Secondary battery and preparation method thereof, and electronic device

Abstract

The present application discloses a secondary battery, a preparation method thereof, and an electronic device. The secondary battery includes a shell and an electrode assembly arranged in the shell; the electrode assembly includes a positive electrode sheet, a first diaphragm, a negative electrode sheet and a second diaphragm stacked in sequence; the first diaphragm includes a first substrate layer and a first hot-melt adhesive layer and a first ceramic layer respectively arranged on both sides of the first substrate layer, the second diaphragm includes a second substrate layer and a second hot-melt adhesive layer and a second ceramic layer respectively arranged on both sides of the second substrate layer, and the first hot-melt adhesive layer and the second hot-melt adhesive layer both face the negative electrode sheet; in a direction perpendicular to the thickness direction of the electrode assembly, the first diaphragm includes a first extension portion extending beyond the negative electrode sheet, and the second diaphragm includes a second extension portion extending beyond the negative electrode sheet; the first extension portion and the second extension portion on both sides of the negative electrode sheet of the outermost layer of the electrode assembly are bonded by the first hot-melt adhesive layer and the second hot-melt adhesive layer, thereby improving the safety performance of the secondary battery.

Description

Technical Field

[0001] The embodiments of the present application relate to the field of energy storage technology, and in particular to a secondary battery and a preparation method thereof, and an electronic device. Background Art

[0002] Currently, electronic devices on the market are usually equipped with secondary batteries, which power electronic devices such as mobile phones and tablets to get rid of the constraints of power cords. The secondary battery includes a shell and an electrode assembly located within the shell. The electrode assembly includes a positive electrode sheet, a negative electrode sheet, and a separator. The separator is located between the positive electrode sheet and the negative electrode sheet. Generally, in the direction perpendicular to the thickness of the electrode assembly, the negative electrode sheet extends beyond the positive electrode sheet to avoid the phenomenon of lithium deposition caused by the negative electrode sheet being able to embed less lithium ions than the positive electrode can de-intercalate lithium ions during the charging process of the secondary battery.

[0003] During the implementation of the present application, the inventors of the present application discovered that the area where the negative electrode sheet extends beyond the edge of the positive electrode sheet is easily affected by external forces, thereby affecting the safety performance of the secondary battery. Summary of the Invention

[0004] In order to solve the above technical problems, embodiments of the present application provide a secondary battery and an electronic device that can improve safety performance.

[0005] The embodiments of the present application solve the technical problems by adopting the following technical solutions:

[0006] A secondary battery comprises a shell and an electrode assembly, wherein the electrode assembly is accommodated in the shell; in the thickness direction of the electrode assembly, the electrode assembly comprises a positive electrode sheet, a negative electrode sheet, a first diaphragm and a second diaphragm arranged in a stacked manner, and the negative electrode sheet is located between the first diaphragm and the second diaphragm; the first diaphragm comprises a first substrate layer, the first substrate layer comprises a first surface and a second surface opposite to each other, the second diaphragm comprises a second substrate layer, the second substrate layer comprises a third surface and a fourth surface opposite to each other, the first surface and the third surface both face the negative electrode sheet, and the second surface and the fourth surface both face the positive electrode sheet; in a direction perpendicular to the thickness direction of the electrode assembly, the negative electrode sheet extends beyond the positive electrode sheet, and the first diaphragm comprises a first main body portion facing the negative electrode sheet and a main body portion connected to the first main body portion and extending beyond the negative electrode sheet. The first extension portion, the second diaphragm includes a second main portion facing the negative electrode sheet and a second extension portion connected to the second main portion and extending beyond the negative electrode sheet, the first extension portion and the second extension portion are located on the same side of the negative electrode sheet; the first diaphragm includes a first hot melt adhesive layer arranged on the first surface and a first ceramic layer arranged on the second surface, the first hot melt adhesive layer is at least located in the first extension portion, and the first ceramic layer is located in the first main portion and the first extension portion; the second diaphragm includes a second hot melt adhesive layer arranged on the third surface and a second ceramic layer arranged on the fourth surface, the second hot melt adhesive layer is at least located in the second extension portion, and the second ceramic layer is located in the second main portion and the second extension portion; the first extension portion and the second extension portion on both sides of the negative electrode sheet located on the outermost layer of the electrode assembly are bonded by the first hot melt adhesive layer and the second hot melt adhesive layer.

[0007] This facilitates the formation of a closed area at the bonding point between the first and second extensions, reducing the risk of material detaching from the negative electrode tab extending beyond the positive electrode tab when subjected to external forces, thereby reducing the risk of detached material shorting with the positive electrode tab, improving the stability of the electrode assembly, and ultimately enhancing the safety of the secondary battery. Furthermore, the first ceramic layer on the first extension and the second ceramic layer on the second extension help prevent deformation of the first and second extensions when subjected to external forces, thereby strengthening the first and second separators, reducing the risk of external forces acting on the portion of the negative electrode tab extending beyond the positive electrode tab, and further improving the stability of the electrode assembly.

[0008] Optionally, the first diaphragm also includes a third extension portion connected to the first main body and extending beyond the negative electrode sheet, and the third extension portion and the first extension portion are located at opposite ends of the first main body. The second diaphragm also includes a fourth extension portion connected to the second main body and extending beyond the negative electrode sheet, and the fourth extension portion and the second extension portion are located at opposite ends of the second main body. The third extension portion and the fourth extension portion on both sides of the negative electrode sheet located at the outermost layer of the electrode assembly are bonded by the first hot melt adhesive layer and the second hot melt adhesive layer.

[0009] This helps reduce the risk of material from falling off at the other end of the negative electrode tab, where it extends beyond the positive electrode tab. This reduces the risk of material from falling off and shorting with the positive electrode tab, further improving the stability of the electrode assembly and enhancing the safety of the secondary battery. Furthermore, the first ceramic layer on the third extension and the second ceramic layer on the fourth extension help prevent deformation of the third and fourth extensions when subjected to external forces, reducing the risk of external forces acting on the portion of the negative electrode tab that extends beyond the positive electrode tab, and improving the stability of the electrode assembly.

[0010] The first extension portion is bonded to the second extension portion while the third extension portion is bonded to the fourth extension portion. The first diaphragm and the second diaphragm form a bag structure in a direction perpendicular to the thickness of the electrode assembly, which helps to suppress the expansion of the negative electrode sheet, indirectly improves the stability of the negative electrode sheet, enhances the safety performance of the electrode assembly, and increases the life of the secondary battery.

[0011] Optionally, the first hot melt adhesive layer includes a first portion and a second portion. Along the thickness direction of the electrode assembly, the projection of the first portion is located within the projection of the negative electrode sheet, and the projection of the second portion is located outside the projection of the negative electrode sheet. The viscosity flow temperature of the first portion is A, and the viscosity flow temperature of the second portion is B, satisfying 10°C < AB ≤ 120°C. This facilitates the hot pressing tip of the hot pressing instrument to apply pressure and heat to the electrode assembly. When the second portion located on the first extension portion and the second extension portion are melted and bonded by heat, the first portion located on the first main portion remains unmelted, thereby reducing the risk of surface micropores in the portion of the first substrate layer overlapping the negative electrode sheet being blocked by the melting of the first portion.

[0012] Optionally, the second hot melt adhesive layer includes a third part and a fourth part. Along the thickness direction of the electrode assembly, the projection of the third part is located inside the projection of the negative electrode sheet, and the projection of the fourth part is located outside the projection of the negative electrode sheet. The viscosity flow temperature of the third part is C, and the viscosity flow temperature of the fourth part is D, satisfying 10℃<CD≤120℃.

[0013] When the viscosity flow temperatures of the third part and the fourth part satisfy the above-mentioned relationship, it is helpful that when the hot pressing end of the hot pressing instrument applies pressure and heat to the electrode assembly, the fourth part located on the second extension part is melted and bonded to the first extension part by heat, while the third part located on the second main body part is not melted, thereby reducing the risk of the surface micropores of the overlapping part of the second substrate layer and the negative electrode sheet being blocked by the melting of the third part.

[0014] Optionally, the first part includes a hot-melt polymer, a binder, and inorganic particles, and satisfies at least one of the following conditions:

[0015] (1) The mass percentage of hot-melt polymer is 50%-97%;

[0016] The higher the hot-melt polymer content, the better the fluidity of the hot-melt polymer when it transitions to a viscous state, making adhesion easier during bonding. When the mass percentage of the hot-melt polymer satisfies the above relationship, it helps improve bonding reliability. The hot-melt polymer includes ethylene-vinyl acetate copolymer and / or polyethylene oxide.

[0017] (2) The mass percentage of the binder is 2%-49%; the binder includes polyvinylidene fluoride.

