Secondary battery and electronic apparatus

Abstract

Provided are a secondary battery and an electronic apparatus. The secondary battery comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the negative electrode sheet comprises a negative electrode current collector and a negative electrode material layer located on at least one surface of the negative electrode current collector; the negative electrode material layer comprises a negative electrode active material; the negative electrode active material comprises a silicon-carbon composite material; on a plane formed by the direction of length of the negative electrode sheet and the direction of thickness thereof, the minimum value of an interior angle of an outer contour of a particle with a maximum circumscribed-circle diameter of greater than 10 μm in the silicon-carbon composite material is A°; and the thickness of the separator is B μm, where 488≤A×B≤3600, and 4≤B≤25. The secondary battery has good self-discharge performance, charging rate performance and cycling performance.

Description

Secondary battery and electronic device

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on March 25, 2024, with application number 202410346301.3 and invention name “A Secondary Battery and Electronic Device”, the entire contents of which are incorporated by reference into this application. Technical Field

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

[0003] Secondary batteries, such as lithium-ion batteries, offer outstanding characteristics such as high energy density, long cycle life, low pollution, and no memory effect. As a clean energy source, their application has gradually expanded from electronic products to large-scale devices such as electric vehicles, aligning with sustainable environmental and energy development strategies. Consequently, higher requirements are being placed on the energy density of secondary batteries.

[0004] Currently, graphite is still the primary negative electrode active material for commercial secondary batteries. However, graphite has a lithium insertion capacity of approximately 372 mAh / g, while silicon, when used as the negative electrode active material, can achieve a lithium insertion capacity of 3579 mAh / g. Using silicon can achieve a higher energy density in secondary batteries, meaning more energy can be stored within the same volume and weight. However, compared to graphite, silicon, when used as the negative electrode active material, undergoes greater volume expansion and contraction during charge and discharge, exerting greater mechanical stress on the separator, increasing the risk of rupture or deformation of the separator, and thus reducing the safety performance of the secondary battery. Summary of the Invention

[0005] The purpose of this application is to provide a secondary battery and electronic device that reduces the risk of self-discharge and thermal runaway caused by puncture of the separator by silicon-carbon composite particles, thereby improving the self-discharge performance, charge rate performance, and cycle performance of the secondary battery. The specific technical solution is as follows:

[0006] It should be noted that, in the invention content of this application, lithium-ion batteries are used as an example of secondary batteries to explain this application, but the secondary batteries of this application are not limited to lithium-ion batteries.

[0007] The first aspect of the present application provides a secondary battery, which includes a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte. The negative electrode sheet includes a negative electrode collector and a negative electrode material layer located on at least one surface of the negative electrode collector. The negative electrode material layer includes a negative electrode active material, and the negative electrode active material includes a silicon-carbon composite material. On the plane formed by the length direction and the thickness direction of the negative electrode sheet, the minimum value 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 A°, the thickness of the separator is B μm, 488≤A×B≤3600, and 4≤B≤25. By limiting the value of A×B and the value of B within the scope of this application, the relative relationship between the minimum value of the 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 isolation membrane is regulated, and the synergistic effect between the inner angle of the outer contour of the silicon-carbon composite material particles and the thickness of the isolation membrane is fully utilized. The isolation membrane has good mechanical strength, which can effectively reduce the risk of the isolation membrane being punctured by particles of the silicon-carbon composite material, thereby reducing the risk of self-discharge and thermal runaway caused by the puncture of the isolation membrane by particles of the silicon-carbon composite material. At the same time, the transmission distance of lithium ions in the secondary battery cycle process is moderate, thereby improving the self-discharge performance, charge rate performance and cycle performance of the secondary battery.

[0008] In one embodiment of the present application, 89≤A≤180. By regulating the value of A within the above range, the risk of the separator being punctured by particles of the silicon-carbon composite material is reduced, thereby reducing the risk of self-discharge and thermal runaway caused by the separator being punctured by particles of the silicon-carbon composite material, thereby improving the self-discharge performance, charge rate performance, and cycle performance of the secondary battery.

[0009] In one embodiment of the present application, the separator includes a base film having a thickness of 4 to 10 μm. By regulating the base film thickness within the above range, the separator has good mechanical strength, which helps reduce the risk of the separator being punctured by particles of the silicon-carbon composite material. This, in turn, reduces the risk of self-discharge and thermal runaway caused by puncture of the separator by particles of the silicon-carbon composite material. Furthermore, the transmission 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.

[0010] In one embodiment of the present application, the separator further includes a bonding layer, with B being 4.5 ≤ B ≤ 15. Selecting the above separator and regulating the value of B within the above range can help increase the bonding force between the separator and the positive electrode sheet and / or between the separator and the negative electrode sheet, shortening the transmission distance of lithium ions, thereby improving the cycle performance and charge rate performance of the secondary battery, while also helping to reduce the risk of the separator being punctured by particles of the silicon-carbon composite material. As a result, the secondary battery has good self-discharge performance, charge rate performance, and cycle performance.

[0011] In one embodiment of the present application, the separator further includes a ceramic coating, with 4.5 ≤ B ≤ 14. By selecting the above separator and regulating 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 the silicon-carbon composite material. Furthermore, the separator exhibits good wettability, allowing for a moderate lithium ion transmission distance, thereby improving the secondary battery's cycle performance and charge rate performance. Consequently, the secondary battery exhibits excellent self-discharge performance, charge rate performance, and cycle performance.

[0012] In one embodiment of the present application, the separator further includes an adhesive layer and a ceramic coating, and 6 ≤ B ≤ 25. Selecting such a separator and regulating the B value within the above range helps reduce the risk of the separator being punctured by silicon-carbon composite material particles. Furthermore, the separator exhibits good wettability, shortening the transmission distance of lithium ions. Consequently, the secondary battery exhibits excellent self-discharge performance, charge rate performance, and cycling performance.

[0013] In one embodiment of the present application, the separator further comprises an adhesive layer, the adhesive layer comprising a binder, and the binder comprises at least one of polyacrylonitrile, polymethyl methacrylate, or polyvinylidene fluoride. By selecting the aforementioned binders, the secondary battery exhibits excellent self-discharge performance, charge rate performance, and cycle performance.

[0014] In one embodiment of the present application, the separator further includes a ceramic coating comprising inorganic particles, wherein the inorganic particles include at least one of aluminum oxide, titanium oxide, silicon oxide, or magnesium oxide. The use of these inorganic particles improves the mechanical strength of the separator, while also providing the separator with good wettability, and the secondary battery exhibits excellent self-discharge performance, charge rate performance, and cycle performance.

[0015] In one embodiment of the present application, the mass percentage of silicon is 44% to 57% based on the mass of the silicon-carbon composite material. By regulating the mass percentage of silicon within the above range, the volume expansion of the silicon-carbon composite material is reduced. When the silicon-carbon composite material is used as the negative electrode active material, the secondary battery has a higher energy density and is also conducive to improving the cycle performance of the secondary battery.

[0016] In one embodiment of the present application, the negative electrode active material further comprises at least one of artificial graphite, natural graphite, mesocarbon microbeads, soft carbon, or hard carbon. By mixing the silicon-carbon composite material with other negative electrode active materials, the mixed specific capacity of the negative electrode active materials is increased, thereby improving the energy density of the secondary battery.

[0017] In one embodiment of the present application, the separator includes a base film, and the base film is made of at least one of polyethylene or polypropylene. By selecting the above-mentioned base films, the separator has good mechanical strength and chemical stability, and the secondary battery has good self-discharge performance, charge rate performance, and cycle performance.

