Secondary battery, method for manufacturing the same, battery module, battery pack, and electric device

Abstract

The application relates to a secondary battery and a preparation method thereof, a battery module, a battery pack and a power utilization device. The secondary battery comprises a negative electrode sheet, a solid-state electrolyte interface film is formed on the surface of the negative electrode sheet, the solid-state electrolyte interface film comprises a first area part and a second area part connected with each other, the impedance value of the first area part determined based on an alternating current impedance spectrum is Z1, the impedance value of the second area part determined based on the alternating current impedance spectrum is Z2, and Z1 / Z2>=1.5. The thickness of the SEI film in the secondary battery is uneven, the impedance value of the first area part is relatively high, and the density of the first area part is relatively high. The impedance value of the second area part is relatively low, and the second area part is relatively loose. Through cooperation of the first area part and the second area part, the application can guarantee the cycle rate and low-temperature performance of the secondary battery.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to secondary batteries and their preparation methods, battery modules, battery packs, and electrical devices. Background Technology

[0002] Rechargeable batteries have advantages such as small size, high energy density, high safety, low self-discharge, and long lifespan, and are widely used in energy storage, communications, electric vehicles, aerospace, and many other fields. A battery consists of multiple secondary cells connected in series, parallel, or a combination thereof.

[0003] Research has found that the negative electrode active material contained in a secondary battery has a significant impact on its performance. During the charging and discharging process of a secondary battery, the negative electrode active material may undergo structural damage, thereby reducing the battery's performance, such as cycle life and cycle rate. Summary of the Invention

[0004] This application is made in view of the above-mentioned issues, and its purpose is to provide a secondary battery that can improve the cycle life and cycle rate of the secondary battery, as well as a method for preparing the same, a battery module, a battery pack, and an electrical device.

[0005] The first aspect of this application provides a secondary battery including a negative electrode sheet, on the surface of which a solid electrolyte interface film is formed. The solid electrolyte interface film includes a first region portion and a second region portion connected to each other. The impedance value of the first region portion is Z1, and the impedance value of the second region portion is Z2, as determined by AC impedance spectroscopy, wherein Z1 / Z2 ≥ 1.5.

[0006] Therefore, the SEI film in the secondary battery of this application embodiment has uneven thickness, comprising a first region and a second region connected to each other. The first and second regions are interconnected, forming a continuous film structure to provide good protection for the negative electrode. The impedance value Z1 of the first region is greater than the impedance value Z2 of the second region, and Z1 / Z2 ≥ 1.5. The relatively high impedance and density of the first region further enhance the protection of the negative electrode, thereby reducing the risk of side reactions and ensuring the structural stability of the negative electrode, as well as the cycle life of the secondary battery. The relatively low impedance and relatively porous nature of the second region facilitate the migration of metal ions. Through the cooperation of the first and second regions, this application embodiment can ensure the cycle rate and low-temperature performance of the secondary battery.

[0007] In any implementation, the impedance value Z1 of the first region satisfies 4mΩ≤Z1≤11mΩ.

[0008] Therefore, the impedance value Z1 of the first region in this embodiment is relatively large, and the thickness of the film layer formed on the negative electrode is relatively thick, which can improve the protection performance of the negative electrode.

[0009] In any implementation, the impedance value Z2 of the second region satisfies 0.7mΩ≤Z2≤7mΩ.

[0010] Therefore, the impedance value Z2 of the second region in this embodiment is relatively small, and the thickness of the film layer formed on the negative electrode is relatively thin, which is beneficial to the migration of metal ions.

[0011] In any embodiment, the first region portion is formed in a continuous layered structure on a portion of the surface of the negative electrode sheet; the second region portion is configured as a plurality of discretely distributed in the first region portion and formed on another portion of the surface of the negative electrode sheet.

[0012] Therefore, in this embodiment, the first region is formed in a continuous layered structure on a portion of the surface of the negative electrode sheet, which can provide good protection for that portion of the negative electrode sheet. Multiple second regions are discretely distributed within the first region, which facilitates the embedding of metal ions in the electrolyte into or extraction from the negative electrode sheet through the second regions located at different positions, ensuring the uniformity of electrochemical reactions throughout the negative electrode sheet.

[0013] In any implementation, multiple second region portions are evenly distributed within the first region portion.

[0014] Therefore, the multiple second regions in this embodiment are evenly distributed in the first region, which enables metal ions to be more evenly embedded in the negative electrode, thereby improving the uniformity of the electrochemical reaction in the negative electrode.

[0015] In any embodiment, based on a mass meter of a first region portion, the first region portion comprises an organic component with a mass percentage of A1 and an inorganic component with a mass percentage of B1; based on a mass meter of a second region portion, the second region portion comprises an organic component with a mass percentage of A2 and an inorganic component with a mass percentage of B2, wherein A1 > A2; and / or B1 < B2.

[0016] Therefore, the higher the content of organic components in the embodiments of this application, the more beneficial it is to form a film layer with better density. The mass percentage A1 of organic components in the first region is greater than the mass percentage A2 of organic components in the second region. The relatively higher content of organic components in the first region results in a relatively better film layer density in the first region, and better protection performance for the negative electrode sheet.

[0017] In any embodiment, the negative electrode sheet includes a negative electrode active material, which includes one or more of artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. This type of negative electrode active material has relatively good structural stability, which is beneficial for ensuring the cycle life of the secondary battery.

[0018] In any embodiment, the electrolyte includes an organic solvent, an electrolyte lithium salt dissolved in the organic solvent, and an additive; optionally, the additive includes one or more of fluoroethylene carbonate, vinylene carbonate, and ethylene sulfate. The above-mentioned additives can form a solid electrolyte interface film on the surface of the negative electrode, thereby protecting the negative electrode.

[0019] The second aspect of this application provides a battery module including a secondary battery according to any embodiment of the first aspect of this application.

[0020] A third aspect of this application provides a battery pack that includes the battery module of the second aspect of this application.

[0021] A fourth aspect of this application provides an electrical device, including at least one of a secondary battery selected from any embodiment of the first aspect of this application, a battery module of the second aspect of this application, or a battery pack of the third aspect of this application.

[0022] The fifth aspect of this application provides a method for preparing a secondary battery, the method comprising:

[0023] A battery structure is provided, the battery structure including a housing assembly and a positive electrode, a separator and a negative electrode disposed within the housing assembly;

[0024] A first electrolyte is added inside the housing assembly so that the first electrolyte forms a first film layer on a portion of the surface of the negative electrode sheet;

[0025] A second electrolyte is added inside the housing assembly so that the second electrolyte forms a second region on another part of the surface of the negative electrode sheet, and a second film layer is formed on the surface of the first film layer opposite to the negative electrode sheet. The second region and the second film layer are continuous film layers, wherein the first film layer and the second film layer constitute the first region.

