Lithium-ion secondary battery and electric apparatus

Abstract

The present disclosure provides a lithium-ion secondary battery and an electric apparatus. A negative electrode sheet of the lithium-ion secondary battery comprises: a negative electrode current collector and negative electrode active material layers. The negative electrode active material layers include a first negative electrode active material layer and a second negative electrode active material layer which are stacked, and the second negative electrode active material layer is located on the side of the first negative electrode active material layer away from the negative electrode current collector. The first negative electrode active material layer comprises a first negative electrode active material, the first negative electrode active material comprises graphite, and the proportion of primary particles in the first negative electrode active material is greater than that of secondary particles; and the second negative electrode active material layer comprises a second negative electrode active material, the second negative electrode active material comprises graphite, and the proportion of secondary particles in the second negative electrode active material is greater than that of primary particles.

Description

Lithium-ion secondary batteries and electrical appliances

[0001] Priority information

[0002] This application is based on and claims priority to Chinese Patent Application No. 202510018184.2, filed on January 6, 2025, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This application relates to the field of battery technology, specifically to lithium-ion secondary batteries and electrical devices. Background Technology

[0004] Secondary batteries are widely used in energy storage systems such as hydropower, thermal power, wind power, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, and electric vehicles. With the gradual expansion of battery applications, the market is placing higher demands on the rapid charging and discharging capabilities of batteries. However, current batteries still have many shortcomings in their applications, and their fast-charging performance needs further improvement.

[0005] It should be noted that the above statements are only used to provide background information related to this application and do not necessarily constitute prior art.

[0006] Application content

[0007] In a first aspect, this application proposes a lithium-ion secondary battery, including a negative electrode sheet comprising: a negative electrode current collector; and 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 thereon, the second negative electrode active material layer being located on the side of the first negative electrode active material layer away from the negative electrode current collector. The first negative electrode active material layer comprises a first negative electrode active material, which includes graphite, and the proportion of primary particles in the first negative electrode active material is greater than the proportion of secondary particles. The second negative electrode active material layer comprises a second negative electrode active material, which also includes graphite, and the proportion of secondary particles in the second negative electrode active material is greater than the proportion of primary particles. Therefore, the lithium-ion secondary battery possesses both superior fast-charging performance and high energy density.

[0008] In some embodiments, the proportion of primary particles in the first negative electrode active material is greater than or equal to 70%, and the oil absorption value of the first negative electrode active material is 45mL / 100g-60mL / 100g. This can improve the specific capacity of the negative electrode sheet.

[0009] In some embodiments, the proportion of secondary particles in the second negative electrode active material is greater than or equal to 70%, and the oil absorption value of the second negative electrode active material is greater than that of the first negative electrode active material. Therefore, the lithium-ion secondary battery has superior fast-charging performance.

[0010] In some embodiments, the oil absorption value of the second negative electrode active material is 47mL / 100g-62mL / 100g.

[0011] In some embodiments, the Dv50 particle size of the second negative electrode active material is smaller than that of the first negative electrode active material. Therefore, the second negative electrode active material layer has more pores, which is beneficial for the electrolyte to wet the negative electrode sheet.

[0012] In some embodiments, at least one of the following conditions is met: (1) the Dv50 particle size of the first negative electrode active material is 14μm-22μm, and (2) the Dv50 particle size of the second negative electrode active material is 12μm-19μm.

[0013] 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)-(4:1). This helps the lithium-ion secondary battery to have both high energy density and excellent fast charging performance.

[0014] In some embodiments, H1 is 30μm-90μm and H2 is 20μm-60μm.

[0015] In some embodiments, the specific surface area of ​​the second negative electrode active material is greater than that of the first negative electrode active material. This helps to improve the wetting of the negative electrode sheet by the electrolyte.

[0016] In some embodiments, under a pressure of 50,000 N, the compaction density of the first negative electrode active material is greater than that of the second negative electrode active material. This helps to improve the specific capacity of the negative electrode sheet.

[0017] In some embodiments, the OI value of the second negative electrode active material powder is lower than that of the first negative electrode active material. This helps to improve the fast charging performance of the negative electrode sheet.

[0018] In some embodiments, the specific capacity of the first negative electrode active material is greater than that of the second negative electrode active material. This helps to improve the specific capacity of the negative electrode sheet.

[0019] In some embodiments, the second negative electrode active material includes the secondary particles having a carbon coating layer, the carbon coating layer being located at least on a portion of the surface of the secondary particles, the secondary particles being formed by bonding at least two primary particles together, the bonding material being a carbon material. This helps to improve the fast-charging performance of the negative electrode sheet.

[0020] In some embodiments, the compaction density of the negative electrode active material layer is 1.65 g / cm³. 3 -1.85g / cm 3 This helps improve the wetting of the negative electrode by the electrolyte.

