Negative electrode sheet and electrochemical device

Abstract

The application provides a negative electrode sheet and an electrochemical device. The negative electrode sheet comprises a negative electrode current collector, a first negative electrode active material layer on at least one surface of the negative electrode current collector, and a second negative electrode active material layer on the surface of the first negative electrode active material layer. The first negative electrode active material layer comprises a first negative electrode active material, and the first negative electrode active material comprises a silicon-based material and graphite. The second negative electrode active material layer comprises a second negative electrode active material, and the second negative electrode active material comprises an amorphous carbon material with a spherical structure. The average particle size D1 of the amorphous carbon material satisfies 0.2 mu m<=D1<=4 mu m. The application can effectively improve the low-temperature performance, power performance, energy density and other qualities of the electrochemical device.

Description

Technical Field

[0001] This invention relates to the field of batteries, and more specifically to a negative electrode and an electrochemical device. Background Technology

[0002] In the field of electrochemical energy storage devices, there are increasingly higher requirements for the low-temperature performance and power performance of electrochemical devices. For example, hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs) have advantages such as energy saving and environmental protection, short driving start-up time, and good driving experience. In recent years, with the rise of electric vehicles, they have received widespread attention. HEVs and PHEVs are driven by a combination of internal combustion engines and electric motors, reducing reliance on fossil fuels. The electric power generally plays a role in providing power for instantaneous start-up and converting potential energy into electrical energy to recover energy when the car is going downhill. Both of these working conditions require the power battery to provide a lot of power. Moreover, the start-stop battery of the car usually needs to work in low-temperature conditions (extreme low temperature conditions can reach about -30°C). Therefore, extremely high requirements are placed on the power performance and low-temperature performance of the battery (cell). However, the current power battery (or start-stop battery) has poor power performance and low-temperature performance, especially unable to meet the working conditions of HEVs and PHEVs under low-temperature conditions. Furthermore, the energy density of electrochemical devices is a crucial factor in ensuring their application. Silicon-based materials possess advantages such as high capacity, and introducing them into the negative electrode of electrochemical devices can improve their energy density. However, silicon-based materials are prone to volume expansion, leading to defects such as high cell expansion rates in electrochemical devices. Therefore, how to balance the low-temperature performance, power performance, energy density, and low expansion characteristics of electrochemical devices is a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0003] This invention provides a negative electrode and an electrochemical energy storage device, which can effectively improve the power performance and low-temperature performance of the electrochemical device, while ensuring the energy density and other characteristics of the electrochemical device, thus effectively overcoming the defects of the prior art.

[0004] This invention provides a negative electrode sheet, comprising a negative electrode current collector, a first negative electrode active material layer located on at least one surface of the negative electrode current collector, and a second negative electrode active material layer located on the surface of the first negative electrode active material layer. The first negative electrode active material layer contains a first negative electrode active material, which comprises a silicon-based material and graphite. The second negative electrode active material layer contains a second negative electrode active material, which comprises an amorphous carbon material with a spherical structure. The average particle size D1 of the amorphous carbon material satisfies 0.2μm≤D1≤4μm.

[0005] According to one embodiment of the present invention, the Raman spectrum Id / Ig peak ratio of the amorphous carbon material is 0.5-1.5; and / or, in the X-ray diffraction analysis results of the amorphous carbon material, the peak position of the diffraction peak is less than 26.5°.

[0006] According to one embodiment of the present invention, the specific surface area of ​​the amorphous carbon material is 2-23 m². 2 / g; and / or, the specific capacity of the amorphous carbon material is 270-360 mAh / g.

[0007] According to one embodiment of the present invention, the mass content of silicon-based material in the first negative electrode active material is 0.5-15%.

[0008] According to one embodiment of the present invention, the silicon-based material includes at least one of silicon material, silicon-oxygen material, and silicon-carbon material.

[0009] According to one embodiment of the present invention, H1≥D2 is satisfied, where H1 is the thickness of the first negative electrode active material layer and D2 is the maximum particle size of the first negative electrode active material.

