Secondary battery and electronic apparatus

Abstract

A secondary battery and an electronic apparatus. The secondary battery comprises a positive electrode sheet, a negative electrode sheet and a separator, the separator comprising a base film and a coating provided on at least one surface of the base film. The coating comprises a coating material, the coating can melt and flow when the temperature is greater than or equal to 90°C, the coating weight of a single-sided coating is CW g / m2, the density of the coating material is ρ g / cm3, the porosity of the base film is P%, the thickness of the base film is H μm, and 50%≤CW / (ρ×P×H)×100%≤120%, wherein 0.9≤ρ≤1.05, 20≤P≤50, and 3≤H≤10. The separator in the secondary battery can improve the thermal safety performance of the secondary battery.

Description

Secondary battery and electronic device

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on December 19, 2023, with application number 202311768461.9 and invention name “A Secondary Battery 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 a secondary battery and an electronic device. Background Art

[0003] Secondary batteries, 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 thermal safety performance of lithium-ion batteries.

[0005] Summary of the Invention

[0006] The purpose of this application is to provide a secondary battery and an electronic device to improve the thermal safety performance of the secondary battery. The specific technical solution is as follows:

[0007] It should be noted that, in the invention content of this application, lithium-ion batteries are used as an example of secondary batteries to explain this application, but the secondary batteries of this application are not limited to lithium-ion batteries.

[0008] The first aspect of the present application provides a secondary battery comprising a positive electrode sheet, a negative electrode sheet and a separator, wherein the separator comprises a base film and a coating provided on at least one surface of the base film, wherein the coating comprises a coating material, and the coating is melt-flowable at a temperature greater than or equal to 90°C; the coating weight of the coating on a single side is CW g / m 2 , the density of the coating material is ρg / cm 3, the porosity of the base film is P%, the thickness of the base film is Hμm, 50%≤CW / (ρ×P×H)×100%≤120%; in some embodiments of the present application, 55%≤CW / (ρ×P×H)×100%≤100%. Among them, 0.9≤ρ≤1.05, 20≤P≤50, 3≤H≤10. Furthermore, 0.5≤CW≤2. The secondary battery provided in the present application includes a diaphragm having the above characteristics, and the values ​​of CW / (ρ×P×H)×100%, ρ, P, and H are regulated within the above ranges. When the secondary battery is in thermal runaway, the coating material meeting the above density range is adapted to the base film meeting the above porosity range and the above thickness range. The coating on the surface of the base film will have suitable fluidity. The coating cooperates with the base film to enable the diaphragm to achieve rapid pore closure and inhibit the occurrence of thermal runaway of the secondary battery, thereby improving the thermal safety performance of the secondary battery. By further adjusting the CW value, the thermal safety performance of the secondary battery can be effectively improved. At the same time, when the secondary battery is operating normally, other performance of the secondary battery, such as dynamic performance and energy density, will not be affected.

[0009] In some embodiments of the present application, the closed-pore temperature T1 of the coating is between 110° C. and 130° C. By regulating the closed-pore temperature of the coating within the above range, the coating can melt and quickly fill the pores of the base film when there is a risk of thermal runaway in the secondary battery, achieving a good closed-pore effect, improving the thermal shutdown performance of the separator at high temperatures, preventing the transmission of lithium ions and the diffusion of heat and substances between the positive and negative electrode sheets, suppressing thermal runaway of the secondary battery, and further improving the thermal safety performance of the secondary battery.

[0010] In some embodiments of the present application, after the separator is placed at 120°C for 20 minutes, the coating, after melting, covers an area of ​​greater than or equal to 80% of the base film. An area ratio within the above range indicates that the coating, after melting, can substantially cover the surface of the base film. Thus, when there is a risk of thermal runaway in the secondary battery, the coating can fill most of the pores on the surface of the base film, achieving a good closed-pore effect, improving the thermal shutdown performance of the separator at high temperatures, preventing the transmission of lithium ions and the diffusion of heat and substances between the positive and negative electrode sheets, suppressing thermal runaway in the secondary battery, and further improving the thermal safety performance of the secondary battery.

[0011] In some embodiments of the present application, the closed-pore temperature T0 of the separator is between 132°C and 142°C, and the closed-pore temperature of the base film is T2, where 3°C ≤ T2 - T0 ≤ 10°C. By regulating the closed-pore temperature T0 of the separator and the difference between the closed-pore temperature of the base film and the closed-pore temperature of the separator within the above range, the low-closed-pore coating can be caused to melt and flow first, blocking the pores of the base film, achieving a good closed-pore effect, improving the thermal shutdown performance of the separator at high temperatures, preventing the transmission of lithium ions and the diffusion of heat and substances between the positive and negative electrode sheets, suppressing thermal runaway of the secondary battery, and further improving the thermal safety performance of the secondary battery.

[0012] In some embodiments of the present application, 135°C ≤ T2 ≤ 150°C. By regulating the closed-pore temperature of the base film within the above range, when the secondary battery is at risk of thermal runaway, the base film can close itself, and the coating can also fill the pores of the base film, achieving a better closed-pore effect, further improving the thermal shutdown performance of the separator at high temperatures, preventing the transmission of lithium ions and the diffusion of heat and substances between the positive and negative electrode sheets, suppressing thermal runaway of the secondary battery, and further improving the thermal safety performance of the secondary battery.

[0013] In some embodiments of the present application, the coating material satisfies at least one of the following characteristics:

[0014] (1) Melting point Tm is 95°C to 125°C;

[0015] (2) The melt index MFR is 12 g / 10 min to 60 g / 10 min.

[0016] The coating material satisfies at least one of the above characteristics, which can further improve the thermal safety performance of the secondary battery.

[0017] In some embodiments of the present application, the coating material includes at least one of polyethylene wax or core-shell structured microspheres; the core-shell structured microspheres include a core material and a shell material, the difference in glass transition temperature between the shell material and the core material is 1.2°C to 10°C, the core material includes at least one of polyethylene wax, polyethylene, polyvinyl acetate, and polymethyl acrylate, and the shell material includes at least one of polymethyl methacrylate, polystyrene, and polyacrylic acid. By selecting the above coating materials, a coating with a closed-cell temperature T1 of 110°C to 130°C can be obtained, thereby cooperating with the base film to achieve a good closed-cell effect when there is a risk of thermal runaway in the secondary battery, improving the thermal shutdown performance of the diaphragm at high temperatures, preventing the transmission of lithium ions, and the diffusion of heat and substances between the positive and negative electrode sheets, inhibiting thermal runaway of the secondary battery, and further improving the thermal safety performance of the secondary battery.

[0018] In some embodiments of the present application, the average particle size D of the coating material is 0.2 μm to 1.5 μm. By regulating the average particle size D of the coating material within the above range, the coating has a suitable thickness, which improves the thermal safety performance of the secondary battery while having little impact on its energy density.

[0019] In some embodiments of the present application, the ratio of the puncture strength of the separator to the thickness of the separator is F / H, where the unit of the ratio is gf / μm, and 40 gf / μm ≤ F / H ≤ 150 gf / μm. A ratio of the puncture strength of the separator to the thickness of the separator within the above range indicates that the separator also has good puncture resistance, thereby improving the thermal safety performance of the secondary battery while also having good mechanical safety performance.

