Negative pole piece, preparation method of negative pole piece, secondary battery and electric device

Abstract

The invention relates to a negative pole piece and a preparation method thereof, a secondary battery and an electric device. The negative pole piece comprises a negative current collector, a negative active layer and flake graphite; the ratio of the thickness t to the flake diameter d of the flake graphite is t / d, and t / d is greater than or equal to 0.005; the negative electrode active layer comprises a negative electrode active material, and the negative electrode active material comprises a silicon-based material. The base material in the negative pole piece is not easy to damage.

Description

Negative electrode sheet and preparation method thereof, secondary battery and electric device Technical Field

[0001] The present application relates to the technical field of lithium batteries, and in particular to a negative electrode plate and a preparation method thereof, a secondary battery and an electrical device. Background Art

[0002] In recent years, as the application scope of lithium-ion batteries has become increasingly wider, lithium-ion batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power stations, as well as power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace and other fields.

[0003] Traditional lithium-ion battery anode active materials are primarily carbon-based. Silicon-based materials have since evolved as energy density requirements continue to increase. However, silicon-based materials are prone to cracking in the substrate near the anode active layer of anode plates. This increases the risk of cell failure due to cracking during battery cycling.

[0004] Summary of the Invention

[0005] This application is made in view of the above-mentioned problems, and its purpose is to provide a negative electrode plate and its preparation method, a secondary battery and an electric device. The substrate in the negative electrode plate is not easily damaged, which can reduce the risk of battery cell failure.

[0006] In a first aspect of the present application, a negative electrode plate is provided, comprising a negative electrode current collector, a negative electrode active layer, and flaky graphite, wherein the negative electrode active layer comprises a negative electrode active material, wherein the negative electrode active material comprises a silicon-based material, and the negative electrode active layer is disposed on the surface of the negative electrode current collector, wherein the ratio of the thickness t of the flaky graphite to the diameter d of the flaky graphite is t / d, and t / d is ≥ 0.005, and the distribution position of the flaky graphite in the negative electrode plate satisfies at least one of the following conditions (1) to (2):

[0007] (1) mixed into the negative electrode active layer;

[0008] (2) forming a flaky graphite layer between the negative electrode current collector and the negative electrode active layer;

[0009] The above-mentioned negative electrode plate introduces flake graphite with a specific t / d value into the negative electrode active layer or between the negative electrode active layer and the current collector, which can reduce the damage of the silicon-based material to the substrate and thus reduce the risk of battery cell failure.

[0010] In a second aspect of the present application, a method for preparing a negative electrode sheet is provided, the method comprising at least one of the following steps (1) to (2):

[0011] (1) mixing flaky graphite with a negative electrode active material to prepare a negative electrode active slurry; applying the negative electrode active slurry to the surface of a negative electrode current collector to form a negative electrode active layer;

[0012] (2) preparing a flaky graphite slurry, applying the flaky graphite slurry on the surface of the negative electrode current collector to prepare a flaky graphite layer, wherein the flaky graphite layer includes flaky graphite; and preparing a negative electrode active layer on the surface of the flaky graphite layer;

[0013] In steps (1) to (2), the ratio of the thickness t of the flake graphite to the flake diameter d is t / d, and t / d≥0.005; and the negative electrode active material includes a silicon-based material.

[0014] In a third aspect of the present application, a secondary battery is provided, comprising the negative electrode sheet described in the first aspect or the negative electrode sheet prepared by the preparation method described in the second aspect.

[0015] A fourth aspect of the present application provides an electrical device, characterized in that it includes the secondary battery described in the third aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is one of the electron microscope images of silicon-based materials;

[0017] Figure 2 is the second electron microscope image of silicon-based materials;

[0018] FIG3 is an electron microscope schematic diagram showing damage to the substrate;

[0019] FIG4 is a schematic structural diagram of a negative electrode sheet according to an example of the present application;

[0020] FIG5 is a schematic diagram of a secondary battery according to an embodiment of the present application;

[0021] FIG6 is an exploded view of the secondary battery according to one embodiment of the present application shown in FIG1 ;

[0022] FIG7 is a schematic diagram of a battery module according to an embodiment of the present application;

[0023] FIG8 is a schematic diagram of a battery pack according to an embodiment of the present application;

[0024] FIG9 is an exploded view of the battery pack according to one embodiment of the present application shown in FIG4 ;

[0025] 10 is a schematic diagram of an electrical device using a secondary battery as a power source according to an embodiment of the present application;

[0026] Explanation of the accompanying symbols: 401, substrate (negative electrode current collector); 402, flaky graphite layer; 403, negative electrode active layer; 1, battery pack; 2, upper case; 3, lower case; 4, battery module; 5, secondary battery; 51, casing; 52, electrode assembly; 53, cover plate; 6, electrical device. DETAILED DESCRIPTION

[0027] Below, the embodiments of the negative electrode sheet and its preparation method, secondary battery and electric device of the present application are described in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0028] " range " disclosed in the present application is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and can be arbitrarily combined, that is, any lower limit can form a range with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3,4 and 5 are listed, then the following range can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0029] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0030] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0031] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0032] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0033] Unless otherwise specified, the term "or" is used in this application to be inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0034] Unless otherwise specified, in this application, the terms "on the surface of..." and "laminated on the surface of..." include direct contact and indirect contact. For example, "preparing a flaky graphite layer on the surface of the negative electrode current collector" means, on the one hand, that the flaky graphite layer is in contact with the negative electrode current collector, that is, directly laminated on the surface of the negative electrode current collector. On the other hand, it can also mean that there is no direct contact between the flaky graphite layer and the negative electrode current collector, for example, a primer layer may be provided between the two.

