Negative pole piece, secondary battery and electronic equipment

Abstract

The invention provides a negative pole piece, a secondary battery and electronic equipment. The negative electrode plate comprises a negative electrode current collector and negative electrode film layers, and the negative electrode film layers comprise a first negative electrode film layer and a second negative electrode film layer; the first negative electrode film layer is positioned on at least one surface of the negative electrode current collector and comprises a first negative electrode active material and a first gel polymer; the second negative electrode film layer is located on the surface, away from the negative electrode current collector, of the first negative electrode film layer, the second negative electrode film layer comprises a second negative electrode active material and a second gel polymer, the ionic conductivity sigma of the second gel polymer is larger than or equal to 0.1 mS / cm and smaller than or equal to 10 mS / cm; the porosity of the first negative electrode film layer is phi 1, the porosity of the second negative electrode film layer is phi 2, and phi 1 is smaller than phi 2. The first negative electrode film layer in the negative electrode plate has high compaction density and improved electrolyte wettability, and the second negative electrode film layer has high porosity and high ionic conductivity, so that the secondary battery has high energy density and high dynamic performance, and also has improved cycle performance and low expansion rate.

Description

Technical Field

[0001] This application relates to the technical field of batteries, and particularly to a negative electrode sheet, a secondary battery, and an electronic device. Background Art

[0002] Lithium-ion batteries are widely used in digital electronic products, energy storage, drones, power tools, electric vehicles and other products due to their high energy density, long cycle life, high safety, fast charging ability and other characteristics. At present, in order to balance high energy density and high kinetic performance, the double-layer coating technology is adopted for the negative electrode of lithium-ion batteries. High-compaction-density graphite is coated on the lower layer of the negative electrode to achieve high energy density, and low-density graphite is coated on the upper layer to provide a higher porosity to achieve high-rate high-current fast charging. However, due to the different requirements of the upper and lower layers of different types of main materials for the electrolyte, it is difficult for the electrolyte to be compatible between the upper and lower layers of the negative electrode, resulting in an increase in the irreversible consumption of the electrolyte and affecting the cycle performance of the secondary battery. Summary of the Invention

[0003] This application provides a negative electrode sheet, a secondary battery, and an electronic device.

[0004] In a first aspect, this application provides a negative electrode sheet, which includes a negative electrode current collector and a negative electrode film layer. The negative electrode film layer includes a first negative electrode film layer and a second negative electrode film layer; the first negative electrode film layer is located on at least one surface of the negative electrode current collector, and the first negative electrode film layer includes a first negative electrode active material and a first gel polymer; the second negative electrode film layer is located on the surface of the first negative electrode film layer away from the negative electrode current collector, and the second negative electrode film layer includes a second negative electrode active material and a second gel polymer. The ionic conductivity of the second gel polymer is σ, and 0.1 mS / cm ≤ σ ≤ 10 mS / cm; the porosity of the first negative electrode film layer is The porosity of the second negative electrode film layer is By adding gel polymers to the first negative electrode film layer and the second negative electrode film layer of the negative electrode sheet respectively and controlling the porosities of the first negative electrode film layer and the second negative electrode film layer, the first negative electrode film layer can have a high compaction density and improved electrolyte wettability, and the second negative electrode film layer can have a high porosity and high ionic conductivity. Thus, while the secondary battery has a high energy density and high kinetic performance, it also takes into account improved cycle performance and low expansion rate.

[0005] In some embodiments, Thus, the negative electrode sheet can take into account a relatively high compaction density and electrolyte wettability, and the secondary battery can take into account a relatively high energy density and improved cycle performance and kinetic performance.

[0006] In some embodiments, Thus, the secondary battery can have a high energy density while improving its cycle performance.

[0007] In some embodiments, the secondary battery can have high kinetic performance while improving its cycling performance.

[0008] In some embodiments, the thickness of the first negative electrode film layer is H1, and the thickness of the second negative electrode film layer is H2, where 1 ≤ H1 / H2 ≤ 2. Thus, the negative electrode sheet can have a relatively high compaction density while taking into account good electrolyte wettability, enabling the secondary battery to have both high energy density and high kinetic performance.

[0009] In some embodiments, the peel strength F between the negative electrode film layer and the negative electrode current collector is 5 N / m - 50 N / m. This can reduce the risk of rupture and detachment of the negative electrode film layer and improve the cycling performance of the secondary battery.

[0010] In some embodiments, the Young's modulus E of the standard gel film made of the first gel polymer is 0.5 MPa - 4 MPa. This can reduce the swelling deformation of the negative electrode sheet during the insertion / extraction process of active ions and further reduce the swelling rate of the secondary battery.

[0011] In some embodiments, the first gel polymer includes one or more of polyethylene glycol dimethacrylate gel, poly(hydroxypropyl methacrylate) gel, poly(hydroxyethyl acrylate) gel, poly(hydroxypropyl acrylate) gel, and polyacrylonitrile gel.

[0012] In some embodiments, the second gel polymer includes one or more of polymethyl methacrylate gel, polyacrylic acid gel, poly(N-isopropylacrylamide) gel, poly(hydroxyethyl methacrylate) gel, poly(N,N-dimethylacrylamide) gel, and poly(2-hydroxyethyl methacrylate) gel.

[0013] In some embodiments, the compaction density of the first negative electrode film layer is PD1 g / cm 3 , and based on the total mass of the first negative electrode film layer, the mass percentage of the first gel polymer is W1%, where 0.57 ≤ W1 / PD1 ≤ 2.86.

[0014] In some embodiments, the compaction density of the second negative electrode film layer is PD2 g / cm 3 , and based on the total mass of the second negative electrode film layer, the mass percentage of the second gel polymer is W2%, where 1 ≤ W2 / PD2 ≤ 5. This can better disperse the swelling stress of the first negative electrode film layer in the high compaction density state, reduce the swelling of the negative electrode sheet, and further reduce the swelling rate of the secondary battery.

[0015] In some embodiments, 1 ≤ W1 ≤ 5; optionally, 1 ≤ W1 ≤ 4.

[0016] In some embodiments, 1 ≤ W2 ≤ 5; optionally, 1 ≤ W2 ≤ 3.

[0017] In some embodiments, the first negative electrode active material includes a graphite material, and the tap density of the graphite material is PD3 g / cm 3 , 2.2 ≤ PD3 ≤ 2.5. This can enable the second negative electrode film layer to have a better porosity and further improve the kinetic performance of the secondary battery.

[0018] In some embodiments, the second negative electrode active material includes a silicon-containing material. Based on the total mass of the second negative electrode film layer, the mass percentage of silicon element is 5% - 50%. This can improve the charging rate and the kinetic performance of the secondary battery.

[0019] In a second aspect, the present application provides a secondary battery, including the negative electrode sheet of the first aspect embodiment of the present application. The secondary battery of the present application satisfies the above characteristics, and can enable the secondary battery to have high energy density and high kinetic performance while taking into account improved cycle performance and low expansion rate.

[0020] In a third aspect, the present application provides an electronic device, including the secondary battery of the second aspect embodiment of the present application.

[0021] Advantages of the present application:

[0022] By adding gel polymers to the upper layer and the lower layer of the negative electrode sheet respectively and controlling the porosity of the first negative electrode film layer and the second negative electrode film layer, the present application can reduce the consumption of the electrolyte while maintaining high kinetic performance in the upper layer of the negative electrode sheet, and enable the lower layer of the negative electrode sheet to have high energy density and improved electrolyte wettability while reducing the expansion of the lower layer of the negative electrode sheet; thus, the secondary battery can have high energy density and high kinetic performance while taking into account improved cycle performance and low expansion rate. Specific embodiments

[0023] The embodiments or implementation schemes in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments.

[0024] In the description of this specification, the description with reference to terms such as "one embodiment", "some embodiments", "illustrative embodiments", "examples", "specific examples", or "some examples" means that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0025] In addition, the terms "first" and "second" are for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, features defined with "first" and "second" may explicitly or implicitly include at least one such feature. In the description of this application, "a plurality" means at least two, such as two, three, etc., unless otherwise specifically defined.

[0026] In the description of this application, "room temperature" refers to 10°C to 35°C.

[0027] In this application, the battery may include a lithium-ion secondary battery, a lithium-ion primary battery, a lithium-sulfur battery, a sodium-lithium ion, a sodium-ion battery, a magnesium-ion battery, etc., and the embodiments of this application do not limit this. The battery may be in a cylindrical shape, a flat shape, a cuboid shape or other shapes, etc., and the embodiments of this application do not limit this either.

