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 composite positive electrode sheet, which includes a positive current collector and a positive active layer provided on at least one side surface of the positive current collector. The positive active layer includes a positive active material, a conductive agent, and a binder. In the direction away from the current collector, the positive active layer includes a first positive active layer and a second positive active layer stacked in sequence; the first positive active layer and the second positive active layer also include heat-conducting nanoparticles. The present application creates a thermal conductivity gradient along the thickness direction by designing the structure and components of the double-layer active layer in the composite positive electrode sheet, significantly improving the overall thermal conductivity of the resulting positive electrode sheet, thereby improving the temperature uniformity of the surface of the resulting positive electrode, and ultimately improving the electrical performance and safety of the battery cell.

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, especially lithium-ion batteries, are the heart of modern mobile electronic devices, electric vehicles, and large-scale energy storage systems. Their performance is directly related to the product's endurance, safety, and cost-effectiveness. However, lithium-ion batteries still face challenges in achieving high power output, long cycle life, and stable operation at extreme temperatures. For lithium-ion batteries, effective thermal management can not only avoid performance degradation due to overheating, but also prevent potential thermal runaway risks and ensure safe operation of the battery. The core of thermal management lies in controlling the temperature distribution within the battery to ensure that each cell in the battery pack operates within the optimal operating temperature range.

[0003] The thermal conductivity of lithium-ion batteries depends in part on the efficiency of the cathode. Existing methods for addressing uneven thermal conductivity in cathodes have significant limitations in practical applications. Furthermore, the development of new high-thermal-conductivity binders is expensive, and their chemical stability and compatibility with traditional active materials remain technical challenges. 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 low energy efficiency and low safety of secondary batteries in the prior art.

[0005] According to some embodiments of the present application, the first aspect of the present application provides a secondary battery, including a composite positive electrode sheet, the composite positive electrode sheet including a positive electrode collector and a positive electrode active layer arranged on the surface of at least one side of the positive electrode collector, the positive electrode active layer including a positive electrode active material, a conductive agent and a binder, and in a direction away from the current collector, the positive electrode active layer includes a first positive electrode active layer and a second positive electrode active layer stacked in sequence; the first positive electrode active layer and the second positive electrode active layer also include thermal conductive nanoparticles; the content of the thermal conductive nanoparticles in the first positive electrode active layer is M1; the content of the thermal conductive nanoparticles in the second positive electrode active layer is M2; M1>M2, and 0.3wt%≤M1≤0.6wt%, 0.1wt%≤M2<0.3wt%; the thermal conductive nanoparticles include aluminum oxide particles and aluminum nitride particles.

[0006] In some embodiments, the thickness of the first positive electrode active layer is 60 μm~90 μm, and / or the thickness of the second positive electrode active layer is 60 μm~90 μm; in the first positive electrode active layer, the weight ratio of the aluminum oxide particles to the aluminum nitride particles is (1.5~2.5):1; and / or, in the second positive electrode active layer, the weight ratio of the aluminum oxide particles to the aluminum nitride particles is (3~4):1.

[0007] In some embodiments, the D50 of the aluminum oxide particles is 20±2 nm, and the D50 of the aluminum nitride particles is 50±5 nm.

[0008] In some embodiments, the surface of the aluminum oxide particles is further coated with a carbon layer having a thickness of 1 nm to 3 nm; and / or the surface of the aluminum nitride particles is further coated with a carbon layer having a thickness of 2 nm to 4 nm.

[0009] In some embodiments, the first positive active layer and the second active layer further independently include 95% to 97% of positive active material, 1% to 3% of conductive agent, and 1% to 2% of binder.

[0010] In some embodiments, the conductive agent is conductive carbon black and / or carbon nanotubes.

[0011] 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 composite positive electrode sheet, and the preparation process of the composite positive electrode sheet includes: step S1, respectively preparing thermally conductive nanoparticles, positive electrode active materials, conductive agents and binders into a first slurry and a second slurry; step S2, coating the first slurry on at least one side surface of the positive electrode collector, and forming a first positive electrode active layer after drying; step S3, coating the second slurry on the surface of the first positive electrode active layer away from the positive electrode collector, and forming a second positive electrode active layer after drying, thereby obtaining a composite positive electrode sheet.

[0012] In some embodiments, step S1 further includes: performing a first mixing of the thermally conductive nanoparticles and the conductive agent to obtain a first mixed material; formulating the first mixed material, the positive electrode active material and the binder into a first slurry; and / or, performing a second mixing of the thermally conductive nanoparticles and the conductive agent to obtain a second mixed material; formulating the second mixed material, the positive electrode active material and the binder into a second slurry.

[0013] In some embodiments, before performing step S1, the thermally conductive nanoparticles are further subjected to a carbon coating treatment, and the carbon coating treatment includes: mixing a carbon source with the thermally conductive nanoparticles to obtain pre-coated nanoparticles, and the pre-coated nanoparticles are sequentially subjected to a first calcination at a temperature of 200°C to 500°C and a second calcination at a temperature of 600°C to 800°C to obtain thermally conductive nanoparticles having a carbon layer coated on the surface; the carbon source is glucose and / or sucrose.

