Secondary battery and electric device

Abstract

Provided in the present disclosure are a secondary battery and an electric device. The secondary battery comprises: a negative electrode sheet, wherein the negative electrode sheet comprises a negative electrode current collector and a negative electrode film layer formed on at least one surface of the negative electrode current collector; the negative electrode film layer has a first surface away from the negative electrode current collector and a second surface arranged opposite the first surface; the thickness of the negative electrode film layer is denoted as H; a region within a thickness range from the second surface of the negative electrode film layer to 0.3H is denoted as a first region of the negative electrode film layer; a region within a thickness range from the first surface of the negative electrode film layer to 0.3H is denoted as a second region of the negative electrode film layer; the first region comprises a first binder, the first binder being a linear binder; and the second region comprises a second binder, the second binder being a dot-like binder. The secondary battery can reduce the cold-press rebound rate and cyclic expansion rate of an electrode sheet, thereby improving the cycling stability of the battery.

Description

Secondary batteries and electrical devices

[0001] Cross-references

[0002] This application claims priority to Chinese Patent Application No. 202311675385.7, filed on December 7, 2023, entitled “Secondary Battery and Electrical Device,” which is incorporated herein by reference in its entirety. Technical Field

[0003] The present disclosure belongs to the field of battery technology, and particularly relates to a secondary battery and an electrical device. Background Art

[0004] In recent years, secondary batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power stations, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace and other fields.

[0005] As the application scope of secondary batteries becomes wider and wider, people have put forward higher requirements on the performance of secondary batteries, such as requiring the service life of secondary batteries to be continuously improved to increase the endurance of electrical devices.

[0006] Summary of the Invention

[0007] The present disclosure is developed in view of the above technical problems, and its purpose is to provide a secondary battery and an electrical device to increase the service life of the secondary battery.

[0008] According to a first aspect of the present disclosure, a secondary battery is provided, comprising a negative electrode plate, the negative electrode plate comprising a negative electrode current collector and a negative electrode film layer formed on at least one surface of the negative electrode current collector, the negative electrode film layer having a first surface away from the negative electrode current collector and a second surface arranged opposite to the first surface, the thickness of the negative electrode film layer being denoted as H, the region within a thickness range from the second surface of the negative electrode film layer to 0.3H being denoted as the first region of the negative electrode film layer, the region within a thickness range from the first surface of the negative electrode film layer to 0.3H being denoted as the second region of the negative electrode film layer, the first region comprising a first binder, which is a linear binder; and the second region comprising a second binder, which is a point-shaped binder.

[0009] By using a linear first binder in the first area of ​​the negative electrode film layer close to the current collector side and using a dot-shaped second binder in the second area of ​​the negative electrode film layer away from the current collector side, the probability of the linear binder coming into contact with moisture can be effectively reduced, the moisture absorption rate of the negative electrode plate can be reduced, and the increase in the thickness of the plate caused by the moisture absorption of the linear binder can be alleviated. The linear binder can give full play to its many polar functional groups and strong bonding force, thereby achieving a low expansion rate of the plate and reducing the probability of a significant deterioration in the defective rate and battery cycle stability.

[0010] In any embodiment of the present disclosure, the weight average molecular weight of the first binder is greater than the weight average molecular weight of the second binder.

[0011] The linear first binder has a higher weight-average molecular weight than the dot-shaped second binder, which is beneficial to improving the cohesion of the negative electrode film layer and the adhesion between the negative electrode film layer and the current collector, further reducing the rebound rate of the electrode after cold pressing and the expansion rate of the battery after full charge, and comprehensively improving the processing performance and cycle stability of the battery.

[0012] In any embodiment of the present disclosure, the mass proportion of the first binder in the first region is greater than the mass proportion of the second binder in the second region; the mass proportion of the first binder in the first region refers to the mass percentage of the first binder based on the total mass of the negative electrode film layer in the first region; the mass proportion of the second binder in the second region refers to the mass percentage of the second binder based on the total mass of the negative electrode film layer in the second region.

[0013] During the drying process of the electrode slurry coating, the binder near the current collector side tends to float to the surface of the negative electrode film layer as the solvent evaporates, that is, it tends to migrate from the first area of ​​the electrode to the second area of ​​the electrode, resulting in insufficient binder content in the first area of ​​the electrode, and significantly deteriorating the cohesion of the negative electrode film layer and the adhesion between the negative electrode film layer and the current collector. At the same time, the floating binder tends to clog the pore structure in the second area of ​​the upper layer of the electrode, making it difficult for the electrolyte to infiltrate, which in turn leads to insufficient battery dynamics. Setting the mass proportion of the first binder in the first area to be greater than the mass proportion of the second binder in the second area can, on the one hand, help improve the cohesion of the negative electrode film layer and the adhesion between the negative electrode film layer and the current collector, restrain the expansion of the negative electrode active material, and improve the battery cycle stability; on the other hand, it can reduce the blockage of pores on the electrode surface and optimize the battery's dynamic performance.

[0014] In any embodiment of the present disclosure, the first adhesive comprises an acrylic adhesive; and / or the second adhesive comprises polystyrene butadiene copolymer.

[0015] In any embodiment of the present disclosure, the acrylic binder includes at least one of polyacrylic acid, polymethacrylic acid, polymethyl acrylate, polyethyl methacrylate, polyhydroxyethyl methacrylate, polyhydroxyethyl acrylate, polyhydroxypropyl acrylate, polyhydroxypropyl methacrylate, polyhydroxyethyl acrylate, polypropyl methacrylate, polyethyl acrylate, and polymethyl methacrylate.

[0016] Acrylic binders have more polar functional groups and are linear binders. They can fully stretch in the solvent and coat the surface of the negative electrode active material, with a large contact area. They can effectively reduce the rebound of the electrode and improve the battery cycle stability through bonding.

[0017] In any embodiment of the present disclosure, the weight average molecular weight of the first binder is 500-1500, optionally 800-1000; the weight average molecular weight of the second binder is 100-200.

[0018] In any embodiment of the present disclosure, the mass proportion of the first adhesive in the first region is 1.0% to 3.0%, optionally 1.0% to 2.5%, and / or the mass proportion of the second adhesive in the second region is 0.6% to 1.5%, optionally 0.6% to 1.2%.

[0019] The binder content within the above range enables the battery to have both good cycle stability and dynamic performance.

[0020] In any embodiment of the present disclosure, the compaction density of the negative electrode film layer is 1.55 g / cm 3 -1.68g / cm 3 .

[0021] In any embodiment of the present disclosure, the negative electrode film layer includes a conductive agent, and based on the total mass of the negative electrode film layer, the mass proportion of the conductive agent is 0.4%-2.0%.

[0022] In any embodiment of the present disclosure, the first region and / or the second region comprises graphite, and the specific surface area of ​​the graphite is 1.2 m 2 / g-1.8m 2 / g, optional 1.25m 2 / g-1.70m 2 / g.

[0023] The specific surface area of ​​the graphite material being within the above range helps to further reduce the degree of side reactions of the graphite material and improve the cycle life of the secondary battery.

[0024] In any embodiment of the present disclosure, the volume distribution particle size Dv50 of the graphite is 7.0 μm-14.0 μm, and optionally 9.0 μm-12.0 μm.

[0025] In any embodiment of the present disclosure, the particle size distribution of graphite (Dv90-Dv10) / Dv50 is 1.10-1.60, and optionally 1.20-1.50.

[0026] Controlling the particle size distribution of the graphite material within the above-mentioned range is conducive to improving the tight packing of the graphite material and increasing the compaction density of the negative electrode. In other words, the cold pressing pressure required for the negative electrode to achieve the same compaction density is smaller, thereby reducing the probability of cracking of the graphite material during the cold pressing process, and further improving the integrity of the graphite material during the processing process; and the small stress inside the graphite material particles is conducive to maintaining the long-period pore structure of the electrode during the cycle, and can maintain the original pore structure of the electrode during the cycle, so that the lithium ion insertion path remains unobstructed, while reducing the re-filming of the graphite material during the charging process, improving the dynamic performance, cycle life and storage stability. Furthermore, the particle size distribution within the above-mentioned range can also improve the uniformity of lithium insertion between the graphite material particles, reduce polarization, avoid lithium precipitation caused by uneven current density, and help achieve long-term cycle stability. In addition, the particle size distribution within the above range can also help improve the processing performance of the electrode, and will not affect the uniformity of slurry stirring due to excessive small particles in the graphite material, which is beneficial to improving the uniformity and stability of the electrode quality and further improving the long-term cycle stability.

[0027] In any embodiment of the present disclosure, the specific capacity of graphite is 345 mAh / g-355 mAh / g, optionally 347 mAh / g-353 mAh / g.

[0028] Graphite materials with gram capacities within this range do not experience significant lattice expansion during charge and discharge due to excessive graphitization, nor do they experience difficulty compacting due to low graphitization, requiring higher cold pressing pressures to achieve the required electrode density. This can lead to cracking during cold pressing and the creation of new interfaces during cycling, which consumes excessive active lithium. Graphite materials with gram capacities within this range can comprehensively improve the cycle life of secondary batteries.

[0029] In any embodiment of the present disclosure, the powder compaction density of graphite under a pressure of 49000N is less than or equal to 1.85g / cm 3 , optional 1.75g / cm 3 -1.84g / cm 3 .

[0030] Graphite materials with a powder compaction density within the above range can easily maintain high particle integrity during the cold pressing process, which helps to improve the cycle life of secondary batteries.

[0031] In any embodiment of the present disclosure, the particle body of graphite includes an internal region and a surface region that at least partially surrounds the internal region, and the surface region refers to a region extending 30 nm from the surface of the particle body to the interior of the particle, and the surface region includes a disordered layer; optionally, the thickness of the disordered layer is 1 nm-20 nm.

[0032] The disordered layer mainly includes amorphous carbon. The carbon atoms in the amorphous carbon structure have no regular arrangement, which makes the interaction between its molecules more complex, resulting in higher strength and hardness than the graphite crystals in the internal area. This reduces the probability of surface damage of the graphite particles due to friction between particles and between particles and the mixing tank during slurry preparation, and reduces the risk of graphite material particles cracking and exposing new interfaces during the cold pressing of the pole piece, so that the graphite material maintains higher particle integrity during the manufacturing process. Graphite materials with high particle integrity have fewer surface defects and a good interface with the electrolyte. They can effectively reduce side reactions between the negative electrode plate and the electrolyte, reduce the loss of active lithium, and are beneficial to improving battery storage stability and cycle stability. At the same time, the disordered layer on the graphite material also helps to improve the infiltration of the electrolyte and the negative electrode plate, improving the battery dynamic performance.