[0028] As described in the background art section, the double-layer coating technology can better meet the different performance requirements of the upper and lower layers of the negative electrode sheet. For example, the upper layer is coated with a low bulk density material (such as low bulk density graphite) and / or Si material, and the high specific capacity of Si and / or the high porosity of low bulk density graphite are used to improve the upper layer kinetics performance, thereby achieving high-rate high-current fast charging; the lower layer is coated with a high bulk density material (such as high bulk density graphite) to achieve the high energy density of the secondary battery, so that the secondary battery takes into account both high kinetics performance and high energy density. Since the upper and lower layers of the negative electrode sheet use different types of active materials, their requirements for the electrolyte are different. The high-kinetics materials in the upper layer will continuously consume the electrolyte during the charge and discharge cycles of the secondary battery as the SEI film (solid electrolyte interface film) is formed and repaired, resulting in a continuous reduction in the electrolyte components required for the lower layer and poor wettability, affecting the cycle performance of the secondary battery. In addition, the high bulk density characteristics of the lower layer result in more contact sites between the active material particles, and the uneven dispersion of the swelling stress in the lower layer during the insertion / extraction process of the active ions will cause serious swelling of the secondary battery.

[0029] In view of this, the embodiments of this application provide a negative electrode sheet. By adding gel polymers to the upper and lower layers of the negative electrode film layer respectively and controlling the porosity of the first negative electrode film layer and the second negative electrode film layer, the upper layer of the secondary battery can have high kinetics characteristics while reducing the consumption of the electrolyte, and at the same time, the swelling pressure of the lower layer can be evenly dispersed, so that the secondary battery has improved cycle performance and a lower swelling rate.

[0030] [Negative electrode sheet]

[0031] An embodiment of the first aspect of the present application provides a negative electrode plate, which includes a negative electrode current collector and a negative electrode film layer. The negative electrode film layer includes a first negative electrode film layer and a second negative electrode film layer; the first negative electrode film layer is located on at least one surface of the negative electrode current collector, and the first negative electrode film layer includes a first negative electrode active material and a first gel polymer; the second negative electrode film layer is located on the surface of the first negative electrode film layer away from the negative electrode current collector, and the second negative electrode film layer includes a second negative electrode active material and a second gel polymer; the porosity of the first negative electrode film layer is The porosity of the second negative electrode film layer is

[0032] According to the embodiments of the present application, by making the porosity of the first negative electrode film layer lower than that of the second negative electrode film layer, the first negative electrode film layer located in the lower layer of the negative electrode plate has a higher compaction density, which is beneficial to making the secondary battery have a high energy density; the second negative electrode film layer located in the upper layer of the negative electrode plate has a higher porosity, which is beneficial to the infiltration of the electrolyte and the rapid transmission of active lithium ions, and can improve the kinetic performance of the secondary battery. On this basis, in the second negative electrode film layer, the second gel polymer can coat at least part of the surface of the second negative electrode active material, thereby reducing the contact sites between the second negative electrode active material and the electrolyte, reducing the consumption of the electrolyte by the upper layer of the negative electrode plate, and then improving the problem of lack of liquid in the lower layer of the negative electrode plate, improving the electrolyte infiltration of the lower layer of the negative electrode plate, and improving the cycle performance of the secondary battery; and the second gel polymer has a high ionic conductivity, which can make up for the kinetic loss caused by the reduction of the contact sites between the second negative electrode film layer and the electrolyte, and keep the negative electrode plate with high kinetic performance. At the same time, by adding a first gel polymer with a spatial network structure to the first negative electrode film layer, the spatial network structure of the first gel polymer has a good liquid locking effect, can adsorb and fix the electrolyte, and then improve the problem of lack of liquid in the lower layer of the negative electrode plate, so that the lower layer of the negative electrode plate maintains good electrolyte infiltration, thereby improving the cycle performance of the secondary battery; and the first gel polymer can coat at least part of the first negative electrode active material, thereby evenly dispersing the expansion stress in the lower layer of the negative electrode plate, uniformly dissipating the expansion stress during the insertion / extraction process of active ions, reducing the cracking of the negative electrode plate, reducing the overall expansion rate of the negative electrode plate, and further reducing the expansion rate of the secondary battery.

[0033] In this application, both the first gel polymer and the second gel polymer refer to organic polymers in a gel form. Such organic polymers in a gel form have a three-dimensional network structure, in which media such as electrolytes can be accommodated. Both the first gel polymer and the second gel polymer can be obtained by polymerization of corresponding gel monomers. For example, the corresponding gel monomers can be separately added to the first negative electrode film layer and the second negative electrode film layer, and then the corresponding gel polymers are formed in the first negative electrode film layer and the second negative electrode film layer by initiating polymerization. It can be understood that the gel monomers in this application refer to organic small molecules that can be polymerized to form gel polymers, and the methods for initiating the polymerization of gel monomers can adopt methods known in the art, such as photoinitiated polymerization, thermally initiated polymerization, initiator-initiated polymerization, etc.

[0034] In some embodiments, the porosity of the first negative electrode film layer and the porosity of the second negative electrode film layer can satisfy: As an example, the value can be 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, or a range composed of any of the above numerical values. Optionally,

[0035] According to the embodiments of the present application, the first negative electrode film layer having a lower porosity can make the lower layer of the negative electrode sheet have a higher compaction density, thereby improving the energy density of the secondary battery; the second negative electrode film layer having a higher porosity can make the negative electrode sheet have a better electrolyte wetting effect and active ion transport rate, which can improve the kinetic performance and cycling performance of the secondary battery. By controlling the ratio of the porosities of the first negative electrode film layer and the second negative electrode film layer within the above range, the negative electrode sheet can have both a higher compaction density and electrolyte wettability, thereby enabling the secondary battery to have both a higher energy density and improved cycling performance and kinetic performance.

[0036] In some embodiments, the porosity of the first negative electrode film layer can satisfy As an example, it can be 5%, 8%, 10%, 12%, 15%, 18%, 20%, 22%, 25%, or a range composed of any of the above numerical values. Optionally, it is

[0037] According to the embodiments of the present application, by limiting the porosity of the first negative electrode film layer within the above range, the first negative electrode film layer can have a higher compaction density and can have a better electrolyte wetting property, which can make the secondary battery have a high energy density while improving its cycling performance.

[0038] In some embodiments, the porosity of the second negative electrode film layer can satisfy As an example, it can be 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, or a range composed of any of the above values. Optionally,

[0039] According to the embodiments of the present application, by limiting the porosity of the second negative electrode film layer within the above range, the negative electrode sheet can have better electrolyte wettability and ion transport performance. At the same time, the negative electrode sheet can maintain an appropriate compaction density, enabling the secondary battery to have high kinetic performance while improving its cycling performance.

[0040] In the present application, the porosities of the first negative electrode film layer and the second negative electrode film layer have meanings well known in the art, and can be measured by methods and instruments known in the art. For example, reference can be made to GB / T21650.1-2008 and a mercury intrusion porosimeter can be used for testing.

[0041] In some embodiments, the thickness of the first negative electrode film layer is H1, and the thickness of the second negative electrode film layer is H2. H1 and H2 can satisfy: 1≤H1 / H2≤2. As an example, the ratio of H1 / H2 can be 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, or a range composed of any of the above values.

[0042] According to the embodiments of the present application, by limiting the ratio of the thicknesses of the first negative electrode film layer and the second negative electrode film layer within the above range, the negative electrode sheet can have a high compaction density while having good electrolyte wettability, enabling the secondary battery to balance high energy density and high kinetic performance.

[0043] In some embodiments, the thickness H1 of the first negative electrode film layer can satisfy: 39μm≤H1≤58.8μm. As an example, the thickness H1 of the first negative electrode film layer can be 39μm, 40μm, 41μm, 42μm, 43μm, 44μm, 45μm, 46μm, 47μm, 48μm, 49μm, 50μm, 51μm, 52μm, 53μm, 54μm, 55μm, 56μm, 57μm, 58μm, 58.5μm, 58.8μm, or a range composed of any of the above values. Optionally, 45μm≤H1≤54μm.

[0044] In some embodiments, the thickness H2 of the second negative electrode film layer may satisfy: 20.5 μm ≤ H2 ≤ 39 μm. As an example, the thickness H2 of the second negative electrode film layer may be 20.5 μm, 21 μm, 22 μm, 23 μm, 24 μm, 25 μm, 26 μm, 27 μm, 28 μm, 29 μm, 30 μm, 31 μm, 32 μm, 33 μm, 34 μm, 35 μm, 36 μm, 37 μm, 38 μm, 39 μm, or a range composed of any of the above values. Optionally, 22.5 μm ≤ H2 ≤ 28.4 μm.

