Pole piece and battery

Abstract

The invention relates to the technical field of lithium ion batteries, and provides a pole piece and a battery. The negative plate comprises a negative current collector and a negative active material layer located on at least one side surface of the negative current collector, and the negative active material layer comprises a negative active material and a filler A; the filler A comprises one or two of filler particles and a high polymer material; wherein the Mohs hardness of the filler particles is recorded as a, the Mohs hardness of the negative electrode active material is recorded as b, and a and b meet the condition that a-b is greater than or equal to 3. The filler particles and / or the high polymer material are mixed in the negative active material layer, so that the overall strength of the negative plate can be improved, the battery can bear higher extrusion force without failure, and the safety of the battery is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and particularly relates to an electrode sheet and a battery. Background Art

[0002] In modern life and production, electric energy is indispensable. Lithium-ion batteries are widely used in the fields of electronic devices, electric vehicles, and energy storage due to their advantages such as long life, high energy density, high voltage platform, and no memory effect. With the diversification of application scenarios, lithium-ion batteries face higher requirements such as faster charging speed and thinner and lighter design. However, the thinner and lighter design leads to a decrease in the compressive strength of the battery, affecting the safety performance. To enhance the compressive strength, existing technologies such as hot pressing treatment, coating the separator with polyvinylidene fluoride, or using a steel shell for encapsulation can improve the hardness, but are accompanied by problems such as capacity loss, decline in electrochemical performance, and high encapsulation difficulty. Summary of the Invention

[0003] The purpose of the present invention is to overcome the above problems existing in the prior art, and provide a negative electrode sheet, in which one or two of filler particles and a polymer material are admixed in the negative electrode active material layer; by controlling the difference in Mohs hardness between the filler particles and the negative electrode active material, and / or forming a crosslinked network between the polymer material and the negative electrode active material, the overall strength of the negative electrode sheet can be improved, so that the battery can withstand a higher extrusion force without failure, and the safety of the battery is improved.

[0004] To achieve the above purpose, in the first aspect of the present invention, a negative electrode sheet is provided. The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer located on at least one surface of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material and filler A; the filler A includes one or two of filler particles and a polymer material; wherein, the Mohs hardness of the filler particles is denoted as a, and the Mohs hardness of the negative electrode active material is denoted as b, and a and b satisfy: a - b ≥ 3.

[0005] In the second aspect of the present invention, a lithium-ion battery is provided. The lithium-ion battery includes the negative electrode sheet provided in the first aspect of the present invention, a positive electrode sheet, and a separator; the yield stress of the lithium-ion battery ≥ 350 N.

[0006] The present invention adopts the above technical solutions and has the following beneficial effects:

[0007] (1) For the negative electrode sheet provided by the present invention, filler particles can be admixed in the negative electrode active material layer. By controlling the difference in Mohs hardness between the filler particles and the negative electrode active material ≥ 3, the compressive strength of the negative electrode sheet can be effectively improved, and the safety performance of the battery is improved;

[0008] (2) The negative electrode sheet provided by the present invention can incorporate a polymer material into the negative electrode active material layer. The polymer material can form a cross-linked network with the negative electrode active material, improving the structural stability of the negative electrode sheet and enhancing the safety performance of the battery.

[0009] (3) The negative electrode sheet provided by the present invention can also incorporate filler particles and a polymer material into the negative electrode active material layer simultaneously. The combined action of the two can further improve the compressive strength and crack resistance of the negative electrode sheet, thereby increasing the threshold value of the battery's resistance to external extrusion and achieving the effect of enhancing the safety performance of the lithium-ion battery.

[0010] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein. In this article, unless otherwise specified, data ranges include endpoints. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 The figure shows a schematic structural diagram of a negative electrode sheet in an example of the present invention.

[0012] Figure 2 The figure shows a schematic structural diagram of a negative electrode sheet in another example of the present invention.

[0013] Figure 3 The figure shows a schematic structural diagram of a negative electrode sheet in yet another example of the present invention.

[0014] Reference numerals: 1 - negative electrode active material; 2 - negative electrode current collector; 3 - filler particles; 4 - polymer material. DETAILED DESCRIPTION OF THE INVENTION

[0015] The following provides a detailed description of the specific embodiments of the present invention. It should be understood that the specific embodiments described herein are only for the purpose of illustrating and explaining the present invention and are not intended to limit the present invention.

[0016] Unless otherwise defined, all scientific and technical terms used in the present invention have the same meaning as commonly understood by those skilled in the technical field to which the present invention pertains.

[0017] In the present invention, the terms "battery", "lithium battery", "lithium-ion battery", and "lithium-ion secondary battery" all have the same meaning, referring to a lithium-ion secondary battery, which generally includes an electrode assembly (such as a positive electrode sheet, a negative electrode sheet, and a separator), a container (housing) for accommodating the electrode assembly, and an electrolyte.

[0018] In the present invention, the term "Mohs hardness" is a test standard for hardness, which is divided into 10 grades according to the magnitude of hardness.