[0018] (3) The mass percentage of the inorganic material is 1%-10%, and the porosity of the inorganic particles of the inorganic material is greater than 50%. The inorganic material may include SiO2, Al2O3 and / or ZrO2.

[0019] Optionally, the thickness of the first substrate layer is T1, the thickness of the first ceramic layer is T2, the thickness of the first hot melt adhesive layer is T3, the thickness of the second substrate layer is T4, the thickness of the second ceramic layer is T5, the thickness of the second hot melt adhesive layer is T6, and the thickness of the negative electrode sheet is t, and at least one of the following conditions is met:

[0020] (1) 3μm≤T1≤9μm;

[0021] The thicker the first substrate layer, the better the first separator's ability to resist deformation. However, this will correspondingly occupy more space within the housing, leaving less room for the positive or negative electrode sheets, which is detrimental to increasing the secondary battery's energy density. When the thickness of the first substrate layer satisfies this relationship, it is considered to be an appropriate thickness, meeting the first separator's required resistance to deformation while also reducing the risk of the first substrate layer occupying excessive space.

[0022] (2) 1 μm ≤ T2 ≤ 5 μm;

[0023] The thicker the first ceramic layer, the more space the first diaphragm takes up within the housing, which is detrimental to increasing the energy density of the secondary battery. On the other hand, the thinner the first ceramic layer, the more difficult it is to improve the first diaphragm's ability to resist deformation, which is detrimental to increasing its strength. When the thickness of the first ceramic layer satisfies the above relationship, the thickness of the first ceramic layer is optimal, ensuring improved overall strength of the first diaphragm while reducing the risk of excessive space occupation, which is beneficial to increasing the energy density of the secondary battery. Furthermore, 1μm ≤ T2 ≤ 2μm is satisfied.

[0024] (3) 0.5 μm<T3≤6 μm;

[0025] The thicker the first hot melt adhesive layer, the stronger the bond between the first and second hot melt adhesive layers. However, the more internal space the secondary battery will occupy, which is not conducive to improving the energy density of the secondary battery and also hinders the transmission of lithium ions. When the thickness of the first hot melt adhesive layer satisfies the above relationship, the thickness of the first hot melt adhesive layer is more appropriate, which helps to improve the energy density of the secondary battery. In addition, the first and second hot melt adhesive layers are firmly bonded, while also reducing the risk of hindering lithium ion transmission. Furthermore, 1.5μm≤T3≤5μm is satisfied.

[0026] (4) 80 μm ≤ t ≤ 300 μm;

[0027] When the thickness of the negative electrode sheet satisfies this relationship, the thickness of the negative electrode sheet is appropriate, which is beneficial to improving the energy density of the secondary battery. Furthermore, it satisfies 80 μm ≤ t ≤ 100 μm.

[0028] (5) 3μm≤T4≤9μm;

[0029] The thicker the second substrate layer, the better the second separator's ability to resist deformation. However, this will correspondingly occupy more space within the housing, leaving less room for the positive or negative electrode sheets, which is detrimental to increasing the secondary battery's energy density. When the thickness of the second substrate layer satisfies this relationship, it is considered to be an appropriate thickness, meeting the second separator's required resistance to deformation while also reducing the risk of the second substrate layer occupying excessive space.

[0030] (6) 1 μm ≤ T5 ≤ 5 μm;

[0031] The thicker the second ceramic layer, the more space the second diaphragm takes up within the housing, which is detrimental to increasing the energy density of the secondary battery. On the other hand, the thinner the second ceramic layer, the less likely it is to improve the second diaphragm's deformation resistance, which is detrimental to increasing its strength. When the thickness of the second ceramic layer satisfies the aforementioned relationship, the thickness is optimal, ensuring increased overall strength of the second diaphragm while reducing the risk of excessive space occupation, thus contributing to increased energy density of the secondary battery.

[0032] (7) 0.5μm<T6≤6μm.

[0033] The thicker the second hot-melt adhesive layer, the more securely it bonds to the first. However, it also occupies more space within the secondary battery, hindering the battery's energy density and hindering lithium ion transmission. When the thickness of the second hot-melt adhesive layer satisfies the aforementioned relationship, it is an appropriate thickness, contributing to increased battery energy density. Furthermore, the second hot-melt adhesive layer maintains a secure bond with the first layer while minimizing the risk of lithium ion transmission obstruction.

[0034] Optionally, the thickness of the negative electrode sheet is t, and when observed along the thickness direction of the electrode assembly, the length of the first extension portion and / or the second extension portion in a direction perpendicular to the thickness direction of the electrode assembly is L, satisfying L≥0.5mm and L≥5t.

[0035] When this relationship is satisfied, the length of the first extension portion and / or the second extension in the direction perpendicular to the thickness of the electrode assembly is more appropriate, reducing the risk of affecting the bonding effect due to the smaller length of the first extension portion and / or the second extension portion in the direction perpendicular to the thickness of the electrode assembly, ensuring the reliability of the bonding between the first extension portion and the second extension portion through the first hot melt adhesive layer and the second hot melt adhesive layer, and at the same time improving the isolation effect between the positive electrode sheet and the negative electrode sheet, reducing the risk of short circuit between the positive electrode sheet and the negative electrode sheet.

[0036] Optionally, the negative electrode sheet includes a negative electrode current collector, a first active material layer, and a second active material layer. Along the thickness direction of the electrode assembly, the negative electrode current collector includes a first surface proximate to the first separator and a second surface proximate to the second separator. The first surface is provided with the first active material layer, and the second surface is provided with the second active material layer. The first active material layer includes a first active portion extending beyond the edge of the positive electrode sheet, and the second active material layer includes a second active portion extending beyond the edge of the positive electrode sheet. The first active portion is bonded to the first hot-melt adhesive layer, and the second active portion is bonded to the second hot-melt adhesive layer. This reduces the risk of the third and fourth active portions detaching from the negative electrode current collector under external force, thereby further improving the stability of the negative electrode sheet.

[0037] The embodiments of the present application also adopt the following technical solutions to solve the technical problems:

[0038] An electronic device includes the secondary battery mentioned above.

[0039] The embodiments of the present application also adopt the following technical solutions to solve the technical problems:

[0040] A method for preparing a secondary battery, applied to the above-mentioned secondary battery, comprises: preparing an electrode assembly, the electrode assembly comprising a stacked positive electrode sheet, a negative electrode sheet and a separator, the separator comprising a first separator and a second separator, the negative electrode sheet being located between the first separator and the second separator; hot pressing the electrode assembly so that the first hot melt adhesive layer and the second hot melt adhesive layer are bonded, thereby the first separator and the second separator jointly wrap the negative electrode sheet; providing a packaging bag, placing the electrode assembly in the packaging bag, and hot pressing and sealing the edges of the packaging bag; injecting electrolyte into the packaging bag, forming and allowing it to stand.

[0041] Optionally, the step of preparing the electrode assembly includes: preparing at least two diaphragms, wherein the at least two diaphragms include a first diaphragm and a second diaphragm; providing a positive electrode sheet and a negative electrode sheet; stacking the positive electrode sheet, the negative electrode sheet, the first diaphragm and the second diaphragm to finally form an electrode assembly, wherein the negative electrode sheet is located between the first diaphragm and the second diaphragm.

[0042] Optionally, the step of preparing at least two diaphragms includes: providing an isolation substrate, a hot melt adhesive raw material and a ceramic raw material; uniformly mixing the hot melt adhesive raw materials to obtain a first mixed slurry; uniformly mixing the ceramic raw materials to obtain a second mixed slurry; applying the first mixed slurry to the first coating surface of the isolation substrate, and applying the second mixed slurry to the second coating surface of the isolation substrate layer, and then performing a drying treatment to obtain a diaphragm. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] One or more embodiments are exemplarily illustrated by pictures in the corresponding drawings. These exemplifications do not constitute limitations on the embodiments. Elements with the same reference numerals in the drawings are represented as similar elements. Unless otherwise stated, the figures in the drawings do not constitute proportional limitations.