[0020] In some embodiments of the present application, the diaphragm satisfies at least one of the following characteristics:

[0021] (1) The base film comprises at least one of a polyethylene base film, a polypropylene base film, a polyethylene / polypropylene mixed film, a polypropylene / polyethylene / polypropylene composite base film, and a polyvinylidene fluoride film;

[0022] (2) The coating further comprises a coating binder, wherein the mass ratio of the coating material to the coating binder is (85 to 97):(3 to 15), and the coating binder comprises at least one of polyacrylic acid, polymethacrylic acid, polymethyl methacrylate, acrylate, styrene-acrylate emulsion, acrylate emulsion, lithium polyacrylate, polyvinyl alcohol, carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, polyimide, polyamide-imide, styrene-butadiene rubber, or polyvinylidene fluoride;

[0023] (3) The diaphragm further includes a ceramic coating, which is disposed on one surface of the base film and faces the positive electrode plate, and the ceramic coating is disposed on the other surface of the base film and faces the negative electrode plate.

[0024] The separator satisfies at least one of the above characteristics, and can further improve the thermal safety performance of the secondary battery.

[0025] A second aspect of the present application provides an electronic device comprising the secondary battery according to any one of the aforementioned embodiments.

[0026] Beneficial effects of this application:

[0027] The present application provides a secondary battery and an electronic device. The secondary battery includes a positive electrode sheet, a negative electrode sheet, and a separator. The separator includes a base film and a coating provided on at least one surface of the base film. The coating comprises a coating material. The coating can melt and flow at a temperature greater than or equal to 90°C. The coating weight of the single-side coating is CW g / m2 , the density of the coating material is ρg / cm 3 The porosity of the base film is P%, the thickness of the base film is H μm, 50% ≤ CW / (ρ×P×H)×100% ≤ 120%; wherein, 0.9 ≤ ρ ≤ 1.05, 20 ≤ P ≤ 50, and 3 ≤ H ≤ 10. The separator of the secondary battery provided in this application includes the above-mentioned features, which can improve the thermal safety performance of the secondary battery.

[0028] 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

[0029] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. 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.

[0030] FIG1 is a DSC curve of the coating material of Example 1-2;

[0031] FIG2 is a temperature-resistance curve of the diaphragm of Example 1-2;

[0032] FIG3 is an electron microscope photograph of a sample obtained after the separator of Example 1-2 was placed in an oven at 120° C. for 20 minutes. DETAILED DESCRIPTION

[0033] The following will be combined with the accompanying drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the embodiments described are only part of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0034] It should be noted that, in the invention content of this application, lithium-ion batteries are used as an example of secondary batteries to explain this application, but the secondary batteries of this application are not limited to lithium-ion batteries.

[0035] The first aspect of the present application provides a secondary battery comprising a positive electrode sheet, a negative electrode sheet and a separator, wherein the separator comprises a base film and a coating provided on at least one surface of the base film, the coating comprising a coating material, the coating being meltable and flowable at a temperature greater than or equal to 90°C; the coating weight of the single-sided coating is CW g / m 2 , the density of the coating material is ρg / cm 3, the porosity of the base film is P%, the thickness of the base film is H μm, and 50% ≤ CW / (ρ×P×H)×100% ≤ 120%. In some embodiments of the present application, 55% ≤ CW / (ρ×P×H)×100% ≤ 100%. For example, the value of CW / (ρ×P×H)×100% can be 50%, 53%, 55%, 59%, 64%, 68%, 72%, 77%, 80%, 86%, 91%, 97%, 103%, 106%, 111%, 118%, 120%, or a range consisting of any two values ​​therebetween. Among them, 0.9≤ρ≤1.05, for example, ρ can be 0.9, 0.93, 0.97, 0.99, 1.01, 1.03, 1.05 or a range consisting of any two values ​​therebetween; 20≤P≤50, for example, it can be 20, 23, 27, 30, 34, 38, 40, 42, 45, 48, 50 or a range consisting of any two values ​​therebetween; 3≤H≤10, for example, it can be 3, 4, 5, 6, 7, 8, 9, 10 or a range consisting of any two values ​​therebetween.

[0036] Normally, the base membrane has a porous structure. After the coating is set on the surface of the base membrane, there are still pores that allow active ions (such as lithium ions) in the secondary battery to pass smoothly. When the temperature of the secondary battery is at risk of thermal runaway, if the above pores are not blocked in time, the secondary battery will experience thermal runaway. This application uses the synergistic effect of the base membrane and the coating. When the temperature of the secondary battery is at risk of thermal runaway, for example, when the temperature is greater than or equal to 90°C, the coating can melt quickly and have a high coverage rate on the surface of the base membrane. At the same time, the fluid after the coating is melted can fill the pores of the base membrane, achieving a good closed-pore effect, improving the thermal shutdown performance of the diaphragm at high temperatures, preventing the transmission of lithium ions, and the diffusion of heat and substances between the positive electrode and the negative electrode, inhibiting the thermal runaway of the secondary battery, thereby improving the thermal safety performance of the secondary battery. The density of the coating material is often related to the type of coating material selected. Typically, if the coating material type remains unchanged and its density is too low, for example, ρ is less than 0.9, the coating material is prone to melt flow at relatively low temperatures (e.g., when the secondary battery is not at risk of thermal runaway), blocking the pores of the base film, thereby affecting the secondary battery's charge and discharge performance, such as at low temperatures. If the coating material type remains unchanged and its density is too high, for example, ρ is greater than 1.05, the coating material exhibits poor fluidity at higher temperatures (e.g., when the secondary battery is at risk of thermal runaway), failing to effectively fill the base film pores and suppressing thermal runaway and improving the secondary battery's safety performance. If the base film porosity is too low, for example, ρ is less than 20, it can affect the shuttle of lithium ions between the positive and negative electrodes, thereby affecting the secondary battery's kinetic performance. If ρ is too high, for example, ρ is greater than 50, it can affect the closed-pore effect, thereby affecting the secondary battery's thermal safety performance. When the thickness of the base film is too small, for example, H is less than 3, the strength of the base film is low, and the processing technology requirements are high, making it difficult to achieve industrial production; when the thickness of the base film is too large, for example, H is greater than 10, the overall thickness of the diaphragm increases, thereby affecting the energy density of the secondary battery. Therefore, the secondary battery provided by the present application includes a diaphragm having the above characteristics, and the values ​​of CW / (ρ×P×H)×100%, ρ, P, and H are regulated within the above ranges. When the secondary battery is at risk of thermal runaway, the coating on the surface of the base film cooperates with the base film to enable the diaphragm to achieve rapid pore closure and suppress the occurrence of thermal runaway of the secondary battery, thereby improving the thermal safety performance of the secondary battery. At the same time, when the secondary battery is operating normally, it basically does not affect other properties of the secondary battery, such as dynamic performance, energy density, low-temperature charge and discharge performance, and processing performance. In this application, the above-mentioned "low temperature" refers to a temperature less than or equal to 30°C, and the above-mentioned "high temperature" refers to a temperature greater than or equal to 90°C.

[0037] In some embodiments of the present application, 0.5≤CW≤2. For example, CW can be 0.5, 0.8, 1.1, 1.3, 1.4, 1.7, 1.9, 2, or a range consisting of any two values ​​therebetween. By regulating the CW value within the above range, the thermal safety performance of the secondary battery can be further improved while taking into account its energy density.

