Secondary battery and electronic device

Abstract

Disclosed are a secondary battery and an electronic device, the secondary battery including an electrode, a separator, and an electrolyte, the electrode including a current collector and a material layer disposed on at least one surface of the current collector, the material layer including composite foamed particles and an electrode active material, the composite foaming particle comprises a foaming particle and a polymer layer arranged on the surface of the foaming particle, the foaming particle comprises a first polymer and a foaming agent, the polymer layer comprises a second polymer, and the melting point of the first polymer is larger than or equal to that of the second polymer. According to the composite foaming particle, the polymer layer is arranged on the surface of the foaming particle to wrap the foaming agent exposed on the surface of the foaming particle, so that the cycle performance of a secondary battery with an electrode containing a foaming material is improved.

Description

Technical Field

[0001] The present application relates to the field of energy storage technology, and more specifically, to a secondary battery and an electronic device. Background Art

[0002] Battery thermal runaway is an extremely dangerous condition that may cause serious safety accidents, such as explosions and fires, posing a huge threat to the safety of personnel and property. The causes of battery thermal runaway can be attributed to mechanical inducements, electrical inducements, thermal inducements and internal short circuits. Among them, mechanical inducements include extrusion, puncture and collision, electrical inducements include external short circuits, overcharging and over-discharging, thermal inducements include inconsistent heat generation inside the battery and frequent use at too high / low temperatures or fast charging conditions, and internal short circuits include battery cycle aging, storage aging and battery defects themselves.

[0003] At present, one of the strategies to solve the thermal runaway of batteries is to add foaming materials to the electrodes. In the case of abuse and abnormal heating of the battery cells, the foaming materials can produce gas at the corresponding temperature, thereby destroying the conductive network of the electrode, rapidly increasing the internal resistance, and breaking the battery cells to prevent further heating and explosion of the battery cells, thereby solving the problem of thermal runaway of the battery. However, adding foaming materials to the electrodes will reduce the cycle performance of the battery.

[0004] Therefore, it is of great significance to develop a secondary battery to improve the cycle performance of the battery containing foaming materials in the electrode. Summary of the invention

[0005] In view of the above problems existing in the prior art, the object of the present application is to provide a secondary battery and an electronic device, which can improve the cycle performance of a secondary battery containing a foaming material in the electrode.

[0006] The first aspect of the present application provides a secondary battery, including an electrode, a separator and an electrolyte, the electrode including a current collector and a material layer arranged on at least one surface of the current collector, the material layer including composite foamed particles and electrode active materials, the composite foamed particles including foamed particles and a polymer layer arranged on the surface of the foamed particles, the foamed particles including a first polymer and a foaming agent, the polymer layer including a second polymer, and the melting point of the first polymer is greater than or equal to the melting point of the second polymer.

[0007] The composite foamed particles of the present application wrap the exposed foaming agent on the surface of the foamed particles by arranging a polymer layer on the surface of the foamed particles, thereby reducing the probability of the electrolyte directly contacting the exposed foaming agent and reducing the possibility of the foaming agent and the electrode having a side reaction during the charge and discharge process of the secondary battery to deteriorate the electrode interface, thereby improving the cycle performance of the secondary battery containing the foaming material in the electrode.

[0008] The reason why the first polymer and the foaming agent are used to form the foaming particles, and then the second polymer is used to wrap the foaming particles to form the composite foaming particles is that when the polymer and the foaming agent are directly combined, the polymer cannot completely wrap the foaming agent, which still leads to a decrease in the cycle performance of the battery; and the composite foaming particles of the present application first aggregate the foaming agent into nuclei through the first polymer, and use the second polymer to wrap the foaming particles, which can improve the cycle performance of the secondary battery. In addition, internal short circuit or external overcharge or heating may cause high temperature in the secondary battery. When the temperature reaches the melting temperature of the polymer and the decomposition temperature of the foaming agent, it begins to melt and foam. The foam will destroy the conductive network inside the pole piece and block the electron path; moreover, the foam will occupy the gap between the pole piece particles to squeeze out the electrolyte, thereby blocking the ion path while avoiding the exothermic reaction between the electrolyte and the active substance at high temperature; the molten polymer will form a layer of polymer film on the surface of the active substance to further protect the active substance from reacting with chemical crosstalk substances, such as oxidizing or reducing gases formed at the positive or negative electrode; thereby improving the problem of thermal runaway of the battery.

[0009] The reason why the melting point of the first polymer is chosen to be greater than or equal to the melting point of the second polymer is that if the melting point of the second polymer is greater than the melting point of the first polymer, when facing thermal runaway of the battery, although the first polymer and the foaming agent have been stimulated by thermal runaway and begin to melt and foam, the barrier formed by the second polymer still firmly wraps the first polymer and the foaming agent, making it impossible for the foaming agent to play a role, thereby failing to improve the problem of thermal runaway of the battery.

[0010] The present application characterizes the cycle performance of a secondary battery by the number of charge and discharge cycles experienced by the secondary battery when its capacity drops to 80% during multiple charge and discharge cycles. A greater number of cycles indicates a higher cycle performance of the secondary battery.

[0011] In some embodiments, the average particle size of the composite foamed particles is R1 μm, 0.1 ≤ R1 ≤ 5. When R1 is within the above range, the composite foamed particles can be better filled into the gaps in the material layer on the electrode sheet, which is beneficial to improving the material stacking and compaction density in the material layer, and is also beneficial to building a conductive network of the electrode, improving the energy density of the secondary battery, and further improving the cycle performance of the secondary battery containing the foamed material in the electrode.

[0012] In some embodiments, the average thickness of the polymer layer is R2 nm, where 5 ≤ R2 ≤ 100. When R2 is within the above range, the second polymer can not only better wrap the blowing agent exposed on the surface of the foamed particles, but also provide a strong barrier to the foamed particles, reducing the probability of the blowing agent being exposed due to damage during processing, thereby further improving the cycling performance of the secondary battery containing the foaming material in the electrode. In addition, it can also increase the content of the blowing agent, give full play to the role of the composite foamed particles, enable the secondary battery to better handle the thermal runaway problem, and have better safety performance.

[0013] In some embodiments, the number of particles of the electrode active material is N1, and the number of particles of the composite foamed particles is N2, where 0.1 ≤ N1 / N2 ≤ 1. When N1 / N2 is within the above range, the composite foamed particles can be evenly filled in the voids in the material layer on the electrode sheet, which not only helps the composite foamed particles effectively play the role of insulation and blocking chemical crosstalk, can better improve the thermal runaway problem, but also helps to further improve the cycling performance of the secondary battery containing the foaming material in the electrode.

[0014] In some embodiments, the average particle size of the composite foamed particles in the material layer is R1 μm, and the average particle size of the electrode active material is R3 μm, where R3 > R1. When R3 > R1, the composite foamed particles can better fill into the voids in the material layer on the electrode sheet, especially into the voids between the electrode active materials, which is beneficial to improving the material packing and compaction density in the material layer, and is also beneficial to constructing the conductive network of the electrode, improving the energy density of the secondary battery, and thus further improving the cycling performance of the secondary battery containing the foaming material in the electrode.

[0015] In some embodiments, the melting point of the polymer layer is T1 °C, where 90 ≤ T1 ≤ 180. When T1 is within the above range, when the secondary battery faces the high temperature of thermal runaway, it can quickly melt, the internal blowing agent decomposes and foams, blocking the electron-ion path, reducing chemical crosstalk, and reducing the probability of exothermic reactions occurring, enabling the secondary battery to better handle the thermal runaway problem and having better safety performance.

[0016] In some embodiments, the melting point of the separator is T2 °C, where 120 ≤ T2.

[0017] In some embodiments, T1 < T2. When T1 < T2, when the composite foamed particles play a role, the probability of the separator melting and collapsing is small, reducing the phenomenon of positive and negative electrode contact short circuit, being able to better improve the thermal runaway problem, and having better safety performance.

[0018] In some embodiments, based on the mass of the composite foamed particles, the mass percentage content of the blowing agent is 0.1% to 10%.

[0019] In some embodiments, at least two material layers stacked in sequence are disposed on at least one surface of the current collector, the content of the composite foamed particles in the material layer close to the current collector is Q1, and the content of the composite foamed particles in the material layer far from the current collector is Q2, and Q1>Q2. When Q1>Q2, the foaming amount of the bottom layer (the pole piece layer close to the current collector) is large, and the foaming amount of the top layer (the pole piece layer far from the current collector) is small, and there are still gaps in the top layer that are not filled with the foaming agent. The electrolyte squeezed out after the foaming agent in the bottom layer is foamed will be discharged through the gaps in the top layer, and at the same time, the foaming material after the foaming agent in the bottom layer is foamed will fill the gaps in the top layer, avoiding the electrolyte remaining in the pole piece and reacting with the active material to release heat, so that the secondary battery can better avoid the problem of thermal runaway and have better safety.

[0020] In some embodiments, in the composite foamed particles, the foaming temperature of the foaming agent is ≥ the melting point of the first polymer.

[0021] In some embodiments, the foaming agent includes at least one of an azo compound, a sulfonylhydrazide compound, a nitroso compound, sodium carbonate, and sodium bicarbonate.

[0022] In some embodiments, the first polymer includes at least one of polypropylene, polyethylene, polylactic acid, polyvinyl chloride, polyethylene terephthalate, polystyrene, polycarbonate, polyvinyl alcohol, polyurethane, polycarbonate, and polyamide.

[0023] In some embodiments, the second polymer includes at least one of polypropylene, polyethylene, polylactic acid, polyvinyl chloride, polyethylene terephthalate, polystyrene, polycarbonate, polyvinyl alcohol, polyurethane, polycarbonate, and polyamide.

[0024] A second aspect of the present application provides an electronic device, comprising the secondary battery according to the first aspect.

[0025] Compared with the prior art, the beneficial effects of this application are:

[0026] The composite foamed particles of the present application wrap the exposed foaming agent on the surface of the foamed particles by arranging a polymer layer on the surface of the foamed particles, thereby reducing the probability of the electrolyte directly contacting the exposed foaming agent and reducing the possibility of the foaming agent and the electrode having a side reaction during the charge and discharge process of the secondary battery to deteriorate the electrode interface, thereby improving the cycle performance of the secondary battery containing the foaming material in the electrode.

