A secondary battery and electronic device

Abstract

This application provides a secondary battery and an electronic device. The secondary battery includes a positive electrode, a negative electrode, a separator, and an electrolyte. The negative electrode includes a negative current collector and a negative electrode material layer located on at least one surface of the negative current collector. The negative electrode material layer includes a negative electrode active material, which is a silicon-carbon composite material. On the plane formed by the length direction and thickness direction of the negative electrode, the minimum interior angle of the outer contour of particles with a longest diameter greater than 10 μm in the silicon-carbon composite material is A°. The thickness of the separator is B μm, where 488 ≤ A × B ≤ 360°, and 4 ≤ B ≤ 25°. With the above configuration, the secondary battery has good self-discharge performance, charge rate performance, and cycle performance.

Description

Technical Field

[0001] This application relates to the field of electrochemical technology, and in particular to a secondary battery and electronic device. Background Technology

[0002] Rechargeable batteries, represented by lithium-ion batteries, possess outstanding characteristics such as high energy density, long cycle life, low pollution, and no memory effect. As a clean energy source, the application of rechargeable batteries has gradually expanded from electronic products to large-scale devices such as electric vehicles, in order to adapt to the sustainable development strategy of environment and energy. Consequently, higher requirements are being placed on the energy density of rechargeable batteries.

[0003] Currently, graphite remains the primary anode active material for commercially available rechargeable batteries. However, graphite has a lithium intercalation capacity of approximately 372 mAh / g, while silicon, as the anode active material, can achieve a lithium intercalation capacity of 3579 mAh / g. Utilizing silicon allows for higher energy density in rechargeable batteries, meaning more energy can be stored within the same volume and weight. However, compared to graphite, silicon, when used as the anode active material, undergoes greater volume expansion and contraction during charging and discharging. This exerts greater mechanical stress on the separator, increasing the risk of separator rupture or deformation, and consequently reducing the safety performance of the rechargeable battery. Summary of the Invention

[0004] The purpose of this application is to provide a secondary battery and electronic device to reduce the risk of self-discharge and thermal runaway caused by the separator being punctured by particles of silicon-carbon composite material, thereby improving the self-discharge performance, charge rate performance, and cycle performance of the secondary battery. The specific technical solution is as follows:

[0005] It should be noted that the invention description in this application uses lithium-ion batteries as an example of secondary batteries to explain this application, but the secondary batteries in this application are not limited to lithium-ion batteries.

[0006] The first aspect of this application provides a secondary battery, which includes a positive electrode, a negative electrode, a separator, and an electrolyte. The negative electrode includes a negative current collector and a negative electrode material layer located on at least one surface of the negative current collector. The negative electrode material layer includes a negative electrode active material, which includes a silicon-carbon composite material. On the plane formed by the length direction and the thickness direction of the negative electrode, the minimum internal angle of the outer contour of the particles with the longest diameter greater than 10 μm in the silicon-carbon composite material is A°. The thickness of the separator is B μm, 488≤A×B≤3600, and 4≤B≤25. By limiting the values ​​of A×B and B within the scope of this application, the relative relationship between the minimum inner angle of the outer contour of particles with a longest diameter greater than 10 μm in silicon-carbon composite material and the thickness of the separator is controlled. This fully leverages the synergistic effect between the inner angle of the outer contour of silicon-carbon composite material particles and the thickness of the separator. The separator has good mechanical strength, which can effectively reduce the risk of the separator being punctured by silicon-carbon composite material particles. This reduces the risk of self-discharge and thermal runaway caused by the separator being punctured by silicon-carbon composite material particles. At the same time, the lithium ion transport distance during the secondary battery cycle is moderate, thereby improving the self-discharge performance, charge rate performance and cycle performance of the secondary battery.

[0007] In one embodiment of this application, 89 ≤ A ≤ 180. By adjusting the value of A within the above range, it is beneficial to reduce the risk of the separator being punctured by particles of silicon-carbon composite material, thereby reducing the risk of self-discharge and thermal runaway caused by the separator being punctured by particles of silicon-carbon composite material, thus improving the self-discharge performance, charge rate performance and cycle performance of the secondary battery.

[0008] In one embodiment of this application, the separator includes a base film with a thickness of 4 μm to 10 μm. By adjusting the thickness of the base film within the above range, the separator has better mechanical strength, which helps reduce the risk of the separator being punctured by particles of silicon-carbon composite material. This reduces the risk of self-discharge and thermal runaway caused by the puncture of the separator by particles of silicon-carbon composite material. At the same time, the lithium ion transport distance during the secondary battery cycle is moderate, thereby improving the self-discharge performance, charge rate performance, and cycle performance of the secondary battery.

[0009] In one embodiment of this application, the separator further includes an adhesive layer, where 4.5 ≤ B ≤ 15. By selecting the above-mentioned separator and adjusting the value of B within the above range, it is beneficial to increase the adhesion between the separator and the positive electrode and / or between the separator and the negative electrode, shortening the lithium-ion transport distance, thereby improving the cycle performance and charge rate performance of the secondary battery. Simultaneously, it helps reduce the risk of the separator being punctured by particles of the silicon-carbon composite material. Therefore, the secondary battery exhibits good self-discharge performance, charge rate performance, and cycle performance.

[0010] In one embodiment of this application, the separator further includes a ceramic coating, where 4.5 ≤ B ≤ 14. By selecting the above-mentioned separator and adjusting the value of B within the above range, the separator exhibits good mechanical strength, which helps reduce the risk of the separator being punctured by particles of silicon-carbon composite material. Simultaneously, the separator has good wetting properties, resulting in a moderate lithium-ion transport distance and improved cycle performance and charge rate performance of the secondary battery. Therefore, the secondary battery possesses good self-discharge performance, charge rate performance, and cycle performance.

[0011] In one embodiment of this application, the separator further includes an adhesive layer and a ceramic coating, where 6 ≤ B ≤ 25. By selecting the above-mentioned separator and controlling the value of B within the above range, it is beneficial to reduce the risk of the separator being punctured by particles of silicon-carbon composite material. Simultaneously, the separator has good wetting properties, shortening the lithium-ion transport distance. Therefore, the secondary battery exhibits good self-discharge performance, charge rate performance, and cycle performance.

[0012] In one embodiment of this application, the separator further includes an adhesive layer comprising an adhesive, which includes at least one selected from polyacrylonitrile, polymethyl methacrylate, or polyvinylidene fluoride. By selecting the above-mentioned types of adhesives, the secondary battery exhibits good self-discharge performance, charge rate performance, and cycle performance.

[0013] In one embodiment of this application, the separator further includes a ceramic coating comprising inorganic particles, including at least one of alumina, titanium oxide, silicon oxide, or magnesium oxide. By selecting the aforementioned types of inorganic particles, the mechanical strength of the separator is improved, and the separator also exhibits good wetting properties, resulting in good self-discharge performance, charge rate performance, and cycle performance of the secondary battery.

[0014] In one embodiment of this application, the silicon mass percentage content is 44% to 57% based on the mass of the silicon-carbon composite material. Controlling the silicon mass percentage content within this range helps reduce the volume expansion of the silicon-carbon composite material. When the silicon-carbon composite material is used as the negative electrode active material, the secondary battery exhibits a higher energy density and also improves its cycle performance.

[0015] In one embodiment of this application, the negative electrode active material further includes at least one of artificial graphite, natural graphite, mesophase carbon microspheres, soft carbon, or hard carbon. By mixing silicon-carbon composite materials with other negative electrode active materials, the mixed specific capacity of the negative electrode active materials is larger, thereby improving the energy density of the secondary battery.

[0016] In one embodiment of this application, the separator includes a base membrane, the material of which includes at least one of polyethylene or polypropylene. By selecting the above-mentioned types of base membranes, the separator exhibits good mechanical strength and chemical stability, and the secondary battery demonstrates excellent self-discharge performance, charge rate performance, and cycle performance.

