Secondary battery and electronic device

Abstract

The invention provides a secondary battery and an electronic device, the secondary battery comprises an electrode assembly of a winding structure, the electrode assembly comprises a positive pole piece, a negative pole piece, a first diaphragm and a second diaphragm, and at least part of the negative pole piece is located between the first diaphragm and the second diaphragm; in one winding circle of the electrode assembly, the first diaphragm is far away from the winding center of the electrode assembly compared with the second diaphragm; the first diaphragm comprises a first base membrane, the second diaphragm comprises a second base membrane, the porosity n1 of the first base membrane is greater than the porosity n2 of the second base membrane, the puncture strength F2 of the second base membrane is greater than the puncture strength F1 of the first base membrane, 30% < = n1 < = 60%, and 300 gf < = F2 < = 650 gf. The secondary battery comprises the structure, and the manufacturing yield and the dynamic performance of the secondary battery can be both considered.

Description

Technical Field

[0001] The present application relates to the field of electrochemical technologies, and particularly to a secondary battery and an electronic device. Background Art

[0002] With the wide application of secondary batteries in various fields, people have higher and higher requirements for secondary batteries, and people expect secondary batteries to have more excellent performance.

[0003] Two identical separators are usually selected for secondary batteries. Due to the characteristics of the separators, it is impossible to simultaneously meet high puncture resistance and high kinetic performance. Therefore, in some cases, secondary batteries cannot take into account both manufacturing yield and kinetic performance. Summary of the Invention

[0004] The purpose of the present application is to provide a secondary battery and an electronic device to take into account both manufacturing yield and kinetic performance.

[0005] It should be noted that in the summary of the invention of the present application, a lithium-ion battery is used as an example of the secondary battery to explain the present application. However, the secondary battery of the present application is not limited to lithium-ion batteries. The specific technical solutions are as follows:

[0006] The first aspect of the present application provides a secondary battery, which includes a wound electrode assembly. The electrode assembly includes a positive electrode tab, a negative electrode tab, a first separator and a second separator. At least a part of the negative electrode tab is located between the first separator and the second separator. The first separator includes a first base film, and the second separator includes a second base film. The porosity n1 of the first base film is greater than the porosity n2 of the second base film, and the puncture resistance F2 of the second base film is greater than the puncture resistance F1 of the first base film, where 30% ≤ n1 ≤ 60% and 300 gf ≤ F2 ≤ 650 gf. Thus, by using the first base film with a higher porosity and the second base film with a higher puncture resistance, and regulating the porosity of the first base film and the puncture resistance of the second base film within the above ranges, it is beneficial to take into account both the kinetic performance and the manufacturing yield of the secondary battery.

[0007] In an embodiment of the present application, the electrode assembly includes a flat region and a corner region. The negative electrode tab includes a negative current collector and a first negative electrode material layer and a second negative electrode material layer located on both sides of the negative current collector in the thickness direction. In the corner region, a corner segment of a first positive electrode tab is adjacent to a corner segment of a first negative electrode material layer. The corner segment of the first positive electrode tab is farther from the winding center of the electrode assembly than the corner segment of the first negative electrode material layer. The first separator is located between the corner segment of the first positive electrode tab and the corner segment of the first negative electrode material layer. By disposing the first separator including the first base film between the corner segment of the first positive electrode tab and the corner segment of the first negative electrode material layer, the first base film with a higher porosity has a higher liquid retention capacity, which is beneficial to improving the problem of insufficient electrolyte in the corner region and is also beneficial to the transmission of active ions to improve the interface problem, thereby improving the lithium deposition problem of the secondary battery, that is, improving the kinetic performance.

[0008] In some embodiments of the present application, 40% ≤ n1 ≤ 50%, and / or, 400 gf ≤ F2 ≤ 500 gf. By regulating the porosity of the first base film and the puncture resistance of the second base film within the scope of the present application, the first separator has good wettability and liquid absorption and retention capabilities, which is beneficial to improving the infiltration performance of the electrolyte, enhancing the transmission rate of lithium ions, further reducing the lithium deposition risk of the negative electrode tab, and further improving the kinetic performance of the secondary battery. At the same time, it is beneficial to enhancing the puncture resistance of the second separator and further increasing the excellent rate of Hi-pot and K value of the secondary battery, thereby further improving the manufacturing excellent rate of the secondary battery.

[0009] In an embodiment of the present application, the first base film includes a first resin material, and the weight average molecular weight of the first resin material is 500,000 to 900,000; the second base film includes a second resin material and a third resin material, the weight average molecular weight of the second resin material is 500,000 to 900,000, and the weight average molecular weight of the third resin material is 1,000,000 to 2,000,000. By regulating the weight average molecular weights of the first resin material, the second resin material, and the third resin material within the scope of the present application, the obtained first base film has appropriate puncture resistance and a higher porosity, which is beneficial to improving lithium ion transmission and reducing the lithium deposition risk of the negative electrode tab; the obtained second base film has higher puncture resistance and appropriate porosity, which is beneficial to improving lithium ion transmission, increasing the excellent rate of Hi-pot and K value of the secondary battery, and further improving the manufacturing excellent rate of the secondary battery while taking into account the kinetic performance.

[0010] In an embodiment of the present application, based on the mass of the second base film, the mass percentage content W1 of the second resin material is 70% to 90%, and the mass percentage content W2 of the third resin material is 10% to 30%. By adjusting the mass percentage contents of the second resin material and the third resin material within the scope of the present application, the obtained second base film has both a suitable porosity and a high puncture resistance strength, which is beneficial to improving lithium ion transport, reducing the risk of the separator being punctured by hard particles of the negative electrode sheet, and increasing the excellent rate of Hi-pot and K value of the secondary battery, thereby further improving the manufacturing excellent rate of the secondary battery while taking into account the kinetic performance.

[0011] In an embodiment of the present application, the first resin material, the second resin material, and the third resin material are each independently selected from at least one of polyethylene, polypropylene, polyimide, polyethyleneimine, or polyethylene terephthalate. When the first resin material, the second resin material, and the third resin material are selected from the materials within the scope of the present application, it is beneficial to obtain a first base film that takes into account a relatively high porosity and a suitable puncture resistance strength, and a second base film that takes into account a suitable porosity and a high puncture resistance strength, thereby further improving the manufacturing excellent rate of the secondary battery while taking into account the kinetic performance.

[0012] In an embodiment of the present application, in the differential scanning calorimetry test spectrum of the first base film, there is a characteristic peak at 128°C to 134°C. In the differential scanning calorimetry test spectrum of the first base film, there is the above-mentioned characteristic peak, and the peak value of this characteristic peak corresponds to the melting temperature of the first resin material.

[0013] In an embodiment of the present application, in the differential scanning calorimetry test spectrum of the second base film, there are characteristic peaks at 128°C to 134°C and 136°C to 140°C. In the differential scanning calorimetry test spectrum of the second base film, there is a characteristic peak at 128°C to 134°C, and the peak value of this characteristic peak corresponds to the melting temperature of the second resin material. In the differential scanning calorimetry test spectrum, there is a characteristic peak at 136°C to 140°C, and the peak value of this characteristic peak corresponds to the melting temperature of the third resin material.

[0014] In an embodiment of the present application, 10% ≤ n2 ≤ 30%, 200 gf ≤ F1 ≤ 300 gf. By regulating the porosity of the second base film and the puncture resistance of the first base film within the scope of the present application, while having a relatively high puncture resistance, the second base film has a relatively appropriate porosity and good ion transport performance, which is beneficial to reducing the risk of the diaphragm being punctured by hard particles of the negative electrode sheet, and improving the excellent rate of Hi-pot and K value of the secondary battery; while having a relatively high porosity, the first base film has a relatively appropriate puncture resistance, which is beneficial to improving lithium ion transport, reducing the risk of lithium deposition on the negative electrode sheet, and at the same time is beneficial to reducing the risk of the diaphragm being punctured by hard particles of the negative electrode sheet, improving the excellent rate of Hi-pot and K value of the secondary battery, thereby further improving the kinetic performance of the secondary battery while taking into account the manufacturing excellent rate.

[0015] In an embodiment of the present application, 15% ≤ n2 ≤ 30%. By regulating the porosity of the second base film within the scope of the present application, while having a relatively high puncture resistance, the second base film has a relatively appropriate porosity, which is beneficial to improving the wettability, liquid absorption and liquid retention ability of the second diaphragm, beneficial to improving the infiltration performance of the electrolyte, improving lithium ion transport, reducing the risk of lithium deposition on the negative electrode sheet, and further improving the manufacturing excellent rate of the secondary battery while taking into account the kinetic performance.

[0016] In an embodiment of the present application, the thicknesses of the first base film and the second base film are each independently 4 μm to 7 μm. When the thicknesses of the first base film and the second base film are within the above range, it is also beneficial to take into account the energy density of the secondary battery.

[0017] In an embodiment of the present application, the first separator further includes a first ceramic layer, and the porosity of the first separator is n3, 40% ≤ n3 ≤ 70%. The setting of the first ceramic layer is beneficial to further improving the liquid absorption and liquid retention ability of the first separator and further improving the kinetic performance of the secondary battery.

[0018] In an embodiment of the present application, the coating weight CW1 of the first ceramic layer is 9 mg / 5000 mm 2 to 15 mg / 5000 mm 2 . When the coating weight CW1 of the first ceramic layer is within the above range, the first ceramic layer has an appropriate thickness, which is beneficial to taking into account its energy density while improving the kinetic performance of the secondary battery.

[0019] In an embodiment of the present application, the first ceramic layer includes first ceramic particles, and the first ceramic particles include at least one of alumina, boehmite, silicon oxide, magnesium oxide, titanium oxide, tin oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, aluminum hydroxide, magnesium hydroxide, or calcium hydroxide. By selecting the first ceramic particles of the above types, it is beneficial to further improve the liquid absorption and retention capabilities of the first separator, and thus improve the kinetic performance of the secondary battery.

[0020] In some embodiments of the present application, the first negative electrode material layer includes a first negative electrode material, the second negative electrode material layer includes a second negative electrode material, and the first negative electrode material and / or the second negative electrode material includes a silicon material. When the first separator and the second separator are used in combination, the first separator has good liquid absorption and retention properties, and the second separator has a high puncture resistance, which is beneficial to taking into account the energy density, kinetic performance, and manufacturing yield of the secondary battery.

[0021] In an embodiment of the present application, the silicon material includes at least one of silicon, silicon carbide compound, silicon oxide compound, or silicon alloy. When the silicon material satisfies the above characteristics, it is beneficial to improve the energy density of the secondary battery.

[0022] The second aspect of the present application provides an electronic device, which includes the secondary battery described in the first aspect of the present application.

[0023] Advantages of the present application:

[0024] The present application provides a secondary battery, which includes a wound electrode assembly. The electrode assembly includes a positive electrode tab, a negative electrode tab, a first separator, and a second separator. At least a part of the negative electrode tab is located between the first separator and the second separator; in one winding turn of the electrode assembly, the first separator is farther from the winding center of the electrode assembly than the second separator; the first separator includes a first base film, the second separator includes a second base film, the porosity n1 of the first base film is greater than the porosity n2 of the second base film, and the puncture resistance F2 of the second base film is greater than the puncture resistance F1 of the first base film, where 30% ≤ n1 ≤ 60% and 300 gf ≤ F2 ≤ 650 gf. By using the first base film with a high porosity and the second base film with a high puncture resistance, and the porosity of the first base film is within the scope of the present application, the first separator has good wettability and liquid absorption and retention capabilities, which is beneficial to improving the infiltration performance of the electrolyte, increasing the transmission rate of lithium ions, reducing the risk of lithium deposition on the negative electrode tab, and improving the kinetic performance of the secondary battery; at the same time, the puncture resistance of the second base film is within the scope of the present application, which can improve the puncture resistance of the second separator, reduce the risk of the separator being punctured by hard particles on the negative electrode tab, and improve the yield of Hi-pot (high voltage withstand test value) and K value (battery voltage drop per unit time) of the secondary battery, thereby taking into account the manufacturing yield and kinetic performance of the secondary battery.