[0026] Therefore, the secondary battery prepared by the method of this application embodiment has a first region and a second region with different thicknesses formed on the surface of the negative electrode sheet. Since the first region is formed by two film depositions, the thickness of the first region is relatively large, which provides relatively good protection for the negative electrode sheet and thus improves the cycle life of the secondary battery. Since the second region is formed by one film deposition, the thickness of the first region is relatively small, which is conducive to the migration of metal ions and thus ensures the cycle rate and low temperature performance of the secondary battery.

[0027] In any embodiment, the step of adding a first electrolyte into the housing assembly to form a first film layer on a portion of the surface of the negative electrode sheet includes: adding the first electrolyte into the housing assembly, and charging the positive electrode sheet and the negative electrode sheet with a first constant current to form a first film layer on a portion of the surface of the negative electrode sheet; optionally, in the step of forming the first film layer, the charging current of the first constant current is 0.3C to 0.5C, and the charging time is 80min to 140min.

[0028] Therefore, under the above charging current and charging time, the first electrolyte can fully form a film on the surface of the negative electrode, thereby improving the protection of the negative electrode.

[0029] In any embodiment, during the step of forming the first film layer, when the positive and negative electrode plates are charged with a first constant current, the interior of the housing assembly is connected to an external vacuum pumping source to extract gas from the interior of the housing assembly until charging is completed, at which point gas extraction stops. Optionally, the housing assembly is connected to an external vacuum pumping source to ensure that the vacuum level inside the housing assembly is -20 kPa to -60 kPa. Maintaining a vacuum level within the aforementioned range ensures the formation of the first film layer.

[0030] Therefore, during the process of the first electrolyte forming a first film layer on a portion of the surface of the negative electrode, gas will inevitably be generated inside the casing assembly. When the positive and negative electrodes are charged with a first constant current, an external vacuum suction source is used to remove some of the gas inside the casing assembly, thereby reducing the risk of secondary battery expansion to a certain extent.

[0031] In any embodiment, during the first constant current charging of the positive and negative electrodes, the housing assembly is vibrated to uniformly distribute bubbles on the surface of the negative electrode; optionally, the housing assembly is ultrasonically vibrated.

[0032] Therefore, in this embodiment of the application, under the action of vibration, multiple bubbles are discretely distributed on the surface of the negative electrode sheet and are uniformly distributed on the surface of the negative electrode sheet, so that the second region formed at the bubble is uniformly dispersed in the first region, which enables the metal ions in the electrolyte to be uniformly embedded on the negative electrode sheet.

[0033] In any embodiment, the step of adding a second electrolyte into the housing assembly to form a second region portion on another part of the surface of the negative electrode plate and forming a second film layer on the surface of the first film layer opposite to the negative electrode plate includes: adding the second electrolyte into the housing assembly, charging the positive electrode plate and the negative electrode plate with a second constant current, so that the second electrolyte forms a second region portion on another part of the surface of the negative electrode plate and forms a second film layer on the surface of the first film layer opposite to the negative electrode plate, wherein the charging current of the second constant current is greater than the charging current of the first constant current; optionally, in the step of forming the second film layer and the second region portion: the charging current of the second constant current is 1C to 2C, and the charging time is 2.5min to 5min.

[0034] Therefore, under the above charging current and charging time, the second electrolyte can be fully formed on the surface of the negative electrode, and the formed film has high porosity, which is conducive to the migration of metal ions in the film, thereby improving the rate performance and low temperature performance of the secondary battery.

[0035] In any embodiment, during the step of forming the second film layer and the second region portion, when the positive electrode and the negative electrode are charged with a second constant current, the inside of the housing assembly is connected to an external vacuum suction source to extract gas from the inside of the housing assembly until the end, at which point the gas extraction is stopped; optionally, the housing assembly is connected to an external vacuum suction source to make the vacuum degree inside the housing assembly -20KPa to -60KPa.

[0036] Therefore, during the formation of the film layer by the second electrolyte, gas will inevitably be generated inside the casing assembly. When the positive and negative electrodes are charged with a second constant current, an external vacuum suction source is used to remove some of the gas inside the casing assembly, thereby reducing the risk of secondary battery expansion to a certain extent.

[0037] In any embodiment, the first electrolyte and the second electrolyte have the same composition. During the two film-forming processes, the bonding force between the formed film layers is relatively strong, resulting in high structural stability of the SEI film.

[0038] In any embodiment, based on the mass of the first membrane layer, the first membrane layer comprises an inorganic component with a mass percentage of C1 and an organic component with a mass percentage of D1; based on the mass of the second membrane layer, the second membrane layer comprises an inorganic component with a mass percentage of C2 and an organic component with a mass percentage of D2, wherein C1 < C2; and / or D1 > D2.

[0039] Therefore, C1 < C2, the second region has a relatively high porosity, which is conducive to the migration of metal ions. D1 > D2, the content of organic components in the first film layer is relatively higher, the density of the first film layer is relatively higher, and the density of the first region formed by the first and second film layers is relatively higher, which is beneficial to the protection of the negative electrode sheet by the first region. Attached Figure Description

[0040] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the embodiments of this application will be briefly introduced 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.

[0041] Figure 1 This is a schematic diagram of a secondary battery according to one embodiment of this application.

[0042] Figure 2 yes Figure 1 An exploded view of a secondary battery according to one embodiment of this application is shown.

[0043] Figure 3 This is a schematic diagram of a battery module according to one embodiment of this application.

[0044] Figure 4 This is a schematic diagram of a battery pack according to one embodiment of this application.

[0045] Figure 5 yes Figure 4 An exploded view of a battery pack according to one embodiment of this application is shown.

[0046] Figure 6 This is a schematic diagram of an electrical device according to one embodiment of this application;

[0047] Figure 7 This is a schematic diagram of the process for preparing a secondary battery according to one embodiment of this application.

[0048] The reference numerals in the attached figures are explained as follows:

[0049] 1. Secondary battery;

[0050] 11. Housing assembly; 111. Top cover assembly; 112. Housing; 12. Electrode assembly;

[0051] 10. Battery module;

[0052] 20. Battery pack; 21. Upper casing; 22. Lower casing;

[0053] 30. Electrical appliances. Detailed Implementation

[0054] The following details embodiments of the secondary battery, its preparation method, 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 to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0055] 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.