[0019] In the present invention, the term "ultimate compressive strength" refers to the maximum pressure value that a material or structure can withstand when subjected to pressure.

[0020] In the present invention, the term "yield stress" refers to the stress value corresponding to the onset of significant plastic deformation when a material is subjected to an external force. Before the stress applied to the material reaches the yield stress, the material mainly undergoes elastic deformation, that is, after the external force is removed, the material can return to its original shape and size; while when the stress reaches the yield stress, the material begins to enter the plastic deformation stage, and at this time, even if the external force is removed, the material cannot completely return to its initial state and will leave permanent deformation.

[0021] In a first aspect of the present invention, a negative electrode sheet is provided. The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer located on at least one surface of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material and filler A; the filler A includes one or both of filler particles and a polymer material; wherein, the Mohs hardness of the filler particles is denoted as a, and the Mohs hardness of the negative electrode active material is denoted as b, and a and b satisfy: a - b ≥ 3.

[0022] In the present invention, by introducing filler particles and / or polymer materials into the negative electrode active material layer, the filler particles have a higher Mohs hardness grade relative to the negative electrode active material. By introducing filler particles into the negative electrode active material layer, the high-hardness filler particles can fill the pores between the negative electrode active materials, making the structure of the negative electrode active material layer more dense, effectively enhancing the structural stability of the negative electrode active material layer, and thus improving the strength of the negative electrode sheet; introducing a polymer material into the negative electrode active material layer can construct a cross-linked network structure with the negative electrode active material, which not only strengthens the connection stability between the negative electrode active materials, but also can effectively transfer and disperse the internal stress generated when the negative electrode sheet is squeezed, avoiding the fracture of the negative electrode current collector, thereby significantly improving the strength of the negative electrode sheet. When the filler particles and the polymer material are simultaneously blended into the negative electrode active material layer, the two cooperate to further improve the compressive resistance and crack resistance of the negative electrode sheet, increase the threshold of the battery's resistance to external extrusion, and achieve the effect of further improving the safety performance of the lithium-ion battery.

[0023] In the present invention, the Mohs hardness of the filler particles is denoted as a, and the Mohs hardness of the negative electrode active material is denoted as b, and a and b satisfy: a - b ≥ 3. Controlling the difference between the Mohs hardness of the filler particles and the Mohs hardness of the negative electrode active material to be ≥ 3 can avoid the situation where when a - b < 3, the Mohs hardness of the filler particles may be relatively small, and it cannot fully play the role of stabilizing the structure of the negative electrode active material layer, resulting in an insignificant improvement in the strength of the negative electrode sheet; on the other hand, since the difference between the Mohs hardness of the filler particles and the Mohs hardness of the negative electrode active material is too small, the difference in mechanical properties between the filler particles and the negative electrode active material is not significant. When the negative electrode sheet is subjected to an external force, the filler particles cannot significantly improve the compressive resistance compared with the negative electrode active material, and thus it is also difficult to improve the overall compressive and crack resistance of the negative electrode sheet.

[0024] In some embodiments, the Mohs hardness a of the filler particles satisfies: 4 ≤ a ≤ 10; for example, it can be 4, 5, 6, 7, 8, 9, 10.

[0025] Exemplarily, the Mohs hardness of the filler particles and the negative electrode active material can be obtained by the standard mineral scratching method, which specifically may include the following steps: Prepare standard minerals: Prepare a set of Mohs hardness standard minerals, including talc (hardness 1), gypsum (hardness 2), calcite (hardness 3), fluorite (hardness 4), apatite (hardness 5), orthoclase (hardness 6), quartz (hardness 7), topaz (hardness 8), corundum (hardness 9), diamond (hardness 10). Conduct scratching tests: Use the standard minerals to scratch the tested samples in turn, starting from the minerals with lower hardness and gradually moving to the minerals with higher hardness. Apply appropriate and uniform pressure during scratching to make the mineral fully contact the surface of the tested sample and produce scratches. Observe the scratch situation: After scratching, observe whether there are scratches on the surface of the tested sample. If the surface of the tested sample is scratched by a standard mineral of a certain hardness and cannot be scratched by a mineral of a lower hardness level, then the Mohs hardness of the tested sample is between the hardnesses of these two minerals. For example, if the tested sample can be scratched by calcite but not by gypsum, then its Mohs hardness is between 2 and 3.

[0026] In some embodiments, as Figure 1 shown, in the present invention, filler particles 3 can be separately admixed in the negative electrode active material layer. The filler particles 3 have a relatively high Mohs hardness and are filled in the gaps between the negative electrode active materials 1, which can improve the overall structural stability of the negative electrode sheet.

[0027] In some embodiments, as Figure 2 shown, in the present invention, a polymer material 4 can also be separately admixed in the negative electrode active material layer. The polymer material 4 can form a cross-linked network structure with the negative electrode active material 1 under certain conditions (such as heating or light irradiation), stabilize the structure of the negative electrode sheet, and improve the battery safety.