[0045] In the present application, the thicknesses of the first negative electrode film layer and the second negative electrode film layer can be measured by methods and instruments known in the art. For example, it can be measured and analyzed by observing the cross-sectional SEM (scanning electron microscope) image of the negative electrode sheet, and a laser thickness gauge can also be used for measurement. In the present application, the cross-section of the negative electrode sheet can be subjected to microscopic morphology detection, and the interfaces between the layers in the negative electrode sheet can be observed to determine the thicknesses of the layers. In the present application, the cross-section of the negative electrode sheet refers to the cross-section formed by slicing along the thickness direction of the negative electrode sheet. Instruments such as a focused electron beam (FIB) electron microscope (such as the FEI Scios2HiVa device, etc.) and an ion cross-section polisher (such as the IB-09010CP type argon ion cross-section polisher of JEOL Co., Ltd., etc.) can be used to polish the cross-section to obtain a clear cross-section. In addition, microscopic morphology observation and composition analysis can be performed on the cross-section of the negative electrode sheet, such as energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) analysis, etc., to determine the element types to confirm the element composition in each layer of the negative electrode sheet.

[0046] In some embodiments, the peel strength F between the negative electrode film layer and the negative electrode current collector can be 5 N / m - 50 N / m. As an example, F can be 5 N / m, 10 N / m, 15 N / m, 20 N / m, 25 N / m, 30 N / m, 35 N / m, 40 N / m, 45 N / m, 50 N / m, or a range composed of any of the above values. Optionally, F can be 10 N / m - 40 N / m.

[0047] According to the embodiments of the present application, when the peel strength between the negative electrode film layer and the negative electrode current collector is within the above range, a relatively high bonding strength can be achieved between the negative electrode film layer and the negative electrode current collector, thereby further evenly dispersing the expansion stress of the negative electrode sheet, reducing the expansion of the secondary battery, and at the same time reducing the risk of rupture and detachment of the negative electrode film layer, and improving the cycling performance of the secondary battery.

[0048] In this application, the peel strength between the negative electrode film layer and the negative electrode current collector can be further calculated by measuring the force required to peel the negative electrode film layer from the negative electrode current collector, and can be measured using methods and instruments known in the art. For example, a tensile testing machine can be used for measurement, and the tensile speed can be 5 mm / min.

[0049] It can be understood that the bonding performance of the first gel polymer can be adjusted by changing parameters such as the type, molecular weight, and crosslinking degree of the first gel polymer, and thus the peel strength between the negative electrode film layer and the negative electrode current collector can be adjusted.

[0050] In some embodiments, the Young's modulus E of the standard adhesive film made of the first gel polymer can be 0.5 MPa - 4 MPa. As an example, E can be 0.5 MPa, 0.6 MPa, 0.7 MPa, 0.8 MPa, 0.9 MPa, 1.0 MPa, 1.2 MPa, 2.0 MPa, 2.5 MPa, 3.0 MPa, 3.5 MPa, 4.0 MPa, or a range composed of any of the above values. Optionally, E can be 1 MPa - 3 MPa.

[0051] According to the embodiments of the present application, when the Young's modulus of the standard adhesive film made of the first gel polymer is within the above range, the first gel polymer has a good ability to resist elastic deformation. Adding it to the first negative electrode film layer can enable the first negative electrode film layer to better disperse the swelling stress, reduce the swelling deformation of the negative electrode plate during the insertion / extraction process of active ions, and further reduce the swelling rate of the secondary battery.

[0052] In some embodiments, the first gel polymer may include one or more of polyethylene glycol dimethacrylate gel, poly(hydroxypropyl methacrylate) gel, poly(hydroxyethyl acrylate) gel, poly(hydroxypropyl acrylate) gel, and polyacrylonitrile gel.

[0053] In some embodiments, the ionic conductivity σ of the second gel polymer can satisfy: 0.1 mS / cm ≤ σ ≤ 10 mS / cm. As an example, σ can be 0.1 mS / cm, 0.2 mS / cm, 0.3 mS / cm, 0.4 mS / cm, 0.5 mS / cm, 0.6 mS / cm, 0.7 mS / cm, 0.8 mS / cm, 0.9 mS / cm, 1.0 mS / cm, 2.0 mS / cm, 3.0 mS / cm, 4.0 mS / cm, 5.0 mS / cm, 6.0 mS / cm, 7.0 mS / cm, 8.0 mS / cm, 9.0 mS / cm, 10.0 mS / cm, or a range composed of any of the above values. Optionally, 0.5 mS / cm ≤ σ ≤ 5 mS / cm.

[0054] According to the embodiments of the present application, limiting the ionic conductivity of the second gel polymer within the above range, it has high ionic conductivity, which is beneficial to further improving the ionic transport performance of the negative electrode sheet, and thus improving the kinetic performance of the secondary battery.

[0055] In the present application, the ionic conductivity of the second gel polymer can be measured by methods and instruments known in the art. For example, the alternating current impedance spectroscopy method can be used. The second gel polymer is made into a standard film, and a symmetrical electrode (such as a stainless steel electrode) is used to clamp the film for testing, and the ionic conductivity of the second gel polymer is calculated according to the test results.

[0056] In some embodiments, the second gel polymer may include one or more of polymethyl methacrylate gel, polyacrylic acid gel, poly(N-isopropylacrylamide) gel, poly(2-hydroxyethyl methacrylate) gel, poly(N,N-dimethylacrylamide) gel, and poly(2-hydroxyethyl methacrylate) gel.

[0057] For the negative electrode sheet provided by the embodiments of the present application, by adding a first gel polymer with high viscosity to the first negative electrode film layer in the lower layer and a second gel polymer with high ionic conductivity to the second negative electrode film layer in the upper layer, the negative electrode sheet and the negative electrode current collector can have high bonding strength, reducing the rupture and shedding of the negative electrode film layer; the liquid absorption and locking ability of the first gel polymer can improve the wettability of the electrolyte in the lower layer of the negative electrode sheet, thereby improving the cycle performance of the secondary battery; the high elastic modulus of the first gel polymer can evenly disperse the expansion stress of the first negative electrode active material during the insertion / extraction of active ions, thereby reducing the expansion of the negative electrode film layer and lowering the expansion rate of the secondary battery.

[0058] The second gel polymer in the second negative electrode film layer reduces the contact sites between the second negative electrode active material and the electrolyte by coating the second negative electrode active material, thereby reducing the consumption of the electrolyte by the upper layer of the negative electrode sheet, thus improving the problem of lack of liquid in the lower layer of the negative electrode sheet, and further improving the cycle performance of the secondary battery; while the high ionic conductivity characteristic of the second gel polymer can increase the ionic transport rate of the upper layer of the negative electrode sheet, which can make up for the loss of kinetic performance caused by the reduction of the contact sites between the active material in the upper layer of the negative electrode sheet and the electrolyte, so that the secondary battery maintains high kinetic performance. In addition, the second gel polymer can adsorb the side reaction products free in the electrolyte, thereby reducing the cross-talk of the side reaction products between the positive and negative electrodes of the secondary battery and reducing the side reactions inside the secondary battery. Therefore, by adding gel polymers with different characteristics to the upper and lower layers of the negative electrode sheet and controlling different porosities of the upper and lower layers in the embodiments of the present application, the secondary battery can have improved cycle performance and low expansion rate while taking into account high energy density and high kinetic performance.

[0059] It can be understood that parameters such as the Young's modulus and ionic conductivity of the first gel polymer and / or the second gel can be adjusted by changing the structural data such as the type, molecular weight, crosslinking degree, and crystallinity of the first gel polymer and / or the second gel. In the embodiments of the present application, if parameters such as the Young's modulus and ionic conductivity of the first gel polymer and / or the second gel change, it can be considered that the above changes are made by adjusting at least one of the parameters such as the type, molecular weight, crosslinking degree, and crystallinity of the first gel polymer and / or the second gel.

[0060] In some embodiments, the tap density of the first negative electrode film layer is PD1 g / cm 3 , based on the total mass of the first negative electrode film layer, the mass percentage of the first gel polymer is W1%, and PD1 and W1 can satisfy: 0.57 ≤ W1 / PD1 ≤ 2.86. As an example, the value of W1 / PD1 can be 0.57, 0.6, 0.7, 0.8, 0.9, 1.0, 1.5, 2.0, 2.5, 2.8, 2.86, or the range composed of any of the above values. Optionally, 1.5 ≤ W1 / PD1 ≤ 2.3.

[0061] According to the embodiments of the present application, since the first negative electrode film layer needs to have a high tap density to meet the requirements of high energy density of the secondary battery, therefore, limiting the ratio of the content of the first gel polymer in the first negative electrode film layer to the tap density of the first negative electrode film layer within the above range can make the first negative electrode film layer have an appropriate content of the gel polymer, thereby better dispersing the expansion stress of the first negative electrode film layer in the high tap density state, reducing the expansion of the negative electrode sheet, and further reducing the expansion rate of the secondary battery.

[0062] In some embodiments, W1 can satisfy: 1 ≤ W1 ≤ 5. For example, W1 can be 1, 2, 3, 4, 5, or the range composed of any of the above values. Optionally, 1 ≤ W1 ≤ 4.