[0038] In some embodiments of the present application, the closed-pore temperature T1 of the coating is 110°C to 130°C. For example, the closed-pore temperature T1 of the coating can be 110°C, 113°C, 115°C, 117°C, 120°C, 122°C, 125°C, 128°C, 130°C, or a range consisting of any two values ​​therebetween. By regulating the closed-pore temperature of the coating within the above range, the coating can melt and quickly fill the pores of the base film when there is a risk of thermal runaway in the secondary battery, thereby achieving a good closed-pore effect, improving the thermal shutdown performance of the diaphragm at high temperatures, preventing the transmission of lithium ions, and the diffusion of heat and substances between the positive electrode and the negative electrode, thereby suppressing thermal runaway of the secondary battery and further improving the thermal safety performance of the secondary battery.

[0039] In some embodiments of the present application, after the diaphragm is placed at 120°C for 20 minutes, the area ratio X of the coating covering the base film after melting is greater than or equal to 80%. For example, the area ratio X can be 80%, 83%, 85%, 88%, 90%, 92%, 95%, 97%, 100% or a range consisting of any two values ​​therebetween. The area ratio X is within the above range, indicating that the coating can basically cover the surface of the base film after melting, so that the coating can fill most of the pores on the surface of the base film when there is a risk of thermal runaway in the secondary battery, achieve a good closed-pore effect, improve the thermal shutdown performance of the diaphragm at high temperatures, prevent the transmission of lithium ions, and the diffusion of heat and substances between the positive electrode and the negative electrode, inhibit the thermal runaway of the secondary battery, and further improve the thermal safety performance of the secondary battery.

[0040] In some embodiments of the present application, the closed-pore temperature T0 of the separator is 132°C to 142°C, the closed-pore temperature of the base film is T2, and 3°C ≤ T2-T0 ≤ 10°C. For example, the closed-pore temperature T0 of the separator can be 132°C, 133°C, 134°C, 135°C, 136°C, 137°C, 138°C, 139°C, 140°C, 141°C, 142°C, or a range consisting of any two values ​​therebetween; for example, the value of T2-T0 can be 3°C, 4°C, 5°C, 6°C, 7°C, 8°C, 9°C, 10°C, or a range consisting of any two values ​​therebetween. By regulating the closed-pore temperature T0 of the separator and the difference between the closed-pore temperature of the base film and the closed-pore temperature of the separator within the above range, the low closed-pore coating can be melted and flowed first, blocking the pores of the base film, thereby further improving the thermal safety performance of the secondary battery.

[0041] In some embodiments of the present application, 135°C ≤ T2 ≤ 150°C. For example, the closed-pore temperature T2 of the base film can be 135°C, 136°C, 137°C, 138°C, 139°C, 140°C, 141°C, 142°C, 143°C, 144°C, 145°C, 146°C, 147°C, 148°C, 149°C, 150°C, or a range consisting of any two values ​​therebetween. By regulating the closed-pore temperature of the base film within the above range, when the secondary battery is at risk of thermal runaway, on the one hand, the base film can close itself, and on the other hand, the coating can also fill the pores of the base film to achieve a better closed-pore effect, further improving the thermal shutdown performance of the diaphragm at high temperatures, preventing the transmission of lithium ions, as well as the diffusion of heat and substances between the positive and negative electrode sheets, inhibiting thermal runaway of the secondary battery, and further improving the thermal safety performance of the secondary battery.

[0042] In some embodiments of the present application, the melting point Tm of the coating material is 95°C to 125°C; for example, the melting point Tm of the coating material can be 95°C, 98°C, 100°C, 103°C, 106°C, 109°C, 110°C, 114°C, 118°C, 120°C, 123°C, 125°C or a range consisting of any two values ​​therebetween. By regulating the melting point of the coating material within the above range, a coating with a closed-pore temperature T1 of 110°C to 130°C can be obtained, and the coating can melt and quickly fill the pores of the base film when there is a risk of thermal runaway in the secondary battery, thereby achieving a good closed-pore effect, improving the thermal shutdown performance of the diaphragm at high temperatures, preventing the transmission of lithium ions, and the diffusion of heat and substances between the positive and negative electrode sheets, inhibiting thermal runaway of the secondary battery, and further improving the thermal safety performance of the secondary battery. The present application does not particularly limit the method of regulating the melting point of the coating material, as long as the purpose of the present application can be achieved. For example, different coating materials can be selected to regulate the melting point of the coating material.

[0043] In some embodiments of the present application, the coating material has a melt index (MFR) of 12 g / 10 min to 60 g / 10 min. For example, the coating material has a melt index (MFR) of 12 g / 10 min, 15 g / 10 min, 18 g / 10 min, 20 g / 10 min, 23 g / 10 min, 37 g / 10 min, 30 g / 10 min, 34 g / 10 min, 39 g / 10 min, 41 g / 10 min, 46 g / 10 min, 50 g / 10 min, 53 g / 10 min, 57 g / 10 min, 60 g / 10 min, or a range consisting of any two values ​​therebetween. By regulating the melt index of the coating material within the above range, a coating with a closed-cell temperature T1 of 110°C to 130°C can be obtained, and the coating can melt and quickly fill the pores of the base film when there is a risk of thermal runaway in the secondary battery, achieving a good closed-cell effect, improving the thermal shutdown performance of the diaphragm at high temperatures, preventing the transmission of lithium ions, and the diffusion of heat and substances between the positive and negative electrodes, inhibiting thermal runaway of the secondary battery, and further improving the thermal safety performance of the secondary battery. This application does not particularly limit the method of regulating the melt index of the coating material, as long as the purpose of this application can be achieved. For example, different coating materials can be selected to regulate the melt index of the coating material.

[0044] In some embodiments of the present application, the coating material includes at least one of polyethylene wax or core-shell structured microspheres. The core-shell structured microspheres include a core material and a shell material, and the difference in glass transition temperature ΔTg between the shell material and the core material is 1.2°C to 10°C. For example, ΔT can be 1.2°C, 1.8°C, 2.3°C, 2.7°C, 3°C, 3.5°C, 3.9°C, 4°C, 4.3°C, 5°C, 5.6°C, 6°C, 6.8°C, 7.5°C, 8°C, 8.7°C, 9.3°C, 10°C, or a range consisting of any two values ​​therebetween. The core material includes at least one of polyethylene wax, polyethylene, polyvinyl acetate, and polymethyl acrylate, and the shell material includes at least one of polymethyl methacrylate, polystyrene, and polyacrylic acid. By selecting the above-mentioned coating materials, a coating with a closed-cell temperature T1 of 110°C to 130°C can be obtained, thereby cooperating with the base film to achieve a good closed-cell effect when the secondary battery is at risk of thermal runaway, improving the thermal shutdown performance of the diaphragm at high temperatures, preventing the transmission of lithium ions, and the diffusion of heat and substances between the positive and negative electrode sheets, inhibiting thermal runaway of the secondary battery, and further improving the thermal safety performance of the secondary battery. This application does not particularly limit the glass transition temperature of the core material and the shell material, as long as the purpose of this application can be achieved. For example, the glass transition temperature Tg1 of the core material can be 108°C to 120°C, and the glass transition temperature Tg2 of the shell material can be 112°C to 125°C.

