Secondary battery, electric device, and method for manufacturing secondary battery

Abstract

The application relates to the technical field of electrochemical devices, in particular to a secondary battery, an electric device and a preparation method of the secondary battery. The secondary battery comprises a negative electrode sheet, the negative electrode sheet comprises a current collector and a first active material layer, the first active material layer is arranged on at least one surface of the current collector, and the first active material layer comprises a silicon-based material and a solid-state electrolyte, and the solid-state electrolyte is at least partially combined to the surface of silicon in the silicon-based material. The secondary battery and the electric device provided by the embodiments of the application can improve the expansion problem of the silicon-based material, and enhance the structural integrity and the structural stability of the negative electrode sheet. In addition, since the solid-state electrolyte is used in the application, compared with the existing technology of coating the silicon-based material with a binder, the lithium intercalation capacity of the silicon-based material can be improved, the ion conduction rate in the negative electrode sheet is improved, and the problem of the deterioration of the kinetic performance of the secondary battery caused by the coating of the silicon-based material with the binder is solved.

Description

Technical Field

[0001] This application relates to the field of electrochemical device technology, and in particular to secondary batteries, electrical equipment, and methods for preparing secondary batteries. Background Technology

[0002] The negative electrode material in lithium-ion batteries is generally graphite. If silicon is added to the negative electrode material, the theoretical specific capacity and energy density of the negative electrode will be greatly improved. Therefore, lithium-ion batteries with silicon-based negative electrode sheets are the future development direction.

[0003] During the charging and discharging process of lithium-ion batteries, silicon particles undergo lithiation and delithiation, causing them to expand and contract, which in turn damages the structural integrity and stability of the negative electrode. Summary of the Invention

[0004] A binder can be simply coated onto the surface of silicon particles to reduce their expansion, thereby mitigating the problem of damage to the negative electrode. However, coating the silicon particles with a binder affects the conductivity of the negative electrode, leading to deterioration of the lithium-ion battery's kinetic performance, reduced cycle performance, and the occurrence of interfacial lithium plating.

[0005] The embodiments of this application aim to provide a secondary battery, an electrical device, and a method for preparing a secondary battery, which can improve the problem of deterioration of the dynamic performance of the secondary battery after the silicon particles in the negative electrode are coated with a binder.

[0006] To solve the above-mentioned technical problems, one technical solution adopted in the embodiments of this application is: to provide a secondary battery, the secondary battery including a negative electrode sheet, the negative electrode sheet including a negative current collector and a first active material layer, the first active material layer being disposed on at least one surface of the negative current collector, the first active material layer including a silicon-based material and a solid electrolyte, at least a portion of the solid electrolyte being bonded to the surface of the silicon-based material.

[0007] By setting the first active material layer in the negative electrode sheet as a silicon-based material and a solid electrolyte, with at least a portion of the solid electrolyte bonded to the surface of the silicon-based material, the solid electrolyte bonded to the silicon-based material can effectively bind the silicon, reduce silicon expansion and the resulting silicon displacement, enhance the structural integrity and stability of the negative electrode sheet, and extend the service life of the secondary battery. On the other hand, using a solid electrolyte to bind silicon, compared to using a binder to coat silicon in the prior art, can improve the lithium intercalation capability of the silicon-based material and increase the ion conduction rate in the negative electrode sheet, thus improving the problem of deterioration in the kinetic performance of the secondary battery caused by binder-coated silicon.

[0008] In some embodiments, the solid electrolyte accounts for 0.1 wt% to 20 wt% of the mass percentage of the first active material layer. Limiting the solid electrolyte content to not less than 0.1 wt% ensures effective bonding between the solid electrolyte and silicon, guarantees the confinement of silicon by the solid electrolyte, and limits silicon expansion and displacement. Limiting the solid electrolyte content to not more than 20 wt% ensures the silicon-based material content in the first active material layer, guarantees the energy density (ED) of the secondary battery, and reduces lithium plating. Furthermore, when the solid electrolyte content exceeds 20 wt%, not only is the limitation on silicon expansion and displacement limited, but the kinetic performance of the secondary battery also deteriorates. Therefore, limiting the solid electrolyte content to not more than 20 wt% not only ensures the performance of the secondary battery but also takes into account the cost of material usage.

[0009] In some embodiments, the solid electrolyte accounts for 5 wt% to 15 wt% of the first active material layer by mass. To balance the binding effect of the solid electrolyte on silicon and the ED and cycle performance of the secondary battery, the solid electrolyte accounts for 5 wt% to 15 wt% of the first active material layer by mass.

[0010] In some embodiments, the solid electrolyte includes at least one of polymethyl methacrylate, polyvinylidene fluoride, and polyacrylonitrile.

[0011] In some embodiments, a solid electrolyte is coated onto the surface of the silicon-based material. This allows the solid electrolyte to effectively bind to the surface of the silicon-based material, reducing expansion.

[0012] In some embodiments, the solid electrolyte comprises polymethyl methacrylate and / or polyacrylonitrile, with silicon in the silicon-based material linked to hydroxyl groups, and the solid electrolyte bonded to the hydroxyl groups. Specifically, the carbonyl groups in the polymethyl methacrylate can form hydrogen bonds with the hydroxyl groups on the surface of the silicon-based material, and the lone pair electrons on the nitrogen atoms in the polyacrylonitrile can form hydrogen bonds with the hydroxyl groups on the surface of the silicon-based material. Thus, the solid electrolyte can be at least partially and more effectively bonded to the surface of the silicon-based material, confining the silicon-based material and reducing its expansion and displacement.

[0013] In some embodiments, the secondary battery also includes a free electrolyte to facilitate lithium-ion transport, ensure charge and discharge rates, and further improve the kinetic performance of the secondary battery.

[0014] In some embodiments, the first active material layer further includes a first graphite material, which is mixed with a silicon-based material. The mixing of the silicon-based material and the first graphite material allows the first graphite material to help bind silicon, further reducing silicon expansion and displacement, and further enhancing the structural integrity and stability of the negative electrode sheet.

[0015] In some embodiments, the mass of the silicon-based material is m1, the mass of the first graphite material is m2, and m1 / (m1+m2) is 5% to 30%. Limiting the ratio of the mass of the silicon-based material to the mass of the first graphite material to not less than 5% can ensure the ED of the secondary battery, and limiting it to not more than 30% can not only reduce lithium plating, but also ensure the binding effect of the first graphite material on silicon, and ensure the structural integrity and structural stability of the negative electrode sheet.

[0016] In some embodiments, the particle size D of the silicon-based material v 50 and the particle size D of the first graphite material v The ratio of 50 is 0.1 to 0.5. The particle size D of the silicon-based material... v 50 and the particle size D of the first graphite material v The ratio of 50 to 1 is limited to no less than 0.1 to reduce side reactions, decrease the cycle decay rate of the secondary battery, and ensure the capacity retention rate of the secondary battery. The particle size D of the silicon-based material is... v 50 and the particle size D of the first graphite material v If the ratio of 50 is limited to no more than 0.5, the relatively small silicon can be bound by the relatively large first graphite material, thereby reducing the expansion and displacement of silicon and enhancing the structural integrity and stability of the negative electrode sheet.

[0017] In some embodiments, the particle size D of the silicon-based material v 50 and the particle size D of the first graphite material v The ratio of 50 is 0.2 to 0.3. By further limiting the particle size relationship between the silicon-based material and the first graphite material, the capacity retention rate of the secondary battery and the binding effect of the first graphite material on silicon can be balanced.

[0018] In some embodiments, the particle size D of the silicon-based material v 50 is 4μm to 10μm, and the particle size D of the first graphite material is... v 50 is 16μm to 40μm.

[0019] In some embodiments, the silicon-based material includes one or more of elemental silicon, silicon oxide, silicon carbide, or silicon alloys. Therefore, by adding a silicon-based material to the negative electrode, the theoretical specific capacity and energy density of the negative electrode will be significantly improved.

[0020] In some embodiments, the secondary battery further includes a second active material layer disposed on the side of the first active material layer opposite to the negative electrode current collector; the second active material layer includes a second graphite material. By further providing a second active material layer in the secondary battery, silicon can be further confined, reducing silicon expansion and displacement, and enhancing the structural integrity and stability of the negative electrode sheet.

[0021] In some embodiments, the mass ratio of the second active material layer to the first active material layer ranges from 0.2:1 to 2:1. When the mass ratio of the second active material layer to the first active material layer is less than 0.2, the second active material layer has limited ability to restrict the expansion of the silicon-based material in the first active material layer, resulting in a rapid increase in the battery's expansion rate. When the mass ratio of the second active material layer to the first active material layer is greater than 2, the wetting time increases significantly, indicating that the first active material layer has a high compaction density and is not easily wetted. Excessive compaction density can lead to particle breakage in the first active material layer. Therefore, the preferred mass ratio of the second active material layer to the first active material layer is between 0.2:1 and 2:1.

[0022] In some embodiments, the second active material layer further includes inorganic materials, wherein the particle size D of the inorganic materials is... v 50 and the particle size D of the second graphite material v The ratio of 50 is less than 0.23. This is due to the addition of inorganic materials, and the limitation of the particle size D of the inorganic materials. v 50 and the particle size D of the second graphite material v If the ratio of 50 to 1 is less than 0.23, the inorganic material can fill the gaps in the second graphite material, acting as a lubricant and buffer, reducing the problem of graphite particle breakage, improving the overvoltage issue of the negative electrode, and enhancing the cycle performance of the secondary battery. Furthermore, when the secondary battery includes a free electrolyte, the addition of inorganic materials can increase the wetting of the free electrolyte and increase the amount of free electrolyte retained in the negative electrode, improving interface issues, increasing ion conduction efficiency, and improving the kinetic performance of the secondary battery.