[0044] Figure 1 is a schematic diagram of a secondary battery according to one embodiment of the present application;

[0045] Figure 2 yes Figure 1 Schematic diagram of the electrode assembly after cutting along the line MM;

[0046] Figure 3 yes Figure 1 Schematic diagram of the electrode assembly after cutting along the line MM;

[0047] Figure 4 is a partial schematic diagram of a first diaphragm and a second diaphragm bonded together in one embodiment;

[0048] Figure 5 is a partial schematic diagram of another embodiment after the first diaphragm and the second diaphragm are bonded together;

[0049] Figure 6 yes Figure 2 Schematic diagram after sectioning along the section line NN;

[0050] Figure 7 is a schematic diagram of the steps of a method for preparing a secondary battery according to another embodiment of the present application;

[0051] Figure 8 yes Figure 7 FIG. 2 is a further detailed step diagram of step S201;

[0052] Figure 9 yes Figure 8 FIG. 2 is a further detailed step diagram of step S2011;

[0053] In the figure: 1, secondary battery; 2, housing; 3, electrode assembly; 31, negative electrode sheet; 32, positive electrode sheet; 33, first separator; 34, second separator; 311, negative electrode current collector; 312, first active material layer; 313, second active material layer; 3111, first surface; 3112, second surface; 321, positive electrode current collector; 322, third active material layer; 323, fourth active material layer; 330, first main body; 331, first extension; 340, second main body; 341, second extension; 31a, first electrode sheet; 31b, second electrode sheet; 332, third extension; 342, fourth extension; 333, fifth extension; 343, sixth extension; 334, seventh extension portion; 344, eighth extension portion; 3121, first active portion; 3131, second active portion; 3122, third active portion; 3132, fourth active portion; 3301, first substrate layer; 33011, first surface; 33012, second surface; 3302, first hot-melt adhesive layer; 3303, first ceramic layer; 3401, second substrate layer; 34011, third surface; 34012, fourth surface; 3402, second hot-melt adhesive layer; 3403, second ceramic layer; 33021, first portion; 33022, second portion; 34021, third portion; 34022, fourth portion; 4, positive electrode tab; 5, negative electrode tab. DETAILED DESCRIPTION

[0054] In order to facilitate the understanding of the present application, the present application will be described in more detail below in conjunction with the accompanying drawings and specific embodiments. It should be noted that when an element is described as being "fixed on" another element, it can be directly on the other element, or there can be one or more centered elements therebetween. When an element is described as being "connected to" another element, it can be directly connected to the other element, or there can be one or more centered elements therebetween. The terms "upper", "lower", "inside", "outside", "vertical", "horizontal", etc. used in this specification indicate an orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing the present application and simplifying the description, rather than indicating or implying 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 present application. In addition, the terms "first", "second", etc. are used for descriptive purposes only and cannot be understood as indicating or implying relative importance.

[0055] Unless otherwise defined, all technical and scientific terms used in this specification have the same meanings as those commonly understood by those skilled in the art to which this application belongs. The terms used in this specification are for the purpose of describing specific embodiments only and are not intended to limit this application. The term "and / or" as used in this specification includes any and all combinations of one or more of the relevant listed items.

[0056] In addition, the technical features involved in different embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0057] like Figure 1 As shown, a secondary battery 1 provided in one embodiment of the present application includes a housing 2 and an electrode assembly 3, wherein the electrode assembly 3 is accommodated in the housing 2. The housing 2 can be a packaging bag or other materials as long as it can accommodate the electrode assembly 3.

[0058] See also Figure 2 As shown, the electrode assembly 3 includes a stacked negative electrode sheet 31, a positive electrode sheet 32, a first separator 33, and a second separator 34. The negative electrode sheet 31 is located between the first separator 33 and the second separator 34. The first separator 33 and the second separator 34 isolate the negative electrode sheet 31 from the positive electrode sheet 32 ​​to reduce the risk of short circuit between the positive electrode sheet 32 ​​and the negative electrode sheet 31. Observing along the thickness direction Z of the electrode assembly 3, in a direction perpendicular to the electrode assembly 3, the negative electrode sheet 31 extends beyond the positive electrode sheet 32. This is beneficial for the secondary battery 1 to have a lithium insertion and removal capacity of the negative electrode sheet 31 greater than that of the positive electrode sheet 32 ​​during charging, thereby reducing the risk of lithium deposition in the electrode assembly 3.

[0059] The negative electrode sheet 31 includes a negative current collector 311, a first active material layer 312, and a second active material layer 313. The negative current collector 311 includes a first surface 3111 near the first separator 33 and a second surface 3112 near the second separator 34. The first active material layer 312 is disposed on the first surface 3111 of the negative current collector 311, and the second active material layer 313 is disposed on the second surface 3112 of the negative current collector 311. The positive electrode sheet 32 ​​includes a positive current collector 321, a third active material layer 322, and a fourth active material layer 323. The third active material layer 322 is disposed on one surface of the positive current collector 321, and the fourth active material layer 323 is disposed on the other surface of the positive current collector 321.

[0060] The electrode assembly 3 can be a winding structure or a laminated structure, which can be set according to the specific needs. Figure 2 As shown, the electrode assembly 3 adopts a laminated structure, and the number of negative electrode sheets 31, positive electrode sheets 32, first separators 33 and second separators 34 is multiple, multiple negative electrode sheets 31, multiple positive electrode sheets 32, multiple first separators 33 and multiple second separators 34 are stacked, and a first separator 33 and a second separator 34 are respectively provided on the two opposite surfaces of each negative electrode sheet 31. Figure 3 As shown, the electrode assembly 3 adopts a wound structure, wherein the negative electrode sheet 31, the positive electrode sheet 32, the first separator 33, and the second separator 34 are stacked and wound, and the negative electrode sheet 31 is located between the first separator 33 and the second separator 34. For ease of description, the following describes the technical solution of the present application using the laminated structure of the electrode assembly 3 as an example, but the structure of the electrode assembly 3 of the present application is not limited to the laminated structure.

[0061] The inventors of the present application have discovered that the portion of the negative electrode sheet 31 that extends beyond the edge of the positive electrode sheet 32 ​​is susceptible to partial detachment of the active material layer on the negative electrode sheet 31 when subjected to external force. This detached material may come into contact with the positive electrode sheet 32, posing a risk of causing a short circuit. Based on this, the inventors of the present application have improved the structure between the negative electrode sheet 31 and the separator. The improved solution is specifically as follows:

[0062] like Figure 2 and Figure 4 As shown, in a direction perpendicular to the thickness direction Z of the electrode assembly 3, with the negative electrode sheet 31 as a reference object, the first diaphragm 33 includes a first main body 330 facing the negative electrode sheet 31 and a first extension portion 331 connected to the first main body 330 and extending beyond the negative electrode sheet 31; the second diaphragm 34 includes a second main body 340 and a second extension portion 341 connected to the second main body 340 and extending beyond the negative electrode sheet 31; the first extension portion 331 and the second extension portion 341 are located on the same side of the negative electrode sheet 31.

[0063] Among them, the first diaphragm 33 includes a first substrate layer 3301, a first hot melt adhesive layer 3302 and a first ceramic layer 3303. The first substrate layer 3301 includes a first surface 33011 and a second surface 33012 arranged opposite to each other. The first hot melt adhesive layer 3302 is set on the first surface 33011, and the first hot melt adhesive layer 3302 is at least located at the first extension portion 331. The second surface 33012 is provided with a second ceramic layer 3403, and the second ceramic layer 3403 is at least located at the first main body portion 330 and the first extension portion 331.

[0064] The second separator 34 includes a second substrate layer 3401, a second hot melt adhesive layer 3402, and a second ceramic layer 3403. The second substrate layer 3401 includes a third surface 34011 and a fourth surface 34012. The second hot melt adhesive layer 3402 is disposed on the third surface 34011, and is at least partially located on the second extension portion 341. The second ceramic layer 3403 is disposed on the fourth surface 34012, and is at least located on the second main portion 340 and the second extension portion 341. The first surface 33011 and the third surface 34011 both face the negative electrode tab 31, while the second surface 33012 and the fourth surface 34012 both face the positive electrode tab 32. In other words, the side of the first substrate layer 3301 facing the negative electrode tab 31 is the first surface 33011, and the side of the second substrate layer 3401 facing the negative electrode tab 31 is the third surface 34011.

[0065] In some embodiments, the first extension 331 and the second extension 341 on either side of the negative electrode tab 31 located on the outermost layer of the electrode assembly 3 are bonded together by a first hot-melt adhesive layer 3302 and a second hot-melt adhesive layer 3402. This means that the first extension 331 and the second extension 341 are bonded together. This facilitates forming a closed area at the bonding location between the first extension 331 and the second extension 341. This reduces the risk of material from the negative electrode tab 31 extending beyond the positive electrode tab 32 falling outside the closed area when subjected to external forces. This reduces the risk of material from falling off and shorting with the positive electrode tab 32, improves the stability of the electrode assembly 3, and enhances the safety of the secondary battery 1. Furthermore, the first ceramic layer 3303 on the first extension 331 and the second ceramic layer 3403 on the second extension 341 help prevent the first and second extensions 331 and 341 from shrinking and deforming when heated, thereby enhancing the strength and thermal stability of the first and second separators 33 and 34, further improving the stability of the electrode assembly 3.