[0033] Unlike the disordered layer coated on the surface of graphite materials in the prior art, the disordered layer of the graphite material provided in the embodiments of the present disclosure is located within the graphite material particles themselves. It is derived from the same raw materials as the graphite material and is not produced through post-processing. Therefore, compared to graphite materials with disordered layers obtained through surface coating, this graphite material has better material consistency, resulting in longer battery cycle life and a balance between battery kinetic performance and cycle and storage stability. This makes it particularly suitable for energy storage batteries with extremely high requirements for cycle and storage life.

[0034] In any embodiment of the present disclosure, the graphite includes both primary particles and secondary particles; optionally, based on the total number of primary particles and secondary particles, the number of secondary particles accounts for less than or equal to 50%.

[0035] Graphite materials with a low proportion of secondary particles are beneficial for maintaining the integrity of the graphite particles during battery preparation, reducing the formation of new interfaces and the consumption of active lithium during cycling, further improving the cycling stability of secondary batteries. At the same time, a certain number of secondary particles can reduce the expansion of the negative electrode while also taking into account the kinetic performance of the secondary battery, thus ensuring that the battery cell has a kinetic window throughout its life cycle. This prevents lithium plating caused by uneven current distribution from causing a sharp decline in battery capacity and a sharp deterioration in battery life, thereby comprehensively improving the battery's cycling stability.

[0036] In any embodiment of the present disclosure, the graphite has a degree of graphitization of 92.0% to 94.0%.

[0037] In any embodiment of the present disclosure, the graphite material I D / I G is 0.05-0.10, where I D / I G is the ratio of the D peak intensity to the G peak intensity obtained from the Raman spectrum, I D Indicates that the Raman spectrum is at 1350±100cm -1 The D peak intensity at I G Indicates that the Raman spectrum is at 1580±100cm -1 The G peak intensity at .

[0038] Graphite Material I D / I G It can be used to characterize the surface disorder of graphite materials. Unlike active materials that coat the surface of the graphite particle body with a disordered layer, the graphite material provided by the embodiment of the present disclosure has a disordered layer on the surface and has a low surface disorder. The disordered layer in the active material that coats the surface of the graphite particle body with a disordered layer is usually derived from the coated organic carbon, so that the disordered layer after carbonization has a high surface disorder; while the disordered layer structure in the surface region of the graphite material provided by the embodiment of the present disclosure and the other parts of the graphite material are derived from the same precursor, and the disordered layer has a high uniformity and an extremely thin thickness, so that it can have a low surface disorder. On the one hand, this enables the graphite material to take advantage of the disordered layer and improve the kinetic performance of the battery, without causing excessive side reactions due to the excessive disorder of the surface layer of the material. It can improve the kinetic performance of the battery while taking into account the cycle stability of the battery.

[0039] A second aspect of the present disclosure provides an electric device including the secondary battery according to the first aspect of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] To more clearly illustrate the technical solutions of the embodiments of the present disclosure, the following briefly introduces the drawings required for use in the embodiments of the present disclosure. Obviously, the drawings described below are only some embodiments of the present disclosure, and those skilled in the art can derive other drawings based on the drawings without inventive effort.

[0041] FIG1 is a schematic diagram of an embodiment of a negative electrode sheet disclosed herein;

[0042] FIG2 is a schematic diagram of another embodiment of the negative electrode sheet disclosed herein;

[0043] FIG3 is a schematic diagram of another embodiment of the negative electrode sheet disclosed herein;

[0044] FIG4 is a schematic diagram of a cross-sectional image of a particle of the graphite material disclosed herein;

[0045] FIG5 is a schematic diagram of an embodiment of a secondary battery disclosed herein;

[0046] FIG6 is an exploded schematic diagram of an embodiment of a secondary battery disclosed herein;

[0047] FIG7 is a schematic diagram of an embodiment of a battery module of the present disclosure;

[0048] FIG8 is a schematic diagram of an embodiment of a battery pack of the present disclosure;

[0049] FIG. 9 is an exploded schematic diagram of the embodiment of the battery pack shown in FIG. 8 .

[0050] FIG. 10 is a schematic diagram of an embodiment of an electric device including the secondary battery of the present disclosure as a power source.

[0051] FIG. 11 is a scanning electron microscope image of a second region of a negative electrode sheet according to an embodiment of the present disclosure.

[0052] FIG. 12 is a scanning electron microscope image of a first region of a negative electrode sheet according to an embodiment of the present disclosure.

[0053] In the accompanying drawings, which are not necessarily drawn to scale, reference numerals are as follows: 1 battery pack, 2 upper housing, 3 lower housing, 4 battery module, 5 secondary battery, 51 housing, 52 electrode assembly, 53 cover plate, 10 negative electrode sheet, 101 negative electrode current collector, 102 negative electrode film layer, 102a first surface, 102b second surface, 1021 first region, 1022 second region, 1023 intermediate region, 200 graphite material, 201 surface region, 202 internal region. DETAILED DESCRIPTION

[0054] Below, the embodiments of the secondary battery and the electrical device disclosed in the present invention are described in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there may be cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present disclosure and are not intended to limit the subject matter described in the claims.

[0055] " scope " disclosed in the present disclosure 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 selected lower limit and upper limit define the boundary of special scope.The scope that this mode limits can be to include end value or not include end value, and can be combined arbitrarily, and promptly any lower limit can form a scope with any upper limit combination.For example, if the scope of 60-120 and 80-110 is listed for specific parameter, 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 listed, and if the maximum range value 3,4 and 5 listed, then the following scope can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5.In the present disclosure, unless otherwise specified, numerical range " ab " represents the abbreviation of any real number combination between a and b, and 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.

[0056] Unless otherwise specified, all embodiments and optional embodiments of the present disclosure can be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure content of the present disclosure.

[0057] Unless otherwise specified, all technical features and optional technical features of the present disclosure can be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure content of the present disclosure.

[0058] Unless otherwise specified, all steps of the present disclosure may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), which means that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0059] Unless otherwise specified, the terms "include" and "comprising" mentioned in this disclosure may be open-ended or closed-ended. For example, the terms "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0060] Unless otherwise specified, the term "or" is inclusive in this disclosure. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0061] Unless otherwise specified, the terms used in the present disclosure have the common meanings that are generally understood by those skilled in the art.

[0062] Unless otherwise specified, the numerical values ​​of the parameters mentioned in the present disclosure can be measured using various test methods commonly used in the art, for example, they can be measured according to the test methods given in the present disclosure.

[0063] Unless otherwise specified, in the present disclosure, the term "active ions" refers to ions that can be intercalated and extracted back and forth between the positive electrode and the negative electrode of a secondary battery, including but not limited to lithium ions.

[0064] In the present disclosure, the terms "plurality" and "multiplicity" refer to two or more.

[0065] Binders are a crucial component of secondary battery electrodes. Although they comprise a relatively small proportion of the electrode, they are closely linked to the electrochemical performance of secondary batteries. The primary function of a binder is to bond the various electrode components, such as the active material, conductive agent, and current collector, together to form a stable electrode structure. This binder also facilitates contact between the active material and the conductive agent, forming a well-developed conductive network. Furthermore, binders mitigate the volume expansion and contraction of the positive and negative electrode materials during lithium intercalation and deintercalation, stabilizing the internal structure of the electrode for optimal cycling performance. For example, during the use of a secondary battery, the intercalation and deintercalation of active ions in the negative electrode active material during charge and discharge causes the negative electrode active material to expand and contract, increasing the interplanar spacing and volume expansion rate. Therefore, the negative electrode binder is required to buffer the expansion and contraction of the negative electrode active material. Styrene-butadiene rubber (SBR), a commonly used negative electrode binder, is an elastomer made from the copolymerization of 1,3-butadiene and styrene. It effectively improves the flexibility of the negative electrode. However, the styrene-butadiene rubber is in the form of dots in the negative electrode film layer, and is connected to the negative electrode active material in a point-bonding manner. The bonding surface is small, and it is difficult to effectively reduce the rebound of the negative electrode plate after cold pressing and the expansion of the negative electrode active material after full charging, which is not conducive to improving the cycle stability of the secondary battery.

[0066] Based on this, the present disclosure provides a secondary battery including a negative electrode sheet. Figures 1 to 3 are schematic diagrams of embodiments of the negative electrode sheet of the present disclosure. As shown in Figures 1 to 3, the negative electrode sheet 10 includes a negative electrode current collector 101 and a negative electrode film 102 formed on at least one surface of the negative electrode current collector 101. The negative electrode film 102 has a first surface 102a distal from the negative electrode current collector 101 and a second surface 102b opposite the first surface 102a. The thickness of the negative electrode film 102 is denoted as H. The region from the second surface 102b of the negative electrode film layer to a thickness of 0.3H is denoted as a first region 1021 of the negative electrode film layer. The region from the first surface 102a of the negative electrode film layer to a thickness of 0.3H is denoted as a second region 1022 of the negative electrode film layer. The first region 1021 includes a first binder, which is a linear binder; the second region 1022 includes a second binder, which is a dot-shaped binder.

[0067] In the present disclosure, the morphology of the binder can be observed by using a scanning electron microscope to obtain a morphology picture of the sample to be tested. As an example, the negative electrode plate is cut into a 10mm×5mm sample to be tested, and the sample to be tested is clamped with two copper plates. The sample to be tested and the copper plates are fixed with double-sided tape, and a flat iron block of a certain mass (500g) is used to press for a certain time of 10 minutes to make the gap between the sample to be tested and the copper foil as small as possible. Then, the edges are trimmed with scissors and glued to a sample table with conductive glue. The sample is slightly protruding from the edge of the sample table. The morphology of the binder can be observed by using a scanning electron microscope to obtain a surface morphology picture of the sample to be tested.

[0068] Figure 11 is a scanning electron microscope image of the second region of one embodiment of the negative electrode sheet disclosed herein; Figure 12 is a scanning electron microscope image of the first region of one embodiment of the negative electrode sheet disclosed herein. As shown in Figure 11, the dot-shaped binder provides bonding force through point contact between the negative electrode active materials, failing to effectively cover and protect the surface of the negative electrode active materials, making it difficult to effectively inhibit the expansion of the negative electrode active materials, leading to deteriorated cycling performance. Linear binders often have a large number of polar functional groups. During the slurrying process, the interaction between the polar functional groups and the aqueous solvent allows the binder to fully expand and coat the surface of the negative electrode active materials. As shown in Figure 12, the coating of the linear second binder on the surface of the negative electrode active materials can significantly improve the bonding strength between the negative electrode active materials and between the negative electrode film layer and the negative electrode current collector, which is beneficial to improving the cohesion of the negative electrode sheet. However, the polar functional groups in linear binders are also prone to forming highly polar hydrogen bonds with water molecules in the air, causing the electrode to easily absorb moisture in the air after cold pressing, resulting in excessive rebound in electrode thickness, affecting processing performance such as battery cell shelling, and even leading to deterioration of battery cell cycle stability.

