Secondary battery and electronic device

Abstract

To provide a secondary battery and an electronic device which improve cycle attenuation and film drop-off due to expansion of a silicon-based material in a silicon-mixed negative electrode, and improve a lithium storage and release capacity of the negative electrode.SOLUTION: A secondary battery comprises a metal case and an electrode assembly arranged in the metal case, the electrode assembly comprises a negative electrode, the negative electrode comprises a negative electrode material layer, the negative electrode material layer comprises binders, the binders comprise a first binder and a second binder. The first binder is selected from styrene-butadiene rubber polymers, the second binder is selected from polyacrylic acid polymers, the weight-average molecular weight of the first binder is 40,000-300,000, the weight-average molecular weight of the second binder is 400,000-2,000,000, and the mass content of the first binder is 10%-90% based on the mass of the binders. The secondary battery provided by the invention has high energy density and high-rate cycle performance.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to the field of energy storage. In particular, the present invention relates to secondary batteries and electronic devices. [Background technology]

[0002] As the power consumption of power-consuming devices such as smartphones, personal computers, power tools, and electric vehicles increases, the demand for higher energy density in lithium-ion batteries is also increasing. The most direct and effective way to increase the energy density of lithium-ion batteries is to increase the capacity of the anode. Currently, high-capacity silicon-based anodes are often used to increase anode capacity, but the expansion of silicon-based materials during cycling causes a series of problems, such as cycle decay and film detachment. To address these issues, the introduction of a coating binder, such as a polyacrylic acid (PAA)-based binder, is necessary to alleviate the problems caused by the expansion of silicon-based materials. However, excessive coating of the binder on graphite particles in a composite anode reduces the lithium ion transport capacity of the graphite particles, thereby slowing the lithium absorption and desorption rate of the anode, making lithium-ion batteries unsuitable for high-rate discharge devices. Summary of the Invention

[0003] In view of the problems of the prior art, the present invention provides a secondary battery and an electronic device including the secondary battery. Based on the fact that the metal case structure of the secondary battery places some constraints on the expansion of the silicon-based material, the present invention improves the lithium absorption / desorption capability of the negative electrode by adjusting the type and blending ratio of the binder in the negative electrode, thereby alleviating the problems caused by the expansion of the silicon-based material in the silicon-mixed negative electrode and providing a secondary battery with both high energy density and high-rate cycle characteristics.

[0004] A first aspect of the present invention provides a secondary battery comprising a metal case and an electrode assembly disposed within the metal case, the electrode assembly including a negative electrode, the negative electrode including a negative electrode material layer, the negative electrode material layer including a binder, the binders including a first binder and a second binder, the first binder being selected from styrene-butadiene rubber-based polymers, and the second binder being selected from polyacrylic acid-based polymers, the first binder having a weight-average molecular weight of 40,000 to 300,000, the second binder having a weight-average molecular weight of 400,000 to 2,000,000, and the mass content of the first binder relative to the mass of the binder being 10% to 90%. The inventors of the present invention have found through research that in secondary batteries constrained by a metal case, such as cylindrical batteries, the metal case naturally inhibits the expansion of silicon during cycling. Based on this, it is possible to use a styrene-butadiene rubber-based point-contact binder in place of a coated binder depending on the content of silicon-based material. This reduces the amount of coating of the graphite particles with the coating-type binder, reduces the binder's inhibition of lithium ion transport, improves the rate of lithium absorption and desorption in the negative electrode, and effectively improves the high-rate cycle characteristics of the secondary battery while maintaining its energy density.

[0005] In some embodiments, the mass content of the first binder is 20% to 80% relative to the mass of the binder. If the content of the first binder is too low, the binder cannot effectively improve the coating of the graphite particles, the lithium absorption / desorption rate of the negative electrode is too low, and the high-rate cycling characteristics of the secondary battery are poor. If the content of the first binder is too high, the expansion of the silicon-based material during cycling increases, which also affects the cycling characteristics of the secondary battery. In some embodiments, the mass content of the first binder is 20% to 80%. In some embodiments, the mass content of the first binder is 40% to 60%.

