Secondary battery and electronic device

Abstract

A secondary battery and an electronic device. The secondary battery comprises an electrode assembly having a wound structure. 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 separator comprises a first base film, and the second separator comprises a second base film. A porosity n1 of the first base film is greater than a porosity n2 of the second base film, a shutdown temperature T12 of the second base film is less than a shutdown temperature T11 of the first base film, and a rupture temperature T22 of the second base film is greater than a rupture temperature T21 of the first base film, where 30% ≤ n1 ≤ 60%, T12 ≤ 140°C, and T22 ≥ 150°C. The secondary battery is capable of achieving both kinetic performance and high-temperature hot-box performance.

Description

A secondary battery and electronic device

[0001] This application claims priority to Chinese Patent Application No. 202510356815.1, filed on March 25, 2025, entitled "A Secondary Battery and Electronic Device", the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to the field of electrochemical technology, and in particular to a secondary battery and electronic device. Background Technology

[0003] Secondary batteries, such as lithium-ion batteries, possess advantages such as high energy density, long cycle life, low self-discharge rate, and environmental friendliness, and are widely used in aviation, aerospace, marine, and electric vehicle fields. The performance of the separator in a lithium-ion battery determines its interface structure and internal resistance, directly affecting its capacity, cycle life, and safety performance. A high-performance separator plays a crucial role in improving the overall performance of lithium-ion batteries. Currently, the two separators in secondary batteries are usually identical, making it difficult to balance various performance aspects, such as kinetic performance and high-temperature thermal performance. Summary of the Invention

[0004] The purpose of this application is to provide a secondary battery and electronic device that balances kinetic performance and high-temperature thermal performance.

[0005] It should be noted that while this application uses lithium-ion batteries as an example of secondary batteries to explain the invention, the secondary batteries in this application are not limited to lithium-ion batteries. The specific technical solution is as follows:

[0006] A first aspect of this application provides a secondary battery comprising a wound electrode assembly, the electrode assembly including a positive electrode, a negative electrode, a first separator, and a second separator, the negative electrode being 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 pore-closing temperature T12 of the second base film is less than the pore-closing temperature T11 of the first base film, and the pore-breaking temperature T22 of the second base film is greater than the pore-breaking temperature T21 of the first base film, wherein 30% ≤ n1 ≤ 60%, T12 ≤ 140℃, and T22 ≥ 150℃. In some embodiments of this application, 40% ≤ n1 ≤ 50%. In some embodiments of this application, 120℃ ≤ T12 ≤ 140℃. In some embodiments of this application, 160℃ ≤ T22 ≤ 180℃. Using a first base membrane with relatively high porosity in combination with a second base membrane with relatively low pore-closing temperature and relatively high membrane rupture temperature, and controlling the porosity of the first base membrane and the pore-closing temperature and membrane rupture temperature of the second base membrane within the above range, is beneficial to balance the dynamic performance of the secondary battery and the performance of the high-temperature thermal chamber.

[0007] In some embodiments of this application, the electrode assembly includes a flat region and a corner region. The negative electrode 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 thickness direction of the negative current collector. In the corner region, a first positive electrode corner segment is adjacent to a first negative electrode material layer corner segment. The first positive electrode corner segment is farther from the winding center of the electrode assembly than the first negative electrode material layer corner segment. The first separator is located between the first positive electrode corner segment and the first negative electrode material layer corner segment. The first separator, containing a first base film, is disposed between the first positive electrode corner segment and the first negative electrode material layer corner segment. The first base film, with its higher porosity, has a higher electrolyte retention capacity, which helps to improve the problem of insufficient electrolyte in the corner region and also facilitates the transport of active ions, thereby improving the interface problem and ultimately improving the lithium plating problem of the secondary battery, i.e., improving the kinetic performance.

[0008] In some embodiments of this application, the first base film comprises a first resin material, wherein the enthalpy of melting 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 comprises a second resin material, a third resin material, and a fourth resin material, wherein the enthalpy of melting H2 of the second resin material is 140 J / g to 150 J / g and the melt index MFR2 is 6 g / 10 min to 14 g / 10 min, the enthalpy of melting H3 of the third resin material is 185 J / g to 195 J / g and the melt index MFR3 is 0.4 g / 10 min to 0.6 g / 10 min, and the enthalpy of melting H4 of the fourth resin material is 130 J / g to 180 J / g and the melt index MFR4 is 0.1 g / 10 min to 100 g / 10 min. The melting enthalpy and melt index of the first, second, third, and fourth resin materials are within the above range. The resulting first base film has a suitable pore-closing temperature and film-breaking temperature, and the resulting second base film has a suitable pore-closing temperature and film-breaking temperature. Thus, the first separator can improve the lithium plating problem of the secondary battery while also taking into account the high-temperature thermal performance of the secondary battery, and the second separator is beneficial to improving the high-temperature thermal performance of the secondary battery.

[0009] In some embodiments of this application, the first base membrane comprises a first resin material with a weight-average molecular weight of 50W to 90W; the second base membrane comprises a second resin material, a third resin material, and a fourth resin material, wherein 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. When the weight-average molecular weights of the first, second, third, and fourth resin materials are within the aforementioned ranges, the resulting first and second base membranes possess suitable porosity, pore-closing temperature, and membrane rupture temperature. Therefore, the first separator is beneficial for improving the kinetic performance of the secondary battery while also considering high-temperature thermal box performance, and the second separator is beneficial for improving the high-temperature thermal box performance of the secondary battery while also considering kinetic performance.

[0010] In some embodiments of this application, based on the mass of the second base membrane, 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%. When the mass percentages of the second, third, and fourth resin materials are within the above ranges, the resulting second base membrane has suitable porosity, pore closure temperature, and membrane rupture temperature. Therefore, the second separator is beneficial for improving the high-temperature thermal performance of the secondary battery while also considering its kinetic performance.

[0011] In some embodiments of this application, 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. Since the types of the first resin material, the second resin material, the third resin material, and the fourth resin material are within the above-mentioned range, the resulting first base membrane and the second base membrane have suitable porosity, pore closure temperature, and membrane rupture temperature. Therefore, the first separator is beneficial for improving the kinetic performance of the secondary battery while also considering high-temperature thermal performance, and the second separator is beneficial for improving the high-temperature thermal performance of the secondary battery while also considering kinetic performance.

[0012] In some embodiments of this application, the fourth resin material is of a different type than the second and third resin materials, which is beneficial to improving the high-temperature thermal performance of the secondary battery.

[0013] In some embodiments of this application, a characteristic peak exists in the differential scanning calorimetry spectrum of the first base film between 128°C and 134°C.

[0014] In some embodiments of this application, characteristic peaks exist at 120°C to 126°C, 128°C to 134°C, and 150°C to 160°C in the differential scanning calorimetry spectrum of the second base film.

[0015] In some embodiments of this application, the secondary battery satisfies at least one of the following characteristics:

[0016] (1) 30% ≤ n2 ≤ 40%;

[0017] (2) 140℃≤T11≤160℃;

[0018] (3) 150℃≤T21≤180℃.

[0019] A secondary battery that meets at least one of the above characteristics is beneficial for balancing the high-temperature thermal performance and kinetic performance of the secondary battery.

[0020] In some embodiments of this application, the thicknesses of the first base film and the second base film are each independently between 4 μm and 7 μm. Having the thicknesses of the first and second base films within this range also helps to balance the energy density of the secondary battery.

[0021] In some embodiments of this application, the first separator further includes a first ceramic coating, wherein the porosity of the first separator is n3, and 40% ≤ n3 ≤ 70%. The presence of the first ceramic coating is beneficial for further improving the liquid absorption and retention capacity of the first separator, and further improving the kinetic performance of the secondary battery.

[0022] In some embodiments of this application, the coating weight CW1 of the first ceramic coating is 9 mg / 5000 mm². 2 Up to 15mg / 5000mm 2 The coating weight CW1 of the first ceramic coating is within the above range, and the first ceramic coating has a suitable thickness, which not only improves the dynamic performance of the secondary battery, but also helps to balance its energy density.

[0023] In some embodiments of this application, the first ceramic coating comprises first ceramic particles, which include at least one of alumina, boehmite, silicon oxide, magnesium oxide, titanium oxide, tin oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, aluminum hydroxide, magnesium hydroxide, or calcium hydroxide. By selecting the above-mentioned types of first ceramic particles, it is beneficial to further improve the liquid absorption and retention capacity of the first separator, thereby improving the kinetic performance of the secondary battery.

[0024] In some embodiments of this 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 silicon material, which is beneficial to balance the energy density, high-temperature thermal performance and kinetic performance of the secondary battery.

[0025] In some embodiments of this application, the silicon material includes at least one of elemental silicon, silicon carbide, silicon oxide, or silicon alloy, which is beneficial for balancing the energy density, high-temperature thermal performance, and kinetic performance of the secondary battery.

[0026] A second aspect of this application provides an electronic device that includes a secondary battery as described in any of the foregoing embodiments.

[0027] The beneficial effects of this application are:

[0028] This application provides a secondary battery and an electronic device. The secondary battery includes a wound electrode assembly, which includes a positive electrode, a negative electrode, a first separator, and a second separator. The negative electrode is located between the first separator and the second separator. The first separator includes a first base membrane, and the second separator includes a second base membrane. The porosity n1 of the first base membrane is greater than the porosity n2 of the second base membrane. The pore-closing temperature T12 of the second base membrane is less than the pore-closing temperature T11 of the first base membrane, and the membrane-breaking temperature T22 of the second base membrane is greater than the membrane-breaking temperature T21 of the first base membrane. Wherein, 30% ≤ n1 ≤ 60%, T12 ≤ 140℃, and T22 ≥ 150℃. Using a first base membrane with relatively high porosity paired with a second base membrane with relatively low pore-closing temperature and relatively high membrane-breaking temperature, and controlling the porosity of the first base membrane and the pore-closing and membrane-breaking temperatures of the second base membrane within the aforementioned ranges, is beneficial for balancing the kinetic performance and high-temperature thermal performance of the secondary battery.