[0010] According to one embodiment of the present invention, H2≥D3 is satisfied, where H2 is the thickness of the second negative electrode active material layer and D3 is the maximum particle size of the second negative electrode active material.

[0011] According to one embodiment of the present invention, the thickness of the first negative electrode active material layer is H1, where 16μm≤H1≤75μm.

[0012] According to one embodiment of the present invention, the particle size of the first negative electrode active material satisfies: D2≤30μm, where D2 is the maximum particle size of the first negative electrode active material.

[0013] According to one embodiment of the present invention, the thickness of the second negative electrode active material is H2, where 2μm≤H2≤65μm.

[0014] According to one embodiment of the present invention, the particle size of the second negative electrode active material satisfies: D3≤15μm, where D3 is the maximum particle size of the second negative electrode active material.

[0015] According to one embodiment of the present invention, the first negative electrode active material layer comprises a conductive agent, a binder, a thickener, and the first negative electrode active material, wherein the mass content of the first negative electrode active material is 88-98%, the mass content of the conductive agent is 0-5%, the mass content of the binder is 0.5-6%, and the mass content of the thickener is 0.5-3%.

[0016] According to one embodiment of the present invention, the second negative electrode active material layer comprises a conductive agent, a binder, a thickener, and the second negative electrode active material, wherein the mass content of the second negative electrode active material is 88-98%, the mass content of the conductive agent is 0-5%, the mass content of the binder is 0.5-6%, and the mass content of the thickener is 0.5-3%.

[0017] In another aspect, the present invention provides an electrochemical device comprising the aforementioned negative electrode.

[0018] In this invention, by combining a first negative electrode active material layer and a second negative electrode active material layer, silicon-based materials, graphite, and the aforementioned amorphous carbon materials with a small particle size (0.2μm≤D1≤4μm) spherical structure are introduced into the negative electrode sheet. This effectively improves the low-temperature performance and power performance of the negative electrode sheet, while also enhancing its energy density and other characteristics. Consequently, it improves the low-temperature performance, power performance, and energy density of the electrochemical device, enabling it to meet the start-up performance requirements of HEVs and PHEVs even under extreme low-temperature conditions of -30℃. Furthermore, the amorphous carbon material has characteristics such as large interlayer spacing, exhibiting excellent low-temperature performance, while also showing minimal expansion during cycling. Thus, while achieving excellent low-temperature performance, it can also suppress the electrode sheet expansion caused by silicon negative electrode materials, which is of great significance for practical industrial applications. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the negative electrode sheet according to one embodiment of the present invention;

[0020] Figure 2 The image shows the Raman spectrum of an amorphous carbon material according to an embodiment of the present invention (the horizontal axis represents the Raman shift, and the vertical axis represents the peak intensity).

[0021] Figure 3 The image shows the XRD pattern of an amorphous carbon material according to an embodiment of the present invention (the horizontal axis is 2θ and the vertical axis is peak intensity).

[0022] Figure 4 A is a microscopic morphology image of an amorphous carbon material according to an embodiment of the present invention, obtained by scanning electron microscopy (SEM); B is a microscopic morphology image of a comparative amorphous carbon material, obtained by SEM.

[0023] Explanation of reference numerals in the attached figures: 1: Negative electrode current collector; 2: First negative electrode active material layer; 3: Second negative electrode active material layer. Detailed Implementation

[0024] To enable those skilled in the art to better understand the present invention, the present invention will be further described in detail below with reference to the accompanying drawings.

[0025] In this invention, the average particle size (e.g., D1) of the negative electrode material A can be determined as follows: Before fabricating the negative electrode sheet, the Dv50 (i.e., the average particle size of the negative electrode material A) of the negative electrode material A is measured using a laser particle size analyzer. Dv50 is the particle size that reaches 50% of the volume accumulation from the smallest particle size side in the particle size distribution based on volume. Alternatively, after fabricating the negative electrode sheet, a coating sample is taken from the negative electrode sheet, and the average particle size of the negative electrode material A in the sample is measured using a focused ion beam microscope (FIB-SIM).

