WO2023097672A1

WO2023097672A1 - Electrochemical device and electronic device

Info

Publication number: WO2023097672A1
Application number: PCT/CN2021/135409
Authority: WO
Inventors: 李志愿; 李娅洁
Original assignee: 宁德新能源科技有限公司
Priority date: 2021-12-03
Filing date: 2021-12-03
Publication date: 2023-06-08
Also published as: CN115398670A

Abstract

The present application relates to an electrochemical device and an electronic device comprising the electrochemical device. The electrochemical device of the present application comprises an electrode sheet. The electrode sheet comprises a current collector, an active material layer arranged on at least one side of the current collector, and a coating arranged between the current collector and the active material layer. The coating comprises a copolymer, and the coefficient of thermal expansion of the copolymer is less than 1% at the temperature of 100°C and greater than 15% at the temperature of 130°C. According to the present application, by adjusting the structure and composition of the copolymer in a PTC coating, the electrochemical device has both high safety performance and high rate performance.

Description

Electrochemical devices and electronic devices

technical field

The present application relates to the technical field of energy storage, and in particular to an electrochemical device and an electronic device.

Background technique

Lithium-ion batteries have the advantages of high energy density, high power, and long cycle life, and are widely used in electric vehicles and consumer electronics. However, the safety issues of lithium-ion batteries limit its application to a certain extent. In addition to normal charge and discharge, there are many potential exothermic side reactions in lithium-ion batteries, such as thermal decomposition of SEI film on the surface of electrode materials, thermal decomposition of positive electrodes in charging state, and electrolyte decomposition. Under normal battery usage conditions, the above-mentioned exothermic side reactions hardly occur. However, when the temperature of the battery is too high due to abuse conditions such as internal and external short circuits or overcharging, the above-mentioned exothermic side reactions may be triggered, and a large amount of heat will be released in a short period of time, resulting in a sharp rise in the internal temperature of the battery, and further Accelerate the occurrence of side reactions, so that the battery enters a state of thermal runaway, causing safety accidents such as battery combustion and explosion. At present, in the existing technology, technologies such as adding external circuit protection, modifying the structure or material of the isolation film, using PTC (Positive Temperature Coefficient) hybrid electrodes or PTC wrapped electrodes, and adding PTC primers to the electrodes are often used. means to improve battery safety performance. However, some of these technical means will degrade the dynamic performance of the battery while improving the safety performance of the battery, and some of the improvement effects on the safety performance of the battery are not ideal.

Contents of the invention

In view of the problems existing in the prior art, the present application provides an electrochemical device and an electronic device including the electrochemical device. The electrochemical device of the present application includes an electrode pole piece with a PTC coating. In this application, by adjusting the structure and composition of the copolymer in the PTC coating, the electrochemical device has high safety performance and rate performance at the same time.

In a first aspect, the present application provides an electrochemical device, which includes an electrode sheet, the electrode sheet includes a current collector, an active material layer disposed on at least one side of the current collector, and an active material layer disposed between the current collector and the active material layer. The interlayer coating, wherein the coating comprises a copolymer having a thermal expansion rate of less than 1% at a temperature of 100°C and a thermal expansion rate of greater than 15% at a temperature of 130°C. In this application, the coefficient of thermal expansion of the copolymer means the rate of change of the length of the copolymer film in the process of temperature programming after soaking the copolymer film in the electrolyte solvent at 25°C for 24 hours in the dynamic thermomechanical analysis (DMA) test , thermal expansion rate=(length of copolymer film at 100 ℃ temperature-length of copolymer film at room temperature)/length of copolymer film at room temperature×100% at 100 ℃ of temperature, thermal expansion rate at 130 ℃ of temperature= (The length of the copolymer film at a temperature of 130°C-the length of the copolymer film at room temperature)/the length of the copolymer film at room temperature×100%. In this application, a DMA850 dynamic mechanical analyzer is used for the DMA test, the tensile mode is selected, the tensile force is 5g, and the heating rate is 3°C/min. In some embodiments, the electrolyte solvent includes ethylene carbonate (EC) and dimethyl carbonate (DMC). In some embodiments, the electrolyte solvent is ethylene carbonate (EC) and dimethyl carbonate (DMC), and the mass ratio EC:DMC=1:1.

When the electrochemical device is thermally abused, the thermal expansion of the copolymer in the coating will destroy the conductive network of the coating and trigger its PTC effect, thereby improving the safety performance of the electrochemical device. Therefore, the high thermal expansion ability of the copolymer at high temperature can effectively improve the thermal safety performance of electrochemical devices. However, since the electrochemical device will also be at a relatively high temperature (such as 40°C to 70°C) during the preparation process (such as the chemical conversion process), therefore, in order to ensure the normal preparation process, it is necessary for the copolymer to have a low temperature during the process. thermal expansion capacity. The thermal expansion rate of the copolymer in the coating of the present application is less than 1% at a temperature of 100°C, and when the thermal expansion rate is greater than 15% at a temperature of 130°C, it can ensure that the coating has excellent PTC effect and stable resistance in the electrolyte environment, and then The electrochemical device has high safety performance and rate performance at the same time. If the thermal expansion rate of the copolymer is less than 15% at a temperature of 130° C., the PTC effect of the coating is limited, and the improvement of the thermal safety performance of the electrochemical device is limited. If the thermal expansion rate of the copolymer is greater than 1% at 100°C, the coating may trigger the PTC effect near 100°C, and the electrochemical device may trigger the PTC effect during formation, which will limit the application of the electrochemical device and affect normal use. experience.

According to some embodiments of the present application, the weight average molecular weight of the copolymer is 200,000 to 1.5 million. When the molecular weight of the copolymer is too low, the entanglement of the macromolecules is weak, and the molecular chain of the copolymer is very easy to move, so that the thermal expansion rate of the copolymer is very high when the temperature is lower than 100 ° C, and it is easy to be used in the preparation process of the electrochemical device (such as the formation process). ) triggers the PTC effect of the coating, which affects the preparation process of the electrochemical device. When the molecular weight of the copolymer is too high, the molecular chain of the copolymer is difficult to move, and the thermal expansion performance at high temperature is reduced, thereby affecting the PTC effect of the coating.