[0066] like Figure 2 and Figure 4As shown, along the thickness direction Z of the electrode assembly 3, the outermost negative electrode sheet 31 of the electrode assembly 3 is defined as the first electrode sheet 31a, while the negative electrode sheet 31 located between the outermost negative electrode sheets 31 of the electrode assembly 3 is the second electrode sheet 31b. Furthermore, the first extension 331 and the second extension 341 located on either side of each second electrode sheet 31b are bonded together by a first hot melt adhesive layer 3302 and a second hot melt adhesive layer 3402. This helps reduce the risk of material from the second electrode sheet 31b extending beyond the positive electrode sheet 32 ​​falling off and shorting with the adjacent positive electrode sheet 32, thereby improving the stability of the electrode assembly 3 and enhancing the safety of the secondary battery 1. Similarly, the first ceramic layer 3303 on the first extension portion 331 and the second ceramic layer 3403 on the second extension portion 341 on both sides of each second electrode piece 31b help prevent the first extension portion 331 and the second extension portion 341 from shrinking and deforming when heated, thereby enhancing the strength and thermal stability of the first diaphragm 33 and the second diaphragm 34, and further improving the stability of the electrode assembly 3.

[0067] It can be understood that in the process of making the electrode assembly 3, it is necessary to hot press both sides of the electrode assembly 3 of the wound structure or the stacked structure in the thickness direction, so that the first hot melt adhesive layer 3302 on the first extension part 331 on both sides of the outermost negative electrode plate 31 and the second hot melt adhesive layer 3402 on the second extension part 341 are melted and bonded to each other. Because the distance between the hot pressing end of the hot pressing equipment and the first extension part 331 and the second extension part 341 on both sides of the thickness of each second electrode plate 31b is relatively far, the first extension part 331 and the second extension part 341 on both sides of each second electrode plate 31b have not yet been bonded. After appropriately adjusting the hot pressing parameters (for example, extending the hot pressing time, increasing the hot pressing temperature), the first extension part 331 and the second extension part 341 on both sides of each second electrode plate 31b can be bonded to each other, which helps to further improve the safety performance of the second electrode plate 31b and enhance the stability of the electrode assembly 3.

[0068] In some embodiments, the thickness of the first substrate layer 3301 is T1, the thickness of the first ceramic layer 3303 is T2, the thickness of the first hot melt adhesive layer 3302 is T3, and the thickness of the negative electrode plate 31 is t, satisfying at least one of the following conditions:

[0069] (1) 3μm≤T1≤9μm;

[0070] The thicker the first substrate layer 3301, the better the first separator 33's ability to resist deformation. However, this, in turn, occupies more space within the housing 2, leaving less room for the positive electrode sheet 32 ​​or the negative electrode sheet 31, which is detrimental to increasing the energy density of the secondary battery 1. When the thickness of the first substrate layer 3301 satisfies this relationship, the thickness of the first substrate layer 3301 is optimal, meeting the required deformation resistance of the first separator 33 while also reducing the risk of the first substrate layer 3301 occupying excessive space.

[0071] (2) 1 μm ≤ T2 ≤ 5 μm;

[0072] The greater the thickness of the first ceramic layer 3303, the more space the first diaphragm 33 occupies within the housing 2, which is detrimental to increasing the energy density of the secondary battery 1. On the other hand, the smaller the thickness of the first ceramic layer 3303, the more difficult it is to improve the deformation resistance and heat resistance of the first diaphragm 33, which is detrimental to enhancing the strength and thermal stability of the first diaphragm 33. When the thickness of the first ceramic layer 3303 satisfies the above relationship, the thickness of the first ceramic layer 3303 is optimal, ensuring improved overall strength and thermal stability of the first diaphragm 33 while reducing the risk of excessive space occupation, thereby increasing the energy density of the secondary battery 1.

[0073] (3) 0.5 μm<T3≤6 μm;

[0074] The thicker the first hot-melt adhesive layer 3302, the more securely it bonds to the second hot-melt adhesive layer 3402. However, this also occupies more internal space within the secondary battery 1, hindering the energy density of the secondary battery 1 and potentially hindering the transmission of lithium ions. When the thickness of the first hot-melt adhesive layer 3302 satisfies the aforementioned relationship, the thickness of the first hot-melt adhesive layer 3302 is optimal, contributing to an increased energy density of the secondary battery 1. Furthermore, the first hot-melt adhesive layer 3302 and the second hot-melt adhesive layer 3402 maintain a secure bond while reducing the risk of hindering lithium ion transmission.

[0075] (4) 80μm≤t≤300μm.

[0076] The thicker the negative electrode sheet 31 is, the farther the distance between the first hot melt adhesive layer 3302 and the second hot melt adhesive layer 3402 is, which is not conducive to the bonding between the first hot melt adhesive layer 3302 and the second hot melt adhesive layer 3402. The thinner the negative electrode sheet 31 is, the higher the processing difficulty is. When the thickness of the negative electrode sheet 31 satisfies this relationship, the thickness of the negative electrode sheet 31 is more appropriate, which is conducive to the bonding between the first hot melt adhesive layer 3302 and the second hot melt adhesive layer 3402, and at the same time improves the energy density of the secondary battery 1.

[0077] In some embodiments, the following conditions are further satisfied: (1) 1.5 μm ≤ T3 ≤ 5 μm; (2) 1 μm ≤ T2 ≤ 2 μm, and (3) 80 μm ≤ t ≤ 100 μm.

[0078] Furthermore, the thickness of the second base material layer 3401 is T4, the thickness of the second ceramic layer 3403 is T5, and the thickness of the second hot melt adhesive layer 3402 is T6, which satisfies at least one of the following conditions:

[0079] (5) 3μm≤T4≤9μm;

[0080] The thicker the second substrate layer 3401, the better the deformation resistance of the second separator 34. However, this will correspondingly occupy more space within the housing 2, leaving less room for the positive electrode sheet 32 ​​or the negative electrode sheet 31, which is not conducive to improving the energy density of the secondary battery 1. When the thickness of the second substrate layer 3401 satisfies this relationship, the thickness of the second substrate layer 3401 is relatively appropriate, meeting the deformation resistance requirements of the second separator 34 while reducing the risk of the second substrate layer 3401 occupying too much space.

[0081] (6) 1 μm ≤ T5 ≤ 5 μm;

[0082] The thicker the second ceramic layer 3403, the more space the second diaphragm 34 occupies within the housing 2, which is detrimental to increasing the energy density of the secondary battery 1. On the other hand, the thinner the second ceramic layer 3403, the more difficult it is to improve the deformation resistance and heat resistance of the second diaphragm 34, which is detrimental to enhancing the strength and thermal stability of the second diaphragm 34. When the thickness of the second ceramic layer 3403 satisfies the aforementioned relationship, the thickness of the second ceramic layer 3403 is optimal, ensuring improved overall strength and thermal stability of the second diaphragm 34 while reducing the risk of excessive space occupation, thereby contributing to increasing the energy density of the secondary battery 1.

[0083] (7) 0.5μm<T6≤6μm.

[0084] The thicker the second hot-melt adhesive layer 3402, the more securely it bonds to the first hot-melt adhesive layer 3302. However, this also occupies more internal space within the secondary battery 1, hindering the energy density of the secondary battery 1 and potentially hindering the transmission of lithium ions. When the thickness of the second hot-melt adhesive layer 3402 satisfies the aforementioned relationship, the thickness is optimal, contributing to increased energy density within the secondary battery 1. Furthermore, the second hot-melt adhesive layer 3402 securely bonds to the first hot-melt adhesive layer 3302 while minimizing the risk of hindering lithium ion transmission.

[0085] In some embodiments, the thickness of the negative electrode tab 31 is t. When viewed along the thickness direction Z of the electrode assembly 3, the length of the first extension portion 331 and / or the second extension portion 341 in a direction perpendicular to the thickness direction Z of the electrode assembly 3 is L, satisfying L ≥ 0.5 mm and L ≥ 5t. When this relationship is satisfied, the length of the first extension portion 331 and / or the second extension portion in the direction perpendicular to the thickness direction Z of the electrode assembly 3 is relatively appropriate, reducing the risk of the bonding effect being affected by the short length of the first extension portion 331 and / or the second extension portion 341 in the direction perpendicular to the thickness of the electrode assembly 3, ensuring the reliability of the bonding between the first extension portion 331 and the second extension portion 341 via the first hot melt adhesive layer 3302 and the second hot melt adhesive layer 3402, and also improving the isolation effect between the positive electrode tab 32 and the negative electrode tab 31, reducing the risk of short circuit between the positive electrode tab 32 and the negative electrode tab 31.