[0027] The reason why the first polymer and the foaming agent are used to form the foaming particles, and then the second polymer is used to wrap the foaming particles to form the composite foaming particles is that when the polymer and the foaming agent are directly combined, the polymer cannot completely wrap the foaming agent, which still leads to a decrease in the cycle performance of the battery; and the composite foaming particles of the present application first aggregate the foaming agent into nuclei through the first polymer, and use the second polymer to wrap the foaming particles, which can improve the cycle performance of the secondary battery. In addition, internal short circuit or external overcharge or heating may cause high temperature in the secondary battery. When the temperature reaches the melting temperature of the polymer and the decomposition temperature of the foaming agent, it begins to melt and foam. The foam will destroy the conductive network inside the pole piece and block the electron path; moreover, the foam will occupy the gap between the pole piece particles to squeeze out the electrolyte, thereby blocking the ion path while avoiding the exothermic reaction between the electrolyte and the active substance at high temperature; the melted polymer will form a layer of polymer film on the surface of the active substance to further protect the active substance from reacting with chemical crosstalk substances, such as oxidizing or reducing gases formed at the positive or negative electrode; thereby improving the problem of thermal runaway of the battery.

[0028] The reason why the melting point of the first polymer is chosen to be greater than or equal to the melting point of the second polymer is that if the melting point of the second polymer is greater than the melting point of the first polymer, when facing thermal runaway of the battery, although the first polymer and the foaming agent have been stimulated by thermal runaway and begin to melt and foam, the barrier formed by the second polymer still firmly wraps the first polymer and the foaming agent, making it impossible for the foaming agent to play a role, thereby failing to improve the problem of thermal runaway of the battery.

[0029] Additional aspects and advantages of the embodiments of the present application will be described, shown, or explained in part in the subsequent description through implementation of the embodiments of the present application. DETAILED DESCRIPTION

[0030] For simplicity, this application only specifically discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form an undefined range; and any lower limit can be combined with other lower limits to form an undefined range, and any upper limit can be combined with any other upper limit to form an undefined range. In addition, each separately disclosed point or single value can itself be combined as a lower limit or upper limit with any other point or single value or with other lower limits or upper limits to form an undefined range.

[0031] Unless otherwise specified, the terms used in this application have the commonly known meanings generally understood by those skilled in the art. Unless otherwise specified, the numerical values ​​of the various parameters mentioned in this application can be measured using various measurement methods commonly used in the art (for example, they can be tested according to the methods given in the examples of this application).

[0032] A list of items connected by the terms "at least one of," "at least one of," "at least one of," or other similar terms may mean any combination of the listed items. For example, if 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 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 contain a single component or multiple components. Item B may contain a single component or multiple components. Item C may contain a single component or multiple components.

[0033] Primary and secondary batteries

[0034] The first aspect of the present application provides a secondary battery, including an electrode, a separator and an electrolyte, the electrode including a current collector and a material layer arranged on at least one surface of the current collector, the material layer including composite foamed particles and electrode active materials, the composite foamed particles including foamed particles and a polymer layer arranged on the surface of the foamed particles, the foamed particles including a first polymer and a foaming agent, the polymer layer including a second polymer, and the melting point of the first polymer is greater than or equal to the melting point of the second polymer.

[0035] The composite foamed particles of the present application wrap the exposed foaming agent on the surface of the foamed particles by arranging a polymer layer on the surface of the foamed particles, thereby reducing the probability of the electrolyte directly contacting the exposed foaming agent and reducing the possibility of the foaming agent and the electrode having a side reaction during the charge and discharge process of the secondary battery to deteriorate the electrode interface, thereby improving the cycle performance of the secondary battery containing the foaming material in the electrode.

[0036] The reason why the first polymer and the foaming agent are used to form the foaming particles, and then the second polymer is used to wrap the foaming particles to form the composite foaming particles is that when the polymer and the foaming agent are directly combined, the polymer cannot completely wrap the foaming agent, which still leads to a decrease in the cycle performance of the battery; and the composite foaming particles of the present application first aggregate the foaming agent into nuclei through the first polymer, and use the second polymer to wrap the foaming particles, which can improve the cycle performance of the secondary battery. In addition, internal short circuit or external overcharge or heating may cause high temperature in the secondary battery. When the temperature reaches the melting temperature of the polymer and the decomposition temperature of the foaming agent, it begins to melt and foam. The foam will destroy the conductive network inside the pole piece and block the electron path; moreover, the foam will occupy the gap between the pole piece particles to squeeze out the electrolyte, thereby blocking the ion path while avoiding the exothermic reaction between the electrolyte and the active substance at high temperature; the melted polymer will form a layer of polymer film on the surface of the active substance to further protect the active substance from reacting with chemical crosstalk substances, such as oxidizing or reducing gases formed at the positive or negative electrode; thereby improving the problem of thermal runaway of the battery.

[0037] The reason why the melting point of the first polymer is chosen to be greater than or equal to the melting point of the second polymer is that if the melting point of the second polymer is greater than the melting point of the first polymer, when facing thermal runaway of the battery, although the first polymer and the foaming agent have been stimulated by thermal runaway and begin to melt and foam, the barrier formed by the second polymer still firmly wraps the first polymer and the foaming agent, making it impossible for the foaming agent to play a role, unable to quickly destroy the conductive network of the electrode, and unable to quickly increase the internal resistance, thereby failing to improve the problem of thermal runaway of the battery.

[0038] The present application characterizes the cycle performance of a secondary battery by the number of charge and discharge cycles experienced by the secondary battery when its capacity drops to 80% during multiple charge and discharge cycles. A greater number of cycles indicates a higher cycle performance of the secondary battery.

[0039] It should be noted that the "surface" in "at least one surface of the current collector" here can be the entire area of ​​the current collector or a partial area of ​​the current collector. This application has no special restrictions as long as the purpose of this application can be achieved.

[0040] The present application has no particular restrictions on the preparation method of the composite foamed particles, as long as the purpose of the present application can be achieved. For example, the preparation method of the composite foamed particles can be specifically as follows:

[0041] S1. Preparation of foamed particles:

[0042] A. mixing a first polymer and a foaming agent in a solvent-1 to obtain a mixed slurry, coating the mixed slurry into a film, and crushing the film to obtain foamed particles;

[0043] or,

[0044] B. mixing the first polymer and the foaming agent, extruding and granulating, to obtain foamed particles;

[0045] S2. Preparation of composite foamed particles:

[0046] The second polymer and solvent-2 are mixed to obtain a mixed solution, and the foamed particles are added into the mixed solution for immersion to obtain composite foamed particles.

[0047] Wherein, in some embodiments, in step A, when mixing the first polymer and the foaming agent in the solvent-1, a mixed additive is also included. In some embodiments, the ratio of the mass of the additive to the total mass of the first polymer and the foaming agent is = (0.5-10): (99.5-90) = additive: (first polymer + foaming agent). In some embodiments, the additive is at least one of a surfactant and a thickener. Wherein, the addition of a surfactant can not only reduce the surface tension of the foaming, making the foaming stable and lasting, but also increase the affinity between the polymer and the active substance, and the polymer is more likely to form a film on the surface of the active substance after melting. Plasticizers can adjust the processing properties of the polymer, such as adjusting the viscosity and fluidity of the polymer after melting when used. Surface surfactants and thickeners commonly used in the art can be used in this application, for example: the surfactant can be at least one of a cationic surfactant, an anionic surfactant, and a non-ionic surfactant. Cationic surfactants include but are not limited to at least one of hexadecyltrimethylammonium bromide, hexadecyltrimethylammonium chloride, and octadecyltrimethylammonium chloride; anionic surfactants include but are not limited to at least one of sodium dodecyl sulfate and sodium dodecylbenzene sulfonate; nonionic surfactants include but are not limited to at least one of octylphenol polyoxyethylene ether, nonylphenol polyoxyethylene ether, and polyoxyethylene sorbitan ester (Tween). Thickeners include but are not limited to at least one of carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, casein, and their salts. In some embodiments, solvent-1 includes but is not limited to at least one of tetrahydrofuran, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, acetone, toluene, chloroform, cyclohexanone, and ethyl acetate. In some embodiments, after coating and film formation, solvent-1 is also removed.

[0048] Wherein, in some embodiments, in step B, when mixing the first polymer and the foaming agent, a mixing additive is also included. In some embodiments, the ratio of the mass of the additive to the total mass of the first polymer and the foaming agent is = (0.5-10): (99.5-90) = additive: (first polymer + foaming agent). In some embodiments, the additive is at least one of a surfactant and a thickener. Wherein, the addition of a surfactant can not only reduce the surface tension of the foaming, making the foaming stable and lasting, but also increase the affinity between the polymer and the active substance, making it easier to form a film on the surface of the active substance after the polymer melts. Plasticizers can adjust the processing properties of the polymer, such as adjusting the viscosity and fluidity of the polymer after melting when used. Surface surfactants and thickeners commonly used in the art can be used in this application, for example: the surfactant can be at least one of a cationic surfactant, an anionic surfactant, and a non-ionic surfactant. Cationic surfactants include but are not limited to at least one of hexadecyltrimethylammonium bromide, hexadecyltrimethylammonium chloride, and octadecyltrimethylammonium chloride; anionic surfactants include but are not limited to at least one of sodium dodecyl sulfate and sodium dodecylbenzene sulfonate; nonionic surfactants include but are not limited to at least one of octylphenol polyoxyethylene ether, nonylphenol polyoxyethylene ether, and polyoxyethylene sorbitan ester (Tween). Thickeners include but are not limited to at least one of carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, casein, and their salts. In some embodiments, the temperature of extrusion granulation is 90-240°C.

