Secondary battery and electric equipment

Abstract

The present invention relates to the technical field of electrochemical devices, and particularly relates to a secondary battery and an electric equipment. The secondary battery (100) comprises a negative electrode sheet (10), and the negative electrode sheet (10) comprises a current collector (1), a first active material layer (2) and a second active material layer (3), wherein the first active material layer (2) is arranged on at least one surface of the current collector (1), and the second active material layer (3) is arranged on the side, away from the current collector (1), of the first active material layer (2). The first active material layer (2) comprises a silicon-based material and a first graphite material; and the second active material layer (3) comprises a second graphite material and an inorganic material, the ratio of the particle size Dv50 of the inorganic material to the particle size Dv50 of the second graphite material being less than 0.23. The secondary battery and the electric equipment can mitigate the problem of the overvoltage of the second active material layer (3), maintain the expansion inhibition effect of the second active material layer (3) on the silicon-based material, and enhance the structural integrity and structural stability of the negative electrode sheet.

Description

Secondary battery and electrical equipment

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to the Chinese patent application filed with the China Patent Office on March 31, 2024, with application number 202410383524.7 and entitled “A Secondary Battery and Electrical Equipment,” the entire contents of which are incorporated herein by reference. Technical Field

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

[0004] The negative electrode active material in lithium-ion batteries is generally graphite. If silicon is added to the negative electrode active 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 undergo lithiation and delithiation, causing significant expansion and contraction, which can compromise the structural integrity and stability of the negative electrode. Prior art solutions have addressed this issue by adding a graphite layer to the silicon-based active material layer to suppress silicon particle expansion. Summary of the Invention

[0006] The inventors discovered that in a scheme of suppressing the expansion of silicon particles by adding a graphite layer to the silicon-based active material layer, the graphite particles in the added graphite layer are easily broken when the negative electrode is rolled, which can easily lead to failure in suppressing the expansion of the silicon particles and cause capacity loss of the negative electrode.

[0007] The embodiments of the present application aim to provide a secondary battery and an electrical device, which can at least improve the problem of failure of the graphite layer to inhibit the expansion of the silicon-based material and the loss of capacity of the negative electrode plate.

[0008] In order to solve the above technical problems, a technical solution adopted in the embodiment of the present application is as follows: providing a secondary battery, the secondary battery including a negative electrode plate, the negative electrode plate including a current collector, a first active material layer and a second active material layer, the first active material layer is arranged on at least one surface of the current collector, the second active material layer is arranged on a side of the first active material layer away from the current collector, the first active material layer includes a silicon-based material and a first graphite material, the second active material layer includes a second graphite material and an inorganic material, and the particle size D of the inorganic material is 0.0447 W / cm2. v 50 and the particle size D of the second graphite material v 50 is less than or equal to 0.23. v50 and the particle size D of the second graphite material v The ratio of 50 is limited to less than or equal to 0.23, which can allow the inorganic material to fill the gaps in the second graphite material, and can produce a lubricating and buffering effect on the second graphite material. When the negative electrode sheet is rolled, the risk of the second graphite material being crushed can be reduced, thereby improving the problem of the weakened expansion inhibition effect of the second active material layer on the silicon-based material, which is beneficial to enhancing the cycle performance of the secondary battery.

[0009] 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. In this case, the particles of the second graphite material are less broken, the secondary battery can obtain a better capacity retention rate, and the kinetic performance of the second active material layer is the best.

[0010] 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.

[0011] In some embodiments, the inorganic material accounts for 0.1 wt% to 5 wt% of the second active material layer. By defining the above, the lubricating and buffering effect of the inorganic material on the second graphite material and the energy density of the secondary battery can be taken into account. The inorganic material can provide a certain lubricating and buffering effect on the second graphite material, and during the rolling process of the negative electrode sheet, the graphite particles in the second graphite material are not easily broken, and the secondary battery can have a higher energy density.

[0012] In some embodiments, the inorganic material preferably accounts for 1 wt % to 3 wt % of the second active material layer.

[0013] 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 compaction density of the first active material layer after rolling is high. Excessive compaction density can cause the first graphite material particles in the first active material layer to break, thereby causing a loss of negative electrode capacity. 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.

[0014] 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 m1 to the mass of the first graphite material m2 to no less than 5% can ensure the energy density 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.

[0015] 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 of silicon and the resulting displacement, and enhance the structural integrity and stability of the negative electrode plate.

[0016] In some embodiments, the particle size D of the silicon-based material v 50 and the particle size D of the first graphite material v The ratio of 50 is 0.2 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.

[0017] In some embodiments, the particle size D of the silicon-based material v 50 is 4um~10um, the particle size D of the first graphite material v 50 is 16um~40um.

[0018] In some embodiments, the inorganic material includes one or more of aluminum oxide, garnet, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium oxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate.

[0019] In some embodiments, the inorganic material is alumina and / or boehmite. It has good electrochemical stability and the raw materials are easily available.

[0020] In some embodiments, the silicon-based material includes one or more of silicon, silicon oxide, silicon carbide, and a silicon alloy.

[0021] In some embodiments, the secondary battery further includes a free electrolyte. By adding the free electrolyte, the transfer rate of lithium ions can be increased, the charge and discharge rates can be guaranteed, and thus the kinetic performance of the secondary battery can be improved.

[0022] In some embodiments, the first active material layer includes an inorganic material. By adding the inorganic material to the first active material layer, the inorganic material can fill gaps in the first graphite material. During roll-pressing of the negative electrode sheet, the inorganic material can act as a buffer and lubricant for the first graphite material, thereby reducing the risk of crushing the first graphite material. This can also mitigate the problem of failure to suppress silicon expansion and negative electrode sheet capacity loss caused by crushing of the first graphite material.

[0023] 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.

