Electrochemical apparatus and electronic apparatus

Abstract

An electrochemical apparatus and an electronic apparatus; the electrochemical apparatus comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, the separator being disposed between the positive electrode sheet and the negative electrode sheet; the separator comprises a porous substrate, and the porous substrate comprises a first polyethylene resin material and a second polyethylene resin material, the melting enthalpy of the first polyethylene resin material being 110 J / g to 160 J / g, the melting enthalpy of the second polyethylene resin material being 170 J / g to 205 J / g, and the pore closing rate of the separator being 10 kΩ / min to 80 kΩ / min. On the basis of the mass of the porous substrate, the mass percentage content of the first polyethylene resin material is 10% to 40%, and the mass percentage content of the second polyethylene resin material is 45% to 88%. The separator has a high pore closing rate, can rapidly close pores at a low temperature and has a good pore closing effect, thereby improving the high-temperature safety performance and electrochemical performance of the electrochemical apparatus.

Description

Electrochemical devices and electronic devices

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on June 9, 2023, with application number 202310685198.0 and invention name “Electrochemical Device and Electronic Device”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present application relates to the field of electrochemical technology, and in particular to an electrochemical device and an electronic device. Background Art

[0003] Electrochemical devices (such as lithium-ion batteries) have many advantages such as high specific energy density, long cycle life, high nominal voltage, low self-discharge rate, small size and light weight, and are widely used in the field of consumer electronics.

[0004] As lithium-ion batteries face increasingly higher requirements for energy density (ED) and fast charging performance, their design is becoming increasingly extreme. Currently, high-voltage cathodes and fast-charging graphite are mostly used to enhance ED and fast-charging performance. However, due to their poor stability and heat resistance, high-voltage cathodes and fast-charging graphite react rapidly with the electrolyte at certain temperatures, generating significant heat. This prevents the separator from closing its pores in a timely and effective manner, resulting in a slow response and inability to prevent ion transfer, heat, and diffusion between the positive and negative electrodes, impacting the battery's high-temperature safety performance.

[0005] Summary of the Invention

[0006] The purpose of the present application is to provide an electrochemical device and an electronic device to improve the high-temperature safety performance and electrochemical performance of the electrochemical device.

[0007] It should be noted that in the invention content of this application, lithium-ion batteries are used as an example of electrochemical devices to explain this application, but the electrochemical devices of this application are not limited to lithium-ion batteries. The specific technical solutions are as follows:

[0008] The first aspect of the present application provides an electrochemical device comprising a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte, wherein the separator is disposed between the positive electrode sheet and the negative electrode sheet, and the separator comprises a porous substrate, wherein the porous substrate comprises a first polyethylene resin material and a second polyethylene resin material. The first polyethylene resin material has a heat of fusion of 110 J / g to 160 J / g, preferably 130 J / g to 160 J / g. The second polyethylene resin material has a heat of fusion of 170 J / g to 205 J / g, preferably 180 J / g to 205 J / g. Based on the mass of the porous substrate, the mass percentage of the first polyethylene resin material is 10% to 40%, preferably 15% to 30%, and the mass percentage of the second polyethylene resin material is 45% to 88%, preferably 60% to 80%. The diaphragm has a closing rate of 10 kΩ / min to 80 kΩ / min, preferably 25 kΩ / min to 80 kΩ / min. The present application regulates the melting enthalpy and mass percentage of the first polyethylene resin material and the second polyethylene resin material within the above ranges, and makes the melting enthalpy of the first polyethylene resin material lower than the melting enthalpy of the second polyethylene resin material. When the electrochemical device is heated, the diaphragm can achieve rapid pore closing at a lower temperature, thereby increasing the pore closing rate of the diaphragm, achieving a better pore closing effect, and having a higher strength, thereby improving the high-temperature safety performance and electrochemical performance of the electrochemical device.

[0009] In some embodiments of the present application, the melt index of the first polyethylene resin material is 5 g / 10 min to 20 g / 10 min, preferably, the melt index of the first polyethylene resin material is 8 g / 10 min to 18 g / 10 min; the melt index of the second polyethylene resin material is 0.1 g / 10 min to 3.0 g / 10 min, preferably, the melt index of the second polyethylene resin material is 0.2 g / 10 min to 1.5 g / 10 min. By regulating the melt index of the first polyethylene resin material and the second polyethylene resin material within the above range, it is beneficial for the separator to achieve rapid pore closure at a lower temperature, thereby improving the high-temperature safety performance of the electrochemical device.

[0010] In some embodiments of the present application, the maximum resistance of the high-temperature closed-pore separator ranges from 10 kΩ to 400 kΩ, preferably from 40 kΩ to 400 kΩ. When the maximum resistance of the high-temperature closed-pore separator is within this range, the separator exhibits a good closed-pore effect, and the electrochemical device exhibits excellent high-temperature safety performance.

[0011] In some embodiments of the present application, the porous substrate further comprises a high-temperature resistant resin, the high-temperature resistant resin comprising one of a crystalline polymer or an amorphous polymer, including at least one of polypropylene, poly-4-methylpentene, polytetrafluoroethylene (PTFE), polyvinylidene fluoride, and cycloolefin copolymer (COC), and the mass percentage of the high-temperature resistant resin is 2% to 15% based on the mass of the porous substrate. When the above-mentioned type of high-temperature resistant resin is added to the porous substrate and the mass percentage of the high-temperature resistant resin is controlled within the above-mentioned range, it is beneficial to improve the thermal stability and strength of the diaphragm, as well as the melting point, thereby improving the high-temperature safety performance of the electrochemical device.

[0012] In some embodiments of the present application, the tortuosity of the separator is 1.1 to 2.0. When the tortuosity of the separator is within the above range, the pore permeability of the separator is good, which is conducive to improving the low-temperature cycling performance and room-temperature discharge performance of the electrochemical device.

[0013] In some embodiments of the present application, the electrolyte includes fluoroethylene carbonate (FEC), and the weight percentage of fluoroethylene carbonate is 0.1% to 12%, preferably 0.1% to 5%, based on the weight of the electrolyte. By regulating the weight percentage of fluoroethylene carbonate in the electrolyte within the above range, the decomposition of the electrolyte can be suppressed at high temperatures, which helps to reduce the reaction between the electrolyte and the positive electrode plate and the reaction between the electrolyte and the negative electrode plate, thereby improving the high-temperature safety performance of the electrochemical device.

[0014] The second aspect of the present application provides an electronic device comprising the electrochemical device of any of the aforementioned embodiments. The electrochemical device provided by the present application has good high-temperature safety performance and electrochemical performance, thereby providing an electronic device with good high-temperature safety performance and electrochemical performance.