[0049] In some embodiments, in step S2, solvent-2 includes but is not limited to at least one of xylene, tetrahydrofuran, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, acetone, toluene, chloroform, cyclohexanone, and ethyl acetate. In some embodiments, the solid-to-liquid ratio of the second polymer to solvent-2 is (0.01-1) g: 1 mL. In some embodiments, after soaking, filtering and drying are also included.

[0050] In some embodiments, the average particle size of the composite foamed particles is R1 μm, and R1 is 0.1, 0.2, 0.5, 0.7, 1, 1.2, 1.5, 1.7, 2, 2.2, 2.5, 2.7, 3, 3.2, 3.5, 3.7, 4, 4.2, 4.5, 4.7, 5 or a range consisting of any two of these values. In some embodiments, 0.1≤R1≤5. When R1 is within the above range, the composite foamed particles can be better filled into the gaps in the material layer on the electrode sheet, which is beneficial to improve the material stacking and compaction density in the material layer, and is also beneficial to constructing the conductive network of the electrode, improving the energy density of the secondary battery, thereby further improving the cycle performance of the secondary battery containing the foamed material in the electrode.

[0051] The present application has no particular restrictions on the method for regulating the average particle size of the composite foamed particles, as long as the purpose of the present application can be achieved. For example, composite foamed particles with different average particle sizes can be achieved by regulating the average particle size of the foamed particles and the average thickness of the polymer layer. Exemplarily, when other conditions remain unchanged, the average particle size of the foamed particles increases, and the average particle size of the composite foamed particles also increases; when other conditions remain unchanged, the solution concentration of the second polymer used for coating in the composite foamed particle preparation method increases, that is, the solid-liquid ratio of the second polymer and solvent-2 in step S2 increases, which will increase the average thickness of the polymer layer, and the average particle size of the composite foamed particles will also increase.

[0052] In some embodiments, the average thickness of the polymer layer is R2 nm, and R2 is 5, 7, 10, 12, 15, 17, 20, 22, 25, 27, 30, 32, 35, 37, 40, 42, 45, 47, 50, 52, 55, 57, 60, 62, 65, 67, 70, 72, 75, 77, 80, 82, 85, 87, 90, 92, 95, 97, 100 or a range consisting of any two of these values. In some embodiments, 5≤R2≤100. When R2 is within the above range, the second polymer can not only better wrap the exposed foaming agent on the surface of the foamed particles, but also provide a strong barrier to the foamed particles, reducing the probability of the foamed particles being exposed to the foaming agent due to damage during processing, thereby further improving the cycle performance of the secondary battery containing the foaming material in the electrode. In addition, the content of the foaming agent can be increased at the same time, and the role of the composite foaming particles can be fully exerted, so that the secondary battery can better handle the thermal runaway problem and have better safety performance.

[0053] In some embodiments, the number of particles of the electrode active material is N1, the number of particles of the composite foamed particles is N2, and N1 / N2 is 0.1, 0.12, 0.15, 0.17, 0.2, 0.22, 0.25, 0.26, 0.27, 0.28, 0.29, 0.3, 0.31, 0.32, 0.35, 0.37, 0.4, 0.42, 0.45, 0.47, 0.5, 0.52, 0.55, 0.57, 0.6, 0.62, 0.65, 0.67, 0.7, 0.72, 0.75, 0.77, 0.8, 0.82, 0.85, 0.9, 0.92, 0.95, 0.97, 1, or a range consisting of any two of these values. In some embodiments, 0.1≤N1 / N2≤1. When N1 / N2 is within the above range, the composite foamed particles can be uniformly filled in the gaps in the material layer on the electrode sheet, which not only helps the composite foamed particles to effectively insulate and block chemical crosstalk, but also better improves the thermal runaway problem, and is also beneficial to further improve the cycle performance of secondary batteries containing foamed materials in the electrodes.

[0054] In some embodiments, the average particle size of the composite foamed particles in the material layer is R1 μm, the average particle size of the electrode active material is R3 μm, and R3>R1. When R3>R1, the composite foamed particles can be better filled into the gaps in the material layer on the electrode sheet, especially into the gaps between the electrode active materials, which is beneficial to improve the material stacking and compaction density in the material layer, and is also beneficial to constructing the conductive network of the electrode, improving the energy density of the secondary battery, and further improving the cycle performance of the secondary battery containing the foamed material in the electrode.

[0055] In some embodiments, the average particle size of the electrode active material is R3 μm, and R3 is 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.2, 1.5, 1.7, 2, 2.2, 2.5, 2.7, 3, 3.2, 3.5, 3.7, 4, 4.2, 4.5, 4.7, 5, 5.5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 22, 25, 27, 30 or a range consisting of any two of these values.

[0056] In some embodiments, the melting point of the polymer layer is T1 °C, where T1 is 90, 92, 95, 97, 100, 102, 105, 107, 110, 112, 115, 117, 120, 122, 125, 127, 130, 132, 135, 137, 140, 142, 145, 147, 150, 152, 155, 157, 160, 162, 165, 167, 170, 172, 175, 177, 180, or a range formed by any two of these values. In some embodiments, 90 ≤ T1 ≤ 180. When T1 is within the above range, when the secondary battery faces the high temperature of thermal runaway, it can quickly melt, the internal blowing agent decomposes and foams, blocking the electron-ion path, reducing chemical crosstalk, and reducing the probability of exothermic reactions occurring, enabling the secondary battery to better handle thermal runaway problems and having better safety performance.

[0057] In this application, the passing rate value of the overheat box test results is used to characterize the safety performance of the secondary battery. The larger the passing rate, the better the secondary battery can handle thermal runaway problems and the higher the safety performance.

[0058] In some embodiments, the melting point of the separator is T2 °C, where 120 ≤ T2. In some embodiments, T2 is 120, 122, 125, 127, 130, 132, 135, 137, 140, 142, 145, 147, 150, 152, 155, 157, 160, 162, 165, 167, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290, 300, or a range formed by any two of these values.

[0059] In some embodiments, T1 < T2. When T1 < T2, when the composite foaming particles play a role, the probability of the separator melting and collapsing is small, reducing the phenomenon of positive and negative electrode contact short circuits, enabling better handling of thermal runaway problems, and having better safety performance.

[0060] In some embodiments, based on the mass of the composite foaming particles, the mass percentage of the foaming agent is 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.2%, 1.5%, 1.7%, 2%, 2.1%, 2.12%, 2.14%, 2.15%, 2.17%, 2.19%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.72%, 2.75%, 2.77%, 2.8%, 2.85%, 2.88%, 2.9%. %, 4.2%, 4.5%, 4.7%, 5%, 5.2%, 5.5%, 5.7%, 6%, 6.2%, 6.5%, 6.7%, 7%, 7.2%, 7.5%, 7.7%, 8%, 8.2%, 8.5%, 8.7%, 9%, 9.1%, 9.2%, 9.3%, 9.4%, 9.5%, 9.6%, 9.7%, 9.8%, 9.9%, 10% or any two of these values. In some embodiments, the weight percentage of the blowing agent is 0.1% to 10% based on the weight of the composite foaming particles.

[0061] In some embodiments, at least two material layers stacked in sequence are disposed on at least one surface of the current collector, the content of the composite foamed particles in the material layer close to the current collector is Q1, and the content of the composite foamed particles in the material layer far from the current collector is Q2, and Q1>Q2. When Q1>Q2, the foaming amount of the bottom layer (the pole piece layer close to the current collector) is large, and the foaming amount of the top layer (the pole piece layer far from the current collector) is small, and there are still gaps in the top layer that are not filled with the foaming agent. The electrolyte squeezed out after the foaming agent in the bottom layer is foamed will be discharged through the gaps in the top layer, and at the same time, the foaming material after the foaming agent in the bottom layer is foamed will fill the gaps in the top layer, avoiding the electrolyte remaining in the pole piece and reacting with the active material to release heat, so that the secondary battery can better avoid the problem of thermal runaway and have better safety.

[0062] The present application has no particular restrictions on the method for regulating the number of material layers on the surface of the current collector, as long as the purpose of the present application can be achieved. For example, a material layer having a composite foamed particle content of Q1 is first coated and / or sprayed on the surface of the current collector, and then a material layer having a composite foamed particle content of Q2 is continuously coated and / or sprayed on the surface of the material layer.

[0063] In some embodiments, the melting point of the first polymer is ≥ 90°C. In some embodiments, the melting point of the first polymer is 90°C, 92°C, 95°C, 97°C, 100°C, 102°C, 105°C, 107°C, 110°C, 112°C, 115°C, 117°C, 120°C, 122°C, 125°C, 127°C, 130°C, 132°C, 135°C, 137°C, 140°C, 142°C, 145°C, 147°C, 150°C, 15 ℃, 2℃, 155℃, 157℃, 160℃, 162℃, 165℃, 167℃, 170℃, 172℃, 175℃, 177℃, 180℃, 185℃, 190℃, 195℃, 200℃, 210℃, 220℃, 230℃, 240℃, 250℃, 260℃, 270℃, 280℃, 290℃, 300℃ or a range consisting of any two of these values.

[0064] In some embodiments, the melting point of the second polymer is ≥ 90°C. In some embodiments, the melting point of the second polymer is 90°C, 92°C, 95°C, 97°C, 100°C, 102°C, 105°C, 107°C, 110°C, 111°C, 112°C, 115°C, 117°C, 120°C, 122°C, 125°C, 127°C, 130°C, 132°C, 135°C, 137°C, 140°C, 142°C, 145°C, 147°C, 150°C, 152°C, 155°C, 157°C, 160°C, 162°C, 165°C, 167°C, 170°C, 172°C, 175°C, 177°C, 180°C, 185°C, 190°C, 195°C, 200°C, 210°C, 220°C, 230°C, 240°C, 250°C, 260°C, 270°C, 280°C, 290°C, 300°C or a range consisting of any two of these values.

