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; 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 larger than the porosity n2 of the second base membrane, the hole closing temperature T12 of the second base membrane is smaller than the hole closing temperature T11 of the first base membrane, the membrane rupture temperature T22 of the second base membrane is larger than the membrane rupture temperature T21 of the first base membrane, 30% < = n1 < = 60%, T12 < = 140 DEG C, and T22 > = 150 DEG C. The secondary battery provided by the invention can give consideration to both the dynamic performance and the high-temperature hot box performance.

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] Secondary batteries, such as lithium-ion batteries, have the advantages of high energy density, long cycle life, low self-discharge rate, environmental protection and no pollution, and have been widely used in the fields of aviation, aerospace, navigation, electric vehicles, etc. The performance of the separator in a lithium-ion battery determines the interface structure, internal resistance, etc. of the lithium-ion battery, and directly affects the capacity, cycle and safety performance of the lithium-ion battery. A separator with excellent performance plays an important role in improving the comprehensive performance of lithium-ion batteries. At present, the two separators in a secondary battery are usually the same separator, so it is difficult to balance various performances, for example, it is difficult to take into account both kinetic performance and high-temperature hot box performance. Summary of the Invention

[0003] The purpose of the present application is to provide a secondary battery and an electronic device to take into account both kinetic performance and high-temperature hot box performance.

[0004] 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, but the secondary battery of the present application is not limited to lithium-ion batteries. The specific technical solutions are as follows:

[0005] In a first aspect of the present application, a secondary battery is provided, which includes a wound electrode assembly. The electrode assembly includes a positive electrode sheet, a negative electrode sheet, a first separator and a second separator. The negative electrode sheet 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. The closed pore temperature T12 of the second base film is less than the closed pore temperature T11 of the first base film. The film breakage temperature T22 of the second base film is greater than the film breakage temperature T21 of the first base film, where 30% ≤ n1 ≤ 60%, T12 ≤ 140°C, and T22 ≥ 150°C. In some embodiments of the present application, 40% ≤ n1 ≤ 50%. In some embodiments of the present application, 120°C ≤ T12 ≤ 140°C. In some embodiments of the present application, 160°C ≤ T22 ≤ 180°C. Using the first base film with a relatively high porosity and matching it with the second base film with a relatively low closed pore temperature and a relatively high film breakage temperature, and regulating the porosity of the first base film and the closed pore temperature and film breakage temperature of the second base film within the above ranges is beneficial to taking into account both the kinetic performance and high-temperature hot box performance of the secondary battery.

[0006] In some embodiments 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 also beneficial to the transport of active ions to improve the interface problem, thereby improving the lithium plating problem of the secondary battery, that is, improving the kinetic performance.

[0007] In some embodiments of the present application, the first base film includes a first resin material, and the melting enthalpy H1 of the first resin material is 185 J / g to 195 J / g, and the melt flow rate MFR1 is 0.4 g / 10 min to 0.6 g / 10 min. The second base film includes a second resin material, a third resin material, and a fourth resin material. The melting enthalpy H2 of the second resin material is 140 J / g to 150 J / g, and the melt flow rate MFR2 is 6 g / 10 min to 14 g / 10 min. The melting enthalpy H3 of the third resin material is 185 J / g to 195 J / g, and the melt flow rate MFR3 is 0.4 g / 10 min to 0.6 g / 10 min. The melting enthalpy H4 of the fourth resin material is 130 J / g to 180 J / g, and the melt flow rate MFR4 is 0.1 g / 10 min to 100 g / 10 min. With the melting enthalpies and melt flow rates of the first resin material, the second resin material, the third resin material, and the fourth resin material within the above ranges, the obtained first base film has appropriate closed pore temperature and film breaking temperature, and the obtained second base film has appropriate closed pore temperature and film breaking temperature. Thus, while the first separator improves the lithium plating problem of the secondary battery, it can also take into account the high-temperature hot box performance of the secondary battery, and the second separator is beneficial to improving the high-temperature hot box performance of the secondary battery.

[0008] In some embodiments of the present application, the first base film comprises a first resin material with a weight-average molecular weight of 500,000 to 900,000; the second base film comprises a second resin material, a third resin material, and a fourth resin material, with the weight-average molecular weight of the second resin material being 200,000 to 400,000, the weight-average molecular weight of the third resin material being 500,000 to 900,000, and the weight-average molecular weight of the fourth resin material being 50,000 to 500,000. When the weight-average molecular weights of the first resin material, the second resin material, the third resin material, and the fourth resin material are within the above ranges, the obtained first base film and second base film have appropriate porosity, closed-cell temperature, and film-breaking temperature. Thus, the first separator is beneficial to improving the kinetic performance of the secondary battery and taking into account the high-temperature hot box performance, and the second separator is beneficial to improving the high-temperature hot box performance of the secondary battery and taking into account the kinetic performance.

[0009] In some embodiments of the present application, based on the mass of the second base film, the mass percentage content W1 of the second resin material is 5% to 15%, the mass percentage content W2 of the third resin material is 83% to 93%, and the mass percentage content W3 of the fourth resin material is 1% to 3%. When the mass percentage contents of the second resin material, the third resin material, and the fourth resin material are within the above ranges, the obtained second base film has appropriate porosity, closed-cell temperature, and film-breaking temperature. Thus, the second separator is beneficial to improving the high-temperature hot box performance of the secondary battery and taking into account the kinetic performance.

[0010] In some embodiments of the present application, the first resin material, the second resin material, the third resin material, and the fourth resin material each independently comprise at least one of polyethylene, polypropylene, polyimide, polyethyleneimine, or polyethylene terephthalate. When the types of the first resin material, the second resin material, the third resin material, and the fourth resin material are within the above ranges, the obtained first base film and second base film have appropriate porosity, closed-cell temperature, and film-breaking temperature. Thus, the first separator is beneficial to improving the kinetic performance of the secondary battery and taking into account the high-temperature hot box performance, and the second separator is beneficial to improving the high-temperature hot box performance of the secondary battery and taking into account the kinetic performance.

[0011] In some embodiments of the present application, the fourth resin material is different in type from the second resin material and the third resin material, which is beneficial to improving the high-temperature hot box performance of the secondary battery.

[0012] In some embodiments of the present application, in the differential scanning calorimetry test spectrum of the first base film, characteristic peaks exist at 128°C to 134°C.

[0013] In some embodiments of the present application, in the differential scanning calorimetry test spectrum of the second base film, characteristic peaks exist at 120°C to 126°C, 128°C to 134°C, and 150°C to 160°C.

[0014] In some embodiments of the present application, the secondary battery satisfies at least one of the following characteristics:

[0015] (1) 30% ≤ n2 ≤ 40%;

[0016] (2) 140°C ≤ T11 ≤ 160°C;

[0017] (3) 150°C ≤ T21 ≤ 180°C.

[0018] The secondary battery satisfying at least one of the above characteristics is beneficial to taking into account both the high-temperature hot box performance and the kinetic performance of the secondary battery.

[0019] In some embodiments 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.

[0020] In some embodiments of the present application, the first separator further includes a first ceramic coating, and the porosity of the first separator is n3, where 40% ≤ n3 ≤ 70%. The setting of the first ceramic coating is beneficial to further improving the liquid absorption and liquid retention capabilities of the first separator and further improving the kinetic performance of the secondary battery.

[0021] In some embodiments of the present application, the coating weight CW1 of the first ceramic coating is 9 mg / 5000 mm 2 to 15 mg / 5000 mm 2 . When the coating weight CW1 of the first ceramic coating is within the above range, the first ceramic coating has a suitable thickness, which is beneficial to taking into account the energy density while improving the kinetic performance of the secondary battery.

[0022] In some embodiments of the present application, the first ceramic coating includes first ceramic particles, and the first ceramic particles include at least one of alumina, boehmite, silica, magnesia, 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 thus improve the kinetic performance of the secondary battery.

[0023] 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, which is beneficial to taking into account the energy density, high-temperature hot box performance, and kinetic performance of the secondary battery.

[0024] In some embodiments of the present application, the silicon material includes at least one of elemental silicon, silicon carbide compound, silicon oxide compound, or silicon alloy, which is beneficial to taking into account the energy density, high-temperature heat box performance, and kinetic performance of the secondary battery.