[0021] In a second aspect, this application proposes an electrical device including the aforementioned lithium-ion secondary battery. Thus, the electrical device possesses all the features and advantages of the aforementioned battery, which will not be repeated here. Attached Figure Description

[0022] The above and / or additional aspects and advantages of this application will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0023] Figure 1 is a schematic diagram of the structure of a negative electrode sheet according to an embodiment of this application;

[0024] Figure 2 is a schematic diagram of a battery cell according to an embodiment of this application;

[0025] Figure 3 is an exploded view of a battery cell according to an embodiment of this application shown in Figure 2;

[0026] Figure 4 is a schematic diagram of a battery module according to an embodiment of this application;

[0027] Figure 5 is a schematic diagram of a battery pack according to an embodiment of this application;

[0028] Figure 6 is an exploded view of a battery pack according to an embodiment of this application, as shown in Figure 5;

[0029] Figure 7 is a schematic diagram of an electrical device according to an embodiment of this application;

[0030] Figure 8 is a scanning electron microscope image of the negative electrode active material containing secondary particles.

[0031] Explanation of reference numerals in the attached drawings: 1 Battery pack; 2 Upper casing; 3 Lower casing; 4 Battery module; 5 Battery cell; 51 Housing; 52 Electrode assembly; 53 Top cover assembly; 100 Negative electrode current collector; 110 First negative electrode active material layer; 120 Second negative electrode active material layer. Detailed Implementation

[0032] The embodiments of this application are described in detail below, with examples of these embodiments shown in the accompanying drawings. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0033] Unless otherwise defined, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used in this application is for the purpose of describing particular embodiments only and is not intended to limit this application; unless otherwise stated, the values ​​of the parameters mentioned in this application can be measured using various measurement methods commonly used in the art (e.g., they can be tested according to the methods given in the embodiments of this application).

[0034] The terms “comprising” and “having”, and any variations thereof, in the specification and claims of this application are open-ended expressions, meaning they include what is specified in this application but do not exclude other aspects.

[0035] In the description of this application, all figures disclosed herein, whether or not the words "approximately" or "about" are used, are approximate values. Each figure may vary by less than 10% or by a difference that is considered reasonable by one of the art, such as 1%, 2%, 3%, 4%, or 5%.

[0036] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 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 construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. "First feature" and "second feature" may include one or more of the indicated feature.

[0038] In the description of this application, "A and / or B" can include any of the cases of A alone, B alone, or A and B, where A and B are merely examples and can be any technical feature connected by "and / or" in this application.

[0039] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0040] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0041] The technical features in this application can be tested by reverse disassembling the battery, or by obtaining fresh materials / batteries that were not yet manufactured during battery production. For example, when testing the characteristics of the negative electrode active material, it can be tested by scraping and burning graphite powder from the negative electrode sheet after battery disassembly, as conventionally understood, or by obtaining fresh graphite that has not yet been made into a battery.

[0042] Taking graphite as an example of anode active material, the structure of primary graphite particles is more stable, especially under high compaction density, it can still maintain good structural stability. Furthermore, because primary graphite particles are not agglomerated, they are easier to disperse evenly during the mixing process of the anode slurry, resulting in a lower oil absorption value. The primary particles have fewer surface irregularities, making it easier for particles to slip during cold pressing and form a tight connection with the current collector, resulting in high compaction and a higher cell volumetric energy density. However, due to the anisotropy of primary graphite particles, lithium ions can only embed into the interlayer structure of graphite along the end faces, leading to a lower lithium ion migration rate. Moreover, the expansion force generated after lithium ions embed into the interlayer structure can only be released in a direction perpendicular to the interlayer structure, resulting in significant thickness changes of the anode sheet during charging and discharging, and noticeable battery expansion. This can easily cause the electrolyte generated by the expansion of the anode sheet to be squeezed out and unable to flow back, leading to a drop in battery cycle life.

[0043] Unlike primary graphite particles, secondary graphite particles are isotropic. The numerous lithium-ion transport channels in secondary particles effectively increase the lithium-ion insertion / extraction entry points in the negative electrode active material, improving lithium-ion interfacial reactions and increasing lithium-ion migration rates. This, in turn, enhances the fast-charging performance of the negative electrode active material. Simultaneously, the expansion force generated during lithium insertion can be released in multiple directions, reducing the thickness change of the negative electrode sheet during charging and discharging, and consequently reducing battery expansion. This effectively alleviates the problem of electrolyte being squeezed out and unable to flow back due to negative electrode sheet expansion. Furthermore, secondary particles are formed by the bonding of primary particles, creating more uneven structures between the particles. These uneven structures strongly adsorb dispersants, resulting in a large amount of dispersant from the negative electrode slurry being adsorbed onto the particle surface. This blocks the lithium insertion / extraction channels on the graphite surface, leading to less free dispersant in the negative electrode slurry and poorer stability. Consequently, the negative electrode active material exhibits a high oil absorption value. To compensate for the excessive adsorption of dispersant by the negative electrode active material with a large oil absorption value and to 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 lithium intercalation / deintercalation, the proportion of negative electrode active material that can provide lithium intercalation / deintercalation sites in the negative electrode slurry will decrease. Consequently, the proportion of negative electrode active material in the negative electrode active material layer formed by the negative electrode slurry coating will decrease accordingly, ultimately resulting in a lower specific capacity of the negative electrode active material layer.

