Negative electrode sheet, battery, battery pack, and electric device

Abstract

The application provides a negative electrode sheet, a battery, a battery pack and an electric device. The negative electrode sheet comprises a negative electrode current collector and a negative electrode coating on at least one side surface of the negative electrode current collector. The negative electrode coating comprises a first coating and a second coating on the side surface of the first coating away from the negative electrode current collector. The first coating and the second coating each comprise a negative electrode active material. The first coating satisfies 0.21<=k1<=0.65, k1 is the ratio of the particle size D50 of the negative electrode active material in the first coating to the OI value, and the unit of the particle size D50 of the negative electrode active material in the first coating is micrometers. The second coating satisfies 18<=k2<=26, k2 is the ratio of the particle size D50 of the negative electrode active material in the second coating to the OI value, and the unit of the particle size D50 of the negative electrode active material in the second coating is micrometers. The application can reduce the battery impedance, improve the fast charging performance of the battery, and improve the cycle life and other performances of the battery.

Description

Technical Field

[0001] This invention relates to the field of batteries, specifically to a negative electrode sheet, a battery, a battery pack, and an electrical device. Background Technology

[0002] Batteries are common electrochemical devices. During charging and discharging, active ions (such as lithium ions in lithium-ion batteries) intercalate and deintercalate between the positive and negative electrodes. However, due to the poor transport capacity of active ions between the positive and negative electrodes (such as the liquid-phase transport capacity of lithium ions in lithium-ion batteries) and the difficulty in intercalating active ions into the electrode materials, the battery has high impedance, which is detrimental to its charging performance and also affects its cycle life and other performance characteristics, thus limiting its applications. Therefore, how to reduce impedance while improving battery cycle life and other performance characteristics is a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0003] This invention provides a negative electrode sheet, a battery, a battery pack, and an electrical device, which can reduce battery impedance and improve battery cycle life and other performance characteristics, effectively overcoming the defects of the prior art.

[0004] In one aspect, the present invention provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode coating located on at least one side surface of the negative electrode current collector, the negative electrode coating comprising a first coating and a second coating located on the side surface of the first coating away from the negative electrode current collector; both the first coating and the second coating comprise a negative electrode active material; the first coating satisfies 0.21≤k1≤0.65, where k1 is the ratio of the particle size D50 of the negative electrode active material in the first coating to the OI value, and the particle size D50 of the negative electrode active material in the first coating is expressed in μm; the second coating satisfies 18≤k2≤26, where k2 is the ratio of the particle size D50 of the negative electrode active material in the second coating to the OI value, and the particle size D50 of the negative electrode active material in the second coating is expressed in μm.

[0005] According to one embodiment of the present invention, 0.21≤k1≤0.45.

[0006] According to one embodiment of the present invention, the ratio of the OI value of the negative electrode active material in the first coating to the OI value of the negative electrode active material in the second coating is 2 to 52, preferably 30 to 52.

[0007] According to one embodiment of the present invention, the OI value of the negative electrode active material in the first coating is greater than or equal to 21.

[0008] According to one embodiment of the present invention, the OI value of the negative electrode active material in the second coating is less than or equal to 12.

[0009] According to one embodiment of the present invention, the particle size D50 of the negative electrode active material in the first coating is 6 to 23 μm.

[0010] According to one embodiment of the present invention, the particle size D50 of the negative electrode active material in the second coating is 9 to 22 μm.

[0011] According to one embodiment of the present invention, the ratio k3 of the thickness of the first coating to the thickness of the second coating satisfies 0.14≤k3≤5.47; preferably 0.14≤k3≤2.6.

[0012] According to one embodiment of the present invention, 0.3 ≤ k3 ≤ 1.

[0013] According to one embodiment of the present invention, the thickness of the first coating is 14 to 96 μm, preferably 25 to 70 μm.

[0014] According to one embodiment of the present invention, the thickness of the second coating is 14 to 96 μm, preferably 40 to 85 μm.

[0015] According to one embodiment of the present invention, the negative electrode active material in the first coating includes graphite, and the negative electrode active material in the second coating includes graphite.

[0016] In another aspect, the present invention provides a battery comprising the aforementioned negative electrode.

[0017] In another aspect, the present invention provides a battery pack including the battery described above.

[0018] In another aspect, the present invention provides an electrical device comprising the aforementioned battery or battery pack.

[0019] The implementation of this invention has at least the following beneficial effects: by providing a first coating and a second coating on at least one side of the surface of the negative electrode current collector, and controlling the first coating to satisfy 0.21≤k1≤0.65 and the second coating to satisfy 18≤k2≤26, the two work together to improve the liquid phase mass transfer capability of the negative electrode sheet, which is beneficial to the transport of active ions such as lithium ions during battery charging and discharging, reduces the liquid phase diffusion resistance of the negative electrode sheet, improves the dynamic performance of the negative electrode sheet, and enhances the fast charging performance (charging performance) of the battery. At the same time, it can also improve the cycle life of the battery. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the negative electrode sheet in one embodiment of the present invention.

[0021] Explanation of reference numerals in the attached figures: 1: Negative electrode current collector; 2: Negative electrode coating; 20: Negative electrode active material; 21: First coating; 22: Second coating; H1: Thickness of the first coating; H2: Thickness of the second coating. Detailed Implementation

[0022] To enable those skilled in the art to better understand the present invention, the present invention will be further described in detail below. The specific embodiments listed below are merely descriptions of the principles and features of the present invention, and the examples are only for explaining the present invention and are not intended to limit the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] In related technologies, batteries generally suffer from problems such as high impedance, poor fast charging performance, and poor cycle performance, which urgently need to be solved.

[0024] For example, currently, there are increasingly higher requirements for power batteries to balance energy density, power density, and charging speed. Increasing the areal density of electrode sheets (negative electrode sheet and positive electrode sheet) to build thicker electrodes can achieve higher energy density. However, due to the inherent electrolyte wettability of high areal density electrode sheets and the excessively long transport path of active ions (such as lithium ions), it is detrimental to the active ion dynamics of the battery. During the charging and discharging process of the battery, the resistance to active ion transport is large, resulting in a large liquid phase diffusion impedance of the battery, which affects the battery's fast charging performance and cycle life.