[0029] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Attached Figure Description

[0030] The accompanying drawings, which are provided to further illustrate this application and form part of this application, illustrate exemplary embodiments of this application and are used to explain this application, but do not constitute an undue limitation of this application.

[0031] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other embodiments can be obtained based on these drawings.

[0032] Figure 1 is a schematic diagram of the electrode assembly in one embodiment of this application;

[0033] Figure 2 is a magnified view of part A in Figure 1;

[0034] Figure 3 is a schematic diagram of the electrode assembly in another embodiment of this application;

[0035] Figure 4 is a magnified view of part B in Figure 1;

[0036] Figure 5 shows the differential scanning calorimetry spectrum of the first base film in Example 1-1;

[0037] Figure 6 shows the differential scanning calorimetry spectrum of the second base film in Example 1-1:

[0038] Figure 7 shows the cycle capacity retention of the lithium-ion batteries in Examples 1-1 and Comparative Examples 1-2;

[0039] Figure 8 shows the cycle thickness expansion rate of the lithium-ion batteries in Examples 1-1 and Comparative Examples 1-2. Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided with reference to the accompanying drawings and embodiments. Obviously, the described embodiments are merely some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0041] It should be noted that, in the specific embodiments of this application, lithium-ion batteries are used as an example of secondary batteries to explain this application, but the secondary batteries in this application are not limited to lithium-ion batteries.

[0042] A first aspect of this application provides a secondary battery comprising a wound electrode assembly. As shown in Figures 1 and 3, the electrode assembly 01 includes a positive electrode 10, a negative electrode 20, a first separator 30, and a second separator 40, with the negative electrode 20 located between the first separator 30 and the second separator 40. The first separator 30 includes a first base membrane (not shown in the figures), and the second separator 40 includes a second base membrane (not shown in the figures). The porosity n1 of the first base membrane is greater than the porosity n2 of the second base membrane, allowing the first base membrane to store more electrolyte. The resulting first separator has good liquid absorption and retention capabilities, which is beneficial for improving the kinetic performance of the secondary battery. The pore-closing temperature T12 of the second base film is lower than that of the first base film T11, and the rupture temperature T22 of the second base film is higher than that of the first base film T21. When the secondary battery is at risk of thermal runaway at high temperatures, the second separator containing the aforementioned second base film can promptly close the pores to hinder the transport of active ions such as lithium ions. Simultaneously, the higher rupture temperature helps reduce short circuits between positive and negative charges, thus improving the high-temperature thermal performance of the secondary battery. Therefore, the secondary battery of this application, by using two different separators in combination, is beneficial in balancing the kinetic performance and high-temperature thermal performance of the secondary battery.

[0043] In some embodiments of this application, 30% ≤ n1 ≤ 60%. In some embodiments of this application, 40% ≤ n1 ≤ 50%. For example, the porosity n1 of the first base membrane can be 30%, 35%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 55%, 60%, or a range of any two of these values. When the porosity of the first base membrane 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 membrane 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 prone to occur, affecting the high-temperature thermal performance of the secondary battery. Therefore, by controlling the porosity of the first base membrane within the above range, it is beneficial to improve the kinetic performance and high-temperature thermal performance of the secondary battery.

[0044] In some embodiments of this application, T12 ≤ 140℃. In some embodiments of this application, 120℃ ≤ T12 ≤ 140℃. For example, the pore-closure temperature T12 of the second base film can be 120℃, 121℃, 122℃, 123℃, 124℃, 125℃, 126℃, 127℃, 128℃, 129℃, 130℃, 131℃, 132℃, 133℃, 134℃, 135℃, 136℃, 137℃, 138℃, 139℃, 140℃, or a range of any two of these values. In some embodiments of this application, T22 ≥ 150℃. In some embodiments of this application, 160℃ ≤ T22 ≤ 180℃. For example, the rupture temperature T22 of the second base film can be 150℃, 151℃, 152℃, 153℃, 154℃, 155℃, 156℃, 157℃, 158℃, 159℃, 160℃, 161℃, 162℃, 163℃, 164℃, 165℃, 166℃, 167℃, 168℃, 169℃, 170℃, 171℃, 172℃, 173℃, 174℃, 175℃, 176℃, 177℃, 178℃, 179℃, 180℃, or a range of any two of these values. When the pore-closing temperature of the second base film is too low or the rupture temperature is too high, for example, T12 greater than 140℃ or T22 less than 150℃, the second base film cannot close its pores in time to hinder the transport of active ions when the secondary battery is at risk of thermal runaway at high temperatures. It is also prone to damage, causing short circuits between the positive and negative electrodes, increasing heat generation, raising the risk of thermal runaway, and affecting the high-temperature thermal performance of the secondary battery. Therefore, by controlling the pore-closing temperature and rupture temperature of the second base film within the above-mentioned ranges, it is beneficial to improve the high-temperature thermal performance of the secondary battery.

[0045] Therefore, using a first base membrane with relatively high porosity in combination with a second base membrane with relatively low pore-closing temperature and relatively high membrane-breaking temperature, and controlling the porosity of the first base membrane and the pore-closing temperature and membrane-breaking temperature of the second base membrane within the above range, is beneficial to balance the dynamic performance of the secondary battery and the performance of the high-temperature thermal chamber.

[0046] In some embodiments of this application, as shown in Figures 1 to 4, the electrode assembly 01 includes a straight region 011 and a corner region 012, the positive electrode 10 includes a positive current collector 110 and a positive material layer 120 disposed on two surfaces of the positive current collector 110, and the negative electrode 20 includes a negative current collector 210 and a first negative material layer 220 and a second negative material layer 230 located on both sides of the thickness direction of the negative current collector 210.

[0047] In some embodiments of this application, as shown in Figures 1 and 2, in the same negative electrode layer 20, the first negative electrode material layer 220 is farther away 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 corner segment 101 is adjacent to a first negative electrode material layer corner segment 221. The first positive electrode corner segment 101 is farther away from the winding center of the electrode assembly 01 than the first negative electrode material layer corner segment 221. The first diaphragm 30 is located between the positive electrode corner segment 101 and the first negative electrode material layer corner segment 221. The second diaphragm 40 is adjacent to the second negative electrode material layer 230. In a wound electrode assembly, a corner segment of a first positive electrode sheet is adjacent to a corner segment of a first negative electrode material layer but far from the winding center of the electrode assembly. In this case, the winding radius of the positive electrode material layer in the positive electrode sheet is larger than that of the first negative electrode material layer, resulting in a smaller positive-to-negative electrode ratio (Cell Balance, CB, the ratio of negative electrode capacity to positive electrode capacity per unit area). This makes the interface problem more likely, leading to lithium plating. Therefore, a sufficient electrolyte is required to transport active ions. Simultaneously, the corner region of the wound structure is prone to insufficient electrolyte or even electrolyte bridging, further exacerbating the interface problem. Therefore, this application places a first separator containing a first base film between the aforementioned corner segments of the first positive electrode sheet and the first negative electrode material layer. The first base film, with its higher porosity, has a higher electrolyte retention capacity, which helps to improve the problem of insufficient electrolyte in the corner region and also facilitates the transport of active ions, thereby improving the interface problem and ultimately improving the lithium plating problem of the secondary battery, i.e., improving the kinetic performance.

[0048] In some embodiments of this application, as shown in Figures 3 and 4, in the same negative electrode layer 20, the first negative electrode material layer 220 is farther away 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 second positive electrode corner segment 102 is adjacent to a second negative electrode material layer corner segment 222. The second positive electrode corner segment 102 is closer to the winding center of the electrode assembly 01 than the second negative electrode material layer corner segment 222. The first separator 30 is located between the second positive electrode corner segment 102 and the second negative electrode material layer corner segment 222. The second separator 40 is located on the side of the second positive electrode corner segment 102 that is away from the first separator 30.

[0049] In some embodiments of this application, the first base film comprises a first resin material, wherein the enthalpy of melting of the first resin material is from 185 J / g to 195 J / g, and the melt index (MFR1) is from 0.4 g / 10 min to 0.6 g / 10 min. For example, the enthalpy of melting of the first resin material H1 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 consisting of any two of these values. For example, the melt flow index (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 of any two of these values. When the enthalpy of fusion (H1) and melt flow index (MFR1) of the first resin material are within the above ranges, the resulting first base film has suitable pore-closing temperature and film-breaking temperature. Therefore, the first separator can improve the lithium plating problem of the secondary battery while also ensuring the high-temperature thermal performance of the secondary battery.

[0050] In some embodiments of this application, the first base film comprises a first resin material, the first resin material having a weight-average molecular weight M. w1 The weight-average molecular weight is between 50W ("W" represents 10,000, so 50W is 500,000, the same applies below) and 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 of any two of these values. The weight-average molecular weight M of the first resin material... w1 Within the aforementioned range, the obtained first base membrane has suitable porosity, pore closure temperature, and membrane rupture temperature, thus the first separator is beneficial for improving the kinetic performance of the secondary battery while also taking into account the performance of the high-temperature thermal chamber.

[0051] In some embodiments of this application, the first resin material includes at least one selected from polyethylene, polypropylene, polyimide, polyethyleneimine, or polyethylene terephthalate. When the type of the first resin material falls within the above range, the resulting first base membrane has suitable porosity, pore-closing temperature, and membrane rupture temperature, thereby the first separator is beneficial for improving the lithium plating problem of the secondary battery while also maintaining high-temperature thermal performance.

[0052] In some embodiments of this application, a characteristic peak A is present in the differential scanning calorimetry (DSC) spectrum of the first base film, ranging from 128°C to 134°C. The peak value of characteristic peak A corresponds to the melting temperature of the first resin material. In this application, the peak value of the characteristic peak in the first base film is between 128°C and 134°C, i.e., characteristic peak A is present.

