Secondary battery, preparation method thereof, energy storage system and electrical equipment

Abstract

The embodiments of the present application relate to the field of secondary batteries, and provide a secondary battery, a preparation method thereof, an energy storage system, and an electrical device. The secondary battery includes a positive electrode sheet, a negative electrode sheet, and a diaphragm. The diaphragm is arranged between the negative electrode sheet and the positive electrode sheet. The diaphragm includes: a first composite layer having a first surface and a second surface arranged opposite to each other; two second composite layers, respectively arranged on the first surface and the second surface; the first composite layer and the second composite layer both have mutually perpendicular length directions and width directions; the second composite layer is flush with both side edges of the first composite layer in the length direction; the width of the second composite layer is smaller than that of the first composite layer, and both sides of the second composite layer in the width direction are located within the edges of the first composite layer. The embodiments of the present application design a diaphragm structure, so that while maintaining its functionality, it exhibits more uniform thermal conductivity during the application of the secondary battery, thereby improving the electrical performance and cycle performance of the battery.

Description

Technical Field

[0001] The present application relates to the field of secondary batteries, and in particular to a secondary battery, a preparation method thereof, an energy storage system, and electrical equipment. Background Art

[0002] Secondary batteries, also known as rechargeable batteries or storage batteries, are devices that can store and release electrical energy through electrochemical reactions. Due to their recyclable nature, they are widely used in modern society. As a key component of aqueous secondary batteries, the diaphragm primarily separates the positive and negative electrodes to prevent short circuits while allowing ions to pass through. Traditional diaphragm materials, such as polymer microporous membranes such as polyethylene (PE) and polypropylene (PP), are widely used in commercial batteries due to their excellent electrolyte wettability and electrical resistance, as well as their high mechanical strength and low cost.

[0003] However, existing secondary batteries, especially under high-power operating conditions, suffer from poor thermal management, which threatens the overall performance and safety of the battery. Summary of the Invention

[0004] The embodiments of the present application provide a secondary battery, a preparation method thereof, an energy storage system, and an electrical device to solve the problems of poor energy efficiency and cyclability of secondary batteries in the prior art.

[0005] According to some embodiments of the present application, in a first aspect of the embodiments of the present application, a secondary battery is provided, comprising a positive electrode sheet, a negative electrode sheet and a diaphragm, wherein the diaphragm is arranged between the negative electrode sheet and the positive electrode sheet, and the diaphragm comprises: a first composite layer having a first surface and a second surface arranged opposite to each other; two second composite layers, respectively arranged on the first surface and the second surface; the first composite layer and the second composite layer both have a length direction and a width direction perpendicular to each other; the second composite layer is flush with both side edges of the first composite layer in the length direction; the width of the second composite layer is smaller than that of the first composite layer, and both sides of the second composite layer in the width direction are located within the edges of the first composite layer; wherein the first composite layer and the second composite layer both contain polymers and heat-conductive inorganic particles, and the thermal conductivity of the second composite layer is greater than that of the first composite layer.

[0006] In some embodiments, the width of the first composite layer is denoted as W1; the width of the second composite layer is denoted as W2; and W2 / W1=0.25-0.75.

[0007] In some embodiments, the diaphragm also includes two third composite layers, and the two third composite layers are arranged in one of the following ways: Way 1, one third composite layer is located between a second composite layer and a first composite layer, wherein another third composite layer is located between another second composite layer and the first composite layer; or, Way 2, the two third composite layers are respectively located on the side of the two second composite layers away from the first composite layer, and the third composite layer simultaneously covers at least part of the surface of the second composite layer and at least part of the surface of the first composite layer; or, Way 3, the two third composite layers are respectively located on the first surface and the second surface, and the third composite layer is flush with the upper surface of the second composite layer away from the first composite layer.

[0008] In some embodiments, in Method 1 and Method 2, the length of the third composite layer is smaller than that of the first composite layer, and one side of the third composite layer in the length direction is flush with the edge of the first composite layer; the third composite layer is flush with both side edges of the first composite layer in the width direction; in Method 3, each third composite layer includes two parts, and the two parts are respectively located on both sides of the second composite layer along the width direction; wherein, each third composite layer is located on one side of the first composite layer and is flush with the edge of the first composite layer on that side; the length of each third composite layer is smaller than that of the first composite layer.

[0009] In some embodiments, in the diaphragm, in the first composite layer, the content of the thermally conductive inorganic particles is 0.1 wt%~2.5 wt%; and / or, in the second composite layer, the content of the thermally conductive inorganic particles is 2.5 wt%~8.0 wt%; and / or, in the third composite layer, the content of the thermally conductive inorganic particles is 0.5 wt%~1.0 wt%.

[0010] In some embodiments, in the diaphragm, the thermally conductive inorganic particles are nano-aluminum oxide and nano-aluminum nitride, and the particle size of the nano-aluminum oxide is 50±10 nm, and the particle size of the nano-aluminum nitride is 20±10 nm.

[0011] In some embodiments, in the first composite layer, the second composite layer, and the third composite layer, the weight ratio of nano-aluminum oxide to nano-aluminum nitride is independently (1-10):1.

[0012] According to some embodiments of the present application, the second aspect of the embodiments of the present application provides a method for preparing the above-mentioned secondary battery, including a preparation process of a diaphragm, and the preparation process of the diaphragm includes: step S1, preparing a first mixed liquid with heat-conductive inorganic particles and a first organic solvent; preparing a second mixed liquid with heat-conductive inorganic particles, a polymer and a second organic solvent; step S2, preparing a polymer-based membrane with a porous structure, and immersing the polymer-based membrane in the first mixed liquid, and forming a first membrane layer through solid-liquid separation; step S3, using a mask to cover partial areas of the surface on both sides of the first membrane layer respectively; coating the second mixed liquid on the surface of the first membrane layer not covered by the mask, and forming a second membrane layer after molding and extraction in sequence, thereby obtaining an intermediate product; step S4, the intermediate product is stretched to obtain a diaphragm.