[0025] In view of this, embodiments of the present invention provide a negative electrode, such as... Figure 1 As shown, the negative electrode sheet includes a negative current collector 1 and a negative electrode coating 2 located on at least one side surface of the negative current collector 1. The negative electrode coating 2 includes a first coating 21 and a second coating 22 located on the side surface of the first coating 21 away from the negative current collector 1. Both the first coating 21 and the second coating 22 include negative electrode active material 20. The first coating 21 satisfies 0.21≤k1≤0.65, where k1 is the particle size D50 (hereinafter referred to as D1) and OI value (hereinafter referred to as V) of the negative electrode active material 20 in the first coating 21. OI1 The ratio of K1 to D1 / V is K1 = D1 / V. OI1 The unit of D1 is μm; the second coating 22 satisfies 18≤k2≤26, where k2 is the particle size D50 (hereinafter referred to as D2) and OI value (hereinafter referred to as V) of the negative electrode active material 20 in the second coating 22. OI2 The ratio of K2 to D2 / V is K2 = D2 / V. OI2 The unit of D2 is μm.

[0026] According to the inventors' research, in the above-mentioned negative electrode structure system, the first coating 21 satisfies 0.21≤k1≤0.65, and the negative electrode active material 20 particles inside it have basically no obvious orientation and are basically isotropic. This can improve the interfacial stability between the negative electrode coating 2 and the negative electrode current collector 1, improve the cycle stability of the negative electrode, and thus improve the cycle life and other performance of the battery. At the same time, the second coating 22 located above the first coating 21 satisfies 18≤k2≤26, and the negative electrode active material 20 particles inside it have obvious orientation in the thickness direction of the negative electrode coating 2, with good orientation. The oriented negative electrode active material 20 particles can optimize the internal channels of the negative electrode, reduce the tortuosity of the negative electrode, and improve the electrolyte wetting ability, which is conducive to the electrolyte wetting of the negative electrode and can improve the liquid phase mass transfer ability of the negative electrode. At the same time, it is conducive to the intercalation and deintercalation of active ions in the negative electrode, improves the reaction kinetics of active ions, thereby reducing the liquid phase diffusion resistance of the negative electrode and improving the fast charging performance of the battery.

[0027] Therefore, the embodiments of the present invention, through the above-mentioned double-layer negative electrode structure design, can reduce the liquid phase diffusion resistance of the negative electrode, improve the fast charging performance of the battery, and at the same time improve the cycle life and other performance of the battery. It can effectively solve the problems of difficulty in balancing the energy density and kinetic performance of the negative electrode and the poor kinetic performance of high areal density negative electrodes.

[0028] For example, k1 can be a range consisting of 0.21, 0.25, 0.28, 0.3, 0.33, 0.35, 0.38, 0.4, 0.43, 0.45, 0.5, 0.55, 0.6, 0.65, or any two of them.

[0029] In some embodiments, 0.21≤k1≤0.45 is beneficial to further improve the cycle life and other performance of the battery while maintaining a low liquid phase diffusion resistance of the negative electrode.

[0030] In this embodiment of the invention, the OI value of the negative electrode active material 20 in the first coating 21 is greater than the OI value of the negative electrode active material 20 in the second coating 22 (i.e., V). OI1 >V OI2 The ratio (VOI) of the OI value of the negative electrode active material 20 in the first coating 21 to the OI value of the negative electrode active material 20 in the second coating 22 is... OI1 / V OI2 The range can be 2 to 52, for example, 2, 5, 10, 20, 25, 30, 35, 40, 45, 50, 52 or any two of them.

[0031] In some embodiments, the ratio (VOI) of the OI value of the negative electrode active material 20 in the first coating 21 to the OI value of the negative electrode active material 20 in the second coating 22 is... OI1 / VOI2 The coating thickness can be 20-52, further 30-52, and even further 40-52. This is beneficial because, under the above-mentioned negative electrode structure system, the first coating 21 improves the interfacial stability between the negative electrode coating 2 and the negative electrode current collector 1, while the second coating 22 improves the mass transfer capability of the negative electrode coating 2 and reduces the liquid phase mass transfer impedance of the negative electrode. Thus, based on the synergistic effect of the first coating 21 and the second coating 22, the fast charging performance and cycle life of the battery can be improved simultaneously.

[0032] Specifically, the OI value of the negative electrode active material 20 in the first coating 21 can be greater than or equal to 21 (i.e., V). OI1 ≥21), V OI1 For example, the range can be 21, 22, 25, 28, 30, 32, 35, 35.4, 36, 37 or any two of them. This range is beneficial because the particles of the negative electrode active material 20 in the first coating 21 have basically no obvious orientation and exhibit better isotropy. This further improves the interfacial stability between the negative electrode coating 2 and the negative electrode current collector 1, thereby reducing battery impedance and further improving battery cycle life and other performance characteristics.

[0033] Specifically, the OI value of the negative electrode active material 20 in the second coating 22 can be less than or equal to 12 (i.e., V). OI2 ≤12), for example, less than or equal to 11, less than or equal to 10, or less than or equal to 8, or less than or equal to 5, or less than or equal to 3, or less than or equal to 1, which is beneficial for the negative electrode active material 20 particles in the second coating 22 to have better orientation in the thickness direction of the negative electrode sheet (also the thickness direction of the negative electrode coating 2), which is beneficial for the deintercalation and deintercalation of active ions in the negative electrode sheet during battery charging and discharging, improving the reaction kinetics of active ions, reducing liquid phase diffusion resistance, and thus further improving the fast charging performance and cycle performance of the battery.