[0053] In some embodiments of this application, 140℃≤T11≤160℃; for example, the pore-closing temperature T11 of the first base film can be 140℃, 142℃, 145℃, 147℃, 149℃, 150℃, 152℃, 154℃, 155℃, 157℃, 159℃, 160℃, or a range of any two of these values. The pore-closing temperature of the first base film is within the above range, meaning the first base film has a relatively low pore-closing temperature. Therefore, the first separator can improve the lithium plating problem of the secondary battery while also maintaining the high-temperature thermal performance of the secondary battery.

[0054] In some embodiments of this application, 150℃≤T21≤180℃. For example, the rupture temperature T21 of the first base film can be 150℃, 152℃, 155℃, 157℃, 160℃, 162℃, 165℃, 167℃, 170℃, 172℃, 175℃, 178℃, 180℃, or a range of any two of these values. That is, the first base film has a high rupture temperature, so that the first separator can improve the lithium plating problem of the secondary battery while also taking into account the high-temperature thermal performance of the secondary battery.

[0055] In some embodiments of this application, the thickness of the first base film is from 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 of any two of these values. A thickness within the above range is also beneficial for balancing the energy density of the secondary battery.

[0056] In some embodiments of this 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 of any two of these values. The addition of the first ceramic coating is beneficial for further improving the liquid absorption and retention capacity of the first separator, and further improving the kinetic performance of the secondary battery.

[0057] In some embodiments of this application, the coating weight CW1 of the first ceramic coating is 9 mg / 5000 mm. 2 Up to 15mg / 5000mm 2 For example, the coating weight CW1 of the first ceramic coating can be 9 mg / 5000 mm. 2 9.5mg / 5000mm 2 10mg / 5000mm 2 10.5mg / 5000mm 2 11mg / 5000mm2 11.5mg / 5000mm 2 12mg / 5000mm 2 12.5mg / 5000mm 2 13mg / 5000mm 2 13.5mg / 5000mm 2 14mg / 5000mm 2 14.5mg / 5000mm 2 15mg / 5000mm 2 Or it can be a range consisting of any two of these values. If the coating weight CW1 of the first ceramic coating is within the above range, the first ceramic coating has a suitable thickness, which improves the kinetic performance of the secondary battery while also balancing its energy density.

[0058] In some embodiments of this application, the first ceramic coating includes first ceramic particles, which include at least one of alumina, boehmite, silicon oxide, magnesium oxide, titanium oxide, tin oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, aluminum hydroxide, magnesium hydroxide, or calcium hydroxide. By selecting the above-mentioned types of first ceramic particles, it is beneficial to further improve the liquid absorption and retention capacity of the first separator, thereby improving the kinetic performance of the secondary battery.

[0059] In some embodiments of this application, the first separator further includes a first adhesive layer disposed on both sides of the first base film, and a first ceramic coating disposed on one side of the first base film, located between the first base film and the first adhesive layer. In some embodiments of this application, in the electrode assembly, the first ceramic coating is disposed facing the positive electrode, which is beneficial for further improving the problem of insufficient electrolyte in the corner area and also for the transport of active ions, thereby improving interface problems and thus improving the lithium plating problem of the secondary battery.

[0060] This application does not impose any particular limitation on the composition of the first adhesive layer, as long as it achieves the purpose of this application. For example, the first adhesive layer includes a first adhesive, and the first adhesive includes at least one selected from polyvinylidene fluoride, polyacrylonitrile, polyethylene oxide, or polyimide. This application does not impose any particular limitation on the thickness of the first adhesive layer, as long as it achieves the purpose of this application. For example, the thickness of the first adhesive layer can be from 1 μm to 5 μm.

[0061] In some embodiments of this application, the second base film includes a second resin material, a third resin material, and a fourth resin material.

[0062] The enthalpy of melting (H2) of the second resin material is from 140 J / g to 150 J / g, and the melt flow index (MFR2) is from 6 g / 10 min to 14 g / 10 min. For example, the enthalpy of melting (H2) of the second resin material can be 140 J / g, 140 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 consisting of any two of these values. For example, the melt flow index (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 consisting of any two of these values. The melting enthalpy H3 of the third resin material is from 185 J / g to 195 J / g, and the melt flow index MFR3 is from 0.4 g / 10 min to 0.6 g / 10 min. For example, the melting enthalpy 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 of any two of these values. For example, the melt flow index (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 of any two of these values. The enthalpy of melting (H4) of the fourth resin material is from 130 J / g to 180 J / g, and the melt flow index (MFR4) is from 0.1 g / 10 min to 100 g / 10 min. For example, the melting enthalpy 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 of any two of these values. For example, the melt index 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 of any two of these values.When the melting enthalpy and melt index of the second, third, and fourth resin materials are within the above range, the resulting second base film has suitable pore-closing temperature and film-breaking temperature, thus the second separator is beneficial to improving the high-temperature thermal performance of the secondary battery.

[0063] In some embodiments of this application, the weight-average molecular weight M of the second resin material w2 The weight-average molecular weight is between 20W and 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 of any two of these values. The weight-average molecular weight M of the third resin material... w3 The weight-average molecular weight of the third resin material 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 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 is... w4 The W values ​​can be 5W, 10W, 15W, 20W, 25W, 30W, 35W, 40W, 45W, 50W, or any combination of two of these values. When the weight-average molecular weights of the second, third, and fourth resin materials are within the above ranges, the resulting second base membrane possesses suitable porosity, pore-closing temperature, and membrane rupture temperature. Therefore, the second separator is beneficial for improving the high-temperature thermal performance of the secondary battery while also considering its kinetic performance.

[0064] In some embodiments of this application, 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%. For example, the mass percentage W1 of the second resin material can be 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, or a range consisting of any two of these values. For example, the mass percentage W2 of the third resin material can be 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, or a range consisting of any two of these values. For example, the mass percentage 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 of any two of these values. When the mass percentages of the second, third, and fourth resin materials are within the above ranges, the resulting second base membrane has suitable porosity, pore closure temperature, and membrane rupture temperature. Therefore, the second separator is beneficial for improving the high-temperature thermal performance of the secondary battery while also considering its kinetic performance.

[0065] In some embodiments of this application, the second, third, and fourth resin materials each independently include at least one of polyethylene, polypropylene, polyimide, polyethyleneimine, or polyethylene terephthalate. When the types of the second, third, and fourth resin materials are within the aforementioned range, the resulting second base film has suitable porosity, pore-closing temperature, and membrane rupture temperature, thereby the first separator is beneficial for improving the lithium plating problem of the secondary battery while also taking into account high-temperature thermal box performance.

[0066] In some embodiments of this application, the fourth resin material is different from the second and third resin materials. In some embodiments of this application, the second and third resin materials include polyethylene, and the fourth resin material includes polypropylene. The resulting second base film has a lower pore-closing temperature and a higher film-breaking temperature, which is beneficial for improving the high-temperature thermal performance of the secondary battery.

[0067] In some embodiments of this application, characteristic peaks exist in the differential scanning calorimetry (DSC) spectrum of the second base film at 120°C to 126°C, 128°C to 134°C, and 150°C to 160°C. The peak value of characteristic peak B between 120°C and 126°C corresponds to the melting temperature of the second resin material; the peak value of characteristic peak C between 128°C and 134°C corresponds to the melting temperature of the third resin material; and the peak value of characteristic peak D between 150°C and 160°C corresponds to the melting temperature of the fourth resin material. In this application, in the second base film, the peak values ​​are between 120°C and 126°C (i.e., characteristic peak B); between 128°C and 134°C (i.e., characteristic peak C); and between 150°C and 160°C (i.e., characteristic peak D).

[0068] In some embodiments of this application, 30% ≤ n2 ≤ 40%. In some embodiments of this application, 30% < n1 ≤ 60%, and 30% ≤ n2 ≤ 40%. For example, the porosity n2 of the second base membrane can be 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, or a range of any two of these values. The porosity of the second base membrane is within the above range, meaning the second base membrane has a high porosity. Therefore, the first separator can improve the performance of the high-temperature heat box of the secondary battery while also considering the kinetic performance of the secondary battery.

[0069] In some embodiments of this 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 of any two of these values. A thickness within the above range is also beneficial for balancing the energy density of the secondary battery.

[0070] In some embodiments of this 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 membrane, and the second ceramic coating is disposed on one side of the second base membrane, located between the second base membrane and the second adhesive layer. In some embodiments of this application, in the electrode assembly, the second ceramic coating is disposed facing the positive electrode, which is beneficial for further improving the problem of insufficient electrolyte in the corner region and also for the transport of active ions, thereby improving interface problems and thus improving the lithium plating problem of the secondary battery.

[0071] In some embodiments of this application, the second diaphragm further includes a second ceramic coating, and the porosity of the second diaphragm is n4, where 40% ≤ n4 ≤ 50%. For example, the porosity n3 of the second diaphragm can be 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, or a range of any two of these values.

[0072] This application does not impose any particular restrictions on the coating quality of the second ceramic coating, as long as the purpose of this application can be achieved. For example, the coating weight CW2 of the second ceramic coating is 9 mg / 5000 mm. 2 Up to 15mg / 5000mm 2 For example, the coating weight CW2 of the second ceramic coating can be 9 mg / 5000 mm². 2 9.5mg / 5000mm 2 10mg / 5000mm 2 10.5mg / 5000mm 2 11mg / 5000mm 2 11.5mg / 5000mm 2 12mg / 5000mm 2 12.5mg / 5000mm 2 13mg / 5000mm 2 13.5mg / 5000mm 2 14mg / 5000mm 2 14.5mg / 5000mm 2 15mg / 5000mm 2 Or it can be a range consisting of any two of these values.

[0073] In some embodiments of this application, the second ceramic coating includes second ceramic particles, which include at least one of alumina, boehmite, silicon oxide, magnesium oxide, titanium oxide, tin oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, aluminum hydroxide, magnesium hydroxide, or calcium hydroxide.