[0006] In some embodiments, the weight average molecular weight of the first binder is 80,000 to 100,000.

[0007] In some embodiments, the weight average molecular weight of the second binder is 800,000 to 1,000,000.

[0008] In some embodiments, the first binder comprises a structural unit B and a structural unit C.

[0009] [ka]

[0010] Here, R4~R 10 is independently selected from hydrogen and a C1-C4 alkyl group. In some embodiments, the first binder is selected from styrene butadiene rubber (SBR).

[0011] In some embodiments, the second binder comprises a structural unit A.

[0012] [ka]

[0013] wherein R1-R3 are independently selected from hydrogen and C1-C4 alkyl groups. In some embodiments, the second binder is selected from polyacrylic acid.

[0014] In some embodiments, the weight content of the binder is 1% to 10% relative to the weight of the negative electrode material layer, and in some embodiments, the weight content of the binder is 3% to 7% relative to the weight of the negative electrode material layer.

[0015] In some embodiments, the negative electrode layer further includes a negative electrode active material, the negative electrode active material including a silicon-based material and a carbon-based material. In some embodiments, the silicon-based material is at least one selected from the group consisting of a silicon-oxygen compound and a silicon-carbon compound. In some embodiments, the carbon-based material is selected from graphite.

[0016] In some embodiments, the mass content of the silicon-based material is 3% to 70% relative to the mass of the negative electrode active material, hi some embodiments, the mass content of the silicon-based material is 10% to 50%.

[0017] In some embodiments, the metal case is selected from a steel case, an aluminum case, and a metal alloy case. In some embodiments, the metal case is selected from a cylindrical case and a rectangular case.

[0018] A second aspect of the present invention provides an electronic device including the secondary battery of the first aspect.

[0019] The secondary battery of the present invention has a metal case that naturally inhibits the expansion of the silicon-based material during cycling, thereby mitigating to some extent the rapid cycle decay and film shedding caused by the destruction of the conductive network due to the expansion of the silicon-based material. Based on this, the type and composition ratio of the binder in the silicon-mixed negative electrode are further adjusted to alleviate the problem of the expansion of the silicon-based material in the silicon-mixed negative electrode and improve the lithium absorption and desorption ability of the negative electrode, thereby achieving a secondary battery with both high energy density and high-rate cycling characteristics. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 shows cycle curves of the secondary batteries of Example 1 and Comparative Example 1 of the present invention. [Figure 2] FIG. 2 shows cycle curves of the secondary batteries of Example 2 and Comparative Example 1 of the present invention. [Figure 3] FIG. 3 shows cycle curves of the secondary batteries of Example 3 and Comparative Example 2 of the present invention. [Figure 4] FIG. 4 shows cycle curves of the secondary batteries of Example 4 and Comparative Example 2 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0021] In the drawing, 1 is Example 1, 2 is Example 2, 3 is Example 3, 4 is Example 4, 5 is Comparative Example 1, and 6 is Comparative Example 2.

[0022] The following examples of the present invention will be described in detail, but the examples should not be construed as limiting the present invention.

[0023] In the present invention, amounts, ratios, and other numerical values ​​may be expressed in range format. Such range format is used for convenience and brevity and should be understood flexibly to include not only the numerical values ​​explicitly recited as the upper and lower limits of the range, but also to include each and every numerical value or subrange included in said range as if each numerical value or subrange were explicitly recited.

[0024] In the detailed description and claims, a list of terms connected by "at least one of," "at least one of," "at least one of," or other similar terminology can refer to any combination of the listed terms. For example, if terms A and B are listed, the phrase "at least one of A and B" means A only, B only, or A and B. In another example, if terms A, B, and C are listed, the phrase "at least one of A, B, and C" means A only, B only, C only, A and B (excluding C), A and C (excluding B), B and C (excluding A), or all of A, B, and C. Term A may include a single element or multiple elements. Term B may include a single element or multiple elements. Term C may include a single element or multiple elements.