[0018] The second aspect of the present application provides a method for preparing a secondary battery, comprising the following steps: preparing a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte, and assembling the resulting secondary battery. The method for preparing the silicon-carbon composite material in the negative electrode sheet comprises the following steps:

[0019] (1) After uniformly mixing the carbon source and the alkali source, heat-treating at 420° C. to 600° C. for 0.5 h to 2 h, then heating to 650° C. to 950° C. and keeping the temperature for 0.5 h to 3 h to obtain a precursor;

[0020] The carbon source comprises at least one of phenolic resin, urea-formaldehyde resin, melamine-formaldehyde resin, polyvinyl chloride or polyacrylonitrile, the alkali source comprises at least one of potassium hydroxide, sodium hydroxide, lithium hydroxide or ammonia water, and the mass ratio of the carbon source to the alkali source is 1:1 to 1:5;

[0021] (2) heating the precursor to 400° C. to 510° C. under an inert atmosphere for 0.2 to 4 hours, then introducing silane gas for 220 to 420 minutes, then heating to 520 to 560° C. and maintaining the temperature for 0.5 to 2 hours, and then introducing a reducing atmosphere for 1.5 to 4 hours to obtain a silicon-carbon composite material;

[0022] The inert atmosphere includes at least one of nitrogen, argon or helium, the silane gas includes at least one of monosilane, disilane, trisilane, phenylsilane or tolylsilane, and the reducing atmosphere includes at least one of acetylene, propylene or toluene.

[0023] The silicon-carbon composite material prepared using the preparation method of this application has a large inner angle value of the outer contour of the particles. When applied to secondary batteries, the prepared silicon-carbon composite material can effectively reduce the risk of the separator being punctured by the silicon-carbon composite material particles, improving the self-discharge performance of the secondary battery. The obtained silicon-carbon composite material also has a high gram capacity and first coulombic efficiency. When the prepared silicon-carbon composite material is applied to secondary batteries, the secondary batteries have good self-discharge performance, charge rate performance, and cycle performance.

[0024] The third aspect of the present application provides an electronic device comprising the secondary battery of any of the aforementioned embodiments. The secondary battery of the present application has good self-discharge performance, charge rate performance, and cycle performance, and therefore, the electronic device of the present application has a long service life.

[0025] Beneficial effects of this application:

[0026] The present application provides a secondary battery and an electronic device. The secondary battery includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. The negative electrode sheet includes a negative electrode current collector and a negative electrode material layer located on at least one surface of the negative electrode current collector. The negative electrode material layer includes a negative electrode active material. The negative electrode active material includes a silicon-carbon composite material. On the plane formed by the length direction of the negative electrode sheet and its own thickness direction, the minimum inner angle of the outer contour of particles in the silicon-carbon composite material with a longest diameter greater than 10 μm is A°. The thickness of the separator is B μm, 488≤A×B≤3600, and 4≤B≤25. By regulating the thickness of the separator and its synergistic effect with the inner angle of the outer contour of the silicon-carbon composite particles, the risk of self-discharge and thermal runaway caused by the separator being punctured by the silicon-carbon composite particles can be reduced, thereby improving the self-discharge performance, charge rate performance, and cycle performance of the secondary battery.

[0027] Of course, it is not necessary to achieve all the advantages described above at the same time when implementing any product or method of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other embodiments can also be obtained based on these drawings.

[0029] FIG1 is a schematic diagram of a test of the minimum inner angle of the outer contour of silicon-carbon composite material particles;

[0030] FIG2 is a scanning electron micrograph of the negative electrode sheet of Example 1-1 in the present application.

[0031] Reference numeral: silicon-carbon composite material 11 . DETAILED DESCRIPTION

[0032] The following will be combined with the accompanying drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the embodiments described are only part of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0033] It should be noted that, in the specific embodiments of the present application, lithium-ion batteries are used as an example of secondary batteries to explain the present application, but the secondary batteries of the present application are not limited to lithium-ion batteries.

[0034] To address the issue of silicon materials easily causing separator rupture, methods such as high-mechanical-strength separators, separator coatings, and optimized silicon particle size distribution are currently commonly used. However, due to the irregular shapes of conventional silicon particles and the high number of sharp corners on their surfaces, these methods are ineffective in reducing the risk of silicon particles puncturing the separator. Therefore, the present application provides a secondary battery that reduces the risk of self-discharge and thermal runaway caused by separator puncture by silicon-carbon composite material particles.

[0035] The first aspect of the present application provides a secondary battery, which includes a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte. The negative electrode sheet includes a negative electrode collector and a negative electrode material layer located on at least one surface of the negative electrode collector. The negative electrode material layer includes a negative electrode active material, and the negative electrode active material includes a silicon-carbon composite material. On the plane formed by the length direction and the thickness direction of the negative electrode sheet, the minimum value 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 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 consisting of any two of the values, and the value of B can be 4, 5, 6, 8, 10, 12, 14, 15, 16, 18, 20, 22, 24, 25, or a range consisting of any two of the values.

[0036] When the value of A×B is too small, that is, below the lower limit of this application, during the processing or use of the secondary battery, the sharp corners of the particles of the silicon-carbon composite material can easily puncture the isolation membrane, which may cause micro-short circuits and self-discharge of the secondary battery, increasing the risk of thermal runaway and fire and explosion of the secondary battery. When the value of A×B is too large, that is, above the upper limit of this application, the value of B is correspondingly too large, that is, the thickness of the isolation membrane is too large, and the energy density of the resulting secondary battery is low, resulting in a weakening of the high energy density advantage brought by the silicon-carbon composite material as the negative electrode active material. In addition, an isolation membrane that is too thick can easily hinder the transmission of lithium ions, affecting the charge rate performance of the secondary battery. By limiting the value of A×B and the value of B to 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 regulated, and the synergistic effect between the inner angle of the outer contour of the silicon-carbon composite material particles and the thickness of the separator is fully utilized. The separator has good mechanical strength, which can effectively reduce the risk of the separator being punctured by particles of the silicon-carbon composite material, thereby reducing the risk of self-discharge and thermal runaway caused by the puncture of the separator by particles of the silicon-carbon composite material, and improving the self-discharge performance of the secondary battery. At the same time, the transmission distance of lithium ions during the cycle of the secondary battery is moderate, thereby improving the self-discharge performance, charge rate performance, and cycle performance of the secondary battery. In this application, the longest diameter greater than 10 μm means that the maximum circumscribed circle diameter of the outer contour of the silicon-carbon composite material particles is greater than 10 μm.

[0037] In one embodiment of the present 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 consisting of any two values ​​therein. By regulating the value of A within the above range, the minimum inner angle of the outer contour of particles with the longest diameter greater than 10 μm in the silicon-carbon composite material is in a moderate range, which is beneficial to reducing the risk of the isolation membrane being punctured by particles of the silicon-carbon composite material, and further reducing the risk of self-discharge and thermal runaway caused by the isolation membrane being punctured by particles of the silicon-carbon composite material, thereby improving the self-discharge performance, charge rate performance and cycle performance of the secondary battery.