[0023] In some embodiments, the particle size D of the inorganic material v 50 and the particle size D of the second graphite material v The ratio of 50 is 0.04 to 0.1. With these limitations, the second graphite material exhibits less particle breakage, resulting in optimal capacity retention in the secondary battery.

[0024] In some embodiments, the particle size D of the inorganic material v 50 is 0.3μm~2μm, and the particle size D of the second graphite material is... v 50 represents 10μm to 15μm.

[0025] In some embodiments, the inorganic material accounts for 0.1 wt% to 5 wt% of the mass percentage of the second active material layer. Excessive addition of inorganic material affects the energy density (ED), while insufficient addition has little effect on improving wettability. When the inorganic material accounts for less than 0.1 wt% of the mass percentage of the second active material layer, the rate of decrease in compaction density accelerates, resulting in a sharp reduction in the lubrication effect of the inorganic material on the second graphite material, and a significant increase in wetting time. When the inorganic material accounts for more than 5 wt% of the mass percentage of the second active material layer, the large proportion of inorganic material in the second active material layer leads to a severe decrease in energy density. Therefore, the preferred mass percentage of inorganic material in the second active material layer is 0.1 wt% to 5 wt%.

[0026] In some embodiments, the inorganic material includes one or more of the following: alumina, garnet, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate. Thus, by incorporating the inorganic material, the aforementioned beneficial effects, such as enhancing the cold-pressing density of the second active material layer and improving the overpressure problem of the second active material layer, can be achieved.

[0027] In some embodiments, the inorganic material is alumina and / or boehmite. It exhibits good electrochemical stability and the raw materials are readily available.

[0028] In some embodiments, the secondary battery satisfies one of the following conditions:

[0029] The mass percentage of the conductive agent in the first active material layer is 0 wt% to 20%.

[0030] The mass percentage of the conductive agent in the first active material layer is 0.1 wt% to 15%.

[0031] The conductive agent in the first active material layer has a mass percentage of 0.1 wt% to 5%.

[0032] The conductive agent in the first active material layer accounts for 1 wt% to 5% of the total mass.

[0033] To solve the above-mentioned technical problems, another technical solution adopted in the embodiments of this application is: to provide an electrical device, the electrical device including a load and a secondary battery as described above, the secondary battery being used to supply power to the load.

[0034] This application also provides a method for preparing a secondary battery, wherein the negative electrode sheet is prepared by mixing a silicon-based material, a first dispersant, a conductive agent, methyl methacrylate and azobisisobutyronitrile, dissolving them in deionized water to form a first active material layer slurry; and uniformly coating the first active material layer slurry on at least one surface of the negative electrode current collector.

[0035] In this process, methyl methacrylate (MMA) can be polymerized to form a solid electrolyte polymethyl methacrylate (PMMA) under the catalysis of azobisisobutyronitrile (AIBN), and the PMMA is then bonded to the surface of the silicon-based material. This preparation method allows for a more uniform distribution of PMMA in the first active material layer.

[0036] Unlike related technologies, in the secondary battery and electrical device of this application embodiment, the solid electrolyte is at least partially bonded to the surface of silicon in the silicon-based material, and a protective layer is formed on the surface of silicon to reduce the expansion and displacement of silicon, thereby improving the problem of silicon-based material expansion and enhancing the structural integrity and structural stability of the negative electrode sheet. In addition, since this application uses a solid electrolyte, compared with the binder coating of silicon-based materials in the prior art, it can enhance the lithium intercalation capability of silicon-based materials and improve the ion conduction rate in the negative electrode sheet, that is, improve the cycle performance of the secondary battery.

[0037] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0038] One or more embodiments are illustrated by way of example with reference to the accompanying drawings. These illustrations do not constitute a limitation on the embodiments. Elements having the same reference numerals in the drawings are denoted as similar elements. Unless otherwise stated, the figures in the drawings are not to be limited by scale.

[0039] Figure 1 This is a schematic diagram of the frame of the electrical equipment according to an embodiment of this application;

[0040] Figure 2 This is a schematic diagram of the negative electrode sheet in an embodiment of this application;

[0041] Figure 3 This is the Raman spectrum of the negative electrode sheet in an embodiment of this application;

[0042] Figure 4 This is a schematic diagram of another implementation of the negative electrode sheet in this application embodiment;

[0043] Figure 5These are SEM images of the negative electrode sheet in embodiments of this application;

[0044] Figure 6 This is another SEM image of the negative electrode sheet in an embodiment of this application;

[0045] Figure 7 This is a cycle life diagram of the secondary battery of Comparative Example 1 and Example 1 of this application;

[0046] Figure 8 This is a graph showing the cycle expansion rate of the secondary battery in Comparative Example 1 and Example 1 of this application;

[0047] Figure 9 These are cycle life diagrams of the secondary batteries in Embodiments 1 and 2 of this application;

[0048] Figure 10 These are the cycle expansion rate diagrams of the secondary batteries in Embodiments 1 and 2 of this application;

[0049] Figure 11 These are cycle life diagrams of the secondary batteries in Embodiments 2 and 3 of this application;

[0050] Figure 12 This is a graph showing the cycle expansion rate of the secondary battery in Embodiments 2 and 3 of this application.

[0051] The reference numerals in the detailed embodiments are as follows:

[0052] 1000 Electrical equipment; 100 Secondary battery; 200 Load; 10 Negative electrode sheet; 1 Negative current collector; 2 First active material layer; 3 Second active material layer. Detailed Implementation

[0053] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application. It should be noted that when an element is described as being "fixed" to another element, it can be directly on the other element, or one or more intermediate elements may exist between them. When an element is described as being "connected" to another element, it can be directly connected to the other element, or one or more intermediate elements may exist between them. It should be noted that, unless otherwise specified, the various features in the embodiments of this application can be combined with each other, all within the scope of protection of this application. Furthermore, although functional modules are divided in the device schematic diagram and a logical order is shown in the flowchart, in some cases, the steps shown or described may be performed in a different module division or in a different order than that shown in the device schematic diagram or the flowchart.

[0054] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0055] In the description of the embodiments of this application, the technical terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the embodiments of this application and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of this application.

[0056] In the description of the embodiments of this application, the terms "first," "second," etc., are used to define components merely for the purpose of distinguishing the corresponding components. Unless otherwise stated, these terms have no special meaning and should not be construed as limiting the scope of protection of this application. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0057] Unless otherwise defined, all technical and scientific terms used in this specification have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the scope of this application. The term "and / or" as used in this specification includes any and all combinations of one or more of the associated listed items.

[0058] Furthermore, the technical features involved in the different embodiments of this application described below can be combined with each other as long as they do not conflict with each other.

[0059] This application provides an electrical device 1000, such as... Figure 1 As shown, the electrical equipment 1000 includes a secondary battery 100 and a load 200, wherein the secondary battery 100 is used to supply power to the load 200.

[0060] The electrical device 1000 in this application embodiment includes, but is not limited to, Bluetooth headsets, mobile phones, tablets, laptops, electric toys, power tools, electric vehicles, electric cars, ships, spacecraft, etc. Among them, electric toys can include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc., and spacecraft can include airplanes, rockets, space shuttles, and spacecraft, etc.

[0061] The load 200 in this embodiment can be a display, speaker, processor, light-emitting device, motor, etc. It is understood that the type of load 200 is not limited to these.

[0062] Please see Figure 2 This application provides a secondary battery 100, which includes a negative electrode 10. The negative electrode 10 includes a negative current collector 1 and a first active material layer 2. The first active material layer 2 is disposed on at least one surface of the negative current collector 1 and includes a silicon-based material and a solid electrolyte, wherein the solid electrolyte is at least partially bonded to the surface of the silicon-based material. By setting the first active material layer in the negative electrode as a silicon-based material and a solid electrolyte, with the solid electrolyte at least partially bonded to the surface of the silicon-based material, on the one hand, the solid electrolyte bonded to the silicon-based material can effectively bind the silicon, reduce silicon expansion and the resulting displacement of the silicon-based material, enhance the structural integrity and stability of the negative electrode, and extend the service life of the secondary battery; on the other hand, using a solid electrolyte to bind the silicon-based material, compared with the use of binders to coat the silicon-based material in the prior art, can improve the lithium intercalation capability of the silicon-based material and increase the ion conduction rate in the negative electrode, that is, improve the problem of deterioration of the kinetic performance of the secondary battery caused by simply coating the silicon-based material with binders.

[0063] The secondary battery 100 mentioned above can be a pouch battery, a prismatic battery, or a cylindrical battery, etc.

[0064] The negative electrode 10 described above can be in a wound or stacked form. In this embodiment, the negative electrode 10 is described as being in a stacked form. The negative electrode 10 includes a negative current collector 1 and a first active material layer 2, which is disposed on at least one surface of the negative current collector 1. That is, the first active material layer 2 can be disposed on one surface of the negative current collector 1 along the thickness direction X, or it can be disposed on two opposite surfaces of the negative current collector 1 along the thickness direction X.

[0065] The direction in which the first active material layer 2 and the negative electrode current collector 1 are stacked is defined as the thickness direction X.

[0066] It is understandable that multiple first active material layers 2 may be spaced apart on one surface of the negative electrode current collector 1.

[0067] For the aforementioned negative electrode current collector 1, the negative electrode current collector 1 may include copper foil, aluminum foil, nickel foil, stainless steel foil, titanium foil, foamed nickel, foamed copper, polymer substrate coated with conductive metal, or any combination thereof.