[0025] The second aspect of the present application provides an electronic device, which includes the secondary battery in any of the foregoing embodiments.

[0026] Advantages of the present application:

[0027] The present application provides a secondary battery and an electronic device. The secondary battery includes a wound electrode assembly. The electrode assembly includes a positive electrode tab, a negative electrode tab, a first separator, and a second separator. The negative electrode tab is located between the first separator and 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, the closed pore temperature T12 of the second base film is less than the closed pore temperature T11 of the first base film, and the film breakage temperature T22 of the second base film is greater than the film breakage temperature T21 of the first base film. Among them, 30% ≤ n1 ≤ 60%, T12 ≤ 140 °C, and T22 ≥ 150 °C. Using the first base film with a relatively high porosity in combination with the second base film with a relatively low closed pore temperature and a relatively high film breakage temperature, and regulating the porosity of the first base film and the closed pore temperature and film breakage temperature of the second base film within the above ranges is beneficial to taking into account the kinetic performance and high-temperature heat box performance of the secondary battery.

[0028] Of course, it is not necessary for any product or method implementing the present application to simultaneously achieve all the above-mentioned advantages. Description of the Drawings

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present application. For those of ordinary skill in the art, other embodiments can also be obtained based on these drawings.

[0030] Figure 1 It is a schematic structural diagram of an electrode assembly in an embodiment of the present application;

[0031] Figure 2 For Figure 1 The partial enlarged view at A in

[0032] Figure 3 It is a schematic structural diagram of an electrode assembly in another embodiment of the present application;

[0033] Figure 4 For Figure 1 The partial enlarged view at B in

[0034] Figure 5 It is the differential scanning calorimetry test spectrogram of the first base film in Example 1-1;

[0035] Figure 6 It is the differential scanning calorimetry test spectrogram of the second base film in Example 1-1:

[0036] Figure 7 It is the cyclic capacity retention rate graph of the lithium-ion batteries in Example 1-1 and Comparative Example 1-2;

[0037] Figure 8 It is the cyclic thickness swelling rate graph of the lithium-ion batteries in Example 1-1 and Comparative Example 1-2. Detailed implementation manners

[0038] Next, the technical solutions in the present application will be clearly and completely described in conjunction with the embodiments of the present application and the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by those skilled in the art based on the present application belong to the scope of protection of the present application.

[0039] It should be noted that in the specific implementation manners of the present application, a lithium-ion battery is used as an example of a secondary battery to explain the present application, but the secondary battery of the present application is not limited to lithium-ion batteries.

[0040] The first aspect of 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. 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 (not shown in the figure), and the second separator 40 includes a second base film (not shown in the figure). 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, and the obtained first separator has good liquid absorption and liquid retention capabilities, which is beneficial to improving the kinetic performance of the secondary battery. The closed pore temperature T12 of the second base film is less than the closed pore temperature T11 of the first base film, and the film breakage temperature T22 of the second base film is greater than the film breakage temperature T21 of the first base film. When there is a risk of thermal runaway in the secondary battery at high temperatures, the second separator including the above-mentioned second base film can close the pores in time to hinder the transmission of active ions such as lithium ions. At the same time, the higher film breakage temperature is beneficial to reducing the short circuit between the positive and negative electrodes, which is beneficial to improving the high-temperature thermal box performance of the secondary battery. Thus, for the secondary battery of the present application, using two different separators in combination is beneficial to taking into account the kinetic performance and high-temperature thermal box performance of the secondary battery.

[0041] In some embodiments of the present application, 30% ≤ n1 ≤ 60%. In some embodiments of the present application, 40% ≤ n1 ≤ 50%. For example, the porosity n1 of the first base film can be 30%, 35%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 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 is not conducive to storing sufficient electrolyte to improve the kinetic performance of the secondary battery; when the porosity of the first base film is too large, for example, greater than 60%, the mechanical properties and structural stability of the first separator are poor, and short circuits between the positive and negative electrodes are likely to occur, affecting the high-temperature thermal box performance of the secondary battery. Therefore, by controlling the porosity of the first base film within the above range, it is beneficial to improve the kinetic performance and high-temperature thermal box performance of the secondary battery.

[0042] In some embodiments of the present application, T12 ≤ 140 °C. In some embodiments of the present application, 120 °C ≤ T12 ≤ 140. For example, the closed-pore temperature T12 of the second base film can be 120 °C, 121 °C, 122 °C, 123 °C, 124 °C, 125 °C, 126 °C, 127 °C, 128 °C, 129 °C, 130 °C, 131 °C, 132 °C, 133 °C, 134 °C, 135 °C, 136 °C, 137 °C, 138 °C, 139 °C, 140 °C or a range composed of any two of these values. In some embodiments of the present application, T22 ≥ 150 °C. In some embodiments of the present application, 160 °C ≤ T22 ≤ 180 °C. For example, the film-breaking temperature T22 of the second base film can be 150 °C, 151 °C, 152 °C, 153 °C, 154 °C, 155 °C, 156 °C, 157 °C, 158 °C, 159 °C, 160 °C, 161 °C, 162 °C, 163 °C, 164 °C, 165 °C, 166 °C, 167 °C, 168 °C, 169 °C, 170 °C, 171 °C, 172 °C, 173 °C, 174 °C, 175 °C, 176 °C, 177 °C, 178 °C, 179 °C, 180 °C or a range composed of any two of these values. When the closed-pore temperature of the second base film is too low or the film-breaking temperature is too high, for example, T12 is greater than 140 °C or T22 is less than 150 °C, when there is a risk of thermal runaway in the secondary battery at high temperature, the second base film cannot close the pores in time to hinder the transport of active ions, and it is also easy to be damaged to cause short circuits between the positive and negative electrodes, which will increase the generation of heat, increase the risk of thermal runaway, and affect the high-temperature thermal box performance of the secondary battery. Therefore, by controlling the closed-pore temperature and film-breaking temperature of the second base film within the above range, it is beneficial to improve the high-temperature thermal box performance of the secondary battery.

[0043] Therefore, by using a first base film with a relatively high porosity in combination with a second base film with a relatively low closing temperature and a relatively high film-breaking temperature, and controlling the porosity of the first base film and the closing temperature and film-breaking temperature of the second base film within the above ranges, it is beneficial to balance the kinetic performance and high-temperature hot box performance of the secondary battery.

[0044] In some embodiments of the present application, as Figures 1 to 4 shown, the electrode assembly 01 includes a flat region 011 and a corner region 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 201.

[0045] In some embodiments of the present application, as Figure 1 and Figure 2 shown, in the same layer of the negative electrode sheet 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 region 012 of the electrode assembly 01, a first positive electrode sheet corner segment 101 is adjacent to a first negative electrode material layer corner segment 221. The first positive electrode sheet 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 sheet 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, a first positive electrode sheet 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 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 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 region 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, by disposing the first separator including the first base film between the first positive electrode sheet corner segment and the first negative electrode material layer corner segment, the first base film with a high porosity has a high liquid retention capacity, which is beneficial to improving the problem of insufficient electrolyte in the corner region and is also beneficial to the transport of active ions, so as to improve the interfacial problems, and further improve the lithium deposition problem of the secondary battery, that is, improve the kinetic performance.

[0046] In some embodiments of the present application, as Figure 3 and Figure 4As shown, in the same layer of the negative electrode sheet 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 corner section 102 of the second positive electrode sheet is adjacent to a corner section 222 of the second negative electrode material layer. The corner section 102 of the second positive electrode sheet is closer to the winding center of the electrode assembly 01 than the corner section 222 of the second negative electrode material layer. The first separator 30 is located between the corner section 102 of the second positive electrode sheet and the corner section 222 of the second negative electrode material layer, and the second separator 40 is located on the side of the corner section 102 of the second positive electrode sheet away from the first separator 30.