[0035] If not otherwise specified, in this application, the term "sheet diameter" refers to the maximum linear dimension of the sheet material, that is, the maximum length of the two-dimensional plane of the sheet material.

[0036] Research has found that cracks in the substrate (usually the current collector) of negative electrode sheets containing silicon-based materials often form during the production process, such as the cold pressing step. These cracks are further exacerbated during cyclic expansion, leading to aging of the battery cell. This may be due to the fact that, as shown in Figures 1-2 (surface morphology of two silicon-based materials tested using a scanning electron microscope) and Table 1, silicon-based materials typically have sharp, angular particles and a high hardness (Mohs hardness, measured by scratching the surface of the tested mineral with a pyramidal diamond needle and measuring the scratch depth). Therefore, the substrate near the negative electrode active material can be damaged during processes such as cold pressing, as shown in Figure 3.

[0037] Table 1

[0038] Based on this, some examples of the present application provide a negative electrode plate, including a negative electrode current collector, a negative electrode active layer, and flaky graphite, wherein the negative electrode active layer includes a negative electrode active material, the negative electrode active material includes a silicon-based material, the negative electrode active layer is disposed on the surface of the negative electrode current collector, the ratio of the thickness t of the flaky graphite to the sheet diameter d is t / d, t / d ≥ 0.005, and the distribution position of the flaky graphite in the negative electrode plate satisfies at least one of the following conditions (1) to (2):

[0039] (1) mixed into the negative electrode active layer;

[0040] (2) forming a flaky graphite layer between the negative electrode current collector and the negative electrode active layer;

[0041] The above-mentioned negative electrode plate introduces flake graphite with a specific t / d value into the negative electrode active layer or between the negative electrode active layer and the current collector, which can reduce the damage of the silicon-based material to the substrate and thus reduce the risk of battery cell failure.

[0042] Specifically, in the case where flake graphite is mixed with the negative electrode active layer, the flake graphite is directly mixed with the negative electrode active material containing silicon-based materials and embedded between the silicon-based materials. During the process, the flake graphite can increase the slip of the silicon-based material in the negative electrode active layer, thereby reducing damage to the substrate.

[0043] In the case of forming a flaky graphite layer, the flaky graphite layer forms a substrate protective layer between the substrate and the negative electrode active material containing a silicon-based material. During the process, the silicon-based material preferentially slides and partially or completely embeds into the flaky graphite layer, thereby reducing damage to the substrate. Without limitation, the structure of the negative electrode plate is shown in Figure 4, including a flaky graphite layer 402 and a negative electrode active layer 403 stacked on the surface of a substrate (negative electrode current collector) 401.

[0044] It can be understood that in the above-mentioned negative electrode sheet, no matter whether the graphite layer is mixed or formed into a flaky layer, the number of layers of the negative electrode active layer can be one or more layers to meet the energy density requirements.

[0045] In some examples, the ratio of the graphite flake thickness t to the flake diameter d, t / d, is 0.005 to 0.4. Specifically, the values ​​of t / d include, but are not limited to, 0.005, 0.01, 0.03, 0.05, 0.07, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, and 0.4. Furthermore, the ratio of the graphite flake thickness t to the flake diameter d, t / d, is 0.1 to 0.4. Properly controlling the value of t / d can further reduce damage to the substrate caused by the silicon-based material.

[0046] In some examples, the flake graphite includes one or more of KS graphite and SFG graphite. It is understood that KS graphite and SFG graphite can be purchased from TIMREX, Switzerland. Without limitation, similar effects can be achieved by using flake graphite similar to KS graphite and SFG graphite.

[0047] Specifically, the KS family graphite includes one or more of KS-6, KS-10 and KS-16.

[0048] Specifically, the SFG family graphite includes one or more of SFG-6, SFG-10 and SFG-15.

[0049] In some examples, the percentage of the flaky graphite in the total mass of the negative electrode active material and the flaky graphite is i (%), the compacted density of the negative electrode sheet is p (g / cm 3 ), the D90 particle size of the silicon-based material is n (μm), the gram capacity of the negative electrode active material is c (Ah / g), and i, p, n and c satisfy the following conditions: [p×n×(0.25-c)-100]×i+c×p×n≤0.