[0063] According to the embodiments of the present application, when the mass percentage of the first gel polymer in the first negative electrode film layer is within the above range, the first gel polymer can better disperse the expansion stress of the first negative electrode active material, reducing the cyclic expansion of the first negative electrode film layer; at the same time, the first negative electrode film layer can have a high content of active material, enabling the secondary battery to have a high energy density.

[0064] In some embodiments, the tap density of the first negative electrode film layer can be 1.6 g / cm 3 -2 g / cm 3 . As an example, the tap density of the first negative electrode film layer can be 1.6 g / cm 3 , 1.7 g / cm 3 , 1.8 g / cm 3 , 1.9 g / cm3 、 2.0 g / cm 3 , or a range composed of any of the above values.

[0065] According to the embodiments of the present application, limiting the compaction density of the first negative electrode film layer within the above range is beneficial to further improving the energy density of the secondary battery.

[0066] In some embodiments, the compaction density of the second negative electrode film layer is PD2 g / cm 3 , based on the total mass of the second negative electrode film layer, the mass percentage of the second gel polymer is W2%, and PD2 and W2 can satisfy: 1 ≤ W2 / PD2 ≤ 5. As an example, the value of W2 / PD2 can be 1, 2, 3, 4, 5, or a range composed of any of the above values. Optionally, 1.2 ≤ W2 / PD2 ≤ 4.

[0067] According to the embodiments of the present application, the second negative electrode film needs to have a lower compaction density to improve the electrolyte wettability, thereby meeting the requirements of high kinetic performance of the secondary battery. Therefore, limiting the ratio of the content of the second gel polymer in the second negative electrode film layer to the compaction density of the second negative electrode film layer within the above range can make the second negative electrode film layer have a better porosity and further improve the kinetic performance of the secondary battery.

[0068] In some embodiments, W2 can satisfy: 1 ≤ W2 ≤ 5. As an example, W2 can be 1, 2, 3, 4, 5, or a range composed of any of the above values. Optionally, 1 ≤ W2 ≤ 3.

[0069] According to the embodiments of the present application, the mass percentage of the second gel polymer in the second negative electrode film layer within the above range can make the second negative electrode film layer have a higher ion transport rate, which is beneficial to further improving the electrolyte wettability on the upper layer of the negative electrode sheet and enhancing the kinetic performance of the secondary battery.

[0070] In some embodiments, the compaction density of the second negative electrode film layer can be 0.8 g / cm 3 - 1.6 g / cm 3 . As an example, the compaction density of the second negative electrode film layer can be 0.8 g / cm 3 、 0.9 g / cm 3 、 1.0 g / cm 3 、 1.0 g / cm 3 、 1.2 g / cm 3 、 1.3 g / cm 3 、 1.4 g / cm 3 、 1.5 g / cm 3 、 1.6 g / cm 3 , or a range composed of any of the above values.

[0071] According to the embodiments of the present application, limiting the compaction density of the second negative electrode film layer within the above range is beneficial to the upper layer of the negative electrode sheet having a higher porosity, which can further improve the electrolyte wettability of the negative electrode sheet and improve the kinetic performance of the secondary battery.

[0072] In the present application, the first negative electrode film layer and the second negative electrode film layer can be cold-pressed simultaneously after coating. Therefore, the compaction density of the first negative electrode film layer and the compaction density of the second negative electrode film layer will change simultaneously with the cold-pressing pressure. That is, the compaction density of the first negative electrode film layer / second negative electrode film layer at different gel polymer contents can be adjusted by adjusting the cold-pressing pressure.

[0073] In the present application, the compaction densities of the first negative electrode film layer and the second negative electrode film layer have the well-known meanings in the art, and can be measured by the methods and instruments known in the art. For example, the compaction density can be calculated by the ratio of the areal density to the thickness of the corresponding film layer.

[0074] In some embodiments, the first negative electrode active material includes a graphite material, and the compaction density of the graphite material is PD3 g / cm 3 , 2.2 g / cm 3 ≤PD3≤2.5 g / cm 3 . As an example, the compaction density of the graphite material can be 2.20 g / cm 3 , 2.22 g / cm 3 , 2.25 g / cm 3 , 2.26 g / cm 3 , 2.28 g / cm 3 , 2.30 g / cm 3 , 1.32 g / cm 3 , 2.35 g / cm 3 , 2.38 g / cm 3 , 2.40 g / cm 3 , 2.42 g / cm 3 , 2.45 g / cm 3 , 2.48 g / cm 3 , 2.50 g / cm 3 , or the range composed of any of the above values.

[0075] According to the embodiments of the present application, compared with ordinary graphite, the graphite material with a high compaction density can have a higher specific capacity, which is beneficial to further improving the energy density of the secondary battery. In the present application, the compaction density can be improved by performing high-pressure treatment on the graphite material.

[0076] In this application, the tap density of the graphite material refers to the density value obtained by compacting the graphite under a certain pressure and can be measured by methods and instruments known in the art. For example, the tap density can be measured with reference to Standard GB / T 24533-2009 using an electronic pressure testing machine (such as UTM7305). An exemplary testing method is as follows: Weigh 1 g of the positive electrode active material and add it to a mold with a bottom area of 1.327 cm 2 . Apply a certain pressure and hold it for 30 s, then release the pressure and keep it for 10 s, and then record and calculate the powder tap density of the positive electrode active material under this pressure. In this application, the tap density of the graphite material is measured under the cold pressing pressure of the negative electrode sheet, and the cold pressing pressure can be 20 tons to 80 tons.

[0077] In some embodiments, the second negative electrode active material may include a silicon-containing material.

[0078] It can be understood that the silicon-containing material refers to a negative electrode active material containing silicon element. As an example, the silicon-containing material may include, but is not limited to, one or more of silicon-carbon composite materials, silicon-oxygen composite materials, and silicon-nitrogen composite materials. For example, in the silicon-carbon composite material used in this application, based on the mass of the silicon-carbon composite material, the mass percentage content of silicon element is 30% to 70%, and the mass percentage content of carbon element is 30% to 70%. There is no particular limitation on the silicon-carbon composite material in this application as long as the purpose of this application can be achieved. For example, the silicon-carbon composite material can be a composite material obtained by deposition. Exemplarily, the silicon-carbon composite material can be a composite material with silicon material deposited on a carbon skeleton or carbon material deposited on a silicon skeleton. The silicon-oxygen composite material includes silicon monoxide SiO x (0 < x < 2); the silicon-nitrogen composite material may include SiN y (0.5 < y < 1.3)).

[0079] According to the embodiments of this application, the silicon-containing material has a high specific capacity, which is beneficial to improving the energy density of the secondary battery. At the same time, it can reduce the polarization voltage, reduce the risk of lithium deposition on the negative electrode, increase the charging rate, and improve the kinetic performance of the secondary battery.

[0080] In some embodiments, based on the total mass of the second negative electrode active material, the mass content of the silicon-containing material can be 3% - 100%. As an example, based on the total mass of the second negative electrode active material, the mass content of the silicon-containing material can be 3%, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, or a range composed of any of the above values.

[0081] According to the embodiments of this application, by limiting the content of the silicon-containing material in the second negative electrode active material within the above range, it is beneficial to further improve the energy density of the secondary battery.

[0082] In some embodiments, based on the total mass of the second negative electrode film layer, the mass content of silicon element can be 5% - 50%. As an example, based on the total mass of the second negative electrode film layer, the mass content of silicon element can be 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or a range composed of any of the above values.

[0083] According to the embodiments of the present application, by limiting the content of silicon element in the second negative electrode film layer within the above range, the second negative electrode film layer can have a higher capacity, which is beneficial to further improving the energy density of the secondary battery.

[0084] In some embodiments, the first negative electrode active material and the second negative electrode active material can also each independently include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, tin-based materials, and lithium titanate, etc. The tin-based material can be selected from at least one of elemental tin, tin oxides, and tin alloys. However, the present application is not limited to these materials, and other conventional materials that can be used as the negative electrode active material of the battery can also be used. These negative electrode active materials can be used alone or in combination of two or more.

[0085] In some embodiments, the negative electrode film layer can further include a binder. The binder can be a commonly used binder in the art, and there is no specific limitation on the specific type.

[0086] As an example, the binder can include at least one of polyvinylidene fluoride, vinylidene fluoride - hexafluoropropylene copolymer, polyhexafluoropropylene, polytetrafluoropropylene, polytrifluoropropylene, polyhexafluorobutadiene, polyhexafluoroisobutene, polytrifluoroethylene, polytrifluorochloroethylene, polytetrafluoroethylene, hydroxyalkyl methyl cellulose, styrene-butadiene rubber, fluorine rubber, and ethylene propylene diene. The alkyl group in the hydroxyalkyl methyl cellulose includes at least one of methyl, ethyl, propyl, and butyl.

