Separator, and secondary battery and electronic device comprising same

Abstract

A separator and a device comprising same. The separator comprises a base material layer, a first bonding layer and a second bonding layer, which are arranged in a stacked manner in sequence, wherein both the first bonding layer and the second bonding layer are arranged on at least one side of the base material layer, and both the first bonding layer and the second bonding layer extend from a first end of the separator toward a second end opposite to the first end. The first bonding layer comprises first bonding particles, and the second bonding layer comprises second bonding particles; and the particle size of the first bonding particles is D1 μm, and the particle size of the second bonding particles is D2 μm, where D2＞ D1. By using the separator, the precipitation of lithium in a crease gap of a coiled battery can be reduced, thereby improving the cycling performance and safety performance of an electrochemical device.

Description

Separator, secondary battery and electronic device including the same Technical Field

[0001] The present application relates to the field of energy storage technology, and more specifically, to a separator and a secondary battery and an electronic device containing the separator. Background Art

[0002] Rechargeable electrochemical devices (such as lithium-ion batteries and sodium-ion batteries) are considered one of the most attractive energy storage systems due to their high energy density, relatively simple reaction mechanism, high operating voltage, long life, and environmental friendliness. Today, electrochemical devices are widely used in various fields, including wearable devices, smartphones, drones, and laptops.

[0003] With the widespread application of electrochemical devices in various fields, people's requirements for electrochemical devices are becoming increasingly higher. For example, people expect electrochemical devices to have higher energy density while also requiring them to have excellent cycling performance and safety. However, existing electrochemical devices struggle to meet these requirements. In view of this, there is an urgent need to improve the cycling performance and safety of electrochemical devices while increasing their energy density.

[0004] Summary of the Invention

[0005] At least in order to solve the above problems, the present application improves the cycle performance and safety performance of the electrochemical device by improving the separator of the electrochemical device.

[0006] According to one aspect of the present application, the present application provides a diaphragm, which includes a substrate layer, a first adhesive layer, and a second adhesive layer stacked in sequence, wherein the first adhesive layer and the second adhesive layer are both arranged on at least one side of the substrate layer, and the first adhesive layer and the second adhesive layer both extend from the first end of the diaphragm toward the second end opposite to the first end of the diaphragm; wherein the first adhesive layer includes first adhesive particles, the second adhesive layer includes second adhesive particles, the particle size of the first adhesive particles is D1μm, the particle size of the second adhesive particles is D2μm, and D2>D1.

[0007] According to an embodiment of the present application, 0.1≤D2-D1≤10.

[0008] According to an embodiment of the present application, 0.1≤D2-D1≤5.

[0009] According to an embodiment of the present application, 0.01≤D1≤1, and / or 0.1≤D2≤10.

[0010] According to an embodiment of the present application, the length L2 of the second adhesive layer is between the length L3 and 1 / 2 L3 of the substrate layer.

[0011] According to an embodiment of the present application, the first adhesive layer extends from the first end to the second end.

[0012] According to an embodiment of the present application, the first bonding particles include at least one of polyethyl acrylate, polybutyl acrylate, and a copolymer of butadiene and isobutyl acrylate.

[0013] According to an embodiment of the present application, the second bonding particles include at least one of polyvinylidene fluoride, polyethyl acrylate, and polybutyl acrylate.

[0014] According to an embodiment of the present application, one of the first adhesive layer and the second adhesive layer includes a dye.

[0015] According to an embodiment of the present application, the dye contains a chromophoric group and an auxiliary color group, the chromophoric group includes at least one of an azo group (—N=N—) and a vinyl group (—C=C—), and the auxiliary color group includes at least one of —COOH, —NH2, and —N(NH3)2.

[0016] According to an embodiment of the present application, the thickness of the first adhesive layer is T1 μm, the thickness of the second adhesive layer is T2 μm, and T1>T2.

[0017] According to an embodiment of the present application, 0.1≤T1-T2≤10.

[0018] According to an embodiment of the present application, 0.1≤T1-T2≤5.

[0019] According to an embodiment of the present application, the first adhesive layer is disposed on at least one surface of the substrate layer.

[0020] According to an embodiment of the present application, the diaphragm further includes a ceramic layer, and the ceramic layer is located between the substrate layer and the first adhesive layer.

[0021] According to an embodiment of the present application, the length L1 of the first adhesive layer is greater than the length L2 of the second adhesive layer so that at least a portion of the second end of the first adhesive layer close to the diaphragm is exposed from the second adhesive layer.

[0022] According to an embodiment of the present application, in the first adhesive layer, at least part of the first adhesive particles are distributed at intervals.

[0023] According to an embodiment of the present application, in the second adhesive layer, at least part of the second adhesive particles are distributed at intervals.

[0024] According to an embodiment of the present application, the second adhesive particles are located on the first adhesive particles.

[0025] According to another aspect of the present application, the present application also provides a secondary battery, which includes: a positive electrode sheet, wherein the positive electrode sheet includes a positive electrode collector and a positive electrode active material layer arranged on the positive electrode collector, and the positive electrode active material layer extends from the first end of the positive electrode sheet toward the second end opposite to the first end of the positive electrode sheet; a negative electrode sheet, wherein the negative electrode sheet includes a negative electrode collector and a negative electrode active material layer arranged on the negative electrode collector, and the negative electrode active material layer extends from the first end of the negative electrode sheet toward the second end opposite to the first end of the negative electrode sheet; and the diaphragm described in the above embodiment, wherein the diaphragm includes the substrate layer and the first adhesive layer and the second adhesive layer arranged on at least one surface of the substrate layer, and the first adhesive layer and the second adhesive layer both extend from the first end of the diaphragm toward the second end opposite to the first end of the diaphragm; wherein the diaphragm is located between the positive electrode sheet and the negative electrode sheet, and the positive electrode sheet, the diaphragm and the negative electrode sheet are stacked and wound around the central axis to form a flat electrode assembly.

[0026] According to an embodiment of the present application, the length dimension of the electrode assembly along the direction of the central axis is L cm, the width dimension of the electrode assembly in a direction perpendicular to the central axis is W cm, and L>W.

[0027] According to an embodiment of the present application, the secondary battery satisfies (L c +L a ) / 2-[(N-1) / 2]W+4≤L2≤(L c +L a ) / 2-2W+4, where N is the number of winding layers of the electrode assembly and is a positive integer greater than or equal to 5, L c is the length of the positive electrode active material layer; L a is the length of the negative electrode active material layer.