[0050] In some of these examples, i, p, n, and c satisfy the following condition: -70≤[p×n×(0.25-c)-100]×i+c×p×n≤-0.01.

[0051] In some examples, the percentage of the flaky graphite to the total mass of the negative electrode active material and the flaky graphite is i, and 0.3% ≤ i ≤ 70%. Specifically, the values ​​of i include, but are not limited to, 0.3%, 1%, 3%, 5%, 10%, 15%, 20%, 30%, 40%, 50%, 60%, and 70%. Furthermore, 15% ≤ i ≤ 70%. Within this range, substrate damage can be reduced when the negative electrode active material is entirely silicon-based.

[0052] In some examples, the mass percentage of the silicon-based material in the negative electrode active material is q, and 20%≤q≤100%. Specifically, the value of q includes but is not limited to: 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, and 100%.

[0053] Furthermore, the silicon-based material includes silicon-oxygen material, and 30%≤q≤100%.

[0054] Furthermore, the silicon-based material includes a silicon-carbon material or a mixture of a silicon-carbon material and a silicon-oxygen material, and 20%≤q≤100%.

[0055] Without limitation, when the silicon-based material accounts for less than 100%, the negative electrode active material may also include conventional negative electrode active materials used in batteries in the art. For example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, tin-based materials, and lithium titanate. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials; other conventional materials used as negative electrode active materials for batteries may also be used.

[0056] In some examples, the negative electrode active material includes a silicon-based material or a combination of a silicon-based material and graphite.

[0057] In some examples, the silicon-based material includes a silicon-oxygen material, a silicon-carbon material, or a mixture of a silicon-carbon material and a silicon-oxygen material.

[0058] Furthermore, according to different addition conditions of flake graphite and types of silicon-based materials, the proportion of flake graphite can be reasonably controlled, which can reduce substrate damage while reducing costs.

[0059] In some examples, the flake graphite is disposed between the negative electrode current collector and the negative electrode active layer to form a flake graphite layer, and satisfies at least one of the following conditions:

[0060] (1) The silicon-based material includes silicon-oxygen material, 0.3%≤i≤70%;

[0061] (2) The silicon-based material includes a silicon-carbon material or a mixture of a silicon-carbon material and a silicon-oxygen material, and 11.3%≤i≤70%.

[0062] It can be understood that the hardness of silicon-carbon materials is higher than that of silicon-oxygen materials. In the solution of forming a flaky graphite layer alone, the amount of flaky graphite can be appropriately increased.

[0063] In some examples, the flake graphite is mixed in the negative electrode active layer and satisfies at least one of the following conditions:

[0064] (1) The silicon-based material includes silicon-oxygen material, 0.3%≤i≤70%;

[0065] (2) The silicon-based material includes a silicon-carbon material or a mixture of a silicon-carbon material and a silicon-oxygen material, with 1%≤i≤70%.

[0066] It can be understood that the higher hardness of silicon-carbon materials compared to silicon-oxygen materials can appropriately increase the amount of flake graphite used. At the same time, compared with the solution of forming a flake graphite layer alone, the increased amount in the direct mixing solution can be slightly smaller.

[0067] In addition, without limitation, the compaction density of the negative electrode sheet is p=0.1-1.65 g / cm 3 Specifically, the value of p includes but is not limited to: 0.1g / cm 3 , 0.4g / cm 3 , 0.8g / cm 3 , 1g / cm 3 , 1.1g / cm 3 , 1.2g / cm 3 , 1.4g / cm 3 , 1.5g / cm 3 , 1.65g / cm 3 .

[0068] Without limitation, the D90 particle size n of the silicon-based material is 3 μm to 24 μm. Specifically, the value of n includes but is not limited to: 3 μm, 4 μm, 8 μm, 10 μm, 12 μm, 16 μm, 18 μm, 21 μm, and 24 μm.

[0069] Without limitation, the gram capacity c of the negative electrode active material is 0.3 to 1.8 Ah / g. Specifically, the value of c includes, but is not limited to, 0.3 Ah / g, 0.35 Ah / g, 0.4 Ah / g, 0.45 Ah / g, 0.5 Ah / g, 0.55 Ah / g, 0.6 Ah / g, 0.65 Ah / g, 0.7 Ah / g, 0.75 Ah / g, 0.8 Ah / g, 0.85 Ah / g, 0.9 Ah / g, 0.95 Ah / g, 1 Ah / g, 1.1 Ah / g, 1.15 Ah / g, 1.2 Ah / g, 1.25 Ah / g, 1.3 Ah / g, 1.35 Ah / g, 1.5 Ah / g, 1.6 Ah / g, and 1.8 Ah / g.

[0070] In some examples, the laminar graphite layer comprises flake graphite, a binder, and a thickener. Furthermore, optionally, the mass ratio of the flake graphite, binder, and thickener is (2-80):1:1. This ensures that the laminar graphite layer has good adhesion and stabilizes the electrode structure.