[0087] In some embodiments, the negative electrode film layer can further include a conductive agent. The conductive agent can be a commonly used conductive agent in the art, and there is no specific limitation on the specific type.

[0088] As an example, the conductive agent can include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0089] In some embodiments, the negative electrode current collector can include a commonly used current collector in the art, and there is no limitation on the specific type.

[0090] Exemplarily, the negative electrode current collector can be a metal foil or a composite current collector. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (such as copper, copper alloy, aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0091] As an example, the negative electrode current collector can be a copper foil.

[0092] [Secondary battery]

[0093] Embodiments of the second aspect of the present application provide a secondary battery, which includes a positive electrode plate, a negative electrode plate, a separator, and an electrolyte. The negative electrode plate includes the negative electrode plate provided by the embodiments of the first aspect of the present application.

[0094] According to the present application, the negative electrode of the secondary battery is the negative electrode plate provided by the embodiments of the first aspect of the present application. The negative electrode plate has been described and explained in detail above and will not be repeated here. It can be understood that the secondary battery of the present application can achieve the beneficial effects of the first aspect of the present application.

[0095] Positive electrode sheet

[0096] The positive electrode plate includes a positive electrode current collector and a positive electrode film layer located on at least one surface of the positive electrode current collector. The positive electrode film layer includes a positive electrode active material.

[0097] As an example, the positive electrode active material may include at least one of the following materials: lithium phosphate with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, the present application is not limited to these materials, and other conventional materials that can be used as battery positive electrode active materials can also be used. These positive electrode active materials can be used alone or in combination of two or more. Among them, examples of lithium transition metal oxides may include but are not limited to lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (which can also be abbreviated as NCM 333 )、LiNi 0.5 Co 0.2 Mn 0.3 O2 (which can also be abbreviated as NCM 523 )、LiNi 0.5Co 0.25 Mn 0.25 O2 (which can also be abbreviated as NCM 211 )、LiNi 0.6 Co 0.2 Mn 0.2 O2 (which can also be abbreviated as NCM 622 )、LiNi 0.8 Co 0.1 Mn 0.1 O2 (which can also be abbreviated as NCM 811 )、lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co 0.15 Al 0.05 O2) and at least one of its modified compounds, etc. Examples of olivine-structured lithium-containing phosphates can include but are not limited to lithium iron phosphate (such as LiFePO4 (which can also be abbreviated as LFP)), composites of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), composites of lithium manganese phosphate and carbon, lithium manganese iron phosphate, and at least one of composites of lithium manganese iron phosphate and carbon.

[0098] In some embodiments, the positive electrode film layer may further include a binder. The binder can be a commonly used binder in the art, and there is no specific limitation on the specific type.

[0099] As an example, the binder can include at least one of polyvinylidene fluoride, vinylidene fluoride - hexafluoropropylene copolymer, hexafluoropropylene, tetrafluoropropylene, trifluoropropylene, hexafluorobutadiene, hexafluoro-isobutene, trifluoroethylene, trifluorochloroethylene, polytetrafluoroethylene, hydroxyalkyl methylcellulose, styrene-butadiene rubber, fluorine rubber, and ethylene propylene diene. The alkyl group in hydroxyalkyl methylcellulose includes at least one of methyl, ethyl, propyl, and butyl.

[0100] In some embodiments, the positive electrode film layer may further include a conductive agent. The conductive agent can be a commonly used conductive agent in the art, and there is no specific limitation on the specific type.

[0101] As an example, the conductive agent can include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes (single-walled carbon nanotubes, multi-walled carbon nanotubes, or few-walled carbon nanotubes), graphene, and carbon nanofibers.

[0102] In some embodiments, the current collector can include a commonly used current collector in the art, and there is no limitation on the specific type.

[0103] Exemplarily, the current collector can be a metal foil or a composite current collector. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (such as copper, copper alloy, aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0104] As an example, the positive current collector is an aluminum foil.

[0105] Separator

[0106] The separator is disposed between the positive electrode sheet and the negative electrode sheet, mainly serving to prevent short circuit between the positive and negative electrodes, and at the same time allowing active ions to pass through. The present application does not particularly limit the type of the separator, and any well-known porous structure separator with good chemical stability and mechanical stability can be selected.

[0107] In some embodiments, the material of the separator can be selected from one or more of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride, but is not limited thereto. Optionally, the material of the separator can include polyethylene and / or polypropylene. The separator can be a single-layer film or a multi-layer composite film. When the separator is a multi-layer composite film, the materials of each layer can be the same or different. In some embodiments, a ceramic coating or a metal oxide coating can also be provided on the separator.

[0108] Electrolyte

[0109] The electrolyte serves to conduct active ions between the positive electrode sheet and the negative electrode sheet. The electrolyte that can be used in the secondary battery of the present application can be an electrolyte known in the prior art.

[0110] In some embodiments, the electrolyte may include an organic solvent, an electrolyte salt, and an optional additive. The types of the organic solvent, lithium salt, and additive are not specifically limited and can be selected according to requirements.

[0111] In some embodiments, the secondary battery is a lithium-ion battery, and the electrolyte salt may include a lithium salt. As an example, the lithium salt includes, but is not limited to, at least one of LiPF6 (lithium hexafluorophosphate), LiBF4 (lithium tetrafluoroborate), LiClO4 (lithium perchlorate), LiFSI (lithium bis(fluorosulfonyl)imide), LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluoro(oxalato)borate), LiBOB (lithium bis(oxalato)borate), LiPO2F2 (lithium difluorophosphate), LiDODFP (lithium difluoro(dioxalato)phosphate), and LiOTFP (lithium tetrafluoro(oxalato)phosphate). The above lithium salts may be used alone or two or more of them may be used simultaneously.

[0112] In some embodiments, the secondary battery is a sodium-ion battery, and the electrolyte salt may include a sodium salt. As an example, the sodium salt may be selected from at least one of NaPF6, NaClO4, NaBCl4, NaSO3CF3, and Na(CH3)C6H4SO3.

[0113] In some embodiments, as an example, the organic solvent includes, but is not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), ethyl methyl sulfone (EMS), and diethyl sulfone (ESE). The above organic solvents may be used alone or two or more of them may be used simultaneously. Optionally, two or more of the above organic solvents are used simultaneously.

[0114] In some embodiments, the additive may include a negative electrode film-forming additive, a positive electrode film-forming additive, and may also include additives that can improve certain battery performance, such as additives that improve the overcharge performance of the battery, additives that improve the high-temperature or low-temperature performance of the battery, and the like.

[0115] As an example, the additive includes but is not limited to at least one of fluoroethylene carbonate (FEC), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), ethylene sulfate (DTD), propylene sulfate, ethylene sulfite (ES), 1,3 - propanesultone (PS), 1,3 - propenesultone (PST), sulfonate cyclic quaternary ammonium salt, succinic anhydride, succinonitrile (SN), adiponitrile (AND), tris(trimethylsilyl) phosphate (TMSP), and tris(trimethylsilyl) borate (TMSB).

[0116] In some embodiments, the electrolyte may include an initiator for initiating the polymerization of the gel monomer in the negative electrode sheet to form a gel polymer. As an example, the thermosensitive initiator generates free radicals in the range of 60°C - 100°C. By initiating the chain reaction at special sites of the polymer monomer, the monomer is polymerized to form a polymer network. By adding the initiator to the electrolyte and injecting the electrolyte into the secondary battery, the gel monomer in the negative electrode sheet will be initiated to polymerize at the appropriate initiation temperature, and then a gel polymer is formed in the negative electrode sheet.

[0117] The electrolyte can be prepared according to the conventional methods in the art. For example, an organic solvent, an electrolyte salt, and an optional additive can be mixed uniformly to obtain the electrolyte. There is no particular limitation on the addition order of each material. For example, the electrolyte salt and the optional additive are added to the organic solvent and mixed uniformly to obtain the electrolyte; or the electrolyte salt is first added to the organic solvent, and then the optional additive is added to the organic solvent and mixed uniformly to obtain the electrolyte.

[0118] [Preparation method of secondary battery]

[0119] The embodiment of the third aspect of the present application provides a preparation method of a secondary battery, which can be used to prepare the secondary battery of the embodiment of the second aspect of the present application.

[0120] The preparation method of the secondary battery may include the following steps:

[0121] S100, preparing a negative electrode sheet:

[0122] S110, respectively providing a first negative electrode slurry containing a first negative electrode active material and a first gel monomer, and a second negative electrode slurry containing a second negative electrode active material and a second gel monomer;

[0123] S120, using a dual - nozzle coater, simultaneously coating the first negative electrode slurry and the second negative electrode slurry on at least one surface of the negative electrode current collector to form a first negative electrode film layer and a second negative electrode film layer, such that the first negative electrode film layer is located between the negative electrode current collector and the second negative electrode film layer, and then obtaining the negative electrode sheet after drying, cold pressing, and slitting.