[0047] In some embodiments of the present application, the first base film includes a first resin material. The heat of fusion enthalpy H1 of the first resin material is 185 J / g to 195 J / g, and the melt flow rate MFR1 is 0.4 g / 10 min to 0.6 g / 10 min. For example, the heat of fusion enthalpy H1 of the first resin material can be 185 J / g, 186 J / g, 187 J / g, 188 J / g, 189 J / g, 190 J / g, 191 J / g, 192 J / g, 193 J / g, 194 J / g, 195 J / g or a range composed of any two of these values. For example, the melt flow rate MFR1 of the first resin material can be 0.4 g / 10 min, 0.42 g / 10 min, 0.45 g / 10 min, 0.48 g / 10 min, 0.5 g / 10 min, 0.52 g / 10 min, 0.55 g / 10 min, 0.58 g / 10 min, 0.6 g / 10 min or a range composed of any two of these values. When the heat of fusion enthalpy H1 and the melt flow rate MFR1 of the first resin material are within the above ranges, the obtained first base film has appropriate closed pore temperature and film breaking temperature, so that while the first separator improves the problem of lithium plating in the secondary battery, it can also take into account the high temperature heat box performance of the secondary battery.

[0048] In some embodiments of the present application, the first base film includes a first resin material, and the weight average molecular weight M w1 of the first resin material is 50W (where "W" represents 10,000, and 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. The weight average molecular weight M w1 of the first resin material is within the above range, and the obtained first base film has appropriate porosity, closed pore temperature and film breaking temperature, so that the first separator is beneficial to improving the kinetic performance of the secondary battery and taking into account the high temperature heat box performance.

[0049] In some embodiments of the present application, the first resin material includes at least one of polyethylene, polypropylene, polyimide, polyethyleneimine, or polyethylene terephthalate. When the type of the first resin material is within the above range, the obtained first base film has appropriate porosity, closed pore temperature, and film breakage temperature, so that the first separator is beneficial to improving the lithium plating problem of the secondary battery and taking into account the high-temperature hot box performance.

[0050] In some embodiments of the present application, in the differential scanning calorimetry (DSC) test spectrum of the first base film, there is a characteristic peak A at 128°C to 134°C, and the peak value of the characteristic peak A 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 a characteristic peak A.

[0051] In some embodiments of the present application, 140°C ≤ T11 ≤ 160°C; for example, the closed pore temperature T11 of the first base film can be 140°C, 142°C, 145°C, 147°C, 149°C, 150°C, 152°C, 154°C, 155°C, 157°C, 159°C, 160°C, or a range composed of any two of these values. When the closed pore temperature of the first base film is within the above range, that is, the first base film has a lower closed pore temperature, so that the first separator can not only improve the lithium plating problem of the secondary battery but also take into account the high-temperature hot box performance of the secondary battery.

[0052] In some embodiments of the present application, 150°C ≤ T21 ≤ 180°C. For example, the film breakage temperature T21 of the first base film can be 150°C, 152°C, 155°C, 157°C, 160°C, 162°C, 165°C, 167°C, 170°C, 172°C, 175°C, 178°C, 180°C, or a range composed of any two of these values. That is, the first base film has a higher film breakage temperature, so that the first separator can not only improve the lithium plating problem of the secondary battery but also take into account the high-temperature hot box performance of the secondary battery.

[0053] In some embodiments of the present application, the thickness of the first base film is 4 μm to 7 μm. For example, the thickness h1 of the first base film can be 4 μm, 5 μm, 6 μm, 7 μm, or a range composed of any two of these values. When the thickness of the first base film is within the above range, it is also beneficial to take into account the energy density of the secondary battery.

[0054] In some embodiments of the present application, the first separator further includes a first ceramic coating, and the porosity of the first separator is n3, where 40% ≤ n3 ≤ 70%. For example, the porosity n3 of the first separator can be 40%, 42%, 45%, 48%, 50%, 52%, 55%, 58%, 60%, 62%, 65%, 68%, 70%, or a range composed of any two of these values. The setting of the first ceramic coating is beneficial to further improving the liquid absorption and retention capabilities of the first separator and further improving the kinetic performance of the secondary battery.

[0055] In some embodiments of the present application, the coating weight CW1 of the first ceramic coating is 9 mg / 5000 mm 2 to 15 mg / 5000 mm 2 For example, the coating weight CW1 of the first ceramic coating 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 mm 2 、15 mg / 5000 mm 2 or a range composed of any two of these values. When the coating weight CW1 of the first ceramic coating is within the above range, the first ceramic coating has an appropriate thickness, which is beneficial to taking into account its energy density while improving the kinetic performance of the secondary battery.

[0056] In some embodiments of the present application, the first ceramic coating includes first ceramic particles, and the first ceramic particles include at least one of alumina, boehmite, silica, magnesia, titania, stannic oxide, calcium oxide, zirconia, yttria, 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.

[0057] In some embodiments of the present application, the first separator further includes a first adhesive layer disposed on both sides of the first base film, and the first ceramic coating is disposed on one side of the first base film, and the first ceramic coating is located between the first base film and the first adhesive layer. In some embodiments of the present application, in the electrode assembly, the first ceramic coating faces the positive electrode sheet, which is beneficial to further improving the problem of insufficient electrolyte in the corner area and also beneficial to the transmission of active ions to improve the interface problem, thereby improving the lithium deposition problem of the secondary battery.

[0058] The present application does not particularly limit the composition of the first adhesive layer as long as the object of the present 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, polyethylene oxide, or polyimide. The present application does not particularly limit the thickness of the first adhesive layer as long as the object of the present application can be achieved. Exemplarily, the thickness of the first adhesive layer can be 1 μm to 5 μm.

[0059] In some embodiments of the present application, the second base film includes a second resin material, a third resin material, and a fourth resin material.

[0060] The heat of fusion H2 of the second resin material is from 140 J / g to 150 J / g, and the melt flow rate MFR2 is from 6 g / 10 min to 14 g / 10 min. For example, the heat of fusion H2 of the second resin material can be 140 J / g, 141 J / g, 142 J / g, 143 J / g, 144 J / g, 145 J / g, 146 J / g, 147 J / g, 148 J / g, 149 J / g, 150 J / g or a range composed of any two of these values. For example, the melt flow rate MFR2 of the second resin material can be 6 g / 10 min, 7 g / 10 min, 8 g / 10 min, 9 g / 10 min, 10 g / 10 min, 11 g / 10 min, 12 g / 10 min, 13 g / 10 min, 14 g / 10 min or a range composed of any two of these values. The heat of fusion H3 of the third resin material is from 185 J / g to 195 J / g, and the melt flow rate MFR3 is from 0.4 g / 10 min to 0.6 g / 10 min. For example, the heat of fusion H3 of the third resin material can be 185 J / g, 186 J / g, 187 J / g, 188 J / g, 189 J / g, 190 J / g, 191 J / g, 192 J / g, 193 J / g, 194 J / g, 195 J / g or a range composed of any two of these values. For example, the melt flow rate MFR3 of the third resin material can be 0.4 g / 10 min, 0.42 g / 10 min, 0.45 g / 10 min, 0.48 g / 10 min, 0.5 g / 10 min, 0.52 g / 10 min, 0.55 g / 10 min, 0.58 g / 10 min, 0.6 g / 10 min or a range composed of any two of these values. The heat of fusion H4 of the fourth resin material is from 130 J / g to 180 J / g, and the melt flow rate MFR4 is from 0.1 g / 10 min to 100 g / 10 min. For example, the heat of fusion H4 of the fourth resin material can be 130 J / g, 135 J / g, 140 J / g, 145 J / g, 150 J / g, 155 J / g, 160 J / g, 165 J / g, 170 J / g, 175 J / g, 180 J / g or a range composed of any two of these values. For example, the melt flow rate MFR4 of the fourth resin material can be 0.1 g / 10 min, 1 g / 10 min, 5 g / 10 min, 10 g / 10 min, 20 g / 10 min, 30 g / 10 min, 40 g / 10 min, 50 g / 10 min, 60 g / 10 min, 70 g / 10 min, 80 g / 10 min, 90 g / 10 min, 100 g / 10 min or a range composed of any two of these values.The melting enthalpy and melt index of the second resin material, the third resin material, and the fourth resin material are within the above ranges, and the obtained second base film has appropriate closed pore temperature and film breaking temperature, so that the second separator is beneficial to improving the high temperature thermal box performance of the secondary battery.