[0028] According to an embodiment of the present application, the secondary battery further satisfies (L c +L a ) / 2-[(N-4) / 2]W+4≤L2≤(L c +L a ) / 2-4.2W+4.

[0029] According to an embodiment of the present application, the diaphragm includes the ceramic layer, the ceramic layer is located between the substrate layer and the first adhesive layer, and the side of the diaphragm including the ceramic layer is arranged toward the positive electrode sheet.

[0030] According to another aspect of the present application, the present application further provides an electronic device, which includes the secondary battery described in the above embodiment of the present application.

[0031] According to another aspect of the present application, the present application also provides a method for preparing a secondary battery, which includes: preparing a positive electrode sheet; preparing a negative electrode sheet; preparing the diaphragm in the above embodiment, wherein the preparation of the diaphragm includes respectively configuring a first slurry containing the first bonding particles and a second slurry containing the second bonding particles, and coating the first slurry on the substrate layer from the first end of the diaphragm toward the second end opposite to the first end of the diaphragm to form the first bonding layer, and coating the second slurry on the first bonding layer from the first end of the diaphragm toward the second end of the diaphragm to form the second bonding layer, and the particle size of the first bonding particles is smaller than the particle size of the second bonding particles.

[0032] According to an embodiment of the present application, the length L1 of the first adhesive layer is greater than the length L2 of the second adhesive layer so that at least a portion of the second end of the first adhesive layer close to the diaphragm is exposed from the second adhesive layer.

[0033] According to an embodiment of the present application, in the first adhesive layer, at least part of the first adhesive particles are distributed at intervals.

[0034] According to an embodiment of the present application, in the second adhesive layer, at least part of the second adhesive particles are distributed at intervals.

[0035] According to an embodiment of the present application, the second adhesive particles are located on the first adhesive particles. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The following briefly describes the drawings necessary to describe the embodiments of the present application or the prior art to facilitate the description of the embodiments of the present application. Obviously, the drawings described below only represent some of the embodiments of the present application. Those skilled in the art can still derive drawings for other embodiments based on the structures illustrated in these drawings.

[0037] FIG1 shows a scanning electron microscope (SEM) image of a wound structure of a lithium-ion battery using the separator in Comparative Example 2 of the present application after 1000 charge and discharge cycles, and a partial magnified image of the tail region of the wound structure;

[0038] FIG2 shows an SEM image of the winding structure of a lithium-ion battery using the separator in Comparative Example 1 of the present application after 1000 charge and discharge cycles, and a partial enlarged image of the middle region of the winding structure;

[0039] FIG3 is a schematic diagram of a diaphragm structure proposed in one embodiment of the present application;

[0040] 4A and 4B are respectively SEM images of a plan view and a cross section of a separator in a folded region of a wound structure according to an embodiment of the present application;

[0041] FIG5 is a schematic diagram of a diaphragm structure proposed in another embodiment of the present application;

[0042] FIG6 is a schematic diagram of the structure of the positive electrode, separator and negative electrode stacked in sequence in the present application. DETAILED DESCRIPTION

[0043] The embodiments of the present application will be described in detail below. The embodiments of the present application should not be interpreted as limiting the present application.

[0044] As used in this application, the terms "including," "containing," and "comprising" are used in their open, non-limiting sense.

[0045] In addition, amounts, ratios, and other numerical values ​​are sometimes presented herein in a range format. It should be understood that such a range format is used for convenience and brevity and should be interpreted flexibly to include not only the values ​​explicitly specified as limits of the range, but also all individual values ​​or sub-ranges encompassed within the range, as if each value and sub-range were explicitly specified.

[0046] In the detailed description and claims, a list of items connected by the terms "one or more of," "one or more of," "at least one of," or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, the phrase "at least one of A or B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, the phrase "at least one of A, B, or C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may contain a single element or multiple elements. Item B may contain a single element or multiple elements. Item C may contain a single element or multiple elements.

[0047] Electrochemical devices (for example, lithium-ion batteries) typically include a positive electrode sheet (also called a cathode sheet), a negative electrode sheet (also called an anode sheet) and a separator, wherein the separator is located between the positive electrode sheet and the negative electrode sheet to prevent the occurrence of short circuits. Traditional lithium-ion batteries mainly have two structures: winding structure and laminated structure. The winding structure is: the positive electrode sheet, the negative electrode sheet and the separator are all long strips, and the separator is placed between the positive and negative electrode sheets; during assembly, the positive electrode sheet, the negative electrode sheet and the separator are wound into an electrode assembly by a winding needle, and then the production process is completed through packaging, injection, formation and other processes. In comparison, the difference of the laminated structure is that multiple positive and negative electrode tabs need to be welded together separately.

[0048] However, the present applicant has discovered that electrochemical devices with a wound structure experience significant capacity decay during charge-discharge cycles (for example, after 1,000 charge-discharge cycles), resulting in deterioration in cycling performance and safety. Further research has revealed that during charge-discharge cycles of wound batteries, purple-spotted lithium-deposited interfaces can form locally on the electrode sheets (particularly in the folds of the wound battery), causing localized interface abnormalities.

[0049] On the one hand, in the process of the pole piece being wound, as the winding is from the inner circle to the outer circle, the radius of curvature of the winding structure will gradually increase. Among them, at the tail of the winding structure (for example, the last 2 to 3 folds of the winding structure), the winding is relatively loose, which results in a larger gap between the positive and negative pole pieces and the diaphragm at the crease area, requiring more electrolyte to fill, and after the battery undergoes hundreds or thousands of charge and discharge cycles, it is often easy to lose electrolyte. In addition, the larger gap will also increase the transmission path of lithium ions, causing a larger mass transfer resistance. Under the dual effect of the lack of electrolyte and the increase in lithium ion mass transfer resistance, lithium ions are easily precipitated at the crease area of ​​the tail of the winding structure, thereby forming a purple spot lithium precipitation interface, causing local interface abnormality. For example, as shown in Figure 1, after the lithium ion battery in Comparative Example 2 of the present application undergoes 1000 charge and discharge cycles, a significant purple spot lithium precipitation interface is formed at the crease area of ​​the last 2-3 folds of the winding structure.

