Secondary battery, electric device and preparation method for secondary battery

Abstract

A secondary battery (100) comprises a negative electrode sheet (10), wherein the negative electrode sheet comprises a current collector and a first active material layer (2); the first active material layer (2) is arranged on at least one surface of the current collector; and the first active material layer (2) comprises a silicon-based material and a solid electrolyte, and the solid electrolyte is at least partially bonded to the surface of silicon in the silicon-based material. In the secondary battery and the electric device, the solid electrolyte is at least partially bonded to the surface of silicon in the silicon-based material; therefore, the problem of the expansion of the silicon-based material can be ameliorated, and the structural integrity and structural stability of the negative electrode sheet (10) can be enhanced. Moreover, compared with a binder-coated silicon-based material in the prior art, due to the utilization of the solid electrolyte, the lithium intercalation capability of the silicon-based material can be improved, and the ion conduction rate of the negative electrode sheet (10) can be improved, that is, the problem of deteriorated dynamic properties of the secondary battery caused by a binder-coated silicon-based material can be ameliorated.

Description

Secondary battery, electrical equipment, and method for preparing secondary battery

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to the Chinese patent application filed with the Patent Office of China on March 31, 2024, with application number 202410383570.7 and application name “Secondary Battery, Electrical Equipment and Method for Preparing Secondary Battery”, all contents of which are incorporated by reference into this application. Technical Field

[0003] The present application relates to the technical field of electrochemical devices, and in particular to a secondary battery, an electrical device, and a method for preparing a secondary battery. Background Art

[0004] 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.

[0005] During the charge and discharge process of lithium-ion batteries, silicon particles will undergo lithiation and delithiation, causing the silicon particles to expand and contract, resulting in the destruction of the structural integrity and stability of the negative electrode.

[0006] Application Contents

[0007] Simply coating the silicon particles with a binder can reduce their expansion, thereby improving the problem of negative electrode damage. However, coating the silicon particles with a binder can affect the conductivity of the negative electrode, leading to deterioration in the kinetic performance of lithium-ion batteries, reduced cycle performance, and the occurrence of interfacial lithium deposition.

[0008] The embodiments of the present 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 surface of the silicon particles in the negative electrode sheet is coated with a binder.

[0009] In order to solve the above technical problems, a technical solution adopted in the embodiment of the present application is: providing a secondary battery, the secondary battery including a negative electrode plate, the negative electrode plate including a negative electrode collector and a first active material layer, the first active material layer is arranged on at least one surface of the negative electrode collector, the first active material layer includes a silicon-based material and a solid electrolyte, and at least part of the solid electrolyte is bonded to the surface of the silicon-based material.

[0010] By setting the first active material layer in the negative electrode plate to a silicon-based material and a solid electrolyte, at least part of the solid electrolyte is 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 silicon, reduce silicon expansion and the resulting displacement of silicon, enhance the structural integrity and structural stability of the negative electrode plate, and extend the service life of the secondary battery; on the other hand, the use of a solid electrolyte to bind silicon can improve the lithium insertion ability of the silicon-based material and increase the ion conduction rate in the negative electrode plate, compared to the use of a binder-coated silicon in the prior art, that is, improve the problem of deterioration of the kinetic performance of the secondary battery caused by the binder-coated silicon.

[0011] In some embodiments, the mass percentage of the solid electrolyte in the first active material layer is 0.1wt% to 20wt%. By limiting the content of the solid electrolyte to no less than 0.1wt%, the effective combination of the solid electrolyte and silicon can be guaranteed, the binding of the solid electrolyte to silicon can be guaranteed, and its restriction on silicon expansion and silicon displacement can be guaranteed. By limiting the content of the solid electrolyte to no more than 20wt%, the content of the silicon-based material in the first active material layer can be guaranteed, the ED (Energy Density) of the secondary battery can be guaranteed, and lithium plating can be reduced. In addition, when the content of the solid electrolyte exceeds 20wt%, not only is the restriction on silicon expansion and silicon displacement limited, but the kinetic performance of the secondary battery is also deteriorated. Therefore, limiting the content of the solid electrolyte to no more than 20wt% can not only guarantee the performance of the secondary battery, but also take into account the cost of material use.

[0012] In some embodiments, the solid electrolyte accounts for 5 wt% to 15 wt% of the first active material layer. To balance the solid electrolyte's binding effect on silicon with the ED and cycle performance of the secondary battery, the solid electrolyte preferably accounts for 5 wt% to 15 wt% of the first active material layer.

[0013] In some embodiments, the solid-state electrolyte includes at least one of polymethyl methacrylate, polyvinylidene fluoride, and polyacrylonitrile.

[0014] In some embodiments, the solid electrolyte is coated on the surface of the silicon-based material, so that the solid electrolyte can be effectively bonded to the surface of the silicon-based material to reduce expansion.

[0015] In some embodiments, the solid electrolyte comprises polymethyl methacrylate and / or polyacrylonitrile, wherein the silicon in the silicon-based material is connected to the hydroxyl groups, and the solid electrolyte is bonded to the hydroxyl groups. 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 pairs of electrons on the nitrogen atoms in the polyacrylonitrile can form hydrogen bonds with the hydroxyl groups on the surface of the silicon-based material. As a result, the solid electrolyte can at least partially bind more effectively to the surface of the silicon-based material, restraining the silicon-based material and reducing its expansion and displacement.

[0016] In some embodiments, the secondary battery further includes a free electrolyte, thereby facilitating the transmission of lithium ions, ensuring the charge and discharge rate, and further improving the kinetic performance of the secondary battery.

[0017] In some embodiments, the first active material layer further includes a first graphite material, which is mixed with a silicon-based material. The silicon-based material mixed with 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.

[0018] 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 no less than 5% can ensure the ED of the secondary battery, while limiting it to no more than 30% can not only reduce lithium plating but also ensure the binding effect of the first graphite material on silicon, thereby ensuring the structural integrity and stability of the negative electrode sheet.

[0019] 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. v 50 and the particle size D of the first graphite material v The ratio of 50 is limited to no less than 0.1, which can reduce side reactions, reduce the cycle attenuation rate of the secondary battery, and ensure the capacity retention rate of the secondary battery. v 50 and the particle size D of the first graphite material v The ratio of 50 is limited to no more than 0.5, so the first graphite material with a relatively large size can be used to constrain the relatively small silicon, reduce the expansion and displacement of silicon, and enhance the structural integrity and stability of the negative electrode plate.

[0020] 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 defining the relationship between the particle sizes of 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.

[0021] In some embodiments, the particle size D of the silicon-based material v 50 is 4 μm to 10 μm, and the particle size D of the first graphite material is v 50 is 16μm~40μm.

[0022] In some embodiments, the silicon-based material includes one or more of silicon, silicon oxide, silicon carbide, or a silicon alloy. Thus, by adding the silicon-based material to the negative electrode plate, the theoretical specific capacity and energy density of the negative electrode plate can be significantly improved.

[0023] In some embodiments, the secondary battery further includes a second active material layer, disposed on a side of the first active material layer facing away from the negative electrode current collector; the second active material layer comprises a second graphite material. The inclusion of the second active material layer in the secondary battery further constrains silicon, reducing its expansion and displacement, and enhancing the structural integrity and stability of the negative electrode sheet.

[0024] 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, causing the battery's expansion rate to increase rapidly. When the mass ratio of the second active material layer to the first active material layer is greater than 2, the infiltration time increases significantly, indicating that the first active material layer has a high compaction density and is not easily infiltrated. Excessive compaction density can cause particles in the first active material layer to break. Therefore, the mass ratio of the second active material layer to the first active material layer is preferably between 0.2:1 and 2:1.

[0025] In some embodiments, the second active material layer further comprises an 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 The ratio of 50 is less than 0.23. Due to the addition of inorganic materials, the particle size D of the inorganic materials is limited. v 50 and the particle size D of the second graphite material v If the ratio of 50 is less than 0.23, the inorganic material can fill the gaps in the second graphite material, providing lubrication and cushioning for the second graphite material, reducing the problem of second graphite material particle breakage, improving the problem of negative electrode overvoltage, and enhancing the cycle performance of the secondary battery. In addition, when the secondary battery includes a free electrolyte, the addition of the inorganic material can increase the infiltration of the free electrolyte and increase the amount of free electrolyte retained in the negative electrode, improving interface problems, increasing ion conduction efficiency, and improving the dynamic performance of the secondary battery.