[0044] In this application, a first negative electrode active material and a second negative electrode active material are used together to form a negative electrode active material layer. Both the first negative electrode active material and the second negative electrode active material include graphite. The proportion of primary particles in the first negative electrode active material is greater than the proportion of secondary particles, and the proportion of secondary particles in the second negative electrode active material is greater than the proportion of primary particles.

[0045] Specifically, in a system with two or more layers of negative electrode active material, the second negative electrode active material is closer to the electrolyte, with a higher surrounding ion concentration, making it more prone to ion accumulation and lithium plating. Therefore, when the proportion of secondary particles in the second negative electrode active material is greater than that of primary particles, the numerous lithium intercalation / deintercalation channels of the secondary particles can be utilized to ensure rapid lithium ion transfer to the bulk phase, improve lithium ion interface reactions, increase lithium ion migration rates, and thus enhance 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 their uneven surfaces make it difficult for particles to slide during cold pressing, resulting in lower compaction density and affecting the volumetric energy density of the negative electrode sheet. Therefore, the second negative electrode active material layer has higher kinetic performance and relatively lower specific capacity. Furthermore, in a system with two or more layers of negative electrode active material, the first negative electrode active material is closer to the negative electrode current collector, where the surrounding ion concentration is relatively low, and the requirement for lithium-ion migration rate is also relatively low. 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, such as larger specific capacity, easier compaction, lower oil absorption value, and less dispersant required in the negative electrode slurry. This results in a higher proportion of high-specific-capacity active material in the first negative electrode active material layer, greatly improving the overall specific capacity of the negative electrode sheet and compensating for the lower specific capacity of the second negative electrode active material layer. Through the combined use of the aforementioned first and second negative electrode active materials, lithium-ion secondary batteries achieve both superior fast-charging performance and high energy density.

[0046] Specifically, during the charging and discharging process of the battery, lithium ions are extracted 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 sheet. Since the number of secondary particles in the second negative electrode active material is relatively high, it helps the lithium ions to be embedded quickly. The negative electrode sheet has better fast charging performance during charging and discharging, 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 first negative electrode active material layer has a high specific capacity, it can provide more lithium insertion sites, which helps the lithium ions to be embedded as completely as possible in the first negative electrode active material layer. This effectively reduces the occurrence of lithium dendrite defects on the surface of the negative electrode sheet and improves the fast charging performance and specific capacity of the negative electrode sheet.

[0047] In this application, "the proportion of secondary particles in the negative electrode active material can be determined using a scanning electron microscope. As an example, the test method for the proportion of secondary particles can be as follows: the negative electrode active material is uniformly laid and adhered to conductive adhesive to form a sample with dimensions of 6cm x 1.1cm; the particle morphology is tested using a scanning electron microscope (such as a ZEISS Sigma 300). The test can be referenced in JY / T010-1996. To ensure the accuracy of the test results, multiple (e.g., 20) different regions can be randomly selected from the sample for scanning tests. At a certain magnification (e.g., 1000x), the percentage of secondary particles in each test region relative to the total number of particles is calculated, which is the proportion of secondary particles in that region. The average of the test results from the corresponding multiple (e.g., 20) test regions is taken as the proportion of secondary particles in the negative electrode active material."

[0048] It should be noted that in this 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 its packing structure, the second negative electrode active material layer has a looser overall structure than 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 in the following way.

[0050] Specifically: ① Cut the negative electrode sheet into 6mm×6mm pieces using ceramic scissors, attach it to the sample stage coated with paraffin wax, ensuring the sample protrudes slightly (<1mm) from the edge of the sample stage to create a polished cross-section section; ② Place the section on a CP-SEM (Cross Section Polisher-Scanning Electron)... In a cross-sectional polishing-scanning electron microscope chamber, using backscatter mode, more than 10 scanning images are captured at 1000x magnification to test the thickness of the negative electrode sheet. Porosity analysis is performed on the scanning images in 10μm thickness steps. 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 H1 of the first negative electrode active material layer. The thickness H2 of the second negative electrode active material layer is the total thickness H of the electrode sheet minus the thickness H1 of the first negative electrode active material layer. ③ Two-thirds of the thickness H2 of powder is scraped off from the surface of the negative electrode sheet and defined as the second negative electrode active material. The remaining negative electrode sheet is scraped off powder, and 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 particle count ratio of the first negative electrode active material powder and the second negative electrode active material is tested twice. For example, the proportion of secondary particles can be calculated as follows: (a) The aforementioned negative electrode active material powder is uniformly dispersed on the conductive tape substrate using a vacuum negative pressure sputtering method; (b) 50 high-speed images are taken at 500x magnification using the mapping function of a scanning electron microscope (such as Thermo Scientific Apreo 2 S); (c) Particles in each image are automatically labeled using an AI deep learning model or manual image recognition to obtain the number of primary and secondary particles. The results are then statistically analyzed to calculate the proportion of secondary particles. A secondary particle is defined as a single particle formed when two or more particles are stacked. For example, a secondary particle can be a stack of multiple particles with different diameters (see ① in Figure 8), a stack of multiple particles with similar diameters (see ② in Figure 8), or a stack of multiple smaller particles on the surface of a larger particle (see ③ in Figure 8). A primary particle is defined as a particle that is not stacked. For example, see ④ and ⑤ in Figure 8.