[0050] On the other hand, in contrast to the tail region of the wound structure, the crease gap in the middle region of the wound structure is smaller. During the electrochemical charge and discharge cycle, due to the continuous insertion and extraction of lithium ions between the positive and negative electrodes, this will cause the volume of the positive and negative pole pieces to expand and contract significantly, squeezing out the electrolyte and causing the loss of electrolyte, thereby forming a purple-spotted lithium precipitation interface in the crease region of the middle part of the wound structure. For example, as shown in Figure 2, after the lithium-ion battery in Comparative Example 1 of the present application has undergone 1000 charge and discharge cycles, a clear purple-spotted lithium precipitation interface is formed in the crease region near the middle position of the wound structure.

[0051] In response to the above technical challenges and findings, the present applicant believes that it is necessary to increase the fold gap in the middle region of the wound structure and reduce the fold gap in the tail region of the wound structure to ensure sufficient electrolyte supply and promote lithium ion transport during the charge and discharge process of the electrochemical device, thereby reducing lithium plating. Accordingly, the present applicant proposes a double-layer coated separator to solve the above technical problems.

[0052] In one embodiment, as shown in FIG3 , the diaphragm proposed in the present application includes a substrate layer 1, a first adhesive layer 2, and a second adhesive layer 3, wherein the first adhesive layer 1 and the second adhesive layer 2 are both arranged on one side of the substrate layer 1. As indicated by the dotted arrows in FIG3 , the first adhesive layer 1 and the second adhesive layer 2 extend from the first end (P) of the diaphragm toward the second end (Q) opposite to the first end (P) of the diaphragm, and at least a portion of the first adhesive layer 1 close to the second end of the diaphragm is exposed from the second adhesive layer 3. The first adhesive layer 1 includes first adhesive particles, and the second adhesive layer 2 includes second adhesive particles. The particle size of the first adhesive particles is D1 μm, and the particle size of the second adhesive particles is D2 μm, where D2>D1. FIG3 only shows that the first adhesive layer and the second adhesive layer are arranged on one side of the substrate layer. However, it should be understood that the first adhesive layer and the second adhesive layer can be arranged on the other side or both sides of the substrate layer.

[0053] The substrate layer has a starting end and a terminal end opposite to the starting end in its length direction (for example, in the direction indicated by the dotted arrow as shown in FIG3 ). In some embodiments, the second adhesive layer and the first adhesive layer extend from the starting end of the substrate layer along its length direction toward the terminal end opposite to the starting end. In some embodiments, at least some of the second adhesive particles in the second adhesive layer are spaced, that is, there are gaps between the second adhesive particles, which is conducive to the infiltration of the electrolyte and the increase of ion channels, and can be formed by spraying the slurry with a spraying device. In some embodiments, the second adhesive particles are uniformly spaced, which is also the second adhesive layer protected by the present application; the second adhesive layer extending from the starting end of the substrate layer toward the terminal end means that the second adhesive particles are spaced from the starting end of the substrate layer toward the terminal end. In some embodiments, at least some of the first adhesive particles in the first adhesive layer are spaced, that is, there are gaps between the first adhesive particles, which is conducive to the infiltration of the electrolyte and the increase of ion channels, and can be formed by spraying the slurry with a spraying device. In some embodiments, the extension of the second adhesive layer and / or the first adhesive layer is continuous and uninterrupted, which is conducive to enhancing the bonding force between the substrate layer and / or the electrode. In some embodiments, the first adhesive layer extends from the starting end of the substrate layer to the end end of the substrate layer.

[0054] In some embodiments, an electrochemical device (e.g., a secondary battery) includes a positive electrode sheet, a negative electrode sheet, and a separator in the above-described embodiments, wherein the separator is located between the positive electrode sheet and the negative electrode sheet. The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector, wherein the positive electrode active material layer extends from a first end of the positive electrode sheet toward a second end opposite to the first end of the positive electrode sheet. The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, wherein the negative electrode active material layer extends from a first end of the negative electrode sheet toward a second end opposite to the first end of the negative electrode sheet. In the process of preparing the electrochemical device, the positive electrode sheet, the separator, and the negative electrode sheet are stacked to form a stacked structure, wherein the stacked structure is wound around a central axis starting from the first end of the separator along the length direction of the stacked structure (e.g., the direction shown in FIG. 3 ) to form a flat wound structure (also referred to as an electrode assembly).

[0055] In the above manner, in the majority of the folding regions (at least the head region and the middle region) at the beginning of the wound structure, a first adhesive layer containing small-particle adhesive particles and a second adhesive layer containing large-particle adhesive particles are simultaneously provided on the substrate layer of the separator, wherein the second adhesive layer is located on the surface of the first adhesive layer to increase the gap in the electrode fold at the middle region of the wound structure to allow more electrolyte to be accommodated therein, thereby reducing the precipitation of metallic lithium. In addition, since the tail region of the wound structure is only coated with an adhesive layer containing small-particle adhesive particles, the gap in the folding region formed therein is smaller, thereby reducing the mass transfer resistance of lithium ions and reducing the precipitation of metallic lithium.

[0056] In some embodiments, the head region, middle region, and tail region described above each occupy one-third of the total length of the substrate layer. In some embodiments, the tail region described above occupies at most 1 / 3, 1 / 4, 1 / 5, 1 / 6, 1 / 7, 1 / 8, 1 / 9, or 1 / 10 of the total length of the substrate layer. In some embodiments, the head region and middle region described above can be collectively referred to as the front region, wherein the front region occupies at least 2 / 3, 3 / 4, 4 / 5, 5 / 6, 6 / 7, 7 / 8, 8 / 9, or 9 / 10 of the total length of the substrate layer.

[0057] Figures 4A and 4B of the present application respectively show the SEM images of the plane and cross section of the diaphragm at the fold area of ​​the winding structure described in the embodiment of the present application. As shown in Figure 4A, many small-particle adhesive particles (for example, small-particle adhesive particles indicated by the box in Figure 4A) are distributed on the substrate, and large-particle adhesive particles (for example, large-particle adhesive particles indicated by the oval box in Figure 4A) are distributed on the small-particle adhesive particles. Figure 4B shows an SEM image of the cross section of the diaphragm proposed in the present application. As shown in Figure 4B, the diaphragm includes a substrate layer 1, a first adhesive layer 2 including small-particle adhesive particles, and a second adhesive layer 3 including large-particle adhesive particles, wherein the first adhesive layer 2 is arranged between the substrate layer 1 and the second adhesive layer 3.