[0045] In some embodiments of the present application, the average particle size D of the coating material is 0.2 μm to 1.5 μm. For example, the average particle size D of the coating material can be 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm or a range consisting of any two values ​​therebetween. By regulating the average particle size D of the coating material within the above range, the coating has a suitable thickness, which has little effect on its energy density while improving the thermal safety performance of the secondary battery. Coating materials with different average particle sizes in this application can be purchased, and the average particle size of the coating material can be tested in combination with the “Average Particle Size Test of Coating Material” test method provided in this application to select a coating material with the required average particle size.

[0046] In some embodiments of the present application, the base film includes at least one of a polyethylene base film, a polypropylene base film, a polyethylene / polypropylene mixed film, a polypropylene / polyethylene / polypropylene composite base film, and a polyvinylidene fluoride based film. By selecting the above base films, the base film has good strength, and at the same time can melt and close the pores at a temperature where the secondary battery is in thermal runaway, thereby further improving the thermal safety performance of the secondary battery. In the present application, the polyethylene / polypropylene mixed film refers to a diaphragm obtained by mixing polyethylene and polypropylene raw materials and undergoing diaphragm production processes such as extrusion, stretching, and extraction; the present application does not limit the content of polyethylene and polypropylene, as long as the purpose of the present application can be achieved. The polypropylene / polyethylene / polypropylene composite base film refers to a base film obtained by compounding a polypropylene film, a polyethylene film, and a polypropylene film; the present application does not limit the thickness of the polypropylene film and the polyethylene film, as long as the purpose of the present application can be achieved.

[0047] In some embodiments of the present application, the coating further comprises a coating binder, and the mass ratio Y of the coating material to the coating binder is (85 to 97): (3 to 15), for example, 85:15, 87:13, 90:10, 92:8, 95:5, 97:3 or a range consisting of any two ratios therebetween. The coating binder comprises at least one of polyacrylic acid, polymethacrylic acid, polymethyl methacrylate, acrylate, styrene-acrylate emulsion, acrylate emulsion, lithium polyacrylate, polyvinyl alcohol, carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, polyimide, polyamide-imide, styrene-butadiene rubber or polyvinylidene fluoride. By selecting the above-mentioned coating binder and regulating the mass ratio of the coating material to the coating binder within the above-mentioned range, the obtained coating has good adhesion to the base film and is not easy to fall off when the secondary battery is working normally, thereby improving the cycle performance of the secondary battery; when there is a risk of thermal runaway in the secondary battery, the coating can quickly melt and fill the pores of the base film, achieving a good closed-pore effect, improving the thermal shutdown performance of the diaphragm at high temperatures, preventing the transmission of lithium ions, and the diffusion of heat and substances between the positive electrode and the negative electrode, thereby suppressing the thermal runaway of the secondary battery and further improving the thermal safety performance of the secondary battery. In the present application, the styrene-acrylate emulsion is obtained by copolymerization of styrene and acrylate, and the acrylate may include but is not limited to at least one of methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate, and butyl methacrylate; the solid content of the styrene-acrylate emulsion and the mass ratio of styrene to acrylate are not particularly limited. For example, the solid content of the styrene-acrylate emulsion may be 30 wt % to 50 wt %, and the mass ratio of styrene to acrylate may be (20 to 50):(50 to 80); specifically, the comonomers in the styrene-acrylate emulsion include styrene, methyl acrylate, and methyl methacrylate, and the mass ratio of styrene, methyl acrylate, and methyl methacrylate is (20 to 50):(20 to 30):(20 to 60). In the application, the acrylic ester emulsion is obtained by copolymerization of two or more acrylic ester monomers, and the acrylic ester monomers may include but are not limited to at least one of methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, acrylic acid or acrylonitrile; the solid content of the acrylic ester emulsion is not particularly limited, and illustratively, the solid content of the acrylic ester emulsion may be 30wt% to 50wt%; specifically, the copolymerized monomers in the acrylic ester emulsion include methyl acrylate, acrylic acid, methyl methacrylate, and acrylonitrile, and the mass ratio of methyl acrylate, acrylic acid, methyl methacrylate, and acrylonitrile is (30 to 40):(15 to 25):(25 to 35):(10 to 20).

[0048] In some embodiments of the present application, the ratio of the puncture strength of the membrane to the thickness of the membrane is F / H, the unit of the ratio is gf / μm, and 40 gf / μm≤F / H≤150 gf / μm. For example, the value of F / H may be 40gf / μm, 47gf / μm, 50gf / μm, 54gf / μm, 60gf / μm, 68gf / μm, 70gf / μm, 73gf / μm, 80gf / μm, 92gf / μm, 100gf / μm, 105gf / μm, 113gf / μm, 120gf / μm, 126gf / μm, 130gf / μm, 137gf / μm, 140gf / μm, 146gf / μm, 150gf / μm or a range consisting of any two values ​​therebetween; for example, it may be or a range consisting of any two values ​​therebetween. The ratio of the puncture strength of the separator to the thickness of the separator is within the above range, indicating that the separator also has good puncture resistance, thereby improving the thermal safety performance of the secondary battery while also having good mechanical safety performance of the secondary battery.

[0049] The present application has no particular limitation on the thickness of the separator, as long as the purpose of the present application can be achieved. For example, the thickness of the separator can be 3 μm to 10 μm.

[0050] The coating materials with different melting points, melt indexes and densities in this application can be purchased, and the melting point, melt index and density of the coating materials can be tested in combination with the test methods of "melting point test of coating materials", "melting index test of coating materials" and "density test of coating materials" provided in this application, and the coating materials with the required melting point, melt index and density can be selected.

[0051] In the present application, base films with different porosities and / or thicknesses can be purchased, and the porosity and thickness of the base films can be tested in combination with the "Porosity Test of Base Film" and "Thickness Test of Base Film" test methods provided in the present application, and the base films with the required porosity and / or thickness can be selected. The present application has no particular restrictions on the method of regulating the closed-pore temperature of the diaphragm, as long as the purpose of the present application can be achieved. For example, the closed-pore temperature of the diaphragm can be regulated by regulating the closed-pore temperature of the base film and / or coating. Exemplarily, the closed-pore temperature of the coating can be regulated by regulating the type of coating material and the mass ratio of the coating material to the coating binder; the closed-pore temperature of the base film can be regulated by selecting different types of base films. In the present application, base films with different closed-pore temperatures can be purchased, and the closed-pore temperature of the base film can be tested in combination with the "Closed-pore Temperature Test of Base Film" test method provided in the present application, and the base film with the required closed-pore temperature can be selected.

[0052] In some embodiments of the present application, the separator further includes a ceramic coating, which is disposed on one surface of the base film, facing the positive electrode tab, and on the other surface of the base film, facing the negative electrode tab. This configuration improves the separator's heat resistance; the ceramic coating further enhances the separator's puncture resistance, thereby further improving the thermal and mechanical safety of the secondary battery.