[0015] Beneficial effects of this application:

[0016] The present application provides an electrochemical device and an electronic device, the electrochemical device comprising a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. The separator is disposed between the positive electrode sheet and the negative electrode sheet. The separator comprises a porous substrate, the porous substrate comprising a first polyethylene resin material and a second polyethylene resin material. The first polyethylene resin material has a heat of fusion of 110 J / g to 160 J / g, and the second polyethylene resin material has a heat of fusion of 170 J / g to 205 J / g. Based on the mass of the porous substrate, the mass percentage of the first polyethylene resin material is 10% to 40%, and the mass percentage of the second polyethylene resin material is 45% to 88%. The separator has a cell closure rate of 10 kΩ / min to 80 kΩ / min. In the present application, by regulating the melting enthalpy and mass percentage of the first polyethylene resin material and the second polyethylene resin material within the above-mentioned range, and making the melting enthalpy of the first polyethylene resin material lower than the melting enthalpy of the second polyethylene resin material, when the electrochemical device is heated up, the diaphragm can achieve rapid pore closing at a lower temperature, the pore closing rate of the diaphragm is large, the pore closing effect is good, and the diaphragm has high strength, thereby improving the high-temperature safety performance and electrochemical performance of the electrochemical device.

[0017] Of course, it is not necessary to achieve all the advantages described above at the same time when implementing any product or method of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other embodiments can also be obtained based on these drawings.

[0019] FIG1 is a time-resistance curve of the diaphragm in Example 1-2. DETAILED DESCRIPTION

[0020] The following will be combined with the drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field based on this application are within the scope of protection of this application.

[0021] It should be noted that, in the specific embodiments of the present application, a lithium-ion battery is used as an example of an electrochemical device to explain the present application, but the electrochemical device of the present application is not limited to a lithium-ion battery.

[0022] The first aspect of the present application provides an electrochemical device comprising a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte, wherein the separator is disposed between the positive electrode sheet and the negative electrode sheet, the separator comprising a porous substrate, the porous substrate comprising a first polyethylene resin material and a second polyethylene resin material. The first polyethylene resin material has a melting enthalpy H1 of 110 J / g to 160 J / g, preferably, a melting enthalpy H1 of 130 J / g to 160 J / g. For example, the melting enthalpy H1 of the first polyethylene resin material may be 110 J / g, 120 J / g, 130 J / g, 140 J / g, 145 J / g, 150 J / g, 160 J / g, or a range consisting of any two of these values. The second polyethylene resin material has a melting enthalpy H2 of 170 J / g to 205 J / g, preferably, a melting enthalpy H2 of 180 J / g to 205 J / g. For example, the melting enthalpy H2 of the second polyethylene resin material is 170 J / g, 175 J / g, 180 J / g, 185 J / g, 190 J / g, 195 J / g, 200 J / g, 205 J / g, or a range consisting of any two of these values. Based on the mass of the porous substrate, the mass percentage x of the first polyethylene resin material is 10% to 40%, preferably, the mass percentage x of the first polyethylene resin material is 15% to 30%. For example, the mass percentage x of the first polyethylene resin material is 10%, 15%, 20%, 25%, 30%, 35%, 40%, or a range consisting of any two of these values. The mass percentage y of the second polyethylene resin material is 45% to 88%, preferably, the mass percentage y of the second polyethylene resin material is 60% to 80%. For example, the mass percentage y of the second polyethylene resin material is 45%, 50%, 60%, 70%, 80%, 88%, or a range consisting of any two of these values. The closed cell rate of the separator is 10 kΩ / min to 80 kΩ / min, preferably, the closed cell rate of the separator is 25 kΩ / min to 80 kΩ / min. For example, the closed cell rate of the separator is 10 kΩ / min, 20 kΩ / min, 25 kΩ / min, 30 kΩ / min, 40 kΩ / min, 60 kΩ / min, 80 kΩ / min, or a range consisting of any two of these values. Generally, the lower the melting enthalpy of a material, the better its fluidity at the same temperature. In the present application, by regulating the melting enthalpy of the first polyethylene resin material and the second polyethylene resin material within the above-mentioned range, and making the melting enthalpy of the first polyethylene resin material lower than the melting enthalpy of the second polyethylene resin material, when the electrochemical device is heated up, the diaphragm can achieve rapid pore closing at a lower temperature, thereby improving the pore closing rate of the diaphragm. The pore closing rate of the diaphragm is within the range of the present application, and the pore closing effect is good, which can effectively prevent the diffusion of heat and substances in the electrochemical device and prevent the exchange of substances between the positive electrode sheet and the negative electrode sheet.At the same time, the diaphragm has high strength, which can improve the high-temperature safety and electrochemical performance of the electrochemical device. When the mass percentage of the first polyethylene resin material is too low (for example, less than 10%) or the mass percentage of the second polyethylene resin material is too high (for example, greater than 88%), the diaphragm's pore closing rate is too slow and the pore closing effect is poor; when the mass percentage of the first polyethylene resin material is too high (for example, greater than 40%) or the mass percentage of the second polyethylene resin material is too low (for example, less than 45%), the diaphragm's strength is reduced, and it is easy to deform or break, which will cause a short circuit between the positive electrode and the negative electrode. Therefore, the present application can take into account the pore closing rate and strength of the diaphragm by regulating the melting enthalpy and mass percentage of the first polyethylene resin material and the second polyethylene resin material within the above range, and making the melting enthalpy of the first polyethylene resin material lower than the melting enthalpy of the second polyethylene resin material, thereby improving the high-temperature safety and electrochemical performance of the electrochemical device. In this application, high temperature refers to a temperature greater than or equal to 125°C. Where, the closing rate = (R2-R1) / (t2-t1). The closing rate of the diaphragm is calculated using the time-resistance curve measured by the temperature-rise internal resistance method. R1 is the resistance value when the fixture resistance begins to increase, R2 is the highest resistance value measured by the fixture, t1 is the time when the resistance value begins to increase, and t2 is the time when the resistance value reaches the maximum.

[0023] In some embodiments of the present application, the melt index MFR1 of the first polyethylene resin material is 5 g / 10 min to 20 g / 10 min, preferably, the melt index MFR1 of the first polyethylene resin material is 8 g / 10 min to 18 g / 10 min, for example, the melt index MFR1 of the first polyethylene resin material is 5 g / 10 min, 8 g / 10 min, 10 g / 10 min, 15 g / 10 min, 18 g / 10 min, 20 g / 10 min or a range consisting of any two values ​​thereof; the melt index MFR1 of the second polyethylene resin material is 5 g / 10 min, 8 g / 10 min, 10 g / 10 min, 15 g / 10 min, 18 g / 10 min, 20 g / 10 min or a range consisting of any two values ​​thereof; The index MFR2 is 0.1g / 10min to 3.0g / 10min. Preferably, the melt index MFR2 of the second polyethylene resin material is 0.2g / 10min to 1.5g / 10min. For example, the melt index MFR2 of the second polyethylene resin material is 0.1g / 10min, 0.2g / 10min, 0.5g / 10min, 1.0g / 10min, 1.5g / 10min, 2.0g / 10min, 2.5g / 10min, 3.0g / 10min or a range consisting of any two values ​​therein. Generally, the higher the melt index of a material, the better its fluidity at the same temperature. By regulating the melt indexes of the first polyethylene resin material and the second polyethylene resin material within the above range and making the melt index of the first polyethylene resin material higher than the melt index of the second polyethylene resin material, when the electrochemical device is heated, it is beneficial for the diaphragm to achieve rapid pore closure at a lower temperature, and at the same time the diaphragm has higher strength, thereby improving the high-temperature safety performance of the electrochemical device.