[0017] The second aspect of this application provides a method for preparing a secondary battery, comprising the following steps: preparing a positive electrode, a negative electrode, a separator, and an electrolyte, and assembling the secondary battery. The method for preparing the silicon-carbon composite material in the negative electrode includes the following steps:

[0018] (1) After the carbon source and the alkali source are mixed evenly, the mixture is heat-treated at 420℃ to 600℃ for 0.5h to 2h, and then heated to 650℃ to 950℃ and held for 0.5h to 3h to obtain the precursor.

[0019] The carbon source includes at least one of phenolic resin, urea-formaldehyde resin, melamine-formaldehyde resin, polyvinyl chloride or polyacrylonitrile, and the alkali source includes at least one of potassium hydroxide, sodium hydroxide, lithium hydroxide or ammonia water. The mass ratio of carbon source to alkali source is 1:1 to 1:5.

[0020] (2) The precursor was heated to 400°C to 510°C for 0.2h to 4h in an inert atmosphere, then silane gas was introduced for 220min to 420min, then the temperature was raised to 520°C to 560°C and held for 0.5h to 2h, and then a reducing atmosphere was introduced for 1.5h to 4h to obtain silicon-carbon composite material.

[0021] The inert atmosphere includes at least one of nitrogen, argon or helium, the silane gas includes at least one of silane, silane, propane, phenylsilane or tolylsilane, and the reducing atmosphere includes at least one of acetylene, propylene or toluene.

[0022] The silicon-carbon composite material prepared using the method described in this application has a large inner angle value on the outer contour of the particles. Applying this silicon-carbon composite material to a secondary battery can effectively reduce the risk of the separator being punctured by the silicon-carbon composite material particles, thus improving the self-discharge performance of the secondary battery. Simultaneously, the obtained silicon-carbon composite material exhibits high specific capacity and initial coulombic efficiency. When the silicon-carbon composite material prepared above is applied to a secondary battery, the secondary battery exhibits good self-discharge performance, charge rate performance, and cycle performance.

[0023] A third aspect of this application provides an electronic device comprising the secondary battery described in any of the foregoing embodiments. The secondary battery of this application exhibits excellent self-discharge performance, charge rate performance, and cycle performance; therefore, the electronic device of this application has a long service life.

[0024] The beneficial effects of this application are:

[0025] This application provides a secondary battery and an electronic device. The secondary battery includes a positive electrode, a negative electrode, a separator, and an electrolyte. The negative electrode includes a negative current collector and a negative electrode material layer located on at least one surface of the negative current collector. The negative electrode material layer includes a negative electrode active material, which is a silicon-carbon composite material. On the plane formed by the length direction and thickness direction of the negative electrode, the minimum interior angle of the outer contour of particles with a longest diameter greater than 10 μm in the silicon-carbon composite material is A°. The thickness of the separator is B μm, where 488 ≤ A × B ≤ 360° and 4 ≤ B ≤ 25°. By controlling the thickness of the separator and its synergistic effect with the interior angle of the outer contour of the silicon-carbon composite material particles, the risk of self-discharge and thermal runaway caused by the separator being punctured by the silicon-carbon composite material particles can be reduced, thereby improving the self-discharge performance, charge rate performance, and cycle performance of the secondary battery.

[0026] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other embodiments can be obtained based on these drawings.

[0028] Figure 1 A schematic diagram showing the test of the minimum internal angle of the outer contour of silicon-carbon composite particles;

[0029] Figure 2 This is a scanning electron microscope image of the negative electrode sheet of Example 1-1 in this application.

[0030] Figure reference numeral: Silicon-carbon composite material 11. Detailed Implementation

[0031] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0032] It should be noted that, in the specific embodiments of this application, lithium-ion batteries are used as an example of secondary batteries to explain this application, but the secondary batteries in this application are not limited to lithium-ion batteries.

[0033] To address the issue of silicon materials easily causing separator membrane rupture, current methods typically employ high-mechanical-strength separator membranes, adding separator membrane coatings, and optimizing the particle size distribution of silicon materials. However, because conventional silicon materials have irregular shapes and numerous sharp edges on their surfaces, these methods cannot effectively reduce the risk of silicon particles puncturing the separator membrane. Therefore, this application provides a secondary battery that can reduce the risk of self-discharge and thermal runaway caused by the separator membrane being punctured by silicon-carbon composite material particles.

[0034] The first aspect of this application provides a secondary battery, which includes a positive electrode, a negative electrode, a separator, and an electrolyte. The negative electrode includes a negative current collector and a negative electrode material layer located on at least one surface of the negative current collector. The negative electrode material layer includes a negative electrode active material, which includes a silicon-carbon composite material. On the plane formed by the length direction and the thickness direction of the negative electrode, the minimum internal angle of the outer contour of the particles with the longest diameter greater than 10 μm in the silicon-carbon composite material is A°. The thickness of the separator is B μm, 488≤A×B≤3600, and 4≤B≤25. For example, the value of A×B can be 488, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500, 1600, 1700, 1800, 1900, 2000, 2100, 2200, 2300, 2400, 2500, 2600, 2700, 2800, 2900, 3000, 3100, 3200, 3300, 3400, 3500, 3600, or a range of any two of these values. The value of B can be 4, 5, 6, 8, 10, 12, 14, 15, 16, 18, 20, 22, 24, 25, or a range of any two of these values.

[0035] When the value of A×B is too small, i.e., below the lower limit of this application, the sharp corners of the silicon-carbon composite material particles can easily puncture the separator during the processing or use of the secondary battery, potentially leading to micro-short circuits and self-discharge, increasing the risk of thermal runaway and fire / explosion. When the value of A×B is too large, i.e., above the upper limit of this application, the value of B is correspondingly too large, meaning the separator thickness is too large. This results in a lower energy density in the secondary battery, weakening the high energy density advantage of the silicon-carbon composite material as the negative electrode active material. Furthermore, an excessively thick separator can obstruct lithium-ion transport, affecting the charging rate performance of the secondary battery. By limiting the values ​​of A×B and B within the scope of this application, the relative relationship between the minimum inner angle of the outer contour of particles with a longest diameter greater than 10 μm in the silicon-carbon composite material and the thickness of the separator is controlled. This fully leverages the synergistic effect between the inner angle of the outer contour of the silicon-carbon composite material particles and the thickness of the separator. The separator exhibits better mechanical strength, effectively reducing the risk of the separator being punctured by the silicon-carbon composite material particles. This, in turn, reduces the risk of self-discharge and thermal runaway caused by the puncture of the separator by the silicon-carbon composite material particles, thus improving the self-discharge performance of the secondary battery. Simultaneously, the lithium ion transport distance during the secondary battery cycle is moderate, thereby improving the self-discharge performance, charge rate performance, and cycle performance of the secondary battery. In this application, a longest diameter greater than 10 μm refers to the maximum circumscribed circle diameter of the outer contour of the silicon-carbon composite material particles being greater than 10 μm.

[0036] In one embodiment of this application, 89 ≤ A ≤ 180. For example, the value of A can be 89, 90, 93, 95, 98, 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, 178, 180, or a range of any two of these values. By adjusting the value of A within the above range, the minimum range of the inner angle of the outer contour of particles with a longest diameter greater than 10 μm in the silicon-carbon composite material is moderate, which helps to reduce the risk of the separator being punctured by the particles of the silicon-carbon composite material. This reduces the risk of self-discharge and thermal runaway caused by the separator being punctured by the particles of the silicon-carbon composite material, thereby improving the self-discharge performance, charging rate performance and cycle performance of the secondary battery.

[0037] In one embodiment of this application, the separator includes a base film with a thickness T ranging from 4 μm to 10 μm. For example, the thickness T of the base film can be 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, or a range consisting of any two of these values. By adjusting the thickness of the base film within the above range, the separator has better mechanical strength, which helps reduce the risk of the separator being punctured by particles of silicon-carbon composite material, thereby reducing the risk of self-discharge and thermal runaway caused by the puncture of the separator by particles of silicon-carbon composite material. At the same time, the transport distance of lithium ions during the secondary battery cycle is moderate, thereby improving the self-discharge performance, charge rate performance, and cycle performance of the secondary battery. This application does not have any particular limitation on the method of adjusting the thickness of the base film, as long as it can achieve the purpose of this application. For example, commercially available base films with different thicknesses can be selected, and the thickness of the base film can be determined by combining the test method of "testing the thickness B of the separator and the thickness T of the base film" in this application, and a base film of the required thickness can be selected.