[0051] In a first aspect of this application, referring to FIG1, this application proposes a lithium-ion secondary battery, including a negative electrode sheet, the negative electrode sheet comprising: a negative electrode current collector 100; and a negative electrode active material layer, the negative electrode active material layer comprising a first negative electrode active material layer 110 and a second negative electrode active material layer 120 stacked thereon, the second negative electrode active material layer 120 being located on the 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 comprises a first negative electrode active material; the first negative electrode active material comprises graphite, and the proportion of primary particles in the first negative electrode active material is greater than the proportion of secondary particles; the second negative electrode active material layer 120 comprises a second negative electrode active material, the second negative electrode active material comprises graphite, and the proportion of secondary particles in the second negative electrode active material is greater than the proportion of primary particles. The first negative electrode active material layer can provide more lithium intercalation sites, improving the specific capacity of the negative electrode sheet, and the second negative electrode active material layer can provide more fast lithium intercalation / deintercalation channels, improving the fast charging performance of the negative electrode sheet. Therefore, lithium-ion secondary batteries have both superior fast-charging performance and high energy density.

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

[0053] As an example, the percentage of primary particles in the first negative electrode active material can be 70%, 75%, 80%, 85%, 90%, 95%, or 100%.

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

[0055] For the first negative electrode active material, the higher the proportion of primary particles, the greater the specific capacity. When the first negative electrode active material consists entirely of primary particles, it has the most lithium intercalation sites, resulting in the optimal specific capacity. Simultaneously, its oil absorption value is low, requiring less dispersant in the negative electrode slurry, and the specific capacity of the formed first negative electrode active material layer is also improved. In this case, lithium ions can only be inserted and extracted from the end faces of the first negative electrode active material, thus exhibiting a high specific capacity and relatively poor kinetic performance. Those skilled in the art can select the proportion of primary particles and the oil absorption value of the first negative electrode active material according to the actual situation.

[0056] In some embodiments, the proportion of secondary particles in the second negative electrode active material is 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 proportion of secondary particles in the second negative electrode active material can be 70%, 75%, 80%, 85%, 90%, 95%, or 100%.

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

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

[0060] The secondary particles in the second anode active material constitute a higher proportion than those in the first anode active material. These secondary particles expose more uneven surfaces, resulting in higher oil absorption and requiring more dispersant to maintain dispersion. This can lead to the obstruction of lithium intercalation / deintercalation channels, affecting ion transport. Therefore, the second anode active material exhibits a significantly higher oil absorption value compared to the first. When the oil absorption value of the second anode active material is within the range of 47 mL / 100g-62 mL / 100g, the secondary particles can be processed with less dispersant, thus balancing the processing dynamics. Therefore, when the oil absorption value of the secondary particles is limited to a specific range, the anode active material requires less dispersant to be processed, resulting in greater exposure of the lithium intercalation end face, achieving a balance between processing and kinetics.

[0061] It is understandable that for the second negative electrode active material, the higher the proportion of secondary particles, the better the kinetics of the second negative electrode active material. When the second negative electrode active material consists entirely of secondary particles, its kinetic performance is optimal. In this case, the second negative electrode active material is mainly composed of secondary particles, resulting in more end faces and fewer lithium storage sites. Therefore, the second negative electrode active material has high kinetic performance and relatively low specific capacity. Furthermore, the oil absorption value of this second negative electrode active material is also high, and the uneven surface makes it more difficult to press during the cold pressing process, resulting in a decrease in the volumetric energy density of the battery. Those skilled in the art can select the proportion of secondary particles and the oil absorption value of the second negative electrode active material according to the actual situation.

[0062] In this 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, 100g of negative electrode active material is placed at the feeding port of the oil absorption value tester, and dibutyl phthalate is automatically titrated onto the negative electrode active material until the negative electrode active material becomes 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 / 100g.

[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 its packing structure, the second negative electrode active material layer has a looser overall structure than 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 in the following way.

[0064] Specifically: ① Cut the negative electrode sheet into 6mm×6mm pieces using ceramic scissors, attach it to the sample stage coated with paraffin wax, ensuring the sample protrudes slightly (<1mm) from the edge of the sample stage to create a polished cross-section section; ② Place the section on a CP-SEM (Cross Section Polisher-Scanning Electron)... In a cross-sectional polishing-scanning electron microscope chamber, using backscatter mode, more than 10 scanning images are captured at 1000x magnification to test the thickness of the negative electrode sheet. Porosity analysis is performed on the scanning images in 10μm thickness steps. 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 H1 of the first negative electrode active material layer. The thickness H2 of the second negative electrode active material layer is the total thickness H of the electrode sheet minus the thickness H1 of the first negative electrode active material layer. ③ Two-thirds of the powder of thickness H2 is scraped off from the surface of the negative electrode sheet and defined as the second negative electrode active material. The remaining negative electrode sheet is scraped 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 is tested respectively.

[0065] In some embodiments, the Dv50 particle size of the second negative electrode active material is smaller than that of the first negative electrode active material. Therefore, the second negative electrode active material is more difficult to press during the cold pressing process, resulting in a film layer with more pores, which is beneficial for the electrolyte to wet the negative electrode sheet.