According to some embodiments of the present application, the copolymer includes a first structural unit derived from a monomer A and a second structural unit derived from a monomer B, and the monomer A includes acrylic acid, acrylonitrile, acrylamide or vinyl acetate. At least one, monomer B includes at least one of vinylidene fluoride, methyl methacrylate or styrene. Compared with the copolymer, the electrolyte solvent is a small molecule. If it wants to enter between the molecular chains of the copolymer, the electrolyte solvent needs to destroy the force between the molecular chains of the copolymer, so that the interaction between the molecular chains is weakened. The channel that houses it. However, there are strong polar groups such as carboxyl, nitrile, and amide groups in the first structural unit of the copolymer of the present application. These strong polar groups make the interaction between the molecular chains of the copolymer and the interior of the molecular chains Stronger, the electrolyte solvent is difficult to destroy it, and it is difficult to enter between the macromolecular chains, so that the swelling of the copolymer in the electrolyte solvent is low. The low swelling of the copolymer is beneficial to maintain the integrity of the conductive network in the coating, reduce the internal resistance of the electrochemical device, and then improve the rate performance of the electrochemical device. The melting point of the copolymer obtained by copolymerization of monomer B is between 120°C and 170°C, which is close to the PTC response temperature of the electrochemical device. At the same time, it has strong molecular chain movement ability, weak intermolecular force, and excellent high-temperature thermal expansion performance. The copolymer containing the second structural unit derived from monomer B can trigger the PTC effect in time and quickly when the electrochemical device reaches the PTC response temperature, increase the resistance and cut off the electronic path, thereby avoiding thermal runaway and improving the performance of the electrochemical device. safety performance. On the one hand, the present application introduces the first structural unit with strong polar groups such as carboxyl, nitrile, and amide groups into the copolymer, so that the copolymer has low swelling property in the electrolyte solvent; The second structural unit of vinyl fluoride, methyl methacrylate or styrene makes the copolymer have a high expansion rate at high temperature. The low swelling property can avoid the destruction of the conductive network of the coating caused by the copolymer absorbing liquid, so that the conductive network in the electrochemical device can be maintained stable, and the rate performance of the electrochemical device can be avoided from being affected. The high thermal expansion rate makes the coating have an excellent PTC effect, which can effectively improve the safety performance of the electrochemical device. Therefore, the electrochemical device of the present application has both high safety performance and rate performance.

According to some embodiments of the present application, based on the total mass of the copolymer, the mass content of the first structural unit is 20% to 80%. According to some embodiments of the present application, based on the total mass of the copolymer, the mass content of the second structural unit is 20% to 80%. As the mass content of the first structural unit increases, the ability of the copolymer to absorb the electrolyte decreases, and the swelling property of the copolymer in the electrolyte decreases. At the same time, the maintenance of the conductive network in the electrolyte is good, and the internal resistance of the electrochemical device is basically Unaffected, thus making the electrochemical device have better rate performance. However, when the mass content of the first structural unit is too high, the mass content of the second structural unit will decrease accordingly, the PTC effect of the coating will be weakened, and the safety performance of the electrochemical device will be correspondingly reduced.

According to some embodiments of the present application, the coating further includes a conductive agent. In some embodiments, based on the total mass of the coating, the mass content of the copolymer is 60% to 98%. In some embodiments, based on the total mass of the coating, the mass content of the conductive agent is 2% to 40%. As the mass content of the copolymer increases, the PTC effect of the coating is enhanced, which is beneficial to improving the safety performance of the electrochemical device. However, when the mass content of the copolymer is too high, the mass content of the conductive agent will be too low, resulting in poor conductivity of the coating and increased internal resistance of the electrochemical device.

According to some embodiments of the present application, the conductive agent includes at least one of conductive carbon black, acetylene black, graphite, graphene, carbon nanotubes, carbon fibers, aluminum powder, nickel powder and gold powder.

According to some embodiments of the present application, the coating satisfies at least one of the following features (a) to (e): (a) the swelling rate of the coating is less than 10%; (b) the dissolution rate of the coating is less than 3%; (c) the melting point of the coating is 100°C to 130°C; (d) the normal temperature resistance of the coating is 0.05Ω to 0.4Ω; (e) the resistance of the coating is 15Ω to 40Ω at a temperature of 130°C.

According to some embodiments of the present application, the swelling rate of the coating is less than 10%. According to some embodiments of the present application, the dissolution rate of the coating is less than 3%. In the present application, the swelling rate of the coating means the weight change rate of the coating before and after immersion in the electrolyte solvent, and the calculation formula can be swelling rate=(weight after immersion-weight before immersion)/weight before immersion×100%. The dissolution rate of the coating indicates the weight change rate of the coating after soaking in the electrolyte solvent and drying, and the calculation formula can be dissolution rate=(weight before soaking-weight after soaking and drying)/weight before soaking×100% . Wherein, the soaking temperature is 60°C, the soaking time is 6 days, the drying time is 2 days, and the drying temperature is 90°C. In some embodiments, the electrolyte solvent includes ethylene carbonate (EC) and dimethyl carbonate (DMC). In some embodiments, the electrolyte solvent is ethylene carbonate (EC) and dimethyl carbonate (DMC), and the mass ratio EC:DMC=1:1. When the swelling rate of the coating exceeds 10%, the coating will destroy its conductive network due to the absorption of too much electrolyte, resulting in an increase in its resistance, thereby affecting the rate performance of the electrochemical device. When the dissolution rate of the coating exceeds 3%, the low molecular weight part of the copolymer in the coating is easily dissolved into the electrolyte, which increases the side reactions during the cycle of the electrochemical device, thereby affecting its performance.

According to some embodiments of the present application, the coating has a melting point of 100°C to 130°C. In this application, a differential scanning calorimeter (DSC) is used to test the melting point of the coating. During the test, the coating is completely immersed in the electrolyte solvent. In some embodiments, the electrolyte solvent includes ethylene carbonate (EC) and dimethyl carbonate (DMC). In some embodiments, the electrolyte solvent is ethylene carbonate (EC) and dimethyl carbonate (DMC), and the mass ratio EC:DMC=1:1. When the melting point of the coating is too low, the PTC effect of the coating is easily triggered during the preparation process of the electrochemical device (such as the formation process), which will seriously limit the application of the electrochemical device and affect the normal use experience. When the melting point of the coating is too high, the PTC effect of the coating responds slowly, which cannot effectively control the thermal runaway of the electrochemical device.

According to some embodiments of the present application, the resistance of the coating is 0.05Ω to 0.4Ω at a temperature of 25°C. In this application, the resistance of the coating at a temperature of 25° C. means the resistance of the coating after soaking in the electrolyte solvent at 25° C. for 24 hours. In some embodiments, the electrolyte solvent includes ethylene carbonate (EC) and dimethyl carbonate (DMC). In some embodiments, the electrolyte solvent is ethylene carbonate (EC) and dimethyl carbonate (DMC), and the mass ratio EC:DMC=1:1. According to some embodiments of the present application, when the resistance of the coating is in the range of 0.05Ω to 0.4Ω at 25° C., the internal resistance of the electrochemical device at room temperature will not be affected, and thus its rate performance will not be affected. When the resistance of the coating is greater than 0.4Ω at a temperature of 25°C, the electronic pathways in the coating will be affected, and the internal resistance of the electrochemical device will increase, thereby affecting the rate performance of the electrochemical device. When the resistance of the coating is less than 0.05Ω at 25°C, the mass content of the conductive agent in the coating increases, and the mass content of the copolymer decreases accordingly, which weakens the PTC effect of the coating and improves the thermal safety performance of the electrochemical device. limited.