[0034] In this embodiment of the invention, the negative electrode active material 20 in the negative electrode coating 2 may include graphite, specifically including primary graphite particles and / or secondary graphite particles. That is, the negative electrode active material 20 in the first coating 21 includes graphite, and the negative electrode active material 20 in the second coating 22 includes graphite. Through the double-layer negative electrode structure design, the above-mentioned conditions such as 0.21≤k1≤0.65 and 18≤k2≤26 are met, which can take into account both reducing the impedance of the negative electrode (graphite negative electrode) and improving the cycle life of the negative electrode.

[0035] Taking the battery charging process as an example, graphite generally has a multi-layered structure. During battery charging, active ions (such as lithium ions) begin to intercalate from the graphite end faces, entering the spaces between the graphite layers to achieve lithium intercalation in the negative electrode. In related technologies, to improve the energy density of the negative electrode, thick electrodes with high areal density are usually used, supplemented by a high compaction process. This makes the liquid phase transport of active ions in the negative electrode and the intercalation of active ions in the graphite difficult, and also hinders the wetting of the negative electrode by the electrolyte, resulting in high liquid phase resistance of the negative electrode, which is detrimental to battery charging performance and cycle performance.

[0036] In this embodiment of the invention, under the above-mentioned negative electrode structure system, the first coating 21 satisfies 0.21≤k1≤0.65, and the graphite particles inside it have basically no obvious orientation and are isotropic as a whole. This can improve the interface stability between the negative electrode coating 2 and the negative electrode current collector 1, improve the cycle stability of the negative electrode, and thus improve the cycle life and other performance of the battery. At the same time, the second coating 22 located on the first coating 21 satisfies 18≤k2≤26, and the graphite particles inside it have obvious orientation in the thickness direction of the negative electrode. The end faces of the oriented graphite particles are arranged in the thickness direction of the negative electrode, which is conducive to the intercalation and deintercalation of active ions, improves the reaction kinetics of active ions, and also facilitates the wetting of the negative electrode by the electrolyte and improves the liquid phase mass transfer capability of the negative electrode, thereby reducing the battery impedance and improving the fast charging performance of the battery.

[0037] In some embodiments, the particle size D50 of the negative electrode active material 20 in the first coating 21 can be 6 to 23 μm, for example, a range of 6 μm, 8 μm, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, 23 μm or any two of these.

[0038] In some embodiments, the particle size D50 of the negative electrode active material 20 in the second coating 22 can be 9 to 22 μm, for example, a range of 9 μm, 11 μm, 12 μm, 13 μm, 15 μm, 17 μm, 18 μm, 19 μm, 22 μm or any combination thereof.

[0039] Furthermore, the ratio k3 of the thickness H1 of the first coating 21 to the thickness H2 of the second coating 22 can satisfy 0.14≤k3≤5.47. By further coordinating and controlling the thicknesses of the first coating 21 and the second coating 22, the ratio K3 (k3=H1 / H2) can be kept within the above range, which is beneficial to further balance the performance of reducing the liquid phase diffusion resistance of the negative electrode and improving the cycle life of the negative electrode.

[0040] For example, K3 can be a range consisting of 0.14, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, 1, 1.5, 2, 2.3, 2.6, 2.7, 3, 4, 5, 5.47 or any two of them.

[0041] In some embodiments, 0.14≤k3≤2.6, and further, 0.3≤k3≤1, is beneficial for further balancing the reduction of liquid phase diffusion resistance of the negative electrode and the improvement of cycle life of the negative electrode.

[0042] In some embodiments, the thickness H1 of the first coating 21 can be 14 to 96 μm, for example, a range of 14 μm, 20 μm, 25 μm, 27 μm, 30 μm, 35 μm, 40 μm, 43 μm, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 75 μm, 80 μm, 85 μm, 90 μm, 96 μm or any two of these, preferably 25 to 70 μm.

[0043] In some embodiments, the thickness H2 of the second coating 22 is 14 to 96 μm, for example, a range of 14 μm, 20 μm, 25 μm, 27 μm, 30 μm, 35 μm, 40 μm, 43 μm, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 75 μm, 80 μm, 85 μm, 90 μm, 96 μm or any two of these, preferably 40 to 85 μm.

[0044] The embodiments of the present invention may employ a conventional negative electrode current collector 1 in the art, for example, the negative electrode current collector 1 includes copper foil.

[0045] In this embodiment of the invention, a negative electrode coating 2 (20 layers of negative electrode active material) can be provided on one side surface of the negative electrode current collector 1, or a negative electrode coating 2 can be provided on both opposite sides of the negative electrode current collector 1 in the thickness direction (e.g., ...). Figure 1 As shown, when negative electrode coatings 2 are respectively provided on the surfaces of opposite sides of the negative electrode current collector 1, the negative electrode coating 2 on one side of the surface can be the negative electrode coating 2 with the first coating 21 and the second coating 22, or the negative electrode coatings 2 on the opposite sides of the negative electrode current collector 1 can be the negative electrode coatings 2 with the first coating 21 and the second coating 22 respectively.

[0046] Generally, the first coating 21 also includes a conductive agent and a binder. Based on the total mass of the first coating 21, the mass fraction of the negative electrode active material 20 (i.e., the ratio of the mass of the negative electrode active material 20 to the total mass of the first coating 21) can be 70% to 99%, for example, 70%, 75%, 80%, 85%, 90%, 93%, 95%, 97%, 99%, or any combination thereof. The mass fraction of the conductive agent can be 0.5% to 15%, for example... The mass fraction of the adhesive can be 0.5% to 15%, for example, a range consisting of 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 8%, 10%, 13%, 15%, or any two of these.

[0047] In addition, the second coating 22 also includes a conductive agent and a binder. Based on the total mass of the second coating 22, the mass fraction of the negative electrode active material 20 (i.e., the ratio of the mass of the negative electrode active material 20 to the total mass of the second coating 22) can be 70% to 99%, for example, 70%, 75%, 80%, 85%, 90%, 93%, 95%, 97%, 99%, or any combination thereof. The mass fraction of the conductive agent can be 0.5% to 15%, for example... The range of 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 8%, 10%, 13%, 15%, or any two of these, is acceptable. The mass fraction of the adhesive can be from 0.5% to 15%, for example, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 8%, 10%, 13%, 15%, or any two of these.

