m 2 / g, or the range composed of any two of them.

[0063] A lower specific surface area means a smaller active surface area in contact with the electrolyte, thus reducing the side reactions of electrolyte decomposition and the formation of solid electrolyte interface film (SEI), which helps to improve the Coulomb efficiency of the first charge and discharge. On the other hand, a lower specific surface area can reduce the consumption of electrolyte and irreversible capacity loss, thus improving the actual available capacity of the battery.

[0064] It can be understood that in addition to the negative electrode active material, the negative electrode active layer may also include a conductive agent. In a specific embodiment, the negative electrode active layer further includes carbon nanotubes. Preferably, based on the mass of the negative electrode active layer, the mass ratio of the carbon nanotubes is 0.1% to 1%.

[0065] Specifically, the mass ratio of the carbon nanotubes includes but is not limited to 0.1%, 0.2%, 0.4%, 0.6%, 0.8%, 1.0%, or the range composed of any two of them.

[0066] Carbon nanotubes have excellent electrical conductivity. Even at low mass percentages, they can significantly improve the overall conductivity of the negative electrode active layer, which helps reduce the internal resistance of the battery, improve the charge-discharge efficiency and rate performance. The network structure of carbon nanotubes can provide more electron transport channels, promoting the rapid diffusion of electrons in the electrode sheet, thereby improving the rate performance and overall electrochemical performance of the battery.

[0067] In addition to carbon nanotubes, the conductive agent can also include other common types of conductive agents, such as conductive carbon black (SP), Ketjen black, acetylene black, graphite conductive agents (KS-6, KS-15, S-O, SEG-6), carbon fibers (VGCG), carbon nanotubes (CNT), graphene, etc.

[0068] In addition to the negative electrode active material, binder, and conductive agent mentioned above, the negative electrode active layer can also include a thickening agent. The present invention does not limit the specific type of thickening agent, and common thickening agents in the art can be selected according to actual situations, such as carboxymethyl cellulose (CMC), lithium carboxymethyl cellulose (CMCLi), sodium carboxymethyl cellulose (CMCNa), etc.

[0069] When the negative electrode active layer includes a negative electrode active material, binder, conductive agent, and thickening agent, the mass relationship between the various components can be adjusted according to conventional conditions.

[0070] In a specific embodiment, the mass ratio of the negative electrode active material is 94% - 98.5%, the mass ratio of the conductive agent is 0.5% - 2%, the mass ratio of the binder is 0.5% - 3%, and the mass ratio of the thickening agent is 0.5% - 1%.

[0071] When each component meets the above conditions, the proportion of the negative electrode active substance can be as high as possible, thereby increasing the areal capacity of the negative electrode sheet, which is beneficial to improving the energy density of the battery. At the same time, it will not cause insufficient flexibility of the negative electrode sheet due to too high content of the active substance and too low content of the binder, resulting in pole piece fracture or demoulding and powder falling during the winding process, nor will it cause insufficient adhesion between the graphite and silicon material particles in the electrode sheet, resulting in negative electrode pulverization and peeling due to large negative electrode expansion during the cycling process; or due to too low content of the conductive agent, resulting in insufficient connection of the conductive network between the graphite and silicon particles in the electrode sheet, resulting in electron path blockage and too high surface resistance due to large negative electrode expansion during the cycling process, thereby damaging the fast charging performance and cycling performance of the battery.

[0072] In another specific embodiment, there is also a bottom coating between the negative electrode active layer and the negative electrode current collector. The bottom coating includes a conductive agent and a binder. The bottom coating can effectively improve the bonding effect between the negative electrode paste and the current collector under a high silicon doping amount, avoiding peeling of the paste from the current collector due to uneven stress and extrusion deformation of the paste caused by the expansion of the electrode sheet and the winding core during the cycling process, thereby improving the cycling performance of the battery.

[0073] Further, in a specific embodiment, the carbon nanotubes are single-walled carbon nanotubes. Preferably, the diameter of the single-walled carbon nanotubes is < 3 nm, the length is 1800 nm to 7200 nm, and the length-to-diameter ratio is 1200 to 3000.