According to some embodiments of the present application, the resistance of the coating is 15Ω to 40Ω at a temperature of 130°C. In this application, the resistance of the coating at a temperature of 130° C. means the resistance of the coating after soaking in an electrolyte solution at 130° C. for 30 minutes. In some embodiments, the electrolyte solvent includes ethylene carbonate (EC) and dimethyl carbonate (DMC). In some embodiments, the electrolyte solvent is ethylene carbonate (EC) and dimethyl carbonate (DMC), and the mass ratio EC:DMC=1:1. According to some embodiments of the present application, when the resistance of the coating is in the range of 15Ω to 40Ω at a temperature of 130° C., the electrochemical device has excellent thermal safety performance under thermal abuse. If the resistance of the coating is less than 15Ω at a temperature of 130 °C, it indicates that the PTC effect of the coating is limited, which is not conducive to the improvement of the thermal safety performance of the electrochemical device. If the resistance of the coating is greater than 40Ω at a temperature of 130° C., the resistance of the coating at room temperature will increase accordingly, which is not conducive to the rate performance of the electrochemical device.

According to some embodiments of the present application, the thickness of the coating is 1 μm to 12 μm. The thickness of the coating in this application is the total thickness of the coating in the pole piece, that is, the sum of the thicknesses of the coatings on both sides of the current collector. When the thickness of the coating is too low, the PTC effect of the coating is weakened, and the safety performance of the electrochemical device is reduced. When the thickness of the coating is too high, the energy density of the electrochemical device will be reduced, which is not conducive to the improvement of the kinetic performance of the electrochemical device.

According to some embodiments of the present application, the electrode sheet is a positive electrode sheet.

In a second aspect, the present application provides an electronic device, which includes the electrochemical device of the first aspect.

The present application provides an electrochemical device and an electronic device including the electrochemical device. The electrochemical device of the present application includes an electrode sheet with a PTC coating. In this application, by adjusting the structure and composition of the copolymer in the PTC coating, the copolymer has both low electrolyte absorption capacity and high thermal expansion performance, so that the electrochemical device has high safety performance and rate performance.

Description of drawings

Fig. 1 is a schematic diagram of an electrode sheet of an electrochemical device according to an embodiment of the present application, wherein 1 is an active material layer, 2 is a coating, and 3 is a current collector.

Detailed ways

In order to make the purpose, technical solutions and advantages of the present application clearer, the technical solutions of the present application will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are part of the embodiments of the present application, rather than all of them. example. The related examples described herein are illustrative in nature and are used to provide a basic understanding of the application. The examples of the present application should not be construed as limiting the present application.

For the sake of brevity, only certain numerical ranges are specifically disclosed herein. However, any lower limit can be combined with any upper limit to form an unexpressed range; and any lower limit can be combined with any other lower limit to form an unexpressed range, just as any upper limit can be combined with any other upper limit to form an unexpressed range. Furthermore, each individually disclosed point or individual value may serve as a lower or upper limit by itself in combination with any other point or individual value or with other lower or upper limits to form an unexpressly recited range.

In the description herein, unless otherwise specified, "above" and "below" include the number.

Unless otherwise stated, the terms used in the present application have the known meanings generally understood by those skilled in the art. Unless otherwise stated, the values of the parameters mentioned in the present application can be measured by various measurement methods commonly used in the art (for example, can be tested according to the methods given in the examples of the present application).

A list of items to which the terms "at least one of", "at least one of", "at least one of" or other similar terms are concatenated can mean any combination of the listed items. For example, if the items A and B are listed, the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if the items A, B, and C are listed, the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may comprise a single component or multiple components. Item B may comprise a single component or multiple components. Item C may comprise a single component or multiple components.

1. Electrochemical device

In a first aspect, the present application provides an electrochemical device, which includes an electrode sheet, the electrode sheet includes a current collector, an active material layer disposed on at least one side of the current collector, and an active material layer disposed between the current collector and the active material layer. The interlayer coating, wherein the coating comprises a copolymer having a thermal expansion rate of less than 1% at a temperature of 100°C and a thermal expansion rate of greater than 15% at a temperature of 130°C.

According to some embodiments of the present application, the coefficient of thermal expansion of the copolymer at a temperature of 100°C is 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.55% , 0.6%, 0.65%, 0.7%, 0.75%, 0.8%, 0.85%, 0.9%, 0.95%, or a range consisting of any two of these values. In some embodiments, the copolymer has a thermal expansion rate of 0.1% to 0.8% at a temperature of 100°C. According to some embodiments of the present application, the thermal expansion rate of the copolymer at a temperature of 130° C. is 16%, 18%, 20%, 22%, 25%, 27%, 29%, 30%, 32%, 35%, 38%. , 40%, or a range consisting of any two of these values. In some embodiments, the copolymer has a thermal expansion rate of 15.5% to 36% at a temperature of 130°C.

When the electrochemical device is thermally abused, the thermal expansion of the copolymer in the coating will destroy the conductive network of the coating and trigger its PTC effect, thereby improving the safety performance of the electrochemical device. Therefore, the high thermal expansion ability of the copolymer at high temperature can effectively improve the thermal safety performance of electrochemical devices. However, since the electrochemical device will also be at a relatively high temperature (such as 40°C to 70°C) during the preparation process (such as the chemical conversion process), therefore, in order to ensure the normal preparation process, it is necessary for the copolymer to have a low temperature during the process. thermal expansion capacity. The thermal expansion rate of the copolymer in the coating of the present application is less than 1% at a temperature of 100°C, and when the thermal expansion rate is greater than 15% at a temperature of 130°C, it can ensure that the coating has excellent PTC effect and stable resistance in the electrolyte environment, thereby making Electrochemical devices have both high safety performance and rate performance. If the thermal expansion rate of the copolymer is less than 15% at a temperature of 130° C., the PTC effect of the coating is limited, and the improvement of the thermal safety performance of the electrochemical device is limited. If the thermal expansion rate of the copolymer is greater than 1% at 100°C, the coating may trigger the PTC effect near 100°C, and the electrochemical device may trigger the PTC effect during formation, which will limit the application of the electrochemical device and affect normal use. experience.