[0048] In this embodiment of the invention, the conductive agent can be a conventional conductive material in the art, such as one or more of carbon black, carbon nanotubes (CNTs), acetylene black, graphene, Ketjen black, and carbon fiber.

[0049] In this embodiment of the invention, the adhesive can be a conventional adhesive material in the art. For example, the adhesive may include one or more of the following: sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamide-imide, polyvinyl alcohol, and sodium polyacrylate.

[0050] In this embodiment of the invention, k1 (or k2) can be adjusted by regulating parameters such as the particle size and / or OI value of the negative electrode active material 20 in the first coating 21 (or the second coating 22) to make it meet the above-mentioned preset value. The OI value of the negative electrode active material 20 in the first coating 21 (or the second coating 22) can be controlled by means of magnetic field induction, etc. For example, in the process of preparing the negative electrode sheet by coating method, after the first negative electrode slurry for forming the first coating 21 is coated on the surface of the negative electrode current collector 1, a magnetic field is applied to the first wet film formed, and then dried (baked) (specifically, dried under the above magnetic field conditions) to form the first coating 21. The magnetic field strength, magnetic field induction time and other conditions can be adjusted to make the negative electrode active material 20 particles in the first wet film reach the preset OI value. Then, the second negative electrode slurry for forming the second coating 22 is coated on the first coating 21, a magnetic field is applied to the second wet film formed, and then dried (baked) (specifically, dried under the above magnetic field conditions) to form the second coating 22 on the first coating 21 to obtain the negative electrode sheet. The magnetic field strength, magnetic field induction time and other conditions can be adjusted to make the negative electrode active material 20 particles in the second wet film reach the preset OI value.

[0051] Generally, after drying the second wet film, it is uniformly rolled and then cut to obtain the negative electrode sheet.

[0052] In this embodiment of the invention, a conventional magnetic field application method can be used to apply a magnetic field and adjust the magnetic field strength and magnetic field induction time, so that the negative electrode active material 20 in the first coating 21 and the second coating 22 can reach a preset OI value.

[0053] For example, in specific implementation, after coating the surface of the negative electrode current collector with a wet film (first wet film, second wet film), the negative electrode current collector coated with the wet film is passed sequentially through a magnetic field region with a magnetic field and then through a magnetic field oven. During this process, the negative electrode active material 20 in the wet film is subjected to magnetic field treatment. Simultaneously, as the negative electrode current collector coated with the wet film passes through the magnetic field oven, the wet film is dried to form the corresponding coatings (first coating, second coating), and the negative electrode active material 20 in the coatings reaches a preset OI value. Specifically, a magnet with a maximum magnetic field strength of 900 mT can be used to provide the magnetic field (i.e., the maximum magnetic field strength on the magnet surface is 900 mT, such as TUNKIA). The TD8620 gaussmeter measures the magnetic field strength on the magnet surface. The OI value of the negative electrode active material 20 in the formed coating is controlled by adjusting the magnetic field state, the speed of the negative electrode current collector coated with wet film as it passes through the magnetic field area and the oven (belt speed), drying conditions, etc. The belt speed can be 0.5 to 4 m / s. These control methods are all conventional operations in the field and are not particularly limited.

[0054] In this embodiment of the invention, the processes involved, such as coating, drying, and rolling, are conventional operations for preparing negative electrode sheets using the coating method, and there are no special limitations on them.

[0055] In this embodiment of the invention, the first negative electrode slurry and the second negative electrode slurry can be prepared by conventional methods in the art. For example, taking the preparation process of the first negative electrode slurry as an example, the components used to form the first coating 21, such as the negative electrode active material 20, conductive agent, and binder, can be dispersed in a first solvent. The first solvent includes, for example, deionized water and / or N-methylpyrrolidone (NMP) to prepare the first negative electrode slurry. Then, it is coated on the surface of the negative electrode current collector 1. After magnetic field induction, drying and other processes, the first coating 21 is formed.

[0056] This invention also provides a battery including the above-described negative electrode sheet, which has advantages corresponding to the above-described negative electrode sheet, and will not be described in detail here.

[0057] The battery in this embodiment of the invention can be a lithium-ion battery (such as a lithium-ion power battery), a solar cell, or other novel energy storage battery.

[0058] Generally, a battery includes an electrolyte, a battery cell, and a casing that encapsulates the battery cell. The electrolyte is injected into the battery cell inside the casing. The battery cell includes a positive electrode, a negative electrode, and a separator located between the positive and negative electrode. The battery cell can be a stacked battery cell, meaning it is composed of a positive electrode, a separator, and a negative electrode stacked together.

[0059] Specifically, the positive electrode sheet includes a positive current collector and a positive electrode coating located on at least one side surface of the positive current collector. Specifically, the positive electrode coating may be provided on one side surface of the negative current collector 1, or the positive electrode coating may be provided on both sides of the positive current collector in the thickness direction.

[0060] Specifically, the positive electrode coating (positive electrode active material layer) may include a positive electrode active material, a conductive agent, and a binder, all of which can be conventional materials in the art. For example, the positive electrode active material may include lithium-containing positive electrode active materials used in lithium-ion batteries, such as one or more of lithium iron phosphate (LFP), lithium cobalt oxide, and ternary positive electrode materials. Ternary positive electrode materials may include, for example, nickel-cobalt-manganese ternary materials and / or nickel-cobalt-aluminum ternary materials. The conductive agent may include one or more of carbon black, carbon nanotubes (CNTs), acetylene black, graphene, Ketjen black, and carbon fiber. The binder may include one or more of polyvinylidene fluoride (PVDF), polyvinylidene fluoride, polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, ethylene oxide-containing polymers, polyvinylpyrrolidone, and polyurethane.

[0061] The embodiments of the present invention may employ conventional positive current collectors in the art, for example, positive current collectors may include aluminum foil.