[0074] This application does not impose any particular limitation on the composition of the second adhesive layer, as long as it achieves the purpose of this application. For example, the second adhesive layer includes a second adhesive, and the second adhesive includes at least one selected from polyvinylidene fluoride, polyacrylonitrile, polyethylene oxide, or polyimide. This application does not impose any particular limitation on the thickness of the second adhesive layer, as long as it achieves the purpose of this application. For example, the thickness of the second adhesive layer can be from 1 μm to 5 μm.

[0075] Currently, silicon materials have a significant advantage in energy density compared to traditional carbon materials. However, silicon materials typically have poor electrical conductivity, thus requiring the use of highly kinetic electrolytes and material layer formulations. However, these highly kinetic electrolytes and material layer formulations often exhibit poor thermal performance at high temperatures. Therefore, when silicon-containing anodes are used in rechargeable batteries, it is generally difficult to simultaneously achieve optimal energy density, kinetic performance, and high-temperature thermal performance. Based on these issues, in some embodiments of this application, the first anode material layer includes a first anode material, which includes silicon. In some embodiments of this application, the second anode material layer includes a second anode material, which includes silicon. Simultaneously, the aforementioned first and second separators are used. The first separator has good liquid absorption and retention properties, while the second separator has a lower pore-closing temperature and a higher membrane rupture temperature, thereby facilitating a balance between energy density, kinetic performance, and high-temperature thermal performance in the rechargeable battery.

[0076] In some embodiments of this application, the silicon material includes at least one of elemental silicon, silicon carbide, silicon oxide, or silicon alloy. Choosing the aforementioned silicon material is more advantageous for obtaining secondary batteries with higher energy density.

[0077] In this application, the features of the above-described embodiments can be combined arbitrarily.

[0078] This application does not impose any particular limitation on the preparation method of the first base film, as long as it can achieve the purpose of this application. For example, the preparation method of the first base film includes, but is not limited to, the following steps: (1) mixing additives and solvents 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) to obtain a first substrate by extrusion, casting, cooling, and casting; (3) stretching the first substrate longitudinally and transversely, and then extracting and drying it to obtain a first porous substrate; (4) stretching the first porous substrate a second time, heat-setting it, and winding it up to obtain the first base film. 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 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 may include, but is not limited to, at least one of dichloromethane or n-hexane. The stretching ratios for longitudinal and transverse stretching may each be independently 6 to 8 times; the stretching ratio for secondary stretching may be 1 to 2 times; the extraction temperature may be 20°C to 30°C, and the extraction time may be 0.5 h to 1 h; the heat setting temperature may be 110°C to 135°C.

[0079] In this application, the porosity n1 of the first base film can be controlled by adjusting the transverse stretching ratio, the longitudinal stretching ratio, the secondary stretching ratio, and the mass ratio of the solvent during the preparation process. For example, when other conditions remain unchanged, the porosity n1 of the first base film increases when the transverse stretching ratio of the first base film increases, and vice versa. When other conditions remain unchanged, the porosity n1 of the first base film increases when the longitudinal stretching ratio of the first base film increases, and vice versa. When other conditions remain unchanged, the porosity n1 of the first base film increases when the secondary stretching ratio of the first base film increases, and vice versa. When other conditions remain unchanged, the porosity n1 of the first base film increases when the mass ratio of the solvent during the preparation process increases, and vice versa.

[0080] In this application, the first resin materials with different enthalpy of melt, melt index, weight-average molecular weight, and types can be purchased. Their enthalpy of melt, melt index, and weight-average molecular weight can be obtained by measurement, and materials with the desired enthalpy of melt, melt index, and weight-average molecular weight can be selected. Specific testing methods can be found in the relevant content of the "Test Methods and Equipment" section.

[0081] In this application, the porosity n3 of the first diaphragm can be controlled by adjusting the porosity of the first base membrane and the coating quality of the first ceramic coating. The method for controlling the porosity of the first base membrane is as described above. For example, when other conditions remain constant, an increase in the porosity of the first base membrane leads to an increase in the porosity n3 of the first diaphragm, and vice versa; similarly, when the coating quality of the first ceramic coating increases, an increase in the porosity n3 of the first diaphragm leads to a decrease.

[0082] This application does not impose any particular limitation on the preparation method of the second base film, as long as it can achieve the purpose of this application. For example, the preparation method of the second base film includes, but is not limited to, the following steps: (1) mixing the second resin material, the third resin material and the fourth resin material uniformly according to the above W1, W2 and W3 to obtain a mixed material; (2) mixing the additive and the solvent uniformly according to the mass ratio (0.1 to 0.3):100 to obtain a mixed solution; (3) mixing the mixed material and the mixed solution uniformly according to the mass ratio (20:80) to (40:60), and extruding, casting and cooling to form a film to obtain a second substrate; (4) stretching the second substrate longitudinally and transversely, and obtaining a second porous substrate after extraction and drying; (5) stretching the second porous substrate a second time, heat setting and winding to obtain the second base film. The solvent may include, but is not limited to, at least one of paraffin oil or dichloromethane; the additives may include, but are 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 may include, but is not limited to, at least one of dichloromethane or n-hexane. The stretching ratios for longitudinal and transverse stretching may each be independently 6 to 8 times; the stretching ratio for secondary stretching may be 1 to 2 times; the extraction temperature may be 20°C to 30°C, and the extraction time may be 0.5 h to 1 h; the heat setting temperature may be 110°C to 135°C.

[0083] In this application, the porosity n2 of the second base membrane can be adjusted in the same way as the porosity n1 of the first base membrane.

[0084] In this application, the pore-closing temperature and rupture temperature of the second base film vary with the melting enthalpy, melt index, weight-average molecular weight, content, and type of the second, third, and fourth resin materials. For example, when other conditions remain constant, an increase in the weight-average molecular weight of the second resin material leads to an increase in the pore-closing temperature, and vice versa; an increase in the mass percentage of the second resin material leads to a decrease in the pore-closing temperature, and vice versa. Similarly, when other conditions remain constant, an increase in the weight-average molecular weight of the third resin material leads to a higher pore-closing temperature and a higher rupture temperature, and vice versa; a higher mass percentage of the third resin material leads to a higher pore-closing temperature, and vice versa. Likewise, when other conditions remain constant, an increase in the weight-average molecular weight of the fourth resin material leads to an increase in the rupture temperature, and vice versa; an increase in the mass percentage of the fourth resin material leads to an increase in the rupture temperature, and vice versa. In this application, the pore-closing temperature and rupture temperature of the first base film can be controlled in the same way as the adjustment of the pore-closing temperature and rupture temperature of the second base film. In this application, the porosity of the second diaphragm can be adjusted in the same way as the porosity n3 of the first diaphragm.

[0085] In this application, the second, third, and fourth resin materials with different enthalpy of melt, melt index, weight-average molecular weight, and types can be purchased. Their enthalpy of melt, melt index, and weight-average molecular weight can be measured, and materials with the desired enthalpy of melt, melt index, and weight-average molecular weight can be selected. Specific testing methods for the weight-average molecular weight of the materials can be found in the relevant content of the "Test Methods and Equipment" section.

[0086] This application does not impose any particular restrictions on the negative electrode current collector, as long as it can achieve the purpose of this application. For example, it may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or composite current collector. For example, the composite current collector may be lithium copper composite current collector, carbon copper composite current collector, nickel copper composite current collector, titanium copper composite current collector, etc.

[0087] In some embodiments of this application, the first negative electrode material layer and the second negative electrode material layer may each independently include a negative electrode conductive agent and a negative electrode binder. This application does not particularly limit the types of negative electrode conductive agents and negative electrode binders, as long as they achieve the purpose of this application. For example, they may be at least one of a positive electrode conductive agent and a positive electrode binder. This application does not particularly limit the mass ratio of negative electrode material, negative electrode conductive agent, and negative electrode binder in the first and second negative electrode material layers. Those skilled in the art can choose according to actual needs, as long as the purpose of this application is achieved.

[0088] Optionally, the negative electrode sheet 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. This application does not particularly limit the composition of the first negative electrode conductive layer; it can be a conductive layer commonly used 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. This application does not particularly limit the first negative electrode conductive layer conductive agent and the first negative electrode conductive layer binder; for example, they can be at least one of a positive electrode conductive agent and a positive electrode binder.

[0089] Optionally, the negative electrode sheet 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. This application does not impose any particular limitation on the composition of the second negative electrode conductive layer, and it can be a conductive layer commonly used in the art. For example, the second negative electrode conductive layer may include a second negative electrode conductive layer conductive agent and a second negative electrode conductive layer binder. This application does not impose any particular limitation on the second negative electrode conductive layer conductive agent and the second negative electrode conductive layer binder; for example, it can be at least one of a positive electrode conductive agent and a positive electrode binder.

[0090] This application does not impose any particular limitation on the thickness of the first negative electrode material layer, as long as the purpose of this application can be achieved. For example, the thickness of the first negative electrode material layer is 30 μm to 120 μm.

[0091] This application does not impose any particular limitation on the thickness of the second negative electrode material layer, as long as it can achieve the purpose of this application. For example, the thickness of the second negative electrode material layer is 30 μm to 120 μm.

[0092] This application does not impose any particular limitation on the thickness of the negative electrode current collector, as long as it can achieve the purpose of this application. For example, the thickness of the negative electrode current collector can be from 4 μm to 20 μm.

[0093] In this application, the positive electrode includes a positive current collector and positive electrode material layers located on both sides of the positive current collector. The aforementioned "positive electrode material layers located on both sides of the positive current collector" means that the positive electrode material layers are disposed on two surfaces of the positive current collector along its thickness direction. It should be noted that the term "surface" here can refer to the entire surface area of ​​the positive current collector or only a portion thereof; this application does not impose any particular limitation, as long as the purpose of this application is achieved.

[0094] This application does not impose any particular restrictions on the positive electrode current collector, as long as it can achieve the purpose of this application. For example, it may include aluminum foil, aluminum alloy foil, or composite current collector (such as aluminum-carbon composite current collector).