[0038] In one embodiment of the present application, the separator includes a base film, and the base film has a thickness T of 4 μm to 10 μm. For example, the base film thickness T 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 regulating the base film thickness within the above range, the separator has good mechanical strength, which helps reduce the risk of the separator being punctured by particles of the silicon-carbon composite material, thereby reducing the risk of self-discharge and thermal runaway caused by puncture of the separator by particles of the silicon-carbon composite material. At the same time, the transmission 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. The present application does not specifically limit the method for regulating the base film thickness, as long as it can achieve the objectives of the present application. For example, commercially available base films of different thicknesses can be selected and combined with the test method of "Testing the Thickness B of the Separator Film and the Thickness T of the Base Film" in this application to determine the base film thickness, and then select a base film of the desired thickness.

[0039] In one embodiment of the present application, the isolation membrane further includes an adhesive layer, 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 isolation membrane and regulating the value of B within the above range, it is beneficial to increase the bonding force between the isolation membrane and the positive electrode sheet and / or between the isolation membrane and the negative electrode sheet, reduce the gap between the isolation membrane and the positive electrode sheet and / or between the isolation membrane and the negative electrode sheet, shorten the transmission distance of lithium ions, and thus improve the cycle performance and charge rate performance of the secondary battery, while helping to reduce the risk of the isolation membrane being punctured by particles of the silicon-carbon composite material, thereby reducing the risk of self-discharge and thermal runaway caused by the isolation membrane being punctured by particles of the 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.

[0040] In one embodiment of the present application, the isolation membrane further includes a ceramic coating, and 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 consisting of any two of these values. By selecting the above isolation membrane and regulating the value of B within the above range, the isolation membrane has good mechanical strength, which is beneficial to reducing the risk of the isolation membrane being punctured by particles of the silicon-carbon composite material, thereby reducing the risk of self-discharge and thermal runaway caused by the isolation membrane being punctured by particles of the silicon-carbon composite material, and improving the self-discharge performance of the secondary battery. At the same time, the isolation membrane has good wettability, and the transmission distance of lithium ions 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.

[0041] In one embodiment of the present application, the separator further includes an adhesive layer and a ceramic coating, and 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 consisting of any two of these values. By selecting the above-mentioned separator and regulating the value of B within the above-mentioned range, the separator has good mechanical strength, which is conducive to reducing the risk of the separator being punctured by particles of the silicon-carbon composite material, thereby reducing the risk of self-discharge and thermal runaway caused by the separator being punctured by particles of the silicon-carbon composite material, and improving the self-discharge performance of the secondary battery. At the same time, the separator has good wettability, and the gap between the separator and the positive electrode sheet and / or between the separator and the negative electrode sheet is small, which shortens the transmission distance of lithium ions and 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.

[0042] In one embodiment of the present application, the separator further includes a bonding layer, the bonding layer includes a binder, and the binder includes at least one of polyacrylonitrile (PAN), polymethyl methacrylate (PMMA) or polyvinylidene fluoride (PVDF). By selecting the above-mentioned types of binders, it is beneficial to increase the bonding force between the separator and the positive electrode sheet and / or between the separator and the negative electrode sheet, reduce the gap between the separator and the positive electrode sheet and / or between the separator and the negative electrode sheet, shorten the transmission distance of lithium ions, and thus improve 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.

[0043] In one embodiment of the present application, the isolation membrane further includes a ceramic coating, which includes inorganic particles, and the inorganic particles include at least one of aluminum oxide, titanium oxide, silicon oxide, or magnesium oxide. By selecting the above-mentioned types of inorganic particles, it is beneficial to improve the mechanical strength of the isolation membrane, reduce the risk of the isolation membrane being punctured by particles of the silicon-carbon composite material, and further reduce the risk of self-discharge and thermal runaway caused by the isolation membrane being punctured by particles of the silicon-carbon composite material, thereby improving the self-discharge performance of the secondary battery. At the same time, the isolation membrane has good wettability, which improves the cycle performance of the secondary battery. Therefore, the secondary battery has good self-discharge performance, charge rate performance, and cycle performance.

[0044] In one embodiment of the present application, the mass percentage of silicon is 44% to 57% based on the mass of the silicon-carbon composite material. 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 consisting of any two of these values. By regulating 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 is also beneficial to improving the cycle performance of the secondary battery. In the present application, when calculating the mass percentage of silicon based on the mass of the silicon-carbon composite material, the impurity elements in the silicon-carbon composite material are excluded and calculated, wherein the content of the impurity elements is generally less than 0.5%. The present application does not limit the types of the above-mentioned impurity elements. For example, the impurity elements may include but are not limited to at least one of oxygen, nitrogen, sulfur, iron, nickel or aluminum.

[0045] In one embodiment of the present application, the negative electrode active material further comprises at least one of artificial graphite, natural graphite, mesocarbon microbeads, soft carbon, or hard carbon. By mixing the silicon-carbon composite material with other negative electrode active materials, the mixed specific capacity of the negative electrode active materials is increased, thereby improving the energy density of the secondary battery.

[0046] In one embodiment of the present application, the isolation membrane includes a base membrane, and the material of the base membrane includes at least one of polyethylene or polypropylene. By selecting the above-mentioned type of base membrane, the isolation membrane has good mechanical strength and chemical stability, which is conducive to reducing the risk of the isolation membrane being punctured by particles of the silicon-carbon composite material, thereby reducing the risk of self-discharge and thermal runaway caused by the isolation membrane being punctured by particles of the 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. The present application has no special restrictions on the method of regulating the material of the base membrane, as long as the purpose of the present application can be achieved. For example, commercially available base membranes of different materials can be selected, and the base membrane of the required material can be selected. The present application has no special restrictions, as long as the purpose of the present application can be achieved.

[0047] In the present application, the above-mentioned "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 thickness direction, or on both surfaces of the negative electrode current collector along its thickness direction. It should be noted that the "surface" here can be the entire area of ​​the surface of the negative electrode current collector, or it can be a partial area of ​​the surface of the negative electrode current collector. This application has no special restrictions, as long as the purpose of this application can be achieved. This application has no special restrictions on the negative electrode current collector, as long as the purpose of this application can be achieved. For example, the negative electrode current collector can include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam or a composite current collector (such as a lithium-copper composite current collector, a carbon-copper composite current collector, a nickel-copper composite current collector, a titanium-copper composite current collector, etc.). In the present application, there is no special restriction on the thickness of the negative electrode current collector, as long as the purpose of this application can be achieved. For example, the thickness of the negative electrode current collector is 4μm to 20μm. Optionally, the negative electrode material layer may also include a negative electrode binder and a conductive agent. The present application does not particularly limit the type of negative electrode binder in the negative electrode material layer, as long as the purpose of the present application can be achieved. For example, the negative electrode binder may include but is not limited to polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene 1,1-difluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylic (ester) styrene-butadiene rubber, epoxy resin or nylon. The present application does not particularly limit the type of conductive agent in the negative electrode material layer, as long as the purpose of the present application can be achieved. For example, the conductive agent may include but is not limited to at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, Ketjen black, graphene, metal materials or conductive polymers. The above-mentioned carbon nanotubes may include but are not limited to single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The above-mentioned carbon fibers may include but are not limited to vapor-grown carbon fibers (VGCF) and / or nanocarbon fibers. The aforementioned metal material may include, but is not limited to, metal powder and / or metal fiber. 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 a polyphenylene derivative, polyaniline, polythiophene, polyacetylene, or polypyrrole. This application does not particularly limit 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 objectives of this application can be achieved.