[0028] In some embodiments, such as Figure 3 shown, in the present invention, filler particles 3 and polymer materials 4 can also be simultaneously blended in the negative electrode active material layer. The filler particles 3 and the polymer materials 4 cooperate with each other to further improve the compressive strength and crack resistance of the negative electrode sheet, increase the threshold value of the battery against external force extrusion, and achieve the effect of further improving the safety performance of the lithium-ion battery.

[0029] In some embodiments, the Dv50 particle size of the filler particles is 0.1 μm - 10 μm, for example, it can be 0.1 μm, 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, or any value within the range composed of any two of the above point values. Preferably, it is 0.5 μm - 5 μm. When the Dv50 particle size of the filler particles is within the above range, the filler particles can better adapt to the gap size between the negative electrode active materials, avoiding the situation that the filler particles are too large to be filled in the gaps, and also avoiding the situation that the filler particles are too small to form effective support and filling in the gaps. As a result, the negative electrode active material layer becomes denser, enhancing the structural stability of the negative electrode sheet and improving the safety of the battery.

[0030] In some embodiments, the Dv50 particle size of the filler particles is 0.5 μm - 5 μm. The inventors of the present invention have found that when the Dv50 particle size of the filler particles is within this range, the filler particles can be more evenly distributed in the negative electrode active material layer. The filler particles are filled between the negative electrode active materials to form lithium-ion transmission channels with appropriate sizes and relatively orderly arrangements. These channels provide a smoother transmission path for lithium ions, enabling lithium ions to be embedded and extracted in the negative electrode sheet more quickly during charge and discharge, reducing the resistance and diffusion distance of lithium-ion transmission, and further improving the charge and discharge rate of the battery while taking into account the improvement of the battery safety.

[0031] Furthermore, when there are polymer materials in the negative electrode active material layer at the same time, with the Dv50 particle size of the filler particles being 0.5 μm - 5 μm, a good synergistic effect can be formed with the negative electrode active materials. In the cross-linked network structure constructed by the polymer materials, the filler particles can act as physical cross-linking points or reinforcing phases to further improve the stability and strength of the cross-linked network, thereby better enhancing the overall mechanical properties of the negative electrode sheet and further improving the safety of the battery.

[0032] Exemplarily, the Dv50 particle size of the filler particles can be tested using a laser particle size analyzer.

[0033] In some embodiments, the polymer material includes one or both of a thermosetting polymer material and a photocuring polymer material.

[0034] In some embodiments, the thermosetting polymer material includes one or both of polyacrylic acid and polymethacrylic acid.

[0035] In some embodiments, the photocuring polymer material includes at least one of acrylate resins, epoxy resins, phenolic resins, and silicone resins.

[0036] In some embodiments, the acrylate resin includes one or both of epoxy acrylate resin and polyurethane acrylate resin.

[0037] When there is a thermosetting polymer material in the polymer material, active groups in the thermosetting polymer material, such as carboxyl groups and hydroxyl groups, can react with surface functional groups or atoms of the negative electrode active material when heated. For example, carboxyl groups and hydroxyl groups can esterify to form ester bonds, and amino groups and carbonyl groups can condense to form amide bonds, forming a crosslinked network structure, improving the stability of the negative electrode sheet and the safety of the battery.

[0038] When there is a photocuring polymer material in the polymer material, the photocuring polymer material further includes a photoinitiator. When the photoinitiator is irradiated with light, free radicals are generated, and these free radicals will initiate the polymerization reaction of the unsaturated double bonds on the molecular chain of the photocuring polymer material; at the same time, active intermediates are generated during the photocuring process, and these active intermediates can react with the functional groups or atoms (such as hydroxyl groups) on the surface of the negative electrode active material, thereby realizing the crosslinking of the polymer material and the negative electrode active material, thus improving the stability of the negative electrode sheet and the safety of the battery.

[0039] In the present invention, the type of the photoinitiator is not specifically limited, and the types of conventional photoinitiators in the art are all within the protection scope of the present invention. The photoinitiator can be, for example, photoinitiator BP (benzophenone), benzophenone, acetophenone, diphenyliodonium, or triphenylmethane.

[0040] In some embodiments, based on the total mass of the negative electrode active material layer, the mass ratio of filler A is 0.1%-5%, and can be, for example, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5% or any value within the range composed of any two of the above values, preferably 0.3%-3%. When the mass ratio of filler A satisfies the above range, it can be avoided that when the mass ratio of filler A is too small, the content of filler particles and / or polymer material in the negative electrode active material layer is too small, and the stability of the negative electrode sheet cannot be effectively improved, and the improvement effect on the battery safety performance is not obvious; it can be avoided that when the mass ratio of filler A is too large, the network crosslinking density formed by excessive filler particle filling and / or polymer material may occupy the channels for lithium ion transmission, or shield the electrochemically active sites of the negative electrode active material, hindering the transmission of lithium ions in the negative electrode active material layer, resulting in a decrease in the charge and discharge performance of the battery; it will also reduce the proportion of the negative electrode active material and affect the energy density of the battery.