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

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

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

[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)-(4:1). This helps the negative electrode sheet to have both high specific capacity and better fast charging performance.

[0070] As an example, the ratio of H1 to H2 can 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 aforementioned range, the negative electrode exhibits superior fast-charging performance, lower volume expansion rate, and higher specific capacity. Specifically, the first negative electrode active material layer effectively enhances the specific capacity of the negative electrode, while the second negative electrode active material layer balances improving fast-charging performance with suppressing negative electrode expansion.

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

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

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

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

[0076] Specific surface area refers to the surface area of ​​a unit mass of material. A larger specific surface area means more channels and shorter paths for lithium ion migration, resulting in better fast-charging performance of the negative electrode active material. However, this also requires more dispersant for dispersion; that is, when the proportion of secondary particles in the negative electrode active material is large, the specific surface area of ​​the negative electrode active material is larger, and the oil absorption value of the negative electrode active material increases accordingly. When the specific surface area of ​​the second negative electrode active material is greater than that of the first negative electrode active material, the improvement in fast-charging performance by the second negative electrode active material is more significant.

[0077] As an example, the specific surface area of ​​the first negative electrode active material can be 1.5 m². 2 / g-3.5m 2 / g, for example, the specific surface area of ​​the first negative electrode active material can be 1.5m². 2 / g-2.5m 2 / g.

[0078] As an example, the specific surface area of ​​the second negative electrode active material can be 3.0 m². 2 / g-4.5m 2 / g, for example, the specific surface area of ​​the second negative electrode active material can be 3.4m². 2 / g-4.0m 2 / g.

[0079] The specific surface area of ​​the negative electrode active material can be measured using the American Mach Gemini VII2390 multi-station fully automated specific surface area and porosity analyzer. 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 15min, and then put it into the main unit for testing to obtain BET data.

[0080] In some embodiments, under a pressure of 50,000 N, the compaction density of the first negative electrode active material is greater than that of the second negative electrode active material. This helps to improve 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 rolling, 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 more compact after rolling, providing more lithium insertion / extraction sites for the negative electrode sheet. Furthermore, the overall pore structure of the negative electrode sheet remains relatively large, which is beneficial for the electrolyte's wetting of the negative electrode sheet and for lithium ion insertion / extraction.

[0082] As an example, the powder compaction density of the first negative electrode active material at 50000N can be 1.85 g / cm³. 3 -2.00g / cm 3 For example, the powder compaction density of the first negative electrode active material at 50000N can be 1.88 g / cm³. 3 -1.98g / cm 3 .

[0083] As an example, the powder compaction density of the second negative electrode active material at 50000N can be 1.78 g / cm³. 3 -1.95g / cm 3 For example, the powder compaction density of the second negative electrode active material at 50000N can be 1.80 g / cm³. 3 -1.90g / cm 3 .

[0084] In some embodiments, the OI value of the second negative electrode active material powder is lower than that of the first negative electrode active material. This helps to improve the fast charging performance of the negative electrode sheet.

[0085] The lower the OI value, the stronger the lithium-ion intercalation capability of the negative electrode active material. Specifically, the greater the proportion of secondary particles in the negative electrode active material, the lower the OI value. When the OI value of the second negative electrode active material powder is less than that of the first negative electrode active material, the proportion of secondary particles in the second negative electrode active material is relatively large. Compared to primary particles, which can only intercalate lithium ions from their end faces, secondary particles are isotropic and can intercalate lithium ions from multiple directions. The second negative electrode active material layer has the ability to quickly complete lithium-ion intercalation, which can effectively improve the fast-charging performance of the negative electrode sheet.

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

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

[0088] When graphite, the active material for the negative electrode, is fabricated into a negative electrode sheet, the orientation of the graphite layered structure has a significant impact on lithium-ion migration. Since graphite can only intercalate and deintercalate lithium ions at its end faces, ideally, the end faces of the graphite in the active material should be perpendicular to the surface of the negative electrode sheet for better lithium-ion diffusion. However, in actual fabrication, it is difficult to precisely control the orientation of each graphite particle. XRD can be used to test the orientation of graphite on the negative electrode sheet. When performing diffraction pattern testing on a horizontally placed negative electrode sheet sample, the diffraction signal of the (110) crystal plane can be collected from the graphite in the active material layer that is perpendicular to the surface of the negative electrode sheet. The diffraction signals of the (002) and (004) crystal planes come from the graphite in the active material that is parallel to the surface of the electrode sheet. Therefore, the orientation of graphite in the negative electrode sheet can be described by the ratio of the intensity (or integrated area) of the (002) or (004) diffraction peak to the intensity (or integrated area) of the (110) diffraction peak. 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 specific capacity of the first negative electrode active material is greater than that of the second negative electrode active material. Since the second negative electrode active material is mainly composed of secondary particles, which have more exposed end faces, but these end faces do not have lithium storage function, its specific capacity is relatively low. The first negative electrode active material is mainly used to improve the specific capacity of the negative electrode sheet. When the specific capacity 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 provides more capacity, and the negative electrode sheet has a higher specific capacity.