[0061] In some embodiments of the present application, the weight average molecular weight M of the second resin material w2 is 20W to 40W. For example, the weight average molecular weight of the second resin material can be 20W, 22W, 25W, 27W, 29W, 30W, 32W, 35W, 37W, 39W, 40W, or a range composed of any two of these values. The weight average molecular weight M of the third resin material w3 is 50W to 90W. For example, the weight average molecular weight of the third 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 fourth resin material is 5W to 50W. For example, the weight average molecular weight M of the fourth resin material w4 can be 5W, 10W, 15W, 20W, 25W, 30W, 35W, 40W, 45W, 50W, or a range composed of any two of these values. When the weight average molecular weights of the second resin material, the third resin material, and the fourth resin material are within the above ranges, the obtained second base film has appropriate porosity, closed pore temperature, and film breaking temperature, so that the second separator is beneficial to improving the high temperature thermal box performance of the secondary battery and taking into account the kinetic performance.

[0062] In some embodiments of the present application, based on the mass of the second base film, the mass percentage content W1 of the second resin material is 5% to 15%, the mass percentage content W2 of the third resin material is 83% to 93%, and the mass percentage content W3 of the fourth resin material is 1% to 3%. For example, the mass percentage content W1 of the second resin material can be 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, or a range composed of any two of these values. For example, the mass percentage content W2 of the third resin material can be 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, or a range composed of any two of these values. For example, the mass percentage content W3 of the fourth resin material can be 1%, 1.2%, 1.4%, 1.5%, 1.7%, 1.9%, 2%, 2.2%, 2.4%, 2.5%, 2.7%, 2.9%, 3%, or a range composed of any two of these values. When the mass percentage contents of the second resin material, the third resin material, and the fourth resin material are within the above ranges, the obtained second base film has appropriate porosity, closed pore temperature, and film breaking temperature, so that the second separator is beneficial to improving the high temperature thermal box performance of the secondary battery and taking into account the kinetic performance.

[0063] In some embodiments of the present application, the second resin material, the third resin material, and the fourth resin material each independently include at least one of polyethylene, polypropylene, polyimide, polyethyleneimine, or polyethylene terephthalate. When the types of the second resin material, the third resin material, and the fourth resin material are within the above range, the obtained second base film has appropriate porosity, closed pore temperature, and film breaking temperature, so that the first separator is beneficial to improving the lithium plating problem of the secondary battery and taking into account the high-temperature hot box performance.

[0064] In some embodiments of the present application, the fourth resin material is different from the second resin material and the third resin material in type. In some embodiments of the present application, the second resin material and the third resin material include polyethylene, and the fourth resin material includes polypropylene. The obtained second base film has a lower closed pore temperature and a higher film breaking temperature, which is beneficial to improving the high-temperature hot box performance of the secondary battery.

[0065] In some embodiments of the present application, in the differential scanning calorimetry test spectrum of the second base film, characteristic peaks exist at 120°C to 126°C, 128°C to 134°C, and 150°C to 160°C. The peak value of the characteristic peak B between 120°C and 126°C corresponds to the melting temperature of the second resin material, the peak value of the characteristic peak C between 128°C and 134°C corresponds to the melting temperature of the third resin material, and the peak value of the characteristic peak D between 150°C and 160°C corresponds to the melting temperature of the fourth resin material. In the present application, in the second base film, the peak value of the characteristic peak is between 120°C and 126°C, that is, the characteristic peak B exists; the peak value of the characteristic peak is between 128°C and 134°C, that is, the characteristic peak C exists; the peak value of the characteristic peak is between 150°C and 160°C, that is, the characteristic peak D exists.

[0066] In some embodiments of the present application, 30% ≤ n2 ≤ 40%. In some embodiments of the present application, 30% < n1 ≤ 60% and 30% ≤ n2 ≤ 40%. For example, the porosity n2 of the second base film can be 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, or a range composed of any two of these values. When the porosity of the second base film is within the above range, that is, the second base film has a relatively high porosity, so that the first separator can take into account the kinetic performance of the secondary battery while improving the high-temperature hot box performance of the secondary battery.

[0067] In some embodiments of the present application, the thickness of the second base film is 4 μm to 7 μm. For example, the thickness h2 of the second base film can be 4 μm, 5 μm, 6 μm, 7 μm, or a range composed of any two of these values. When the thickness of the second base film is within the above range, it is also beneficial to take into account the energy density of the secondary battery.

[0068] In some embodiments of the present application, the second separator includes a second ceramic coating and a second adhesive layer. The second adhesive layer is disposed on both sides of the second base film, and the second ceramic coating is disposed on one side of the second base film. The second ceramic coating 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 plate, 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 plating problem of the secondary battery.

[0069] In some embodiments of the present application, the second separator further includes a second ceramic coating, and the porosity of the second separator is n4, where 40% ≤ n4 ≤ 50%. For example, the porosity n3 of the second separator can be 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50% or a range composed of any two of these values.

[0070] The present application does not particularly limit the coating quality of the second ceramic coating, as long as the purpose of the present application can be achieved. Exemplarily, the coating weight CW2 of the second ceramic coating is 9 mg / 5000 mm 2 to 15 mg / 5000 mm 2 . For example, the coating weight CW2 of the second ceramic coating 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 mm 2 , 15 mg / 5000 mm 2 or a range composed of any two of these values.

[0071] In some embodiments of the present application, the second ceramic coating includes second ceramic particles, and the second ceramic particles include at least one of alumina, boehmite, silica, magnesia, titania, stannic oxide, calcium oxide, zirconia, yttria, silicon carbide, aluminum hydroxide, magnesium hydroxide, or calcium hydroxide.

[0072] 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.

[0073] At present, silicon materials have outstanding advantages over traditional carbon materials in terms of energy density, but silicon materials generally have poor electrical conductivity, so they usually need to be paired with high-kinetic electrolytes and material layer formulations, but high-kinetic electrolytes and material layer formulations have poor hot box performance at high temperatures. Therefore, negative electrodes containing silicon materials are used in secondary batteries, and it is usually difficult to take into account energy density, kinetic performance, and high-temperature hot box performance. 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, in combination 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 lower closed-cell temperature and a higher membrane rupture temperature, which is beneficial to take into account the energy density, kinetic performance and high-temperature hot box performance of the secondary battery.

[0074] In some embodiments of the present application, the silicon material includes at least one of silicon, silicon-carbon compound, silicon-oxygen compound or silicon alloy. Selecting the above silicon material is more conducive to obtaining a secondary battery with higher energy density.

[0075] In the present application, the features in the above-mentioned various embodiments can be combined arbitrarily.

[0076] The present application has no particular limitation on the preparation method of the first base film, as long as the purpose of the present application can be achieved. For example, the preparation method of the first base film includes but is not limited to the following steps: (1) mixing an additive and a solvent in a mass ratio of (0.1 to 0.3):100 to obtain a mixed solution; (2) mixing the first resin material and the mixed solution in a mass ratio of (10:90) to (30:70), and obtaining a first base material through extrusion, casting, cooling, and film forming; (3) longitudinally stretching and transversely stretching the first base material, and then extracting and drying to obtain a first porous base material; (4) performing secondary stretching on the first porous base material, performing heat setting, and winding to obtain the first base film. Among them, the solvent may include but is not limited to at least one of paraffin oil or dichloromethane; the additive may 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 may include but is not limited to at least one of dichloromethane or n-hexane. The stretching multiples of longitudinal stretching and transverse stretching can each independently be 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 0.5 h to 1 h; the heat setting temperature can be 110°C to 135°C.

[0077] In the present application, the porosity n1 of the first base film can be adjusted 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.

[0078] In the present application, the first resin materials with different melting enthalpies, melt indices, weight average molecular weights, and types can be obtained by purchase. Their melting enthalpies, melt indices, and weight average molecular weights can be measured, and materials with the required melting enthalpies, melt indices, and weight average molecular weights can be selected. Among them, the specific test methods can refer to the relevant content in the "Test Methods and Equipment" section.

[0079] 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 quality of the first ceramic coating. Among them, the method for regulating 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; conversely, n3 decreases. When other conditions remain unchanged, when the coating quality of the first ceramic coating increases, the porosity n3 of the first separator increases; conversely, n3 decreases.

