Electrode assembly, secondary battery, battery module, battery pack, and electric device

Abstract

The application provides an electrode assembly, a secondary battery, a battery module, a battery pack and a power utilization device. The electrode assembly comprises a negative electrode sheet, a positive electrode sheet and a separator film between the negative electrode sheet and the positive electrode sheet. The negative electrode sheet comprises a negative electrode current collector and a first negative electrode film layer and a second negative electrode film layer arranged on at least one surface of the negative electrode current collector. The first negative electrode film layer is between the negative electrode current collector and the second negative electrode film layer and comprises a first negative electrode active material. The first negative electrode active material comprises graphite. The second negative electrode film layer comprises a second negative electrode active material. The second negative electrode active material comprises hard carbon. The separator film comprises a base film and a functional coating layer on at least one side of the base film opposite to the negative electrode sheet. The functional coating layer comprises a ferroelectric material. The electrode assembly provided by the application can make the secondary battery have high energy density, high charging rate and long cycle life at the same time.

Description

Technical Field

[0001] This application belongs to the field of battery technology, specifically relating to an electrode assembly, a secondary battery, a battery module, a battery pack, and an electrical device. Background Technology

[0002] In recent years, rechargeable batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, and many other fields. With the application and promotion of rechargeable batteries, the requirements for their energy density, cycle performance, and high-rate charging performance are becoming increasingly stringent. As a crucial component of rechargeable batteries, the performance of the negative electrode active material significantly impacts the overall battery performance. Graphite is one of the most commonly used negative electrode active materials in rechargeable batteries, possessing advantages such as low polarization and high cycle stability; however, its theoretical specific capacity is only 372 mAh / g. Currently, the performance of commercial graphite has been almost fully developed, with very limited room for improvement in its reversible specific capacity and energy density. Furthermore, the small interlayer spacing of graphite also limits its high-rate charging performance. Hard carbon, as a novel negative electrode active material, enables rapid insertion and extraction of active ions during the charging and discharging process of rechargeable batteries, thus showing great promise. However, commercial hard carbon has low compaction density and initial coulombic efficiency, limiting its contribution to improving the energy density of rechargeable batteries. Summary of the Invention

[0003] The purpose of this application is to provide an electrode assembly, a secondary battery, a battery module, a battery pack, and an electrical device, which aims to enable the secondary battery to have high energy density while also having high charging rate and long cycle life.

[0004] A first aspect of this application provides an electrode assembly, including a negative electrode sheet, a positive electrode sheet, and a separator membrane located between the negative electrode sheet and the positive electrode sheet. The negative electrode sheet includes a negative current collector and a first negative electrode film layer and a second negative electrode film layer disposed on at least one surface of the negative current collector. The first negative electrode film layer is located between the negative current collector and the second negative electrode film layer and includes a first negative electrode active material, which includes graphite. The second negative electrode film layer includes a second negative electrode active material, which includes hard carbon. The separator membrane includes a base film and a functional coating located at least on the side of the base film opposite to the negative electrode sheet. The functional coating includes a ferroelectric material.

[0005] In the electrode assembly of this application, a two-layer design, in which a first negative electrode film layer comprising graphite and a second negative electrode film layer comprising hard carbon are sequentially disposed on the surface of the negative electrode current collector, can compensate for the respective defects of graphite and hard carbon and highlight their respective advantages. The larger interlayer spacing of hard carbon allows for a higher charging rate; the graphite, disposed between the hard carbon and the negative electrode current collector, compensates for the initial coulombic efficiency of hard carbon. Furthermore, since the potential for active ions to embed in the microporous structure of hard carbon is around 0V, which is close to the deposition potential of active ions on the graphite surface, the role of the hard carbon microporous structure as a storage site for active ions cannot be fully utilized. However, the inventors of this application have surprisingly discovered that by disposing a functional coating comprising a ferroelectric material on at least one side of the separator close to the second negative electrode film layer, the above problem can be solved. The ferroelectric material can regulate the deposition mode of active ions, thereby highlighting the large capacity advantage of the hard carbon microporous structure and inhibiting the continuous reduction and deposition of active ions on the graphite surface, thus improving the cycle life of the secondary battery. Therefore, the secondary battery using the electrode assembly of this application has a long cycle life and can be charged at a high rate. At the same time, the secondary battery also has a high output voltage and thus a high energy density.

[0006] In any embodiment of this application, the thickness of the functional coating is H1 μm, the thickness of the second negative electrode film is H2 μm, the thickness of the first negative electrode film is H3 μm, and the electrode assembly satisfies H1 / (H2+H3) as 0.01 to 0.15, optionally 0.01 to 0.08. This results in better overall performance of the secondary battery, enabling high-rate charging at high energy density.

[0007] In any embodiment of this application, the thickness of the functional coating is H1 μm, where H1 is 2 to 10, optionally 4 to 6. This results in better overall performance of the secondary battery, enabling high-rate charging at high energy density.

[0008] In any embodiment of this application, the thickness of the second negative electrode film is H2 μm, the thickness of the first negative electrode film is H3 μm, and the negative electrode sheet satisfies H2 / H3 = 0.10 to 5, optionally 0.5 to 4. Thus, the second and first negative electrode films can exert a better synergistic effect.

[0009] In any embodiment of this application, the volume average particle size Dv50 of the ferroelectric material is d1 μm, where d1 is less than 1, and optionally from 0.05 to 0.8. This results in better overall performance of the secondary battery, enabling high-rate charging at high energy density while reducing production costs.

[0010] In any embodiment of this application, the volume average particle size Dv50 of the second negative electrode active material is d2 μm, the volume average particle size Dv50 of the first negative electrode active material is d3 μm, and the d2 / d3 ratio is 0.1 to 1, optionally 0.2 to 0.8. This is beneficial for fully utilizing the role of the second negative electrode film in improving the high-rate charging capability of the secondary battery and the first negative electrode film in improving the initial coulombic efficiency and cycle life of the secondary battery. Therefore, the overall performance of the secondary battery is better, enabling high-rate charging at high energy density.

[0011] In any embodiment of this application, the ferroelectric material content in the functional coating is W1, which is 70% to 95% based on the total mass of the functional coating, and optionally 80% to 95%. This results in better overall performance of the secondary battery, enabling high-rate charging at high energy density.

[0012] In any embodiment of this application, the functional coating further includes an adhesive, optionally comprising one or more combinations selected from styrene-butadiene rubber, polyacrylic acid, sodium polyacrylate, polyacrylamide, polyvinyl alcohol, sodium alginate, polymethacrylic acid, carboxymethyl chitosan, fluorinated acrylate resins, polytetrafluoroethylene, vinylidene fluoride homopolymers and copolymers thereof.

[0013] In any embodiment of this application, the separator further includes an adhesive layer disposed on the surface of the functional coating. Optionally, the adhesive layer comprises one or more combinations selected from vinylidene fluoride homopolymers and copolymers thereof. This can improve the winding effect of the electrode assembly, resulting in a longer cycle life for the secondary battery.

[0014] In any embodiment of this application, the dielectric constant of the ferroelectric material is 50 or more, and optionally 50 to 100,000.

[0015] In any embodiment of this application, the ferroelectric material includes one or more combinations selected from inorganic ferroelectric materials and organic ferroelectric materials. Optionally, the inorganic ferroelectric material includes one or more combinations selected from perovskite structure oxides, tungsten bronze type compounds, bismuth oxide type layered structure compounds, lithium niobate, lithium tantalate, lead metaniobate, and lead barium lithium niobate. Optionally, the organic ferroelectric material may include one or more combinations selected from vinylidene fluoride homopolymers or copolymers, 2,5-dibromo-3,6-dihydroxy-p-benzoquinone, phenazine-chlororanic acid, and ketone acid.

[0016] In any embodiment of this application, the mass percentage of hard carbon in the second negative electrode film is W2, which is 68% or more based on the total mass of the second negative electrode film, and optionally 90% to 98%. This is beneficial for the secondary battery to have a higher charging rate.

[0017] In any embodiment of this application, the graphite mass percentage in the first negative electrode film layer is W3, which is 78% or more based on the total mass of the first negative electrode film layer, and optionally 90% to 98%. This is beneficial for the secondary battery to have higher initial coulombic efficiency and longer cycle life.

[0018] In any embodiment of this application, the volume average particle size Dv50 of the second negative electrode active material is d2μm, where d2 is 3 to 11, optionally 3 to 7. This is beneficial for improving the capacity utilization and energy density of the secondary battery.

[0019] In any embodiment of this application, the particle size distribution index (Dv90-Dv10) / Dv50 of the second negative electrode active material is α1, where α1 is 0.6 to 5, and optionally 1 to 4. This is beneficial for improving the high-rate charging capability and charge-discharge efficiency of the secondary battery.

[0020] In any embodiment of this application, the specific surface area of ​​the second negative electrode active material is 3m². 2 / g to 7m 2 / g, optionally 4m 2 / g to 6m 2 / g. This helps to improve the high-rate charging capability of secondary batteries.

[0021] In any embodiment of this application, the powder compaction density of the second negative electrode active material at 20000N is 0.9 g / cm³. 3 Up to 1.3 g / cm 3 Optionally, it can be 1g / cm 3 Up to 1.2 g / cm 3 This is beneficial for increasing the energy density of secondary batteries.

[0022] In any embodiment of this application, the second negative electrode active material includes primary particles, secondary particles, or a combination thereof. Optionally, the proportion of primary particles in the second negative electrode active material is 90% to 100%. This is beneficial for improving the high-rate charging capability of the secondary battery.

[0023] In any embodiment of this application, the volume average particle size Dv50 of the first negative electrode active material is d3 μm, where d3 is 9 to 18, optionally 11 to 15. This is beneficial for the secondary battery to have high initial coulombic efficiency, high energy density, and long cycle life.

[0024] In any embodiment of this application, the particle size distribution index (Dv90-Dv10) / Dv50 of the first negative electrode active material is α2, where α2 is 0.2 to 5, and optionally 0.3 to 4. This is beneficial for improving the cycle performance of the secondary battery.

[0025] In any embodiment of this application, the specific surface area of ​​the first negative electrode active material is 0.6 m². 2 / g to 1.5m 2 / g, optionally 0.8m 2 / g to 1.4m 2 / g. This is beneficial for improving the cycle performance of secondary batteries.

[0026] In any embodiment of this application, the powder compaction density of the first negative electrode active material at 20000N is 1.4 g / cm³. 3 Up to 1.85 g / cm 3 Optionally, it can be 1.6 g / cm³. 3 Up to 1.75 g / cm 3 This is beneficial for increasing the energy density of secondary batteries.

[0027] In any embodiment of this application, the graphitization degree of the first negative electrode active material is 91% to 95%, optionally 92% to 94%. This is beneficial for improving the cycle performance of the secondary battery.

[0028] In any embodiment of this application, the first negative electrode active material includes artificial graphite, natural graphite, or a combination thereof. Optionally, the artificial graphite has a carbon coating layer on its surface. This is beneficial for improving the cycle performance and high-rate charging capability of the secondary battery.

[0029] In any embodiment of this application, the first negative electrode active material includes primary particles, secondary particles, or a combination thereof. Optionally, the proportion of secondary particles in the first negative electrode active material is 90% to 100%. This is beneficial for improving the cycle performance, storage performance, and high-rate charging capability of the secondary battery.

[0030] A second aspect of this application provides a secondary battery that includes the electrode assembly of the first aspect of this application.

[0031] A third aspect of this application provides a battery module that includes the secondary battery of the second aspect of this application.

[0032] The fourth aspect of this application provides a battery pack, which includes one of the secondary battery of the second aspect of this application and the battery module of the third aspect.

[0033] The fifth aspect of this application provides an electrical device that includes at least one of the secondary battery of the second aspect of this application, the battery module of the third aspect, and the battery pack of the fourth aspect.