[0086] In some embodiments, please refer again to Figure 2 and Figure 4 The first diaphragm 33 also includes a third extension portion 332 connected to the first main body 330 and extending beyond the negative electrode sheet 31. The third extension portion 332 and the first extension portion 331 are located at opposite ends of the first main body 330, and the third extension portion 332 is provided with a first hot melt adhesive layer 3302; the second diaphragm 34 also includes a fourth extension portion 342 connected to the second main body 340 and extending beyond the negative electrode sheet 31. The fourth extension portion 342 and the second extension portion 341 are located at opposite ends of the second main body 340, and the fourth extension portion 342 is provided with a second hot melt adhesive layer 3402. The third extension portion 332 and the fourth extension portion 342 on both sides of the negative electrode sheet 31 located at the outermost layer of the electrode assembly 3 are bonded by the first hot melt adhesive layer 3302 and the second hot melt adhesive layer 3402. This helps reduce the risk of material from falling off at the other end of the negative electrode tab 31 that extends beyond the positive electrode tab 32, and reduces the risk of short-circuiting between the falling material and the positive electrode tab 32. This further improves the stability of the electrode assembly 3 and enhances the safety of the secondary battery 1. Furthermore, the first ceramic layer 3303 on the third extension 332 and the second ceramic layer 3403 on the fourth extension 342 help prevent the third and fourth extensions 332, 342, from deforming when subjected to external forces, reducing the risk of external forces acting on the portion of the negative electrode tab 31 that extends beyond the positive electrode tab 32, and improving the stability of the electrode assembly 3.

[0087] The first extension portion 331 is bonded to the second extension portion 341 while the third extension portion 332 is bonded to the fourth extension portion 342. The first diaphragm 33 and the second diaphragm 34 form a bag structure in the thickness direction Z perpendicular to the electrode assembly 3, which helps to suppress the expansion of the negative electrode plate 31, indirectly improves the stability of the negative electrode plate 31, enhances the safety performance of the electrode assembly 3, and increases the life of the secondary battery 1.

[0088] Furthermore, the third extensions 332 and fourth extensions 342 located on both sides of each second electrode sheet 31b are bonded together by the first and second hot melt adhesive layers 3302 and 3402. This helps reduce the risk of material shedding from the portion of each second electrode sheet 31b extending beyond the positive electrode sheet 32, leading to a short circuit with the adjacent positive electrode sheet 32 ​​and causing thermal imbalance, thereby further improving the stability of the electrode assembly 3. Similarly, when bonding the third extensions 332 and fourth extensions 342 on both sides of the outermost negative electrode sheet 31, whether the third extensions 332 and fourth extensions 342 on both sides of each second electrode sheet 31b are bonded together depends on the hot pressing parameters of the hot pressing equipment used during the processing of the electrode assembly 3, and will not be further described here.

[0089] In some embodiments, the direction from the first extension portion 331 to the third extension portion 332 and the direction from the second extension portion 341 to the fourth extension portion 342 are both the first direction X, and the direction perpendicular to the first direction X and the thickness direction Z of the electrode assembly 3 is the second direction Y. When the electrode assembly 3 adopts a laminated structure, such as Figure 6 As shown, Figure 6 It is along Figure 5 In the schematic diagram taken along the section line NN of FIG, along the second direction Y, the first separator 33 further includes a fifth extension portion 333 connected to the first main portion 330 and extending beyond the negative electrode tab 31. The fifth extension portion 333 is provided with a first hot melt adhesive layer 3302. The second separator 34 further includes a sixth extension portion 343 connected to the first main portion 330 and extending beyond the negative electrode tab 31. The sixth extension portion 343 is provided with a second hot melt adhesive layer 3402. The fifth extension portion 333 and the sixth extension portion 343 on either side of the outermost negative electrode tab 31 of the electrode assembly 3 are bonded to each other by the first hot melt adhesive layer 3302 and the second hot melt adhesive layer 3402. This helps reduce the risk of material from the outermost negative electrode tab 31 extending beyond the positive electrode tab 32 falling off, thereby improving the stability of the electrode assembly 3. Furthermore, the fifth extension 333 and the sixth extension on either side of each second electrode sheet 31b are bonded together by a first hot-melt adhesive layer 3302 and a second hot-melt adhesive layer 3402. This helps reduce the risk of material shedding in the portion of the second electrode sheet 31b that extends beyond the positive electrode sheet 32, thereby improving the safety of the second electrode sheet 31b. Furthermore, the first ceramic layer 3303 on the fifth extension 333 and the second ceramic layer 3403 on the sixth extension 343 on either side of each second electrode sheet 31b help prevent deformation of the fifth and sixth extensions 333, 343 when subjected to external forces. This strengthens the first and second separators 33, 34, reduces the risk of external forces acting on the portion of the negative electrode sheet 31 that extends beyond the positive electrode sheet 32, and further improves the stability of the electrode assembly 3.

[0090] Furthermore, the first separator 33 also includes a seventh extension 334 connected to the first main body 330 and extending beyond the negative electrode tab 31. The seventh extension 334 and the fifth extension 333 are arranged opposite each other in the second direction Y. The seventh extension 334 is provided with a first hot melt adhesive layer 3302. The second separator 34 also includes an eighth extension 344 connected to the second main body 340 and extending beyond the negative electrode tab 31. The eighth extension 344 and the sixth extension 343 are arranged opposite each other in the second direction Y. The eighth extension 344 is provided with a second hot melt adhesive layer 3402. In some embodiments, the seventh extension 334 and the eighth extension 344 on both sides of the outermost negative electrode tab 31 are bonded together, which helps reduce the risk of material shedding in the portion of the negative electrode tab 31 that extends beyond the positive electrode tab 32, thereby improving the stability of the electrode assembly 3. In other embodiments, the seventh extension 334 and the eighth extension 344 on either side of the second electrode sheet 31b are bonded to each other, reducing the risk of material shedding from the portion of the negative electrode sheet 31 extending beyond the positive electrode sheet 32, thereby improving the stability of the electrode assembly 3. Furthermore, the pouch structure formed by the bond between the fifth and sixth extensions 343, and the bond between the seventh and eighth extensions 334 and 344, helps the first and second separators 33 and 34 enclose the negative electrode sheet 31, further reducing the risk of material shedding from the portion of the negative electrode sheet 31 extending beyond the positive electrode sheet 32, thereby improving the stability of the electrode assembly 3 and suppressing expansion of the negative electrode sheet 31, thereby enhancing the safety of the electrode assembly 3.

[0091] It should be understood, please combine Figure 4 and Figure 5 The first surface 33011 may be provided with the first hot melt adhesive layer 3302 on the entire surface; or the first hot melt adhesive layer 3302 may be provided on only a partial area. For example, the first surface 33011 may be provided with the first hot melt adhesive layer 3302 in the area of ​​the first extension portion 331 and / or the third extension portion 332, which is specifically set according to needs.

[0092] In some embodiments, as Figure 4As shown, the entire surface of the first surface 33011 is provided with a first hot melt adhesive layer 3302. The first hot melt adhesive layer 3302 includes a first portion 33021 and a second portion 33022. Along the thickness direction Z of the electrode assembly 3, the projection of the first portion 33021 is located within the projection of the negative electrode tab 31, while the projection of the second portion 33022 is located outside the projection of the negative electrode tab 31. The viscosity flow temperature of the first portion 33021 is A, and the viscosity flow temperature of the second portion 33022 is B, satisfying 10°C < AB ≤ 120°C. That is, along the thickness direction Z of the electrode assembly 3, the first portion 33021 is provided in the area where the first separator 33 overlaps with the negative electrode tab 31, and the second portion 33022 is provided in the area where the first separator 33 does not contact the negative electrode tab 31. In this way, when the hot pressing end of the hot pressing equipment applies pressure and heat to the electrode assembly 3, the second portion 33022 located on the first extension portion 331 and the second extension portion 341 are melted and bonded by heat, while the first portion 33021 located on the first main body portion 330 does not melt, thereby reducing the risk of the overlapping portion of the first substrate layer 3301 and the negative electrode plate 31 being blocked by the melting of the first portion 33021 and the clogging of the surface micropores. The viscous flow temperature refers to the transition temperature of a polymer from a highly elastic state to a viscous flow state, and the high or low temperature reflects the heat deformation resistance of the polymer. It is understandable that if the pores of the first substrate layer 3301 are blocked, the lithium ions extracted from the positive electrode plate 32 will be blocked and cannot be embedded into the negative electrode plate 31 through the first separator 33, thereby causing lithium deposition.

[0093] The difference in viscous flow temperature between first portion 33021 and second portion 33022 may be caused by the different mass percentages of the same components, the different hot-melt molecular weights of first portion 33021 and second portion 33022, or the different types of hot-melt polymers in first portion 33021 and second portion 33022. In this embodiment, the difference in viscous flow temperature between first portion 33021 and second portion 33022 is caused by the different mass percentages of the same components.