[0014] 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.

[0015] 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.

[0016] The technical solution provided in the embodiments of the present application has at least the following advantages: by designing the structure and components of the double-layer active layer in the composite positive electrode sheet, a thermal conductivity gradient along the thickness direction is created, wherein the specific weight percentage ranges of M1 (the content of thermally conductive nanoparticles in the first layer) and M2 (the content of thermally conductive nanoparticles in the second layer) are strictly limited, which significantly improves the overall thermal conductivity of the resulting positive electrode sheet, thereby improving the temperature uniformity of the surface of the resulting positive electrode, and ultimately improving the electrical performance and safety of the battery cell. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] 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.

[0018] Figure 1 This is a process flow chart of the method for preparing a composite positive electrode sheet in a secondary battery provided in an embodiment of the present application. DETAILED DESCRIPTION

[0019] As can be seen from the background, existing positive electrode sheets suffer from poor surface thermal uniformity, which in turn leads to low energy efficiency and safety in secondary batteries. Currently, common cathode materials for lithium-ion batteries include lithium iron phosphate (LFP), lithium nickel cobalt manganese oxide (NCM), and lithium nickel cobalt aluminum oxide (NCA). LFP, due to its stable chemical properties and excellent cycling performance, has been widely used in energy storage and electric vehicles. However, the low thermal conductivity of LFP cathode materials limits effective heat conduction. This is particularly true in large-scale battery production and applications, such as in large modules of energy storage systems. Heat accumulation can lead to localized overheating, compromising the stability of the entire battery system. Thermal management performance is often overlooked in the design of existing lithium-ion battery cathode sheets. In traditional cathode sheet formulations, the ratio of LFP, conductive agent, and binder is strictly controlled to optimize electrochemical performance, but insufficient consideration is given to thermal conductivity. The low thermal conductivity of the electrodes, especially the positive electrode, prevents efficient heat conduction, leading to heat accumulation. This dramatic temperature rise directly impacts the intensity of the electrochemical reactions during the battery's charge and discharge processes. In more serious cases, hot spots can form on the positive electrode at high temperatures, accelerating battery aging and even causing safety incidents.

[0020] A first aspect of an embodiment of the present application provides a secondary battery, comprising a composite positive electrode sheet, the composite positive electrode sheet comprising a positive current collector and a positive active layer arranged on at least one side surface of the positive current collector, the positive active layer comprising a positive active material, a conductive agent and a binder, and in a direction away from the current collector, the positive active layer comprising a first positive active layer and a second positive active layer stacked in sequence; the first positive active layer and the second positive active layer also comprise thermally conductive nanoparticles; the content of the thermally conductive nanoparticles in the first positive active layer is denoted as M1; the content of the thermally conductive nanoparticles in the second positive active layer is denoted as M2; M1>M2, and 0.3wt%≤M1≤0.6wt%, 0.1wt%≤M2<0.3wt%; the thermally conductive nanoparticles comprise aluminum oxide particles and aluminum nitride particles.

[0021] This application designs the structure and components of the double-layer active layer in the composite positive electrode sheet to create a thermal conductivity gradient along the thickness direction, thereby significantly improving the overall thermal conductivity of the resulting positive electrode sheet, thereby improving the temperature uniformity of the surface of the resulting positive electrode, and ultimately improving the electrical performance and safety of the battery cell. Specifically, by strictly limiting M1 (the content of thermally conductive nanoparticles in the first layer) to be greater than M2 (the content of thermally conductive nanoparticles in the second layer), and controlling both within a specific weight percentage range, a high concentration of thermally conductive nanoparticles is concentrated in the first positive electrode active layer, quickly conducting the generated heat to the current collector and further diffusing it to the battery cell structure, reducing the accumulation of heat inside the positive electrode sheet and improving the heat dissipation efficiency. Because heat can be more effectively conducted and quickly diffused inside the positive electrode sheet, the temperature distribution of the entire battery cell is more uniform. This avoids electrochemical side reactions caused by local overheating and enhances the operating stability and cycle life of the battery.

[0022] In particular, this application strictly limits the use of thermally conductive nanoparticles to aluminum oxide and aluminum nitride. This is because, while both aluminum oxide and aluminum nitride particles are highly thermally conductive materials, they differ in their thermal conduction mechanisms. Aluminum oxide particles are stable at high temperatures, while aluminum nitride has higher thermal conductivity. The combination of these two improves thermal conductivity while ensuring stable battery operation under varying temperature conditions, ultimately resulting in superior electrical performance and safety.