[0062] In this embodiment of the invention, the positive electrode sheet can be prepared by conventional methods in the art, such as by coating. Specifically, the positive electrode active material, conductive agent, binder and other components used to form the positive electrode coating can be dispersed in a second solvent, such as N-methylpyrrolidone (NMP), to prepare a positive electrode slurry, which is then coated on the surface of the positive electrode current collector. After drying, rolling and other processes, the positive electrode sheet is obtained.

[0063] The electrolyte in this embodiment of the invention can be a conventional electrolyte in the art. For example, the electrolyte is a non-aqueous electrolyte, which may specifically include an organic solvent and an electrolyte salt. The organic solvent may include one or more of ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and vinylene carbonate (VC). The electrolyte salt may include lithium salt, such as lithium hexafluorophosphate (LiPF6), but is not limited thereto.

[0064] In this embodiment of the invention, the separator is used to separate the positive electrode and the negative electrode to prevent the positive electrode and the negative electrode from short-circuiting due to contact. Conventional separators in the art can be used in this embodiment of the invention, and there are no special limitations on this.

[0065] In this embodiment of the invention, the battery cell can be packaged using conventional housing materials in the art, such as flexible packaging materials like aluminum-plastic film, but is not limited thereto.

[0066] The embodiments of the present invention can assemble components such as positive electrode, separator and negative electrode into a battery using conventional methods in the art. For example, positive electrode, separator and negative electrode can be stacked to obtain a stacked cell; then the cell is placed in a shell (outer packaging), baked and injected with electrolyte, and then packaged, impregnated and other processes are performed to obtain a battery.

[0067] This invention also provides a battery pack including the battery described above, which has advantages corresponding to the negative electrode sheet described above, and will not be described in detail here.

[0068] Generally, a battery pack includes multiple batteries as individual cells, which are connected to form the battery pack. These batteries can be electrically connected using methods conventional in the art, such as series connection, parallel connection, or a combination of these connection methods, without any particular limitation.

[0069] This invention also provides an electrical device, including the battery or battery pack described above. This electrical device has advantages corresponding to the negative electrode sheet described above, which will not be elaborated further.

[0070] The electrical equipment in the embodiments of the present invention can be conventional electrical equipment in the art, such as power equipment (e.g., electric vehicles, electric cars), electronic equipment (e.g., mobile phones, tablets, laptops, digital cameras, etc.), wearable devices (e.g., watches, bracelets, VR glasses, etc.), energy storage power stations, etc., and there are no particular limitations.

[0071] In this embodiment of the invention, after obtaining the negative electrode sheet, the cross-section of the negative electrode coating 2 (which is basically parallel to the thickness direction of the negative electrode sheet) can be observed by scanning electron microscope (SEM). Based on the particle size of different regions and their orientation in the thickness direction of the negative electrode coating 2, the first coating 21 and the second coating 22 are determined, and the thickness H1 of the first coating 21 and the thickness H2 of the second coating 22 are measured.

[0072] In practice, a sampler can be used to cut the negative electrode sample, and then an ion milling machine can be used to grind and polish the cross section of the negative electrode sample (which is basically parallel to the thickness direction of the negative electrode). Then, the cross section of the negative electrode coating 2 can be observed by SEM, and the thickness H1 of the first coating 21 and the thickness H2 of the second coating 22 can be measured.

[0073] In this embodiment of the invention, X-ray diffraction (XRD) analysis can be performed on each region (such as the first coating 21 and the second coating 22) in the negative electrode coating 2 to measure the OI value (e.g., V) of the negative electrode active material 20 in each region. OI1 V OI2 Specifically, the OI value represents the orientation degree of the negative electrode active material 20 particles in the negative electrode coating 2. The smaller the OI value, the more the negative electrode active material 20 particles in the negative electrode coating 2 tend to be arranged vertically (i.e., the negative electrode active material 20 particles are more parallel to the thickness direction of the negative electrode coating 2). In the XRD analysis results, I1 is the peak intensity (peak area (integrated area) of the characteristic peak in the direction in which active ions (such as lithium ions) are embedded in the crystal structure of the negative electrode active material 20 particles, and I2 is the peak intensity (peak area (integrated area)) of the characteristic peak in the direction in which active ions are more difficult to embed in the crystal structure, which is perpendicular to I1. That is, the direction in which active ions are more difficult to embed in the crystal structure is perpendicular to the direction in which active ions are embedded in the crystal structure of the negative electrode active material 20 particles, so the OI value of the negative electrode active material 20 = I2 / I1. Taking graphite as an example, in the XRD analysis results, I1 is the peak area of ​​the (110) peak of graphite, and I2 is the peak area of ​​the (004) peak of graphite. Then the OI value of graphite in the negative electrode coating 2 is I2 / I1, that is, the OI value of graphite in the negative electrode coating 2 is the ratio of the intensity of the (004) peak to the intensity of the (110) peak of graphite.

[0074] In this embodiment of the invention, after obtaining the negative electrode sheet, the average particle size D50 of the negative electrode active material 20 in each region (first coating 21, second coating 22) of the negative electrode coating 2 can be measured using a laser particle size analyzer. Specifically, a Mastersizer 3000 laser particle size analyzer can be used.

[0075] In practice, the battery can be disassembled to obtain the negative electrode sheet, which is then cleaned with an organic solvent to remove electrolyte salts and other electrolyte components. The negative electrode sheet is then subjected to SEM and XRD analyses to determine the thickness H1 of the first coating 21, the thickness H2 of the second coating 22, and the OI value (V) of the negative electrode active material 20 in the first coating 21. OI1 The OI value (V) of the negative electrode active material 20 in the second coating 22 OI2 Features such as the particle size D50 (D1) of the negative electrode active material 20 in the first coating 21 and the particle size D50 (D2) of the negative electrode active material 20 in the second coating 22.