[0026] 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. By the above definition, the particles of the second graphite material are less broken, and the secondary battery can obtain the best capacity retention rate.

[0027] In some embodiments, the particle size D of the inorganic material v 50 is 0.3 μm to 2 μm, and the particle size D of the second graphite material is v 50 is 10μm~15μm.

[0028] In some embodiments, the mass percentage of the inorganic material in the second active material layer is 0.1wt% to 5wt%. Too much inorganic material will affect ED (Energy Density), and too little addition will have little effect on wettability. When the mass percentage of the inorganic material in the second active material layer is less than 0.1wt%, the rate of decrease in compaction density is accelerated, causing the lubrication effect of the inorganic material on the second graphite material to decrease sharply, and the infiltration time is also significantly increased; when the mass percentage of the inorganic material in the second active material layer is greater than 5wt%, the inorganic material accounts for a large proportion of the second active material layer, causing the energy density to be severely reduced. Therefore, the mass percentage of the inorganic material in the second active material layer is preferably 0.1wt% to 5wt%.

[0029] In some embodiments, the inorganic material includes one or more of 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. Thus, by providing the inorganic material, the aforementioned beneficial effects of increasing the cold-pressed density of the second active material layer and improving the overpressure problem of the second active material layer can be achieved.

[0030] In some embodiments, the inorganic material is alumina and / or boehmite, which have good electrochemical stability and are readily available as raw materials.

[0031] In some embodiments, the secondary battery satisfies one of the following conditions:

[0032] The conductive agent in the first active material layer accounts for 0 wt% to 20% by mass;

[0033] The conductive agent in the first active material layer accounts for 0.1 wt% to 15% by weight;

[0034] The mass proportion of the conductive agent in the first active material layer is 0.1wt% to 5wt%;

[0035] The conductive agent in the first active material layer accounts for 1 wt % to 5 wt %.

[0036] In order to solve the above technical problems, another technical solution adopted in the embodiment of the present application is: providing an electrical device, the electrical device comprising a load and a secondary battery as described in any one of the above items, wherein the secondary battery is used to power the load.

[0037] The present application also provides a method for preparing a secondary battery, to prepare the negative electrode plate, a silicon-based material, a first dispersant, a conductive agent, methyl methacrylate and azobisisobutyronitrile are mixed, dissolved in deionized water to form a first active material layer slurry; the first active material layer slurry is evenly coated on at least one surface of the negative electrode current collector.

[0038] Under the catalytic action of azobisisobutyronitrile (AIBN), methyl methacrylate (MMA) polymerizes to form a solid electrolyte, polymethyl methacrylate (PMMA), which then bonds 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.

[0039] Different from the related art, in the secondary batteries and electrical equipment of the embodiments of the present application, the solid electrolyte is at least partially bonded to the surface of silicon in the silicon-based material, and forms a protective layer on the surface of the silicon to reduce the expansion and displacement of the silicon, thereby improving the problem of expansion of the silicon-based material and enhancing the structural integrity and structural stability of the negative electrode plate; in addition, since the present application uses a solid electrolyte, compared with the binder-coated silicon-based material in the prior art, it can enhance the lithium insertion ability of the silicon-based material and increase the ion conduction rate in the negative electrode plate, that is, improve the cycle performance of the secondary battery.

[0040] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] One or more embodiments are exemplarily illustrated by corresponding drawings, which do not constitute limitations on the embodiments. Elements with the same reference numerals in the drawings are represented as similar elements, and unless otherwise stated, the figures in the drawings do not constitute proportional limitations.

[0042] FIG1 is a schematic diagram of a framework of an electrical device according to an embodiment of the present application;

[0043] FIG2 is a schematic structural diagram of a negative electrode sheet according to an embodiment of the present application;

[0044] FIG3 is a Raman spectrum of the negative electrode sheet of an embodiment of the present application;

[0045] FIG4 is a schematic structural diagram of another implementation of the negative electrode sheet of an embodiment of the present application;

[0046] FIG5 is a SEM photograph of the negative electrode sheet of an embodiment of the present application;

[0047] FIG6 is another SEM photograph of the negative electrode sheet according to an embodiment of the present application;

[0048] FIG7 is a cycle life diagram of the secondary batteries of Comparative Example 1 and Example 1 of the present application;

[0049] FIG8 is a graph showing the cycle expansion rate of the secondary batteries of Comparative Example 1 and Example 1 of the present application;

[0050] FIG9 is a cycle life diagram of the secondary batteries of Examples 1 and 2 of the present application;

[0051] FIG10 is a graph showing the cycle expansion rate of the secondary batteries of Examples 1 and 2 of the present application;

[0052] FIG11 is a cycle life diagram of the secondary batteries of Examples 2 and 3 of the present application;

[0053] FIG12 is a graph showing the cycle expansion rate of the secondary batteries of Examples 2 and 3 of the present application.

[0054] The reference numerals in the specific embodiments are as follows: 1000, electric device; 100, secondary battery; 200, load; 10, negative electrode sheet; 1, negative electrode current collector; 2, first active material layer; 3, second active material layer. DETAILED DESCRIPTION

[0055] In order to make the purpose, technical solutions and advantages of the present application clearer, the present application is further described in detail below with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are only used to explain the present application and are not intended to limit the present application. It should be noted that when an element is described as "fixed on" another element, it can be directly on the other element, or there can be one or more centered elements therebetween. When an element is described as "connected to" another element, it can be directly connected to the other element, or there can be one or more centered elements therebetween. It should be noted that, if there is no conflict, the various features in the embodiments of the present application can be combined with each other, all within the scope of protection of the present application. In addition, although the functional modules are divided in the device schematic and the logical order is shown in the flow chart, in some cases, the steps shown or described can be performed in a different module division than in the device schematic, or in the order in the flow chart.

[0056] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" and any variations thereof in the specification and claims of this application and the above-mentioned figure descriptions are intended to cover non-exclusive inclusions.

[0057] In the description of the embodiments of the present application, the technical terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing the embodiments of the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the embodiments of the present application.

[0058] In the description of the embodiments of this application, the use of terms such as "first" and "second" to define components is solely for the purpose of distinguishing the corresponding components. Unless otherwise stated, these terms have no special meanings and therefore should not be construed as limiting the scope of protection of this application. In the description of the embodiments of this application, the meaning of "plurality" is two or more, unless otherwise specifically defined.

[0059] Unless otherwise defined, all technical and scientific terms used in this specification have the same meanings as those commonly understood by those skilled in the art to which this application belongs. The terms used in this specification are intended only to describe specific embodiments and are not intended to limit this application. The term "and / or" as used in this specification includes any and all combinations of one or more of the relevant listed items.

[0060] In addition, the technical features involved in different embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0061] An embodiment of the present application provides an electric device 1000 . As shown in FIG1 , the electric device 1000 includes a secondary battery 100 and a load 200 . The secondary battery 100 is used to supply power to the load 200 .

[0062] The electric device 1000 of the embodiment of the present application includes, but is not limited to, a Bluetooth headset, a mobile phone, a tablet, a laptop computer, an electric toy, an electric tool, a battery-powered vehicle, an electric car, a ship, a spacecraft, etc. The electric toy may include a fixed or mobile electric toy, such as a game console, an electric car toy, an electric ship toy, and an electric airplane toy, etc. The spacecraft may include an airplane, a rocket, a space shuttle, and a spacecraft, etc.

[0063] The load 200 in the embodiment of the present application may be a display, a speaker, a processor, a light emitting device, a motor, etc. It is understood that the type of the load 200 is not limited thereto.

[0064] Referring to FIG. 2 , a first embodiment of the present application provides a secondary battery 100 . The secondary battery 100 includes a negative electrode plate 10 . The negative electrode plate 10 includes a negative electrode 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 electrode current collector 1 . The first active material layer 2 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 configuring the first active material layer in the negative electrode plate as a silicon-based material and a solid electrolyte, and the solid electrolyte being at least partially bonded to the surface of the silicon-based material, the solid electrolyte bonded to the silicon-based material effectively binds silicon, reducing silicon expansion and the resulting displacement of the silicon-based material, thereby enhancing the structural integrity and stability of the negative electrode plate and extending the service life of the secondary battery. Furthermore, the use of the solid electrolyte to bind the silicon-based material improves the lithium insertion capacity of the silicon-based material and increases the ion conductivity in the negative electrode plate, compared to the prior art method of using a binder to coat the silicon-based material. This improves the deterioration in the secondary battery's kinetic performance caused solely by the binder coating the silicon-based material.