[0074] When the carbon nanotubes meet the above length and length-to-diameter ratio, the single-walled carbon nanotubes have better dispersion stability and higher electronic conductivity, can maintain a more uniform dispersion in the slurry without agglomeration. Since the silicon particles themselves have poorer conductivity, the single-walled carbon nanotubes can uniformly coat the surfaces of the graphite and silicon particles, forming a three-dimensional conductive network between the particles, which can effectively improve the conductivity of the negative electrode sheet, thereby reducing the polarization of the electrode sheet during charging at high current and high rate, and enhancing the fast charging performance.

[0075] It can be understood that in addition to the negative electrode sheet, the cylindrical battery also includes a positive electrode sheet. In a specific embodiment, the positive electrode sheet includes a positive electrode current collector and a positive electrode active layer covering at least one side of the positive electrode current collector. The positive electrode active layer includes a positive electrode active material having a layered structure, and the chemical formula of the positive electrode active material is Li a Ni x Co y Mn z A k O2, where 0.9 ≤ a ≤ 1.1, 0.8 ≤ x ≤ 0.95, 0 < y ≤ 0.2, 0 < z ≤ 0.2, 0 ≤ k ≤ 0.05, and A includes at least one of Al, Mg, Ti, Y, B, P, Zr. Preferably, 0.9 ≤ x ≤ 0.95.

[0076] Among them, the positive electrode sheet includes a positive electrode current collector and a positive electrode active layer covering one or both sides of the positive electrode current collector, and the positive electrode active layer includes a positive electrode active material having a layered structure.

[0077] When the chemical formula of the positive electrode active material meets the above conditions, a higher Ni element content is beneficial to improving the specific capacity of the positive electrode material, thereby enhancing the energy density of the battery.

[0078] The present invention does not limit the specific type of the positive electrode current collector, and common positive electrode current collectors in the art, such as aluminum foil, can be selected.

[0079] In a specific embodiment, the positive electrode active material includes single crystal particles and polycrystalline particles. Preferably, based on the mass of the positive electrode active material, the mass ratio of the polycrystalline particles is 60% - 90%; preferably, the mass ratio of the polycrystalline particles is 70% - 80%.

[0080] In the present invention, single-crystal particles mean that only a single crystal grain is contained in a single particle. Its crystal phase structure is complete and has strong stress resistance. It can better maintain a complete morphological structure in the battery core. However, due to its single ion channel, its ion conduction performance is poor, which is not conducive to the extraction of ions from the positive electrode and their transmission to the negative electrode during fast charging of the battery; polycrystalline particles are secondary particles formed by the aggregation of multiple primary single-crystal particles. Multiple crystal grains are contained in a single particle. It has more ion channels and a larger contact area with the electrolyte, which is conducive to the rapid transmission of lithium ions. However, the structure of its secondary particles has poor stress resistance and is prone to particle fragmentation when the stress in the core is large, thus destroying the complete structure of the particles and being unfavorable for the long-term cycling performance of the battery.

[0081] When polycrystalline particles are combined with silicon-carbon materials with a certain sphericity, the silicon-carbon materials can reduce the volume expansion during overcharging and discharging, relieve the fragmentation of polycrystalline particles, and improve the long-term cycling performance and high-temperature performance of the battery. At this time, by mixing single-crystal and polycrystalline particles and having a relatively large mass proportion of polycrystalline particles, it is beneficial to improve and promote the diffusion and transmission of ions in the positive electrode, thereby improving the overall ion conduction performance of the positive electrode, improving the kinetic performance of the battery. At the same time, an appropriate amount of single-crystal particles can improve the particle integrity of the positive electrode sheet when subjected to stress, thereby improving the long-term cycling performance of the electrode sheet.

[0082] More preferably, based on the total mass of graphite and the silicon-carbon material, the mass proportion of the silicon-carbon material is D, and C and D satisfy: 4.6 ≤ C / D ≤ 800. In a specific embodiment, 4.6 ≤ C / D ≤ 7.