[0024] In some embodiments of the present application, the maximum resistance of the high-temperature closed-pore diaphragm ranges from 10 kΩ to 400 kΩ, preferably from 40 kΩ to 400 kΩ. For example, the maximum resistance of the high-temperature closed-pore diaphragm ranges from 10 kΩ, 40 kΩ, 100 kΩ, 200 kΩ, 300 kΩ, 400 kΩ, or a range consisting of any two of these values. When the maximum resistance of the high-temperature closed-pore diaphragm is greater than or equal to 10 kΩ, the diaphragm has a good closed-pore effect and can effectively prevent material exchange between the positive electrode plate and the negative electrode plate, thereby providing the electrochemical device with good high-temperature safety performance.

[0025] In some embodiments of the present application, the closed-pore temperature T1 of the diaphragm is 120°C to 145°C, and the melting temperature T2 is 155°C to 250°C. For example, the closed-pore temperature T1 of the diaphragm is 120°C, 125°C, 130°C, 135°C, 140°C, 145°C, or a range consisting of any two of these values, and the melting temperature T2 of the diaphragm is 155°C, 175°C, 200°C, 220°C, 240°C, 250°C, or a range consisting of any two of these values. This indicates that the diaphragm has a lower closed-pore temperature and a higher melting temperature. The diaphragm can achieve rapid closure at a lower temperature, and the diaphragm is not prone to rupture at a higher temperature, thereby improving the high-temperature safety performance of the electrochemical device. Among them, the lower temperature refers to a temperature less than or equal to 140°C, and the higher temperature refers to a temperature greater than or equal to 155°C. Among them, the closed-pore temperature refers to the temperature at which the diaphragm begins to melt and seal part of the originally formed pores during the heating process; the melting temperature refers to the temperature at which the diaphragm melts to a certain extent and breaks, resulting in a local or full short circuit.

[0026] In some embodiments of the present application, the average pore size of the diaphragm is 20 nm to 50 nm, and the pore size distribution is 10 nm to 60 nm. For example, the average pore size of the diaphragm is 20 nm, 30 nm, 40 nm, 50 nm, or a range consisting of any two of these values, and the pore size distribution of the diaphragm is 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, or a range consisting of any two of these values. By regulating the average pore size and pore size distribution of the diaphragm within the above ranges, the pore size of the diaphragm is smaller, which is conducive to improving the closed-pore effect of the diaphragm, thereby improving the safety performance of the electrochemical device.

[0027] In some embodiments of the present application, the ionic conductivity S of the separator is 0.3 mS / cm to 2.0 mS / cm. For example, the ionic conductivity of the separator is 0.3 mS / cm, 0.5 mS / cm, 1.0 mS / cm, 1.5 mS / cm, 2.0 mS / cm, or a range consisting of any two of these values. The ionic conductivity of the separator is related to the average pore size of the separator. When the ionic conductivity of the separator is within the above range, it indicates that the separator has good ionic conductivity, which is beneficial to improving the electrochemical performance of the electrochemical device, such as cycling performance.

[0028] In some embodiments of the present application, the tortuosity of the diaphragm is 1.1 to 2.0. For example, the tortuosity of the diaphragm is 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, or a range consisting of any two of these values. The tortuosity of the diaphragm is related to the porosity of the diaphragm, the conductivity of the electrolyte, and the ionic conductivity of the diaphragm. When the tortuosity of the diaphragm is within the above range, it indicates that the pore permeability of the diaphragm is good, which is beneficial to improving the electrochemical performance of the electrochemical device, such as low-temperature capacity retention and cycle performance. The present application has no particular restrictions on the porosity of the diaphragm, as long as the purpose of the present application can be achieved. For example, the porosity of the diaphragm can be 20% to 60%. The present application has no particular restrictions on the conductivity of the electrolyte, as long as the purpose of the present application can be achieved. For example, the conductivity of the electrolyte can be 5.0 mS / cm to 20 mS / cm.

[0029] in,

[0030] In some embodiments of the present application, the porous substrate further comprises a high temperature resistant resin, the high temperature resistant resin comprising one of a crystalline polymer or an amorphous polymer, the high temperature resistant resin comprising at least one of polypropylene, poly-4-methylpentene, polytetrafluoroethylene, polyvinylidene fluoride and a cycloolefin copolymer. The crystalline polymer comprises at least one of polypropylene, poly-4-methylpentene, polytetrafluoroethylene or polyvinylidene fluoride, and the amorphous polymer comprises a cycloolefin copolymer. Based on the mass of the porous substrate, the mass percentage z of the high temperature resistant resin is 2% to 15%. For example, the mass percentage z of the high temperature resistant resin is 2%, 3%, 5%, 7%, 8%, 10%, 12%, 13%, 15% or a range consisting of any two of these values. When the above-mentioned type of high temperature resistant resin is added to the porous substrate and the mass percentage of the high temperature resistant resin is controlled within the above range, it is beneficial to increase the melting point of the diaphragm, improve the thermal stability and strength of the diaphragm, and thus improve the high temperature safety performance of the electrochemical device.

[0031] In some embodiments of the present application, the electrolyte includes fluoroethylene carbonate. Based on the mass of the electrolyte, the mass percentage of fluoroethylene carbonate is 0.1% to 12%, preferably 0.1% to 5%. For example, the mass percentage of fluoroethylene carbonate can be 0.1%, 3%, 5%, 6%, 9%, 10%, 12% or a range composed of any two resins therein. By regulating the mass percentage of fluoroethylene carbonate in the electrolyte within the above range, it is beneficial to improve the stability of the solid electrolyte interface film (SEI film) on the surface of the negative electrode plate, inhibit the decomposition of the electrolyte at high temperature, reduce the reaction between the electrolyte and the positive electrode plate and the reaction between the electrolyte and the negative electrode plate, and reduce the generation of heat, thereby improving the high temperature safety performance of the electrochemical device.

[0032] In some embodiments of the present application, the melting point of the first polyethylene resin material is 115°C to 135°C, and the melting point of the second polyethylene resin material is 135°C to 145°C. For example, the melting point of the first polyethylene resin material is 115°C, 120°C, 125°C, 130°C, 135°C, or a range consisting of any two of these values, and the melting point of the second polyethylene resin material is 135°C, 138°C, 140°C, 143°C, 145°C, or a range consisting of any two of these values. Generally, the lower the melting point of a material, the lower the degree of crystal structure of the material in the molten state, the lower the crystallinity, and the better the fluidity at the same temperature. The present application regulates the melting points of the first polyethylene resin material and the second polyethylene resin material within the above range, and makes the melting point of the first polyethylene resin material lower than the melting point of the second polyethylene resin material. When the electrochemical device is heated, it is beneficial for the diaphragm to achieve rapid pore closure at a lower temperature, and the diaphragm has higher strength, thereby improving the high-temperature safety performance of the electrochemical device.

[0033] In some embodiments of the present application, the puncture strength of the diaphragm ranges from 153 gf to 245 gf, and the puncture strength / thickness ranges from 10.2 gf / μm to 30 gf / μm. This indicates that the diaphragm has good puncture resistance, which helps reduce the probability of internal short circuit problems caused by diaphragm rupture when the electrochemical device is punctured by foreign objects, thereby improving the high-temperature safety performance of the electrochemical device. The puncture strength refers to the force required for a spherical steel needle with a diameter of 1.0 mm to penetrate the diaphragm at a speed of 300±10 mm / min.