[0025] Of course, it is not necessary for any product or method implementing the present application to achieve all the above-mentioned advantages simultaneously. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the accompanying drawings required for use in the embodiments will be briefly introduced below. Obviously, the accompanying drawings in the following description are only some embodiments of the present application, and those of ordinary skill in the art can also obtain other embodiments based on these drawings.

[0027] Figure 1 Schematic structural diagram of an electrode assembly in a specific implementation solution of the present application along its own thickness direction;

[0028] Figure 2 is Figure 1 partial enlarged view of A in

[0029] Figure 3 Schematic structural diagram of an electrode assembly in another implementation solution of the present application along its own thickness direction;

[0030] Figure 4 is Figure 3 partial enlarged view of B in

[0031] Figure 5 Schematic cross-sectional structural diagram of a first separator in a specific implementation solution of the present application along its own thickness direction;

[0032] Figure 6 Cyclic capacity retention rate curve diagram of lithium-ion batteries prepared in Example 1-1 and Comparative Example 2 of the present application;

[0033] Figure 7 Cyclic expansion rate curve diagram of lithium-ion batteries prepared in Example 1-1 and Comparative Example 2 of the present application;

[0034] Figure 8 Differential scanning calorimetry test spectrum diagram of the first base film in Example 1-1 of the present application;

[0035] Figure 9 Differential scanning calorimetry test spectrum diagram of the second base film in Example 1-1 of the present application. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0036] The technical solutions in the present application will be clearly and completely described below in conjunction with the embodiments of the present application and the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, rather than all embodiments. All other embodiments obtained by those skilled in the art based on the present application fall within the scope of protection of the present application.

[0037] It should be noted that in the specific embodiments of the present application, a lithium-ion battery is taken as an example of the secondary battery to explain the present application. However, the secondary battery of the present application is not limited to lithium-ion batteries.

[0038] The present application provides a secondary battery, which includes a wound electrode assembly. As Figure 1 and Figure 3 shown, the electrode assembly 01 includes a positive electrode tab 10, a negative electrode tab 20, a first separator 30, and a second separator 40. At least a part of the negative electrode tab 20 is located between the first separator 30 and the second separator 40. The first separator 30 includes a first base film, and the second separator 40 includes a second base film. The porosity n1 of the first base film is greater than the porosity n2 of the second base film. The first base film can store more electrolyte, which is beneficial to improving the kinetic performance of the secondary battery. The puncture resistance F2 of the second base film is greater than the puncture resistance F1 of the first base film. The puncture resistance performance of the second base film is improved, which can improve the excellent rate of Hi-pot and K value of the secondary battery and is beneficial to improving the manufacturing excellent rate of the secondary battery.

[0039] In an embodiment of the present application, 30% ≤ n1 ≤ 60%. For example, the porosity n1 of the first base film can be 30%, 35%, 40%, 45%, 50%, 55%, 60% or a range composed of any two of these values. When the porosity of the first base film is too small, for example, less than 30%, it will lead to poor electrolyte infiltration, an increased risk of lithium deposition on the negative electrode tab, and poor kinetic performance of the secondary battery. When the porosity of the first base film is too large, for example, greater than 60%, it will lead to a decrease in the puncture resistance of the first base film and a decrease in the manufacturing excellent rate. Therefore, by regulating the porosity of the first base film within the scope of the present application, the first separator has good wettability, liquid absorption, and liquid retention capabilities, which is beneficial to improving the electrolyte infiltration performance, increasing the lithium ion transmission rate, reducing the risk of lithium deposition on the negative electrode tab, improving the kinetic performance of the secondary battery, and taking into account the manufacturing excellent rate.

[0040] In an embodiment of the present application, 300 gf ≤ F2 ≤ 650 gf. For example, the puncture resistance F2 of the second base film can be 300 gf, 350 gf, 400 gf, 450 gf, 500 gf, 550 gf, 600 gf, 650 gf, or a range composed of any two of these values. When the puncture resistance of the second base film is too small, such as less than 300 gf, the puncture resistance of the second base film is too low, and the hard particles in the negative electrode material layer may pierce the separator, posing a safety risk and affecting the manufacturing yield of the secondary battery; when the puncture resistance of the second base film increases, the tendency of the separator to undergo physical short circuit during the preparation of the secondary battery will decrease, but if the puncture resistance is too high, such as greater than 650 gf, it will affect the kinetic performance of the secondary battery, thereby affecting the degree of lithium plating of the secondary battery. Therefore, adjusting the puncture resistance of the second base film within the scope of this application can improve the puncture resistance performance of the second separator, thereby improving the manufacturing yield of the secondary battery, such as the Hi-pot yield and the K-value yield.

[0041] Thus, using the first base film with a higher porosity and the second base film with a higher puncture resistance, and adjusting the porosity of the first base film and the puncture resistance of the second base film within the above ranges is beneficial to balance the kinetic performance and manufacturing yield of the secondary battery.

[0042] In an embodiment of the present application, as Figures 1 to 4 shown, the electrode assembly 01 includes a flat area 011 and a corner area 012. The positive electrode sheet 10 includes a positive electrode current collector 110 and positive electrode material layers 120 provided on two surfaces of the positive electrode current collector 110. The negative electrode sheet 20 includes a negative electrode current collector 210 and a first negative electrode material layer 220 and a second negative electrode material layer 230 located on both sides in the thickness direction of the negative electrode current collector.

[0043] In some embodiments of the present application, as Figure 1 and Figure 2As shown, in the same layer of the negative electrode tab 20, the first negative electrode material layer 220 is farther from the winding center of the electrode assembly 01 than the second negative electrode material layer 230. In the corner area 012 of the electrode assembly 01, a first positive electrode tab corner segment 101 is adjacent to a first negative electrode material layer corner segment 221. The first positive electrode tab corner segment 101 is farther from the winding center of the electrode assembly 01 than the first negative electrode material layer corner segment 221. The first separator 30 is located between the first positive electrode tab corner segment 101 and the first negative electrode material layer corner segment 221, and the second separator 40 is adjacent to the second negative electrode material layer 230. In the wound electrode assembly structure, a first positive electrode tab corner segment is adjacent to a first negative electrode material layer corner segment and is far from the winding center of the electrode assembly. At this time, the winding radius of the positive electrode material layer in the positive electrode tab is greater than that of the first negative electrode material layer, and the positive-negative ratio (Cell Balance, CB, the ratio of the negative electrode capacity per unit area to the positive electrode capacity per unit area) is small, and interfacial problems are likely to occur, resulting in lithium deposition. Therefore, a relatively sufficient electrolyte is required to transport active ions. At the same time, the corner area of the wound structure itself is prone to problems such as insufficient electrolyte or even electrolyte breakage, which will further exacerbate the interfacial problems. Therefore, in the present application, the first separator including the first base film is disposed between the first positive electrode tab corner segment and the first negative electrode material layer corner segment. The first base film with a higher porosity has a higher liquid retention capacity, which is beneficial to improving the problem of insufficient electrolyte in the corner area and is also beneficial to the transport of active ions, so as to improve the interfacial problem, and further improve the lithium deposition problem of the secondary battery, that is, to improve the kinetic performance. At the same time, matching with the second separator is beneficial to taking into account the kinetic performance and manufacturing yield of the secondary battery. In the present application, the flat area refers to the flat part of the electrode assembly, and the corner area refers to the bent part of the electrode assembly.

[0044] In some embodiments of the present application, as Figure 3 and Figure 4 shown, in the same layer of the negative electrode tab 20, the first negative electrode material layer 220 is farther from the winding center of the electrode assembly 01 than the second negative electrode material layer 230. In the corner area 012 of the electrode assembly 01, a second positive electrode tab corner segment 102 is adjacent to a second negative electrode material layer corner segment 222. The second positive electrode tab corner segment 102 is closer to the winding center of the electrode assembly 01 than the second negative electrode material layer corner segment 222. The first separator 30 is located between the second positive electrode tab corner segment 102 and the second negative electrode material layer corner segment 222, and the second separator 40 is located between the first positive electrode tab corner segment 101 and the first negative electrode material layer corner segment 221.

[0045] In some embodiments of the present application, 40% ≤ n1 ≤ 50%. For example, the porosity n1 of the first base film can be 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, or a range composed of any two of these values. By adjusting the porosity of the first base film within the scope of the present application, the first separator has good wettability and liquid absorption and retention capabilities, which is beneficial to improving the infiltration performance of the electrolyte, enhancing the transmission rate of lithium ions, further reducing the risk of lithium deposition on the negative electrode sheet, and further improving the kinetic performance of the secondary battery.

[0046] In one embodiment of the present application, the first base film includes a first resin material, and the weight average molecular weight of the first resin material is from 50W (where "W" represents 10,000, 50W is 500,000, the same below) to 90W. For example, the weight average molecular weight of the first resin material can be 50W, 55W, 60W, 65W, 70W, 75W, 80W, 85W, 90W, or a range composed of any two of these values; by adjusting the weight average molecular weight of the first resin material within the scope of the present application, the obtained first base film has appropriate puncture resistance and a relatively high porosity, which is beneficial to improving lithium ion transmission, reducing the risk of lithium deposition on the negative electrode sheet, and further improving the kinetic performance of the secondary battery while taking into account the manufacturing yield.

[0047] In one embodiment of the present application, the first resin material is selected from at least one of polyethylene, polypropylene, polyimide, polyethyleneimine, or polyethylene terephthalate. When the first resin material is selected from the materials within the scope of the present application, it is beneficial to obtain a first base film that takes into account a relatively high porosity and appropriate puncture resistance, thereby further improving the kinetic performance of the secondary battery while taking into account the manufacturing yield.

[0048] In one embodiment of the present application, in the differential scanning calorimetry test spectrum of the first base film, there is a characteristic peak at 128°C to 134°C. In the differential scanning calorimetry (DSC) spectrum of the first base film, there is the above-mentioned characteristic peak, and the peak value of this characteristic peak corresponds to the melting temperature of the first resin material. In the present application, in the first base film, the peak value of the characteristic peak is between 128°C and 134°C, that is, there is characteristic peak A.

[0049] In an embodiment of the present application, 200 gf ≤ F1 ≤ 300 gf. For example, the puncture resistance F1 of the first base film can be 200 gf, 210 gf, 220 gf, 230 gf, 240 gf, 250 gf, 260 gf, 270 gf, 280 gf, 290 gf, 300 gf, or a range composed of any two of these values. By regulating the puncture resistance of the first base film within the scope of this application, while having a relatively high porosity, the first base film has a relatively appropriate puncture resistance, which is beneficial to improving lithium ion transport, reducing the risk of lithium deposition on the negative electrode sheet, and at the same time is beneficial to reducing the risk of the separator being punctured by hard particles on the negative electrode sheet, improving the excellent rate of Hi-pot and K value of the secondary battery, and further improving the kinetic performance of the secondary battery while taking into account the manufacturing excellent rate.

[0050] In an embodiment of the present application, the thickness of the first base film is 4 μm to 7 μm. For example, the thickness of the first base film can be 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, 6.5 μm, 7 μm, or a range composed of any two of these values. By regulating the thickness of the first base film within the scope of this application, while further improving the kinetic performance of the secondary battery, the energy density is also taken into account.