[0053] In some embodiments of the present application, ceramic coating inorganic particles and ceramic coating binders, the present application has no particular restrictions on inorganic particles, for example, inorganic particles can include aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide or barium sulfate at least one. The present application has no particular restrictions on ceramic coating binders, for example, ceramic coating binders can include but are not limited to 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 at least one. The present application has no particular restrictions on the mass ratio of inorganic particles and ceramic coating binders, as long as the purpose of the present application can be achieved, for example, the mass ratio of inorganic particles and ceramic coating binders can be (90 to 97): (3 to 10).

[0054] In the present application, the features in the above embodiments can be combined arbitrarily, and the embodiments covered by the above combinations are all within the protection scope of the present application.

[0055] In the present application, the positive electrode plate may include 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 it can be arranged 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. This application has no special restrictions, as long as the purpose of this application can be achieved. This application has no special restrictions on the positive electrode current collector, as long as the purpose of this application can be achieved. For example, it can include aluminum foil, aluminum alloy foil or a composite current collector (such as an aluminum-carbon composite current collector).

[0056] The positive electrode material layer includes a positive electrode active material. The present application has no particular restrictions on the 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 but is not limited to lithium nickel cobalt manganese oxide (such as NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide (LiCoO2), lithium manganate, lithium iron manganese phosphate or lithium titanate. At least one of the positive electrode material layer may also include a positive electrode conductor and a positive electrode binder. The present application has no particular restrictions on the types of positive electrode conductors and positive electrode binders, as long as the purpose of the present application can be achieved. The present application has no particular restrictions on the positive electrode conductor, as long as the purpose of the present application can be achieved. For example, the positive electrode conductor may include but is not limited to at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, graphene, metal materials or conductive polymers. The above-mentioned carbon nanotubes may include but are not limited to single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The carbon fibers may include, but are not limited to, vapor-grown carbon fibers (VGCFs) and / or nanocarbon fibers. The metal materials may include, but are not limited to, metal powders and / or metal fibers. Specifically, the metal may include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The conductive polymers may include, but are not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole. The positive electrode binder may be at least one of the ceramic coating binders described above. This application does not impose any particular restrictions on the mass ratio of the positive electrode active material, positive electrode conductive agent, and positive electrode 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 this application are achieved. This application does not impose any particular restrictions on the thickness of the positive electrode current collector and the positive electrode material layer, as long as the objectives of this application are achieved. For example, the thickness of the positive electrode current collector is 6 to 15 μm, and the thickness of the positive electrode material layer is 30 to 120 μm. This application does not impose any particular restrictions on the thickness of the positive electrode plate, as long as the objectives of this application are achieved. For example, the thickness of the positive electrode plate is 50 to 250 μm.

[0057] Optionally, the positive electrode sheet may further include a conductive layer positioned between the positive electrode current collector and the positive electrode material layer. The composition of the conductive layer is not particularly limited and may be any commonly used conductive layer in the art. The conductive layer includes a conductive agent and a binder. The present application does not particularly limit the conductive agent and binder in the conductive layer; for example, the conductive layer may be at least one of the aforementioned positive electrode conductive agents and binders.

[0058] In the present application, the negative electrode plate may include a negative electrode current collector and a negative electrode material layer arranged on at least one surface of the negative electrode current collector. The above-mentioned "negative electrode material layer is arranged on at least one surface of the negative electrode current collector" means that the negative electrode material layer can be arranged on one surface of the negative electrode current collector along the thickness direction of itself, or it can be arranged on two surfaces of the negative 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 negative electrode current collector, or it can be a partial area of ​​the surface of the negative electrode current collector. This application has no special restrictions, as long as the purpose of this application can be achieved. This application has no special restrictions on the negative electrode current collector, as long as the purpose of this application can be achieved. For example, it can include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foam nickel, foam copper or composite current collector (such as lithium copper composite current collector, carbon copper composite current collector, nickel copper composite current collector, titanium copper composite current collector, etc.).

[0059] The negative electrode material layer includes a negative electrode active material. The present application has no particular limitation on the negative electrode active material, as long as the purpose of the present application can be achieved. For example, the negative electrode active material may include but is not limited to natural graphite, artificial graphite, mesophase microcarbon beads, hard carbon, soft carbon, silicon, silicon-carbon composite, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel structured lithiated TiO2-Li4Ti5O 12 or at least one of Li-Al alloys.

[0060] The negative electrode material layer may also include a negative electrode conductor, a negative electrode binder and a thickener. The present application has no particular restrictions on the types of the negative electrode conductor, the negative electrode binder and the thickener, as long as the purpose of the present application can be achieved. For example, it can be at least one of the above-mentioned positive electrode conductor and the above-mentioned ceramic coating binder. The thickener may include but is not limited to at least one of sodium carboxymethyl cellulose or lithium carboxymethyl cellulose. The present application has no particular restrictions on the mass ratio of the negative electrode active material, the negative electrode conductor, the negative electrode binder and the negative electrode thickener in the negative electrode material layer. Those skilled in the art can choose according to actual needs, as long as the purpose of the present application can be achieved. The present application has no particular restrictions on the thickness of the negative electrode material layer, as long as the purpose of the present application can be achieved. For example, the thickness of the negative electrode material layer is 30μm to 120μm. The present application has no particular restrictions on the thickness of the negative electrode current collector, as long as the purpose of the present application can be achieved. For example, the thickness of the negative electrode current collector is 6μm to 12μm. The application has no particular limitation on the thickness of the negative electrode sheet, as long as the purpose of the application can be achieved. For example, the thickness of the negative electrode sheet is 50 μm to 250 μm.

[0061] Optionally, the negative electrode sheet may further include a conductive layer positioned between the negative electrode current collector and the negative electrode material layer. The present application does not particularly limit the composition of the conductive layer, and it may be any conductive layer commonly used in the art. For example, the conductive layer may include a conductive agent and a binder. The present application does not particularly limit the conductive agent and binder in the conductive layer, and for example, it may be at least one of the aforementioned negative electrode conductive agents and binders.

[0062] In the present application, the secondary battery also includes an electrolyte, and the electrolyte includes 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, LiAsF6, 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 concentration of the lithium salt in the electrolyte, as long as the purpose of the present application can be achieved. The present application has no particular restrictions on the non-aqueous solvent, as long as the purpose of the present application can be achieved, for example, it may include but is not limited to at least one of a carbonate compound, a carboxylate compound, an ether compound or other organic solvents. The above-mentioned carbonate compound may include but is not limited to at least one of a chain carbonate compound, a cyclic carbonate compound or a fluorinated carbonate compound. The above-mentioned linear carbonate compound may include but is 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). The above-mentioned cyclic carbonate may include but is 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 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 above-mentioned carboxylate 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, decanoic acid, valerolactone or caprolactone. The above-mentioned ether compounds may include but are not limited to at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran or tetrahydrofuran. The above-mentioned other organic solvents may include but are not limited to at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate or trioctyl phosphate. The present application does not particularly limit the proportion of the non-aqueous solvent in the electrolyte, as long as the purpose of the present application can be achieved.

[0063] The preparation process of the secondary battery of the present application is well known to those skilled in the art and is not particularly limited in the present application. For example, the preparation process of the secondary battery may include, but is not limited to, the following steps: stacking the diaphragm, positive electrode sheet, diaphragm, and negative electrode sheet in order, 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 an electrochemical device. Alternatively, stacking the diaphragm, positive electrode sheet, diaphragm, and negative electrode sheet in order, and then fixing the four corners of the entire stacked structure 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 an electrochemical device. In addition, as needed, overcurrent protection elements, guide plates, etc. may be placed in the packaging bag to prevent pressure rise and overcharging and discharging inside the electrochemical device. The packaging bag is a packaging bag known in the art and is not limited in the present application.