In the present application, the coefficient of thermal expansion of the copolymer represents the rate of change of the length of the copolymer film during the ascending temperature rise process after the copolymer film is soaked in the electrolyte solvent at 25°C for 24 hours in the dynamic thermomechanical analysis (DMA) test, Wherein at 100 DEG C of temperature thermal expansion rate=(the length of copolymer adhesive film at 100 DEG C of temperature-the length of copolymer adhesive film at room temperature)/the length of copolymer adhesive film at room temperature×100%, at 130 DEG C of temperature thermal expansion rate=( The length of the copolymer film at 130°C - the length of the copolymer film at room temperature)/the length of the copolymer film at room temperature × 100%. In this application, a DMA850 dynamic mechanical analyzer is used for the DMA test, the tensile mode is selected, the tensile force is 5g, and the heating rate is 3°C/min. In some embodiments, the electrolyte solvent includes ethylene carbonate (EC) and dimethyl carbonate (DMC). In some embodiments, the electrolyte solvent is ethylene carbonate (EC) and dimethyl carbonate (DMC), and the mass ratio EC:DMC=1:1.

According to some embodiments of the present application, the weight average molecular weight of the copolymer is 200,000 to 1.5 million. In some embodiments, the weight average molecular weight of the copolymer is 300,000, 450,000, 500,000, 600,000, 700,000, 800,000, 900,000, 1 million, 1.1 million, 1.15 million, 1.25 million, 1.3 million, 140 Ten thousand or a range of any two of these values. In some embodiments, the copolymer has a weight average molecular weight of 400,000 to 1.2 million. When the molecular weight of the copolymer is too low, the entanglement of the macromolecules is weak, and the molecular chain of the copolymer is very easy to move, so that the thermal expansion rate of the copolymer is very high when the temperature is lower than 100 ° C, and it is easy to be used in the preparation process of the electrochemical device (such as the formation process). ) triggers the PTC effect of the coating, which affects the preparation process of the electrochemical device. When the molecular weight of the copolymer is too high, the movement of the molecular segments of the copolymer is difficult, and the thermal expansion performance at high temperature is reduced, thereby affecting the PTC effect of the coating.

According to some embodiments of the present application, the copolymer includes a first structural unit derived from a monomer A and a second structural unit derived from a monomer B, and the monomer A includes acrylic acid, acrylonitrile, acrylamide or vinyl acetate. At least one, monomer B includes at least one of vinylidene fluoride, methyl methacrylate or styrene. Compared with the copolymer, the electrolyte solvent is a small molecule. If it wants to enter between the molecular chains of the copolymer, the electrolyte solvent needs to destroy the force between the molecular chains of the copolymer, so that the interaction between the molecular chains is weakened. The channel that houses it. However, there are strong polar groups such as carboxyl, nitrile, and amide groups in the first structural unit of the copolymer of the present application. These strong polar groups make the interaction between the molecular chains of the copolymer and the interior of the molecular chains Stronger, the electrolyte solvent is difficult to destroy it, and it is difficult to enter between the macromolecular chains, so that the swelling of the copolymer in the electrolyte solvent is low. The low swelling of the copolymer is beneficial to maintain the integrity of the conductive network in the coating, reduce the internal resistance of the electrochemical device, and then improve the rate performance of the electrochemical device. The melting point of the copolymer obtained by copolymerization of monomer B is between 120°C and 170°C, which is close to the PTC response temperature of the electrochemical device. At the same time, it has strong molecular chain movement ability, weak intermolecular force, and excellent high-temperature thermal expansion performance. The copolymer containing the second structural unit derived from monomer B can trigger the PTC effect in time and quickly when the electrochemical device reaches the PTC response temperature, increase the resistance and cut off the electronic path, thereby improving thermal runaway and improving the performance of the electrochemical device. safety performance. On the one hand, the present application introduces the first structural unit with strong polar groups such as carboxyl, nitrile, and amide groups in the copolymer, so that the copolymer has low swelling in the electrolyte solvent, and on the other hand introduces The second structural unit of vinylidene fluoride, methyl methacrylate or styrene, makes the copolymer have a high expansion rate at high temperature. The low swelling property can avoid the destruction of the conductive network of the coating caused by the copolymer absorbing liquid, so that the conductive network in the electrochemical device can be maintained stable, and the rate performance of the electrochemical device can be avoided from being affected. The high thermal expansion rate makes the coating have an excellent PTC effect, which can effectively improve the safety performance of the electrochemical device. Therefore, the electrochemical device of the present application has both high safety performance and rate performance.

According to some embodiments of the present application, based on the total mass of the copolymer, the mass content of the first structural unit is 20% to 80%. In some embodiments, the mass content of the first structural unit is 30%, 35%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, or a range consisting of any two of these values . In some embodiments, the mass content of the first structural unit is 40% to 60%. According to some embodiments of the present application, based on the total mass of the copolymer, the mass content of the second structural unit is 20% to 80%. In some embodiments, the mass content of the second structural unit is 30%, 35%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, or a range consisting of any two of these values . In some embodiments, the mass content of the second structural unit is 40% to 60%. As the mass content of the first structural unit increases, the ability of the copolymer to absorb the electrolyte decreases, the swelling of the copolymer in the electrolyte decreases, the maintenance of the conductive network in the electrolyte is good, and the internal resistance of the electrochemical device is basically not affected. Influence, which in turn makes the electrochemical device have better rate performance. However, when the mass content of the first structural unit is too high, the mass content of the second structural unit will decrease accordingly, the PTC effect of the copolymer will be weakened, and the safety performance of the electrochemical device will be correspondingly reduced.

According to some embodiments of the present application, the coating further includes a conductive agent. In some embodiments, based on the total mass of the coating, the mass content of the copolymer is 60% to 98%. In some embodiments, the mass content of the copolymer is 65%, 70%, 77%, 80%, 83%, 85%, 87%, 90%, 95%, or a range consisting of any two of these values. In some embodiments, the mass content of the copolymer is 75% to 90%. In some embodiments, based on the total mass of the coating, the mass content of the conductive agent is 2% to 40%. In some embodiments, the mass content of the conductive agent is 35%, 30%, 23%, 20%, 17%, 15%, 13%, 10%, 5%, or a range consisting of any two of these values. In some embodiments, the mass content of the copolymer is 10% to 25%. As the mass content of the copolymer increases, the PTC effect of the copolymer is enhanced, which is beneficial to improving the safety performance of the electrochemical device. However, when the mass content of the copolymer is too high, the mass content of the conductive agent will decrease accordingly, the conductivity of the coating will deteriorate, and the internal resistance of the electrochemical device will increase.

According to some embodiments of the present application, the swelling rate of the coating is less than 10%. In some embodiments, the coating has a swelling rate of 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or a range consisting of any two of these values. In some embodiments, the coating has a swelling rate of 0.5% to 9.5%. According to some embodiments of the present application, the dissolution rate of the coating is less than 3%. In some embodiments, the dissolution rate of the coating is 0.1%, 0.3%, 0.5%, 0.7%, 0.9%, 1.0%, 1.2%, 1.5%, 1.7%, 2.0%, 2.5%, 2.7%, or these values The range consisting of any two of them. In some embodiments, the coating has a dissolution rate of 0.1% to 2.5%. When the swelling rate of the coating exceeds 10%, the coating will destroy its conductive network due to the absorption of too much electrolyte, resulting in an increase in its resistance, thereby affecting the rate performance of the electrochemical device. When the dissolution rate of the coating exceeds 3%, the low molecular weight part of the copolymer in the coating is easily dissolved into the electrolyte, which increases the side reactions during the cycle of the electrochemical device, thereby affecting its performance.