[0051] In an embodiment of the present application, the first separator further includes a first ceramic layer, and the porosity of the first separator is n3, 40% ≤ n3 ≤ 70%. For example, the porosity of the first separator can be 40%, 45%, 50%, 55%, 60%, 65%, 70%, or a range composed of any two of these values. The setting of the first ceramic layer is beneficial to further improving the liquid absorption and retention capabilities of the first separator, thereby being beneficial to further improving the liquid absorption and retention capabilities of the first separator, increasing the lithium ion transport rate, reducing the risk of lithium deposition on the negative electrode sheet, and further improving the kinetic performance of the secondary battery.

[0052] In an embodiment of the present application, the coating weight CW1 of the first ceramic layer is 9 mg / 5000 mm 2 to 15 mg / 5000 mm 2 . The coating weight CW1 of the first ceramic layer can be 9 mg / 5000 mm 2 , 10 mg / 5000 mm 2 , 11 mg / 5000 mm 2 , 12 mg / 5000 mm 2 , 13 mg / 5000 mm 2 , 14 mg / 5000 mm 2 , 15 mg / 5000 mm 2Or a range composed of any two of these values. By controlling the coating weight CW1 of the first ceramic layer within the scope of this application, the first ceramic layer has an appropriate thickness, which is beneficial to improving the kinetic performance of the secondary battery while taking into account the energy density.

[0053] In one embodiment of this application, the first ceramic layer includes first ceramic particles, and the first ceramic particles include at least one of alumina, boehmite, silicon oxide, magnesium oxide, titanium oxide, tin oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, aluminum hydroxide, magnesium hydroxide, or calcium hydroxide. By selecting the first ceramic particles of the above types, it is beneficial to further improve the liquid absorption and liquid retention capabilities of the first separator and further improve the kinetic performance of the secondary battery.

[0054] In one embodiment of this application, the first separator further includes a first adhesive layer. The first adhesive layer is disposed on both sides of the first base film, and the first ceramic layer is disposed on one side of the first base film. The first ceramic layer is located between the first base film and the first adhesive layer. In some embodiments of this application, in the electrode assembly, the first ceramic layer is disposed facing the positive electrode sheet, which is beneficial to further improving the problem of insufficient electrolyte in the corner area and is also beneficial to the transmission of active ions to improve the interface problem, and further improve the lithium deposition problem of the secondary battery.

[0055] This application does not particularly limit the composition of the first adhesive layer, as long as the purpose of this application can be achieved. Exemplarily, the first adhesive layer includes a first binder, and the first binder includes at least one of polyvinylidene fluoride, polyacrylonitrile, poly(ethylene oxide), or polyimide. This application does not particularly limit the thickness of the first adhesive layer, as long as the purpose of this application can be achieved. Exemplarily, the thickness of the first adhesive layer can be from 1 μm to 5 μm.

[0056] In some embodiments of this application, as Figure 5 shown, the first separator 30 includes a first base film 310, a first ceramic layer 330, and a first adhesive layer 320. The first ceramic layer 330 is disposed on one side of the first base film 310 close to the positive electrode material layer 120. The first adhesive layer 320 is respectively disposed on one surface of the first base film 310 away from the positive electrode material layer 120 and one surface of the first ceramic layer 330 close to the positive electrode material layer 120. The first ceramic layer 330 is located between the first base film 310 and the first adhesive layer 320. That is, the first separator 30 includes one layer of the first ceramic layer 330 and two layers of the first adhesive layer 320.

[0057] In some embodiments of the present application, 400 gf ≤ F2 ≤ 500 gf. For example, the puncture resistance F2 of the second base film can be 400 gf, 410 gf, 420 gf, 430 gf, 440 gf, 450 gf, 460 gf, 470 gf, 480 gf, 490 gf, 500 gf, or a range composed of any two of these values. By controlling the puncture resistance of the second base film within the scope of the present application, it is beneficial to improve the puncture resistance of the second separator, further improve the excellent rate of Hi-pot and K value of the secondary battery, and thus further improve the manufacturing excellent rate of the secondary battery.

[0058] In one embodiment of the present application, the second base film includes a second resin material and a third resin material. The weight average molecular weight of the second resin material is 50W to 90W, and the weight average molecular weight of the third resin material is 100W to 200W. For example, the weight average molecular weight of the second resin material can be 50W, 55W, 60W, 65W, 70W, 75W, 80W, 85W, 90W, or a range composed of any two of these values; the weight average molecular weight of the third resin material is 100W, 110W, 120W, 130W, 140W, 150W, 160W, 170W, 180W, 190W, 200W, or a range composed of any two of these values; by controlling the weight average molecular weights of the second resin material and the third resin material within the scope of the present application, the obtained second base film has a high puncture resistance and a suitable porosity, which is beneficial to improving lithium ion transport, improving the excellent rate of Hi-pot and K value of the secondary battery, and further improving the manufacturing excellent rate of the secondary battery while taking into account the kinetic performance.

[0059] In one embodiment of the present application, based on the mass of the second base film, the mass percentage content W1 of the second resin material is 70% to 90%, and the mass percentage content W2 of the third resin material is 10% to 30%. For example, the mass percentage content W1 of the second resin material can be 70%, 72%, 74%, 76%, 78%, 80%, 82%, 84%, 86%, 88%, 90%, or a range composed of any two of these values; the mass percentage content W2 of the third resin material can be 10%, 12%, 14%, 16%, 8%, 20%, 2%, 24%, 26%, 28%, 30%, or a range composed of any two of these values; by adjusting the mass percentage contents of the second resin material and the third resin material within the scope of the present application, the obtained second base film has a suitable porosity and a high puncture resistance at the same time, which is beneficial to improving lithium ion transport, reducing the risk of the separator being punctured by hard particles of the negative electrode sheet, improving the excellent rate of Hi-pot and K value of the secondary battery, and thus further improving the manufacturing excellent rate of the secondary battery while taking into account the kinetic performance.

[0060] In an embodiment of the present application, the second resin material and the third resin material are each independently selected from at least one of polyethylene, polypropylene, polyimide, polyethyleneimine, or polyethylene terephthalate. When the second resin material and the third resin material are selected from the materials within the scope of the present application, it is beneficial to obtain a second base film that takes into account a suitable porosity and a high puncture resistance, thereby further improving the manufacturing yield of the secondary battery while taking into account the kinetic performance.

[0061] In an embodiment of the present application, in the differential scanning calorimetry test spectrum of the second base film, characteristic peaks exist at 128 °C to 134 °C and 136 °C to 140 °C. In the DSC spectrum of the second base film, a characteristic peak exists at 128 °C to 134 °C, and the peak value of this characteristic peak corresponds to the melting temperature of the second resin material. In the DSC spectrum, a characteristic peak exists at 136 °C to 140 °C, and the peak value of this characteristic peak corresponds to the melting temperature of the third resin material. In the present application, in the second base film, the peak value of the characteristic peak is between 128 °C and 134 °C, that is, there is a characteristic peak A; the peak value of the characteristic peak is between 136 °C and 140 °C, that is, there is a characteristic peak B.

[0062] In an embodiment of the present application, 10% ≤ n2 ≤ 30%. For example, the porosity n2 of the second base film can be 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, 30%, or a range composed of any two of these values. By adjusting the porosity of the second base film within the scope of the present application, the second base film has a relatively suitable porosity and good ion transport performance while having a high puncture resistance, which is beneficial to reducing the risk of the hard particles of the negative electrode sheet piercing the separator, improving the yield of Hi-pot and K value of the secondary battery, and further improving the manufacturing yield of the secondary battery while taking into account the kinetic performance.

[0063] In an embodiment of the present application, 15% ≤ n2 ≤ 30%. For example, the porosity n2 of the second base film can be 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, or a range composed of any two of these values. By adjusting the porosity of the second base film within the scope of the present application, the second base film has a relatively suitable porosity while having a high puncture resistance, which is beneficial to improving the wettability, liquid absorption and liquid retention ability of the second separator, improving the infiltration performance of the electrolyte, improving lithium ion transport, reducing the risk of lithium deposition on the negative electrode sheet, and further improving the manufacturing yield of the secondary battery while taking into account the kinetic performance.

[0064] In an embodiment of the present application, the thickness of the second base film is 4 μm to 7 μm. For example, the thickness of the second base film can be 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, 6.5 μm, 7 μm, or a range composed of any two of these values. By controlling the thickness of the second base film within the scope of the present application, the manufacturing yield of the secondary battery is further improved while taking into account the energy density.

[0065] In some embodiments of the present application, the second separator includes a second ceramic layer and a second adhesive layer. The second adhesive layer is disposed on both sides of the second base film, and the second ceramic layer is disposed on one side of the second base film. The second ceramic layer is located between the second base film and the second adhesive layer. In some embodiments of the present application, in the electrode assembly, the second ceramic coating faces the positive electrode sheet, which is beneficial to further improving the problem of insufficient electrolyte in the corner area, and is also beneficial to the transmission of active ions to improve the interface problem, thereby improving the lithium deposition problem of the secondary battery.

[0066] In an embodiment of the present application, the second separator further includes a second ceramic layer, and the porosity of the second separator is n4, where 20% ≤ n4 ≤ 40%. For example, the porosity of the second separator can be 20%, 22%, 25%, 28%, 30%, 32%, 35%, 38%, 40%, or a range composed of any two of these values. The setting of the second ceramic layer is beneficial to further improving the liquid absorption and liquid retention capabilities of the second separator, thereby being beneficial to improving the lithium ion transmission performance, reducing the risk of lithium deposition on the negative electrode sheet, and further improving the kinetic performance of the secondary battery.

[0067] The present application does not particularly limit the coating weight CW2 of the second ceramic layer, as long as the object of the present application can be achieved. Exemplarily, the coating weight CW2 of the second ceramic layer is 9 mg / 5000 mm 2 to 15 mg / 5000 mm 2 . For example, the coating weight CW2 of the second ceramic layer can be 9 mg / 5000 mm 2 , 9.5 mg / 5000 mm 2 , 10 mg / 5000 mm 2 , 10.5 mg / 5000 mm 2 , 11 mg / 5000 mm 2 , 11.5 mg / 5000 mm 2 , 12 mg / 5000 mm 2 , 12.5 mg / 5000 mm 2 , 13 mg / 5000 mm 2 , 13.5 mg / 5000 mm 2 , 14 mg / 5000 mm 2 , 14.5 mg / 5000 mm2 、15mg / 5000mm 2 Or a range consisting of any two values.

[0068] In one embodiment of the present application, the second ceramic layer includes second ceramic particles, and the second ceramic particles include at least one of aluminum oxide, boehmite, silicon oxide, magnesium oxide, titanium oxide, tin oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, aluminum hydroxide, magnesium hydroxide, or calcium hydroxide. By selecting the above-mentioned types of second ceramic particles, the second ceramic layer has good wettability and liquid absorption and retention capabilities, improves lithium ion transmission performance, reduces the risk of lithium deposition of the negative electrode sheet, and further improves the kinetic performance of the secondary battery.

[0069] The present application has no particular restrictions on the composition of the second bonding layer, as long as the purpose of the present application can be achieved. For example, the second bonding layer includes a second bonding agent, and the second bonding agent includes at least one of polyvinylidene fluoride, polyacrylonitrile, polyethylene oxide, or polyimide. The present application has no particular restrictions on the thickness of the second bonding layer, as long as the purpose of the present application can be achieved. For example, the thickness of the second bonding layer can be 1 μm to 5 μm.