[0025] 1. Secondary battery The secondary battery provided by the present invention includes a metal case and an electrode assembly provided inside the metal case. The electrode assembly includes a negative electrode. The negative electrode includes a negative electrode material layer. The negative electrode material layer includes a binder. The binder includes a first binder and a second binder. The first binder is selected from styrene-butadiene rubber-based polymers, and the second binder is selected from polyacrylic acid-based polymers. The weight-average molecular weight of the first binder is 40,000 to 300,000, and the weight-average molecular weight of the second binder is 400,000 to 2,000,000. The mass content of the first binder is 10% to 90% of the mass of the binder.

[0026] In the prior art, silicon-containing negative electrodes inevitably increase anode expansion to improve the energy density of secondary batteries. To mitigate the expansion of silicon-based materials during cycling, it is often necessary to incorporate polyacrylic acid (PAA)-based, mostly coated binders. However, excessive coating of graphite particles in a mixed negative electrode with an excessive amount of coated binder reduces the lithium ion transport ability of the graphite particles, thereby affecting the high-rate cycling characteristics of secondary batteries. The inventors of the present invention have found through research that in secondary batteries constrained by a metal case, such as a cylindrical battery, the metal case naturally inhibits the expansion of silicon during cycling. Based on this, it is possible to replace some of the coated binder with a styrene-butadiene rubber-based point-contact binder depending on the content of silicon-based materials. This reduces the amount of coating of the graphite particles with the coating-type binder, reduces the binder's inhibition of lithium ion transport, improves the rate of lithium absorption and desorption in the negative electrode, and effectively improves the high-rate cycle characteristics of the secondary battery while maintaining its energy density.

[0027] In some embodiments, the mass content of the first binder relative to the mass of the binder is 15%, 20%, 25%, 30%, 33%, 35%, 37%, 40%, 45%, 47%, 50%, 53%, 55%, 57%, 60%, 63%, 65%, 67%, 70%, 75%, 80%, 85%, or a range consisting of any two of these values. If the content of the first binder is too low, the binder cannot effectively improve the coverage of the graphite particles, resulting in a low lithium absorption / desorption rate of the negative electrode and poor high-rate cycling performance of the secondary battery. If the content of the first binder is too high, the silicon-based material expands during cycling, which also affects the cycling performance of the secondary battery. In some embodiments, the mass content of the first binder relative to the mass of the binder is 20% to 83%. In some embodiments, the mass content of the first binder is 20% to 80%. In some embodiments, the mass content of the first binder is 40% to 60%.

[0028] In some embodiments, the negative electrode layer further includes a negative electrode active material, the negative electrode active material including a silicon-based material and a carbon-based material. In some embodiments, the silicon-based material is at least one selected from the group consisting of a silicon oxygen compound and a silicon carbon compound. In some embodiments, the silicon-based material includes at least one of a silicon compound, elemental silicon, and a mixture thereof. In some embodiments, the silicon-based material includes a silicon oxide (SiOx), where x is 0.6 to 1.5.

[0029] In some embodiments, the mass content of the silicon-based material is 3% to 70% relative to the mass of the negative electrode active material. In some embodiments, the mass content of the silicon-based material is 5%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, or a range consisting of any two of these values. In some embodiments, the mass content of the silicon-based material is 10% to 50%.

[0030] In some embodiments, the mass content of the silicon-based material is 3% to 25%, for example, 10% to 25%, relative to the mass of the negative electrode active material, and the mass content of the first binder is 40% to 60%, for example, 45%, 50%, or 55%, relative to the mass of the binder.

[0031] In some embodiments, the mass content of the silicon-based material relative to the mass of the negative electrode active material is 30% to 70%, for example, 30% to 50%, and the mass content of the first binder relative to the mass of the binder is 20% to 40%, for example, 25%, 30%, or 35%.

[0032] In some embodiments, the carbon-based material is selected from graphite, hi some embodiments, the graphite is at least one selected from the group consisting of artificial graphite and natural graphite.

[0033] In some embodiments, the weight average molecular weight of the first binder is 60,000, 80,000, 85,000, 90,000, 95,000, 100,000, 120,000, 140,000, 160,000, 180,000, 200,000, 220,000, 240,000, 260,000, 280,000, or a range consisting of any two of these values. In some embodiments, the weight average molecular weight of the first binder is 80,000 to 100,000.