[0023] In some embodiments, the thickness of the first positive electrode active layer is 60 μm to 90 μm, and / or the thickness of the second positive electrode active layer is 60 μm to 90 μm. By optimizing the layer thickness, the electrochemical performance and thermal management capabilities of the battery are more effectively balanced, and the positive electrode materials in the two active layers are fully utilized without affecting the energy density and power density of the battery. In addition, in order to achieve a better synergistic effect in thermal and electrical conductivity of the two active layers, the weight ratio of aluminum oxide particles to aluminum nitride particles in the first positive electrode active layer is preferably (1.5-2.5):1; and / or the weight ratio of aluminum oxide particles to aluminum nitride particles in the second positive electrode active layer is (3-4):1.

[0024] In some embodiments, the D50 of the aluminum oxide particles is 20±2 nm, and the D50 of the aluminum nitride particles is 50±5 nm. By optimizing the D50 values of the aluminum oxide and aluminum nitride particles, the two thermally conductive particles are evenly dispersed in the active layer, increasing the contact area between the particles, thereby significantly improving the efficiency of the formed thermal conductive network and enhancing the electrical performance of the resulting secondary battery.

[0025] In order to further increase the conductivity of the two thermally conductive particles, reduce the charge transfer resistance, and improve the compatibility of the particles with surrounding materials, the surface of the aluminum oxide particles is preferably coated with a carbon layer with a thickness of 1nm to 3nm; and / or the surface of the aluminum nitride particles is preferably coated with a carbon layer with a thickness of 2nm to 4nm, so that the aluminum oxide particles and the aluminum nitride particles can conduct heat efficiently and conduct electricity well, thereby more significantly improving the overall performance of the battery.

[0026] In some embodiments, the first positive electrode active layer and the second active layer each independently comprise 95% to 97% positive electrode active material, 1% to 3% conductive agent, and 1% to 2% binder. The high proportion of positive electrode active material maintains the energy output of the secondary battery, while the appropriate proportions of conductive agent and binder support the transmission of electrons and ions, respectively, improving the stability and reliability of the secondary battery.

[0027] In some embodiments, the positive electrode active material is lithium iron phosphate; the conductive agent is conductive carbon black and / or carbon nanotubes; and the binder is PVDF. Of course, in practical applications, these are not limited to the above.

[0028] In some embodiments, the conductive agent is a mixture of conductive carbon black and carbon nanotubes, with the weight ratio of conductive carbon black to carbon nanotubes in the conductive mixture being (1.5-2):1. In this solution, the conductive carbon black provides a stable conductive framework, while the carbon nanotubes enhance the connectivity and thermal conductivity of the conductive network. Using these two materials in this ratio allows for better coordination with the two thermally conductive particles in the active layer, significantly optimizing both electrical and thermal conductivity.

[0029] The second aspect of the embodiment of the present application provides a method for preparing the above-mentioned secondary battery, including a preparation process of a composite positive electrode sheet, and the preparation process of the composite positive electrode sheet includes: step S1, respectively preparing thermally conductive nanoparticles, positive electrode active materials, conductive agents and binders into a first slurry and a second slurry; step S2, coating the first slurry on at least one side surface of the positive electrode collector, and forming a first positive electrode active layer after drying; step S3, coating the second slurry on the surface of the first positive electrode active layer away from the positive electrode collector, and forming a second positive electrode active layer after drying, thereby obtaining a composite positive electrode sheet.

[0030] With respect to the above-mentioned secondary battery, the present application accordingly provides a preparation method thereof. By adding two different amounts of thermally conductive nanoparticles, first and second positive electrode active layer slurries are prepared respectively, and then these two slurries are sequentially applied to the positive electrode current collector to form a composite positive electrode sheet. This method constructs a thermal conductivity network gradient along the thickness direction of the positive electrode sheet, that is, the positive electrode active layer close to the current collector (the first positive electrode active layer) has a higher amount of thermal conductive auxiliary material added, while the positive electrode active layer away from the current collector (the second positive electrode active layer) has a lower amount of thermal conductive auxiliary material added. This allows heat to be transferred more quickly from the active layer to the current collector, and then distributed to the entire battery cell structure, significantly reducing the temperature difference on the battery cell surface and improving the thermal stability of the battery cell during charging and discharging.

[0031] In some embodiments, step S1 further includes: performing a first mixing of the thermally conductive nanoparticles and the conductive agent to obtain a first mixed material; formulating the first mixed material, the positive electrode active material and the binder into a first slurry; and / or performing a second mixing of the thermally conductive nanoparticles and the conductive agent to obtain a second mixed material; formulating the second mixed material, the positive electrode active material and the binder into a second slurry. In other words, during the preparation of the first slurry and the second slurry, by pre-mixing the thermally conductive nanoparticles and the conductive agent, the two can be better combined to form a more stable and continuous heat conduction path, further enhancing the heat conduction ability along the positive electrode sheet and improving the various performances of the resulting secondary battery. During the pre-mixing process, it is preferred that the rotation speeds of the first mixing and the second mixing are independently 20 rpm to 30 rpm, so that the thermally conductive nanoparticles and the conductive agent are more fully mixed, forming a denser heat conduction network, further improving the thermal conductivity uniformity of the positive electrode sheet, making the heat conduction more efficient, and the resulting secondary battery safer.