[0069] By using a linear first binder in the first area of ​​the negative electrode film layer close to the current collector side and using a dot-shaped second binder in the second area of ​​the negative electrode film layer away from the current collector side, the probability of the linear binder coming into contact with moisture can be effectively reduced, the moisture absorption rate of the negative electrode plate can be reduced, and the increase in the thickness of the plate caused by the moisture absorption of the linear binder can be alleviated. The linear binder can give full play to its many polar functional groups and strong bonding force, thereby achieving a low expansion rate of the plate and reducing the probability of a significant deterioration in the defective rate and battery cycle stability.

[0070] In some embodiments, the weight average molecular weight of the first binder is greater than the weight average molecular weight of the second binder.

[0071] As used herein, the term "weight average molecular weight" refers to the sum of the weight fractions of molecules of different molecular weights in a polymer multiplied by their corresponding molecular weights.

[0072] In the present disclosure, the weight-average molecular weight of the binder can be tested by methods known in the art, such as gel chromatography, such as using a Waters 2695 Isocratic HPLC gel chromatograph (differential refractive index detector 2141). A polystyrene solution sample with a mass fraction of 3.0% is used as a reference, and a matching chromatographic column is selected (oily: Styragel HT5DMF7.8*300mm+Styragel HT4). A 3.0% binder glue solution is prepared with purified N-methylpyrrolidone (NMP) solvent, and the prepared solution is allowed to stand for one day for use. During the test, tetrahydrofuran is first drawn into a syringe and rinsed, and repeated several times. Then 5 mL of the experimental solution is drawn, the air in the syringe is expelled, and the needle tip is wiped dry. Finally, the sample solution is slowly injected into the injection port. After the reading stabilizes, the data is acquired and the weight-average molecular weight is read.

[0073] The linear first binder has a higher weight-average molecular weight than the dot-shaped second binder, which is beneficial to improving the cohesion of the negative electrode film layer and the adhesion between the negative electrode film layer and the current collector, further reducing the rebound rate of the electrode after cold pressing and the expansion rate of the battery after full charge, and comprehensively improving the processing performance and cycle stability of the battery.

[0074] In some embodiments, the mass proportion of the first binder in the first region is greater than the mass proportion of the second binder in the second region; the mass proportion of the first binder in the first region refers to the mass percentage of the first binder based on the total mass of the negative electrode film layer in the first region; the mass proportion of the second binder in the second region refers to the mass percentage of the second binder based on the total mass of the negative electrode film layer in the second region.

[0075] During the drying process of the electrode slurry coating, the binder near the current collector side tends to float to the surface of the negative electrode film layer as the solvent evaporates, that is, it tends to migrate from the first area of ​​the electrode to the second area of ​​the electrode, resulting in insufficient binder content in the first area of ​​the electrode, and significantly deteriorating the cohesion of the negative electrode film layer and the adhesion between the negative electrode film layer and the current collector. At the same time, the floating binder tends to clog the pore structure in the second area of ​​the upper layer of the electrode, making it difficult for the electrolyte to infiltrate, which in turn leads to insufficient battery dynamics. Setting the mass proportion of the first binder in the first area to be greater than the mass proportion of the second binder in the second area can, on the one hand, help improve the cohesion of the negative electrode film layer and the adhesion between the negative electrode film layer and the current collector, restrain the expansion of the negative electrode active material, and improve the battery cycle stability; on the other hand, it can reduce the blockage of pores on the electrode surface and optimize the battery's dynamic performance.

[0076] In some embodiments, the first binder comprises an acrylic binder; and / or the second binder comprises polystyrene butadiene copolymer.

[0077] In this article, acrylic adhesive refers to a polymer with acrylic monomer as the main polymerization monomer, which can be a homopolymer or a copolymer. Acrylic monomer includes acrylic acid monomer or its derivatives.

[0078] In some embodiments, the acrylic binder includes at least one of polyacrylic acid, polymethacrylic acid, polymethyl acrylate, polyethyl methacrylate, polyhydroxyethyl methacrylate, polyhydroxyethyl acrylate, polyhydroxypropyl acrylate, polyhydroxypropyl methacrylate, polyhydroxyethyl acrylate, polypropyl methacrylate, polyethyl acrylate, and polymethyl methacrylate.

[0079] Acrylic binders have more polar functional groups and are linear binders. They can fully stretch in the solvent and coat the surface of the negative electrode active material, with a large contact area. They can effectively reduce the rebound of the electrode and improve the battery cycle stability through bonding.

[0080] As used herein, "styrene butadiene copolymer" refers to a polymer obtained by copolymerizing butadiene monomers with styrene monomers.

[0081] In some embodiments, the weight average molecular weight of the first binder is 500 to 1500. In some embodiments, the weight average molecular weight of the first binder is 800 to 1000; and the weight average molecular weight of the second binder is 100 to 200.

[0082] In some embodiments, the weight average molecular weight of the first binder is 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500 or any range therebetween.

[0083] In some embodiments, the weight average molecular weight of the second binder is 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, or any range therebetween.

[0084] In some embodiments, the first binder accounts for 1.0% to 3.0% by weight in the first region. In some embodiments, the first binder accounts for 1.0% to 2.5% by weight in the first region, and / or the second binder accounts for 0.6% to 1.5% by weight in the second region. In some embodiments, the first binder accounts for 0.6% to 1.2% by weight in the first region.

[0085] The mass content of the binder in different areas of the negative electrode can be tested using any method known in the art. As an example, take a clean copper foil and lay it flat on the table. Cut the negative electrode to be tested into 20cm×10cm pieces. Prepare a clean blade. Hold the electrode with your left hand and the blade with your right hand. Set the blade at a 45-degree angle to the electrode. Select the middle of the electrode and scrape the powder from left to right. The scraping length is 10cm and the width is 5cm. If there are 10 times from the beginning of the powder scraping to the copper foil leaking, take the first 3 scraping times as the second area sample and the last 3 scraping times as the first area sample. Store the collected samples in a sealed bottle. Weigh 50mg of the collected sample, place it in an alumina crucible and shake it flat. Use a thermogravimetric analyzer to detect the binder content in the sample (nitrogen atmosphere, flow rate 20mL / min). Heat the sample from 25℃ to 600℃ at a rate of 10℃ / min. The mass percentage of the sample lost in different areas is the mass percentage of the binder in different areas.

[0086] In some embodiments, the mass proportion of the first binder in the first region is 1.0%, 1.3%, 1.6%, 1.9%, 2.2%, 2.5%, 2.7%, 3.0% or any range therebetween.

[0087] In some embodiments, the mass percentage of the second binder in the second region is 0.6%, 0.7%, 0.9%, 1.1%, 1.3%, 1.5%, or any range therebetween.

[0088] When the mass proportion of the first binder in the first region is 1.0% to 2.5%, the battery has both good cycle stability and dynamic performance.

[0089] When the mass proportion of the second binder in the second region is 0.6% to 1.2%, the battery has both good cycle stability and dynamic performance.

[0090] In some embodiments, the compaction density of the negative electrode film layer is 1.55 g / cm 3 -1.68g / cm 3 .

[0091] In the present disclosure, the compaction density of the negative electrode film layer can be tested using methods known in the art. As an example, an electronic balance is used to weigh a negative electrode sheet with an area of ​​S, and the weight is recorded as W1. The thickness of the negative electrode sheet T1 is measured using a caliper. The weighed electrode film layer is then wiped off, the weight of the negative electrode current collector is weighed, recorded as W2, and the thickness of the negative electrode current collector T2 is measured using a caliper. The compaction density of the negative electrode film layer PD = (W1-W2) / [(T1-T2)×S].

[0092] In some embodiments, the compaction density of the negative electrode film layer is 1.55 g / cm 3 、1.58g / cm 3 , 1.61g / cm 3 , 1.64g / cm 3 , 1.68g / cm 3 or any range of values ​​between them.

[0093] In some embodiments, the negative electrode film layer includes a conductive agent, and based on the total mass of the negative electrode film layer, the mass proportion of the conductive agent is 0.4%-2.0%.

[0094] In some embodiments, based on the total mass of the negative electrode film layer, the mass proportion of the conductive agent is 0.4%, 0.6%, 0.8%, 1.0%, 1.2%, 1.4%, 1.6%, 1.8%, 2.0% or any numerical range therebetween.

[0095] In some embodiments, the first region and / or the second region comprises graphite. It is understood that the first region / the second region may comprise at least one graphite material, and the first region and the second region may comprise the same or different graphite materials.

[0096] In some embodiments, the specific surface area of ​​the graphite is 1.2 m 2 / g-1.8m 2 In some embodiments, the specific surface area of ​​the graphite is 1.25 m 2 / g-1.70m 2 / g.

[0097] In the present disclosure, the specific surface area of ​​the graphite material can be measured using methods known in the art. For example, the specific surface area of ​​the graphite material can be measured using the nitrogen adsorption specific surface area analysis method according to GB / T 19587-2017 and calculated using the BET (Brunauer Emmett Teller) method. The testing instrument can be a Tri-Star 3020 specific surface area pore size analyzer from Micromeritics, Inc., USA.

[0098] In some embodiments, the specific surface area of ​​the graphite is 1.2 m 2 / g, 1.3m 2 / g, 1.4m 2 / g, 1.5m 2 / g, 1.6m 2 / g, 1.7m 2 / g, 1.8m 2 / g or any range of values ​​between them.

[0099] The low specific surface area of ​​graphite materials helps to further reduce the degree of side reactions of graphite materials and improve the cycle life of secondary batteries.

[0100] In some embodiments, the volume distribution particle size Dv50 of the graphite is 7.0 μm to 14.0 μm. In some embodiments, the volume distribution particle size Dv50 of the graphite is 9.0 μm to 12.0 μm.

[0101] As used herein, the term "volume distribution particle size Dv50" refers to the particle size corresponding to when the cumulative volume distribution number of particles reaches 50% in the particle size distribution curve.

[0102] In the present disclosure, the volume distribution particle size Dv50 of the graphite material can be measured using methods known in the art. For example, referring to GB / T 19077-2016, it can be measured using a laser particle size analyzer. The testing instrument can be a Mastersizer 3000 laser particle size analyzer manufactured by Malvern Instruments Ltd., UK.

