Lithium-ion secondary batteries and electrical devices

Abstract

The present application discloses a lithium-ion secondary battery and an electrical device. The negative electrode sheet of the lithium-ion secondary battery includes: a negative electrode current collector; a negative electrode active material layer, the negative electrode active material layer including a first negative electrode active material layer and a second negative electrode active material layer stacked, the second negative electrode active material layer being located on a side of the first negative electrode active material layer away from the negative electrode current collector, wherein the first negative electrode active material layer includes a first negative electrode active material, the first negative electrode active material includes graphite, and the number of primary particles in the first negative electrode active material accounts for a greater proportion than the number of secondary particles; the second negative electrode active material layer includes a second negative electrode active material, the second negative electrode active material includes graphite, and the number of secondary particles in the second negative electrode active material accounts for a greater proportion than the number of primary particles. As a result, the lithium-ion secondary battery has both excellent fast charging performance and high specific capacity.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to lithium-ion secondary batteries and electrical devices. Background Art

[0002] Secondary batteries are widely used in energy storage systems such as hydropower, thermal, wind, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, and electric vehicles. As battery applications expand, the market is placing higher demands on batteries' rapid charge and discharge capabilities. However, current batteries still have many deficiencies in their application, and their fast-charging performance needs to be further improved.

[0003] It should be noted that the above statements are only used to provide background technical information related to this application and do not necessarily constitute prior art. Summary of the Invention

[0004] In the first aspect of the present application, the present application proposes a lithium-ion secondary battery, comprising a negative electrode plate, the negative electrode plate comprising: a negative electrode current collector; a negative electrode active material layer, the negative electrode active material layer comprising a first negative electrode active material layer and a second negative electrode active material layer stacked, the second negative electrode active material layer being located on a side of the first negative electrode active material layer away from the negative electrode current collector, wherein the first negative electrode active material layer comprises a first negative electrode active material, the first negative electrode active material comprises graphite, and the number of primary particles in the first negative electrode active material accounts for a greater proportion than the number of secondary particles; the second negative electrode active material layer comprises a second negative electrode active material, the second negative electrode active material comprises graphite, and the number of secondary particles in the second negative electrode active material accounts for a greater proportion than the number of primary particles. As a result, the lithium-ion secondary battery has both excellent fast charging performance and high energy density.

[0005] In some embodiments, the primary particles in the first negative electrode active material account for greater than or equal to 70%, and the oil absorption value of the first negative electrode active material is 45 mL / 100 g to 60 mL / 100 g. This can increase the specific capacity of the negative electrode sheet.

[0006] In some embodiments, the secondary particles in the second negative electrode active material account for greater than or equal to 70%, and the oil absorption value of the second negative electrode active material is greater than the oil absorption value of the first negative electrode active material. As a result, the lithium-ion secondary battery has excellent fast charging performance.

[0007] In some embodiments, the oil absorption value of the second negative active material is 47 mL / 100 g-62 mL / 100 g.

[0008] In some embodiments, the Dv50 particle size of the second negative electrode active material is smaller than the Dv50 particle size of the first negative electrode active material. As a result, the second negative electrode active material layer has more pores, which is conducive to the infiltration of the electrolyte into the negative electrode sheet.

[0009] In some embodiments, the Dv50 particle size of the first negative electrode active material is 14 μm-22 μm, and / or the Dv50 particle size of the second negative electrode active material is 12 μm-19 μm.

[0010] In some embodiments, the thickness of the first negative electrode active material layer is H1, the thickness of the second negative electrode active material layer is H2, and the ratio of H1 to H2 is in the range of (2:3) to (4:1). This helps the lithium-ion secondary battery achieve both high energy density and excellent fast charging performance.

[0011] In some embodiments, H1 is 30 μm-90 μm, and H2 is 20 μm-60 μm.

[0012] In some embodiments, the specific surface area of ​​the second negative electrode active material is greater than the specific surface area of ​​the first negative electrode active material, thereby helping to improve the wetting of the negative electrode plate by the electrolyte.

[0013] In some embodiments, the compaction density of the first negative electrode active material is greater than the compaction density of the second negative electrode active material under a pressure of 50,000 N. This helps to increase the specific capacity of the negative electrode sheet.

[0014] In some embodiments, the OI value of the second negative electrode active material is less than the OI value of the first negative electrode active material, thereby improving the fast charging performance of the negative electrode plate.

[0015] In some embodiments, the gram capacity of the first negative electrode active material is greater than the gram capacity of the second negative electrode active material, thereby helping to increase the specific capacity of the negative electrode sheet.

[0016] In some embodiments, the second negative electrode active material comprises secondary particles having a carbon coating, the carbon coating being located on at least a portion of the surface of the secondary particles, and the secondary particles being formed by bonding at least two primary particles together, the bonding material comprising a carbon material. This helps improve the fast-charging performance of the negative electrode sheet.

[0017] In some embodiments, the compaction density of the negative electrode active material layer is 1.65 g / cm 3 -1.85g / cm 3 This helps improve the wetting of the negative electrode by the electrolyte.

[0018] In a second aspect of the present application, an electrical device is provided, comprising the aforementioned lithium-ion secondary battery. Thus, the electrical device has all the features and advantages of the aforementioned battery, which will not be described in detail here. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The above and / or additional aspects and advantages of the present application will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0020] Figure 1 This is a schematic structural diagram of a negative electrode sheet according to an embodiment of the present application;

[0021] Figure 2 is a schematic diagram of a battery cell according to an embodiment of the present application;

[0022] Figure 3 yes Figure 2 An exploded view of a battery cell according to an embodiment of the present application is shown;

[0023] Figure 4 is a schematic diagram of a battery module according to an embodiment of the present application;

[0024] Figure 5 is a schematic diagram of a battery pack according to an embodiment of the present application;

[0025] Figure 6 yes Figure 5 An exploded view of a battery pack according to an embodiment of the present application is shown;

[0026] Figure 7 is a schematic diagram of an electrical device according to an embodiment of the present application;

[0027] Figure 8 This is a scanning electron microscope image of the negative electrode active material containing secondary particles.

[0028] Description of reference numerals:

[0029] 1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 battery cell;

[0030] 51 housing; 52 electrode assembly; 53 top cover assembly;

[0031] 100 negative electrode current collector; 110 first negative electrode active material layer; 120 second negative electrode active material layer. DETAILED DESCRIPTION

[0032] The following describes the embodiments of the present application in detail. Examples of the embodiments are shown in the accompanying drawings, but unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repeated descriptions of substantially identical structures may be omitted. This is to avoid unnecessary lengthiness in the following description and to facilitate understanding by those skilled in the art. In addition, the drawings and the following description are provided to enable those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0033] Unless otherwise defined, all technical and scientific terms used in this application have the same meanings as those commonly understood by technicians in the technical field to which this application belongs; the terms used in this application are only for the purpose of describing specific embodiments and are not intended to limit this application; unless otherwise specified, the numerical values ​​of the various parameters mentioned in this application can be measured using various measurement methods commonly used in the art (for example, they can be tested according to the methods given in the embodiments of this application).