[0026] In this invention, the maximum particle size (e.g., D2, D3) of the negative electrode material A can be determined as follows: Before fabricating the negative electrode sheet, the Dv99 (i.e., the maximum particle size of the negative electrode material A) of the negative electrode material A is determined using a laser particle size analyzer. Dv99 refers to the particle size that reaches 99% of the volume accumulation from the smallest particle size side in the particle size distribution based on volume. Alternatively, after fabricating the negative electrode sheet, a coating sample is taken from the negative electrode sheet, and the maximum particle size of the negative electrode material A in the sample is determined by focusing ion beam microscopy (FIB-SIM).

[0027] like Figure 1 As shown, the negative electrode sheet of the present invention includes a negative electrode current collector 1, a first negative electrode active material layer 2 located on at least one surface of the negative electrode current collector 1, and a second negative electrode active material layer 3 located on the surface of the first negative electrode active material layer 2. The first negative electrode active material layer 2 contains a first negative electrode active material, which contains silicon-based material and graphite. The second negative electrode active material layer 3 contains a second negative electrode active material, which contains an amorphous carbon material with a spherical structure. The average particle size D1 of the amorphous carbon material satisfies 0.2μm≤D1≤4μm.

[0028] The microstructure of the aforementioned amorphous carbon materials is an irregular, disordered layer structure with large interlayer spacing, which contrasts with the irregular morphology of other amorphous carbon materials (such as...). Figure 4 Compared to the amorphous carbon material shown in B, this amorphous carbon material has a spherical structure with a small particle size (0.2μm≤D1≤4μm). It has many end faces, high isotropy, and a large number of channels on its surface for ions to enter and exit, which is conducive to the insertion / extraction / transport of ions (such as lithium ions). Therefore, it has excellent power performance and low-temperature kinetic performance, specifically manifested in high discharge power and high low-temperature discharge lower limit voltage. Through the structural design of the first negative electrode active material layer 2 and the second negative electrode active material layer 3, the low-temperature high-power characteristics of amorphous carbon material can be used to improve the electrical performance of the negative electrode / electrochemical device, while also taking into account the energy density and other characteristics of the negative electrode / electrochemical device. At the same time, it can suppress the cycle expansion caused by silicon negative electrode, thus enabling the negative electrode / electrochemical device to have good low-temperature performance, power performance, energy density, and resistance to expansion.

[0029] Specifically, the macroscopic appearance of the aforementioned amorphous carbon material can be a powder composed of particles with the aforementioned spherical structure. According to the research of this invention, the Raman spectrum Id / Ig peak ratio of the amorphous carbon material (or spherical carbon material) is 0.5-1.5, and the peak position of the diffraction peak in its X-ray diffraction (XRD) analysis is less than 26.5°. It is a high-power carbon-based active material. By introducing it into the negative electrode, the low-temperature start-up and shutdown performance and power performance of the negative electrode / electrochemical device can be significantly improved.

[0030] In some embodiments, the average particle size D1 of the amorphous carbon material can be, for example, a range of 0.2 μm, 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, or any combination of two of these values.

[0031] In some embodiments, the specific surface area (BET) of the amorphous carbon material is 2-23 m². 2 / g, for example 2m 2 / g、3m 2 / g、4m 2 / g、5m 2 / g、6m 2 / g、7m 2 / g、8m 2 / g、9m 2 / g, 10m 2 / g、11m 2 / g、12m 2 / g、13m 2 / g、14m 2 / g, 15m 2 / g, 16m 2 / g、17m 2 / g、18m 2 / g、19m 2 / g、20m 2 / g、21m 2 / g、22m 2 / g、23m 2 / g or a range of any two of these values ​​is beneficial for improving the power performance and low-temperature performance of the negative electrode.