[0083] When the ratio of C / D satisfies the above range, when the mass fraction of the silicon-carbon material in the negative electrode sheet increases, the mass fraction of polycrystalline particles in the positive electrode sheet can be appropriately increased. At this time, the increased polycrystalline particles can promote the transmission of lithium ions, thereby reducing the polarization caused by the increase in the content of the silicon-carbon material, which is beneficial to improving the fast charging performance of the battery. Moreover, after the proportion of polycrystalline particles increases, it can also improve the specific capacity of the material, which is beneficial to improving the energy density of the battery. On the contrary, when the mass fraction of the silicon-carbon material in the negative electrode sheet decreases, the proportion of polycrystalline particles can be appropriately reduced. At this time, the decrease in the content of the silicon-carbon material will reduce the polarization of the negative electrode sheet (the requirement for the proportion of polycrystalline particles will also decrease), and the appropriate increase in the proportion of single-crystal particles in the positive electrode sheet can improve the cycling stability of the positive electrode sheet at high temperatures.

[0084] It can be understood that in addition to the positive electrode active material in the positive electrode active layer, in order to ensure conductivity, a conductive agent can also be included. In a specific embodiment, the positive electrode active layer further includes carbon nanotubes. Based on the mass of the positive electrode active layer, the mass proportion of the carbon nanotubes is 0.5% - 1.5%, and the aspect ratio of the carbon nanotubes is 1800 - 2400.

[0085] The selection of carbon nanotubes can have better electronic conductivity. By regulating their aspect ratio, a good conductive network can be formed even at a low addition amount, thereby reducing the resistivity of the positive electrode, improving the conductivity of the positive electrode sheet, and being beneficial to enhancing the fast charging performance and cycling performance of the positive electrode.

[0086] In addition to carbon nanotubes, other common types of conductive agents can also be doped, such as conductive carbon black (SP), Ketjen black, acetylene black, graphite conductive agents (KS-6, KS-15, S-O, SEG-6), carbon fibers (VGCG), carbon nanotubes (CNT), graphene, etc.

[0087] In order to improve the structural stability of the positive electrode active layer, a binder can also be doped in the positive electrode active layer. The present invention does not limit the specific type of the binder, and common binders in the art can be selected, such as polyvinylidene fluoride (PVDF), copolymer of polyvinylidene fluoride - hexafluoropropylene (PVDF-HFP), hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate salt, polyvinylpyrrolidone, polyethylene ether, polymethyl methacrylate, polytetrafluoroethylene, etc.

[0088] When the positive electrode active layer includes a positive electrode active material, a conductive agent, and a binder, the appropriate mass relationship can be selected according to the actual situation.

[0089] In a specific embodiment, the mass ratio of the positive electrode active material is 95% - 99%, the mass ratio of the conductive agent is 0.5% - 2.5%, and the mass ratio of the binder is 0.5% - 2.5%.

[0090] When each component in the positive electrode active layer meets the above conditions, a relatively high and appropriate mass ratio of the positive electrode active material can increase the areal capacity of the positive electrode sheet, thereby increasing the energy density of the battery. At the same time, it will not cause insufficient particle adhesion and powder falling due to too high content of the positive electrode active material and too low content of the binder, or insufficient electronic conductivity and too high surface resistance of the electrode sheet due to too low content of the conductive agent, thereby damaging the fast charging performance and cycling performance of the battery.

[0091] Further, in a specific embodiment, the single-sided areal density of the negative electrode sheet is 6mg / cm 2 ~8mg / cm 2 and the tap density is 1.4g / cm 3 ~1.8g / cm 3 .

[0092] The single-sided areal density of the negative electrode sheet refers to the mass of the negative electrode sheet per unit area, while the tap density refers to the mass of the negative electrode sheet per unit volume, reflecting the packing situation of the active material.