[0024] Different from the related art, in the secondary battery and the electrical equipment of the present invention, the particle size D of the inorganic material is v 50 and the particle size D of the second graphite material v 50 is less than 0.23, the inorganic material can fill the gaps in the second graphite material, thereby lubricating and buffering the second graphite material. When the negative electrode sheet is rolled, the problem of graphite particle breakage can be reduced, the problem of overpressure of the second active material layer can be improved, the expansion inhibition effect of the second active material layer on the silicon-based material can be maintained, the structural integrity and structural stability of the negative electrode sheet can be enhanced, and the problem of deterioration of the performance of the negative electrode sheet can be improved.

[0025] 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

[0026] 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.

[0027] FIG1 is a schematic diagram of a framework of an electrical device according to an embodiment of the present application;

[0028] FIG2 is a schematic structural diagram of a negative electrode sheet according to an embodiment of the present application;

[0029] FIG3 is a SEM photograph of the negative electrode sheet of an embodiment of the present application;

[0030] FIG4 is another SEM photograph of the negative electrode sheet according to an embodiment of the present application;

[0031] FIG5 is a cycle life diagram of the secondary batteries of Comparative Example 1 and Example 1 of the present application;

[0032] FIG6 is a graph showing the cyclic expansion ratio of the secondary batteries of Comparative Example 1 and Example 1 of the present application;

[0033] FIG7 is a cycle life diagram of the secondary batteries of Examples 1 and 2 of the present application;

[0034] FIG8 is a graph showing the cyclic expansion ratios of the secondary batteries of Examples 1 and 2 of the present application.

[0035] The reference numerals in the specific embodiments are as follows: 1000, electric device; 100, secondary battery; 200, load; 10, negative electrode sheet; 1, current collector; 2, first active material layer; 3, second active material layer. DETAILED DESCRIPTION

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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 .

[0043] 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.

[0044] 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.

[0045] The secondary battery 100 may be a soft-pack battery, a square-shell battery, a cylindrical battery, etc. The secondary battery 100 includes a negative electrode sheet 10 , which may be in a wound form or a laminated form.

[0046] In the embodiment of the present application, the negative electrode plate 10 is described as a laminated plate.

[0047] As shown in Figure 2, the negative electrode sheet 10 includes a current collector 1, a first active material layer 2, and a second active material layer 3. The first active material layer 2 is disposed on at least one surface of the current collector 1, and the second active material layer 3 is disposed on the side of the first active material layer 2 facing away from the current collector 1. The first active material layer 2 can be disposed on one surface of the current collector 1 along the thickness direction X, or it can be disposed on two opposing surfaces of the current collector 1 along the thickness direction X, that is, the two first active material layers are disposed on opposing surfaces of the current collector 1. When the two first active material layers 2 are disposed on opposing surfaces of the current collector 1 along the thickness direction, the two second active material layers 3 are disposed on one surface of the two first active material layers 2 facing away from the current collector 1. It is understood that multiple first active material layers 2 can be disposed at intervals on one surface of the current collector 1, and multiple second active material layers 3 can be disposed at intervals on one surface of the first active material layer 2.

[0048] Regarding the above-mentioned current collector 1, the current collector 1 is a conductor and is in the form of a thin sheet. The current collector 1 may include copper foil, aluminum foil, nickel foil, stainless steel foil, titanium foil, foamed nickel, foamed copper, a polymer substrate covered with a conductive metal, or any combination thereof.

[0049] Regarding the above-mentioned first active material layer 2 , the active material in the first active material layer 2 includes a silicon-based material and a first graphite material.

[0050] Regarding the second active material layer 3 , the active material in the second active material layer 3 includes a second graphite material, and the second active material layer 3 also includes an inorganic material.

[0051] It should be noted that during charging and discharging of the secondary battery 100, the silicon-based material in the first active material layer 2 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 first active material layer 2; it will also damage the solid electrolyte interface (SEI), causing free electrolyte to penetrate into the first active material layer 2, triggering a series of unstable reactions such as the decomposition of the free electrolyte and the repeated destruction of the SEI film, which significantly reduces the cycle performance of the secondary battery 100. To address this issue, by adding a second active material layer 3 to the side of the first active material layer 2 facing away from the current collector 1, and the second active material layer contains a second graphite material, the expansion displacement of the silicon-based material in the first active material layer 2 can be suppressed, thereby improving the above-mentioned problem.

[0052] In order to compact the first active material layer 2 and the second active material layer 3, the negative electrode sheet 10 needs to be rolled during the production process. During rolling, the roller directly contacts the second active material layer 3, resulting in a large pressure on the second active material layer 3, making the particles in the second active material layer 3 (such as the second graphite material) easily broken under rolling, which in turn easily leads to the second active material layer 3 weakening the expansion inhibition effect of the silicon-based material in the first active material layer 2, and may even cause complete failure, which also affects the capacity of the negative electrode sheet 10. In order to improve this problem, in the embodiment of the present application, the second active material layer 3 includes a second graphite material and 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 50 is less than or equal to 0.23, the inorganic material can be filled in the gaps of the second graphite material. Please refer to Figure 3, which is a SEM (Scanning Electron Microscope) photo of the negative electrode plate 10. The larger particles in Figure 3 are the second graphite material, and the smaller particles are the inorganic material. As can be seen from Figure 3, 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. When the negative electrode plate 10 is rolled or other second active material layers 3 are subjected to pressure, the risk of graphite material particles breaking can be reduced, thereby improving the problem of the second active material layer 3 weakening the expansion inhibition effect on the silicon-based material and the capacity loss of the negative electrode plate, which is beneficial to enhancing the cycle performance of the secondary battery 100.

[0053] 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. v50 can be measured by a laser particle size analyzer with reference to GB / T 19077-2016 "Particle size distribution by laser diffraction method".