[0032] Furthermore, the specific capacity of the amorphous carbon material can be 270-360 mAh / g, for example, 270 mAh / g, 280 mAh / g, 290 mAh / g, 300 mAh / g, 310 mAh / g, 320 mAh / g, 330 mAh / g, 340 mAh / g, 350 mAh / g, 360 mAh / g, or any combination of these values.

[0033] Amorphous carbon materials can generally be prepared by a hydrothermal method, whereby a polymer undergoes a hydrothermal reaction in a homogeneous dispersion system to obtain a spherical precursor, which is then carbonized to obtain the amorphous carbon material. Alternatively, a precursor with a spherical structure can be directly carbonized (heat-treated) to obtain the amorphous carbon material. In some preferred embodiments of the present invention, the aforementioned amorphous carbon material can be prepared by carbonizing a phenolic resin with the aforementioned spherical structure (0.2 μm ≤ D1 ≤ 4 μm). The aforementioned hydrothermal reaction and carbonization processes are conventional procedures in the art and will not be described in detail further.

[0034] The introduction of silicon-based materials is beneficial for improving the energy density of the negative electrode. Taking into account factors such as the energy density, low-temperature performance and power performance of the negative electrode, in some preferred embodiments, the mass content of silicon-based materials in the first negative electrode active material can be 0.5-15%, for example, 0.5%, 1%, 3%, 5%, 7%, 10%, 12%, 15% or any combination thereof, and the balance can be graphite.

[0035] In this invention, the first negative electrode active material can specifically be a mixture formed by physically mixing silicon-based materials and graphite. In some embodiments, the silicon-based material includes at least one of silicon materials, silicon-oxygen materials, and silicon-carbon materials. For example, silicon-oxygen materials include silicon suboxide.

[0036] In some embodiments, the first negative electrode active material layer 2 comprises a conductive agent, a binder, a thickener, and a first negative electrode active material, wherein the mass content of the first negative electrode active material is 88-98%, the mass content of the conductive agent is 0-5%, the mass content of the binder is 0.5-6%, and the mass content of the thickener is 0.5-3%, which is beneficial to improving the low-temperature performance, power performance, and energy density of the negative electrode sheet.

[0037] In some embodiments, the second negative electrode active material layer 3 comprises a conductive agent, a binder, a thickener, and a second negative electrode active material, wherein the mass content of the second negative electrode active material is 88-98%, the mass content of the conductive agent is 0-5%, the mass content of the binder is 0.5-6%, and the mass content of the thickener is 0.5-3%, which is beneficial to further improve the low-temperature performance, power performance, and energy density of the negative electrode sheet.

[0038] In some embodiments, H1≥D2 is satisfied, where H1 is the thickness of the first negative electrode active material layer 2 and D2 is the maximum particle size of the first negative electrode active material, which is more conducive to improving the performance of the negative electrode sheet and ensuring its function.

[0039] Furthermore, H2 ≥ D3 is satisfied, where H2 is the thickness of the second negative electrode active material layer 3 and D3 is the maximum particle size of the second negative electrode active material.

[0040] High-capacity silicon-based materials are prone to volume expansion. In this invention, a first negative electrode active material layer 2 (lower layer) is formed by combining high-capacity silicon-based materials with graphite. A second negative electrode active material layer 3 is formed on the first negative electrode active material layer 2 using the aforementioned spherical amorphous carbon material. This effectively suppresses the volume expansion of the lower silicon-based material, thereby improving the structural stability of the negative electrode sheet and preventing volume expansion. Relatively speaking, the thicker the first negative electrode active material layer 2, if the thickness of the first negative electrode active material layer is not less than the thickness of the second negative electrode active material layer 3, it is beneficial to improve the energy density of the negative electrode sheet. Conversely, the thicker the second negative electrode active material layer 3, if the thickness of the second negative electrode active material layer 3 is not less than the thickness of the first negative electrode active material layer 2, it is more beneficial to improve the low-temperature performance and power performance of the negative electrode sheet. In specific implementation, the choice can be made according to the needs.