[0103] In some embodiments, the volume distribution particle size Dv50 of the graphite is 7.0 μm, 8.0 μm, 9.0 μm, 10.0 μm, 11.0 μm, 12.0 μm, 13.0 μm, 14.0 μm, or any range therebetween.

[0104] In some embodiments, the particle size distribution of the graphite (Dv90-Dv10) / Dv50 is 1.10-1.60. In some embodiments, the particle size distribution of the graphite (Dv90-Dv10) / Dv50 is 1.20-1.50.

[0105] In this article, the terms "Dv90" and "Dv10" refer to the particle sizes corresponding to when the cumulative volume distribution number of particles reaches 90% and 10% in the particle size distribution curve, respectively.

[0106] In the present disclosure, the volume distribution particle sizes Dv90 and Dv10 of the graphite material can be measured using methods known in the art. For example, referring to GB / T 19077-2016, the particle size distribution can be measured using a laser particle size analyzer. The testing instrument can be a Mastersizer 3000 laser particle size analyzer manufactured by Malvern Instruments Ltd., UK.

[0107] In some embodiments, the particle size distribution of the graphite (Dv90-Dv10) / Dv50 is 1.10, 1.15, 1.20, 1.25, 1.30, 1.35, 1.40, 1.50, 1.60, or any range therebetween.

[0108] Controlling the particle size distribution of the graphite material within the above-mentioned range is conducive to improving the tight packing of the graphite material and increasing the compaction density of the negative electrode. In other words, the cold pressing pressure required for the negative electrode to achieve the same compaction density is smaller, thereby reducing the probability of cracking of the graphite material during the cold pressing process, and further improving the integrity of the graphite material during the processing process; and the small stress inside the graphite material particles is conducive to maintaining the long-period pore structure of the electrode during the cycle, and can maintain the original pore structure of the electrode during the cycle, so that the lithium ion insertion path remains unobstructed, while reducing the re-filming of the graphite material during the charging process, improving the dynamic performance, cycle life and storage stability. Furthermore, the particle size distribution within the above-mentioned range can also improve the uniformity of lithium insertion between the graphite material particles, reduce polarization, avoid lithium precipitation caused by uneven current density, and help achieve long-term cycle stability. In addition, the particle size distribution within the above range can also help improve the processing performance of the electrode, and will not affect the uniformity of slurry stirring due to excessive small particles in the graphite material, which is beneficial to improving the uniformity and stability of the electrode quality and helping to achieve long-cycle stability.

[0109] In some embodiments, the graphite has a gram capacity of 345 mAh / g to 355 mAh / g. In some embodiments, the graphite has a gram capacity of 347 mAh / g to 353 mAh / g.

[0110] As used herein, the term "gram capacity" refers to the ratio of the amount of electricity that an active material can release to the mass of the active material.

[0111] In the present disclosure, the gram capacity of graphite can be tested using methods known in the art. As an example, a graphite sample is mixed with a conductive agent, carbon black, and polyvinylidene fluoride (PVDF) in a mass ratio of 91.6:1.8:6.6 in an appropriate amount of solvent NMP to form a uniform negative electrode slurry; the negative electrode slurry is evenly coated on the surface of the negative electrode current collector copper foil and dried and cold pressed; then, a metal lithium sheet is used as the counter electrode and a polypropylene (PP) film is used as the separator. A CR2430 button cell is assembled in an argon-protected glove box, wherein the electrolyte given in the embodiment of the present disclosure is used. At 25°C, the button cell prepared above is first discharged to 0.005V at a current of 0.05C, then discharged to 0.005V at a constant current of 10μA, and allowed to stand for 5 minutes. The first cycle discharge capacity of the button cell is recorded; then, the button cell is charged to 2.0V at a current of 0.1C, and the charge capacity of the button cell is recorded. The ratio of the charging capacity of the button cell to the mass of the graphite sample is the gram capacity of the graphite material.

[0112] In some embodiments, the specific capacity of the graphite is 345 mAh / g, 346 mAh / g, 347 mAh / g, 348 mAh / g, 349 mAh / g, 350 mAh / g, 351 mAh / g, 352 mAh / g, 353 mAh / g, 354 mAh / g, 355 mAh / g, or any range therebetween.

[0113] Graphite materials with gram capacities within this range do not experience significant lattice expansion during charge and discharge due to excessive graphitization, nor do they experience difficulty compacting due to low graphitization, requiring higher cold pressing pressures to achieve the required density for the electrode. This can lead to cracking during cold pressing and the creation of new interfaces during cycling that consume excessive active lithium. Graphite materials with gram capacities within this range can comprehensively improve the cycle life of secondary batteries.

[0114] In some embodiments, the graphite powder compaction density under a pressure of 49000N is less than or equal to 1.85g / cm 3 In some embodiments, the graphite powder compaction density under a pressure of 49000N is 1.75g / cm 3 -1.84g / cm 3 .

[0115] In this article, the term "powder compaction density" refers to the mass of powder particles per unit volume under a certain pressure.

[0116] In the present disclosure, the powder compaction density of the active material under a pressure of 49000N can be tested by methods known in the art. As an example, referring to GB / T 24533-2009, 1g of graphite material powder is weighed and added to a bottom area of ​​1.327cm 2 In the mold, pressurize to 4900kg (equivalent to 49000N), maintain pressure for 30s, then release the pressure and maintain for 10s. The powder compaction density of the graphite material under a pressure of 49000N is measured by an electronic pressure testing machine (such as UTM7305 electronic pressure testing machine).

[0117] In some embodiments, the powder compaction density of the graphite under a pressure of 49000N can be 1.69g / cm 3 , 1.72g / cm 3 , 1.75g / cm 3 , 1.78g / cm 3 , 1.82g / cm 3 , 1.85g / cm 3 or any range of values ​​between them.

[0118] Graphite materials with a powder compaction density within the above range can easily maintain high particle integrity during the cold pressing process, which helps to improve the cycle life of secondary batteries.

[0119] In some embodiments, the graphite particle body includes an internal region and a surface region that at least partially surrounds the internal region, wherein the surface region refers to a region extending 30 nm from the surface of the particle body to the interior of the particle, and the surface region includes a disordered layer.

[0120] Figure 4 is a schematic diagram of a cross-sectional image of a particle of graphite material 200 of the present disclosure. As shown in Figure 4, the region extending 30 nm inward from the surface of the particle body of graphite material 200 is surface region 201, and the region inside surface region 201 is interior region 202. Surface region 201 includes a disordered layer.

[0121] The disordered layer can be detected through transmission electron microscopy (TEM). Using a focused ion beam (FIB) to slice a 20-50 nm thick slice from the center of the graphite particle, TEM testing can reveal that the surface region includes a layer whose TEM morphology, lattice fringes, and electron diffraction patterns are different from those of the disordered layer in the interior. The electron diffraction pattern in the disordered layer appears halo-like.

[0122] In some embodiments, the disordered layer has a thickness of 1 nm to 20 nm.

[0123] In the present disclosure, the thickness of the disordered layer can be tested by methods known in the art. As an example, it can be obtained by transmission electron microscopy (TEM) testing. A thin slice with a thickness of about 20 to 50 nm is cut from the middle of the graphite material particle body by a focused ion beam (FIB), and then the thin slice is subjected to TEM testing to obtain the original TEM test image. The original image obtained by the above TEM test is opened in Digital Micrograph software, and the disordered layer is identified by diffraction stripes or lattice spacing, and its thickness is measured. Normally, the disordered layer has no diffraction stripes, and the lattice spacing is larger than that of the ordered layer.

[0124] In some embodiments, the thickness of the disordered layer is 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, 11 nm, 12 nm, 13 nm, 14 nm, 15 nm, 16 nm, 17 nm, 18 nm, 19 nm, 20 nm, or any range therebetween.

[0125] The disordered layer mainly includes amorphous carbon. The carbon atoms in the amorphous carbon structure have no regular arrangement, which makes the interaction between its molecules more complex, resulting in higher strength and hardness than the graphite crystals in the internal area. This reduces the probability of surface damage of the graphite particles due to friction between particles and between particles and the mixing tank during slurry preparation, and reduces the risk of graphite material particles cracking and exposing new interfaces during the cold pressing of the pole piece, so that the graphite material maintains higher particle integrity during the manufacturing process. Graphite materials with high particle integrity have fewer surface defects and a good interface with the electrolyte. They can effectively reduce side reactions between the negative electrode plate and the electrolyte, reduce the loss of active lithium, and are beneficial to improving battery storage stability and cycle stability. At the same time, the disordered layer on the graphite material also helps to improve the infiltration of the electrolyte and the negative electrode plate, improving the battery dynamic performance.

[0126] Unlike the disordered layer coated on the surface of graphite materials in the prior art, the disordered layer of the graphite material provided in the embodiments of the present disclosure is located within the graphite material particles themselves. It is derived from the same raw materials as the graphite material and is not produced through post-processing. Therefore, compared to graphite materials with disordered layers obtained through surface coating, this graphite material has better material consistency, resulting in longer battery cycle life and a balance between battery kinetic performance and cycle and storage stability. This makes it particularly suitable for energy storage batteries with extremely high requirements for cycle and storage life.

[0127] In some embodiments, the graphite includes both primary particles and secondary particles. In some embodiments, based on the total number of the primary particles and the secondary particles, the secondary particles account for less than or equal to 50%.

[0128] As used herein, the term "primary particles" refers to particles in a non-agglomerated state.

[0129] As used herein, the term "secondary particles" refers to particles in an agglomerated state formed by the aggregation of two or more primary particles.

[0130] Primary particles and secondary particles can be distinguished by observing the particle cross-section of the graphite material using a scanning electron microscope (SEM). In the present disclosure, the proportion of the number of secondary particles in the graphite material can be tested using methods known in the art. As an example, a cross-section polisher (such as the IB-09010 CP argon ion cross-section polisher of Japan's JEOL company) can be used to prepare the cross-section of the negative electrode sheet; then, referring to JY / T010-1996, a scanning electron microscope (such as the Sigma 300 scanning electron microscope of Germany's ZEISS company) is used to scan the cross-section of the negative electrode sheet, and multiple test areas are randomly selected in the test sample. Images of the multiple test areas are obtained using a scanning electron microscope, and the number of graphite materials with secondary particle morphology in each image is counted as the ratio of the total number of graphite material particles. The average value of the multiple statistical results is the number of secondary particles in the graphite material.

[0131] In some embodiments, the graphite includes both primary particles and secondary particles, and the amount of the secondary particles accounts for 50%, 40%, 30%, 20%, 10% or any range therebetween based on the total amount of the primary particles and the secondary particles.