[0038] In one embodiment of this application, the separator further includes an adhesive layer, where 4.5 ≤ B ≤ 15. For example, the value of B can be 4.5, 4.8, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, or a range consisting of any two of these values. By selecting the above-mentioned separator and adjusting the value of B within the above range, it is beneficial to increase the adhesion between the separator and the positive electrode and / or between the separator and the negative electrode, reduce the gap between the separator and the positive electrode and / or between the separator and the negative electrode, shorten the lithium-ion transport distance, and thus improve the cycle performance and charge rate performance of the secondary battery. Simultaneously, it helps to reduce the risk of the separator being punctured by silicon-carbon composite material particles, thereby reducing the risk of self-discharge and thermal runaway caused by such punctures and improving the self-discharge performance of the secondary battery. Therefore, the secondary battery exhibits good self-discharge performance, charge rate performance, and cycle performance.

[0039] In one embodiment of this application, the separator further includes a ceramic coating, where 4.5 ≤ B ≤ 14. For example, the value of B can be 4.5, 4.8, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or a range of any two values ​​therein. By selecting the above-mentioned separator and adjusting the value of B within the above range, the separator has better mechanical strength, which helps reduce the risk of the separator being punctured by particles of silicon-carbon composite material, thereby reducing the risk of self-discharge and thermal runaway caused by the separator being punctured by particles of silicon-carbon composite material, and improving the self-discharge performance of the secondary battery. At the same time, the separator has good wetting properties, and the lithium ion transport distance is moderate, which improves the cycle performance and charge rate performance of the secondary battery. Therefore, the secondary battery has good self-discharge performance, charge rate performance, and cycle performance.

[0040] In one embodiment of this application, the separator further includes an adhesive layer and a ceramic coating, where 6 ≤ B ≤ 25. For example, the value of B can be 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, or a range of any two of these values. By selecting the above-mentioned separator and adjusting the value of B within the above range, the separator has better mechanical strength, which helps reduce the risk of the separator being punctured by particles of silicon-carbon composite material, thereby reducing the risk of self-discharge and thermal runaway caused by the separator being punctured by particles of silicon-carbon composite material, and improving the self-discharge performance of the secondary battery. At the same time, the separator has better wetting properties, and the gap between the separator and the positive electrode and / or between the separator and the negative electrode is smaller, shortening the lithium ion transport distance and improving the cycle performance and charge rate performance of the secondary battery. Therefore, the secondary battery has good self-discharge performance, charge rate performance, and cycle performance.

[0041] In one embodiment of this application, the separator further includes an adhesive layer comprising an adhesive, which includes at least one of polyacrylonitrile (PAN), polymethyl methacrylate (PMMA), or polyvinylidene fluoride (PVDF). By selecting such adhesives, the adhesion between the separator and the positive electrode and / or between the separator and the negative electrode is increased, reducing the gap between the separator and the positive electrode and / or between the separator and the negative electrode, shortening the lithium-ion transport distance, and thus improving the cycle performance and charge rate performance of the secondary battery. Therefore, the secondary battery exhibits good self-discharge performance, charge rate performance, and cycle performance.

[0042] In one embodiment of this application, the separator further includes a ceramic coating comprising inorganic particles, including at least one of alumina, titanium oxide, silicon oxide, or magnesium oxide. By selecting the aforementioned types of inorganic particles, the mechanical strength of the separator is improved, reducing the risk of puncture by silicon-carbon composite material particles. This, in turn, reduces the risk of self-discharge and thermal runaway caused by puncture of the separator by silicon-carbon composite material particles, thus improving the self-discharge performance of the secondary battery. Simultaneously, the separator exhibits good wetting properties, improving the cycle performance of the secondary battery. Therefore, the secondary battery possesses excellent self-discharge performance, charge rate performance, and cycle performance.

[0043] In one embodiment of this application, the mass percentage of silicon based on the mass of the silicon-carbon composite material is 44% to 57%. For example, the mass percentage of silicon can be 44%, 15%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, or a range of any two of these values. By controlling the mass percentage of silicon within the above range, it is beneficial to reduce the volume expansion of the silicon-carbon composite material. When the silicon-carbon composite material is used as the negative electrode active material, the secondary battery has a higher energy density, and it is also beneficial to improve the cycle performance of the secondary battery. In this application, when calculating the mass percentage of silicon based on the mass of the silicon-carbon composite material, it is calculated after excluding impurity elements in the silicon-carbon composite material, wherein the content of impurity elements is typically less than 0.5%. This application does not limit the types of the above-mentioned impurity elements; for example, impurity elements may include, but are not limited to, at least one of oxygen, nitrogen, sulfur, iron, nickel, or aluminum.

[0044] In one embodiment of this application, the negative electrode active material further includes at least one of artificial graphite, natural graphite, mesophase carbon microspheres, soft carbon, or hard carbon. By mixing silicon-carbon composite materials with other negative electrode active materials, the mixed specific capacity of the negative electrode active materials is larger, thereby improving the energy density of the secondary battery.

[0045] In one embodiment of this application, the separator includes a base membrane, the material of which includes at least one of polyethylene or polypropylene. By selecting the above-mentioned type of base membrane, the separator has better mechanical strength and chemical stability, which helps to reduce the risk of the separator being punctured by particles of silicon-carbon composite material, thereby reducing the risk of self-discharge and thermal runaway caused by the puncture of the separator by particles of silicon-carbon composite material, and improving the self-discharge performance of the secondary battery. Therefore, the secondary battery has good self-discharge performance, charge rate performance, and cycle performance. This application does not have any particular limitations on the method of controlling the material of the base membrane, as long as the purpose of this application can be achieved. For example, commercially available base membranes with different materials can be selected, and a base membrane with the desired material can be selected. This application has no particular limitations, as long as the purpose of this application can be achieved.

[0046] In this application, the aforementioned "negative electrode material layer located on at least one surface of the negative electrode current collector" means that the negative electrode material layer can be located on one surface of the negative electrode current collector along its own thickness direction, or on two surfaces of the negative electrode current collector along its own thickness direction. It should be noted that the "surface" here can be the entire surface area of ​​the negative electrode current collector, or only a portion of the surface area; this application has no particular limitation, as long as the purpose of this application is achieved. This application has no particular limitation on the negative electrode current collector, as long as the purpose of this application is achieved. For example, the negative electrode current collector can include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foamed nickel, foamed copper, or composite current collectors (e.g., lithium copper composite current collector, carbon copper composite current collector, nickel copper composite current collector, titanium copper composite current collector, etc.). In this application, there is no particular limitation on the thickness of the negative electrode current collector, as long as the purpose of this application is achieved. For example, the thickness of the negative electrode current collector is 4 μm to 20 μm. Optionally, the negative electrode material layer may also include a negative electrode binder and a conductive agent. This application does not impose any particular limitation on the type of negative electrode binder in the negative electrode material layer, as long as it can achieve the purpose of this application. For example, the negative electrode binder may include, but is not limited to, at least one of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon. This application does not impose any particular limitation on the type of conductive agent in the negative electrode material layer, as long as it can achieve the purpose of this application. For example, the conductive agent may include, but is not limited to, at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, Ketjen black, graphene, metallic materials, or conductive polymers. The aforementioned carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The aforementioned carbon fibers may include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or carbon nanofibers. The aforementioned metallic materials may include, but are not limited to, metal powder and / or metal fibers. Specifically, the metal may include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The aforementioned conductive polymer may include, but is not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole. This application does not impose any particular limitation on the mass ratio of the negative electrode active material, conductive agent, and negative electrode binder in the negative electrode material layer, as long as the purpose of this application can be achieved.