[0056] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0057] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0058] Unless otherwise specified, all steps of this application may be performed sequentially or randomly, preferably sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if the method may also include step (c), it means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0059] 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.

[0060] 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).

[0061] [Rechargeable Battery]

[0062] This application discloses a secondary battery including a negative electrode. A solid electrolyte interface (SEI) film is formed on the surface of the negative electrode. The SEI film includes a first region and a second region adjacent to each other. The impedance value of the first region, measured by AC impedance spectroscopy, is Z1, and the impedance value of the second region, measured by AC impedance spectroscopy, is Z2, where Z1 / Z2 ≥ 1.5. The first region of the SEI film protects the negative electrode, ensuring the cycle life of the secondary battery. The second region facilitates the migration of metal ions, thereby ensuring the cycle rate and low-temperature performance of the secondary battery. The synergistic effect of the first and second regions effectively improves the performance of the secondary battery.

[0063] [Negative electrode plate]

[0064] The negative electrode includes a negative electrode active material, which may be one or more of carbon-based, tin-based, silicon-based, and lithium carbonate materials. Metal ions can be inserted into and extracted from the negative electrode active material, thereby enabling the charging and discharging of the secondary battery.

[0065] Although the mechanism is not yet clear, the SEI film formed on the negative electrode can passivate the negative electrode active material, thereby improving the electrochemical stability of the negative electrode; and the SEI film can ensure the migration of metal ions, thereby ensuring the smooth charging and discharging.

[0066] The impedance value of the SEI film is positively correlated with its thickness; in other words, the larger the impedance value, the thicker the film, and vice versa. Therefore, the impedance value can be used to indirectly characterize the thickness. In this embodiment, the SEI film includes a first region and a second region connected to each other. The impedance value Z1 of the first region is greater than the impedance value Z2 of the second region, and Z1 / Z2 ≥ 1.5. Optionally, 1.5 ≤ Z1 / Z2 ≤ 12.5. In other words, the thickness of the first region is greater than the thickness of the second region. The SEI film is composed of regions of different thicknesses, and the migration characteristics of metal ions and the protection capabilities for the negative electrode differ between the first and second regions. The first region, with its relatively large impedance value, is thicker and provides better protection for the negative electrode, thus ensuring the cycle life of the secondary battery. The second region, with its relatively small impedance value, is thinner and facilitates the migration of metal ions, thereby ensuring the cycle rate and low-temperature performance of the secondary battery.

[0067] In this paper, the impedance value can be determined using electrochemical impedance spectroscopy (EIS).

[0068] According to the embodiments of this application, the secondary battery has an uneven thickness of SEI film, comprising a first region and a second region connected to each other. The first and second regions are interconnected, forming a continuous film structure to provide good protection for the negative electrode. The impedance value Z1 of the first region is greater than the impedance value Z2 of the second region, and Z1 / Z2 ≥ 1.5. The relatively high impedance of the first region further enhances the protection of the negative electrode, reducing the risk of side reactions and ensuring the structural stability of the negative electrode, thus guaranteeing the cycle life of the secondary battery. The relatively low impedance of the second region facilitates the migration of metal ions, thereby ensuring the cycle rate and low-temperature performance of the secondary battery.

[0069] In some embodiments, the impedance value Z1 of the first region satisfies 4mΩ ≤ Z1 ≤ 11mΩ. The relatively large impedance value Z1 of the first region indicates a relatively thick film layer formed on the negative electrode, which improves the protection performance of the negative electrode. The impedance value Z1 of the first region must be greater than the impedance value Z2 of the second region. For example, if the impedance value Z1 of the first region is 11mΩ, then the impedance value Z2 of the second region is less than 11mΩ.

[0070] In some embodiments, the impedance value Z2 of the second region satisfies 0.7mΩ ≤ Z2 ≤ 7mΩ. The relatively small impedance value Z2 of the second region indicates a relatively thin film layer formed on the negative electrode, which facilitates the migration of metal ions. The impedance value Z2 of the second region must be less than the impedance value Z1 of the first region. For example, the impedance value Z1 of the first region can be 5mΩ, and the impedance value Z2 of the second region can be less than 5mΩ, satisfying 0.7mΩ ≤ Z2 < 7mΩ, and Z1 / Z2 ≥ 1.5; for example, Z2 can be 2.5mΩ or 3mΩ. Alternatively, the impedance value Z1 of the first region can be 10.8mΩ, and the impedance value Z2 of the second region can be less than 10.8mΩ, and Z1 / Z2 ≥ 1.5, satisfying 0.7mΩ ≤ Z2 ≤ 7mΩ; for example, Z2 can be 6.5mΩ, 6mΩ, or 5mΩ.

[0071] In some embodiments, a first region is formed in a continuous layered structure on a portion of the surface of the negative electrode sheet; a plurality of second regions are provided, which are discretely distributed within the first region and formed on another portion of the surface of the negative electrode sheet. In embodiments of this application, a portion of the surface of the negative electrode sheet and another portion of the surface are different regions of the same surface of the negative electrode sheet.

[0072] The first region is formed in a continuous layered structure on a portion of the surface of the negative electrode sheet, providing good protection for that portion of the negative electrode sheet. Multiple second regions are discretely distributed within the first region, which facilitates the insertion of metal ions in the electrolyte into or extraction from the negative electrode sheet through the second regions located at different positions, ensuring the uniformity of electrochemical reactions throughout the negative electrode sheet.

[0073] The SEI film in the embodiments of this application can be formed by multiple film deposition processes.

[0074] For example, the SEI film can be formed in two stages. In the first stage, a first film layer can be formed, which can cover a part of the surface of the negative electrode sheet. Then, a second film layer is formed on the basis of the first film layer. In the second stage, films can be formed on the surface of the first film layer away from the negative electrode sheet and on the other part of the surface of the negative electrode sheet. The surface of the first film layer away from the negative electrode sheet forms a second film layer. The first film layer and the second film layer constitute a first region, and the other part of the surface of the negative electrode sheet forms a second region. The second film layer and the second region are continuous film layer structures.

[0075] Of course, the SEI film can also be formed in three stages. In the first stage, a first film layer is formed, which covers a portion of the surface of the negative electrode. Then, a second film layer is formed on top of the first film layer. During the second film layer, films can be formed on both the surface of the first film layer facing away from the negative electrode and the other portion of the negative electrode surface. The surface of the first film layer facing away from the negative electrode forms the second film layer, and the other portion of the negative electrode surface forms the first sub-section. The second film layer and the first sub-section form a continuous film structure. Then, a third film layer is formed on top of the second film layer and the first sub-section. During the third film layer, the portion of the second film layer facing away from the negative electrode forms the third film layer, and the portion of the first sub-section facing away from the negative electrode forms the second sub-section. The third film layer and the second sub-section form a continuous film structure. The first, second, and third film layers constitute the first region, and the first and second sub-sections constitute the second region.