[0093] Specifically, the single-sided areal density of the negative electrode sheet includes but is not limited to 6mg / cm2 , 6.2 mg / cm 2 , 6.4 mg / cm 2 , 6.6 mg / cm 2 , 6.8 mg / cm 2 , 7.0 mg / cm 2 , 7.2 mg / cm 2 , 7.4 mg / cm 2 , 7.6 mg / cm 2 , 7.8 mg / cm 2 , 8.0 mg / cm 2 or the range formed between any two of them.

[0094] The compacted density includes but is not limited to 1.4 g / cm 3 , 1.45 g / cm 3 , 1.5 g / cm 3 , 1.55 g / cm 3 , 1.6 g / cm 3 , 1.65 g / cm 3 , 1.7 g / cm 3 , 1.75 g / cm 3 , 1.8 g / cm 3 or the range formed between any two of them.

[0095] When the areal density of the negative electrode sheet meets the above conditions, a moderate areal density can balance the energy density and kinetic performance of the battery. It will neither cause difficulty in the infiltration of the electrolyte into the negative electrode sheet after winding due to an excessive areal density, resulting in excessive negative electrode polarization and lithium deposition during charging, nor lead to too low an energy density of the battery due to an excessively low areal density. When the compacted density of the negative electrode sheet meets the above conditions, effective physical contact can be maintained between the negative electrode active material particles, and a good conductive network can be formed, which is beneficial to improving the fast charging performance of the battery at high rates. It will neither cause too few internal voids after winding due to excessive compaction, resulting in the inability of the electrolyte to infiltrate, nor lead to poor particle contact or too low an energy density of the battery due to too low compaction.

[0096] In a specific embodiment, the negative electrode active layer includes at least one groove;

[0097] Preferably, as Figure 2 shown, the minimum distance between the groove and the edge of the negative electrode sheet is L, and L is 10 mm to 20 mm.

[0098] The specific shape of the groove is not limited in the present invention, and the shape of the groove can be adjusted according to actual needs. For example, along the lamination direction of the negative electrode current collector and the negative electrode active layer, the cross-section of the groove can be circular, triangular, fan-shaped, etc. Conventional methods in the art can be used to provide the negative electrode sheet with grooves. For example, mechanical punching, laser punching, electro- or thermo-optical melting, radiation melting, chemical etching, friction punching and other processing methods can be used.

[0099] The distance between the groove and the edge of the negative electrode sheet refers to the distance between the groove and the edges at both ends of the negative electrode sheet along the length extension direction of the negative electrode sheet. Specifically, L includes but is not limited to 10mm, 12mm, 14mm, 16mm, 18mm, 20mm or the range composed of any two of them.

[0100] The groove can increase the specific surface area of the negative electrode sheet, promote the faster entry of the electrolyte into the interior of the negative electrode sheet, facilitate the rapid diffusion and transmission of lithium ions, reduce the polarization difference between the side of the negative electrode sheet close to the electrolyte and the side far from the electrolyte, and improve the fast charging performance and cycling performance of the negative electrode sheet.

[0101] When the distance between the groove and the edge of the negative electrode sheet is within the above range, it can avoid the problems of decreased mechanical strength and unstable structure of the negative electrode sheet caused by the setting of the groove.

[0102] In a specific embodiment, the depth of the groove is 3μm to 40μm, the width is 50μm to 500μm, and the distance between adjacent grooves is 400μm to 5000μm.

[0103] When the depth, width and distance between adjacent grooves of the groove are within the above range, the appropriate size can better balance the relationship between the electrolyte infiltration effect and the mechanical strength of the negative electrode sheet, and further ensure the fast charging performance and cycling performance of the negative electrode sheet.

[0104] Further, as Figure 2 shown, the negative electrode sheet sequentially includes a first region, a second region, and a first region along the width extension direction;

[0105] The width of the first region is less than or equal to 20mm;

[0106] The compaction density of the first region is E, and the compaction density of the second region is F, satisfying: E > F, E is 1.65 g / cm 3 to 1.8 g / cm 3 , and F is 1.4 g / cm 3 to 1.65 g / cm 3 .