[0068] The first active material layer 2 described above contains a negative electrode active material comprising silicon. In the embodiments of this application, the first active material layer 2 includes a silicon-based material and a solid electrolyte.

[0069] The silicon-based materials mentioned above can include one or more of elemental silicon, silicon oxide, silicon carbide, and silicon alloys. Silicon oxide includes silicon monoxide, silicon dioxide, and other silicon oxides. Adding silicon-based materials to the negative electrode will significantly improve the theoretical specific capacity and energy density of the negative electrode.

[0070] It should be noted that during charging and discharging, the negative electrode 10 of the secondary battery 100 will release and insert lithium ions, causing the silicon-based material to expand by more than 100% in volume. Such a high volume expansion rate will destroy the structural stability and integrity of the negative electrode 10; it will also damage the solid electrolyte interface (SEI) film, causing free electrolyte to penetrate into the interior of the negative electrode 10, triggering a series of unstable reactions such as the decomposition of free electrolyte and repeated destruction of the SEI film, which will significantly reduce the cycle performance of the secondary battery 100.

[0071] For the aforementioned solid electrolyte, the solid electrolyte is at least partially bonded to the surface of the silicon-based material, thereby forming a protective layer on the surface of the silicon-based material. This reduces the expansion and displacement of the silicon-based material, decreases the expansion or contraction of the negative electrode sheet, and thus enhances the structural integrity and stability of the negative electrode sheet. Furthermore, the solid electrolyte can also enhance the lithium intercalation capability of the silicon-based material and accelerate the ion conduction rate in the negative electrode sheet 10, thereby improving the kinetic performance and cycle performance of the secondary battery 100.

[0072] In some embodiments, the solid electrolyte accounts for 0.1 wt% to 20 wt% of the mass percentage of the first active material layer. Limiting the solid electrolyte content to not less than 0.1 wt% ensures effective bonding between the solid electrolyte and the silicon-based material, guarantees the confinement of the solid electrolyte on the silicon-based material, and limits its expansion and displacement. Limiting the solid electrolyte content to not more than 20 wt% ensures the silicon-based material content in the first active material layer, guarantees the energy density (ED) of the secondary battery, and reduces lithium plating. Furthermore, when the solid electrolyte content exceeds 20 wt%, not only is the limitation on silicon expansion and displacement limited, but the kinetic performance of the secondary battery also deteriorates. Therefore, limiting the solid electrolyte content to not more than 20 wt% not only ensures the performance of the secondary battery but also takes into account the cost of material usage.

[0073] In some embodiments, the solid electrolyte accounts for 5 wt% to 15 wt% of the first active material layer by mass. To balance the binding effect of the solid electrolyte on the silicon-based material and the ED and cycle performance of the secondary battery, the solid electrolyte accounts for 5 wt% to 15 wt% of the first active material layer by mass.

[0074] In some embodiments, the solid electrolyte comprises at least one of polymethyl methacrylate, polyvinylidene fluoride, and polyacrylonitrile. In some embodiments, the solid electrolyte coats the surface of the silicon-based material. In some embodiments, the solid electrolyte comprises polymethyl methacrylate and / or polyacrylonitrile, wherein silicon in the silicon-based material is linked to hydroxyl groups, and the solid electrolyte is bonded to the hydroxyl groups. The hydroxyl groups on the silicon surface in the silicon-based material can be obtained through modification treatment. It is understood that the solid electrolyte can be obtained using conventional methods in the art.

[0075] When the solid electrolyte is PMMA (polymethyl methacrylate), the oxygen atom in the carbonyl group of PMMA forms a hydrogen bond with the hydroxyl group on the silicon surface, as shown in chemical reaction formula ① below. In this formula, R represents the portion of PMMA excluding the -COOCH3 group. Furthermore, under high-temperature conditions, PMMA can also form a covalent bond with the O (oxygen) atom in the hydroxyl group, as shown in chemical reaction formula ② below. Again, R represents the portion of PMMA excluding the -COOCH3 group. The covalent bond formed between PMMA and the O (oxygen) atom in the hydroxyl group enhances the bonding force between the silicon-based material and the solid electrolyte, further reducing silicon expansion and displacement, and minimizing the expansion or contraction of the negative electrode.

[0076]

[0077] Please see Figure 3 , Figure 3This is the Raman spectrum of the negative electrode sheet with PMMA as the solid electrolyte and subjected to high-temperature treatment. The horizontal axis represents the Raman shift, i.e., the wavenumber difference between the scattered light and the incident light, and the vertical axis represents the intensity of the scattered light, i.e., the photon count. Figure 3 As can be seen from the image, the Raman spectrum of the negative electrode has characteristic peaks of SiOOC(C6H3) and SiOOC(C6H4), proving that under high temperature conditions, PMMA forms covalent bonds with the O (oxygen) atoms in the hydroxyl groups on the surface of the silicon-based material.

[0078] The PMMA mentioned above is produced by free radical polymerization of MMA (methyl methacrylate). It should be noted that PMMA not only functions as a binder, but also reacts with lithium ions (Li...). + It forms electronic coordination, plays a role in ion transport, improves ion transport efficiency, and improves the problem of deterioration of the dynamic performance of secondary batteries caused by the binder coating silicon in the existing technology.

[0079] Furthermore, in some embodiments, the first active material layer 2 also includes AIBN (azobisisobutyronitrile), which can promote the free radical polymerization of MMA (methyl methacrylate), that is, improve the efficiency and conversion rate of MMA polymerization to form PMMA.

[0080] The PMMA-containing negative electrode prepared using the above method exhibits a more uniform distribution of PMMA in the first active material layer compared to negative electrode prepared by directly adding PMMA. This is because, when PMMA is directly added, it tends to aggregate on the surface of the first active material layer away from the negative electrode current collector. However, by adding MMA monomers and polymerizing them under stirring to form PMMA, the distribution of PMMA in the first active material layer becomes more uniform. This results in a more uniform coating of the silicon-based material in the first active material layer by PMMA, and a more significant effect on inhibiting the expansion of the silicon-based material.

[0081] When the solid electrolyte is PVDF (polyvinylidene fluoride), PVDF can physically coat the surface of the silicon-based material, thus forming a protective layer. When the solid electrolyte is PAN (polyacrylonitrile), the lone pair electrons on the nitrogen atoms in PAN can form hydrogen bonds with the hydroxyl groups on the silicon surface. Both PVDF and PAN have similar functions to PMMA, namely, they can at least partially bind to the surface of silicon in the silicon-based material, thereby effectively confining silicon, reducing silicon expansion and displacement, and improving the lithium intercalation capability of the silicon-based material and increasing the ion conduction rate in the negative electrode, thus mitigating the problem of degraded kinetic performance of secondary batteries caused by binder-coated silicon.

[0082] In some embodiments, the secondary battery also includes a free electrolyte to facilitate lithium-ion transport, ensure charge and discharge rates, and further improve the kinetic performance of the secondary battery.

[0083] It is worth noting that the free electrolyte can be filled into the secondary battery in liquid form, so that at least part of the free electrolyte can wet the first active material layer 2, thereby facilitating the transport of lithium ions.

[0084] In some embodiments, the first active material layer 2 further includes a first graphite material, which is mixed with a silicon-based material. The first graphite material can help bind silicon, further reducing silicon expansion and displacement, and further enhancing the structural integrity and stability of the negative electrode sheet.

[0085] It is worth noting that the first graphite material can be one or more of hard carbon, soft carbon, or graphite. The OI value of the first graphite material is less than or equal to 10.

[0086] The OI (Orientation Index) value of graphite materials represents the degree of uniformity in the orientation of graphite grains and is an important indicator of the low expansion characteristics of graphite anode active materials. Generally, a lower OI value indicates a stronger isotropic degree of graphite particles, which is beneficial for suppressing expansion during cycling. The OI value can be measured by XRD (X-ray Diffraction) to detect the 110 and 002 or 004 characteristic peaks of graphite in the electrode, and then the OI value can be calculated. When performing diffraction pattern testing on a horizontally placed electrode sample, the diffraction signal of the (110) crystal plane that can be collected comes from the graphite with a layer structure perpendicular to the electrode sheet, and the diffraction signals of the (002) and (004) crystal planes come from the graphite with a layer structure parallel to the electrode sheet. Therefore, the orientation of the graphite electrode can be described by the ratio of the intensity (or integrated area) of the (002) or (004) diffraction peak to the intensity (or integrated area) of the (110) diffraction peak, i.e., OI = I(002) / I(110) or OI = I(004) / I(110), where I(002) represents the intensity of the (002) diffraction peak, I(004) represents the intensity of the (004) diffraction peak, and I(110) represents the intensity of the (110) diffraction peak.

[0087] In some embodiments, the mass of the silicon-based material is m1, the mass of the first graphite material is m2, and m1 / (m1+m2) is 5% to 30%. Limiting the ratio of the mass of the silicon-based material to the mass of the first graphite material to not less than 5% can ensure the ED of the secondary battery, and limiting it to not more than 30% can not only reduce lithium plating, but also ensure the binding effect of the first graphite material on silicon, and ensure the structural integrity and structural stability of the negative electrode sheet.

[0088] In some embodiments, the particle size D of the silicon-based material v 50 and the particle size D of the first graphite material v The ratio of 50 is 0.1 to 0.5.

[0089] Among them, particle size D v 50 refers to the particle size that, in the volumetric particle size distribution of a material, reaches 50% of the cumulative volume from the smallest particle size. Particle size D v 50 can be obtained by measuring with a laser particle size analyzer in accordance with GB / T 19077-2016 "Particle size distribution by laser diffraction".