[0076] Optionally, multiple second regions are evenly distributed within the first region, thereby enabling metal ions to be more uniformly embedded in the negative electrode, thus improving the uniformity of the electrochemical reaction occurring on the negative electrode.

[0077] In some embodiments, based on a mass meter of a first region portion, the first region portion comprises an organic component with a mass percentage of A1 and an inorganic component with a mass percentage of B1. Based on a mass meter of a second region portion, the second region portion comprises an organic component with a mass percentage of A2 and an inorganic component with a mass percentage of B2, wherein A1 > A2; and / or B1 < B2.

[0078] The higher the content of organic components, the better the film density will be. The mass percentage of organic components A1 in the first region is greater than that in the second region A2. The higher the content of organic components in the first region, the better the film density in the first region, and the better the protection performance for the negative electrode.

[0079] The higher the content of inorganic components, the higher the porosity of the membrane. The mass percentage of inorganic components B1 in the first region is lower than that in the second region B2. The higher the content of inorganic components in the second region, the higher the porosity of the membrane, which is more conducive to the migration of metal ions.

[0080] In some embodiments, the negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including a negative electrode active material.

[0081] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0082] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0083] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. Silicon-based materials may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0084] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0085] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0086] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0087] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0088] [Positive electrode plate]

[0089] In some implementations, the secondary battery may also include a positive electrode.

[0090] 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, the positive electrode film layer including the positive electrode active material of the first aspect of this application.

[0091] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0092] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0093] In some embodiments, the positive electrode active material may be a known battery positive electrode active material. As an example, the positive electrode active material may include at least one of the following materials: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, this application is not limited to these materials, and other conventional materials that can be used as battery positive electrode active materials may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as LiNi). 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also known as NCM) 333 LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811 ), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.15 Al 0.05 At least one of O2 and its modified compounds. Examples of lithium phosphates with an olivine structure include, but are not limited to, lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0094] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0095] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0096] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0097] [Electrolytes]

[0098] In some implementations, the secondary battery may also include an electrolyte.

[0099] The electrolyte acts as a conductor of metal ions between the positive and negative electrodes. This application does not specify any particular type of electrolyte; it can be selected according to requirements.

[0100] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0101] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0102] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0103] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc. The negative electrode film-forming additive can undergo an electrochemical reaction on the surface of the negative electrode active material during the first charge of the secondary battery to form an SEI film. The SEI film formed by the negative electrode film-forming additive may simultaneously include inorganic and organic components. For example, the negative electrode film-forming additive may include one or more of fluoroethylene carbonate (FEC), vinylene carbonate (VC), and ethylene sulfate (DTD). For example, based on the same mass percentage of VC and FEC, the SEI film formed with VC has a relatively higher content of organic components, but it may also contain inorganic components. The SEI film formed with FEC has a relatively higher content of inorganic components, but it may also contain organic components.

[0104] [Isolation membrane]

[0105] In some embodiments, the secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0106] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0107] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0108] In some embodiments, the secondary battery may include a housing assembly. This housing assembly can be used to encapsulate the electrode assembly and electrolyte described above.

[0109] In some embodiments, the casing assembly of the secondary battery can be a rigid casing, such as a hard plastic casing, an aluminum casing, or a steel casing. The casing assembly of the secondary battery can also be a pouch, such as a pouch-type pouch. The pouch material can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0110] In some embodiments, such as Figure 1 and Figure 2 As shown, the secondary battery may include a housing assembly 11. The housing assembly 11 can be used to encapsulate the aforementioned electrode assembly and electrolyte.

[0111] In some embodiments, the casing assembly 11 of the secondary battery can be a rigid casing, such as a hard plastic casing, an aluminum casing, a steel casing, etc. The casing assembly 11 of the secondary battery can also be a pouch, such as a pouch-type pouch. The material of the pouch can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0112] 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. For example, Figure 1 and Figure 2 This is an example of a square-structured secondary battery 1.

[0113] In some embodiments, the secondary battery 1 includes a housing assembly 11. The housing assembly 11 includes a top cover assembly 111 and a housing 112. A positive electrode, a negative electrode, and a separator constitute an electrode assembly 12 housed within the housing 112, which also contains an electrolyte. The positive or negative electrode includes tabs. During the charging and discharging process of the secondary battery 1, active ions repeatedly insert and extract between the positive and negative electrode. The electrolyte acts as a conductor between the positive and negative electrode. The separator is disposed between the positive and negative electrode, primarily to prevent short circuits between the positive and negative electrodes, while allowing active ions to pass through. Specifically, the secondary battery 1 can be a wound or stacked battery, such as a lithium-ion battery or a sodium-ion battery, but is not limited to these.

[0114] Optionally, the housing 112 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 112 has an opening communicating with the receiving cavity, and a top cover assembly 111 can be placed over the opening to close the receiving cavity. The positive electrode, negative electrode, and separator can be formed into an electrode assembly 12 by a winding process or a stacking process. The electrode assembly 12 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 12. The secondary battery 1 may contain one or more electrode assemblies 12, which can be selected by those skilled in the art according to specific practical needs.

[0115] In some embodiments, the secondary battery 1 can be assembled into a battery. The battery can be a battery module or a battery component. For example, the number of secondary batteries 1 contained in a battery module can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0116] Figure 3 This is battery module 10 as an example. (See reference...) Figure 3 In the battery module 10, multiple secondary batteries 1 can be arranged sequentially along the length of the battery module 10. Of course, they can also be arranged in any other manner. Furthermore, the multiple secondary batteries 1 can be fixed in place using fasteners.

[0117] Optionally, the battery module 10 may also include a housing with a receiving space in which a plurality of secondary batteries 1 are received.

[0118] In some embodiments, the battery module 10 described above can also be assembled into a battery pack. The battery pack may contain one or more battery modules 10, and the specific number can be selected by those skilled in the art based on the application and capacity of the battery pack. Of course, the battery pack may also be directly composed of multiple secondary batteries 1.

[0119] Figure 4 and Figure 5 This is a sample battery pack 20. (See reference) Figure 4 and Figure 5 The battery pack 20 may include a battery box and multiple battery modules 10 disposed within the battery box. The battery box includes an upper box 21 and a lower box 22, with the upper box 21 covering the lower box 22 to form a closed space for accommodating the battery modules 10. The multiple battery modules 10 may be arranged in any manner within the battery box.