[0107] When the compaction density of the negative electrode sheet meets the above conditions, the compaction density of the second region of the negative electrode sheet is relatively low, which can result in more voids inside the wound core, facilitating the penetration and infiltration of the electrolyte into the interior, thereby enhancing the infiltration of lithium ions inside the wound core. The compaction density of the edge region of the negative electrode sheet is relatively high, which can make the infiltration of the electrolyte in the edge region into the negative electrode sheet relatively slow, thus delaying the lithium deposition in the edge region of the negative electrode during fast charging.

[0108] It can be understood that the cylindrical battery provided by the present invention includes a separator and an electrolyte in addition to the above-mentioned negative electrode sheet and positive electrode sheet.

[0109] In a specific embodiment, the separator includes a base film and a ceramic layer, where the ceramic layer is coated on the base film, and can be coated on one side of the base film or both sides of the base film. The thickness of the base film is 5 μm to 9 μm, and the single-layer thickness of the ceramic layer is 1 μm to 4 μm.

[0110] The present invention does not limit the specific type of the base film, and common base films such as PP and PE can be selected.

[0111] Coating the ceramic layer on the surface of the base film can enhance the thermal stability of the separator. When the battery is charged quickly, the internal temperature rises relatively high, and the separator coated with ceramic is not prone to heat shrinkage or wrinkling, thereby improving the safety of the battery interior under high-temperature conditions and high-rate charging. The above-mentioned separator does not contain glue, which can reduce the thickness of the separator and thus improve the energy density of the battery.

[0112] The present invention also does not limit the specific type of the electrolyte, and common electrolytes in the art can be selected. In a specific embodiment, the electrolyte includes a lithium salt and an organic solvent. The lithium salt includes at least one of lithium bis(fluorosulfonyl)imide, lithium hexafluorophosphate, lithium difluorophosphate (LiPO2F2), lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), lithium tetrafluoroborate (LiBF4), lithium difluoro(oxalato)phosphate (LiDFOP), and lithium bis(trifluoromethylsulfonyl)imide; the organic solvent includes ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), dimethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, dipropyl carbonate, diethyl carbonate (DMC), ethyl formate, ethyl acetate (EA), ethyl propionate (EP), and propyl propionate (PP).

[0113] In another specific embodiment, the electrolyte further includes an additive fluoroethylene carbonate (FEC), and based on the total mass of the electrolyte, the mass fraction of the additive is 0.1% to 25%.

[0114] When the electrolyte contains FEC, it is beneficial for the electrolyte to react with the negative electrode surface to form a relatively stable and dense SEI film, thereby improving the cycle stability of the negative electrode.

[0115] The present invention does not limit the specific preparation method of the cylindrical battery. In a specific embodiment, the negative electrode sheet, the separator, and the positive electrode sheet are laminated in sequence, and then one end of the laminate is used as the winding center to wind and obtain a cylindrical winding core. Subsequently, after the cylindrical winding core is placed in the battery case, the tabs of the positive electrode and the negative electrode are welded to the case structural member, and then the case structural member is fixed by welding, and after liquid injection, formation, and sorting, the finished product is obtained.

[0116] The present invention does not limit the specific type of the battery case, as long as it can improve the structural strength and suppress the swelling of the winding core, such as an aluminum case, a steel case, etc.

[0117] When the cylindrical battery meets the above conditions, the cylindrical winding core and the battery case can be closely attached, thereby increasing the space utilization rate inside the battery case, and the tabs extending from the positive and negative electrodes are connected to the structural member, which can reduce the ohmic impedance inside the battery and improve the overcurrent capacity of the battery.

[0118] Hereinafter, the cylindrical battery provided by the present invention will be introduced in detail through specific examples.