[0048] The present application has no special restrictions on the positive electrode sheet, as long as the purpose of the present application can be achieved. For example, the positive electrode sheet includes a positive electrode current collector and a positive electrode material layer located on at least one surface of the positive electrode current collector. The above-mentioned "positive electrode material layer located on at least one surface of the positive electrode current collector" means that the positive electrode material layer can be located on one surface of the positive electrode current collector along the thickness direction of itself, or on two surfaces of the positive electrode current collector along the thickness direction of itself. It should be noted that the "surface" here can be the entire area of ​​the surface of the positive electrode current collector, or it can be a partial area of ​​the surface of the positive electrode current collector. The present application has no special restrictions, as long as the purpose of the present application can be achieved. The present application has no special restrictions on the positive electrode current collector, as long as the purpose of the present application can be achieved. For example, the positive electrode current collector can include aluminum foil, aluminum alloy foil or a composite current collector (such as an aluminum-carbon composite current collector). The positive electrode material layer of the present application includes a positive electrode active material. The present application has no special restrictions on the type of positive electrode active material, as long as the purpose of the present application can be achieved. For example, the positive electrode active material can include lithium nickel cobalt manganese oxide (LiNi 0.90 Co 0.05 Mn 0.05 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 iron manganese phosphate or lithium titanate, etc. In the present application, the positive electrode active material may also contain non-metallic elements, for example, the non-metallic elements include at least one of fluorine, phosphorus, boron, chlorine, silicon or sulfur. In the present application, there is no particular restriction on the thickness of the positive electrode current collector and the positive electrode material layer, as long as the purpose of the present application can be achieved. For example, the thickness of the positive electrode current collector is 5μm to 20μm, and the thickness of the single-sided positive electrode material layer is 30μm to 120μm. In the present application, the positive electrode material layer may also include a positive electrode binder and a conductive agent. The present application has no particular restrictions on the type of positive electrode binder in the positive electrode material layer, as long as the purpose of the present application can be achieved. 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. The present application has no particular restrictions on the type of conductive agent in the positive electrode material layer, as long as the purpose of the present application can be achieved. For example, the conductive agent may be the same as the conductive agent type in the above-mentioned negative electrode material layer. The present application has no particular restrictions on the mass ratio of the positive active material, conductive agent and positive electrode binder in the positive electrode material layer. Those skilled in the art can choose according to actual needs, as long as the purpose of the present application can be achieved.

[0049] In the present application, the electrolyte includes a lithium salt and a non-aqueous solvent. 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(oxalatoborate) (LiBOB), lithium bis(trifluoromethanesulfonyl imide) (LiTFSI) or lithium difluoroborate. The present application does not limit the content of the lithium salt in the electrolyte, as long as the purpose of the present application can be achieved. The present application has no special restrictions on the non-aqueous solvent, as long as the purpose of the present application can be achieved. For example, the non-aqueous solvent may include but is not limited to at least one of a carbonate compound, a carboxylate compound, an ether compound or other organic solvents. The above-mentioned carbonate compound may include but is not limited to at least one of a chain carbonate compound, a cyclic carbonate compound or a fluorinated carbonate compound. Above-mentioned linear carbonate compound can include but not limited to at least one of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate or methyl ethyl carbonate.Above-mentioned cyclic carbonate can include but not limited to at least one of ethylene carbonate, propylene carbonate (PC), butylene carbonate or vinyl ethylene carbonate.Fluorinated carbonate compound can include but not limited to at least one of fluoroethylene carbonate, 1,2-difluoro ethylene carbonate, 1,1-difluoro ethylene carbonate, 1,1,2-trifluoro ethylene carbonate, 1,1,2,2-tetrafluoro ethylene carbonate, 1-fluoro-2-methyl ethylene carbonate, 1-fluoro-1-methyl ethylene carbonate, 1,2-difluoro-1-methyl ethylene carbonate, 1,1,2-trifluoro-2-methyl ethylene carbonate or trifluoromethyl ethylene carbonate. The carboxylate compound may include but is not limited to at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanoic acid, 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. The other organic solvents may include but are not limited to at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate or trioctyl phosphate.

[0050] The secondary battery of this application also includes a packaging bag for containing the positive electrode sheet, the negative electrode sheet, the separator, and the electrolyte, as well as other components of the secondary battery known in the art. This application does not limit these other components. This application does not specifically limit the packaging bag and can be any packaging bag known in the art, as long as it can achieve the purpose of this application.

[0051] The secondary battery of the present application is not particularly limited and may include any device that undergoes an electrochemical reaction. In one embodiment of the present application, the secondary battery may include, but is not limited to, a lithium ion secondary battery (lithium ion battery), a sodium ion battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery.

[0052] The second aspect of the present application provides a method for preparing a secondary battery, comprising the following steps: preparing a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte, and assembling the resulting secondary battery. The method for preparing the silicon-carbon composite material in the negative electrode sheet comprises the following steps:

[0053] (1) After the carbon source and the alkali source are uniformly mixed, heat treatment is carried out at 420°C to 600°C for 0.5h to 2h. For example, after the carbon source and the alkali source are uniformly mixed, heat treatment can be carried out at 420°C, 430°C, 440°C, 450°C, 460°C, 470°C, 480°C, 490°C, 500°C, 510°C, 520°C, 530°C, 540°C, 550°C, 560°C, 570°C, 580°C, 590°C, 600°C or a range consisting of any two values ​​therein. 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 a range consisting of any two values ​​therein. Then the temperature is raised to 650°C to 950°C and kept warm for 0.5h to 3h. For example, the temperature can be raised to 650°C, 670°C, 700°C, 720°C, 750°C, 780°C, 800°C, 820°C, 850°C, 880°C, 900°C, 920°C, 950°C or a range consisting of any two of the values ​​therein. 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 a range consisting of any two of the values ​​therein to obtain a precursor.

[0054] The carbon source includes at least one of phenolic resin, urea-formaldehyde resin, melamine formaldehyde resin, polyvinyl chloride or polyacrylonitrile, the alkali source includes at least one of potassium hydroxide, sodium hydroxide, lithium hydroxide or ammonia water, and 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 values ​​therein.

[0055] (2) The precursor is heated to 400°C to 510°C under 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 a range consisting of any two of the values ​​therein, and 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 a range consisting of any two of the values ​​therein. 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 a range consisting of any two values ​​therein. Then the temperature is raised to 520°C to 560°C and then kept warm for 0.5h to 2h. For example, the temperature can be raised to 520°C, 522°C, 525°C, 528°C, 530°C, 532°C, 535°C, 538°C, 540°C, 542°C, 545°C, 548°C, 550°C, 552°C, 555°C, 558°C, 560°C or a range consisting of any two values ​​therein, and 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 a range consisting of any two values ​​therein. 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 a range consisting of any two values ​​therein to obtain a silicon-carbon composite material.

[0056] The inert atmosphere includes at least one of nitrogen, argon or helium, the silane gas includes at least one of monosilane, disilane, trisilane, phenylsilane or tolylsilane, and the reducing atmosphere includes at least one of acetylene, propylene or toluene.