[0070] In some embodiments of the present application, Figure 5 As shown, the second diaphragm 40 can be understood as having the same structure as the first diaphragm 30 , that is, the second diaphragm 40 includes a second ceramic layer 430 and two second bonding layers 420 .

[0071] The negative electrode material of the secondary battery contains silicon material. Since the silicon material particles are relatively hard, they may pierce the diaphragm, resulting in poor Hi-pot and K value quality, and a decrease in manufacturing quality. At the same time, Si is a semiconductor with poor conductivity. Therefore, the negative electrode plate containing silicon material is used in secondary batteries, which is usually difficult to take into account energy density, kinetic performance and manufacturing quality. Based on the above problems, in some embodiments of the present application, the first negative electrode material layer includes a first negative electrode material, and the first negative electrode material includes a silicon material. In some embodiments of the present application, the second negative electrode material layer includes a second negative electrode material, and the second negative electrode material includes a silicon material. At the same time, with the above-mentioned first diaphragm and the second diaphragm, the first diaphragm has good liquid absorption and liquid retention properties, and the second diaphragm has a high puncture resistance, which is beneficial to take into account the energy density, kinetic performance and manufacturing quality of the secondary battery.

[0072] In one embodiment of the present application, the silicon material includes at least one of silicon, silicon-carbon compound, silicon-oxygen compound or silicon alloy. When the silicon material meets the above characteristics, it is beneficial to improve the energy density of the secondary battery.

[0073] In the present application, the features in the above-mentioned various embodiments can be combined arbitrarily.

[0074] The preparation method of the first base film in this application is not particularly limited as long as the purpose of this application can be achieved. For example, the preparation method of the first base film includes but is not limited to the following steps: (1) Mix the additive and the solvent evenly according to the mass ratio of (0.1 to 0.3):100 to obtain a mixed solution; (2) Mix the first resin material and the mixed solution evenly according to the mass ratio of (10:90) to (30:70), and obtain the first base material through extrusion, casting, cooling, and film casting; (3) Perform longitudinal stretching and transverse stretching on the first base material, and then obtain the first porous base material after extraction and drying; (4) Perform secondary stretching on the first porous base material, perform heat setting, and wind up to obtain the first base film. Among them, the solvent can include but is not limited to at least one of paraffin oil or dichloromethane; the additive can include but is not limited to at least one of pentaerythritol tetra[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], N-methylpyrrolidone (NMP), dimethylacetamide (DMAc), dimethylformamide (DMF), or dimethyl sulfoxide (DMSO); the extractant used during extraction can include but is not limited to at least one of dichloromethane or n-hexane. The stretching multiples of longitudinal stretching and transverse stretching can be independently 6 to 8 times; the multiple of secondary stretching can be 1 to 2 times; the extraction temperature can be 20°C to 30°C, and the time can be 0.5h to 1h; the heat setting temperature can be 110°C to 135°C.

[0075] The preparation method of the second base film in this application is not particularly limited as long as the purpose of this application can be achieved. For example, the preparation method of the second base film includes but is not limited to the following steps: (1) Mix the second resin material and the third resin material evenly according to the above W1 and W2 to obtain a mixed material; (2) Mix the additive and the solvent evenly according to the mass ratio of (0.1 to 0.3):100 to obtain a mixed solution; (3) Mix the mixed material and the mixed solution evenly according to the mass ratio of (10:90) to (30:70), and form a film by extrusion, casting, and cooling to obtain a second base material; (4) Perform longitudinal stretching and transverse stretching on the second base material, and obtain a second porous base material after extraction and drying; (5) Perform secondary stretching on the second porous base material, perform heat setting, and wind up to obtain a second base film. Among them, the solvent may include but is not limited to at least one of paraffin oil or dichloromethane; the additive includes but is not limited to at least one of [β-(3,5-ditert-butyl-4-hydroxyphenyl)propionic acid] pentaerythritol ester, N-methylpyrrolidone (NMP), dimethylacetamide (DMAc), dimethylformamide (DMF), or dimethyl sulfoxide (DMSO); the extraction agent used during extraction may include but is not limited to at least one of dichloromethane or n-hexane; the stretching multiples of longitudinal stretching and transverse stretching may each independently be 6 to 8 times; the multiple of secondary stretching may be 1 to 2 times; the extraction temperature may be 20°C to 30°C, and the time may be 0.5 h to 1 h; the heat setting temperature may be 110°C to 135°C.

[0076] In this application, the porosity n1 of the first base film can be regulated by adjusting the transverse stretching multiple, longitudinal stretching multiple, secondary stretching multiple, and the mass ratio of the solvent during the preparation process. For example, when other conditions remain unchanged, when the transverse stretching multiple of the first base film increases, the porosity n1 of the first base film increases, and vice versa, n1 decreases; when other conditions remain unchanged, when the longitudinal stretching multiple of the first base film increases, the porosity n1 of the first base film increases, and vice versa, n1 decreases; when other conditions remain unchanged, when the secondary stretching multiple of the first base film increases, the porosity n1 of the first base film increases, and vice versa, n1 decreases; when other conditions remain unchanged, when the mass ratio of the solvent during the preparation process increases, the porosity n1 of the first base film increases, and vice versa, n1 decreases.

[0077] In this application, the porosity n2 of the second base film can be regulated in the same way as the adjustment method of the porosity n1 of the first base film.

[0078] In this application, the puncture resistance F2 of the second base film can be regulated by adjusting the porosity of the second base film, the weight-average molecular weight of the second resin material, the mass percentage content of the second resin material, the weight-average molecular weight of the third resin material, the mass percentage content of the third resin material, and the thickness of the second base film. Among them, the regulation method of the porosity of the second base film is as described above. For example, when other conditions remain unchanged, when the porosity of the second base film increases, the puncture resistance F2 of the second base film decreases, and vice versa, F2 increases; when other conditions remain unchanged, when the mass percentage content of the second resin material increases, the puncture resistance F2 of the second base film decreases, and vice versa, F2 increases; when other conditions remain unchanged, when the mass percentage content of the third resin material increases, the puncture resistance F2 of the second base film increases, and vice versa, F2 decreases; when other conditions remain unchanged, when the weight-average molecular weight of the second resin material increases, the puncture resistance F2 of the second base film increases, and vice versa, F2 decreases; when other conditions remain unchanged, when the weight-average molecular weight of the third resin material increases, the puncture resistance F2 of the second base film increases, and vice versa, F2 decreases; when other conditions remain unchanged, when the thickness of the second base film increases, the puncture resistance F2 of the second base film increases, and vice versa, F2 decreases.

[0079] In this application, the puncture resistance F1 of the first base film can be regulated in the same way as the adjustment method of the puncture resistance F2 of the second base film.

[0080] In this application, the porosity n3 of the first separator can be regulated by adjusting the porosity of the first base film and the coating mass of the first ceramic layer. Among them, the regulation method of the porosity of the first base film is as described above. For example, when other conditions remain unchanged, when the porosity of the first base film increases, the porosity n3 of the first separator increases, and vice versa, n3 decreases; when other conditions remain unchanged, when the coating mass of the first ceramic layer increases, the porosity n3 of the first separator increases, and vice versa, n3 decreases.

[0081] In this application, the porosity n4 of the second separator can be regulated in the same way as the adjustment method of the porosity n3 of the first separator.

[0082] In this application, the first resin material, the second resin material, and the third resin material with different weight-average molecular weights and types can be obtained by purchase. Their weight-average molecular weights can be measured, and the materials with the required weight-average molecular weights can be selected. Among them, the specific test method can refer to the relevant content in the "Test Methods and Equipment" section.

[0083] In the present application, the negative electrode sheet includes a negative electrode current collector and a first negative electrode material layer and a second negative electrode material layer located on both sides of the negative electrode current collector. The above-mentioned "first negative electrode material layer and second negative electrode material layer located on both sides of the negative electrode current collector" means that the first and second negative electrode material layers are respectively disposed on two surfaces of the negative electrode current collector along its own thickness direction. It should be noted that the "surface" here can be the entire area of the surface of the negative electrode current collector or a partial area of the surface of the negative electrode current collector. There is no particular limitation in the present application as long as the purpose of the present application can be achieved.

[0084] There is no particular limitation on the negative electrode current collector in the present application as long as the purpose of the present application can be achieved. For example, it can include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam or a composite current collector. Exemplarily, the composite current collector can be a lithium-copper composite current collector, a carbon-copper composite current collector, a nickel-copper composite current collector, a titanium-copper composite current collector, etc.

[0085] In some embodiments of the present application, the first negative electrode material layer and the second negative electrode material layer may each independently include a negative electrode conductive agent and a negative electrode binder. There is no particular limitation on the types of the negative electrode conductive agent and the negative electrode binder in the present application as long as the purpose of the present application can be achieved. For example, it can be at least one of a positive electrode conductive agent and a positive electrode binder. There is no particular limitation on the mass ratio of the negative electrode material, the negative electrode conductive agent, and the negative electrode binder in the first negative electrode material layer and the second negative electrode material layer. Those skilled in the art can select according to actual needs as long as the purpose of the present application can be achieved.

[0086] Optionally, the negative electrode sheet may further include a first negative electrode conductive layer located between the negative electrode current collector and the first negative electrode material layer. There is no particular limitation on the composition of the first negative electrode conductive layer in the present application, and it can be a commonly used conductive layer in the art. For example, the first negative electrode conductive layer includes a first negative electrode conductive layer conductive agent and a first negative electrode conductive layer binder. There is no particular limitation on the first negative electrode conductive layer conductive agent and the first negative electrode conductive layer binder in the present application. For example, it can be at least one of a positive electrode conductive agent and a positive electrode binder.

[0087] Optionally, the negative electrode sheet may further include a second negative electrode conductive layer located between the negative electrode current collector and the second negative electrode material layer. There is no particular limitation on the composition of the second negative electrode conductive layer in the present application, and it can be a commonly used conductive layer in the art. For example, the second negative electrode conductive layer includes a second negative electrode conductive layer conductive agent and a second negative electrode conductive layer binder. There is no particular limitation on the second negative electrode conductive layer conductive agent and the second negative electrode conductive layer binder in the present application. For example, it can be at least one of a positive electrode conductive agent and a positive electrode binder.

[0088] The present application has no particular limitation on the thickness of the first negative electrode material layer, as long as the object of the present application can be achieved. For example, the thickness of the first negative electrode material layer is 30 μm to 120 μm.

[0089] The present application has no particular limitation on the thickness of the second negative electrode material layer, as long as the object of the present application can be achieved. For example, the thickness of the second negative electrode material layer is 30 μm to 120 μm.

[0090] The present application has no particular limitation on the thickness of the negative electrode current collector, as long as the object of the present application can be achieved. For example, the thickness of the negative electrode current collector is 4 μm to 20 μm.

[0091] In the present application, the positive electrode plate includes a positive electrode current collector and positive electrode material layers located on both sides of the positive electrode current collector. The above-mentioned "positive electrode material layers located on both sides of the positive electrode current collector" means that the positive electrode material layers are disposed on two surfaces of the positive electrode current collector along its own thickness direction. It should be noted that the "surface" here can be the entire area of the positive electrode current collector surface or a partial area of the positive electrode current collector surface. The present application has no particular limitation, as long as the object of the present application can be achieved.

[0092] The present application has no particular limitation on the positive electrode current collector, as long as the object of the present application can be achieved. For example, it can include aluminum foil, aluminum alloy foil or composite current collector (such as aluminum-carbon composite current collector), etc.