[0034] In some embodiments of the present application, the thickness of the separator is between 2 μm and 15 μm. For example, the thickness of the separator is 2 μm, 5 μm, 8 μm, 10 μm, 12 μm, 15 μm, or a range consisting of any two of these values. By regulating the separator thickness within the above range, the electrochemical device has good high-temperature safety performance and high energy density.

[0035] The present application does not particularly limit the preparation method of the diaphragm, as long as the purpose of the present application can be achieved. For example, the preparation method of the diaphragm may include but is not limited to the following steps: uniformly mixing a first polyethylene resin material, a second polyethylene resin material, and a high-temperature resistant resin to obtain a polyolefin material, uniformly mixing an additive with a solvent to obtain a mixed solvent, then uniformly mixing the polyolefin material and the mixed solvent, adding the mixture to an extruder system, extruding through a die, casting and cooling the film to obtain a substrate; longitudinally and transversely stretching the substrate, extracting with dichloromethane, and drying to obtain a porous substrate; and secondary stretching, heat-setting, and winding the porous substrate to obtain the desired diaphragm. Among them, the present application has no special restrictions on additives, as long as the purpose of the present application can be achieved. For example, the additive can be at least one of tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate] pentaerythritol ester, tris(2,4-di-tert-butylphenyl) phosphite, β(3,5-di-tert-butyl-4-hydroxybenzene) propionate octadecyl alcohol ester, 2,5-di-tert-butylhydroquinone or 2,5-di-tert-butylhydroquinone. The present application has no special restrictions on the amount of additives added, as long as the purpose of the present application can be achieved. For example, based on the total mass of the first polyethylene resin material, the second polyethylene resin material and the high temperature resistant resin, the amount of additives added can be 0.1% to 1%. When the additives include multiple types, the present application does not limit the added content of different additives, as long as the purpose of the present application can be achieved. The present application has no special restrictions on the solvent, as long as the purpose of the present application can be achieved. For example, the solvent can be at least one of paraffin oil, mineral oil, diphenyl ether or xylene. The present application has no particular restrictions on the amount of solvent added, as long as the purpose of the present application can be achieved. The present application has no particular restrictions on the standards for transverse stretching and longitudinal stretching, as long as the purpose of the present application can be achieved. For example, the transverse stretching and longitudinal stretching ratios can be 4 to 10 times. The present application has no particular restrictions on the temperature and time of extraction, as long as the purpose of the present application can be achieved. For example, the extraction temperature can be 20°C to 35°C, and the extraction time can be 20 min to 40 min. The present application has no particular restrictions on the standards for secondary stretching, as long as the purpose of the present application can be achieved. For example, the secondary stretching ratio can be 1 to 3 times. The present application has no particular restrictions on the temperature for heat setting, as long as the purpose of the present application can be achieved. For example, the temperature for heat setting can be 100°C to 125°C.

[0036] The present application has no special restrictions on the positive electrode plate, as long as the purpose of the present application can be achieved. For example, the positive electrode plate includes a positive electrode current collector and a positive electrode material layer arranged on at least one surface of the positive electrode current collector. The above-mentioned "positive electrode material layer arranged on at least one surface of the positive electrode current collector" means that the positive electrode material layer can be arranged on one surface of the positive electrode current collector along the thickness direction of itself, or on two surfaces of the positive electrode current collector along the thickness direction of itself. It should be noted that the "surface" here can be the entire area of ​​the surface of the positive electrode current collector, or it can be a partial area of ​​the surface of the positive electrode current collector. The present application has no special restrictions, as long as the purpose of the present application can be achieved. The present application has no special restrictions on the positive electrode current collector, as long as the purpose of the present application can be achieved. For example, the positive electrode current collector may include aluminum foil, aluminum alloy foil or a composite current collector (such as an aluminum-carbon composite current collector), etc.

[0037] The positive electrode material layer of the present application includes a positive electrode active material. The present application does not particularly limit the type of positive electrode active material, as long as the purpose of the present application can be achieved. For example, the positive electrode active material may include at least one of lithium nickel cobalt manganese oxide (NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based material, lithium cobalt oxide (LiCoO2), lithium manganese oxide, lithium iron manganese phosphate or lithium titanate. In the present application, the positive electrode active material may also include non-metallic elements, for example, non-metallic elements include at least one of fluorine, phosphorus, boron, chlorine, silicon or sulfur. In the present application, there is no particular restriction on the thickness of the positive electrode current collector and the positive electrode material layer, as long as the purpose of the present application can be achieved. For example, the thickness of the positive electrode current collector is 5μm to 20μm, preferably 6μm to 18μm. The thickness of the single-sided positive electrode material layer is 30μm to 120μm.

[0038] In the present application, the positive electrode material layer may further include a conductive agent and a binder. The present application does not particularly limit the type of binder in the positive electrode material layer, as long as the purpose of the present application can be achieved. For example, the binder may include but is not limited to at least one of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinyl pyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene or polyhexafluoropropylene. The present application does not particularly limit the type of conductive agent in the positive electrode material layer, as long as the purpose of the present application can be achieved. For example, the 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, Ketjen black, graphene, metal materials or conductive polymers. The above-mentioned carbon nanotubes may include but are not limited to single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The above-mentioned carbon fibers may include but are not limited to vapor-grown carbon fibers (VGCF) and / or nano-carbon fibers. The above-mentioned metal materials may include but are not limited to metal powder and / or metal fibers. Specifically, the metal may include but is not limited to at least one of copper, nickel, aluminum or silver. The conductive polymer may include, but is not limited to, at least one of a polyphenylene derivative, polyaniline, polythiophene, polyacetylene, or polypyrrole. The present application does not particularly limit the mass ratio of the positive electrode active material, conductive agent, and binder in the positive electrode material layer. Those skilled in the art may select the ratio based on actual needs, as long as the objectives of the present application are achieved.

[0039] The present application has no particular restrictions on the negative electrode sheet, as long as the purpose of the present application can be achieved. For example, the negative electrode sheet includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. The above-mentioned "negative electrode material layer disposed on at least one surface of the negative electrode current collector" means that the negative electrode material layer can be disposed on one surface of the negative electrode current collector along its own thickness direction, or on two surfaces of the negative electrode current collector along its own thickness direction. It should be noted that the "surface" here can be the entire area of ​​the surface of the negative electrode current collector, or it can be a partial area of ​​the surface of the negative electrode current collector. The present application has no particular restrictions, as long as the purpose of the present application can be achieved. The present application has no particular restrictions on the negative electrode current collector, as long as the purpose of the present application can be achieved. For example, the negative electrode current collector can include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foamed nickel, foamed copper or a composite current collector (such as a lithium-copper composite current collector, a carbon-copper composite current collector, a nickel-copper composite current collector, a titanium-copper composite current collector, etc.). The negative electrode material layer of the present application includes a negative electrode active material. The present application has no particular restrictions on the type of negative electrode active material, as long as it can achieve the purpose of the present application. For example, the negative electrode active material can include natural graphite, artificial graphite, mesophase microcarbon beads (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, SiO x(0 < x < 2), at least one of Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel-structured lithium titanate Li4Ti5O 12 , Li-Al alloy or metallic lithium. In this application, there is no particular limitation on the thickness of the negative electrode current collector and the negative electrode material layer, as long as the object of this application can be achieved. For example, the thickness of the negative electrode current collector is 4 μm to 20 μm, and the thickness of the negative electrode active material layer is 30 μm to 130 μm. Optionally, the negative electrode active material layer may further include at least one of a conductive agent, a stabilizer, and a binder. In this application, there is no particular limitation on the types of the conductive agent, the stabilizer, and the binder in the negative electrode active material layer, as long as the object of this application can be achieved. In this application, there is no particular limitation on the mass ratio of the negative electrode active material, the conductive agent, the stabilizer, and the binder in the negative electrode material layer, as long as the object of this application can be achieved.