[0047] This application does not impose any particular limitation on the positive electrode sheet, as long as it achieves the purpose of this application. For example, the positive electrode sheet includes a positive current collector and a positive electrode material layer located on at least one surface of the positive current collector. The aforementioned "positive electrode material layer located on at least one surface of the positive current collector" means that the positive electrode material layer can be located on one surface of the positive current collector along its own thickness direction, or on two surfaces of the positive current collector along its own thickness direction. It should be noted that the "surface" here can be the entire surface area of ​​the positive current collector, or only a part of the surface area; this application does not impose any particular limitation, as long as it achieves the purpose of this application. This application does not impose any particular limitation on the positive current collector, as long as it achieves the purpose of this application. For example, the positive current collector can include aluminum foil, aluminum alloy foil, or composite current collectors (e.g., aluminum-carbon composite current collectors). The positive electrode material layer of this application includes a positive electrode active material; this application does not impose any particular limitation on the type of positive electrode active material, as long as it achieves the purpose of this application. For example, the positive electrode active material can include lithium nickel cobalt manganese oxide (LiNi). 0.90 Co 0.05 Mn 0.05 At least one of the following: O2 (NCM955), NCM811, NCM622, NCM523, NCM111, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide (LiCoO2), lithium manganese oxide, lithium manganese iron phosphate, or lithium titanate. In this application, the positive electrode active material may also contain non-metallic elements, such as at least one of fluorine, phosphorus, boron, chlorine, silicon, or sulfur. In this application, there are no particular limitations on the thickness of the positive electrode current collector and the positive electrode material layer, as long as the purpose of this application is achieved. For example, the thickness of the positive electrode current collector is 5 μm to 20 μm, and the thickness of the single-sided positive electrode material layer is 30 μm to 120 μm. In this application, the positive electrode material layer may also include a positive electrode binder and a conductive agent. This application does not impose any particular limitation on the type of positive electrode binder in the positive electrode material layer, as long as it can achieve the purpose of this application. For example, the positive electrode binder may include, but is not limited to, at least one of polyvinylidene fluoride, polytetrafluoroethylene, polyolefins, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, modified polyvinylidene fluoride, modified styrene-butadiene rubber (SBR), or polyurethane. This application does not impose any particular limitation on the type of conductive agent in the positive electrode material layer, as long as it can achieve the purpose of this application. For example, the conductive agent may be the same type as the conductive agent in the negative electrode material layer. This application does not impose any particular limitation on the mass ratio of positive electrode active material, conductive agent, and positive electrode binder in the positive electrode material layer. Those skilled in the art can select according to actual needs, as long as the purpose of this application can be achieved.

[0048] In this application, the electrolyte includes lithium salts and non-aqueous solvents. The lithium salt may include at least one of LiPF6, LiPO2F2, LiNO3, LiBF4, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalato)borate (LiBOB), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), or lithium difluoroborate. This application does not limit the content of lithium salts in the electrolyte, as long as the purpose of this application is achieved. This application does not particularly limit the non-aqueous solvent, as long as the purpose of this application is achieved. For example, the non-aqueous solvent may include, but is not limited to, at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents. The aforementioned carbonate compounds may include, but are not limited to, at least one of chain carbonate compounds, cyclic carbonate compounds, or fluorocarbonate compounds. The aforementioned chain carbonate compounds may include, but are not limited to, at least one of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, or methyl ethyl carbonate. The aforementioned cyclic carbonates may include, but are not limited to, at least one of ethylene carbonate, propylene carbonate (PC), butylene carbonate, or vinylene carbonate. Fluorinated carbonate compounds may include, but are not limited to, at least one of fluoroethylene carbonate, 1,2-difluoroethylene carbonate, 1,1,2-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, or trifluoromethylethylene carbonate. The aforementioned carboxylic acid ester compounds may include, but are not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valproic acid lactone, or caprolactone. The aforementioned ether compounds may include, but are not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. The aforementioned other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate.

[0049] The secondary battery of this application also includes a packaging bag for containing the positive electrode, negative electrode, separator, and electrolyte, as well as other components known in the art for secondary batteries. This application does not limit the aforementioned other components. This application does not impose any particular limitation on the packaging bag; it can be any packaging bag known in the art, as long as it can achieve the purpose of this application.

[0050] The secondary battery described in this application is not particularly limited and may include any device in which an electrochemical reaction occurs. In one embodiment of this application, the secondary battery may include, but is not limited to, lithium-ion secondary batteries (lithium-ion batteries), sodium-ion batteries, lithium polymer secondary batteries, or lithium-ion polymer secondary batteries.

[0051] The second aspect of this application provides a method for preparing a secondary battery, comprising the following steps: preparing a positive electrode, a negative electrode, a separator, and an electrolyte, and assembling the secondary battery. The method for preparing the silicon-carbon composite material in the negative electrode includes the following steps:

[0052] (1) After the carbon source and the alkali source are mixed evenly, heat treatment is carried out at 420℃ to 600℃ for 0.5h to 2h. For example, after the carbon source and the alkali source are mixed evenly, heat treatment can be carried out at 420℃, 430℃, 440℃, 450℃, 460℃, 470℃, 480℃, 490℃, 500℃, 510℃, 520℃, 530℃, 540℃, 550℃, 560℃, 570℃, 580℃, 590℃, 600℃ or any two of these values. The heat treatment time can be 0.5h, 0.6h, 0.7h, 0.8h, 0.9h, 1h, 1.1h, 1.2h, 1.3h, 1.4h, 1.5h, 1.6h, 1.7h, 1.8h, 1.9h, 2h or any two of these values. Then, the temperature is raised to 650℃ to 950℃ and held for 0.5h to 3h. For example, the temperature can be raised to 650℃, 670℃, 700℃, 720℃, 750℃, 780℃, 800℃, 820℃, 850℃, 880℃, 900℃, 920℃, 950℃ or any two of these values. The holding time can be 0.5h, 0.8h, 1h, 1.2h, 1.5h, 1.8h, 2h, 2.2h, 2.5h, 2.8h, 3h or any two of these values ​​to obtain the precursor.

[0053] The carbon source includes at least one of phenolic resin, urea-formaldehyde resin, melamine-formaldehyde resin, polyvinyl chloride, or polyacrylonitrile, and the alkali source includes at least one of potassium hydroxide, sodium hydroxide, lithium hydroxide, or ammonia water. The mass ratio of the carbon source to the alkali source is 1:1 to 1:5. For example, the mass ratio of the carbon source to the alkali source can be 1:1, 1:2, 1:3, 1:4, 1:5, or a range consisting of any two of these values.

[0054] (2) The precursor is heated to 400°C to 510°C in an inert atmosphere for 0.2h to 4h. For example, the precursor can be heated to 400°C, 410°C, 420°C, 430°C, 440°C, 450°C, 460°C, 470°C, 480°C, 490°C, 500°C, 510°C or any two of these values. The treatment time can be 0.2h, 0.5h, 0.8h, 1h, 1.2h, 1.5h, 1.8h, 2h, 2.2h, 2.5h, 2.8h, 3h, 3.2h, 3.5h, 3.8h, 4h or any two of these values. Then silane gas is introduced for 220 to 420 minutes. For example, the time for introducing silane gas can be 220 minutes, 240 minutes, 250 minutes, 260 minutes, 280 minutes, 300 minutes, 320 minutes, 340 minutes, 350 minutes, 360 minutes, 380 minutes, 400 minutes, 420 minutes, or any range of two of these values. Then, raise the temperature to 520℃ to 560℃ and hold it for 0.5h to 2h. For example, the temperature can be raised to 520℃, 522℃, 525℃, 528℃, 530℃, 532℃, 535℃, 538℃, 540℃, 542℃, 545℃, 548℃, 550℃, 552℃, 555℃, 558℃, 560℃ or any two of these values. The holding time can be 0.5h, 0.6h, 0.7h, 0.8h, 0.9h, 1h, 1.1h, 1.2h, 1.3h, 1.4h, 1.5h, 1.6h, 1.7h, 1.8h, 1.9h, 2h or any two of these values. The reducing atmosphere is then introduced for 1.5 to 4 hours. For example, the reducing atmosphere can be introduced for 1.5 hours, 1.8 hours, 2 hours, 2.2 hours, 2.5 hours, 2.8 hours, 3 hours, 3.2 hours, 3.5 hours, 3.8 hours, 4 hours, or any two of these values ​​to obtain a silicon-carbon composite material.