[0119] Example 1

[0120] The preparation method of the cylindrical battery provided in this example includes the following steps:

[0121] 1. Preparation of the negative electrode sheet:

[0122] Mix 97% of the negative electrode active material (89 wt% graphite + 11 wt% SiC), 0.5% of the thickener CMCLi, 1.5% of the binder SBR, and 1% of the conductive agent (0.5% of conductive carbon black SP, 0.5% of carbon nanotubes), and obtain a uniformly dispersed mixture through high-speed stirring, and use water as the solvent to make the surface negative electrode slurry, and the solid content in the slurry is 50 wt%;

[0123] During coating, the slurry should be evenly sprayed on the negative electrode current collector copper foil to form a wet film, and then a single-sided negative electrode is obtained after baking. After coating both sides in the same way, it is baked, rolled, slit, and a groove structure is provided on both sides of the negative electrode active layer of the negative electrode sheet by laser drilling to obtain the negative electrode sheet.

[0124] 2. Preparation of the positive electrode sheet:

[0125] Mix 97% of the positive electrode active material (the proportion of single crystal particles is 20%, the proportion of polycrystalline particles is 80%, the chemical formula is single crystal LiNi 0.9 Co 0.04 Mn 0.04 Al 0.02 O2, polycrystalline particles LiNi 0.92 Co 0.02 Mn 0.04 Al 0.02O2), 1.4% binder and 1.6% conductive agent (0.8% conductive carbon black SP, 0.8% carbon nanotubes) were mixed, and a uniformly dispersed mixture was obtained by high-speed stirring. Then, NMP was used as the solvent to make the positive electrode active material slurry, and the solid content in the slurry was 70 wt%.

[0126] The slurry was evenly coated on the positive electrode current collector aluminum foil to form a wet film, which was then baked to obtain a single-sided positive electrode. The double-sided positive electrode was coated in the same way and then baked, rolled, and slit to obtain the positive electrode sheet.

[0127] 3. Preparation of electrolyte:

[0128] 1 mol / L LiPF6 was selected, the solvent was EC / DMC / DEC (mass ratio 1:1:1), and 5% FEC additive was added as the electrolyte.

[0129] Battery preparation: The positive electrode, separator, and negative electrode can be wound into a core, and then made into a cylindrical battery after being put into the shell, welded, encapsulated, injected with electrolyte, formed, and sorted.

[0130] Examples 2 - 38

[0131] The preparation methods of the cylindrical batteries provided in Examples 2 - 38 were basically the same as those in Example 1, but some parameters were adjusted. The specific parameter information is shown in Table 1 and Table 2.

[0132] Table 1

[0133] Serial number Sphericity A graphite Dv50 (μm) B silicon carbide material Dv50 (μm) A / B Example 1 0.94 14.6 7.3 2 Example 2 0.8 14.6 7.3 2 Example 3 0.85 14.6 7.3 2 Comparative example 1 0.45 14.6 7.3 2 Example 4 0.94 13 5 2.6 Example 5 0.94 16 9 1.78 Example 6 0.94 10 3 3.33 Example 7 0.94 20 15 1.33 Example 8 0.94 14.6 7.3 2 Example 9 0.94 14.6 7.3 2 Example 10 0.94 14.6 7.3 2 Example 11 0.94 14.6 7.3 2 Example 12 0.94 14.6 7.3 2 Example 13 0.94 14.6 7.3 2 Example 14 0.94 14.6 7.3 2 Example 15 0.94 14.6 7.3 2 Example 16 0.94 14.6 7.3 2 Example 17 0.94 14.6 7.3 2 Example 18 0.94 14.6 7.3 2 Example 19 0.94 14.6 7.3 2 Example 20 0.94 14.6 7.3 2

[0134] Table 1 - continued

[0135]

[0136] Table 2

[0137]

[0138]

[0139]

[0140] Comparative Example 1

[0141] The preparation method of the cylindrical battery provided in this comparative example was basically the same as that in Example 1, except that non-spherical silicon carbide with a sphericity Q of 0.45 was used in the preparation process of the negative electrode sheet.

[0142] Test Example

[0143] 1. Room temperature cycle test:

[0144] The cylindrical batteries provided in all the examples and comparative examples were tested. At 25°C, within the charge-discharge window of 4.2V to 2.5V, 1C / 1C charge-discharge cycles were carried out. The test process was as follows: First, constant current charge at 1C to 4.2V, then constant voltage charge, with the cut-off current of 0.05C, and finally constant current discharge at 1C to 2.5V, and such cycles were tested. The number of cycles when the ratio of the discharge capacity to the first discharge capacity (capacity retention rate) reached 80% was recorded in Table 3.