[0064] The second aspect of the present application provides an electronic device, which includes the electrochemical device according to any of the aforementioned embodiments. Thus, the electronic device provided by the present application has good performance.

[0065] The present application does not particularly limit the type of electronic device, and it can be any electronic device known in the prior art. In some embodiments of the present application, 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 head-mounted stereo 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, a car, 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.

[0066] Example

[0067] The following examples and comparative examples are provided to more specifically illustrate the embodiments of the present invention. Various tests and evaluations were performed according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0068] Test methods and equipment:

[0069] Density test of coating materials:

[0070] Calculate the density of the coating material using the weighing method. First, use a balance to weigh the coating material and record it as M. Suspend the weighed coating material in an ethylene glycol solution in a graduated container, ensuring that the coating material is completely submerged and avoiding the formation of bubbles during suspension. Measure the height of the solution rise and record the difference in the solution surface between the unsuspended and suspended sample states (ΔH). The total volume of the container is V, and the height is H.

[0071] Density of coating material = M / (VΔH / H).

[0072] Melting point test of coating materials:

[0073] The melting point of the coating material was measured using a differential scanning calorimeter. The coating material was placed in a crucible and heated from 30°C to 300°C at a rate of 10°C / min under a nitrogen atmosphere to obtain a DSC curve. The peak temperature in the DSC curve is the melting point of the coating material.

[0074] Melt index test of coating materials:

[0075] The coating material's melt index (MFR) was measured using a melt flow rate (MFR) tester. The instrument was heated to 110°C, and a standard die (φ2.095±0.005mm) was placed in the barrel. The coating material was placed into the barrel and compacted. The temperature was maintained for 10 minutes. The coating material was extruded with a pressure of 2.16kg and a cutting time of 60 seconds. The coating material's melt index (MFR) was calculated. MFR = mT / t, where t is the cutting time (60 seconds), m is the mass of the coating material, and T is 600 seconds.

[0076] Average particle size test of coating materials:

[0077] Disperse 5g of the coating material in 100mL of water and ultrasonically disperse for 30 minutes at 53Hz. Then, use a Malvern 3000 particle size analyzer to measure the coating material's particle size. The particle size distribution (D50) is read from the instrument, representing the average particle size. D50 is the particle size at which the cumulative volumetric particle size, measured from the smallest particle size, reaches 50%.

[0078] Closure temperature test of coating:

[0079] The coating material is coated on a dry-stretched polypropylene membrane (manufacturer: Xingyuan Material, thickness of 16 μm), and the coating slurry in each embodiment or comparative example is coated, and the coating weight CW is the same as that of each embodiment or comparative example. After drying, the test membrane of each embodiment or comparative example is obtained.

[0080] The test diaphragm was cut into a fixed size of 5cm×5cm and placed in a component consisting of a ceramic and stainless steel fixture. 10mL of electrolyte was injected. The above fixture was placed in a 250℃ oven and heated at a rate of 15℃ / min. The resistance and temperature of the fixture were monitored at the same time, and the data of the fixture resistance and temperature, oven temperature and time were output.

[0081] According to the time-resistance curve, the temperature when the resistance suddenly increases to 1000Ω is the closed-cell temperature of the test diaphragm. Since the melting point of the polypropylene diaphragm is higher than 150°C, the closed-cell temperature of the test diaphragm is the closed-cell temperature T1 of the coating.

[0082] Test of the area ratio of the base film covered by the coating after melting:

[0083] The diaphragm was placed in an oven at 120°C for 20 minutes, taken out and placed on a transparent plastic paper with a grid coordinate. The area not covered by the molten coating was marked and its area was calculated as S1. Then, the area S0 of the diaphragm was calculated, and the area ratio X = S1 / S0×100%.

[0084] Base film thickness test:

[0085] Use a 10,000-degree thickness gauge to evenly test the thickness of any 10 points of the base film, and take the average value as the thickness of the base film.

[0086] Porosity test of base film:

[0087] The porosity of the base film = 1-W / ρ'V. Where W is the mass of the base film, ρ' is the density of the base film, and V is the volume of the base film. V = base film length × base film width × base film thickness. The thickness of the base film is measured according to the method in "Base Film Thickness Test". The length and width of the base film are measured with a ruler. When the base film is a polyethylene base film, ρ' = 0.95g / cm 3 When the base film is a polypropylene base film, ρ'=0.91g / cm 3 When the base film is a polyvinylidene fluoride film, ρ' = 1.76g / cm 3 When the base film is a polyethylene / polypropylene mixed film, ρ'=0.93g / cm 3 When the base film is a polypropylene / polyethylene / polypropylene composite base film, ρ'=0.92g / cm 3 When the base film is an aramid base film, ρ'=1.37g / cm 3 .

[0088] Closed-cell temperature test of base film:

[0089] The test base film was cut into a fixed size of 5cm×5cm and placed in a component consisting of a ceramic and stainless steel fixture. 10mL of electrolyte was injected, and the above fixture was placed in a 250℃ oven and heated at a rate of 15℃ / min. The resistance and temperature of the fixture were monitored at the same time, and the data of the fixture resistance and temperature, oven temperature and time were output.

[0090] According to the time-resistance curve, the temperature when the resistance suddenly increases to 1000Ω is the closed-pore temperature T2 of the base film.

[0091] Diaphragm closed cell temperature test:

[0092] The temperature-rise internal resistance method is used to test the closed-pore temperature of the diaphragm. First, the diaphragm is cut into a fixed size of 5cm×5cm, and the diaphragm size is larger than the size of the fixture test area. The diaphragm is placed in a component consisting of a ceramic and stainless steel fixture, and 10mL of electrolyte is injected. The above fixture is placed in an oven with a set temperature of 250℃. The temperature is increased at a constant rate of 15℃ / min. The resistance and temperature of the fixture are monitored at the same time. The data of the fixture temperature and resistance, oven temperature and time are output to obtain the temperature-resistance curve. According to the temperature-resistance curve, the temperature when the resistance suddenly increases to 1000Ω is the closed-pore temperature T0 of the diaphragm.

[0093] The ratio of the puncture strength of the diaphragm to the thickness of the diaphragm F / H test:

[0094] The diaphragm is fixed to a test fixture with a 10mm hole in the center. A hemispherical steel nail with a diameter of 1mm is lowered at a speed of 120mm / min. The maximum force required to puncture the diaphragm is the puncture strength F. The thickness of the diaphragm is uniformly measured at any 10 points using a 10,000-degree thickness gauge, and the average value is taken as the diaphragm thickness H. The F / H ratio is calculated based on the measured F and H.

[0095] Hot box test:

[0096] Charge at 25±3°C with a constant current of 1.0C to a full charge voltage of 4.5V, and further charge at a constant voltage of 4.5V to a cutoff current of 0.02C to make it in a fully charged state. Place the fully charged battery cell in an oven and heat it at a rate of 5°C / min until it reaches the specified hot box test temperature of 135°C and maintain it for one hour. Observe the state of the lithium-ion battery during this process. If the lithium-ion battery does not catch fire or explode, it passes the hot box test. If the lithium-ion battery catches fire or explodes, it fails the hot box test. Test 10 lithium-ion batteries for each embodiment and comparative example, and record the number of passed batteries / 10 as the final result.