[0041] It should be noted that in the present invention, based on the total mass of the negative electrode active material layer, the mass ratio of filler A refers to the ratio of the addition amount of filler A (filler particles and / or polymer material) to the total mass of the negative electrode active material layer slurry, that is, the total mass of the negative electrode active material layer slurry does not include the mass of the negative electrode current collector. For example, when preparing the negative electrode active material layer slurry, if 1 g of filler A is added and the total mass of the negative electrode active material layer slurry is 100 g, then the mass ratio of filler A is 1%.

[0042] In some embodiments, based on the total mass of the negative electrode active material layer, the ratio of the mass ratio of the filler particles to the mass ratio of the polymer material is 0.4 - 9, and can be, for example, 0.4, 0.43, 0.5, 0.8, 1, 2, 3, 4, 5, 6, 7, 8, 9 or any value composed of any two of the above point values. When the filler particles and the polymer material are simultaneously blended in the negative electrode active material layer, the two cooperate to further improve the compressive resistance and crack resistance of the negative electrode sheet, and increase the threshold of the battery's resistance to external force extrusion, achieving the effect of further improving the safety performance of the lithium-ion battery.

[0043] In some embodiments, the filler particles include at least one of carbides, nitrides, borides, and inorganic oxides.

[0044] In some embodiments, the carbide includes at least one of boron carbide, silicon carbide, tungsten carbide, titanium carbide, zirconium carbide, or their modified products.

[0045] In some embodiments, the nitride includes at least one of cubic boron nitride, silicon nitride, titanium nitride, aluminum nitride, or their modified products.

[0046] In some embodiments, the boride includes at least one of titanium boride, zirconium boride, aluminum boride, or their modified products.

[0047] In some embodiments, the inorganic oxide includes at least one of zirconia, titania, alumina, silica, magnesia, zinc oxide, or their modified products.

[0048] The above-mentioned "modified product" can be understood as a substance obtained by changing the structure, properties or surface properties of the original carbide, nitride, boride or inorganic oxide through physical or chemical methods. For example, physical modification such as surface coating can be carried out. For example, a carbon material is coated on the surface of the filler particles, which can change the surface properties of the filler particles, enhance the compatibility with the anode active material or polymer material, reduce the agglomeration phenomenon, and improve the dispersion uniformity of the filler particles in the anode active material layer, etc.; or chemical modification such as element doping and functional group modification can be carried out. For example, a small amount of other elements are doped into the crystal lattice of the filler particles. Element doping can change the electronic structure of the material, improve the conductivity and chemical stability of the filler particles, and thus improve the charge and discharge performance of the battery, etc.

[0049] In the present invention, the mass percentage of the filler particles in the anode active material layer can be characterized by element M, and then the mass percentage of the filler particles in the anode active material layer can be obtained. Element M is an element existing in the filler particles and can be distinguished from other components in the anode active material layer. For example, when the filler particles are alumina, element M is aluminum. The test method for the mass percentage of the filler particles in the anode active material layer may specifically include the following steps: after the battery is discharged, the negative electrode sheet is disassembled and taken out, and the negative electrode sheet is washed with dimethyl carbonate (DMC) and then dried. The negative electrode material is scraped, and the content of element M is measured using an X-ray fluorescence analyzer, and the amount of substance containing element M is calculated according to the chemical formula to obtain the mass of the filler, and then divided by the mass of the tested sample to obtain the mass percentage of the filler particles.

[0050] In some embodiments, the anode active material layer includes a first active material layer and a second active material layer, and the second active material layer is located between the first active material layer and the anode current collector; the first active material layer and the second active material layer each independently include an anode active material and filler A; based on the total mass of the first active material layer, the mass percentage of filler A is n%; based on the total mass of the second active material layer, the mass percentage of filler A is m%; n and m satisfy 0 ≤ m - n ≤ 4. The damage process of the negative electrode sheet under stress extrusion usually starts from the deformation of the anode active material layer, and then drives the anode current collector to collapse and break together. In the present invention, a double-layer anode active material layer can also be added on at least one surface of the anode current collector, and by controlling the mass percentage of filler A in the second active material layer close to the anode current collector to be greater than or equal to the mass percentage of filler A in the first active material layer, the compressive capacity of the second active material layer is relatively high or at a comparable level with the compressive capacity of the first active material layer. When the first active material layer is deformed and damaged, the second active material layer can also serve as an effective "buffer barrier" to buffer the extrusion stress, reduce the risk of fracture of the anode current collector to a certain extent, avoid internal short circuit of the battery caused by the fracture of the anode current collector, and improve the safety of the battery.