[0034] The terms "include" and "have" in the description and claims of this application and any variations thereof are open expressions, that is, including the contents specified in this application but not excluding other contents.

[0035] In the description of this application, regardless of whether the word "about" or "approximately" is used, all numbers disclosed herein are approximate values. The value of each number may vary by less than 10% or by a reasonable difference considered by a person skilled in the art, such as 1%, 2%, 3%, 4% or 5%.

[0036] " range " disclosed in the present application is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and can be arbitrarily combined, that is, any lower limit can form a range with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3,4 and 5 are listed, then the following range can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0037] In the description of this application, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly specifying the quantity of the technical features indicated. "First feature" and "second feature" may include one or more of the features.

[0038] In the description of this application, "A and / or B" may include the case of A alone, the case of B alone, or any of the cases of A and B, where A and B are only used for example, and may be any technical feature connected by "and / or" in this application.

[0039] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0040] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0041] The technical features of this application can be tested by reverse disassembly of the battery or by obtaining fresh materials from the battery manufacturing process / batteries that have not yet been shipped. For example, when testing the characteristics of the negative electrode active material, the test can be performed by scraping and burning the conventional negative electrode sheet after disassembly of the battery, or by obtaining fresh graphite that has not yet been used to make a battery.

[0042] Taking graphite as an example, the structure of graphite primary particles is more stable, especially at high compaction densities. Since the graphite primary particles are not agglomerated, they are more easily dispersed during the mixing process of the negative electrode slurry, resulting in lower oil absorption. The primary particles have fewer surface irregularities, making it easier for particles to slip between each other during the cold pressing process, forming a tight connection with the current collector, resulting in high compaction and a higher volumetric energy density in the battery cell. However, due to the anisotropic nature of graphite primary particles, lithium ions can only embed into the graphite interlayer structure along the end faces, resulting in a low lithium ion migration rate in graphite. Furthermore, the expansion force generated by lithium ions embedded in the graphite interlayer structure can only be released in a direction perpendicular to the interlayer structure. This results in significant variations in the thickness of the negative electrode sheet during charge and discharge, and significant battery expansion. This can easily cause the electrolyte generated by the expansion of the negative electrode sheet to be squeezed out and unable to flow back, leading to battery cycle failure.

[0043] Unlike graphite primary particles, graphite secondary particles are isotropic. The larger number of lithium ion transmission channels in the secondary particles can effectively increase the lithium ion insertion and extraction entrances of the negative electrode active material, improve the lithium ion interface reaction, increase the migration rate of lithium ions, and then improve the fast charging performance of the negative electrode active material. At the same time, the expansion force generated during the lithium insertion process can be released in multiple directions, thereby reducing the thickness change of the negative electrode plate during the charge and discharge process, and the battery expansion is correspondingly reduced, which can effectively alleviate the electrolyte being squeezed out and unable to flow back due to the expansion of the negative electrode plate. Furthermore, the secondary particles are formed by the bonding of primary particles, and more concave-convex structures will be formed between the particles. The concave-convex structure has a strong adsorption effect on the dispersant, which will adsorb a large amount of dispersant in the negative electrode slurry on the particle surface, causing the lithium insertion and extraction channels on the graphite surface to be blocked. There is less free dispersant in the negative electrode slurry, and the stability of the negative electrode slurry is poor. At this time, the negative electrode active material exhibits a higher oil absorption value. In order to compensate for the excessive adsorption of the dispersant by the negative electrode active material with a larger oil absorption value and achieve uniform dispersion of the negative electrode slurry, more dispersant needs to be added to the negative electrode slurry. However, since the dispersant itself does not have the function of inserting and extracting lithium, the proportion of negative electrode active materials that can provide lithium insertion and extraction sites in the negative electrode slurry will decrease, and the proportion of negative electrode active materials in the negative electrode active material layer formed by the negative electrode slurry coating will be reduced accordingly, which will ultimately lead to a lower specific capacity of the negative electrode active material layer.

[0044] In the present application, a first negative electrode active material and a second negative electrode active material are used in combination to form a negative electrode active material layer, wherein the first negative electrode active material and the second negative electrode active material both include graphite, the number of primary particles in the first negative electrode active material accounts for a greater proportion than the number of secondary particles, and the number of secondary particles in the second negative electrode active material accounts for a greater proportion than the number of primary particles.

[0045] Specifically, in a double-layer or multi-layer negative electrode active material system, the second negative electrode active material is closer to the electrolyte, and the surrounding ion concentration is higher, making it more likely for ion accumulation to occur and trigger lithium precipitation. Therefore, when the number of secondary particles in the second negative electrode active material is relatively large compared to the number of primary particles, the secondary particles can use more lithium insertion and extraction channels to ensure that lithium ions are quickly transferred to the particle bulk, improve the lithium ion interface reaction, increase the migration rate of lithium ions, and then improve the fast charging performance of the second negative electrode active material. However, the secondary particles have more end faces and fewer lithium storage sites, and the concave and convex surfaces of the secondary particles make it difficult for the particles to slide between each other during the cold pressing process, and the compaction density is low, which affects the volume energy density of the negative electrode sheet. Therefore, the second negative electrode active material layer has higher kinetic performance and relatively lower gram capacity. Furthermore, in a double-layer or multi-layer negative electrode active material layer system, the first negative electrode active material is closer to the negative electrode current collector, with a relatively low surrounding ion concentration and a relatively low requirement for lithium ion migration rate. When the proportion of primary particles in the first negative electrode active material is higher than that of secondary particles, the first negative electrode active material layer can fully utilize the characteristics of primary particles with large gram capacity, easier compaction, lower oil absorption value, and less dispersant required in the negative electrode slurry. This makes the proportion of active materials with high gram capacity in the first negative electrode active material layer higher, greatly improving the overall specific capacity of the negative electrode sheet and compensating for the low specific capacity of the second negative electrode active material layer. By using the above-mentioned first negative electrode active material and the second negative electrode active material together, the lithium-ion secondary battery has both excellent fast charging performance and high energy density.

[0046] Specifically, during the charge and discharge process of the battery, lithium ions are released from the positive electrode active material and first embedded in the second negative electrode active material layer located on the outside of the negative electrode plate. Since the number of secondary particles in the second negative electrode active material accounts for a relatively high proportion, it is conducive to the rapid embedding of lithium ions. The negative electrode plate has excellent fast charging performance during the charge and discharge process, which helps to meet the battery's usage requirements in fast charging scenarios. Furthermore, after the lithium ions are embedded in the second negative electrode active material layer, they will continue to migrate to the first negative electrode active material layer. Since the specific capacity of the first negative electrode active material layer is relatively high, it can provide more lithium embedding sites, which helps to embed lithium ions in the first negative electrode active material layer as much as possible, effectively reducing the occurrence of lithium dendrites on the surface of the negative electrode plate, and improving the fast charging performance and specific capacity of the negative electrode plate.