[0071] Without limitation, the binder and thickener may be conventional in the art. For example, the binder may include at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS). The thickener may include sodium carboxymethyl cellulose (CMC-Na), etc.

[0072] In addition, a primer layer may be provided between the negative electrode active layer and the negative electrode current collector. Without limitation, the primer layer may include one or more of a conductive agent, a binder, and a thickener.

[0073] Specifically, in the case where the flake graphite is mixed in the negative electrode active layer, a primer layer may be provided according to conventional methods in the art.

[0074] Specifically, for the case where the flaky graphite is disposed between the negative electrode current collector and the negative electrode active layer to form a flaky graphite layer:

[0075] On the one hand, a primer layer can be additionally provided between the flaky graphite layer and the negative electrode current collector according to conventional methods in the art. In this case, in some examples, the mass ratio of the flaky graphite, binder, and thickener in the flaky graphite layer is (60-80):(0.5-1.5):(0.5-1.5); in this way, a higher proportion of flaky graphite can be used to further reduce damage to the substrate by the silicon-based material;

[0076] Alternatively, the flaky graphite layer can be used directly as a primer layer, i.e., the negative electrode current collector is in direct contact with the flaky graphite layer without an additional primer layer disposed therebetween. In this case, in some examples, the mass ratio of the flaky graphite, binder, and thickener in the flaky graphite layer is (2-10):(0.5-1.5):(0.5-1.5). This can reduce damage to the substrate caused by the silicon-based material while ensuring good adhesion to the current collector.

[0077] Without limitation, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material on a polymer material substrate. The metal material includes but is not limited to stainless steel, copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, and the polymer material substrate includes but is not limited to polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE) and other substrates.

[0078] In some examples, the negative electrode current collector includes one of copper foil, stainless steel, and nickel foil.

[0079] Some examples of the present application also provide a method for preparing a negative electrode sheet, comprising the following steps:

[0080] Mixing the flake graphite with the negative electrode active material to prepare a negative electrode active slurry;

[0081] Applying the negative electrode active slurry to the surface of the negative electrode current collector to form a negative electrode active layer;

[0082] Wherein, the ratio of the thickness t of the flake graphite to the flake diameter d is t / d, t / d≥0.005;

[0083] The negative electrode active material includes a silicon-based material.

[0084] Some examples of the present application also provide a method for preparing a negative electrode sheet, comprising the following steps:

[0085] preparing a flake graphite slurry, and applying the flake graphite slurry on the surface of a negative electrode current collector to prepare a flake graphite layer, wherein the flake graphite layer comprises flake graphite;

[0086] preparing a negative electrode active layer on the surface of the flaky graphite layer;

[0087] Wherein, the ratio of the thickness t of the flake graphite to the flake diameter d is t / d, t / d≥0.005;

[0088] The negative electrode active material includes a silicon-based material.

[0089] It can be understood that the negative electrode sheet prepared by the above preparation method has the same technical solutions and advantages as the aforementioned negative electrode sheet, which will not be described in detail here.

[0090] Some examples of the present application further provide a secondary battery comprising the negative electrode sheet as described above, wherein the energy density of the secondary battery is 360 to 500 Wh / kg.

[0091] Some examples of the present application further provide an electric device including the secondary battery as described above.

[0092] The secondary battery, battery module, battery pack, and electric device of the present application are described below with reference to the accompanying drawings as appropriate.

[0093] In one embodiment of the present application, a secondary battery is provided.

[0094] Typically, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During the battery's charge and discharge processes, active ions are inserted and removed between the positive and negative electrodes. The electrolyte conducts ions between the positive and negative electrodes. The separator, located between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

[0095] Positive electrode

[0096] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector, wherein the positive electrode film layer includes the positive electrode active material of the first aspect of the present application.

[0097] As an example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive electrode current collector.

[0098] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector may be formed by forming a metal material on a polymer material substrate. Among them, the metal material includes but is not limited to stainless steel, aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc. Polymer material substrate (such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.)

[0099] In some embodiments, the positive electrode active material may include a positive electrode active material for a battery that is well known in the art. As an example, the positive electrode active material may include at least one of the following materials: a lithium-containing phosphate with an olivine structure, a lithium transition metal oxide, and their respective modified compounds. However, the present application is not limited to these materials, and other traditional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also referred to as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2 (also referred to as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2 (also referred to as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2 (also referred to as NCM 622 ), LiNi 0.8 Co0.1 Mn 0.1 O2 (also referred to as NCM 811 ), lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co 0.15 Al 0.05 O2) and its modified compounds. Examples of olivine-structured lithium-containing phosphates may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon. The weight ratio of the positive electrode active material in the positive electrode film layer is 80-100 weight%, based on the total weight of the positive electrode film layer.

[0100] In some embodiments, the positive electrode active material includes LiNi x Co y Mn z O2, x is 0.90~0.99, y is 0.01~0.1, and z is 0.01~0.1.