[0054] It is worth noting that during the rolling process of the negative electrode sheet 10, 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 affects the compaction density of the negative electrode and the capacity retention rate of the secondary battery. The so-called capacity retention rate is: after the secondary battery has undergone several cycles of charge and discharge, the capacity of the secondary battery is divided by the capacity at the first charge and discharge, and the capacity retention rate of the secondary battery after several cycles of charge and discharge is obtained. v 50 and the particle size D of the second graphite material v When the ratio of 50 is too large, the compaction density is likely to decrease, resulting in a narrower processing window of the second graphite material, increased processing difficulty, and a decrease in capacity retention rate. Therefore, the particle size D of the inorganic material is selected. v 50 and the particle size D of the second graphite material v 50 is less than 0.23, the inorganic material can fill the gaps in the second graphite material, can play a lubricating and buffering role for the second graphite material, reduce the problem of particle breakage of the second graphite material, improve the problem of overvoltage of the negative electrode plate, enhance the cycle performance of the secondary battery, and ensure that the second graphite material has a wider processing window.

[0055] Furthermore, 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. The applicant has found through experiments that when the particle size D v 50 and the particle size D of the second graphite material v When the ratio of 50 is 0.04 to 0.1, the negative electrode sheet can have a suitable compaction density and ensure that the secondary battery has a better capacity retention rate. v 50 and the particle size D of the second graphite material v The relationship between the ratio of 50 and the compaction density of the second active material layer 3 and the capacity retention rate of the secondary battery is described in detail in experimental data later.

[0056] Please refer to Figure 4, which is an SEM photograph of the negative electrode sheet 10 after roller pressing. In Figure 4, 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 4, after the inorganic material is added to the second active material layer 3, there is no particle breakage in the first active material layer 2 and the second active material layer 3, indicating that there is no overpressure on the first and second graphite materials.

[0057] In some embodiments, the particle size D of the inorganic material v 50 is 0.3 μm to 2 μm. The particle size D of the second graphite material v 50 is 10μm~15μm.

[0058] In some embodiments, the mass percentage of the inorganic material in the second active material layer 3 is 0.1wt% to 5wt%. When the mass percentage of the inorganic material in the second active material layer 3 is less than 0.1wt%, the lubricating effect of the inorganic material on the second graphite material is sharply reduced. When the mass percentage of the inorganic material in the second active material layer 3 is greater than 5wt%, the inorganic material accounts for a larger proportion of the second active material layer 3, and the proportion of the second graphite material decreases accordingly, which seriously reduces the energy density of the secondary battery 100. Therefore, in this application, the mass percentage of the inorganic material in the second active material layer 3 is selected to be 0.1wt% to 5wt%. Regarding the relationship between the mass percentage of the inorganic material in the second active material layer 3 and the energy density of the secondary battery 100, specific experimental data are recorded later.

[0059] Preferably, the inorganic material accounts for 1 wt% to 3 wt% of the second active material layer 3. By making the inorganic material account for 1 wt% to 3 wt% of the second active material layer 3, the inorganic material can have a better lubricating and buffering effect on the second graphite material and the secondary battery 100 can have a better energy density.

[0060] 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 compaction density of the first active material layer after rolling will be large. Excessive compaction density will cause the first graphite material particles in the first active material layer to break, thereby causing a loss of negative electrode capacity. 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. 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 m1 of the silicon-based material to the mass of the first graphite material m2 to no less than 5% can ensure the energy density of the secondary battery. 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 plate 10.

[0061] In some embodiments, the particle size D of the silicon-based material in the first active material layer 2 is v50 and the particle size D of the first graphite material v The ratio of 50 is 0.1 to 0.5, whereby the silicon-based material can fill the gaps in the first graphite material, so that the first graphite material can restrain the expansion and displacement of the silicon-based material, thereby improving the problem of damage to the negative electrode plate 10.

[0062] 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 cycle decay of the secondary battery. 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 secondary 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 negative electrode. 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 selected as 0.1 to 0.5.

[0063] 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 ratio 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, 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.

[0064] 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.

[0065] It should be noted that, in the embodiment of the present application, the particle size D of the second graphite material is v 50 is 10-15um, which is smaller than the particle size D of the first graphite material vThe value range of 50 is 16μm to 40μm. This is because when rolling the negative electrode sheet 10, the roller directly contacts the second active material layer 3, and the pressure exerted on the second active material layer 3 is greater than that on the first active material layer 2. The second graphite material with a small particle size is less likely to be crushed than the first graphite material with a large particle size, thereby improving the problem of the second graphite material in the second active material layer 3 being easily broken during rolling.

[0066] In some embodiments, the inorganic material can be 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, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate.

[0067] Furthermore, the inorganic material is alumina and / or boehmite, which have good electrochemical stability and the raw materials are easily available. Furthermore, the inorganic material is alumina, which has high hardness.

[0068] In some embodiments, the second graphite material may be one or more of hard carbon, soft carbon, or graphite.

[0069] In some embodiments, the OI value of the second graphite material is less than or equal to 10. 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, 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 calculating the OI value. 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.

[0070] In some embodiments, the second active material layer 3 further includes a second binder. The second binder may be a polymer such as PAA (polyacrylic acid), SBR (styrene-butadiene rubber), CMC (sodium carboxymethyl cellulose), PVDF (polyvinylidene fluoride), PAN (polyacrylonitrile), PVA (polyvinyl alcohol), or a functionalized derivative or monomer copolymer thereof. The addition of the second binder can strengthen the bond between the particles in the second graphite material, thereby enabling the second active material layer 3 to better bind the silicon-based material in the first active material layer 2 and suppress expansion of the negative electrode 10.

[0071] In some embodiments, the second active material layer 3 further includes a second dispersant, which may be CMC (sodium carboxymethyl cellulose).