[0080] This application does not particularly limit the preparation method of the second base film, 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, the third resin material, and the fourth resin material evenly according to the above W1, W2, and W3 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 (20:80) to (40:60), and form a film by extrusion, casting, and cooling to obtain a second substrate; (4) Perform longitudinal stretching and transverse stretching on the second substrate, and obtain a second porous substrate after extraction and drying; (5) Perform secondary stretching on the second porous substrate, perform heat setting, and wind up to obtain the second base film. Among them, the solvent can 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 pentaerythritol tetrakis[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 each independently be 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 0.5 h to 1 h; the heat setting temperature can be 110°C to 135°C.

[0081] 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.

[0082] In this application, the closed pore temperature and film breaking temperature of the second base film vary with the melting enthalpy, melt index, weight average molecular weight, content, and type of the second resin material, the third resin material, and the fourth resin material. For example, when other conditions remain unchanged, as the weight average molecular weight of the second resin material increases, the closed pore temperature rises, and vice versa; as the mass percentage content of the second resin material increases, the closed pore temperature decreases, and vice versa. Another example is that when other conditions remain unchanged, as the weight average molecular weight of the third resin material increases, the closed pore temperature and the film breaking temperature are higher, and vice versa, the closed pore temperature and the film breaking temperature are lower; as the mass percentage content of the third resin material is higher, the closed pore temperature is higher, and vice versa. Still another example is that when other conditions remain unchanged, as the weight average molecular weight of the fourth resin material increases, the film breaking temperature rises, and vice versa; as the mass percentage content of the fourth resin material increases, the film breaking temperature increases, and vice versa. In this application, the closed pore temperature and the film breaking temperature of the first base film can be regulated in the same way as those of the second base film. In this application, the porosity of the second separator can be regulated in the same way as the porosity n3 of the first separator.

[0083] In this application, the second resin material, the third resin material, and the fourth resin material with different melting enthalpies, melt indexes, weight average molecular weights, and types can be obtained by purchase, and their melting enthalpies, melt indexes, and weight average molecular weights can be measured, and the materials with the desired melting enthalpies, melt indexes, and weight average molecular weights can be selected. Among them, the specific test method for the weight average molecular weight of the material can refer to the relevant content in the "Test Methods and Equipment" section.

[0084] This application has no particular limitation on the negative electrode current collector, as long as the purpose of this 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 this application, the first negative electrode material layer and the second negative electrode material layer can also independently include a negative electrode conductive agent and a negative electrode binder. This application has no particular limitation on the types of the negative electrode conductive agent and the negative electrode binder, as long as the purpose of this application can be achieved. For example, it can be at least one of a positive electrode conductive agent and a positive electrode binder. This application has 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, and those skilled in the art can select according to actual needs as long as the purpose of this application can be achieved.

[0086] Optionally, the negative electrode tab may further include a first negative electrode conductive layer, which is located between the negative electrode current collector and the first negative electrode material layer. The present application places no particular limitation on the composition of the first negative electrode conductive layer, and it may 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. The present application places no particular limitation on the first negative electrode conductive layer conductive agent and the first negative electrode conductive layer binder, and for example, it may be at least one of a positive electrode conductive agent and a positive electrode binder.

[0087] Optionally, the negative electrode tab may further include a second negative electrode conductive layer, which is located between the negative electrode current collector and the second negative electrode material layer. The present application places no particular limitation on the composition of the second negative electrode conductive layer, and it may 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. The present application places no particular limitation on the second negative electrode conductive layer conductive agent and the second negative electrode conductive layer binder, and for example, it may be at least one of a positive electrode conductive agent and a positive electrode binder.

[0088] The present application places 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 places 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 places 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 tab 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 thickness direction. It should be noted that the "surface" here may 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 places no particular limitation, as long as the object of the present application can be achieved.

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

[0093] The positive electrode material layer includes a positive electrode active material. There are no particular limitations on the positive electrode active material in this application, as long as the objectives of this application can be achieved. For example, the positive electrode active material may include, but is not limited to, lithium nickel cobalt manganate (such as NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminate, lithium iron phosphate, lithium-rich manganese-based material, lithium cobaltate (LiCoO2), lithium manganate, or lithium manganese iron phosphate, or at least one of them.

[0094] The positive electrode material layer may also include a positive electrode conductive agent and a positive electrode binder. There are no particular limitations on the types of the positive electrode conductive agent and the positive electrode binder in this application, as long as the objectives of this 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] There are no particular limitations on the positive electrode binder in this application, as long as the objectives of this 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] There are no particular limitations on 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 in this application. Those skilled in the art can select according to actual needs, as long as the objectives of this application can be achieved.

[0097] There are no particular limitations on the thickness of the positive electrode current collector and the positive electrode material layer in this application, as long as the objectives of this 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, which 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 places no particular restrictions on 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-mentioned positive electrode conductive agents and the above-mentioned positive electrode binders.

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

[0100] The present application places no particular restrictions 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 places no particular restrictions 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 places no particular restrictions 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 has no particular limitation on 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 has no particular limitation on 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 electrode assembly with a wound structure, 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 electrode assembly with a wound structure, 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. may 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 may 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, a laptop computer, a pen input computer, a mobile computer, an e-book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a headset stereo, a video recorder, a liquid crystal TV, a portable cleaner, a portable CD player, a mini disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, a car, a motorcycle, a moped, a bicycle, a lighting fixture, a toy, a game console, a clock, a power tool, a flash, a camera, a large household battery, and a lithium ion capacitor, 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 embodiments 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 for 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-sized and -shaped 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, and 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] Closed pore temperature and film rupture temperature of the first base film and the second base film:

[0118] The closed pore temperature and film rupture temperature of the sample are tested by the temperature rise internal resistance method. First, cut the sample into a fixed size of 5 cm × 5 cm, and the size of the base film sample is larger than the size of the fixture test area. Place the sample in a component composed of a ceramic and a stainless steel fixture, inject 10 mL of electrolyte, place the above fixture in an oven, set the oven temperature to 250 °C, and heat it at a certain rate of 15 °C / min. At the same time, monitor the resistance and temperature of the fixture, and output the data of the fixture temperature and resistance, oven temperature and time to obtain the temperature-resistance curve. Among them, the sample is the first base film or the second base film.

[0119] According to the temperature-resistance curve, the temperature at which the resistance suddenly increases (for example, reaches 1000 Ω) is the closed pore temperature of the separator; the maximum value of the resistance is the highest resistance of the separator.

[0120] On the basis of the above test of the diaphragm's closing temperature, continue to increase the test time. When the resistance suddenly drops to the same value as the resistance at the closing temperature, the temperature at this time is the diaphragm's film-breaking temperature.

[0121] Among them, the composition and preparation method of the electrolyte in the test are the same as those in Example 1-1.

[0122] Thickness test of the first base film and the second base film:

[0123] 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.

[0124] DSC spectrum test of the first base film and the second base film:

[0125] 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 60 °C to 190 °C. 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 of the first base film, the characteristic peak corresponding to the peak value of 128 °C to 134 °C is recorded as characteristic peak A; in the obtained DSC spectrum of the second base film, the characteristic peak corresponding to the peak value of 120 °C to 126 °C is recorded as characteristic peak B, the characteristic peak corresponding to the peak value of 128 °C to 134 °C is recorded as characteristic peak C, and the characteristic peak corresponding to the peak value of 150 °C to 160 °C is recorded as characteristic peak D.

[0126] According to the DSC spectrum of the second base film, calculate the ratio of the peak areas of characteristic peak B, characteristic peak C, and characteristic peak D. The peak area ratio is also the mass ratio of the second resin material, the third resin material, and the fourth resin material. After conversion, the mass percentage content of the second resin material, the third resin material, and the fourth resin material can be obtained.

[0127] Weight-average molecular weight test:

[0128] 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%.

[0129] 2) Gel column selection: The smaller the pore size of the gel column, the slower the flow rate, and the larger the high molecular weight 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.

[0130] 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.

[0131] 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.

[0132] 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 based on a reference substance (polyethylene), and obtain the weight-average molecular weight of the first resin material.