[0034] The secondary battery using the electrode assembly of this application has a long cycle life and can be charged at high rates. It also has a high output voltage and, consequently, high energy density. The battery module, battery pack, and power device of this application include the secondary battery provided in this application and therefore have at least the same advantages as the described secondary battery. Attached Figure Description

[0035] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments of this application will be briefly described below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on the drawings without creative effort.

[0036] Figure 1 This is a schematic diagram of one embodiment of the electrode assembly of this application.

[0037] Figure 2 This is a schematic diagram of another embodiment of the electrode assembly of this application.

[0038] Figure 3 This is a schematic diagram of one embodiment of the secondary battery of this application.

[0039] Figure 4 yes Figure 3 An exploded view of the implementation method of the secondary battery.

[0040] Figure 5 This is a schematic diagram of one embodiment of the battery module of this application.

[0041] Figure 6 This is a schematic diagram of one embodiment of the battery pack of this application.

[0042] Figure 7 yes Figure 6 An exploded view of an embodiment of the battery pack shown.

[0043] Figure 8 This is a schematic diagram of one embodiment of an electrical device that uses a secondary battery as a power source, as described in this application.

[0044] In the accompanying drawings, the figures may not be drawn to scale. The reference numerals are explained as follows: 1 Battery pack, 2 Upper casing, 3 Lower casing, 4 Battery module, 5 Secondary battery, 51 Housing, 52 Electrode assembly, 53 Cover plate, 6 Electrode assembly, 10 Negative electrode sheet, 100 Negative current collector, 101 First negative electrode film, 102 Second negative electrode film, 20 Positive electrode sheet, 200 Positive current collector, 201 Positive electrode film, 30 Separator, 300 Base film, 301 Functional coating, 302 Adhesive layer. Detailed Implementation

[0045] The following detailed description, with appropriate reference to the accompanying drawings, specifically discloses embodiments of the electrode assembly, secondary battery, battery module, battery pack, and power-consuming device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0046] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0047] Unless otherwise specified, all embodiments and optional embodiments of this application may be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of this application.

[0048] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions, and such technical solutions shall be deemed to be included in the disclosure of this application.

[0049] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the method may also include step (c), indicating that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0050] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0051] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: 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).

[0052] In this application, the terms "multiple" or "various" refer to two or more kinds.

[0053] In this application, the terms "primary particle" and "secondary particle" have meanings known in the art. A primary particle refers to a particle that has not formed an aggregate, while a secondary particle refers to an aggregated particle formed by the aggregation of two or more primary particles. Primary and secondary particles can be easily distinguished by taking SEM images using a scanning electron microscope.

[0054] Hard carbon refers to carbon that is difficult to graphitize, even at temperatures above 2500℃. Hard carbon is typically obtained by pyrolyzing precursors such as polymers. During pyrolysis, the cross-linked structure of carbon atoms in the precursor hinders the planar growth of carbon layers. Therefore, hard carbon structures contain a large number of disordered graphite-like microcrystals (referred to as graphite microcrystals). The complex structure of hard carbon includes not only graphite microcrystals but also defect structures (such as surface defects and lattice defects) and microporous structures (such as open-pore and closed-pore structures). Therefore, active ions can be inserted and extracted from various angles of hard carbon, giving secondary batteries excellent high-rate charging capabilities. Hard carbon has unique advantages, especially in the field of power batteries. Compared to graphite, hard carbon (002) has a larger interlayer spacing, resulting in higher structural stability during charging and discharging, without significant volume expansion or contraction effects.

[0055] Hard carbon exhibits numerous defective structures, which catalyze the decomposition of the electrolyte, forming a thicker solid electrolyte interphase (SEI) film and increasing the irreversible loss of active ions. Furthermore, the porous surface of hard carbon readily adsorbs moisture and oxygen from the air, forming various CH functional groups. These functional groups react with active ions, further increasing their irreversible consumption. Therefore, compared to graphite, hard carbon exhibits lower initial coulombic efficiency and poorer cycling performance; for example, its initial coulombic efficiency is typically below 80%, failing to fully realize its high capacity advantage.

[0056] The inventors of this application, through extensive research, have proposed an electrode assembly that can improve the capacity utilization of hard carbon, enabling secondary batteries to have high energy density while also having enhanced high-rate charging capability and cycle life.

[0057] Electrode assembly

[0058] Specifically, the electrode assembly of this application includes a negative electrode sheet, a positive electrode sheet, and a separator membrane located between the negative electrode sheet and the positive electrode sheet. The negative electrode sheet includes a negative current collector and a first negative electrode film layer and a second negative electrode film layer disposed on at least one surface of the negative current collector. The first negative electrode film layer is located between the negative current collector and the second negative electrode film layer and includes a first negative electrode active material, which includes graphite. The second negative electrode film layer includes a second negative electrode active material, which includes hard carbon. The separator membrane includes a base film and a functional coating located at least on the side of the base film opposite to the negative electrode sheet. The functional coating includes a ferroelectric material. In some embodiments, the positive electrode sheet, the separator membrane, and the negative electrode sheet can be fabricated into the electrode assembly using a winding process and / or a stacking process.

[0059] Graphite possesses advantages such as high initial coulombic efficiency and high cycle stability, but its performance during continuous high-rate charging is poor. Hard carbon, on the other hand, has advantages such as high active ion insertion and extraction rates, resulting in excellent high-rate charging capabilities, but its initial coulombic efficiency is low and its capacity decays rapidly, leading to poor actual capacity utilization in rechargeable batteries. In their actual research, the inventors of this application discovered that a two-layer design, sequentially placing graphite and hard carbon on the surface of the negative electrode current collector, can compensate for the respective shortcomings of graphite and hard carbon. Placing graphite closer to the negative electrode current collector compensates for the low initial coulombic efficiency of hard carbon, while placing hard carbon further away allows more active ions to be inserted into the negative electrode more quickly, thus avoiding the poor continuous high-rate charging performance of graphite.

[0060] However, during further research, the inventors of this application discovered that combining graphite and hard carbon sequentially on the surface of the negative electrode current collector in a two-layer configuration did not ideally improve the high-rate charging capability and energy density of the secondary battery. Further investigation revealed that the possible reason is that, during secondary battery charging, especially towards the end of the charging process, the formation of dendrites is a significant factor affecting the improvement of the high-rate charging capability and energy density of the secondary battery.

[0061] During the charging process of a secondary battery, when abnormal situations occur such as insufficient space for active ions to embed in the negative electrode, excessive resistance to active ions embedding in the negative electrode, or active ions rapidly detaching from the positive electrode but failing to embed an equal amount into the negative electrode, the active ions that cannot embed in the negative electrode can only gain electrons on the surface of the negative electrode and precipitate as metallic elements, forming "dendrites." Therefore, the higher the charging rate of the secondary battery, the more severe the "dendritic" problem becomes. The formation of dendrites not only degrades the performance of the secondary battery, such as shortening cycle life, but in severe cases, sharp dendrites can pierce the separator, causing internal short circuits and potentially leading to catastrophic consequences such as combustion or explosion, increasing the safety risks of the secondary battery. Furthermore, continuously deposited dendrites can detach from the surface of the negative electrode, causing it to lose electrical contact with the negative electrode current collector, thus preventing it from continuing to participate in the charging and discharging reaction and contributing capacity, thereby reducing the energy density of the secondary battery.

[0062] After combining graphite and hard carbon in a two-layer configuration on the surface of the negative electrode current collector, during the initial charging phase of the secondary battery, active ions are first adsorbed at the defects on the hard carbon surface and at lattice defects. This process corresponds to the "high-potential slope region" in the hard carbon charge-discharge curve, where the negative electrode potential is typically between ~2V and ~0.1V. As charging continues, active ions intercalate between the graphite microcrystals of the hard carbon, exhibiting graphite-like intercalation behavior. This process corresponds to the "low-potential plateau region" in the hard carbon charge-discharge curve, where the negative electrode potential is typically between ~0.1V and 0V. Because hard carbon also possesses a rich microporous structure, during secondary charging, when the negative electrode potential drops to around 0V, active ions can still be stored in the microporous structure of the hard carbon, providing additional active ion storage sites and improving the capacity utilization and initial coulombic efficiency of the hard carbon. Furthermore, the longer the discharge plateau in the low-potential plateau region, the higher the reversible capacity of the hard carbon and the better its capacity utilization. However, the potential of the low-potential plateau region of hard carbon is close to the dendrite formation potential of the graphite surface, both around 0V. Therefore, to prevent dendrite formation, the capacity utilization of the low-potential plateau region of hard carbon is limited, and the role of the hard carbon microporous structure as an active ion storage site cannot be fully utilized. At the same time, to prevent dendrite formation, the negative electrode cutoff voltage is usually set relatively high, which also leads to a decrease in the overall output voltage of the secondary battery, and consequently a decrease in the energy density of the secondary battery. Therefore, the effect of combining graphite and hard carbon sequentially on the surface of the negative electrode current collector in a two-layer combination has limited effect on improving the high-rate charging capability, energy density, and / or cycle life of the secondary battery.

[0063] The electrode assembly of this application comprises a first negative electrode film layer including graphite and a second negative electrode film layer including hard carbon, sequentially disposed on the surface of the negative electrode current collector in a two-layer combination design. Furthermore, a functional coating including ferroelectric material is also disposed on the side of the separator near the second negative electrode film layer. Ferroelectric materials exhibit spontaneous polarization; when electrons accumulate on the dendrite surface, the ferroelectric material undergoes spontaneous polarization under the influence of an electric field. This causes the positive charge center of the ferroelectric material to move towards the electron accumulation region, thereby encapsulating the electron accumulation region. Simultaneously, because the positive charge center of the ferroelectric material carries a positive charge, it repels the positively charged active ions on the dendrite surface, thus balancing the electron density and reducing the enrichment of active ions, thereby inhibiting the continuous growth of dendrites in the direction perpendicular to the electrode.

[0064] Therefore, in the electrode assembly of this application, the two-layer combination design, which sequentially deposits a first negative electrode film layer including graphite and a second negative electrode film layer including hard carbon on the surface of the negative electrode current collector, can compensate for the respective defects of graphite and hard carbon and highlight their respective advantages. The larger interlayer spacing of hard carbon allows for a higher charging rate; the graphite, placed between the hard carbon and the negative electrode current collector, compensates for the initial coulombic efficiency of hard carbon. Furthermore, since the potential for active ions to embed in the microporous structure of hard carbon is around 0V, which is close to the deposition potential of active ions on the graphite surface, the role of the hard carbon microporous structure as a storage site for active ions cannot be fully utilized. However, the inventors of this application have surprisingly discovered that by depositing a functional coating including a ferroelectric material on at least one side of the separator near the second negative electrode film layer, the above problem can be solved. The ferroelectric material can regulate the deposition mode of active ions, thereby highlighting the large capacity advantage of the hard carbon microporous structure and inhibiting the continuous reduction and deposition of active ions on the graphite surface, thus improving the cycle life of the secondary battery.

[0065] Specifically, in the early stage of secondary battery charging, active ions are rapidly adsorbed at the defects on the hard carbon surface and at the lattice defect sites, thereby giving the negative electrode sheet high kinetic performance. In the later stage of secondary battery charging, the overpotential can be precisely controlled by the functional coating, allowing the hard carbon microporous structure to store a large number of active ions, increasing the length of the low-potential plateau region, and increasing the reversible capacity of the secondary battery, thereby improving the high-rate charging capability and energy density of the secondary battery. In addition, under overpotential, the functional coating can also generate a reverse electric field, reducing the enrichment of active ions, inhibiting the continuous growth of dendrites in the direction perpendicular to the electrode sheet, and improving the safety and cycle performance of the secondary battery.