[0034] In some embodiments, the weight average molecular weight of the second binder is 600,000, 800,000, 850,000, 900,000, 950,000, 1,000,000, 1,200,000, 1,400,000, 1,600,000, 1,800,000, or a range consisting of any two of these values. In some embodiments, the weight average molecular weight of the second binder is 800,000 to 1,000,000.

[0035] In some embodiments, the first binder comprises a structural unit B and a structural unit C.

[0036] [ka]

[0037] Here, R4~R 10 are each independently selected from hydrogen and a C1-C4 alkyl group.

[0038] In some embodiments, R4 to R7 are each independently selected from hydrogen, a methyl group, an ethyl group, an n-propyl group, and an isopropyl group. 10 are each independently selected from hydrogen, a methyl group, an ethyl group, an n-propyl group, and an isopropyl group. In some embodiments, the first binder is selected from styrene butadiene rubber.

[0039] In some embodiments, the second binder comprises a structural unit A.

[0040] [ka]

[0041] Here, R1 to R3 are each independently selected from hydrogen and a C1-C4 alkyl group.

[0042] In some embodiments, R1-R3 are each independently selected from hydrogen, methyl, ethyl, n-propyl, and isopropyl. In some embodiments, the second binder is selected from polyacrylic acid and polymethacrylic acid.

[0043] In some embodiments, the binder content is 1% to 10% by weight of the negative electrode layer. In some embodiments, the binder content is 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, or a range consisting of any two of these values. In some embodiments, the binder content is 3% to 7% by weight of the negative electrode layer.

[0044] In some embodiments, the metal case is selected from an aluminum case and a metal alloy case, hi some embodiments, the metal case is selected from a steel case.

[0045] In some embodiments, the metal case is selected from a cylindrical case and a prismatic case, hi some embodiments, the secondary battery is a cylindrical battery.

[0046] In some embodiments, the negative electrode further comprises a negative electrode current collector, wherein the negative electrode current collector comprises copper foil, aluminum foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, or any combination thereof.

[0047] In some embodiments, the negative electrode layer further comprises a conductive agent. In some embodiments, the conductive agent includes, but is not limited to, a carbon-based material, a metal-based material, a conductive polymer, and a mixture thereof. In some examples, the carbon-based material is selected from natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, and any combination thereof. In some examples, the metal-based material is selected from metal powder, metal fiber, copper, nickel, aluminum, and silver. In some examples, the conductive polymer is a polyphenylene derivative.

[0048] The electrode assembly of the present invention further includes a positive electrode, which includes a positive electrode current collector and a positive electrode material layer, which includes a positive electrode active material, a binder, and a conductive agent.

[0049] According to some embodiments of the present invention, the positive electrode current collector may be a metal foil or a composite current collector. For example, the positive electrode current collector may be an aluminum foil. The composite current collector is formed by depositing a metal material (such as copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, or silver alloy) on a polymer substrate.

[0050] According to some embodiments of the present invention, the positive electrode active material includes at least one of lithium cobalt oxide, lithium nickel manganese cobalt oxide, lithium nickel manganese aluminate, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium iron silicate, lithium vanadium silicate, lithium cobalt silicate, lithium manganese silicate, lithium manganese spinel oxide, lithium nickel manganese spinel oxide, and lithium titanate. In some embodiments, the binder includes at least one of a binder polymer, such as polyvinylidene fluoride, polytetrafluoroethylene, polyolefin, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, modified polyvinylidene fluoride, modified SBR rubber, and polyurethane. In some embodiments, the polyolefin binder includes at least one of polyethylene, polypropylene, polyalkenyl ester, polyalkenyl alcohol, and polyacrylic acid. In some embodiments, the conductive agent includes a carbon-based material such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, or carbon fiber; a metal-based material such as a metal powder or metal fiber of copper, nickel, aluminum, silver, or the like; a conductive polymer such as a polyphenylene derivative, or a mixture thereof.