[0040] In the present application, the electrochemical device includes an electrolyte, and the electrolyte includes, in addition to fluoroethylene carbonate, a lithium salt and a non-aqueous solvent. The present application has no particular restrictions on the lithium salt, as long as the purpose of the present application can be achieved. For example, the lithium salt may include, but is not limited to, at least one of LiPF6, LiBF4, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalatoborate) (LiBOB) or lithium difluoroborate. The present application has no particular restrictions on the content of the lithium salt in the electrolyte, as long as the purpose of the present application can be achieved. For example, based on the mass of the electrolyte, the mass percentage of the lithium salt is 5% to 23%, for example, the concentration of the lithium salt in the electrolyte can be 5%, 8%, 12%, 16%, 20%, 23% or a range consisting of any two of the above values. The application has no particular restrictions on non-aqueous solvents, as long as the purpose of the application can be achieved. For example, non-aqueous solvents may include but are not limited to at least one of carbonate compounds, carboxylate compounds, ether compounds or other organic solvents. Above-mentioned carbonate compounds may include but are not limited to at least one of linear carbonate compounds, cyclic carbonate compounds or fluorinated carbonate compounds. Above-mentioned linear carbonate compounds may include but are not limited to at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC) or methyl ethyl carbonate (MEC). Above-mentioned cyclic carbonates may include but are not limited to at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) or vinyl ethylene carbonate (VEC). The fluorinated carbonate compound may include, but is not limited to, at least one of 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 above-mentioned carboxylate compound may include, but is 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, decanoic acid lactone, valerolactone, or caprolactone. The ether compound may include, but is not limited to, at least one of dibutyl ether, tetraglyme, diglyme, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran.The aforementioned other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate. The present application does not particularly limit the content of the non-aqueous solvent in the electrolyte, as long as the objectives of the present application are achieved. The non-aqueous solvent content is calculated as follows: 100% - lithium salt content - fluoroethylene carbonate content.

[0041] The electrochemical device of the present application is not particularly limited and may include any device that undergoes an electrochemical reaction. In one embodiment of the present application, the electrochemical device may include, but is not limited to, a lithium ion secondary battery (lithium ion battery), a lithium polymer secondary battery, or a lithium ion polymer secondary battery.

[0042] The preparation process of the electrochemical device of the present application is well known to those skilled in the art and is not particularly limited herein. For example, it may include, but is not limited to, the following steps: stacking the positive electrode sheet, separator, and negative electrode sheet in sequence, and winding, folding, and other operations as needed to obtain a wound electrode assembly; placing the electrode assembly in a packaging bag, injecting an electrolyte into the packaging bag, and sealing the bag to obtain the electrochemical device; or stacking the positive electrode sheet, separator, and negative electrode sheet in sequence, and then securing the four corners of the entire stack with tape to obtain a stacked electrode assembly; placing the electrode assembly in a packaging bag, injecting an electrolyte into the packaging bag, and sealing the bag to obtain the electrochemical device. Furthermore, overcurrent protection elements, guide plates, and the like may be placed in the packaging bag as needed to prevent pressure buildup and overcharging and discharging within the electrochemical device. The present application does not limit the packaging bag, and those skilled in the art may select a suitable packaging bag based on actual needs, as long as it achieves the objectives of the present application. For example, an aluminum-plastic film packaging bag may be used.

[0043] The second aspect of the present application provides an electronic device, which includes the electrochemical device of any of the aforementioned embodiments. The electrochemical device provided by the present application has good high-temperature safety performance, and thus the electronic device provided by the present application has good high-temperature safety performance.

[0044] The electronic device of the present application is not particularly limited and can be any electronic device known in the art. For example, the electronic device can include, but is not limited to, a laptop computer, a pen-type computer, a mobile computer, an electronic book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a headset, a video recorder, an LCD television, a portable cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, a power tool, a flashlight, a camera, a large household battery, and a lithium-ion capacitor.

[0045] Example

[0046] Hereinafter, the embodiments of the present application will be described in more detail with reference to Examples and Comparative Examples. Various tests and evaluations were performed according to the following methods.

[0047] Test methods and equipment:

[0048] Melting enthalpy test:

[0049] The heat of fusion and melting point of the first polyethylene resin material, the second polyethylene resin material, and the high-temperature resistant resin in each embodiment and comparative example were measured using a differential scanning calorimeter. 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 to obtain a differential scanning calorimetry curve (DSC curve). The heat of fusion of the sample material was calculated based on the area enclosed by the melting peak in the DSC curve. The peak temperature of the melting peak in the DSC curve was used as the melting point of the sample material.

[0050] Melting enthalpy = integrated area of ​​melting endotherm curve / mass.

[0051] Tests on diaphragm closing rate, closing temperature, maximum resistance and melting temperature:

[0052] The closed-cell temperature (T1) of the diaphragms in each example and comparative example was tested using the temperature-rise internal resistance method. The diaphragms were cut into a fixed size of 5 cm x 5 cm and placed in a fixture consisting of ceramic and stainless steel. 5 g of electrolyte was injected into the fixture. The fixture was then placed in a 250°C oven and heated at a rate of 10°C / min. The fixture's resistance and temperature were monitored simultaneously, and data such as the fixture's resistance and temperature, oven temperature, and time were output.

[0053] According to the time-resistance curve, the diaphragm closing rate is calculated using the following formula:

[0054] Closure rate = (R2-R1) / (t2-t1); where R1 is the resistance value when the fixture starts to increase, R2 is the highest resistance value measured by the fixture, t1 is the time when the resistance value starts to increase, and t2 is the time when the resistance value reaches the maximum.

[0055] According to the temperature-resistance curve, the temperature when the resistance suddenly increases (reaches 1000Ω) is the diaphragm's closed-pore temperature T1; the maximum resistance value is the diaphragm's maximum resistance.

[0056] On the basis of the above test of the diaphragm closed-cell temperature, the test time is continued to increase. When the resistance value suddenly drops to the same as the closed-cell temperature resistance, the temperature at this time is the diaphragm's melting temperature T2.

[0057] The composition and preparation method of the electrolyte in the test are the same as those in Example 1-1.

[0058] Melt index test:

[0059] The melt index of the first polyethylene resin material in each example and comparative example was measured using a melt flow rate meter. The instrument was heated to 190°C, a standard die with a diameter of 2.095 ± 0.005 mm was placed in the barrel, and the resin material was compacted. The temperature was maintained for 10 minutes. The sample material cutting time was set to 60 seconds, and the resin material was extruded under pressure (21.6 kg). The melt index of the first polyethylene resin material was calculated as: MFR1 = mT / t. Where t is the sample material cutting time (60 seconds), m is the sample mass, and T is 600 seconds.