[0120] In addition, this application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided in this application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device or as an energy storage unit of the electrical device. The electrical device may include, 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. The secondary battery, battery module, or battery pack can be selected as the electrical device according to its usage requirements.

[0121] Figure 6 This is an example of an electrical device 30. This electrical device 30 can be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of the secondary battery for this electrical device 30, a battery pack or battery module can be used.

[0122] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0123] [Methods for preparing secondary batteries]

[0124] Based on the same inventive concept, this application also provides a method for preparing a secondary battery. This method can be used to prepare the secondary batteries described in the above embodiments. Figure 7 As shown, the method includes:

[0125] Step S100: A battery structure is provided, the battery structure including a housing assembly and a positive electrode, a separator and a negative electrode disposed within the housing assembly.

[0126] Step S200: Add a first electrolyte into the housing assembly so that the first electrolyte forms a first film layer on a portion of the surface of the negative electrode sheet;

[0127] Step 300: Add a second electrolyte into the housing assembly so that the second electrolyte forms a second region on another part of the surface of the negative electrode sheet, and the first film layer forms a second film layer away from the surface of the negative electrode sheet. The second region and the second film layer are continuous film layers, wherein the first film layer and the second film layer constitute the first region.

[0128] The positive electrode, separator, and negative electrode constitute the electrode assembly, which is the core component for rechargeable batteries. The battery structure includes a casing assembly and an electrode assembly, with the electrode assembly housed within the casing assembly. In this case, the electrolyte has not yet been added to the battery structure, i.e., the battery structure before formation.

[0129] A first electrolyte is added inside the outer casing assembly. The first electrolyte can undergo an oxidation-reduction reaction with the negative electrode active material of the negative electrode sheet to form a first film layer on the surface of the negative electrode active material. This first film layer can serve as a component of the SEI film to protect the negative electrode active material.

[0130] The first membrane layer can be a continuous membrane layer, but it may only cover a portion of the surface of the negative electrode, or it may not cover another portion. Because bubbles adhere to the surface of the negative electrode, the first electrolyte is less likely to form a first membrane layer at the bubble locations, thus causing the first membrane layer to cover only a portion of the negative electrode surface. In this paper, the portion of the negative electrode surface and the other portion are different areas of the same surface of the negative electrode. Given the potential for fluctuations during actual operation, the negative electrode may also include a further portion of its surface that may not be covered by the SEI membrane.

[0131] A second electrolyte is added inside the outer casing assembly. The second electrolyte can undergo a redox reaction with the negative electrode active material of the negative electrode sheet to form a film layer. Specifically, the second electrolyte can be formed on another part of the surface of the negative electrode sheet to form a second region, and the second electrolyte can be formed on the surface of the first film layer away from the negative electrode sheet to form a second film layer. The second region and the second film layer are continuous film layer structures, that is, the second region and the second film layer are formed simultaneously. The composition of the second region and the second film layer are basically the same, and the thickness is basically the same.

[0132] The second film layer is formed on the surface of the first film layer. The second film layer and the first film layer may contain the same or similar components. Therefore, there may be no obvious interface between the first film layer and the second film layer. From a macroscopic perspective, the first film layer and the second film layer constitute the whole film layer of the first region.

[0133] The second region and the first region may contain the same or similar components, and there may be no obvious interface between the second region and the first region. From a macroscopic perspective, the second region and the first region constitute the entire SEI film.

[0134] Since the first region comprises a first film layer and a second film layer formed sequentially along its own thickness direction, and the thickness of the second region is approximately the same as the thickness of the second film layer, the thickness of the first region is greater than the thickness of the second region. Given that the impedance value measured based on AC impedance spectroscopy is proportional to the thickness value, the impedance value of the film layer can be used to indirectly reflect the thickness of the film layer.

[0135] The first electrolyte and the second electrolyte in the embodiments of this application may have the same or different compositions.

[0136] The secondary battery prepared according to the method of the embodiments of this application has a first region and a second region with different thicknesses formed on the surface of the negative electrode. Since the first region is formed by two film depositions, the thickness of the first region is relatively large, which provides relatively good protection for the negative electrode and thus improves the cycle life of the secondary battery. Since the second region is formed by one film deposition, the thickness of the first region is relatively small, which is conducive to the migration of metal ions and thus ensures the cycle rate and low temperature performance of the secondary battery.

[0137] In some embodiments, step S200 may include: adding a first electrolyte into the housing assembly, and charging the positive electrode and the negative electrode with a first constant current to form a first film layer on a portion of the surface of the negative electrode with the first electrolyte.

[0138] The positive and negative electrode plates are charged with a first constant current. The secondary battery is in the formation stage. An oxidation-reduction reaction can occur between the first electrolyte and the negative electrode active material of the negative electrode plate, thereby forming a first film layer on the surface of the negative electrode active material to protect the negative electrode plate.

[0139] For example, the charging current of the first constant current charging is 0.3C to 0.5C, and the charging time is 80 min to 140 min; optionally, the charging time can be 80 min to 133.33 min. Under the above charging current and charging time, the first electrolyte can be fully formed on the surface of the negative electrode, thereby improving the protection of the negative electrode.

[0140] In some embodiments, step S200 includes: adding a first electrolyte into the housing assembly, charging the positive electrode and the negative electrode with a first constant current, so that the first electrolyte forms a first film layer on a portion of the surface of the negative electrode, wherein, while charging the positive electrode and the negative electrode with the first constant current, the inside of the housing assembly is connected to an external vacuum suction source to extract gas from the inside of the housing assembly until the charging is finished, and then the gas extraction is stopped.

[0141] During the formation of the first film layer on a portion of the surface of the negative electrode by the first electrolyte, gas inevitably occurs within the casing assembly. When the positive and negative electrodes are charged with a first constant current, an external vacuum suction source is used to remove some of the gas from the casing assembly, thereby reducing the risk of secondary battery expansion to a certain extent.

[0142] To ensure that air bubbles adhere to another portion of the negative electrode surface, so that the first film layer is formed on a portion of the negative electrode surface, the external vacuum suction source does not remove all the gas inside the housing assembly during the gas extraction process; some gas will remain.

[0143] For example, the housing assembly is connected to an external vacuum suction source to achieve a vacuum level of -20 kPa to -60 kPa inside the housing assembly. When the interior of the housing assembly is within the aforementioned vacuum range, the formation of the first film layer can be guaranteed.

[0144] The time for the vacuum suction source to remove gas can be 140 min to 80 min.