[0095] The positive electrode material layer includes a positive electrode active material. This application does not impose any particular limitation on the positive electrode active material, as long as it can achieve the purpose of this application. For example, the positive electrode active material may include, but is not limited to, lithium nickel cobalt manganese oxide (e.g., NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide (LiCoO2), lithium manganese oxide, or lithium manganese iron phosphate, at least one of these.

[0096] The positive electrode material layer may further include a positive electrode conductive agent and a positive electrode binder. This application does not impose any particular limitation on the types of positive electrode conductive agents and positive electrode binders, as long as they can achieve the purpose of this application. 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, metallic materials, or conductive polymers. Conductive carbon black may include, but is not limited to, at least one of acetylene black or Ketjen black. The aforementioned carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The aforementioned carbon fibers may include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or carbon nanofibers. The aforementioned metallic 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 aforementioned conductive polymer may include, but is not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole.

[0097] This application does not impose any particular restrictions on the positive electrode binder, as long as it can achieve the purpose of this application. 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, polyamide-imide, styrene-butadiene rubber, or polyvinylidene fluoride.

[0098] This application does not impose any particular restrictions on the mass ratio of positive electrode active material, positive electrode conductive agent, and positive electrode binder in the positive electrode material layer. Those skilled in the art can choose according to actual needs, as long as the purpose of this application can be achieved.

[0099] This application does not impose any particular limitations on the thickness of the positive electrode current collector and the positive electrode material layer, as long as the purpose of this application can be achieved. For example, the thickness of the positive electrode current collector can be 5 μm to 20 μm, and the thickness of the positive electrode material layer can be 30 μm to 120 μm.

[0100] Optionally, the positive electrode may further include a positive conductive layer, which is located between the positive current collector and the positive electrode material layer. The composition of the positive conductive layer is not particularly limited and can be any conductive layer commonly used in the art. The positive conductive layer includes a positive conductive layer conductive agent and a positive conductive layer binder. This application does not particularly limit the positive conductive layer conductive agent and the positive conductive layer binder; for example, it can be at least one of the above-described positive conductive agents and positive conductive binders.

[0101] In this application, the secondary battery also includes an electrolyte, which includes lithium salts and non-aqueous solvents.

[0102] This application does not impose any particular limitation on lithium salts, as long as they achieve the purpose of this application. For example, lithium salts may include, but are not limited to, at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), lithium perchlorate (LiClO4), lithium tetraphenylboron (LiB(C6H5)4), lithium methanesulfonate (LiCH3SO3), lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis(trifluoromethanesulfonyl)imide (LiN(SO2CF3)2), tris(trifluoromethanesulfonyl)methyl lithium (LiC(SO2CF3)3), lithium hexafluorosilicate (LiSiF6), lithium bis(oxalateborate)borate (LiBOB), and lithium difluoroborate (LiF2OB). This application does not impose any particular limitation on the content of lithium salts in the electrolyte, as long as they achieve the purpose of this application.

[0103] This application does not impose any particular restrictions on non-aqueous solvents, as long as they can achieve the purpose of this application. For example, non-aqueous solvents may include, but are not limited to, at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents.

[0104] The aforementioned carbonate compounds may include, but are not limited to, at least one of chain carbonate compounds, cyclic carbonate compounds, or fluorocarbonate compounds. The aforementioned 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 aforementioned cyclic carbonates may include, but are not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), or vinyl ethylene carbonate (VEC). Fluorocarbonate compounds may include, but are not limited to, at least one of fluoroethylene 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 trifluoromethylethylene carbonate. The aforementioned carboxylic acid 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, decanolactone, valproic acid lactone, or caprolactone. The aforementioned 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. Other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate. This application does not impose any particular limitation on the content of non-aqueous solvents in the electrolyte, as long as the purpose of this application is achieved.

[0105] The secondary battery also includes a casing for housing a second separator, a positive electrode, a first separator, a negative electrode, and an electrolyte, as well as other components known in the field of secondary batteries. This application does not limit the scope of these other components. This application does not impose any particular limitation on the casing; it can be a casing known in the art, as long as it achieves the purpose of this application. For example, the casing can be a rigid casing or a flexible casing. The material of the rigid casing can be metal; this application does not limit the type of metal and can use known metal rigid casings, as long as they achieve the purpose of this application. The flexible casing can be a metal plastic film, such as aluminum-plastic film, steel-plastic film, etc.

[0106] The fabrication process of the secondary battery described in this application is well known to those skilled in the art, and this application does not impose any particular limitations. For example, the fabrication process of the secondary battery may include, but is not limited to, the following steps: stacking the positive electrode, the first separator, the negative electrode, and the second separator in sequence, and performing operations such as winding and folding as needed to obtain a wound electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain the secondary battery. Alternatively, stacking the negative electrode, the second separator, the positive electrode, and the first separator in sequence, and performing operations such as winding and folding as needed to obtain a wound electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain the secondary battery. In addition, overcurrent protection elements, conductive plates, etc., may be placed in the housing as needed to prevent pressure rise and overcharging / discharging inside the secondary battery.

[0107] In some embodiments of this application, the secondary battery 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. In some embodiments of this application, the secondary battery includes lithium-ion batteries.

[0108] A second aspect of this application provides an electronic device comprising the secondary battery described in the first aspect of this application.

[0109] This application does not specifically limit the type of electronic device; it can be any electronic device known in the prior art. In some embodiments of this application, the electronic device may include, but is not limited to, laptops, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors, etc.

[0110] Example

[0111] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0112] Test methods and equipment:

[0113] Sampling methods for the first and second septa:

[0114] The lithium-ion batteries in the tested examples and comparative examples were disassembled, and the first and second separators were removed. The separators were soaked in dimethyl carbonate (DMC) for 20 minutes to remove electrolyte residue. Then, the first and second separators were placed in an oven and dried at 60°C for 12 hours to obtain the first and second separator samples. Unless otherwise specified, the first and second separators were obtained using the above method in the following tests.

[0115] Sampling methods for the first and second base films:

[0116] The first and second septa obtained from the above sampling were placed in separate containers, N-methylpyrrolidone (NMP) was added, and the containers were placed in an ultrasonic instrument with heating function for ultrasonication at 45°C for 3 hours. Once the first and second septa became completely transparent, they were removed to obtain the first and second base films. Unless otherwise specified, the following tests will use the same method to obtain the first and second base films.

[0117] Porosity testing of the first base membrane, first diaphragm, second base membrane, and second diaphragm:

[0118] The gas displacement method was used for testing. Samples were prepared by punching the first base membrane, the first diaphragm, the second base membrane, and the second diaphragm using molds (those skilled in the art can select molds of common sizes and shapes in the field according to factors such as the size and shape of the test object and the requirements of the test equipment). The true volume V0 of the sample was measured using a true density meter. The apparent volume V of the sample can be calculated by measuring the area and thickness of the sample. The percentage of the sample pore volume to the total volume is (V-V0) / V×100%, which gives the porosity of the first base membrane, the first diaphragm, the second base membrane, and the second diaphragm.

[0119] Closing temperature and rupture temperature of the first and second base films:

[0120] The closed-cell temperature and rupture temperature of the samples were tested using the temperature rise resistance method. The samples were first cut into fixed sizes of 5cm × 5cm, with the base film sample size larger than the test area of ​​the fixture. The samples were placed in a component consisting of a ceramic and stainless steel fixture, and 10mL of electrolyte was injected. The fixture was then placed in an oven set to 250℃, and the temperature was increased at a rate of 15℃ / min. Simultaneously, the resistance and temperature of the fixture were monitored, and the data for fixture temperature and resistance, oven temperature, and time were output to obtain a temperature-resistance curve. The sample was either the first or second base film.

[0121] According to the temperature-resistance curve, the temperature at which the resistance suddenly increases (e.g., reaching 1000Ω) is the pore-closing temperature of the diaphragm; the maximum resistance value is the highest resistance value of the diaphragm.

[0122] Based on the above test of the diaphragm pore-closure temperature, the test time is increased. When the resistance suddenly drops to the same value as the pore-closure temperature, the temperature at this point is the diaphragm rupture temperature.

[0123] The composition and preparation method of the electrolyte used in the test were the same as those in Example 1-1.

[0124] Thickness testing of the first and second base films:

[0125] The first base film was argon-ion polished to obtain its cross-section. The morphology of the cross-section along the thickness direction of the first base film was observed and scanned electron microscopy (SEM) images were taken using a field emission scanning electron microscope (Philips XL-30). The thickness of the first base film was then measured using the SEM. By replacing the first base film with a second base film, the thickness of the second base film could be measured.

[0126] DSC spectrum analysis of the first and second base films:

[0127] The DSC spectrum of 8 mg of the first base film was measured using a differential scanning spectroscopy (ASTM D3418-15) at a heating rate of 10 °C / min, with a temperature range of 60 °C to 190 °C. The second base film was then replaced with the first base film to obtain its DSC spectrum. In the obtained DSC spectrum of the first base film, the characteristic peak corresponding to the peak at 128 °C to 134 °C was designated as characteristic peak A. In the obtained DSC spectrum of the second base film, the characteristic peak corresponding to the peak at 120 °C to 126 °C was designated as characteristic peak B, the characteristic peak corresponding to the peak at 128 °C to 134 °C as characteristic peak C, and the characteristic peak corresponding to the peak at 150 °C to 160 °C as characteristic peak D.

[0128] Based on the DSC spectrum of the second base film, the ratio of the peak areas of characteristic peaks B, C and D is calculated. 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.

[0129] Weight-average molecular weight test:

[0130] 1) Sample preparation: Dissolve 5g of the first base film to be tested in 100g of 1,2,4-trichlorobenzene at 150℃ to prepare a 2wt% solution.

[0131] 2) Gel column selection: The smaller the pore size of the gel column, the slower the flow rate, and the larger the molecular weight of the polymer that can be separated. By selecting a suitable gel column, the first resin material can pass through the gel column at a suitable flow rate.