[0032] In some embodiments, before performing step S1, the step further includes carbon coating the thermally conductive nanoparticles, the carbon coating comprising: mixing a carbon source with the thermally conductive nanoparticles to obtain pre-coated nanoparticles, the pre-coated nanoparticles sequentially undergoing a first calcination at a temperature of 200°C to 500°C and a second calcination at a temperature of 600°C to 800°C to obtain thermally conductive nanoparticles with a carbon layer coated on the surface. The pre-coated nanoparticles are subjected to a first calcination and a second calcination at different temperatures to form thermally conductive nanoparticles with a carbon layer coated on the surface. Each coated carbon layer formed by this method can better serve as an intermediate phase, enhancing the bonding force between the thermally conductive nanoparticles and the conductive agent, the positive active material, and the binder, making the structure of the composite positive electrode sheet more stable, reducing the peeling or separation of the active layer during the charge and discharge process, and thus making the resulting secondary battery safer. As for the carbon source, in order to obtain a denser and more conductive carbon layer, thereby more significantly optimizing the various properties of the resulting secondary battery, it is preferably glucose and / or sucrose.

[0033] A third aspect of the present invention provides an energy storage system comprising at least one secondary battery, such as the aforementioned secondary battery. By utilizing the aforementioned secondary battery with optimized thermal conductivity provided herein, the energy storage system significantly improves its thermal management capabilities, reduces safety risks caused by localized overheating, and improves the overall operational stability and safety of the system.

[0034] The fourth aspect of the embodiment of the present application provides an electrical device, including an energy storage system. The energy storage system is the above-mentioned energy storage system, and its efficient heat conduction reduces the temperature rise of the electrical equipment during operation, helps to extend the service life of the equipment, and reduce maintenance and replacement costs.

[0035] 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.

[0036] References to "embodiments" herein 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.

[0037] 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.

[0038] 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).

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] Example 1

[0046] A method for preparing a secondary battery:

[0047] (1) Carbon coating treatment of thermally conductive nanoparticles:

[0048] (1-1) In the first active layer, two types of thermally conductive nanoparticles are carbon-coated: glucose is used as a carbon source and mixed with alumina particles with a D50 of 20 nm to obtain pre-coated nano-alumina particles. The obtained pre-coated nano-alumina particles are sequentially subjected to a first calcination at a temperature of 450°C and a second calcination at a temperature of 800°C to obtain first thermally conductive nano-alumina particles with a surface carbon layer thickness of 1 nm; glucose is used as a carbon source and mixed with aluminum nitride particles with a D50 of 50 nm to obtain pre-coated nano-aluminum nitride particles. The obtained pre-coated nano-aluminum nitride particles are sequentially subjected to a first calcination at a temperature of 400°C and a second calcination at a temperature of 800°C to obtain first thermally conductive nano-aluminum nitride particles with a surface carbon layer thickness of 3 nm.

[0049] (1-2) In the second active layer, two types of thermally conductive nanoparticles are carbon-coated: sucrose is used as a carbon source and mixed with alumina particles with a D50 of 20 nm to obtain pre-coated nano-alumina particles. The obtained pre-coated nano-alumina particles are sequentially subjected to a first calcination at a temperature of 250°C and a second calcination at a temperature of 600°C to obtain second thermally conductive nano-alumina particles with a surface carbon layer thickness of 2 nm; sucrose is used as a carbon source and mixed with aluminum nitride particles with a D50 of 50 nm to obtain pre-coated nano-aluminum nitride particles. The obtained pre-coated nano-aluminum nitride particles are sequentially subjected to a first calcination at a temperature of 300°C and a second calcination at a temperature of 600°C to obtain second thermally conductive nano-aluminum nitride particles with a surface carbon layer thickness of 4 nm.

[0050] (2) Preparation of coating slurry:

[0051] (2-1) Preparation of the first slurry for the first active layer: Weigh the required materials in a ratio of LFP:SP:PVDF:CNT:thermally conductive nanoparticles = 96:1.0:2.0:0.5:0.5. The positive electrode active material is LFP (lithium iron phosphate), the conductive agent is a mixture of SP (conductive carbon black) and CNT (carbon nanotubes) (with SP:CNT = 2:1), the binder is PVDF, and the thermally conductive nanoparticles include the first thermally conductive nano-alumina particles and the first thermally conductive nano-aluminum nitride particles obtained above. To prepare the first slurry, first mix the thermally conductive nanoparticles and the conductive agent at a rotation speed of 20 rpm to obtain a first mixed material. The first mixed material, the positive electrode active material, and the binder are then combined to form the first slurry.