[0058] In some embodiments, the particle size D1 μm of the first adhesive particles satisfies 0.01≤D1≤1. In some embodiments, D1 can be, but is not limited to, 0.01, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, or 1, or within a range formed by any two of the above values.

[0059] In some embodiments, the particle size D2 μm of the second adhesive particles satisfies 0.1≤D2≤10. In some embodiments, D2 can be, but is not limited to, 0.1, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10, or within a range formed by any two of the above values.

[0060] It is worth noting that the particle sizes of the first and second adhesive particles mentioned above are calculated by statistical simulation using a scanning electron microscope (SEM). For example, the specific test method can be found in the following specific examples.

[0061] In some embodiments, the particle size difference between the first and second adhesive particles satisfies 0.1 ≤ D2 - D1 ≤ 10. In some embodiments, the particle size difference can be, but is not limited to, 0.1, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10, or within a range comprised of any two of the foregoing values. In some embodiments, 0.1 ≤ D2 - D1 ≤ 5.

[0062] In one embodiment, the lengths of the first adhesive layer and the second adhesive layer are L1 mm and L2 mm, respectively, where L1>L2. In this manner, at least a portion of the first adhesive layer near the second end (e.g., the terminal end) of the diaphragm is exposed from the second adhesive layer. The exposed portion of the first adhesive layer forms the tail end of the wound structure or the outer layer of the wound structure.

[0063] In some embodiments, the substrate layer has a length of L3 mm, wherein the second adhesive layer occupies at least half of the total length of the substrate layer, i.e., 1 / 2L3 ≤ L2 < L3. In some embodiments, L2 can be, but is not limited to, 1 / 2L3, 2 / 3L3, 3 / 4L3, 4 / 5L3, 5 / 6L3, 6 / 7L3, 7 / 8L3, 8 / 9L3, or 9 / 10L3, or within a range between any two of the foregoing values. For example, 3 / 4L3 ≤ L2 ≤ 9 / 10L3.

[0064] In some embodiments, the lengths of the first adhesive layer and the substrate layer satisfy L1 ≤ L3. In some embodiments, the first adhesive layer occupies at least 60%, 70%, 75%, 80%, 85%, 90%, 95%, or 98% of the total length of the substrate layer. In some embodiments, the first adhesive layer completely occupies the total length of the substrate layer.

[0065] In some embodiments, the first adhesive particles include, but are not limited to, at least one of polyethyl acrylate, polybutyl acrylate (PAB), butadiene, and isobutyl acrylate copolymers. In some embodiments, the second adhesive particles include, but are not limited to, at least one of polyvinylidene fluoride (PVDF), polyethyl acrylate, and polybutyl acrylate (PAB). Polyethyl acrylate and polybutyl acrylate (PAB) can have different particle sizes at different polymerization levels, and thus can be selected for the first and second adhesive layers based on their particle size.

[0066] In some embodiments, one of the first adhesive layer and the second adhesive layer includes a colorant. For example, the first adhesive layer includes a colorant while the second adhesive layer does not. In other embodiments, the second adhesive layer includes a colorant while the first adhesive layer does not. The colorant serves to determine the position and length of the second adhesive so as to ensure that at least a portion of the first adhesive layer is exposed from the second adhesive layer.

[0067] In some embodiments, the dye can be a dye commonly used in the art. For example, the dye comprises a chromophore and an auxochrome, wherein the chromophore includes but is not limited to at least one of an azo group (—N═N—) and a vinyl group (—C═C—), and the auxochrome includes but is not limited to at least one of an acidic group such as —COOH and a basic group such as —NH 2 or —N(NH 3 ) 2 . In some embodiments, the dye can be, but is not limited to, at least one of fuchsin, Congo red, methyl blue, methyl green, and gentian violet.

[0068] In some embodiments, the thickness of the first adhesive layer is T1 μm, and the thickness of the second adhesive layer is T2 μm, wherein T1>T2.

[0069] In some embodiments, 0.5≤T1≤15. In some embodiments, T1 can be, but is not limited to, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15, or is within a range formed by any two of the foregoing values.

[0070] In some embodiments, 0.01≤T2≤5. In some embodiments, T2 can be, but is not limited to, 0.01, 0.1, 1, 2, 3, 4, or 5, or within a range formed by any two of the foregoing values.

[0071] In some embodiments, the difference between the thickness of the first adhesive layer and the thickness of the second adhesive layer satisfies 0.1 ≤ T1-T2 ≤ 10. In some embodiments, the difference can be, but is not limited to, 0.1, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10, or within a range formed by any two of the foregoing values. In some embodiments, 0.1 ≤ T1-T2 ≤ 5.

[0072] In some embodiments, as shown in FIG5 , the separator further includes a ceramic layer 4 positioned between the substrate layer 1 and the first adhesive layer 2. The ceramic layer primarily protects the separator, improves its oxidation resistance and strength, and reduces the risk of localized micro-shorts. The ceramic layer may be composed of, but is not limited to, at least one of aluminum oxide, boehmite, zirconium oxide, boron nitride, silicon nitride, or aluminum nitride.

[0073] In some embodiments, the substrate layer described in the present application may include, but is not limited to, at least one of the following: polyethylene (PE), ultra-high molecular weight polyethylene (UHMWPE), high-density polyethylene (HDPE), polypropylene (PP), polyethylene terephthalate (PET), polyphenylene sulfide (PPS), cellulose, polyimide, polystyrene (PS), poly-4-methyl-1-pentene (TPX), polymethyl methacrylate (PMMA), polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polytetrafluoroethylene (PTFE) and polysulfone.

[0074] In some embodiments, the thickness of the substrate layer described herein is H1 μm, where 3 ≤ H1 ≤ 10. In some embodiments, H1 can be, but is not limited to, 3, 4, 5, 6, 7, 8, 9, or 10, or within a range consisting of any two of the foregoing values.

[0075] electrochemical devices

[0076] The present application further provides an electrochemical device comprising the separator of the above embodiment of the present application. In some embodiments, the electrochemical device further comprises a negative electrode, a positive electrode, and an electrolyte, wherein the separator is located between the positive electrode and the negative electrode.

[0077] The electrochemical device of the present application includes any device that generates an electrochemical reaction, and specific examples thereof include all kinds of primary batteries, secondary batteries, fuel cells, solar cells or capacitors. In particular, the electrochemical device is a lithium secondary battery, including a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery or a lithium ion polymer secondary battery.