[0066] Furthermore, in the electrode assembly of this application, the functional coating is disposed on the surface of the separator, thereby avoiding additional manufacturing processes and preventing damage to the negative electrode sheet or impact on its processing performance, thus contributing to better overall performance of the secondary battery. The functional coating on the separator surface also increases the separator's heat resistance and reduces its thermal shrinkage rate, thereby improving the safety performance of the secondary battery, particularly its thermal safety performance.

[0067] Therefore, the secondary battery using the electrode assembly of this application has a long cycle life and can be charged at a high rate. At the same time, the secondary battery also has a high output voltage and thus a high energy density.

[0068] [Isolation membrane]

[0069] The separator of this application includes a base film and a functional coating located at least on the side of the base film opposite to the negative electrode sheet. The base film has two surfaces opposite each other in its thickness direction. The functional coating may be disposed on both surfaces of the base film, or it may be disposed only on the surface of the base film opposite to the negative electrode sheet. Optionally, the functional coating may be disposed on both surfaces of the base film.

[0070] This application does not impose any particular limitation on the type of base membrane; any well-known porous membrane with good chemical and mechanical stability can be selected. In some embodiments, the material of the base membrane may include one or more combinations selected from glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The base membrane may be a single-layer membrane or a multilayer composite membrane. When the base membrane is a multilayer composite membrane, the materials of each layer may be the same or different.

[0071] In some embodiments, the thickness of the base film can be from 7 μm to 12 μm, but this application is not limited thereto.

[0072] The functional coating comprises a ferroelectric material. Optionally, in some embodiments, the dielectric constant of the ferroelectric material is 50 or higher. The higher the dielectric constant of the ferroelectric material, the better its effect in suppressing the continuous growth of dendrites perpendicular to the electrode direction; however, its effect does not increase indefinitely. Furthermore, a higher dielectric constant places increasingly higher demands on the fabrication process of the ferroelectric material, thereby increasing production costs. Optionally, in some embodiments, the dielectric constant of the ferroelectric material can be from 50 to 100,000, for example, 50 to 50,000, 50 to 25,000, 50 to 10,000, 50 to 5,000, 50 to 4,000, 50 to 3,000, 50 to 2,000, 100 to 100,000, 100 to 50,000, or 100 to 25,000. 100 to 10000, 100 to 5000, 100 to 4000, 100 to 3000, 100 to 2000, 200 to 100000, 200 to 50000, 200 to 25000, 200 to 10000, 200 to 5000, 200 to 4000, 200 to 3000, 200 to 2000 or 200 to 1000.

[0073] In this application, the dielectric constant of ferroelectric materials refers to the dielectric constant at room temperature (25±5℃), which has a meaning known in the art and can be tested using instruments and methods known in the art. For example, after preparing the ferroelectric material into a circular sample, the capacitance C can be measured using an LCR meter and calculated according to the formula: dielectric constant ε=(C×d) / (ε0×A). C represents the capacitance, in farads (F); d represents the sample thickness, in cm; and A represents the sample area, in cm². 2; ε0 represents the vacuum permittivity, ε0 = 8.854×10 -14 F / cm. In this application, the test conditions can be 1KHz, 1.0V, 25±5°C. The test standard can be based on GB / T 11297.11-2015. When preparing the specimen, reference can be made to Chinese Patent Application CN114217139A.

[0074] In some embodiments, the ferroelectric material may include a combination of one or more selected from inorganic ferroelectric materials and organic ferroelectric materials. Optionally, the ferroelectric material may include a combination of one or more selected from inorganic ferroelectric materials.

[0075] In some embodiments, optionally, the inorganic ferroelectric material may include a combination of one or more selected from perovskite structure oxides, tungsten bronze type compounds, bismuth oxide type layered structure compounds, lithium niobate (LiNbO3), lithium tantalate (LiTaO3), lead metaniobate, and lead barium lithium niobate. More optionally, the inorganic ferroelectric material is selected from perovskite structure oxides.

[0076] Optionally, the perovskite structure oxide has the molecular formula Ba 1-x A x Ti 1-y B y O3. A may include a combination of one or more selected from Pb, Sr, Ca, K, Na, and Cd, and B may include a combination of one or more selected from Sn, Hf, Zr, Ce, Nb, and Th, where 0≤x≤1 and 0≤y≤1. For example, the perovskite structure oxide may include a combination of one or more selected from BaTiO3, Ba 1- x1 Sr x1 TiO3 (0≤x1≤1), SrTiO3, PbTiO3, PbZr y1 Ti 1-y1 O3 (0≤y1≤1), BaZr y2 Ti 1-y2 O3 (0<y2<1), KNbO3, NaNbO3.

[0077] Optionally, the tungsten bronze type compound may have the molecular formula M z WO3. M may include a combination of one or more selected from Na, K, Rb, and Cs, where 0<z<1. For example, the tungsten bronze type compound may include a combination of one or more selected from Na z1 WO3 (0<z1<1), K z2 WO3 (0<z2<1).

[0078] Optionally, the bismuth oxide-type layered compound has the molecular formula (Bi2O2)(C n-1 D n O 3n+1 C may include one or more combinations selected from Na, K, Ba, Sr, Pb, Ca, Ln, and Bi; D may include one or more combinations selected from Zr, Cr, Nb, Ta, Mo, W, Fe, Ti, and V; 2 ≤ n ≤ 5. For example, the bismuth oxide-type layered compound may be SrBi₂Nb₂O₉, SrBi₂Ta₂O₉, SrBi₂Nb₂O₉, or Bi₄Ti₃O₉. 12 One or more combinations thereof.

[0079] In some embodiments, the organic ferroelectric material may include one or more combinations selected from vinylidene fluoride homopolymers or copolymers, 2,5-dibromo-3,6-dihydroxy-p-benzoquinone, phenazine-chlororanic acid, and ketone acid.

[0080] In some embodiments, the volume average particle size Dv50 of the ferroelectric material is d1 μm, where d1 is less than 1, for example, less than 0.8, less than 0.7, less than 0.6, less than 0.5, less than 0.4, less than 0.3, or less than 0.2. Optionally, d1 is 0.01 to 1, 0.02 to 1, 0.03 to 1, 0.04 to 1, 0.05 to 1, 0.06 to 1, 0.07 to 1, 0.08 to 1, 0.01 to 0.8, 0.02 to 0.8, 0.03 to 0.8, 0.04 to 0.8, 0.05 to 0.8, 0.06 to 0.8, 0.07 to 0.8, 0.08 to 0.8, 0.09 to 0.8, or 0.1 to 0.8.

[0081] When the Dv50 of ferroelectric materials is within a suitable range, the overall performance of the secondary battery is better, enabling high-rate charging at high energy density while reducing production costs. Furthermore, it effectively avoids the following situations: When the Dv50 of the ferroelectric material is large, the resulting reverse electric field interference is significant, potentially failing to balance electron density and suppress dendrite growth along the perpendicular electrode direction. Therefore, increasing the output voltage of the secondary battery to improve energy density increases safety risks, while decreasing the output voltage to reduce safety risks fails to fully utilize the hard carbon microporous structure as active ion storage sites. When the Dv50 of the ferroelectric material is small, its fabrication process is complex, thus increasing production costs.

[0082] In some embodiments, the thickness of the functional coating is H1 μm, the thickness of the second negative electrode film is H2 μm, the thickness of the first negative electrode film is H3 μm, and the electrode assembly satisfies that H1 / (H2+H3) is 0.01 to 0.15, optionally 0.01 to 0.08.

[0083] Further research by the inventors revealed that when the thicknesses of the functional coating (H1μm), the second negative electrode film (H2μm), and the first negative electrode film (H3μm) satisfy H1 / (H2+H3) of 0.01 to 0.15, the overall performance of the secondary battery is better, enabling high-rate charging at high energy density. Furthermore, this effectively avoids the following situations: when the thickness of the functional coating is low while the total thickness of the first and second negative electrode films is high, the reverse electric field strength generated by the functional coating is insufficient, potentially failing to balance electron density and suppress dendrite growth perpendicular to the electrode. Consequently, increasing the output voltage of the secondary battery to improve energy density increases safety risks, while decreasing the output voltage to reduce safety risks fails to fully utilize the hard carbon microporous structure as active ion storage sites. Conversely, when the thickness of the functional coating is high while the total thickness of the first and second negative electrode films is low, the functional coating lacks electrochemical activity and cannot contribute to capacity, thus occupying a larger volumetric and mass share, negatively impacting the energy density of the secondary battery.

[0084] In some embodiments, the thickness of the functional coating is H1 μm, where H1 is 2 to 10, optionally 4 to 6. When the thickness of the functional coating is within a suitable range, the overall performance of the secondary battery is better, enabling high-rate charging at high energy density. Furthermore, it effectively avoids the following situations: when the functional coating is too thin, the reverse electric field strength it generates is insufficient, which may not be able to balance the electron density and suppress the continuous growth of dendrites in the direction perpendicular to the electrode. Therefore, when increasing the output voltage of the secondary battery to improve energy density, the safety risk of the secondary battery increases; conversely, when reducing the output voltage of the secondary battery to reduce safety risk, the role of the hard carbon microporous structure as active ion storage sites cannot be fully utilized. When the functional coating is too thick, since it lacks electrochemical activity and cannot contribute to capacity, it occupies a larger volume and mass share, thus affecting the energy density of the secondary battery.

[0085] In some embodiments, the ferroelectric material in the functional coating has a mass percentage W1, which is 70% to 95% based on the total mass of the functional coating, and optionally 80% to 95%.

[0086] Further research by the inventors revealed that when the content of ferroelectric material is within a suitable range, the overall performance of the secondary battery is better, enabling high-rate charging at high energy density. Furthermore, it effectively avoids the following situations: when the content of ferroelectric material is low, the reverse electric field strength it generates is insufficient, which may not be able to balance the electron density and suppress the continuous growth of dendrites in the direction perpendicular to the electrode. Therefore, increasing the output voltage of the secondary battery to improve energy density increases the safety risk, while decreasing the output voltage to reduce the safety risk fails to fully utilize the hard carbon microporous structure as active ion storage sites. When the content of ferroelectric material is high, other components in the functional coating (such as binder content) are less, which may lead to poor adhesion between the functional coating and the base film, making it prone to detachment.

[0087] In some embodiments, the functional coating may further include an adhesive to bond the ferroelectric materials to each other and to bond the functional coating to the second negative electrode film layer of the negative electrode sheet, thereby improving the winding effect of the electrode assembly and giving the secondary battery a longer cycle life. Optionally, in some embodiments, the adhesive includes one or more combinations selected from styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), carboxymethyl chitosan (CMCS), fluorinated acrylate resins, polytetrafluoroethylene (PTFE), vinylidene fluoride homopolymers, and copolymers thereof. As an example, the vinylidene fluoride copolymer may include one or more combinations selected from vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-trifluoroethylene copolymer, vinylidene fluoride-chlorotrifluoroethylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, and vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer.

[0088] In some embodiments, the functional coating may further include other additives. As an example, the other additives may include dispersants, such as sodium carboxymethyl cellulose (CMC).

[0089] In some embodiments, the separator may further include an adhesive layer disposed on the surface of the functional coating, thereby further improving the winding effect of the electrode assembly and giving the secondary battery a longer cycle life. Optionally, the adhesive layer comprises one or more combinations selected from vinylidene fluoride homopolymers and copolymers thereof. Thus, in addition to its bonding function, the adhesive layer also possesses a certain dielectric constant, thereby better suppressing the continuous growth of dendrites perpendicular to the electrode direction. As an example, the vinylidene fluoride copolymer may include one or more combinations selected from vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-trifluoroethylene copolymer, vinylidene fluoride-chlorotrifluoroethylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, and vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer. Optionally, the thickness of the adhesive layer is less than 1 μm.