[0072] In a specific embodiment, the mass ratio of the second graphite material, the inorganic material, the second binder, and the second dispersant is 97:0.5:1.5:1.

[0073] In some embodiments, the first graphite material in the first active material layer 2 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. Similar to the OI value of the second graphite material, selecting an OI value of less than or equal to 10 for the first graphite material helps suppress expansion during cycling.

[0074] In some embodiments, the silicon-based material includes one or more of silicon, silicon oxide, and silicon carbide.

[0075] In some embodiments, the first active material layer 2 further includes a solid electrolyte, which is at least partially bonded to the surface of the silicon-based material. Thus, the solid electrolyte can form a protective layer on the silicon surface, further reducing the expansion displacement of the silicon and the expansion or contraction of the negative electrode plate 10, thereby enhancing the structural integrity and stability of the negative electrode plate 10. Furthermore, the solid electrolyte can enhance the lithium insertion capability of the silicon-based material and accelerate the ion conduction rate in the negative electrode plate 10, thereby improving the kinetic performance and cycle performance of the secondary battery 100.

[0076] In some embodiments, the solid-state electrolyte includes at least one of polymethyl methacrylate, polyvinylidene fluoride, and polyacrylonitrile.

[0077] In some embodiments, the first active material layer 2 further includes a first binder, a first dispersant, and a conductive agent.

[0078] For the above-mentioned first binder, the first binder can be polymers such as PAA (polyacrylic acid), SBR (styrene-butadiene rubber), CMC (sodium carboxymethyl cellulose), PVDF (polyvinylidene fluoride), PAN (polyacrylonitrile), PVA (polyvinyl alcohol) and their functionalized derivatives or monomer copolymers.

[0079] As for the first dispersant, the first dispersant may be CMC (sodium carboxymethyl cellulose).

[0080] For the above-mentioned conductive agent, the conductive agent can be one or more of conductive carbon materials, SP (conductive carbon black), CNT (carbon nanotube), VGCF (vinyl glass carbon fiber), metal particles or metal fibers, and the added amount of the conductive agent accounts for 0 to 20 wt% of the mass percentage of the first active material layer 2.

[0081] In a specific embodiment, the mass ratio of the first graphite material, the silicon-based material, the first binder, the first dispersant and the conductive agent is 80:10:5.5:2:2.5.

[0082] In some embodiments, the secondary battery further includes a free electrolyte, which can increase the transmission rate of lithium ions and ensure the charge and discharge rate, thereby improving the kinetic performance of the secondary battery.

[0083] Since the second active material layer 3 is added to the first active material layer 2, when the secondary battery 100 also includes free electrolyte, it is more difficult for the free electrolyte to penetrate the first active material layer 2, which may lead to poor wettability of the negative electrode plate 10, a decrease in the free electrolyte retention of the negative electrode plate 10, and thus a decrease in the electrochemical performance of the secondary battery 100. However, in the present application, due to the addition of the inorganic material, the porosity of the second active material layer 3 is increased. Under the premise that the second active material layer 3 can suppress the expansion and displacement of the silicon-based material, the impact of the second active material layer 3 on the wettability of the free electrolyte can be reduced. Specifically, it can be seen from Figure 4 that the porosity of the second active material layer 3 is greater than the porosity of the first active material layer 2, so the free electrolyte can easily penetrate from the second active material layer 3 into the first active material layer 2, which is beneficial to reducing the influence of the second active material layer 3 on the wettability of the free electrolyte, reducing concentration polarization, and thus reducing the internal resistance of the secondary battery 100, improving the charge and discharge efficiency and capacity retention rate of the secondary battery 100, and extending the cycle life of the secondary battery 100.

[0084] It can be understood that the addition of inorganic materials can form more pores in the negative electrode plate 10, which is beneficial to enhance the wetting effect of the free electrolyte in the negative electrode plate 10, reduce the wetting time of the free electrolyte, increase the retention of the free electrolyte in the negative electrode plate 10, increase the ion transmission speed of the free electrolyte, enhance the fast charging capability of the secondary battery 100, and improve the electrochemical performance of the secondary battery 100.

[0085] It is worth noting that the wetting time is measured by dropping an equal volume of electrolyte, for example, 0.05 ml of free electrolyte, onto different negative electrode sheets 10 and observing the time it takes for the free electrolyte to be completely absorbed. This time is the free electrolyte wetting time. The wetting time reflects the ease with which the negative electrode sheet 10 is wetted; the shorter the wetting time, the easier it is for the free electrolyte to wet the negative electrode sheet 10.

[0086] In some embodiments, the first active material layer 2 may also include the above-mentioned inorganic material. Since the particle size of the first graphite material is larger than the particle size of the second graphite material, the inorganic material can fill the gaps in the first graphite material. When the negative electrode sheet 10 is rolled, the inorganic material can also buffer and lubricate the first graphite material, which helps to reduce the risk of the first graphite material being crushed.

[0087] In some embodiments, the particle size D of the inorganic material v 50 and the particle size D of the first graphite material v 50 is less than or equal to 0.23.

[0088] In some embodiments, the inorganic material accounts for 0.1 wt % to 5 wt % of the first active material layer 2 .

[0089] In some embodiments, the secondary battery 100 further includes a separator and a positive electrode plate. The positive electrode plate 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, polyacrylic acid salt, 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, polyacrylic acid salt, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride or poly (vinylidene fluoride and hexafluoropropylene).

[0090] In order to evaluate the beneficial effects of the present invention, a lithium-ion secondary battery is used as an example for testing. In the following examples and comparative examples, all reagents, materials, and instruments used are commercially available unless otherwise specified.