[0132] Graphite materials with a low proportion of secondary particles are beneficial for maintaining the integrity of the graphite particles during battery preparation, reducing the formation of new interfaces and the consumption of active lithium during cycling, further improving the cycling stability of secondary batteries. At the same time, a certain number of secondary particles can reduce the expansion of the negative electrode while also taking into account the kinetic performance of the secondary battery, thus ensuring that the battery cell has a kinetic window throughout its life cycle. This prevents lithium plating caused by uneven current distribution from causing a sharp decline in battery capacity and a sharp deterioration in battery life, thereby comprehensively improving the battery's cycling stability.

[0133] In some embodiments, the graphite has a degree of graphitization of 92.0% to 94.0%.

[0134] As used herein, the term "degree of graphitization" refers to an indicator measuring the degree to which carbon atoms form a close-packed hexagonal graphite crystal structure.

[0135] In the present disclosure, the degree of graphitization of a graphite material can be tested using methods known in the art. As an example, high-purity silicon powder (purity ≥ 99.99%) is used as an internal standard for calibration. The graphite material and silicon are mixed in a weight ratio of 5:1, ground uniformly, and pressed into a pellet. Testing is performed using an X-ray diffractometer (e.g., a Bruker D8 Discover), and with reference to JIS K 0131-1996 and JB / T 4220-2011, the average interlayer spacing d002 of the (002) plane in the crystal structure of the graphite material is obtained. The degree of graphitization is then calculated using the formula g = (0.344 - d002) / (0.344 - 0.3354) × 100%. In the above formula, d002 is the average interlayer spacing of the (002) plane in the crystal structure of the graphite material expressed in nanometers (nm).

[0136] In some embodiments, the graphite has a degree of graphitization of 92%, 92.2%, 92.4%, 92.6%, 92.8%, 93%, 93.2%, 93.4%, 93.6%, 93.8%, 94%, or any range therebetween.

[0137] Graphite materials with a degree of graphitization within the above range can achieve both cycle stability and high capacity.

[0138] In some embodiments, the graphite material I D / I G is 0.05-0.10, where I D / I G is the ratio of the D peak intensity to the G peak intensity obtained from the Raman spectrum, I D Indicates that the Raman spectrum is at 1350±100cm -1 The D peak intensity at I G Indicates that the Raman spectrum is at 1580±100cm -1 The G peak intensity at .

[0139] In the present disclosure, the graphite material I D / I G Any known mode of Raman spectroscopy can be used for testing. As an example, referring to GB / T 40219-2021, an In Via Qontor (Reflex) Raman spectrometer is used for testing; a solid laser with a wavelength of 523 nm is used as the light source, 100 points are sampled in an area of ​​100 μm × 100 μm, and I D / I G The median value of graphite material I D / I G , the median is the collected I arranged in order of size D / I G The middle number in the data.

[0140] In some embodiments, the graphite material I D / I G 0.05, 0.06, 0.07, 0.08, 0.09, 0.10 or any numerical range therebetween.

[0141] Graphite Material I D / I G It can be used to characterize the surface disorder of graphite materials. Different from the active material with a disordered layer coated on the surface of the graphite particle body, the graphite material provided by the embodiment of the present disclosure has a disordered layer on the surface and has a low surface disorder. The disordered layer in the active material with a disordered layer coated on the surface of the graphite particle body is usually derived from the coated organic carbon, so that the disordered layer after carbonization has a high surface disorder; while the disordered layer structure in the surface area of ​​the graphite material provided by the embodiment of the present disclosure and the other parts of the graphite material are derived from the same precursor, and the disordered layer has a high uniformity and an extremely thin thickness, so that it can have a low surface disorder. On the one hand, this enables the graphite material to take advantage of the disordered layer and improve the kinetic performance of the battery, without causing too many side reactions due to the excessive disorder of the surface layer of the material. It can improve the kinetic performance of the battery while taking into account the cycle stability of the battery.

[0142] The present disclosure has no particular restrictions on the types of secondary batteries. For example, the secondary battery can be a lithium-ion battery, etc. In general, a secondary battery includes a positive electrode plate, a negative electrode plate, and an electrolyte, etc. During the charge and discharge process of the secondary battery, active ions are embedded and extracted back and forth between the positive electrode plate and the negative electrode plate, and the electrolyte plays a role in conducting active ions between the positive electrode plate and the negative electrode plate. The present disclosure has no particular restrictions on the type of the electrolyte, and it can be selected according to actual needs. For example, the electrolyte can be selected from at least one of a solid electrolyte and a liquid electrolyte (i.e., an electrolyte). Secondary batteries using electrolytes and some secondary batteries using solid electrolytes can also include an isolation membrane, which is arranged between the positive electrode plate and the negative electrode plate to play an isolation role.

[0143] The present disclosure also provides a method for preparing a graphite material, comprising the following steps: providing raw materials; processing the raw materials to obtain an intermediate product; graphitizing the intermediate product to obtain a graphite material; the particle body of the graphite material includes an internal region and a surface region that at least partially surrounds the internal region, the surface region refers to a region extending 30 nm from the surface of the particle body of the graphite material to the interior of the particle, and the surface region includes a disordered layer.

[0144] In this article, the term "graphitization treatment" refers to the heat treatment process of carbon materials. Under the action of high temperature, the carbon material transforms from a two-dimensional carbon network structure to a three-dimensional ordered structure through "microcrystal" growth.

[0145] In some embodiments, the maximum power of the graphitization process is 70% to 90% of the rated power of the graphitization process equipment.

[0146] In some embodiments, the maximum power of the graphitization treatment can be selected as 70%, 75%, 80%, 85%, 90% or any numerical range therebetween of the rated power of the graphitization treatment equipment. It is understood that the graphitization treatment device refers to any device capable of performing graphitization treatment, including but not limited to Acheson furnace, box furnace, internal string furnace, continuous graphitization, electric calcining furnace, medium frequency furnace, tubular furnace and other devices. Among them, the rated power of graphitization treatment equipment produced by different manufacturers may be different, and can be selected according to actual conditions. The maximum power of the graphitization treatment adopted in the present disclosure needs to be lower than the rated power of the graphitization treatment device to achieve uniformity of the thermal field during the graphitization treatment process.

[0147] The maximum power of the graphitization treatment adopted in the present disclosure needs to be lower than the rated power of the graphitization treatment equipment to achieve uniformity of the thermal field during the graphitization treatment.

[0148] In some embodiments, the graphitization treatment equipment is an internal string furnace with a rated power of 25,000-32,000 W. The graphitization treatment time can be 10 hours to 30 hours. In some embodiments, the graphitization treatment time can be 15 hours to 25 hours.

[0149] In some embodiments, the graphitization treatment equipment is an Acheson furnace with a rated power of 28,000-30,000 W. The graphitization treatment time may be 30 hours to 50 hours. In some embodiments, the graphitization treatment time may be 40 hours to 50 hours.

[0150] In some embodiments, the maximum power of the graphitization process is 22,000W-25,000W.

[0151] In some embodiments, the maximum power of the graphitization process may be 22,000 W, 22,500 W, 23,000 W, 23,500 W, 24,000 W, 25,000 W, or any range therebetween.

[0152] By controlling the maximum power of the graphitization treatment, the degree of graphitization of the graphite material during the heat treatment process can be effectively controlled. While the internal area of ​​the graphite material particles is highly graphitized, a uniform disordered layer is formed on the surface of the body, which is beneficial to improving the cycle stability of the secondary battery.

[0153] In some embodiments, the temperature of the graphitization treatment is 2600°C to 3000°C.

[0154] In some embodiments, the temperature of the graphitization treatment is 2600° C., 2700° C., 2800° C., 2900° C., 3000° C., or any range therebetween.

[0155] The appropriate graphitization treatment temperature and the appropriate graphitization treatment time are not likely to cause excessive rearrangement of the precursor, resulting in a high specific surface area of ​​the graphite material after graphitization and deterioration of high-temperature performance; they can also effectively improve the graphitization degree of the graphite material, thereby facilitating the simultaneous improvement of the high-temperature storage and cycle life of the secondary battery.

[0156] In some embodiments, the feedstock comprises at least one of petroleum coke, needle coke, and pitch coke. In some embodiments, the feedstock is needle coke.

[0157] In this article, the term "petroleum coke" refers to the coke formed by high-temperature carbonization of petroleum residue or petroleum asphalt.

[0158] In this article, the term "needle coke" refers to coal tar pitch or petroleum pitch, which, after undergoing liquid phase carbonization to generate an anisotropic mesophase, can produce coke with a needle-like texture through processes such as high-temperature carbonization.

[0159] As used herein, the term "pitch coke" refers to the solid material produced by carbonizing coal tar pitch at high temperatures.

[0160] Needle coke has a range of advantages, including low thermal expansion coefficient, low porosity, low sulfur, low ash, low metal content, high electrical conductivity, and easy graphitization. The graphite material after graphitization can achieve a high ultimate compaction density and a low cyclic expansion rate.

[0161] In some embodiments, the volume of the fiber-type structure in the raw material accounts for greater than or equal to 55% based on the total volume of the raw material structure. In some embodiments, the volume of the fiber-type structure in the raw material accounts for 58%-70% based on the total volume of the raw material structure.

[0162] In this article, "fibrous structure" is also called streamlined structure, which refers to the structure of the raw material with obvious fibrous texture observed under a microscope.

[0163] Generally, the microstructure of the raw material can be divided into mosaic, regional, and fibrous types based on its morphological characteristics and the size of the isochromatic zones under a polarizing microscope. Generally, isochromatic zones with a size of 30 μm or more are classified as mosaic, isochromatic zones with a size greater than 30 μm are classified as regional, and anisotropic banded isochromatic zones are classified as fibrous.

[0164] In the present disclosure, the volume percentage of the fiber structure in the raw material can be tested using methods known in the art. As an example, according to the provisions of GB 1997-89, the raw material is crushed to 1mm and mixed, and 40g to 50g is separated. A square hole sieve is used to take 4g to 5g of a 0.07mm to 1.0mm grade sample for film making; according to the provisions of MT 116.1-86, powder coke and block coke optical films are prepared. The diameter of the powder coke optical film shall not be less than 22mm, and the volume occupied by the cement shall be less than 1 / 3; the sample is placed on a slide with clay, flattened, and then placed on the stage for focusing. After calibrating the microscope, the polarizer and analyzer are adjusted to make them orthogonal. Insert the azurite inspection plate (1λ) so that the field of view shows the interference color of the first-order red; determine the step length of the moving ruler to ensure that more than 400 effective measuring points are evenly distributed, with a point spacing of 0.3 to 0.5mm and a line spacing of 0.5 to 0.8mm. Starting from one end of the sample, determine the microstructure category at the intersection of the crosshairs, and divide the number of effective measurement points of the fiber-type optical structure by the total number of test points as the volume proportion of the fiber-type structure in the raw material.