[0090] The method for preparing the separator membrane of this application is well known. In some embodiments, the separator membrane can be prepared by dispersing ferroelectric materials, binders, and any other components in a solvent to form a slurry; coating the slurry onto at least one surface of a base membrane, and drying it to obtain the separator membrane. The solvent can be N-methylpyrrolidone (NMP) or deionized water, but is not limited thereto. In some embodiments, the method for preparing the separator membrane may further include the step of coating the surface of the functional coating with a slurry for forming an adhesive layer.

[0091] It should be noted that the functional coating parameters given in this application refer to the parameter range of the functional coating on one side of the base film. When the functional coating is disposed on both surfaces of the base film, if the functional coating parameters on either surface meet the requirements of this application, it is considered to fall within the protection scope of this application.

[0092] The thickness of the functional coating of the separator is a well-known concept in the art and can be measured using instruments and methods known in the art, such as scanning electron microscopy (e.g., ZEISS Sigma 300). This allows for a more accurate determination of the boundary between the functional coating and the base film. The testing standard can be found in JY / T010-1996. To ensure the accuracy of the test results, multiple (e.g., more than 5) different regions can be randomly selected from the sample to be tested for scanning. The thicknesses of the functional coating and the base film in each test region can be read at a certain magnification (e.g., 500x or higher). For greater accuracy, multiple test regions can be tested and the average value taken.

[0093] [Negative electrode plate]

[0094] In some embodiments, the thickness of the second negative electrode film is H2μm, the thickness of the first negative electrode film is H3μm, and the negative electrode sheet satisfies H2 / H3 of 0.10 to 5, optionally 0.5 to 4.

[0095] Further research by the inventors revealed that when the ratio of the thickness H2μm of the second negative electrode film to the thickness H3μm of the first negative electrode film is within a suitable range, the second and first negative electrode films can exert a better synergistic effect. The second negative electrode film mainly bears the high-rate charging pressure, ensuring a high charging rate for the secondary battery; the first negative electrode film mainly provides a long cycle life while ensuring a high initial coulombic efficiency. This results in better overall performance of the secondary battery, enabling high-rate charging at high energy density. Furthermore, it effectively avoids the following situations: when the second negative electrode film is thinner and the first negative electrode film is thicker, the hard carbon in the second negative electrode film bears less of the high-rate charging pressure during charging, and the graphite in the first negative electrode film still plays a dominant role, resulting in a less significant improvement in the charging rate of the secondary battery; when the second negative electrode film is thicker and the first negative electrode film is thinner, the presence of a large number of inactive functional groups on the surface of the hard carbon increases the irreversible consumption of active ions, affecting the initial coulombic efficiency of the secondary battery, and potentially reducing the cycle life of the secondary battery.

[0096] In some embodiments, the thickness of the second negative electrode film is H2 μm, where H2 is 10 to 120, optionally 35 to 110, and more preferably 40 to 100. A thickness within a suitable range is beneficial for the secondary battery to have a high charge rate.

[0097] In some embodiments, the thickness of the first negative electrode film is H3 μm, where H3 is 20 to 100, optionally 25 to 70, and more preferably 28 to 60. A thickness within a suitable range is beneficial for the secondary battery to have high initial coulombic efficiency and long cycle life.

[0098] The second negative electrode film layer includes a second negative electrode active material, which includes hard carbon. Optionally, the hard carbon accounts for 70% to 100% by mass, and more preferably 80% to 98%, of the second negative electrode active material, based on the total mass of the second negative electrode active material. This results in the second negative electrode film layer containing a higher amount of hard carbon, which is beneficial for the secondary battery to have a higher charge rate. In some embodiments, the second negative electrode active material may consist only of hard carbon; in other embodiments, the second negative electrode active material may also include other negative electrode active materials besides hard carbon, such as graphite, soft carbon, mesophase carbon microspheres, silicon-based materials, tin-based materials, etc. Optionally, the other negative electrode active materials account for less than 30% by mass, and more preferably less than 20%, of the second negative electrode active material, based on the total mass of the second negative electrode active material.

[0099] In some embodiments, the mass percentage of hard carbon in the second negative electrode film is W2, which is more than 68% based on the total mass of the second negative electrode film, and optionally from 90% to 98%. When the content of hard carbon is within a suitable range, it is beneficial for the secondary battery to have a higher charging rate.

[0100] In some embodiments, the second negative electrode film may further include a conductive agent to collect microcurrents between the second negative electrode active material (e.g., hard carbon), reduce electrode contact resistance, and accelerate electron mobility; it can also reduce polarization and improve the charge-discharge efficiency of the secondary battery. As an example, the conductive agent may include one or more combinations selected from superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. In some embodiments, the mass percentage of the conductive agent is less than 5% based on the total mass of the second negative electrode film.

[0101] In some embodiments, the second negative electrode film layer may further include an adhesive to bond the second negative electrode active material (e.g., hard carbon) to each other and to bond the second negative electrode film layer and the first negative electrode film layer. As an example, the adhesive may include one or more combinations selected from styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, waterborne acrylic resins (e.g., polyacrylic acid PAA, polymethacrylic acid PMAA, sodium polyacrylate PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS). In some embodiments, the adhesive content by mass percentage is less than 5% based on the total mass of the second negative electrode film layer.

[0102] In some embodiments, the second negative electrode film layer may further include other additives. As an example, the other additives may include thickeners, such as sodium carboxymethyl cellulose (CMC), PTC thermistor materials, etc. In some embodiments, the mass percentage of the other additives is less than 2% based on the total mass of the second negative electrode film layer.

[0103] The first negative electrode film layer includes a first negative electrode active material, which includes graphite. Optionally, the graphite content in the first negative electrode active material is 80% to 100% by mass, and optionally 90% to 98% by mass, based on the total mass of the first negative electrode active material. Thus, the first negative electrode film layer contains a relatively high amount of graphite, which is beneficial for the secondary battery to have a higher initial coulombic efficiency and a longer cycle life. In some embodiments, the first negative electrode active material may consist only of graphite; in other embodiments, the first negative electrode active material may also include other negative electrode active materials besides graphite, such as hard carbon, soft carbon, mesophase carbon microspheres, silicon-based materials, tin-based materials, etc. Optionally, the other negative electrode active materials content in the first negative electrode active material is less than 20% by mass, more preferably less than 10% by mass, based on the total mass of the first negative electrode active material.

[0104] In some embodiments, the graphite content in the first negative electrode film layer is W3, which is more than 78% based on the total mass of the first negative electrode film layer, and optionally from 90% to 98%. A suitable graphite content is beneficial for the secondary battery to have higher initial coulombic efficiency and longer cycle life.

[0105] In some embodiments, the first negative electrode film layer may further include a conductive agent to collect microcurrents between the first negative electrode active materials (e.g., graphite), reduce the contact resistance of the electrodes, and accelerate the electron mobility; it can also reduce polarization and improve the charge-discharge efficiency of the secondary battery. As an example, the conductive agent may include one or more combinations selected from superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. In some embodiments, the mass percentage of the conductive agent is less than 5% based on the total mass of the first negative electrode film layer.

[0106] In some embodiments, the first negative electrode film layer may further include an adhesive for bonding the first negative electrode active material (e.g., graphite) to each other, bonding the first negative electrode film layer and the second negative electrode film layer, and bonding the first negative electrode film layer and the negative electrode current collector. As an example, the adhesive may include one or more combinations selected from styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, waterborne acrylic resins (e.g., polyacrylic acid PAA, polymethacrylic acid PMAA, sodium polyacrylate PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS). In some embodiments, the adhesive content by mass percentage is less than 5% based on the total mass of the first negative electrode film layer.

[0107] In some embodiments, the first negative electrode film layer may further include other additives. As an example, the other additives may include thickeners, such as sodium carboxymethyl cellulose (CMC), PTC thermistor materials, etc. In some embodiments, the mass percentage of the other additives is less than 2% based on the total mass of the first negative electrode film layer.

[0108] In some embodiments, the volume average particle size Dv50 of the second negative electrode active material is d2μm, the volume average particle size Dv50 of the first negative electrode active material is d3μm, and the d2 / d3 ratio is 0.1 to 1, optionally 0.2 to 0.8.

[0109] In the negative electrode sheet of this application, the first negative electrode film layer uses a first negative electrode active material with a larger particle size, resulting in a higher compaction density of the first negative electrode film layer, which is beneficial for the secondary battery to have high initial coulombic efficiency, high energy density, and long cycle life. In the negative electrode sheet of this application, the second negative electrode film layer uses a second negative electrode active material with a smaller particle size, resulting in a higher specific surface area, which can increase the contact area with the electrolyte and shorten the liquid-phase diffusion path and solid-phase diffusion path of active ions; at the same time, the integrity of the pore structure of the second negative electrode film layer is better maintained, which is beneficial for the secondary battery to have a higher charge rate. In addition, because the particle size of the second negative electrode active material is smaller, it is more pressure resistant, thus the microporous structure of the second negative electrode active material, especially hard carbon, is better maintained, thereby fully utilizing the role of the hard carbon microporous structure as active ion storage sites, improving the capacity utilization and energy density of the secondary battery; at the same time, the lower compaction density of the second negative electrode film layer is beneficial for the electrolyte to quickly wet the first negative electrode film layer, reducing negative electrode polarization and improving the charge and discharge efficiency of the secondary battery.

[0110] Further research by the inventors revealed that when the ratio d2 / d3 of the particle size of the second negative electrode active material to that of the first negative electrode active material is within a suitable range, it is beneficial to fully utilize the role of the second negative electrode film in improving the high-rate charging capability of the secondary battery and the first negative electrode film in improving the initial coulombic efficiency and cycle life of the secondary battery. As a result, the overall performance of the secondary battery is better, and it can achieve high-rate charging at high energy density.

[0111] In some embodiments, the volume average particle size Dv50 of the second negative electrode active material is d2μm, where d2 is 3 to 11, optionally 3 to 7. When the particle size of the second negative electrode active material is within a suitable range, its microporous structure is better preserved, thereby fully utilizing the role of the hard carbon microporous structure as active ion storage sites, improving the capacity utilization and energy density of the secondary battery; at the same time, when the particle size of the second negative electrode active material is within a suitable range, the second negative electrode film layer can have a smooth pore structure, which facilitates the electrolyte to pass smoothly through the pore structure of the second negative electrode film layer and quickly wet the first negative electrode film layer, thereby reducing negative electrode polarization and improving the charge and discharge efficiency of the secondary battery.

[0112] In some embodiments, the particle size distribution index (Dv90-Dv10) / Dv50 of the second negative electrode active material is α1, where α1 is 0.6 to 5, and optionally 1 to 4. When the particle size distribution index of the second negative electrode active material is within a suitable range, it is beneficial to improve the processing performance of the second negative electrode film, giving the second negative electrode film a high degree of particle distribution uniformity and unobstructed pore structure. This, in turn, is beneficial to ensure that different regions of the second negative electrode film have high active ion transport performance, thereby further improving the high-rate charging capability of the secondary battery. At the same time, it is also beneficial to facilitate the rapid wetting of the first negative electrode film by the electrolyte, reducing negative electrode polarization and improving the charge and discharge efficiency of the secondary battery.

[0113] In some embodiments, the specific surface area of ​​the second negative electrode active material is 3m². 2 / g to 7m 2 / g, optionally 4m 2 / g to 6m 2 / g. When the specific surface area of ​​the second negative electrode active material is within a suitable range, it can reduce charge exchange resistance and give the second negative electrode film a more open pore structure, thereby exhibiting higher active ion transport performance and further improving the high-rate charging capability of the secondary battery. When the specific surface area of ​​the second negative electrode active material is within a suitable range, it can also improve the film formation efficiency of the SEI film, avoid the formation of an excessively thick SEI film, reduce irreversible consumption of active ions, and thus further improve the capacity utilization and cycle performance of the secondary battery.