In the present application, the swelling rate of the coating means the weight change rate of the coating before and after immersion in the electrolyte solvent, and the calculation formula can be swelling rate=(weight before immersion−weight after immersion)/weight before immersion×100%. The dissolution rate of the coating represents the weight change rate of the coating after soaking in the electrolyte solvent and drying, and the calculation formula can be dissolution rate=(weight before soaking-weight after drying)/weight before soaking×100%. Wherein, the soaking temperature is 60°C, the soaking time is 6 days, the drying time is 2 days, and the drying temperature is 90°C. In some embodiments, the electrolyte solvent includes ethylene carbonate (EC) and dimethyl carbonate (DMC). In some embodiments, the electrolyte solvent is ethylene carbonate (EC) and dimethyl carbonate (DMC), and the mass ratio EC:DMC=1:1.

According to some embodiments of the present application, the coating has a melting point of 100°C to 130°C. In some embodiments, the coating has a melting point in the range of 110°C, 115°C, 123°C, 125°C, 129°C, or a combination of any two of these values. In some embodiments, the coating has a melting point of 117°C to 127°C. When the melting point of the coating is too low, the PTC effect of the coating is easily triggered in the preparation process of the electrochemical device, such as the formation process, which will seriously limit the application of the electrochemical device and affect the normal use experience. When the melting point of the coating is too high, the PTC effect of the coating responds slowly, which cannot effectively control the thermal runaway of the electrochemical device.

In this application, a differential scanning calorimeter (DSC) is used to test the melting point of the coating. During the test, the coating is completely immersed in the electrolyte solvent. In some embodiments, the electrolyte solvent includes ethylene carbonate (EC) and dimethyl carbonate (DMC). In some embodiments, the electrolyte solvent is ethylene carbonate (EC) and dimethyl carbonate (DMC), and the mass ratio EC:DMC=1:1.

According to some embodiments of the present application, the resistance of the coating is 0.05Ω to 0.4Ω at a temperature of 25°C. In some embodiments, the coating has an electrical resistance of 0.1 Ω, 0.15 Ω, 0.2 Ω, 0.25 Ω, 0.3 Ω, 0.35 Ω, or a range consisting of any two of these values at a temperature of 25°C. When the resistance of the coating is in the range of 0.05Ω to 0.4Ω at a temperature of 25°C, the internal resistance of the electrochemical device at room temperature will not be affected, and thus its rate performance will not be affected. When the resistance of the coating is greater than 0.4Ω at a temperature of 25°C, the electronic pathways in the coating will be affected, and the internal resistance of the electrochemical device will increase, thereby affecting the rate performance of the electrochemical device. When the resistance of the coating is less than 0.05Ω at 25°C, the mass content of the conductive agent in the coating increases, and the mass content of the copolymer decreases accordingly, which weakens the PTC effect of the coating and improves the thermal safety performance of the electrochemical device. limited.

In this application, the resistance of the coating at a temperature of 25° C. means the resistance of the coating after soaking in the electrolyte solvent at 25° C. for 24 hours. In some embodiments, the electrolyte solvent includes ethylene carbonate (EC) and dimethyl carbonate (DMC). In some embodiments, the electrolyte solvent is ethylene carbonate (EC) and dimethyl carbonate (DMC), and the mass ratio EC:DMC=1:1.

According to some embodiments of the present application, the resistance of the coating is 15Ω to 40Ω at a temperature of 130°C. In some embodiments, the coating has an electrical resistance of 17Ω, 20Ω, 23Ω, 25Ω, 27Ω, 30Ω, 33Ω, 35Ω, 37Ω, 39Ω, or a range consisting of any two of these values at a temperature of 130°C. With the coating resistance ranging from 15 Ω to 40 Ω at 130 °C, the electrochemical device has excellent thermal safety performance under thermal abuse conditions. If the resistance of the coating is less than 15Ω at a temperature of 130 °C, it indicates that the PTC effect of the coating is limited, which is not conducive to the improvement of the thermal safety performance of the electrochemical device. If the resistance of the coating is greater than 40Ω at a temperature of 130° C., the resistance of the coating at room temperature will increase accordingly, which is not conducive to the rate performance of the electrochemical device.

In this application, the resistance of the coating at a temperature of 130° C. means the resistance of the coating after soaking in an electrolyte solution at 130° C. for 30 minutes. In some embodiments, the electrolyte solvent includes ethylene carbonate (EC) and dimethyl carbonate (DMC). In some embodiments, the electrolyte solvent is ethylene carbonate (EC) and dimethyl carbonate (DMC), and the mass ratio EC:DMC=1:1.

According to some embodiments of the present application, the normal temperature resistance of the coating is 0.05Ω to 0.3Ω. In some embodiments, the room temperature resistance of the coating is in the range of 0.1Ω, 0.15Ω, 0.2Ω, 0.25Ω, 0.27Ω, or a combination of any two of these values.

According to some embodiments of the present application, the thickness of the coating is 1 μm to 12 μm. In some embodiments, the thickness of the coating is in the range of 2.5 μm, 3.5 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, or a combination of any two of these values. In some embodiments, the coating has a thickness of 2 μm to 5 μm. When the thickness of the coating is too low, the PTC effect of the coating is weakened, and the safety performance of the electrochemical device is reduced. When the thickness of the coating is too high, the energy density of the electrochemical device will be reduced, which is not conducive to the improvement of the kinetic performance of the electrochemical device. The thickness of the coating in this application is the total thickness of the coating in the pole piece, that is, the sum of the thicknesses of the coatings on both sides of the current collector.

According to some embodiments of the present application, the electrode sheet is a positive electrode sheet and/or a negative electrode sheet. In some embodiments, the electrode tab is a positive electrode tab. In some embodiments, the electrode tab is a negative electrode tab.