[0145] At higher vacuum levels, the gas extraction time can be relatively shorter; at lower vacuum levels, the gas extraction time can be relatively longer. For example, at a vacuum level of -20 kPa, the gas extraction time can be 140 minutes. At a vacuum level of -60 kPa, the gas extraction time can be 80 minutes.

[0146] In some embodiments, step S200 includes: while charging the positive electrode and the negative electrode with a first constant current, vibrating the housing assembly to uniformly distribute bubbles on another portion of the surface of the negative electrode.

[0147] Multiple bubbles are discretely and uniformly distributed on the surface of the negative electrode sheet. This ensures that the second region formed at the bubble location is uniformly dispersed within the first region, allowing metal ions in the electrolyte to be uniformly embedded in the negative electrode sheet.

[0148] For example, ultrasonic vibration of the housing assembly can be used to disperse the bubbles evenly. Of course, other vibration methods can also be used, such as using a robotic arm to shake the housing assembly.

[0149] In some embodiments, step S300 includes: adding a second electrolyte into the housing assembly, charging the positive electrode and the negative electrode with a second constant current, so that the second electrolyte forms a second region on another part of the surface of the negative electrode, and the first film layer forms a second film layer away from the surface of the negative electrode, wherein the charging current of the second constant current is greater than the charging current of the first constant current.

[0150] The positive and negative electrode plates are charged with a second constant current. The secondary battery is in the formation stage. An oxidation-reduction reaction can occur between the second electrolyte and the negative electrode active material of the negative electrode plate, thereby forming a second film layer and a second region to protect the negative electrode plate.

[0151] The second constant current charging current is greater than the first constant current charging current, which is more conducive to the formation of inorganic components on the surface of the negative electrode active material, thereby making the second region more porous and the impedance value relatively low.

[0152] For example, the second constant current charging current is 1C to 2C, and the charging time is 2.5min to 5min. Under the above charging current and charging time, the second electrolyte can be fully formed on the surface of the negative electrode, and the formed film has high porosity, which is conducive to the migration of metal ions in the film, thereby improving the rate performance and low temperature performance of the secondary battery.

[0153] In some embodiments, step S300 includes: adding a second electrolyte into the housing assembly, charging the positive and negative electrode plates with a second constant current, so that the second electrolyte forms a second region on another part of the surface of the negative electrode plate, and the first film layer forms a second film layer away from the surface of the negative electrode plate, the charging current of the second constant current is greater than the charging current of the first constant current, and when charging the positive and negative electrode plates with the second constant current, connecting the inside of the housing assembly with an external vacuum suction source to extract the gas inside the housing assembly, until the end, and stopping the gas extraction.

[0154] During the formation of the second electrolyte film, gas inevitably forms within the casing assembly. When charging the positive and negative electrodes with a second constant current, an external vacuum pump source is used to remove some of the gas from the casing assembly, thereby reducing the risk of secondary battery expansion to some extent.

[0155] For example, the housing assembly is connected to an external vacuum suction source to achieve a vacuum level of -20 kPa to -60 kPa inside the housing assembly. When the interior of the housing assembly is within this vacuum range, the formation of the second film layer and the second region portion can be guaranteed.

[0156] The time for the vacuum suction source to remove gas can be from 5 min to 2.5 min.

[0157] At higher vacuum levels, the gas extraction time can be relatively shorter; at lower vacuum levels, the gas extraction time can be relatively longer. For example, at a vacuum level of -20 kPa, the gas extraction time can be 5 minutes. At a vacuum level of -60 kPa, the gas extraction time can be 2.5 minutes.

[0158] In some embodiments, the first electrolyte and the second electrolyte have the same composition. During the two film-forming processes, the bonding force between the resulting film layers is relatively strong, resulting in high structural stability of the SEI film.

[0159] In other embodiments, the first electrolyte and the second electrolyte have different compositions, and the composition of the first electrolyte and the second electrolyte can be selected according to the performance of the SEI membrane to be formed.

[0160] Based on the mass of the first membrane layer, the first membrane layer comprises an inorganic component with a mass percentage of C1 and an organic component with a mass percentage of D1.

[0161] Based on the mass of the second membrane layer, the second membrane layer comprises an inorganic component with a mass percentage of C2 and an organic component with a mass percentage of D2.

[0162] As examples, where C1 < C2, the inorganic component content in the second membrane layer is relatively higher, resulting in a relatively higher porosity of the second membrane layer. Since the second membrane layer and the second region have the same composition, the relatively higher porosity of the second region facilitates the migration of metal ions. Exemplarily, the second electrolyte may include a relatively high content of FEC.

[0163] As other examples, D1 > D2, the organic component content in the first membrane layer is relatively higher, the density of the first membrane layer is relatively higher, and the density of the first region formed by the first and second membrane layers is relatively higher, which is beneficial for the protection of the negative electrode sheet by the first region. For example, the first electrolyte may include a relatively high content of VC.

[0164] In this application, D1+D2 can be considered as A1, and C1+C2 can be considered as B1. D2 is equivalent to A2, and the inorganic component of C2 is equivalent to B2.

[0165] Example

[0166] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0167] Examples 1 to 5 and Comparative Examples 1 to 3

[0168] 1. Preparation of positive electrode sheet

[0169] An 8μm thick aluminum foil was used as the positive electrode current collector.

[0170] LiNi, the positive electrode active material 1 / 3 Co 1 / 3 Mn 1 / 3 O2(NCM 333 Conductive carbon black and binder polyvinylidene fluoride (PVDF) are mixed in an appropriate amount of N-methylpyrrolidone (NMP) solvent at a weight ratio of 93:2:5 to form a uniform positive electrode slurry. The positive electrode slurry is coated on the surface of the positive electrode current collector and then dried to obtain the positive electrode sheet.

[0171] 2. Preparation of negative electrode sheet

[0172] The negative electrode active material graphite, conductive carbon black, thickener sodium carboxymethyl cellulose (CMC), and binder styrene-butadiene rubber latex (SBR) are thoroughly mixed in an appropriate amount of deionized water at a weight ratio of 96.5:1.0:1.0:1.5 to form a uniform negative electrode slurry. The negative electrode slurry is coated onto the negative electrode current collector, and after drying and other processes, the negative electrode sheet is obtained.

[0173] 3. Preparation of electrolytes

[0174] The electrolytes in Examples 1 to 5, and Comparative Examples 1 and 3, all include a first electrolyte and a second electrolyte.