[0060] The first polyethylene resin material is replaced with a second polyethylene resin material and the MFR2 of the second polyethylene resin material is obtained according to the above test method.

[0061] Ionic conductivity test of the diaphragm:

[0062] (1) Diaphragm preparation: Cut the diaphragms to be tested into the same size (45.3 mm × 33.7 mm), dry them at 60°C for more than 4 h, and then transfer them to a glove box for later use;

[0063] (2) Preparation of blank symmetrical lithium-ion battery packaging bags: Use copper foil against copper foil as the current collector to assemble blank symmetrical lithium-ion battery packaging bags without a separator. The packaging bags must be dried at 60°C for more than 4 hours before use, and then transferred to a glove box for standby use.

[0064] (3) Assembly of symmetrical lithium-ion batteries: Assemble symmetrical batteries with graphite electrodes against graphite electrodes in a blank symmetrical lithium-ion battery packaging bag, and assemble different layers (1, 2, 3, 4, 5 layers) of separators in situ in the middle of the symmetrical electrodes in a glove box to form symmetrical lithium-ion batteries (5 parallel samples of symmetrical batteries with each layer number are assembled); use a simple packaging machine to seal the packaging bag on the side, inject the electrolyte (300 μL) with a pipette, and seal the bottom;

[0065] (4) Symmetrical battery loading fixture: Place the assembled symmetrical lithium-ion battery in a glove box overnight to allow the electrolyte to fully infiltrate the diaphragm;

[0066] (5) Electrochemical Impedance Spectroscopy (EIS) Test: Before measuring the EIS, symmetrical lithium-ion batteries with different numbers of separator layers were placed in a temperature chamber at 25°C for half an hour. The EIS at the set temperature was measured; the EIS conditions were set to 1 kHz and the perturbation voltage was set to 5 mV. The ionic conductivity S of the separator was calculated based on the test results.

[0067] The resistance values ​​obtained from symmetrical batteries with different numbers of diaphragm layers are plotted against the number of layers n, and then a linear fit is performed. The slope is the resistance of the diaphragm. The ionic conductivity S of the diaphragm can be calculated according to the following formula.

[0068] Ionic conductivity S = membrane thickness / (membrane resistance × effective area of ​​the membrane that can transmit ions when measuring impedance).

[0069] Among them, the effective area of ​​the diaphragm that can transmit ions when testing impedance is 153.86mm 2 The composition and preparation method of the negative electrode plate and electrolyte in the test are the same as those in Example 1-1.

[0070] Diaphragm porosity:

[0071] Porosity = 1-W / ρV.

[0072] Wherein, W is the test membrane weight; ρ is the density of the porous substrate, which is obtained by weighted average of the densities of the various materials, ρ = polyethylene ratio × polyethylene material density + high temperature resistant resin ratio × high temperature resistant resin density; V is the volume when the membrane weight is tested; V = membrane surface area × membrane thickness.

[0073] Conductivity of electrolyte:

[0074] The conductivity of the electrolyte is measured using a conductivity tester. Two parallel platinum electrodes with a distance L and a cross-sectional area A are placed in the electrolyte to be measured. The resistance R between the two electrodes is measured and the conductivity of the electrolyte is calculated using the formula: L / (R×A); where L is 1 cm and A is 0.25 cm 2 .

[0075] Diaphragm tortuosity test:

[0076] After measuring the ionic conductivity of the diaphragm, the tortuosity value is calculated according to the following formula:

[0077] Hot box test:

[0078] The lithium-ion batteries in each example and comparative example were charged at room temperature at a constant current rate of 0.5C to a full charge voltage of 4.5V. Charging was continued at a constant voltage of 4.5V to a cutoff current of 0.05C, bringing them to a fully charged state. The appearance was inspected to ensure that the lithium-ion batteries were in normal usable condition. The fully charged batteries were placed in an oven and heated at a rate of 5°C / min until reaching the designated hot box test temperature of 135°C. The temperature was maintained for one hour, during which the battery condition was observed.

[0079] Judgment standard: The battery does not catch fire or explode.

[0080] Hot box test pass rate = number of hot box tests passed / total number.

[0081] Low temperature discharge capacity test:

[0082] The lithium-ion batteries in each embodiment and each comparative example were tested using the following method, and the low-temperature discharge capacity of the lithium-ion batteries was calculated.

[0083] The lithium-ion battery was placed at 25°C for 30 minutes and discharged at a constant current rate of 0.2C until the final voltage was 3.0V. The lithium-ion battery was further fully charged and charged at a constant current of 1.5C to a full charge voltage of 4.45V. Subsequently, constant voltage charging was performed at the maximum voltage until the current was 0.02C, and then constant current discharge was performed at a discharge current of 1.0C until the final voltage was 3.0V. The discharge capacity at 25°C was recorded as C0.

[0084] The battery was fully charged at 25°C and charged at a constant current of 1.5C to a full charge voltage of 4.45V. It was then charged at a constant voltage at the maximum voltage until the current reached 0.02C. The fully charged battery was placed at -20°C for 60 minutes and then discharged at a constant current of 1.0C until the final voltage reached 3.0V. The discharge capacity at -20°C was C1.

[0085] Discharge capacity rate at -20°C = C1 / C0×100%.

[0086] Normal temperature cycle performance test:

[0087] The lithium ion batteries of the embodiments and comparative examples were repeatedly charged and discharged by the following steps, and the discharge capacity retention rates of the lithium ion batteries were calculated.

[0088] In an environment of 25°C, the first charge and discharge were carried out. Constant current charging was performed at a charging current of 2C to the full charge voltage (the battery was designed for a maximum voltage of 4.45V). Then, constant voltage charging was performed at the maximum voltage until the current was 0.02C. Then, constant current discharge was performed at a discharge current of 0.5C until the final voltage was 3.0V. The discharge capacity of the first cycle was recorded. After that, the above steps were repeated for 1000 charge and discharge cycles, and the discharge capacity of the 1000th cycle was recorded.

[0089] Cycle capacity retention rate=(discharge capacity at the 1000th cycle / discharge capacity at the first cycle)×100%.

[0090] Example 1-1

[0091] <Preparation of Separator>

[0092] A first polyethylene resin material (mass percentage x=10%, melting enthalpy H1=145 J / g, melt index MFR1=15 g / 10 min), a second polyethylene resin material (mass percentage y=88%, melting enthalpy H2=190 J / g, melt index MFR2=1 g / 10 min) and polypropylene (mass percentage z=2%) were uniformly mixed to obtain a polyolefin material, and an additive tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid] pentylene glycol was added. Tetraol ester and solvent paraffin oil are evenly mixed in a ratio of 0.2:100 to obtain a mixed solvent; then the polyolefin material and the mixed solvent are evenly mixed in a ratio of 30:70, added to the extruder system, extruded through a T-die, and cast and cooled to form a film to obtain a substrate; the above-mentioned substrate is longitudinally stretched and transversely stretched to 7 times, extracted with dichloromethane at 25°C for 0.5h, and dried to obtain a porous substrate; the porous substrate is stretched twice by 1 times, heat-set at 120°C, and rolled to obtain the desired diaphragm with a thickness of 7μm.