[0065] In some embodiments, the foaming temperature of the foaming agent is ≥ 90°C. In some embodiments, the foaming temperature of the foaming agent is 90°C, 92°C, 95°C, 97°C, 100°C, 102°C, 105°C, 107°C, 110°C, 112°C, 115°C, 117°C, 120°C, 122°C, 125°C, 127°C, 130°C, 132°C, 135°C, 137°C, 140°C, 142°C, 145°C, 147°C, 150°C, 15 ℃, 2℃, 155℃, 157℃, 160℃, 162℃, 165℃, 167℃, 170℃, 172℃, 175℃, 177℃, 180℃, 185℃, 190℃, 195℃, 200℃, 210℃, 220℃, 230℃, 240℃, 250℃, 260℃, 270℃, 280℃, 290℃, 300℃ or a range consisting of any two of these values.

[0066] In some embodiments, in the composite foamed particles, the foaming temperature of the foaming agent is ≥ the melting point of the first polymer.

[0067] In some embodiments, the foaming agent includes at least one of an azo compound, a sulfonylhydrazide compound, a nitroso compound, sodium carbonate, and sodium bicarbonate. The present application has no particular limitation on the types of azo compounds, sulfonylhydrazide compounds, and nitroso compounds, as long as the purpose of the present application can be achieved. For example, the azo compound may include but is not limited to at least one of azodicarbonamide, azobisisobutyronitrile, diisopropyl azodicarboxylate, barium azodicarboxylate, diethyl azodicarboxylate, and azoaminobenzene; the sulfonylhydrazide compound may include but is not limited to at least one of benzenesulfonylhydrazide, p-toluenesulfonylhydrazide, 4-chlorobenzenesulfonylhydrazide, 4-nitrobenzenesulfonylhydrazide, 4,4'-oxidized bisbenzenesulfonylhydrazide, 3,3'-disulfonylhydrazide diphenyl sulfone, 1,3-benzenedisulfonylhydrazide, p-toluenesulfonylsemicarbazide, 4,4',-oxybis(benzenesulfonylsemicarbazide), and diphenyl ether sulfonylhydrazide; the nitroso compound may include but is not limited to at least one of N,N'-dimethyl-N,N'-dinitrosoterephthalamide and dinitrosopentamethylenetetramine.

[0068] In some embodiments, the first polymer includes at least one of polypropylene, polyethylene, polylactic acid, polyvinyl chloride, polyethylene terephthalate, polystyrene, polycarbonate, polyvinyl alcohol, polyurethane, polycarbonate and polyamide. In some embodiments, polyethylene includes at least one of low-density polyethylene (LDPE), high-density polyethylene, medium-density polyethylene, and linear low-density polyethylene. In some embodiments, polypropylene includes at least one of homopolymer polypropylene and copolymer polypropylene. In some embodiments, copolymer polypropylene includes at least one of random copolymer polypropylene and block copolymer polypropylene. In some embodiments, polyurethane includes but is not limited to thermoplastic polyurethane.

[0069] In some embodiments, the second polymer includes at least one of polypropylene, polyethylene, polylactic acid, polyvinyl chloride, polyethylene terephthalate, polystyrene, polycarbonate, polyvinyl alcohol, polyurethane, polycarbonate and polyamide. In some embodiments, polyethylene includes at least one of low density polyethylene (LDPE), high density polyethylene, medium density polyethylene, linear low density polyethylene. In some embodiments, polypropylene includes at least one of homopolymer polypropylene and copolymer polypropylene. In some embodiments, copolymer polypropylene includes at least one of random copolymer polypropylene and block copolymer polypropylene. In some embodiments, polyurethane includes but is not limited to thermoplastic polyurethane.

[0070] In some embodiments, the material layer has an average thickness of 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm, 150 μm, 160 μm, 170 μm, 180 μm, 190 μm, 200 μm, 210 μm, 220 μm, 230 μm, 240 μm, 250 μm, 260 μm, 270 μm, 280 μm, 290 μm, 300 μm, 350 μm, 400 μm, 450 μm, 500 μm, 550 μm, 650 μm, 700 μm, or a range consisting of any two of these values.

[0071] In some embodiments, the battery includes a positive electrode and a negative electrode. At least one of the positive electrode and the negative electrode adopts any of the electrodes described above in the present application. In other words, the electrode can be used as at least one of the positive electrode and the negative electrode of the battery.

[0072] <Positive electrode>

[0073] In some embodiments, when the electrode is a positive electrode, the current collector is a positive electrode current collector. The present application has no particular restrictions on the positive electrode current collector, 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 aluminum foil, aluminum alloy foil, composite current collector, carbon cloth or carbon paper. In some embodiments, the composite current collector may include but is not limited to an aluminum-carbon composite current collector.

[0074] In some embodiments, when the electrode is a positive electrode, the electrode active material is a positive electrode active material. The present application has no particular limitation on the positive electrode active material, as long as the purpose of the present application can be achieved. For example, it may include but is not limited to lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide (LiCoO 2 ), at least one of lithium manganese oxide, lithium iron manganese phosphate, lithium titanate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium iron silicate, lithium vanadium silicate, lithium cobalt silicate or lithium manganese silicate. The chemical formula of lithium-rich manganese-based materials is γLi 2 MnO 3 (1-γ)LiGO 2, 0<γ<1, G is a transition metal such as nickel, cobalt or iron. In some embodiments, the lithium nickel cobalt manganese oxide includes at least one of NCM811, NCM622, NCM523 or NCM111. In the present application, the surface of the positive electrode active material may be attached with a substance having a different composition from that of the positive electrode active material. For example, the substance attached to the surface may include but is not limited to at least one of aluminum oxide, silicon dioxide, titanium dioxide, zirconium oxide, magnesium oxide, calcium oxide, boron oxide, antimony oxide, bismuth oxide, lithium sulfate, sodium sulfate, potassium sulfate, magnesium sulfate, calcium sulfate, aluminum sulfate, lithium carbonate, calcium carbonate, magnesium carbonate or carbon. By attaching the above substances to the surface of the positive electrode active material, the oxidation reaction of the electrolyte on the surface of the positive electrode active material can be suppressed, and the service life of the electrochemical device can be improved.

[0075] In some embodiments, when the electrode is a positive electrode, the material layer is a positive electrode material layer, and the positive electrode material layer further includes at least one of a conductive agent, a binder, or a thickener; the present application has no particular restrictions on the conductive agent and the binder, as long as the purpose of the present application can be achieved. For example, the conductive agent may include but is not limited to at least one of a carbon-based material, a metal-based material, or a conductive polymer. In some embodiments, the carbon-based material may include but is not limited to at least one of natural graphite, artificial graphite, carbon black, acetylene black, Super-P, Ketjen black, carbon fiber, carbon nanotubes, graphene, or amorphous carbon. In some embodiments, the metal-based material may include but is not limited to at least one of metal powder and metal fiber, and the metal may include but is not limited to at least one of copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer may include but is not limited to at least one of polyphenylene derivatives, polythiophene, polypyrrole, polyaniline, polyacetylene, polyparaphenylene, and polyfluorene. For another example, the binder may include but is not limited to at least one of polyvinyl alcohol, hydroxypropyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride (PVDF), polyethylene, polypropylene, polyolefin esters, polyolefin alcohols, polyacrylic acid, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resins or nylon. For another example, the thickener may include but is not limited to at least one of carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, casein and their salts.

[0076] In some embodiments, the structure of the positive electrode is a positive electrode structure known in the art that can be used in electrochemical devices (including secondary batteries).

[0077] In some embodiments, the method for preparing the positive electrode is a method well-known to those skilled in the art and can be used for preparing the positive electrode of an electrochemical device (including a secondary battery). For example, the positive electrode can be obtained by the following method: mixing the components in the positive electrode material layer [including composite foamed particles, positive electrode active material (i.e., electrode active material), and optional conductive agent, binder, thickener, etc.] in a solvent, and heating the thickener before use as needed to prepare a positive electrode active paste, and coating and / or spraying the positive electrode active paste on a current collector. In some embodiments, the solvent can include, but is not limited to, at least one of water, N-methylpyrrolidone, dimethylformamide, dimethylacetamide, dimethyl sulfoxide, trimethyl phosphate, acetone, or dipropylene glycol dimethyl ether.

[0078] It can be understood that when the positive electrode of the battery adopts any of the above electrodes of the present application, the negative electrode of the battery can adopt a negative electrode with other structures, such as a common negative electrode in the art.

[0079] <Negative electrode>

[0080] In some embodiments, when the electrode is a negative electrode, the current collector is a negative electrode current collector. The present application has no particular limitation on the negative electrode current collector, as long as the purpose of the present application can be achieved. For example, it can include, but is not limited to, at least one of copper foil, aluminum foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or a composite current collector. In some embodiments, the composite current collector can include, but is not limited to, at least one of a carbon-copper composite current collector, a nickel-copper composite current collector, or a titanium-copper composite current collector.

[0081] In some embodiments, when the electrode is a negative electrode, the electrode active material is 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, it can include, but is not limited to, at least one of natural graphite, artificial graphite, mesocarbon microbeads (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, SiO x (0.5 < x < 1.6), Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO 2 、spinel-structured lithiated TiO 2 -Li 4 Ti 5 O 12 、lithium metal, structured lithium metal, or Li-Al alloy.

[0082] In some embodiments, when the electrode is a negative electrode, the material layer is a negative electrode material layer, and the negative electrode material layer also includes at least one of a conductive agent, a binder, or a thickener; the present application has no particular restrictions on the conductive agent and the binder, as long as the purpose of the present application can be achieved. For example, the conductive agent may include but is not limited to at least one of a carbon-based material, a metal-based material, or a conductive polymer. In some embodiments, the carbon-based material may include but is not limited to at least one of natural graphite, artificial graphite, carbon black, acetylene black, Super-P, Ketjen black, carbon fiber, carbon nanotubes, graphene, or amorphous carbon. In some embodiments, the metal-based material may include but is not limited to at least one of metal powder and metal fiber, and the metal may include but is not limited to at least one of copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer may include but is not limited to at least one of polyphenylene derivatives, polythiophene, polypyrrole, polyaniline, polyacetylene, polyparaphenylene, and polyfluorene. For another example, the binder may include but is not limited to at least one of polyvinyl alcohol, hydroxypropyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride (PVDF), polyethylene, polypropylene, polyolefin esters, polyolefin alcohols, polyacrylic acid, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resins or nylon. For another example, the thickener may include but is not limited to at least one of carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, casein and their salts.