[0091] Example 1

[0092] Preparation of negative electrode slurry

[0093] Second active material layer slurry: Mix the second graphite material, inorganic material, second binder and second dispersant in a mass ratio of 97:0.5:1.5:1, dissolve in deionized water to form a negative electrode slurry with a solid content of 50%, stir evenly, and prepare for coating. v 50 is 15μm graphite particles, and the inorganic material is D v 50 is 0.3 μm aluminum oxide (Al2O3) (ie, D of inorganic materials v 50 and D of the second graphite material v50 is 0.02), the second binder is SBR (styrene-butadiene rubber), and the second dispersant is CMC (sodium carboxymethyl cellulose).

[0094] First active material layer slurry: Mix the first graphite material, silicon-based material, first binder, first dispersant, and conductive agent in a mass ratio of 80:10:5.5:2:2.5, dissolve in deionized water to form a negative electrode slurry with a solid content of 40%. Stir evenly and prepare for coating. The silicon-based material is SiO2 (silicon dioxide), the first graphite material is graphite particles, the first binder is PAA (polyacrylic acid), the first dispersant is CMC (sodium carboxymethyl cellulose), and the conductive agent is conductive carbon black.

[0095] Preparation of negative electrode sheet 10

[0096] A double-layer coating machine is used to evenly coat the second active material layer slurry and the first active material layer slurry onto one surface of a negative electrode current collector copper foil having a thickness of 8 μm. The coating is then dried at 110 degrees Celsius to obtain a negative electrode sheet 10 coated on one side with a double layer of negative electrode active material. After drying, the coating thickness of the double layer of negative electrode active material is 150 μm. In this step, since the second active material layer slurry is coated onto the undried first active material layer slurry, the first active material layer 2 and the second active material layer 3 are mutually soluble, and there is no obvious interface, which can reduce the impedance between the first active material layer 2 and the second active material layer 3.

[0097] Then, the above coating steps are repeated on the other surface of the negative electrode sheet 10 to obtain a negative electrode sheet 10 with double-layer negative electrode active material layers coated on both sides.

[0098] After coating, the negative electrode sheet 10 is dried, cold-pressed, and cut into sheets measuring 74 mm x 867 mm for later use. The bonding strength between the active material layer and the current collector 1 must be 10 to 100 N / m, and the total porosity of the negative electrode sheet 10 must be 10 to 30%.

[0099] Preparation of positive electrode

[0100] The positive electrode active material LiCoO2 (lithium cobalt oxide), the conductive agent SuperP (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.

[0101] 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.

[0102] Then, the above coating steps are repeated on the other surface of the positive electrode sheet to obtain a positive electrode sheet with positive electrode active material coated on both sides.

[0103] After coating, the positive electrode sheets were dried, cold pressed, and cut into sheets of 74 mm x 867 mm for later use. The compacted density of the positive electrode sheets was 4.15 g / cm3.

[0104] Preparation of free electrolyte

[0105] 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 free electrolyte. The concentration of LiPF6 is 1 mol / L.

[0106] The present application does not impose any specific restrictions on the free electrolyte, and it can be selected according to actual needs. As an example, the additive in the free 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).

[0107] Preparation of isolation membrane

[0108] The isolation membrane includes a substrate layer and a coating. The substrate layer is 5μm thick PE (polyethylene), and then a 2μm alumina ceramic layer is coated on both sides of the substrate layer. 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.

[0109] Preparation of lithium-ion secondary batteries

[0110] 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.

[0111] The electrode assembly was placed in an outer packaging aluminum-plastic film, dried in a vacuum oven at 85 degrees Celsius for 12 hours to remove moisture, and the prepared free electrolyte was injected. After 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, a lithium-ion secondary battery was obtained.

[0112] Comparative Example 1

[0113] The difference from Example 1 is that the slurry of the second active material layer 3 does not include inorganic materials, and the mass ratio of the second graphite material, the second binder and the second dispersant is 97:1.5:1.5.

[0114] The lithium-ion secondary batteries of Comparative Example 1 and Example 1 were subjected to the following tests. If the test temperature is not clearly stated during the test, the normal temperature of 25° C. is used as an example.

[0115] (1) Secondary battery capacity retention test

[0116] Charging: In a high-temperature furnace at 45 degrees Celsius, charge the secondary battery to the upper voltage window at a constant current of 2C, then charge it to 0.05C at a constant voltage, and then let it stand for 5 minutes.

[0117] Discharge: Discharge to 3.0V using 0.7C.

[0118] According to the above charge and discharge system, the discharge capacity of each cycle was recorded during the cycle and divided by the initial capacity to obtain the capacity retention rate. Among them, the discharge capacity of the first discharge was the initial discharge capacity, which was calculated as 100%.

[0119] For example, the 300-cycle capacity retention rate = (discharge capacity at the 300th cycle / discharge capacity at the first cycle) × 100%.

[0120] The test results are shown in Figure 5.

[0121] FIG5 is a cycle life diagram of the secondary batteries of Example 1 and Example 1. As can be seen from FIG5, under the same number of cycles, the capacity retention rate of the secondary battery of Example 1 is improved compared with the secondary battery of Comparative Example 1, that is, by adding an inorganic material in the second active material layer 3 and the particle size D of the inorganic material is increased. 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 lubricating and buffering role on the second graphite material, reduce the breakage of graphite material particles, reduce the capacity loss of the secondary battery 100, improve the capacity retention rate of the secondary battery 100, and improve the cycle performance of the secondary battery 100.

[0122] (2) Secondary battery expansion rate test

[0123] Test the thickness of the lithium-ion secondary battery at three locations and take the average value. Note that different lithium-ion secondary batteries should take the same three locations.