[0076] In this embodiment of the invention, tortuosity is a parameter describing the pore structure of the negative electrode sheet. The greater the tortuosity, the greater the tortuosity of the negative electrode coating 2, that is, the more tortuous its internal pores are, and the longer the path that active ions take to diffuse from the surface of the negative electrode coating 2 to the negative electrode current collector 1.

[0077] The present invention will be further described below through specific embodiments.

[0078] Example 1

[0079] (1) Graphite, carbon black and SBR are mixed in a mass ratio of 100:4.8:1.1 and deionized water is added to prepare a first negative electrode slurry; the first negative electrode slurry is coated on both the front and back surfaces of the copper foil, a magnetic field is applied to the first wet film formed, and then dried under the same magnetic field conditions so that the graphite in the first coating 21 reaches the preset OI value, and a copper foil with the first coating 21 formed on the surface is obtained (hereinafter referred to as the negative electrode precursor).

[0080] (2) Graphite, carbon black and SBR are mixed in a mass ratio of 100:4.1:0.9 and deionized water is added to prepare a second negative electrode slurry. The second negative electrode slurry is coated on the front and back surfaces of the negative electrode sheet precursor. A magnetic field is applied to the second wet film formed. Then, the film is dried under the same magnetic field conditions so that the graphite in the second coating 22 reaches the preset OI value. After rolling, slitting and other processes, the negative electrode sheet is obtained.

[0081] The structural diagram of the negative electrode is shown below. Figure 1As shown, it includes a negative current collector 1 (copper foil), negative electrode coatings 2 on both the positive and negative surfaces of the negative current collector 1, and each side of the negative electrode coating 2 includes a first coating 21 and a second coating 22 on the side of the first coating 21 away from the negative current collector 1.

[0082] Comparative Examples 1 to 4: The only difference from Example 1 is that in the negative electrode sheet, the negative electrode coating 2 is a single layer (i.e., only the first coating 21 or only the second coating 22), and the graphite OI value (V) of the negative electrode coating 2 is... OI1 、 or V OI2 The conditions such as the ratio of graphite particle size D50 to graphite OI value (K1 or K2) are different, as shown in Table 1. Except for the differences shown in Table 1, the other conditions are the same as in Example 1.

[0083] Examples 5 to 17, Comparative Examples 5 to 9: The difference from Example 1 is that the graphite OI value (V) of the first coating 21 is... OI1 K1 of the first coating 21 and the graphite OI value (V) of the second coating 22 OI2 V OI1 and V OI2 The ratio (V) OI1 / V OI2 The conditions such as K2 of the second coating 22, H1 of the first coating 21, H2 of the second coating 22, and K3 of the ratio of the thickness H1 of the first coating 21 to the thickness H2 of the second coating 22 are different, as shown in Table 1. Except for the differences shown in Table 1, the other conditions are the same as in Example 1.

[0084] The negative electrode sheets of each embodiment and comparative example were subjected to the following performance tests, and the performance test results are shown in Table 2.

[0085] 1. Liquid phase diffusion impedance test

[0086] Liquid phase diffusion impedance is measured using electrochemical impedance spectroscopy (EIS). This involves observing the impedance of the electrode system as a function of the sinusoidal wave frequency when a small-amplitude potential signal of a specific frequency is applied to the electrode at a reference potential. The resulting spectrum is then analyzed and fitted to obtain information on electrode process kinetics and interface structure, further characterizing the internal impedance of the battery. The specific testing process for liquid phase diffusion impedance is as follows:

[0087] (1) Assemble two negative electrode plates and a separator in sequence to form an electrode core (the separator is placed between the two negative electrode plates); place the electrode core in an aluminum-plastic film, bake it, inject electrolyte, and after encapsulation, wetting and other processes, the battery to be tested is obtained; the composition of the electrolyte is as follows: the organic solvent is composed of EC, EMC, DMC, DEC and VC in a mass ratio of 6:6:2:2:1 (EC:EMC:DMC:DEC:VC=6:6:2:2:1), and the concentration of LiPF6 in the electrolyte is 1mol / L;

[0088] (2) At room temperature (25℃), using the CHI Chenhua electrochemical workstation, the IMP-ACImpedance electrochemical testing method was used to perform liquid phase diffusion impedance testing on the battery under test in the frequency range of 300000Hz~0.05Hz (liquid phase diffusion impedance was obtained by processing the real and imaginary parts of the resistance). The results of the liquid phase diffusion impedance test are shown in Table 2.

[0089] 2. Cyclic performance test

[0090] (1) Preparation of positive electrode

[0091] LFP, conductive agents (carbon black and CNTs), and PVDF are mixed in a mass ratio of 100:7.5:2.5, and NMP is added. The mixture is stirred evenly to prepare a positive electrode slurry.

[0092] The positive electrode slurry is coated on both sides of the aluminum foil. After drying and rolling, a positive electrode coating is formed on both sides of the aluminum foil, thus producing a positive electrode sheet.

[0093] (2) Battery assembly

[0094] Positive electrode, separator, and negative electrode are alternately stacked to assemble a stacked cell; the stacked cell is placed in an aluminum-plastic film, and after baking, electrolyte injection, and encapsulation, it is assembled into a soft-pack lithium-ion battery; the composition of the electrolyte is as follows: the organic solvent is composed of EC, EMC, DMC, DEC, and VC in a mass ratio of 6:6:2:2:1 (EC:EMC:DMC:DEC:VC=6:6:2:2:1), and the concentration of LiPF6 in the electrolyte is 1 mol / L.

[0095] (3) Battery cycle performance test

[0096] (1) After letting the battery stand for 30 minutes, charge it to 3.8V with a constant current of 2C, and then charge it to 0.05C with a constant voltage of 3.8V.

[0097] (2) After letting the battery stand for 30 minutes, discharge it to 2.0V at 0.33C;

[0098] (3) Following the charging and discharging process in steps (1) and (2), cycle 500 times. Divide the discharge capacity of the 500th cycle by the discharge capacity of the 1st cycle to calculate the capacity retention rate. The results are shown in Table 2.

[0099] Table 1. Relevant parameters of the negative electrode.