[0013] In some embodiments, in step S1, in the first mixed liquid, the solid content of the thermally conductive inorganic particles is 20%-25%; in the second mixed liquid, the weight ratio of the thermally conductive inorganic particles, the polymer, and the second organic solvent is (2.5-8.0):100:100.

[0014] In some embodiments, before or after preparing the second film layer, the preparation process of the diaphragm also includes the preparation of a third film layer. The preparation process of the third film layer includes: preparing a third mixed liquid with thermally conductive inorganic particles, a polymer and a third organic solvent; and preparing the third film layer by coating, forming and extracting in sequence according to the third composite layer to be formed, thereby obtaining an intermediate product.

[0015] In some embodiments, in the third mixed liquid, the weight ratio of the thermally conductive inorganic particles, the polymer, and the third organic solvent is (1.5-3.0):100:100.

[0016] In some embodiments, in step S3, the extraction is performed using dichloromethane as the extraction liquid, and the extraction is performed at 42±2°C for 5 min to 20 min; in step S4, the stretching is performed at 100°C to 120°C.

[0017] In some embodiments, the method for preparing a secondary battery further includes a process of preparing a roll core by winding the positive electrode sheet, the negative electrode sheet, and the separator; the winding is started from the edge of the side where the third composite layer is provided.

[0018] According to some embodiments of the present application, a third aspect of the embodiments of the present application provides an energy storage system, comprising at least one secondary battery, which is the secondary battery described above.

[0019] According to some embodiments of the present application, a fourth aspect of the embodiments of the present application provides an electrical device, including an energy storage system, which is the above-mentioned energy storage system.

[0020] The technical solution provided by the embodiments of this application has at least the following advantages: by designing a separator composed of a first composite layer and two second composite layers, while strictly controlling the structure, size, position, and thermal conductivity relationship of the two composite layers, the resulting separator has enhanced thermal conductivity in the central region, while maintaining lower thermal conductivity and higher structural stability in the edge regions. This allows for more uniform thermal conductivity during secondary battery use while maintaining functionality, thereby improving the battery's electrical performance and cycling performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] One or more embodiments are exemplified by the figures in the corresponding drawings. These exemplified descriptions do not constitute a limitation on the embodiments. Unless otherwise stated, the figures in the drawings do not constitute a scale limitation. In order to more clearly illustrate the embodiments of the present application or the technical solutions in the traditional technology, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0022] Figure 1 This is a schematic top view of the structure of the diaphragm obtained in Example 1 of the present application;

[0023] Figure 2 This is a schematic front view of the structure of the diaphragm obtained in Example 1 of the present application;

[0024] Figure 3 This is a schematic top view of the structure of the diaphragm obtained in Example 2 of this application using the first method;

[0025] Figure 4 This is a schematic front view of the structure of the diaphragm obtained in Example 2 of this application using the first method;

[0026] Figure 5 This is a schematic top view of the structure of the diaphragm obtained in Example 3 of this application using the second method;

[0027] Figure 6 This is a schematic front view of the structure of the diaphragm obtained in Example 3 of this application using the second method;

[0028] Figure 7 This is a schematic top view of the structure of the diaphragm obtained in Example 4 of this application using the third method;

[0029] Figure 8 This is a schematic front view of the structure of the diaphragm obtained in Example 4 of this application using the third method.

[0030] The above drawings include the following reference numerals:

[0031] 10. First composite layer; 20. Second composite layer; 30. Third composite layer. DETAILED DESCRIPTION

[0032] As can be seen from the background technology, in order to solve the problem of inefficient heat conduction caused by the above-mentioned difference in thermal conductivity, researchers are actively exploring new diaphragm materials, such as ceramic coated diaphragms, composite diaphragms, etc. These new materials have higher mechanical strength and better thermal stability, which can improve thermal conductivity to a certain extent and reduce the risk of thermal runaway. On the other hand, the optimization of processing technology has also become the key to improving diaphragm performance, including enhancing the thermal conductivity of the diaphragm by controlling the pore structure and introducing thermal conductive additives. Despite this, existing diaphragm materials and processing technologies still have limitations. For example, although ceramic coating can improve thermal stability, it often sacrifices the battery's cycle performance and energy density; the thermal conductivity of traditional diaphragm materials is difficult to significantly improve; and in large-size batteries, due to the increase in the contact area between the electrode and the diaphragm, the thermal conductivity of the diaphragm has a more significant impact on the overall thermal management of the battery. In other words, the secondary batteries in the prior art have uneven internal heat conduction, resulting in poor energy efficiency and cyclability.

[0033] The first aspect of the embodiment of the present application provides a secondary battery, comprising a positive electrode sheet, a negative electrode sheet and a separator, wherein the separator is arranged between the negative electrode sheet and the positive electrode sheet. Figure 1 As shown, the diaphragm includes: a first composite layer 10 having a first surface and a second surface arranged opposite to each other; two second composite layers 20, respectively arranged on the first surface and the second surface; the first composite layer 10 and the second composite layer 20 both have a length direction and a width direction perpendicular to each other; the second composite layer 20 is flush with the two side edges of the first composite layer 10 in the length direction; the width of the second composite layer 20 is smaller than that of the first composite layer 10, and both sides of the second composite layer 20 in the width direction are located within the edges of the first composite layer 10; wherein, the first composite layer 10 and the second composite layer 20 both contain polymers and thermally conductive inorganic particles, and the thermal conductivity of the second composite layer 20 is greater than that of the first composite layer 10.

[0034] The present application designs a diaphragm composed of a first composite layer and two second composite layers, and strictly defines the structure, size, position and thermal conductivity relationship of the two composite layers, so that the thermal conductivity of the central area of ​​the resulting diaphragm is enhanced, while the edge area maintains lower thermal conductivity and higher structural stability. Specifically, the first composite layer provides the basic mechanical strength and pore structure for the diaphragm, while the design of the second composite layer achieves the enhancement of the thermal conductivity of the central area of ​​the diaphragm without affecting the stability and safety of the overall structure. In particular, since the second composite layer has a higher thermal conductivity, the heat distribution between the electrode sheets is more uniform. This helps to reduce the temperature gradient inside the battery, thereby avoiding local overheating during the charge and discharge process and improving the overall thermal management performance of the battery. The improvement in the uniformity of the thermal conductivity of the diaphragm directly promotes the consistency of the battery's electrochemical reactions, especially in large-capacity energy storage batteries, which helps to improve the cycle life and energy density of the battery.