[0124] S200, provide an electrolyte solution containing a first initiator and a second initiator, where the first initiator is used to initiate the polymerization of a first gel monomer, and the second initiator is used to initiate the polymerization of a second gel monomer;

[0125] S300, stack the negative electrode plate, separator, and positive electrode plate in sequence and wind them to obtain an electrode assembly; or, stack the positive electrode plate, separator, and negative electrode plate in sequence with the separator located between the positive electrode plate and the negative electrode plate to obtain a laminated electrode assembly; place the electrode assembly in a housing, inject the electrolyte solution and then encapsulate it; let it stand at a first temperature to initiate the polymerization of the first gel monomer, and let it stand at a second temperature to initiate the polymerization of the second gel monomer to obtain a secondary battery.

[0126] According to the embodiments of the present application, when preparing a secondary battery, by adding the first gel monomer and the second gel monomer into the first negative electrode film layer and the second negative electrode film layer respectively, a raw material basis for forming a gel polymer through polymerization can be introduced into the first negative electrode film layer and the second negative electrode film layer. During the process of the electrolyte solution infiltrating the negative electrode plate after injecting the electrolyte solution, the initiator contained in the electrolyte solution can initiate the polymerization of the gel monomer at the corresponding initiation temperature to form the corresponding gel polymer. Thus, a first gel polymer can be formed in the first negative electrode film layer of the negative electrode plate, and a second gel polymer can be formed in the second negative electrode film layer. And because the gel monomer and the negative electrode active material are uniformly dispersed, the formed gel polymer can well coat the negative electrode active material, making the negative electrode active material uniformly dispersed in the spatial network structure of the gel polymer. Thus, the gel polymer can better disperse the swelling stress of the negative electrode active material during the insertion / extraction process of active ions, reducing swelling; at the same time, the electrolyte solution adsorbed in the spatial network structure of the gel polymer can better contact and infiltrate the negative electrode active material, improving the problem of lack of liquid on the upper layer of the negative electrode plate, making the negative electrode plate have better electrolyte wettability, and further enabling the secondary battery to have both improved cycle performance and a low swelling rate. Therefore, the preparation method provided by the third aspect embodiments of the present application can be used to prepare the secondary battery of the second aspect embodiments of the present application.

[0127] It can be understood that the first temperature and the second temperature in the embodiments of the present application can be determined according to the specific type of gel monomer, and different gel monomers may have different initiation polymerization temperatures; the first temperature can be higher than the second temperature, or the first temperature can be lower than the second temperature, or the first temperature is the same as the second temperature.

[0128] As an example, the first temperature can be 60°C - 100°C. For example, it can be 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, 90°C, 95°C, 100°C, or the range composed of any of the above values.

[0129] As an example, the second temperature can be 50°C - 80°C. For example, it can be 50°C, 55°C, 60°C, 65°C, 70°C, 75°C, 80°C, or a range composed of any of the above values.

[0130] In this application, the encapsulated secondary battery can be left standing at the first temperature first, and then at the second temperature; alternatively, it can be left standing at the second temperature first, and then at the first temperature.

[0131] In some embodiments, the first temperature can be higher than the second temperature. The secondary battery is left standing at the second temperature first, and then at the first temperature. Thereby, the second gel monomer in the second negative electrode film layer can be polymerized first to form a second gel polymer, adsorbing and fixing part of the electrolyte in the first negative electrode film layer. Thus, the upper layer of the negative electrode plate has better electrolyte wettability, which can improve the liquid shortage problem in the upper layer of the negative electrode plate during the charge and discharge cycle, and further improve the cycle performance and kinetic performance of the secondary battery.

[0132] In some embodiments, the first gel monomer can include one or more of ethylene glycol dimethacrylate, hydroxypropyl methacrylate, hydroxyethyl acrylate, and hydroxypropyl acrylate.

[0133] In some embodiments, the second gel monomer can include one or more of methyl methacrylate, acrylic acid, N-isopropylacrylamide, hydroxyethyl methacrylate, N,N-dimethylacrylamide, and 2-hydroxyethyl methacrylate.

[0134] In some embodiments, the first initiator and the second initiator can be the same or different. The first initiator and the second initiator can be selected respectively according to the types of the first gel monomer and the second gel monomer.

[0135] As an example, the first initiator can include one or more of azobisisobutyronitrile (AIBN), azobiscyclohexanecarbonitrile (ACCN), dicumyl peroxide (DCP), and benzoyl peroxide (BPO).

[0136] As an example, the second initiator can include one or more of azobisisobutyronitrile (AIBN), ammonium persulfate (APS), and potassium persulfate (KPS).

[0137] It can be understood that the dosages of the first initiator and the second initiator in the embodiments of this application can be determined respectively according to the contents of the first gel monomer and the second gel monomer, so that the gel monomers in the negative electrode plate are completely polymerized, and at the same time, the residual amount of the initiator in the electrolyte is as small as possible.

[0138] In some embodiments, based on the total mass of the electrolyte, the mass percentage of the first initiator may be 0.1%-0.5%. As an example, based on the total mass of the electrolyte, the mass percentage of the first initiator may be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, or a range composed of any of the above values.

[0139] In some embodiments, based on the total mass of the electrolyte, the mass percentage of the second initiator may be 0.1%-1%. As an example, based on the total mass of the electrolyte, the mass percentage of the second initiator may be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, or a range composed of any of the above values.

[0140] [Electronic device]

[0141] An embodiment of the fourth aspect of the present application provides an electronic device, which includes the secondary battery of the embodiment of the second aspect of the present application or the secondary battery obtained by the preparation method according to the embodiment of the third aspect of the present application.

[0142] According to the present application, since the electronic device includes the secondary battery of any one of the second aspect or the third aspect of the present application, the electronic device has the beneficial effects of the second aspect or the third aspect.

[0143] The electronic device of the present application is not particularly limited, and it may be any electronic device known in the prior art. In some embodiments, the electronic device may include, but is not limited to, laptop computers, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, head-mounted stereo headphones, video recorders, liquid crystal televisions, portable cleaners, portable CD players, mini discs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, motorized bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors, etc.

[0144] Embodiment

[0145] Hereinafter, embodiments of the present application will be described. The embodiments described below are exemplary and are only used to explain the present application and should not be construed as a limitation to the present application. For those not specified in the embodiments regarding specific technologies or conditions, they shall be carried out according to the technologies or conditions described in the literature in the art or according to the product specifications. For the reagents or instruments not specified for the manufacturers, they are all conventional products that can be obtained through commercial purchases.

[0146] In the embodiments of the present application, some tests and evaluations are carried out according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0147] Negative electrode sheet sampling

[0148] At 25°C, the secondary battery is discharged at a constant current of 0.5C until the discharge cut-off voltage. The lithium-ion battery is disassembled in an argon atmosphere, and the negative electrode plate is soaked in dimethyl carbonate solvent for 2 hours and dried at 60°C for 1 hour to obtain the negative electrode plate. Among them, the discharge cut-off voltage of the lithium-ion batteries in the embodiments of the present application and the comparative examples is 3.0V. It can be understood that when the voltage range marked on the outer package of the factory battery is 3.0V to 4.5V, the charge cut-off voltage is 4.5V and the discharge cut-off voltage is 3.0V. Unless otherwise specified, the charge cut-off voltage of the secondary battery as an example in the present application is 4.5V and the discharge cut-off voltage is 3.0V.

[0149] The following test methods use the negative electrode plates obtained in the above manner unless otherwise specified.

[0150] Negative electrode film layer thickness test

[0151] Use laser cutting (CP) to prepare a flat cross-section along the vertical direction of the electrode plate lamination interface. After polishing and coating, it is placed in the SEM sample chamber. Select the backscattered electron mode, locate the clear interface at 3000 times magnification, and there is a light and dark contrast between the upper and lower layers due to differences in composition and porosity. During measurement, use image analysis software to uniformly select at least 5 measurement points along the vertical direction of the interface, avoiding the cutting edge or defect area. After marking the upper and lower layer boundaries, the software automatically generates spacing data and calculates the average value. Repeat the measurement of 3 different cross-sectional areas to verify the reliability of the data.

[0152] Test of peeling strength F of negative electrode film layer

[0153] Cut the negative electrode plate into a spline with a size of 100mm×20mm and stick it on double-sided tape. Roll the spline back and forth 4 times with a roller. Clamp one end of the spline on the tensile machine fixture, stretch it at 180°C, turn on the tensile machine test, and pull the negative electrode plate at a constant speed of 5mm / min until the negative electrode film layer peels off from the negative electrode current collector. Visual inspection shows that the current collector surface is exposed after peeling, and the test is completed. After the test is completed, the peeling strength of the negative electrode film layer is calculated.