[0093] The positive electrode material layer includes a positive electrode active material. The present application has no particular limitation on the positive electrode active material, as long as the object of the present application can be achieved. For example, the positive electrode active material can include but is not limited to at least one of lithium nickel cobalt manganese oxide (such as NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminate, lithium iron phosphate, lithium-rich manganese-based material, lithium cobalt oxide (LiCoO2), lithium manganese oxide or lithium manganese iron phosphate.

[0094] The positive electrode material layer may further include a positive electrode conductive agent and a positive electrode binder. The present application does not particularly limit the types of the positive electrode conductive agent and the positive electrode binder, as long as the purpose of the present application can be achieved. For example, the positive electrode conductive agent may include, but is not limited to, at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, graphene, metal materials, or conductive polymers. The conductive carbon black may include, but is not limited to, at least one of acetylene black or Ketjen black. The above carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The above carbon fibers may include, but is not limited to, vapor-grown carbon fibers (VGCF) and / or nanofibers. The above metal materials may include, but are not limited to, metal powders and / or metal fibers. Specifically, the metal may include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The above conductive polymers may include, but is not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole.

[0095] The present application does not particularly limit the positive electrode binder, as long as the purpose of the present application can be achieved. For example, the positive electrode binder may include, but is not limited to, at least one of polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyimide, polyvinyl alcohol, carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, polyimide, polyamideimide, styrene-butadiene rubber, or polyvinylidene fluoride.

[0096] The present application does not particularly limit the mass ratio of the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder in the positive electrode material layer. Those skilled in the art can select according to actual needs, as long as the purpose of the present application can be achieved.

[0097] The present application does not particularly limit the thickness of the positive electrode current collector and the positive electrode material layer, as long as the purpose of the present application can be achieved. For example, the thickness of the positive electrode current collector is 5 μm to 20 μm, and the thickness of the positive electrode material layer is 30 μm to 120 μm.

[0098] Optionally, the positive electrode sheet may further include a positive electrode conductive layer, and the positive electrode conductive layer is located between the positive electrode current collector and the positive electrode material layer. The composition of the positive electrode conductive layer is not particularly limited and may be a commonly used conductive layer in the art. The positive electrode conductive layer includes a positive electrode conductive layer conductive agent and a positive electrode conductive layer binder. The present application does not particularly limit the positive electrode conductive layer conductive agent and the positive electrode conductive layer binder. For example, it may be at least one of the above positive electrode conductive agent and the above positive electrode binder.

[0099] In the present application, the secondary battery further includes an electrolyte, and the electrolyte includes a lithium salt and a non-aqueous solvent.

[0100] The present application has no particular limitation on the lithium salt, as long as the object of the present application can be achieved. For example, the lithium salt may include, but is not limited to, at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), lithium perchlorate (LiClO4), lithium tetraphenylborate (LiB(C6H5)4), lithium methylsulfonate (LiCH3SO3), lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis(trifluoromethanesulfonyl)imide (LiN(SO2CF3)2), lithium tris(trifluoromethylsulfonyl)methyl (LiC(SO2CF3)3), lithium hexafluorosilicate (LiSiF6), lithium bis(oxalato)borate (LiBOB), and lithium difluoroborate (LiF2OB). The present application has no particular limitation on the content of the lithium salt in the electrolyte, as long as the object of the present application can be achieved.

[0101] The present application has no particular limitation on the non-aqueous solvent, as long as the object of the present application can be achieved. For example, the non-aqueous solvent may include, but is not limited to, at least one of carbonate compounds, carboxylate compounds, ether compounds, or other organic solvents.

[0102] The above-mentioned carbonate compounds may include, but are not limited to, at least one of chain carbonate compounds, cyclic carbonate compounds, or fluorinated carbonate compounds. The above-mentioned chain carbonate compounds may include, but are not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), or methyl ethyl carbonate (MEC). The above-mentioned cyclic carbonates may include, but are not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), or vinylene ethylene carbonate (VEC). The fluorinated carbonate compounds may include, but are not limited to, at least one of fluorinated ethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-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, or trifluoromethyl ethylene carbonate. The above-mentioned carboxylic ester compounds may include, but are not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolide, valerolactone, or caprolactone. The above-mentioned ether compounds may include, but are not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. The above-mentioned other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate. The present application does not particularly limit the content of the non-aqueous solvent in the electrolyte, as long as the object of the present application can be achieved.

[0103] The secondary battery further includes a housing for accommodating the second separator, the positive electrode sheet, the first separator, the negative electrode sheet, and the electrolyte, as well as other components known in the field of secondary batteries. The present application does not limit the above-mentioned other components. The present application does not particularly limit the housing, and it may be a housing well-known in the art, as long as the object of the present application can be achieved. For example, the housing may be a hard-shell housing or a flexible housing. The material of the hard-shell housing may be metal. The present application does not limit the type of metal, and a metal hard-shell housing known in the art may be used, as long as the object of the present application can be achieved. The flexible housing may be a metal plastic film, such as an aluminum plastic film, a steel plastic film, etc.

[0104] The preparation process of the secondary battery of the present application is well-known to those skilled in the art, and the present application has no special limitations. For example, the preparation process of the secondary battery may include but is not limited to the following steps: stacking the positive electrode sheet, the first separator, the negative electrode sheet, and the second separator in sequence, and winding, folding, etc. as needed to obtain a wound structure electrode assembly, placing the electrode assembly into a housing, injecting an electrolyte into the housing and sealing it to obtain a secondary battery. Alternatively, stack the negative electrode sheet, the second separator, the positive electrode sheet, and the first separator in sequence, and wind, fold, etc. as needed to obtain a wound structure electrode assembly, place the electrode assembly into a housing, inject an electrolyte into the housing and seal it to obtain a secondary battery. In addition, an overcurrent protection element, a guide plate, etc. can be placed in the housing as needed to prevent the pressure inside the secondary battery from rising and overcharging and discharging.

[0105] In some embodiments of the present application, the secondary battery of the present application may include but is not limited to: lithium metal secondary batteries, lithium ion secondary batteries (lithium ion batteries), lithium polymer secondary batteries, or lithium ion polymer secondary batteries, etc. In some embodiments of the present application, the secondary battery includes a lithium ion battery.

[0106] The second aspect of the present application provides an electronic device, which includes the secondary battery described in the first aspect of the present application.

[0107] The present application does not particularly limit the type of the electronic device, and it can be any electronic device known in the prior art. In some embodiments of the present application, the electronic device may include but is not limited to laptop computers, pen input computers, mobile computers, e-book players, portable phones, portable fax machines, portable copiers, portable printers, head-mounted stereo headphones, video recorders, liquid crystal TVs, portable cleaners, portable CD players, mini discs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, motorized bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium ion capacitors, etc.

[0108] Examples

[0109] Hereinafter, examples and comparative examples are given to more specifically illustrate the embodiments of the present application. Various tests and evaluations are carried out according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0110] Testing method and equipment:

[0111] Sampling method for the first separator and the second separator:

[0112] Disassemble the lithium-ion batteries in the tested examples and comparative examples, take out the first separator and the second separator, soak them in dimethyl carbonate (DMC) for 20 min to remove the residual electrolyte, and then place the first separator and the second separator in an oven and dry them at 60 °C for 12 h to obtain the first separator sample and the second separator sample. Unless otherwise specified, the above method is used to obtain the first separator and the second separator for the following tests.

[0113] Sampling method for the first base film and the second base film:

[0114] Put the first separator and the second separator obtained by the above sampling into containers respectively, add N-methylpyrrolidone (NMP), place them in an ultrasonic instrument with a heating function for ultrasonic treatment, control the temperature at 45 °C, and perform ultrasonic treatment for 3 h. Take them out after the first separator and the second separator become completely transparent to obtain the first base film and the second base film. Unless otherwise specified, the above method is used to obtain the first base film and the second base film for the following tests.

[0115] Porosity test of the first base film, the first separator, the second base film, and the second separator:

[0116] The gas displacement method is used for the test. Use a mold to punch and prepare samples of the first base film, the first separator, the second base film, and the second separator respectively (those skilled in the art can select common sizes and shapes of molds in the art according to factors such as the size and shape of the test object and the requirements of the test equipment). Use a true density tester to measure the true volume V0 of the sample. The apparent volume V of the sample can be calculated by measuring the area and thickness of the sample. Then the percentage of the pore volume of the sample in the total volume = (V - V0) / V × 100%, that is, the porosities of the first base film, the first separator, the second base film, and the second separator are obtained respectively.

[0117] Puncture resistance of the first base film and the second base film:

[0118] The mass applied to a given needle to pierce the first base film and the second base film of the example or the comparative example is recorded as the puncture resistance.

[0119] The test method is as follows:

[0120] 1. Prepare a puncture fixture and a blade before measurement;

[0121] 2. Use a blade to cut the first base film and the second base film respectively, and the sample is a circular specimen with a diameter of 100 mm. Precautions: The first base film and the second base film should be free of defects and any flaws;

[0122] 3. After checking the cleanliness of the high-speed tensile machine, install the puncture fixture, place the first base film and the second base film in the center of the fixture respectively, and cover the upper cover. Note: The sample must be placed flat without any wrinkles; the sample size should be larger than the puncture fixture, that is, the sample is tightly pressed by the fixture on all sides;

[0123] 4. Set the test speed to 50 mm / min on the computer operation panel of the high-speed tensile machine;

[0124] 5. Click "Start" and conduct the puncture test in sequence. Stop the puncture when the base film sample is punctured, and save the force-displacement curve; perform at least three parallel tests for each group. If the repeatability of three force-displacement curves is good, then test the next group of samples. Note: The unit of force F is uniformly N. Puncture resistance strength = force F / 9.8 × 10 3 , with the unit of gf, and obtain the puncture resistance strengths of the first base film and the second base film respectively.

[0125] Thickness test of the first base film and the second base film:

[0126] Perform argon ion polishing on the first base film to obtain the cross-section of the first base film. Observe the morphology of the cross-section of the first base film along the thickness direction through a field emission scanning electron microscope (Philips, model XL-30) and take a scanning electron microscope photo. Measure the thickness of the first base film through the scanning electron microscope. Replace the first base film with the second base film to measure the thickness of the second base film.

[0127] DSC spectrum test of the first base film and the second base film:

[0128] Use a differential scanner (model: ASTM D3418-15) to test the DSC spectrum of 8 mg of the first base film. The heating rate is 10 °C / min, and the temperature range is from 60 °C to 190 °C to obtain the DSC spectrum of the first base film. Replace the first base film with the second base film to measure the DSC spectrum of the second base film. In the obtained DSC spectrum, the characteristic peak corresponding to the peak value between 128 °C and 134 °C is denoted as characteristic peak A, and the characteristic peak corresponding to the peak value between 136 °C and 140 °C is denoted as characteristic peak B.

[0129] Based on the DSC spectrum of the second base film, calculate the ratio of the peak areas of characteristic peak A and characteristic peak B. The peak area ratio is also the mass ratio of the second resin material and the third resin material. After conversion, the mass percentage contents of the second resin material and the third resin material can be obtained.

[0130] Weight-average molecular weight test of the first resin material, the second resin material, and the third resin material:

[0131] 1) Sample preparation: Dissolve 5 g of the first base film to be tested in 100 g of 1,2,4-trichlorobenzene at 150 °C to prepare a solution with a concentration of 2 wt%.

[0132] 2) Gel column selection: The smaller the pore size of the gel column, the slower the flow rate, and the larger the molecular weight of the polymer that can be separated. By selecting a suitable gel column, the first resin material can pass through the gel column at a suitable flow rate;

[0133] 3) Sample loading and elution: Inject the prepared sample solution into an ultra-high performance polymer chromatograph (APC, ACQUITY), and pump the solution through the gel column at a certain flow rate. Meanwhile, elute the gel column with an appropriate eluent to elute the separated polymers in sequence.