[0035] Furthermore, to avoid ambiguity, the length direction of the aforementioned diaphragm and each composite layer provided herein is essentially the length direction of the diaphragm after unwinding from a rolled state. Furthermore, in actual applications, due to the limitations of current production processes, there will inevitably be some error in the alignment of the edges of the composite layers. Therefore, even slight gaps and deviations are considered to be aligned.

[0036] Based on the above thermal conductivity design, the width of the first composite layer 10 is further denoted as W1; the width of the second composite layer 20 is denoted as W2. In order to more effectively optimize the distribution of the thermally conductive material and achieve a better thermal conduction balance, W2 / W1 is preferably 0.25~0.75, and more preferably 0.5±0.02.

[0037] Furthermore, the central axes of the second composite layer 20 and the first composite layer 10 along the length direction are parallel to each other, and the common plane of the two central axes is perpendicular to the first surface. In other words, the two composite layers are preferably arranged neatly along the length direction, which helps to further improve the overall symmetry, structural consistency, and thermal conductivity uniformity of the diaphragm.

[0038] In some embodiments, the diaphragm further includes two third composite layers 30, and the two third composite layers 30 are arranged in one of the following ways: Way 1, one third composite layer 30 is located between a second composite layer 20 and a first composite layer 10, wherein another third composite layer 30 is located between another second composite layer 20 and the first composite layer 10; or, Way 2, the two third composite layers 30 are respectively located on the side of the two second composite layers 20 away from the first composite layer 10, and the third composite layer 30 simultaneously covers at least part of the surface of the second composite layer 20 and at least part of the surface of the first composite layer 10; or, Way 3, the two third composite layers 30 are respectively located on the first surface and the second surface, and the third composite layer 30 is flush with the upper surface of the second composite layer 20 away from the first composite layer 10.

[0039] Among the three preferred structural settings mentioned above, Method 1 further enhances the interface bonding between the second composite layer and the first composite layer, while introducing an additional heat conduction path, thereby significantly improving the overall thermal conductivity of the diaphragm. Method 2 not only enhances heat conduction by covering the second composite layer and part of the surface of the first composite layer with a third composite layer, but also optimizes the edge sealing performance of the diaphragm and reduces electrolyte leakage during application. Method 3 forms a new and simpler surface structure for industrial application by setting a third composite layer flush with the second composite layer. It not only optimizes the heat conduction path, but also further improves the flatness of the diaphragm and the contact performance of the electrode sheet, and the electrical performance of the secondary battery obtained subsequently is more superior.

[0040] With respect to the above-mentioned preferred configurations, in order to further enhance the optimization and enhancement effect of the obtained diaphragm structure on the overall uniformity of thermal conductivity inside the battery, thereby further enhancing the various performances of the obtained secondary battery, especially the cycle stability, it is specifically preferred that: in Method 1 and Method 2, the length of the third composite layer 30 is less than the first composite layer 10, and one side of the third composite layer 30 in the length direction is flush with the edge of the first composite layer 10; the third composite layer 30 is flush with the two side edges of the first composite layer 10 in the width direction; in Method 3, each third composite layer 30 includes two parts, and the two parts are respectively located on both sides of the second composite layer 20 along the width direction; wherein, each third composite layer 30 is located on one side of the first composite layer 10, and is flush with the edge of the first composite layer 10 on that side; the length of each third composite layer 30 is less than the first composite layer 10.

[0041] In some embodiments, the length of the first composite layer 10 is L1, and the length of each third composite layer 30 is L3, where L3 / L1 = 0.2-0.25. During battery operation, ion migration and re-migration are accompanied by energy conversion, which in turn releases heat. Because the inner region of the core is where the cell winding begins, the material packing density is relatively high, and electrochemical reactions are more concentrated, making it more susceptible to heat generation. In other words, the inner region of the wound cell is often a concentrated heat source. The third composite layer, in this way, precisely covers these critical areas, thereby more efficiently accelerating the conduction of heat energy in these areas while maintaining mechanical properties, reducing the formation of hot spots, and thus significantly improving the thermal management performance of the entire cell. Furthermore, to create a more continuous and effective heat conduction path and reduce unnecessary backflow or stagnation of heat flow within the cell, for method three, the gap between the third composite layer 30 and the second composite layer 20 is preferably zero.

[0042] In some embodiments, the content of the thermally conductive inorganic particles in the first composite layer 10 of the separator is 0.1 wt% to 2.5 wt%; and / or the content of the thermally conductive inorganic particles in the second composite layer 20 is 2.5 wt% to 8.0 wt%; and / or the content of the thermally conductive inorganic particles in the third composite layer 30 is 0.5 wt% to 1.0 wt%. This hierarchical filling strategy of thermally conductive particles, optimized by the inventors through extensive experimentation, improves the overall performance of the separator while specifically enhancing the thermal conductivity of the electrode interface and key heat source areas, thereby more effectively balancing the temperature within the battery cell and improving the stability and efficiency of the battery.

[0043] Furthermore, to promote uniform particle dispersion within the separator, reduce pore clogging, and effectively improve the thermal conductivity and uniformity of the resulting separator, the thermally conductive inorganic particles are preferably nano-alumina and nano-aluminum nitride, with the particle size of the nano-alumina being 50±10nm and the particle size of the nano-aluminum nitride being 20±10nm. The polymer is preferably polyethylene. To better balance the mechanical properties and thermal conductivity uniformity of the resulting separator, the weight ratio of nano-alumina to nano-aluminum nitride in the first composite layer 10, the second composite layer 20, and the third composite layer 30 is preferably (1-10):1.