[0065] Regarding the secondary battery 100 , the secondary battery 100 may be a soft-pack battery, a square-shell battery, a cylindrical battery, or the like.

[0066] The negative electrode sheet 10 can be in a wound or laminated form. In the embodiments of the present application, the laminated negative electrode sheet 10 is used as an example for illustration. The negative electrode sheet 10 includes a negative electrode 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 electrode current collector 1. Specifically, the first active material layer 2 can be disposed on one surface of the negative electrode current collector 1 along the thickness direction X, or on two opposing surfaces of the negative electrode current collector 1 along the thickness direction X.

[0067] 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.

[0068] It is understood that a plurality of first active material layers 2 may be spaced apart and arranged on one surface of the negative electrode current collector 1 .

[0069] Regarding the 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, a polymer substrate coated with a conductive metal, or any combination thereof.

[0070] Regarding the first active material layer 2, the active material therein is a negative electrode active material containing silicon. In the embodiment of the present application, the first active material layer 2 includes a silicon-based material and a solid electrolyte.

[0071] The aforementioned silicon-based materials can include one or more of silicon, silicon oxide, silicon carbide, and silicon alloys. Silicon oxide includes silicon oxides such as silicon monoxide and silicon dioxide. Adding silicon-based materials to the negative electrode significantly increases the theoretical specific capacity and energy density of the negative electrode.

[0072] It should be noted that when the secondary battery 100 is charging and discharging, the negative electrode plate 10 will release and embed lithium ions, causing the silicon-based material to expand in volume by more than 100%. Such a high volume expansion rate will destroy the structural stability and integrity of the negative electrode plate 10; it will also damage the solid electrolyte interface (SEI) membrane, causing free electrolyte to penetrate into the interior of the negative electrode plate 10, triggering a series of unstable reactions such as the decomposition of the free electrolyte and the repeated destruction of the SEI membrane, which will significantly reduce the cycle performance of the secondary battery 100.

[0073] The solid electrolyte at least partially bonds to the surface of the silicon-based material, forming a protective layer on the surface of the silicon-based material. This reduces the expansion and displacement of the silicon-based material, and thus reduces the expansion or contraction of the negative electrode, thereby enhancing the structural integrity and stability of the negative electrode. Furthermore, the solid electrolyte enhances the lithium insertion capacity of the silicon-based material, accelerating the ion conduction rate in the negative electrode 10, thereby improving the dynamic performance and cycling performance of the secondary battery 100.

[0074] In some embodiments, the mass percentage of the solid electrolyte in the first active material layer is 0.1wt% to 20wt%. By limiting the content of the solid electrolyte to no less than 0.1wt%, the effective combination of the solid electrolyte and the silicon-based material can be guaranteed, the binding of the solid electrolyte to the silicon-based material can be guaranteed, and its restriction on silicon expansion and silicon displacement can be guaranteed. By limiting the content of the solid electrolyte to no more than 20wt%, the content of the silicon-based material in the first active material layer can be guaranteed, the ED (Energy Density) of the secondary battery can be guaranteed, and lithium plating can be reduced. In addition, when the content of the solid electrolyte exceeds 20wt%, not only is the restriction on silicon expansion and displacement limited, but the kinetic performance of the secondary battery is also deteriorated. Therefore, limiting the content of the solid electrolyte to no more than 20wt% can not only guarantee the performance of the secondary battery, but also take into account the cost of material use.

[0075] In some embodiments, the solid electrolyte accounts for 5 wt% to 15 wt% of the first active material layer. 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 preferably accounts for 5 wt% to 15 wt% of the first active material layer.

[0076] In some embodiments, the solid electrolyte comprises at least one of polymethyl methacrylate, polyvinylidene fluoride, and polyacrylonitrile. In some embodiments, the solid electrolyte is coated on the surface of the silicon-based material. In some embodiments, the solid electrolyte comprises polymethyl methacrylate and / or polyacrylonitrile, the silicon in the silicon-based material is connected to the hydroxyl group, and the solid electrolyte is combined with the hydroxyl group. The hydroxyl group on the silicon surface of the silicon-based material can be obtained by modification. It is understood that the solid electrolyte can be obtained using conventional methods in the art.

[0077] 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 the chemical reaction formula ① below, where R represents the portion of PMMA other than the -COOCH3 group. Furthermore, at high temperatures, PMMA can also form a covalent bond with the O (oxygen) atom in the hydroxyl group, as shown in the chemical reaction formula ② below, where R represents the portion of PMMA other than the -COOCH3 group. The covalent bond formed by PMMA and the O (oxygen) atom in the hydroxyl group helps strengthen the bonding between the silicon-based material and the solid electrolyte, further reducing the expansion and displacement of silicon, and thus the expansion or contraction of the negative electrode.

[0078] Please refer to Figure 3, which shows the Raman spectrum of a high-temperature treated negative electrode with a PMMA solid electrolyte. The horizontal axis represents the Raman shift, i.e., the wavenumber difference of the scattered light relative to the incident light, while the vertical axis represents the intensity of the scattered light, i.e., the photon count. As can be seen from Figure 3, the Raman spectrum of this negative electrode exhibits characteristic peaks for SiOOC (C6H3) and SiOOC (C6H4), demonstrating 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.

[0079] Regarding the aforementioned PMMA, PMMA is produced by free radical polymerization of MMA (methyl methacrylate). It should be noted that PMMA not only functions as a binder, but also forms electron coordination with lithium ions (Li+), acting as an ion transporter, improving ion transport efficiency and addressing the deterioration in secondary battery dynamics caused by binder-coated silicon in existing technologies.

[0080] Furthermore, in some embodiments, the first active material layer 2 further 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.

[0081] Compared to negative electrode sheets prepared by directly adding PMMA, the PMMA-containing negative electrode sheet prepared using the above method has a more uniform distribution of PMMA in the first active material layer. This is because when PMMA is directly added, PMMA tends to accumulate on the surface of the first active material layer away from the negative electrode current collector. However, by adding MMA monomer and polymerizing it during stirring to form PMMA, the distribution of PMMA in the first active material layer is more uniform, resulting in more uniform PMMA coating of the silicon-based material in the first active material layer and a more significant effect on suppressing the expansion of the silicon-based material.

[0082] When the solid electrolyte is PVDF (polyvinylidene fluoride), PVDF can be physically coated on the surface of the silicon-based material to form a protective layer. When the solid electrolyte is PAN (polyacrylonitrile), the lone pair of electrons on the N (nitrogen) atom in PAN can form hydrogen bonds with the hydroxyl groups on the silicon surface. Both PVDF and PAN have similar functions to PMMA, that is, they are at least partially bound to the surface of silicon in the silicon-based material, thereby effectively binding silicon, reducing the expansion and displacement of silicon, and compared with the binders used in the prior art, they can improve the lithium insertion ability of the silicon-based material and increase the ion conduction rate in the negative electrode, that is, improve the problem of deterioration of the dynamic performance of the secondary battery caused by the binder coating silicon.

[0083] In some embodiments, the secondary battery further includes a free electrolyte, thereby facilitating the transmission of lithium ions, ensuring the charge and discharge rate, and further improving the kinetic performance of the secondary battery.

[0084] It is worth noting that the free electrolyte can be filled in the secondary battery in the form of liquid, so that at least part of the free electrolyte can infiltrate the first active material layer 2, thereby facilitating the transmission of lithium ions.

[0085] In some embodiments, the first active material layer 2 further includes a first graphite material 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.

[0086] 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.

[0087] Among them, the OI value (Orientation Index) of the graphite material indicates the degree of consistency of the orientation of the grains in the graphite material, and is an important consideration for the low expansion characteristics of the graphite negative electrode active material. Generally speaking, when the OI value is low, the isotropy of the graphite particles is strong, which is beneficial to suppress expansion during the cycle. The OI value can be measured by XRD (X-ray Diffraction) testing the 110 and 002 or 004 characteristic peaks of the graphite in the pole piece, and then the OI value is calculated. When the diffraction pattern test is performed on the 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 (002) or (004) diffraction peak intensity (or integrated area) to the (110) diffraction peak intensity (or integrated area), that is, 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.