[0134] 4) Detection and recording: During the elution process, detect the concentration of the effluent through a detector and record the change of the concentration over time.

[0135] 5) Data processing and analysis: Import the detected concentration data into computer software, convert the concentration data into information on molecular weight and its distribution with reference to a standard substance (polyethylene), and obtain the weight-average molecular weight of the first resin material.

[0136] Replace the first base film with the second base film. Among them, by selecting a suitable gel column, the second resin material and the third resin material pass through the gel column at appropriate flow rates respectively, so that the second resin material and the third resin material can be separated. Repeat the above steps to obtain the weight-average molecular weights of the second resin material and the third resin material respectively.

[0137] Hi-pot yield test:

[0138] Conduct the Hi-pot test on the lithium-ion battery and calculate the test passing rate.

[0139] The test method is: Detect the leakage current generated by the lithium-ion battery prepared in the example or comparative example under the 100V test voltage output by the high-voltage machine, and then calculate the resistance value = test voltage / leakage current.

[0140] Compare the calculated resistance value with the set judgment resistance. In this application, the preset value of the judgment resistance is 5 mΩ.

[0141] If the detected resistance value is greater than or equal to the preset value of 5 mΩ, it is determined that the tested product passes the test (OK);

[0142] If the detected resistance value is less than the preset value of 5 mΩ, the test voltage is instantly cut off and the tested product is determined to fail the test (NG).

[0143] For each group of examples or comparative examples, 100 lithium-ion batteries are tested. The number of lithium-ion batteries passing the test is X1, and the test passing rate is X1 / 100×100%, that is, the Hi-pot yield.

[0144] K-value yield test:

[0145] The lithium-ion batteries prepared in the examples or comparative examples were charged at a constant current of 0.5C until the voltage reached 3.95V. When the lithium-ion batteries stored 60% of the electricity (60% SOC), their voltages were measured at 25 ± 5°C and recorded as the first voltage. After standing for 48 hours, the voltages were measured again and recorded as the second voltage. The voltage drop per unit time was denoted as K, and K = (the first voltage - the second voltage) / the standing time.

[0146] When the K value is less than 0.006 mV / h, it is determined to pass the K value test (OK);

[0147] When the K value is greater than or equal to 0.006 mV / h, it is determined to fail the K value test (NG).

[0148] For each group of comparative examples and each group of examples, 100 lithium-ion batteries were tested, and the passing rate of the K value test for each group was recorded.

[0149] The number of lithium-ion batteries passing the test was X2, and the passing rate of the test was X2 / 100 × 100%, which is also the excellent rate of the K value.

[0150] Degree of lithium deposition on the negative electrode sheet after 1000 charge-discharge cycles:

[0151] The lithium-ion battery was placed in an environment at 25°C and charged at a constant current of 0.5C until the voltage reached 4.5V, then charged at a constant voltage of 4.5V until the cut-off current was 0.05C, and left standing for 5 minutes. Then it was discharged at a constant current of 0.5C until the voltage reached 3.0V and left standing for 5 minutes. This was one charge-discharge cycle. Then the charging and discharging cycles were carried out 1000 times in the same steps.

[0152] Then it was charged at a constant current of 0.5C to 4.5V, then charged at a constant voltage of 4.5V until the cut-off current was 0.05C, and left standing for 5 minutes; then the lithium-ion battery was disassembled, and the lithium deposition situation on the negative electrode sheet, the lithium deposition situation on the surface of the negative electrode sheet opposite to the first separator, and the lithium deposition situation on the surface of the negative electrode sheet opposite to the second separator were observed.

[0153] For the surface of the negative electrode sheet opposite to the first separator, if no white lithium metal is deposited, it is recorded as "no lithium deposition"; if white lithium metal is deposited and the proportion of the lithium deposition area is greater than 0 and less than 5%, it is recorded as "slight lithium deposition"; if white lithium metal is deposited and the proportion of the lithium deposition area is greater than or equal to 5% and less than or equal to 10%, it is recorded as "moderate lithium deposition"; if white lithium metal is deposited and the proportion of the lithium deposition area is greater than 10%, it is recorded as "severe lithium deposition". Among them, the proportion of the lithium deposition area is the percentage of the lithium deposition area in the surface area of the negative electrode material layer of the negative electrode sheet opposite to the first separator.

[0154] Similarly, for the surface of the negative electrode tab opposite to the second separator, if no white lithium metal is deposited, it is recorded as "no lithium deposition"; if white lithium metal is deposited and the area ratio of lithium deposition is greater than 0 and less than 5%, it is recorded as "mild lithium deposition"; if white lithium metal is deposited and the area ratio of lithium deposition is greater than or equal to 5% and less than or equal to 10%, it is recorded as "moderate lithium deposition"; if white lithium metal is deposited and the area ratio of lithium deposition is greater than 10%, it is recorded as "severe lithium deposition". Among them, the area ratio of lithium deposition is the percentage of the lithium deposition area to the surface area of the negative electrode material layer of the negative electrode tab opposite to the second separator.

[0155] For the entire negative electrode tab, if no white lithium metal is deposited, it is recorded as "no lithium deposition"; if white lithium metal is deposited and the area ratio of lithium deposition is greater than 0 and less than 5%, it is recorded as "mild lithium deposition"; if white lithium metal is deposited and the area ratio of lithium deposition is greater than or equal to 5% and less than or equal to 10%, it is recorded as "moderate lithium deposition"; if white lithium metal is deposited and the area ratio of lithium deposition is greater than 10%, it is recorded as "severe lithium deposition". Among them, the area ratio of lithium deposition is the percentage of the lithium deposition area to the total area of the negative electrode material layers on both surfaces of the negative electrode tab.

[0156] The lithium deposition test is used to characterize the kinetic performance of the lithium-ion battery. The lighter the degree of lithium deposition, the better the kinetic performance, and vice versa.

[0157] Cycling performance test:

[0158] At 25°C, the lithium-ion battery is charged at a constant current of 3C to 4.5V and then at a constant voltage to 0.05C, and then discharged at 0.7C to 3.0V to complete one cycle, which is recorded as the first cycle, and the discharge capacity of the first cycle is recorded. Then, the above steps are repeated for 1500 cycles, and the discharge capacity after each cycle is recorded separately. When cycling to 50, 100, 150, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400 cycles, the charge and discharge are carried out according to the following steps: constant current charge at 0.5C to 4.5V, constant voltage charge to 0.02C, and discharge at 0.2C to 3.0V.

[0159] Cycling capacity retention rate (%) = (discharge capacity after cycling N times / discharge capacity of the first cycle) × 100%. Among them, N is a positive integer from 1 to 1500.

[0160] During the cycling process, a press plate thickness gauge is used to measure the thickness of the lithium-ion battery, and the initial thickness is recorded. Then, after cycling 50, 100, 150, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500 cycles, the thickness of the lithium-ion battery after cycling Q times is recorded respectively.

[0161] Cyclic swelling ratio (%) = {(Thickness after Q cycles - Thickness of the first cycle) / Thickness of the first cycle} × 100%. Where Q is 50, 100, 150, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500.

[0162] Example 1-1

[0163] <Preparation of the first separator>

[0164] (1) Mix the additive pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and the solvent paraffin oil evenly at a mass ratio of 0.2:100 to obtain a mixed solution;

[0165] (2) Mix the first resin material polyethylene and the mixed solution evenly at a mass ratio of 20:80, add them to an extrusion system, extrude through a T-shaped die, and cast, cool, and sheet to form a first substrate; among them, the weight-average molecular weight Mw1 of the first resin material polyethylene = 70W;

[0166] (3) Stretch the first substrate 7 times longitudinally and 7 times transversely, extract it with dichloromethane at 25°C for 0.5 h, and dry it to obtain a first porous substrate;

[0167] (4) Stretch the first porous substrate 1.5 times longitudinally and 1.5 times transversely for the second time, heat-set it at 115°C, and wind it up to obtain a first base film, and the thickness of the first base film is 5 μm.

[0168] (5) Mix boehmite, an inorganic particle with a Dv50 of 1 μm, and polyacrylate in a mass ratio of 90:10, and dissolve them in deionized water to form a first ceramic layer slurry with a solid content of 50%. Subsequently, use the microgravure coating method to evenly coat the inorganic coating slurry on one surface of the first base film, and dry it to obtain a first ceramic layer. Among them, the coating weight CW1 of the first ceramic layer is 12 mg / 5000 mm 2 . Among them, the weight-average molecular weight Mw4 of polyacrylate = 40W.

[0169] (6) The first binder polyvinylidene fluoride (PVDF), with a weight-average molecular weight of 8.5×10 6, add it to a stirrer, then add deionized water and stir to adjust the viscosity of the slurry to 3500 mPa·s and the solid content to 75 wt% to obtain the first binder layer slurry; use screen printing to evenly coat the first binder layer slurry on the surface of the first ceramic layer away from the first base film, dry it, and then repeat the above coating process on the other surface of the first base film to obtain the first separator. Among them, the single-layer coating thickness of the first binder layer is 2 μm.

[0170] <Preparation of the second separator>

[0171] (1) Mix the second resin material polyethylene and the third resin material polyethylene evenly to form a mixed material; among them, the weight-average molecular weight Mw2 of the second resin material polyethylene is 70W, and the mass ratio W1 of the second resin material is 80%; the weight-average molecular weight Mw3 of the third resin material polyethylene is 150W, and the mass ratio W2 of the third resin material is 20%.

[0172] (2) Mix the additive pentaerythritol tetra[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and the solvent paraffin oil evenly according to the mass ratio of 0.2:100 to obtain a mixed solution.

[0173] (3) Mix the mixed material and the mixed solution evenly according to the mass ratio of 20:80, add them to an extrusion machine system, extrude through a T-shaped die, and then cast, cool, and form a film to obtain the second substrate.

[0174] (4) Stretch the second substrate 6 times longitudinally and 6 times transversely, extract it with dichloromethane at 25°C for 0.5 h, and dry it to obtain the second porous substrate.

[0175] (5) Stretch the second porous substrate 1.5 times longitudinally and 1.5 times transversely for the second time, heat-set it at 125°C, and wind it up to obtain the second base film with a thickness of 5 μm.

[0176] (6) Dissolve the inorganic particles boehmite with a Dv50 of 1 μm and polyacrylate in deionized water according to the mass ratio of 90:10 to form a second ceramic layer slurry with a solid content of 50 wt%. Then, use the microgravure coating method to evenly coat the second ceramic layer slurry on one surface of the second base film and dry it to obtain the second ceramic layer. Among them, the coating weight CW2 of the second ceramic layer is 12 mg / 5000 mm 2 . Among them, the weight-average molecular weight Mw5 of polyacrylate is 40W.

[0177] (7) The second binder PVDF, with a weight-average molecular weight of 8.5×10 6, and then deionized water is added and stirred to adjust the viscosity of the slurry to 3500 mPa·s and the solid content to 75 wt% to obtain the second binder layer slurry; the second binder layer slurry is uniformly coated on one surface of the second ceramic layer away from the second base film by screen printing, and then dried in an oven. The above coating process is repeated on the other surface of the second base film to obtain the second separator. Among them, the single-layer coating thickness of the second binder layer is 2 μm.