[0051] Meanwhile, controlling n and m to satisfy the range of 0 ≤ m - n ≤ 4 can avoid the situation where when m - n > 4, the mass ratio of filler A in the first active material layer and the second active material layer varies too much, which may lead to poor interfacial bonding performance between the two layers, and may also cause stress concentration, affecting the integrity and structural stability of the entire negative electrode sheet; it may also lead to a difference in the lithium ion transport rate between the first active material layer and the second active material layer. The transport resistance of lithium ions in the second active material layer is relatively large, which will not only reduce the charge-discharge efficiency of the battery, but may also cause non-uniform reactions inside the battery, having an adverse impact on the cycle performance and safety of the battery.

[0052] In some embodiments, based on the total mass of the first active material layer, the mass ratio (n%) of the filler A satisfies: 0 ≤ n ≤ 3, and the mass ratio can be, for example, 0%, 1%, 2%, 3% or any value within the range composed of any two of the above values.

[0053] In some embodiments, based on the total mass of the second active material layer, the mass ratio (m%) of the filler A satisfies: 0.2 ≤ m ≤ 7, and the mass ratio can be, for example, 0.2%, 1%, 2%, 3%, 4%, 5%, 6%, 7% or any value within the range composed of any two of the above values. Further, the mass ratio of the filler A is preferably 2% - 5%.

[0054] Further controlling the mass ratio of the filler A in the first active material layer and the second active material layer to satisfy the above range can further improve the overall stability of the negative electrode sheet while avoiding adverse effects on the charge-discharge performance of the battery.

[0055] It should be noted that in the present invention, based on the total mass of the first active material layer / second active material layer, the mass ratio of the filler A refers to the ratio of the addition amount of the filler A (filler particles and / or polymer material) to the total mass of the slurry of the first active material layer / second active material layer, that is, the total mass of the slurry of the first active material layer / second active material layer does not include the mass of the negative electrode current collector. For example, when preparing the slurry of the first active material layer, if 0.5 g of the filler A is added and the total mass of the slurry of the first active material layer is 50 g, then the mass ratio of the filler A is 1%.

[0056] In some embodiments, the negative electrode active material includes at least one of graphite, hard carbon, soft carbon, activated carbon, graphene, carbon nanotubes, silicon carbon, lithium titanate, boron nitride or their modified products.

[0057] In some embodiments, the ultimate compressive strength of the negative electrode sheet is 1.5 KN - 2.5 KN, and for example, it can be 1.5 KN, 2 KN, 2.5 KN, or any value within the range composed of any two of the above values. By blending filler particles and / or polymer materials in the negative electrode active material layer in the present invention, the ultimate compressive strength of the negative electrode sheet can reach 1.5 KN - 2.5 KN, effectively improving the compressive resistance and crack resistance of the negative electrode sheet, and further improving the safety performance of the battery.

[0058] Exemplarily, the method for testing the ultimate compressive strength of the electrode sheet includes the following steps: Discharge the battery until it is completely discharged, disassemble the positive and negative electrode sheets from the battery respectively, fold the positive electrode sheet or the negative electrode sheet alone into 20 layers and compact them. Keep the temperature of the test environment at 25 ± 2 °C. Align a steel needle with a diameter of 6 mm (the top is a hemisphere with a radius of 3 mm) with the middle position of the stacked electrode sheet, and press it down at a speed of 5 N / sec. Stop the test after the pressure no longer rises; Record the force value and displacement at the critical point of electrode sheet rupture, and monitor the curve of force value changing with displacement and the original data.

[0059] In the second aspect of the present invention, a lithium-ion battery is provided. The lithium-ion battery includes the negative electrode sheet, positive electrode sheet, and separator provided in the first aspect of the present invention; the yield stress of the lithium-ion battery ≥ 350 N. For a battery including the negative electrode sheet, the threshold for resisting external force extrusion is higher. The yield stress of the lithium-ion battery ≥ 350 N, and the lithium-ion battery has higher safety performance.

[0060] Exemplarily, the method for testing the yield stress includes the following steps: Discharge the battery at 0.2 C to 3.0 V, then charge it at 0.5 C to 3.88 V. The temperature of the test environment is 25 ± 2 °C. Set the descending speed of the equipment to 10 mm / min, adjust the span of the fixture base to 0.7 times the length of the battery, press down on the long side of the battery, with a downward displacement of 2 mm, measure the force during the downward pressing process, and the maximum force value recorded is the yield stress.

[0061] Next, the technical solutions in the embodiments of the present invention will be described clearly and completely in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the scope of protection of the present invention.

[0062] The materials, reagents, etc. used in the following embodiments can be obtained from commercial channels without special instructions.

[0063] The present invention will be described in detail below in conjunction with specific embodiments, and these embodiments are for understanding rather than limiting the present invention.

[0064] Example 1 - 1

[0065] Step 1: Prepare the slurry for the positive electrode active material layer. The composition of the active material layer slurry is lithium cobalt oxide (LCO): conductive carbon black (SP): polyvinylidene fluoride (PVDF) = 98:1:1. The solvent used is N-methylpyrrolidone (NMP). After stirring and mixing evenly, the slurry is coated on the surface of the current collector, dried, rolled, and slit to obtain the positive electrode sheet.