[0088] 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 no less than 5% can ensure the ED of the secondary battery, while limiting it to no more than 30% can not only reduce lithium plating but also ensure the binding effect of the first graphite material on silicon, thereby ensuring the structural integrity and stability of the negative electrode sheet.

[0089] 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.

[0090] Among them, the particle size D v 50 refers to the particle size at which the volume accumulation reaches 50% in the volume-based particle size distribution of the material, starting from the smallest particle size. v 50 can be measured by a laser particle size analyzer with reference to GB / T 19077-2016 "Particle size distribution by laser diffraction method".

[0091] 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 D to 50 is less than 0.1, the particle size of the silicon-based material is small and the specific surface area is large, which leads to excessive side reactions and accelerated battery cycle decay.v 50 and the particle size D of the first graphite material v When the ratio of D to 50 is greater than 0.5, the expansion rate of the battery increases rapidly. This is because the particle size of the silicon-based material is close to that of the first graphite material, resulting in poor co-embedding effect between the silicon-based material and the first graphite material, which causes a sharp increase in the expansion rate of the battery. Therefore, the particle size D of the silicon-based material is v 50 and the particle size D of the first graphite material v The ratio of 50 is preferably 0.1 to 0.5.

[0092] 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 50 is 0.2~0.3. When the particle size D of 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 capacity retention rate increases more slowly with the increase of the ratio. Therefore, when the ratio is greater than 0.3, the benefit of increasing the ratio on the capacity retention rate is small. When the ratio is less than 0.2, the expansion rate decreases more slowly with the decrease of the ratio. Therefore, when the expansion rate is less than 0.2, the benefit of reducing the ratio on the expansion rate is small. Considering the expansion rate and capacity retention rate comprehensively, the particle size D of the silicon-based material is v 50 and the particle size D of the first graphite material v The ratio of 50 is preferably 0.2 to 0.3.

[0093] Among them, the particle size D of silicon-based material v 50 can be 4 μm to 10 μm, and the particle size D of the first graphite material v 50 can be 16μm to 40μm.

[0094] In some embodiments, the first active material layer 2 further includes a first dispersant and a conductive agent, wherein the first dispersant, the conductive agent, the first graphite material, the silicon-based material and the solid electrolyte are mixed.

[0095] As for the first dispersant, the first dispersant may be CMC (sodium carboxymethyl cellulose).

[0096] For the above-mentioned conductive agent, the conductive agent can adhere to the surface of the silicon-based material, thereby increasing the electron transfer efficiency of the silicon-based material. The conductive agent can be one or more of a conductive carbon material such as SP (conductive carbon black), CNT (carbon nanotube), VGCF (vinyl glass carbon fiber), metal particles or metal fibers. The amount of conductive agent added and the mass percentage of the first active material layer 2 are 0 to 20 wt%. 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 as the mass percentage of the conductive agent in the first active material layer 2 increases, the energy density of the secondary battery decreases. Therefore, the mass percentage of the conductive agent in the first active material layer 2 is preferably 0 to 20 wt%.

[0097] In some embodiments, referring to FIG. 4 , the secondary battery 100 further includes a second active material layer 3 . The second active material layer 3 is disposed on a side of the first active material layer 2 facing away from the negative electrode current collector 1 .

[0098] 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 may be disposed. The two second active material layers 3 may be respectively disposed on a surface of the two first active material layers 2 facing away from the negative electrode current collector 1 .

[0099] It can be understood that a plurality of second active material layers 3 may be spaced apart on one surface of the first active material layer 2 .

[0100] The second active material layer 3 includes a second graphite material. The second active material layer 3 is further provided in the secondary battery 100 to further constrain silicon, reduce silicon expansion and displacement, and enhance the structural integrity and stability of the negative electrode.

[0101] The second graphite material may be one or more of hard carbon, soft carbon, or graphite. The second graphite material may 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.

[0102] 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 rapid 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 infiltration time increases significantly, indicating that the first active material layer 2 has a high compaction density and is not easily infiltrated. Excessive compaction density can cause particles in the first active material layer 2 to break. Therefore, the mass ratio of the second active material layer 3 to the first active material layer 2 is preferably between 0.2:1 and 2:1.

[0103] In some embodiments, the second active material layer 3 further includes a binder and a second dispersant.

[0104] For the above-mentioned binder, the binder 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.

[0105] As for the second dispersant, the second dispersant may be CMC (sodium carboxymethyl cellulose).

[0106] In a specific embodiment, the mass ratio of the second graphite material, the binder, and the second dispersant may be 97.5:1.5:1.

[0107] In some embodiments, the second active material layer 3 further includes an inorganic material. The inorganic material may include one or more of 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.

[0108] 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 relatively high hardness.

[0109] It should be noted that when the negative electrode sheet 10 is rolled, the second active material layer 3 is subjected to a greater pressure than 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 are broken, for example, the first graphite material and the second graphite material are broken, the capacity of the secondary battery 100 is lost, thereby reducing the cycle performance of the secondary battery 100.

[0110] In the present application, the negative electrode sheet 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 v50 is limited to less than 0.23, then the inorganic material can be filled in the gaps of the second graphite material. When the ratio is greater than 0.23, the processing window of the second graphite material begins to narrow. Please refer to Figure 5, which is a SEM (Scanning Electron Microscope) photo of the negative electrode plate 10. The larger particles in Figure 5 are the second graphite material, and the smaller particles are the inorganic material. It can be seen from Figure 5 that the inorganic material is mainly distributed in the gaps of the second graphite material. Since the inorganic material can be filled in the gaps of the second graphite material, it can play a lubricating and buffering role for the second graphite material, reduce the problem of graphite material particle breakage, improve the problem of overvoltage of the negative electrode plate 10, and enhance the cycle performance of the secondary battery 100.

[0111] Please refer to Figure 6, which is another SEM photograph of the negative electrode sheet 10. The larger particles in Figure 6 are the second graphite material, the smaller particles are the inorganic material, the upper side is the second active material layer 3, and the lower side is the first active material layer 2. As can be seen from Figure 6, after the inorganic material is added to the second active material layer 3, the second graphite material of the negative electrode sheet 10 as a whole is free of 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 free electrolyte, the free electrolyte easily penetrates from the second active material layer 3 into the first active material layer 2, reducing concentration polarization and, in turn, internal resistance, extending the cycle life of the secondary battery 100, and improving the utilization rate of the secondary battery 100.

[0112] It can be understood that when the secondary battery 100 includes a free electrolyte, the inorganic material can form more pores in the negative electrode plate 10, which is beneficial to enhancing the free electrolyte infiltration effect of the negative electrode plate 10, and can increase the amount of free electrolyte retained in the negative electrode plate 10, increase the ion transmission speed of the free electrolyte, and improve the fast charging capability.

[0113] 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. By this definition, the particles of the second graphite material are less broken, and the secondary battery 100 can obtain the best capacity retention rate. At this time, the processing window of the second graphite material is wider and the dynamic performance is better. Optionally, the particle size D of the inorganic material is v 50 is 0.3 μm to 2 μm, and the particle size D of the second graphite material is v 50 is 10μm~15μm.

[0114] In some embodiments, the inorganic material accounts for a mass percentage of 0.1 wt% to 5 wt% in the second active material layer 3. Excessive amounts of inorganic material can affect ED (Energy Density), while too low an amount has little effect on wettability. When the inorganic material accounts for a mass percentage of less than 0.1 wt% in the second active material layer 3, the rate of decrease in compaction density accelerates, causing the lubrication effect of the inorganic material on the second graphite material to decrease sharply and significantly increasing the wetting time. When the inorganic material accounts for a mass percentage of greater than 5 wt% in the second active material layer 3, the inorganic material accounts for a significant proportion of the second active material layer 3, severely reducing the energy density. Therefore, the inorganic material preferably accounts for a mass percentage of 0.1 wt% to 5 wt% in the second active material layer 3.

[0115] 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 may be 97:0.5:1.5:1.