[0100] Example <![CDATA[V OI1 ]]> <![CDATA[k1]]> <![CDATA[V OI2 ]]> <![CDATA[k2]]> <![CDATA[V OI1 and V OI2 The ratio <![CDATA[H1(μm)]]> <![CDATA[H2(μm)]]> <![CDATA[k3]]> Example 1 35.4 0.25 0.7 25.71 50.57 55 55 1.000 Example 2 35.4 0.25 0.7 21.43 50.57 55 55 1.000 Example 3 35.4 0.25 0.7 18.57 50.57 55 55 1.000 Example 4 35.4 0.34 0.7 25.71 50.57 55 55 1.000 Example 5 35.4 0.42 0.7 25.71 50.57 55 55 1.000 Example 6 35.4 0.51 0.7 25.71 50.57 55 55 1.000 Example 7 35.4 0.65 0.7 25.71 50.57 55 55 1.000 Example 8 35.4 0.25 0.7 25.71 50.57 14 96 0.146 Example 9 35.4 0.25 0.7 25.71 50.57 27 83 0.325 Example 10 35.4 0.25 0.7 25.71 50.57 43 67 0.642 Example 11 35.4 0.25 0.7 25.71 50.57 68 42 1.619 Example 12 35.4 0.25 0.7 25.71 50.57 79 31 2.548 Example 13 35.4 0.25 0.7 25.71 50.57 93 17 5.471 Example 14 28.9 0.25 0.7 25.71 41.29 55 55 1.000 Example 15 21.3 0.25 0.7 25.71 30.43 55 55 1.000 Example 16 35.4 0.25 11.6 0.78 3.05 55 55 1.000 Example 17 35.4 0.25 5.7 1.53 6.21 55 55 1.000 Comparative Example 1 35.4 0.25 - - - 110 - - Comparative Example 2 35.4 0.51 - - - 110 - - Comparative Example 3 23.5 0.77 - - - 110 - - Comparative Example 4 - - 0.7 25.71 - - 110 - Comparative Example 5 0.7 25.71 35.4 0.25 0.02 55 55 1.000 Comparative Example 6 35.4 0.17 0.7 25.71 50.57 55 55 1.000 Comparative Example 7 35.4 0.68 0.7 25.71 50.57 55 55 1.000 Comparative Example 8 35.4 0.25 0.7 28.57 50.57 55 55 1.000 Comparative Example 9 35.4 0.25 0.7 14.29 50.57 55 55 1.000

[0101] Table 2 Performance Test Results

[0102] Example Liquid phase diffusion resistance (Ω) Capacity retention rate (%) Example 1 2.55 90.7 Example 2 2.61 89.7 Example 3 2.68 88.2 Example 4 2.53 88.3 Example 5 2.49 86.8 Example 6 2.47 85.9 Example 7 2.44 85.1 Example 8 2.38 88.2 Example 9 2.42 92.6 Example 10 2.47 93.4 Example 11 2.67 88.9 Example 12 2.82 87.1 Example 13 3.06 86.6 Example 14 2.63 88.9 Example 15 2.71 87.9 Example 16 2.81 86.2 Example 17 2.77 85.7 Comparative Example 1 3.52 81.3 Comparative Example 2 3.46 78.5 Comparative Example 3 3.22 75.9 Comparative Example 4 2.66 79.2 Comparative Example 5 3.41 79.1 Comparative Example 6 2.79 81.8 Comparative Example 7 2.43 82.7 Comparative Example 8 2.74 83.5 Comparative Example 9 2.83 85.3

[0103] As can be seen from Comparative Examples 1 to 4, when the negative electrode is coated with a single layer, it is difficult to simultaneously reduce the liquid phase diffusion resistance of the negative electrode and improve the battery capacity retention rate. Specifically, in Comparative Examples 1 to 3, the OI value (V) of graphite in the negative electrode coating is... OI1 The negative electrode has a relatively large OI value (VI) and a high liquid phase diffusion resistance (not less than 3.22Ω), while its capacity retention is low (not more than 81.3%). In Comparative Example 4, the OI value of graphite in the negative electrode coating is relatively high. OI2 The resistance of the negative electrode was reduced to 0.7, which could reduce the liquid phase diffusion resistance of the negative electrode to some extent, but the cycle life of the battery was severely deteriorated (capacity retention rate was less than 80%).

[0104] Compared to Comparative Examples 1 to 4, the negative electrode sheets in Examples 1 to 17 are coated with two layers, and the first coating layer (lower layer) is controlled to satisfy 0.21≤k1≤0.65 and the second coating layer (upper layer) is controlled to satisfy 18≤k2≤26. This is beneficial for balancing the liquid phase diffusion resistance and cycle life of the negative electrode sheet, and for maintaining a low liquid phase diffusion resistance (not higher than 3.16Ω) and a high cycle life of the battery (capacity retention rate not lower than 85%).

[0105] Furthermore, as can be seen from Example 1 and Comparative Example 5, the relationship between K1 of the first coating and K2 of the second coating affects the liquid phase diffusion resistance of the negative electrode and the cycle life of the battery. Specifically, although Comparative Example 5 performed double coating and kept the k value of one layer in the range of 0.21 to 0.65 and the k value of the other layer in the range of 18 to 26, in the negative electrode of Comparative Example 5, the K1 of the lower layer (first coating) is greater than the K2 of the upper layer (second coating) (equivalent to using the second coating in Example 1 as the lower layer of Comparative Example 5). The first coating in Example 1 (used as the upper layer of Comparative Example 5, the second coating) has a significantly higher liquid phase diffusion resistance than the negative electrode in Example 1, and a significantly lower battery cycle capacity retention rate than the battery cycle capacity retention rate in Example 1. Therefore, compared to Comparative Example 5, Example 1 can significantly reduce the liquid phase diffusion resistance of the negative electrode and improve the battery cycle life by controlling the first coating (lower layer) to satisfy 0.21≤k1≤0.65 and the second coating (upper layer) to satisfy 18≤k2≤26.