[0097] Low temperature discharge capacity test:

[0098] Place the lithium-ion battery in a 25°C environment for 30 minutes and discharge it at a constant current of 0.2C until the final voltage reaches 3.0V. Then fully charge the battery and perform constant current charging at a charging current of 1.5C to a full charge voltage of 4.45V. Then perform constant voltage charging at a maximum voltage of 4.45V until the current cuts off at 0.02C. Then perform constant current discharge at a discharge current of 1.0C until the final voltage reaches 3.0V. Record the discharge capacity at 25°C as C0.

[0099] Fully charge the lithium-ion battery at 25°C, then perform constant-current charging at a charge current of 1.5C to a full charge voltage of 4.45V. Then, perform constant-voltage charging at a maximum voltage of 4.45V until the current cutoff reaches 0.02C. Place the battery in a -10°C environment for 60 minutes, then perform constant-current discharge at a discharge current of 1.0C until the final voltage reaches 3.0V. Record the discharge capacity at -10°C as C1.

[0100] Discharge capacity retention rate at -10°C = C1 / C0×100%.

[0101] Example 1-1

[0102] <Preparation of Separator>

[0103] A polyethylene base film was used as the base film, the viscosity average molecular weight of the polyethylene was 800,000, the thickness Hμm of the base film was 6μm, the porosity P% was 30%, and the closed cell temperature T2 was 142°C.

[0104] The coating material polyethylene wax (weight average molecular weight of 2000) and the coating binder acrylic emulsion were mixed in a mass ratio Y of 95:5, deionized water was added, and a slurry with a solid content of 40 wt% was prepared. The coating slurry was obtained after stirring evenly. The density of the coating material is ρg / cm 3 0.9g / cm 3 The melting point Tm is 95°C and the melt index MFR is 60g / 10min. The solvent of the acrylic emulsion is water, the solid content is 45wt%, and the comonomers in the acrylic emulsion include methyl acrylate, acrylic acid, methyl methacrylate, and acrylonitrile. The mass ratio of methyl acrylate, acrylic acid, methyl methacrylate, and acrylonitrile is 35:20:30:15.

[0105] The coating slurry was evenly coated on one surface of the base film using a micro-concave coating method and dried at 55°C to obtain a diaphragm. The coating weight of the single-side coating was 1.5 g / m 2 .

[0106] <Preparation of positive electrode sheet>

[0107] The positive electrode active material lithium cobalt oxide (LiCoO2), conductive carbon black, and polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 96:2:2, and N-methylpyrrolidone (NMP) is added as a solvent to prepare a slurry with a solid content of 75wt%, which is stirred evenly to obtain a positive electrode. The positive electrode slurry is evenly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 10μm, and dried at 90°C to obtain a positive electrode sheet with a single-sided coating of a negative electrode active material layer with a coating thickness of 100μm. Then repeat the above steps on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of positive electrode active material. After drying at 90°C, cold pressing is performed, and then cutting and welding of the tabs are performed to obtain a positive electrode sheet with a specification of 74mm×867mm for standby use. The density of the positive electrode material layer is 4.2g / cm 3 .

[0108] <Preparation of negative electrode sheet>

[0109] The negative electrode active materials artificial graphite, conductive carbon black, styrene-butadiene rubber, and sodium carboxymethyl cellulose are mixed in a mass ratio of 95:1:3:1, and deionized water is added as a solvent to prepare a slurry with a solid content of 45wt%. After stirring evenly, a negative electrode slurry is obtained. The negative electrode slurry is evenly coated on one surface of a negative electrode current collector copper foil with a thickness of 10μm and dried at 90°C to obtain a negative electrode sheet with a single-sided negative electrode active material layer coated with a coating thickness of 100μm. The above steps are then repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided negative electrode active material layer coated. After drying at 90°C, cold pressing is performed, and then cutting and welding of the tabs are performed to obtain a negative electrode sheet with a specification of 78mm×875mm for use. The density of the negative electrode material layer is 1.75g / cm 3 .

[0110] <Preparation of Electrolyte>

[0111] In an argon atmosphere glove box with a water content of <10 ppm, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) were mixed in a ratio of EC:PC:DEC:DMC = 1:1:1:1 to prepare an organic solvent. Lithium hexafluorophosphate was then dissolved in the organic solvent and mixed thoroughly to obtain an electrolyte. The lithium hexafluorophosphate content was 12% by weight based on the mass of the electrolyte.

[0112] <Preparation of lithium-ion batteries>

[0113] The prepared separator, positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator positioned between the positive and negative electrode sheets to provide insulation, and then wound to form an electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag, dehydrated at 80°C, and then injected with the prepared electrolyte. The lithium-ion battery is produced through vacuum packaging, standing, forming, degassing, and trimming. The upper limit of the formation voltage is 4.15V, the formation temperature is 80°C, and the formation standing time is 1 hour.

[0114] Example 1-2 to Example 1-19

[0115] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1.

[0116] Example 2-1 to Example 2-16

[0117] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-2.

[0118] Comparative Examples 1 to 10

[0119] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1.

[0120] The preparation parameters and performance tests of each embodiment and comparative example are shown in Table 1 and Table 2.

[0121] It can be seen from Examples 1-1 to 1-19 and Comparative Examples 1 to 10 that the values ​​of CW / (ρ×P×H)×100%, ρ, P, H, and further, CW in the Examples are all within the scope of the present application, while at least one of the values ​​of CW / (ρ×P×H)×100%, ρ, P, and H in the Comparative Examples is outside the scope of the present application. The lithium-ion batteries obtained in the Examples have both a higher number of hot box test passes and a higher low-temperature discharge capacity, indicating that the lithium-ion batteries provided in the present application have both better safety performance and low-temperature performance.

[0122] The closed-pore temperature T1 of the coating and the closed-pore temperature T2 of the base film will affect the closed-pore temperature T0 and T2-T0 values ​​of the diaphragm, thereby affecting the safety performance and low-temperature performance of the lithium-ion battery. It can be seen from Examples 1-1 to 1-19 that when the closed-pore temperature T1 of the coating, the closed-pore temperature T2 of the base film and the values ​​of T2-T0 are within the scope of this application, the value of F / H and the area ratio X of the diaphragm are also within the scope of this application, the obtained lithium-ion battery has a high number of hot box test passes and low-temperature discharge capacity, indicating that the lithium-ion battery provided by this application has good safety performance and low-temperature performance.

[0123] The melting point Tm and melt index MFR of the coating material vary with the type of coating material, which usually affects the closed-pore temperature T1 of the coating, and thus affects the safety performance and low-temperature performance of the lithium-ion battery. It can be seen from Examples 1-1 to 1-19 that when the type of coating material, melting point Tm and melt index MFR, and the closed-pore temperature T1 of the coating are within the scope of this application, the value of F / H and the area ratio X of the diaphragm are also within the scope of this application, and the obtained lithium-ion battery has a high number of hot box test passes and low-temperature discharge capacity, indicating that the lithium-ion battery provided by this application has good safety performance and low-temperature performance.