[0175] Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7 were mixed uniformly to obtain an organic solvent. Then, the negative electrode film-forming additive and 1 mol / L LiPF6 were uniformly dissolved in the organic solvent to form the first electrolyte. The types and mass percentages of the negative electrode film-forming additives are shown in Table 1. The mass percentages are calculated based on the total mass of the first electrolyte.

[0176] Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7 were mixed evenly to obtain an organic solvent. Then, the negative electrode film-forming additive and 1 mol / L LiPF6 were uniformly dissolved in the above organic solvent to form a second electrolyte. The types and mass percentages of the negative electrode film-forming additive are shown in Table 1.

[0177] Electrolyte for Comparative Example 2: Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7 were mixed uniformly to obtain an organic solvent. Then, the negative electrode film-forming additive and 1 mol / L LiPF6 were uniformly dissolved in the organic solvent to form a conventional electrolyte. The types and mass percentages of the negative electrode film-forming additives are shown in Table 1. The mass percentages are calculated based on the total mass of the first electrolyte.

[0178] 4. Preparation of lithium-ion batteries

[0179] The positive electrode, separator (PP / PE / PP composite film), and negative electrode are stacked in sequence, then wound into a cell and packaged in a casing. The electrolyte is injected into the cell, and then the cell undergoes sealing, settling, hot and cold pressing, and formation processes to obtain a lithium-ion battery.

[0180] The formation in Examples 1 to 5 includes a first formation and a second formation.

[0181] First transformation:

[0182] After the first electrolyte is injected, the battery is allowed to stand and soak. The lithium-ion battery is charged in a stepped charging manner within the charging current range of 0.3C to 0.5C. During the formation process, gas is simultaneously extracted, and some of the gas adheres to the surface of the negative electrode sheet in the form of bubbles. After standing and aging, the first film layer is formed.

[0183] Second transformation:

[0184] After the second electrolyte is injected, the battery is allowed to stand and soak. The lithium-ion battery is charged in a stepped charging manner within the charging current range of 1C to 2C. During the formation process, gas is simultaneously removed (bubbles are removed), and the battery is allowed to stand and age. The second electrolyte forms a second region at the original location of the bubbles and forms a second film on the surface of the first film.

[0185] Unlike Examples 1 to 5, in the first formation of Comparative Example 1, the gas was simultaneously extracted during the formation process, and there were basically no bubbles adhering to the surface of the negative electrode sheet. After static aging, a film layer was formed.

[0186] Unlike Examples 1 to 5, Comparative Example 2 uses a single formation process, during which gas is simultaneously extracted, resulting in virtually no bubbles adhering to the surface of the negative electrode sheet. After static aging, a film layer is formed.

[0187] Table 1

[0188]

[0189] Test section

[0190] 1. Method for EIS measurement of impedance values ​​Z1 and Z2

[0191] Based on the positive and negative electrode sheets included in the embodiments, a first electrolyte is injected into the battery and charged at 0.3C to 0.5C for 80 to 140 minutes to form a first film layer; a second electrolyte is injected into the battery and charged at 1C to 2C for 2.5 to 5 minutes to form a second film layer, thus preparing a lithium-ion battery. The lithium-ion battery is adjusted to a 50% SOC state, and then the negative electrode sheet is disassembled to create a symmetrical battery. Its EIS is tested and used as the impedance value of Z1. During the formation process, gas is removed to prevent air bubbles from adhering to the negative electrode sheet.

[0192] Based on the positive and negative electrode sheets included in the embodiments, a second electrolyte is used, and the battery is charged at 1C to 2C for 2.5 to 5 minutes to form a lithium-ion battery. The lithium-ion battery is then adjusted to 50% SOC, and the negative electrode sheet is removed to create a symmetrical battery. Its EIS is measured and used as the impedance value of Z2.

[0193] EIS testing was performed using an electrochemical workstation with a test frequency of 3mHz-500kHz and a perturbation voltage of 5mV.

[0194] 2. Cycle performance testing of lithium-ion batteries

[0195] At 45°C, the lithium-ion battery was left to stand for 30 minutes, then charged at a constant current of 3C to a voltage of 4.2V, and further charged at a constant voltage of 4.2V to a current of 0.05C. After standing for 5 minutes, it was discharged at a constant current of 3C to a voltage of 2.8V. This is one charge-discharge cycle. The discharge capacity of this cycle is the battery's first discharge capacity. The lithium-ion battery was subjected to 1000 charge / discharge cycles according to the above method, and the discharge capacity of the lithium-ion battery after the 1000th cycle was recorded.

[0196] The capacity retention rate (%) of a lithium-ion battery after 1000 cycles = (discharge capacity of the 1000th cycle / discharge capacity of the first cycle) × 100%.

[0197] 3. DC discharge internal resistance (DCR) test of lithium-ion batteries

[0198] The test conditions were -10℃. The lithium-ion battery was discharged at 50% SOC for 10 seconds at a 10C rate (constant current denoted as I). The voltage drop before and after discharge was ΔV. DCR = ΔV / I, where I is the current.

[0199] 4. Interface observation of the negative electrode sheet

[0200] The lithium-ion battery that has completed its first formation is fully charged and disassembled to observe the gap at the interface of the negative electrode and whether the gap distribution is uniform.

[0201] Performance results:

[0202] Table 2

[0203]

[0204] As shown in Tables 1 and 2, the DCR of Examples 2 to 5 and Comparative Examples 1 to 3 was calculated with the DCR of Example 1 as 100% as the baseline.

[0205] In Comparative Example 1, during the formation process, air bubbles on the surface of the negative electrode sheet were largely removed. Due to the absence of gaps, films were formed on all surfaces of the negative electrode sheet during both formation processes. Z1 and Z2 show that the film layer is relatively dense, resulting in a high DCR; however, this film layer also provides good protection for the negative electrode sheet and exhibits excellent cycle life. Z1 and Z2 are samples taken from different regions of the SEI film for testing.

[0206] Comparative Example 2 uses only the first electrolyte for formation, resulting in relatively poor density and relatively poor protection for the negative electrode. Bubbles may exist on the surface of the negative electrode, and because a second formation is not performed, film is formed only in areas of the negative electrode that do not contain bubbles, while areas containing bubbles do not form a film. Therefore, areas containing bubbles continuously consume active lithium during cycling, leading to a poor lifespan.

[0207] In Comparative Example 3, because bubbles are attached to part of the surface of the negative electrode, during the first formation using the first electrolyte, the first electrolyte does not easily form a film in the bubble region, but mainly forms a film in the region without bubbles, forming the first film layer. During the second formation using the second electrolyte, a second film layer forms on the surface of the first film layer away from the negative electrode, and a second region is formed in the original bubble region. The first film layer and the second film layer constitute the first region.