[0041] Taking into account the volume suppression effect on the negative electrode, as well as factors such as capacity, low-temperature performance, and power performance, in some embodiments, the thickness of the first negative electrode active material layer is H1, where 16μm ≤ H1 ≤ 75μm, and the particle size of the first negative electrode active material satisfies: D2 ≤ 30μm, where D2 is the maximum particle size of the first negative electrode active material. H1 can be, for example, a range of 16μm, 20μm, 25μm, 30μm, 35μm, 40μm, 45μm, 50μm, 55μm, 60μm, 65μm, 70μm, 75μm, or any combination thereof.

[0042] Furthermore, the thickness of the second negative electrode active material is H2, where 2μm ≤ H2 ≤ 65μm, and the particle size of the second negative electrode active material satisfies: D3 ≤ 15μm, where D3 is the maximum particle size of the second negative electrode active material. H2 can be, for example, a range of 2μm, 5μm, 10μm, 15μm, 20μm, 25μm, 30μm, 35μm, 40μm, 45μm, 50μm, 55μm, 60μm, 65μm, or any combination thereof.

[0043] In this invention, the thickness of the negative electrode active material layer (such as the thickness of the first negative electrode active material layer 2 or the thickness of the second negative electrode active material layer 3) refers to the thickness of a single-sided coating, that is, the thickness of the negative electrode active material layer located on one surface of the negative electrode current collector, excluding the thickness of the negative electrode current collector, and not the sum of the thickness of the negative electrode active material layer on one side of the negative electrode current collector and the thickness of the negative electrode active material layer on the other side.

[0044] In this invention, the first negative electrode active material layer and the second negative electrode active material layer can be provided on only one surface of the negative electrode current collector, or the first negative electrode active material layer and the second negative electrode active material layer can be provided on both the positive and negative surfaces of the current collector. In specific implementation, the choice can be made according to the needs.

[0045] The electrochemical device of the present invention includes the above-mentioned negative electrode. Optionally, the electrochemical device is, for example, a battery, specifically a power battery, such as a power battery used in HEVs and PHEVs, and especially a lithium-ion battery.

[0046] The aforementioned electrochemical device further includes a positive electrode sheet, which includes a positive current collector and a positive active material layer located on at least one surface of the positive current collector. The average particle size of the positive active material in the positive active material layer can be, for example, 2.5-10 μm.

[0047] Specifically, the raw materials for the aforementioned positive electrode active material layer may include a conductive agent, a binder, and the aforementioned positive electrode active material. Optionally, the positive electrode active material may include at least one of lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium nickel cobalt manganese oxide, lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium vanadium oxide phosphate, lithium-rich manganese-based materials, and lithium nickel cobalt aluminum oxide, wherein the general formula of lithium nickel cobalt manganese oxide (NCM) is LiNi. x Co y Mn 1-x-y O2, 0 <x≤1,0<y≤1,1> 1-xy>0, specifically including at least one of NCM111, NCM523, NCM532, NCM622, and NCM811. Relatively speaking, NCMs with lower nickel content have a better effect on improving the low-temperature performance and power performance of the battery.

[0048] The aforementioned positive electrode active material can be a single crystal material and / or a polycrystalline material. Generally, single crystal materials have smaller particles, which makes the diffusion distance of ions (such as lithium ions) smaller, which is more conducive to the low temperature performance and power performance of the battery.

[0049] The aforementioned electrochemical device also includes a separator (located between the positive and negative electrodes) for separating the positive and negative electrodes. The separator has a porosity of 30-60%, which facilitates ion (such as lithium ion) transport, thereby improving the low-temperature performance and power performance of the electrochemical device.