[0165] In some embodiments, based on the total volume of the raw material structure, the volume proportion of the fibrous structure in the raw material can be selected to be 55%, 58%, 60%, 65%, 70% or any numerical range therebetween.

[0166] Raw materials with a high volume fraction of fiber-type structures help increase the compaction density and specific capacity of graphite materials, allowing the graphite material to retain a high degree of integrity during the compaction process, resulting in batteries with both a long cycle life and good energy density. However, an excessively high percentage of fiber-type structures increases the cost and expansion rate of the graphite material, and deteriorates its dynamic performance. Raw materials with a volume fraction of fiber-type structures within the above range offer both lower costs and good specific capacity for the graphite, providing a full lifecycle kinetic window for the battery cell, thereby comprehensively improving the battery's long-term cycle life and electrochemical performance.

[0167] In some embodiments, the maximum gram capacity achievable by the feedstock is greater than the gram capacity of the graphite material.

[0168] By using high-grade raw materials and controlling the degree of graphitization so that the maximum gram capacity that the raw materials can achieve is not fully utilized, a graphite material including a disordered layer in the surface area is obtained, achieving a balance between battery cycle life and kinetic performance.

[0169] In some embodiments, the processing of raw materials specifically includes: crushing, shaping and grading the raw materials to obtain a first precursor; granulating the first precursor to obtain a second precursor; and low-temperature carbonizing a mixture of the first precursor and the second precursor to obtain the intermediate product.

[0170] Crushing is the process of reducing the particle size of raw materials. The raw materials can be crushed by any mechanical device such as crusher, mechanical mill, etc.

[0171] Shaping and grading are the processes of adjusting the particle size distribution of the raw materials to obtain a first precursor that meets the particle size requirements. The particle size and particle size distribution of the first precursor can be controlled by adjusting the grading frequency and air volume.

[0172] It can be understood that low-temperature carbonization of the first precursor and the second precursor to obtain the intermediate product includes low-temperature carbonization of a mixture of the first precursor and the second precursor to obtain the intermediate product; it also includes low-temperature carbonization of the first precursor and the second precursor separately to obtain the first intermediate product and the second intermediate product respectively.

[0173] In some embodiments, the Dv50 particle size of the first precursor is 6.5 μm to 10.0 μm.

[0174] In some embodiments, the Dv50 particle size of the first precursor is 6.5 μm, 7.0 μm, 8.0 μm, 9.0 μm, 10.0 μm, or any range therebetween.

[0175] In some embodiments, the particle size distribution of the first precursor (Dv90-Dv10) / Dv50 is 1.05-1.75.

[0176] In some embodiments, the particle size distribution of the first precursor (Dv90-Dv10) / Dv50 is 1.05, 1.15, 1.25, 1.35, 1.45, 1.55, 1.65, 1.75, or any range therebetween.

[0177] In some embodiments, the tap density of the first precursor is 0.5 g / cm 3 ~0.7g / cm 3 .

[0178] As used herein, the term "tap density" refers to the mass per unit volume of powder in a container measured after being tapped under specified conditions.

[0179] In the present disclosure, the tap density of the first precursor can be measured using methods known in the art. For example, GB / T 5162-2006 can be used for the tap density test using a powder tap density tester. The test instrument can be a Dandong Better BT-301, with the following test parameters: vibration frequency of 250 ± 15 times / minute, amplitude of 3 ± 0.2 mm, number of vibrations of 5000 times, and a 25 mL graduated cylinder.

[0180] In some embodiments, the tap density of the first precursor is 0.5 g / cm 3 , 0.55g / cm 3, 0.6g / cm 3 , 0.65g / cm 3 , 0.7g / cm 3 or any range of values ​​between them.

[0181] In some embodiments, the Dv50 particle size of the second precursor is 11.0 μm to 14.0 μm.

[0182] In some embodiments, the Dv50 particle size of the second precursor is 11.0 μm, 12.0 μm, 13.0 μm, 14.0 μm, or any range therebetween.

[0183] The second precursor is obtained by granulating the first precursor, and therefore, the second precursor mainly forms secondary particles in the graphite material.

[0184] Controlling the particle size of the first precursor and the second precursor helps to regulate the particle size and particle size distribution of the graphite material and improve the cycle stability of the battery.

[0185] In some embodiments, the crushing, shaping and grading of the raw materials to obtain the first precursor includes: crushing, shaping and grading the raw materials to obtain secondary raw materials; removing fine powder accounting for 10%-35% of the total mass of the secondary raw materials to obtain the first precursor; the Dv50 of the fine powder is 3μm-7μm, and Dv99 is less than or equal to 30μm.

[0186] In some embodiments, the fine powder has a particle size distribution (Dv90-Dv10) / Dv50 greater than 1.50.

[0187] In some embodiments, the temperature of low-temperature carbonization is 900° C.-1300° C., and the time of low-temperature carbonization is 24 hours-240 hours.

[0188] In some embodiments, the temperature of the low-temperature carbonization may be selected to be 900° C., 1000° C., 1100° C., 1200° C., 1300° C., or any range therebetween.

[0189] In some embodiments, the low-temperature carbonization time may be 24 h, 50 h, 75 h, 100 h, 150 h, 200 h, 240 h, or any range therebetween.

[0190] [Negative electrode]

[0191] In some embodiments, the negative electrode sheet includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector. For example, the negative electrode current collector has two surfaces that are opposite to each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposing surfaces of the negative electrode current collector.

[0192] As shown in FIG. 1 to FIG. 3 , the negative electrode film layer 102 further includes a middle region 1023 located between the first region 1021 and the second region 1022 of the negative electrode film layer and having a thickness of 0.4H (H represents the thickness of the negative electrode film layer 102 ).

[0193] In some embodiments, the intermediate region includes the first binder and / or the second binder. For example, as shown in FIG2 , the intermediate region 1023 may be the same in composition as the first region 1021, whereby the first binder is distributed in the thickness direction of the negative electrode film layer 102 within a range from the second surface 102b of the negative electrode film layer to a thickness of 0.7H; or, as shown in FIG3 , the intermediate region 1023 may be the same in composition as the second region 1022, whereby the second binder is distributed in the thickness direction of the negative electrode film layer 102 within a range from the first surface 102a of the negative electrode film layer to a thickness of 0.7H; or, as shown in FIG1 , the intermediate region 1023 includes both the first binder and the second binder. In this case, the intermediate region 1023 includes both a layer structure having the first binder and a layer structure having the second binder, and the two-layer structure may further have a layer interface.

[0194] In some embodiments, the negative electrode film layer may further include other negative electrode active materials in addition to the above-mentioned active materials. In some embodiments, the other negative electrode active materials include, but are not limited to, one or more of conventional natural graphite, other artificial graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based materials may include one or more of elemental silicon, silicon oxide, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy materials. The tin-based materials may include one or more of elemental tin, tin oxide, and tin alloy materials.

[0195] In some embodiments, the negative electrode film layer may further include other additives. For example, the other additives may include a thickener, such as sodium carboxymethyl cellulose (CMC), a PTC thermistor material, and the like.

[0196] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. As an example of a metal foil, copper foil may be used. The composite current collector may include a polymer material base layer and a metal material layer formed on at least one surface of the polymer material base layer. As an example, the metal material may include one or more of copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy. As an example, the polymer material base layer may include one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS) and polyethylene (PE).

[0197] The negative electrode film layer is typically formed by coating a negative electrode slurry onto a negative electrode current collector, drying, and cold pressing. The negative electrode slurry is typically formed by dispersing the negative electrode active material, an optional conductive agent, an optional binder, and other optional additives in a solvent and stirring until uniformly mixed. The solvent may be, but is not limited to, N-methylpyrrolidone (NMP) or deionized water.

[0198] The negative electrode plate does not exclude other additional functional layers in addition to the negative electrode film layer. For example, in some embodiments, the negative electrode plate of the present disclosure further includes a conductive primer layer (e.g., composed of a conductive agent and a binder) sandwiched between the negative electrode current collector and the negative electrode film layer and disposed on the surface of the negative electrode current collector; in some embodiments, the negative electrode plate of the present disclosure further includes a protective layer covering the surface of the negative electrode film layer.

[0199] [Positive electrode]

[0200] In some embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector. For example, the positive electrode current collector has two surfaces that are opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive electrode current collector.

[0201] The positive electrode current collector may be a metal foil or a composite current collector. As an example of a metal foil, aluminum foil may be used. The composite current collector may include a polymer material base layer and a metal material layer formed on at least one surface of the polymer material base layer. As an example, the metal material may include one or more of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy. As an example, the polymer material base layer may include one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS) and polyethylene (PE).

[0202] The positive electrode film layer generally includes a positive electrode active material, an optional binder, and an optional conductive agent. The positive electrode film layer is generally formed by coating a positive electrode slurry on the positive electrode current collector and then drying and cold pressing. The positive electrode slurry is generally formed by dispersing a positive electrode active material, an optional conductive agent, an optional binder, and any other components in a solvent and stirring evenly. The solvent can be N-methylpyrrolidone (NMP), but is not limited thereto. As an example, the binder for the positive electrode film layer may include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin. As an example, the conductive agent for the positive electrode film layer includes one or more of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0203] The positive electrode active material can adopt the positive electrode active materials for secondary batteries well-known in the art.

[0204] When the secondary battery of the present disclosure is a lithium-ion battery, the positive electrode active material may include, but is not limited to, one or more of lithium-containing transition metal oxides, lithium-containing phosphates, and their respective modified compounds. Examples of the lithium transition metal oxides may include, but are not limited to, one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and their respective modified compounds. Examples of the lithium-containing phosphates may include, but are not limited to, lithium iron phosphate, a composite material of lithium iron phosphate and carbon, lithium manganese phosphate, a composite material of lithium manganese phosphate and carbon, lithium manganese iron phosphate, a composite material of lithium manganese iron phosphate and carbon, and their respective modified compounds.

[0205] In some embodiments, in order to further improve the energy density of the secondary battery, the positive electrode active material for the lithium-ion battery may include a lithium transition metal oxide having the general formula Li a Ni b Co c M d O e A f and its modified compounds. 0.8 ≤ a ≤ 1.2, 0.5 ≤ b < 1, 0 < c < 1, 0 < d < 1, 1 ≤ e ≤ 2, 0 ≤ f ≤ 1, M is selected from one or more of Mn, Al, Zr, Zn, Cu, Cr, Mg, Fe, V, Ti, and B, and A is selected from one or more of N, F, S, and Cl.