[0124] Specifically, FIG1 is a DSC curve of the coating material in Example 1-2. As can be seen from the figure, the temperature corresponding to the peak is 115° C., and the melting point of the coating material in Example 1-2 is 115° C.

[0125] FIG2 is a temperature-resistance curve of the diaphragm in Example 1-2. As can be seen from the figure, at 137° C., the resistance suddenly increases to 1000Ω, so the closed-cell temperature T0 of the diaphragm in Example 1-2 is 137° C.

[0126] Figure 3 is an electron microscope photograph of the sample obtained after the diaphragm in Example 1-2 was placed in an oven at 120°C for 20 minutes. It can be seen from the electron microscope photograph that the low closed-pore coating has a high coverage rate after high-temperature closed-pores, and the pore structure of the base membrane is basically invisible.

[0127] Table 2 Note: In Table 2, the weight-average molecular weights of the coating binders "polyacrylic acid" and "polymethyl methacrylate" are 200,000 and 150,000, respectively. The comonomers in the coating binder "styrene-acrylate emulsion" include styrene, methyl acrylate, and methyl methacrylate in a mass ratio of 40:30:30. The solvent of the emulsion is water, and the solids content is 45 wt%. The mass ratio of polyethylene to polypropylene in the base film "polyethylene and polypropylene mixed film" is 90:10. The thickness ratio of the three layers in the polypropylene / polyethylene / polypropylene composite base film is 1:1:1. The viscosity-average molecular weight of the polypropylene is 400,000, and the viscosity-average molecular weight of the polyethylene is 200,000. The aramid base film is meta-aramid.

[0128] The type of binder in the coating and the mass ratio Y of the coating material to the coating binder usually affect the closed-pore temperature T1 of the coating and the closed-pore temperature T0 of the diaphragm, thereby affecting the safety performance, low-temperature performance and cycle performance at room temperature of the lithium-ion battery. It can be seen from Examples 1-2, 2-1 to 2-3, 2-11 and 2-14 that when the type of binder in the coating and the mass ratio Y of the coating material to the coating binder are within the scope of this application, the value of F / H and the area ratio X of the diaphragm are also within the scope of this application, the obtained lithium-ion battery has a high number of hot box test passes and low-temperature discharge capacity, indicating that the lithium-ion battery provided in this application has good safety performance and low-temperature performance.

[0129] The average particle size D of the coating material usually affects the closed-pore temperature T1 of the coating and the closed-pore temperature T0 of the diaphragm, thereby affecting the safety performance, low-temperature performance and cycle performance at room temperature of the lithium-ion battery. It can be seen from Examples 1-2, 2-4 and 2-5 that when the average particle size D of the coating material is within the range of this application, the value of F / H and the area ratio X of the diaphragm are also within the range of this application, and the obtained lithium-ion battery has a higher number of hot box test passes and low-temperature discharge capacity, indicating that the lithium-ion battery provided by this application has good safety performance and low-temperature performance.

[0130] The type of base film will affect the closed-pore temperature T2 of the base film, which usually affects the safety performance, low-temperature performance and cycle performance at room temperature of the lithium-ion battery. It can be seen from Examples 1-2, 2-8 to 2-10, 2-15 and 2-16 that when the type of base film and the closed-pore temperature T2 of the base film are within the range of this application, the value of F / H and the area ratio X of the diaphragm are also within the range of this application, and the obtained lithium-ion battery has a higher number of hot box test passes and low-temperature discharge capacity, indicating that the lithium-ion battery provided by this application has good safety performance and low-temperature performance.

[0131] It should be noted that, in this article, the terms "comprises", "includes" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method or article that includes a series of elements includes not only those elements, but also includes other elements that are not explicitly listed, or also includes elements that are inherent to such process, method or article.

[0132] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application shall be included in the scope of protection of the present application.

Claims

1. A secondary battery comprising a positive electrode sheet, a negative electrode sheet and a separator, wherein the separator comprises a base film and a coating disposed on at least one surface of the base film, wherein the coating comprises a coating material, and the coating can melt and flow at a temperature greater than or equal to 90°C; the coating weight of the coating on a single side is CW g / m 2 , the density of the coating material is ρg / cm 3 , the porosity of the base film is P%, the thickness of the base film is H μm, 50%≤CW / (ρ×P×H)×100%≤120%; wherein, 0.9≤ρ≤1.05, 20≤P≤50, 3≤H≤10.

2. The secondary battery according to claim 1, wherein 0.5≤CW≤2.

3. The secondary battery according to claim 1, wherein 55%≤CW / (ρ×P×H)×100%≤100%.

4. The secondary battery according to claim 1, wherein The closed-cell temperature T1 of the coating is 110°C to 130°C.

5. The secondary battery according to claim 1, wherein After the diaphragm is placed at 120° C. for 20 minutes, the coating melts and covers an area of ​​the base film that is greater than or equal to 80%.

6. The secondary battery according to claim 1, wherein The closed-cell temperature T0 of the separator is 132° C. to 142° C., the closed-cell temperature of the base film is T2, and 3° C. ≤ T2-T0 ≤ 10° C.

7. The secondary battery according to claim 6, wherein 135℃≤T2≤150℃。 8. The secondary battery according to claim 1, wherein The coating material satisfies at least one of the following characteristics: (1) Melting point Tm is 95°C to 125°C; (2) The melt index MFR is 12 g / 10 min to 60 g / 10 min.

9. The secondary battery according to claim 8, wherein The coating material includes at least one of polyethylene, polyethylene wax or core-shell structured microspheres; the core-shell structured microspheres include a core material and a shell material, the difference in glass transition temperature between the shell material and the core material is 1.2°C to 10°C, the core material includes at least one of polyethylene wax, polyethylene, polyvinyl acetate, and polymethyl acrylate, and the shell material includes at least one of polymethyl methacrylate, polystyrene, and polyacrylic acid.

10. The secondary battery according to claim 1, wherein The average particle size D of the coating material is 0.2 μm to 1.5 μm.

11. The secondary battery according to claim 1, wherein The ratio of the puncture strength of the diaphragm to the thickness of the diaphragm is F / H, the unit of the ratio is gf / μm, and 40gf / μm≤F / H≤150gf / μm.

12. The secondary battery according to claim 1, wherein The diaphragm satisfies at least one of the following characteristics: (1) The base film comprises at least one of a polyethylene base film, a polypropylene base film, a polyethylene / polypropylene mixed film, a polypropylene / polyethylene / polypropylene composite base film, and a polyvinylidene fluoride film; (2) The coating further comprises a coating binder, the mass ratio of the coating material to the coating binder is (85 to 97):(3 to 15), and the coating binder comprises at least one of polyacrylic acid, polymethacrylic acid, polymethyl methacrylate, acrylate, styrene-acrylate emulsion, acrylate emulsion, lithium polyacrylate, polyvinyl alcohol, carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, polyimide, polyamide-imide, styrene-butadiene rubber or polyvinylidene fluoride; (3) The separator further includes a ceramic coating, which is disposed on one surface of the base film and faces the positive electrode plate, and the ceramic coating is disposed on the other surface of the base film and faces the negative electrode plate. 13 . An electronic device comprising the secondary battery according to claim 1 .

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