[0083] In some embodiments, the structure of the negative electrode is a negative electrode structure known in the art that can be used in electrochemical devices (including secondary batteries).

[0084] In some embodiments, the preparation method of the negative electrode is a method for preparing a negative electrode that can be used for an electrochemical device (including a secondary battery) that is well known in the art. For example, the negative electrode can be obtained by the following method: mixing the components in the negative electrode material layer [including composite foamed particles, negative electrode active materials (i.e., electrode active materials), and optional conductive agents, binders, and thickeners, etc.] in a solvent, and heating the thickener before use as needed to prepare a negative electrode active slurry, and coating and / or spraying the negative electrode active slurry on the current collector. In some embodiments, the solvent may include, but is not limited to, at least one of water, N-methylpyrrolidone, dimethylformamide, dimethylacetamide, dimethyl sulfoxide, trimethyl phosphate, acetone, or dipropylene glycol dimethyl ether.

[0085] It is understandable that when the negative electrode of the battery adopts any of the electrodes described above in the present application, the positive electrode of the battery may adopt positive electrodes of other structures, such as positive electrodes commonly used in the art.

[0086] <Electrolyte>

[0087] The present application has no particular limitation on the electrolyte, as long as the purpose of the present application can be achieved. The electrolyte used in the present application can be an electrolyte known in the prior art. For example, the electrolyte can be divided into an aqueous electrolyte and a non-aqueous electrolyte. Compared with an aqueous electrolyte, an electrochemical device (secondary battery) using a non-aqueous electrolyte can operate in a wider voltage window, thereby achieving a higher energy density.

[0088] In some embodiments, the non-aqueous electrolyte includes an organic solvent and an electrolyte.

[0089] In some embodiments, the present application has no particular restrictions on organic solvents, as long as the purpose of the present application can be achieved, the organic solvent used in the present application can be an organic solvent known in the prior art, for example, the organic solvent can include but is not limited to at least one of carbonate compounds, carboxylate compounds, ether compounds or other organic solvents. Wherein, carbonate compounds can include but are not limited to at least one of linear carbonate compounds, cyclic carbonate compounds or fluorinated carbonate compounds. Linear carbonate compounds can include but are not limited to at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC) or ethyl methyl carbonate (EMC). Cyclic carbonates can include but are not limited to at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) or vinyl ethylene carbonate (VEC). Fluorinated carbonate compounds may include, but are not limited to, at least one of fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, or trifluoromethylethylene carbonate. 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, decalactone, valerolactone, or caprolactone. The ether compound may include, but is not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran or tetrahydrofuran. Other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,3-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate or trioctyl phosphate.

[0090] In some embodiments, the electrolyte may include but is not limited to at least one of an inorganic lithium salt, a fluorine-containing organic lithium salt, or a dicarboxylic acid complex lithium salt. Among them, the inorganic lithium salt may include but is not limited to LiClO 4 、LiAsF 6 、LiPF 6 , LiBF 4 、LiSbF 6 、LiSO 3 F. LiPO 2 F 2 or LiN(FSO 2 ) 2 At least one of the following. Fluorine-containing organic lithium salts may include but are not limited to LiCF 3 SO 3 、LiN(FSO 2 )(CF 3 SO 2 )、LiN(CF 3 SO 2 ) 2 、LiN(C 2 F 5 SO 2 ) 2 、Cyclic 1,3-hexafluoropropane disulfonyl imide lithium, cyclic 1,2-tetrafluoroethane disulfonyl imide lithium, LiPF 4 (CF 3 ) 2 、LiN(CF 3 SO 2 )(C 4 F 9 SO 2 )、LiC(CF 3 SO 2 )、LiPF 4 (CF 3 SO 2 ) 2 、LiPF 4 (C 2 F 5 ) 2 、LiPF 4 (C 2 F 5 SO 2 ) 2 , LiBF 2 (CF 3 ) 2 , LiBF 2 (C 2 F 5 ) 2 , LiBF2 (CF 3 SO 2 ) 2 or LiBF 2 (C 2 F 5 SO 2 ) 2 At least one of the following. The lithium salt containing the dicarboxylic acid complex may include but is not limited to lithium bis(oxalato)borate, lithium difluorooxalatoborate [LiBF 2 (C 2 O 4 )], at least one of lithium tri(oxalato)phosphate, lithium difluorobis(oxalato)phosphate or lithium tetrafluoro(oxalato)phosphate.

[0091] In some embodiments, the mass proportion of the electrolyte is 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 12.5%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30% based on the mass of the electrolyte.

[0092] In some embodiments, the non-aqueous electrolyte further includes an additive.

[0093] In some embodiments, the present application has no particular restrictions on additives, as long as the purpose of the present application can be achieved, and the additives used in the present application can be additives known in the prior art, for example, the additives can include but are not limited to at least one of polynitrile compounds, sulfur-containing additives, fluoroethylene carbonate (FEC), 1,3-propane sultone (PS) or 1,4-butane sultone. Among them, the polynitrile compound includes at least one of a dinitrile compound or a trinitrile compound. The dinitrile compound is a compound containing two cyano groups (-CN), and may include, but is not limited to, malononitrile, succinonitrile, glutaronitrile, adiponitrile, pimelonitrile, suberonitrile, azelaic acid dinitrile, methyl malononitrile, ethyl malononitrile, isopropyl malononitrile, tert-butyl malononitrile, methyl succinonitrile, 2-methylene glutaronitrile, 1,4-dicyano-3-butene, 2,2-dimethylsuccinonitrile, 2,3-dimethylsuccinonitrile, 2,3,3-trimethylsuccinonitrile, 2,2,3,3-tetramethylsuccinonitrile, 2,3-diethyl-2,3-dimethylsuccinonitrile, 2,2-diethyl-3,3-dimethylsuccinonitrile, dicyclohexyl-1,1-dicarbonitrile, dicyclohexyl-2,2-dicarbonitrile, dicyclohexyl-3,3-dicarbonitrile, 2,5-dimethyl-2,5-hexane dicarbonitrile, 2 ,3-diisobutyl-2,3-dimethylsuccinonitrile, 2,2-diisobutyl-3,3-dimethylsuccinonitrile, 2-methylglutaronitrile, 2,3-dimethylglutaronitrile, 2,4-dimethylglutaronitrile, 2,2,3,3-tetramethylglutaronitrile, 2,2,4,4-tetramethylglutaronitrile, 2,2,3,4-tetramethylglutaronitrile, 2,3,3,4-tetramethylglutaronitrile, 1, At least one of 4-dicyanopentane, 2,6-dicyanoheptane, 2,7-dicyanooctane, 2,8-dicyanononane, 1,6-dicyanodecane, 1,2-dicyanobenzene, 1,3-dicyanobenzene, 1,4-dicyanobenzene, 3,3'-(ethylenedioxy)dipropionitrile, 3,3'-(ethylenedisulfide)dipropionitrile, 1,4-dicyano-2-butene or butylene dinitrile. The trinitrile compound is a compound containing three cyano groups (-CN).

[0094] In some embodiments, the preparation method of the electrolyte is a method for preparing an electrolyte that can be used in an electrochemical device (including a secondary battery) that is well known in the art. For example, the electrolyte can be obtained by the following method: mixing the components of the electrolyte [including an organic solvent, an electrolyte, and optional additives, etc.].

[0095] <Diaphragm>

[0096] The present application has no particular restrictions on the diaphragm, as long as the purpose of the present application can be achieved. The diaphragm used in the present application can be a diaphragm known in the prior art. For example, the type of the diaphragm can include but is not limited to at least one of a woven membrane, a non-woven membrane, a microporous membrane, a composite membrane, a rolled membrane or a spun membrane. The material of the diaphragm can include but is not limited to at least one of polyethylene (PE), polypropylene (PP)-based polyolefins (PO), polyester, cellulose, polyimide (PI), polyamide (PA), spandex or aramid. Polyester can include but is not limited to polyethylene terephthalate (PET) film.

[0097] In some embodiments, the diaphragm includes a substrate layer. The substrate layer may include but is not limited to at least one of a nonwoven fabric, a film or a composite film having a porous structure. The material of the substrate layer may include but is not limited to at least one of polyethylene, polypropylene, polyethylene terephthalate or polyimide. In some embodiments, the substrate layer may include but is not limited to at least one of a polypropylene porous film, a polyethylene porous film, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric or a polypropylene-polyethylene-polypropylene porous composite film.

[0098] In some embodiments, the diaphragm further includes a surface treatment layer disposed on at least one surface of the substrate layer. The surface treatment layer may include but is not limited to at least one of a polymer layer, an inorganic layer, or a layer formed by a mixed polymer and an inorganic substance. The polymer layer includes a polymer. The present application has no particular restrictions on the polymer, as long as the purpose of the present application can be achieved, for example, the polymer includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinyl pyrrolidone, polyvinyl ether, polyvinylidene fluoride or poly (vinylidene fluoride-hexafluoropropylene). The inorganic layer includes inorganic particles and a binder. The present application has no particular restrictions on inorganic particles and binders, as long as the purpose of the present application can be achieved, for example, inorganic particles may include but are not limited to 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. For another example, the binder may include but is not limited to at least one of the binders used in the above-mentioned positive electrode material layer or negative electrode material layer.

[0099] In some embodiments, the present application has no particular limitation on the average thickness of the separator, as long as the purpose of the present application can be achieved. For example, the average thickness of the separator is 1 μm to 500 μm.

[0100] <Packaging bag>

[0101] In some embodiments, the secondary battery further includes a packaging bag for containing electrodes (including positive electrodes and negative electrodes), a separator and an electrolyte, and other components known in the art in the secondary battery, and the present application has no particular restrictions on other components. The present application has no particular restrictions on the packaging bag, as long as the purpose of the present application can be achieved, and the packaging bag used in the present application can be a packaging bag known in the prior art, for example, the packaging bag can include but is not limited to an aluminum-plastic film packaging bag.