[0114] In some embodiments, the powder compaction density of the second negative electrode active material at 20000N is 0.9 g / cm³. 3 Up to 1.3 g / cm 3 Optionally, it can be 1g / cm 3 Up to 1.2 g / cm 3 When the compaction density of the second negative electrode active material is within a suitable range, it is beneficial to improve the energy density of the secondary battery.

[0115] In some embodiments, the second negative electrode active material comprises primary particles, secondary particles, or a combination thereof. Optionally, the proportion of primary particles in the second negative electrode active material is 90% to 100%. When the second negative electrode active material contains an appropriate proportion of primary particles, it is beneficial for the second negative electrode film to have a shorter active ion transport path, thereby further improving the high-rate charging capability of the secondary battery. In addition, it can also reduce negative electrode polarization and electrolyte side reactions, thereby further improving the cycle performance and storage performance of the secondary battery.

[0116] In some embodiments, the volume average particle size Dv50 of the first negative electrode active material is d3 μm, where d3 is 9 to 18, optionally 11 to 15. When the particle size of the first negative electrode active material is within a suitable range, it is beneficial for the first negative electrode film to have a high compaction density, thereby enabling the secondary battery to have high initial coulombic efficiency, high energy density, and long cycle life.

[0117] In some embodiments, the particle size distribution index (Dv90-Dv10) / Dv50 of the first negative electrode active material is α2, where α2 is 0.2 to 5, and optionally 0.3 to 4. When the particle size distribution index of the first negative electrode active material is within a suitable range, it is beneficial to improve the processing performance of the first negative electrode film, enabling the first negative electrode film to have high particle distribution uniformity and unobstructed pore structure as a whole. This, in turn, is beneficial to ensure that different regions of the first negative electrode film have high active ion transport performance, thereby further improving the cycle performance of the secondary battery.

[0118] In some embodiments, the specific surface area of ​​the first negative electrode active material is 0.6 m². 2 / g to 1.5m 2 / g, optionally 0.8m 2 / g to 1.4m 2 / g. When the specific surface area of ​​the first negative electrode active material is within a suitable range, it can reduce the charge exchange resistance and make the first negative electrode film layer have a more unobstructed pore structure, thereby having higher active ion transport performance and further improving the cycle performance of the secondary battery.

[0119] In some embodiments, the powder compaction density of the first negative electrode active material at 20000N is 1.4 g / cm³.3 Up to 1.85 g / cm 3 Optionally, it can be 1.6 g / cm³. 3 Up to 1.75 g / cm 3 When the compaction density of the first negative electrode active material powder is within a suitable range, it is beneficial to improve the energy density of the secondary battery.

[0120] In some embodiments, the graphitization degree of the first negative electrode active material is 91% to 95%, optionally 92% to 94%. When the graphitization degree of the first negative electrode active material is within a suitable range, it is beneficial to improve the cycle performance of the secondary battery.

[0121] In some embodiments, the first negative electrode active material comprises artificial graphite, natural graphite, or a combination thereof. Optionally, the mass percentage of artificial graphite in the first negative electrode active material is 50% or more, optionally 60% to 100%, based on the total mass of the first negative electrode active material. Optionally, the surface of the artificial graphite further has a carbon coating layer, thereby further reducing charge transfer resistance and improving the cycle performance and high-rate charging capability of the secondary battery. Optionally, the carbon coating layer comprises amorphous carbon.

[0122] In some embodiments, the first negative electrode active material comprises primary particles, secondary particles, or a combination thereof. Optionally, the proportion of secondary particles in the first negative electrode active material is 90% to 100%. When the first negative electrode active material contains an appropriate proportion of secondary particles, its isotropy is improved, which is beneficial for the first negative electrode film to have more active ion transport channels, thereby further improving the high-rate charging capability of the secondary battery; at the same time, it is beneficial for the first negative electrode film to have a high compaction density, thereby improving the energy density of the secondary battery; in addition, it can also reduce negative electrode polarization and electrolyte side reactions, thereby further improving the cycle performance and storage performance of the secondary battery.

[0123] In some embodiments, the compaction density of the negative electrode sheet is 1.4 g / cm³. 3 Up to 1.85 g / cm 3 Optionally, it can be 1.6 g / cm³. 3 Up to 1.75 g / cm 3 When the compaction density of the negative electrode sheet is within a suitable range, it is beneficial to improve the high-rate charging capability, cycle performance, and energy density of the secondary battery.

[0124] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. Examples of metal foils include copper foil or copper alloy foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one surface of the polymer base layer. Examples of the metal material include one or more combinations selected from copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. The polymer base layer may include one or more combinations selected from polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0125] The method for preparing the negative electrode sheet of this application is well known. In some embodiments, the negative electrode sheet can be prepared by the following method: dispersing a first negative electrode active material, a conductive agent, a binder, and any other components in a solvent (e.g., deionized water) to form a first slurry; dispersing a second negative electrode active material, a conductive agent, a binder, and any other components in a solvent (e.g., deionized water) to form a second slurry; coating the first slurry onto a negative electrode current collector and drying it to form a first negative electrode film layer; coating the second slurry onto the first negative electrode film layer, and after drying, cold pressing, and other processes, the negative electrode sheet is obtained.

[0126] The negative electrode current collector has two surfaces opposite each other in its thickness direction. The first negative electrode film layer and the second negative electrode film layer can be disposed on either or both of the two opposite surfaces of the negative electrode current collector. Optionally, the functional coating can be disposed only on the surface opposite to the second negative electrode film layer of the base film and the negative electrode sheet, thereby reducing the mass of inactive components and increasing the energy density of the secondary battery.

[0127] It should be noted that the parameters of the first and second negative electrode films given in this application refer to the parameter range of the coating layer on one side of the negative electrode current collector. When the first and second negative electrode films are disposed on two surfaces of the negative electrode current collector, if the coating layer parameters on either surface meet the requirements of this application, it is considered to fall within the protection scope of this application.

[0128] In the negative electrode of this application, Dv90, Dv50, and Dv10 of the material have meanings known in the art and can be measured using instruments and methods known in the art. For example, they can be conveniently measured using a laser particle size analyzer, such as the Mastersizer 2000E laser particle size analyzer from Malvern Instruments Ltd., UK, according to GB / T 19077-2016 Particle Size Distribution Laser Diffraction Method. Dv90 is the particle size corresponding to a cumulative volume distribution percentage of 90%; Dv50 is the particle size corresponding to a cumulative volume distribution percentage of 50%; and Dv10 is the particle size corresponding to a cumulative volume distribution percentage of 10%.

[0129] In the negative electrode of this application, the specific surface area of ​​the material has a meaning known in the art and can be measured using instruments and methods known in the art. For example, it 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 nitrogen adsorption specific surface area analysis can be performed using a Tri-Star 3020 specific surface area and pore size analyzer from Micromeritics, USA.

[0130] In the negative electrode sheet of this application, the degree of graphitization of the material has a meaning known in the art and can be measured using instruments and methods known in the art. For example, it can be tested using an X-ray diffractometer (such as a Bruker D8 Discover), and the testing can be performed with reference to JIS K 0131-1996 and JB / T 4220-2011 to obtain d. 002 Then, according to the formula g = (0.344 - d) 002 The degree of graphitization of the material is calculated as d / (0.344-0.3354)×100%. In the above formula, d 002 It is the interlayer spacing of the (002) crystal plane in the crystal structure of a material, expressed in nanometers (nm).

[0131] In the negative electrode sheet of this application, the powder compaction density of the material has a meaning known in the art and can be determined using instruments and methods known in the art. For example, it can be tested using an electronic pressure testing machine (e.g., UTM7305 type) according to GB / T24533-2009. An exemplary test method includes the following steps: weigh 1g of material, add a bottom area of ​​1.327cm²... 2 In the mold, pressure is applied to 2000 kg (equivalent to 20000 N), held for 30 seconds, then depressurized and held for 10 seconds. The compaction density of the powder under a force of 20000 N is then recorded and calculated.

[0132] In the negative electrode sheet of this application, the proportion of primary and secondary particles can be determined using instruments and methods known in the art, such as scanning electron microscopy. To ensure the accuracy of the test results, multiple (e.g., more than 5) different regions can be randomly selected from the test sample for scanning tests. At a certain magnification (e.g., 1000x or higher), the percentage of primary and secondary particles in each region relative to the total number of particles is calculated; this percentage represents the proportion of primary and secondary particles in that region. To ensure the accuracy of the test results, the above test can be repeated on multiple test samples (e.g., more than 10), and the average value of each test sample is taken as the final test result. The testing standard can be found in JY / T010-1996.

[0133] In the negative electrode sheet of this application, the thicknesses of the first negative electrode film layer and the second negative electrode film layer have meanings known in the art and can be measured using instruments and methods known in the art, such as scanning electron microscopy (e.g., ZEISS Sigma 300). This allows for a more accurate determination of the boundary region between the second and first negative electrode film layers. An exemplary testing method includes the following steps: cutting the negative electrode sheet into a sample of a certain size (e.g., 2cm × 2cm), fixing the negative electrode sheet onto the sample stage with paraffin wax; installing the sample stage into the sample holder and locking it in place; turning on the power of the argon ion cross-section polisher (e.g., IB-19500CP) and drawing a vacuum (e.g., 10... -4 Set the argon flow rate (e.g., 0.15 MPa), voltage (e.g., 8 kV), and polishing time (e.g., 2 hours), and adjust the sample stage to swing mode to begin polishing. The test standard can be found in JY / T010-1996. To ensure the accuracy of the test results, multiple (e.g., more than 5) different regions can be randomly selected from the sample to be tested for scanning. At a certain magnification (e.g., 500x or higher), the thicknesses of the first and second negative electrode films in the scale test areas can be read. For greater accuracy, multiple test areas can be tested and the average value taken.

[0134] In the negative electrode sheet of this application, the compaction density of the negative electrode sheet has a meaning known in the art and can be measured using instruments and methods known in the art. The compaction density of the negative electrode sheet = the areal density of the negative electrode sheet / the coating thickness on one side of the negative current collector. The areal density of the negative electrode sheet has a meaning known in the art and can be measured using instruments and methods known in the art. An exemplary test method is as follows: Take a negative electrode sheet coated on one side and cold-pressed (if it is a double-sided coated negative electrode sheet, the coating on one side can be wiped off first), punch it into a small circular piece with an area of ​​S1, weigh it, and record its weight as M1; then wipe off the coating layer of the weighed negative electrode sheet, weigh the negative current collector, and record it as M0; the areal density of the negative electrode sheet = (M1-M0) / S1.

[0135] It should be noted that the above-mentioned parameter tests for the first and second negative electrode active materials can be performed by sampling before coating, or by sampling from the cold-pressed negative electrode sheet, battery assembly, or secondary battery. When the test sample is taken from the cold-pressed negative electrode sheet, battery assembly, or secondary battery, as an example, the sampling can be carried out in the following steps: (1) Select any one side of the negative current collector after cold pressing and take a sample of the second negative electrode active material (for example, a blade can be used to scrape the powder). The scraping depth does not exceed the boundary between the second negative electrode film and the first negative electrode film; (2) Take a sample of the first negative electrode active material (for example, a blade can be used to scrape the powder). Since there may be a fusion layer between the second negative electrode film and the first negative electrode film during the cold pressing process, in order to ensure the accuracy of the test, when taking a sample of the first negative electrode active material, the fusion layer can be scraped off first, and then the powder of the first negative electrode active material can be scraped off; (3) Place the first negative electrode active material and the second negative electrode active material collected above into deionized water, then filter and dry them. Then sinter the dried powder at a certain temperature and time (for example, 400°C, 2h) to remove the binder and conductive agent, and the test sample is obtained.