[0044] In some embodiments, the weight ratio of nano-aluminum oxide to nano-aluminum nitride in the first composite layer 10 is (4-9):1, preferably (8-9):1; the weight ratio of nano-aluminum oxide to nano-aluminum nitride in the second composite layer 20 is (1.5-4):1, more preferably (3-4):1; and the weight ratio of nano-aluminum oxide to nano-aluminum nitride in the third composite layer 30 is (2-3):1, more preferably (2-2.5):1. Through extensive experimentation, the inventors have optimized the weight ratio of the different thermally conductive particles described above based on the functions and positions of the different composite layers, thereby achieving a better improvement in thermal conductivity uniformity and ultimately significantly enhancing the cycling stability of the resulting secondary battery.

[0045] Regarding the thickness of each composite layer, the preferred thickness of the first composite layer 10 is 13±3 μm, the thickness of the second composite layer 20 is 1 μm to 3 μm, and the thickness of the third composite layer 30 is 0.5 μm to 1 μm. The above-selected thickness setting scheme can better balance the mechanical strength of the resulting separator and the overall thermal management effect, so that the resulting battery cell can still maintain a stable temperature state during high power density discharge, further improving the reliability and service life of the battery cell.

[0046] The second aspect of the embodiment of the present application provides a preparation method of the above-mentioned secondary battery, including a preparation process of the diaphragm, and the preparation process of the diaphragm includes: step S1, preparing a first mixed liquid with heat-conductive inorganic particles and a first organic solvent; preparing a second mixed liquid with heat-conductive inorganic particles, a polymer and a second organic solvent; step S2, preparing a polymer-based membrane with a porous structure, and immersing the polymer-based membrane in the first mixed liquid, and forming a first membrane layer through solid-liquid separation; step S3, using a mask to cover partial areas of the surface on both sides of the first membrane layer respectively; coating the second mixed liquid on the surface of the first membrane layer not covered by the mask, and forming a second membrane layer after molding and extraction in sequence, thereby obtaining an intermediate product; step S4, stretching the intermediate product to obtain a diaphragm.

[0047] With respect to the diaphragm in the aforementioned secondary battery, the present application accordingly provides a preparation method thereof. After preparing the two mixed solutions, a non-uniform heat-conducting layer is formed on the surface of the diaphragm using a masking technique, thereby significantly improving the thermal conductivity of the diaphragm. This diaphragm preparation method can effectively reduce the temperature gradient within the battery cell and improve the thermal stability of the battery cell, thereby improving the battery's charge and discharge efficiency and extending the battery's service life. At the same time, the above-mentioned preparation method can also be well adapted to existing diaphragm production processes, facilitating industrial scale-up and practical application.

[0048] In some embodiments, in step S1, the solid content of the thermally conductive inorganic particles in the first mixed liquid is 20%-25%; and / or, in the second mixed liquid, the weight ratio of the thermally conductive inorganic particles, polymer, and second organic solvent is (2.5-8.0):100:100. By optimizing the relationship between the component amounts in the two mixed liquids, the uniform distribution of the thermally conductive inorganic particles in each composite layer of the separator can be further promoted, thereby forming a separator with more uniform thermal conductivity, higher mechanical strength, and better electrolyte compatibility. Furthermore, preferably, the first organic solvent is dichloromethane; and / or the second organic solvent is white wax oil, but these are not limited to these two types.

[0049] In some embodiments, before or after preparing the second film layer, the preparation process of the diaphragm also includes the preparation of a third film layer. The preparation process of the third film layer includes: preparing a third mixed liquid with thermally conductive inorganic particles, a polymer and a third organic solvent; and preparing the third film layer by coating, forming and extracting in sequence according to the third composite layer 30 to be formed, thereby obtaining an intermediate product.

[0050] During the formation of the third composite layer, while taking into account the mechanical properties of the first and second composite layers, in order to more efficiently optimize the thermal conductivity uniformity of the resulting separator and thereby extend the service life of the resulting secondary battery, the weight ratio of the thermally conductive inorganic particles, polymer, and third organic solvent in the third mixed solution is preferably (1.5-3.0):100:100. Furthermore, the third organic solvent is preferably white wax oil, but is not limited to this.

[0051] In the process of extracting the second film layer and / or the third film layer, in order to reduce the structural damage of the diaphragm during the extraction process, better maintain the porosity and thermal conductivity of the diaphragm, and thus more effectively improve the overall performance and safety of the secondary battery, it is preferred that in step S3, the extraction is carried out with dichloromethane as the extraction liquid, and the extraction is carried out at 42±2°C, and the extraction time is 5min~20min. In step S4, stretching is carried out at 100°C~120°C. This preferred stretching temperature can more effectively maintain the good structural stability and pore morphology of the diaphragm during the stretching process, while repairing the internal defects that may be generated in the previous coating extraction process, thereby more significantly improving the thermal conductivity of the obtained diaphragm and the safety of the subsequent secondary battery.

[0052] In some embodiments, the secondary battery preparation method further includes a process of preparing a core by winding the positive electrode sheet, the negative electrode sheet, and the separator; the winding is performed starting from the edge of the side where the third composite layer 30 is disposed. Because the side where the third composite layer 30 is disposed has superior thermal conductivity, starting the winding from this edge ensures that the core portion of the resulting battery cell has superior thermal conductivity, thereby providing more sustained, efficient, and stable thermal conductivity and mechanical support during the battery's charge and discharge processes, significantly improving the performance and reliability of the resulting secondary battery.

[0053] In a third aspect, embodiments of the present application provide an energy storage system comprising at least one secondary battery, the secondary battery being the aforementioned secondary battery. By utilizing the secondary battery having the high thermal conductivity separator provided above, the energy storage system can more effectively manage internal heat, reduce the risk of thermal runaway, and extend the overall service life of the system.

[0054] In a fourth aspect of the embodiments of the present application, an electrical device is provided, including an energy storage system, which is the above-mentioned energy storage system. This secondary battery with a high thermal conductivity separator, when combined with the optimized design of the energy storage system, can exhibit excellent performance in high-power output and high-energy density applications. The corresponding energy storage system, due to the high thermal conductivity and overall performance optimization of its internal battery separator, can significantly improve the energy efficiency and safety of the electrical device.