[0102] <Method for preparing secondary battery>

[0103] The preparation method of the secondary battery is well known to those skilled in the art. The present application does not have any particular restrictions on the preparation method of the secondary battery, as long as the purpose of the present application can be achieved. For example, the preparation method of the secondary battery may include but is not limited to the following steps: stacking the electrodes (including the positive electrode and the negative electrode) and the diaphragm in order, and winding and folding them as needed to obtain an electrode assembly of a winding structure, placing the electrode assembly in a packaging bag, injecting the electrolyte into the packaging bag and sealing it to obtain a secondary battery; or stacking the electrodes (including the positive electrode and the negative electrode) and the diaphragm in order, and then fixing the four corners of the entire laminated structure with tape to obtain an electrode assembly of a laminated structure, placing the electrode assembly in a packaging bag, injecting the electrolyte into the packaging bag and sealing it to obtain a secondary battery. In addition, overcurrent protection components, guide plates, etc. may also be placed in the packaging bag as needed.

[0104] 2. Electronic Devices

[0105] A second aspect of the present application provides an electronic device, comprising the secondary battery according to the first aspect.

[0106] The present application has no particular restrictions on electronic devices, as long as the purpose of the present application can be achieved. The electronic devices used in the present application can be electronic devices known in the prior art. For example, the electronic devices can include but are not limited to laptop computers, pen-input computers, mobile computers, e-book players, portable phones, portable fax machines, portable copiers, portable printers, head-mounted stereo headphones, video recorders, LCD televisions, portable cleaners, portable CD players, mini CDs, transceivers, electronic notepads, calculators, memory cards, portable recorders, radios, backup power supplies, motors, cars, motorcycles, power-assisted bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, and at least one of large household batteries or lithium-ion capacitors.

[0107] 3. Embodiment

[0108] The present application is further clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by those skilled in the art based on the present application belong to the scope of protection of the present application.

[0109] It should be noted that in the specific implementation manner of the present application, the present application is explained by taking a lithium-ion battery as an example of a secondary battery, but the secondary battery of the present application is not limited to a lithium-ion battery.

[0110] In the following examples and comparative examples, the reagents, materials and instruments used are all commercially available unless otherwise specified. In addition, "part" and "%" are by mass unless otherwise specified.

[0111] Test methods and equipment:

[0112] (1) Test on the mass percentage of the foaming agent in the composite foaming particles:

[0113] 1 g of the composite foamed particles was fully dissolved in toluene to prepare a solution, the content of the foaming agent in the solution was measured by high performance liquid chromatography, and the mass percentage of the foaming agent in the composite foamed particles was calculated.

[0114] (2) Test on average particle size of composite foamed particles or foamed particles or electrode active materials:

[0115] The particle size distribution of the composite foamed particles or foamed particles or electrode active materials is tested by a Malvern particle size tester (instrument model: MasterSizer2000). The sample preparation method is as follows: add about 0.02g of the powder sample of the composite foamed particles or foamed particles or electrode active materials to a 50mL clean beaker, add 20mL of deionized water, and ultrasonicate for 5min in a 120W ultrasonic cleaning machine to obtain a sample dispersion. In the volume-based particle size distribution of the composite foamed particles or foamed particles or electrode active materials, starting from the small particle size, the particle size reaching 50% of the volume accumulation is Dv50, and the particle size reaching 10% of the volume accumulation is Dv10, and the Dv50 size is used as the average particle size of the first or second negative electrode material.

[0116] (3) Average thickness test of polymer layer:

[0117] The composite foamed particles were sliced ​​using a focused ion beam (FIB) and then observed using a high-resolution transmission electron microscope (HRTEM, model Talos F200X). Five areas were randomly selected to record the average thickness of the polymer layer in the composite foamed particles, and the arithmetic mean was calculated to obtain the average thickness of the polymer layer in the composite foamed particles.

[0118] (4) Melting point test of the first polymer or the second polymer or the polymer layer or the membrane:

[0119] The melting point test uses an aluminum crucible. After the sample crucible is stable and cleared at room temperature, take it out, add 300 mg of sample (first polymer or second polymer or polymer layer or diaphragm) into the crucible, and buckle the edge of the crucible. Gently place the crucible with the sample on the bracket, call the latest calibration baseline → enter the sample number, sample name, sample weight according to the software process → edit the heating rate and heating range → save the file name, confirm that the input parameters are correct, and click Start. The instrument starts the heating test. Among them, the heating rate is 10℃ / min, and the test temperature is RT-400℃. Open the raw data of the completed test with the X-axis as the reference temperature, unit: ℃, and the Y-axis as the heat flow power caused by the unit weight sample at a certain temperature, unit: mW / mg. The X-axis value at the highest point of the peak is the melting point of the sample (first polymer or second polymer or polymer layer or diaphragm).

[0120] (5) Foaming temperature test of foaming agent:

[0121] Using differential scanning calorimetry (DSC), accurately weigh 10 mg of the blowing agent and place it in the DSC tray, flatten it, and place the sample tray and reference tray in the instrument; select a suitable heating rate (usually 10°C / min), set the temperature range from room temperature to 300°C, observe the endothermic or exothermic peak through the DSC curve, and determine the decomposition temperature, which is used as the foaming temperature of the blowing agent.

[0122] (6) Test on the number of composite foam particles or electrode active materials:

[0123] Use a scanning electron microscope (SEM) to observe the cross-section of the electrode sheet, randomly select 5 test areas (25μm×25μm), record the number of composite foam particles and electrode active material particles in the selected test areas, take the arithmetic mean, and then calculate the N1 / N2 value, where the number of electrode active material particles is N1, and the number of composite foam particles is N2.

[0124] (7) Cycle performance test of secondary batteries:

[0125] In an environment of 25°C, the secondary battery is subjected to multiple charge and discharge cycles. One charge and discharge cycle means that the secondary battery is first charged at a constant current and constant voltage with a current of 0.5C until the upper limit voltage is 4.4V, and then the secondary battery is discharged at a constant current with a current of 0.5C until the cut-off voltage is 3.0V; then the number of charge and discharge cycles experienced when the secondary battery capacity drops to 80% is recorded, and recorded as F, unit: times, one time equals one charge and discharge cycle.

[0126] Since the present application characterizes the cycle performance of a secondary battery by the number of charge and discharge cycles experienced when the capacity of the secondary battery drops to 80% during multiple charge and discharge cycles, a larger F value indicates a greater number of charge and discharge cycles, indicating a higher cycle performance of the secondary battery.

[0127] (8) Overheat box test of secondary batteries:

[0128] The secondary battery is charged to 4.5V at a constant current of 0.5C, and then charged to 0.02C at a constant voltage of 4.5V to fully charge the secondary battery. Then the secondary battery is placed in a constant temperature box for 1h, and the temperature of the constant temperature box is 130℃ or 140℃. During the observation period, whether the secondary battery catches fire or explodes. If there is no fire or explosion, it is considered to have passed. The experiment is repeated 10 times, and the pass rate = the number of passes / 10.

[0129] The present application uses the pass rate value of the overheat box test result to characterize the safety performance of the secondary battery. The larger the pass rate, the better the secondary battery can handle the thermal runaway problem and has higher safety performance.

[0130] Example 1

[0131] 1. Preparation of composite foamed particles

[0132] S1. Preparation of foamed particles:

[0133] A first polymer (low-density polyethylene LDPE), a foaming agent (azodicarbonamide) and an additive (surfactant hexadecyltrimethylammonium bromide and thickener carboxymethyl cellulose in a mass ratio of 1:1) are mixed in a mass ratio of 95:3:2, and extruded and granulated at 180° C. using a twin-screw extruder to obtain foamed particles;

[0134] S2. Preparation of composite foamed particles:

[0135] The second polymer (low-density polyethylene LDPE) and xylene are mixed at a solid-liquid ratio of 0.02 g:1 mL to obtain a mixed solution, the foamed particles are added to the mixed solution and soaked for 10 seconds, filtered and dried, and then dispersed to obtain composite foamed particles;

[0136] Wherein, based on the mass of the composite foamed particles, the mass percentage of the foaming agent is E%, E=2.88; the average particle size of the composite foamed particles is R1 μm, R1=1; the average thickness of the polymer layer (second polymer low-density polyethylene LDPE) disposed on the surface of the foamed particles is R2 nm, R2=40; the melting point of the polymer layer (second polymer low-density polyethylene LDPE) is T1°C, T1=120; the melting point of the first polymer (low-density polyethylene LDPE) is T0°C, T0=120; the foaming temperature of the foaming agent is 120°C, and the foaming temperature of the foaming agent=the melting point of the first polymer;

[0137] 2. Preparation of positive electrode

[0138] The composite foamed particles, the positive electrode active material (i.e., the electrode active material) lithium cobalt oxide, the conductive agent Super-P, the conductive agent carbon nanotube CNT, and the binder polyvinylidene fluoride PVDF in a mass ratio of 1:96:1:0.5:1.5 are mixed in a solvent N-methylpyrrolidone to prepare a positive electrode active slurry, and the positive electrode active slurry is coated on one surface of the positive electrode current collector aluminum foil, and dried to obtain a positive electrode coated with a positive electrode material layer on one side; the above steps are repeated on the other surface of the positive electrode current collector aluminum foil to obtain a positive electrode coated with a positive electrode material layer on both sides; after cold pressing, cutting, slitting, and drying, a positive electrode with a specification of 74 mm×867 mm is obtained;

[0139] Among them, in the positive electrode material layer, the number of particles of the electrode active material (i.e., the positive electrode active material lithium cobalt oxide) is N1, the number of particles of the composite foamed particles is N2, N1 / N2=0.3; the average particle size of the electrode active material (i.e., the positive electrode active material lithium cobalt oxide) is R3 μm, R3=5, R3>R1; the average thickness of the positive electrode material layer is 100 μm;