[0136] [Positive electrode plate]

[0137] In some embodiments, the positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector. For example, the positive current collector has two surfaces opposite 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 current collector.

[0138] The positive electrode film layer includes a positive electrode active material, which may be a positive electrode active material known in the art for use in secondary batteries. For example, the positive electrode active material may include one or more combinations selected from lithium transition metal oxides, lithium phosphates, and their respective modified compounds. Examples of lithium transition metal oxides may include those selected from lithium cobalt oxides (e.g., LiCoO2), lithium nickel oxides (e.g., LiNiO2), lithium manganese oxides (e.g., LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (e.g., LiNi). 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.5 Co 0.25 Mn 0.25 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2), lithium nickel cobalt aluminum oxide (e.g., LiNi) 0.85 Co 0.15 Al 0.05 O2) and combinations of one or more of their respective modified compounds. Examples of lithium phosphates may include combinations of one or more of lithium iron phosphate, lithium iron phosphate and carbon composites, lithium manganese phosphate, lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, lithium manganese iron phosphate and carbon composites, and combinations of their respective modified compounds.

[0139] In some embodiments, to further improve the energy density of the secondary battery, the positive electrode active material may include one or more combinations of lithium transition metal oxides and their modified compounds as shown in Formula 1.

[0140] Li a Ni b Co c M d O e A f Formula 1

[0141] In Equation 1, 0.8≤a≤1.2, 0.5≤b<1, 0<c<1, 0<d<1, 1≤e≤2, 0≤f≤1, M may include one or more combinations selected from Mn, Al, Zr, Zn, Cu, Cr, Mg, Fe, V, Ti and B, and A may include one or more combinations selected from N, F, S and Cl.

[0142] In this application, the modified compounds of the above-mentioned positive electrode active materials may be those that have been doped or / or surface-coated to modify the positive electrode active materials.

[0143] In some embodiments, the positive electrode film may further include a conductive agent to collect microcurrents between the positive electrode active materials, reduce electrode contact resistance, and accelerate electron mobility; it can also reduce polarization and improve the charge-discharge efficiency of the secondary battery. As an example, the conductive agent may include one or more combinations selected from superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. In some embodiments, the mass percentage of the conductive agent is less than 5% based on the total mass of the positive electrode film.

[0144] In some embodiments, the positive electrode film layer may further include an adhesive to bond the positive electrode active materials to each other and to bond the positive electrode film layer and the positive electrode current collector. As an example, the adhesive may include one or more combinations selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resins. In some embodiments, the adhesive content by mass percentage is less than 5% based on the total mass of the positive electrode film layer.

[0145] In some embodiments, the positive current collector may be a metal foil or a composite current collector. Examples of metal foils include aluminum foil or aluminum alloy foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one surface of the polymer base layer. Examples of the metal material include one or more combinations selected from aluminum, aluminum alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. The polymer base layer may include one or more combinations selected from polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0146] The positive electrode film layer is typically formed by coating a positive electrode slurry onto a positive electrode current collector, followed by drying and cold pressing. The positive electrode slurry is typically formed by dispersing positive electrode active materials, conductive agents, binders, and any other components in a solvent and stirring until homogeneous. The solvent can be N-methylpyrrolidone (NMP), but is not limited to this.

[0147] The electrode assembly 6 of this application is described below with reference to the accompanying drawings.

[0148] Figure 1 This is a schematic diagram of one embodiment of the electrode assembly 6 of this application. Figure 1As shown, the electrode assembly 6 includes a negative electrode 10, a positive electrode 20, and a separator 30 located between the negative electrode 10 and the positive electrode 20. The negative electrode 10 includes a negative current collector 100 and a first negative electrode film layer 101 and a second negative electrode film layer 102 located on both sides of the negative current collector 100, with the first negative electrode film layer 101 situated between the negative current collector 100 and the second negative electrode film layer 102. The separator 30 includes a base film 300 and functional coatings 301 located on both sides of the base film 300. The positive electrode 20 includes a positive current collector 200 and a positive electrode film layer 201 located on both sides of the positive current collector 200.

[0149] Figure 2 This is a schematic diagram of one embodiment of the electrode assembly 6 of this application. Figure 2 As shown, the electrode assembly 6 includes a negative electrode 10, a positive electrode 20, and a separator 30 located between the negative electrode 10 and the positive electrode 20. The negative electrode 10 includes a negative current collector 100 and a first negative electrode film layer 101 and a second negative electrode film layer 102 located on both sides of the negative current collector 100, with the first negative electrode film layer 101 situated between the negative current collector 100 and the second negative electrode film layer 102. The separator 30 includes a base film 300 and a functional coating layer 301 and an adhesive layer 302 located on both sides of the base film 300. The positive electrode 20 includes a positive current collector 200 and a positive electrode film layer 201 located on both sides of the positive current collector 200.

[0150] like Figure 1 and Figure 2 As shown, the first negative electrode film layer 101 and the second negative electrode film layer 102 are disposed on both surfaces of the negative electrode current collector 100. Of course, in some embodiments, the first negative electrode film layer 101 and the second negative electrode film layer 102 may also be disposed on only one surface of the negative electrode current collector 100. The positive electrode film layer 201 is disposed on both surfaces of the positive electrode current collector 200. Of course, in some embodiments, the positive electrode film layer 201 may also be disposed on only one surface of the positive electrode current collector 200. The functional coating 301 is disposed on both surfaces of the base film 300. Of course, in some embodiments, the functional coating 301 may also be disposed on only one surface of the base film 300 opposite to the second negative electrode film layer 102 of the negative electrode sheet 10.

[0151] Secondary batteries

[0152] This application also provides a secondary battery. A secondary battery, also known as a rechargeable battery or storage battery, is a battery that can be recharged after discharge to activate its active materials and continue to be used. The secondary battery of this application includes the electrode assembly and electrolyte. The secondary battery of this application can be a lithium-containing secondary battery, particularly a lithium-ion secondary battery.

[0153] The electrolyte comprises a lithium salt and a solvent. As an example, the lithium salt may comprise one or more combinations selected from lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalate borate (LiDFOB), lithium dioxalate borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorodioxalate phosphate (LiDFOP), and lithium tetrafluorooxalate phosphate (LiTFOP). As an example, the organic solvent may include one or more combinations selected from ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butyl ester carbonate (BC), 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), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE).

[0154] In some embodiments, the electrolyte may also include additives, such as negative electrode film-forming additives and positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance and additives that improve battery high-temperature or low-temperature performance.

[0155] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte of this application.

[0156] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic, such as one or more combinations of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0157] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. Figure 3 This is an example of a square-structured secondary battery 5.

[0158] In some embodiments, such as Figure 4As shown, the outer packaging may include a housing 51 and a cover 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 is used to cover the opening to close the receiving cavity. A positive electrode, a negative electrode, and a separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated in the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52, which can be adjusted according to requirements.

[0159] The method for preparing the secondary battery described in this application is well known. In some embodiments, a positive electrode, a separator, a negative electrode, and an electrolyte can be assembled to form a secondary battery. As an example, the positive electrode, separator, and negative electrode can be formed into an electrode assembly through a winding process and / or a stacking process. The electrode assembly is placed in an outer packaging, dried, and then injected with an electrolyte. After vacuum sealing, settling, formation, and shaping processes, a secondary battery is obtained.

[0160] In some embodiments of this application, the secondary battery according to this application can be assembled into a battery module. The number of secondary batteries contained in the battery module can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module.

[0161] Figure 5 This is a schematic diagram of battery module 4 as an example. Figure 5 As shown, in battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple secondary batteries 5 can be fixed in place using fasteners.

[0162] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0163] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0164] Figure 6 and Figure 7 This is a schematic diagram of battery pack 1 as an example. Figure 6 and Figure 7 As shown, the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3. The upper body 2 covers the lower body 3, forming a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0165] Electrical appliances

[0166] This application also provides an electrical device, which includes at least one of the secondary battery, battery module, and battery pack described in this application. The secondary battery, battery module, and battery pack can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device can be, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0167] The electrical device can be equipped with a secondary battery, battery module, or battery pack according to its usage requirements.

[0168] Figure 8 This is a schematic diagram of an example electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the device's requirements for high power and high energy density, a battery pack or battery module can be used.

[0169] Another example of an electrical device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use rechargeable batteries as their power source.

[0170] Example

[0171] The following embodiments describe the disclosure of this application in more detail. These embodiments are merely illustrative, as various modifications and variations will be apparent to those skilled in the art within the scope of the disclosure of this application. Unless otherwise stated, all parts, percentages, and ratios reported in the following embodiments are based on mass, and all reagents used in the embodiments are commercially available or synthesized by conventional methods and can be used directly without further processing, and the instruments used in the embodiments are commercially available.

[0172] Example 1-1

[0173] Step (1): Preparation of negative electrode sheet

[0174] S10, Preparation of Artificial Graphite

[0175] Petroleum non-needle coke raw material powder was pretreated to remove impurities, then mixed with coal tar pitch and granulated to obtain secondary particles with a Dv50 of 10 μm. The granulated product was then graphitized in an Atchison graphitization furnace at 3000℃ for 24 h to obtain graphite particles. These graphite particles were then mixed with petroleum tar pitch and carbonized at 1000℃ for 15 h to obtain artificial graphite. The artificial graphite had a Dv50 of 12 μm, a Dv90 of 20 μm, a Dv10 of 6.5 μm, a graphitization degree of 92%, and a specific surface area of ​​0.86 m². 2 / g, the compacted density of the powder is 1.6g / cm³. 3 The morphology is that of secondary particles.

[0176] S20, Preparation of Hard Carbon

[0177] A precursor was prepared by heating phenolic resin at 500℃ for 30 min. The precursor was then pulverized and heat-treated at 1200℃ for 20 h under nitrogen atmosphere, followed by further pulverization to obtain hard carbon. The hard carbon had a Dv50 of 5 μm, a Dv90 of 10 μm, a Dv10 of 2.5 μm, and a specific surface area of ​​5 m². 2 / g, with the morphology of primary particles.

[0178] S30, Preparation of the first slurry

[0179] The artificial graphite, conductive carbon black, styrene-butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC) prepared above were mixed in a mass ratio of 96.8:0.8:1.2:1.2 and then added to deionized water. The mixture was stirred under vacuum until the system was homogeneous, resulting in a first slurry with a solid content of 66%.

[0180] S40, Preparation of the second slurry

[0181] The hard carbon, conductive carbon black, styrene-butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC) prepared above were mixed in a mass ratio of 95:1.5:3.1:0.4 and then added to deionized water. The mixture was stirred under vacuum until the system was homogeneous, resulting in a second slurry with a solid content of 66%.

[0182] S50, coating of slurry

[0183] A first slurry is coated onto one surface of an 8 μm thick copper foil used as a negative electrode current collector, and dried to form a first negative electrode film. A second slurry is then coated onto the first negative electrode film, and dried to form a second negative electrode film. The above steps are repeated on the other surface of the copper foil used as a negative electrode current collector, followed by cold pressing and other processes to obtain the negative electrode sheet. The thickness of the first negative electrode film on one side of the negative electrode current collector is 59 μm, and the thickness of the second negative electrode film is 82 μm. The compaction density of the negative electrode sheet is 1.55 g / cm³. 3 .

[0184] Step (2): Preparation of the positive electrode sheet

[0185] LiNi, the positive electrode active material 0.8 Co 0.1 Mn 0.1 O2 (NCM811), conductive carbon black, and polyvinylidene fluoride (PVDF) binder are mixed in a mass ratio of 98:1:1 and then added to NMP solvent. The mixture is stirred under vacuum until the system is homogeneous, resulting in a positive electrode slurry with a solid content of 75%. The positive electrode slurry is uniformly coated on both surfaces of an aluminum foil with a thickness of 13 μm. After drying at 90°C, it is cold-pressed to obtain a positive electrode sheet with a positive electrode film thickness of 114 μm on one side of the positive current collector.