[0047] In this application, "the proportion of secondary particles in the negative electrode active material can be determined by scanning electron microscopy. As an example, the test method for the proportion of secondary particles can be: the negative electrode active material is evenly laid and adhered to the conductive glue to form a sample to be tested with a length × width = 6 cm × 1.1 cm; the particle morphology is tested using a scanning electron microscope (such as ZEISS Sigma 300). The test can refer to JY / T010-1996. In order to ensure the accuracy of the test results, a plurality of (for example, 20) different areas can be randomly selected from the sample to be tested for scanning testing, and at a certain magnification (for example, 1000 times), the percentage of the number of secondary particles in each test area to the total number of particles is calculated, that is, the proportion of the number of secondary particles in the area, and the average value of the test results of the corresponding plurality of (for example, 20) test areas is taken as the proportion of the number of secondary particles in the negative electrode active material.

[0048] It should be noted that, in the present application, the percentage of primary particles in the negative electrode active material (%) + the percentage of secondary particles in the negative electrode active material = 100%.

[0049] The first and second negative electrode active material layers have different compaction densities due to their different material compositions. For example, when the compaction density of the first negative electrode active material layer is greater than that of the second negative electrode active material layer, since the compaction density of the negative electrode active material layer is related to the stacking structure of the negative electrode active material layer, the overall structure of the second negative electrode active material layer is looser than that of the first negative electrode active material layer. Therefore, the first and second negative electrode active material layers can be distinguished and the first and second negative electrode active materials can be obtained by the following method.

[0050] Specifically: ① Use ceramic scissors to cut the negative electrode into 6mm×6mm size, stick it on the sample stage coated with paraffin, and make the sample slightly protrude (<1mm) from the edge of the sample stage, and then polish the cross section; ② Place the slice on the CP-SEM (Cross Section Polisher-Scanning Electron ③ In a cross-sectional polishing scanning electron microscope (SEM), at least 10 scan images are taken at 1000x magnification in backscattering mode to test the thickness of the negative electrode sheet. Porosity analysis is performed on the scan images with a step size of 10 μm. The area where the porosity changes significantly is defined as the interface between the first negative electrode active material layer and the second negative electrode active material layer. The distance between the interface layer and the copper foil is the thickness of the first negative electrode active material layer H1. The thickness of the second negative electrode active material layer H2 is the thickness of the entire electrode sheet H minus the thickness of the first negative electrode active material layer H1. ③ The powder scraped off from the surface of the negative electrode sheet by two-thirds of the thickness H2 is defined as the second negative electrode active material. The remaining negative electrode sheet is further scraped. During the scraping process, the remaining thickness of the negative electrode sheet is measured using a micrometer or laser thickness gauge until the thickness of the remaining negative electrode active material layer is 2 / 3 of H1. The powder in this area is defined as the first negative electrode active material. ④ The secondary particle number ratio of the aforementioned first negative electrode active material powder and the second negative active material are respectively tested. For example, the number ratio of secondary particles can be calculated as follows: (a) the aforementioned negative electrode active material powder is evenly dispersed on the conductive tape matrix by vacuum negative pressure sputtering; (b) the mapping function of a scanning electron microscope (such as ThermoScientific Apreo 2S) is used to take 50 high-speed photos at 500 times; (c) the particles in each picture are automatically marked using an AI deep learning model or manual image recognition to obtain the number of primary and secondary particles, the results are statistically analyzed, and the number ratio of secondary particles is calculated. Among them, the definition of secondary particles is: when two or more particles are stacked, they are regarded as a secondary particle. For example, secondary particles can be a stack of multiple particles with different particle sizes (see Figure 8 ① in ), multiple particles with similar particle sizes are stacked (see Figure 8 ② in the above), multiple smaller particles stacked on the surface of a larger particle (see Figure 8 ③ in the text). Primary particles are defined as particles that are not stacked. For example, see Figure 8 ④ and ⑤ in the .

[0051] In the first aspect of this application, reference is made to Figure 1The present application proposes a lithium-ion secondary battery, including a negative electrode plate, the negative electrode plate including: a negative electrode current collector 100; a negative electrode active material layer, the negative electrode active material layer including a first negative electrode active material layer 110 and a second negative electrode active material layer 120 arranged in a stacked manner, the second negative electrode active material layer 120 being located on a side of the first negative electrode active material layer 110 away from the negative electrode current collector 100, wherein the first negative electrode active material layer 110 includes a first negative electrode active material; the first negative electrode active material includes graphite, and the number of primary particles in the first negative electrode active material accounts for a greater proportion than the number of secondary particles; the second negative electrode active material layer 120 includes a second negative electrode active material, the second negative electrode active material includes graphite, and the number of secondary particles in the second negative electrode active material accounts for a greater proportion than the number of primary particles. The first negative electrode active material layer can provide more lithium insertion sites, thereby improving the specific capacity of the negative electrode plate, and the second negative electrode active material layer can provide more rapid lithium insertion and extraction channels, thereby improving the fast charging performance of the negative electrode plate. Therefore, lithium-ion secondary batteries have both excellent fast charging performance and high energy density.

[0052] In some embodiments, the primary particles in the first negative electrode active material account for greater than or equal to 70%, and the oil absorption value of the first negative electrode active material is 45 mL / 100 g-60 mL / 100 g.

[0053] As an example, the amount of primary particles in the first negative electrode active material may account for 70%, 75%, 80%, 85%, 90%, 95% or 100%.

[0054] As an example, the oil absorption value of the first negative active material can be 45 mL / 100 g, 46 mL / 100 g, 47 mL / 100 g, 48 mL / 100 g, 49 mL / 100 g, 50 mL / 100 g, 51 mL / 100 g, 52 mL / 100 g, 53 mL / 100 g, 54 mL / 100 g, 55 mL / 100 g, 56 mL / 100 g, 57 mL / 100 g, 58 mL / 100 g, 59 mL / 100 g or 60 mL / 100 g.

[0055] For the first negative electrode active material, the greater the proportion of primary particles in the first negative electrode active material, the greater the gram capacity of the first negative electrode active material. When the first negative electrode active material is composed entirely of primary particles, the first negative electrode active material has the most lithium insertion sites, and the gram capacity of the first negative electrode active material is optimal. At the same time, the oil absorption value of the first negative electrode active material is also low, and less dispersant is required when dispersed in the negative electrode slurry, and the specific capacity of the first negative electrode active material layer formed is also improved. At this time, lithium ions can only be inserted and removed from the end face of the first negative electrode active material, so the first negative electrode active material has a higher gram capacity and relatively poor kinetic performance. Those skilled in the art can select the primary particle proportion and oil absorption value of the first negative electrode active material according to actual conditions.

[0056] In some embodiments, the secondary particles in the second negative electrode active material account for greater than or equal to 70%, and the oil absorption value of the second negative electrode active material is greater than the oil absorption value of the first negative electrode active material.