[0055] In the description of the embodiments of this application, the technical terms "first" and "second" are used only to distinguish different objects and should not be understood to indicate or imply relative importance or implicitly specify the quantity, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, the meaning of "plurality" is more than two, unless otherwise clearly and specifically defined.

[0056] References herein to "embodiments" mean that a particular feature, structure, or characteristic described in connection with the embodiments may be included in at least one embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute an independent or alternative embodiment that is mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0057] In the description of the embodiments of this application, the term "and / or" is simply a description of the association relationship between associated objects, indicating that three relationships can exist. For example, A and / or B can represent the following three situations: A exists, A and B exist at the same time, and B exists. In addition, the character " / " in this document generally indicates that the associated objects are in an "or" relationship.

[0058] In the description of the embodiments of the present application, the term "multiple" refers to more than two (including two). Similarly, "multiple groups" refers to more than two groups (including two groups), and "multiple pieces" refers to more than two pieces (including two pieces).

[0059] In the description of the embodiments of the present application, the technical terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing the embodiments of the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the embodiments of the present application.

[0060] In the description of the embodiments of the present application, unless otherwise expressly specified or limited, technical terms such as "installed," "connected," "connected," and "fixed" should be understood in a broad sense. For example, they can refer to fixed connections, detachable connections, or integration; mechanical connections or electrical connections; direct connections or indirect connections through an intermediate medium; internal connections between two components or interactions between two components. Those skilled in the art can understand the specific meanings of the above terms in the embodiments of the present application based on specific circumstances.

[0061] In the accompanying drawings corresponding to the embodiments of the present application, the thickness and area of ​​the layers are exaggerated for better understanding and ease of description. When describing a component (such as a layer, film, region, or substrate) as being on another component or on the surface of another component, the component may be "directly" located on the surface of the other component, or a third component may be present between the two components. Conversely, when describing a component as being on the surface of another component or as being formed or provided on the surface of a component, it means that there is no third component between the two components. In addition, when describing a component as being "substantially" formed on another component, it means that the component is not formed on the entire surface (or front surface) of the other component, nor is it formed on a portion of the edge of the entire surface.

[0062] In the description of the embodiments of this application, when a component "includes" another component, unless otherwise specified, other components are not excluded, and other components may be further included. In addition, when a component such as a layer, film, region, or plate is referred to as being "on / located on" another component, it can be "directly on" the other component (i.e., located on the surface of the other component with no other components between them) or another component can be present between them. In addition, when a component such as a layer, film, region, or plate is "directly on" another component, or when a component such as a layer, film, region, or plate is located on the surface of another component, it means that no other components are located between them.

[0063] The terms used herein in the description of the various embodiments are intended only to describe the specific embodiments and are not intended to be limiting. As used in the description of the various embodiments and the appended claims, "part" is intended to include the plural form unless the context clearly indicates otherwise. A component includes a layer, film, region, or plate.

[0064] The following detailed description of the various embodiments of the present application is provided in conjunction with the accompanying drawings. However, those skilled in the art will appreciate that many technical details are provided in the various embodiments of the present application to facilitate a better understanding of the present application. However, even without these technical details and the various variations and modifications based on the following embodiments, the technical solutions claimed in the present application can still be implemented.

[0065] Example 1

[0066] A method for preparing a secondary battery:

[0067] (1) Preparation of diaphragm:

[0068] (1-1) Preparing a first mixed solution: Weighing nano-alumina with a particle size of 50 nm and nano-aluminum nitride with a particle size of 20 nm in a weight ratio of nano-alumina:nano-aluminum nitride = 4:1, and adding them together as thermally conductive inorganic particles into dichloromethane to obtain a first mixed solution having a solid content of thermally conductive inorganic particles of 21%. Preparing a second mixed solution: Weighing nano-alumina with a particle size of 50 nm and nano-aluminum nitride with a particle size of 20 nm in a weight ratio of nano-alumina:nano-aluminum nitride = 1.5:1, and adding them together as thermally conductive inorganic particles into white wax oil. Simultaneously, PE particles were added to the white wax oil to obtain a second mixed solution. In the resulting second mixed solution, the weight ratio of the thermally conductive inorganic particles, PE polymer particles, and white wax oil was 8.0:100:100.

[0069] (1-2) Using white wax oil as a solvent, PE particles are extruded into a cast sheet by high-temperature extrusion. The white wax oil is then extruded through a dichloromethane solution to form a PE polymer-based membrane having a porous structure. The PE polymer-based membrane is immersed in the first mixed solution obtained above, and solid-liquid separation is performed to form a first membrane layer.

[0070] (1-3) According to Figure 1 、 2 In the structure shown, masks are used to cover portions of both sides of the resulting first film layer. The second mixed solution is then applied to the surface of the first film layer not covered by the mask. Forming and extraction are then performed to form a second film layer, thereby obtaining an intermediate product. The extraction is performed at 42°C for 15 minutes.

[0071] (1-4) The intermediate product was stretched at 110°C to obtain a separator, the top view of which is shown in FIG. Figure 1 , the front view shows Figure 2 .

[0072] The resulting diaphragm structure includes a 13μm-thick first composite layer containing 2.5wt% of thermally conductive inorganic particles. The first composite layer has a first surface and a second surface facing each other, each with two 2μm-thick second composite layers disposed on each surface. Both second composite layers contain 1.5wt% of thermally conductive inorganic particles. Furthermore, the widths W1 and W2 of the first and second composite layers satisfy the following relationship: W2 / W1 = 0.5.

[0073] (2) Assembly of secondary batteries: Using lithium iron phosphate as the positive electrode, graphite as the negative electrode, and the above-obtained diaphragm as the battery diaphragm, the positive electrode, negative electrode, and diaphragm are wound to form a core, with the winding being carried out along the length of the diaphragm. Then, using lithium hexafluorophosphate as the electrolyte, a lithium-ion battery with a capacity of 314 Ah is assembled.