[0132] 3) Sample loading and elution: The prepared sample solution is injected into the ultra-high performance polymer chromatograph (APC, ACQUITY), and the solution is pumped through the gel column at a certain flow rate. At the same time, the gel column is eluted with an appropriate eluent to elute the separated polymers sequentially.

[0133] 4) Detection and recording: During the elution process, the concentration of the effluent is detected by a detector and the change in concentration over time is recorded.

[0134] 5) Data processing and analysis: The detected concentration data is imported into computer software, and the concentration data is converted into molecular weight and its distribution information based on the standard substance (polyethylene) as a reference, so as to obtain the weight-average molecular weight of the first resin material.

[0135] The first base membrane is replaced with the second base membrane. By selecting a suitable gel column, the second resin material and the third resin material are allowed to pass through the gel column at a suitable flow rate, thereby separating the second resin material and the third resin material. By repeating the above steps, the weight-average molecular weights of the second resin material, the third resin material and the fourth resin material can be obtained respectively.

[0136] Melt flow index test:

[0137] The melt flow rate (MFR) of the material was tested using a melt flow rate meter. The instrument was heated to 110℃, a standard die with a diameter of φ2.095±0.005mm was placed inside, the material was placed into the barrel and compacted, and held at this temperature for 10 minutes. The material cutting time was set to 60 seconds, and 2.16 kg of pressure was applied to extrude the material. The MFR was then calculated. MFR = mT / t, where t is the cutting time (60 seconds), m is the mass of the material, and T is 600 seconds. The material can be a first resin material, a second resin material, a third resin material, or a fourth resin material.

[0138] Enthalpy of fusion test:

[0139] The enthalpy of fusion of the resin materials in each embodiment and comparative example was tested using differential scanning calorimetry (DSC). The sample material was placed in a crucible and heated from room temperature to 250°C at a rate of 10°C / min under a nitrogen atmosphere, yielding a DSC curve. The enthalpy of fusion of the sample material was calculated based on the area enclosed by the melting peak in the DSC curve, and the peak temperature of the melting peak in the DSC curve was taken as the melting point of the sample material. The materials used in the above calculations can be the first, second, third, or fourth resin materials.

[0140] Enthalpy of fusion = Integral area of ​​the melting endothermic curve / mass.

[0141] The degree of lithium deposition on the negative electrode after 1000 cycles:

[0142] The lithium-ion battery was placed in an environment of 25°C and charged at a constant current of 0.5C to a voltage of 4.5V. Then, it was charged at a constant voltage of 4.5V to a cutoff current of 0.05C, allowed to stand for 5 minutes, and then discharged at a constant current of 0.5C to a voltage of 3.0V, allowed to stand for 5 minutes. This constitutes one charge-discharge cycle. Then, the same steps were repeated for 1000 charge-discharge cycles.

[0143] Then charge at a constant current of 0.5C to 4.5V, and then charge at a constant voltage of 4.5V to the cutoff current of 0.05C, and let stand for 5 minutes; then disassemble the lithium-ion battery and observe the degree of lithium plating on the negative electrode, the degree of lithium plating on the surface of the negative electrode opposite to the first separator, and the degree of lithium plating on the surface of the negative electrode opposite to the second separator.

[0144] For the surface of the negative electrode sheet opposite to the first separator, if there is no white lithium metal deposition, it is recorded as "no lithium deposition"; if there is white lithium metal deposition and the lithium deposition area percentage is greater than 0 and less than 5%, it is recorded as "slight lithium deposition"; if there is white lithium metal deposition and the lithium deposition area percentage is greater than or equal to 5% and less than or equal to 10%, it is recorded as "moderate lithium deposition"; if there is white lithium metal deposition and the lithium deposition area percentage is greater than 10%, it is recorded as "heavy lithium deposition". Here, the lithium deposition area percentage is the percentage of the lithium deposition area relative to the surface area of ​​the negative electrode material layer of the negative electrode sheet opposite the first separator.

[0145] Similarly, for the surface of the negative electrode opposite the second separator, if there is no white lithium metal deposition, it is recorded as "no lithium deposition"; if there is white lithium metal deposition and the lithium deposition area percentage is greater than 0 and less than 5%, it is recorded as "slight lithium deposition"; if there is white lithium metal deposition and the lithium deposition area percentage is greater than or equal to 5% and less than or equal to 10%, it is recorded as "moderate lithium deposition"; and if there is white lithium metal deposition and the lithium deposition area percentage is greater than 10%, it is recorded as "heavy lithium deposition". Here, the lithium deposition area percentage is the percentage of the lithium deposition area relative to the surface area of ​​the negative electrode material layer of the negative electrode opposite the second separator.

[0146] For the entire negative electrode sheet, if there is no white lithium metal deposition, it is recorded as "no lithium deposition"; if there is white lithium metal deposition and the lithium deposition area percentage is greater than 0 and less than 5%, it is recorded as "slight lithium deposition"; if there is white lithium metal deposition and the lithium deposition area percentage is greater than or equal to 5% and less than or equal to 10%, it is recorded as "moderate lithium deposition"; if there is white lithium metal deposition and the lithium deposition area percentage is greater than 10%, it is recorded as "heavy lithium deposition". The lithium deposition area percentage is the percentage of the lithium deposition area relative to the total area of ​​the two surfaces of the negative electrode material layer of the negative electrode sheet.

[0147] The lithium plating test is used to characterize the kinetic performance of lithium-ion batteries. The less lithium plating, the better the kinetic performance, and vice versa.

[0148] Cyclic performance test:

[0149] At 25°C, the lithium-ion battery is charged at a constant current of 3C to 4.5V, then charged at a constant voltage of 0.05C, and finally discharged at 0.7C to 3.0V, completing one cycle, which is recorded as the first cycle. The discharge capacity of the first cycle is recorded. This process is repeated for 1500 cycles, and the discharge capacity after each cycle is recorded. At cycles of 50, 100, 150, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, and 1400, the following charge-discharge procedure is followed: constant current charging at 0.5C to 4.5V, then constant voltage charging at 0.02C, and finally discharge at 0.2C to 3.0V.

[0150] Cycle capacity retention (%) = (Discharge capacity after N cycles / Discharge capacity in the first cycle) × 100%. Where N is a positive integer from 1 to 1500.

[0151] During the cycling process, the thickness of the lithium-ion battery was measured using a pressure plate thickness gauge, and the initial thickness was recorded. Then, after cycling for 50, 100, 150, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, and 1500 cycles, the thickness of the lithium-ion battery after Q cycles was recorded.

[0152] Cyclic expansion rate (%) = (thickness after Q cycles - thickness of the first cycle) / thickness of the first cycle × 100%, where Q is 50, 100, 150, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500.

[0153] Hot box test:

[0154] Under conditions of 25℃, lithium-ion batteries that have been charged at a constant current of 2C to 4.5V and then at a constant voltage of 4.5V to a current of 0.02C are placed in a test chamber with circulating air convection, a temperature of 25℃, and a humidity of 80%. After standing for 5 minutes, the test chamber is heated to 130℃ at a rate of 5℃ / min and maintained at 130℃. After 1 hour, the test is stopped, and the lithium-ion batteries are checked for fire or explosion. A battery that does not fire or explode is considered to have passed the test. The pass rate for the 130℃ hot chamber test is calculated as: (Number of batteries passing the 130℃ hot chamber test) / (Total number of batteries tested at 130℃, 5 batteries).

[0155] Under conditions of 25℃, lithium-ion batteries that have been charged at a constant current of 2C to 4.5V and then at a constant voltage of 4.5V to a current of 0.02C are placed in a test chamber with circulating air convection, a temperature of 25℃, and a humidity of 80%. After standing for 5 minutes, the test chamber is heated to 132℃ at a rate of 5℃ / min and maintained at 132℃. After 1 hour, the test is stopped, and the lithium-ion batteries are checked for fire or explosion. The pass rate of the 132℃ hot chamber test = number of batteries that pass the 132℃ hot chamber test / total number of batteries tested at 132℃ (5 batteries).

[0156] Example 1-1

[0157] <Preparation of the first diaphragm>

[0158] (1) The additive pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid] and the solvent paraffin oil were mixed evenly at a mass ratio of 0.2:100 to obtain a mixed solution;

[0159] (2) The first resin material, polyethylene, and the mixed solution are mixed evenly at a mass ratio of 20:80, added to an extruder system, extruded through a T-die, cast, cooled, and cast into a film to obtain the first substrate; wherein, M w1 =70W, H1=190J / g, MFR1=0.5g / 10min.

[0160] (3) The first substrate was stretched 7 times longitudinally and 7 times transversely, extracted with dichloromethane at 25°C for 0.5 h, and dried to obtain the first porous substrate.

[0161] (4) The first porous substrate is stretched longitudinally by 1.5 times and transversely by 1.5 times, heat-set at 115°C, and wound up to obtain the first base film. The thickness h1 of the first base film is 5 μm.

[0162] (5) Inorganic boehmite particles with a Dv50 of 1 μm and polyacrylate (Mw = 40W) were mixed at a mass ratio of 90:10 and dissolved in deionized water to form a first ceramic coating slurry with a solid content of 50 wt%. The first ceramic coating slurry was then uniformly coated onto one surface of a first base film using a microgravure coating method and dried to obtain a first base film with the first ceramic coating. The coating weight CW1 of the first ceramic coating was 12 mg / 5000 mm². 2 .

[0163] (6) The first adhesive, polyvinylidene fluoride (PVDF, weight average molecular weight 8.5 × 10⁻⁶), is added. 6The first adhesive layer slurry was obtained by adding deionized water to a mixer and stirring to adjust the viscosity to 3500 mPa·s and the solid content to 75 wt%. The first adhesive layer slurry was then uniformly coated onto the surface of the first ceramic coating away from the first base film using screen printing. After drying, the coating process was repeated on the other surface of the first base film to obtain the first diaphragm. The single-layer coating thickness of the first adhesive layer was 2 μm.