[0145] The ratio of the DCR value of the battery at 50% SOC at the end of the cycle to the DCR value in the initial state was recorded, which was the DCR growth rate. The specific calculation results are shown in Table 3.

[0146] 2. Fast charging window test:

[0147] The cylindrical batteries provided in all the examples and comparative examples were tested. At room temperature, the battery cells were charged to 4.2V at nC (n = 1, 2, 3, 4, 5), then constant voltage charged with the cut-off current of 0.05C, and left standing for 30 min; discharged at 1C to 2.5V, and left standing for 30 min; such charge-discharge cycles were carried out 20 times. Finally, after the battery cells were fully charged, the test was ended and the cells were dissected to observe whether lithium plating occurred at the edge of the negative electrode sheet. The maximum rate at which the battery did not show lithium plating was recorded, which was the fast charging window of the battery. The temperature change during the charging process of the battery was recorded, and its maximum value was the maximum temperature rise of the battery.

[0148] The specific results are shown in Table 3.

[0149] Table 3

[0150]

[0151]

[0152] After testing:

[0153] 1. Referring to Table 1 and Table 3, compared with Comparative Example 1, for the cylindrical batteries prepared in Examples 1-3, when the negative electrode active layer included graphite and silicon-carbon material, and the sphericity of the silicon-carbon material was limited to be greater than or equal to 0.8 and less than or equal to 1, the cycle life of the cylindrical battery was greatly improved (improved by more than 800 cycles), the DCR growth rate was significantly reduced. At the same time, the maximum rate of the cylindrical battery was significantly increased and the maximum temperature rise was significantly decreased, indicating that the cylindrical battery of the present application had more excellent fast charging performance. Therefore, the technical solution provided by the present invention can give full play to the kinetic performance of the negative electrode sheet while improving the energy density.

[0154] Further, compared with Examples 6 and 7, it can be seen from Examples 1, 4, and 5 that when the Dv50 particle size of graphite, the Dv50 particle size of the silicon-carbon material, and the ratio of the Dv50 particle size of graphite to the Dv50 particle size of the silicon-carbon material are limited within the corresponding ranges, the cycle life of the cylindrical battery is improved. It can be known from Examples 8-11 that when the graphitization degree of graphite is limited to 92% - 94%, the cycle life of the cylindrical battery is improved.

[0155] Further, compared with Examples 14 and 15, it can be seen from Examples 1, 12, and 13 that when carbon nanotubes are introduced into the negative electrode active layer and the mass ratio of the carbon nanotubes is limited within the corresponding range, the cycle life of the cylindrical battery is improved. At the same time, when the carbon nanotubes are single-walled carbon nanotubes and the diameter, length, and aspect ratio of the single-walled carbon nanotubes are limited within the corresponding ranges, the cycle life and fast charging performance of the cylindrical battery are improved.

[0156] 2. Referring to Tables 2 and 3, it can be known from Examples 21 and 22 that when the chemical formula of the positive electrode active material is different from the chemical formula of the present application, the cycle life of the cylindrical battery decreases significantly, the DCR growth rate increases significantly. At the same time, the maximum rate of the cylindrical battery decreases significantly and the maximum temperature rise increases significantly.

[0157] Further, it can be known from Examples 23-26 that when the mass ratio of the polycrystalline particles in the positive electrode active material is too high or too low, the cycle life of the cylindrical battery decreases significantly and the DCR growth rate increases significantly.

[0158] Further, it can be known from Examples 27-38 that when carbon nanotubes are further introduced into the positive electrode active layer and their content and aspect ratio are limited within a suitable range, the cycle life of the cylindrical battery can be improved, and the DCR growth rate can be reduced. When the single-sided surface density and compaction density of the negative electrode sheet are maintained within a suitable range, the cycle life of the cylindrical battery can be improved, the DCR growth rate can be reduced, and the maximum rate can also be increased to a certain extent.