[0066] Step 2: Prepare the slurry for the negative electrode active material layer. The composition of the negative electrode active material layer slurry is graphite: boron carbide (filler particles): styrene-butadiene rubber (SBR): carboxymethyl cellulose (CMC) = 95:2:1.5:1.5. The solvent used is deionized water. After stirring and mixing evenly, the slurry is coated on the surface of the current collector, dried, rolled, and slit to obtain the negative electrode sheet.

[0067] Step 3: Stack and wind the positive electrode sheet, separator, and negative electrode sheet in sequence to obtain a wound core.

[0068] Use a stamping die to stamp the aluminum-plastic film, and then use the stamped aluminum-plastic film to encapsulate the wound core to obtain an electric core. Bake it until the moisture is qualified, and inject the electrolyte; use a lithium-ion battery formation device to charge and discharge the electric core to harden the electric core and sort out the capacity of the electric core; perform secondary sealing on the electric core and fold the edges to form the electric core. After OCV testing, measure the K value of the battery, and select the products with qualified K values to obtain lithium-ion batteries.

[0069] Battery performance testing

[0070] i) Indentation test (Dent test)

[0071] Charge the battery fully, keep the test environment temperature at 25 ± 2°C, align a steel needle with a diameter of 6 mm (the top is a hemisphere with a radius of 3 mm) with the middle position of the battery, and press it down at a speed of 5 N / sec. Stop the test when the pressure no longer rises; record the force value and displacement at the critical point of battery failure, and monitor the curve of force value changing with displacement and the original data.

[0072] ii) Yield stress test

[0073] Discharge the battery at 0.2C to 3.0V, then charge it at 0.5C to 3.88V. The test environment temperature is 25 ± 2°C. Set the descending speed of the equipment at 10 mm / min, adjust the span of the fixture base to 0.7 times the length of the battery, press down on the long side of the battery, with a downward displacement of 2 mm, measure the force during the downward pressing process, and record the maximum force value as the yield stress.

[0074] iii) Pressure resistance test of the electrode sheet

[0075] Discharge the battery until it is completely discharged. Disassemble the positive and negative electrode plates of the battery separately. Fold the positive or negative electrode plate alone into 20 layers and compact it. Keep the temperature of the test environment at 25 ± 2 °C. Align a steel needle with a diameter of 6 mm (the top is a hemisphere with a radius of 3 mm) with the middle position of the stacked electrode plate, and press it down at a speed of 5 N / sec. Stop the test after the pressure no longer rises; record the force value and displacement at the critical point of electrode plate rupture, and monitor the curve of force value changing with displacement and the original data.

[0076] iv) Cycle performance test

[0077] Place the battery in a constant temperature environment of 45 ± 3 °C, charge it to the upper limit voltage at 1.5C, cut off at 0.05C, after standing for 10 min, discharge it at 0.5C to 3.0V, and record the discharge capacity as C0. Perform multiple charge and discharge cycles according to the above charge and discharge regime. The 500th discharge capacity is C1, and C1 / C0 is the capacity retention rate after 500 cycles.

[0078] v) Fast charging performance test

[0079] Place the battery in a constant temperature environment of 25 ± 3 °C, charge it to the upper limit voltage at a certain rate, cut off at 0.05C, after standing for 10 min, discharge it at 0.5C to 3.0V, which is one charge and discharge cycle. After the battery undergoes 20 charge and discharge cycles, charge it to full at a certain rate. Disassemble the battery within 24 h (the disassembly environment is a drying room with a dew point below -30 °C), and observe whether lithium is deposited on the negative electrode plate and the separator. The maximum charging rate without lithium deposition is the fast charging window of the battery at room temperature.

[0080] The Example 1 group and Comparative Examples 1 - 2 were carried out with reference to Example 1 - 1, and the main differences are shown in Table 1. Among them, the Example 1 group changed the types of filler particles.

[0081] Table 1

[0082]

[0083] Note: " / " indicates that the corresponding parameter was not tested.

[0084] As can be seen from Table 1, in the present invention, by blending filler particles in the negative active material layer and controlling that the Mohs hardness (a) of the filler particles and the Mohs hardness (b) of the negative active material satisfy a - b ≥ 3, the compressive resistance of the negative electrode plate can be significantly improved, enabling the battery to withstand a higher extrusion force without failure and improving the safety of the battery.

[0085] The Example 2 group and Comparative Examples 1 - 2 were carried out with reference to Example 1 - 1, and the main differences are shown in Table 2. Among them, the Example 2 group changed the proportion of filler particles.

[0086] Table 2

[0087]

[0088] As can be seen from Table 2, by controlling the blending ratio of filler A in the negative active material layer, the present invention can avoid the excessive content of filler A from deteriorating the cycle performance of the battery, improve the compressive resistance of the negative electrode sheet while ensuring the cycle performance of the battery, and enhance the safety of the battery.