[0093] <Preparation of positive electrode sheet>

[0094] The active material, lithium cobalt oxide, conductive carbon, and binder polyvinylidene fluoride (PVDF), were mixed in a mass ratio of 96:2:2. N-methylpyrrolidone (NMP) was added as a solvent and thoroughly stirred to form a slurry with a solid content of 75 wt%. The slurry was evenly coated on one surface of a 10 μm thick positive electrode current collector aluminum foil and dried at 90°C to obtain a positive electrode sheet with a single-sided positive electrode material layer of 48 μm thickness. After completing the above steps, the single-sided coating of the positive electrode sheet was completed. Then, the above steps were repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided positive electrode material layer. After coating, the positive electrode sheet was cut into 74 mm × 86 mm specifications for future use.

[0095] <Preparation of negative electrode sheet>

[0096] Active material artificial graphite, conductive agent conductive carbon black (Super P), binder styrene-butadiene rubber (SBR), and thickener sodium carboxymethyl cellulose (CMC) were mixed in a mass ratio of 95:1:3:1. Deionized water was added as a solvent and thoroughly stirred to form a slurry with a solid content of 70 wt%. The slurry was evenly coated on one surface of an 8 μm thick negative electrode current collector copper foil and dried at 110°C to produce a negative electrode sheet with a coating thickness of 50 μm. After completing these steps, the negative electrode sheet was coated on one side. The above steps were then repeated on the other side of the negative electrode sheet to obtain a negative electrode sheet coated on both sides with the negative electrode active material layer. After coating, the negative electrode sheet was cut into 76 mm × 90 mm squares for future use.

[0097] <Preparation of Electrolyte>

[0098] In an argon atmosphere glove box with a water content of less than 0.001%, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) were mixed in a mass ratio of 1:1:1:1 to form a base solvent. Fluorinated ethylene carbonate and lithium hexafluorophosphate were then added and mixed thoroughly to form an electrolyte. The lithium hexafluorophosphate content was 12.5% ​​by mass based on the mass of the electrolyte, with the remainder being the base solvent.

[0099] <Preparation of lithium-ion batteries>

[0100] The positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator positioned between the positive and negative electrodes to provide insulation. The wound electrode assembly is then produced through a winding process. The electrode assembly is packaged in an aluminum-plastic film bag and dehydrated at 80°C. The prepared electrolyte is then injected and vacuum packaged, allowed to stand, formed, and shaped to produce a lithium-ion battery. The upper limit of the formation voltage is 4.15V, the formation temperature is 85°C, and the formation stand time is 2 hours.

[0101] Example 1-2 to Example 1-15

[0102] Except for adjusting the relevant preparation parameters in <Preparation of Separator> according to Table 1, the rest is the same as Example 1-1.

[0103] Example 2-1 to Example 2-16

[0104] Except for adjusting the relevant preparation parameters according to Table 2 in <Preparation of Separator>, the rest is the same as Example 1-4.

[0105] Example 3-1 to Example 3-6

[0106] Except that the content of fluoroethylene carbonate added in <Preparation of Electrolyte> was adjusted according to Table 3, the content of the base solvent was changed accordingly, and the mass percentage of the lithium salt remained unchanged, the rest was the same as in Examples 1-5.

[0107] Comparative Example 1

[0108] Except that the separator is a polyethylene (PE) single-layer film with a thickness of 7 μm, the rest is the same as Example 1-1.

[0109] Comparative Example 2

[0110] Except that the separator is a polypropylene (PP) single-layer film with a thickness of 10 μm, the rest is the same as Example 1-1.

[0111] Comparative Examples 3 to 11

[0112] Except for adjusting the relevant preparation parameters in <Preparation of Separator> according to Table 1, the rest is the same as Example 1-1.

[0113] The preparation parameters and performance parameters of each embodiment and comparative example are shown in Tables 1 to 3.

[0114] Referring to Table 1, it can be seen from Examples 1-1 to 1-15 and Comparative Examples 1 to 11 that when only one resin material is used in the separator, the closed cell rate and maximum resistance of the separators in Comparative Examples 1 and 2 are low, the closed cell effect is poor, and the low closed cell temperature and high melt temperature of the separator cannot be achieved at the same time. Although the separators have high puncture strength, the hot box test pass rates of the lithium ion batteries in Comparative Examples 1 and 2 are lower than those of the examples, and the low-temperature discharge capacity rate and cycle capacity retention rate are also lower than those of the examples. When the melting heat enthalpy H1 of the first polyethylene resin material and / or the melting heat enthalpy H2 of the second polyethylene resin material are too low, the closed cell rate and maximum resistance of the separators in Comparative Examples 4 and 6 are high, the closed cell temperature is low, and the melt temperature is high, but the puncture strength of the separator is low. The hot box test pass rates of the lithium ion batteries in Comparative Examples 4 and 6 are lower than those of the examples, and the low-temperature discharge capacity rate and cycle capacity retention rate are also lower than those of the examples. When the heat of fusion H1 of the first polyethylene resin material and / or the heat of fusion H2 of the second polyethylene resin material are too high, the diaphragms in Comparative Examples 3, 5, and 7 have low cell closing rates and maximum resistance values, resulting in poor cell closing effects. Although the diaphragms have low cell closing temperatures, high melt-down temperatures, and high puncture strengths, the hot box test pass rates of the lithium-ion batteries in Comparative Examples 3, 5, and 7 are all lower than those in the Examples, and the low-temperature discharge capacity rate and cycle capacity retention rates are also lower than those in the Examples. When the mass percentage x of the first polyethylene resin material is too low and the mass percentage y of the second polyethylene resin material is too high, the diaphragm in Comparative Example 8 has low cell closing rates and maximum resistance values, resulting in poor cell closing effects. Although the diaphragm has low cell closing temperatures, high melt-down temperatures, and high puncture strengths, the hot box test pass rates of the lithium-ion batteries in Comparative Example 8 are all lower than those in the Examples, and the low-temperature discharge capacity rate and cycle capacity retention rates are also lower than those in the Examples. When the mass percentage x of the first polyethylene resin material is too high and the mass percentage y of the second polyethylene resin material is too low, the diaphragm in Comparative Example 9 has a higher closed cell rate and maximum resistance, a lower closed cell temperature, and a higher meltdown temperature, but the diaphragm has a lower puncture strength. The hot box test pass rates of the lithium-ion batteries in Comparative Examples 4 and 6 are lower than those in the examples, and the low-temperature discharge capacity rate and cycle capacity retention rate are also lower than those in the examples. When the porous substrate contains only one polyethylene resin and a high-temperature-resistant resin, the diaphragms in Comparative Examples 10 and 11 have a lower closed cell rate and maximum resistance, and a poor closed cell effect. Although the diaphragm has a lower closed cell temperature, a higher meltdown temperature, and a higher puncture strength, the hot box test pass rates of the lithium-ion batteries in Comparative Examples 10 and 11 are lower than those in the examples, and the low-temperature discharge capacity rate and cycle capacity retention rate are also lower than those in the examples.