[0094] In some embodiments, the second portion 33022 includes a hot-melt polymer, a binder, and inorganic particles, and the sum of the mass percentages of the hot-melt polymer, the binder, and the inorganic particles is 100%, satisfying at least one of the following conditions:

[0095] (1) The mass percentage of hot-melt polymer is 50%-97%;

[0096] The higher the content of the hot-melt polymer, the better the fluidity of the hot-melt polymer when it transforms into a viscous state, and the easier it is to adhere during bonding. When the mass percentage of the hot-melt polymer satisfies the above relationship, it helps to improve the reliability of the bonding.

[0097] (2) The mass percentage of the binder is 2%-49%;

[0098] (3) The mass percentage of inorganic matter is 1%-10%, and the porosity of inorganic particles of the inorganic matter is greater than 50%.

[0099] The higher the mass percentage of the inorganic matter and the porosity, the more conducive it is to improving the lithium ion transmission performance, that is, the lithium ions can pass through the first part 33021 better.

[0100] In some embodiments, the hot-melt polymer includes one or more of ethylene-vinyl acetate copolymer (EVA resin) and polyethylene oxide (PEO). The binder can be, for example, polyvinylidene fluoride (PVDF), although other binders are also possible. The inorganic material can include one or more of silica, alumina, zirconium dioxide, or boehmite, although other materials are also possible. The porosity of the inorganic particles can be measured using the BET (Brunauer-Emmett-Teller) law.

[0101] Similarly, the third surface 34011 may have the second hot melt adhesive layer 3402 set on the entire surface; or the second hot melt adhesive layer 3402 may be set only in a partial area. For example, the second hot melt adhesive layer 3402 may be set in the area of ​​the second extension portion 341 and / or the fourth extension portion 342 of the third surface 34011, and the specific setting is more based on needs.

[0102] In some embodiments, the entire third surface 34011 is provided with a second hot melt adhesive layer 3402. The second hot melt adhesive layer 3402 includes a third portion 34021 and a fourth portion 34022. Along the thickness direction Z of the electrode assembly 3, the projection of the third portion 34021 is located within the projection of the negative electrode tab 31, while the projection of the fourth portion 34022 is located outside the projection of the negative electrode tab 31. The viscosity flow temperature of the third portion 34021 is C, and the viscosity flow temperature of the fourth portion 34022 is D, satisfying 10°C < CD ≤ 120°C. That is, along the thickness direction Z of the electrode assembly 3, the third portion 34021 is provided in the area where the second separator 34 overlaps with the negative electrode tab 31, and the fourth portion 34022 is provided in the area where the second separator 34 does not contact the negative electrode tab 31. In this way, when the hot pressing end of the hot pressing equipment applies pressure and heat to the electrode assembly 3, the third part 34021 located on the second main body 340 does not melt, reducing the risk of the overlapping part of the second substrate layer 3401 and the negative electrode plate 31 being blocked by the melting of the third part 34021, which is beneficial for the lithium ions of the positive electrode plate 32 to pass smoothly through the second diaphragm 34 and be embedded in the negative electrode plate 31, thereby improving the stability of the electrode assembly 3.

[0103] Similarly, the reason for the difference in viscous flow temperature between the third part 34021 and the fourth part 34022 can be referred to the reason for the difference in viscous flow temperature between the first part 33021 and the second part 33022 mentioned above, and will not be repeated here.

[0104] In some embodiments, the fourth portion 34022 is the same as the second portion 33022, that is, the fourth portion 34022 includes a hot-melt polymer, a binder, and inorganic particles, the sum of the mass percentages of the hot-melt polymer, the binder, and the inorganic particles is 100%, and at least one of the following conditions is met:

[0105] (1) The mass percentage of hot-melt polymer is 50%-97%;

[0106] The higher the content of the hot-melt polymer, the better the fluidity of the hot-melt polymer when it transforms into a viscous state, and the easier it is to adhere during bonding. When the mass percentage of the hot-melt polymer satisfies the above relationship, it helps to improve the reliability of the bonding.

[0107] (2) The mass percentage of the binder is 2%-49%;

[0108] (3) The mass percentage of inorganic matter is 1%-10%, and the porosity of inorganic particles of the inorganic matter is greater than 50%.

[0109] The higher the mass percentage and porosity of the inorganic material, the better the lithium ion transmission performance is, that is, the better the lithium ions can pass through the first portion 33021. When the mass percentage of the inorganic material and the porosity of the inorganic particles meet the above relationship, the inorganic material can better play its role.

[0110] In some embodiments, the first active material layer 312 includes a first active portion 3121 extending beyond the edge of the positive electrode tab 32, and the second active material layer 313 includes a second active portion 3131 extending beyond the edge of the positive electrode tab 32. The first active portion 3121 is bonded to the first hot-melt adhesive layer 3302, and the second active portion 3131 is bonded to the second hot-melt adhesive layer 3402. This reduces the risk of the first active portion 3121 and the second active portion 3131 separating from the negative electrode current collector 311 under external force, thereby improving the stability of the negative electrode tab 31.

[0111] Furthermore, the first active material layer 312 also includes a third active portion 3122 extending beyond the edge of the positive electrode tab 32. The third active portion 3122 and the first active portion 3121 are located on either side of the positive electrode tab 32. The second active material layer 313 also includes a fourth active portion 3132 extending beyond the edge of the positive electrode tab 32. The fourth active portion 3132 and the second active portion 3131 are located on either side of the positive electrode tab 32. The third active portion 3122 is bonded to the first hot melt adhesive layer 3302, and the fourth active portion 3132 is bonded to the second hot melt adhesive layer 3402. This reduces the risk of the third active portion 3122 and the fourth active portion 34022 separating from the negative electrode current collector 311 under external force, thereby further improving the stability of the negative electrode tab 31.

[0112] In some embodiments, the secondary battery 1 further includes a positive electrode tab 4 and a negative electrode tab 5. One end of each of the positive electrode tab 4 and the negative electrode tab 5 is connected to the electrode assembly 3, and the other ends of each of the positive electrode tab 4 and the negative electrode tab 5 extend outside the housing 2. The positive electrode tab 4 is connected to the positive electrode sheet 32 ​​of the electrode assembly 3, and the negative electrode tab 5 is connected to the negative electrode sheet 31 of the electrode assembly 3. The positive electrode tab 4 and the negative electrode tab 5 are used to connect to electrical equipment.

[0113] The secondary battery 1 provided in the embodiment of the present application includes a housing 2 and an electrode assembly 3, wherein the electrode assembly 3 is accommodated in the housing 2; in the thickness direction Z of the electrode assembly 3, the electrode assembly 3 includes a positive electrode sheet 32, a negative electrode sheet 31, a first separator 33 and a second separator 34 arranged in a stacked manner, wherein the negative electrode sheet 31 is located between the first separator 33 and the second separator 34; the first separator 33 includes a first substrate layer 3301, the first substrate layer 3301 includes a first surface 33011 and a second surface 33012 opposite to each other, and the second separator 34 includes a second substrate layer 34 01, the second substrate layer 3401 includes a third surface 34011 and a fourth surface 34012 opposite to each other, the first surface 33011 and the third surface 34011 both face the negative electrode sheet 31, and the second surface 33012 and the fourth surface 34012 both face the positive electrode sheet 32; in a direction perpendicular to the thickness direction Z of the electrode assembly 3, the negative electrode sheet 31 extends beyond the positive electrode sheet 32, and the first separator 33 includes a first main body 330 facing the negative electrode sheet 31 and a first extension portion connected to the first main body 330 and extending beyond the negative electrode sheet 31. The second diaphragm 34 includes a second main body 340 facing the negative electrode sheet 31 and a second extension 341 connected to the second main body 340 and extending beyond the negative electrode sheet 31. The first extension 331 and the second extension 341 are located on the same side of the negative electrode sheet 31. The first diaphragm 33 includes a first hot melt adhesive layer 3302 provided on the first surface 33011 and a first ceramic layer 3303 provided on the second surface 33012. The first hot melt adhesive layer 3302 is at least located on the first extension 331, and the first ceramic layer 3303 is located on the first main body. The body 330 and the first extension 331 are provided. The second separator 34 includes a second hot-melt adhesive layer 3402 provided on the third surface 34011 and a second ceramic layer 3403 provided on the fourth surface 34012. The second hot-melt adhesive layer 3402 is located at least on the second extension 341, and the second ceramic layer 3403 is located on the second body 340 and the second extension 341. The first extension 331 and the second extension 341 on either side of the negative electrode tab 31, located on the outermost layer of the electrode assembly 3, are bonded together by the first and second hot-melt adhesive layers 3302 and 3402. This facilitates forming a closed area at the bonding location between the first extension 331 and the second extension 341, reducing the risk of material detaching from the side of the negative electrode tab 31 extending beyond the positive electrode tab 32 when subjected to external force, thereby reducing the risk of detached material short-circuiting with the positive electrode tab 32, improving the stability of the electrode assembly 3, and enhancing the safety performance of the secondary battery 1.At the same time, the first ceramic layer 3303 on the first extension portion 331 and the second ceramic layer 3403 on the second extension portion 341 help prevent the first extension portion 331 and the second extension portion 341 from deforming when subjected to external force, thereby enhancing the strength of the first diaphragm 33 and the second diaphragm 34, reducing the risk of the portion of the negative electrode plate 31 extending beyond the positive electrode plate 32 being subjected to external force, and further improving the stability of the electrode assembly 3.