[0090] It is worth noting that when the particle size D of the silicon-based material... v 50 and the particle size D of the first graphite material v When the ratio of 50 to 1 is less than 0.1, the silicon-based material has a small particle size and a large specific surface area, leading to excessive side reactions and accelerated battery cycle degradation. When the particle size D of the silicon-based material... v 50 and the particle size D of the first graphite material v When the ratio of 50 to 1 is greater than 0.5, the battery expansion rate increases rapidly. This is because the particle size of the silicon-based material is very close to that of the first graphite material, resulting in poor co-intercalation between the two materials and causing a sharp increase in the battery expansion rate. Therefore, the particle size D of the silicon-based material... v The ratio of 50 to the particle size D of the first graphite material v The ratio of 50 is preferably 0.1 to 0.5.

[0091] In some embodiments, the particle size D of the silicon-based material v 50 and the particle size D of the first graphite material v The ratio of 50 is 0.2 to 0.3. When the particle size D of the silicon-based material... v 50 and the particle size D of the first graphite material v When the ratio of 50 is greater than 0.3, the increase in capacity retention is relatively gradual with increasing ratio; therefore, increasing the ratio has little benefit on capacity retention when the ratio is greater than 0.3. When the ratio is less than 0.2, the decrease in expansion rate is relatively gradual with decreasing ratio; therefore, decreasing the ratio has little benefit on expansion rate when expansion rate is less than 0.2. Considering both expansion rate and capacity retention, the particle size D of silicon-based materials... v 50 and the particle size D of the first graphite material v The ratio of 50 is preferably 0.2 to 0.3.

[0092] Among them, the particle size D of silicon-based materials v 50 can be 4μm to 10μm, the first graphite material particle size D v 50 can range from 16μm to 40μm.

[0093] In some embodiments, the first active material layer 2 further includes a first dispersant and a conductive agent. The first dispersant, conductive agent, first graphite material, silicon-based material, and solid electrolyte are mixed.

[0094] For the first dispersant mentioned above, the first dispersant can be CMC (sodium carboxymethyl cellulose).

[0095] The conductive agent described above can adhere to the surface of the silicon-based material, thereby increasing the electron transport efficiency of the silicon-based material. The conductive agent can be one or more of conductive carbon materials such as SP (conductive carbon black), CNT (carbon nanotubes), VGCF (vinyl glass fiber), metal particles, or metal fibers. The amount of conductive agent added is 0–20 wt% of the mass percentage of the first active material layer 2. When the mass percentage of the conductive agent in the first active material layer 2 is greater than 20 wt%, the capacity retention rate of the secondary battery no longer increases, and the energy density of the secondary battery decreases as the mass percentage of the conductive agent in the first active material layer 2 increases. Therefore, the mass percentage of the conductive agent in the first active material layer 2 is preferably 0–20 wt%.

[0096] In some embodiments, please refer to Figure 4 The secondary battery 100 also includes a second active material layer 3, which is disposed on the side of the first active material layer 2 away from the negative electrode current collector 1.

[0097] When two first active material layers 2 are disposed on two opposite surfaces of the negative electrode current collector 1 along the thickness direction X, two second active material layers 3 can be disposed. The two second active material layers 3 can be disposed on a surface of the two first active material layers 2 that is away from the negative electrode current collector 1.

[0098] It is understandable that multiple second active material layers 3 may be spaced apart on one surface of the first active material layer 2.

[0099] The second active material layer 3 includes a second graphite material. By further providing the second active material layer 3 in the secondary battery 100, silicon can be further confined, reducing silicon expansion and displacement, and enhancing the structural integrity and stability of the negative electrode sheet.

[0100] The second graphite material can be one or more of hard carbon, soft carbon, or graphite. The second graphite material can be the same as the first graphite material; for example, the OI value of the second graphite material is also less than or equal to 10.

[0101] In some embodiments, the mass ratio of the second active material layer 3 to the first active material layer 2 ranges from 0.2:1 to 2:1. When the mass ratio of the second active material layer 3 to the first active material layer 2 is less than 0.2, the second active material layer 3 has limited ability to restrict the expansion of the silicon-based material in the first active material layer 2, resulting in a faster increase in the battery's expansion rate. When the mass ratio of the second active material layer 3 to the first active material layer 2 is greater than 2, the wetting time increases significantly, indicating that the first active material layer 2 has a high compaction density and is not easily wetted. Excessive compaction density can lead to particle breakage in the first active material layer 2. Therefore, the preferred mass ratio range of the second active material layer 3 to the first active material layer 2 is between 0.2:1 and 2:1.

[0102] In some embodiments, the second active material layer 3 further includes a binder and a second dispersant.

[0103] For the above-mentioned adhesives, the adhesives can be PAA (polyacrylic acid), SBR (styrene-butadiene rubber), CMC (sodium carboxymethyl cellulose), PVDF (polyvinylidene fluoride), PAN (polyacrylonitrile), PVA (polyvinyl alcohol), etc., as well as functionalized derivatives or monomer copolymers of the above polymers.

[0104] For the second dispersant mentioned above, the second dispersant can be CMC (sodium carboxymethyl cellulose).

[0105] In one specific embodiment, the mass ratio of the second graphite material, the binder, and the second dispersant can be 97.5:1.5:1.

[0106] In some embodiments, the second active material layer 3 further includes inorganic materials. The inorganic materials may include one or more of the following: alumina, garnet, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate.

[0107] In some embodiments, the inorganic material is alumina and / or boehmite, which have good electrochemical stability and are readily available. In some embodiments, the inorganic material is alumina, which has high hardness.

[0108] It should be noted that when the negative electrode sheet 10 is rolled, the pressure on the second active material layer 3 is greater than that on the first active material layer 2. Therefore, the compaction density of the second active material layer 3 is higher than that of the first active material layer 2, and the particles in the second active material layer 3 are more easily broken than the particles in the first active material layer 2. When the particles in the first active material layer 2 and the second active material layer 3 break, for example, when the first graphite material and the second graphite material break, it will lead to a capacity loss in the secondary battery 100 and reduce the cycle performance of the secondary battery 100.

[0109] In this application, the negative electrode 10 is made of inorganic material, and the particle size D of the inorganic material is... v 50 and the particle size D of the second graphite material v When the ratio of 50 is less than 0.23, the inorganic material can fill the gaps in the second graphite material. When the ratio is greater than 0.23, the processing window for the second graphite material begins to narrow. Please refer to [link / reference]. Figure 5 , Figure 5 This is an SEM (Scanning Electron Microscope) image of the negative electrode plate 10. Figure 5 The larger particles are secondary graphite materials, and the smaller particles are inorganic materials. From Figure 5 As can be seen, the inorganic material is mainly distributed within the gaps of the second graphite material. Since the inorganic material can fill the gaps of the second graphite material, it can lubricate and buffer the second graphite material, reduce the problem of graphite particle breakage, improve the overvoltage problem of the negative electrode 10, and enhance the cycle performance of the secondary battery 100.

[0110] Please see Figure 6 , Figure 6 This is another SEM image of the negative electrode plate 10. Figure 6 The larger particles are the second graphite material, and the smaller particles are inorganic materials. The upper side is the second active material layer 3, and the lower side is the first active material layer 2. From Figure 6 As can be seen, after the addition of inorganic materials to the second active material layer 3, the second graphite material of the negative electrode 10 as a whole does not exhibit overvoltage, and the porosity of the second active material layer 3 is greater than that of the first active material layer 2. Therefore, when the secondary battery 100 includes a free electrolyte, the free electrolyte can easily permeate from the second active material layer 3 into the first active material layer 2, reducing concentration polarization, thereby reducing internal resistance, extending the cycle life of the secondary battery 100, and improving the utilization rate of the secondary battery 100.

[0111] It is understandable that when the secondary battery 100 includes a free electrolyte, the inorganic material can form more pores in the negative electrode 10, which is beneficial to enhance the free electrolyte wetting effect of the negative electrode 10, increase the amount of free electrolyte in the negative electrode 10, increase the ion transport speed of the free electrolyte, and improve the fast charging capability.

[0112] In some embodiments, the particle size D of the inorganic material v 50 and the particle size D of the second graphite material v The ratio of 50 is 0.04 to 0.1. This limitation minimizes particle breakage of the second graphite material, allowing the secondary battery 100 to achieve optimal capacity retention. At this point, the processing window for the second graphite material is wider, and its kinetic properties are better. Optionally, the particle size D of the inorganic material... v 50 is 0.3μm~2μm, and the particle size D of the second graphite material is... v 50 represents 10μm to 15μm.

[0113] In some embodiments, the inorganic material accounts for 0.1 wt% to 5 wt% of the mass percentage of the second active material layer 3. Excessive addition of inorganic material affects the energy density (ED), while insufficient addition has little effect on improving wettability. When the inorganic material accounts for less than 0.1 wt% of the mass percentage of the second active material layer 3, the rate of decrease in compaction density accelerates, resulting in a sharp reduction in the lubrication effect of the inorganic material on the second graphite material, and a significant increase in wetting time. When the inorganic material accounts for more than 5 wt% of the mass percentage of the second active material layer 3, the large proportion of inorganic material in the second active material layer 3 leads to a severe decrease in energy density. Therefore, the preferred mass percentage of inorganic material in the second active material layer 3 is 0.1 wt% to 5 wt%.

[0114] When the second active material layer 3 further includes a binder and a second dispersant, in a specific embodiment, the mass ratio of the second graphite material, the inorganic material, the binder, and the second dispersant can be 97:0.5:1.5:1.