[0074] Example 2

[0075] A method for preparing a secondary battery:

[0076] (1) Preparation of diaphragm:

[0077] (1-1) Preparing a first mixed solution: Weighing nano-alumina with a particle size of 50 nm and nano-aluminum nitride with a particle size of 20 nm in a weight ratio of 9:1, and adding them together as thermally conductive inorganic particles to dichloromethane, thereby obtaining a first mixed solution having a solid content of 21% thermally conductive inorganic particles. Preparing a second mixed solution: Weighing nano-alumina with a particle size of 50 nm and nano-aluminum nitride with a particle size of 20 nm in a weight ratio of 4:1, and adding them together as thermally conductive inorganic particles to white wax oil, while simultaneously adding PE particles to the white wax oil, thereby obtaining a second mixed solution. In the resulting second mixed solution, the weight ratio of the thermally conductive inorganic particles, PE polymer particles, and white wax oil is 2.5:100:100. Prepare the third mixed solution: 50nm nano-alumina and 20nm nano-aluminum nitride were weighed in a weight ratio of 2:1 and added to white wax oil as thermally conductive inorganic particles. PE particles were also added to the white wax oil to produce a third mixed solution. The weight ratio of the thermally conductive inorganic particles, PE polymer particles, and white wax oil in this third mixed solution was 1.5:100:100.

[0078] (1-2) Using white wax oil as a solvent, PE particles are extruded into a cast sheet by high-temperature extrusion. The white wax oil is then extruded through a dichloromethane solution to form a PE polymer-based membrane having a porous structure. The PE polymer-based membrane is immersed in the first mixed solution obtained above, and solid-liquid separation is performed to form a first membrane layer.

[0079] (1-3) According to Figure 3 、 4 The structure shown in FIG. 1 is constructed by using a mask to cover the partial areas of the surfaces of both sides of the obtained first film layer; and then the mask is used to cover the partial areas of the surfaces of the ... Figure 3 and Figure 4 The third composite layer 30 to be formed is then coated on the surface of the first film layer not covered by the mask. The third mixed solution is then formed and extracted to form the third film layer. The extraction is performed at 42°C for 15 minutes. The second mixed solution is then applied, formed, and extracted in the same manner to form the second film layer, thereby obtaining an intermediate product.

[0080] (1-4) The intermediate product was stretched at 110°C to obtain a separator, the top view of which is shown in FIG. Figure 3 , the front view shows Figure 4 .

[0081] The resulting diaphragm structure included a 13μm-thick first composite layer containing 1.5wt% of thermally conductive inorganic particles. The first composite layer had a first surface and a second surface facing each other, with two second composite layers disposed on each surface. Both second composite layers contained 8.0wt% of thermally conductive inorganic particles. Furthermore, the widths W1 and W2 of the first and second composite layers satisfied the relationship W2 / W1 = 0.5.

[0082] Based on the above structure, the resulting diaphragm structure also includes two third composite layers, each 1 μm thick. One third composite layer is positioned between one second composite layer and the first composite layer, and the other is positioned between the other second composite layer and the first composite layer. The thermally conductive inorganic particle content in each third composite layer is 0.5 wt %. The length L1 of the resulting first composite layer and the length L3 of the resulting third composite layer satisfy the following relationship: L3 / L1=0.2.

[0083] And, for the thickness of the two second composite layers, Figure 4 The portion directly in contact with the first composite layer has a thickness of 1 μm; the portion covering the third composite layer has a thickness of 2 μm.

[0084] (2) Assembly of the secondary battery: The assembly is consistent with that in Example 1, but the winding is started from the edge of the side where the third composite layer is provided.

[0085] Example 3

[0086] A method for preparing a secondary battery:

[0087] The only difference between this embodiment and embodiment 2 is that the positions of the two third composite layers are set differently. Figure 5 、 Figure 6 Set as shown.

[0088] In the resulting diaphragm structure, the thickness and positional relationship of the first and second composite layers remain consistent with those of Example 2. Furthermore, the resulting diaphragm structure further includes two third composite layers, each 0.5 μm thick, located on the side of the two second composite layers away from the first composite layer, and the third composite layers simultaneously cover at least a portion of the surface of the second composite layer and at least a portion of the surface of the first composite layer. The content of the thermally conductive inorganic particles in both third composite layers is 0.8 wt %. The length L1 of the resulting first composite layer and the length L3 of the resulting third composite layer satisfy the following relationship: L3 / L1=0.2.

[0089] Example 4

[0090] A method for preparing a secondary battery:

[0091] The only difference between this embodiment and embodiment 2 is that the positions of the two third composite layers are set differently. Figure 7 、 Figure 8 Set as shown.

[0092] In the resulting diaphragm structure, the thickness and positional relationship of the first and second composite layers remained consistent with those in Example 2. Furthermore, the resulting diaphragm structure further included two third composite layers, each of which was flush with the upper surface of the two second composite layers away from the first composite layer (i.e., the thickness of the third composite layer remained consistent with that of the second composite layer).

[0093] Each third composite layer consists of two sections, one located on either side of the second composite layer along the width direction. Each third composite layer is located on one side of the first composite layer and is flush with the edge of the first composite layer on that side. The thermally conductive inorganic particles in each third composite layer comprise 1 wt %. The length L1 of the resulting first composite layer and the length L3 of the resulting third composite layer satisfy the following equation: L3 / L1 = 0.25.

[0094] Furthermore, the third composite layer and the second composite layer are closely arranged, that is, the gap between them is zero.

[0095] Example 5

[0096] A method for preparing a secondary battery:

[0097] The only difference between this embodiment and embodiment 2 is that the relationship between the width W1 of the obtained first composite layer and the width W2 of the obtained second composite layer is changed to: W2 / W1=0.25.

[0098] Example 6

[0099] A method for preparing a secondary battery:

[0100] The only difference between this embodiment and embodiment 2 is that the relationship between the width W1 of the obtained first composite layer and the width W2 of the obtained second composite layer is changed to: W2 / W1=0.75.