[0116] In some embodiments, a secondary battery 100 includes a negative electrode sheet 10, a separator, and a positive electrode sheet. The positive electrode sheet includes a positive electrode active material layer, and the positive electrode active material includes 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 iron manganese phosphate, lithium vanadium phosphate, and lithium iron phosphate. The separator may include a polymer or inorganic material formed from a material that is stable to the free electrolyte. For example, the separator may include a substrate layer and a surface treatment layer. The substrate layer is a non-woven fabric, film, or composite film having 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 film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric, or a polypropylene-polyethylene-polypropylene porous composite film may be selected. A surface treatment layer is provided on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer, an inorganic layer, or a layer formed by a mixed polymer and an inorganic material. The inorganic layer includes inorganic particles and a binder, wherein the inorganic particles include at least one of aluminum oxide, 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 vinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene or polyhexafluoropropylene. The polymer layer includes a polymer, and the material of the polymer includes at least one of polyamide, polyacrylonitrile, an acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride or poly (vinylidene fluoride and hexafluoropropylene).

[0117] In order to evaluate the beneficial effects of the secondary battery 100 of the present application, a test was conducted on the secondary battery 100, wherein PMMA was used as the solid electrolyte. In the following embodiments and comparative examples, all reagents, materials, and instruments used are commercially available unless otherwise specified.

[0118]

Preparation method

[0119] Example 1

[0120] Preparation of first active material layer 2 slurry

[0121] 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, dissolved in deionized water to form a first active material layer slurry with a solid content of 40%. The mixture is stirred evenly and then 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.

[0122] Preparation of negative electrode sheet 10

[0123] The first active material layer 2 slurry was evenly coated onto one surface of a 10 μm thick copper foil of the negative electrode current collector 1 using an extrusion coater and then dried at 110°C to obtain a negative electrode sheet 10 coated on one side with the first active material layer 2. After drying, the coating thickness of the first active material layer 2 was 150 μm. The coating steps were then repeated on the other surface of the negative electrode current collector 1 to obtain a negative electrode sheet 10 coated on both sides with the first active material layer 2.

[0124] After coating, the negative electrode sheet 10 is dried, cold pressed, and cut into sheets with a size of 74 mm × 867 mm for later use. The compacted density of the negative electrode sheet 10 is 1.7 g / cm 3 The bonding force between the first active material layer 2 and the negative electrode current collector 1 must meet 10 to 100 N / m; the total porosity of the negative electrode sheet 10 must meet 10% to 30%.

[0125] Preparation of positive electrode active material layer slurry

[0126] The positive electrode active material LiCoO2 (lithium cobalt oxide), the conductive agent conductive carbon black, and the binder PVDF (polyvinylidene fluoride) are dissolved in NMP (N-methylpyrrolidone) solution in a mass ratio of 97:1.4:1.6 to form a positive electrode slurry with a solid content of 75%, and stirred evenly for coating use.

[0127] Preparation of positive electrode

[0128] The positive electrode slurry was evenly coated on one surface of a 10 μm thick positive electrode current collector aluminum foil and dried at 110°C to obtain a positive electrode sheet coated with a positive electrode active material layer on one side. After drying, the positive electrode active material layer had a coating thickness of 110 μm.

[0129] Then, the above coating steps are repeated on the other surface of the positive electrode current collector to obtain a positive electrode sheet with positive electrode active material coated on both sides.

[0130] After coating, the positive electrode sheet is dried, cold pressed, and cut into sheets with a size of 74mm×867mm for later use. The compacted density of the positive electrode sheet is 4.15g / cm 3 .

[0131] Preparation of electrolyte

[0132] In an environment with a water content of less than 10 ppm, the organic solvents EC (ethylene carbonate), PC (propylene carbonate), DEC (diethyl carbonate), and EP (ethyl propionate) are mixed in a mass ratio of 3:1:3:3. Lithium hexafluorophosphate (LiPF6) is then added to the mixed organic solvent, dissolved, and mixed evenly to obtain the desired electrolyte. The concentration of LiPF6 is 1 mol / L.

[0133] The present application does not impose any specific restrictions on the electrolyte, and it can be selected according to actual needs. As an example, the additive in the electrolyte may include one or more of vinylene carbonate (VC), vinyl ethylene carbonate (VEC), succinonitrile (SN), adiponitrile (ADN), 1,3-propylene sultone (PST), tris(trimethylsilyl) phosphate (TMSP), trimethyl borate (TMB), or tris(trimethylsilyl) borate (TMSB).

[0134] Preparation of isolation membrane

[0135] The isolation membrane includes 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, and then 2.5mg of adhesive PVDF (polyvinylidene fluoride) is coated on the side of the ceramic layer facing away from the substrate layer, and finally dried.

[0136] Preparation of Secondary Battery 100

[0137] The positive electrode sheet, separator, and negative electrode sheet 10 prepared above are stacked in sequence, with the separator placed between the positive electrode sheet and the negative electrode sheet 10 to play an isolating role, and then hot-pressed to form an electrode assembly with a laminated structure.

[0138] The electrode assembly is placed in a packaging bag and sealed on the side, placed in a vacuum oven at 85 degrees Celsius to dry for 12 hours to remove moisture, and the prepared electrolyte is injected. The secondary battery 100 is obtained through vacuum packaging, standing, formation (0.02C constant current charging to 3.5V, then 0.1C constant current charging to 3.9V), shaping, and capacity testing.

[0139] Example A1 to Example A10

[0140] To facilitate the reader's understanding of the design concept of the present application, the following lists 10 specific secondary batteries provided by the present application based on Example 1. These 10 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 in Example 1 are performed to obtain a solid electrolyte with a mass percentage of 0.01wt%, 0.05wt%, 0.1wt%, 1wt%, 5.5wt%, 10wt%, 15wt%, 20wt%, 25wt% and 30wt% in the first active material layer 2, respectively. The remaining steps are the same as in Example 1.

[0141] Comparative Example 1

[0142] The difference from Example 1 is that the first active material layer 2 slurry does not include MMA (methyl methacrylate) and AIBN (azobisisobutyronitrile), PAA (polyacrylic acid) is used as a binder in the first active material layer slurry, and 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.

[0143] Example B1 to Example B9

[0144] To explore the preferred mass ratio range of the silicon-based material to the first graphite material, nine specific secondary batteries provided herein were listed based on Example 1. These nine specific secondary batteries correspond to specific Examples B1 to B9. In Examples B1 to B9, the mass percentages of the silicon-based material to the sum of the mass of the first graphite material and the silicon-based material were 1%, 3%, 5%, 10%, 15%, 20%, 25%, 30%, and 35%, respectively. The remaining steps were the same as in Example 1. The battery was cycled 300 times.

[0145] Example C1 to Example C11

[0146] To explore the particle size D of silicon-based materials v 50 and the particle size D of the first graphite material vThe preferred ratio range of 50 is as follows. Based on Example 1, 11 specific secondary batteries provided by this application are listed. 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 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, and the remaining steps are the same as those in Example 1. The number of cycles of the battery is 300.

[0147] Example D1 to Example D10

[0148] To explore the preferred range of the mass ratio of the conductive agent to the first active material layer 2, 10 specific secondary batteries provided herein were listed based on Example 1. These 10 specific secondary batteries correspond to specific Examples D1 to D10. In Examples D1 to D10, the amount of conductive agent added relative to the mass percentage of the first active material layer 2 was 0 wt%, 0.1 wt%, 1 wt%, 2 wt%, 5 wt%, 10 wt%, 15 wt%, 20 wt%, 30 wt%, and 40 wt%, respectively. The remaining steps were the same as in Example 1. The battery was cycled 300 times.

[0149] Example 2

[0150] The difference from Example 1 is that:

[0151] The negative electrode sheet 10 further includes a second active material layer 3 , which includes a second graphite material, a binder, and a second dispersant. The mass ratio of the second graphite material, the binder, and the second dispersant is 97.5:1.5:1.

[0152] The preparation of negative electrode slurry also includes

[0153] Second active material layer slurry: Mix the second graphite material, binder, and second dispersant in a mass ratio of 97.5:1.5:1, dissolve in deionized water, and form a negative electrode slurry with a solid content of 50%. Stir evenly before coating. The second graphite material is graphite particles, the binder is SBR (styrene-butadiene rubber), and the second dispersant is CMC (sodium carboxymethyl cellulose).