[0115] In some embodiments, the secondary battery 100 includes a negative electrode 10, a separator, and a positive electrode. The positive electrode includes a positive active material layer, which may be at least one of lithium cobalt oxide, lithium nickel oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium manganese oxide, lithium nickel oxide, lithium manganese iron phosphate, lithium vanadium phosphate, and lithium iron phosphate. The separator may include a polymer or inorganic material formed from a material stable to the free electrolyte. For example, the separator may include a substrate layer and a surface treatment layer. The substrate layer is a nonwoven fabric, membrane, or composite membrane with a porous structure, and the material of the substrate layer includes at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Specifically, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane may be selected. At least one surface of the substrate layer is provided with a surface treatment layer, which may be a polymer layer, an inorganic layer, or a layer formed by mixing polymers and inorganic materials. The inorganic layer comprises inorganic particles and a binder. The inorganic particles include at least one of alumina, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. The binder includes at least one of polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene. The polymer layer contains a polymer, the polymer material of which includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride, or poly(vinylidene fluoride-hexafluoropropylene).

[0116] To evaluate the beneficial effects of the secondary battery 100 of this application, tests were conducted, wherein PMMA was used as the solid electrolyte. Unless otherwise specified, all reagents, materials, and instruments used in the following examples and comparative examples are commercially available.

[0117]

Preparation Method

[0118] Example 1

[0119] Preparation of the first active material layer 2 slurry

[0120] The first graphite material, silicon-based material, first dispersant, conductive agent, MMA (methyl methacrylate), and AIBN (azobisisobutyronitrile) are mixed in a mass ratio of 80:10:2:2:5.5:0.5 and dissolved in deionized water to form a first active material slurry with a solid content of 40%. The slurry is stirred evenly and prepared for coating. The silicon-based material is SiO2 (silicon dioxide), the first graphite material is graphite particles, the first dispersant is CMC (sodium carboxymethyl cellulose), and the conductive agent is conductive carbon black.

[0121] Preparation of negative electrode 10

[0122] The first active material layer 2 slurry is uniformly coated onto one surface of a copper foil of the negative electrode current collector 1 with a thickness of 10 μm using an extrusion coating machine. The coating is then dried at 110 degrees Celsius to obtain a negative electrode sheet 10 with the first active material layer 2 coated on one side only. After drying, the coating thickness of the first active material layer 2 is 150 μm. The coating process is then repeated on the other surface of the negative electrode current collector 1 to obtain a negative electrode sheet 10 with the first active material layer 2 coated on both sides.

[0123] After coating, the negative electrode sheet 10 is dried, cold-pressed, and cut into sheets with dimensions of 74mm × 867mm for later use. The compacted density of the negative electrode sheet 10 is 1.7g / cm³. 3 The adhesion force between the first active material layer 2 and the negative electrode current collector 1 must be 10-100 N / m; the total porosity of the negative electrode sheet 10 must be 10%-30%.

[0124] Preparation of positive electrode active material layer slurry

[0125] The positive electrode active material LiCoO2 (lithium cobalt oxide), conductive agent conductive carbon black, and binder PVDF (polyvinylidene fluoride) are dissolved in NMP (N-methylpyrrolidone) solution at a mass ratio of 97:1.4:1.6 to form a positive electrode slurry with a solid content of 75%. The slurry is stirred evenly and is ready for coating and use.

[0126] Preparation of positive electrode sheet

[0127] The positive electrode slurry was uniformly coated onto one surface of a 10 μm thick aluminum foil for the positive electrode current collector, and then dried at 110°C to obtain a positive electrode sheet with a positive electrode active material layer coated on one side. After drying, the coating thickness of the positive electrode active material layer was 110 μm.

[0128] Next, repeat the above coating steps on the other surface of the positive current collector to obtain a positive electrode sheet with positive active material coated on both sides.

[0129] After coating, the positive electrode sheet is dried, cold-pressed, and cut into sheets with dimensions of 74mm × 867mm for later use. The compacted density of the positive electrode sheet is 4.15g / cm3.

[0130] Preparation of electrolytes

[0131] In an environment with a water content of less than 10 ppm, organic solvents EC (ethylene carbonate), PC (propylene carbonate), DEC (diethyl carbonate), and EP (ethyl propionate) were mixed in a mass ratio of 3:1:3:3. Lithium hexafluorophosphate (LiPF6) was then added to this mixed organic solvent to dissolve and mix thoroughly to obtain the desired electrolyte. The concentration of LiPF6 was 1 mol / L.

[0132] This application does not impose specific limitations on the electrolyte, which can be selected according to actual needs. As an example, the additives in the electrolyte may include one or more of the following: vinylene carbonate (VC), ethylene ethylene carbonate (VEC), succinate (SN), adiponitrile (ADN), 1,3-propenesulfonate lactone (PST), tris(trimethylsilane) phosphate (TMSP), trimethyl borate (TMB), or tris(trimethylsilane) borate (TMSB).

[0133] Preparation of the separating membrane

[0134] The separator consists of a substrate layer and a coating. The substrate layer is 5μm thick PE (polyethylene). A 2μm thick alumina ceramic layer is coated on both sides of the substrate layer. Then, 2.5mg of PVDF (polyvinylidene fluoride) binder is coated on the side of the ceramic layer away from the substrate layer. Finally, it is dried.

[0135] Preparation of secondary battery 100

[0136] The positive electrode, separator, and negative electrode 10 prepared above are stacked in sequence, with the separator positioned between the positive and negative electrode 10 to provide isolation. Then, they are hot-pressed together to form a stacked electrode assembly.

[0137] The electrode assembly was placed in a packaging bag and sealed from the side and top. It was then placed in a vacuum oven at 85 degrees Celsius for 12 hours to remove moisture. The prepared electrolyte was then injected. After vacuum sealing, settling, formation (0.02C constant current charging to 3.5V, then 0.1C constant current charging to 3.9V), shaping, and capacity testing, a secondary battery 100 was obtained.

[0138] Examples A1 to A10

[0139] To facilitate the reader's understanding of the inventive concept of this application, ten specific secondary batteries provided by this application are listed below based on Example 1. These ten specific secondary batteries correspond to specific Examples A1 to A10. In Examples A1 to A10, the mass percentages of the first graphite material, silicon-based material, first dispersant, conductive agent, MMA (methyl methacrylate), and AIBN (azobisisobutyronitrile) in the slurry of the first active material layer 2 are adjusted, and then the coating and drying steps as described in Example 1 are performed to obtain solid electrolytes accounting for 0.01 wt%, 0.05 wt%, 0.1 wt%, 1 wt%, 5.5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, and 30 wt% of the first active material layer 2, respectively. The remaining steps are the same as in Example 1.

[0140] Comparative Example 1

[0141] The difference from Example 1 is that the first active material layer 2 slurry does not contain MMA (methyl methacrylate) and AIBN (azobisisobutyronitrile), and PAA (polyacrylic acid) is used as a binder in the first active material layer slurry. The mass ratio of the first graphite material, silicon-based material, first dispersant, conductive agent and PAA is 80:10:2:2.5:5.5.

[0142] Examples B1 to B9

[0143] To explore the preferred mass ratio range of silicon-based material to first graphite material, nine specific secondary batteries provided in this application are listed based on Example 1. These nine specific secondary batteries correspond to specific Examples B1 to B9. In Examples B1 to B9, the mass percentage of silicon-based material in the sum of the mass of first graphite material and silicon-based material is 1%, 3%, 5%, 10%, 15%, 20%, 25%, 30%, and 35%, respectively. The remaining steps are the same as in Example 1. The battery cycle count is 300 revolutions.

[0144] Examples C1 to C11

[0145] To investigate the particle size D of silicon-based materials v 50 and the particle size D of the first graphite material v The preferred ratio range of 50 is listed in Example 1, outlining 11 specific secondary batteries provided in this application. These 11 specific secondary batteries correspond to specific Examples C1 to C11. The particle size D of the silicon-based material in Examples C1 to C11 is... v 50 and the particle size D of the first graphite material vThe ratios of 50 were 0.03, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.4, 0.5, 0.6, and 0.7, respectively, and the remaining steps were the same as in Example 1. The battery cycle count was 300 revolutions.

[0146] Examples D1 to D10

[0147] To explore the preferred range of the mass ratio of the conductive agent to the first active material layer 2, ten specific secondary batteries provided in this application are listed based on Example 1. These ten specific secondary batteries correspond to specific Examples D1 to D10. In Examples D1 to D10, the mass percentage of the conductive agent added to the first active material layer 2 is 0 wt%, 0.1 wt%, 1 wt%, 2 wt%, 5 wt%, 10 wt%, 15 wt%, 20 wt%, 30 wt%, and 40 wt%, respectively, and the remaining steps are the same as in Example 1. The battery cycle count is 300 revolutions.

[0148] Example 2

[0149] The difference from Example 1 is that:

[0150] The negative electrode 10 also includes a second active material layer 3, which includes a second graphite material, a binder, and a second dispersant, with a mass ratio of 97.5:1.5:1.

[0151] The preparation of negative electrode slurry also includes

[0152] Second active material layer slurry: The second graphite material, binder, and second dispersant are mixed at a mass ratio of 97.5:1.5:1, dissolved in deionized water, to form a negative electrode slurry with a solid content of 50%. The mixture is stirred evenly and prepared for coating. The second graphite material is made of graphite particles, the binder is SBR (styrene-butadiene rubber), and the second dispersant is CMC (sodium carboxymethyl cellulose).