[0101] In some embodiments, the positive electrode film layer may further optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin. The weight ratio of the binder in the positive electrode film layer is 0-20 weight%, based on the total weight of the positive electrode film layer.

[0102] In some embodiments, the positive electrode film layer may further optionally include a conductive agent. For example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The weight ratio of the conductive agent in the positive electrode film layer is 0-20% by weight, based on the total weight of the positive electrode film layer.

[0103] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry, wherein the positive electrode slurry has a solid content of 40-80 wt %, and the viscosity at room temperature is adjusted to 5000-25000 mPa·s, the positive electrode slurry is coated on the surface of the positive electrode current collector, and after drying, the positive electrode sheet is formed by cold rolling; the positive electrode powder coating unit area density is 150-350 mg / m 2 The compaction density of the positive electrode is 3.0-3.6g / cm 3, optional 3.3-3.5g / cm 3 The calculation formula of the compacted density is

[0104] Compaction density = coating surface density / (thickness of the electrode after extrusion - thickness of the current collector).

[0105] Negative electrode

[0106] As mentioned above, the negative electrode sheet includes a negative electrode current collector, a negative electrode active layer and flake graphite.

[0107] As an example, the negative electrode current collector has two surfaces facing each other in its thickness direction, and the negative electrode active layer is disposed on either or both of the two facing surfaces of the negative electrode current collector.

[0108] In some embodiments, the negative electrode active layer may further include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS). The binder may be present in the negative electrode active layer in an amount of 0-30% by weight, based on the total weight of the negative electrode active layer.

[0109] In some embodiments, the negative electrode active layer may further include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The weight ratio of the conductive agent in the negative electrode active layer is 0-20% by weight, based on the total weight of the negative electrode active layer.

[0110] In some embodiments, the negative electrode active layer may further include other additives, such as a thickener (e.g., sodium carboxymethyl cellulose (CMC-Na)). The weight ratio of the other additives in the negative electrode film layer is 0-15% by weight, based on the total weight of the negative electrode active layer.

[0111] electrolytes

[0112] The electrolyte conducts ions between the positive and negative electrodes. This application does not specify the type of electrolyte, and the electrolyte can be selected based on the needs. For example, the electrolyte can be liquid, gel, or solid.

[0113] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0114] In some embodiments, the electrolyte salt may be selected from one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalatoborate (LiDFOB), lithium bis(oxalatoborate) (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorobis(oxalatophosphate) (LiDFOP), and lithium tetrafluorooxalatophosphate (LiTFOP). The concentration of the electrolyte salt is typically 0.5-5 mol / L.

[0115] In some embodiments, the solvent can be selected from one or more of fluoroethylene carbonate (FEC), ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS) and diethyl sulfone (ESE).

[0116] In some embodiments, the electrolyte may further include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0117] Isolation film

[0118] In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

[0119] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0120] In some embodiments, the isolation film has a thickness of 6-40 μm, and optionally 12-20 μm.

[0121] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0122] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0123] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the secondary battery can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0124] The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square, or any other shape. For example, FIG5 shows a secondary battery 5 having a square structure as an example.

[0125] In some embodiments, referring to Figure 6, the outer packaging may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can be formed into an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0126] In some embodiments, secondary batteries can be assembled into a battery module. The number of secondary batteries contained in the battery module can be one or more. The specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0127] Figure 7 shows an example battery module 4. Referring to Figure 7 , within the battery module 4, multiple secondary batteries 5 may be arranged sequentially along the length of the battery module 4. Of course, any other arrangement is also possible. Furthermore, the multiple secondary batteries 5 may be secured together using fasteners.

[0128] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0129] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art based on the application and capacity of the battery pack.

[0130] Figures 8 and 9 illustrate an example battery pack 1. Referring to Figures 8 and 9 , the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box comprises an upper case 2 and a lower case 3. The upper case 2 can be positioned over the lower case 3 to form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0131] In addition, the present application also provides an electric device, which includes at least one of the secondary battery, battery module, or battery pack provided in the present application. The secondary battery, battery module, or battery pack can be used as a power source for the electric device, and can also be used as an energy storage unit for the electric device. The electric device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but is not limited thereto.

[0132] As the electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0133] Figure 10 shows an example of an electric device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery, a battery pack or battery module can be used.

[0134] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a secondary battery as a power source.

[0135] Example

[0136] In order to make the technical problems, technical solutions and beneficial effects solved by this application clearer, the application will be further described in detail below with reference to the embodiments and drawings. Obviously, the described embodiments are only a part of the embodiments of this application, rather than all the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is in no way intended to limit the present application and its applications. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0137] If no specific techniques or conditions are specified in the examples, the experiments were carried out according to the techniques or conditions described in the literature in the field or according to the product instructions. All reagents or instruments used without specifying the manufacturer are commercially available conventional products.