[0154] The preparation process of the negative electrode sheet 10 is changed to:

[0155] A double-layer coating machine is used to evenly coat the first active material layer slurry and the second active material layer slurry on one surface of a 10μm thick copper foil of a negative electrode current collector 1, and dried at 110 degrees Celsius to obtain a negative electrode sheet 10 coated with a double layer of negative electrode active material layer on one side. After drying, the coating thickness of the double layer of negative electrode active material layer is 150μm. 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, there is no obvious interface, and the interface impedance is small, which is conducive to improving 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 coated with a double layer of negative electrode active material layer on both sides.

[0156] After coating is completed, the negative electrode sheet 10 is dried, cold-pressed, and cut into sheets with a size of 74 mm×867 mm for later use.

[0157] Example E1 to Example E10

[0158] To explore the preferred range of the mass ratio of the second active material layer 3 to the first active material layer 2, 10 specific secondary batteries provided herein were listed based on Example 2. These 10 specific secondary batteries correspond to specific Examples E1 to E10. In Examples E1 to E10, the mass ratios of the second active material layer 3 to the first active material layer 2 were 0.05, 0.1, 0.2, 0.3, 0.5, 1, 1.5, 2, 2.5, and 3, respectively. The remaining steps were the same as in Example 2. The batteries were cycled 300 times.

[0159] Example 3

[0160] The difference from Example 2 is that the second active material layer 3 further includes an inorganic material, and the mass ratio of the second graphite material, the inorganic material, the binder and the second dispersant is 97:0.5:1.5:1.

[0161] Example F1 to Example F13

[0162] To explore 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 of 50 is as follows. Based on Example 3, 13 specific secondary batteries provided by the present application are listed. 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 is v 50 and the particle size D of the second graphite material vThe 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, and the remaining steps are the same as those in Example 3. The number of cycles of the battery is 300.

[0163] Example G1 to Example G11

[0164] To explore the preferred range of the mass percentage of the inorganic material in the second active material layer 3, 11 specific secondary batteries provided by this application were listed based on Example 3. These 11 specific secondary batteries correspond to specific Examples G1 to G11. In Examples G1 to G11, the mass percentages of the inorganic material in the second active material layer 3 were 0.01wt%, 0.05wt%, 0.1wt%, 0.5wt%, 1wt%, 2wt%, 3wt%, 4wt%, 5wt%, 6wt%, and 7wt%, respectively. The remaining steps were the same as in Example 3. The battery was cycled 300 times.

[0165]

Test method

[0166] The secondary batteries 100 of Example 1, Examples A1-A10, Comparative Example 1, Examples B1-B9, Examples C1-C11, Examples D1-D10, Example 2, Examples E1-E10, Example 3, Examples F1-F13, and Examples G1-G11 were subjected to the following tests. If the test temperature is not explicitly stated during the test, the room temperature of 25° C. is used as an example.

[0167] (1) Capacity retention test

[0168] 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 rest for 5 minutes.

[0169] Discharge: Discharge to 3.0V using 0.7C.

[0170] According to the above charge and discharge method, the discharge capacity of each cycle was recorded during the cycle, and divided by the initial capacity to obtain the capacity retention rate of each cycle. Among them, the discharge capacity of the first discharge was the initial discharge capacity, which was calculated as 100%.

[0171] For example, the 300-cycle capacity retention rate = (discharge capacity at the 300th cycle / discharge capacity at the first cycle) × 100%.

[0172] (2) Secondary battery 100% expansion rate test

[0173] The thickness of three locations of the secondary battery 100 was measured and an average value was taken.

[0174] During the cycling process, the average value of the battery thickness is recorded. The difference between the average battery thickness after each cycle and the average initial battery thickness is divided by the average initial battery thickness to obtain the battery expansion rate. The average battery thickness when the battery cycle number is 0 is the average initial thickness.

[0175] For example, the battery expansion rate after 300 cycles = [(average value of battery thickness after the 300th cycle - average value of initial battery thickness) / average value of initial battery thickness] × 100%.

[0176] (3) Lithium deposition test of negative electrode sheet 10

[0177] The secondary battery 100 with 300 cycles was disassembled, and the lithium deposition on the negative electrode 10 was visually observed. The degree of lithium deposition was determined to be "mild", "moderate" or "severe". The determination criteria were as follows:

[0178] 1) There are less than 3 lithium deposition locations on each negative electrode sheet 10, and / or the lithium deposition area accounts for less than 10% of the area of ​​the negative electrode sheet 10 - slight;

[0179] 2) There are greater than or equal to 3 and less than or equal to 10 lithium deposition locations on each negative electrode sheet 10, and / or the lithium deposition area accounts for greater than or equal to 10% and less than or equal to 50% of the area of ​​the negative electrode sheet 10 - moderate;

[0180] 3) There are more than 10 lithium deposition locations on each negative electrode sheet 10, and / or the lithium deposition area accounts for more than 50% of the area of ​​the negative electrode sheet 10 - severe

[0181] (4) Ionic conductivity test

[0182] EIS testing was performed using a symmetrical SS / SE / SS cell assembled with stainless steel (SS) and electrolyte (SE). The resistance R was obtained, and the test results were calculated using the ionic conductivity formula. Here, resistivity ρ = RS / L, conductivity = 1 / ρ = L / RS, S is the cell's cross-sectional area, and L is the cell's thickness.

[0183] (5) Adhesion test

[0184] 1) hot pressing the negative electrode sheet 10 at 85° C. and 2 MPa for 10 min, and after cooling, performing a peeling force test between the first active material layer 2 and the current collector 1 ;

[0185] 2) Attach a layer of double-sided tape to the steel plate, gently press the negative electrode sheet 10 flat on the double-sided tape, then attach a layer of test tape above the negative electrode sheet 10, and roll a handheld roller (2000g) back and forth on the test tape three times;

[0186] 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. The peel speed is 50 mm / min and the peel test duration is 1 minute.

[0187] 4) Divide the test tensile force value by the width of the corresponding tape to obtain the final peel force (N / m).

[0188] (6) Wetting time test

[0189] An equal volume of free electrolyte, for example, 0.05 ml of free electrolyte, is dropped onto different negative electrode sheets 10 , and the time required for the free electrolyte to be completely absorbed is observed.

[0190] (7) Secondary battery 100 volume energy density test

[0191] Place secondary battery 100 in a thermostat at 25°C ± 2°C for 30 minutes to allow the secondary battery to reach a constant temperature. Charge the battery at a constant current of 0.5C to the full charge voltage. Then, charge the battery at the full charge voltage to a current of 0.05C, and discharge it at 0.2C to a voltage of 3.0V. Record the discharge energy.

[0192] Volume energy density=discharge energy / (length of the secondary battery 100×width of the secondary battery 100×thickness of the secondary battery 100).

[0193]

Experimental results

[0194] The effects of solid electrolytes and binders in existing technologies on the performance of secondary batteries

[0195] FIG7 is a graph showing the cycle life of the secondary batteries of Comparative Example 1 and Example 1. FIG7 shows that, at the same number of cycles, the capacity retention of the secondary battery of Example 1 is improved compared to that of the secondary battery of Comparative Example 1. This indicates that by replacing the binder in the first active material layer 2 with PMMA, the expansion displacement of silicon can be reduced, thereby increasing the capacity retention of the secondary battery 100 and improving the cycle performance of the secondary battery 100.

[0196] Figure 8 is a graph showing the cyclic expansion rate of the secondary batteries of Comparative Example 1 and Example 1. As can be seen from Figure 8, at the same number of cycles, the expansion rate of the secondary battery of Example 1 is lower than that of the secondary battery of 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. Because the capacity of the secondary battery of Comparative Example 1 decreased significantly and its expansion rate was relatively large, testing was stopped after 300 cycles.

[0197] The lithium plating degree of comparative example 1 and embodiment 1 was tested. The lithium plating degree of the negative electrode sheet 10 of comparative example 1 was "severe", and the lithium plating degree of the negative electrode sheet 10 of embodiment 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 sheet 10 and improve the cycle performance of the secondary battery 100.