[0089] Example 3-1

[0090] First step: Prepare the slurry for the positive active material layer. The composition of the active material layer slurry is lithium cobalt oxide (LCO): conductive carbon black (SP): polyvinylidene fluoride (PVDF) = 98:1:1. The solvent used is N-methylpyrrolidone (NMP). After stirring and mixing evenly, the slurry is coated on the surface of the current collector, dried, rolled, and slit to obtain the positive electrode sheet.

[0091] Second step: Prepare the slurry for the negative active material layer. The composition of the negative active material layer slurry is graphite: polyacrylic acid (polymer material): styrene-butadiene rubber (SBR): carboxymethyl cellulose (CMC) = 95:3:1:1. The solvent used is deionized water. After stirring and mixing evenly, the slurry is coated on the surface of the current collector, dried, rolled, and slit. Then, it is baked at 150 °C for 8 h to crosslink the polyacrylic acid, and a crosslinked polymer is formed to obtain the negative electrode sheet.

[0092] Third step: Stack and wind the positive electrode sheet, separator, and negative electrode sheet in sequence to obtain a wound core;

[0093] Use a punching die to punch the aluminum-plastic film, and then use the punched aluminum-plastic film to encapsulate the wound core to obtain an electric core. Bake it until the moisture is qualified and inject the electrolyte; Use lithium-ion battery formation equipment to charge and discharge the electric core to harden the electric core and sort out the capacity of the electric core; Perform secondary sealing on the electric core and fold the edges to form the electric core. After OCV testing, test the K value of the battery, and select the products with qualified K values to obtain lithium-ion batteries.

[0094] The Example 3 group is carried out with reference to Example 3-1, and the main differences are shown in Table 3. Among them, the type of polymer material is changed in the Example 3 group, and the mass ratio of filler A in the Example 3 group is 3% for all.

[0095] Table 3

[0096]

[0097] In the present invention, blending the polymer material in the negative active material layer can also improve the compressive resistance of the negative electrode sheet, enable the battery to withstand higher extrusion pressure, and enhance the safety of the battery.

[0098] Example 4-1

[0099] Step 1: Prepare the slurry for the positive electrode active material layer. The composition of the active material layer slurry is lithium cobalt oxide (LCO): conductive carbon black (SP): polyvinylidene fluoride (PVDF) = 98:1:1. The solvent used is N-methylpyrrolidone (NMP). After stirring and mixing evenly, the slurry is coated on the surface of the current collector, dried, rolled, and slit to obtain the positive electrode sheet.

[0100] Step 2: Prepare the slurry for the negative electrode active material layer. The composition of the negative electrode active material layer slurry is graphite: boron carbide (filler particles): polyacrylic acid (polymer material): styrene-butadiene rubber (SBR): carboxymethyl cellulose (CMC) = 93:2:3:1:1. The solvent used is deionized water. After stirring and mixing evenly, the slurry is coated on the surface of the current collector, dried, rolled, and slit, and then baked at a high temperature of 150 °C for 8 h to crosslink the polyacrylic acid, and the negative electrode sheet is obtained after forming the crosslinked polymer.

[0101] Step 3: Stack and wind the positive electrode sheet, separator, and negative electrode sheet in sequence to obtain the wound core;

[0102] Use a stamping die to stamp the aluminum-plastic film, and then use the stamped aluminum-plastic film to encapsulate the wound core to obtain the battery cell. Bake it until the moisture is qualified and inject the electrolyte; Use lithium-ion battery formation equipment to charge and discharge the battery cell to harden the battery cell and sort out the capacity of the battery cell; Perform secondary sealing on the battery cell and fold the edges to form the battery cell. After OCV testing, measure the K value of the battery, and select the products with qualified K values to obtain the lithium-ion battery.

[0103] Examples 4-5 are carried out with reference to Example 4-1, and the main differences are shown in Table 4. Among them, in Example 4, the types of polymer materials and filler particles are changed. In Example 5, the proportions of polymer materials and filler particles are changed.

[0104] Table 4

[0105]

[0106] Note: " / " indicates that the corresponding parameter is not tested.

[0107] As can be seen from Table 4, when the filler particles and the polymer material are simultaneously blended in the negative electrode active material layer, the increase in the compressive strength and yield stress of the negative electrode sheet is greater, and the critical Dent failure force is further improved.

[0108] In Example 6-1, the negative electrode is a double-layer material, including a first active material layer and a second active material layer. Among them, the composition of the second active material is the same as that in Example 1, the composition of the first active material layer is the same as that in Comparative Example 1, the material masses of the first active material layer and the second active material layer are each half of those in Comparative Example 1 and Example 1-1, and the total mass of the negative electrode active material layer is the same as that in Example 1 and Comparative Example 1. The Example 6 group is carried out with reference to Example 6-1, and the main differences are shown in Table 5. Among them, the Example 6 group changes the proportion of filler A in the first active material layer or the second active material layer.