[0055] The inert atmosphere includes at least one of nitrogen, argon or helium, the silane gas includes at least one of silane, silane, propane, phenylsilane or tolylsilane, and the reducing atmosphere includes at least one of acetylene, propylene or toluene.

[0056] This application does not impose any particular limitation on the heat treatment method used in preparing the precursor, as long as it achieves the purpose of this application. For example, heat treatment can be performed in a rotary kiln. This application does not impose any particular limitation on the heating rate used in preparing the precursor, as long as it achieves the purpose of this application. For example, the heating rate can be from 2°C / min to 10°C / min. This application does not impose any particular limitation on the heat treatment method used to prepare silicon-carbon composite materials from the precursor, as long as it achieves the purpose of this application. For example, the precursor can be placed in a fluidized bed for heat treatment. This application does not impose any particular limitation on the heating rate used in preparing silicon-carbon composite materials from the precursor, as long as it achieves the purpose of this application. For example, the heating rate can be from 5°C / min to 15°C / min. This application does not impose any particular limitation on the flow rate of the inert atmosphere, as long as it achieves the purpose of this application. For example, the flow rate of the inert atmosphere can be from 5 L / min to 15 L / min. This application does not impose any particular limitation on the flow rate of silane gas, as long as the purpose of this application can be achieved. For example, the flow rate of silane gas can be from 1 L / min to 5 L / min. This application also does not impose any particular limitation on the flow rate of the reducing atmosphere, as long as the purpose of this application can be achieved. For example, the flow rate of the reducing atmosphere can be from 2 L / min to 10 L / min.

[0057] The inventors discovered that when the inner angle of the outer contour of silicon-carbon composite material particles is too small, there is a risk that the particles may puncture the separator during the preparation or use of secondary batteries, leading to self-discharge and thermal runaway. Using the aforementioned method to prepare silicon-carbon composite materials, by controlling the alkali-to-carbon ratio, performing heat treatment at different temperatures, and adjusting the conditions for gaseous silicon deposition, a larger inner angle value was obtained for particles with a longest diameter greater than 10 μm in the resulting silicon-carbon composite material. Applying the prepared silicon-carbon composite material to secondary batteries effectively reduces the risk of the separator being punctured by silicon-carbon composite material particles, thereby reducing the risk of self-discharge and thermal runaway caused by such punctures and improving the self-discharge performance of the secondary battery. Introducing silane gas ensures uniform distribution of silicon material within the precursor. By controlling the timing of silane gas introduction, the mass percentage of silicon in the silicon-carbon composite material is kept within a suitable range. This increases the energy density of the silicon-carbon composite material while reducing its volume expansion, resulting in a silicon-carbon composite material with high specific capacity and initial coulombic efficiency. When the silicon-carbon composite material prepared above is applied to a secondary battery, the secondary battery exhibits good self-discharge performance, charge rate performance, and cycle performance.

[0058] A third aspect of this application provides an electronic device comprising the secondary battery described in any of the foregoing embodiments. The secondary battery of this application exhibits excellent self-discharge performance, charge rate performance, and cycle performance; therefore, the electronic device of this application has a long service life.

[0059] The electronic device described in this application is not particularly limited and can be any electronic device known in the prior art. For example, the electronic device may include, but is not limited to, laptops, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, household large-capacity batteries, and lithium-ion capacitors.

[0060] Example

[0061] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0062] Test methods and equipment:

[0063] Test A for the minimum interior angle of the outer contour of particles with a longest diameter greater than 10 μm:

[0064] The negative electrode sheet was sliced ​​using an argon-ion cross-section polisher (JEOL, model: IB-09010CP) to obtain a cross-section along the thickness direction. Microscopic images of the silicon-carbon composite material cross-section obtained above were captured using a field emission scanning electron microscope (Zeiss, model: Sigma-02-33). The cross-sectional profiles of 50 silicon-carbon composite particles with a longest diameter greater than 10 μm were randomly selected for analysis, such as... Figure 1 As shown, the outline of a single silicon-carbon composite material particle 11 is traced, tangents are drawn along the edges of the sharp corners, and the angle between the two tangents is measured. The measured angle values ​​are compared, and the minimum value is taken as the minimum internal angle of the outer contour of the silicon-carbon composite material particle 11 (e.g., ...). Figure 1 If angle 1 is 115°, angle 2 is 105°, angle 3 is 142°, and angle 4 is 140°, then angle 2 is denoted as the minimum interior angle of the outer contour of the silicon-carbon composite particle. The minimum interior angles of the outer contours of particles with a longest diameter greater than 10 μm among 50 silicon-carbon composite materials are obtained, and the average value is taken to obtain A.

[0065] Tests on the thickness B of the separator and the thickness T of the base film:

[0066] Argon ion polishing was performed on the separator to obtain its cross-section. The morphology of the separator's cross-section along the thickness direction was observed and scanned electron micrographs were taken using a field emission scanning electron microscope (Philips XL-30). The thickness B of the separator and the thickness T of the base film were measured using the scanning electron microscope.

[0067] Test of silicon mass percentage in silicon-carbon composite materials:

[0068] The negative electrode sheet was sliced ​​using an argon ion cross-section polisher (JEOL, model: IB-09010CP) to obtain a cross-section along the thickness direction. The silicon-carbon composite material in the obtained negative electrode sheet cross-section was observed using a field emission scanning electron microscope (Zeiss, model: Sigma-02-33). The silicon content of the silicon-carbon composite material particles was measured using energy dispersive spectroscopy (EDS), and the silicon content of 50 particles was statistically analyzed and averaged.

[0069] Gram capacity and first coulomb efficiency test:

[0070] Silicon-carbon composite material, conductive carbon black (SP) as a conductive agent, lithium-modified polyacrylic acid (PAA-Li) as a negative electrode binder, carbon nanotubes (CNTs), and carboxymethyl cellulose (CMC) as a dispersant were mixed in a mass ratio of 84:10:5:0.4:0.6. Deionized water was added and mixed thoroughly to obtain a negative electrode slurry with a solid content of 48 wt%. The negative electrode slurry was uniformly coated onto one surface of a 10 μm thick copper foil, dried at 85 °C, and then cold-pressed and stamped to obtain the negative electrode sheet.

[0071] In a glove box where the water and oxygen content are both less than 10 ppm, ethyl methyl carbonate (EMC), ethylene carbonate (EC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1 to obtain a mixed solvent. Then, fluoroethylene carbonate (FEC) at a volume fraction of 10% of the mixed solvent is added, and finally, lithium salt LiPF6 is added to obtain the electrolyte. The concentration of lithium salt LiPF6 is 1 mol / L.

[0072] In a glove box where the water and oxygen content are both less than 10 ppm, the above-mentioned negative electrode sheet is cut into a circular piece with a diameter of 14 mm and used as the working electrode. A lithium metal sheet is used as the counter electrode, and a polypropylene (PP) film with a thickness of 7 μm is used as the separator. The above-mentioned electrolyte is injected and assembled into a button cell.

[0073] After the button cell was left to stand for 6 hours at 25°C, it was discharged at a constant current of 0.05C to 5mV, then discharged at a constant current of 50μA to 5mV. After standing for 5 minutes, it was discharged at a constant current of 10μA to 5mV. The initial discharge specific capacity G0 of the button cell was recorded. After standing for 5 minutes, it was charged at a rate of 0.05C to 0.8V, and the initial charge specific capacity G1 of the button cell was recorded. The mass of the silicon-carbon composite material in the negative electrode sheet was calculated based on the coating weight and area of ​​the negative electrode slurry during the preparation of the negative electrode sheet.

[0074] The specific capacity (mAh / g) of silicon-carbon composite material = G1 / mass of silicon-carbon composite material;

[0075] Initial Coulomb efficiency (%) = G1 / G0 × 100%.