[0052] (2-2) Preparation of the first slurry for the second active layer: Weigh the required materials according to a ratio of LFP:SP:PVDF:CNT:thermally conductive nanoparticles = 96.3:1.0:2.0:0.5:0.2. The positive electrode active material is LFP, the conductive agent is a mixture of SP and CNT (where SP:CNT = 2:1), the binder is PVDF, and the thermally conductive nanoparticles include the second thermally conductive nano-alumina particles and second thermally conductive nano-aluminum nitride particles obtained above. To prepare the second slurry, first mix the thermally conductive nanoparticles and the conductive agent at a rotation speed of 20 rpm to obtain a second mixed material. Then, the second mixed material, the positive electrode active material, and the binder are combined to form a second slurry.

[0053] (3) Coating to obtain a composite positive electrode sheet:

[0054] (3-1) A first slurry is applied to one side surface of a positive electrode current collector and dried to form a first positive electrode active layer having a thickness of 60 μm, wherein the content of the thermally conductive nanoparticles is 0.5%, wherein the weight ratio of the first thermally conductive nano-aluminum oxide particles to the first thermally conductive nano-aluminum nitride particles is 6:4.

[0055] (3-2) The second slurry was applied to the surface of the first positive electrode active layer away from the positive electrode current collector. After drying, a second positive electrode active layer having a thickness of 60 μm was formed. The second positive electrode active layer contained 0.2% of the thermally conductive nanoparticles, wherein the weight ratio of the second thermally conductive nano-alumina particles to the second thermally conductive nano-aluminum nitride particles was 8:2. Thus, a composite positive electrode sheet was obtained.

[0056] Also, the process flow of the preparation method of the composite positive electrode sheet is shown in Figure 1 .

[0057] (4) Assembly of secondary battery: The composite positive electrode sheet obtained above was used as the positive electrode, the graphite system was used as the negative electrode, the PE coated diaphragm was used as the diaphragm, and lithium hexafluorophosphate was used as the electrolyte to assemble a lithium-ion battery with a capacity of 314Ah.

[0058] Example 2

[0059] A method for preparing a secondary battery:

[0060] (1) Carbon coating treatment of thermally conductive nanoparticles:

[0061] (1-1) In the first active layer, two types of thermally conductive nanoparticles are carbon-coated: sucrose is used as a carbon source and mixed with alumina particles with a D50 of 20 nm to obtain pre-coated nano-alumina particles, and the obtained pre-coated nano-alumina particles are sequentially subjected to a first calcination at a temperature of 250°C and a second calcination at a temperature of 600°C to obtain first thermally conductive nano-alumina particles with a surface carbon layer thickness of 2 nm; sucrose is used as a carbon source and mixed with aluminum nitride particles with a D50 of 50 nm to obtain pre-coated nano-aluminum nitride particles, and the obtained pre-coated nano-aluminum nitride particles are sequentially subjected to a first calcination at a temperature of 250°C and a second calcination at a temperature of 600°C to obtain first thermally conductive nano-aluminum nitride particles with a surface carbon layer thickness of 4 nm.

[0062] (1-2) In the second active layer, two types of thermally conductive nanoparticles are carbon-coated: glucose is used as a carbon source and mixed with alumina particles with a D50 of 20 nm to obtain pre-coated nano-alumina particles. The obtained pre-coated nano-alumina particles are sequentially subjected to a first calcination at a temperature of 450°C and a second calcination at a temperature of 800°C to obtain second thermally conductive nano-alumina particles with a surface carbon layer thickness of 1 nm; glucose is used as a carbon source and mixed with aluminum nitride particles with a D50 of 50 nm to obtain pre-coated nano-aluminum nitride particles. The obtained pre-coated nano-aluminum nitride particles are sequentially subjected to a first calcination at a temperature of 400°C and a second calcination at a temperature of 800°C to obtain second thermally conductive nano-aluminum nitride particles with a surface carbon layer thickness of 3 nm.

[0063] (2) Preparation of coating slurry:

[0064] (2-1) Preparation of the first slurry for the first active layer: Weigh the required materials in a ratio of LFP:SP:PVDF:CNT:thermally conductive nanoparticles = 95.2:1.5:2.0:1.0:0.3. The positive electrode active material is LFP (lithium iron phosphate), the conductive agent is a mixture of SP (conductive carbon black) and CNT (carbon nanotubes) (with SP:CNT = 1.5:1), the binder is PVDF, and the thermally conductive nanoparticles include the first thermally conductive nano-alumina particles and the first thermally conductive nano-aluminum nitride particles obtained above. To prepare the first slurry, first mix the thermally conductive nanoparticles and the conductive agent at a rotation speed of 30 rpm to obtain a first mixed material. The first mixed material, the positive electrode active material, and the binder are then combined to form the first slurry.