[0198] The ionic conductivity of Comparative Example 1 and Example 1 was tested. The ionic conductivity of the negative electrode sheet of Example 1 was 1.2x10-3~6x10-3 S / cm, and the ionic conductivity of the negative electrode sheet of Comparative Example 1 was 1.2x10-5~8x10-4. By adding PMMA to the first active material layer 2, the dynamic performance of the secondary battery 100 can be improved.

[0199] Effect of the mass percentage of solid electrolyte in the first active material layer 2 on the performance of secondary batteries

[0200] Table 1 below shows the performance of the secondary batteries corresponding to Examples A1 to A10. The mass percentages of the solid electrolyte in the first active material layer 2 in Examples A1 to A10 are 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.

[0201] Table 1

[0202] 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, and the mass ratio of the required MMA to each substance in the first active material layer slurry is calculated based on the mass percentage of the solid electrolyte PMMA in Table 1.

[0203] As can be seen from Table 1, when the mass percentage of the solid electrolyte in the first active material layer 2 is less than 0.1wt%, the solid electrolyte has limited ability to limit the expansion of the silicon-based material, resulting in a large expansion rate of the secondary battery and a low capacity retention rate; and the bonding force between the isolation membrane and the negative electrode plate 10 does not meet the industry's minimum requirement of 10N / m, which can easily cause the first active material layer 2 to pulverize or fall off. This is because the solid electrolyte in this application is not only used to inhibit the expansion and displacement of the silicon material and serve as an electrolyte, but also has the function of a binder. When the solid electrolyte is not added enough, it will affect the bonding force between the first active material layer 2 and the negative electrode current collector 1; when the mass percentage of the solid electrolyte in the first active material layer 2 is greater than 20wt%, the negative electrode plate will seriously precipitate lithium, resulting in the expansion of the secondary battery. The expansion rate increases and the capacity retention rate decreases. It should be noted that when the mass percentage of the solid electrolyte in the first active material layer 2 is less than or equal to 20wt%, the expansion rate of the negative electrode plate 10 decreases. This is because as the mass percentage of the solid electrolyte increases, its inhibitory effect on the expansion and displacement of silicon becomes more significant, which is easy to understand. When the mass percentage of the solid electrolyte in the first active material layer 2 is greater than 20wt%, the expansion rate of the negative electrode plate 10 increases instead. This is not because the solid electrolyte's inhibition of silicon expansion is weakened, but because when the solid electrolyte accounts for too large a proportion, the proportion of active material in the negative electrode plate decreases accordingly, resulting in severe lithium deposition. The deposited lithium dendrites cause the volume of the secondary battery 100 to increase, which is manifested as an increase in the expansion rate. Therefore, the mass percentage of the solid electrolyte in the first active material layer 2 is preferably 0.1wt% to 20wt%.

[0204] Effect of the mass percentage of silicon-based material and first graphite material on the performance of secondary batteries

[0205] 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 the silicon-based material to the sum of the mass of the first graphite material and the silicon-based material were 1%, 3%, 5%, 10%, 15%, 20%, 25%, 30%, and 35%, respectively.

[0206] Table 2

[0207] It should be noted that: in Table 2, the first dispersant, the conductive agent, MMA (methyl methacrylate) and AIBN (azobisisobutyronitrile) are added in a mass ratio of 2:2:5.5:0.5, and the mass ratio of each substance in the silicon-based material and the first graphite material to the first active material layer slurry is calculated by m1 / (m1+m2) in Table 2.

[0208] As can be seen from Table 2, when m1 / (m1+m2) is greater than 30, the solid electrolyte's ability to restrict the expansion of the silicon-based material is limited due to the increased content of the silicon-based material, resulting in a larger expansion rate and lower capacity retention of the secondary battery. Furthermore, lithium plating is severe, deteriorating the battery's kinetic performance. When m1 / (m1+m2) is less than 5, the battery's energy density (ED) drops sharply. Therefore, the ratio of the mass of the silicon-based material to the sum of the masses of the first graphite material and the silicon-based material is preferably 5% to 30%.

[0209] Effect of the particle size ratio of silicon-based material to first graphite material on secondary battery performance

[0210] Table 3 below shows the performance of the secondary batteries corresponding to the above-mentioned Examples C1 to C11. v 50 and the particle size D of the first graphite material v 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.

[0211] Table 3

[0212] It can be seen from Table 3 that 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 D to 50 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, resulting in excessive side reactions and accelerated cycle attenuation of the secondary battery 100. v 50 and the particle size D of the first graphite material v When the ratio D 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 close to that of the first graphite material, resulting in poor co-embedding effect 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 is v 50 and the particle size D of the first graphite material v The ratio of 50 is preferably 0.1 to 0.5.

[0213] Further observation of the data in Table 3 shows that the particle size D of the silicon-based material v 50 and the particle size D of the first graphite material vWhen the ratio of Dv50 to 50 is greater than 0.25, the capacity retention rate increases more slowly with the increase of the ratio. Therefore, when the ratio is greater than 0.25, the benefit of increasing the ratio on the capacity retention rate is small. When the ratio is less than 0.25, the expansion rate decreases more slowly with the decrease of the ratio. Therefore, when the expansion rate is less than 0.25, the benefit of reducing the ratio on the expansion rate is small. Considering the expansion rate and capacity retention rate comprehensively, the particle size Dv50 of the silicon-based material and the particle size Dv50 of the first graphite material are v 50 is preferably 0.25, wherein the particle size D of the silicon-based material is v 50 can be 4 to 10 μm, then the particle size D of the first graphite material is v 50 is 16~40μm.

[0214] Effect of the mass ratio of the conductive agent to the first active material layer 2 on the performance of the secondary battery

[0215] Table 4 below shows the performance of the secondary batteries corresponding to Examples D1 to D10. In Examples D1 to D10, the amount of the conductive agent added relative to the mass percentage of 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.

[0216] Table 4

[0217] It should be noted that: in Table 4, the mass ratio of the first graphite material, the silicon-based material, MMA (methyl methacrylate), AIBN (azobisisobutyronitrile) and the first dispersant is 80:10:5.5:0.5:2, and the mass ratio of the conductive agent to each substance in the first active material layer slurry is calculated by the mass percentage of the conductive agent in Table 4.

[0218] As can be seen from Table 4, when the mass percentage of the conductive agent in the first active material layer 2 exceeds 20 wt%, the capacity retention rate no longer increases, and as the mass percentage of the conductive agent in the first active material layer 2 increases, the energy density of the secondary battery 100 decays faster. Therefore, the mass percentage of the conductive agent in the first active material layer 2 is preferably 0-20 wt%. Furthermore, the mass percentage of the conductive agent in the first active material layer 2 is preferably 0.1-15 wt%. Furthermore, the mass percentage of the conductive agent in the first active material layer 2 is preferably 0.1-5 wt%. Furthermore, the mass percentage of the conductive agent in the first active material layer 2 is preferably 1-5 wt%.

[0219] Influence of the Second Active Material Layer 3 on the Performance of the Secondary Battery

[0220] Figure 9 is a graph showing the cycle life of the secondary batteries of Examples 1 and 2. As can be seen from Figure 9, at the same number of cycles, the capacity retention of the secondary battery of Example 2 is improved compared to that of the secondary battery of Example 1. This indicates that the addition of a second active material layer 3 comprising a second graphite material to the surface of the first active material layer 2 further reduces silicon expansion displacement, thereby improving the cycle performance of the secondary battery 100.

[0221] Figure 10 is a cyclic expansion rate diagram of the secondary batteries of Example 1 and Example 2. It can be seen from Figure 10 that under the same number of cycles, the expansion rate of the secondary battery of Example 2 is reduced compared with the secondary battery of Example 1. That is, by adding a second active material layer 3 having 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.

[0222] The degree of lithium deposition of the negative electrode sheets 10 of Example 1 and Example 2 was tested. The degree of lithium deposition of Example 1 was "moderate", and the degree of lithium deposition of the negative electrode sheet 10 of Example 2 was "mild". By adding a second active material layer 3 having a second graphite material to the surface of the first active material layer 2, the lithium deposition problem of the negative electrode sheet 10 can be improved, and the cycle performance of the secondary battery 100 can be improved.

[0223] Effect of the Mass Ratio of the Second Active Material Layer 3 to the First Active Material Layer 2 on Secondary Battery Performance

[0224] Table 5 below shows the performance of the secondary batteries corresponding to Examples E1 to E10. In Examples E1 to E10, the mass ratios of the second active material layer 3 to the first active material layer 2 are 0.05, 0.1, 0.2, 0.3, 0.5, 1, 1.5, 2, and 2.5, respectively.