[0124] During the cycle, the average value of the secondary battery thickness is recorded. The difference between the average value of the secondary battery thickness at each cycle and the average value of the initial thickness of the secondary battery is divided by the average value of the initial thickness of the secondary battery to obtain the secondary battery expansion rate. The average thickness of the secondary battery when the cycle number of the secondary battery is 0 is the average value of the initial thickness.

[0125] For example, the expansion rate of the secondary battery after 300 cycles = [(average thickness of the secondary battery at the 300th cycle - average thickness of the secondary battery at the beginning) / average thickness of the secondary battery at the beginning] × 100%.

[0126] The test results are shown in Figure 6.

[0127] FIG6 is a graph of the cyclic expansion rates of the secondary batteries of Comparative Example 1 and Example 1. As can be seen from FIG6, under the same number of cycles, the expansion rate of the secondary battery of Example 1 is reduced compared to that of the secondary battery of Comparative Example 1, that is, the addition of inorganic materials to the second active material layer 3 can reduce the expansion rate of the secondary battery 100. It should be noted that the reason for the difference in expansion rate between Comparative Example 1 and Example 1 is that in Comparative Example 1, no inorganic particles are added to the second active material layer, resulting in insufficient wetting of the first active material layer by the free electrolyte, insufficient free electrolyte retention in the first active material layer, and more severe lithium deposition in the negative electrode sheet than in Example 1. The deposited lithium dendrites cause the volume of the secondary battery 100 to increase, which manifests as an increased expansion rate.

[0128] (3) Lithium deposition test of negative electrode sheet 10

[0129] The secondary battery with 300 cycles was disassembled, and the lithium deposition on the negative electrode 10 was observed visually. The degree of lithium deposition was judged as "no lithium deposition", "mild", "moderate" or "severe". The evaluation criteria are as follows:

[0130] 1) There is no visible lithium deposition on the negative electrode sheet 10 - no lithium deposition;

[0131] 2) 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 - minor;

[0132] 3) There are more 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 more than or equal to 10% and less than or equal to 50% of the area of ​​the negative electrode sheet 10 - moderate;

[0133] 4) 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

[0134] Test results: The degree of lithium plating of the negative electrode plate 10 of comparative example 1 is "moderate", and the degree of lithium plating of the negative electrode plate 10 of embodiment 1 is "mild". By adding inorganic materials to the second active material layer 3, the lithium plating problem of the negative electrode plate 10 can be improved, thereby reducing the expansion rate of the secondary battery 100 and improving the cycle performance of the secondary battery 100.

[0135] (4) Porosity test of negative electrode sheet 10

[0136] The porosity of the electrode was tested using a true density tester (AccuPycⅡ1340).

[0137] Analysis Principle: By applying Archimedes' principle (density = mass / volume) and Boyle's law (PV = nRT), and replacing the liquid in Archimedes' principle with gas, the true volume of the material being measured can be accurately measured using the gas displacement method. The gas can be an inert gas with a small molecular diameter to improve measurement accuracy.

[0138] Sample preparation: Punch the negative electrode sheet 10 into discs with a diameter of 10 mm or 14 mm, with the quantity being greater than 40 discs.

[0139] Result calculation: The apparent volume V1 of the sample is calculated based on the shape of the negative electrode plate 10 = S × H × number of samples (S is the bottom area of ​​the sample, H is the thickness of the plate). The above test results can be used to obtain the actual volume V2 of the sample, so the porosity = (V1-V2) × 100% / V1.

[0140] Test results: The total porosity of the negative electrode sheet 10 of comparative example 1 is 37.5%, the porosity of the first active material layer 2 is 38%, and the porosity of the second active material layer 3 is 37%; the total porosity of the negative electrode sheet 10 of embodiment 1 is 40.5%, the porosity of the first active material layer 2 is 38%, and the porosity of the second active material layer 3 is 43%. That is, by adding inorganic materials to the second active material layer 3, the porosity of the second active material layer 3 can be increased, thereby increasing the overall porosity of the negative electrode sheet 10, which is beneficial to enhancing the wetting effect of the free electrolyte.

[0141] The total volume of pores with a pore diameter of less than 3 μm in the negative electrode sheet 10 accounts for 67% of the total volume of the pores.

[0142] (5) Compaction density test of negative electrode sheet 10

[0143] Prepare the negative electrode sheet 10 after cold pressing and cutting, weigh the mass m of the negative electrode sheet 10, measure the length L, width W, and thickness H of the negative electrode sheet 10, and calculate the compaction density ρ of the negative electrode sheet 10 using the following formula: ρ = m / (L×W×H)

[0144] (6) Secondary battery 100 volume energy density test

[0145] 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.

[0146] Volume energy density=discharge energy / (length of the secondary battery 100×width of the secondary battery 100×thickness of the secondary battery 100).

[0147] (7) Free electrolyte infiltration time test

[0148] 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.

[0149] To explore the particle size D of inorganic materials v 50 and the particle size D of the second graphite material v 50, the particle size of the second graphite material and the particle size of the inorganic material used in Example 1 were adjusted to conduct the test. The test results are shown in Table 1 below. In Examples A1 to A13, except for the particle size D of the inorganic material listed in Table 1, v 50 and the particle size D of the second graphite material v Except that the ratio of 50 is different from that in Example 1, other parameters are the same as those in Example 1, and the number of cycles of the secondary battery is 300.

[0150] Table 1

[0151] As can be seen from Table 1, 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 50 is greater than 0.23, the compaction density begins to decrease, which will lead to a narrowing of the processing window of the second graphite material; when the ratio is less than 0.1, the capacity retention rate increases faster, so the particle size D of the inorganic material v 50 and the particle size D of the second graphite material vThe ratio of 50 should not be higher than 0.23 to make the processing window of the second graphite material wider. In this application, considering the capacity retention rate, the particle size D of the inorganic material is selected. v 50 and the particle size D of the second graphite material v 50 is less than 0.1. Furthermore, it can be seen from Table 1 that the particle size D v 50 and the particle size D of the second graphite material v When the ratio of 50 is 0.04 to 0.1, the capacity retention rate is better. This is because when the ratio is within this range, the overall overvoltage 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. Therefore, in this application, 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 preferably 0.04 to 0.1.