[0057] As an example, the amount of the secondary particles in the second negative electrode active material may account for 70%, 75%, 80%, 85%, 90%, 95% or 100%.

[0058] In some embodiments, the oil absorption value of the second negative active material is 47 mL / 100 g-62 mL / 100 g.

[0059] As an example, the oil absorption value of the second negative active material can be 47 mL / 100 g, 48 mL / 100 g, 49 mL / 100 g, 50 mL / 100 g, 51 mL / 100 g, 52 mL / 100 g, 53 mL / 100 g, 54 mL / 100 g, 55 mL / 100 g, 56 mL / 100 g, 57 mL / 100 g, 58 mL / 100 g, 59 mL / 100 g, 60 mL / 100 g, 61 mL / 100 g or 62 mL / 100 g.

[0060] The proportion of secondary particles in the second negative electrode active material is higher than that in the first negative electrode active material. The surface of the secondary particles will expose more concave and convex surfaces, and the oil absorption value is larger. More dispersants are required to maintain dispersion, which in turn causes the lithium insertion and extraction channels to be covered, affecting ion transport. The second negative electrode active material has a larger oil absorption value than the first negative electrode active material. When the oil absorption value of the second negative electrode active material is in the range of 47mL / 100g-62mL / 100g, the secondary particles can be processed with less dispersant, thereby balancing the dynamics. Therefore, when the oil absorption value of the secondary particles is limited to a specific range, the negative electrode active material requires less dispersant to be processed, and the lithium insertion end face is exposed more, achieving a balance between processing and dynamics.

[0061] It is understandable that for the second negative electrode active material, the greater the proportion of secondary particles in the second negative electrode active material, the better the kinetics of the second negative electrode active material. When the second negative electrode active material is composed entirely of secondary particles, the kinetic performance of the second negative electrode active material is optimal. At this time, the second negative electrode active material is mainly composed of secondary particles, so that it has more end faces and fewer lithium storage sites. Therefore, the second negative electrode active material has higher kinetic performance and relatively low gram capacity. Furthermore, at this time, the oil absorption value of the second negative electrode active material is also higher, and there are more concave and convex surfaces, which makes it more difficult to press during the cold pressing process, and the volume energy density of the battery is reduced. Those skilled in the art can select the proportion of secondary particles in the second negative electrode active material and the oil absorption value according to actual conditions.

[0062] In the present application, the "oil absorption value of the negative electrode active material" can be obtained based on the amount of dibutyl phthalate used when the mixture of the negative electrode active material and dibutyl phthalate changes from a free-flowing state to a semi-plastic agglomerate. Specifically, 100 g of the negative electrode active material is placed in the feed port of an oil absorption value tester, and dibutyl phthalate is automatically titrated toward the negative electrode active material through the equipment until the negative electrode active material changes into a semi-plastic agglomerate. The amount of dibutyl phthalate added at this time (a mL) is recorded, thereby obtaining the oil absorption value of the negative electrode active material as a mL / 100 g.

[0063] The first and second negative electrode active material layers have different compaction densities due to their different material compositions. For example, when the compaction density of the first negative electrode active material layer is greater than that of the second negative electrode active material layer, since the compaction density of the negative electrode active material layer is related to the stacking structure of the negative electrode active material layer, the overall structure of the second negative electrode active material layer is looser than that of the first negative electrode active material layer. Therefore, the first and second negative electrode active material layers can be distinguished and the first and second negative electrode active materials can be obtained by the following method.

[0064] Specifically: ① Use ceramic scissors to cut the negative electrode into 6mm×6mm size, stick it on the sample stage coated with paraffin, and make the sample slightly protrude (<1mm) from the edge of the sample stage, and then polish the cross section; ② Place the slice on the CP-SEM (Cross Section Polisher-Scanning Electron ③ In a cross-sectional polishing-scanning electron microscope cavity, more than 10 scanning images are taken at 1000x magnification using backscattering mode to test the thickness of the negative electrode sheet. The porosity analysis is performed on the scanning images with a step size of 10 μm. The area where the porosity changes significantly is defined as the interface between the first negative electrode active material layer and the second negative electrode active material layer. The distance between the test interface layer and the copper foil is the thickness H1 of the first negative electrode active material layer. The thickness H2 of the second negative electrode active material layer is the thickness H of the entire electrode sheet minus the thickness H1 of the first negative electrode active material layer. ③ The powder scraped off two-thirds of the thickness H2 from the surface of the negative electrode sheet is defined as the second negative electrode active material. The powder scraping is continued on the remaining negative electrode sheet until the thickness of the remaining negative electrode active material layer is 2 / 3 of H1. The powder in this area is defined as the first negative electrode active material. ④ The oil absorption value of the aforementioned first negative electrode active material powder and the second negative electrode active material are tested separately.

[0065] In some embodiments, the Dv50 particle size of the second negative electrode active material is smaller than the Dv50 particle size of the first negative electrode active material. This makes the second negative electrode active material more difficult to press during the cold pressing process, and the resulting film layer has more pores, which facilitates the infiltration of the electrolyte into the negative electrode sheet.

[0066] In some embodiments, the Dv50 particle size of the first negative electrode active material is 14 μm-22 μm, and / or the Dv50 particle size of the second negative electrode active material is 12 μm-19 μm.

[0067] The aforementioned Dv50 particle size refers to the particle size corresponding to when the cumulative volume distribution percentage of the particles reaches 50%.

[0068] The "particle size" in this application can be measured by laser diffraction particle size analysis. Specifically, the particle size of the negative electrode active material can be measured with reference to the standard GB / T 19077-2016 using a laser particle size analyzer (eg, Malvern Master Size 3000).

[0069] In some embodiments, the thickness of the first negative electrode active material layer is H1, the thickness of the second negative electrode active material layer is H2, and the ratio of H1 to H2 is in the range of (2:3) to (4:1). This helps the negative electrode sheet achieve both a higher specific capacity and better fast charging performance.

[0070] As examples, the ratio of H1 to H2 may be 2:3, 1:1, 1.5:1, 2:1, 2.5:1, 3:1, 3.5:1, or 4:1.

[0071] When the ratio of H2 to H1 falls within the above range, the negative electrode sheet exhibits excellent fast-charging performance, low volume expansion, and high specific capacity. The first negative electrode active material layer can effectively increase the specific capacity of the negative electrode sheet, while the second negative electrode active material layer can balance improving fast-charging performance and suppressing negative electrode sheet expansion.

[0072] In some embodiments, H1 is 30 μm-90 μm, and H2 is 20 μm-60 μm.

[0073] As an example, H1 may be 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, or 90 μm.

[0074] As an example, H2 may be 20 μm, 30 μm, 40 μm, 50 μm, or 60 μm.

[0075] In some embodiments, the specific surface area of ​​the second negative electrode active material is greater than the specific surface area of ​​the first negative electrode active material, thereby helping to improve the wetting of the negative electrode plate by the electrolyte.