[0109] Table 5

[0110]

[0111] Note: " / " indicates that the corresponding parameter is not tested.

[0112] As can be seen from Table 5, when the negative electrode is a double-layer active material layer, the filler particles and the polymer material are admixed in the negative electrode active material layer, and in the thickness direction of the negative electrode sheet, when the filler particles are located in the second active material layer (bottom layer), the compressive performance of the negative electrode sheet is better, the overall strength of the negative electrode sheet is better, and the compressive performance of the battery is further improved.

[0113] The Example 7 group is carried out with reference to Example 1-1, and the main differences are shown in Table 6. Among them, the Example 7 group changes the particle size Dv50 of the filler particles.

[0114] Table 6

[0115]

[0116] As can be seen from Table 6, by further optimizing the particle size Dv50 of the filler particles in the present invention, the fast charging performance of the battery can be further improved.

[0117] It should be noted that in this text, the term "including", "comprising" or any other variant thereof is intended to cover non-exclusive inclusion, such that a process, method, article or device including a series of elements not only includes those elements but also other elements not expressly listed, or elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "including one..." does not exclude the existence of additional identical elements in the process, method, article or device including such element. In addition, it should be pointed out that the scope of the methods and devices in the embodiments of the present application is not limited to performing functions in the order shown or discussed, and may also include performing functions in a substantially simultaneous manner or in a reverse order according to the functions involved. For example, the described methods may be performed in an order different from that described, and various steps may be added, omitted, or combined. Additionally, features described with reference to certain examples may be combined in other examples.

[0118] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A negative electrode sheet, characterized in that: The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer located on at least one side of the negative electrode current collector, wherein the negative electrode active material layer includes a negative electrode active material and filler A; the filler A includes one or two of filler particles and polymer materials; wherein the Mohs hardness of the filler particles is denoted as a, the Mohs hardness of the negative electrode active material is denoted as b, and a and b satisfy: ab≥3.

2. The negative electrode sheet according to claim 1, characterized in that: The particle size Dv50 of the filler particles is 0.1 μm-10 μm, preferably 0.5 μm-5 μm.

3. The negative electrode sheet according to claim 1, characterized in that: The polymer material includes one or both of a heat-curing polymer material and a light-curing polymer material; Preferably, the heat-curing polymer material includes one or both of polyacrylic acid and polymethacrylic acid; Preferably, the photocurable polymer material includes at least one of acrylic resin, epoxy resin, polyurethane resin and silanized polymer.

4. The negative electrode sheet according to claim 1, characterized in that: Based on the total mass of the negative electrode active material layer, the mass proportion of filler A is 0.1%-5%, preferably 0.3%-3%; And / or, based on the total mass of the negative electrode active material layer, the ratio of the mass proportion of the filler particles to the mass proportion of the polymer material is 0.4-9.

5. The negative electrode sheet according to claim 1, characterized in that: The negative electrode active material layer includes a first active material layer and a second active material layer, the second active material layer is located between the first active material layer and the negative electrode current collector; the first active material layer and the second active material layer each independently include a negative electrode active material and a filler A; based on the total mass of the first active material layer, the mass proportion of the filler A is n%; based on the total mass of the second active material layer, the mass proportion of the filler A is m%; n and m satisfy 0≤mn≤4.

6. The negative electrode sheet according to claim 5, characterized in that: n satisfies: 0≤n≤3; And / or, m satisfies: 0.2≤m≤7, preferably 2≤m≤5.

7. The negative electrode sheet according to any one of claims 1 to 6, characterized in that: The filler particles include at least one of carbides, nitrides, borides, and inorganic oxides; Preferably, the carbide comprises at least one of boron carbide, silicon carbide, tungsten carbide, titanium carbide, zirconium carbide or modified products thereof; Preferably, the nitride comprises at least one of cubic boron nitride, silicon nitride, titanium nitride, aluminum nitride or modified substances thereof; Preferably, the boride comprises at least one of titanium boride, zirconium boride, aluminum boride or modified products thereof; Preferably, the inorganic oxide includes at least one of zirconium oxide, titanium oxide, aluminum oxide, silicon dioxide, magnesium oxide, zinc oxide or modified substances thereof.

8. The negative electrode sheet according to any one of claims 1 to 6, characterized in that: The negative electrode active material includes at least one of graphite, hard carbon, soft carbon, activated carbon, graphene, carbon nanotubes, silicon carbon, lithium titanate, boron nitride or modified products thereof.

9. The negative electrode sheet according to any one of claims 1 to 6, characterized in that: The ultimate compressive strength of the negative electrode sheet is 1.5KN-2.5KN.

10. A lithium ion battery, characterized in that: The lithium-ion battery comprises the negative electrode sheet, the positive electrode sheet and the separator according to any one of claims 1 to 9; the yield stress of the lithium-ion battery is ≥350N.

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