[0152] To explore the optimal range of the mass percentage of the inorganic material in the second active material layer 3, the amount of inorganic material added in Example 1 was adjusted to achieve a mass percentage of 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% in the second active material layer 3, respectively. Tests were conducted on these materials. The test results are shown in Table 2 below. In Examples B1 to B10, except for the mass percentage of the inorganic material in the second active material layer 3, which differed from that in Example 1, all other parameters were the same as in Example 1. The secondary batteries were cycled 300 times.

[0153] Table 2

[0154] It should be noted that: in Table 2, the mass ratio of the second graphite material, the second binder and the second dispersant is 97:1.5:1, and the mass ratio of the inorganic material to each substance in the second active material layer slurry is calculated based on the mass percentage of the inorganic material in the second active material layer 3 in Table 2.

[0155] As can be seen from Table 2, when the inorganic material's mass percentage in the second active material layer 3 is less than 0.1 wt%, the inorganic material's lubricating and buffering effect on the second graphite material is insignificant. During the cold pressing process, a significant number of active material particles are crushed, resulting in a loss of battery capacity, leading to a lower energy density of the secondary battery 100. Furthermore, the compaction density of the negative electrode 10 is higher, significantly increasing the time it takes for the free electrolyte to soak. When the inorganic material's mass percentage in the second active material layer 3 is greater than 5 wt%, the inorganic material accounts for a significant proportion of the second active material layer 3, resulting in a significant loss of energy density. Therefore, to balance the inorganic material's lubricating and buffering effect on the second graphite material with the energy density of the secondary battery 100, the inorganic material's mass percentage in the second active material layer 3 is set between 0.1 wt% and 5 wt%. Furthermore, the inorganic material's mass percentage in the second active material layer 3 is preferably between 1 wt% and 3 wt%.

[0156] To explore the particle size D of silicon-based materials v 50 and the particle size D of the first graphite material v The optimal ratio range is 50, and the particle size D of the silicon-based material in Example 1 is adjusted. v 50 and the particle size D of the first graphite material v The test results are shown in Table 3 below, where the particle size D of the silicon-based material in Examples C1 to C11 is v 50 and the particle size D of the first graphite material v Except that the value of 50 is different from that in Example 1, other parameters are the same as those in Example 1, and the number of cycles of the secondary battery is 300.

[0157] Table 3

[0158] 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 50 is less than 0.1, the capacity retention rate of the secondary battery decreases rapidly. This is because 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 cycle attenuation of the secondary battery. v 50 and the particle size D of the first graphite material v When the ratio D of the silicon-based material is greater than 0.5, the expansion rate of the secondary battery increases rapidly. This is because the particle size of the silicon-based material is close to the particle size 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. Therefore, in this application, 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 selected from 0.1 to 0.5.

[0159] 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 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, at which point the silicon-based material and the first graphite material are most densely packed and the porosity of the first active material layer is the smallest. v 50 can be 4 to 10 μm, then the particle size D of the first graphite material is v 50 is 16~40um.

[0160] To explore the optimal range for the mass ratio of the second active material layer 3 to the first active material layer 2, experiments were conducted by adjusting the coating thicknesses of the first and second active material layer slurries in Example 1. The test results are shown in Table 4 below. Except for the differences in the mass of the first active material layer 2 and the second active material layer 3, all other parameters in Examples D1 to D10 were the same as in Example 1. The secondary batteries were cycled 300 times.

[0161] Table 4

[0162] It should be noted that in Table 4, 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.

[0163] Table 4 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 effect on suppressing 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. When the mass ratio of the second active material layer 3 to the first active material layer 2 is greater than 2, the free electrolyte 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 particle breakage in the first active material layer 2. Therefore, in this embodiment, the mass ratio of the second active material layer 3 to the first active material layer 2 is selected within a range of 0.2:1 to 2:1, which balances the second active material layer 3's effect on suppressing the expansion of the silicon-based material and the free electrolyte infiltration time.

[0164] To explore the optimal mass ratio range of the silicon-based material to the first graphite material, experiments were conducted by adjusting the amounts of the silicon-based material and the first graphite material used in Example 1. The experimental results are shown in Table 5 below. In Examples E1 to E10, except for the differences in the amounts of the silicon-based material and the first graphite material used in Example 1, all other parameters were the same as in Example 1. The secondary battery was cycled 300 times.

[0165] Table 5

[0166] It should be noted that: in Table 5, the first dispersant, the conductive agent, MMA (methyl methacrylate) and AIBN (azobisisobutyronitrile) are 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 5.

[0167] As can be seen from Table 5, when m1 / (m1+m2) is greater than 30, due to the increased content of the silicon-based material, the first graphite material has limited ability to suppress the expansion of the silicon-based material, resulting in a larger expansion rate of the secondary battery, a lower capacity retention rate, and severe lithium plating, which deteriorates the kinetic performance of the secondary battery. When m1 / (m1+m2) is less than 5, the energy density of the secondary battery drops sharply. Therefore, in this application, the mass ratio of the silicon-based material to the mass of the first graphite material and the silicon-based material is selected to be 5% to 30%. This can maintain the energy density of the secondary battery at a high level while ensuring the first graphite material's inhibitory effect on the silicon-based material.

[0168] In order to evaluate the beneficial effects of adding inorganic materials to the first active material layer 2 , a test is performed using a lithium-ion secondary battery as an example.