[0065] (2-2) Preparation of the first slurry for the second active layer: Weigh the required materials in a ratio of LFP:SP:PVDF:CNT:thermally conductive nanoparticles = 95.4:1.5:2.0:1.0:0.1. The positive electrode active material is LFP, the conductive agent is a mixture of SP and CNT (where SP:CNT = 1.5:1), the binder is PVDF, and the thermally conductive nanoparticles include the second thermally conductive nano-alumina particles and the second thermally conductive nano-aluminum nitride particles obtained above. To prepare the second slurry, first mix the thermally conductive nanoparticles and the conductive agent at 30 rpm to obtain a second mixed material. The second mixed material, the positive electrode active material, and the binder are then combined to form a second slurry.

[0066] (3) Coating to obtain a composite positive electrode sheet:

[0067] (3-1) A first slurry is applied to one side surface of a positive electrode current collector and dried to form a first positive electrode active layer having a thickness of 90 μm, wherein the content of the thermally conductive nanoparticles is 0.3%, wherein the weight ratio of the first thermally conductive nano-aluminum oxide particles to the first thermally conductive nano-aluminum nitride particles is 6:4.

[0068] (3-2) The second slurry is applied to the surface of the first positive electrode active layer away from the positive electrode current collector, and dried to form a second positive electrode active layer having a thickness of 90 Å. The second positive electrode active layer contains 0.1% of the thermally conductive nanoparticles, wherein the weight ratio of the second thermally conductive nano-alumina particles to the second thermally conductive nano-aluminum nitride particles is 8:2. Thus, a composite positive electrode sheet is obtained.

[0069] (4) Assembly of secondary battery: same as in Example 1.

[0070] Example 3

[0071] A method for preparing a secondary battery:

[0072] The only difference between this embodiment and embodiment 1 is that the thickness of the prepared first active layer and the second active layer are both changed to 50 μm.

[0073] Example 4

[0074] A method for preparing a secondary battery:

[0075] The only difference between this embodiment and embodiment 1 is that the thickness of the prepared first active layer and the second active layer are both changed to 100 μm.

[0076] Example 5

[0077] A method for preparing a secondary battery:

[0078] The only difference between this embodiment and embodiment 1 is that the weight ratio of the first thermally conductive nano-aluminum oxide particles to the first thermally conductive nano-aluminum nitride particles in the first positive electrode active layer is changed to 1:1; and the weight ratio of the second thermally conductive nano-aluminum oxide particles to the second thermally conductive nano-aluminum nitride particles in the second positive electrode active layer is changed to 5:1.

[0079] Example 6

[0080] A method for preparing a secondary battery:

[0081] The only difference between this embodiment and embodiment 1 is that the weight ratio of the first thermally conductive nano-aluminum oxide particles to the first thermally conductive nano-aluminum nitride particles in the first positive electrode active layer is changed to 3:1; and the weight ratio of the second thermally conductive nano-aluminum oxide particles to the second thermally conductive nano-aluminum nitride particles in the second positive electrode active layer is changed to 2:1.

[0082] Example 7

[0083] A method for preparing a secondary battery:

[0084] The only difference between this embodiment and embodiment 1 is that the D50 of the aluminum oxide particles in the first active layer is changed to 10 nm, and the D50 of the aluminum nitride particles is changed to 70 nm.

[0085] Example 8

[0086] A method for preparing a secondary battery:

[0087] The only difference between this embodiment and embodiment 1 is that the D50 of the aluminum oxide particles in the second active layer is changed to 30 nm, and the D50 of the aluminum nitride particles is changed to 30 nm.

[0088] Example 9

[0089] A method for preparing a secondary battery:

[0090] The only difference between this embodiment and embodiment 1 is that:

[0091] During the preparation process of the first thermally conductive nano-alumina particles, the thickness of the surface carbon layer is changed to 0.5 nm, thereby obtaining the first thermally conductive nano-alumina particles;

[0092] During the preparation process of the first thermally conductive nano-aluminum nitride particles, the thickness of the surface carbon layer is changed to 5 nm, thereby obtaining the first thermally conductive nano-aluminum nitride particles.

[0093] Example 10

[0094] A method for preparing a secondary battery:

[0095] The only difference between this embodiment and embodiment 1 is that:

[0096] During the preparation process of the first thermally conductive nano-alumina particles, the thickness of the surface carbon layer is changed to 4 nm, thereby obtaining the first thermally conductive nano-alumina particles;

[0097] During the preparation of the first thermally conductive nano-aluminum nitride particles, the thickness of the surface carbon layer is changed to 1 nm, thereby obtaining the first thermally conductive nano-aluminum nitride particles.

[0098] Example 11

[0099] A method for preparing a secondary battery:

[0100] The only difference between this embodiment and embodiment 1 is that during the preparation of the first slurry and the second slurry, no CNTs were added, but an equal weight of SP was used instead.

[0101] Example 12

[0102] A method for preparing a secondary battery:

[0103] The only difference between this embodiment and embodiment 1 is that the two thermally conductive nanoparticles are not subjected to any carbon coating, but are directly used as thermally conductive particles to prepare the first active layer and the second active layer.