[0051] The electrode assembly of the present invention further includes a separator. The material and shape of the separator used in the secondary battery of the present invention are not particularly limited and may be any technology disclosed in the prior art. In some embodiments, the separator includes a polymer or inorganic material that is stable against the electrolyte solution of the present invention.

[0052] For example, the separator may include a substrate layer and a surface treatment layer. The substrate layer is a nonwoven fabric, a membrane, or a composite membrane having a porous structure, and the material of the substrate layer is at least one selected from the group consisting of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. Specifically, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane may be selected and used.

[0053] A surface treatment layer is provided on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer, an inorganic layer, or a layer formed by mixing a polymer and an inorganic material. The inorganic layer includes inorganic particles and a binder. The inorganic particles are at least one selected from the group consisting of alumina, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium oxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate. The binder is at least one selected from the group consisting of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyamide, polyacrylonitrile, polyacrylic acid ester, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl alkoxide, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene. The polymer layer includes a polymer, and the polymer material is at least one selected from the group consisting of polyamide, polyacrylonitrile, acrylic ester polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinylalkoxide, polyvinylidene fluoride, and poly(vinylidene fluoride-hexafluoropropylene).

[0054] The secondary battery of the present invention further comprises an electrolyte solution. The electrolyte solution that can be used in the present invention may be any electrolyte solution known in the prior art.

[0055] According to some embodiments of the present invention, the electrolyte solution includes an organic solvent, a lithium salt, and optional additives. The organic solvent included in the electrolyte solution of the present invention may be any organic solvent known in the prior art that can be used as a solvent for an electrolyte solution. The electrolyte used in the electrolyte solution of the present invention is not particularly limited and may be any electrolyte known in the prior art. The additive included in the electrolyte solution of the present invention may be any additive known in the prior art that can be used as an additive for an electrolyte solution. In some examples, the organic solvent includes, but is not limited to, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), propylene carbonate, or ethyl propionate. In some examples, the organic solvent includes an ether-based solvent, such as at least one of 1,3-dioxolane (DOL) and ethylene glycol dimethyl ether (DME). In some examples, the lithium salt includes at least one of an organic lithium salt and an inorganic lithium salt. In some embodiments, the lithium salt includes, but is not limited to, lithium hexafluorophosphate (LiPF), lithium tetrafluoroborate (LiBF), lithium difluorophosphate (LiPOF), lithium bis(trifluoromethanesulfonyl)imide LiN(CFSO) (LiTFSI), lithium bis(fluorosulfonyl)imide Li(N(SOF)) (LiFSI), lithium bis(oxalato)borate LiB(CO) (LiBOB), or lithium difluoro(oxalato)borate LiBF(CO) (LiDFOB). In some embodiments, the additive includes at least one of fluoroethylene carbonate and adiponitrile.

[0056] In some embodiments, the secondary battery of the present invention includes, but is not limited to, a lithium ion battery or a sodium ion battery. In some examples, the secondary battery includes a lithium ion battery.

[0057] 2.Electronic equipment The present invention further provides an electronic device comprising the secondary battery of the first aspect of the present invention.

[0058] The electronic device or device of the present invention is not particularly limited. In some embodiments, the electronic device of the present invention includes, but is not limited to, a laptop computer, a pen-input computer, a mobile computer, an electronic book player, a mobile phone, a portable facsimile machine, a portable copier, a portable printer, a stereo headset, a video recorder, an LCD television, a portable cleaner, a portable CD player, a mini CD, a walkie-talkie, an electronic notebook, a calculator, a memory card, a portable tape recorder, a radio, a backup power supply, a motor, an automobile, a motorcycle, an auxiliary bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, a power tool, a flashlight, a camera, a large household storage battery, and a lithium ion capacitor.

[0059] In the following examples and comparative examples, the reagents, materials and instruments used are commercially available unless otherwise specified.