[0138] Example 1-1

[0139] 1) Preparation of positive electrode sheet

[0140] Nickel-cobalt-manganese (NCM) ternary materials (LiNi 0.94 Co 0.03 Mn 0.03 O2), conductive agent carbon black, and binder polyvinylidene fluoride (PVDF) are dispersed in a solvent N-methylpyrrolidone at a mass ratio of 97%:1%:2%, and mixed and stirred for 3 hours to obtain a positive electrode slurry; the positive electrode slurry is evenly coated on the positive electrode collector aluminum foil, and after drying and cold pressing, a positive electrode sheet is obtained, and its coating amount per unit area is 0.27g / 1540.25mm 2 .

[0141] 2) Preparation of negative electrode sheet

[0142] Preparation of primer: Conductive carbon, binder styrene-butadiene rubber (SBR), and thickener sodium hydroxymethyl cellulose (CMC) were added to deionized water in a weight ratio of 2:1:1, mixed and stirred for 3 hours to obtain a primer slurry. The primer slurry was applied to the negative electrode current collector copper foil by gravure, dried, and formed into a primer layer. The coating amount was about 2 mg / 1540.25 mm 2 ;

[0143] Flake graphite (t / d = 0.2, KS-10, purchased from TIMREX, Switzerland), binder styrene-butadiene rubber (SBR), and thickener sodium carboxymethyl cellulose (CMC) were added to deionized water in a weight ratio of 97.5%:1.25%:1.25% (78:1:1), and mixed and stirred for 3 h to obtain negative electrode slurry ①;

[0144] Add silicon-based material, conductive agent carbon black, binder styrene-butadiene rubber (SBR), and thickener sodium carboxymethyl cellulose (CMC) in a weight ratio of 97%: 0.5%: 1.25%: 1.25% (the proportion of silicon-based material in the negative electrode active material q = 100%) into deionized water and mix and stir for 3 hours to obtain negative electrode slurry ②;

[0145] Apply the negative electrode slurry ① on the base coating and dry it. The coating amount is 0.051g / 1540.25mm 2 , forming a flake graphite layer, and then coating the negative electrode slurry ② on the flake graphite layer and drying it. The coating amount is 0.119g / 1540.25mm 2 , and then cold pressed and cut to obtain the negative electrode sheet, the coating amount per unit area is 0.17g / 1540.25mm 2 .

[0146] In the above-mentioned negative electrode sheet:

[0147] The percentage of flake graphite in the total mass of the negative electrode active material (silicon oxide material) and flake graphite is i (%) = 97.5*0.051 / (97*0.119+97.5*0.051) = 30%;

[0148] The compaction density of the negative electrode is p = 1.5g / cm 3 ;

[0149] The D90 particle size of the silicon oxide material is n=16 μm;

[0150] Gram capacity of negative electrode active material c = 0.9773 Ah / g;

[0151] Calculations show that the following formula is satisfied: [p×n×(0.25-c)-100]×i+c×p×n=-11.78.

[0152] 3) Isolation film

[0153] A 12μm thick polypropylene isolation film was selected.

[0154] 4) Preparation of electrolyte

[0155] The organic solvent is a mixture of ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate in a volume ratio of 1:1:1. In an argon atmosphere glove box with a water content of <10 ppm, fully dried lithium salt LiPF6 is dissolved in the organic solvent and mixed thoroughly to obtain an electrolyte solution. The concentration of the lithium salt is 1 mol / L.

[0156] 5) Battery Preparation

[0157] The positive electrode sheet, isolation film, and negative electrode sheet are stacked in order, with the isolation film placed between the positive and negative electrode sheets to serve as an isolation. After winding into a square bare battery cell, an aluminum-plastic film is placed in it. After baking at 80°C to remove water, 10g of the corresponding non-aqueous electrolyte is injected and sealed. After standing, hot and cold pressing, formation, clamping, capacity division and other processes, a finished battery with a capacity of 4000mAh is obtained.

[0158] The battery preparation methods for Examples 1-2 to 1-25 and Comparative Examples 1-2 were the same as those for Example 1-1. The main differences were the t / d value and type of flake graphite, the mass percentage of flake graphite in the negative electrode active material (i), the percentage of silicon-based material in the negative electrode active material (q), and the type of silicon-based material. The details are shown in Table 2 below.

[0159] Example 2-1

[0160] 1) Preparation of positive electrode sheet

[0161] Same as Example 1;

[0162] 2) Preparation of negative electrode sheet

[0163] Flake graphite (t / d = 0.2, KS-10, purchased from TIMREX, Switzerland), binder styrene-butadiene rubber (SBR), and thickener sodium carboxymethyl cellulose (CMC) were added to deionized water in a weight ratio of 2:1:1, and mixed and stirred for 3 hours to obtain a primer slurry; the primer slurry was applied to the negative electrode current collector copper foil by gravure to form a primer layer (flake graphite layer) with a weight of about 2 mg / 1540.25 mm 2 ;

[0164] Silicon material, artificial graphite, conductive agent carbon black, binder styrene-butadiene rubber (SBR), and thickener sodium hydroxymethyl cellulose (CMC) were added to deionized water in a weight ratio of 67.9%: 29.1%: 0.5%: 1.25%: 1.25% (the proportion of silicon-based materials in the negative electrode active material q = 30%), and mixed and stirred for 3 hours to obtain a negative electrode slurry; the negative electrode slurry was coated on the primer layer, dried, and then cold pressed and cut to obtain a negative electrode sheet, and the coating amount per unit area was 0.17g / 1540.25mm 2 .