[0057] The present application has no special restrictions on the heat treatment method when preparing the precursor, as long as the purpose of the present application can be achieved. For example, the heat treatment can be carried out in a rotary furnace. The present application has no special restrictions on the heating rate when preparing the precursor, as long as the purpose of the present application can be achieved. For example, the heating rate when preparing the precursor can be 2°C / min to 10°C / min. The present application has no special restrictions on the heat treatment method when preparing the silicon-carbon composite material from the precursor, as long as the purpose of the present application can be achieved. For example, the precursor can be placed in a fluidized bed for heat treatment. The present application has no special restrictions on the heating rate when preparing the silicon-carbon composite material from the precursor, as long as the purpose of the present application can be achieved. For example, the heating rate when preparing the silicon-carbon composite material from the precursor can be 5°C / min to 15°C / min. The present application has no special restrictions on the flow rate of the inert atmosphere, as long as the purpose of the present application can be achieved. For example, the flow rate of the inert atmosphere can be 5L / min to 15L / min. The flow rate of the silane gas is not particularly limited in this application, as long as the purpose of this application can be achieved. For example, the flow rate of the silane gas can be 1 L / min to 5 L / min. The flow rate of the reducing atmosphere is not particularly limited in this application, as long as the purpose of this application can be achieved. For example, the flow rate of the reducing atmosphere can be 2 L / min to 10 L / min.

[0058] The inventors have discovered that when the inner angle of the outer contour of silicon-carbon composite particles is too small, there is a risk of the particles puncturing the separator during the preparation or use of a secondary battery, potentially causing self-discharge and thermal runaway. The silicon-carbon composite material prepared using the above method, by controlling the alkali-carbon ratio, performing heat treatment at different temperatures, and regulating the conditions for gaseous silicon deposition, results in a silicon-carbon composite material with particles having a maximum diameter greater than 10 μm having a larger inner angle. Application of the prepared silicon-carbon composite material in a secondary battery effectively reduces the risk of the separator being punctured by silicon-carbon composite particles, thereby reducing the risk of self-discharge and thermal runaway caused by puncture of the separator, and improving the self-discharge performance of the secondary battery. By introducing silane gas, the silicon material is uniformly distributed within the precursor. By regulating the time for introducing silane gas, the mass percentage of silicon in the silicon-carbon composite material is kept within an appropriate 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 a high gram capacity and initial coulombic efficiency. The silicon-carbon composite material prepared above is applied to a secondary battery, and the secondary battery has good self-discharge performance, charge rate performance and cycle performance.

[0059] The third aspect of the present application provides an electronic device comprising the secondary battery of any of the aforementioned embodiments. The secondary battery of the present application has good self-discharge performance, charge rate performance, and cycle performance, and therefore, the electronic device of the present application has a long service life.

[0060] The electronic device of the present application is not particularly limited and can be any electronic device known in the art. For example, the electronic device can include, but is not limited to, a laptop computer, a pen-type computer, a mobile computer, an electronic book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a headset, a video recorder, an LCD television, a portable cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, a power tool, a flashlight, a camera, a large household battery, and a lithium-ion capacitor.

[0061] Example

[0062] The following examples and comparative examples are provided to more specifically illustrate the embodiments of the present invention. Various tests and evaluations were performed according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0063] Test methods and equipment:

[0064] Test of the minimum inner angle of the outer contour of particles with the longest diameter greater than 10 μm:

[0065] The negative electrode sheet was sliced ​​using an argon ion cross-section polisher (JEOL, model: IB-09010CP) to obtain a cross section of the negative electrode sheet along the thickness direction. A field emission scanning electron microscope (Zeiss, model: sigma-02-33) was used to capture a microscopic image of the cross section of the silicon-carbon composite material in the cross section of the negative electrode sheet obtained above. The cross-sectional profiles of 50 particles of the silicon-carbon composite material with the longest diameter greater than 10 μm were randomly selected for analysis. As shown in FIG1 , the particle profile of a single silicon-carbon composite material 11 was outlined, tangents were made along the edges on both sides of the sharp corner, and the angle between the two tangents was measured. The values ​​of the measured angles were compared, and the minimum value was taken as the minimum value of the inner angle of the outer contour of the silicon-carbon composite material 11 particle (as shown in FIG1 , angle 1 is 115°, angle 2 is 105°, angle 3 is 142°, and angle 4 is 140°, then angle 2 is recorded as the minimum value of the inner angle of the outer contour of the silicon-carbon composite material particle). The minimum value of the inner angle of the outer contour of 50 particles with the longest diameter greater than 10 μm in the silicon-carbon composite material is obtained, and the average value is obtained to obtain A.

[0066] Test of the thickness B of the isolation film and the thickness T of the base film:

[0067] The isolation membrane was subjected to argon ion polishing to obtain a cross-section of the isolation membrane. The cross-sectional morphology of the isolation membrane along the thickness direction was observed using a field emission scanning electron microscope (Philips, XL-30 model) and scanning electron microscope photos were taken. The thickness B of the isolation membrane and the thickness T of the base membrane were measured using the scanning electron microscope.

[0068] Test of the mass percentage of silicon in silicon-carbon composite materials:

[0069] 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 of the negative electrode sheet. The silicon-carbon composite material in the cross-section of the negative electrode sheet obtained above was observed using a field emission scanning electron microscope (Zeiss, Model: Sigma-02-33). The silicon content of the silicon-carbon composite particles was measured using an energy dispersive spectrometer (EDS). The silicon content of 50 particles was counted and the average value was calculated.

[0070] Gram capacity and first coulombic efficiency test:

[0071] A silicon-carbon composite material, conductive carbon black (SP), lithiated 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, and deionized water was added and mixed thoroughly to produce a negative electrode slurry with a solid content of 48 wt%. The negative electrode slurry was evenly coated onto one surface of a 10 μm thick copper foil, dried at 85°C, and then cold pressed and punched to produce a negative electrode sheet.

[0072] In a glove box with a water and oxygen content of less than 10 ppm, ethyl methyl carbonate (EMC), ethylene carbonate (EC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to form a mixed solvent. Fluoroethylene carbonate (FEC) was then added at a 10% volume fraction of the mixed solvent. Finally, lithium salt LiPF6 was added to form an electrolyte solution. The concentration of the lithium salt LiPF6 was 1 mol / L.

[0073] In a glove box with a water and oxygen content of less than 10 ppm, the above-mentioned negative electrode sheet was cut into a disc with a diameter of 14 mm and used as the working electrode, a metal lithium sheet was used as the counter electrode, and a polypropylene (PP) film with a thickness of 7 μm was used as the isolation membrane. The above-mentioned electrolyte was injected into the battery to assemble it into a button cell.

[0074] After standing for 6 hours in a 25°C environment, the button cell 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 then 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 above negative electrode sheet preparation process.

[0075] Gram capacity of silicon-carbon composite material (mAh / g) = G1 / mass of silicon-carbon composite material;

[0076] First coulombic efficiency (%) = G1 / G0×100%.

[0077] Voltage drop (K value) test of lithium-ion battery per unit time:

[0078] A lithium-ion battery with an initial voltage of 3.85V was placed at 45°C for 24 hours, then at 25°C for 24 hours. The voltage OCV1 of the lithium-ion battery was measured. The battery was then placed at 25°C for another 48 hours, and the voltage OCV2 of the battery was measured.

[0079] K value (mV / h) = (OCV1 - OCV2) / 48.

[0080] The K value is used to measure the self-discharge rate of the lithium-ion battery. When the K value is less than 0.09mV / h, it means that the self-discharge rate of the lithium-ion battery is small and the self-discharge performance of the lithium-ion battery is good.

[0081] Cyclic performance test:

[0082] Place the lithium-ion battery in a 25°C constant temperature test chamber and let it stand for 30 minutes to allow the lithium-ion battery to reach a constant temperature of 25°C. Charge it to 4.53V at a constant current of 1C, charge it to a current of 0.025C at a constant voltage of 4.53V, let it stand for 5 minutes, and discharge it to 3.0V at a constant current of 0.5C. This is the first cycle, and the initial discharge capacity is recorded as C0. Perform charge and discharge cycles on the lithium-ion battery according to the above process. When the cycle reaches 400 cycles (cls), stop the test and record the discharge capacity after 400 cycles (cls) as C1. Calculate the capacity retention rate after 400cls as an indicator for evaluating the cycle performance of the lithium-ion battery.