[0153] The preparation process of the negative electrode 10 is changed to:

[0154] A double-layer coating machine is used to simultaneously and uniformly coat the first and second active material layer slurries onto one surface of a 10 μm thick copper foil of the negative electrode current collector 1, and then dry it at 110 degrees Celsius to obtain a negative electrode sheet 10 with a double-layer negative electrode active material layer on one side. After drying, the coating thickness of the double-layer negative electrode active material layer is 150 μm. In this step, since the second active material layer slurry is coated on the undried first active material layer slurry, the first active material layer 2 and the second active material layer 3 are mutually soluble, with no obvious interface and low interfacial impedance, thus helping to improve the interface problem between the first active material layer 2 and the second active material layer 3. Then, the above coating steps are repeated on the other surface of the negative electrode sheet 10 to obtain a negative electrode sheet 10 with a double-layer negative electrode active material layer coated on both sides.

[0155] After coating, the negative electrode sheet 10 is dried, cold-pressed, and cut into sheets with a specification of 74mm×867mm for later use.

[0156] Examples E1 to E10

[0157] To explore the preferred range of the mass ratio of the second active material layer 3 to the first active material layer 2, ten specific secondary batteries provided in this application are listed based on Example 2. These ten specific secondary batteries correspond to specific Examples E1 to E10. In Examples E1 to E10, the mass ratio of the second active material layer 3 to the first active material layer 2 is 0.05, 0.1, 0.2, 0.3, 0.5, 1, 1.5, 2, 2.5, and 3, respectively, and the remaining steps are the same as in Example 2. The battery cycle count is 300 revolutions.

[0158] Example 3

[0159] The difference from Example 2 is that the second active material layer 3 also includes inorganic materials, and the mass ratio of the second graphite material, inorganic materials, binder and second dispersant is 97:0.5:1.5:1.

[0160] Examples F1 to F13

[0161] To investigate the particle size D of inorganic materials v 50 and the particle size D of the second graphite material v The preferred range of the ratio 50 is illustrated in Example 3, which lists 13 specific secondary batteries provided in this application. These 13 specific secondary batteries correspond to specific Examples F1 to F13. The particle size D of the inorganic material in Examples F1 to F13... v 50 and the particle size D of the second graphite material vThe ratios of 50 were 0.03, 0.035, 0.04, 0.045, 0.05, 0.1, 0.15, 0.2, 0.21, 0.22, 0.23, 0.24, and 0.25, respectively, and the remaining steps were the same as in Example 3. The battery cycle count was 300 revolutions.

[0162] Examples G1 to G11

[0163] To investigate the preferred range of the mass percentage of inorganic material in the second active material layer 3, 11 specific secondary batteries provided in this application are listed based on Example 3. These 11 specific secondary batteries correspond to specific Examples G1 to G11. In Examples G1 to G11, the mass percentage of inorganic material in the second active material layer 3 is 0.01 wt%, 0.05 wt%, 0.1 wt%, 0.5 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, and 7 wt%, respectively, and the remaining steps are the same as in Example 3. The battery cycle count is 300 cycles.

[0164] [Testing Method]

[0165] The secondary batteries 100 of Examples 1, A1 to A10, Comparative Examples 1, B1 to B9, C1 to C11, D1 to D10, 2, E1 to E10, 3, F1 to F13 and G1 to G11 were subjected to the following related tests. If the test temperature is not explicitly stated during the test, the room temperature of 25°C is used as an example.

[0166] (1) Capacity retention test

[0167] Charging: In a high-temperature furnace at 45 degrees Celsius, charge the battery to the upper voltage window at a constant current of 2C, then charge it to 0.05C at a constant voltage, and then let it stand for 5 minutes.

[0168] Discharge: Discharge to 3.0V using 0.7C.

[0169] Following the above charge-discharge method, record the discharge capacity for each cycle, divide it by the initial capacity, and obtain the capacity retention rate for each cycle. The discharge capacity of the first discharge is the initial discharge capacity, which is set to 100%.

[0170] For example, the capacity retention rate after 300 cycles = (discharge capacity of the 300th cycle / discharge capacity of the first cycle) × 100%.

[0171] (2) Secondary battery expansion rate test

[0172] The thickness of the secondary battery was tested at three locations, and the average value was taken.

[0173] The average battery thickness is recorded during the cycling process. The difference between the average battery thickness after each cycle and the initial average battery thickness is divided by the initial average battery thickness to obtain the battery expansion rate. The average battery thickness at cycle number 0 is the initial average thickness.

[0174] For example, the battery expansion rate after 300 cycles = [(average battery thickness after 300 cycles - average initial battery thickness) / average initial battery thickness] × 100%.

[0175] (3) Lithium deposition degree test of negative electrode sheet 10

[0176] The secondary battery 100, which has undergone 300 cycles, was disassembled. The lithium plating on the negative electrode 10 was visually inspected, and the degree of lithium plating was judged as "slight," "moderate," or "severe." The judgment criteria are as follows:

[0177] 1) There are fewer than 3 lithium plating sites on each negative electrode 10, and / or the area of ​​lithium plating accounts for less than 10% of the area of ​​the negative electrode 10 – slight;

[0178] 2) There are 3 or more lithium deposition sites and 10 or fewer sites on each negative electrode 10, and / or the lithium deposition area accounts for 10% or more and 50% or less of the area of ​​the negative electrode 10 - moderate.

[0179] 3) There are more than 10 lithium plating sites on each negative electrode 10, and / or the area of ​​lithium plating accounts for more than 50% of the area of ​​the negative electrode 10 – severe.

[0180] (4) Ionic conductivity test

[0181] A symmetrical SS / SE / SS battery was assembled using stainless steel (SS) and an electrolyte (SE) for EIS testing to obtain the resistance R. The test result was then calculated using the ionic conductivity formula. Wherein, resistivity ρ = RS / L, conductivity = 1 / ρ = L / RS, S is the cross-sectional area of ​​the battery, and L is the thickness of the battery.

[0182] (5) Adhesion test

[0183] 1) The negative electrode 10 was hot-pressed at 85°C and 2MPa for 10 minutes. After cooling, the peel force between the first active material layer 2 and the current collector 1 was tested.

[0184] 2) Stick a layer of double-sided tape on the steel plate, place the negative electrode 10 flat and lightly press it on the double-sided tape, then stick a layer of test tape on top of the negative electrode 10, and hold the pressure roller (2000g) and roll it back and forth on the test tape three times.

[0185] 3) Tear one end of the tape to the middle of the sample, and perform a 180-degree peel test using a tensile testing machine. Peeling speed: 50 mm / min, peeling test duration: 1 min.

[0186] 4) Divide the test tensile force value by the width of the corresponding tape to obtain the final peel force N / m.

[0187] (6) Immersion time test

[0188] Equal volumes of free electrolyte, such as 0.05 ml of free electrolyte, were dropped onto different negative electrode plates 10, and the time it took for the free electrolyte to be completely absorbed was observed.

[0189] (7) Secondary battery volume energy density test

[0190] Place the secondary battery 100 in a constant temperature chamber at 25℃±2℃ and let it stand for 30 minutes to allow it to reach a constant temperature. Charge the battery at a constant current of 0.5C until it reaches the full charge voltage, then charge it at a constant voltage of 0.05C and discharge it at 0.2C until it reaches 3.0V. Record the discharge energy.

[0191] Volumetric energy density = Discharge energy / (Length of secondary battery 100 × Width of secondary battery 100 × Thickness of secondary battery 100).

[0192]

Experimental Results

[0193] The impact of solid electrolytes and binders in existing technologies on the performance of secondary batteries.

[0194] Figure 7 This is a cycle life graph of the secondary batteries in Comparative Example 1 and Example 1. (From...) Figure 7 It can be seen that, under the same number of cycles, the secondary battery of Example 1 has a higher capacity retention rate than the secondary battery of Comparative Example 1. That is, by replacing the binder in the first active material layer 2 with PMMA, the expansion displacement of silicon can be reduced, thereby improving the capacity retention rate of the secondary battery 100 and improving the cycle performance of the secondary battery 100.

[0195] Figure 8 This is a cycle expansion rate graph for the secondary batteries in Comparative Example 1 and Example 1. (From...) Figure 8It can be seen that, under the same number of cycles, the expansion rate of the secondary battery in Example 1 is reduced compared to that of the secondary battery in Comparative Example 1. Therefore, by replacing the binder in the first active material layer 2 with PMMA, the expansion displacement of silicon can be reduced, thereby lowering the expansion rate of the secondary battery 100. Since the capacity of the secondary battery in Comparative Example 1 decreased significantly and its expansion rate was large, the test was stopped after 300 cycles.

[0196] The degree of lithium plating in Comparative Example 1 and Example 1 was tested. The degree of lithium plating in the negative electrode 10 of Comparative Example 1 was "severe" and the degree of lithium plating in the negative electrode 10 of Example 1 was "moderate". This proves that replacing the binder in the first active material layer 2 with PMMA can improve the lithium plating problem of the negative electrode 10 and improve the cycle performance of the secondary battery 100.

[0197] The ionic conductivity of Comparative Example 1 and Example 1 was tested. The ionic conductivity of the negative electrode in Example 1 was 1.2 x 10⁻⁶. -3 ~6x10 -3 The ionic conductivity of the negative electrode in Comparative Example 1 is 1.2 x 10⁻⁶ S / cm. -5 ~8x10 -4 By adding PMMA to the first active material layer 2, the kinetic performance of the secondary battery 100 can be improved.

[0198] The influence of the mass percentage of solid electrolyte in the first active material layer 2 on the performance of secondary batteries

[0199] Table 1 below shows the performance of the secondary batteries corresponding to Examples A1 to A10. In Examples A1 to A10, the mass percentage of the solid electrolyte in the first active material layer 2 is 0.01 wt%, 0.05 wt%, 0.1 wt%, 1 wt%, 5.5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, and 30 wt%, respectively.