[0078] In some embodiments, as shown in Figure 6, in the process of preparing an electrochemical device, the positive electrode C, the separator S and the negative electrode A are stacked in sequence to form a stacked structure, and the stacked structure is wound into a flat wound structure (also referred to as an electrode assembly) around the central axis along the length direction of the stacked structure (i.e., toward the second end (Q)) starting from the first end (P) of the separator. It is worth noting that in the actual production process of the electrode assembly, the length of the separator is generally greater than the length of the positive and negative electrodes to achieve complete isolation between the positive and negative electrodes, and compared to the positive electrode active material carried by the positive electrode C, the negative electrode A is generally required to contain an excess of negative electrode active material to avoid lithium precipitation. In addition, it should be understood that Figure 6 only shows a schematic diagram of the stacked structure formed by the separator S, the negative electrode A and the positive electrode C, but those skilled in the art can appropriately adjust the arrangement and length of the positive and negative electrodes and the separator according to the actual conditions in which they are located to achieve the spirit taught in this application. The length of the rolled structure along the central axis is L cm, and the width of the rolled structure is W cm, perpendicular to the central axis (but not in the thickness direction), where L>W. In this manner, at least in the head and middle regions of the rolled structure, the separator includes both the second adhesive layer and the first adhesive layer to increase the gap between the folded regions, thereby reducing the precipitation of metallic lithium. In the tail region of the rolled structure, the separator includes only the first adhesive layer to reduce the gap between the folded regions at the tail, thereby reducing the precipitation of metallic lithium.

[0079] 6 shows that the second adhesive layer and the first adhesive layer are provided on both sides of the substrate layer of the separator. However, it should be understood that the second adhesive layer and the first adhesive layer can also be provided on only one side of the substrate layer.

[0080] The present application further discovered that by designing and matching the dimensions of the positive electrode, negative electrode, and first adhesive layer in the electrochemical device, the cycling stability of the electrochemical device can be further improved. Specifically, the present application discovered that when the electrochemical device further satisfies the following formula, even better cycling performance and safety performance can be achieved: (L c +L a ) / 2-[(N-1) / 2]W+4≤L2≤(L c +L a ) / 2-2W+4,

[0081] Among them L c L is the length of the positive electrode coated with the positive electrode active material; a is the length of the negative electrode coated with the negative electrode active material, W is the width of the wound structure; L1 is the length of the first adhesive layer; N is the number of winding layers of the wound structure and is a positive integer greater than or equal to 5.

[0082] In some embodiments, when the electrochemical device further satisfies the following formula, the performance of the electrochemical device can be further improved: (L c +L a ) / 2-[(N-4) / 2]W+4≤L2≤(L c +L a ) / 2-4.2W+4.

[0083] In some embodiments, for example, as shown in FIG6 , when a ceramic layer is included on only one side of the separator substrate, the side of the separator containing the ceramic layer is disposed toward the positive electrode tab. This is because the present applicant has found that the side of the separator facing the positive electrode tab is more susceptible to damage, and therefore, coating the separator with a ceramic layer on the side facing the positive electrode tab can provide significant protection.

[0084] As follows, the present application will describe in detail the compositions of the positive electrode, the negative electrode, and the electrolyte.

[0085] positive electrode

[0086] The positive electrode includes a positive electrode material, and the positive electrode material includes a positive electrode material capable of absorbing and releasing lithium (Li) (hereinafter, sometimes referred to as a "positive electrode material capable of absorbing / releasing lithium Li"). Examples of the positive electrode material capable of absorbing / releasing lithium (Li) may include lithium cobaltate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium manganese oxide, lithium manganese iron phosphate, lithium vanadium phosphate, lithium vanadium oxyphosphate, lithium iron phosphate, lithium titanate, and lithium-rich manganese-based materials.

[0087] Specifically, the chemical formula of lithium cobalt oxide can be as shown in Chemical Formula 1: Li x1 Co a1 M1 b1 O 2-c1 Chemical formula 1

[0088] wherein M1 represents at least one selected from nickel (Ni), manganese (Mn), magnesium (Mg), aluminum (Al), boron (B), titanium (Ti), vanadium (V), chromium (Cr), iron (Fe), copper (Cu), zinc (Zn), molybdenum (Mo), tin (Sn), calcium (Ca), strontium (Sr), tungsten (W), yttrium (Y), lanthanum (La), zirconium (Zr) and silicon (Si), and the values ​​of x1, a1, b1 and c1 are respectively in the following ranges: 0.8≤x1≤1.2, 0.8≤a1≤1, 0≤b1≤0.2, and -0.1≤c1≤0.2;

[0089] The chemical formula of lithium nickel cobalt manganese oxide or lithium nickel cobalt aluminum oxide can be as shown in Chemical Formula 2: Li y1 Ni d1 M2 e1 O 2-f1 Chemical formula 2

[0090] wherein M2 represents at least one selected from the group consisting of cobalt (Co), manganese (Mn), magnesium (Mg), aluminum (Al), boron (B), titanium (Ti), vanadium (V), chromium (Cr), iron (Fe), copper (Cu), zinc (Zn), molybdenum (Mo), tin (Sn), calcium (Ca), strontium (Sr), tungsten (W), zirconium (Zr) and silicon (Si), and the values ​​of y1, d1, e1 and f1 are respectively within the following ranges: 0.8≤y1≤1.2, 0.3≤d1≤0.98, 0.02≤e1≤0.7, and -0.1≤f1≤0.2;

[0091] The chemical formula of lithium manganate can be as shown in Chemical Formula 3: Li z1 Mn 2-g1 M 3g1 O 4-h1 Chemical formula 3

[0092] Wherein M3 represents at least one selected from cobalt (Co), nickel (Ni), magnesium (Mg), aluminum (Al), boron (B), titanium (Ti), vanadium (V), chromium (Cr), iron (Fe), copper (Cu), zinc (Zn), molybdenum (Mo), tin (Sn), calcium (Ca), strontium (Sr) and tungsten (W), and the values ​​of z1, g1 and h1 are respectively in the following ranges: 0.8≤z1≤1.2, 0≤g1<1.0 and -0.2≤h1≤0.2.

[0093] negative electrode

[0094] The negative electrode includes a negative electrode material, which includes a negative electrode material capable of absorbing and releasing lithium (Li) (hereinafter, sometimes referred to as "negative electrode material capable of absorbing / releasing lithium Li"). Examples of the negative electrode material capable of absorbing / releasing lithium (Li) may include carbon materials, metal compounds, oxides, sulfides, lithium nitrides such as LiN3, lithium metal, metals that form alloys with lithium, and polymer materials.