[0169] Example 2

[0170] The difference from Example 1 is that the first active material layer 2 slurry also includes the inorganic material aluminum oxide (Al2O3), and the mass ratio of the first graphite material, silicon-based material, inorganic material, first binder, first dispersant and conductive agent is 80:10:0.5:5:2:2.5.

[0171] The following tests were performed on the lithium-ion secondary batteries of Examples 1 and 2. If the test temperature is not clearly stated during the test, the normal temperature of 25° C. was used as an example.

[0172] (1) Capacity retention test

[0173] Figure 7 is a cycle life diagram of the secondary batteries of Example 1 and Example 2. It can be seen from Figure 7 that under the same number of cycles, the capacity retention rate of the secondary battery of Example 2 is improved compared with the secondary battery of Example 1. That is, when there is an inorganic material in the second active material layer 3, an inorganic material is also added to the first active material layer 2, which can lubricate and buffer the first graphite material, reduce the breakage of the graphite material particles, reduce the capacity loss of the secondary battery 100, further improve the capacity retention rate of the secondary battery, and improve the cycle performance of the secondary battery.

[0174] (2) Secondary battery expansion rate test

[0175] Figure 8 shows the cyclic expansion rate of the secondary batteries of Examples 1 and 2. As shown in Figure 8, when the number of cycles reaches 50 or more, the expansion rate of the secondary battery of Example 2 is reduced compared to that of the secondary battery of Example 1 at the same number of cycles. This means that, while the second active material layer 3 contains an inorganic material, the addition of the inorganic material to the first active material layer 2 can further reduce the expansion rate of the secondary battery. One reason for the difference in expansion rate between Examples 2 and 1 is that the addition of the inorganic material to the first active material layer 2 alleviates the graphite particle breakage in the first graphite material. The increased amount of first graphite material can better suppress the expansion displacement of the silicon-based material.

[0176] (3) Secondary battery lithium deposition degree test

[0177] Test results: The degree of lithium deposition in the negative electrode sheet 10 of Example 2 was "no lithium deposition", while the degree of lithium deposition in the negative electrode sheet 10 of Example 1 was "slight". In other words, by adding an inorganic material to the first active material layer 2, the wettability of the first active material layer 2 by the free electrolyte can be further increased, thereby increasing the free electrolyte retention in the negative electrode sheet 10, thereby improving the lithium deposition problem of the negative electrode sheet 10 and improving the cycle performance of the secondary battery 100. In particular, because the degree of lithium deposition in the negative electrode sheet 10 of Example 2 is lower than that of Example 1, the expansion rate of the secondary battery 100 is also reduced.

[0178] In the secondary battery 100 and the electric device 1000 of the embodiment of the present application, the particle size D of the inorganic material is v 50 and the particle size D of the second graphite material vIf the ratio of 50 is less than 0.1, the inorganic material can fill the gaps in the second graphite material, thereby providing lubrication and cushioning for the second graphite material, reducing the problem of graphite particle breakage, improving the overpressure problem of the second active material layer 3, maintaining the second active material layer 3's effect of inhibiting the expansion of the silicon-based material and the capacity of the negative electrode plate 10, enhancing the structural integrity and stability of the negative electrode plate, and improving the problem of performance deterioration of the negative electrode plate 10. At the same time, when the secondary battery also includes free electrolyte, the inorganic material filling the gaps in the second graphite material can increase the porosity of the second active material layer 3, allowing the free electrolyte to easily penetrate from the second active material layer 3 into the first active material layer 2, thereby reducing concentration polarization, reducing internal resistance, increasing the amount of free electrolyte retained in the negative electrode plate 10, increasing ion transmission speed, improving fast charging capability, and extending the cycle life of the secondary battery 100. Inorganic materials can also be added to the first active material layer 2 to improve the problem of graphite particle breakage in the first active material layer 2 and further improve the wettability of the free electrolyte to the negative electrode plate 10, thereby increasing the capacity of the negative electrode plate 10 and the fast charging capability of the secondary battery 100.

[0179] 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 negative electrode sheet includes a current collector, a first active material layer and a second active material layer, wherein the first active material layer is disposed on at least one surface of the current collector, and the second active material layer is disposed on a side of the first active material layer away from the current collector, wherein the first active material layer includes a silicon-based material and a first graphite material, and the second active material layer includes a second graphite material and an inorganic material, wherein the particle size D of the inorganic material is v 50 and the particle size D of the second graphite material v 50 is less than or equal to 0.

23.

2. The secondary battery according to claim 1, 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.

3. The secondary battery according to claim 2, 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.

4. The secondary battery according to any one of claims 1 to 3, characterized in that The mass percentage of the inorganic material in the second active material layer is 0.1 wt % to 5 wt %.

5. The secondary battery according to claim 4, wherein The mass percentage of the inorganic material in the second active material layer is 1 wt % to 3 wt %.

6. The secondary battery according to any one of claims 1 to 5, 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.

7. The secondary battery according to any one of claims 1 to 6, characterized in that: The mass of the silicon-based material is m1, the mass of the first graphite material is m2, and m1 / (m1+m2) is 5% to 30%.

8. The secondary battery according to any one of claims 1 to 7, characterized in that The particle size D of the silicon-based material v 50 and the particle size D of the first graphite material v The ratio of 50 is 0.1 to 0.

5.

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

3.

10. The secondary battery according to claim 9, wherein 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.

11. The secondary battery according to any one of claims 1 to 10, characterized in that: 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, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide and barium sulfate.

12. The secondary battery according to claim 11, wherein The inorganic material is alumina and / or boehmite.

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 free electrolyte.

15. The secondary battery according to any one of claims 1 to 14, characterized in that: The first active material layer includes the inorganic material.

16. An electrical device, characterized in that: The utility model comprises the secondary battery according to any one of claims 1 to 15 and a load, wherein the secondary battery is used to supply power to the load.

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