[0106] Furthermore, as can be seen from Examples 1, 4-7, Comparative Examples 6 and 7, both excessively small (Comparative Example 6) and excessively large (Comparative Example 7) K1 of the first coating (lower layer) in the negative electrode sheet will affect the liquid phase diffusion resistance of the negative electrode sheet and the battery cycle life. Specifically, in Comparative Example 6, the first coating K1 < 0.21, the liquid phase diffusion resistance of the negative electrode sheet is high (2.79Ω), and the battery cycle capacity retention rate is low (81.8%). In Comparative Example 7, the first coating K1 > 0.65, although it can reduce the liquid phase diffusion resistance of the negative electrode sheet to a certain extent, the battery cycle life is still severely deteriorated (capacity retention rate is 82.7%). Compared with Comparative Examples 6 and 7, Examples 1 and 4-7, by controlling the K1 of the first coating in the range of 0.21≤k1≤0.65, can significantly improve the battery cycle life (capacity retention rate is not less than 85.1%), while maintaining a low liquid phase diffusion resistance of the negative electrode sheet (not higher than 2.55Ω).

[0107] Furthermore, compared to Comparative Example 8 (K2 > 26) and Comparative Example 9 (K2 < 18), Examples 1 to 3, by controlling the k2 of the second coating (upper layer) within the range of 18 ≤ k2 ≤ 26, were able to reduce the liquid phase diffusion resistance of the negative electrode and improve the cycle capacity retention of the battery.

[0108] Furthermore, as can be seen from Examples 8 to 13, compared with Example 13, Examples 8 to 12 further control the ratio K3 of the thickness H1 of the first coating (lower layer) to the thickness H2 of the second coating (upper layer) in the negative electrode sheet within the range of 0.14 to 2.6, which is beneficial to further reduce the liquid phase diffusion resistance of the negative electrode sheet and improve the cycle life of the battery. In particular, Examples 9 and 10 further control K3 within the range of 0.3 to 1, resulting in lower liquid phase diffusion resistance of the negative electrode sheet and higher cycle capacity retention of the battery.

[0109] Furthermore, as can be seen from Examples 1 to 3 and Examples 14 to 17, compared to Examples 16 and 17, Examples 1 to 3, Examples 14 and 15 further control the graphite OI value (V) of the first coating 21 in the negative electrode sheet. OI1 ) and the graphite OI value (V) of the second coating 22 OI2 The ratio (V) OI1 / V OI2 Within the range of 30 to 52, it is beneficial to further reduce the liquid phase diffusion resistance of the negative electrode and improve the cycle capacity retention rate of the battery.

[0110] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A negative electrode sheet, characterized in that, The device includes a negative electrode current collector and a negative electrode coating located on at least one side surface of the negative electrode current collector. The negative electrode coating includes a first coating and a second coating located on the side surface of the first coating away from the negative electrode current collector. Both the first coating and the second coating include a negative electrode active material. The first coating satisfies 0.21≤k1≤0.65, where k1 is the ratio of the particle size D50 of the negative electrode active material in the first coating to the OI value, and the unit of the particle size D50 of the negative electrode active material in the first coating is μm. The second coating satisfies 18≤k2≤26, where k2 is the ratio of the particle size D50 of the negative electrode active material in the second coating to the OI value, and the unit of the particle size D50 of the negative electrode active material in the second coating is μm. The ratio k3 of the thickness of the first coating to the thickness of the second coating satisfies 0.14≤k3≤5.

47.

2. The negative electrode sheet according to claim 1, characterized in that, 0.21≤k1≤0.45。 3. The negative electrode sheet according to claim 1, characterized in that, The ratio of the OI value of the negative electrode active material in the first coating to the OI value of the negative electrode active material in the second coating is 2 to 52.

4. The negative electrode sheet according to claim 3, characterized in that, The ratio of the OI value of the negative electrode active material in the first coating to the OI value of the negative electrode active material in the second coating is 30 to 52.

5. The negative electrode sheet according to claim 1, characterized in that, The OI value of the negative electrode active material in the first coating is greater than or equal to 21.

6. The negative electrode sheet according to claim 1, characterized in that, The OI value of the negative electrode active material in the second coating is less than or equal to 12.

7. The negative electrode sheet according to claim 1, characterized in that, The particle size D50 of the negative electrode active material in the first coating is 6~23 μm.

8. The negative electrode sheet according to claim 1, characterized in that, The particle size D50 of the negative electrode active material in the second coating is 9~22μm.

9. The negative electrode sheet according to claim 1, characterized in that, The ratio k3 of the thickness of the first coating to the thickness of the second coating satisfies 0.14≤k3≤2.

6.

10. The negative electrode sheet according to claim 7, characterized in that, 0.3≤k3≤1。 11. The negative electrode sheet according to claim 1 or 9, characterized in that, The thickness of the first coating is 14~96μm.

12. The negative electrode sheet according to claim 11, characterized in that, The thickness of the first coating is 25~70μm.

13. The negative electrode sheet according to claim 1 or 7, characterized in that, The thickness of the second coating is 14~96μm.

14. The negative electrode sheet according to claim 13, characterized in that, The thickness of the second coating is 40~85μm.

15. The negative electrode sheet according to any one of claims 1-10, 12, and 14, characterized in that, The negative electrode active material in the first coating includes graphite, and the negative electrode active material in the second coating includes graphite.

16. The negative electrode sheet according to claim 11, characterized in that, The negative electrode active material in the first coating includes graphite, and the negative electrode active material in the second coating includes graphite.

17. The negative electrode sheet according to claim 13, characterized in that, The negative electrode active material in the first coating includes graphite, and the negative electrode active material in the second coating includes graphite.

18. A battery, characterized in that, Includes the negative electrode sheet as described in any one of claims 1-17.

19. A battery pack, characterized in that, Includes the battery as described in claim 18.

20. An electrical appliance, characterized in that, Includes the battery of claim 18 or the battery pack of claim 19.

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