[0200] Table 1

[0201]

[0202]

[0203] It should be noted that in Table 1, the mass ratio of the first graphite material, silicon-based material, first dispersant, conductive agent and AIBN (azobisisobutyronitrile) in the first active material layer slurry is 80:10:2:2:0.5. The required mass ratio of MMA to each substance in the first active material layer slurry is calculated from the mass percentage of solid electrolyte PMMA in Table 1.

[0204] As shown in Table 1, when the solid electrolyte accounts for less than 0.1 wt% of the first active material layer 2, its ability to restrict the expansion of the silicon-based material is limited, resulting in a larger expansion rate and lower capacity retention of the secondary battery. Furthermore, the adhesion between the separator and the negative electrode 10 does not meet the industry minimum requirement of 10 N / m, easily causing the first active material layer 2 to pulverize or detach. This is because, in this invention, the solid electrolyte, besides suppressing the expansion displacement of the silicon material and acting as an electrolyte, also functions as a binder; insufficient addition of solid electrolyte will affect the adhesion between the first active material layer 2 and the negative electrode current collector 1. When the solid electrolyte accounts for more than 20 wt% of the first active material layer 2, severe lithium plating occurs on the negative electrode, leading to a larger expansion rate of the secondary battery. An increase in the expansion rate leads to a decrease in capacity retention. It's important to note that when the solid electrolyte accounts for less than or equal to 20 wt% of the first active material layer 2, the expansion rate of the negative electrode 10 decreases. This is because as the mass percentage of the solid electrolyte increases, its effect on suppressing silicon expansion and displacement becomes more significant, which is easily understood. However, when the solid electrolyte accounts for more than 20 wt% of the first active material layer 2, the expansion rate of the negative electrode 10 actually increases. This is not because the solid electrolyte's suppression of silicon expansion weakens, but because when the solid electrolyte percentage is too high, the proportion of active material in the negative electrode decreases accordingly, resulting in severe lithium plating. The deposited lithium dendrites cause an increase in the volume of the secondary battery 100, manifested as an increased expansion rate. Therefore, the preferred mass percentage of the solid electrolyte in the first active material layer 2 is 0.1 wt% to 20 wt%.

[0205] The impact of the mass percentage of silicon-based materials and primary graphite materials on the performance of secondary batteries.

[0206] Table 2 below shows the performance of the secondary batteries corresponding to Examples B1 to B9. In Examples B1 to B9, the mass percentages of silicon-based material in the sum of the mass of the first graphite material and the silicon-based material are 1%, 3%, 5%, 10%, 15%, 20%, 25%, 30%, and 35%, respectively.

[0207] Table 2

[0208]

[0209] It should be noted that in Table 2, the first dispersant, conductive agent, MMA (methyl methacrylate) and AIBN (azobisisobutyronitrile) are added in a mass ratio of 2:2:5.5:0.5. The mass ratio of each substance in the slurry of silicon-based material and first graphite material to the first active material layer is calculated by m1 / (m1+m2) in Table 2.

[0210] As shown in Table 2, when m1 / (m1+m2) is greater than 30, the increased content of silicon-based material limits the ability of the solid electrolyte to restrict the expansion of the silicon-based material, resulting in a larger expansion rate and lower capacity retention of the secondary battery, as well as severe lithium plating, thus deteriorating the battery's kinetic performance. Conversely, when m1 / (m1+m2) is less than 5, the battery's energy density (ED) drops sharply. Therefore, the preferred ratio of the mass of silicon-based material to the sum of the masses of the first graphite material and the silicon-based material is 5%–30%.

[0211] The influence of the particle size ratio of silicon-based materials to graphite-based materials on the performance of secondary batteries.

[0212] Table 3 below shows the performance of the secondary batteries corresponding to Examples C1 to C11. The particle size D of the silicon-based material in Examples C1 to C11 is... v 50 and the particle size D of the first graphite material v The ratios of 50 are 0.03, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.4, 0.5, 0.6 and 0.7, respectively.

[0213] Table 3

[0214]

[0215] As can be seen from Table 3, the particle size D of silicon-based materials v The ratio of 50 to the particle size D of the first graphite material v When the ratio of 50 to 1 is less than 0.1, the capacity of the secondary battery 100 decreases rapidly. This is because when the particle size of the silicon-based material is small, its specific surface area is large, leading to excessive side reactions and accelerating the cycle degradation of the secondary battery 100. The particle size D of the silicon-based material v The ratio of 50 to the particle size D of the first graphite material v When the ratio of 50 to 1 is greater than 0.5, the expansion rate of the secondary battery 100 increases rapidly. This is because the particle size of the silicon-based material is very close to that of the first graphite material, resulting in poor co-intercalation between the silicon-based material and the first graphite material, which causes a sharp increase in the expansion rate of the secondary battery 100. Therefore, the particle size D of the silicon-based material... v The ratio of 50 to the particle size D of the first graphite material v The ratio of 50 is preferably 0.1 to 0.5.

[0216] Further observation of the data in Table 3 shows that, in the silicon-based material particle size D... v 50 and the particle size D of the first graphite material vWhen the ratio of 50 to capacity retention is greater than 0.25, the increase in capacity retention is relatively gradual with increasing ratio; therefore, increasing the ratio has little benefit on capacity retention when the ratio is greater than 0.25. When the ratio is less than 0.25, the decrease in expansion rate is relatively gradual with decreasing ratio; therefore, decreasing the ratio has little benefit on expansion rate when expansion rate is less than 0.25. Considering both expansion rate and capacity retention, the particle size D of silicon-based materials... v 50 and the particle size D of the first graphite material v The ratio of 50 is preferably 0.25, wherein the particle size D of the silicon-based material is... v 50 can be 4-10 μm, then the particle size D of the first graphite material v 50 is 16-40 μm.

[0217] The influence of the mass ratio of conductive agent to the first active material layer 2 on the performance of secondary batteries

[0218] Table 4 below shows the performance of the secondary batteries corresponding to Examples D1 to D10. In Examples D1 to D10, the mass percentage of the conductive agent added to the first active material layer 2 is 0 wt%, 0.1 wt%, 1 wt%, 2 wt%, 5 wt%, 10 wt%, 15 wt%, 20 wt%, 30 wt%, and 40 wt%, respectively.

[0219] Table 4

[0220] Conductive agent mass percentage / wt% Energy density / Wh / L Capacity retention rate / % Example D1 0 788 65.3 Example D2 0.1 787.7 70.1 Example D3 1 784 76.5 Example D4 2 778 81.7 Example D5 5 764 84.4 Example D6 10 737 86.1 Example D7 15 707 87.0 Example D8 20 680 87.5 Example D9 30 617 87.5 Example D10 40 550 87.5

[0221] It should be noted that in Table 4, the mass ratio of the first graphite material, silicon-based material, MMA (methyl methacrylate), AIBN (azobisisobutyronitrile), and the first dispersant is 80:10:5.5:0.5:2. The mass ratio of the conductive agent to each substance in the first active material layer slurry is calculated from the mass percentage of the conductive agent in Table 4.

[0222] As can be seen from Table 4, when the mass percentage of the conductive agent in the first active material layer 2 is greater than 20 wt%, the capacity retention rate no longer increases, and the energy density decay of the secondary battery 100 accelerates as the mass percentage of the conductive agent in the first active material layer 2 increases. Therefore, the preferred mass percentage of the conductive agent in the first active material layer 2 is 0–20 wt%. Further, the preferred mass percentage of the conductive agent in the first active material layer 2 is 0.1–15 wt%. Further, the preferred mass percentage of the conductive agent in the first active material layer 2 is 0.1–5 wt%. Further, the preferred mass percentage of the conductive agent in the first active material layer 2 is 1–5 wt%.

[0223] The impact of the second active material layer 3 on the performance of secondary batteries

[0224] Figure 9 This is a cycle life diagram of the secondary batteries from Examples 1 and 2. Figure 9 It can be seen that, under the same number of cycles, the capacity retention rate of the secondary battery in Example 2 is improved compared with that of the secondary battery in Example 1. That is, by attaching a second active material layer 3 with a second graphite material to the surface of the first active material layer 2, the expansion displacement of silicon can be further reduced, thereby improving the cycle performance of the secondary battery 100.

[0225] Figure 10 These are cycle expansion rate diagrams of the secondary batteries in Examples 1 and 2, from... Figure 10 It can be seen that, under the same number of cycles, the expansion rate of the secondary battery in Example 2 is reduced compared to that in Example 1. That is, by attaching a second active material layer 3 with a second graphite material to the surface of the first active material layer 2, the expansion displacement of silicon can be further reduced, and the expansion rate of the secondary battery 100 can be further reduced.

[0226] The degree of lithium plating of the negative electrode 10 in Examples 1 and 2 was tested. The degree of lithium plating in Example 1 was "moderate" and the degree of lithium plating in the negative electrode 10 in Example 2 was "slight". By attaching a second active material layer 3 with a second graphite material to the surface of the first active material layer 2, the lithium plating problem of the negative electrode 10 can be improved and the cycle performance of the secondary battery 100 can be improved.

[0227] The influence of the mass ratio of the second active material layer 3 to the first active material layer 2 on the performance of the secondary battery

[0228] Table 5 below shows the performance of the secondary batteries corresponding to Examples E1 to E10. In Examples E1 to E10, the mass ratio of the second active material layer 3 to the first active material layer 2 is 0.05, 0.1, 0.2, 0.3, 0.5, 1, 1.5, 2, and 2.5, respectively.

[0229] Table 5

[0230]

[0231]

[0232] It should be noted that in Table 5, the quality of the second active material layer 3 and the first active material layer 2 is controlled by controlling the coating thickness of the first active material layer slurry and the second active material layer slurry. The total coating thickness of the second active material layer 3 and the first active material layer 2 is 150 μm.