[0090] As an example, the specific capacity of the first negative electrode active material can be greater than or equal to 358 mAh / g, for example, the specific capacity of the first negative electrode active material can be 360 ​​mAh / g-367 mAh / g. It can be used internally for lithium intercalation.

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

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

[0093] The carbon coating layer can cover the active sites on the surface of graphite secondary particles, reducing the direct contact between secondary particles in the negative electrode active material and the electrolyte, reducing the occurrence of irreversible side reactions, and also restricting and buffering the volume expansion of graphite, increasing the stability during cycling.

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

[0095] It is understandable that the primary purpose of the first negative electrode active material is to provide lithium intercalation sites for the negative electrode sheet. The amorphous carbon coating layer can only improve the lithium ion intercalation channel and cannot be used as an effective lithium intercalation site itself. Therefore, in order to meet the high specific capacity requirement, the carbon coating layer on the surface of the primary particles may not be required for the first negative electrode active material.

[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 surface of the negative electrode active material particles has fewer irregularities, the particles adhere more tightly, resulting in a higher tap density. Furthermore, the amount of dispersant required in the negative electrode slurry is also less. Therefore, when the tap density of the first negative electrode active material is greater than that of the second negative electrode active material, the particles of the first negative electrode active material adhere more tightly, which helps to increase the specific capacity of the first negative electrode active material layer, thereby increasing 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 / cm³. 3 -1.30g / cm 3 For example, the tap density of the first negative electrode active material can be 1.00 g / cm³. 3 -1.20g / cm 3 .

[0099] As an example, the tap density of the second negative electrode active material can be 0.90 g / cm³. 3 -1.20g / cm 3 For example, the tap density of the second negative electrode active material can be 0.95 g / cm³. 3 -1.10g / cm 3 .

[0100] Tapped density refers to the mass per unit volume of powder in a container after it has been tapped under specified conditions, and is expressed in g / cm³. 3 It is measured by a specialized tap density instrument.

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

[0102] In some embodiments, the compaction density of the negative electrode active material layer is 1.65 g / cm³. 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, resulting in better wetting of the negative electrode active material layer by the electrolyte and a faster liquid phase conduction rate of lithium ions. Furthermore, when the second negative electrode active material has a surface carbon coating layer, the solid phase conduction rate of lithium ions can be further improved through the carbon coating layer, thereby enhancing the fast charging performance of the negative electrode sheet.

[0104] The aforementioned compaction density of the negative electrode active material layer refers to the compaction density of the negative electrode active material layer after roll forming. Specifically, the compaction density of the negative electrode active material layer after roll forming and formation, the compaction density of the negative electrode active material layer when the battery is fully charged or fully discharged, and the compaction density of the negative electrode active material layer after the battery has been left to stand for a long time are all within the aforementioned range.

[0105] As an example, the compaction density of the negative electrode active material layer can be 1.65 g / cm³. 3 1.70g / cm 3 1.75g / cm 31.80g / cm 3 Or 1.85g / cm 3 .

[0106] It should be noted that the compaction density of the aforementioned negative electrode active material layer is the designed compaction density after roll forming. After the battery undergoes charge-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, which in turn leads to a decrease in 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 roll forming is 1.65 g / cm³. 3 -1.85g / cm 3 At that time, after charge-discharge cycles, the compaction density of the negative electrode active material layer may be 1.55 g / cm³. 3 -1.75g / cm 3 .

[0107] In some embodiments, the areal density of the negative electrode active material layer may be greater than or equal to 7 mg / cm³. 2 For example, it could be 8 mg / cm³ 2 -15mg / cm 2 .

[0108] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode active material layer is disposed on either or both of the two opposite 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 polymeric material substrate and a metal layer formed on at least one surface of the polymeric 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 polymeric 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 material, the negative electrode active material layer may also contain negative electrode active materials known in the art for use in batteries.

[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, this application is not limited to these materials, and other conventional 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 optionally include a binder. The binder includes 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 optionally include a conductive agent. The conductive agent includes at least one selected from 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 also optionally include other additives, such as dispersants (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0115] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0116] Typically, a battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active metal ions repeatedly insert and extract between the positive and negative electrodes. The electrolyte acts as a conductor of ions between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits between them while allowing active metal ions to pass through.

[0117] [Positive electrode plate]

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

[0119] In some embodiments, the positive 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 substrate and a metal layer formed on at least one surface of the polymer substrate. 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 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 known in the art for lithium-ion batteries.

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

[0122] During the charging and discharging process of a battery, lithium (Li) undergoes insertion / extraction and consumption, resulting in varying molar Li content at different discharge states. In the examples of positive electrode active materials in this application, the molar Li content refers to the initial state of the material, i.e., before feeding. After charge-discharge cycles, the molar Li content changes when the positive electrode active material is applied to the battery system.

[0123] In the examples of positive electrode active materials in this application, the molar content of O is only a theoretical value. Oxygen release from the crystal lattice will cause changes in the molar content of oxygen, and the actual molar content of O will fluctuate.