[0104] Comparative Example 1

[0105] A method for preparing a secondary battery:

[0106] (1) Preparation of coating slurry: Weigh the required materials and prepare the coating slurry according to the ratio of LFP:SP:PVDF:CNT = 95.2:1.5:2.0:1.0.

[0107] (2) The obtained coating slurry is coated on one side surface of the positive electrode current collector, and after drying, a positive electrode active layer with a thickness of 150 μm is formed, thereby obtaining a composite positive electrode sheet.

[0108] (3) Assembly of secondary battery: same as in Example 1.

[0109] Comparative Example 2

[0110] A method for preparing a secondary battery:

[0111] The difference between this comparative example and Example 1 is only in step (3), specifically:

[0112] (3) Coating to obtain a composite positive electrode sheet:

[0113] (3-1) The second slurry is applied to one surface of the positive electrode current collector and dried to form a first positive electrode active layer having the same thickness as the first positive electrode active layer in Example 1, except that the content of the thermally conductive nanoparticles therein is 0.2%, wherein the weight ratio of the first thermally conductive nano-aluminum oxide particles to the first thermally conductive nano-aluminum nitride particles is 6:4.

[0114] (3-2) The first slurry is applied to the surface of the first positive electrode active layer away from the positive electrode current collector, and after drying, a second positive electrode active layer having the same thickness as the second positive electrode active layer in Example 1 is formed, except that the content of the thermally conductive nanoparticles therein is 0.5%, wherein the weight ratio of the second thermally conductive nano-aluminum oxide particles to the second thermally conductive nano-aluminum nitride particles is 8:2.

[0115] Then, a composite positive electrode sheet was obtained.

[0116] Comparative Example 3

[0117] A method for preparing a secondary battery:

[0118] The only difference between this comparative example and the embodiment is that the content of the thermally conductive nanoparticles in the obtained first positive electrode active layer is changed to 0.8%, and the content of the thermally conductive nanoparticles in the second positive electrode active layer is changed to 0.05%.

[0119] Comparative Example 4

[0120] A method for preparing a secondary battery:

[0121] The only difference between this comparative example and Example 1 is that: in the first positive electrode active layer, the first thermally conductive aluminum nitride particles are not added, but the first thermally conductive aluminum oxide particles of equal weight are used instead; in the second positive electrode active layer, the second thermally conductive aluminum oxide particles are not added, but the second thermally conductive aluminum nitride particles of equal weight are used instead.

[0122] That is, in the composite positive electrode sheet obtained in this comparative example, the thermally conductive particles of the first positive electrode active layer are only the first thermally conductive aluminum oxide particles, whose content is 0.5%; the thermally conductive particles of the second positive electrode active layer are only the second thermally conductive aluminum nitride particles, whose content is 0.2%.

[0123] Comparative Example 5

[0124] A method for preparing a secondary battery:

[0125] The only difference between this comparative example and Example 1 is that: in the first positive electrode active layer, the first thermally conductive aluminum oxide particles are not added, but the first thermally conductive aluminum nitride particles of equal weight are used instead; in the second positive electrode active layer, the second thermally conductive aluminum nitride particles are not added, but the second thermally conductive aluminum oxide particles of equal weight are used instead.

[0126] That is, in the composite positive electrode sheet obtained in this comparative example, the thermally conductive particles of the first positive electrode active layer are only first thermally conductive aluminum nitride particles, whose content is 0.5%; the thermally conductive particles of the second positive electrode active layer are only second thermally conductive aluminum oxide particles, whose content is 0.2%.

[0127] Test Method

[0128] 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 samples obtained in each embodiment and comparative example were charged and discharged at 1P 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.

[0129] 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%.

[0130] The test results are shown in Table 1.

[0131] Table 1

[0132]

[0133] From the above description, it can be seen that compared with Comparative Examples 1 to 5, the above-mentioned embodiments of the present application design the structure and components of the double-layer active layer in the composite positive electrode sheet, and strictly limit the specific weight percentage range of M1 (the content of thermally conductive nanoparticles in the first layer) and M2 (the content of thermally conductive nanoparticles in the second layer), thereby significantly improving the temperature uniformity of the surface of the obtained positive electrode and ultimately improving the electrical performance and safety of the battery cell.

[0134] Specifically, by comparing Examples 3 and 4 with Example 1, it can be seen that by optimizing the thickness of the two active layers, the electrochemical performance and thermal management capability of the battery can be more effectively balanced.

[0135] Comparing Examples 5 and 6 with Example 1, it can be seen that by optimizing the weight ratio of aluminum oxide particles and aluminum nitride particles in the two active layers, the two active layers can more significantly exhibit better synergistic effects in thermal conductivity and electrical conductivity, thereby enabling the corresponding battery cell samples to have better thermal management capabilities.

[0136] Comparing Examples 7 and 8 with Example 1, it can be seen that by optimizing the D50 values of the thermally conductive particles in the two active layers, the efficiency of the formed thermally conductive network can be significantly improved, thereby enhancing the electrical performance of the resulting secondary battery.