[0186] Step (3): Preparation of electrolyte

[0187] In an environment with a water content of less than 10 ppm, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1 to obtain an organic solvent. Then, fully dried LiPF6 is dissolved in the above organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0188] Step (4): Preparation of the separating membrane

[0189] Barium titanate (dielectric constant above 3000) with a Dv50 of 100 nm, styrene-butadiene rubber (SBR) binder, and sodium carboxymethyl cellulose (CMC) dispersant were mixed in a mass ratio of 85:10:5 and then added to deionized water. The mixture was stirred under vacuum until the system was homogeneous, resulting in a slurry with a solid content of 40%. The slurry was then coated on both surfaces of a porous polyethylene membrane (10 μm thick) and dried to form a functional coating with a single-sided thickness of 2 μm.

[0190] Step (5): Preparation of secondary battery

[0191] The positive electrode, separator, and negative electrode are stacked in sequence so that the side of the separator coated with the functional coating is opposite to the second functional layer of the negative electrode. The electrode assembly is then wound to obtain an electrode assembly. The electrode assembly is placed in an outer package, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping, a secondary battery is obtained.

[0192] Examples 1-2 to 1-7

[0193] The secondary battery was prepared in a similar manner to that in Examples 1-1, except that the coating thickness of the slurry and the thickness of the resulting functional coating were different in the preparation of the separator. Specific parameters are detailed in Table 1.

[0194] Comparative Example 1-1

[0195] The secondary battery was prepared in a similar manner to that in Example 1-1, except that the separator was made of porous polyethylene membrane.

[0196] Comparative Examples 1-2

[0197] The secondary battery was prepared in a similar manner to that in Example 1-1, except that the separator was made of porous polyethylene membrane and the order of slurry coating was different in the preparation of the negative electrode sheet.

[0198] First, a second slurry is coated onto one surface of an 8 μm thick copper foil used as a negative electrode current collector, and then dried to form a first negative electrode film. Next, the first slurry is coated onto the first negative electrode film, and dried to form a second negative electrode film. The above steps are then repeated on the other surface of the copper foil used as a negative electrode current collector. After cold pressing and other processes, the negative electrode sheet is obtained. The thickness of the first negative electrode film on one side of the negative electrode current collector is 82 μm, and the thickness of the second negative electrode film is 59 μm.

[0199] Comparative Examples 1-3

[0200] The secondary battery was prepared in a similar manner to that in Example 1-1, except that the separator was made of porous polyethylene membrane and the second slurry was not coated during the preparation of the negative electrode sheet.

[0201] The first slurry is coated on both surfaces of an 8μm thick copper foil for the negative electrode current collector. After drying and cold pressing, the negative electrode sheet is obtained. The thickness of the negative electrode film layer on one side of the negative electrode current collector is 141μm.

[0202] Comparative Examples 1-4

[0203] The secondary battery was prepared in a similar manner to that in Example 1-1, except that the separator was made of porous polyethylene membrane and the first slurry was not coated during the preparation of the negative electrode sheet.

[0204] The second slurry is coated on both surfaces of an 8μm thick copper foil for the negative electrode current collector. After drying and cold pressing, the negative electrode sheet is obtained. The thickness of the negative electrode film layer on one side of the negative electrode current collector is 141μm.

[0205] Comparative Examples 1-5

[0206] The secondary battery was prepared in a similar manner to that in Example 1-1, except that the separator was made of porous polyethylene membrane and the preparation process of the negative electrode was different.

[0207] S10 to S20 are the same as in Example 1-1.

[0208] S30, Slurry Preparation

[0209] The artificial graphite, hard carbon, conductive carbon black, styrene-butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC) prepared above were mixed in a mass ratio of 40:55:1.5:3.1:0.4 and then added to deionized water. The mixture was stirred under vacuum until the system was homogeneous, resulting in a slurry with a solid content of 66%.

[0210] S40, coating of slurry

[0211] The slurry is coated on both surfaces of an 8μm thick copper foil for the negative electrode current collector. After drying and cold pressing, the negative electrode sheet is obtained. The thickness of the negative electrode film layer on one side of the negative electrode current collector is 141μm.

[0212] Comparative Examples 1-6

[0213] The secondary battery was prepared in a similar manner to that in Example 1-1, except that the separator was made of porous polyethylene membrane and the preparation process of the negative electrode was different.

[0214] S10 to S30 are the same as in Example 1-1.

[0215] S40, Preparation of the second slurry

[0216] The hard carbon, barium titanate (Dv50 of 100 nm and dielectric constant of 3000 or higher), conductive carbon black, styrene-butadiene rubber (SBR) binder, and sodium carboxymethyl cellulose (CMC) thickener prepared above were mixed in a mass ratio of 93:1:1.5:3.1:0.4 and then added to deionized water. The mixture was stirred under vacuum until the system was homogeneous, resulting in a second slurry with a solid content of 66%.

[0217] S50, coating of slurry

[0218] A first slurry is coated onto one surface of an 8 μm thick copper foil used as a negative electrode current collector, and dried to form a first negative electrode film. A second slurry is then coated onto the first negative electrode film, and dried to form a second negative electrode film. The above steps are repeated on the other surface of the copper foil used as a negative electrode current collector, followed by cold pressing and other processes to obtain the negative electrode sheet. The thickness of the first negative electrode film on one side of the negative electrode current collector is 59 μm, and the thickness of the second negative electrode film is 82 μm.

[0219] Test section

[0220] (1) Maximum charging rate test

[0221] At 25°C, the prepared secondary battery was fully discharged and fully charged at a rate of 0.33C. After being fully charged, the battery was allowed to stand for 5 minutes before being fully discharged at a rate of 0.33C. The discharge capacity obtained at this point is the actual capacity of the secondary battery at a rate of 1C, denoted as C0. The secondary battery was fully charged at x C0 rates (representing gradient charging rates, e.g., 1C0, 1.1C0, 1.2C0, 1.3C0, 1.4C0, etc.), allowed to stand for 5 minutes, and then fully discharged at 1C. This cycle was repeated 10 times before the secondary battery was fully charged at 1C again. The secondary battery was disassembled to observe the lithium deposition on the surface of the negative electrode. If no lithium deposition was observed on the surface of the negative electrode, the charging rate was increased and the measurement was repeated until lithium deposition was observed on the surface of the negative electrode. The maximum charging rate at which no lithium deposition was observed on the surface of the negative electrode was recorded. The higher the maximum charging rate of the secondary battery, the better its high-rate charging capability.

[0222] (2) Energy density test

[0223] At 25°C, the prepared secondary battery was fully charged and fully discharged at a rate of 0.33C for three cycles, and the discharge energy of the secondary battery was recorded. The energy density of the secondary battery = the discharge energy of the secondary battery / the mass of the secondary battery. In the embodiments and comparative examples of this application, the energy density of the secondary battery prepared in Comparative Example 1-1 is 100%, representing the energy density of the secondary batteries in other embodiments and comparative examples.

[0224] (3) Cyclic life test

[0225] At 25°C, the prepared secondary battery was fully discharged and fully charged at a 1C rate. After being allowed to stand for 5 minutes, it was fully discharged at a 1C rate. The discharge capacity C0 obtained at this point was taken as the initial capacity of the secondary battery. The secondary battery was then fully charged at a 3C rate and fully discharged at a 1C rate, and this cycle charge-discharge test was performed. The discharge capacity after each cycle was recorded until the discharge capacity of the secondary battery decreased to 80% of the initial capacity. The number of cycles at this point was used to characterize the cycle life of the secondary battery. The higher the number of cycles, the longer the expected cycle life.

[0226] Table 1

[0227]

[0228] Based on the test results of Examples 1-1 to 1-7 and Comparative Examples 1-1 to 1-6, it can be seen that the secondary battery using the electrode assembly of this application simultaneously exhibits high charge rate and long cycle life. Comparative Example 1-1 did not have a functional coating on the surface of the separator, therefore the secondary battery prepared therecannot simultaneously possess both high charge rate and long cycle life. Comparative Example 1-2 did not have a functional coating on the surface of the separator, and a hard carbon layer and a graphite layer were sequentially formed on the surface of the negative electrode current collector, thus the secondary battery could not simultaneously possess both high charge rate and long cycle life. Comparative Example 1-3 did not have a functional coating on the surface of the separator and only formed a graphite layer on the surface of the negative electrode current collector, thus the secondary battery could have a long cycle life, but it was difficult to have a high charge rate. Comparative Example 1-4 did not have a functional coating on the surface of the separator and only formed a hard carbon layer on the surface of the negative electrode current collector, thus the secondary battery could have a relatively high charge rate, but it was difficult to have a long cycle life. Comparative Examples 1-5 lacked a functional coating on the separator surface and featured a mixed layer of graphite and hard carbon on the negative electrode current collector surface. Consequently, the secondary batteries struggled to simultaneously achieve both high charge rates and long cycle lives. Compared to Comparative Example 1-1, the hard carbon layer in Comparative Example 1-6 also included barium titanate. However, barium titanate's effect on suppressing the continuous growth of lithium dendrites in the direction perpendicular to the electrode was relatively poor, making it difficult to significantly improve the charge rate and cycle life of the secondary batteries.

[0229] The test results from Examples 1-1 to 1-7 also show that increasing the thickness of the functional coating on the separator increases both the maximum charging rate and cycle life of the secondary battery. This is likely because the increased reverse electric field strength generated by the functional coating enhances its ability to suppress the continuous growth of lithium dendrites in the direction perpendicular to the electrode, thus improving the high-rate charging capability and cycle performance of the secondary battery.

[0230] Based on the test results of Examples 1-1 to 1-7, it can also be seen that when H1 / (H2+H3) is less than 0.01, the functional coating of the separator is set relatively small relative to the total thickness of the first negative electrode film and the second negative electrode film. Its effect of inhibiting the continuous growth of lithium dendrites in the direction perpendicular to the electrode is not obvious, and thus the effect of improving the maximum charging rate of the secondary battery is not obvious.

[0231] Based on the test results of Examples 1-6 and 1-7, it can be seen that when H1 is greater than 10, the effect of the functional coating of the separator in inhibiting the continuous growth of lithium dendrites in the direction perpendicular to the electrode does not continue to increase. At the same time, since the functional coating does not have electrochemical activity and cannot contribute to the capacity, when its thickness is greater than 10 μm, the energy density of the secondary battery is significantly reduced.

[0232] The inventors then investigated the effect of the mass percentage of ferroelectric material in the functional coating of the separator on the performance of the secondary battery. The secondary batteries of Examples 2-1 to 2-6 were prepared in a similar manner to those of Examples 1-2, except that the mass percentage of ferroelectric material in the functional coating of the separator was different.

[0233] Table 2

[0234] Serial Number Ferroelectric material content W1 Maximum charging rate Number of cycles Energy density Example 2-1 70％ 5.1C0 2100 98.8％ Example 2-2 75％ 5.3C0 2250 98.5％ Example 2-3 80％ 5.4C0 2310 98.3％ Examples 2-4 90％ 5.6C0 2415 98.0％ Examples 2-5 95％ 5.7C0 2460 97.8％ Examples 2-6 98％ 6.0C0 1600 97.5％

[0235] Based on the test results of Examples 1-2 and 2-1 to 2-6, it can be seen that when the thickness of the functional coating of the separator is fixed at 4 μm, the maximum charge rate and cycle life of the secondary battery both increase with the increase of the mass percentage of ferroelectric material in the functional coating. This may be because the reverse electric field strength generated by the functional coating increases at this point, thereby enhancing its effect in suppressing the continuous growth of lithium dendrites in the direction perpendicular to the electrode. Therefore, the high-rate charging capability and cycle performance of the secondary battery are improved.