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

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

[0126] In some embodiments, the positive electrode active material layer may optionally 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 dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0129] [Electrolytes]

[0130] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0131] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes 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 bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0133] In some embodiments, the solvent includes at least one selected from ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

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

[0135] In some embodiments, the battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0136] [Isolation membrane]

[0137] In some embodiments, the material of the separator includes at least one selected from glass fiber, nonwoven 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 batteries of this application include battery cells, battery modules, and battery packs. The battery cells, battery modules, and battery packs of this application will be described below with appropriate reference to the accompanying drawings.

[0139] In some embodiments, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly by a winding process or a stacking process.

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

[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 pack, such as a pouch. The material of the soft pack can be plastic, for example, it can include polypropylene, polybutylene terephthalate, polybutylene succinate, etc.

[0142] This application does not impose any particular limitation on the shape of the battery; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 2 shows a square battery cell 5 as an example.

[0143] In some embodiments, referring to FIG3, the outer packaging may include a housing 51 and a top cover assembly 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the top cover assembly 53 can cover the opening to close the receiving cavity. A positive electrode sheet, a negative electrode sheet, and a separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in a single battery cell may be one or more, which can be selected by those skilled in the art according to specific practical needs.

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

[0145] Figure 4 shows a battery module 4 as an example. Referring to Figure 4, in the battery module 4, multiple battery cells 5 can be arranged sequentially along the length of the battery module 4. Of course, they can also be arranged in any other manner. Furthermore, the multiple battery cells 5 can be fixed in place using fasteners.

[0146] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0147] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0148] Figures 5 and 6 show a battery pack 1 as an example. Referring to Figures 5 and 6, the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper box 2 and a lower box 3, with the upper box 2 covering the lower box 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0149] In a second aspect, this application proposes an electrical device comprising the aforementioned lithium-ion secondary battery. Thus, the electrical device possesses all the features and advantages of the aforementioned battery, which will not be repeated here.

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

[0151] As an electrical device, batteries, battery modules, or battery packs can be selected according to their usage requirements.

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

[0153] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a battery as their power source.

[0154] The following specific embodiments illustrate the solution of this application. It should be noted that these embodiments are for illustrative purposes only and should not be considered as limiting the scope of this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0155] Example 1

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

[0157] Preparation of the first negative electrode slurry: The first negative electrode active material, conductive carbon, sodium carboxymethyl cellulose stabilizer, and SBR binder were dispersed in deionized water at 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 was 45 ml / 100 g, the primary particle ratio was 90%, and the Dv50 particle size was 16 μm.

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

[0159] Preparation of the negative electrode sheet: The first and second negative electrode slurries are uniformly coated on both sides of the copper foil of the negative electrode current collector using an extrusion coating machine, and the coating weights of the upper and lower layers are controlled to be 0.076g / 1540mm. 2 and 0.114g / 1540mm 2 After being dried in an oven, it is then compacted using a cold press to control the density at 1.7 g / cm³. 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 to 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 60ml / 100g and 70%, respectively, and the oil absorption value and the proportion of secondary particles of the second negative electrode active material are 62ml / 100g 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 52ml / 100g and 80%, respectively, and the oil absorption value and the proportion of secondary particles of the second negative electrode active material are 55ml / 100g 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 particle sizes of the first negative electrode active material and the second negative electrode active material Dv50 are 14 μm and 12 μm, respectively.

[0170] Example 7

[0171] Example 7 is consistent with Example 3, except that the particle sizes of the first negative electrode active material and the second negative electrode active material Dv50 are 16 μm and 15 μm, respectively.

[0172] Comparative Example 1

[0173] 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.

[0174] Comparative Example 2

[0175] 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.

[0176] Comparative Example 3

[0177] 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.

[0178] Assembling the aforementioned negative electrode sheet into a battery specifically includes:

[0179] Positive electrode sheet: LiNi is used as the positive electrode active material. 0.8 Co 0.1 Mn 0.1 O2, conductive agent Super-P, and binder polyvinylidene fluoride were dispersed in N-methylpyrrolidone at a mass ratio of 97.4:1.5:1.1 to prepare a positive electrode slurry. This slurry was then coated onto 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, the material is further cut to obtain the positive electrode sheet, which has a compacted density of 3.45 g / cm³. 3 .

[0180] Separating membrane: A 12μm thick porous polyethylene membrane was selected.

[0181] Electrolyte: Ethyl carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed at a volume ratio of 1:1:1 at 25°C to obtain a mixed solvent. LiPF6 is then dissolved in the mixed solvent to obtain the electrolyte, wherein the concentration of LiPF6 is 1 mol / L.

[0182] Battery assembly: The prepared negative electrode sheet, separator, and positive electrode sheet are stacked in sequence, with the separator in the middle of the positive and negative electrode sheets to provide isolation. The cells are then wound to obtain bare cells, which are then inserted into the battery casing. The battery is obtained through processes such as baking, electrolyte injection, settling, encapsulation, formation, and capacity testing.