[0076] Specific surface area refers to the surface area of ​​an object per unit mass. A larger specific surface area provides more channels for lithium ion migration and a shorter path, resulting in better fast-charging performance for the negative electrode active material. However, this also requires more dispersant for dispersion. That is, when the number of secondary particles in the negative electrode active material accounts for a large proportion, the larger the specific surface area of ​​the negative electrode active material, the higher the oil absorption value of the negative electrode active material. When the specific surface area of ​​the second negative electrode active material is greater than that of the first negative electrode active material, the second negative electrode active material significantly improves fast-charging performance.

[0077] As an example, the specific surface area of ​​the first negative electrode active material may be 1.5 m 2 / g-3.5m 2 / g, for example, the specific surface area of ​​the first negative electrode active material can be 1.5m 2 / g-2.5m 2 / g.

[0078] As an example, the specific surface area of ​​the second negative electrode active material may be 3.0 m 2 / g-4.5m 2 / g, for example, the specific surface area of ​​the second negative electrode active material can be 3.4m 2 / g-4.0m 2 / g.

[0079] The "specific surface area" of the negative electrode active material can be measured using the American Microelectronics multi-station fully automatic specific surface area and pore analyzer GeminiVII2390. Take about 7g of the negative electrode active material sample and put it into a 9cc long tube with a bulb. Degas at 150℃ for 15 minutes, and then put it into the host test to obtain BET data.

[0080] In some embodiments, the compaction density of the first negative electrode active material is greater than the compaction density of the second negative electrode active material under a pressure of 50,000 N. This helps to increase the specific capacity of the negative electrode sheet.

[0081] The higher the compaction density of the negative electrode active material powder, the easier it is to compact after roller pressing, resulting in a higher specific capacity of the negative electrode active material layer. When the compaction density of the first negative electrode active material is greater than that of the second negative electrode active material, the first negative electrode active material layer is denser after roller pressing, providing more lithium insertion and extraction sites for the negative electrode plate. The overall porous structure of the negative electrode plate is also relatively high, which is conducive to the infiltration of the electrolyte into the negative electrode plate and the insertion and extraction of lithium ions.

[0082] As an example, the powder compaction density of the first negative electrode active material at 50,000 N may be 1.85 g / cm 3 -2.00g / cm 3 For example, the powder compaction density of the first negative electrode active material under 50000N can be 1.88g / cm 3 -1.98g / cm 3 .

[0083] As an example, the powder compaction density of the second negative electrode active material at 50,000 N may be 1.78 g / cm 3 -1.95g / cm 3 For example, the powder compaction density of the second negative electrode active material under 50000N can be 1.80g / cm 3 -1.90g / cm 3 .

[0084] In some embodiments, the OI value of the second negative electrode active material is less than the OI value of the first negative electrode active material, thereby improving the fast charging performance of the negative electrode plate.

[0085] The smaller the OI value, the stronger the lithium ion embedding ability of the negative electrode active material. Specifically, the greater the proportion of secondary particles in the negative electrode active material, the smaller the OI value of the negative electrode active material. When the OI value of the powder of the second negative electrode active material is less than the OI value of the first negative electrode active material, the number of secondary particles in the second negative electrode active material accounts for a large proportion. Compared with the primary particles that can only embed lithium ions from the end face, the secondary particles are isotropic and can embed lithium ions from multiple directions. The second negative electrode active material layer has the ability to quickly complete lithium ion embedding, which can effectively improve the fast charging performance of the negative electrode sheet.

[0086] As an example, the OI value of the powder of the first negative electrode active material may be 30-45, for example, the OI value of the powder of the first negative electrode active material may be 35-40.

[0087] As an example, the OI value of the powder of the second negative electrode active material may be 2-10, for example, the OI value of the powder of the second negative electrode active material may be 3-8.

[0088] When the negative electrode active material graphite is made into a negative electrode sheet, the arrangement direction (orientation) of the graphite layered structure has a great influence on the migration of lithium ions. Since only the end faces of graphite can insert and extract lithium ions, ideally, it is more beneficial for lithium ion diffusion when the end faces of the graphite in the negative electrode active material are perpendicular to the surface of the negative electrode sheet. However, it is difficult to accurately control the orientation of each graphite particle during actual preparation. The orientation of the graphite on the negative electrode sheet can be tested by XRD. When the diffraction pattern of the horizontally placed negative electrode sheet sample is tested, the diffraction signal of the (110) crystal plane that can be collected comes from the graphite in the negative electrode active material layer that is perpendicular to the surface of the negative electrode sheet, and the diffraction signals of the (002) and (004) crystal planes come from the graphite in the negative electrode active material that is parallel to the surface of the electrode sheet. Therefore, the orientation of the graphite in the negative electrode sheet can be described by the ratio of the (002) or (004) diffraction peak intensity (or integrated area) to the (110) diffraction peak intensity (or integrated area). The formula is described as: OI=I(002) / I(110) or OI=I(004) / I(110), where OI (orientation index) is the orientation of graphite in the negative electrode active material layer.

[0089] In some embodiments, the gram capacity of the first negative electrode active material is greater than the gram capacity of the second negative electrode active material. Because the second negative electrode active material is primarily composed of secondary particles, which have a relatively large number of exposed end faces, but these end faces do not have lithium storage function, the gram capacity is relatively low. The first negative electrode active material is primarily used to increase the specific capacity of the negative electrode plate. When the gram capacity of the first negative electrode active material is greater than the gram capacity of the second negative electrode active material, the first negative electrode active material layer provides more capacity, and the negative electrode plate has a higher specific capacity.

[0090] As an example, the gram capacity of the first negative electrode active material may be greater than or equal to 358 mAh / g, for example, the gram capacity of the first negative electrode active material may be 360 ​​mAh / g-367 mAh / g.

[0091] As an example, the gram capacity of the second negative electrode active material may be greater than or equal to 354 mAh / g. For example, the gram capacity of the second negative electrode active material may be 355 mAh / g-361 mAh / g.

[0092] In some embodiments, the second negative electrode active material comprises secondary particles having a carbon coating, the carbon coating being located on at least a portion of the surface of the secondary particles, and the secondary particles being formed by bonding at least two primary particles together, the bonding material comprising a carbon material. This helps improve the fast-charging performance of the negative electrode sheet.

[0093] The carbon coating layer can cover the active sites on the surface of the graphite secondary particles, reduce the direct contact between the secondary particles in the negative electrode active material and the electrolyte, reduce the occurrence of irreversible side reactions, and restrain and buffer the volume expansion of graphite, thereby increasing stability during the cycle.

[0094] As an example, the carbon coating layer may include amorphous carbon, for example, soft carbon and / or hard carbon. The amorphous carbon coating layer has a larger interlayer spacing, which helps to inhibit the expansion of the second negative electrode active material, improve the diffusion rate of lithium ions within the second negative electrode active material, enhance high current charge and discharge capabilities, and improve the performance of the battery at high rates.