[0050] The separator used in this invention can be a conventional separator in the art. For example, in some embodiments, the separator includes a substrate and a coating layer located on at least one surface of the substrate. The thickness of the substrate is 5-22 μm, and the thickness of the coating layer is 0-10 μm. When the thickness of the coating layer is 0, the separator is an uncoated separator. When the thickness of the coating layer is not 0, the separator is a coated separator. Generally, the porosity of an uncoated separator is greater than that of a coated separator. Higher separator porosity is more conducive to the transport of ions (such as lithium ions) and is beneficial to the low-temperature performance and power performance of the battery. Therefore, in some embodiments, an uncoated separator is preferred.

[0051] Specifically, in some embodiments, the substrate may include at least one of polyethylene (PP) film, polypropylene (PE) film, and a composite film made of PP film and PE film. For example, the composite film may be a PP / PE / PP composite film formed by sequentially combining PP film, PE film, and PP film. The coating layer may include an adhesive coating layer on the surface of the substrate and a ceramic coating layer on the surface of the adhesive coating layer. The adhesive coating may be made from an adhesive, and the ceramic coating may be made from ceramic particles and an adhesive. The ceramic particles may include alumina, but are not limited thereto.

[0052] The aforementioned electrochemical device also includes an electrolyte. For example, the electrolyte used may include a non-aqueous electrolyte, the components of which may include a non-aqueous solvent, a lithium salt, and additives. The non-aqueous solvent includes at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, ethyl acetate, ethyl propionate, propyl propionate, and propyl acetate. The lithium salt includes at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium difluorooxalate borate, lithium bis(trifluoromethylsulfonyl)imide, and lithium bis(oxalate borate). The additives include at least one of ethylene sulfite, lithium bis(oxalate borate), ethylene sulfate, tri(trimethylsilane)borate, 1,3-propenesulfonyl lactone, 1,3-propanesulfonyl lactone, ethylene carbonate, ethylene sulfite, lithium difluorophosphate, lithium difluorobis(oxalate borate), lithium tetrafluorooxalate, lithium difluorophosphate, ethylene sulfate, and boron phosphate lithium oxalate, and ethyl 3-methoxypropionate.

[0053] The electrochemical device of the present invention can be manufactured according to conventional methods in the art. For example, the positive electrode, separator and negative electrode can be stacked in sequence and then wound (or stacked) to form a cell. Then, after processes such as encapsulation, cell baking, electrolyte injection (i.e., injection of electrolyte), and hot pressing formation, a battery is obtained. These steps / processes are all conventional operations in the art and will not be described in detail.

[0054] The electrochemical device of the present invention can be used as an energy storage device for an electronic device, such as an automobile, specifically a HEV or PHEV.

[0055] Unless otherwise specified, the conductive agent used in this invention may include at least one of superconducting carbon black (SP), graphene, carbon nanotubes, fullerene, graphene nanoribbons, and sheet graphite, wherein the carbon nanotubes may include single-walled carbon nanotubes and / or multi-walled carbon nanotubes; the binder may include at least one of polyvinylidene fluoride (PVDF), a copolymer of polyvinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, polyhexafluoropropylene, and styrene-butadiene rubber (SBR); the thickener may include sodium carboxymethyl cellulose (CMC). However, this invention is not limited to these materials, and other suitable conductive agents, binders, thickeners, etc., may also be selected.

[0056] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described below through specific embodiments and comparative examples.

[0057] Unless otherwise specified, in the following examples, the specific surface area (BET) of the material was measured using a Tristar 3020 McMillite surface area analyzer; and the thickness of the negative electrode active material layer was measured using a micrometer.

[0058] In the following embodiments, the amorphous carbon material used has the following properties: its Raman spectrum Id / Ig peak ratio is 1.09, and the elution position of the diffraction peak in the X-ray diffraction (XRD) analysis is approximately 22.445. The Raman spectrum of the amorphous carbon material is shown below. Figure 2 ( Figure 2 (The results are from three tests; see XRD patterns below.) Figure 3 SEM image as follows Figure 4 As shown in A.