[0076] Test of voltage drop (K value) of lithium-ion battery per unit time:

[0077] A lithium-ion battery with an initial voltage of 3.85V was left to stand at 45℃ for 24 hours, then at 25℃ for 24 hours. The voltage OCV1 of the lithium-ion battery was measured at this time. The battery was then left to stand at 25℃ for another 48 hours, and the voltage OCV2 of the battery was measured at this time.

[0078] K value (mV / h) = (OCV1 - OCV2) / 48.

[0079] The K-value is used to measure the self-discharge rate of lithium-ion batteries. When the K-value is less than 0.09 mV / h, it indicates that the self-discharge rate of the lithium-ion battery is relatively small and the self-discharge performance of the lithium-ion battery is good.

[0080] Cyclic performance test:

[0081] The lithium-ion battery was placed in a 25°C constant-temperature test chamber and allowed to stand for 30 minutes to reach a constant temperature of 25°C. It was then charged at a constant current of 1C to 4.53V, followed by constant voltage charging at 4.53V to a current of 0.025C. After standing for 5 minutes, it was discharged at a constant current of 0.5C to 3.0V. This was the first cycle, and the initial discharge capacity was recorded as C0. The lithium-ion battery was cycled according to the above process. The test was stopped after 400 cycles (cls), and the discharge capacity after 400 cycles (cls) was recorded as C1. The capacity retention rate after 400 cls was calculated as an indicator of the lithium-ion battery's cycle performance.

[0082] Capacity retention rate (%) after 400cls = C1 / C0 × 100%.

[0083] The higher the capacity retention rate after 400cls, the better the cycle performance of the lithium-ion battery.

[0084] Charging rate performance test:

[0085] Under normal pressure and at 25°C, the lithium-ion battery was discharged at a constant current rate of 0.2C to 3.0V and allowed to stand for 5 minutes; then charged at a constant current rate of 0.5C to 4.53V, and then charged at a constant voltage of 4.53V to 0.05C, and allowed to stand for 5 minutes; finally, it was discharged at a constant current rate of 0.2C to 3.0V and allowed to stand for 5 minutes. The discharge capacity of this step was recorded as C. 10 Charge at a constant current rate of 2C to 4.53V, then charge at a constant voltage of 4.53V to 0.05C, let stand for 5 minutes, and record the charging capacity of this step as C. 20 .

[0086] Charging efficiency (%) = C 20 / C 10 ×100%.

[0087] Higher charging efficiency indicates better charging rate performance of lithium-ion batteries.

[0088] Example 1-1

[0089] <Preparation of Silicon-Carbon Composite Materials>

[0090] (1) Crosslinking reaction: 1000g of linear phenolic resin and 120g of hexamethylenetetramine were added to 5L of water and stirred for 5h. The solution was then placed in a high-pressure reactor and reacted at 100℃ for 48h. The product was washed with water and dried to obtain phenolic resin microspheres.

[0091] (2) Activation and carbonization: 1000g of the above phenolic resin microspheres and potassium hydroxide were mixed at a mass ratio of 1:3. After the mixture was evenly mixed, it was heat-treated at 460℃ for 0.5h in a rotary kiln. Then the rotary kiln was heated to 750℃ and held for 0.75h. The product was taken out, acid-washed with 2mol / L dilute hydrochloric acid solution, washed with water, and then dried at 80℃ to obtain the precursor.

[0092] (3) Silicon deposition and carbon coating: The above precursor was added to a fluidized bed reactor and heated to 480°C for 3 hours under a nitrogen atmosphere of 10 L / min. Then, silane gas was introduced at a rate of 2.5 L / min for 300 minutes. After stopping the silane gas introduction, the fluidized bed was heated to 520°C and held for 2 hours. Finally, acetylene gas was introduced at a rate of 5 L / min for 4 hours. After the reaction, a silicon-carbon composite material with an A value of 122° was obtained. Based on the mass of the silicon-carbon composite material, the silicon mass percentage was 47.5%.

[0093] <Preparation of Negative Electrode Sheets>

[0094] Silicon-carbon composite material and artificial graphite were mixed at a mass ratio of 1:9 to form the negative electrode active material. The negative electrode active material, carbon nanotubes, lithium carboxymethyl cellulose, and lithium polyacrylate were mixed at a mass ratio of 97.4:0.2:0.4:2, with deionized water added as a solvent. Under vacuum stirring, a negative electrode slurry with a solid content of 45 wt% and a viscosity of 6000 mPa·s was obtained. The negative electrode slurry was uniformly coated onto one surface of a 6 μm thick copper foil current collector and dried at 80 °C, yielding a coating weight of 100.1 mg / 1540.25 mm. 2 A negative electrode sheet with a negative electrode material layer coated on one side is obtained. The above steps are then repeated on the other surface of the negative electrode current collector copper foil to obtain a negative electrode sheet with a negative electrode material layer coated on both sides. After cold pressing, cutting, and slitting, a negative electrode sheet with a size of 661mm × 78mm is obtained. The specific capacity of the artificial graphite is 360mAh / g.

[0095] <Preparation of the separating membrane>

[0096] A porous polypropylene film (PP, provided by Celgard) with a thickness T of 4 μm was used as the separator.

[0097] <Preparation of the positive electrode>

[0098] Lithium cobalt oxide (LiCoO2), conductive carbon black, and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 96.7:1.7:1.6. N-methylpyrrolidone (NMP) was added as a solvent, and the mixture was stirred under vacuum to obtain a positive electrode slurry with a solid content of 76 wt%. The positive electrode slurry was then uniformly coated onto one surface of a 9 μm thick aluminum foil current collector and dried at 120 °C, yielding a coating weight of 260 mg / 1540.25 mm. 2 A positive electrode sheet with a positive electrode material layer coated on one side is obtained. Then, the above steps are repeated on the other surface of the positive electrode current collector aluminum foil to obtain a positive electrode sheet with a positive electrode material layer coated on both sides. After cold pressing, cutting, and slitting, a positive electrode sheet with a size of 661mm×76.5mm is obtained.

[0099] <Preparation of Electrolyte>

[0100] In an argon-atmospheric glove box with a water content of less than 10 ppm, fluoroethylene carbonate (FEC), ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a mass ratio of 5:10:15:20:50 to obtain an organic solvent. Then, lithium hexafluorophosphate (LiPF6) was added to the organic solvent to obtain an electrolyte. The lithium salt LiPF6 comprised 12.5% ​​by mass, with the remainder being the organic solvent.

[0101] <Preparation of Lithium-ion Batteries>

[0102] The separator, positive electrode, separator, and negative electrode prepared above are stacked in sequence, with the separator positioned between the positive and negative electrode to provide isolation. The electrode assembly is then wound to obtain the electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, degassing, and edge trimming, a lithium-ion battery is obtained.

[0103] Examples 1-2

[0104] Except for adjusting the relevant preparation parameters according to Tables 1 and 2, the rest is the same as in Example 1-1.

[0105] Examples 1-3

[0106] Except for the steps described below for preparing the separator membrane, the rest is the same as in Example 1-1.

[0107] <Preparation of the separating membrane>

[0108] A porous polypropylene film with a thickness T of 10 μm was used as the base film. Polyvinylidene fluoride (PVDF, Mw = 5 × 10⁻⁶) was used as the binder. 6 ) and thickener sodium carboxymethyl cellulose (Mw = 8 × 10) 5 Mix the inorganic alumina particles and the ceramic coating adhesive styrene-butadiene rubber (Mw = 7 × 10⁻⁶) at a mass ratio of 98.5:1.5, add deionized water as a solvent, and stir until homogeneous to form a bonding slurry with a solid content of 75 wt%. 6 Solvent and deionized water are mixed in a mass ratio of 35:10:55 to obtain a ceramic coating slurry.

[0109] A ceramic coating slurry is applied to one surface of a base film and dried at 60°C to form a ceramic coating on that surface. An adhesive layer slurry is then applied to the surface of the ceramic coating away from the base film and dried at 60°C to obtain a release film with a single-sided ceramic coating and adhesive layer. The above steps are then repeated on the other surface of the base film to obtain the release film. The thickness of the single-layer adhesive layer is 0.5 μm, the thickness of the single-layer ceramic coating is 0.5 μm, and the thickness B of the release film is 12 μm.