[0140] 3. Preparation of negative electrode

[0141] Mixing artificial graphite, a negative electrode active material (i.e., electrode active material), conductive agent acetylene black, binder styrene-butadiene rubber, and thickener carboxymethyl cellulose in a mass ratio of 97:1:1:1 in a solvent N-methylpyrrolidone to prepare a negative electrode active slurry, and coating the negative electrode active slurry on one surface of a negative electrode current collector copper foil, drying, and obtaining a negative electrode coated with a negative electrode material layer on one side; repeating the above steps on the other surface of the negative electrode current collector copper foil, and obtaining a negative electrode coated with a negative electrode material layer on both sides; and drying after cold pressing, cutting, slitting, and obtaining a negative electrode with a specification of 78 mm×875 mm;

[0142] The average particle size of the artificial graphite of the negative electrode active material (i.e., the electrode active material) is 10 μm, and the average thickness of the negative electrode material layer is 100 μm;

[0143] 4. Preparation of electrolyte

[0144] In a dry argon atmosphere glove box, organic solvents ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed uniformly at a mass ratio of EC:PC:EMC:DEC=10:30:30:30, and then fluoroethylene carbonate (FEC) and 1,3-propane sultone (PS) were added, dissolved and stirred thoroughly, and then the inorganic lithium salt LiPF was added. 6 , and mix well to obtain an electrolyte;

[0145] Among them, based on the mass of the electrolyte, the mass of fluoroethylene carbonate (FEC) accounts for 2%, the mass of 1,3-propane sultone (PS) accounts for 2%, and the mass of LiPF accounts for 2%. 6 (electrolyte) accounts for 12.5% ​​by mass;

[0146] 5. Diaphragm

[0147] A polyethylene porous membrane with an average thickness of 15 μm was used as the separator;

[0148] Among them, the melting point of the diaphragm is T2℃, T2=135, T1 <T2;

[0149] 6. Preparation of secondary batteries

[0150] The positive electrode and negative electrode prepared as above are respectively connected to the electrode ears, the positive electrode, separator, and negative electrode are stacked in order, and they are wound, folded, and the like as needed to obtain an electrode assembly with a wound structure, the electrode assembly is placed in an aluminum-plastic film packaging bag, the electrolyte is injected into the packaging bag and the bag is sealed, allowed to stand, formed (charged at a constant current of 0.2C for 120s, then charged at a constant current of 1C for 180s, and finally charged to 4.6V at a constant current of 1.5C), degassed, and trimmed to obtain a secondary battery.

[0151] Example 2

[0152] 1. Preparation of composite foamed particles

[0153] Consistent with Example 1;

[0154] 2. Preparation of positive electrode

[0155] The positive electrode active material (i.e., electrode active material) lithium cobalt oxide, conductive agent Super-P, conductive agent carbon nanotube CNT, and binder polyvinylidene fluoride PVDF are mixed in a solvent N-methylpyrrolidone in a mass ratio of 97:1:0.5:1.5 to prepare a positive electrode active slurry, and the positive electrode active slurry is coated on one surface of a positive electrode current collector aluminum foil, and dried to obtain a positive electrode coated with a positive electrode material layer on one side; the above steps are repeated on the other surface of the positive electrode current collector aluminum foil to obtain a positive electrode coated with a positive electrode material layer on both sides; after cold pressing, cutting, slitting, and drying, a positive electrode with a specification of 74 mm×867 mm is obtained;

[0156] The average particle size of the positive electrode active material (i.e., the electrode active material) lithium cobalt oxide is 5 μm; the average thickness of the positive electrode material layer is 100 μm;

[0157] 3. Preparation of negative electrode

[0158] The composite foamed particles, the negative electrode active material (i.e., the electrode active material) artificial graphite, the conductive agent acetylene black, the binder styrene butadiene rubber, and the thickener carboxymethyl cellulose are mixed in a solvent N-methylpyrrolidone in a mass ratio of 1:96:1:1:1 to prepare a negative electrode active slurry, and the negative electrode active slurry is coated on one surface of the negative electrode current collector copper foil, and dried to obtain a negative electrode coated with a negative electrode material layer on one side; the above steps are repeated on the other surface of the negative electrode current collector copper foil to obtain a negative electrode coated with a negative electrode material layer on both sides; after cold pressing, cutting, slitting, and drying, a negative electrode with a specification of 78 mm×875 mm is obtained;

[0159] Among them, in the negative electrode material layer, the number of particles of the electrode active material (i.e., the negative electrode active material artificial graphite) is N1, the number of particles of the composite foamed particles is N2, N1 / N2=0.06; the average particle size of the electrode active material (i.e., the negative electrode active material artificial graphite) is R3μm, R3=10, R3>R1; the average thickness of the negative electrode material layer is 100μm;

[0160] 4. Preparation of electrolyte

[0161] Consistent with Example 1;

[0162] 5. Diaphragm

[0163] Consistent with Example 1;

[0164] 6. Preparation of secondary batteries

[0165] The same as Example 1.

[0166] Example 3

[0167] 1. Preparation of composite foamed particles

[0168] Consistent with Example 1;

[0169] 2. Preparation of positive electrode

[0170] Consistent with Example 1;

[0171] 3. Preparation of negative electrode

[0172] Consistent with Example 2;

[0173] 4. Preparation of electrolyte

[0174] Consistent with Example 1;

[0175] 5. Diaphragm

[0176] Consistent with Example 1;

[0177] 6. Preparation of secondary batteries

[0178] The same as Example 1.

[0179] Comparative Example 1

[0180] The difference between Comparative Example 1 and Example 1 is that in step 1, foamed particles with an average particle size of 1 μm are used instead of composite foamed particles with an average particle size of 1 μm, that is, the foamed particles are not coated with the second polymer, and the rest are the same as in Example 1.

[0181] Comparative Example 2

[0182] The difference between Comparative Example 2 and Example 1 is that the first polymer, foaming agent, surfactant, thickener and second polymer of the same mass as those in the composite foaming particles are used to directly replace the composite foaming particles, that is, the first polymer, foaming agent, surfactant, thickener and second polymer in step 1 do not need to be prepared into composite foaming particles before use, but can be directly used in the form of raw materials to replace the composite foaming particles in step 2. The rest is referenced to Example 1.

[0183] In the cycle performance test, the number of charge and discharge cycles that the secondary battery undergoes when its capacity drops to 80% is recorded as F, unit: times. In the overheat box test, the "130℃ pass rate" refers to the pass rate when the thermostat temperature is 130℃. In the overheat box test, the "140℃ pass rate" refers to the pass rate when the thermostat temperature is 140℃.

[0184] Table 1 Test results of Examples 1 to 3 and Comparative Examples 1 to 2

[0185] Pass rate at 130℃ Pass rate at 140℃ F / times Example 1 10 / 10 10 / 10 1504 Example 2 10 / 10 10 / 10 1512 Example 3 10 / 10 10 / 10 1520 Comparative Example 1 10 / 10 8 / 10 610 Comparative Example 2 0 / 10 0 / 10 200

[0186] It can be seen from Table 1 that the composite foamed particles of the present application can improve the cycle performance of the secondary battery containing the foamed material in the electrode, and also improve the safety performance.

[0187] Example 4 and Comparative Example 3

[0188] Except for adjusting the melting point of the second polymer according to Table 2, the rest is consistent with Example 1. The melting point of the second polymer can be adjusted as shown in Table 2 by using a second polymer (low-density polyethylene LDPE) with different molecular weights. The melting point of the polymer layer (or the second polymer) is T1°C, and the melting point of the first polymer is T0°C.

[0189] Table 2 Condition parameters and test results of Example 1, Example 4 and Comparative Example 3

[0190]

[0191]

[0192] It can be seen from Table 2 that when the melting point of the first polymer is greater than or equal to the melting point of the second polymer, the composite foamed particles of the present application can improve the cycle performance of the secondary battery containing the foaming material in the electrode, while also improving the safety performance.

[0193] Examples 5 to 10

[0194] Except for adjusting the average particle size R1μm of the composite foamed particles and the average thickness R2 nm of the polymer layer according to Table 3, the rest is consistent with Example 1. The average thickness R2 nm of the polymer layer can be adjusted as shown in Table 3 by adjusting the amount of the polymer layer (second polymer low-density polyethylene LDPE), that is, adjusting the solid-liquid ratio of the second polymer and xylene in the preparation method of the composite foamed particles. The average particle size of the foamed particles is adjusted by crushing and grading, so that the average particle size R1μm of the composite foamed particles is shown in Table 3. Based on the mass of the composite foamed particles, the mass percentage of the foaming agent is E%; in the positive electrode material layer, the number of particles of the electrode active material (that is, the positive electrode active material lithium cobalt oxide) is N1, and the number of particles of the composite foamed particles is N2. Changes in the values ​​of R1 and / or R2 will cause changes in the values ​​of E and N1 / N2.

[0195] Table 3 Condition parameters and test results of Example 1, Examples 5 to 10

[0196]

[0197] As can be seen from Table 3, when the average particle size R1 of the composite foamed particles and the average thickness R2 of the polymer layer are within the range of the present application, the secondary battery containing the foamed material in the electrode has good cycle performance and safety performance. In particular, when R1 or R2 is further adjusted to meet 1≤R1≤3 or 10≤R2≤40, the cycle performance of the secondary battery containing the foamed material in the electrode can be further improved.

[0198] Examples 11 to 13

[0199] Except for adjusting N1 / N2 according to Table 4, the rest is consistent with Example 1. In the positive electrode material layer, the number of particles of the electrode active material (i.e., the positive electrode active material lithium cobalt oxide) is N1, and the number of particles of the composite foamed particles is N2. By adjusting the mass ratio of the composite foamed particles and lithium cobalt oxide (i.e., keeping the total mass of the composite foamed particles and lithium cobalt oxide, the mass of Super-P, the mass of CNT and the mass of PVDF unchanged at 97:1:0.5:1.5, only changing the mass ratio of the composite foamed particles and lithium cobalt oxide), N1 / N2 is as shown in Table 4.