[0178] <Preparation of negative electrode sheet>

[0179] The negative electrode active materials artificial graphite, silicon carbide compound, acetylene black, styrene-butadiene rubber and sodium carboxymethyl cellulose are mixed according to a mass ratio of 90:6:1:1.5:1.5, and then deionized water is added as a solvent to prepare a slurry with a solid content of 70%. The slurry is stirred evenly by a vacuum mixer to obtain the first negative electrode slurry. At the same time, the first negative electrode slurry is also used as the second negative electrode slurry. The first negative electrode slurry is uniformly coated on one surface of a copper foil with a thickness of 8 μm and dried at 110°C to obtain a negative electrode sheet with a coating thickness of 50 μm for the first negative electrode material layer. The second negative electrode slurry is coated on the other surface of the negative electrode current collector copper foil. After drying, the thickness of the second negative electrode material layer coated on this surface is 50 μm, and then a negative electrode sheet with a total thickness of 108 μm is obtained. The coated negative electrode sheet is cold-pressed and then cut into a size of 74 mm × 867 mm for standby. Among them, the compaction density of the first negative electrode material layer is 1.735 g / cm 3 , with a length of 720 mm, and the compaction density of the second negative electrode material layer is 1.735 g / cm 3 , with a length of 680 mm. Among them, the mass ratio of silicon element to carbon element in the silicon carbide compound is 2:8.

[0180] <Preparation of positive electrode sheet>

[0181] The positive electrode active materials lithium cobaltate, acetylene black, and polyvinylidene fluoride (PVDF) are mixed according to a mass ratio of 94:3:3, and then NMP is added as a solvent to prepare a slurry with a solid content of 75%. Subsequently, the slurry is stirred evenly by a vacuum mixer to obtain the positive electrode slurry. The positive electrode slurry is uniformly coated on one surface of an aluminum foil with a thickness of 12 μm and dried at 90°C to obtain a negative electrode sheet with a single-sided positive electrode material layer coating thickness of 55 μm. The positive electrode slurry is coated on the other surface of the positive electrode current collector aluminum foil. After drying, the thickness of the positive electrode material layer coated on this surface is 55 μm, and then a positive electrode sheet with a total thickness of 122 μm is obtained. After cold pressing, a positive electrode sheet with a single-sided coating of a positive electrode material layer with a thickness of 122 μm is obtained. The coated positive electrode sheet is cold-pressed and then cut into a size of 70 mm × 800 mm, and the tab is welded and then used for standby. Among them, the compaction density of the positive electrode material layer is 4.23 g / cm 3 .

[0182] <Preparation of electrolyte>

[0183] In an environment with a water content of less than 10 ppm, ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), propyl propionate (PP), and vinylene carbonate (VC) as non-aqueous organic solvents are mixed in a mass ratio of 20:30:20:28:2. Then, lithium hexafluorophosphate (LiPF6) is added to the non-aqueous organic solvent and dissolved and mixed evenly to obtain an electrolyte. Based on the total mass of the electrolyte, the mass content of LiPF6 is 8%, and the balance is the base solvent.

[0184] <Preparation of lithium-ion battery>

[0185] The above-prepared positive electrode sheet, first separator, negative electrode sheet, and second separator are stacked in sequence and then wound to obtain an electrode assembly, specifically referring to Figure 1 and Figure 2 , the corner section of the first positive electrode sheet is adjacent to the corner section of the first negative electrode material layer and the corner section of the first positive electrode sheet is farther from the winding center of the electrode assembly than the corner section of the first negative electrode material layer. The first separator is located between the corner section of the first positive electrode sheet and the first negative electrode material layer, and the second separator is adjacent to the second negative electrode material layer; and the first ceramic layer of the first separator faces the positive electrode sheet, the second ceramic layer of the second separator faces the positive electrode sheet. In the same negative electrode sheet, the first negative electrode material layer is farther from the winding center of the electrode assembly than the second negative electrode material layer. This structure is denoted as Structure A. The electrode assembly is placed in an aluminum-plastic film packaging bag, dehydrated at 80 °C, injected with the prepared electrolyte, and subjected to processes such as vacuum packaging, standing, forming, and shaping to obtain a lithium-ion battery.

[0186] Examples 1-2 to Examples 1-5

[0187] Except that the weight-average molecular weight of the first resin material is adjusted according to Table 1, and the stretching multiples of the first base film in transverse stretching, longitudinal stretching, secondary transverse stretching, and secondary longitudinal stretching are adjusted so that the porosity and puncture resistance of the first base film are as shown in Table 1, the rest is the same as Example 1-1.

[0188] Examples 1-6 to Examples 1-9

[0189] Except that the mass percentage contents of the second resin material and the third resin material are adjusted according to Table 1, and the stretching multiples of the second base film in transverse stretching, longitudinal stretching, secondary transverse stretching, and secondary longitudinal stretching are adjusted so that the puncture resistance and porosity of the second base film are as shown in Table 1, the rest is the same as Example 1-1.

[0190] Example 1-10

[0191] Except for adjusting <Preparation of Lithium-Ion Battery> according to the following steps, the rest is the same as in Example 1-1.

[0192] <Preparation of Lithium-Ion Battery>

[0193] Stack the above-prepared positive electrode sheet, second separator, negative electrode sheet, and first separator in sequence, and then wind them to obtain an electrode assembly, specifically referring to Figure 3 and Figure 4 , the corner section of the second positive electrode sheet is adjacent to the corner section of the second negative electrode material layer, and the corner section of the second positive electrode sheet is closer to the winding center of the electrode assembly than the corner section of the second negative electrode material layer. The first separator is located between the corner section of the second positive electrode sheet and the corner section of the second negative electrode material layer, and the second separator is located on the side of the positive electrode sheet away from the first separator; and the first ceramic layer of the first separator is close to the positive electrode sheet, the second ceramic layer of the second separator is close to the positive electrode sheet. In the same layer of negative electrode sheet, the first negative electrode material layer is farther from the winding center of the electrode assembly than the second negative electrode material layer. This structure is denoted as Structure B. Place the electrode assembly into an aluminum-plastic film packaging bag, remove moisture at 80 °C, inject the prepared electrolyte, and obtain a lithium-ion battery through processes such as vacuum packaging, standing, forming, and shaping.

[0194] Examples 1-11 to 1-17

[0195] Except for adjusting the relevant parameters according to Table 1 and adjusting the stretching multiples of the first base film in transverse stretching, longitudinal stretching, secondary transverse stretching, and secondary longitudinal stretching so that the porosity and puncture resistance of the first base film are as shown in Table 1, the rest is the same as in Example 1-1.

[0196] Examples 2-1 to 2-13

[0197] Except for adjusting the relevant parameters according to Table 2 and adjusting the stretching multiples of the second base film in transverse stretching, longitudinal stretching, secondary transverse stretching, and secondary longitudinal stretching so that the porosity and puncture resistance of the second base film are as shown in Table 2, the rest is the same as in Example 1-1.

[0198] Comparative Example 1

[0199] Except for replacing the second base film with the first base film, the rest is the same as in Example 1-1.

[0200] Comparative Example 2

[0201] Except for replacing the first base film with the second base film, the rest is the same as in Example 1-1.

[0202] Comparative Examples 3 to 4

[0203] Except for adjusting the weight-average molecular weight of the first resin material according to Table 1 and adjusting the draw ratios of the first base film in transverse stretching, longitudinal stretching, secondary transverse stretching, and secondary longitudinal stretching so that the porosity and puncture resistance of the first base film are as shown in Table 1, the rest is the same as in Example 1-1.

[0204] Comparative Example 5

[0205] Except for adjusting the weight-average molecular weight of the third resin material to 900,000 and adjusting the draw ratios of the second base film in transverse stretching, longitudinal stretching, secondary transverse stretching, and secondary longitudinal stretching so that the porosity and puncture resistance of the second base film are as shown in Table 1, the rest is the same as in Example 1-1.

[0206] Comparative Example 6

[0207] Except for adjusting the weight-average molecular weight of the third resin material to 2,060,000 and adjusting the draw ratios of the second base film in transverse stretching, longitudinal stretching, secondary transverse stretching, and secondary longitudinal stretching so that the porosity and puncture resistance of the second base film are as shown in Table 1, the rest is the same as in Example 1-1.

[0208] The preparation parameters and performance parameters of each example and comparative example are shown in Tables 1 to 2.

[0209]

[0210]

[0211] The porosity of the first base film and the puncture resistance of the second base film will affect the kinetic performance and manufacturing yield of the secondary battery. It can be seen from Examples 1-1 to 1-17 and Comparative Examples 1 to 6 that when the porosity of the first base film and the puncture resistance of the second base film are within the scope of this application, the Hi-pot yield and K-value yield of the lithium-ion battery are relatively high, and the degree of lithium deposition on the negative electrode sheet, the surface of the negative electrode sheet facing the first separator, and the surface of the negative electrode sheet facing the second separator after 1000 cycles is relatively low, indicating that the lithium-ion battery prepared in this application can take into account both the manufacturing yield and the kinetic performance. Among them, it can be seen from Examples 1-1 to 1-5 that as the porosity of the first base film increases, the degree of lithium deposition on the negative electrode sheet, the surface of the negative electrode sheet facing the first separator after 1000 cycles is improved, indicating that the kinetic performance of the lithium-ion battery is improved. However, due to the increase in porosity, the contact probability between the first base film and the negative electrode material layer increases, and the Hi-pot yield and K-value yield of the lithium-ion battery are affected; similarly, in Comparative Example 4, due to the porosity of the first base film being higher than the scope defined in this application, the Hi-pot yield and K-value yield of the lithium-ion battery are affected, indicating that the manufacturing yield of the lithium-ion battery is poor; in Comparative Example 6, due to the puncture resistance of the second base film being higher than the scope defined in this application, the degree of lithium deposition on the negative electrode sheet, the surface of the negative electrode sheet facing the second separator after 1000 cycles is relatively high, indicating that the kinetic performance of the lithium-ion battery is affected.

[0212] The puncture resistance of the second base film will be affected by the mass percentage content of the second resin material and the mass percentage content of the third resin material. It can be seen from Examples 1-1, 1-6 to 1-9 that when the mass percentage content of the second resin material and the mass percentage content of the third resin material are within the scope of this application, the puncture resistance of the second base film is within the scope of this application.

[0213] It can be seen from Examples 1-1, 1-6 to 1-9 that when the puncture resistance of the second base film increases, the manufacturing yield of the lithium-ion battery is improved. Since the porosity of the first base film and the second base film does not change, the degree of lithium deposition on the negative electrode sheet, the surface of the negative electrode sheet facing the first separator, and the surface of the negative electrode sheet facing the second separator after 1000 cycles does not change significantly, indicating that the kinetic performance of the lithium-ion battery is less affected; it can be seen from Examples 1-1, 1-12 to 1-13 that when the puncture resistance of the first base film increases, the manufacturing yield of the lithium-ion battery is improved. Since the porosity of the first base film and the second base film does not change, the degree of lithium deposition on the negative electrode sheet, the surface of the negative electrode sheet facing the first separator, and the surface of the negative electrode sheet facing the second separator after 1000 cycles does not change significantly, indicating that the kinetic performance of the lithium-ion battery is less affected.