[0183] The batteries in the aforementioned embodiments and comparative examples were tested as follows, and the test results are shown in Table 1:

[0184] Test of 25℃ fast charging capability: The prepared battery was placed at room temperature (25℃) and charged to 4.25V using a constant current rate of 0.33C. Then, it was charged to 0.05C using a constant voltage rate, allowed to rest for 5 minutes, and then discharged to 2.5V using a constant current rate of 0.33C. The constant current discharge capacity was recorded as the initial capacity C0. The battery was then sequentially charged at constant current rates of 0.5C0, 1C0, 1.5C0, 2C0, 2.5C0, 3C0, and 3.5C0 until the full cell potential of 4.25V or the negative electrode cutoff potential of 0mV (reaching either condition indicates completion of charging). After each charging, it was discharged to 2.5V using a constant current rate of 0.33C0. At 10% SOC intervals (from 10% SOC to 80% SOC), the corresponding negative electrode potential at different charging rates was recorded. Rate-negative electrode potential curves at different SOCs were plotted. After linear fitting, the charging rate corresponding to a negative electrode potential of 0mV at different SOCs was obtained, denoted 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, the charging time T (min) for the battery to charge from 10% SOC to 80% SOC is calculated. The shorter this time, the better the battery's fast charging performance.

[0185] Capacity retention: At 25°C, the battery was charged at a constant current of 0.33C to the charging cutoff voltage of 4.25V, then charged at a constant voltage to a current of 0.05C, allowed to stand for 5 minutes, and then discharged at a constant current of 0.33C to the discharge cutoff voltage of 2.5V. The initial capacity was recorded as C0. Then, the charging strategy described above was followed, with discharge at 0.33C0. The discharge capacity C of each cycle was recorded. n (n is the number of cycles, n is 1-1000), until 1000 cycles are completed, then calculate the cycle capacity retention rate (i.e., C). 1000 / C0×100%). A higher cycle retention rate indicates a better lifespan.

[0186] Table 1

[0187] Test results show that the fast-charging performance and cycle performance of the batteries in Examples 1-7 are superior to those of the batteries in the comparative examples. Specifically, the first negative electrode active material layer of the batteries in Examples 1-7 can provide more lithium insertion / extraction sites, and the second negative electrode active material layer can provide rapid lithium insertion / extraction channels, which helps to facilitate the rapid insertion of lithium ions and improve the fast-charging performance of the negative electrode sheet. Therefore, the batteries have superior fast-charging performance and superior cycle performance.

[0188] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

Lithium-ion secondary batteries, among which... include: The negative electrode plate includes: Negative electrode current collector; The negative electrode active material layer includes a first negative electrode active material layer and a second negative electrode active material layer stacked together, wherein the second negative electrode active material layer is located on the side of the first negative electrode active material layer away from the negative electrode current collector. The first negative electrode active material layer includes a first negative electrode active material, which includes graphite, and the proportion of primary particles in the first negative electrode active material is greater than the proportion of secondary particles. The second negative electrode active material layer includes a second negative electrode active material, which includes graphite, and the proportion of secondary particles in the second negative electrode active material is greater than the proportion of primary particles. According to claim 1, the lithium-ion secondary battery, wherein, The proportion of primary particles in the first negative electrode active material is greater than or equal to 70%, and the oil absorption value of the first negative electrode active material is 45mL / 100g-60mL / 100g. The lithium-ion secondary battery according to claim 1 or 2, wherein, The proportion of secondary particles in the second negative electrode active material is greater than or equal to 70%, and the oil absorption value of the second negative electrode active material is greater than that of the first negative electrode active material. The lithium-ion secondary battery according to any one of claims 1-3, wherein, The oil absorption value of the second negative electrode active material is 47mL / 100g-62mL / 100g. The lithium-ion secondary battery according to any one of claims 1-4, wherein, The Dv50 particle size of the second negative electrode active material is smaller than that of the first negative electrode active material. According to claim 5, the lithium-ion secondary battery, wherein, At least one of the following conditions must be met: (1) The Dv50 particle size of the first negative electrode active material is 14μm-22μm. (2) The particle size of the second negative electrode active material Dv50 is 12μm-19μm. The lithium-ion secondary battery according to any one of claims 1-6, wherein, 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). The lithium-ion secondary battery according to claim 7, wherein, H1 is 30μm-90μm, and H2 is 20μm-60μm. The lithium-ion secondary battery according to any one of claims 1-8, wherein, The specific surface area of ​​the second negative electrode active material is greater than that of the first negative electrode active material. The lithium-ion secondary battery according to any one of claims 1-9, wherein, Under a pressure of 50,000 N, the compaction density of the first negative electrode active material is greater than that of the second negative electrode active material. The lithium-ion secondary battery according to any one of claims 1-10, wherein, The OI value of the second negative electrode active material is less than that of the first negative electrode active material. The lithium-ion secondary battery according to any one of claims 1-11, wherein, The specific capacity of the first negative electrode active material is greater than that of the second negative electrode active material. The lithium-ion secondary battery according to any one of claims 1-12, wherein, The second negative electrode active material includes the secondary particles having a carbon coating layer, the carbon coating layer being located at least on a portion of the surface of the secondary particles, the secondary particles being formed by bonding at least two primary particles together, the bonding material being a carbon material. The lithium-ion secondary battery according to any one of claims 1-13, wherein, The compaction density of the negative electrode active material layer is 1.65 g / cm³. 3 -1.85g / cm 3 . Electrical appliances, among which Includes the lithium-ion secondary battery according to any one of claims 1-14.

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