[0060] Examples and Comparative Examples Example 1 <Preparation of negative electrode piece> A negative electrode active material (15 wt % silicon material SiO and 85 wt % graphite), a binder (50 wt % PAA and 50 wt % SBR), and other components (conductive material and dispersant) were mixed in a mass ratio of 94.1 wt %:5 wt %:0.9 wt % to obtain a negative electrode slurry composition, to which deionized water was added and blended in a vacuum blender (wherein the PAA was added before high-speed dispersion and the SBR was added after high-speed dispersion) to obtain a negative electrode slurry, where the solids content of the negative electrode slurry was 38%. The negative electrode slurry was uniformly applied onto one surface of a copper foil serving as a negative electrode current collector with a thickness of 8 μm, and the copper foil was dried at 85°C to obtain a negative electrode piece having a coating layer thickness of 30 μm and a negative electrode material layer applied to one side thereof. The above procedure was repeated on the other surface of the negative electrode current collector to obtain a negative electrode piece having a negative electrode material layer applied to both sides thereof, and the resulting negative electrode piece was cold-pressed.

[0061] <Preparation of positive electrode piece> The positive electrode active material, lithium nickel-cobalt manganese oxide, conductive carbon black, and the binder, polyvinylidene fluoride, were mixed in a mass ratio of 96.7:1.7:1.6, N-methylpyrrolidone (NMP) was added, and the mixture was vacuum blended to obtain a positive electrode slurry. The solids content of the positive electrode slurry was 76 wt%. The positive electrode slurry was uniformly applied to one surface of a 10 μm-thick aluminum foil current collector, and the aluminum foil was dried at 120°C to obtain a positive electrode piece coated with a positive electrode material layer on one side and with a coating layer thickness of 45 μm. The same procedure was repeated on the other surface of the aluminum foil, i.e., a positive electrode piece coated with a positive electrode material layer on both sides was obtained, and then cold-pressed.

[0062] <Preparation of electrolyte> In a glove box under a dry argon atmosphere, propylene carbonate, ethylene carbonate, and diethyl carbonate were mixed in a mass ratio of 1:1:1 to obtain an organic solvent, and then lithium salt LiPF6 was added to the organic solvent, dissolved, and mixed uniformly to obtain an electrolyte solution with a LiPF6 concentration of 1 mol / L.

[0063] <Preparation of separator> A 7 μm thick porous polyethylene film (provided by Celgard) is used, with an opening of 0.1 μm. Here, a polyacrylic acid ester binder layer is provided on the surface of the separator, and the coating mass of the binder layer is 10±2 mg / 5000 mm. 2 and the thickness was 3±1 μm.

[0064] <Preparation of Lithium-ion Cylindrical Batteries> The positive electrode pieces, separator, and negative electrode pieces prepared as described above were stacked in order and wound up to obtain an electrode assembly, with the separator interposed between the positive electrode pieces and the negative electrode pieces to act as an insulator. The electrode assembly was placed in a cylindrical steel case, dried, and then an electrolyte was poured into it. After leaving it to stand, the battery underwent formation, capacity grading, and K value measurement, and a lithium-ion cylindrical battery was obtained.

[0065] Examples 2 to 11, Comparative Examples 1 and 2 The same as in Example 1, except that the relevant parameters were adjusted as shown in Table 1.

[0066] Measurement method 1. Determination of binder composition and content The composition of the binder can be detected by employing infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR).

[0067] The binder content can be detected by employing simultaneous thermogravimetry-mass spectrometry (TG-MS).

[0068] 2. Measurement of cycle characteristics of lithium-ion batteries The measurement temperature was 25°C, and the battery was charged at a constant current of 2C to 4.25V, then charged at a constant voltage of 0.025C, and allowed to stand for 5 minutes before discharging at 7C to 2.5V. The capacity obtained at each step was taken as the initial capacity, and cycle measurements were performed by charging at 2C and discharging at 7C. The ratio of the capacity at each step to the initial capacity was calculated, and a capacity decay curve was obtained. Here, the capacity retention after n cycles = (discharge capacity at nth cycle / discharge capacity at 1st cycle) × 100%.