[0095] The example of carbon material can comprise low graphitization carbon, easy graphitization carbon, artificial graphite, natural graphite, mesophase carbon microbeads, soft carbon, hard carbon, pyrolytic carbon, coke, glassy carbon, organic polymer compound sintered body, carbon fiber and activated carbon.Wherein, coke can comprise pitch coke, needle coke and petroleum coke.Organic polymer compound sintered body refers to the material obtained by calcining polymer material such as phenol plastic or furan resin so that carbonization at an appropriate temperature, and some of these materials are divided into low graphitization carbon or easy graphitization carbon.The example of polymer material can comprise polyacetylene and polypyrrole.

[0096] Among these negative electrode materials that can absorb / release lithium (Li), furthermore, materials with a charge and discharge voltage close to that of lithium metal are selected. This is because the lower the charge and discharge voltage of the negative electrode material, the easier it is for the electrochemical device (such as a lithium ion battery) to have a higher energy density. Among them, carbon materials can be selected as negative electrode materials because their crystal structure only changes slightly during charge and discharge, and therefore, good cycle characteristics and large charge and discharge capacity can be obtained. Graphite can be selected in particular because it can provide a large electrochemical equivalent and high energy density.

[0097] In addition, the negative electrode material capable of absorbing / releasing lithium (Li) may include single substance lithium metal, metallic elements and semi-metallic elements that can form alloys with lithium (Li), alloys and compounds of such elements, and the like. In particular, they are used together with carbon materials because in this case, good cycle characteristics and high energy density can be obtained. In addition to alloys comprising two or more metallic elements, the alloys used here also include alloys comprising one or more metallic elements and one or more semi-metallic elements. The alloy may be in the following states: solid solution, eutectic crystal (eutectic mixture), intermetallic compound, and mixtures thereof.

[0098] Examples of metal elements and semi-metal elements may include tin (Sn), lead (Pb), aluminum (Al), indium (In), silicon (Si), zinc (Zn), antimony (Sb), bismuth (Bi), cadmium (Cd), magnesium (Mg), boron (B), gallium (Ga), germanium (Ge), arsenic (As), silver (Ag), zirconium (Zr), yttrium (Y), and hafnium (Hf). Examples of the above alloys and compounds may include those having the chemical formula: Ma s Mb t Li u The material has the chemical formula: Ma p Mc q Md rIn these chemical formulas, Ma represents at least one element of a metal element and a semi-metal element capable of forming an alloy with lithium; Mb represents at least one element of a metal element and a semi-metal element other than lithium and Ma; Mc represents at least one element of a non-metal element; Md represents at least one element of a metal element and a semi-metal element other than Ma; and s, t, u, p, q, and r satisfy s>0, t≥0, u≥0, p>0, q>0, and r≥0.

[0099] In addition, inorganic compounds that do not include lithium (Li), such as MnO2, V2O5, V6O 13 , NiS and MoS.

[0100] electrolytes

[0101] The electrolyte may be one or more of a gel electrolyte, a solid electrolyte, and an electrolyte solution, wherein the electrolyte solution includes a lithium salt and a non-aqueous solvent.

[0102] The lithium salt is selected from one or more of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, LiSiF6, LiBOB, and lithium difluoroborate. For example, LiPF6 is selected as the lithium salt because it can provide high ionic conductivity and improve cycle characteristics.

[0103] The non-aqueous solvent may be a carbonate compound, a carboxylate compound, an ether compound, other organic solvents, or a combination thereof.

[0104] The carbonate compound may be a linear carbonate compound, a cyclic carbonate compound, a fluorinated carbonate compound, or a combination thereof.

[0105] Examples of chain carbonate compounds are diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), methylethyl carbonate (MEC), and combinations thereof. Examples of cyclic carbonate compounds are ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylethylene carbonate (VEC), and combinations thereof. Examples of fluorocarbonate compounds are fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, trifluoromethylethylene carbonate, and combinations thereof.

[0106] Examples of the carboxylic acid ester compound are methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolide, valerolactone, mevalonolactone, caprolactone, methyl formate, and combinations thereof.

[0107] Examples of the ether compound are dibutyl ether, tetraglyme, diglyme, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, and combinations thereof.

[0108] Examples of other organic solvents are dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, trioctyl phosphate, and phosphoric acid esters and combinations thereof.

[0109] The present application also provides a method for preparing a secondary battery, which includes: stacking a positive electrode, a separator and a negative electrode in sequence to form a stacked structure, wherein the positive electrode, the separator and the negative electrode have a common starting end; winding the stacked structure from the starting end to form a wound structure; placing the wound structure in a packaging shell, injecting an electrolyte and encapsulating it to obtain the secondary battery.

[0110] The present application further provides an electronic device, which includes the electrochemical device (eg, a secondary battery) according to the present application.

[0111] The use of the electrochemical device of the present application is not particularly limited, and it can be used in any electronic device known in the prior art. In some embodiments, the electrochemical device of the present application can be used in, but is not limited to, laptop computers, pen-type computers, mobile computers, e-book players, portable phones, portable fax machines, portable copiers, portable printers, head-mounted stereo headphones, video recorders, LCD televisions, portable cleaners, portable CD players, mini-discs, transceivers, electronic notepads, calculators, memory cards, portable recorders, radios, backup power supplies, motors, cars, motorcycles, power-assisted bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors.

[0112] The preparation of lithium-ion batteries is described below using lithium-ion batteries as an example and in combination with specific embodiments. Those skilled in the art will understand that the preparation method described in this application is only an example, and any other suitable preparation method is within the scope of this application.

[0113] Example

[0114] The following describes the performance evaluation of the lithium-ion battery according to the embodiments and comparative examples of the present application.

[0115] 1. Preparation of lithium-ion batteries

[0116] 1. Preparation method of diaphragm

[0117] A boehmite ceramic layer is first coated on the PE substrate, and then a first bonding layer is continuously coated on both sides of the substrate. The coating weight of the first bonding layer is 1.1 mg / 5000 mm 2 After coating, a second adhesive layer is applied on both sides of the substrate. The coating weight of the second adhesive layer is 5mg / 5000mm 2 The second adhesive layer contains a dye at an amount equal to 0.5% of the amount of the large-particle adhesive. After coating, the separator is slit into specific widths. Both the first and second adhesive layers extend from the starting end of the separator toward the terminal end opposite the starting end, with at least a portion of the first adhesive layer proximal to the terminal end protruding from the second adhesive layer. The adhesives used in the first and second adhesive layers and the coating lengths will be described in the specific examples.