[0115] It can be seen from Examples 1-1 to 1-15 that by regulating the melting heat enthalpy H1 of the first polyethylene resin material, the melting heat enthalpy H2 of the second polyethylene resin material, the mass percentage x of the first polyethylene resin material and the mass percentage y of the second polyethylene resin material within the scope of this application, and making the melting heat enthalpy H1 of the first polyethylene resin material lower than the melting heat enthalpy H2 of the second polyethylene resin material, the closed cell rate of the diaphragm is within the scope of this application, the maximum resistance of the diaphragm is higher, the closed cell effect is better, and the low closed cell temperature and high melting temperature of the diaphragm can be taken into account at the same time. The puncture strength of the diaphragm is higher, and the lithium ion battery in the embodiment also has a higher hot box test pass rate, a higher low-temperature discharge capacity rate and a higher cycle capacity retention rate, indicating that the lithium ion battery in the embodiment of the present application has better high-temperature safety performance and electrochemical performance.

[0116] The closed-cell rate of the diaphragm = (R2-R1) / (t2-t1). As can be seen from Figure 1, the resistance value of Example 1-2 surged to 135kΩ within 6 minutes, and the closed-cell rate of the diaphragm was 25kΩ / min, which is relatively high. The mass percentage z of the high-temperature resistant resin usually affects the high-temperature safety performance of the lithium-ion battery. From Examples 1-1 to 1-7, and 1-12 to 1-13, it can be seen that when the mass percentage z of the high-temperature resistant resin is within the scope of this application, the closed-cell rate of the diaphragm is large, the maximum resistance of the diaphragm is large, the closed-cell temperature of the diaphragm is low and the melting temperature is high, the puncture strength is high, the hot box test pass rate of the lithium-ion battery is high, and the low-temperature discharge capacity rate and cycle capacity retention rate are high, which shows that the lithium-ion battery in this embodiment has good high-temperature safety performance and electrochemical performance.

[0117] The type of high-temperature-resistant resin typically affects the high-temperature safety performance of lithium-ion batteries. As can be seen from Examples 1-5, 1-14, and 1-15, and from Example 1-3 to 2-15 and 2-16, when the high-temperature-resistant resins described in this application are used, the separators exhibit a higher cell closure rate, a higher maximum resistance, a lower cell closure temperature, a higher meltdown temperature, and higher puncture strength. The lithium-ion batteries also exhibit a higher hot box test pass rate, and higher low-temperature discharge capacity and cycle capacity retention rates. This demonstrates that the lithium-ion batteries of these examples exhibit excellent high-temperature safety and electrochemical performance.

[0118] Referring to Table 2, the melt index (MFR1) of the first polyethylene resin material and the melt index (MFR2) of the second polyethylene resin material generally affect the high-temperature safety performance of lithium-ion batteries. As can be seen from Examples 1-4 and 2-1 to 2-16, when the melt index (MFR1) of the first polyethylene resin material and the melt index (MFR2) of the second polyethylene resin material are within the range of this application, and the melt index (MFR1) of the first polyethylene resin material is higher than the melt index (MFR2) of the second polyethylene resin material, the separator exhibits a higher cell closure rate, a higher maximum resistance, a lower cell closure temperature, a higher melt-rupture temperature, and higher puncture strength. The lithium-ion battery also exhibits a higher hot box test pass rate, and higher low-temperature discharge capacity rate and cycle capacity retention. This demonstrates that the lithium-ion battery in this embodiment exhibits excellent high-temperature safety and electrochemical performance.

[0119] Table 3

[0120] The mass percentage of fluoroethylene carbonate in the electrolyte generally affects the high-temperature safety performance of lithium-ion batteries. As can be seen from Examples 1-5 and Examples 3-1 to 3-6, when the mass percentage of fluoroethylene carbonate in the electrolyte is within the range of this application, the lithium-ion batteries have a high hot box test pass rate and high low-temperature discharge capacity rate and cycle capacity retention rate, demonstrating that the lithium-ion batteries in these examples have good high-temperature safety and electrochemical performance.

[0121] It should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or article comprising a list of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, or article.

[0122] Each embodiment in this specification is described in a related manner. The same or similar parts between the embodiments can be referred to each other. Each embodiment focuses on the differences from other embodiments.

[0123] The above description is only a preferred embodiment of the present application and is not intended to limit the scope of protection of the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application are included in the scope of protection of the present application.

Claims

1. An electrochemical device, comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the separator is disposed between the positive electrode sheet and the negative electrode sheet, the separator comprises a porous substrate, and the porous substrate comprises a first polyethylene resin material and a second polyethylene resin material; in, The melting enthalpy of the first polyethylene resin material is 110 J / g to 160 J / g, and the melting enthalpy of the second polyethylene resin material is 170 J / g to 205 J / g; Based on the mass of the porous substrate, the mass percentage of the first polyethylene resin material is 10% to 40%, and the mass percentage of the second polyethylene resin material is 45% to 88%; the closure rate of the diaphragm is 10 kΩ / min to 80 kΩ / min.

2. The electrochemical device according to claim 1, wherein: The melting enthalpy of the first polyethylene resin material is 130 J / g to 160 J / g, and the melting enthalpy of the second polyethylene resin material is 180 J / g to 205 J / g.

3. The electrochemical device according to claim 1, wherein Based on the mass of the porous substrate, the mass percentage of the first polyethylene resin material is 15% to 30%, and the mass percentage of the second polyethylene resin material is 60% to 80%.

4. The electrochemical device according to claim 1, wherein The melt index of the first polyethylene resin material is 5 g / 10 min to 20 g / 10 min; the melt index of the second polyethylene resin material is 0.1 g / 10 min to 3.0 g / 10 min.

5. The electrochemical device according to claim 1, wherein The maximum resistance of the diaphragm after high-temperature pore closure is in the range of 10 kΩ to 400 kΩ.

6. The electrochemical device according to claim 1, wherein The maximum resistance of the diaphragm after high-temperature pore closure is in the range of 40 kΩ to 400 kΩ.

7. The electrochemical device according to claim 1, wherein: The diaphragm has a closed-cell temperature of 120°C to 145°C and a melting point of 155°C to 250°C.

8. The electrochemical device according to claim 1, wherein The porous substrate also includes a high temperature resistant resin, which includes one of a crystalline polymer or an amorphous polymer. The high temperature resistant resin includes at least one of polypropylene, poly-4-methylpentene, polytetrafluoroethylene, polyvinylidene fluoride or a cycloolefin copolymer. Based on the mass of the porous substrate, the mass percentage of the high temperature resistant resin is 2% to 15%.

9. The electrochemical device according to claim 1, which satisfies at least one of the following characteristics: (a) the melt index of the first polyethylene resin material is 8 g / 10 min to 18 g / 10 min; (b) The melt index of the second polyethylene resin material is 0.2 g / 10 min to 1.5 g / 10 min.

10. The electrochemical device according to claim 1, wherein The tortuosity of the diaphragm is 1.1 to 2.

0.

11. The electrochemical device according to claim 1, wherein The closure rate of the separator is 25 kΩ / min to 80 kΩ / min.

12. The electrochemical device according to claim 1, wherein The electrolyte includes fluoroethylene carbonate, and based on the mass of the electrolyte, the mass percentage of the fluoroethylene carbonate is 0.1% to 12%.

13. The electrochemical device according to claim 1, wherein The electrolyte includes fluoroethylene carbonate, and based on the mass of the electrolyte, the mass percentage of the fluoroethylene carbonate is 0.1% to 5%.

14. An electronic device comprising the electrochemical device according to any one of claims 1 to 13.

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