[0069] 3. Energy density of lithium-ion batteries Leave the lithium-ion battery at room temperature (25°C ± 2°C) for 30 minutes or more, charge it using the shipping standard charging method up to the shipping standard cut-off condition (charging time must be 8 hours or less), leave it for 30 minutes or more, measure the discharge energy E (Wh), measure the diameter and maximum height of the lithium-ion battery using a micrometer or vernier caliper, and measure the volume V (L), and the volumetric energy density of the battery discharge VED (Wh / L) = E / V.

[0070] Measurement results [Table 1]

[0071] The data for Comparative Example 1 and Example 1 in Table 1 and Figure 1 show that the binder used in Comparative Example 1 was a 100% coated binder (PAA). This indicates that during high-rate charge-discharge cycles (2C and 7C) at 25°C, excessive PAA coating on the graphite particles reduces the lithium ion transport capacity of the graphite surface, thereby affecting the lithium absorption / desorption rate of the anode, resulting in rapid capacity fade during high-rate cycles. In Example 1, replacing some of the PAA with 50% point-contact binder SBR reduces the binder coverage on the graphite particle surface, improving the lithium absorption / desorption rate of the anode, thereby improving high-rate cycle performance. In Example 2, the ratio of PAA to SBR was adjusted. A comparison of the performance of the lithium-ion batteries of Example 2 and Comparative Example 1 is shown in Figure 2, and the high-rate cycle performance is also significantly improved.

[0072] In Examples 4 and 5, the mixing ratio of the silicon-based material was changed. A comparison of the cycle characteristics of Examples 4 and 5 with Comparative Example 2 is shown in Figures 3 and 4. Similarly, compared to Comparative Example 2, by using a different ratio of the coated binder (PAA) and the point contact binder (SBR), the coating of the coated binder on the graphite particles was reduced, improving the lithium absorption and desorption ability of the negative electrode, and the high-rate cycle characteristics were clearly improved.

[0073] While several exemplary embodiments of the present invention have been illustrated and described, the present invention is not limited to the disclosed embodiments. Rather, those skilled in the art will recognize that various modifications and changes can be made to the described embodiments without departing from the spirit and scope of the present invention, as set forth in the appended claims.

Claims

1. a metal case and an electrode assembly provided inside the metal case; the electrode assembly includes a negative electrode, the negative electrode includes a negative electrode material layer, the negative electrode material layer includes a binder, the binder includes a first binder and a second binder; The first binder is selected from a styrene-butadiene rubber-based polymer, and the second binder is selected from a polyacrylic acid-based polymer; The weight average molecular weight of the first binder is 80,000 to 100,000, and the weight average molecular weight of the second binder is 800,000 to 1,000,000, The mass content of the first binder is 10% to 90% relative to the mass of the binder; the negative electrode material layer further contains a negative electrode active material, the negative electrode active material includes a silicon-based material and a carbon-based material, The silicon-based material includes silicon oxide SiOx, where x is 0.6 to 1.5; the carbon-based material is selected from graphite; A secondary battery, characterized in that the mass content of the silicon-based material is 10% to 50% relative to the mass of the negative electrode active material.

2. The mass content of the first binder is 20% to 80% based on the mass of the binder. The secondary battery according to claim 1 .

3. The mass content of the first binder is 40% to 60% relative to the mass of the binder. The secondary battery according to claim 1 .

4. the first binder includes a structural unit B and a structural unit C, and the second binder includes a structural unit A; 【Chemistry 1】 Here, R １ ~R １０ is independently selected from hydrogen and a C1-C4 alkyl group.

5. the first binder is selected from styrene butadiene rubber and the second binder is selected from polyacrylic acid; and / or The mass content of the binder is 1% to 10% relative to the mass of the negative electrode material layer. The secondary battery according to claim 1 .

6. The mass content of the binder is 3% to 7% relative to the mass of the negative electrode material layer. The secondary battery according to claim 1 .

7. the metal case is selected from an aluminum case and a metal alloy case; and / or The metal case is selected from a cylindrical case and a rectangular case. The secondary battery according to claim 1 .

8. The secondary battery according to claim 7, wherein the metal case is selected from a steel case.

9. An electronic device comprising the secondary battery according to claim 1.

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