[0114] Another embodiment of the present application provides an electronic device, comprising the secondary battery 1 of the above embodiment, wherein the secondary battery 1 provides electrical energy for the electronic device. The electronic device may be any electronic device known in the prior art. For example, the electronic device includes, but is not limited to, a laptop computer, a pen-type computer, a mobile computer, an e-book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a head-mounted stereo headset, a video recorder, an LCD television, a portable cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, lighting fixtures, toys, a game console, a clock, a power tool, a flashlight, a camera, a large household battery, and a lithium-ion capacitor.

[0115] Please combine Figure 7-9 Another embodiment of the present application provides a method for preparing a secondary battery, comprising the following steps:

[0116] Step 201: preparing an electrode assembly, wherein the electrode assembly includes a stacked positive electrode sheet, a negative electrode sheet, and a separator, wherein the separator includes a first separator and a second separator, and the negative electrode sheet is located between the first separator and the second separator;

[0117] In some embodiments, see Figure 8 , step S201 includes the following steps:

[0118] Step S2011: preparing at least two diaphragms, wherein the at least two diaphragms include a first diaphragm and a second diaphragm;

[0119] In some embodiments, see Figure 9 , step S2011 includes the following steps:

[0120] Step S20111: providing an isolation substrate, hot melt adhesive raw materials, and ceramic raw materials;

[0121] Step S20112: uniformly mixing the hot melt adhesive raw materials to obtain a first mixed slurry;

[0122] The hot melt adhesive raw material is the component of the hot melt adhesive layer, such as the hot melt polymer, binder and inorganic substance mentioned in the above embodiment. These components are mixed together according to a certain mass percentage to obtain the hot melt adhesive raw material.

[0123] Step S20113: uniformly mixing the ceramic raw materials to obtain a second mixed slurry;

[0124] The ceramic raw material includes one or more of silicon dioxide, aluminum oxide, zirconium dioxide or boehmite mentioned in the above embodiments, and is obtained by stirring and mixing according to the weight of the required ceramic raw material components.

[0125] Step S20114: applying the first mixed slurry to the first coating surface of the isolation substrate, and applying the second mixed slurry to the second coating surface of the isolation substrate layer, and then performing drying to obtain a diaphragm.

[0126] The purpose of the drying process is to dry out excess water so that the first mixed slurry and the second mixed slurry can adhere to the surface of the isolation substrate, thereby obtaining a separator.

[0127] Step S2012: providing a positive electrode sheet and a negative electrode sheet;

[0128] Step S2013: stacking the positive electrode sheet, the negative electrode sheet, the first separator, and the second separator to finally form an electrode assembly, wherein the negative electrode sheet is located between the first separator and the second separator.

[0129] Step 202: hot pressing the electrode assembly to bond the first hot melt adhesive layer to the second hot melt adhesive layer, so that the first separator and the second separator together cover the negative electrode plate;

[0130] Step S203: providing a packaging bag, placing the electrode assembly in the packaging bag, and heat-pressing and sealing the packaging bag;

[0131] Step S204: injecting electrolyte into the packaging bag, forming electrolyte and allowing the electrolyte to stand.

[0132] To facilitate readers' understanding of the technical solution of the present application, the following is an experimental comparison conducted by the inventor on multiple secondary batteries made with hot melt adhesive layers and ceramic layers of different thicknesses, as follows:

[0133] Example 1

[0134] Preparation of secondary batteries

[0135] (1) Preparation of electrode assembly

[0136] 1.1 Preparation of diaphragm:

[0137] Preparation of ceramic raw materials: 95%wt silicon dioxide (particle size 50-150nm, porosity 70%-80%) and 5%wt sodium carboxymethyl cellulose are uniformly mixed with deionized water to form slurry 1, with a solid content of 40%;

[0138] Prepare hot melt adhesive raw materials: 80% wt EVA (VA content 40%-60%, flow point ~60°C), 15% polyvinylidene fluoride, and 5% silica (particle size 50-150 nm, porosity 50%-80%) are uniformly mixed with deionized water to form slurry 2. The slurry has a solid content of 20%.

[0139] Slurry 1 was coated on a polyethylene substrate (5 μm thick) using gravure coating to a coating thickness of 1.5 μm. The coated surface was designated as side B. Slurry 2 was coated on the polyethylene substrate using gravure coating repeatedly to a coating thickness of 4 μm. The coated surface was the other side of side B, designated as side A. After drying, a finished diaphragm was obtained, designated as the first diaphragm. At this point, the dried first diaphragm formed a hot melt adhesive layer, a substrate layer, and a ceramic layer.

[0140] Repeat the above steps to obtain a second diaphragm;

[0141] 1.2 Provide a positive electrode sheet and a negative electrode sheet, stack the positive electrode sheet, the negative electrode sheet, a first separator, and a second separator, with the negative electrode sheet located between the first separator and the second separator, to form an electrode assembly.

[0142] (2) The electrode assembly is placed on the hot pressing end of a hot pressing instrument and hot pressed to form the electrode assembly so that the hot melt adhesive layers of the first and second separators are bonded to each other. The thickness of the negative electrode sheet is 150 μm, and the length L of the first and second separators extending beyond the negative electrode sheet is 0.8 mm.

[0143] (3) Providing a packaging bag, placing the electrode assembly in the packaging bag, and heat-pressing the packaging bag to seal the edges.

[0144] (4) After injecting the electrolyte into the packaging bag, forming and standing it, the preparation of the secondary battery is finally completed.

[0145] Examples 2-6

[0146] The difference from Example 1 is that the length L of the hot melt adhesive layer, the ceramic layer, the first separator and the second separator extending beyond the negative electrode sheet and the thickness of the negative electrode sheet are different.

[0147] The main body (including the first and second main bodies) of the separator of the secondary battery in Example 6 is not coated with slurry 2 , while the main body (including the first and second main bodies) of the separator of the secondary battery in Example 5 is coated with slurry 2 .

[0148] Table 1 Comparison of thickness of hot melt adhesive layer and ceramic layer in Examples 1-6

[0149]

[0150] Table 2 Experimental table of heating, drop test, and lithium deposition test of the secondary batteries of Examples 1-6 under full charge

[0151]

[0152] In the above experiment, the comparative example uses a conventional secondary battery. The passing condition for the constant temperature heating experiment in the fully charged state is that the secondary battery does not catch fire or explode during the constant temperature heating time. The passing condition for the drop test is that 1.5m, 1.8m and 2.0m represent the drop height of the secondary battery in the drop test, and the experimental conditions of the drop test are 6 sides, 4 corners and 5 rounds, that is, each secondary battery is dropped 5 times on six sides and four corners respectively. The secondary battery passes the test if it is not damaged, leaks and the voltage drop is less than 50mV. In the lithium plating test, 1.5C, 1.6C and 1.7C represent that the secondary battery is charged at different charging rates.

[0153] The experimental results of the comparative example and Examples 1-6 show that the qualified rate of secondary batteries in the constant temperature heating experiment under full charge is high, indicating that the installation of a ceramic layer is beneficial to improving the thermal stability of the secondary batteries. The results of the drop test show that the secondary batteries with both hot melt adhesive layers and ceramic layers have a higher pass rate, indicating that the installation of a hot melt adhesive layer helps to improve the connection between the separators, thereby improving the qualified rate of secondary batteries.

[0154] It can be seen from the experimental results of Example 1 and Example 2 that the thicker the ceramic layer, the better the thermal stability of the secondary battery. However, increasing the thickness of the ceramic layer will affect the bonding effect of the hot melt adhesive layer.

[0155] From the comparison of the experimental results of Example 2 and Example 3, it can be seen that increasing the thickness of the hot melt adhesive layer is more conducive to the bonding between the separators, and is more conducive to improving the thermal stability of the secondary battery. It is also conducive to improving the bonding strength between the separators and improving the reliability of the secondary battery.