[0164] <Preparation of the Second Diaphragm>

[0165] (1) The second resin material (polyethylene), the third resin material (polyethylene), and the fourth resin material (polypropylene) are mixed evenly at a mass ratio of 10:88:2 to obtain a mixed material; wherein, M w2 =30W, H2=145J / g, MFR2=10g / 10min; M w3 =70W, H3=190J / g, MFR3=0.5g / 10min; M w4 =28W, H4=155J / g, MFR4=50g / 10min.

[0166] (2) The additive pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid] and the solvent paraffin oil were mixed evenly at a mass ratio of 0.2:100 to obtain a mixed solution;

[0167] (3) Mix the mixed materials and mixed solution evenly at a mass ratio of 20:80, add them to the extruder system, extrude them through a T-die, cast them, cool them, and cast them into a film to obtain the second substrate.

[0168] (4) The second substrate was stretched 7 times longitudinally and 7 times transversely, extracted with dichloromethane at 25°C for 0.5 h, and dried to obtain the second porous substrate.

[0169] (5) The second porous substrate is stretched longitudinally by 1.5 times and transversely by 1.5 times, heat-set at 125°C, and wound up to obtain the second base film. The thickness h2 of the second base film is 5 μm.

[0170] (6) Inorganic boehmite particles with a Dv50 of 1 μm and polyacrylate (Mw = 40W) were mixed at a mass ratio of 90:10 and dissolved in deionized water to form a first ceramic coating slurry with a solid content of 50 wt%. The first ceramic coating slurry was then uniformly coated onto one surface of the first base film using a microgravure coating method and dried to obtain a second base film with a second ceramic coating. The coating weight CW2 of the second ceramic coating was 12 mg / 5000 mm². 2 .

[0171] (7) The weight-average molecular weight of the second adhesive PVDF is 8.5 × 10⁻⁶. 6 Then, deionized water was added and stirred to adjust the viscosity of the slurry to 3500 mPa·s and the solid content to 75 wt%, thus obtaining the second adhesive layer slurry. The second adhesive layer slurry was then uniformly coated onto one surface of the second ceramic coating away from the second base film using screen printing, and subsequently dried in an oven. The coating process was repeated on the other surface of the second base film to obtain the second diaphragm. The single-layer coating thickness of the second adhesive layer was 2 μm.

[0172] <Preparation of Negative Electrode Sheets>

[0173] Artificial graphite, silicon carbide, acetylene black, styrene-butadiene rubber, and sodium carboxymethyl cellulose (CMC) were mixed in a mass ratio of 90:6:1:1.5:1.5. Deionized water was added as a solvent to prepare a slurry with a solid content of 70 wt%. The mixture was stirred evenly in a vacuum mixer to obtain the first negative electrode slurry, which was also used as the second negative electrode slurry. The first negative electrode slurry was uniformly coated onto one surface of an 8 μm thick copper foil and dried at 110°C to obtain a negative electrode sheet with a first negative electrode material layer coating thickness of 50 μm. The second negative electrode slurry was coated onto the other surface of the copper foil used as the negative electrode current collector. After drying, the second negative electrode material layer on this surface was 50 μm thick, resulting in a negative electrode sheet with a total thickness of 108 μm. The coated negative electrode sheet was cold-pressed and then cut into 74 mm × 867 mm dimensions for later use. The compaction density of the first negative electrode material layer was 1.735 g / cm³. 3 The length is 720mm, and the compaction density of the second negative electrode material layer is 1.735g / cm³. 3 It has a length of 680 mm. The mass ratio of silicon to carbon in the silicon-carbon compound is 2:8.

[0174] <Preparation of the positive electrode>

[0175] Lithium cobalt oxide, acetylene black, and PVDF (positive electrode active materials) were mixed in a mass ratio of 94:3:3, and NMP was added as a solvent to prepare a slurry with a solid content of 75 wt%. The mixture was then stirred evenly in a vacuum mixer to obtain the positive electrode slurry. The positive electrode slurry was uniformly coated onto one surface of a 12 μm thick aluminum foil and dried at 90°C to obtain a negative electrode sheet with a single-sided positive electrode material layer coating thickness of 55 μm. The positive electrode slurry was coated onto the other surface of the positive electrode current collector aluminum foil, and after drying, the positive electrode material layer coating on this surface was also 55 μm thick, resulting in a positive electrode sheet with a total thickness of 122 μm. Cold pressing yielded a positive electrode sheet with a single-sided positive electrode material layer thickness of 122 μm. The coated positive electrode sheet was then cold-pressed, cut into 70 mm × 800 mm dimensions, and had tabs welded on for later use. The compaction density of the positive electrode material layer was 4.23 g / cm³. 3 .

[0176] <Preparation of Electrolyte>

[0177] In an environment with a water content of less than 10 ppm, non-aqueous organic solvents ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), propyl propionate (PP), and vinylene carbonate (VC) were mixed in a mass ratio of 20:30:20:28:2. Lithium hexafluorophosphate (LiPF6) was then added to the non-aqueous organic solvents, dissolved, and mixed thoroughly to obtain the electrolyte. Based on the total mass of the electrolyte, the mass content of LiPF6 was 8%, with the remainder being non-aqueous organic solvent.

[0178] <Preparation of Lithium-ion Batteries>

[0179] The prepared positive electrode, first separator, negative electrode, and second separator are stacked in sequence and then wound to obtain an electrode assembly. Referring specifically to Figures 1 and 2, the corner segment of the first positive electrode is adjacent to the corner segment of the first negative electrode material layer, and the corner segment of the first positive electrode is further away 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 segments of the first positive electrode and the first negative electrode material layer. The first ceramic coating of the first separator faces the positive electrode, and the second ceramic coating of the second separator faces the positive electrode. ； In the same negative electrode layer, 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. The prepared electrolyte is injected, and the secondary battery is obtained through vacuum sealing, settling, formation, and shaping processes.

[0180] Examples 1-2 to Examples 1-5

[0181] Except for adjusting the stretching ratios of the first base film in transverse stretching, longitudinal stretching, secondary transverse stretching, and secondary longitudinal stretching, so that the porosity, closure temperature, and rupture temperature of the first base film are as shown in Table 1, the rest is the same as in Example 1-1.

[0182] Examples 1-6 to 1-9, Examples 1-15

[0183] Except for adjusting the type of the first resin material according to Table 1, and adjusting the stretching ratio of the first base film in transverse stretching, longitudinal stretching, secondary transverse stretching, and secondary longitudinal stretching so that the porosity, closure temperature and rupture temperature of the first base film are as shown in Table 1, the rest is the same as in Example 1-1.

[0184] Examples 1-10 to Examples 1-13

[0185] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Example 1-1.

[0186] Examples 1-14

[0187] Except for adjusting the <Preparation of Lithium-ion Batteries> according to the following steps, the rest is the same as in Example 1-1.

[0188] <Preparation of Lithium-ion Batteries>

[0189] The prepared positive electrode, second separator, negative electrode, and first separator are stacked in sequence and then wound to obtain an electrode assembly. Referring to Figures 3 and 4, the corner segment of the second positive electrode is adjacent to the corner segment of the second negative electrode material layer, and the corner segment of the second positive electrode is closer to the winding center of the electrode assembly than the corner segment of the second negative electrode material layer. The first separator is located between the corner segments of the second positive electrode and the second negative electrode material layer. The first ceramic coating of the first separator faces the positive electrode, and the second ceramic coating of the second separator faces the positive electrode. In the same negative electrode layer, the first negative electrode material layer is farther from the winding center of the electrode assembly than the second negative electrode material layer, and the second separator is located on the side of the corner segment of the second positive electrode that is farther from the first separator. This structure is denoted as Structure B. The electrode assembly is placed in an aluminum-plastic film packaging bag and dehydrated at 80°C. A prepared electrolyte is injected, and the lithium-ion battery is obtained through vacuum sealing, settling, formation, and shaping processes.

[0190] Examples 2-1 to 2-12, Example 2-27

[0191] Except for adjusting the types of the second, third, or fourth resin materials according to Table 2, and adjusting the stretching ratio of the second base film in transverse stretching, longitudinal stretching, secondary transverse stretching, and secondary longitudinal stretching so that the porosity, closure temperature, and rupture temperature of the second base film are as shown in Table 2, the rest is the same as in Examples 1-1.

[0192] Examples 2-13 to 2-19, Examples 2-22 to 2-26

[0193] Except for adjusting the relevant preparation parameters according to Table 2, the rest is the same as in Examples 1-1.

[0194] Examples 2-20 to 2-21

[0195] Except for adjusting the stretching ratios of the second base film in transverse stretching, longitudinal stretching, secondary transverse stretching, and secondary longitudinal stretching, so that the porosity, closure temperature, and rupture temperature of the first base film are as shown in Table 2, the rest are the same as in Examples 1-1.

[0196] Comparative Examples 1-1 to 1-2

[0197] Except for adjusting the stretching ratios of the first base film in transverse stretching, longitudinal stretching, secondary transverse stretching, and secondary longitudinal stretching, so that the porosity, closure temperature, and rupture temperature of the first base film are as shown in Table 1, the rest is the same as in Example 1-1.

[0198] Comparative Examples 1-3

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

[0200] Comparative Examples 2-1 to 2-2

[0201] Except for adjusting the relevant preparation parameters according to Table 2, the rest is the same as in Examples 1-1.

[0202] Comparative Examples 2-3

[0203] Except for replacing the first base film with the second base film, the rest is the same as in Example 1-1.

[0204] The preparation parameters and performance tests of each embodiment and comparative example are shown in Tables 1 and 2.