[0110] Examples 1-4 to Examples 1-6

[0111] Except for adjusting the relevant preparation parameters according to Tables 1 and 2, the rest is the same as in Examples 1-3. Among them, when the thickness B of the separator changes, the thickness T of the base film remains unchanged, while the thickness of the single-layer adhesive layer and the thickness of the single-layer ceramic coating change accordingly, and the change in the thickness of the single-layer adhesive layer is equal to the change in the thickness of the single-layer ceramic coating.

[0112] Examples 1-7 to Examples 1-9

[0113] Except for adjusting the relevant preparation parameters according to Tables 1 and 2, the rest is the same as in Examples 1-5.

[0114] Examples 1-10

[0115] Except for adjusting the relevant preparation parameters according to Tables 1 and 2, the rest is the same as in Examples 1-3.

[0116] Examples 1-11

[0117] Except for using a porous polypropylene film (PP, provided by Celgard) with a thickness T of 4 μm as the separator, the rest is the same as in Examples 1-9.

[0118] Example 2-1

[0119] Except for adjusting the relevant preparation parameters according to Tables 1 and 3, the rest is the same as in Example 1-1.

[0120] Example 2-2

[0121] Except for the preparation of the separator membrane according to the following steps, the rest is the same as in Example 1-1.

[0122] <Preparation of the separating membrane>

[0123] A porous polypropylene film with a thickness T of 4 μm was used as the base film. The binder was polyacrylonitrile (PAN, Mw = 1.5 × 10⁻⁶). 5 ) and thickener sodium carboxymethyl cellulose (Mw = 8 × 10) 5 Mix them at a mass ratio of 98.5:1.5, add deionized water as a solvent, and stir evenly to form a bonding layer slurry with a solid content of 75wt%.

[0124] An adhesive slurry is coated on one surface of the base film and dried at 60°C to obtain a release film with an adhesive layer coated on one side. The above steps are then repeated on the other surface of the base film to obtain the release film. The thickness of the single adhesive layer is 0.25 μm, and the thickness B of the release film is 4.5 μm.

[0125] Examples 2-3 to Examples 2-6

[0126] Except for adjusting the relevant preparation parameters according to Tables 1 and 3, the rest is the same as in Examples 2-2.

[0127] Examples 2-7

[0128] Except for adjusting the relevant preparation parameters according to Tables 1 and 3, the rest is the same as in Example 1-1.

[0129] <Preparation of the separating membrane>

[0130] A porous polypropylene film with a thickness T of 4 μm was used as the base film. Inorganic silica particles and ceramic coating binder styrene-butadiene rubber (Mw = 7 × 10⁻⁶) were then used. 6 Solvent and deionized water are mixed in a mass ratio of 35:10:55 to obtain a ceramic coating slurry.

[0131] A ceramic coating slurry is coated on one surface of a base film and dried at 60°C to obtain a separator film with a ceramic coating on one side. The above steps are then repeated on the other surface of the base film to obtain the separator film. The thickness of the single-layer ceramic coating is 0.25 μm, and the thickness B of the separator film is 4.5 μm.

[0132] Examples 2-8 to 2-11

[0133] Except for adjusting the relevant preparation parameters according to Tables 1 and 3, the rest is the same as in Examples 2-7.

[0134] Examples 2-12 to 2-13

[0135] Except for adjusting the relevant preparation parameters according to Tables 1 and 3, the rest is the same as in Examples 1-3.

[0136] Examples 3-1 to 3-2

[0137] Except for adjusting the relevant preparation parameters according to Table 1 so that the mass percentage of silicon is as shown in Table 4, the rest is the same as in Example 1-1.

[0138] Examples 3-3 to 3-4

[0139] Except for adjusting the relevant preparation parameters according to Tables 1 and 4, the rest is the same as in Example 1-1.

[0140] Comparative Example 1

[0141] Except for adjusting the relevant preparation parameters according to Tables 1 and 2, the rest is the same as in Example 1-1.

[0142] Comparative Example 2

[0143] Except for using a porous polyethylene film (PP, provided by Celgard) with a thickness T of 4 μm as the separator, the rest is the same as in Examples 1-10.

[0144] Comparative Example 3

[0145] Except for using a porous polyethylene film (PP, provided by Celgard) with a thickness T of 4 μm as the separator, the rest is the same as in Examples 1-8.

[0146] Comparative Example 4

[0147] Except for adjusting the relevant preparation parameters according to Tables 1 and 2, the rest is the same as in Examples 1-2.

[0148] Comparative Example 5

[0149] Except for adjusting the relevant preparation parameters according to Tables 1 and 2, the rest is the same as in Examples 1-3.

[0150] Comparative Example 6

[0151] Except for the use of the separator membranes prepared in Examples 1-6, the rest is the same as in Examples 1-9.

[0152] Comparative Example 7

[0153] Except for adjusting the relevant preparation parameters according to Tables 1 and 2, the rest is the same as in Examples 1-3. Among them, when the thickness B of the separator changes, the thickness T of the base film remains unchanged, while the thickness of the single-layer adhesive layer and the thickness of the single-layer ceramic coating change accordingly, and the change in the thickness of the single-layer adhesive layer is equal to the change in the thickness of the single-layer ceramic coating.

[0154] The preparation and performance parameters of each embodiment and comparative example are shown in Tables 1 to 4.

[0155] Table 1

[0156]

[0157]

[0158] Table 2

[0159]

[0160]

[0161] As can be seen from Examples 1-1 to 1-11 and Comparative Examples 1 to 7, by limiting the values ​​of A×B and B to within the scope of this application, the silicon-carbon composite material exhibits higher specific capacity and initial coulombic efficiency, indicating higher energy density of the lithium-ion battery. The resulting lithium-ion battery has a smaller K value, indicating improved self-discharge performance. The lithium-ion battery exhibits higher capacity retention and charging efficiency after 400cls, demonstrating that the lithium-ion battery of this application has good self-discharge performance, cycle performance, and charging rate performance. In Comparative Examples 1 to 6, the values ​​of A×B are not within the scope of this application. Among them, the lithium-ion batteries in Comparative Examples 1 to 5 have larger K values, indicating poorer self-discharge performance; the lithium-ion batteries have lower capacity retention and charging efficiency after 400cls, indicating poorer cycle performance and charging rate performance. Although Comparative Example 6 shows a smaller K-value and higher capacity retention after 400cls, its charging efficiency is lower. While it exhibits good self-discharge and cycle performance, it cannot simultaneously achieve good charging rate performance. Similarly, Comparative Example 7 shows a smaller K-value and higher capacity retention after 400cls, but its charging efficiency is lower. While it also demonstrates good self-discharge and cycle performance, it cannot simultaneously achieve good charging rate performance. As can be seen from Examples 1-1 to 1-11, the silicon-carbon composite material in the lithium-ion battery of this application exhibits higher specific capacity and initial coulombic efficiency, indicating higher energy density. The resulting lithium-ion battery has a smaller K-value and higher capacity retention and charging efficiency after 400cls, indicating that the lithium-ion battery of this application possesses good self-discharge and cycle performance while also achieving good charging rate performance.

[0162] from Figure 2 It can be seen that the minimum inner angle of the outer contour of the particles with the longest diameter greater than 10 μm in the silicon-carbon composite material of Example 1-1 is relatively large. Referring to Table 2, the value of A is 122°. When the silicon-carbon composite material of Example 1-1 is applied to lithium-ion batteries, the value of K is relatively small, indicating that the resulting lithium-ion battery has good self-discharge performance.

[0163] The minimum internal angle A of the outer contour of particles with a longest diameter greater than 10 μm in silicon-carbon composite materials usually affects the self-discharge performance, charge rate performance, and cycle performance of lithium-ion batteries. As can be seen from Examples 1-3, 1-5, 1-7 to 1-10, when the value of A is within the range of this application, the specific capacity and initial coulombic efficiency of the silicon-carbon composite material are relatively high, indicating that the energy density of the lithium-ion battery is relatively high. The resulting lithium-ion battery has a small K value, and the capacity retention rate and charging efficiency after 400 cls are relatively high, indicating that the lithium-ion battery of this application has good self-discharge performance, cycle performance, and charge rate performance.