[0133] 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, and the weight-average molecular weights of the second resin material, the third resin material, and the fourth resin material can be obtained respectively.

[0134] Testing of melt index:

[0135] Use a melt flow rate tester to test the melt index of the material. Heat the instrument to 110 °C, place a standard die with a diameter of φ2.095 ± 0.005 mm, put the material into the barrel and compact it, keep it warm for 10 min, set the material cutting time to 60 s, apply a pressure of 2.16 kg to extrude the material, and calculate the melt index MFR of the material melt. MFR = mT / t, where t is the cutting time of 60 s, m is the mass of the material, and T is 600 s. The above material can be the first resin material, the second resin material, the third resin material, or the fourth resin material.

[0136] Testing of melting enthalpy:

[0137] Use a differential scanning calorimeter to test the melting enthalpy of the resin materials in each example and comparative example. Put the sample material into a crucible, and under a nitrogen atmosphere, heat it from room temperature to 250 °C at a rate of 10 °C / min to obtain a differential scanning calorimetry curve (DSC curve). Calculate the melting enthalpy of the sample material according to the area enclosed by the melting peak in the DSC curve, and take the peak temperature of the melting peak in the DSC curve as the melting point of the sample material. The above numerical materials can be the first resin material, the second resin material, the third resin material, or the fourth resin material.

[0138] Melting enthalpy = integral area of the melting endothermic curve / mass.

[0139] Degree of lithium deposition on the negative electrode plate after 1000 cycles:

[0140] The lithium-ion battery is placed in an environment at 25 °C and charged at a constant current of 0.5C until the voltage reaches 4.5V. Then it is charged at a constant voltage of 4.5V until the cut-off current is 0.05C, and left standing for 5 minutes. After that, it is discharged at a constant current of 0.5C until the voltage reaches 3.0V and left standing for 5 minutes. This is one charge-discharge cycle. Then, the charging and discharging cycles are carried out 1000 times in the same steps.

[0141] Then it is 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 is 0.05C, and left standing for 5 minutes. Then the lithium-ion battery is disassembled, and the degree of lithium deposition on the negative electrode plate is observed, including the degree of lithium deposition on the surface of the negative electrode plate opposite to the first separator and the degree of lithium deposition on the surface of the negative electrode plate opposite to the second separator.

[0142] For the surface of the negative electrode plate 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 "mild 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". Here, the proportion of the lithium deposition area is the percentage of the lithium deposition area in the area of the negative electrode material layer surface of the negative electrode plate opposite to the first separator.

[0143] Similarly, for the surface of the negative electrode plate 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 proportion of the lithium deposition area is greater than 0 and less than 5%, it is recorded as "mild 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". Here, the proportion of the lithium deposition area is the percentage of the lithium deposition area in the area of the negative electrode material layer surface of the negative electrode plate opposite to the second separator.

[0144] For the entire negative electrode plate, 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 "mild 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". Here, the proportion of the lithium deposition area is the percentage of the lithium deposition area in the total area of the two surfaces of the negative electrode material layer of the negative electrode plate.

[0145] 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.

[0146] Cycling performance test:

[0147] 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. Then, it is 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, repeat the above steps 1500 times, and record the discharge capacity after each cycle. When cycling to 50, 100, 150, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400 cycles, charge and discharge according to the following steps: charge at a constant current of 0.5C to 4.5V, and then at a constant voltage to 0.02C, and discharge at 0.2C to 3.0V.

[0148] Cycling capacity retention rate (%) = (discharge capacity after cycling N times / discharge capacity of the first cycle) × 100%. Where N is a positive integer from 1 to 1500.

[0149] During the cycling process, use a platen thickness gauge to measure the thickness of the lithium-ion battery, record the initial thickness, and then record the thickness of the lithium-ion battery after cycling Q times at 50, 100, 150, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500.

[0150] Cycling expansion rate (%) = (thickness after cycling Q times - 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.

[0151] Hot box test:

[0152] Under the condition of 25 °C, put the lithium-ion battery charged at a constant current of 2C to 4.5V and then at a constant voltage of 4.5V until the current is 0.02C into a test chamber with circulating air convection, a temperature of 25 °C, and a humidity of 80%. After standing for 5 minutes, heat the test chamber at a rate of 5 °C / min to 130 °C, keep it at 130 °C unchanged, stop the test after 1 hour, and check whether the lithium-ion battery catches fire or explodes. Pass if there is no fire or explosion. Pass rate of hot box test at 130 °C = number of passes of hot box test at 130 °C / total number of hot box tests at 130 °C (5).

[0153] At 25°C, a lithium-ion battery charged at a constant current of 2C to 4.5V and then at a constant voltage of 4.5V until the current reached 0.02C was placed in a test chamber with circulating air convection, a temperature of 25°C, and a humidity of 80%. After standing for 5 minutes, the test chamber was heated to 132°C at a rate of 5°C / min and maintained at 132°C. After 1 hour, the test was stopped, and the lithium-ion battery was checked for ignition or explosion. The passing rate of the hot box test at 132°C = the number of lithium-ion batteries passing the hot box test at 132°C / the total number of lithium-ion batteries in the hot box test at 132°C (5).

[0154] Example 1-1

[0155] <Preparation of the first separator>

[0156] (1) The additive pentaerythritol tetra[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and the solvent paraffin oil were mixed evenly at a mass ratio of 0.2:100 to obtain a mixed solution;

[0157] (2) The first resin material polyethylene and the mixed solution were mixed evenly at a mass ratio of 20:80, added to an extrusion system, extruded through a T-shaped die, and cast, cooled, and sheeted to form a first substrate; where M w1 = 70W, H1 = 190J / g, MFR1 = 0.5g / 10min.

[0158] (3) The first substrate was longitudinally stretched 7 times and transversely stretched 7 times, extracted with dichloromethane at 25°C for 0.5 h, and dried to obtain a first porous substrate;

[0159] (4) The first porous substrate was longitudinally stretched 1.5 times and transversely stretched 1.5 times for the second time, heat-set at 115°C, and wound up to obtain a first base film, and the thickness h1 of the first base film was 5μm.

[0160] (5) The inorganic particle boehmite with a Dv50 of 1μm and polyacrylate (Mw = 40W) were mixed at a mass ratio of 90:10 and then dissolved in deionized water to form a first ceramic coating slurry with a solid content of 50wt%. Subsequently, the first ceramic coating slurry was evenly coated on one surface of the first base film by microgravure coating and dried to obtain a first base film provided with a first ceramic coating. Among them, the coating weight CW1 of the first ceramic coating was 12mg / 5000mm 2 .

[0161] (6) The first binder polyvinylidene fluoride (PVDF, 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. The first binder layer slurry is uniformly coated on the surface of the first ceramic coating away from the first base film by screen printing, dried, and then the above coating process is repeated 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.

[0162] <Preparation of the second separator>

[0163] (1) Mix the second resin material polyethylene, the third resin material polyethylene, and the fourth resin material polypropylene evenly according to a mass ratio of 10:88:2 to obtain a mixed material; among them, M w2 = 30 W, H2 = 145 J / g, MFR2 = 10 g / 10 min; M w3 = 70 W, H3 = 190 J / g, MFR3 = 0.5 g / 10 min; M w4 = 28 W, H4 = 155 J / g, MFR4 = 50 g / 10 min.

[0164] (2) Mix the additive pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and the solvent paraffin oil evenly according to a mass ratio of 0.2:100 to obtain a mixed solution;

[0165] (3) Mix the mixed material and the mixed solution evenly according to a mass ratio of 20:80, add them to an extrusion system, extrude through a T-shaped die, and cast, cool, and form a film to obtain a second substrate;

[0166] (4) Stretch the second 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 second porous substrate;

[0167] (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 a second base film, and the thickness h2 of the second base film is 5 μm.

[0168] (6) Dissolve boehmite, an inorganic particle with a Dv50 of 1 μm, and polyacrylate (Mw = 40 W) in deionized water according to a mass ratio of 90:10 to form a first ceramic coating slurry with a solid content of 50 wt%. Subsequently, the first ceramic coating slurry is uniformly coated on one surface of the first base film by microgravure coating and dried to obtain a second base film provided with a second ceramic coating. Among them, the coating weight CW2 of the second ceramic coating is 12 mg / 5000 mm 2 .