[0095] It can be understood that for the first negative electrode active material, it is mainly used to provide lithium insertion sites for the negative electrode plate, and the amorphous carbon coating layer can only improve the insertion channel of lithium ions, and it itself cannot be used as an effective lithium insertion site. Therefore, for the first negative electrode active material, in order to meet the higher gram capacity requirements, the carbon coating layer may not be set on the surface of the primary particle.

[0096] In some embodiments, the tap density of the second negative electrode active material is less than the tap density of the first negative electrode active material.

[0097] When the negative electrode active material particles have fewer concave and convex structures on their surface, the negative electrode active materials adhere more closely to each other, resulting in a higher tap density. In this case, less dispersant is required in the negative electrode slurry. Therefore, when the tap density of the first negative electrode active material is greater than the tap density of the second negative electrode active material, the particles of the first negative electrode active material adhere more closely to each other, helping to increase the specific capacity of the first negative electrode active material layer, and subsequently, the specific capacity of the negative electrode sheet.

[0098] As an example, the tap density of the first negative electrode active material can be 0.95 g / cm 3 -1.30g / cm 3 For example, the tap density of the first negative electrode active material can be 1.00 g / cm 3 -1.20g / cm 3 .

[0099] As an example, the tap density of the second negative electrode active material can be 0.90 g / cm 3 -1.20g / cm 3 For example, the tap density of the second negative electrode active material can be 0.95 g / cm 3 -1.10g / cm 3 .

[0100] Tap density refers to the mass per unit volume of powder in a container measured after tapping under specified conditions, and is expressed in g / cm 3 . Measured by a special tap density instrument.

[0101] It is understood that the tap density is affected by the particle size of the material. When the particle size of the negative electrode active material meets the aforementioned limitations, the tap density within the aforementioned range can be obtained.

[0102] In some embodiments, the compaction density of the negative electrode active material layer is 1.65 g / cm 3 -1.85g / cm 3 This helps improve the wetting of the negative electrode by the electrolyte.

[0103] At this compaction density, the negative electrode active material layer has more pores, allowing for better electrolyte wetting of the negative electrode active material layer and faster liquid-phase lithium ion conduction. Furthermore, when the second negative electrode active material has a surface carbon coating, the carbon coating can further enhance the solid-phase lithium ion conduction rate, thereby improving the fast-charging performance of the negative electrode sheet.

[0104] The aforementioned compacted density of the negative electrode active material layer refers to the compacted density of the negative electrode active material layer after roller pressing. Specifically, the compacted density of the negative electrode active material layer after roller pressing and formation treatment, the compacted density of the negative electrode active material layer when the battery is fully charged or fully discharged, and the compacted density of the negative electrode active material layer after the battery has been stationary for a long time are all within the aforementioned ranges.

[0105] As an example, the compaction density of the negative electrode active material layer can be 1.65 g / cm 3 , 1.70g / cm 3 , 1.75g / cm 3, 1.80g / cm 3 or 1.85g / cm 3 .

[0106] It should be noted that the compaction density of the negative electrode active material layer is the designed compaction density after roller pressing. After the battery undergoes charge and discharge cycles, the negative electrode active material will expand due to the insertion of lithium ions, causing the negative electrode active material layer to rebound, thereby reducing the compaction density of the negative electrode active material layer. For example, when the designed compaction density of the negative electrode active material layer after roller pressing is 1.65 g / cm 3 -1.85g / cm 3 When the negative electrode active material layer is subjected to charge and discharge cycles, the compact density may be 1.55 g / cm 3 -1.75g / cm 3 .

[0107] In some embodiments, the surface density of the negative electrode active material layer may be greater than or equal to 7 mg / cm 2 , for example, it can be 8 mg / cm 2 -15mg / cm 2 .

[0108] As an example, the negative electrode current collector has two surfaces facing each other in its thickness direction, and the negative electrode active material layer is provided on either or both of the two facing surfaces of the negative electrode current collector.

[0109] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0110] In some embodiments, in addition to the aforementioned negative electrode active materials, the negative electrode active material layer may also contain negative electrode active materials for batteries known in the art.

[0111] As an example, the negative electrode active material may also include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, lithium titanate, etc. Silicon-based materials include at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials include at least one of elemental tin, tin oxide compounds, and tin alloys. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0112] In some embodiments, the negative electrode active material layer may further include a binder. The binder may include at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0113] In some embodiments, the negative electrode active material layer may further include a conductive agent, which may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0114] In some embodiments, the negative electrode active material layer may further optionally include other auxiliary agents, such as a dispersant (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0115] In some embodiments, the negative electrode sheet can be prepared by the following method: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0116] Typically, a battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During the battery's charge and discharge processes, metal active ions are inserted and removed between the positive and negative electrodes. The electrolyte conducts ions between the positive and negative electrodes. The separator, located between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing the metal active ions to pass through.

[0117] [Positive electrode]

[0118] As an example, the positive electrode current collector has two surfaces facing each other in its thickness direction, and the positive electrode active material layer is provided on either or both of the two facing surfaces of the positive electrode current collector.

[0119] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0120] In some embodiments, when the battery is a lithium-ion battery, the positive electrode active material may be a positive electrode active material for lithium-ion batteries known in the art.

[0121] As an example, the positive electrode active material may include at least one of the following materials: lithium-containing phosphates with an olivine structure, lithium transition metal oxides and their respective modified compounds. However, the present application is not limited to these materials, and other traditional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also referred to as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2 (also referred to as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2 (also referred to as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2 (also referred to as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2 (also referred to as NCM 811 ), lithium nickel cobalt aluminum oxide (such as LiNi 0.8 Co 0.15 Al 0.05O2) and its modified compounds. Examples of olivine-structured lithium-containing phosphates include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon. The modified compounds of the above materials may be modified by doping and / or surface coating the materials.

[0122] The battery's charge and discharge processes involve the insertion and removal of lithium, leading to different molar contents of lithium at different discharge states. The molar contents of lithium in the positive electrode active materials listed in this application refer to the initial state of the material, i.e., the state before the materials are added. When the positive electrode active materials are used in a battery system, the molar contents of lithium will change after charge and discharge cycles.

[0123] In the list of positive electrode active materials in this application, the molar content of O is only a theoretical value. Lattice oxygen release will cause the molar content of oxygen to change, and the actual molar content of O will fluctuate.

[0124] In some embodiments, the positive active material layer may optionally further include a binder.

[0125] As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorine-containing acrylate resin.

[0126] In some embodiments, the positive active material layer may optionally further include a conductive agent.

[0127] As an example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0128] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0129] [Electrolytes]

[0130] The electrolyte conducts ions between the positive and negative electrodes. This application does not specify the type of electrolyte, and the electrolyte can be selected based on the needs. For example, the electrolyte can be liquid, gel, or solid.

[0131] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0132] In some embodiments, the electrolyte salt includes at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonyl imide, lithium bistrifluoromethanesulfonyl imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0133] In some embodiments, the solvent includes at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclopentane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0134] In some embodiments, the electrolyte may optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0135] In some embodiments, the battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

[0136] [Isolation film]

[0137] In some embodiments, the material of the separator includes at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0138] The battery of the present application includes a battery cell form, a battery module form and a battery pack form. The battery cell, battery module and battery pack of the present application are described below with reference to the accompanying drawings as appropriate.