[0118] 2. Preparation method of lithium-ion battery

[0119] The positive electrode active material, lithium cobalt oxide, conductive carbon (Super P), and binder polyvinylidene fluoride (PVDF), were dissolved in an N-methylpyrrolidone solvent system at a weight ratio of 96:2:2 and thoroughly stirred to prepare a positive electrode slurry. Using aluminum foil as the positive electrode current collector, the positive electrode slurry was applied from the starting end of the foil toward the opposite end, dried, cold-pressed, and then slit to produce the positive electrode.

[0120] The negative electrode active material, artificial graphite, a conductive agent (Super P), a binder of styrene-butadiene rubber, and a thickener of sodium carboxymethyl cellulose were dissolved in a deionized water solvent system at a weight ratio of 98:0.5:1:0.5 and thoroughly stirred to prepare a negative electrode slurry. Using copper foil as the negative electrode current collector, the negative electrode slurry was coated from the starting end of the foil toward the opposite end, dried, cold-pressed, and then stripped to produce the negative electrode. The coating lengths for the positive and negative electrodes will be described in the specific examples.

[0121] A solution prepared by mixing lithium salt LiPF6 and a non-aqueous organic solvent (ethylene carbonate (EC): diethyl carbonate (DEC): propylene carbonate (PC): propyl propionate (PP): vinylene carbonate (VC)) in a mass ratio of 8:92 was used as the electrolyte for lithium-ion batteries.

[0122] The positive electrode sheets and the separator-containing negative electrode sheets are stacked to form a laminated structure. This laminated structure is then wound around a central axis from the first end of the separator to form a flat electrode assembly. The electrode assembly is placed in an outer packaging, injected with electrolyte, and then sealed. After forming, the final lithium-ion battery product is produced. The resulting electrode assembly has a width W of 43 mm and a winding layer number N of 17.

[0123] 2. Test Method

[0124] 1. Test of bonding particle size

[0125] Testing instrument: SEM (scanning electron microscope)

[0126] Test Method and Procedure: The particle size of small-sized adhesive particles can be determined by observing the folded area of ​​the coiled structure using a SEM. Within a field of view of 5μm×5μm or 10μm×10μm, select 10 small-sized adhesive particles, measure their respective maximum dimensions as their particle size, and then take the average value as D1. The particle size of large-sized adhesive particles can be determined by observing the folded area of ​​the coiled structure using a SEM. Within a field of view of 15μm×15μm, select 10 large-sized adhesive particles, measure their respective maximum dimensions as their particle size, and then take the average value as D2.

[0127] 2. Electrochemical testing

[0128] The battery was subjected to a charge and discharge cycle test: the battery was charged to 4.5V at a constant current of 1.5C, charged to 0.05C at a constant voltage, and left to stand for 3 minutes; the battery was discharged to 3.0V at a constant current of 0.7C, and the charge and discharge cycle was repeated 1000 times according to the above process.

[0129] The initial thickness of the battery before the charge and discharge cycle is measured using a flat plate thickness gauge, which is T0 / mm. The thickness of the battery in the fully charged state after 500 and 1000 charge and discharge cycles is measured, which are recorded as T 500 / mm and T 1000 / mm.

[0130] 3. Test Results

[0131] Table 1 shows the effect of the diaphragm made by the double-layer adhesive coating process described in the present application on the electrochemical properties of the lithium-ion battery, wherein the length of the second adhesive layer is 632 mm, the length of the first adhesive layer is 325 mm, the length of the coated positive electrode active material in the positive electrode is 622 mm, and the length of the coated negative electrode active material in the negative electrode is 628 mm. The initial thickness of the lithium-ion batteries in Table 1 is 3.8 mm.

[0132] Table 1

[0133] As can be seen from Table 1, compared with Comparative Examples 1-3, after the diaphragm in the embodiment is double-coated with the first adhesive layer and the second adhesive layer, the thickness of the battery is significantly reduced after 500 and 1000 charge and discharge cycles. This shows that the diaphragm proposed in this application can effectively inhibit the precipitation of metallic lithium in the fold gap of the winding structure, thereby improving the cycle stability and safety performance of the electrochemical device. In addition, it can be seen from Table 1 that further adjusting the particle size of the adhesive particles to meet 0.1≤D2-D1≤10 can further improve the performance of the electrochemical device. When the particle size of the adhesive particles further meets 0.1≤D2-D1≤5, the improvement effect of the electrochemical device is more obvious.

[0134] Table 2 discusses the effects of the thickness of the first adhesive layer and the second adhesive layer on the electrochemical properties of lithium-ion batteries. The examples in Table 2 are adjustments made based on Examples 1-3 in Table 1. The only difference is the thickness of the first adhesive layer and the second adhesive layer.

[0135] Table 2

[0136] As can be seen from Table 2, adjusting the thickness of the first and second adhesive layers to meet the relationship 0.1 ≤ T1-T2 ≤ 10 can further improve the performance of the electrochemical device. Furthermore, when the particle size of the adhesive particles further meets the relationship 0.1 ≤ T1-T2 ≤ 5, the improvement in the electrochemical device is even more pronounced.

[0137] Table 3 discusses the effect of the length of the first bonding layer in the separator on the electrochemical properties of the lithium-ion battery when the lengths of the positive and negative electrodes in the battery are fixed. The examples in Table 3 are adjustments made based on Examples 1-3 in Table 1, and the only difference lies in the parameters listed in Table 3.

[0138] Table 3

[0139] As can be seen from Table 3, further adjusting the length of the second adhesive layer in the diaphragm to meet (L c +L a ) / 2-[(N-1) / 2]W+4≤L2≤(L c +L a ) / 2-2W+4, the performance of the electrochemical device can be further improved. In addition, when the length of the second adhesive layer meets (L c +L a ) / 2-[(N-4) / 2]W+4≤L2≤(L c +L a) / 2-4.2W+4, which can further optimize the performance of electrochemical devices.