[0169] (7) The weight-average molecular weight of the second binder PVDF is 8.5×10 6 ), 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%, obtaining the second binder layer slurry; the second binder layer slurry is uniformly coated on one surface of the second ceramic coating away from the second base film by means of 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.

[0170] <Preparation of negative electrode sheet>

[0171] The negative electrode active materials artificial graphite, silicon carbide compound, acetylene black, styrene-butadiene rubber and sodium carboxymethyl cellulose are mixed in a mass ratio of 90:6:1:1.5:1.5, and then deionized water is added as a solvent to formulate a slurry with a solid content of 70 wt%. After being stirred evenly by a vacuum mixer, the first negative electrode slurry is obtained. 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 , the length is 720 mm, and the compaction density of the second negative electrode material layer is 1.735 g / cm 3 , the length is 680 mm. Among them, the mass ratio of silicon element to carbon element in the silicon carbide compound is 2:8.

[0172] <Preparation of positive electrode sheet>

[0173] The cathode active material lithium cobalt oxide, acetylene black, and PVDF are mixed at 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 wt%. Subsequently, it is stirred evenly by a vacuum mixer to obtain a cathode slurry. The cathode slurry is evenly 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 cathode material layer coating thickness of 55 μm. The cathode slurry is coated on the other surface of the cathode current collector aluminum foil. After drying, the thickness of the cathode material layer coated on this surface is 55 μm, and then a cathode electrode sheet with a total thickness of 122 μm is obtained. After cold pressing, a cathode electrode sheet with a single-sided coating of a cathode material layer with a thickness of 122 μm is obtained. The coated cathode electrode sheet is cold pressed and then cut into a specification of 70 mm × 800 mm and welded with electrode tabs for later use. Among them, the tap density of the cathode material layer is 4.23 g / cm 3 .

[0174] <Preparation of electrolyte>

[0175] 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) in non-aqueous organic solvents are mixed at a mass ratio of 20:30:20:28:2. Then, lithium hexafluorophosphate (LiPF6) is added to the non-aqueous organic solvents 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 non-aqueous organic solvents.

[0176] <Preparation of lithium-ion battery>

[0177] The above-prepared cathode electrode sheet, the first separator, the negative electrode sheet, and the 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 cathode electrode sheet is adjacent to the corner section of the first negative electrode material layer. The corner section of the first cathode 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 cathode electrode sheet and the corner section of the first negative electrode material layer; the first ceramic coating of the first separator faces the cathode electrode sheet, and the second ceramic coating of the second separator faces the cathode electrode sheet,; in the same layer of the negative electrode sheet, the first negative electrode material layer is farther from the winding center than the second negative electrode material layer, and the second separator is adjacent to 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, and the moisture is removed at 80 °C, and the prepared electrolyte is injected. After processes such as vacuum packaging, standing, formation, and shaping, a secondary battery is obtained.

[0178] Examples 1-2 to Examples 1-5

[0179] Except for 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, closed pore temperature, and film breaking temperature of the first base film are as shown in Table 1, the rest is the same as in Example 1-1.

[0180] Examples 1-6 to 1-9, Example 15

[0181] Except for adjusting the type of the first resin material 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, closed pore temperature, and film breaking temperature of the first base film are as shown in Table 1, the rest is the same as in Example 1-1.

[0182] Examples 1-10 to 1-13

[0183] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Example 1-1.

[0184] Example 1-14

[0185] Except for adjusting <the preparation of lithium-ion batteries> according to the following steps, the rest is the same as in Example 1-1.

[0186] <the preparation of lithium-ion batteries>

[0187] 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, refer 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. 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; the first ceramic coating of the first separator faces the positive electrode sheet, and the second ceramic coating of the second separator faces the positive electrode sheet; in the same layer of negative electrode sheets, the first negative electrode material layer is farther from the winding center of the electrode assembly than the second negative electrode material layer. The second separator is located on the side of the corner section of the second positive electrode sheet away from the first separator. 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.

[0188] Examples 2-1 to 2-12, Example 2-27

[0189] Except for adjusting the types of the second resin material, the third resin material, or the fourth resin material 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, closed pore temperature, and film breaking temperature of the second base film are as shown in Table 2, the rest is the same as in Example 1-1.

[0190] Examples 2-13 to 2-19, Examples 2-22 to 2-26

[0191] Except for adjusting the relevant preparation parameters according to Table 2, the rest is the same as in Example 1-1.

[0192] Examples 2-20 to 2-21

[0193] Except for 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, closed pore temperature, and film breaking temperature of the first base film are as shown in Table 2, the rest is the same as in Example 1-1.

[0194] Comparative Examples 1-1 to 1-2

[0195] Except for 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, closed pore temperature, and film breaking temperature of the first base film are as shown in Table 1, the rest is the same as in Example 1-1.

[0196] Comparative Example 1-3

[0197] Except for replacing the second base film with the first base film, the rest is the same as in Example 1-1.

[0198] Comparative Examples 2-1 to 2-2

[0199] Except for adjusting the relevant preparation parameters according to Table 2, the rest is the same as in Example 1-1.

[0200] Comparative Example 2-3

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

[0202] The preparation parameters and performance tests of each example and comparative example are shown in Tables 1 to 2.

[0203] Table 1

[0204]

[0205]

[0206]

[0207]

[0208] It can be seen from Examples 1-1 to 1-15, Examples 2-1 to 2-27, Comparative Examples 1-1 to 1-3, and Comparative Examples 2-1 to 2-3 that when the porosity n1 of the first base film in the lithium-ion battery is greater than the porosity n2 of the second base film, the closed-pore temperature T12 of the second base film is less than the closed-pore temperature T11 of the first base film, the film-breaking temperature T22 of the second base film is greater than the film-breaking temperature T21 of the first base film, and n1, T12, and T22 are simultaneously within the scope of this application, the obtained lithium-ion battery has a high passing rate in the hot box test at 130 °C and 132 °C, and the degree of lithium deposition on the surface of the negative electrode sheet opposite to the first separator and the second separator and the overall negative electrode sheet is relatively light, thus indicating that the lithium-ion battery obtained in this application can balance the kinetic performance and the high-temperature hot box performance. In Comparative Examples 1-1 to 1-2, the porosity of the first base film is not within the scope of this application. In Comparative Example 1-3, the second base film is not used. For the obtained lithium-ion batteries, although Comparative Example 1-1 has good high-temperature hot box performance, its degree of lithium deposition is also more serious; although Comparative Examples 1-2 and 1-3 have a relatively light degree of lithium deposition, their high-temperature hot box performance is poor. In Comparative Example 2-1, the closed-pore temperature of the second base film is not within the scope of this application. In Comparative Example 2-2, the film-breaking temperature of the second base film is not within the scope of this application. In Comparative Example 1-3, the first base film is not used. For the obtained lithium-ion batteries, although Comparative Examples 2-1 and 2-2 have a relatively light degree of lithium deposition, their high-temperature hot box performance is poor; for Comparative Example 2-3, although it has good high-temperature hot box performance, its degree of lithium deposition is also more serious. It can be seen that the lithium-ion batteries in Comparative Examples 1-1 to 1-3 and Comparative Examples 2-1 to 2-3 are difficult to balance the kinetic performance and the high-temperature hot box performance.

[0209] There is a linked change among the melt enthalpy, melt index, and weight-average molecular weight of the first resin material. It can be seen from Examples 1-1, 1-6 to 1-9 that when the above parameters are within the scope of this application, the obtained lithium-ion battery has a high passing rate in the hot box test at 130 °C and 132 °C, and the degree of lithium deposition on the surface of the negative electrode sheet opposite to the first separator and the second separator and the overall negative electrode sheet is relatively light, thus indicating that the obtained lithium-ion battery can balance the kinetic performance and the high-temperature hot box performance.

[0210] Whether there is a characteristic peak A in the DSC spectrum of the first base film is affected by the melt enthalpy, melt index, and weight-average molecular weight of the first resin material. It can be seen from Examples 1-1 to 1-15 that when the above parameters of the first resin material are within the scope of this application, the DSC spectrum of the first base film all has the characteristic peak A. Specifically, from Figure 5It 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.