[0059] In the following embodiments, the negative electrode, positive electrode, and battery are all prepared according to the following process:

[0060] (1) Prepare the first negative electrode slurry by placing the first negative electrode active material, SBR, superconducting carbon black, and CMC in deionized water at a mass ratio of 94.3:1.5:1.5:2.7; prepare the second negative electrode slurry by placing the spherical amorphous carbon material, SBR, superconducting carbon black, and CMC in deionized water at a mass ratio of 94.3:1.5:1.5:2.7; and apply the first and second negative electrode slurries to the surface of the negative electrode current collector (copper foil) using a double-layer coating machine (on both sides of the negative electrode current collector). Each surface of the negative electrode current collector is coated with a first negative electrode slurry and a second negative electrode slurry. After drying and rolling, a negative electrode active material layer is formed on both the positive and negative surfaces of the negative electrode current collector (the negative electrode active material layer includes a first negative electrode active material layer formed by the first negative electrode slurry and a second negative electrode active material layer formed by the second negative electrode slurry, with the first negative electrode active material layer located between the surface of the negative electrode current collector and the second negative electrode active material layer), thus obtaining a negative electrode sheet; wherein, the first negative electrode active material is a mixture of silicon-based materials and graphite.

[0061] (2) NCM111 (single crystal), SBR and superconducting carbon black are placed in N-methylpyrrolidone at a mass ratio of 94:3.5:2.5 to prepare a positive electrode slurry. The positive electrode slurry is coated on both the front and back surfaces of the positive electrode current collector (aluminum foil). After drying and rolling, a positive electrode active material layer is formed on both the front and back surfaces of the positive electrode current collector to obtain a positive electrode sheet.

[0062] (3) After stacking the positive electrode, separator and negative electrode in sequence, they are wound to form a cell. Then, after the processes of encapsulation, cell baking, liquid injection and hot pressing, a battery (with a cell capacity of about 2Ah) is obtained. The separator used is a PP / PE / PP composite film with a thickness of 9μm and a porosity of about 43.43%.

[0063] In each embodiment and comparative example, the average particle size D1, specific capacity, specific surface area (BET), type of silicon-based material, mass content of silicon-based material in the first negative electrode active material, thickness H1 of the first negative electrode active material layer, thickness H2 of the second negative electrode active material layer, maximum particle size D2 of the first negative electrode active material, and maximum particle size D3 of the second negative electrode active material are shown in Table 1; among them,

[0064] The difference between Comparative Example 1 and Example 5 is that the negative electrode active material in the second negative electrode active material layer has a particulate morphology as shown in Example 5. Figure 4 The amorphous carbon material with a non-spherical structure shown in B has parameters such as average particle size, specific capacity, and BET as shown in Table 1.

[0065] The difference between Comparative Example 2 and Example 5 is that there is only a first negative electrode active material layer (i.e. there is no second negative electrode active material layer);

[0066] The difference between Comparative Example 3 and Example 5 is that there is only a second negative electrode active material layer (i.e., no first negative electrode active material layer).

[0067] Apart from the differences mentioned above and those shown in Table 1, the remaining conditions of each embodiment and comparative example are basically the same.

[0068] In each embodiment and comparative example, battery performance was tested through the following process:

[0069] (1) -30℃ discharge lower limit voltage test: Adjust the battery capacity to 50% SOC, stand at -30℃ for 4h, then discharge at 3C for 2s, and record the battery lower limit voltage. The test results (-30℃ discharge lower limit voltage) of each embodiment are shown in Table 2.

[0070] (2) Power test: With a fixed discharge time of 10s, different current rates were adjusted for discharge. When the lower limit voltage after 10s of discharge was exactly 2.7V, the current was the maximum discharge current. The actual discharge end voltage was the lower limit voltage. The power calculation formula was: Power = Lower limit voltage × Maximum discharge current. The test results (discharge power) of each embodiment and comparative example are shown in Table 2.

[0071] (3) The mass energy density of each embodiment and comparative battery was determined according to the formula: energy density = capacity × open circuit voltage / cell mass, as shown in Table 2;

[0072] (4) The mass energy density of each embodiment and comparative battery was measured according to the formula: energy density = capacity × open circuit voltage / cell volume (cell volume = cell length × cell width × cell thickness). See Table 2.