[0225] Table 5

[0226] 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, wherein the total coating thickness of the second active material layer 3 and the first active material layer 2 is 150 μm.

[0227] Table 5 shows that 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 infiltration time increases significantly, indicating that the first active material layer 2 has a high compaction density and is not easily infiltrated. Furthermore, excessive compaction density can cause particles in the first active material layer 2 to break. Therefore, the mass ratio of the second active material layer 3 to the first active material layer 2 is preferably between 0.2:1 and 2:1.

[0228] Influence of the Inorganic Material in the Second Active Material Layer 3 on the Performance of the Secondary Battery

[0229] FIG11 is a cycle life graph of the secondary battery 100 of Example 2 and Example 3. As can be seen from FIG11, under the same number of cycles, the capacity retention rate of the secondary battery 100 of Example 3 is improved compared with the secondary battery 100 of Example 2, that is, by adding an inorganic material in the second active material layer 3 and the particle size D of the inorganic material is larger than that of the secondary battery 100 of Example 2, the capacity retention rate of the secondary battery 100 of Example 3 is improved. v 50 and the particle size D of the second graphite material v The ratio of 50 is less than 0.1, which can play a role in lubrication and buffering for 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.

[0230] FIG12 is a graph showing the cyclic expansion rates of the secondary batteries 100 of Examples 2 and 3. As can be seen from FIG12 , under the same number of cycles, the expansion rate of the battery of Example 3 is reduced compared to that of the battery of 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.

[0231] The degree of lithium deposition of the negative electrode sheets 10 of Examples 2 and 3 was tested. The degree of lithium deposition of the negative electrode sheet 10 of Example 2 was "mild", and the degree of lithium deposition of the negative electrode sheet 10 of Example 3 was also "mild". However, the secondary battery of Example 3 had a lower degree of lithium deposition than the secondary battery of Example 2. That is, by adding a second active material layer 3 having a second graphite material and inorganic particles to the surface of the first active material layer 2, the lithium deposition problem of the negative electrode sheet 10 can be further improved, thereby improving the cycle performance of the secondary battery 100. In particular, since the degree of lithium deposition of the negative electrode sheet 10 is reduced, the expansion rate of the secondary battery 100 is also reduced.

[0232] Regarding the particle size D of inorganic materials v 50 and the particle size D of the second graphite material v Effect of the ratio of 50 on the performance of secondary batteries

[0233] Table 6 below shows the performance of the secondary batteries corresponding to the above-mentioned Examples F1 to F13. v 50 and the particle size D of the second graphite material v 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.

[0234] Table 6

[0235] It can be seen from Table 6 that the particle size D of the inorganic material v 50 and the particle size D of the second graphite material v When the ratio of 0 to 50 is 0.045, the capacity retention rate is the best. This is because when the ratio is 0.045, the overall overpressure of the first active material layer 2 and the second active material layer 3 is better, the particle breakage is less, the capacity loss is less, and the dynamic performance of the secondary battery 100 is better. 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 below 0.23, preferably 0.045.

[0236] Effect of the mass percentage of inorganic material in the second active material layer 3 on the performance of the secondary battery

[0237] Table 7 below shows the performance of the secondary batteries corresponding to Examples G1 to G11. In Examples G1 to G11, the mass percentages of the inorganic material in the second active material layer 3 were 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.

[0238] Table 7

[0239] 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, and the amount of inorganic material added is calculated by the mass percentage of the inorganic material in the second active material layer 3 in Table 7.

[0240] As can be seen from Table 7, when the inorganic material accounts for less than 0.1 wt% of the second active material layer 3, the inorganic material's lubricating and buffering effect on the second graphite material is insignificant. During the cold pressing process of the electrode, many active material particles are crushed, resulting in a loss of battery capacity, which leads to a lower energy density of the secondary battery 100. In addition, the compaction density of the negative electrode 10 is higher, significantly increasing the infiltration time of the free electrolyte. When the inorganic material accounts for more than 5 wt% of the second active material layer 3, the inorganic material accounts for a large proportion of the second active material layer 3, resulting in a significant loss of energy density. Therefore, the inorganic material preferably accounts for a mass percentage of 0.1 to 5 wt% of the second active material layer 3.

[0241] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Based on the idea of ​​the present application, the technical features in the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations in different aspects of the present application as above, which are not provided in detail for the sake of simplicity. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A secondary battery, characterized in that: The present invention comprises a negative electrode plate, wherein the negative electrode plate comprises a negative electrode current collector and a first active material layer, wherein the first active material layer is arranged on at least one surface of the negative electrode current collector, and the first active material layer comprises a silicon-based material and a solid electrolyte, and at least a portion of the solid electrolyte is bonded to the surface of the silicon-based material.

2. The secondary battery according to claim 1, wherein The solid electrolyte accounts for 0.1 wt% to 20 wt% of the first active material layer.

3. The secondary battery according to claim 2, wherein The solid electrolyte accounts for 5 wt % to 15 wt % of the first active material layer.

4. The secondary battery according to claim 1, wherein The solid electrolyte includes at least one of polymethyl methacrylate, polyvinylidene fluoride, and polyacrylonitrile.

5. The secondary battery according to claim 4, wherein The solid electrolyte is coated on the surface of the silicon-based material.

6. The secondary battery according to claim 4, characterized in that The solid electrolyte includes polymethyl methacrylate and / or polyacrylonitrile, silicon in the silicon-based material is connected to hydroxyl groups, and the solid electrolyte is combined with the hydroxyl groups.

7. The secondary battery according to any one of claims 1 to 6, characterized in that: The secondary battery further includes a free electrolyte.

8. The secondary battery according to any one of claims 1 to 6, characterized in that: The first active material layer further includes a first graphite material mixed with the silicon-based material.

9. The secondary battery according to claim 8, 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% to 30%.

10. The secondary battery according to claim 8, wherein 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.

11. The secondary battery according to claim 10, wherein: 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.

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

13. The secondary battery according to any one of claims 1 to 12, characterized in that: The silicon-based material includes one or more of silicon, silicon oxide, silicon carbide and silicon alloy.

14. The secondary battery according to any one of claims 1 to 13, characterized in that: The secondary battery further includes a second active material layer, the second active material layer being disposed on a side of the first active material layer away from the negative electrode current collector; The second active material layer includes a second graphite material.

15. The secondary battery according to claim 14, characterized in that The mass ratio of the second active material layer to the first active material layer is in a range from 0.2:1 to 2:

1.

16. The secondary battery according to claim 14, characterized in that The second active material layer further comprises an inorganic material, the particle size D of the inorganic material v 50 and the particle size D of the second graphite material v A ratio of 50 is less than 0.

23.

17. The secondary battery according to claim 16, wherein: 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.

18. The secondary battery according to claim 17, wherein: The particle size D of the inorganic material v 50 is 0.3 μm to 2 μm, and the particle size D of the second graphite material is v 50 is 10μm~15μm.

19. The secondary battery according to claim 16, wherein The mass percentage of the inorganic material in the second active material layer is 0.1 wt % to 5 wt %.

20. The secondary battery according to claim 16, wherein The inorganic material includes one or more of 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.

21. The secondary battery according to claim 20, characterized in that The inorganic material is alumina and / or boehmite.

22. The secondary battery according to any one of claims 1 to 21, characterized in that: The secondary battery satisfies one of the following conditions: The conductive agent in the first active material layer accounts for 0 wt% to 20% by mass; The conductive agent in the first active material layer accounts for 0.1 wt% to 15% by weight; The mass proportion of the conductive agent in the first active material layer is 0.1wt% to 5wt%; The conductive agent in the first active material layer accounts for 1 wt % to 5 wt %.

23. An electrical device, characterized in that: The utility model comprises the secondary battery according to any one of claims 1 to 22 and a load, wherein the secondary battery is used to supply power to the load.

24. A method for preparing a secondary battery according to any one of claims 1 to 22, characterized in that: To prepare the negative electrode sheet, a silicon-based material, a first dispersant, a conductive agent, 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 evenly coated on at least one surface of the negative electrode current collector.

Images