[0156] From the comparison of the experimental results of Example 3 and Example 4, it can be seen that the higher the passing rate of the secondary battery drop test of Example 4, the thinner the thickness of the negative electrode plate is, the more conducive it is to improving the firm bonding of the hot melt adhesive layer and improving the reliability of the secondary battery.

[0157] From the comparison of the experimental results of Example 4 and Example 5, it can be seen that in the drop test experiment, the pass rate of the secondary battery of Example 5 is higher than the pass rate of the secondary battery of Example 4, indicating that the greater the length L of the diaphragm extending beyond the negative electrode sheet, the more conducive it is to the firm bonding between the diaphragms, thereby improving the reliability of the secondary battery.

[0158] From the comparison of the experimental results of Example 5 and Example 6, it can be seen that in the lithium deposition experiment, the secondary battery of Example 6 did not exhibit lithium deposition, indicating that not coating the main body of the diaphragm with a hot-melt high layer will be beneficial to lithium ion transmission and reduce the risk of lithium deposition in the secondary battery.

[0159] The above description is only an implementation method of the present application and does not limit the patent scope of the present application. Any equivalent structure or equivalent process transformation made using the contents of the description and drawings of this application, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present application.

Claims

1. A secondary battery comprising a housing and an electrode assembly, wherein the electrode assembly is housed in the housing; In the thickness direction of the electrode assembly, the electrode assembly includes a positive electrode sheet, a negative electrode sheet, a first separator and a second separator that are stacked, and the negative electrode sheet is located between the first separator and the second separator; The first separator includes a first substrate layer, the first substrate layer includes a first surface and a second surface facing each other, the second separator includes a second substrate layer, the second substrate layer includes a third surface and a fourth surface facing each other, the first surface and the third surface both face the negative electrode sheet, and the second surface and the fourth surface both face the positive electrode sheet; Its characteristics are: In a direction perpendicular to the thickness direction of the electrode assembly, the negative electrode sheet extends beyond the positive electrode sheet, the first separator includes a first main portion facing the negative electrode sheet and a first extension portion connected to the first main portion and extending beyond the negative electrode sheet, and the second separator includes a second main portion facing the negative electrode sheet and a second extension portion connected to the second main portion and extending beyond the negative electrode sheet, wherein the first extension portion and the second extension portion are located on the same side of the negative electrode sheet; The first diaphragm includes a first hot-melt adhesive layer arranged on the first surface and a first ceramic layer arranged on the second surface, the first hot-melt adhesive layer is located at least in the first extension portion, and the first ceramic layer is located in the first main body portion and the first extension portion; the second diaphragm includes a second hot-melt adhesive layer arranged on the third surface and a second ceramic layer arranged on the fourth surface, the second hot-melt adhesive layer is located at least in the second extension portion, and the second ceramic layer is located in the second main body portion and the second extension portion; the first extension portion and the second extension portion on both sides of the negative electrode sheet located on the outermost layer of the electrode assembly are bonded by the first hot-melt adhesive layer and the second hot-melt adhesive layer.

2. The secondary battery according to claim 1, wherein The first diaphragm also includes a third extension portion connected to the first main body and extending beyond the negative electrode sheet. The third extension portion and the first extension portion are located at opposite ends of the first main body. The second diaphragm also includes a fourth extension portion connected to the second main body and extending beyond the negative electrode sheet. The fourth extension portion and the second extension portion are located at opposite ends of the second main body. The third extension portion and the fourth extension portion on both sides of the negative electrode sheet located at the outermost layer of the electrode assembly are bonded by the first hot melt adhesive layer and the second hot melt adhesive layer.

3. The secondary battery according to claim 1, wherein The first hot melt adhesive layer includes a first part and a second part. Along the thickness direction of the electrode assembly, the projection of the first part is located within the projection of the negative electrode sheet, and the projection of the second part is located outside the projection of the negative electrode sheet. The viscosity flow temperature of the first part is A, and the viscosity flow temperature of the second part is B, satisfying 10℃<AB≤120℃.

4. The secondary battery according to claim 3, wherein The second hot melt adhesive layer includes a third part and a fourth part. Along the thickness direction of the electrode assembly, the projection of the third part is located within the projection of the negative electrode sheet, and the projection of the fourth part is located outside the projection of the negative electrode sheet. The viscosity flow temperature of the third part is C, and the viscosity flow temperature of the fourth part is D, satisfying 10℃<CD≤120℃.

5. The secondary battery according to claim 3, wherein The first part includes a hot-melt polymer, a binder, and inorganic particles, the sum of the mass percentages of the hot-melt polymer, the mass percentages of the binder, and the mass percentages of the inorganic particles is 100%, and at least one of the following conditions is met: (1) The mass percentage of hot-melt polymer is 50%-97%; (2) The mass percentage of the binder is 2%-49%; (3) The mass percentage of inorganic matter is 1%-10%, and the porosity of inorganic particles of the inorganic matter is greater than 50%.

6. The secondary battery according to claim 5, characterized in that At least one of the following conditions is met: (1) Hot-melt polymers include ethylene-vinyl acetate copolymer and / or polyethylene oxide; (2) The binder includes polyvinylidene fluoride; (3) The inorganic substance may include at least one of silicon dioxide, aluminum oxide, zirconium dioxide or boehmite.

7. The secondary battery according to claim 1, wherein The thickness of the first substrate layer is T1, the thickness of the first ceramic layer is T2, the thickness of the first hot melt adhesive layer is T3, the thickness of the second substrate layer is T4, the thickness of the second ceramic layer is T5, the thickness of the second hot melt adhesive layer is T6, and the thickness of the negative electrode sheet is t, and at least one of the following conditions is met: (1) 3μm≤T1≤9μm; (2) 1 μm ≤ T2 ≤ 5 μm; (3) 0.5 μm<T3≤6 μm; (4) 80 μm ≤ t ≤ 300 μm; (5) 3μm≤T4≤9μm; (6) 1 μm ≤ T5 ≤ 5 μm; (7) 0.5μm<T6≤6μm.

8. The secondary battery according to claim 7, wherein: At least one of the following conditions is met: (1) 1.5 μm ≤ T3 ≤ 5 μm; (2) 1 μm ≤ T2 ≤ 2 μm; (3) 80μm≤t≤100μm.

9. The secondary battery according to claim 1, wherein The thickness of the negative electrode plate is t. When observed along the thickness direction of the electrode assembly, the length of the first extension portion and / or the second extension portion in a direction perpendicular to the thickness direction of the electrode assembly is L, satisfying L≥0.5mm and L≥5t.

10. The secondary battery according to claim 1, wherein The negative electrode plate includes a negative electrode current collector, a first active material layer and a second active material layer. Along the thickness direction of the electrode assembly, the negative electrode current collector includes a first surface close to the first separator and a second surface close to the second separator. The first active material layer is provided on the first surface, and the second active material layer is provided on the second surface. The first active material layer includes a first active portion extending beyond the edge of the positive electrode plate, and the second active material layer includes a second active portion extending beyond the edge of the positive electrode plate. The first active portion is bonded to the first hot melt adhesive layer, and the second active portion is bonded to the second hot melt adhesive layer.

11. An electronic device, characterized in that: The secondary battery comprises the secondary battery according to any one of claims 1 to 10.

12. A method for preparing a secondary battery, applied to the secondary battery according to any one of claims 1 to 10, characterized in that: include: Prepare an electrode assembly, the electrode assembly comprising a stacked positive electrode sheet, a negative electrode sheet, and a separator, the separator comprising a first separator and a second separator, the negative electrode sheet being located between the first separator and the second separator; Hot pressing the electrode assembly to bond the first hot melt adhesive layer to the second hot melt adhesive layer, so that the first separator and the second separator jointly wrap the negative electrode sheet; Providing a packaging bag, placing the electrode assembly in the packaging bag, and heat-pressing and sealing the packaging bag; The electrolyte is injected into the packaging bag, and the electrolyte is formed and allowed to stand.

13. The preparation method according to claim 12, characterized in that The steps of preparing the electrode assembly include: preparing at least two diaphragms, wherein the at least two diaphragms include a first diaphragm and a second diaphragm; Provide positive electrode sheets and negative electrode sheets; The positive electrode sheet, the negative electrode sheet, the first separator and the second separator are stacked to finally form an electrode assembly, wherein the negative electrode sheet is located between the first separator and the second separator.

14. The preparation method according to claim 13, characterized in that The step of preparing at least two diaphragms comprises: Provide isolation substrates, hot melt adhesive raw materials and ceramic raw materials; uniformly mixing the hot melt adhesive raw materials to obtain a first mixed slurry; uniformly mixing the ceramic raw materials to obtain a second mixed slurry; The first mixed slurry is coated on the first coating surface of the isolation substrate, and the second mixed slurry is coated on the second coating surface of the isolation substrate layer, and then dried to obtain a separator.

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