[0211] It can be seen from Examples 1-1, 1-10 to 1-13 that when the thickness of the first base film is within the scope of the present application, the obtained lithium-ion battery has a high passing rate in the hot box test at 130°C and 132°C, and the degree of lithium deposition on the surface of the negative electrode tab and the overall negative electrode tab opposite to the first separator and the second separator is relatively light, indicating that the obtained lithium-ion battery can balance the kinetic performance and the high-temperature hot box performance.

[0212] It can be seen from Example 1-1 and Example 1-14 that the first separator is located between the corner section of the first positive electrode tab and the corner section of the first negative electrode material layer. Compared with the second separator located between the corner section of the second positive electrode tab 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 tab is adjacent to a corner section of a 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 tab is larger 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 problems. Therefore, in the present application, by arranging the first separator including the first base film between the corner section of the first positive electrode tab 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 transmission of active ions, so as to improve the interface problem, and further improve the lithium deposition problem of the secondary battery, that is, to improve the kinetic performance.

[0213] The co-variation among the melt enthalpy, melt index, and weight-average molecular weight of the second resin material, the third resin material, and the fourth resin material will affect the closed-cell temperature and film-breaking temperature of the second base film, and further affect the high-temperature hot-box performance of the lithium-ion battery. It can be seen from Examples 1-1, 2-1 to 2-12, and 2-22 that when the above parameters are within the scope of this application, the obtained lithium-ion battery has a high passing rate in the hot-box test at 130 °C and 132 °C, indicating that the obtained lithium-ion battery has good high-temperature hot-box performance. In addition, the degree of lithium deposition on the surface of the negative electrode sheet opposite to the first separator and the second separator and the overall negative electrode sheet is relatively light, and the kinetic performance can be taken into account at the same time. In Examples 1-1, 2-1 to 2-4, the weight-average molecular weight and melt enthalpy of the second resin material in Example 2-3 are less than those in Example 1-1, and the melt index is greater than that in Example 1-1. The closed-cell temperature of the obtained second base film is also smaller, so its high-temperature hot-box performance is better, but the mechanical properties of the second base film are poor, which will affect the manufacturing yield of the lithium-ion battery.

[0214] Whether there are characteristic peaks B, C, and D in the DSC spectrum of the second base film is affected by the melt enthalpy, melt index, and weight-average molecular weight of the second resin material, the third resin material, and the fourth resin material. It can be seen from Examples 2-1 to 2-27 that when the above parameters are within the scope of this application, the DSC spectrum of the second base film has characteristic peaks B, C, and D. Specifically, as can be seen from Figure 6 it can be seen that there are characteristic peaks in the DSC spectrum of the second base film in Example 1-1 at 120 °C to 126 °C, 128 °C to 134 °C, and 150 °C to 160 °C, that is, there are characteristic peaks B, C, and D.

[0215] The mass percentage contents of the second resin material, the third resin material, and the fourth resin material will affect the closed-cell temperature and film-breaking temperature of the second base film, and further affect the high-temperature hot-box performance of the lithium-ion battery. It can be seen from Examples 1-1, 2-13 to 2-19 that when the above parameters are within the scope of this application, the obtained lithium-ion battery has a high passing rate in the hot-box test at 130 °C and 132 °C, indicating that the obtained lithium-ion battery has good high-temperature hot-box performance. In addition, the degree of lithium deposition on the surface of the negative electrode sheet opposite to the first separator and the second separator and the overall negative electrode sheet is relatively light, and the kinetic performance can be taken into account at the same time. Among them, compared with Example 1-1 in Example 2-17, the mass percentage content of the second resin material is too high and the mass percentage content of the third resin material is too low. The mass percentage content of the third resin material is the main factor affecting the strength of the second separator. When the mass percentage content of the third resin material is too low, it will lead to insufficient puncture resistance of the second separator, affecting the use safety and service life of the secondary battery.

[0216] As can be seen from Examples 1-1, 2-20 to 2-21, when the porosity of the second base film is within the scope of the present application, the obtained lithium-ion battery has a high passing rate in the hot box test at 130 °C and 132 °C, and the degree of lithium deposition on the surface of the negative electrode sheet opposite to the first separator and the second separator and the overall negative electrode sheet is relatively light, indicating that the obtained lithium-ion battery can balance the kinetic performance and the high-temperature hot box performance.

[0217] As can be seen from Examples 1-1, 2-23 to 2-26, when the thickness of the second base film is within the scope of the present application, the obtained lithium-ion battery has a high passing rate in the hot box test at 130 °C and 132 °C, and the degree of lithium deposition on the surface of the negative electrode sheet opposite to the first separator and the second separator and the overall negative electrode sheet is relatively light, indicating that the obtained lithium-ion battery can balance the kinetic performance and the high-temperature hot box performance.

[0218] From Figure 7 and Figure 8 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 1-2, and the cycle capacity retention rate is higher than that of the lithium-ion battery prepared in Comparative Example 1-2, indicating that the lithium-ion battery prepared in the example is beneficial to improving lithium-ion transport, that is, improving the kinetic performance of the lithium-ion battery.

[0219] 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 closed pore temperature T12 of the second base film is less than the closed pore temperature T11 of the first base film, and the rupture temperature T22 of the second base film is greater than the rupture temperature T21 of the first base film, wherein, 30%≤n1≤60%, T12≤140℃, T22≥150℃.

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, which satisfies at least one of the following characteristics: (1)40％≤n1≤50％； (2)120℃≤T12≤140℃； (3)160℃≤T22≤180℃。 4. The secondary battery according to claim 1, wherein The first base film includes a first resin material, the melting enthalpy H1 of the first resin material is 185 J / g to 195 J / g, and the melt index MFR1 is 0.4 g / 10 min to 0.6 g / 10 min; The second base film includes a second resin material, a third resin material and a fourth resin material. The second resin material has a melting enthalpy H2 of 140 J / g to 150 J / g, and a melt index MFR2 of 6 g / 10min to 14 g / 10min. The third resin material has a melting enthalpy H3 of 185 J / g to 195 J / g, and a melt index MFR3 of 0.4 g / 10min to 0.6 g / 10min. The fourth resin material has a melting enthalpy H4 of 130 J / g to 180 J / g, and a melt index MFR4 of 0.1 g / 10min to 100 g / 10min.

5. The secondary battery according to claim 4, wherein The weight average molecular weight of the first resin material is 50W to 90W; The weight average molecular weight of the second resin material is 20W to 40W, the weight average molecular weight of the third resin material is 50W to 90W, and the weight average molecular weight of the fourth resin material is 5W to 50W.

6. 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 5% to 15%, the mass percentage W2 of the third resin material is 83% to 93%, and the mass percentage W3 of the fourth resin material is 1% to 3%.

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

8. The secondary battery according to claim 7, wherein The fourth resin material is of a different type from the second resin material and the third resin material.

9. The secondary battery according to any one of claims 1 to 8, wherein In the differential scanning calorimetry test spectrum of the first base film, a characteristic peak exists at 128° C. to 134° C.

10. The secondary battery according to any one of claims 1 to 8, wherein In the differential scanning calorimetry test spectrum of the second base film, characteristic peaks exist at 120°C to 126°C, 128°C to 134°C, and 150°C to 160°C.

11. The secondary battery according to any one of claims 1 to 8, which satisfies at least one of the following characteristics: (1)30％≤n2≤40％； (2)140℃≤T11≤160℃； (3)150℃≤T21≤180℃。 12. The secondary battery according to any one of claims 1 to 8, wherein The thickness of the first base film and the second base film is each independently 4 μm to 7 μm.

13. The secondary battery according to any one of claims 1 to 8, wherein The first diaphragm further includes a first ceramic coating. The porosity of the first diaphragm is n3, and 40%≤n3≤70%.

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

15. The secondary battery according to claim 13, wherein The first ceramic coating 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.

16. 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.

17. The secondary battery according to claim 16, wherein The silicon material includes at least one of silicon element, silicon carbon compound, silicon oxygen compound or silicon alloy. 18 . An electronic device comprising the secondary battery according to claim 1 .

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