[0206] In some embodiments, as examples, the positive electrode active material for lithium ion batteries may include LiCoO2, LiNiO2, LiMnO2, LiMn2O4, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2(NCM333),LiNi 0.5 Co 0.2 Mn 0.3 O2(NCM523), LiNi 0.6 Co 0.2 Mn 0.2 O2(NCM622),LiNi 0.8 Co 0.1 Mn 0.1 O2(NCM811), LiNi 0.85 Co 0.15 Al 0.05 One or more of O2, LiFePO4 and LiMnPO4.

[0207] In the present disclosure, the modified compounds of the above-mentioned positive electrode active materials may be the ones subjected to doping modification and / or surface coating modification on the positive electrode active materials.

[0208] [Electrolytes]

[0209] In some embodiments, the electrolyte is an electrolyte solution including an electrolyte salt and a solvent.

[0210] The type of the electrolyte salt is not particularly limited and can be selected according to actual needs.

[0211] When the secondary battery of the present disclosure is a lithium ion battery, as an example, the electrolyte salt may include one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bisfluorosulfonyl imide (LiFSI), lithium bistrifluoromethanesulfonyl imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalatoborate (LiDFOB), lithium dioxalatoborate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorobisoxalatophosphate (LiDFOP) and lithium tetrafluorooxalatophosphate (LiTFOP).

[0212] The type of the solvent is not specifically limited and can be selected according to actual needs. In some embodiments, as an example, the solvent may include ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), ethyl methyl sulfone (EMS) and diethyl sulfone (ESE) One or more.

[0213] In some embodiments, the electrolyte may further include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, or additives capable of improving certain properties of the secondary battery, such as additives that improve the overcharge performance of the secondary battery, additives that improve the high-temperature performance of the secondary battery, and additives that improve the low-temperature power performance of the secondary battery.

[0214] [Isolation film]

[0215] The present disclosure has no particular limitation on the type of the isolation membrane, and any known isolation membrane with a porous structure having good chemical stability and mechanical stability can be selected.

[0216] In some embodiments, the material of the isolation membrane may include one or more of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The isolation membrane may be a single-layer film or a multi-layer composite film. When the isolation membrane is a multi-layer composite film, the materials of each layer may be the same or different.

[0217] In some embodiments, the positive electrode sheet, the separator, and the negative electrode sheet may be formed into an electrode assembly through a winding process or a lamination process.

[0218] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0219] In some embodiments, the outer packaging can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging can also be a soft bag, such as a bag-type soft bag. The material of the soft bag can be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0220] The present disclosure has no particular limitation on the shape of the secondary battery, which can be cylindrical, square, or any other shape. FIG5 shows a secondary battery 5 with a square structure as an example.

[0221] In some embodiments, as shown in FIG6 , the outer packaging may include a shell 51 and a cover plate 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 cover plate 53 is used to cover the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the separator can be formed into 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 impregnated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, which can be adjusted according to demand.

[0222] The preparation method of the secondary battery disclosed herein is well known. In some embodiments, a positive electrode sheet, a separator, a negative electrode sheet, and an electrolyte can be assembled to form a secondary battery. As an example, the positive electrode sheet, separator, and negative electrode sheet can be wound or laminated to form an electrode assembly. The electrode assembly is then placed in an outer package, dried, and then injected with electrolyte. The secondary battery is then vacuum packaged, allowed to stand, formed, and shaped to obtain the secondary battery.

[0223] In some embodiments of the present disclosure, the secondary batteries according to the present disclosure may be assembled into a battery module. The battery module may contain multiple secondary batteries, and the specific number may be adjusted according to the application and capacity of the battery module.

[0224] Figure 7 is a schematic diagram of an exemplary battery module 4. As shown in Figure 7 , within the battery module 4, multiple secondary batteries 5 may be arranged sequentially along the length of the battery module 4. Of course, any other arrangement is also possible. Furthermore, the multiple secondary batteries 5 may be secured together using fasteners.

[0225] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0226] In some embodiments, the battery modules described above may also be assembled into a battery pack, and the number of battery modules contained in the battery pack may be adjusted according to the application and capacity of the battery pack.

[0227] Figures 8 and 9 are schematic diagrams of an exemplary battery pack 1. As shown in Figures 8 and 9, the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the box. The battery box comprises an upper case 2 and a lower case 3. The upper case 2 covers the lower case 3 and forms an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 may be arranged in any manner within the battery box.

[0228] The present disclosure also provides an electric device, which includes at least one of the secondary battery, battery module, or battery pack of the present disclosure. The secondary battery, battery module, or battery pack can be used as a power source for the electric device, and can also be used as an energy storage unit for the electric device. The electric device can be, but is not limited to, a mobile device (such as a mobile phone, a tablet computer, a laptop computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship, a satellite, an energy storage system, etc.

[0229] The electrical device may select a secondary battery, a battery module or a battery pack according to its usage requirements.

[0230] Figure 10 is a schematic diagram of an exemplary electric 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 may be used.

[0231] As another example, the electric device may be a mobile phone, a tablet computer, a laptop computer, etc. Such an electric device is usually required to be lightweight and thin, and may use a secondary battery as a power source.

[0232] Example

[0233] The following examples describe the present disclosure in more detail. These examples are intended to be illustrative only, as various modifications and variations within the scope of the present disclosure will be apparent to those skilled in the art. Unless otherwise stated, all parts, percentages, and ratios reported in the following examples are by mass, and all reagents used in the examples are commercially available or synthesized according to conventional methods and can be used directly without further processing, and all instruments used in the examples are commercially available.

[0234] In the following examples and comparative examples, the graphite materials used can be purchased commercially or prepared by the following method disclosed herein.

[0235] Example 1

[0236] (1) Preparation of graphite:

[0237] The needle coke with a fiber structure of 63.7% was crushed; the crushed material was shaped to remove fine powder to obtain the first precursor. Some fine powder was removed from the first precursor. The mass of the removed fine powder accounted for 21% of the total mass of the crushed material. The Dv50 of the fine powder was 3-7μm and Dv99 was less than or equal to 30μm. The particle size distribution of the fine powder (Dv90-Dv10) / Dv50 was greater than 1.6. Among them, the Dv50 particle size of the first precursor was 9.0μm, the particle size distribution (Dv90-Dv10) / Dv50 was 1.35, and the tap density of the first precursor was 0.65g / cm 3 .

[0238] The first precursor is granulated and shaped in a reactor to obtain a second precursor with a particle size Dv50 of 14.1 μm;

[0239] The first precursor and the second precursor were placed in a kiln for carbonization at a temperature of 1100°C and a carbonization time of 24 hours to obtain a first intermediate product and a second intermediate product. The tap density of the first intermediate product was 0.98 g / cm 3 The tap density of the second intermediate product is 0.91g / cm 3 ;

[0240] The first intermediate product and the second intermediate product were respectively placed in an inner series furnace for graphitization treatment at a temperature of 2800°C. The rated power of the inner series furnace is 28000W, the maximum power of the graphitization treatment is 22400W, and the constant power time of the maximum power is maintained for 24 hours to obtain primary particles and secondary particles respectively.

[0241] The primary particles and secondary particles are mixed evenly in a mass ratio of 1:1 and sieved to obtain the final graphite material.

[0242] The thickness of the disordered layer of the graphite material is 11.3 nm. D / I G The interlayer spacing d1 of the disordered layer is 0.3368 nm, the interlayer spacing d2 of the inner region is 0.3361 nm, the particle size distribution (Dv90-Dv10) / Dv50 is 1.28, and the specific surface area is 1.65 m 2 / g, the volume distribution particle size Dv50 is 10.2μm, and the powder compaction density under a pressure of 49000N is 1.81g / cm 3 , the gram capacity is 350.4mAh / g.

[0243] (2) Preparation of negative electrode sheet

[0244] The prepared graphite negative electrode active material, conductive agent carbon black (Super P), thickener sodium carboxymethyl cellulose, and linear binder polyacrylic acid were fully stirred and mixed in an appropriate amount of solvent deionized water at a mass ratio of 96.3:1:1.2:1.5 to form a first slurry.

[0245] The prepared graphite negative electrode active material, conductive agent carbon black (Super P), thickener sodium carboxymethyl cellulose, and point binder styrene-butadiene rubber were fully stirred and mixed in an appropriate amount of solvent deionized water at a mass ratio of 96.8:1:1.2:1.0 to form a second slurry.

[0246] The second slurry and the first slurry are extruded simultaneously through a dual-chamber coating device. The first slurry is coated on the negative electrode current collector copper foil, and the second slurry is coated on the first slurry. After drying and cold pressing, the negative electrode sheet is obtained. The coating weight of the first and second slurries is the same, both 5.0mg / cm 2 .

[0247] (3) Preparation of positive electrode sheet

[0248] The positive electrode active material lithium iron phosphate (LFP), the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 97:1:2, and the solvent NMP was added. The mixture was stirred in a vacuum mixer until the system became uniform to obtain a positive electrode slurry. The positive electrode slurry was evenly coated on the positive electrode current collector aluminum foil, and the positive electrode sheets were obtained after drying, cold pressing, and slitting. The compacted density of the positive electrode film layer is 2.50g / cm 3 , the surface density is 19.47 mg / cm 2 .

[0249] (4) Preparation of electrolyte

[0250] In an argon atmosphere glove box with a water content of less than 10 ppm, diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and ethylene carbonate (EC) were mixed in a weight ratio of 1:1:1 to obtain an organic solvent. LiPF6 was then dissolved in the organic solvent to prepare an electrolyte with a concentration of 1.0 mol / L. Vinylene carbonate (VC) was then added, and the VC content was 2% of the total mass of the electrolyte.

[0251] (5) Preparation of isolation membrane

[0252] Polypropylene film is used as the isolation film.

[0253] (6) Preparation of lithium-ion batteries

[0254] The positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator placed between the positive and negative electrode sheets to serve as an isolation, and then wound to obtain an electrode assembly; the electrode assembly is placed in an outer packaging shell, dried, and then injected with electrolyte, and after vacuum packaging, standing, forming, shaping and other processes, a lithium-ion battery is obtained.

[0255] In Examples 2-8, the type of binder in the first slurry and / or the second slurry and / or the mass content of the binder in the first slurry or the second slurry were adjusted; see Table 1 for details.

[0256] Table 1

[0257] The preparation methods of Comparative Example 1 are basically the same as those of Example 1, except that a dot-shaped adhesive is used in both the first region and the second region of the negative electrode film layer.