[0200] Table 4 Condition parameters and test results of Example 1, Examples 11 to 13

[0201] N1 / N2 Pass rate at 130℃ Pass rate at 140℃ F / times Example 1 0.3 10 / 10 10 / 10 1504 Embodiment 11 0.5 10 / 10 10 / 10 1515 Example 12 0.1 10 / 10 10 / 10 1121 Embodiment 13 1 10 / 10 5 / 10 1498

[0202] As can be seen from Table 4, when the value of N1 / N2 is within the range of the present application, the secondary battery containing the foaming material in the electrode has good cycle performance. In particular, when N1 / N2 is further adjusted to meet 0.3 to 0.5, the cycle performance and safety performance of the secondary battery containing the foaming material in the electrode can be further improved.

[0203] Examples 14 to 16

[0204] Except for adjusting the average particle size R3 μm of the positive electrode active material (i.e., electrode active material) lithium cobalt oxide according to Table 5, the rest is consistent with Example 1. The average particle size R3 μm of the positive electrode active material (i.e., electrode active material) lithium cobalt oxide is obtained by crushing and grading as shown in Table 5. The change of R3 value will cause the change of N1 / N2 value.

[0205] Table 5 Condition parameters and test results of Example 1, Examples 14 to 16

[0206]

[0207] It can be seen from Table 5 that when R3 is further adjusted to satisfy R3>R1, the cycle performance and safety performance of the secondary battery containing the foaming material in the electrode can be further improved.

[0208] Examples 17 to 19

[0209] Except for adjusting the melting point T1°C of the polymer layer according to Table 6, the rest is the same as Example 1. By adjusting the material type of the polymer layer (second polymer), the melting point T1°C of the polymer layer can be as shown in Table 6. The melting point of the separator is T2°C.

[0210] Table 6 Condition parameters and test results of Example 1, Examples 17 to 19

[0211]

[0212] It can be seen from Table 6 that when the value of the melting point T1 of the polymer layer is within the range of the present application, the secondary battery has good cycle performance and good safety performance.

[0213] Embodiment 20

[0214] 1. Preparation of composite foamed particles

[0215] Consistent with Example 1;

[0216] 2. Preparation of positive electrode

[0217] In a solvent N-methylpyrrolidone, composite foamed particles, positive electrode active material (i.e., electrode active material) lithium cobalt oxide, conductive agent Super-P, conductive agent carbon nanotube CNT, and binder polyvinylidene fluoride PVDF in a mass ratio of 1.3:95.7:1:0.5:1.5 are mixed to prepare a positive electrode active slurry A, and the positive electrode active slurry A is coated on one surface of a positive electrode current collector aluminum foil and dried to obtain a positive electrode coated with a first positive electrode material layer on one side; then, the composite foamed particles, positive electrode active material (i.e., electrode active material) lithium cobalt oxide, conductive agent Super-P, conductive agent carbon nanotube CNT, and binder polyvinylidene fluoride PVDF are changed. The mass ratio of er-P, conductive agent carbon nanotube CNT, and binder polyvinylidene fluoride PVDF is 0.7:96.3:1:0.5:1.5. The positive electrode active slurry B is prepared by referring to the above method. The positive electrode active slurry B is coated on the surface of the first positive electrode material layer, and dried to form a second positive electrode material layer to obtain a positive electrode with a single-sided positive electrode material layer (two layers); the above steps are repeated on the other surface of the positive electrode current collector aluminum foil to obtain a positive electrode with a double-sided positive electrode material layer; after cold pressing, cutting, slitting and drying, a positive electrode with a specification of 74 mm×867 mm is obtained;

[0218] Among them, in the positive electrode material layer (two layers), the number of particles of the electrode active material (i.e., the positive electrode active material lithium cobalt oxide) is N1, and the number of particles of the composite foamed particles is N2, N1 / N2=0.3; the average particle size of the electrode active material (i.e., the positive electrode active material lithium cobalt oxide) is R3μm, R3=5, R3>R1; the thickness of the first positive electrode material layer is 50μm, and the thickness of the second positive electrode material layer is 50μm; the content of the composite foamed particles in the first material layer (the material layer close to the current collector) is Q1, and the content of the composite foamed particles in the second material layer (the material layer far away from the current collector) is Q2, and Q1>Q2;

[0219] 3. Preparation of negative electrode

[0220] Consistent with Example 1;

[0221] 4. Preparation of electrolyte

[0222] Consistent with Example 1;

[0223] 5. Diaphragm

[0224] Consistent with Example 1;

[0225] 6. Preparation of secondary batteries

[0226] The same as Example 1.

[0227] Embodiment 21

[0228] 1. Preparation of composite foamed particles

[0229] Consistent with Example 1;

[0230] 2. Preparation of positive electrode

[0231] The composite foamed particles, the positive electrode active material (i.e., the electrode active material) lithium cobalt oxide, the conductive agent Super-P, the conductive agent carbon nanotube CNT, and the binder polyvinylidene fluoride PVDF in a mass ratio of 0.7:96.3:1:0.5:1.5 were mixed in a solvent N-methylpyrrolidone to prepare a positive electrode active slurry C, and the positive electrode active slurry C was coated on one surface of the positive electrode current collector aluminum foil and dried to obtain a positive electrode coated with a first positive electrode material layer on one side; then, the composite foamed particles, the positive electrode active material (i.e., the electrode active material) lithium cobalt oxide, the conductive agent Super-P, the conductive agent carbon nanotube CNT, and the binder polyvinylidene fluoride PVDF were changed. The mass ratio of er-P, conductive agent carbon nanotube CNT, and binder polyvinylidene fluoride PVDF is 1.3:95.7:1:0.5:1.5. The positive electrode active slurry D is prepared by referring to the above method. The positive electrode active slurry D is coated on the surface of the first positive electrode material layer, and dried to form a second positive electrode material layer to obtain a positive electrode with a single-sided positive electrode material layer (two layers); the above steps are repeated on the other surface of the positive electrode current collector aluminum foil to obtain a positive electrode with a double-sided positive electrode material layer; after cold pressing, cutting, slitting and drying, a positive electrode with a specification of 74 mm×867 mm is obtained;

[0232] Among them, in the positive electrode material layer (two layers), the number of particles of the electrode active material (i.e., the positive electrode active material lithium cobalt oxide) is N1, and the number of particles of the composite foamed particles is N2, N1 / N2=0.3; the average particle size of the electrode active material (i.e., the positive electrode active material lithium cobalt oxide) is R3μm, R3=5, R3>R1; the thickness of the first positive electrode material layer is 50μm, and the thickness of the second positive electrode material layer is 50μm; the content of the composite foamed particles in the first material layer (the material layer close to the current collector) is Q1, and the content of the composite foamed particles in the second material layer (the material layer far away from the current collector) is Q2, and Q1<Q2;

[0233] 3. Preparation of negative electrode

[0234] Consistent with Example 1;

[0235] 4. Preparation of electrolyte

[0236] Consistent with Example 1;

[0237] 5. Diaphragm

[0238] Consistent with Example 1;

[0239] 6. Preparation of secondary batteries

[0240] The same as Example 1.

[0241] Table 7 Condition parameters and test results of Example 1, Example 20 to 21

[0242]

[0243] It can be seen from Table 7 that when the present application further regulates Q1 and Q2 to satisfy Q1≥Q2, the safety performance of the secondary battery containing foaming material in the electrode can be further improved.

[0244] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present application rather than to limit the scope of protection of the present application. Although the present application has been described in detail with reference to the preferred embodiments, ordinary technicians in this field should understand that the technical solution of the present application can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present application.

Claims

1. A secondary battery comprising an electrode, a separator and an electrolyte, characterized in that: The electrode includes a current collector and a material layer arranged on at least one surface of the current collector, the material layer includes composite foamed particles and electrode active materials, the composite foamed particles include foamed particles and a polymer layer arranged on the surface of the foamed particles, the foamed particles include a first polymer and a foaming agent, the polymer layer includes a second polymer, and the melting point of the first polymer is greater than or equal to the melting point of the second polymer.

2. The secondary battery according to claim 1, characterized in that: Satisfy at least one of the following conditions (a)-(b): (a) the average particle size of the composite foamed particles is R1 μm, 0.1≤R1≤5; (b) The average thickness of the polymer layer is R2 nm, 5≤R2≤100.

3. The secondary battery according to claim 1, characterized in that: The number of particles of the electrode active material is N1, the number of particles of the composite foamed particles is N2, and 0.1≤N1 / N2≤1.

4. The secondary battery according to claim 1, wherein: The average particle size of the composite foamed particles in the material layer is R1 μm, and the average particle size of the electrode active material is R3 μm, where R3>R1.

5. The secondary battery according to claim 1, characterized in that: Satisfy at least one of the following conditions (a)-(b): (a) the melting point of the polymer layer is T1°C, 90≤T1≤180°C; (b) The melting point of the diaphragm is T2°C, 120≤T2.

6. The secondary battery according to claim 5, characterized in that: T1 <T2。 7. The secondary battery according to claim 1, characterized in that: Based on the mass of the composite foamed particles, the mass percentage of the foaming agent is 0.1% to 10%.

8. The secondary battery according to claim 1, characterized in that: At least two material layers stacked in sequence are arranged on at least one surface of the current collector, the content of composite foamed particles in the material layer close to the current collector is Q1, and the content of composite foamed particles in the material layer far from the current collector is Q2, Q1>Q2.

9. The secondary battery according to claim 1, characterized in that: Satisfy at least one of the following conditions (a)-(c): (a) the foaming agent comprises at least one of an azo compound, a sulfonylhydrazide compound, a nitroso compound, sodium carbonate and sodium bicarbonate; (b) the first polymer comprises at least one of polypropylene, polyethylene, polylactic acid, polyvinyl chloride, polyethylene terephthalate, polystyrene, polycarbonate, polyvinyl alcohol, polyurethane, polycarbonate and polyamide; (c) The second polymer includes at least one of polypropylene, polyethylene, polylactic acid, polyvinyl chloride, polyethylene terephthalate, polystyrene, polycarbonate, polyvinyl alcohol, polyurethane, polycarbonate and polyamide.

10. An electronic device, characterized in that: The invention comprises a secondary battery as claimed in any one of claims 1 to 9.

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