[0154] Test of porosity of negative electrode sheet

[0155] Refer to the test method of the national standard GB / T 24586-2009 "Determination of Apparent Density, True Density and Porosity of Iron Ore", and the equipment model is: The fully automatic true density meter of II 1340 uses the gas displacement method to test the porosity of the negative electrode plate, obtaining the overall porosity of the negative electrode film layer. The second negative electrode film layer on the upper layer of the negative electrode film layer is peeled off to obtain the first negative electrode film layer, and then the porosity of the first negative electrode film layer is tested by the above method. The porosity of the second negative electrode film layer is measured and calculated by the density method.

[0156] Determination of gel polymer type

[0157] The negative electrode film layer on the negative electrode plate is bisected in the thickness direction, and the part with the closest half thickness to the surface of the negative electrode plate is collected as the upper layer sample, and the remaining is the lower layer sample. Referring to JY T 0589.5-2020 "General Rules for Thermal Analysis Methods", a thermal analyzer is used to test the change of the mass of the negative electrode film layer powder with temperature. The model of the thermal analyzer equipment is STA449F3, using an N2 inert atmosphere, the test temperature is from 35°C to 1700°C, the heating rate is 5°C / min, the purge gas flow rate is 60 mL / min, and the protective gas flow rate is 20 mL / min.

[0158] After drying the scraped upper layer sample and lower layer sample, an infrared spectroscopy detector is used to test the characteristic functional groups respectively to reverse the major categories of polymers, and then the Young's modulus of this type of substance is tested.

[0159] Determination of gel polymer content

[0160] Obtain the thermal decomposition temperature range T1 and mass loss m u of the upper layer sample (M u ); take the mass loss m u within the decomposition temperature range of 200°C to 450°C and calculate the mass fraction W1 of the gel polymer = m u / M u * 100%. Similarly, after scraping off the film layer on the upper layer of the electrode plate, according to the mass decomposition curve of the lower layer sample (M d ) at different temperatures, take the mass loss m d within the decomposition temperature range of 200°C to 450°C and calculate the mass fraction W2 of the gel polymer = m d / M d * 100%.

[0161] Mass percentage of silicon element

[0162] Scrape the material on the upper layer of the electrode sheet, and use the chemical dissolution-ICP method to determine the Si content in the silicon composite material: After weighing the sample, digest it with a mixed acid of HF-HNO3, convert silicon into a soluble state, and filter and remove other residues. After diluting the filtrate, measure the intensity of the Si characteristic spectrum by ICP-OES, calculate the concentration by comparing with the standard curve, and obtain the content by combining the volume of the digestion solution and the mass of the sample.

[0163] Energy density test

[0164] Under the condition of 25°C, measure the length, width and thickness of the secondary battery and calculate the volume V (L) of the secondary battery; charge the lithium-ion secondary battery at a constant current of 0.05C until the voltage reaches 4.5V (i.e., the full charge voltage), and then charge it at a constant voltage of 4.5V until the current reaches 0.025C (cut-off current) to make the lithium-ion battery reach the full charge state. Let the battery stand for 10 minutes, and then discharge it at a rate of 0.2C until the voltage reaches 3.0V, and stand for 5 minutes to obtain the discharge energy Q of the secondary battery. The volume energy density (Wh / L) of the secondary battery = Q / V.

[0165] Kinetics performance test

[0166] Under the condition of 25°C, charge the lithium-ion battery to be tested at a constant current of 0.05C until the voltage reaches 4.5V (i.e., the full charge voltage), and then charge it at a constant voltage of 4.5V until the current reaches 0.025C (cut-off current) to make the lithium-ion battery reach the full charge state. Let the battery stand for 10 minutes, and then discharge it at a rate of 0.2C until the voltage reaches 3.0V, and stand for 5 minutes. At this time, the obtained discharge capacity is D0. Charge the lithium-ion battery to be tested at a constant current of 0.05C until the voltage reaches 4.5V (i.e., the full charge voltage), and then charge it at a constant voltage of 4.5V until the current reaches 0.025C (cut-off current) to make the lithium-ion battery reach the full charge state. Let the battery stand for 10 minutes, and then discharge it at a rate of 3.0C until the voltage reaches 3.0V, and stand for 5 minutes. At this time, the obtained discharge capacity is D1. Then the 3C discharge rate = D1 / D0×100%.

[0167] Cycle performance test

[0168] Under the condition of 25°C, charge the secondary battery at a constant current of 3C to 4.5V, then charge it at a constant voltage of 4.5V until the current reaches 0.05C, and then discharge it at a constant current of 0.5C to 3.0V. Charge and discharge cyclically three times according to the above method, and record the discharge capacity and the thickness of the secondary battery after the third cycle; continue to perform charge and discharge cycles, and record the discharge capacity and the thickness of the secondary battery after the 500th cycle.

[0169] Cycle capacity retention rate = (discharge capacity after 500 cycles / discharge capacity after 3 cycles)×100%;

[0170] Swelling rate = (thickness of the secondary battery after 500 cycles / thickness of the secondary battery after 3 cycles) × 100%.

[0171] Example 1-1

[0172] Preparation of positive electrode sheet

[0173] The positive electrode active material lithium cobalt oxide (LiCoO₂), the conductive agent conductive carbon black, and the binder polyvinylidene fluoride (PVDF) were mixed at a mass ratio of 97.6:1.2:1.2, and added to the solvent N-methylpyrrolidone (NMP). The solid content was controlled to be 70%, and the mixture was stirred evenly to obtain the positive electrode slurry. The positive electrode slurry was coated on the positive electrode current collector aluminum foil with a thickness of 10 μm, and after drying and cold pressing, a positive electrode film layer was formed to obtain a positive electrode plate. Among them, the thickness of the positive electrode film layer was 92.4 μm, and the width of the positive electrode plate was 77.4 mm.

[0174] Preparation of negative electrode sheet

[0175] The first negative electrode active material high-pressure compact graphite (compacted density of 2.3 g / cm 3 )), the binder styrene-butadiene rubber, the dispersant sodium carboxymethyl cellulose, and the first gel monomer hydroxypropyl methacrylate were mixed at a mass ratio of 94.5:1.3:1.2:3 and then dispersed in deionized water to form the first negative electrode slurry;

[0176] The second negative electrode active material silicon-carbon composite material, the conductive agent carbon nanotubes, the binder styrene-butadiene rubber, the dispersant sodium carboxymethyl cellulose, and the second gel monomer methyl methacrylate were mixed at a mass ratio of 95.3:0.2:1.3:1.2:2 and then dispersed in deionized water to form the second negative electrode slurry;

[0177] Using a double-layer coating nozzle, the first negative electrode slurry was sprayed on the surface of the negative electrode current collector copper foil, and the second negative electrode slurry was sprayed on the surface of the first negative electrode slurry. After drying, a first negative electrode film layer and a second negative electrode film layer were formed respectively; after cold pressing and slitting, a negative electrode plate was obtained. Among them, the total thickness of the negative electrode film layer was 78 μm, the width of the negative electrode plate was 78.8 mm, and the thickness ratio of the first negative electrode film layer to the second negative electrode film layer was 2:1.

[0178] Preparation of electrolyte

[0179] In an environment with a water content of less than 10 ppm, ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), propyl propionate (PP), and vinylene carbonate (VC) are mixed in a mass ratio of 20:30:20:28:2 to obtain an organic solvent. Then, lithium hexafluorophosphate, the above-prepared organic solvent, a first initiator azobiscyclohexanecarbonitrile (ACCN), and a second initiator azobisisobutyronitrile (AIBN) are mixed to form an electrolyte. The concentration of the lithium salt in the electrolyte is 1 mol / L; based on the total mass of the electrolyte, the mass ratio of the first initiator is 0.3%, and the mass ratio of the second initiator is 0.2%.

[0180] Preparation of separator

[0181] A 7-μm-thick polyethylene (PE) separator is selected.

[0182] Preparation of secondary battery

[0183] The positive electrode plate, the separator, and the negative electrode plate are stacked in sequence, and the separator is placed between the positive and negative electrode plates to play an insulating role. Then, it is wound 14 layers to obtain an electrode assembly; after welding the electrode tabs, the bare battery cell is placed in an outer packaging aluminum-plastic film. After removing moisture at 80 °C, the electrolyte is injected and encapsulated. After standing at room temperature for 48 h, forming, standing at 50 °C for 24 h, standing at 80 °C for 24 h, degassing, shaping and other technological processes, a lithium-ion secondary battery is obtained. The thickness of the secondary battery body is 52 mm, the width is 33 mm, and the length is 85 mm.

[0184] Examples 1-2 to Examples 1-21

[0185] Except for adjusting the parameters of the negative electrode plate according to Table 1, the rest are the same as in Example 1-1.