[0233] As shown in Table 5, when the mass ratio of the second active material layer 3 to the first active material layer 2 is less than 0.2, the second active material layer 3 has limited ability to restrict the expansion of the silicon-based material in the first active material layer 2, resulting in a rapid increase in the expansion rate of the secondary battery 100. When the mass ratio of the second active material layer 3 to the first active material layer 2 is greater than 2, the wetting time increases significantly, indicating that the compaction density of the first active material layer 2 is relatively high, making it difficult to be wetted. Moreover, excessive compaction density can lead to the breakage of particles in the first active material layer 2. Therefore, the preferred mass ratio of the second active material layer 3 to the first active material layer 2 is between 0.2:1 and 2:1.

[0234] The influence of inorganic materials in the second active material layer 3 on the performance of secondary batteries

[0235] Figure 11 This is a cycle life diagram of the secondary battery 100 in Examples 2 and 3. Figure 11 It can be seen that, under the same number of cycles, the capacity retention rate of the secondary battery 100 in Example 3 is improved compared to that in Example 2, that is, by adding inorganic materials to the second active material layer 3, and the particle size D of the inorganic materials is... v 50 and the particle size D of the second graphite material v The ratio of 50 to 0.1 can lubricate and buffer the second graphite material, reduce the breakage of the second graphite material particles, reduce the capacity loss of the secondary battery 100, and improve the cycle performance of the secondary battery 100.

[0236] Figure 12 These are cycle expansion rate diagrams of the secondary battery 100 in Examples 2 and 3, derived from... Figure 12 It can be seen that, under the same number of cycles, the expansion rate of the battery in Example 3 is reduced compared to that in Example 2. That is, by adding inorganic materials to the second active material layer 3, the expansion rate of the secondary battery 100 can be further reduced.

[0237] The degree of lithium plating on the negative electrode 10 of Examples 2 and 3 was tested. The degree of lithium plating on the negative electrode 10 of Example 2 was "slight," and the degree of lithium plating on the negative electrode 10 of Example 3 was also "slight." However, the secondary battery of Example 3 had a lower degree of lithium plating than that of Example 2. That is, by attaching a second active material layer 3 containing a second graphite material and inorganic particles to the surface of the first active material layer 2, the lithium plating problem of the negative electrode 10 can be further improved, and the cycle performance of the secondary battery 100 can be improved. Furthermore, because the degree of lithium plating on the negative electrode 10 is reduced, the expansion rate of the secondary battery 100 is also reduced.

[0238] Regarding the particle size D of inorganic materials v 50 and the particle size D of the second graphite materialv The effect of the 50 ratio on the performance of secondary batteries

[0239] Table 6 below shows the performance of the secondary batteries corresponding to Examples F1 to F13. In Examples F1 to F13, the particle size D of the inorganic material is... v 50 and the particle size D of the second graphite material v The ratios of 50 are 0.03, 0.035, 0.04, 0.045, 0.05, 0.1, 0.15, 0.2, 0.21, 0.22, 0.23, 0.24 and 0.25, respectively.

[0240] Table 6

[0241]

[0242]

[0243] As can be seen from Table 6, the particle size D of inorganic materials v 50 and the particle size D of the second graphite material v When the ratio of 50 to 1 is 0.045, the capacity retention is optimal. This is because at a ratio of 0.045, the overall overpressure of the first active material layer 2 and the second active material layer 3 is better, with less particle breakage and less capacity loss, resulting in better kinetic performance of the secondary battery 100. When the ratio is greater than 0.23, the compaction density begins to decrease, and the processing window of the second graphite material begins to narrow. Therefore, the particle size D of the inorganic material... v 50 and the particle size D of the second graphite material v The ratio of 50 should be less than 0.23, preferably 0.045.

[0244] The influence of the mass percentage of inorganic materials in the second active material layer 3 on the performance of secondary batteries

[0245] Table 7 below shows the performance of the secondary batteries corresponding to Examples G1 to G11. In Examples G1 to G11, the mass percentage of inorganic material in the second active material layer 3 is 0.01 wt%, 0.05 wt%, 0.1 wt%, 0.5 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, and 7 wt%, respectively.

[0246] Table 7

[0247]

[0248]

[0249] It should be noted that in Table 7, the mass ratio of the second graphite material, the binder, and the second dispersant is 97:1.5:1. The amount of inorganic material added is calculated from the mass percentage of inorganic material in the second active material layer 3 in Table 7.

[0250] As shown in Table 7, when the inorganic material accounts for less than 0.1 wt% of the second active material layer 3, its lubricating and buffering effect on the second graphite material is not significant. During the cold pressing of the electrode, many active material particles are crushed, leading to a loss of battery capacity and a lower energy density of the secondary battery 100. Furthermore, the compaction density of the negative electrode 10 is relatively high, significantly increasing the wetting time of the free electrolyte. When the inorganic material accounts for more than 5 wt% of the second active material layer 3, its large proportion results in severe energy density loss. Therefore, the preferred mass percentage of the inorganic material in the second active material layer 3 is 0.1–5 wt%.

[0251] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and not to limit them; under the concept of this application, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of different aspects of this application as described above. For the sake of brevity, they are not provided in detail; although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or make equivalent substitutions for some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A secondary battery, characterized in that, The device includes a negative electrode sheet, which comprises a negative current collector and a first active material layer. The first active material layer is disposed on at least one surface of the negative current collector. The first active material layer comprises a silicon-based material and a solid electrolyte, with at least a portion of the solid electrolyte bonded to the surface of the silicon-based material. The solid electrolyte comprises at least one of polymethyl methacrylate, polyvinylidene fluoride, and polyacrylonitrile. The solid electrolyte accounts for 0.1 wt% to 20 wt% of the mass percentage of the first active material layer.

2. The secondary battery according to claim 1, characterized in that, The solid electrolyte accounts for 5 wt% to 15 wt% of the mass percentage of the first active material layer.

3. The secondary battery according to claim 1, characterized in that, The solid electrolyte is coated on the surface of the silicon-based material.

4. The secondary battery according to claim 1, characterized in that, The solid electrolyte comprises polymethyl methacrylate and / or polyacrylonitrile, wherein silicon in the silicon-based material is linked to hydroxyl groups, and the solid electrolyte is bonded to the hydroxyl groups.

5. The secondary battery according to claim 1, characterized in that, The secondary battery also includes a free electrolyte.

6. The secondary battery according to claim 1, characterized in that, The first active material layer further includes a first graphite material, which is mixed with the silicon-based material.

7. The secondary battery according to claim 6, characterized in that, The mass of the silicon-based material is m1, the mass of the first graphite material is m2, and m1 / (m1+m2) is 5%~30%.

8. The secondary battery according to claim 6, characterized in that, The particle size D of the silicon-based material v 50 and the particle size D of the first graphite material v The ratio of 50 is 0.1 to 0.

5.

9. The secondary battery according to claim 8, characterized in that, The particle size D of the silicon-based material v 50 and the particle size D of the first graphite material v The ratio of 50 is 0.2 to 0.

3.

10. The secondary battery according to claim 8, characterized in that, The particle size D of the silicon-based material v 50 is 4μm~10μm, and the particle size D of the first graphite material is... v 50 ranges from 16μm to 40μm.

11. The secondary battery according to claim 1, characterized in that, The silicon-based material includes one or more of elemental silicon, silicon oxide, silicon carbide, and silicon alloys.

12. The secondary battery according to any one of claims 1 to 11, characterized in that, The secondary battery further includes a second active material layer, which is disposed on the side of the first active material layer away from the negative electrode current collector; The second active material layer includes a second graphite material.

13. The secondary battery according to claim 12, characterized in that, The mass ratio of the second active material layer to the first active material layer is between 0.2:1 and 2:

1.

14. The secondary battery according to claim 12, characterized in that, The second active material layer also includes inorganic materials, wherein the particle size D of the inorganic materials is... v 50 and the particle size D of the second graphite material v The ratio of 50 is less than 0.

23.

15. The secondary battery according to claim 14, characterized in that, The particle size D of the inorganic material v 50 and the particle size D of the second graphite material v The ratio of 50 is 0.04 to 0.

1.

16. The secondary battery according to claim 15, characterized in that, The particle size D of the inorganic material v 50 is 0.3μm~2μm, and the particle size D of the second graphite material is... v 50 represents 10μm~15μm.

17. The secondary battery according to claim 14, characterized in that, The inorganic material accounts for 0.1wt% to 5wt% of the mass percentage of the second active material layer.

18. The secondary battery according to claim 14, characterized in that, The inorganic materials include one or more of the following: aluminum oxide, garnet, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate.

19. The secondary battery according to claim 18, characterized in that, The inorganic material is alumina and / or boehmite.

20. The secondary battery according to claim 1, characterized in that, The secondary battery satisfies one of the following conditions: The mass percentage of the conductive agent in the first active material layer is 0 wt% to 20%. The conductive agent in the first active material layer accounts for 0.1 wt% to 15% by mass; The conductive agent in the first active material layer accounts for 0.1 wt% to 5% by mass; The conductive agent in the first active material layer accounts for 1 wt% to 5% of the total mass.

21. An electrical appliance, characterized in that, Includes a secondary battery and a load as described in any one of claims 1 to 20, wherein the secondary battery is used to power the load.

22. A method for preparing a secondary battery according to any one of claims 1-20, characterized in that, To prepare the negative electrode sheet, a silicon-based material, methyl methacrylate, and azobisisobutyronitrile are mixed and dissolved in deionized water to form a first active material layer slurry; the first active material layer slurry is then uniformly coated onto at least one surface of the negative electrode current collector.

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