[0165] In the above-mentioned negative electrode sheet:

[0166] The percentage of flake graphite in the total mass of the negative electrode active material (silicon oxide material and artificial graphite) and flake graphite is i (%) = 50*0.002 / (97*0.17+50*0.002) = 0.6%;

[0167] The compaction density of the negative electrode is p = 0.4g / cm 3 ;

[0168] The D90 particle size of the silicon oxide material is n=4 μm;

[0169] Gram capacity of negative electrode active material c = 0.334 Ah / g;

[0170] Calculations show that the following formula is satisfied: [p×n×(0.25-c)-100]×i+c×p×n=-0.07.

[0171] 3) Isolation film

[0172] Same as Example 1;

[0173] 4) Preparation of electrolyte

[0174] Same as Example 1;

[0175] 5) Battery Preparation

[0176] Same as Example 1.

[0177] Example 3-1

[0178] 1) Preparation of positive electrode sheet

[0179] Same as Example 1;

[0180] 2) Preparation of negative electrode sheet

[0181] Silicon-carbon material, artificial graphite, flake graphite (t / d = 0.2, KS-10, purchased from TIMREX, Switzerland), conductive agent carbon black, binder styrene-butadiene rubber (SBR), and thickener sodium hydroxymethyl cellulose (CMC) were added into deionized water in a weight ratio of 57.4%: 24.6%: 15%: 0.5%: 1.25%: 1.25% (the proportion of silicon-based materials in the negative electrode active material q = 70%), and mixed and stirred for 3 hours to obtain a negative electrode slurry. The negative electrode slurry was coated on the negative electrode current collector copper foil, dried, and then cold pressed and cut to obtain a negative electrode sheet. The coating amount per unit area was 0.17g / 1540.25mm 2 .

[0182] In the above-mentioned negative electrode sheet:

[0183] The percentage of flake graphite in the total mass of the negative electrode active material (silicon-carbon material and artificial graphite) and flake graphite is i (%) = 15 / (57.4 + 24.6 + 15) = 15.5%;

[0184] The compaction density of the negative electrode is p = 1.2g / cm 3 ;

[0185] The D90 particle size of the silicon oxide material is n=21 μm;

[0186] Gram capacity of negative electrode active material c = 0.5672 Ah / g;

[0187] Calculations show that the following formula is satisfied: [p×n×(0.25-c)-100]×i+c×p×n=-2.45.

[0188] 3) Isolation film

[0189] Same as Example 1;

[0190] 4) Preparation of electrolyte

[0191] Same as Example 1;

[0192] 5) Battery Preparation

[0193] Same as Example 1.

[0194] The battery preparation method of Example 3-2 is the same as that of Example 3-1, with the main difference being the mass percentage i of the flaky graphite in the negative electrode active material, as shown in Table 2 below.

[0195] Test Case

[0196] The electrode cross section is photographed by a scanning electron microscope. Assuming that the thickness of the substrate is a (um), ① the thickness of the substrate in the visible range is uniform within a±0.5%*a, which is judged as no damage; ② the presence of particles in the visible range of the substrate compresses or cuts into the substrate to a thickness of >85%*a, which is judged as partial damage; ③ the presence of particles in the visible range of the substrate compresses or cuts into the substrate to a thickness of <85%*a, which is judged as severe damage.

[0197] The test results are shown in Table 2 below:

[0198] Table 2

[0199] By comparing the above embodiments and comparative examples, it can be seen that the embodiments of the present application can reduce the damage of the silicon-based material to the substrate.

[0200] Furthermore, from the comparison between Examples 1-1 to 1-7, it can be seen that reasonable control of the value of t / d can further reduce the damage of the silicon-based material to the substrate.

[0201] From the comparison between Example 1-1 and Examples 1-8 to 1-12, it can be seen that the use of different types of flake graphite can effectively reduce the damage of the silicon-based material to the substrate.

[0202] From the comparison between Example 1-1 and Examples 1-13 to 1-19, it can be seen that reasonable control of the value of i can further improve the protection of the substrate and reduce damage to the substrate when the negative electrode active materials are all silicon-based materials.

[0203] From the comparison between Example 1-1 and Examples 1-20 to 1-23, it can be seen that the proportions of different silicon-based materials can effectively reduce the damage of the silicon-based material to the substrate.

[0204] From the comparison between Example 1-1 and Examples 1-24 to 1-25, it can be seen that even for silicon-carbon materials having a harder hardness than silicon-oxygen materials, the damage caused by the silicon-based material to the substrate can still be reduced to a certain extent.