[0214] The structure of the electrode assembly affects the kinetic performance and manufacturing yield of the secondary battery. As can be seen from Example 1-1 and Example 1-10, the first separator is located between the corner section of the first positive electrode sheet and the corner section of the first negative electrode material layer. Compared with the first separator located between the corner section of the second positive electrode sheet and the corner section of the second negative electrode material layer, the degree of lithium deposition on the first negative electrode material layer is reduced, and the interface condition on one side of the first negative electrode material layer is optimized. This is because the corner section of the first positive electrode sheet is adjacent to a corner section of the first negative electrode material layer and is far from the winding center of the electrode assembly. At this time, the winding radius of the positive electrode material layer in the positive electrode sheet is greater than that of the first negative electrode material layer, and the positive-negative ratio (Cell Balance, CB, the ratio of the negative electrode capacity per unit area to the positive electrode capacity per unit area) is small, and interface problems are likely to occur, resulting in lithium deposition. Therefore, a relatively sufficient electrolyte is required to transport active ions. At the same time, the corner area of the winding structure itself is prone to problems such as insufficient electrolyte or even electrolyte breakage, which will further exacerbate the interface problem. Therefore, in this application, the first separator including the first base film is disposed between the corner section of the first positive electrode sheet and the corner section of the first negative electrode material layer. The first base film with a higher porosity has a higher liquid retention capacity, which is beneficial to improving the problem of insufficient electrolyte in the corner area and is also beneficial to the transport of active ions, so as to improve the interface problem, and further improve the lithium deposition problem of the secondary battery, that is, improve the kinetic performance.

[0215] The material of the first base film affects the kinetic performance and manufacturing yield of the secondary battery. As can be seen from Example 1-1 and Example 1-11, when the first resin material is within the scope of this application, the Hi-pot yield and K-value yield of the lithium-ion battery are relatively high, and the degree of lithium deposition on the negative electrode sheet, the surface of the negative electrode sheet opposite to the first separator, and the surface of the negative electrode sheet opposite to the second separator after 1000 cycles is relatively low, indicating that the lithium-ion battery prepared in this application can balance the manufacturing yield and kinetic performance.

[0216] The puncture resistance of the first base film is affected by the weight-average molecular weight of the first resin material. As can be seen from Example 1-1, Example 1-12 to Example 1-13, when the weight-average molecular weight of the first resin material is within the scope of this application and the puncture resistance of the first base film is within the scope of this application, the Hi-pot yield and K-value yield of the lithium-ion battery are relatively high, and the degree of lithium deposition on the negative electrode sheet, the surface of the negative electrode sheet opposite to the first separator, and the surface of the negative electrode sheet opposite to the second separator after 1000 cycles is relatively low, indicating that the lithium-ion battery prepared in this application can balance the manufacturing yield and kinetic performance. When the weight-average molecular weight of the first resin material is higher than the scope of this application, although the manufacturing yield and kinetic performance of the secondary battery are good, due to the too high weight-average molecular weight of the first resin material, the closed-pore temperature and film-breaking temperature of the obtained first separator are affected, so the thermal box performance of the secondary battery will be affected.

[0217] The thickness of the first base film affects the kinetic performance and manufacturing yield of the secondary battery. It can be seen from Examples 1-1, 1-14 to 1-17 that when the thickness of the first base film is within the scope of this application, the Hi-pot yield and K-value yield of the lithium-ion battery are relatively high, and after 1000 cycles, the degree of lithium deposition on the negative electrode sheet, the surface of the negative electrode sheet opposite the first separator, and the surface of the negative electrode sheet opposite the second separator is relatively low, indicating that the lithium-ion battery prepared in this application can take into account both the manufacturing yield and the kinetic performance. When the thickness of the first base film is higher than the scope of this application, the kinetic performance of the prepared lithium-ion battery is affected; when the thickness of the first base film is lower than the scope of this application, the manufacturing yield of the prepared lithium-ion battery is affected. The porosity of the first separator is affected by the porosity of the first base film. It can be seen from Examples 1-1 to 1-5 that when the porosity of the first base film is within the scope of this application, the porosity of the first separator is within the scope of this application.

[0218] From Figure 6 and Figure 7 it can be seen that during the 1500 cycles of the lithium-ion battery prepared in Example 1-1, the cycle expansion rate is lower than that of the lithium-ion battery prepared in Comparative Example 2, and the cycle capacity retention rate is higher than that of the lithium-ion battery prepared in Comparative Example 2, indicating that the lithium-ion battery prepared in the example is beneficial to improving lithium-ion transmission, that is, improving the kinetic performance of the secondary battery.

[0219] Whether there is a characteristic peak A in the DSC spectrum of the first base film is affected by the type and weight-average molecular weight of the first resin material. It can be seen from Examples 1-1 to 1-17 that when the type and weight-average molecular weight of the first resin material are within the scope of this application, the DSC spectra of the prepared first base films all have the characteristic peak A. Specifically, from Figure 8 it can be seen that there is a characteristic peak in the DSC spectrum of the first base film in Example 1-1 at 128 °C to 134 °C, that is, there is a characteristic peak A.

[0220] Whether there are characteristic peak A and characteristic peak B in the DSC spectrum of the second base film is affected by the types and weight-average molecular weights of the second resin material and the third resin material. It can be seen from Examples 1-1 to 1-17 that when the types and weight-average molecular weights of the second resin material and the third resin material are within the scope of this application, the DSC spectra of the prepared second base films all have the characteristic peak A and the characteristic peak B. Specifically, from Figure 9 it can be seen that in the DSC spectrum of the second base film, there are characteristic peaks at 128 °C to 134 °C and 136 °C to 140 °C, that is, there are characteristic peaks A and B.

[0221]

[0222]

[0223] The material of the second base film can affect the kinetic performance and manufacturing yield of the secondary battery. It can be seen from Example 1-1, Example 2-1, and Example 2-4 that when the second resin material and the third resin material are within the scope of this application, the Hi-pot yield and K-value yield of the lithium-ion battery are relatively high, and the degree of lithium deposition on the negative electrode sheet, the surface of the negative electrode sheet facing the first separator, and the surface of the negative electrode sheet facing the second separator after 1000 cycles is relatively low, indicating that the lithium-ion battery prepared in this application can take into account both the manufacturing yield and the kinetic performance.

[0224] The puncture resistance of the second base film is affected by the weight-average molecular weight of the second resin material and the weight-average molecular weight of the third resin material. It can be seen from Example 1-1, Example 2-2 to Example 2-3, and Example 2-5 to Example 2-6 that when the weight-average molecular weight of the second resin material and the weight-average molecular weight of the third resin material are within the scope of this application, the puncture resistance of the second base film is within the scope of this application. When the weight-average molecular weights of the second resin material and the third resin material are higher than the scope of this application, although the manufacturing yield and kinetic performance of the secondary battery are good, due to the too high weight-average molecular weights of the second resin material and the third resin material, the closed-pore temperature and film-breaking temperature of the obtained second separator are affected, so the thermal box performance of the secondary battery will be affected.

[0225] The porosity of the second base film can affect the kinetic performance and manufacturing yield of the secondary battery. It can be seen from Example 1-1, Example 2-7 to Example 2-9 that when the porosity of the second base film increases, the degree of lithium deposition on the negative electrode sheet and the surface of the negative electrode sheet facing the second separator after 1000 cycles is improved, indicating that the kinetic performance of the lithium-ion battery is improved.

[0226] The thickness of the second base film can affect the kinetic performance and manufacturing yield of the secondary battery. It can be seen from Example 1-1, Example 2-10 to Example 2-13 that when the thickness of the second base film is within the scope of this application, the Hi-pot yield and K-value yield of the lithium-ion battery are relatively high, and the degree of lithium deposition on the negative electrode sheet, the surface of the negative electrode sheet facing the first separator, and the surface of the negative electrode sheet facing the second separator after 1000 cycles is relatively low, indicating that the lithium-ion battery prepared in this application can take into account both the manufacturing yield and the kinetic performance. When the thickness of the second base film is higher than the scope of this application, the kinetic performance of the prepared lithium-ion battery is affected; when the thickness of the second base film is lower than the scope of this application, the manufacturing yield of the prepared lithium-ion battery is affected.

[0227] It should be noted that in this text, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method or article comprising a series of elements not only includes those elements, but also includes other elements not expressly listed, or also includes elements inherent in such process, method or article.

[0228] The above are only the preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present application shall be included within the scope of protection of the present application.

Claims

1. A secondary battery comprising an electrode assembly of a wound structure, wherein the electrode assembly comprises a positive electrode sheet, a negative electrode sheet, a first separator and a second separator, and at least a portion of the negative electrode sheet is located between the first separator and the second separator; The first diaphragm includes a first base film, the second diaphragm includes a second base film, the porosity n1 of the first base film is greater than the porosity n2 of the second base film, the puncture resistance F2 of the second base film is greater than the puncture resistance F1 of the first base film, wherein, 30%≤n1≤60%, 300gf≤F2≤650gf.

2. The secondary battery according to claim 1, wherein The electrode assembly comprises a straight region and a corner region, and the negative electrode plate comprises a negative electrode current collector and a first negative electrode material layer and a second negative electrode material layer located on both sides of the negative electrode current collector in a thickness direction; In the corner area, a first positive electrode sheet corner segment is adjacent to a first negative electrode material layer corner segment, the first positive electrode sheet corner segment is farther away from the winding center of the electrode assembly than the first negative electrode material layer corner segment, and the first separator is located between the first positive electrode sheet corner segment and the first negative electrode material layer corner segment.

3. The secondary battery according to claim 1, wherein 40%≤n1≤50%, and / or, 400gf≤F2≤500gf.

4. The secondary battery according to claim 1, wherein The first base film includes a first resin material, and the weight average molecular weight of the first resin material is 50W to 90W; The second base film includes a second resin material and a third resin material, the second resin material has a weight average molecular weight of 50W to 90W, and the third resin material has a weight average molecular weight of 100W to 200W.

5. The secondary battery according to claim 4, wherein Based on the mass of the second base film, the mass percentage W1 of the second resin material is 70% to 90%, and the mass percentage W2 of the third resin material is 10% to 30%.

6. The secondary battery according to claim 4, wherein The first resin material, the second resin material and the third resin material are each independently selected from at least one of polyethylene, polypropylene, polyimide, polyethyleneimine or polyethylene terephthalate.

7. The secondary battery according to any one of claims 1 to 6, wherein In the differential scanning calorimetry test spectrum of the first base film, a characteristic peak exists at 128° C. to 134° C.

8. The secondary battery according to any one of claims 1 to 6, wherein In the differential scanning calorimetry test spectrum of the second base film, characteristic peaks exist at 128° C. to 134° C. and 136° C. to 140° C.

9. The secondary battery according to any one of claims 1 to 6, wherein 10%≤n2≤30%, 200gf≤F1≤300gf.

10. The secondary battery according to claim 9, wherein 15％≤n2≤30％。 11. The secondary battery according to any one of claims 1 to 6, wherein The thickness of the first base film and the second base film is each independently 4 μm to 7 μm.

12. The secondary battery according to any one of claims 1 to 6, wherein The first diaphragm further includes a first ceramic layer. The porosity of the first diaphragm is n3, and 40%≤n3≤70%.

13. The secondary battery according to claim 12, wherein: The coating weight CW1 of the first ceramic layer is 9 mg / 5000 mm 2 Up to 15mg / 5000mm 2 .

14. The secondary battery according to claim 12, wherein: The first ceramic layer includes first ceramic particles, and the first ceramic particles include at least one of aluminum oxide, boehmite, silicon oxide, magnesium oxide, titanium oxide, tin oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, aluminum hydroxide, magnesium hydroxide or calcium hydroxide.

15. The secondary battery according to claim 2, wherein The first negative electrode material layer includes a first negative electrode material, the second negative electrode material layer includes a second negative electrode material, and the first negative electrode material and / or the second negative electrode material include a silicon material.

16. The secondary battery according to claim 15, wherein The silicon material includes at least one of silicon element, silicon carbon compound, silicon oxygen compound or silicon alloy. 17 . An electronic device comprising the secondary battery according to claim 1 .

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