[0101] Example 7

[0102] A method for preparing a secondary battery:

[0103] The only difference between this embodiment and embodiment 2 is that the relationship between the length L1 of the first composite layer and the length L32 of the third composite layer is changed to: L3 / L1=0.1.

[0104] Example 8

[0105] A method for preparing a secondary battery:

[0106] The only difference between this embodiment and embodiment 2 is that the relationship between the length L1 of the obtained first composite layer and the length L3 of the obtained third composite layer is changed to: L3 / L1=0.5.

[0107] Example 9

[0108] A method for preparing a secondary battery:

[0109] The only difference between this embodiment and embodiment 2 is that the content of the thermally conductive inorganic particles in the first composite layer is changed to 5.0 wt %; the content of the thermally conductive inorganic particles in the two second composite layers is changed to 10.0 wt %; and the content of the thermally conductive inorganic particles in the two third composite layers is changed to 2.0 wt %.

[0110] Example 10

[0111] A method for preparing a secondary battery:

[0112] The only difference between this embodiment and embodiment 2 is that the weight ratio of nano-aluminum oxide to nano-aluminum nitride in the first mixed solution is changed to 3:1; the weight ratio of nano-aluminum oxide to nano-aluminum nitride in the second mixed solution is changed to 1:1; and the weight ratio of nano-aluminum oxide to nano-aluminum nitride in the third mixed solution is changed to 4:1.

[0113] Comparative Example 1

[0114] A method for preparing a secondary battery:

[0115] The only difference between this comparative example and Example 1 is that no second composite layer is provided.

[0116] Comparative Example 2

[0117] A method for preparing a secondary battery:

[0118] The only difference between this comparative example and Example 1 is that the lengths and widths of the two oppositely disposed second composite layers are changed to be consistent with those of the first composite layer.

[0119] Comparative Example 3

[0120] A method for preparing a secondary battery:

[0121] The only difference between this comparative example and Example 1 is that the thermal conductivity coefficients of the two oppositely disposed second composite layers are changed to be consistent with that of the first composite layer.

[0122] Comparative Example 4

[0123] A method for preparing a secondary battery:

[0124] The only difference between this comparative example and Example 1 is that the thermal conductivity coefficients of the two oppositely disposed second composite layers are both changed to be smaller than the thermal conductivity coefficient of the first composite layer.

[0125] Comparative Example 5

[0126] A method for preparing a secondary battery:

[0127] The only difference between this comparative example and Example 1 is that an existing commercial PE coated separator is used instead of the separator prepared above, and a lithium ion battery with a capacity of 314 Ah is obtained by assembling.

[0128] Secondary battery cell sample test method

[0129] In a 45°C constant temperature box, using a Nebula charge and discharge device (with a voltage and current accuracy of ±0.05% FS), the lithium-ion battery cell samples obtained in each embodiment and comparative example were charged and discharged at 0.5P with a cut-off voltage of 3.65V / 2.5V. The temperature range of the large surface of the aluminum shell of the battery cell and the energy efficiency of the battery cell of each sample were measured.

[0130] The calculation method of the temperature range of the large surface of the aluminum shell of the battery cell is as follows: at 45±2℃, the initial temperature T of the large surface of the aluminum shell of the battery cell of each battery cell sample is obtained by testing 初始 Then charge and discharge at 0.5P, and test the maximum temperature T of the aluminum shell of each battery sample during the discharge process. max The temperature difference of the large surface of the battery core aluminum shell is T max -T 初始 ; During the charging and discharging process, the calculation formula for the battery cell energy efficiency is: discharge energy / charging energy × 100%.

[0131] The above test results are shown in Table 1.

[0132] Table 1

[0133]

[0134] From the above description, it can be seen that, compared to Comparative Examples 1 to 4, the above-mentioned embodiments of the present application achieve the preparation of a composite separator with a unique structure. The resulting separator has enhanced thermal conductivity in the central region, while the peripheral regions maintain lower thermal conductivity and higher structural stability. Therefore, while maintaining its functionality, it exhibits more uniform thermal conductivity during secondary battery use, improving the battery's electrical performance and cycle performance.

[0135] Specifically, by comparing Examples 5 and 6 with Example 2, it can be seen that preferably setting the relationship between the width of the first composite layer W1 and the width of the second composite layer W2 to W2 / W1 = 0.5±0.02 can more effectively optimize the distribution of the thermally conductive material to achieve a better heat conduction balance, thereby making the final battery cell sample more uniform in heat conduction and higher in energy efficiency.

[0136] Comparing Examples 7 and 8 with Example 2, it can be seen that on the basis of introducing the third composite layer, the relationship between its length L3 and the length L1 of the first composite layer is further preferably L3 / L1=0.2~0.25, which can enable the obtained diaphragm to more efficiently accelerate the conduction of heat energy in these areas while maintaining mechanical properties, reduce the formation of hot spots, and reduce the temperature difference of the large surface of the aluminum shell of the battery cell where it is located, thereby more significantly improving the thermal management performance of the entire battery cell.

[0137] Comparing Example 9 with Example 2, it can be seen that for the three composite layers, by optimizing the graded filling strategy of the thermally conductive particles therein, it is possible to more effectively balance the temperature inside the battery cell while improving the overall performance of the diaphragm, thereby improving the stability and efficiency of the battery.

[0138] Comparing Example 10 with Example 2, it can be seen that for the three composite layers, further optimizing the weight matching relationship between different thermally conductive particles therein can achieve a better effect of improving thermal conductivity uniformity, and ultimately more significantly improve the thermal conductivity uniformity of the obtained secondary battery.

[0139] Those skilled in the art will appreciate that the above-described embodiments are specific examples for implementing the present application, and that in actual applications, various changes in form and detail may be made thereto without departing from the spirit and scope of the present application. Any person skilled in the art may make various changes and modifications without departing from the spirit and scope of the present application. Therefore, the scope of protection of the present application shall be based on the scope defined in the claims.