[0139] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0140] In some embodiments, the battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0141] In some embodiments, the outer packaging of the battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the battery can also be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, polybutylene succinate, etc.

[0142] The present application has no particular restrictions on the shape of the battery, which can be cylindrical, square or any other shape. For example, Figure 2 The battery cell 5 is a square structure as an example.

[0143] In some embodiments, reference Figure 3 The outer packaging may include a shell 51 and a top cover assembly 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the top cover assembly 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can form an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the battery cell can be one or more, and those skilled in the art can select according to specific actual needs.

[0144] In some embodiments, batteries may be assembled into a battery module. The number of batteries contained in the battery module may be one or more. The specific number may be selected by those skilled in the art based on the application and capacity of the battery module.

[0145] Figure 4 4 is an example of a battery module. Figure 4 In the battery module 4, the plurality of battery cells 5 may be arranged in sequence along the length direction of the battery module 4. Of course, they may also be arranged in any other manner. Furthermore, the plurality of battery cells 5 may be fixed by fasteners.

[0146] Optionally, the battery module 4 may further include a housing having an accommodation space, and the plurality of battery cells 5 are accommodated in the accommodation space.

[0147] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art based on the application and capacity of the battery pack.

[0148] Figure 5 and Figure 6 The battery pack 1 is used as an example. Figure 5 and Figure 6The battery pack 1 may include a battery box and multiple battery modules 4 disposed in the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 can cover the lower box body 3 and form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner in the battery box.

[0149] In a second aspect of the present application, an electrical device is provided, comprising the aforementioned lithium-ion secondary battery. Thus, the electrical device has all the features and advantages of the aforementioned battery, which will not be described in detail here.

[0150] Batteries, battery modules, and battery packs can be used as power sources or energy storage units for electrical devices. Electrical devices may include, but are not limited to, mobile devices (e.g., mobile phones, laptops), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks), electric trains, ships, satellites, and energy storage systems.

[0151] As an electrical device, a battery, battery module or battery pack can be selected according to its usage requirements.

[0152] Figure 7 This is an example of an electric device. This device is a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of the battery, a battery pack or battery module can be used.

[0153] Another example device may be a mobile phone, a tablet computer, a laptop computer, etc. Such a device is usually required to be light and thin, and may use a battery as a power source.

[0154] The present invention will be described below by way of specific examples. It should be noted that the following examples are intended only to illustrate the present invention and are not intended to limit the scope of the present invention. Where specific techniques or conditions are not specified in the examples, they are determined according to the techniques or conditions described in the literature in this area or according to the product specifications. Reagents or instruments not specified by manufacturer are all commercially available conventional products.

[0155] Example 1

[0156] The negative electrode sheet preparation process is as follows:

[0157] Preparation of the first negative electrode slurry: The first negative electrode active material, conductive agent conductive carbon, stabilizer sodium carboxymethyl cellulose and binder SBR are dispersed in deionized water in a mass ratio of 96.5:0.4:1.1:2.0 to form a negative electrode slurry. The oil absorption value of the first negative electrode active material is 45 ml / 100 g, the primary particle ratio is 90%, and the Dv50 particle size is 16 μm.

[0158] Preparation of the second negative electrode slurry: The second negative electrode active material, conductive agent conductive carbon, stabilizer sodium carboxymethyl cellulose and binder SBR are dispersed in deionized water in a mass ratio of 97.3:0.7:1.2:0.8 to form a negative electrode slurry. The oil absorption value of the second negative electrode active material is 47 ml / 100 g, the number of secondary particles accounts for 70%, and the Dv50 particle size is 13 μm.

[0159] Preparation of negative electrode sheet: The first negative electrode slurry and the second negative electrode slurry are evenly coated on both sides of the negative electrode current collector copper foil through an extrusion coater, and the coating weight of the upper and lower layers is controlled to be 0.076g / 1540mm respectively. 2 and 0.114g / 1540mm 2 After drying in an oven, it is compacted using a cold press, with the density controlled at 1.7g / cm 3 The thickness H1 of the first negative electrode active material layer is 30 μm, the thickness H2 of the second negative electrode active material layer is 45 μm, and the ratio of H1:H2 is 2:3.

[0160] Example 2

[0161] Example 2 is consistent with Example 1, except that the oil absorption value and the proportion of secondary particles of the first negative electrode active material are 60 ml / 100 g and 70% respectively, and the oil absorption value and the proportion of secondary particles of the second negative electrode active material are 62 ml / 100 g and 80% respectively.

[0162] Example 3

[0163] Example 3 is consistent with Example 1, except that the oil absorption value and the proportion of secondary particles of the first negative electrode active material are 52 ml / 100 g and 80%, respectively, and the oil absorption value and the proportion of secondary particles of the second negative electrode active material are 55 ml / 100 g and 72%, respectively.

[0164] Example 4

[0165] Example 4 is consistent with Example 3, except that the thickness H1 of the first negative electrode active material layer and the thickness H2 of the second negative electrode active material layer are 2:1.

[0166] Example 5

[0167] Example 5 is consistent with Example 3, except that the thickness H1 of the first negative electrode active material layer and the thickness H2 of the second negative electrode active material layer are 4:1.

[0168] Example 6

[0169] Example 6 is consistent with Example 3, except that the Dv50 particle sizes of the first negative electrode active material and the second negative electrode active material are 14 μm and 12 μm, respectively.

[0170] s

[0171] Example 7

[0172] Example 7 is consistent with Example 3, except that the particle sizes Dv50 of the first negative electrode active material and the second negative electrode active material are 16 μm and 15 μm, respectively.

[0173] Comparative Example 1

[0174] Comparative Example 1 is consistent with Example 1, except that only the first negative electrode slurry is used to form the negative electrode active material layer.

[0175] Comparative Example 2

[0176] Comparative Example 2 is consistent with Example 1, except that only the second negative electrode slurry is used to form the negative electrode active material layer.

[0177] Comparative Example 3

[0178] Comparative Example 3 is consistent with Example 1, except that the second negative electrode active material layer is located on the surface of the negative electrode current collector, and the first negative electrode active material layer is located on the side of the second negative electrode active material layer away from the current collector.

[0179] Assembling the aforementioned negative electrode sheets into a battery specifically includes:

[0180] Positive electrode sheet: Take the positive electrode active material LiNi 0.8 Co 0.1 Mn 0.1 O2, conductive agent Super-P, and binder polyvinylidene fluoride were stirred and dispersed in N-methylpyrrolidone in a mass ratio of 97.4:1.5:1.1 to prepare a positive electrode slurry, which was then coated on the positive electrode current collector aluminum foil with a coating weight of 0.302 g / 1540 mm 2 After being compacted by a cold press, it is further cut to obtain the positive electrode sheet. The compaction density of the positive electrode sheet is 3.45g / cm 3 .