[0083] Capacity retention rate after 400 cls (%) = C1 / C0×100%.

[0084] The higher the capacity retention rate after 400cls, the better the cycle performance of the lithium-ion battery.

[0085] Charging rate performance test:

[0086] At 25°C, the lithium-ion battery is discharged at a constant current rate of 0.2C to 3.0V and allowed to stand for 5 minutes. It is then charged at a constant current rate of 0.5C to 4.53V and charged at a constant voltage rate of 4.53V to 0.05C and allowed to stand for 5 minutes. It is then 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 is recorded as C 10 Charge at a constant current of 2C to 4.53V, and charge at a constant voltage of 4.53V to 0.05C, let it stand for 5 minutes, and record the charging capacity of this step as C 20 .

[0087] Charging efficiency (%) = C 20 / C 10 ×100%.

[0088] The higher the charging efficiency, the better the charging rate performance of the lithium-ion battery.

[0089] Example 1-1

[0090] <Preparation of Silicon-Carbon Composite Material>

[0091] (1) Cross-linking reaction: 1000 g of linear phenolic resin and 120 g of hexamethylenetetramine were added to 5 L of water and stirred for 5 h. The solution was placed in an autoclave and reacted at 100°C for 48 h. The product was washed with water and dried to obtain phenolic resin microspheres.

[0092] (2) Activation and carbonization: 1000 g of the above-mentioned phenolic resin microspheres were mixed with potassium hydroxide in a mass ratio of 1:3. After uniform mixing, the mixture was heat-treated at 460° C. in a rotary kiln for 0.5 h. The rotary kiln was then heated to 750° C. and kept warm for 0.75 h. The product was taken out, pickled with a 2 mol / L dilute hydrochloric acid solution, washed with water, and then dried at 80° C. to obtain a precursor.

[0093] (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 at 10 L / min. Then, monosilane gas was introduced at 2.5 L / min for 300 minutes. After the introduction of monosilane gas was stopped, the fluidized bed was heated to 520°C and kept at this temperature for 2 hours. Then, acetylene atmosphere was introduced at 5 L / min for 4 hours. After the reaction was completed, a silicon-carbon composite material with an A value of 122° was obtained. The mass percentage of silicon based on the mass of the silicon-carbon composite material was 47.5%.

[0094] <Preparation of negative electrode sheet>

[0095] The silicon-carbon composite material and artificial graphite were mixed in 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 in a mass ratio of 97.4:0.2:0.4:2. Deionized water was added as a solvent. A negative electrode slurry with a solid content of 45 wt% was obtained under the action of a vacuum mixer. The viscosity of the negative electrode slurry was 6000 mPa·s. The negative electrode slurry was evenly coated on one surface of a negative electrode current collector copper foil with a thickness of 6 μm and dried at 80°C to obtain a coating weight of 100.1 mg / 1540.25 mm 2 A negative electrode sheet coated on one side with a negative electrode material layer is produced. The above steps are then repeated on the other surface of the negative electrode current collector copper foil to obtain a negative electrode sheet coated on both sides with a negative electrode material layer. After cold pressing, cutting, and slitting, a negative electrode sheet measuring 661 mm x 78 mm is obtained. The gram capacity of the artificial graphite is 360 mAh / g.

[0096] <Preparation of Separator>

[0097] A porous polypropylene film (PP, provided by Celgard Company) with a thickness T of 4 μm was used as the isolation membrane.

[0098] <Preparation of positive electrode sheet>

[0099] The positive electrode active material lithium cobalt oxide (LiCoO2), conductive agent conductive carbon black, and binder polyvinylidene fluoride were mixed in a mass ratio of 96.7:1.7:1.6, and N-methylpyrrolidone (NMP) was added as a solvent. A positive electrode slurry with a solid content of 76 wt% was obtained under the action of a vacuum mixer. The positive electrode slurry was evenly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 9 μm and dried at 120°C to obtain a coating weight of 260 mg / 1540.25 mm 2 A single-sided positive electrode sheet coated with a positive electrode material layer is produced. The above steps are then repeated on the other surface of the positive electrode current collector aluminum foil to obtain a double-sided positive electrode sheet coated with a positive electrode material layer. After cold pressing, cutting, and slitting, a positive electrode sheet with a size of 661 mm x 76.5 mm is obtained.

[0100] <Preparation of Electrolyte>

[0101] In an argon atmosphere 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. Lithium hexafluorophosphate (LiPF6) was then added to the organic solvent to produce an electrolyte. The mass percentage of the LiPF6 was 12.5%, with the balance being the organic solvent.

[0102] <Preparation of lithium-ion batteries>

[0103] The separator, positive electrode sheet, separator, and negative electrode sheet prepared above are stacked in order, with the separator positioned between the positive and negative electrode sheets to provide insulation, and then wound to form an electrode assembly. The electrode assembly is then placed in an aluminum-plastic film packaging bag, dried, and then injected with electrolyte. The lithium-ion battery is then produced through vacuum packaging, resting, formation, degassing, and trimming.

[0104] Example 1-2

[0105] Except for adjusting the relevant preparation parameters according to Table 1 and Table 2, the rest is the same as Example 1-1.

[0106] Examples 1-3

[0107] Except for using the following steps to prepare the isolation membrane, the rest is the same as Example 1-1.

[0108] <Preparation of Separator>

[0109] A porous polypropylene film with a thickness of 10 μm was used as the base film. The binder polyvinylidene fluoride (PVDF, Mw = 5×10 6 ) and thickener sodium carboxymethyl cellulose (Mw = 8 × 10 5 ) were mixed in a mass ratio of 98.5:1.5, deionized water was added as a solvent, and the mixture was stirred to form a bonding layer slurry with a solid content of 75 wt%. The inorganic particles of aluminum oxide and the ceramic coating binder styrene butadiene rubber (Mw = 7 × 10 6 ), solvent and deionized water were mixed in a mass ratio of 35:10:55 to obtain a ceramic coating slurry.

[0110] A ceramic coating slurry is applied to one surface of a base film and dried at 60°C to form a ceramic coating on one surface of the base film. A bonding layer slurry is applied to the surface of the ceramic coating facing away from the base film and dried at 60°C to form a separator film coated with both a ceramic coating and a bonding layer on one side. The above steps are then repeated on the other surface of the base film to form a separator film. The thickness of the single bonding layer is 0.5 μm, the thickness of the single ceramic coating layer is 0.5 μm, and the thickness B of the separator film is 12 μm.

[0111] Example 1-4 to Example 1-6

[0112] The preparation parameters were the same as in Examples 1-3, except that they were adjusted according to Tables 1 and 2. When the thickness B of the isolation film changed, the thickness T of the base film remained unchanged, and the thickness of the single-layer adhesive layer and the thickness of the single-layer ceramic coating changed accordingly. The change in thickness of the single-layer adhesive layer was equal to the change in thickness of the single-layer ceramic coating.

[0113] Example 1-7 to Example 1-9

[0114] Except for adjusting the relevant preparation parameters according to Table 1 and Table 2, the rest is the same as Example 1-5.

[0115] Examples 1-10

[0116] Except for adjusting the relevant preparation parameters according to Table 1 and Table 2, the rest is the same as Example 1-3.