[0258] The preparation method of Comparative Example 2 is basically the same as that of Example 1, except that a linear binder is used in both the first region and the second region of the negative electrode film layer.

[0259] The preparation method of Comparative Example 3 is basically the same as that of Example 1, except that the second slurry is coated on the negative electrode current collector copper foil, and the first slurry is coated on the second slurry.

[0260] Performance Testing

[0261] (1) Electrode rebound rate and cycle expansion rate

[0262] Rebound rate of the electrode: The thickness of the negative electrode after cold pressing is L0. The thickness of the negative electrode after cold pressing is measured at 25°C and 10% humidity for 24 hours, and the thickness of the negative electrode is L1. The rebound rate of the electrode is V0 = (L1-L0) / L0;

[0263] Cyclic expansion rate of the electrode: The batteries of the above-mentioned embodiments and comparative examples are charged at a constant current of 1C to a voltage of 3.65V, and then charged at a constant voltage of 3.65V to a current ≤ 0.05C. The batteries are then discharged at a constant current of 1C to a voltage of 2.5V. This is one charge and discharge process. After repeating the charge and discharge cycle for 100 times, the thickness L2 of the negative electrode is tested. The cyclic expansion rate of the electrode is V2 = (L2-L0) / L0.

[0264] (2) Cohesive strength test

[0265] Cut the negative electrode into strips with a length of 20 cm and a width of 20 mm, and evenly stick 3M tape on the surface of the strip (make sure the surface is smooth and wrinkle-free after sticking). Keep a small piece of copper foil tape on the paper, and roll the tape back and forth with a roller 4 times. Clamp the white paper on the tensile testing machine fixture, and start the tensile testing machine test to separate the single-sided tape that sticks to the negative active material layer of the strip from the strip. After the test is completed, the cohesive force value is obtained, and the force is divided by the width of the test strip (20 mm) as the cohesive strength (N / m) between the particles in the negative electrode film layer.

[0266] (3) Cyclic performance test

[0267] At 60°C, the batteries of the above examples and comparative examples were charged at a constant current of 1C to a voltage of 3.65V. They were then charged at a constant voltage of 3.65V to a current ≤ 0.05C. The batteries were then discharged at a constant current of 1C to a voltage of 2.5V. This constituted one charge and discharge cycle, and the discharge capacity (C1) of the first cycle was recorded. This charge and discharge cycle was repeated until the battery capacity decayed to 80% of the initial capacity (C1). The test was then stopped, and the number of test cycles was recorded.

[0268] (4) Dynamic performance test

[0269] At 25°C, the secondary battery was charged to 3.65V at a constant current of 0.33C, then charged to a current of 0.05C at a constant voltage. After standing for 5 minutes, the secondary battery was discharged to 2.50V at a constant current of 0.33C, and its actual capacity was recorded as C0.

[0270] Then the secondary battery is charged with a constant current of 1.0 C0, 1.3 C0, 1.5 C0, 1.8 C0, 2.0 C0, 2.3 C0, 2.5 C0, 3.0 C0, to a negative electrode cutoff potential of 3.5V or 0V (whichever is reached first). After each charging is completed, it is discharged to 2.5V with 1 C0. The negative electrode potential corresponding to charging to 10%, 20%, 30%, ..., 80% SOC (State of Charge) at different charging rates is recorded, and the charging rate-negative electrode potential curve under different SOC states is drawn. After linear fitting, the charging rate corresponding to the negative electrode potential of 0V under different SOC states is obtained. The charging rate is the charging window under the SOC state, which is recorded as C10% SOC, C20% SOC, C30% SOC, C40% SOC, C50% SOC, C60% SOC, C70% SOC, C80% SOC, C90% SOC, ...110% SOC, C120% SOC, C130% SOC, C140% SOC, C150% SOC, C160% SOC, C170% SOC, C180% SOC, C190% SOC, C20% SOC, C210% SOC, C220% SOC, C230% SOC, C240% SOC, C250% SOC, C260% SOC, C270% SOC, C300% SOC, C310% SOC, C320% SOC, C330% S At 80% SOC, the charging time T (in minutes) required for the secondary battery to charge from 10% SOC to 80% SOC (assuming lithium deposition does not occur in the secondary battery) is calculated using the formula (60 / C10% SOC + 60 / C20% SOC + 60 / C30% SOC + 60 / C40% SOC + 60 / C50% SOC + 60 / C60% SOC + 60 / C70% SOC + 60 / C80% SOC) × 10%. The shorter the charging time, the better the secondary battery's kinetic performance.

[0271] Test results

[0272] The test results are shown in Tables 2 and 3. As shown in Table 2, the secondary battery includes a negative electrode plate, which includes a negative electrode current collector and a negative electrode film formed on at least one surface of the negative electrode current collector. The negative electrode film has a first surface remote from the negative electrode current collector and a second surface opposite the first surface. The thickness of the negative electrode film is denoted as H. The region from the second surface of the negative electrode film to a thickness of 0.3H is denoted as the first region of the negative electrode film layer, and the region from the first surface of the negative electrode film to a thickness of 0.3H is denoted as the second region of the negative electrode film layer. The first region includes a first binder, which is a linear binder; the second region includes a second binder, which is a point-shaped binder, which helps reduce the cold press rebound rate and cycle expansion rate of the plate and improve the cycle stability of the battery.

[0273] Table 2

[0274] As can be seen from Table 3, when the mass proportion of the first binder in the first region is 1.0% to 2.5% and / or the mass proportion of the second binder in the second region is 0.6% to 1.2%, the battery has both good cycle stability and dynamic performance.

[0275] Table 3

[0276] It should be noted that the present disclosure is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and within the scope of the technical solution of the present disclosure, embodiments having substantially the same structure as the technical concept and exerting the same effects are all included in the technical scope of the present disclosure. In addition, within the scope of the present disclosure, various modifications that can be imagined by those skilled in the art to the embodiments, and other methods constructed by combining some of the constituent elements of the embodiments are also included in the scope of the present disclosure.

Claims

1. A secondary battery, characterized in that: The invention comprises a negative electrode plate, wherein the negative electrode plate comprises a negative electrode current collector and a negative electrode film layer formed on at least one surface of the negative electrode current collector, wherein the negative electrode film layer has a first surface away from the negative electrode current collector and a second surface arranged opposite to the first surface, the thickness of the negative electrode film layer is recorded as H, the area within the thickness range from the second surface of the negative electrode film layer to 0.3H is recorded as the first area of ​​the negative electrode film layer, the area within the thickness range from the first surface of the negative electrode film layer to 0.3H is recorded as the second area of ​​the negative electrode film layer, the first area comprises a first binder, and the first binder is a linear binder; the second area comprises a second binder, and the second binder is a point binder.

2. The secondary battery according to claim 1, characterized in that: The weight average molecular weight of the first binder is greater than the weight average molecular weight of the second binder.

3. The secondary battery according to claim 1 or 2, characterized in that: The mass proportion of the first binder in the first region is greater than the mass proportion of the second binder in the second region; the mass proportion of the first binder in the first region refers to the mass percentage of the first binder based on the total mass of the negative electrode film layer in the first region; the mass proportion of the second binder in the second region refers to the mass percentage of the second binder based on the total mass of the negative electrode film layer in the second region.

4. The secondary battery according to any one of claims 1 to 3, characterized in that: The first adhesive includes an acrylic adhesive; and / or the second adhesive includes polystyrene butadiene copolymer.

5. The secondary battery according to claim 4, characterized in that: The acrylic adhesive includes at least one of polyacrylic acid, polymethacrylic acid, polymethyl acrylate, polyethyl methacrylate, polyhydroxyethyl methacrylate, polyhydroxyethyl acrylate, polyhydroxypropyl acrylate, polyhydroxypropyl methacrylate, polyhydroxyethyl acrylate, polypropyl methacrylate, polypropyl methacrylate, polyethyl acrylate, and polymethyl methacrylate.

6. The secondary battery according to any one of claims 1 to 5, characterized in that: The weight average molecular weight of the first binder is 500-1500, and can be 800-1000; the weight average molecular weight of the second binder is 100-200.

7. The secondary battery according to any one of claims 1 to 6, characterized in that: The mass proportion of the first binder in the first region is 1.0% to 3.0%, optionally 1.0% to 2.5%, and / or the mass proportion of the second binder in the second region is 0.6% to 1.5%, optionally 0.6% to 1.2%.

8. The secondary battery according to any one of claims 1 to 7, characterized in that: The negative electrode film layer satisfies at least one of the following conditions: (1) The compaction density of the negative electrode film layer is 1.55 g / cm 3 -1.68g / cm 3 ; (2) The negative electrode film layer includes a conductive agent, and based on the total mass of the negative electrode film layer, the mass proportion of the conductive agent is 0.4%-2.0%.

9. The secondary battery according to any one of claims 1 to 8, characterized in that: The first region and / or the second region comprises graphite, and the graphite satisfies at least one of the following conditions: (1) The specific surface area of ​​the graphite is 1.2 m 2 / g-1.8m 2 / g, optional 1.25m 2 / g-1.70m 2 / g; (2) The volume distribution particle size Dv50 of the graphite is 7.0 μm-14.0 μm, and can be optionally 9.0 μm-12.0 μm; (3) The particle size distribution of the graphite (Dv90-Dv10) / Dv50 is 1.10-1.60, and can be optionally 1.20-1.50; (4) The gram capacity of the graphite is 345 mAh / g-355 mAh / g, and can be optionally 347 mAh / g-353 mAh / g; (5) The graphite powder compaction density under a pressure of 49000N is less than or equal to 1.85g / cm 3 , optional 1.75g / cm 3 -1.84g / cm 3 ; (6) The graphite particle body includes an internal region and a surface region at least partially surrounding the internal region, wherein the surface region refers to a region extending 30 nm from the surface of the particle body to the interior of the particle, and the surface region includes a disordered layer; optionally, the thickness of the disordered layer is 1 nm-20 nm; (7) The graphite includes both primary particles and secondary particles; optionally, based on the total number of the primary particles and the secondary particles, the number of the secondary particles accounts for less than or equal to 50%; (8) The graphitization degree of the graphite is 92.0%-94.0%; (9) I of the graphite material D / I G is 0.05-0.10, among which I D / I G is the ratio of the D peak intensity to the G peak intensity obtained from the Raman spectrum, I D Indicates that the Raman spectrum is at 1350±100cm -1 The D peak intensity at I G Indicates that the Raman spectrum is at 1580±100cm -1 The G peak intensity at .

10. An electrical device comprising the secondary battery according to any one of claims 1 to 9.

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