[0164] The thickness T of the base film typically affects the self-discharge performance, charge rate performance, and cycle performance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-3, when the thickness T of the base film is within the range of this application, the specific capacity and initial coulombic efficiency of the silicon-carbon composite material are relatively high, indicating that the energy density of the lithium-ion battery is relatively high. The resulting lithium-ion battery has a smaller K value, and the capacity retention rate and charging efficiency after 400cls are relatively high, indicating that the lithium-ion battery of this application has good self-discharge performance, cycle performance, and charge rate performance.

[0165] Table 3

[0166]

[0167] Note: " / " in Table 3 indicates that there are no relevant parameters.

[0168] The material of the base film typically affects the self-discharge performance, charge rate performance, and cycle performance of lithium-ion batteries. As can be seen from Examples 1-1 and 2-1, when the base film material is within the scope of this application, the silicon-carbon composite material has a higher specific capacity and initial coulombic efficiency, indicating that the lithium-ion battery has a higher energy density. The resulting lithium-ion battery has a smaller K value, and the lithium-ion battery has a higher capacity retention rate and charging efficiency after 400cls, indicating that the lithium-ion battery of this application has good self-discharge performance, cycle performance, and charge rate performance.

[0169] Different compositions of separators typically affect the self-discharge performance, charge rate performance, and cycle performance of lithium-ion batteries. As can be seen from Examples 1-1, 1-3, 2-3 to 2-6, and 2-8 to 2-13, when separators of different compositions are used within the scope of this application, the silicon-carbon composite material exhibits higher specific capacity and initial coulombic efficiency, indicating a higher energy density of the lithium-ion battery. The resulting lithium-ion battery has a smaller K value, and the lithium-ion battery exhibits higher capacity retention and charging efficiency after 400cls, indicating that the lithium-ion battery of this application has good self-discharge performance, cycle performance, and charge rate performance.

[0170] The type of binder typically affects the self-discharge performance, charge rate performance, and cycle performance of lithium-ion batteries. As can be seen from Examples 2-2 and 2-3, when the type of binder is within the scope of this application, the silicon-carbon composite material has a higher specific capacity and initial coulombic efficiency, indicating that the lithium-ion battery has a higher energy density. The resulting lithium-ion battery has a smaller K value, and the lithium-ion battery has a higher capacity retention rate and charging efficiency after 400cls, indicating that the lithium-ion battery of this application has good self-discharge performance, cycle performance, and charge rate performance.

[0171] The type of inorganic particles typically affects the self-discharge performance, charge rate performance, and cycle performance of lithium-ion batteries. As can be seen from Examples 2-7 and 2-8, when the type of inorganic particles is within the scope of this application, the silicon-carbon composite material has a higher specific capacity and initial coulombic efficiency, indicating that the lithium-ion battery has a higher energy density. The resulting lithium-ion battery has a smaller K value, and the lithium-ion battery has a higher capacity retention rate and charging efficiency after 400cls, indicating that the lithium-ion battery of this application has good self-discharge performance, cycle performance, and charge rate performance.

[0172] Table 4

[0173]

[0174] The mass percentage of silicon typically affects the self-discharge performance, charge rate performance, and cycle performance of lithium-ion batteries. As can be seen from Examples 1-1, 3-1 to 3-2, when the mass percentage of silicon is within the range of this application, the silicon-carbon composite material has a higher specific capacity and initial coulombic efficiency, indicating that the lithium-ion battery has a higher energy density. The resulting lithium-ion battery has a smaller K value, and the lithium-ion battery has a higher capacity retention rate and charging efficiency after 400cls, indicating that the lithium-ion battery of this application has good self-discharge performance, cycle performance, and charge rate performance.

[0175] The types of other negative electrode active materials usually affect the self-discharge performance, charge rate performance, and cycle performance of lithium-ion batteries. As can be seen from Examples 1-1, 3-3 to 3-4, when the types of other negative electrode active materials are within the scope of this application, the silicon-carbon composite material has a higher specific capacity and initial coulombic efficiency, indicating that the lithium-ion battery has a higher energy density. The resulting lithium-ion battery has a smaller K value, and the lithium-ion battery has a higher capacity retention rate and charging efficiency after 400cls, indicating that the lithium-ion battery of this application has good self-discharge performance, cycle performance, and charge rate performance.

[0176] The terms “comprising,” “including,” or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or article that comprises a list of elements includes not only those elements but also other elements not expressly listed or inherent to such process, method, or article.

[0177] The various embodiments in this specification are described in a related manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

[0178] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the negative electrode comprises a negative current collector and a negative electrode material layer located on at least one surface of the negative current collector, the negative electrode material layer comprises a negative electrode active material, the negative electrode active material comprises a silicon-carbon composite material, wherein on a plane formed by the length direction and the thickness direction of the negative electrode, the minimum inner angle of the outer contour of the particles with the longest diameter greater than 10 μm in the silicon-carbon composite material is A°, the thickness of the separator is B μm, 488≤A×B≤3600, 4≤B≤25, 89≤A≤180; Based on the mass of the silicon-carbon composite material, the silicon mass percentage is 44% to 50%.

2. The secondary battery according to claim 1, wherein, The isolation membrane includes a base membrane with a thickness of 4 μm to 10 μm.

3. The secondary battery according to claim 2, wherein it satisfies any one of the following characteristics: (1) The isolation membrane further includes an adhesive layer, 4.5≤B≤15; (2) The isolation membrane also includes a ceramic coating, 4.5≤B≤14; (3) The isolation membrane also includes an adhesive layer and a ceramic coating, 6≤B≤25.

4. The secondary battery according to claim 2, wherein it satisfies at least one of the following characteristics: (1) The separator further includes an adhesive layer, the adhesive layer including an adhesive, the adhesive including at least one of polyacrylonitrile, polymethyl methacrylate or polyvinylidene fluoride; (2) The isolation membrane further includes a ceramic coating, which includes inorganic particles, including at least one of alumina, titanium oxide, silicon oxide or magnesium oxide.

5. The secondary battery according to claim 1, wherein, Based on the mass of the silicon-carbon composite material, the silicon mass percentage is 44% to 49%.

6. The secondary battery according to claim 1, wherein, The negative electrode active material also includes at least one of artificial graphite, natural graphite, mesophase carbon microspheres, soft carbon, or hard carbon.

7. The secondary battery according to claim 1, wherein, The separator membrane includes a base membrane, and the material of the base membrane includes at least one of polyethylene or polypropylene.

8. A method for preparing a secondary battery according to any one of claims 1 to 7, comprising the following steps: The positive electrode, the negative electrode, the separator, and the electrolyte are prepared and then assembled to obtain the secondary battery. The method for preparing the silicon-carbon composite material in the negative electrode sheet includes the following steps: (1) After the carbon source and the alkali source are mixed evenly, the mixture is heat-treated at 420℃ to 600℃ for 0.5h to 2h, and then heated to 650℃ to 950℃ and held for 0.5h to 3h to obtain the precursor; The carbon source includes at least one of phenolic resin, urea-formaldehyde resin, melamine-formaldehyde resin, polyvinyl chloride or polyacrylonitrile, and the alkali source includes at least one of potassium hydroxide, sodium hydroxide, lithium hydroxide or ammonia water. The mass ratio of the carbon source to the alkali source is 1:1 to 1:

5. (2) The precursor is heated to 400°C to 510°C for 0.2h to 4h in an inert atmosphere, then silane gas is introduced for 220min to 420min, then the temperature is raised to 520°C to 560°C and held for 0.5h to 2h, and then a reducing atmosphere is introduced for 1.5h to 4h to obtain the silicon-carbon composite material. The inert atmosphere includes at least one of nitrogen, argon, or helium; the silane gas includes at least one of silane, silane, propane, phenylsilane, or tolylsilane; and the reducing atmosphere includes at least one of acetylene, propylene, or toluene.

9. An electronic device comprising a secondary battery according to any one of claims 1 to 7 or a secondary battery prepared by the preparation method described in claim 8.

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