[0236] Based on the test results of Comparative Example 1-1 and Example 2-1, it can be seen that when the mass percentage of ferroelectric material in the functional coating of the separator is small, the functional coating obtained thereby has little effect on inhibiting the continuous growth of lithium dendrites in the direction perpendicular to the electrode, and thus has little effect on improving the maximum charging rate of the secondary battery.

[0237] The test results from Examples 2-5 and 2-6 also show that when the mass percentage of ferroelectric material in the functional coating of the separator exceeds 95%, the cycle performance of the secondary battery deteriorates significantly. This may be because the binder content in the functional coating of the separator is too low, and the functional coating may detach from the base film surface during long-term charge-discharge cycles of the secondary battery.

[0238] The inventors then investigated the effect of the volume average particle size Dv50 of the ferroelectric material in the functional coating of the separator on the performance of the secondary battery. The secondary batteries of Examples 3-1 to 3-6 were prepared in a similar manner to those of Examples 1-2, except that the volume average particle size Dv50 of the ferroelectric material in the functional coating of the separator was different.

[0239] Table 3

[0240] Serial Number Dv50 of ferroelectric materials Maximum charging rate Number of cycles Example 3-1 0.05μm 5.8C0 2415 Example 3-2 0.3μm 5.4C0 2350 Example 3-3 0.5μm 5.3C0 2330 Examples 3-4 0.8μm 5.2C0 2310 Examples 3-5 1.0μm 5.1C0 2100 Examples 3-6 1.5μm 5.0C0 1750

[0241] Based on the test results of Examples 1-2 and 3-1 to 3-6, it can be seen that when the thickness of the functional coating of the separator is fixed at 4 μm and the mass percentage of ferroelectric material is fixed at 85%, the maximum charge rate and cycle life of the secondary battery decrease as the volume average particle size Dv50 of the ferroelectric material increases. The possible reason is that as the volume average particle size Dv50 of the ferroelectric material increases, the reverse electric field interference it generates increases, thereby reducing the effectiveness of suppressing the continuous growth of lithium dendrites in the direction perpendicular to the electrode. Therefore, the maximum charge rate and cycle life of the secondary battery both decrease.

[0242] The inventors then investigated the effects of the thickness of the first negative electrode film and the thickness of the second negative electrode film on the performance of the secondary battery. The secondary batteries of Examples 4-1 to 4-7 were prepared in a similar manner to those of Examples 1-2, except that the thicknesses of the first and second negative electrode films were different.

[0243] Table 4

[0244]

[0245] Based on the test results of Examples 4-1 to 4-7, it can be seen that when the thickness of the second negative electrode film increases, the maximum charging rate of the secondary battery increases.

[0246] Based on the test results of Examples 4-1 to 4-7, it can be seen that when the thickness H2μm of the second negative electrode film and the thickness H3μm of the first negative electrode film satisfy the condition that H2 / H3 is between 0.25 and 4, the secondary battery can simultaneously exhibit a high maximum charge rate and a long cycle life. When H2 / H3 is less than 0.25, the second negative electrode film is relatively thin, and its effect on improving the maximum charge rate of the secondary battery is not significant; when H2 / H3 is greater than 4, the second negative electrode film is relatively thick, and due to the poor cycle performance of hard carbon itself, the cycle performance of the secondary battery is poor. Based on the test results of Examples 4-7 and Comparative Examples 1-4, it can be seen that when H2 / H3 is greater than 4, the improvement on the cycle life of the secondary battery is not significant.

[0247] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. An electrode assembly comprising a negative electrode, a positive electrode, and a separator located between the negative electrode and the positive electrode, wherein, The negative electrode sheet includes a negative current collector and a first negative electrode film layer and a second negative electrode film layer disposed on at least one surface of the negative current collector. The first negative electrode film layer is located between the negative current collector and the second negative electrode film layer and includes a first negative electrode active material, which includes graphite. The second negative electrode film layer includes a second negative electrode active material, which includes hard carbon. The separator includes a base film and a functional coating located at least on the side of the base film opposite to the negative electrode sheet, the functional coating comprising a ferroelectric material. Wherein, the thickness of the functional coating is H1 μm, the thickness of the second negative electrode film is H2 μm, the thickness of the first negative electrode film is H3 μm, and the electrode assembly satisfies H1 / (H2+H3) as 0.01 to 0.

15.

2. The electrode assembly according to claim 1, wherein, H1 / (H2+H3) ranges from 0.01 to 0.

08.

3. The electrode assembly according to any one of claims 1-2, wherein, The thickness of the functional coating is H1 μm, where H1 is 2 to 10.

4. The electrode assembly according to claim 3, wherein, The thickness of the functional coating is H1 μm, where H1 is 4 to 6.

5. The electrode assembly according to any one of claims 1-4, wherein, The thickness of the second negative electrode film is H2 μm, the thickness of the first negative electrode film is H3 μm, and the negative electrode sheet satisfies H2 / H3 = 0.10 to 5.

6. The electrode assembly according to claim 5, wherein, The thickness of the second negative electrode film is H2 μm, the thickness of the first negative electrode film is H3 μm, and the negative electrode sheet satisfies H2 / H3 = 0.5 to 4.

7. The electrode assembly according to any one of claims 1-6, wherein, The volume average particle size Dv50 of the ferroelectric material is d1 μm, where d1 is less than 1.

8. The electrode assembly according to claim 7, wherein, The volume average particle size Dv50 of the ferroelectric material is d1 μm, where d1 is 0.05 to 0.

8.

9. The electrode assembly according to any one of claims 1-8, wherein, The volume average particle size Dv50 of the second negative electrode active material is d2 μm, and the volume average particle size Dv50 of the first negative electrode active material is d3 μm, with d2 / d3 ranging from 0.1 to 1.

10. The electrode assembly according to claim 9, wherein, The ratio of d2 / d3 is 0.2 to 0.

8.

11. The electrode assembly according to any one of claims 1-10, wherein, The ferroelectric material in the functional coating has a mass percentage of W1, which is 70% to 95% based on the total mass of the functional coating.

12. The electrode assembly according to claim 11, wherein, W1 is 80% to 95%.

13. The electrode assembly according to any one of claims 1-12, wherein, The functional coating also includes an adhesive.

14. The electrode assembly according to claim 13, wherein, The adhesive comprises one or more of the following: styrene-butadiene rubber, polyacrylic acid, sodium polyacrylate, polyacrylamide, polyvinyl alcohol, sodium alginate, polymethacrylic acid, carboxymethyl chitosan, fluorinated acrylate resins, polytetrafluoroethylene, vinylidene fluoride homopolymers and copolymers.

15. The electrode assembly according to any one of claims 1-14, wherein, The isolation membrane also includes an adhesive layer disposed on the surface of the functional coating.

16. The electrode assembly of claim 15, wherein, The adhesive layer comprises one or more combinations selected from vinylidene fluoride homopolymers and copolymers thereof.

17. The electrode assembly according to any one of claims 1-16, wherein, The dielectric constant of the ferroelectric material is above 50.

18. The electrode assembly according to claim 17, wherein, The dielectric constant of the ferroelectric material is between 50 and 100,000.

19. The electrode assembly according to any one of claims 1-18, wherein, The ferroelectric material includes one or more combinations selected from inorganic ferroelectric materials and organic ferroelectric materials.

20. The electrode assembly of claim 19, wherein, The inorganic ferroelectric material includes one or more combinations selected from perovskite structure oxides, tungsten bronze type compounds, bismuth oxide type layered structure compounds, lithium niobate, lithium tantalate, lead metaniobate, and lead barium lithium niobate.

21. The electrode assembly according to claim 19, wherein, The organic ferroelectric material may include one or more combinations selected from vinylidene fluoride homopolymers or copolymers, 2,5-dibromo-3,6-dihydroxy-p-benzoquinone, phenazine-chlororanic acid, and ketone acid.

22. The electrode assembly according to any one of claims 1-21, wherein, The mass percentage of hard carbon in the second negative electrode film is W2, which, based on the total mass of the second negative electrode film, is 68% or more; and / or, The graphite content in the first negative electrode film layer is W3, which is more than 78% based on the total mass of the first negative electrode film layer.

23. The electrode assembly according to claim 22, wherein, W2 is 90% to 98%.

24. The electrode assembly according to claim 22, wherein, W3 is 90% to 98%.

25. The electrode assembly according to any one of claims 1-24, wherein, The second negative electrode active material satisfies at least one of the following conditions (1) to (5): (1) The volume average particle size Dv50 of the second negative electrode active material is d2 μm, and d2 is 3 to 11; (2) The particle size distribution index (Dv90-Dv10) / Dv50 of the second negative electrode active material is α1, and α1 is 0.6 to 5; (3) The specific surface area of ​​the second negative electrode active material is 3 m². 2 / g to 7 m 2 / g; (4) The compaction density of the second negative electrode active material at 20000N is 0.9 g / cm³. 3 Up to 1.3 g / cm 3 ; (5) The second negative electrode active material includes primary particles, secondary particles or a combination thereof.

26. The electrode assembly of claim 25, wherein, d2 ranges from 3 to 7.

27. The electrode assembly of claim 25, wherein, α1 ranges from 1 to 4.

28. The electrode assembly according to claim 25, wherein, The specific surface area of ​​the second negative electrode active material is 4 m². 2 / g to 6 m 2 / g.

29. The electrode assembly according to claim 25, wherein, The second negative electrode active material has a powder compaction density of 1 g / cm³ at 20000 N. 3 Up to 1.2 g / cm 3 .

30. The electrode assembly according to claim 25, wherein, The primary particles account for 90% to 100% of the second negative electrode active material.

31. The electrode assembly according to any one of claims 1-30, wherein, The first negative electrode active material satisfies at least one of the following conditions (1) to (7): (1) The volume average particle size Dv50 of the first negative electrode active material is d3 μm, where d3 is 9 to 18; (2) The particle size distribution index (Dv90-Dv10) / Dv50 of the first negative electrode active material is α2, and α2 is 0.2 to 5; (3) The specific surface area of ​​the first negative electrode active material is 0.6 m². 2 / g to 1.5 m 2 / g; (4) The compacted density of the first negative electrode active material at 20000 N is 1.4 g / cm³. 3 Up to 1.85 g / cm 3 ; (5) The degree of graphitization of the first negative electrode active material is 91% to 95%; (6) The first negative electrode active material includes artificial graphite, natural graphite, or a combination thereof; (7) The first negative electrode active material includes primary particles, secondary particles or a combination thereof.

32. The electrode assembly according to claim 31, wherein, The volume average particle size Dv50 of the first negative electrode active material is d3 μm, where d3 is 11 to 15.

33. The electrode assembly according to claim 31, wherein, α2 ranges from 0.3 to 4.

34. The electrode assembly according to claim 31, wherein, The specific surface area of ​​the first negative electrode active material is 0.8 m². 2 / g to 1.4 m 2 / g.

35. The electrode assembly according to claim 31, wherein, The first negative electrode active material has a powder compaction density of 1.6 g / cm³ at 20000 N. 3 Up to 1.75 g / cm 3 .

36. The electrode assembly according to claim 31, wherein, The graphitization degree of the first negative electrode active material is 92% to 94%.

37. The electrode assembly according to claim 31, wherein, The artificial graphite has a carbon coating layer on its surface.

38. The electrode assembly according to claim 31, wherein, The proportion of secondary particles in the first negative electrode active material is 90% to 100%.

39. A secondary battery comprising an electrode assembly according to any one of claims 1-38.

40. A battery module comprising the secondary battery according to claim 39.

41. A battery pack comprising one of the secondary battery according to claim 39 and the battery module according to claim 40.

42. An electrical device comprising at least one of the secondary battery according to claim 39, the battery module according to claim 40, and the battery pack according to claim 41.

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