[0205] Table 1

[0206] As 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, 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 pore-closing temperature T12 of the second base film is less than the pore-closing 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 all within the range of this application, the obtained lithium-ion battery has a high pass rate in the hot box test at 130°C and 132°C, and the degree of lithium plating on the surface of the negative electrode sheet opposite to the first separator and the second separator, as well as on the negative electrode sheet as a whole, is relatively light. This shows that the lithium-ion battery obtained in this application can take into account both kinetic performance and high-temperature hot box performance. In Comparative Examples 1-1 to 1-2, the porosity of the first base film is outside the range of this application. Comparative Examples 1-3 did not use a second base film. The resulting lithium-ion batteries, while exhibiting better high-temperature thermal performance in Comparative Example 1-1, also showed more severe lithium plating. Comparative Examples 1-2 and 1-3, while showing less lithium plating, exhibited poorer high-temperature thermal performance. In Comparative Example 2-1, the pore-closing temperature of the second base film was outside the range of this application. In Comparative Example 2-2, the film-breaking temperature of the second base film was outside the range of this application. Comparative Examples 1-3 did not use a first base film. The resulting lithium-ion batteries, while showing less lithium plating in Comparative Examples 2-1 and 2-2, exhibited poorer high-temperature thermal performance. Comparative Example 2-3, while exhibiting better high-temperature thermal performance, also showed more severe lithium plating. Therefore, it is evident that the lithium-ion batteries in Comparative Examples 1-1 to 1-3 and Comparative Examples 2-1 to 2-3 struggle to balance kinetic performance and high-temperature thermal performance.

[0207] The melting enthalpy, melting index, and weight-average molecular weight of the first resin material change in a linked manner. As can be seen from Examples 1-1, 1-6 to 1-9, when the above parameters are within the range of this application, the obtained lithium-ion battery has a high pass rate in the hot box test at 130°C and 132°C, and the degree of lithium plating on the surface of the negative electrode opposite to the first separator and the second separator, as well as on the negative electrode as a whole, is relatively light. This shows that the obtained lithium-ion battery can take into account both kinetic performance and high-temperature hot box performance.

[0208] The presence of characteristic peak A in the DSC spectrum of the first base film is affected by the melting enthalpy, melt index, and weight-average molecular weight of the first resin material. As can be seen from Examples 1-1 to 1-15, when the above parameters of the first resin material are within the range of this application, the DSC spectrum of the first base film always contains characteristic peak A. Specifically, as shown in Figure 5, the DSC spectrum of the first base film in Example 1-1 shows a characteristic peak between 128°C and 134°C, i.e., characteristic peak A is present.

[0209] As can be seen from Examples 1-1, 1-10 to 1-13, when the thickness of the first base film is within the range of this application, the obtained lithium-ion battery has a high pass rate in the hot box test at 130°C and 132°C, and the degree of lithium plating on the surface of the negative electrode sheet opposite to the first separator and the second separator, as well as on the negative electrode sheet as a whole, is relatively light, thus indicating that the obtained lithium-ion battery can take into account both dynamic performance and high-temperature hot box performance.

[0210] As can be seen from Examples 1-1 and 1-14, the first separator is located between the corner segment of the first positive electrode and the corner segment of the first negative electrode material layer. Compared to the second separator being located between the corner segment of the second positive electrode and the corner segment of the second negative electrode material layer, the degree of lithium plating 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 segment of the first positive electrode is adjacent to a corner segment of the first negative electrode material layer and is far from the winding center of the electrode assembly. At this time, the winding radius of the positive electrode material layer in the positive electrode is larger than the winding radius of the first negative electrode material layer, and the positive-negative electrode ratio (Cell Balance, CB, the ratio of negative electrode capacity per unit area to positive electrode capacity per unit area) is smaller, making it easy for interface problems to occur and lead to lithium plating. Therefore, a sufficient amount of electrolyte is required to transport active ions. At the same time, the corner area of ​​the winding structure itself is prone to insufficient electrolyte or even electrolyte breakage, which will further aggravate the interface problem. Therefore, this application provides a first separator containing a first base film between the corner section of the first positive electrode and the corner section of the first negative electrode material layer. The first base film with higher porosity has a higher electrolyte retention capacity, which is beneficial to improving the problem of insufficient electrolyte in the corner area and also beneficial to the transport of active ions, thereby improving the interface problem and thus improving the lithium plating problem of the secondary battery, that is, improving the kinetic performance.

[0211] The interconnected changes in the melting enthalpy, melt index, and weight-average molecular weight of the second, third, and fourth resin materials affect the pore-closing temperature and rupture temperature of the second base film, thereby influencing the high-temperature thermal performance of the lithium-ion battery. As can be seen from Examples 1-1, 2-1 to 2-12, and 2-22, when the above parameters are within the range of this application, the obtained lithium-ion battery exhibits a high thermal test pass rate at 130°C and 132°C, indicating that the obtained lithium-ion battery has good high-temperature thermal performance. Furthermore, the degree of lithium plating on the surface of the negative electrode opposite the first and second separators, as well as on the negative electrode as a whole, is relatively mild, while also maintaining good kinetic performance. In Examples 1-1, 2-1 to 2-4, the weight-average molecular weight and melting enthalpy of the second resin material in Example 2-3 are lower than those in Example 1-1, while the melt index is higher. The resulting second base film has a lower pore-closing temperature, thus exhibiting better high-temperature thermal performance. However, the mechanical properties of the second base film are poor, which affects the manufacturing yield of the lithium-ion battery.

[0212] The presence of characteristic peaks B, C, and D in the DSC spectrum of the second base film is influenced by the melting enthalpy, melt index, and weight-average molecular weight of the second, third, and fourth resin materials. As can be seen from Examples 2-1 to 2-27, when the above parameters are within the range of this application, the DSC spectrum of the second base film exhibits characteristic peaks B, C, and D. Specifically, as shown in Figure 6, the DSC spectrum of the second base film in Example 1-1 shows characteristic peaks at 120°C to 126°C, 128°C to 134°C, and 150°C to 160°C, i.e., characteristic peaks B, C, and D are present.

[0213] The mass percentages of the second, third, and fourth resin materials affect the pore-closing and rupture temperatures of the second base film, thus influencing the high-temperature thermal performance of the lithium-ion battery. As seen in Examples 1-1, 2-13 to 2-19, when the parameters are within the range specified in this application, the resulting lithium-ion battery exhibits a high thermal test pass rate at 130°C and 132°C, indicating good high-temperature thermal performance. Furthermore, the lithium plating on the surface of the negative electrode opposite the first and second separators, as well as on the negative electrode as a whole, is relatively mild, while also maintaining good kinetic performance. In Example 2-17, compared to Example 1-1, the mass percentage of the second resin material is too high, and the mass percentage of the third resin material is too low. The mass percentage of the third resin material is the main factor affecting the strength of the second separator. When the mass percentage of the third resin material is too low, the puncture resistance of the second separator is insufficient, affecting the safety and lifespan of the secondary battery.

[0214] As can be seen from Examples 1-1, 2-20 to 2-21, when the porosity of the second base film is within the range of this application, the obtained lithium-ion battery has a high pass rate in the hot box test at 130°C and 132°C, and the degree of lithium plating on the surface of the negative electrode sheet opposite to the first separator and the second separator, as well as on the negative electrode sheet as a whole, is relatively light, thus indicating that the obtained lithium-ion battery can take into account both dynamic performance and high-temperature hot box performance.

[0215] As can be seen from Examples 1-1, 2-23 to 2-26, when the thickness of the second base film is within the range of this application, the obtained lithium-ion battery has a high pass rate in the hot box test at 130°C and 132°C, and the degree of lithium plating on the surface of the negative electrode sheet opposite to the first separator and the second separator, as well as on the negative electrode sheet as a whole, is relatively light, thus indicating that the obtained lithium-ion battery can take into account both dynamic performance and high-temperature hot box performance.

[0216] As can be seen from Figures 7 and 8, the lithium-ion battery prepared in Example 1-1 has a lower cycle expansion rate and a higher cycle capacity retention rate than the lithium-ion battery prepared in Comparative Example 1-2 after 1500 cycles. This indicates that the lithium-ion battery prepared in the examples is beneficial to improving lithium-ion transport, that is, improving the kinetic performance of the lithium-ion battery.

[0217] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A secondary battery comprising a wound electrode assembly, the electrode assembly comprising a positive electrode, a negative electrode, a first separator, and a second separator, wherein at least a portion of the negative electrode is located between the first separator and the second separator; The first separator comprises a first base film, and the second separator comprises a second base film, the porosity nl of the first base film is greater than the porosity n2 of the second base film, the closure temperature T12 of the second base film is less than the closure temperature T11 of the first base film, the burst temperature T22 of the second base film is greater than the burst 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 includes a flat region and a corner region, and the negative electrode sheet includes a negative electrode current collector and a first negative electrode material layer and a second negative electrode material layer located on both sides of the thickness direction of the negative electrode current collector. In the corner region, a first positive electrode corner segment is adjacent to a first negative electrode material layer corner segment. The first positive electrode corner segment is farther away from the winding center of the electrode assembly than the first negative electrode material layer corner segment. The first separator is located between the first positive electrode corner segment and the first negative electrode material layer corner segment.

3. The secondary battery according to claim 1, wherein it 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 comprises a first resin material, wherein 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 comprises 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 index 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 index 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 index MFR4 is 0.1 g / 10 min to 100 g / 10 min.

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

6. The secondary battery according to claim 4, wherein Based on the quality 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 different from the second and third resin materials.

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

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

11. The secondary battery according to any one of claims 1 to 8, wherein it 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 are each independently between 4 μm and 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, and the porosity of the first diaphragm is n3, 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 to 15 mg / 5000 mm 2 .

15. The secondary battery according to claim 13, wherein, The first ceramic coating comprises first ceramic particles, which include at least one of alumina, boehmite, silicon oxide, magnesium oxide, titanium oxide, tin oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, aluminum hydroxide, magnesium hydroxide, or calcium hydroxide.

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 includes silicon material.

17. The secondary battery according to claim 16, wherein, The silicon material includes at least one of elemental silicon, silicon carbide, silicon oxide, or silicon alloy.

18. An electronic device comprising a secondary battery as claimed in any one of claims 1 to 17.

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