[0186] Comparative Examples 1-1 to Comparative Examples 1-2

[0187] Except for adjusting the parameters of the negative electrode plate according to Table 1, the rest are the same as in Example 1-1.

[0188] Comparative Example 1-3

[0189] The difference from Example 1-1 is that in the first negative electrode film layer, an equal mass of the first negative electrode active material is used to replace the first gel monomer, specifically referring to Table 1.

[0190] Comparative Example 1-4

[0191] The difference from Example 1-1 is that in the second negative electrode film layer, an equal mass of the second negative electrode active material is used to replace the second gel monomer, specifically referring to Table 1.

[0192] Comparative Example 1-5

[0193] The difference from Example 1-1 is that in the first negative electrode film layer, an equal mass of the first negative electrode active material is used to replace the first gel monomer, and in the second negative electrode film layer, an equal mass of the second negative electrode active material is used to replace the second gel monomer. Specifically, refer to Table 1.

[0194] Examples 2-1 to 2-18

[0195] Except for adjusting the parameters of the secondary battery according to Table 2, the rest are the same as those in Example 1-1.

[0196] Comparative Examples 2-1 to 2-2

[0197] Except for adjusting the parameters of the secondary battery according to Table 2, the rest are the same as those in Example 1-1.

[0198]

[0199]

[0200]

[0201]

[0202] The test results of the secondary batteries in each example and comparative example are shown in Table 3 in detail.

[0203] Table 3

[0204]

[0205]

[0206] Referring to Tables 1 to 3, it can be seen from Examples 1-1 to 1-21, Examples 2-1 to 2-18, Comparative Examples 1-1 to 1-5, and Comparative Examples 2-1 to 2-6 that in the examples, different types of gel polymers are added to the upper and lower layers of the negative electrode sheet, and the ionic conductivity of the gel polymer in the upper layer is within the scope of the present application, which can make the negative electrode sheet take into account high tap density and good electrolyte wettability, make the negative electrode sheet have high ionic conductivity and ion transport rate, and reduce the swelling of the secondary battery while improving the cycle performance and rate performance of the secondary battery. When the ionic conductivity of the gel polymer is too large, although it is beneficial to the improvement of the rate performance, the gel polymer itself is not stable, the electrolyte consumption increases, and it is not conducive to the cycle performance.

[0207] As can be seen from Examples 1-1, 1-22 and Comparative Examples 1-1 to 1-5, when the same gel polymer is added to the upper and lower layers of the negative electrode sheet and the conductivity of the gel polymer in the upper layer is controlled within the range of this application, the negative electrode sheet can have good ionic conductivity and ion transport rate, which is beneficial to improving the rate performance of the secondary battery. Since the gel polymer with good ionic conductivity in the lower layer has a low Young's modulus and a high electrolyte consumption capacity, it is not conducive to making the negative electrode sheet have both high tap density and good electrolyte wettability, affecting the cycle performance and swelling of the secondary battery.

[0208] The tap density and porosity of the negative electrode film layer will affect the electrolyte wettability effect and ion transport rate of the negative electrode sheet, thus affecting the cycle performance and rate performance of the secondary battery. Combining Examples 1-1 to 1-12, it can be seen that when the porosity of the negative electrode film layer is within the range of this application, the secondary battery can maintain a high energy density, and at the same time its discharge rate and cycle capacity retention rate are relatively high, indicating that the secondary battery has high rate performance and improved cycle performance while taking into account high energy density.

[0209] The content of the gel polymer in the negative electrode film layer will affect the electrolyte wettability of the negative electrode sheet and the liquid retention effect of the bottom layer of the negative electrode film layer, and the content of the gel polymer in the negative electrode film layer will also affect the porosity of the negative electrode film layer, thereby affecting the cycle performance and rate performance of the secondary battery. Combining Examples 1-13 to 1-16, it can be seen that when the content of the gel polymer in the negative electrode film layer is within the range of this application, the secondary battery can have high rate performance and high cycle performance.

[0210] Combining Examples 2-15 to 2-18, it can be seen that controlling the proportion of the initiator in the electrolyte and the initiation temperature within the range of this application is beneficial to avoiding the residual excess gel monomer or initiator in the negative electrode sheet, deteriorating the cycle performance or rate performance of the secondary battery. It is beneficial to avoid rapid initiation caused by too high temperature, resulting in uneven polymerization of the gel monomer, increasing the local ion transport impedance and affecting the cycle performance of the secondary battery; at the same time, avoiding too low initiation temperature resulting in incomplete initiation and excessive remaining gel monomer, affecting the cycle performance and rate performance of the secondary battery.

[0211] Combining the data in Table 1 and Table 3, it can be seen that by adjusting the content of the gel polymer in the negative electrode film layer so that the ratio of the content of the gel polymer to the tap density of the negative electrode film layer is within the range of this application, the liquid retention effect and electrolyte wettability of the negative electrode sheet can also be improved, further enhancing the cycle performance and rate performance of the secondary battery.

[0212] From the data in Table 2 and Table 3, it can be seen that by adjusting the type of gel polymer, the liquid retention effect of the negative electrode sheet and the electrolyte wettability can also be adjusted, thereby improving the cycle performance and rate performance of the secondary battery. In addition, during the preparation of the secondary battery, by adjusting the dosage of the initiator and the initiation temperature in the electrolyte, the gel monomers added in the negative electrode film layer can be completely polymerized to form a polymer, which is beneficial to further improving the electrolyte wettability and liquid retention effect of the negative electrode sheet, and improving the cycle performance and rate performance of the secondary battery.

[0213] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than limiting them; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A negative electrode plate, characterized in that, The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer, and the negative electrode film layer includes a first negative electrode film layer and a second negative electrode film layer; The first negative electrode film layer is located on at least one surface of the negative electrode current collector, and the first negative electrode film layer includes a first negative electrode active material and a first gel polymer; The second negative electrode film layer is located on the surface of the first negative electrode film layer away from the negative electrode current collector. The second negative electrode film layer includes a second negative electrode active material and a second gel polymer, and the ionic conductivity of the second gel polymer is σ, 0.1 mS / cm ≤ σ ≤ 10 mS / cm; The porosity of the first negative electrode film layer is The porosity of the second negative electrode film layer is 2. The negative electrode sheet according to claim 1, characterized in that, 3. The negative electrode sheet according to claim 2, wherein and / or 4. The negative electrode sheet according to claim 1, characterized in that, The thickness of the first negative electrode film layer is H1, and the thickness of the second negative electrode film layer is H2, 1 ≤ H1 / H2 ≤ 2.

5. The negative electrode sheet according to claim 1, characterized in that, The peel strength F between the negative electrode film layer and the negative electrode current collector is 5 N / m - 50 N / m.

6. The negative electrode sheet according to claim 1, characterized in that, The Young's modulus E of the standard gel film made of the first gel polymer is 0.5 MPa - 4 MPa.

7. The negative electrode sheet according to claim 6, wherein The first gel polymer includes one or more of polyethylene glycol dimethacrylate gel, poly(hydroxypropyl methacrylate) gel, poly(2-hydroxyethyl acrylate) gel, poly(hydroxypropyl acrylate) gel, and polyacrylonitrile gel.

8. The negative electrode sheet according to claim 1, characterized in that, The second gel polymer includes one or more of polymethyl methacrylate gel, polyacrylic acid gel, poly(N-isopropylacrylamide) gel, poly(2-hydroxyethyl methacrylate) gel, poly(N,N-dimethylacrylamide) gel, and poly(2-hydroxyethyl methacrylate) gel.

9. The negative electrode sheet according to any one of claims 1-8, wherein The compaction density of the first negative electrode film layer is PD1 g / cm 3 , based on the total mass of the first negative electrode film layer, the mass percentage of the first gel polymer is W1%, and 0.57 ≤ W1 / PD1 ≤ 2.86; and / or The compaction density of the second negative electrode film layer is PD2 g / cm 3 , based on the total mass of the second negative electrode film layer, the mass percentage of the second gel polymer is W2%, and 1 ≤ W2 / PD2 ≤ 5.

10. The negative electrode sheet according to claim 9, wherein 1 ≤ W1 ≤ 5; optionally, 1 ≤ W1 ≤ 4; and / or 1 ≤ W2 ≤ 5; optionally, 1 ≤ W2 ≤ 3.

11. The negative electrode sheet according to claim 1, wherein, The first negative electrode active material includes a graphite material, and the tap density of the graphite material is PD3 g / cm 3 , and 2.2 ≤ PD3 ≤ 2.

5.

12. The negative electrode sheet according to claim 1, wherein The second negative electrode active material includes a silicon-containing material, and based on the total mass of the second negative electrode film layer, the mass percentage content of silicon element is 5% - 50%.

13. A secondary battery, characterized in that, A negative electrode sheet including any one of claims 1-12.

14. An electronic device, characterized in that, A secondary battery including the negative electrode sheet according to claim 13.

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