[0159] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A cylindrical battery, characterized in that: The invention comprises a shell and an electrode assembly, wherein the electrode assembly is accommodated in the shell, the electrode assembly comprises a negative electrode sheet, the negative electrode sheet comprises a negative electrode collector and a negative electrode active layer covering at least one side of the negative electrode collector, the negative electrode active layer comprises graphite and silicon-carbon material, and the sphericity of the silicon-carbon material is Q, 0.8≤Q≤1.

2. The cylindrical battery according to claim 1, characterized in that: The Dv50 particle size of the graphite is A μm, the Dv50 particle size of the silicon-carbon material is B μm, and A and B satisfy: 1.4≤A / B≤3.

2.

3. The cylindrical battery according to claim 1 or 2, characterized in that: The Dv50 particle size of the silicon-carbon material is 5 μm to 9 μm; and / or, The specific surface area of ​​the silicon-carbon material is 2 m 2 / g~3m 2 / g; and / or, The graphitization degree of the graphite is 92% to 94%; and / or, The Dv50 particle size of the graphite is 13 μm to 16 μm; and / or, The specific surface area of ​​the graphite is 1.2 m 2 / g~1.7m 2 / g.

4. The cylindrical battery according to claim 1, characterized in that: The negative electrode active layer also includes carbon nanotubes; Based on the mass of the negative electrode active layer, the mass of the carbon nanotubes accounts for 0.1% to 1%; and / or, The carbon nanotubes are single-walled carbon nanotubes; the diameter of the single-walled carbon nanotubes is less than 3 nm, the length is 1800 nm to 7200 nm, and the aspect ratio is 1200 to 3000.

5. The cylindrical battery according to claim 1, characterized in that: The positive electrode sheet includes a positive electrode current collector and a positive electrode active layer covering at least one side of the positive electrode current collector. The positive electrode active layer includes a positive electrode active material with a layered structure. The chemical formula of the positive electrode active material is Li a Ni x Co y Mn z A k O2, wherein 0.9≤a≤1.1, 0.8≤x≤0.95, 0<y≤0.2, 0<z≤0.2, 0≤k≤0.05, and A includes at least one of Al, Mg, Ti, Y, B, P, and Zr.

6. The cylindrical battery according to claim 5, characterized in that: The positive electrode active material includes single crystal particles and polycrystalline particles; Based on the mass of the positive electrode active material, the mass proportion C of the polycrystalline particles is 60%-90%; and / or, Based on the total mass of the graphite and the silicon-carbon material, the mass proportion of the silicon-carbon material is D, and C and D satisfy: 4.6≤C / D≤800.

7. The cylindrical battery according to claim 6, characterized in that: The positive electrode active layer also includes carbon nanotubes; Based on the mass of the positive electrode active layer, the mass of the carbon nanotubes accounts for 0.5% to 1.5%, and the aspect ratio of the carbon nanotubes is 1800-2400.

8. The cylindrical battery according to claim 1, characterized in that: The single surface density of the negative electrode sheet is 6 mg / cm 2 ~8mg / cm 2 , compacted density is 1.4g / cm 3 ~1.8g / cm 3 .

9. The cylindrical battery according to claim 1, characterized in that: The negative electrode active layer comprises at least one groove; The minimum distance between the groove and the edge of the negative electrode sheet is 10 mm to 20 mm; and / or, The groove has a depth of 3 μm to 40 μm and a width of 50 μm to 500 μm.

10. The cylindrical battery according to claim 9, characterized in that: The negative electrode sheet includes a first region, a second region, and a first region in sequence along a width extension direction; The width of the first area is less than or equal to 20 mm; and / or, The compaction density of the first region is greater than the compaction density of the second region; and / or, The compacted density of the first region is 1.65 g / cm 3 ~1.8g / cm 3 The compacted density of the second region is 1.4 g / cm 3 ~1.65g / cm 3 .

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