[0117] Examples 1-11

[0118] Except that a porous polypropylene film (PP, provided by Celgard Company) with a thickness T of 4 μm was used as the separator, the rest was the same as that of Example 1-9.

[0119] Example 2-1

[0120] Except for adjusting the relevant preparation parameters according to Table 1 and Table 3, the rest is the same as Example 1-1.

[0121] Example 2-2

[0122] Except for preparing the isolation film according to the following steps, the rest is the same as Example 1-1.

[0123] <Preparation of Separator>

[0124] A porous polypropylene film with a thickness of 4 μm was used as the base film. The binder polyacrylonitrile (PAN, Mw = 1.5 × 10 5 ) and thickener sodium carboxymethyl cellulose (Mw = 8 × 10 5 ) were mixed in a mass ratio of 98.5:1.5, deionized water was added as a solvent, and the mixture was stirred evenly to form a bonding layer slurry with a solid content of 75 wt%.

[0125] An adhesive layer slurry was applied to one surface of a base film and dried at 60°C to produce a separator film coated with an adhesive layer on one side. The above steps were then repeated on the other surface of the base film to produce a separator film. The thickness of the single adhesive layer was 0.25 μm, and the thickness B of the separator film was 4.5 μm.

[0126] Example 2-3 to Example 2-6

[0127] Except for adjusting the relevant preparation parameters according to Table 1 and Table 3, the rest is the same as Example 2-2.

[0128] Examples 2-7

[0129] Except for adjusting the relevant preparation parameters according to Table 1 and Table 3, the rest is the same as Example 1-1.

[0130] <Preparation of Separator>

[0131] A porous polypropylene film with a thickness of 4 μm was used as the base film. Inorganic silica particles and ceramic coating binder styrene-butadiene rubber (Mw = 7×10 6 ), solvent and deionized water were mixed in a mass ratio of 35:10:55 to obtain a ceramic coating slurry.

[0132] A ceramic coating slurry was applied to one surface of a base film and dried at 60°C to produce a single-sided ceramic-coated separator. The above steps were then repeated on the other surface of the base film to form a separator. The thickness of the single ceramic coating layer was 0.25 μm, and the thickness B of the separator was 4.5 μm.

[0133] Example 2-8 to Example 2-11

[0134] Except for adjusting the relevant preparation parameters according to Table 1 and Table 3, the rest is the same as Example 2-7.

[0135] Example 2-12 to Example 2-13

[0136] Except for adjusting the relevant preparation parameters according to Table 1 and Table 3, the rest is the same as Example 1-3.

[0137] Example 3-1 to Example 3-2

[0138] 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 Example 1-1.

[0139] Example 3-3 to Example 3-4

[0140] Except for adjusting the relevant preparation parameters according to Table 1 and Table 4, the rest is the same as Example 1-1.

[0141] Comparative Example 1

[0142] Except for adjusting the relevant preparation parameters according to Table 1 and Table 2, the rest is the same as Example 1-1.

[0143] Comparative Example 2

[0144] Except that a porous polyethylene film (PP, provided by Celgard Company) with a thickness T of 4 μm is used as the separator, the rest is the same as that of Example 1-10.

[0145] Comparative Example 3

[0146] Except that a porous polyethylene film (PP, provided by Celgard Company) with a thickness T of 4 μm was used as the separator, the rest was the same as that of Example 1-8.

[0147] Comparative Example 4

[0148] Except for adjusting the relevant preparation parameters according to Table 1 and Table 2, the rest is the same as Example 1-2.

[0149] Comparative Example 5

[0150] Except for adjusting the relevant preparation parameters according to Table 1 and Table 2, the rest is the same as Example 1-3.

[0151] Comparative Example 6

[0152] Except for using the isolation membrane prepared in Example 1-6, the rest is the same as Example 1-9.

[0153] Comparative Example 7

[0154] The preparation parameters were the same as in Examples 1-3, except that they were adjusted according to Tables 1 and 2. When the thickness B of the isolation film changed, the thickness T of the base film remained unchanged, and the thickness of the single-layer adhesive layer and the thickness of the single-layer ceramic coating changed accordingly. The change in thickness of the single-layer adhesive layer was equal to the change in thickness of the single-layer ceramic coating.

[0155] The preparation parameters and performance parameters of each embodiment and comparative example are shown in Tables 1 to 4.

[0156] Table 1

[0157] Table 2

[0158] From Examples 1-1 to 1-11 and Comparative Examples 1 to 7, it can be seen that by limiting the values ​​of A×B and B to within the range of this application, the gram capacity and first coulombic efficiency of the silicon-carbon composite material are high, indicating that the energy density of the lithium-ion battery is high; the K value of the resulting lithium-ion battery is small, indicating that the self-discharge performance of the lithium-ion battery is improved; the capacity retention rate and charging efficiency of the lithium-ion battery after 400 cls are high, indicating that the lithium-ion battery of the present application has good self-discharge performance, cycle performance, and charge rate performance. The A×B values ​​in Comparative Examples 1 to 6 are not within the range of this application. Among them, the K values ​​of the lithium-ion batteries in Comparative Examples 1 to 5 are large, indicating that the self-discharge performance of the lithium-ion battery is poor; the capacity retention rate and charging efficiency of the lithium-ion battery after 400 cls are low, indicating that the cycle performance and charge rate performance of the lithium-ion battery are poor. In Comparative Example 6, although the K value of the lithium-ion battery is small and the capacity retention rate after 400cls is high, the charging efficiency of the lithium-ion battery is lower. Although it has good self-discharge performance and cycle performance, it cannot take into account the charging rate performance of the lithium-ion battery. In Comparative Example 7, although the K value of the lithium-ion battery is small and the capacity retention rate after 400cls is high, the charging efficiency of the lithium-ion battery is lower. Although it has good self-discharge performance and cycle performance, it cannot take into account the charging rate performance of the lithium-ion battery. From Examples 1-1 to 1-11, it can be seen that the gram capacity and the first coulomb efficiency of the silicon-carbon composite material in the lithium-ion battery of the present application are high, indicating that the energy density of the lithium-ion battery is high; the K value of the obtained lithium-ion battery is small, and the capacity retention rate and charging efficiency of the lithium-ion battery after 400cls are high, indicating that the lithium-ion battery of the present application has good self-discharge performance and cycle performance while taking into account good charging rate performance.

[0159] As can be seen from Figure 2, the minimum value of the inner angle of the outer contour of the particles with the longest diameter greater than 10 μm in the silicon-carbon composite material in Example 1-1 is large. Compared with Table 2, the value of A is 122°. When the silicon-carbon composite material in Example 1-1 is applied to a lithium-ion battery, the value of K is small, indicating that the self-discharge performance of the obtained lithium-ion battery is good.

[0160] The minimum value A of the inner angle of the outer contour of particles with the longest diameter greater than 10 μm in the silicon-carbon composite material usually affects the self-discharge performance, charge rate performance and cycle performance of the lithium-ion battery. It can be seen from Examples 1-3, 1-5, 1-7 to 1-10 that when the value of A is within the range of this application, the gram capacity and the first coulombic efficiency of the silicon-carbon composite material are high, indicating that the energy density of the lithium-ion battery is high; the K value of the obtained lithium-ion battery is small, and the capacity retention rate and charging efficiency of the lithium-ion battery after 400cls are high, indicating that the lithium-ion battery of the present application has good self-discharge performance, cycle performance and charge rate performance.