According to some embodiments of the present application, the active material layer includes positive active material or negative active material. In some embodiments, the positive electrode active material may include lithium cobaltate, lithium nickel manganese cobaltate, lithium nickel manganese aluminate, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium manganese iron phosphate, silicic acid At least one of lithium iron, lithium vanadium silicate, lithium cobalt silicate, lithium manganese silicate, spinel lithium manganese oxide, spinel lithium nickel manganese oxide, and lithium titanate. In some embodiments, the negative electrode active material may include materials that can reversibly intercalate/deintercalate lithium ions, lithium metal, lithium metal alloys, materials capable of doping/dedoping lithium, or transition metal oxides, such as Si, SiO _x (0<x<2), silicone and other materials. The material that reversibly intercalates/deintercalates lithium ions may be a carbon material. The carbon material can be any carbon-based negative active material commonly used in lithium-ion rechargeable electrochemical devices. Examples of carbon materials include crystalline carbon, amorphous carbon, and combinations thereof. The crystalline carbon may be amorphous or plate-shaped, platelet-shaped, spherical or fibrous natural or artificial graphite. The amorphous carbon may be soft carbon, hard carbon, mesophase pitch carbonization product, fired coke, or the like. Both low-crystalline carbon and high-crystalline carbon can be used as the carbon material. As the low-crystalline carbon material, soft carbon and hard carbon may be generally included. As highly crystalline carbon materials, natural graphite, crystalline graphite, pyrolytic carbon, mesophase pitch-based carbon fibers, mesocarbon microbeads, mesophase pitch, and high-temperature calcined carbons (such as petroleum or coke derived from coal tar pitch) may generally be included. ).

According to some embodiments of the present application, the active material layer further includes a binder. In some embodiments, the binder may include various binder polymers such as polyvinylidene fluoride, polytetrafluoroethylene, polyolefins, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, modified At least one of polyvinylidene fluoride, modified SBR rubber or polyurethane. In some embodiments, the polyolefin-based binder includes at least one of polyethylene, polypropylene, polyalkene, polyenol, or polyacrylic acid.

According to some embodiments of the present application, the active material layer further includes a conductive material to improve electrode conductivity. Any conductive material can be used as the conductive material as long as it does not cause a chemical change. Examples of conductive materials include: carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fibers, etc.; metal-based materials such as metal powder or metal fibers including copper, nickel, aluminum, silver, etc. ; Conductive polymers, such as polyphenylene derivatives, etc.; or their mixtures.

According to some embodiments of the present application, the current collector includes a positive electrode current collector or a negative electrode current collector. In some embodiments, a metal foil or a composite current collector can be used as the positive electrode current collector. For example, aluminum foil can be used. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate. In some embodiments, the negative electrode current collector can be copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with conductive metal, or a combination thereof.

The electrochemical device of the present application also includes an electrolytic solution, and the electrolytic solution includes a lithium salt and a non-aqueous solvent.

In some embodiments of the present application, the lithium salt is selected from LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ , LiB(C ₆ H ₅ ) ₄ , LiCH ₃ SO ₃ , LiCF ₃ SO ₃ , LiN(SO ₂ CF ₃ ) _2. One or more of LiC(SO ₂ CF ₃ ) ₃ , LiSiF ₆ , LiBOB and lithium difluoroborate. For example, LiPF ₆ may be selected as a lithium salt because it can give high ion conductivity and improve cycle characteristics.

The non-aqueous solvent can be carbonate compound, carboxylate compound, ether compound, other organic solvent or their combination.

The above-mentioned carbonate compound can be a chain carbonate compound, a cyclic carbonate compound, a fluorocarbonate compound or a combination thereof.

Examples of the aforementioned chain carbonate compounds are dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), carbonic acid Methyl ethyl ester (MEC) and combinations thereof. Examples of cyclic carbonate compounds are ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylethylene carbonate (VEC), and combinations thereof. Examples of fluorocarbonate compounds are fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate Ethyl carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-dicarbonate Fluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, trifluoromethylethylene carbonate, and combinations thereof.

Examples of the above carboxylate compounds are methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone , decanolactone, valerolactone, mevalonolactone, caprolactone, and combinations thereof.

Examples of the aforementioned ether compounds are dibutyl ether, tetraglyme, diglyme, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethyl Oxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, and combinations thereof.

Examples of the aforementioned other organic solvents are dimethylsulfoxide, 1,2-dioxolane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, Formamide, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, trioctyl phosphate and phosphate esters and combinations thereof.

The electrochemical device of the present application further includes a separator. The present application has no particular limitation on the material and shape of the isolation membrane, which can be any technology disclosed in the prior art. In some embodiments, the separator includes a polymer or an inorganic substance formed of a material stable to the electrolyte of the present application.

For example, a release film may include a substrate layer and a surface treatment layer. The substrate layer is non-woven fabric, film or composite film with a porous structure, and the material of the substrate layer is at least one selected from polyethylene, polypropylene, polyethylene terephthalate and polyimide. Specifically, polypropylene porous film, polyethylene porous film, polypropylene non-woven fabric, polyethylene non-woven fabric or polypropylene-polyethylene-polypropylene porous composite film can be selected.

At least one surface of the substrate layer is provided with a surface treatment layer, and the surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by mixing polymers and inorganic materials.

The inorganic layer includes inorganic particles and a binder, and the inorganic particles are selected from aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium oxide, tin oxide, cerium oxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, At least one of yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide and barium sulfate. The binder is selected from polyvinylidene fluoride, copolymer of vinylidene fluoride-hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinyl pyrrolidone, polyvinyl alkoxy , polymethyl methacrylate, polytetrafluoroethylene and polyhexafluoropropylene at least one.

Polymer is contained in the polymer layer, and the material of polymer is selected from polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinyl pyrrolidone, polyvinyl alkoxide, polyvinylidene fluoride, At least one of poly(vinylidene fluoride-hexafluoropropylene).

According to some embodiments of the present application, the electrochemical device of the present application includes, but is not limited to: all kinds of primary batteries, secondary batteries, fuel cells, solar cells or capacitors. In some embodiments, the electrochemical device is a lithium secondary battery. In some embodiments, lithium secondary batteries include, but are not limited to: lithium metal secondary batteries, lithium ion secondary batteries, lithium polymer secondary batteries, or lithium ion polymer secondary batteries.

2. Electronic devices

The present application further provides an electronic device comprising the electrochemical device of the present application.

The electronic device or device of the present application is not particularly limited. In some embodiments, electronic devices of the present application include, but are not limited to, notebook computers, pen-input computers, mobile computers, e-book players, cellular phones, portable fax machines, portable copiers, portable printers, headsets , VCRs, LCD TVs, Portable cleaners, Portable CD players, Mini CDs, Transceivers, Electronic organizers, Calculators, Memory cards, Portable tape recorders, Radios, Backup power supplies, Motors, Automobiles, Motorcycles, Power-assisted bicycles, Bicycles , Lighting appliances, toys, game consoles, clocks, electric tools, flashlights, cameras or large batteries for household use, etc.

Below in conjunction with embodiment, further elaborate the present application. It should be understood that these examples are only used to illustrate the present application and are not intended to limit the scope of the present application.