Claims

1. A secondary battery comprising a positive electrode sheet, a negative electrode sheet, and a separator, wherein the separator is disposed between the negative electrode sheet and the positive electrode sheet, wherein: The diaphragm comprises: A first composite layer (10) having a first surface and a second surface arranged opposite to each other; Two second composite layers (20), respectively arranged on the first surface and the second surface; The first composite layer (10) and the second composite layer (20) both have a length direction and a width direction that are perpendicular to each other; The second composite layer (20) is flush with both side edges of the first composite layer (10) in the length direction; the width of the second composite layer (20) is smaller than that of the first composite layer (10), and both sides of the second composite layer (20) in the width direction are located within the edges of the first composite layer (10); The width of the first composite layer (10) is denoted as W1; the width of the second composite layer (20) is denoted as W2, W2 / W1= 0.5±0.02; The diaphragm further comprises two third composite layers (30), and the two third composite layers (30) are arranged in one of the following ways: Mode 1: One of the third composite layers (30) is located between one of the second composite layers (20) and the first composite layer (10), wherein another of the third composite layers (30) is located between another of the second composite layers (20) and the first composite layer (10); or Mode 2: The two third composite layers (30) are respectively located on the side of the two second composite layers (20) away from the first composite layer (10), and the third composite layer (30) simultaneously covers at least part of the surface of the second composite layer (20) and at least part of the surface of the first composite layer (10); or, Mode three: the two third composite layers (30) are respectively located on the first surface and the second surface, and the third composite layer (30) is flush with the upper surface of the second composite layer (20) away from the first composite layer (10); The length of the first composite layer (10) is denoted as L1, the length of the third composite layer (30) is denoted as L3, and L3 / L1=0.2~0.25; The first composite layer (10) and the second composite layer (20) both contain polymers and thermally conductive inorganic particles, and the thermal conductivity of the second composite layer (20) is greater than the thermal conductivity of the first composite layer (10).

2. The secondary battery according to claim 1, wherein In the first and second methods, the length of the third composite layer (30) is smaller than that of the first composite layer (10), and one side of the third composite layer (30) in the length direction is flush with the edge of the first composite layer (10); the third composite layer (30) is flush with both side edges of the first composite layer (10) in the width direction; In the third method, each of the third composite layers (30) includes two parts, and the two parts are respectively located on both sides of the second composite layer (20) along the width direction; wherein, each of the third composite layers (30) is located on one side of the first composite layer (10) and is flush with the edge of the first composite layer (10) on that side; and the length of each of the third composite layers (30) is smaller than that of the first composite layer (10).

3. The secondary battery according to claim 1, wherein In the diaphragm, In the first composite layer (10), the content of the thermally conductive inorganic particles is 0.1 wt% to 2.5 wt%; and / or, In the second composite layer (20), the content of the thermally conductive inorganic particles is 2.5 wt% to 8.0 wt%; and / or, In the third composite layer (30), the content of the thermally conductive inorganic particles is 0.5 wt% to 1.0 wt%.

4. The secondary battery according to claim 3, wherein In the diaphragm, the thermally conductive inorganic particles are nano-aluminum oxide and nano-aluminum nitride, and the particle size of the nano-aluminum oxide is 50±10 nm, and the particle size of the nano-aluminum nitride is 20±10 nm.

5. The secondary battery according to claim 4, wherein In the first composite layer (10), the second composite layer (20), and the third composite layer (30), the weight ratio of the nano-aluminum oxide to the nano-aluminum nitride is independently (1-10):

1.

6. A method for preparing a secondary battery according to any one of claims 1 to 5, comprising a process for preparing the separator, characterized in that: The preparation process of the diaphragm includes: Step S1, preparing the thermally conductive inorganic particles and a first organic solvent into a first mixed liquid; preparing the thermally conductive inorganic particles, a polymer and a second organic solvent into a second mixed liquid; Step S2, preparing a polymer-based membrane with a porous structure, and immersing the polymer-based membrane in the first mixed liquid to form a first membrane layer through solid-liquid separation; Step S3, using a mask to cover partial areas of both sides of the first film layer; applying the second mixed liquid on the surface of the first film layer not covered by the mask, and forming and extracting the second film layer to obtain an intermediate product; Step S4: the intermediate product is subjected to stretching treatment to obtain the diaphragm.

7. The method for preparing a secondary battery according to claim 6, wherein: In the step S1, In the first mixed liquid, the solid content of the thermally conductive inorganic particles is 20% to 25%; In the second mixed liquid, the weight ratio of the thermally conductive inorganic particles, the polymer, and the second organic solvent is (2.5-8.0):100:

100.

8. The method for preparing a secondary battery according to claim 7, wherein: Before or after the second film layer is prepared, the preparation process of the diaphragm further includes the preparation of a third film layer, and the preparation process of the third film layer includes: preparing a third mixed liquid by preparing the thermally conductive inorganic particles, a polymer and a third organic solvent; According to the third composite layer (30) to be formed, coating, forming and extraction are carried out in sequence to prepare a third film layer, and then obtain the intermediate product.

9. The method for preparing a secondary battery according to claim 8, wherein: In the third mixed liquid, the weight ratio of the thermally conductive inorganic particles, the polymer, and the third organic solvent is (1.5-3.0):100:

100.

10. The method for preparing a secondary battery according to claim 9, wherein: In step S3, the extraction is performed using dichloromethane as the extraction liquid, and the extraction is performed at 42±2° C. for a time of 5 min to 20 min; In the step S4, the stretching is performed at 100°C to 120°C.

11. The method for preparing a secondary battery according to any one of claims 8 to 10, characterized in that: The method for preparing a secondary battery further includes a process of preparing a winding core by winding the positive electrode sheet, the negative electrode sheet and the separator; the winding is performed starting from the edge of the side where the third composite layer (30) is provided.

12. An energy storage system comprising at least one secondary battery, characterized in that: The secondary battery is the secondary battery according to any one of claims 1 to 5.

13. An electrical device comprising an energy storage system, characterized in that: The energy storage system is the energy storage system according to claim 12.

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