[0208] Compared to Comparative Example 3, in Examples 1 to 5, Z1 / Z2≥1.5, the film density of the first region in Examples 1 to 5 is relatively high and the film is thicker; the film of the second region is relatively loose and the film is thinner, so that the first region and the second region work together to balance the cycle life and DCR of the lithium-ion battery.

[0209] Especially during the first formation process, vibrating the lithium-ion battery can make the bubbles attached to the surface of the negative electrode uniformly distributed, which can further improve the DCR and cycle life of the lithium-ion battery.

[0210] Although this application has been described with reference to preferred embodiments, various modifications can be made thereto and components can be replaced with equivalents without departing from the scope of this application. In particular, the technical features mentioned in the various embodiments can be combined in any manner, provided there is no structural conflict. This application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A secondary battery, characterized in that, The device includes a negative electrode sheet, on the surface of which a solid electrolyte interface film is formed. The solid electrolyte interface film includes a first region portion and a second region portion that are connected to each other. The impedance value of the first region portion is Z1, and the impedance value of the second region portion is Z2, as determined by AC impedance spectroscopy, wherein Z1 / Z2≥1.

5. The first region is formed in a continuous layered structure on a portion of the surface of the negative electrode sheet; The second region is configured as a plurality of portions, which are discretely distributed in the first region and formed on another portion of the surface of the negative electrode sheet.

2. The secondary battery according to claim 1, characterized in that, The impedance value Z1 of the first region satisfies 4mΩ ≤ Z1 ≤ 11mΩ; and / or The impedance value Z2 of the second region satisfies 0.7mΩ≤Z2≤7mΩ.

3. The secondary battery according to claim 1, characterized in that, Multiple second region portions are evenly distributed within the first region portion.

4. The secondary battery according to claim 1, characterized in that, Based on the mass meter of the first region portion, the first region portion contains an organic component with a mass percentage of A1 and an inorganic component with a mass percentage of B1. Based on the mass meter of the second region, the second region comprises an organic component with a mass percentage of A2 and an inorganic component with a mass percentage of B2. Where A1 > A2; and / or B1 < B2.

5. The secondary battery according to claim 1, characterized in that, The negative electrode sheet includes a negative electrode active material, which includes one or more of artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate; and / or, The secondary battery also includes an electrolyte, which comprises an organic solvent, an electrolyte lithium salt dissolved in the organic solvent, and additives.

6. The secondary battery according to claim 5, characterized in that, The electrolyte lithium salt is selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate; and / or, The organic solvent is selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone; and / or, The additives include one or more of fluoroethylene carbonate, vinylene carbonate, and ethylene sulfate.

7. A battery module, characterized in that, Includes the secondary battery as described in any one of claims 1 to 6.

8. A battery pack, characterized in that, Includes the battery module as described in claim 7.

9. An electrical device, characterized in that, It includes the secondary battery as described in any one of claims 1 to 6, the battery module as described in claim 7, or the battery pack as described in claim 8.

10. A method for preparing a secondary battery, characterized in that, include: A battery structure is provided, the battery structure including a housing assembly and a positive electrode, a separator and a negative electrode disposed within the housing assembly; A first electrolyte is added inside the housing assembly so that the first electrolyte forms a first film layer on a portion of the surface of the negative electrode sheet; A second electrolyte is added within the housing assembly, such that the second electrolyte forms a second region on another portion of the surface of the negative electrode sheet, and a second film layer is formed on the surface of the first film layer opposite to the negative electrode sheet. The second region and the second film layer together constitute a continuous film layer. The first film layer and the second film layer constitute the first region.

11. The method according to claim 10, characterized in that, The step of adding a first electrolyte into the housing assembly to form a first film layer on a portion of the surface of the negative electrode sheet includes: A first electrolyte is added into the housing assembly, and the positive electrode and the negative electrode are charged with a first constant current so that the first electrolyte forms a first film layer on a portion of the surface of the negative electrode.

12. The method according to claim 11, characterized in that, In the step of forming the first film layer, The charging current of the first constant current is 0.3C~0.5C, and the charging time is 80min~140min.

13. The method according to claim 11, characterized in that, In the step of forming the first film layer, when the positive electrode and the negative electrode are charged with a first constant current, the inside of the housing assembly is connected to an external vacuum suction source to extract the gas inside the housing assembly until the charging is finished, at which point the gas extraction stops.

14. The method according to claim 13, characterized in that, The outer casing assembly is connected to an external vacuum suction source to make the vacuum level inside the outer casing assembly -20KPa to -60KPa.

15. The method according to claim 11, characterized in that, During the first constant current charging of the positive electrode and the negative electrode, the housing assembly is vibrated to make bubbles evenly distributed on the surface of the negative electrode.

16. The method according to claim 15, characterized in that, The housing assembly is subjected to ultrasonic vibration.

17. The method according to claim 11, characterized in that, The step of adding a second electrolyte into the housing assembly to form a second region on another portion of the surface of the negative electrode sheet, and forming a second film layer on the surface of the first film layer opposite to the negative electrode sheet, includes: A second electrolyte is added inside the housing assembly, and the positive electrode and the negative electrode are charged with a second constant current, so that the second electrolyte forms a second region on another part of the surface of the negative electrode, and the first film layer forms a second film layer opposite to the surface of the negative electrode, and the charging current of the second constant current is greater than the charging current of the first constant current.

18. The method according to claim 17, characterized in that, In the steps of forming the second film layer and the second region portion: The second constant current charging current is 1C~2C, and the charging time is 2.5min~5min.

19. The method according to claim 17, characterized in that, In the steps of forming the second film layer and the second region portion, when the positive electrode and the negative electrode are charged with a second constant current, the inside of the housing assembly is connected to an external vacuum suction source to extract gas from the inside of the housing assembly until the end, at which point the gas extraction is stopped.

20. The method according to claim 19, characterized in that, The outer casing assembly is connected to an external vacuum suction source to make the vacuum level inside the outer casing assembly -20KPa to -60KPa.

21. The method according to any one of claims 10 to 20, characterized in that, The first electrolyte and the second electrolyte have the same composition.

22. The method according to any one of claims 10 to 20, characterized in that, Based on the mass of the first membrane layer, the first membrane layer comprises an inorganic component with a mass percentage of C1 and an organic component with a mass percentage of D1. Based on the mass of the second membrane layer, the second membrane layer comprises an inorganic component with a mass percentage of C2 and an organic component with a mass percentage of D2. Where C1 < C2; and / or D1 > D2.

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