[0140] References throughout this specification to "an embodiment," "part of an embodiment," "one embodiment," "another example," "an example," "a specific example," or "a portion of an example" mean that at least one embodiment or example in this application includes the specific features, structures, materials, or characteristics described in that embodiment or example. Therefore, descriptions appearing throughout this specification, such as, "in some embodiments," "in an embodiment," "in another example," "in an example," "in a specific example," or "an example," are not necessarily references to the same embodiment or example in this application. In addition, the specific features, structures, materials, or characteristics described herein may be combined in any suitable manner in one or more embodiments or examples.

[0141] Although illustrative embodiments have been shown and described, those skilled in the art should understand that the above embodiments should not be construed as limitations on the present application, and that changes, substitutions, and modifications may be made to the embodiments without departing from the spirit, principles, and scope of the present application.

Claims

1. A diaphragm, comprising a substrate layer, a first adhesive layer, and a second adhesive layer stacked in sequence, wherein the first adhesive layer and the second adhesive layer are both arranged on at least one side of the substrate layer, and the first adhesive layer and the second adhesive layer both extend from a first end of the diaphragm toward a second end opposite to the first end of the diaphragm; in, The first adhesive layer includes first adhesive particles, and the second adhesive layer includes second adhesive particles. The particle size of the first adhesive particles is D1 μm, and the particle size of the second adhesive particles is D2 μm, where D2>D1. The separator according to claim 1 , wherein 0.1≤D2-D1≤10. The separator according to claim 1 , wherein 0.1≤D2-D1≤5.

4. The diaphragm according to claim 1, wherein 0.01≤D1≤1, and / or 0.1≤D2≤10. The separator according to claim 1 , wherein a length L2 of the second adhesive layer is between a length L3 of the substrate layer and 1 / 2 L3. The diaphragm of claim 1 , wherein the first adhesive layer extends from the first end to the second end.

7. The diaphragm according to claim 1, wherein The first bonding particles include at least one of polyethyl acrylate, polybutyl acrylate and butadiene-isobutyl acrylate copolymer.

8. The diaphragm according to claim 1, wherein The second bonding particles include at least one of polyvinylidene fluoride, polyethyl acrylate and polybutyl acrylate.

9. The diaphragm according to claim 1, wherein: One of the first adhesive layer and the second adhesive layer includes a coloring agent.

10. The diaphragm according to claim 9, wherein The dye comprises a chromophoric group and an auxiliary color group, wherein the chromophoric group comprises at least one of an azo group (—N=N—) and a vinyl group (—C=C—), and the auxiliary color group comprises at least one of —COOH, —NH2, and —N(NH3)2.

11. The diaphragm according to claim 1, wherein The thickness of the first adhesive layer is T1 μm, the thickness of the second adhesive layer is T2 μm, and T1>T2.

12. The diaphragm according to claim 11, wherein 0.1≤T1-T2≤10.

13. The diaphragm according to claim 11, wherein 0.1≤T1-T2≤5.

14. The diaphragm according to claim 1, wherein The first adhesive layer is disposed on at least one surface of the substrate layer. 15 . The separator according to claim 1 , further comprising a ceramic layer, wherein the ceramic layer is located between the substrate layer and the first bonding layer.

16. The diaphragm according to claim 1, wherein The length L1 of the first adhesive layer is greater than the length L2 of the second adhesive layer so that at least a portion of the first adhesive layer close to the second end is exposed from the second adhesive layer.

17. The diaphragm according to claim 1, wherein In the first adhesive layer, at least part of the first adhesive particles are distributed at intervals; and / or In the second adhesive layer, at least some of the second adhesive particles are distributed in an interval.

18. The diaphragm according to claim 17, wherein: The second bonding particles are located on the first bonding particles.

19. A secondary battery comprising: A positive electrode sheet, the positive electrode sheet comprising a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector, the positive electrode active material layer extending from a first end of the positive electrode sheet toward a second end opposite to the first end of the positive electrode sheet; A negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, the negative electrode active material layer extending from a first end of the negative electrode sheet toward a second end opposite to the first end of the negative electrode sheet; as well as The diaphragm according to any one of claims 1 to 18; The separator is located between the positive electrode sheet and the negative electrode sheet, and the positive electrode sheet, the separator and the negative electrode sheet are stacked and wound around a central axis to form a flat electrode assembly. 20 . The secondary battery according to claim 19 , wherein the length of the electrode assembly along the central axis is L cm, the width of the electrode assembly perpendicular to the central axis is W cm, and L>W.

21. The secondary battery according to claim 20, wherein the secondary battery satisfies (L c +L a ) / 2-[(N-1) / 2]W+4≤L2≤(L c +L a ) / 2-2W+4, wherein N is the number of winding layers of the electrode assembly and is a positive integer greater than or equal to 5, L c is the length of the positive electrode active material layer; L a is the length of the negative electrode active material layer.

22. The secondary battery according to claim 21, wherein the secondary battery further satisfies (L c +L a ) / 2-[(N-4) / 2]W+4≤L2≤(L c +L a ) / 2-4.2W+4. 23 . The secondary battery according to claim 19 , wherein the separator comprises the ceramic layer, the ceramic layer is located between the substrate layer and the first adhesive layer, and a side of the separator including the ceramic layer is disposed toward the positive electrode sheet.

24. An electronic device comprising the secondary battery according to any one of claims 19 to 23.

25. A method for preparing a secondary battery, comprising: preparing a positive electrode sheet; preparing a negative electrode sheet; Prepare a diaphragm as described in any one of claims 1 to 18, wherein the preparation of the diaphragm includes respectively configuring a first slurry containing the first bonding particles and a second slurry containing the second bonding particles, coating the first slurry on the substrate layer from the first end of the diaphragm toward the second end opposite to the first end of the diaphragm to form the first bonding layer, coating the second slurry on the first bonding layer from the first end of the diaphragm toward the second end of the diaphragm to form the second bonding layer, and the particle size of the first bonding particles is smaller than the particle size of the second bonding particles.

26. The method for preparing a secondary battery according to claim 25, wherein: The length L1 of the first adhesive layer is greater than the length L2 of the second adhesive layer so that at least a portion of the second end of the first adhesive layer close to the diaphragm is exposed from the second adhesive layer.

27. The method for preparing a secondary battery according to claim 25, wherein: In the first adhesive layer, at least part of the first adhesive particles are distributed at intervals; and / or In the second adhesive layer, at least some of the second adhesive particles are distributed in an interval.

28. The diaphragm according to claim 27, wherein The second bonding particles are located on the first bonding particles.

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