[0181] Separator: A polyethylene porous membrane with a thickness of 12 μm was selected.

[0182] Electrolyte: At 25°C, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain a mixed solvent, and then LiPF6 was dissolved in the above mixed solvent to obtain an electrolyte, wherein the concentration of LiPF6 was 1 mol / L.

[0183] Battery assembly: The prepared negative electrode sheet, separator, and positive electrode sheet are stacked in order, with the separator placed between the positive and negative electrode sheets to serve as an isolation. The bare battery cell is then wound and inserted into the battery casing. The battery is obtained through processes such as baking, liquid injection, standing, packaging, formation, and capacity division.

[0184] The batteries in the aforementioned embodiments and comparative examples were tested as follows. The test results are shown in Table 1:

[0185] Test of 25℃ fast charging capability: The prepared battery was placed at room temperature of 25℃, charged to 4.25V at a constant current rate of 0.33C, then charged at a constant voltage to a current of 0.05C, left for 5 minutes, and then discharged to 2.5V at a constant current of 0.33C. The constant current discharge capacity was recorded as the initial capacity C0. The battery was sequentially charged at a constant current of 0.5C0, 1C0, 1.5C0, 2C0, 2.5C0, 3C0, and 3.5C0 to a full battery potential of 4.25V or a negative electrode cutoff potential of 0mV (reaching any one of these conditions indicates that charging is complete), and discharged to 2.5V at 0.33C0 after each charge is completed. At every interval of 10% SOC (from 10% SOC-80% SOC), the corresponding negative electrode potential at different charging rates was recorded, and the rate-negative electrode potential curve at different SOCs was drawn. After linear fitting, the charging rate corresponding to the negative electrode potential of 0mV at different SOCs was obtained, which was recorded as C x (x = 2-8). Using the formula (1 / C2 + 1 / C3 + 1 / C4 + 1 / C5 + 1 / C6 + 1 / C7 + 1 / C8) × 0.1 × 60, we calculate the charging time (T (min)) required to charge the battery from 10% SOC to 80% SOC. The shorter this time, the better the battery's fast-charging performance.

[0186] Capacity retention rate: At 25°C, the battery was charged at a constant current of 0.33C to a charge cut-off voltage of 4.25V, then charged at a constant voltage to a current of 0.05C, left to stand for 5 minutes, and then discharged at a constant current of 0.33C to a discharge cut-off voltage of 2.5V. The initial capacity was recorded as C0. Then, the battery was charged according to the charging strategy described above and discharged at 0.33C0, and the discharge capacity of each cycle was recorded as C0. n (n is the number of cycles, n is 1-1000), until the cycle is 1000 cycles, calculate the cycle capacity retention rate (i.e. C 1000 / C0×100%). A higher cycle retention rate indicates a better lifespan.

[0187] Table 1

[0188] serial number Fast charging performance / min Capacity retention rate / % Example 1 20 87.5 Example 2 18 88.6 Example 3 17 90.7 Example 4 16 91.4 Example 5 19 88.1 Example 6 16.2 91.5 Example 7 17.9 89.9 Comparative Example 1 25 80.1 Comparative Example 2 30 75.3 Comparative Example 3 34 69.5

[0189] The test results show that the fast-charging performance and cycling performance of the batteries in Examples 1-7 are superior to those of the batteries in the comparative example. Specifically, the first negative electrode active material layer of the batteries in Examples 1-7 can provide a large number of lithium insertion and extraction sites, and the second negative electrode active material layer can provide fast lithium insertion and extraction channels, which facilitates the rapid insertion of lithium ions and improves the fast-charging performance of the negative electrode sheet. As a result, the batteries have better fast-charging performance and better cycling performance.

[0190] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A lithium-ion secondary battery, characterized in that: include: A negative electrode plate, the negative electrode plate comprising: negative electrode current collector; A negative electrode active material layer, the negative electrode active material layer comprising a first negative electrode active material layer and a second negative electrode active material layer stacked, the second negative electrode active material layer being located on a side of the first negative electrode active material layer away from the negative electrode current collector, wherein: The first negative electrode active material layer includes a first negative electrode active material, the first negative electrode active material includes graphite, and the amount of primary particles in the first negative electrode active material is greater than the amount of secondary particles; The second negative electrode active material layer includes a second negative electrode active material, the second negative electrode active material includes graphite, and the amount of secondary particles in the second negative electrode active material is greater than the amount of primary particles. The primary particles in the first negative electrode active material account for greater than or equal to 70%, and the oil absorption value of the first negative electrode active material is 45 mL / 100 g to 60 mL / 100 g. The amount of secondary particles in the second negative electrode active material accounts for greater than or equal to 70%, the oil absorption value of the second negative electrode active material is greater than the oil absorption value of the first negative electrode active material, and the oil absorption value of the second negative electrode active material is 47 mL / 100 g to 62 mL / 100 g; The Dv50 particle size of the second negative electrode active material is smaller than the Dv50 particle size of the first negative electrode active material.

2. The lithium-ion secondary battery according to claim 1, wherein The Dv50 particle size of the first negative electrode active material is 14 μm-22 μm, and / or the Dv50 particle size of the second negative electrode active material is 12 μm-19 μm.

3. The lithium-ion secondary battery according to claim 1 or 2, characterized in that The thickness of the first negative electrode active material layer is H1, the thickness of the second negative electrode active material layer is H2, and the ratio of H1 to H2 is in the range of (2:3)-(4:1).

4. The lithium-ion secondary battery according to claim 3, wherein H1 is 30μm-90μm, and H2 is 20μm-60μm.

5. The lithium-ion secondary battery according to claim 1 or 2, characterized in that: The specific surface area of ​​the second negative electrode active material is greater than the specific surface area of ​​the first negative electrode active material.

6. The lithium-ion secondary battery according to claim 1 or 2, characterized in that Under a pressure of 50,000 N, the compaction density of the first negative electrode active material is greater than the compaction density of the second negative electrode active material.

7. The lithium-ion secondary battery according to claim 1 or 2, characterized in that The OI value of the second negative electrode active material is smaller than the OI value of the first negative electrode active material.

8. The lithium-ion secondary battery according to claim 1 or 2, characterized in that The gram capacity of the first negative electrode active material is greater than the gram capacity of the second negative electrode active material.

9. The lithium-ion secondary battery according to claim 1 or 2, characterized in that: The second negative electrode active material includes the secondary particle having a carbon coating layer, wherein the carbon coating layer is located at least partially on the surface of the secondary particle. The secondary particle is formed by bonding at least two primary particles, and the bonding material includes a carbon material.

10. The lithium-ion secondary battery according to claim 1 or 2, characterized in that The compaction density of the negative electrode active material layer is 1.65 g / cm 3 -1.85g / cm 3 .

11. An electrical device, characterized in that: A lithium ion secondary battery comprising the lithium ion secondary battery according to any one of claims 1 to 10.

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