[0137] Comparing Examples 9 and 10 with Example 1, it can be seen that further refining the thickness range of the surface carbon layer of the two types of thermally conductive particles with respect to the function and position of the first positive electrode active layer and the second positive electrode active layer helps to more accurately optimize the thermal conductivity while ensuring electrical conductivity, thereby enabling the resulting secondary battery to exhibit more superior overall performance.

[0138] Comparing Example 11 with Example 1, it can be seen that using conductive carbon black and carbon nanotubes together as conductive agents in a certain ratio can better coordinate with the two thermally conductive particles in the active layer and more significantly achieve dual optimization of electrical and thermal conductivity.

[0139] Comparing Example 12 with Example 1, it can be seen that by adding a carbon layer on the surface of the aluminum oxide particles and the aluminum nitride particles, the two types of thermally conductive particles can be both highly efficient in thermal conductivity and good in electrical conductivity, thereby significantly improving the overall performance of the battery.

[0140] 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 composite positive electrode sheet, the composite positive electrode sheet comprising a positive electrode current collector and a positive electrode active layer disposed on at least one side of the positive electrode current collector, the positive electrode active layer comprising a positive electrode active material, a conductive agent, and a binder, characterized in that: Along a direction away from the current collector, the positive electrode active layer includes a first positive electrode active layer and a second positive electrode active layer stacked in sequence; the first positive electrode active layer and the second positive electrode active layer further include thermally conductive nanoparticles; The content of the thermally conductive nanoparticles in the first positive electrode active layer is denoted as M1; The content of the thermally conductive nanoparticles in the second positive electrode active layer is denoted as M2; M1>M2, and 0.3wt%≤M1≤0.6wt%, 0.1wt%≤M2<0.3wt%; The thermally conductive nanoparticles include aluminum oxide particles and aluminum nitride particles.

2. The secondary battery according to claim 1, wherein The thickness of the first positive electrode active layer is 60 μm to 90 μm, and / or the thickness of the second positive electrode active layer is 60 μm to 90 μm; In the first positive electrode active layer, the weight ratio of the aluminum oxide particles to the aluminum nitride particles is (1.5-2.5):1; and / or, in the second positive electrode active layer, the weight ratio of the aluminum oxide particles to the aluminum nitride particles is (3-4):

1.

3. The secondary battery according to claim 2, wherein The D50 of the aluminum oxide particles is 20±2 nm, and the D50 of the aluminum nitride particles is 50±5 nm.

4. The secondary battery according to any one of claims 1 to 3, characterized in that The surface of the aluminum oxide particles is further coated with a carbon layer with a thickness of 1 nm to 3 nm; and / or, The surface of the aluminum nitride particles is also covered with a carbon layer with a thickness of 2nm to 4nm.

5. The secondary battery according to any one of claims 1 to 3, characterized in that Calculated by weight percentage, the first positive electrode active layer and the second positive electrode active layer further independently include 95% to 97% of the positive electrode active material, 1% to 3% of the conductive agent, and 1% to 2% of the binder.

6. The secondary battery according to any one of claims 1 to 3, characterized in that The conductive agent is conductive carbon black and / or carbon nanotubes.

7. A method for preparing a secondary battery according to any one of claims 1 to 6, comprising a process for preparing the composite positive electrode sheet, characterized in that: The preparation process of the composite positive electrode sheet includes: Step S1, respectively preparing the thermally conductive nanoparticles, the positive electrode active material, the conductive agent and the binder into a first slurry and a second slurry; Step S2, coating the first slurry on at least one side of the positive electrode current collector, and drying to form the first positive electrode active layer; Step S3: coating the second slurry on the surface of the first positive electrode active layer away from the positive electrode current collector, and drying to form the second positive electrode active layer, thereby obtaining the composite positive electrode sheet.

8. The method for preparing a secondary battery according to claim 7, wherein: The step S1 further includes: The thermally conductive nanoparticles are first mixed with the conductive agent to obtain a first mixed material; the first mixed material, the positive electrode active material and the binder are formulated into a first slurry; and / or, The thermally conductive nanoparticles are mixed with the conductive agent for a second time to obtain a second mixed material; the second mixed material, the positive electrode active material and the binder are formulated into a second slurry.

9. The method for preparing a secondary battery according to claim 8, wherein: Before performing step S1, the heat-conducting nanoparticles are subjected to a carbon coating treatment, wherein the carbon coating treatment comprises: mixing a carbon source with the heat-conducting nanoparticles to obtain pre-coated nanoparticles, and sequentially subjecting the pre-coated nanoparticles to a first calcination at a temperature of 200° C. to 500° C. and a second calcination at a temperature of 600° C. to 800° C. to obtain the heat-conducting nanoparticles having a carbon layer coated on the surface; The carbon source is glucose and / or sucrose.

10. 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 6.

11. An electrical device comprising an energy storage system, characterized in that: The energy storage system is the energy storage system according to claim 10.

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