[0161] The thickness T of the base film usually affects the self-discharge performance, charge rate performance and cycle performance of the lithium-ion battery. It can be seen from Examples 1-1 to 1-3 that when the thickness T of the base film is within the range of this application, the gram capacity and the first coulombic efficiency of the silicon-carbon composite material are high, indicating that the energy density of the lithium-ion battery is high; the K value of the obtained lithium-ion battery is small, and the capacity retention rate and charging efficiency of the lithium-ion battery after 400cls are high, indicating that the lithium-ion battery of this application has good self-discharge performance, cycle performance and charge rate performance.

[0162] Table 3 Note: “ / ” in Table 3 indicates no relevant parameters.

[0163] The material of the base film usually affects the self-discharge performance, charge rate performance and cycle performance of the lithium-ion battery. It can be seen from Example 1-1 and Example 2-1 that when the material of the base film is within the scope of this application, the gram capacity and the first coulomb efficiency of the silicon-carbon composite material are high, indicating that the energy density of the lithium-ion battery is high; the K value of the obtained lithium-ion battery is small, and the capacity retention rate and charging efficiency of the lithium-ion battery after 400cls are high, indicating that the lithium-ion battery of this application has good self-discharge performance, cycle performance and charge rate performance.

[0164] Separators of different compositions usually affect the self-discharge performance, charge rate performance, and cycle performance of lithium-ion batteries. It can be seen from Examples 1-1, 1-3, 2-3 to 2-6, and 2-8 to 2-13 that when the separators of different compositions are within the scope of this application, the gram capacity and first coulombic efficiency of the silicon-carbon composite material are high, indicating that the energy density of the lithium-ion battery is high; the K value of the obtained lithium-ion battery is small, and the capacity retention rate and charging efficiency of the lithium-ion battery after 400cls are high, indicating that the lithium-ion battery of the present application has good self-discharge performance, cycle performance, and charge rate performance.

[0165] The type of binder usually affects the self-discharge performance, charge rate performance and cycle performance of the lithium-ion battery. It can be seen from Examples 2-2 and 2-3 that when the type of binder is within the scope of this application, the gram capacity and first coulombic efficiency of the silicon-carbon composite material are high, indicating that the energy density of the lithium-ion battery is high; the K value of the obtained lithium-ion battery is small, and the capacity retention rate and charging efficiency of the lithium-ion battery after 400cls are high, indicating that the lithium-ion battery of the present application has good self-discharge performance, cycle performance and charge rate performance.

[0166] The type of inorganic particles usually affects the self-discharge performance, charge rate performance and cycle performance of lithium-ion batteries. It can be seen from Examples 2-7 and 2-8 that when the type of inorganic particles is within the scope of this application, the gram capacity and first coulomb efficiency of the silicon-carbon composite material are high, indicating that the energy density of the lithium-ion battery is high; the K value of the obtained lithium-ion battery is small, and the capacity retention rate and charging efficiency of the lithium-ion battery after 400cls are high, indicating that the lithium-ion battery of the present application has good self-discharge performance, cycle performance and charge rate performance.

[0167] Table 4

[0168] The mass percentage of silicon usually affects the self-discharge performance, charge rate performance and cycle performance of lithium-ion batteries. It can be seen from Examples 1-1, 3-1 and 3-2 that when the mass percentage of silicon is within the range of this application, the gram capacity and the first coulombic efficiency of the silicon-carbon composite material are high, indicating that the energy density of the lithium-ion battery is high; the K value of the obtained lithium-ion battery is small, and the capacity retention rate and charging efficiency of the lithium-ion battery after 400cls are high, indicating that the lithium-ion battery of the present application has good self-discharge performance, cycle performance and charge rate performance.

[0169] The types of other negative electrode active materials usually affect the self-discharge performance, charge rate performance and cycle performance of lithium-ion batteries. It can be seen from Examples 1-1, 3-3 and 3-4 that when the types of other negative electrode active materials are within the scope of this application, the gram capacity and first coulombic efficiency of the silicon-carbon composite material are high, indicating that the energy density of the lithium-ion battery is high; the K value of the obtained lithium-ion battery is small, and the capacity retention rate and charging efficiency of the lithium-ion battery after 400cls are high, indicating that the lithium-ion battery of the present application has good self-discharge performance, cycle performance and charge rate performance.

[0170] The terms "comprises," "comprising," 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.

[0171] Each embodiment in this specification is described in a related manner. The same or similar parts between the embodiments can be referred to each other. Each embodiment focuses on the differences from other embodiments.

[0172] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application shall be included in the scope of protection of the present application.

Claims

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

2. The secondary battery according to claim 1, wherein 89≤A≤180。 3. The secondary battery according to claim 1 or 2, wherein The isolation film includes a base film, and the base film has a thickness of 4 μm to 10 μm.

4. The secondary battery according to claim 3, which satisfies at least one of the following characteristics: (1) The isolation film includes an adhesive layer, 4.5≤B≤15; (2) The isolation membrane includes a ceramic coating, 4.5≤B≤14; (3) The isolation film includes a bonding layer and a ceramic coating, 6≤B≤25.

5. The secondary battery according to claim 3 or 4, which satisfies at least one of the following characteristics: (1) The isolation film includes an adhesive layer, the adhesive layer includes an adhesive, and the adhesive includes at least one of polyacrylonitrile, polymethyl methacrylate, or polyvinylidene fluoride; (2) The isolation film includes a ceramic coating, the ceramic coating includes inorganic particles, and the inorganic particles include at least one of aluminum oxide, titanium oxide, silicon oxide, or magnesium oxide.

6. The secondary battery according to any one of claims 1 to 5, wherein Based on the mass of the silicon-carbon composite material, the mass percentage of silicon is 44% to 57%.

7. The secondary battery according to any one of claims 1 to 6, wherein The negative electrode active material includes at least one of artificial graphite, natural graphite, mesocarbon microbeads, soft carbon or hard carbon.

8. The secondary battery according to any one of claims 1 to 7, wherein The isolation film includes a base film, and a material of the base film includes at least one of polyethylene and polypropylene.

9. A method for preparing a secondary battery according to any one of claims 1 to 8, comprising the following steps: preparing the positive electrode sheet, the negative electrode sheet, the separator and the electrolyte, and assembling them to obtain the secondary battery; The method for preparing the silicon-carbon composite material in the negative electrode plate comprises the following steps: (1) After uniformly mixing the carbon source and the alkali source, heat-treating at 420° C. to 600° C. for 0.5 h to 2 h, then heating to 650° C. to 950° C. and keeping the temperature for 0.5 h to 3 h to obtain a precursor; The carbon source comprises at least one of phenolic resin, urea-formaldehyde resin, melamine-formaldehyde resin, polyvinyl chloride or polyacrylonitrile, the alkali source comprises at least one of potassium hydroxide, sodium hydroxide, lithium hydroxide or ammonia water, and the mass ratio of the carbon source to the alkali source is 1:1 to 1:5; (2) heating the precursor to 400° C. to 510° C. under an inert atmosphere for 0.2 to 4 hours, then introducing silane gas for 220 to 420 minutes, then heating to 520 to 560° C. and keeping the temperature for 0.5 to 2 hours, and then introducing a reducing atmosphere for 1.5 to 4 hours 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 monosilane, disilane, trisilane, phenylsilane or tolylsilane, and the reducing atmosphere includes at least one of acetylene, propylene or toluene. 10 . An electronic device comprising the secondary battery according to claim 1 or the secondary battery prepared by the preparation method according to claim 9 .

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