[0073] (5) According to the thickness expansion rate = (Tx-T0) / T0, T0 is the cell thickness before cycling, and Tx is the cell thickness corresponding to the number of cycles x.

[0074] Table 1

[0075]

[0076] * indicates the mass content of silicon-based material in the first negative electrode active material, which is composed of graphite and silicon-based materials, with the remainder being graphite.

[0077] Table 2

[0078]

[0079] As shown in Table 2, by forming a first negative electrode active material layer (silicon-based material + graphite) and a second negative electrode active material layer (high-power amorphous carbon material) sequentially on the surface of the negative electrode current collector through multi-layer coating, the battery's low-temperature discharge, power performance, and energy density can be improved simultaneously. Among them, the energy density increases with the increase of silicon-based material content in the first negative electrode active material layer, while not significantly affecting the battery's low-temperature performance. In addition, due to the large interlayer spacing of the high-power amorphous carbon material, the expansion during cycling is small, and the above structural design also effectively suppresses the expansion problem during battery cycling.

[0080] The embodiments of the present invention have been described above. However, the present invention is not limited to the above embodiments. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A negative electrode sheet, characterized in that, The device includes a negative electrode current collector, a first negative electrode active material layer located on at least one surface of the negative electrode current collector, and a second negative electrode active material layer located on the surface of the first negative electrode active material layer. The first negative electrode active material layer contains a first negative electrode active material, which comprises a silicon-based material and graphite. The second negative electrode active material layer contains a second negative electrode active material, which comprises an amorphous carbon material with a spherical structure. The average particle size D1 of the amorphous carbon material satisfies 2μm≤D1≤2.3μm. The amorphous carbon is used to provide ion entry and exit channels. The Raman spectrum Id / Ig peak ratio of the amorphous carbon material is 0.5-1.09; the X-ray diffraction analysis results of the amorphous carbon material show that the peak position of the diffraction peak is less than 22.445°. The following conditions must be met: H1≥D2, where H1 is the thickness of the first negative electrode active material layer and D2 is the maximum particle size of the first negative electrode active material; 16μm≤H1≤75μm; The following conditions must be met: H2≥D3, where H2 is the thickness of the second negative electrode active material layer and D3 is the maximum particle size of the second negative electrode active material. The particle size of the first negative electrode active material satisfies: D2≤30μm; The specific surface area of ​​the amorphous carbon material is 4-23 m². 2 / g; the specific capacity of the amorphous carbon material is 270-360mAh / g.

2. The negative electrode sheet according to claim 1, characterized in that, In the first negative electrode active material, the mass content of silicon-based material is 0.5-15%.

3. The negative electrode sheet according to claim 1 or 2, characterized in that, The silicon-based material includes at least one of silicon materials, silicon-oxygen materials, and silicon-carbon materials.

4. The negative electrode sheet according to claim 1, characterized in that, The thickness of the second negative electrode active material is H2, where 2μm≤H2≤65μm; And / or, The particle size of the second negative electrode active material satisfies: D3≤15μm, where D3 is the maximum particle size of the second negative electrode active material.

5. The negative electrode sheet according to claim 1, characterized in that, The first negative electrode active material layer comprises a conductive agent, a binder, a thickener, and the first negative electrode active material, wherein the mass content of the first negative electrode active material is 88-98%, the mass content of the conductive agent is 0-5%, the mass content of the binder is 0.5-6%, and the mass content of the thickener is 0.5-3%. And / or, The second negative electrode active material layer comprises a conductive agent, a binder, a thickener, and the second negative electrode active material, wherein the mass content of the second negative electrode active material is 88-98%, the mass content of the conductive agent is 0-5%, the mass content of the binder is 0.5-6%, and the mass content of the thickener is 0.5-3%.

6. An electrochemical device, characterized in that, Includes the negative electrode sheet as described in any one of claims 1-5.

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