Test Methods

Coating thickness: The battery is disassembled to obtain the pole piece, and the active material is peeled off to obtain the current collector + coating sample. Take a 40cm long and 20cm wide sample (current collector + coating) to ensure that the sample is flat and wrinkle-free. Use Mitutoyo Measure the thickness of 12 different positions within this size range, and take the average value of the thickness of 12 different positions, and count it as D1. Then the coating is peeled off from the current collector, and the average thickness of 12 points of the current collector is measured by the same method, which is calculated as D0. Coating thickness: D1-D0.

Copolymer thermal expansion rate: Cut the copolymer film into a strip with a width of 5mm and a length of 50mm. Use DMA850 dynamic mechanical analyzer to test. Specifically, the tensile jig is selected, the electrolyte solvent (EC:DMC=1:1) is placed in the solvent tank, and then the cut sample is soaked in the solvent at 25° C. for 24 hours before testing. The distance between the fixtures means that the length of the test section is 15mm, the tensile force is 5g, the electrolyte solvent is raised from room temperature to 150°C at a rate of 3°C/min, the length of the test section at 100°C is L1, and the length of the test section at 130°C To count as L2, the thermal expansion rate at 100°C = (L1-15)/15×100%, and the thermal expansion rate at 130°C = (L2-15)/15×100%. The average value of 10 splines was tested.

Swelling rate and dissolution rate of the coating: The battery is disassembled to obtain the pole piece, the active material is peeled off to obtain the current collector + coating sample, the coating is dried and cut into square pieces of 5cm×5cm, weighed, and the weight is m1. Then the square sheet was sealed in the electrolyte solvent (EC:DMC=1:1), and placed in a blast oven at 60°C. After 7 days, wipe off the electrolyte solvent on the surface of the square sheet and weigh it, and the weight is m2. Then the square piece was dried in an oven at 85°C, and after removing the electrolyte solvent, the weight was measured as m3. Then the swelling rate of the coating=(m2-m1)/m1×100%, the dissolution rate=(m1-m3)/m1×100%. The average value of 5 samples was tested.

Melting point of the coating: The melting point of the coating was measured using a differential scanning calorimeter (DSC). Specifically, 5 mg to 20 mg of the coating is placed in a closed DSC crucible, and the electrolyte solvent (EC:DMC=1:1) is added dropwise to ensure that the coating is completely immersed in the electrolyte solvent and sealed. The melting point of the coating was tested at a heating rate of 5°C/min. The average value of 5 samples was tested.

The normal temperature resistance of the coating: the battery is disassembled to obtain the pole piece, and the active material is peeled off to obtain the current collector + coating sample. Take 10 pieces of coating with a length of 15 cm and a width of 5 cm to ensure that it is smooth and wrinkle-free. At room temperature, use the Yuanneng Technology BER1200 pole piece resistance meter to test the resistance of 12 different positions of the single coating along the vertical direction, and take the average value of the resistance of 10 coatings.

The resistance of the coating in the electrolyte solvent at 25°C: the battery is disassembled to obtain the pole piece, and the active material is peeled off to obtain the current collector + coating sample. Take 10 pieces of coating with a length of 15 cm and a width of 5 cm to ensure that they are smooth and wrinkle-free. After soaking in electrolyte solvent (EC:DMC=1:1) at 25°C for 24 hours, take out and wipe off the surface electrolyte. At room temperature, use Yuanneng Technology BER1200 pole sheet resistance meter to test the resistance of 12 different positions of a single coating along the vertical direction, take the average value of the resistance of 10 coatings, and calculate it as the coating at 25°C in the electrolyte solvent resistance at temperature.

The resistance of the coating in the electrolyte solvent at 130°C: the battery is disassembled to obtain the pole piece, the active material is peeled off, and the current collector + coating sample is obtained; the positive aluminum shell, the negative aluminum shell, and the steel sheet matching the size of the aluminum shell are taken. The current collector + coating sample is prepared as a disc with the same size as the steel sheet, and the buckle is assembled. The buckle structure is the negative electrode aluminum shell + steel sheet + coating sample + steel sheet + positive aluminum shell, and the electrolyte solvent is added dropwise (EC:DMC=1:1), so that the coating is completely immersed in the electrolyte solvent. Use Yuanneng Technology BER1200 plate resistance meter, place the resistance meter in a blast oven, place the button battery in the resistance meter test fixture, and connect it to a multi-channel thermometer to test the actual temperature of the button battery. Raise the temperature in the blast oven from room temperature to 130°C at a rate of 5°C/min, and keep it warm for 30 minutes, then test the resistance of the coin cell, which is the resistance of the coating in the electrolyte solvent at 130°C. The average value of 10 samples was tested.

Lithium-ion battery internal resistance: Use TH2829C resistance tester to apply a current of frequency 1KHz and 10mA to the lithium-ion battery to test its internal resistance.

Lithium-ion battery discharge rate performance: at 25°C, a lithium-ion battery with an SOC of 0% is charged to 100% SOC at 0.2C constant current, charged to 0.05V at constant voltage, and then discharged to 0% SOC at 0.2C DC. The discharge capacity was measured as D0. Then charge to 100% SOC with 0.2C constant current, charge to 0.05V with constant voltage, and then discharge to 0% SOC with 3C DC, the obtained discharge capacity is D1. The 3C discharge rate performance of lithium-ion batteries = D1/D0×100%.

Lithium-ion battery hot box test: Lithium-ion battery is subjected to constant current (CC) at 0.5C to SOC% of the state of charge, and then placed in a thermal shock box at 130°C and 150°C respectively, and stops after 1 hour of storage or stops immediately after thermal runaway , collect voltage and surface temperature changes, and record experimental phenomena. The battery passes the test if it does not catch fire, explode or emit smoke. For each test condition, 10 lithium-ion batteries were tested.

Lithium-ion battery overcharge test: Charge the lithium-ion battery with 3C constant current (CC) to 5V, and then charge it with 5V constant voltage (CV) for 2h, collect voltage and surface temperature changes, and record experimental phenomena. The battery passes the test if it does not catch fire, explode or emit smoke.

Charge the lithium-ion battery with 3C constant current (CC) to 6V, and then charge it with 6V constant voltage (CV) for 2h, collect the voltage and surface temperature changes, and record the experimental phenomena. The battery passes the test if it does not catch fire, explode or emit smoke. For each test condition, 10 lithium-ion batteries were tested.

Examples and comparative examples

1. Preparation of positive pole piece: Dissolve the copolymer into NMP to prepare a solution, add a conductive agent and stir evenly to obtain a PTC slurry. The mass content of the conductive agent and copolymer is shown in the table below. At least one surface of the aluminum foil current collector is coated with PTC slurry and dried to prepare a PTC primer layer. Fully stir and mix lithium cobaltate, polyvinylidene fluoride (PVDF), and conductive carbon black in an appropriate amount of NMP solvent in a weight ratio of 98:1:1 to form a uniform slurry. The prepared slurry is coated on the above-mentioned PTC undercoat layer, dried, and cold-pressed to obtain a positive electrode sheet.

2. Preparation of negative electrode sheet: Fully stir and mix graphite, polymethacrylic acid and styrene-butadiene rubber in an appropriate amount of deionized water solvent in a weight ratio of 98:1:1 to form a uniform negative electrode slurry. The prepared negative electrode slurry is coated on the above-mentioned copper foil current collector, dried, and cold pressed to obtain a negative electrode sheet.

3. Preparation of lithium-ion battery: stack the positive pole piece, separator, and negative pole piece in order, so that the separator is between the positive electrode and the negative electrode to play the role of isolation. Winding to obtain an electrode assembly. The electrode assembly is placed in the outer package, and after vacuum drying, the electrolyte is injected and packaged. Lithium-ion batteries are obtained through processes such as formation, degassing, and trimming. Among them, 7 μm PE is used as the isolation film. The electrolyte solution includes a solvent mixed with ethylene carbonate (EC) and dimethyl carbonate (DMC) (about 1:1 by weight) and LiPF ₆ , the concentration of LiPF ₆ is about 1.15mol/L.

Table 1 shows the effect of monomer A, monomer B, the content of the first structural unit derived from monomer A, and the molecular weight of the copolymer on the performance of the coating and the lithium-ion battery comprising the coating.

Wherein, in the coatings of each example and comparative example, the conductive agent is conductive carbon black, the thickness of the coating is 3 μm, and the mass content of the copolymer is 85% based on the total mass of the coating. The electrode sheet of Comparative Example 3 does not contain a coating. In the monomer A of Example 10, the mass ratio of acrylic acid to acrylonitrile is 1:1. In the monomer A of Example 11, the mass ratio of acrylic acid to vinyl acetate is 1:1. In the monomer B of Example 20, the mass ratio of vinylidene fluoride to methyl methacrylate is 1:1. In the monomer B of Example 21, the mass ratio of vinylidene fluoride to styrene is 1:1.

Table 1-1

Table 1-2

According to the data of Examples 1 to 5, it can be seen that as the molecular weight of the copolymer increases, the internal resistance of the lithium-ion battery decreases correspondingly, and the rate performance becomes better. However, the weight-average molecular weight of the copolymer is too high, the molecular segment movement of the copolymer is difficult, and the thermal expansion performance of the copolymer at high temperature is reduced, which is not conducive to the PTC effect of the coating.

According to the data of Examples 12 to 16, it can be seen that as the mass content of the first structural unit increases, the internal resistance of the lithium-ion battery decreases correspondingly, and the rate performance becomes better. However, when the mass content of the first structural unit is too high, the mass content of the second structural unit will decrease accordingly, the PTC effect of the coating will be weakened, and the safety performance of the lithium-ion battery will be correspondingly reduced.

According to the data of Example 3 and Comparative Example 1 and Comparative Example 2, it can be seen that the copolymer prepared by Monomer A and Monomer B has both low electrolyte absorption capacity and high thermal expansion performance, and is effective in improving lithium-ion batteries. While ensuring safety performance, it can ensure that the rate performance of the battery is not affected.

The content of copolymer in coating, the kind of conductive agent and the thickness of coating are shown in table 2 to coating and the performance influence of the lithium-ion battery that comprises coating.

Wherein, the composition, molecular weight, and thermal expansion coefficients at temperatures of 100° C. and 130° C. of the copolymers in Examples 22 to 35 are the same as those in Example 3.

table 2-1

Table 2-2

According to the data of Examples 22 to 26, it can be seen that as the mass content of the copolymer increases, the PTC effect of the coating is enhanced, and the safety performance of the lithium-ion battery is improved. However, when the mass content of the copolymer is too high, the mass content of the conductive agent will decrease accordingly, the conductivity of the coating will deteriorate, the internal resistance of the lithium-ion battery will increase, and the rate performance will deteriorate.

According to the data of Example 31 to Example 35, it can be seen that when the thickness of the coating is too low, the PTC effect of the coating is weakened, and the safety performance of the lithium ion battery is reduced. When the thickness of the coating is too high, the internal resistance of the lithium-ion battery is too high, and the rate performance is deteriorated. At the same time, if the coating is too thick, the energy density of the lithium-ion battery will be reduced, which is not conducive to the improvement of its kinetic performance.

Although illustrative embodiments have been shown and described, those skilled in the art should understand that the foregoing embodiments are not to be construed as limitations on the present application, and that changes may be made in the embodiments without departing from the spirit, principle and scope of the application. , substitution and modification.

Claims

An electrochemical device, comprising an electrode pole piece, the electrode pole piece comprising a current collector, an active material layer disposed on at least one side of the current collector, and a coating disposed between the current collector and the active material layer layer, wherein the coating comprises a copolymer having a thermal expansion rate of less than 1% at a temperature of 100°C and a thermal expansion rate of the copolymer greater than 15% at a temperature of 130°C.
The electrochemical device according to claim 1, wherein the copolymer has a weight average molecular weight of 200,000 to 1.5 million.
The electrochemical device according to claim 1, wherein the copolymer comprises a first structural unit derived from a monomer A and a second structural unit derived from a monomer B, the monomer A comprising acrylic acid, acrylonitrile , at least one of acrylamide or vinyl acetate, and the monomer B includes at least one of vinylidene fluoride, methyl methacrylate or styrene.
The electrochemical device according to claim 3, wherein, based on the total mass of the copolymer, the mass content of the first structural unit is 20% to 80%, and/or the mass content of the second structural unit 20% to 80%.
The electrochemical device according to claim 1, wherein the coating further comprises a conductive agent, the mass content of the copolymer is 60% to 98% based on the total mass of the coating, and/or the The mass content of the conductive agent is 2% to 40%.
The electrochemical device according to claim 5, wherein the conductive agent comprises at least one of conductive carbon black, acetylene black, graphite, graphene, carbon nanotubes, carbon fibers, aluminum powder, nickel powder and gold powder.
The electrochemical device according to claim 1, wherein the coating satisfies at least one of the following features (a) to (e):

(a) the swelling rate of the coating is less than 10%;

(b) the dissolution rate of the coating is less than 3%;

(c) the melting point of the coating is 100°C to 130°C;

(d) the resistance of the coating is 0.05Ω to 0.4Ω at a temperature of 25°C;

(e) The resistance of the coating is 15Ω to 40Ω at a temperature of 130°C.
The electrochemical device according to claim 1, wherein the coating has a thickness of 1 μm to 12 μm.
The electrochemical device according to claim 1, wherein the electrode tab is a positive pole tab.
An electronic device comprising the electrochemical device according to any one of claims 1 to 9.