[0205] From the comparison between Example 1-1 and Example 2-1, Example 3-1, and Example 3-2, it can be seen that placing flake graphite at different positions in the negative electrode, such as directly mixing it into the negative electrode active layer, forming a separate active layer, or mixing it into the primer layer, can reduce the damage of the silicon-based material to the substrate to a certain extent.

[0206] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A negative electrode sheet, comprising a negative electrode current collector, a negative electrode active layer and flaky graphite, wherein the negative electrode active layer comprises a negative electrode active material, wherein the negative electrode active material comprises a silicon-based material, wherein the negative electrode active layer is disposed on the surface of the negative electrode current collector, wherein the ratio of the thickness t of the flaky graphite to the sheet diameter d is t / d, wherein t / d≥0.005, and wherein the distribution position of the flaky graphite in the negative electrode sheet satisfies at least one of the following conditions (1) to (2): (1) mixed in the negative electrode active layer; (2) forming a flaky graphite layer between the negative electrode current collector and the negative electrode active layer.

2. The negative electrode sheet according to claim 1, characterized in that: The ratio t / d of the thickness t of the flake graphite to the flake diameter d is 0.005 to 0.4; optionally, t / d is 0.1 to 0.

4.

3. The negative electrode sheet according to any one of claims 1 to 2, characterized in that: The percentage of the flaky graphite in the total mass of the negative electrode active material and the flaky graphite is i (%), and the compaction density of the negative electrode sheet is p (g / cm 3 ), the D90 particle size of the silicon-based material is n (μm), the gram capacity of the negative electrode active material is c (Ah / g), i, p, n and c satisfy the following conditions: [p×n×(0.25-c)-100]×i+c×p×n≤0.

4. The negative electrode sheet according to claim 3, characterized in that: i, p, n and c satisfy the following condition: -70≤[p×n×(0.25-c)-100]×i+c×p×n≤-0.

01.

5. The negative electrode sheet according to any one of claims 1 to 4, characterized in that: The percentage of the flaky graphite to the total mass of the negative electrode active material and the flaky graphite is i, 0.3%≤i≤70%; optionally, 15%≤i≤70%.

6. The negative electrode sheet according to any one of claims 1 to 5, characterized in that: In the negative electrode active material, the mass percentage of the silicon-based material is q, 20%≤q≤100%; Optionally, the silicon-based material includes silicon-oxygen material, 30%≤q≤100%; Optionally, the silicon-based material includes a silicon-carbon material or a mixture of a silicon-carbon material and a silicon-oxygen material, 20%≤q≤100%.

7. The negative electrode sheet according to any one of claims 1 to 6, characterized in that: The flake graphite is disposed between the negative electrode current collector and the negative electrode active layer to form a flake graphite layer, and satisfies at least one of the following conditions: (1) The silicon-based material includes silicon-oxygen material, 0.3%≤i≤70%; (2) The silicon-based material includes a silicon-carbon material or a mixture of a silicon-carbon material and a silicon-oxygen material, 11.3%≤i≤70%.

8. The negative electrode sheet according to claim 6, characterized in that: The flake graphite is mixed in the negative electrode active layer and satisfies at least one of the following conditions: (1) The silicon-based material includes silicon-oxygen material, 0.3%≤i≤70%; (2) The silicon-based material includes a silicon-carbon material or a mixture of a silicon-carbon material and a silicon-oxygen material, 1%≤i≤70%.

9. The negative electrode sheet according to any one of claims 1 to 8, characterized in that: The composition of the flake graphite layer includes the flake graphite, a binder and a thickener.

10. The negative electrode sheet according to claim 9, characterized in that: A primer layer is also provided between the negative electrode current collector and the flaky graphite layer; optionally, the mass ratio of the flaky graphite, the binder and the thickener is (60-80):(0.5-1.5):(0.5-1.5).

11. The negative electrode sheet according to claim 9, characterized in that: The negative electrode current collector is in direct contact with the flaky graphite layer; optionally, the mass ratio of the flaky graphite, the binder and the thickener is (2-10):(0.5-1.5):(0.5-1.5).

12. A method for preparing a negative electrode sheet, the method comprising at least one of the following steps (1) to (2): (1) mixing graphite flakes with negative electrode active materials to prepare negative electrode active slurry; applying the negative electrode active slurry on the surface of the negative electrode current collector to form a negative electrode active layer; (2) preparing a flaky graphite slurry, applying the flaky graphite slurry on the surface of a negative electrode current collector to prepare a flaky graphite layer, wherein the flaky graphite layer comprises flaky graphite; and preparing a negative electrode active layer on the surface of the flaky graphite layer; In steps (1) to (2), the ratio of the thickness t of the flake graphite to the flake diameter d is t / d, and t / d≥0.005; and the negative electrode active material includes a silicon-based material.

13. The method for preparing a negative electrode sheet according to claim 12, characterized in that: The ratio t / d of the thickness t of the flake graphite to the flake diameter d is 0.005 to 0.4; optionally, t / d is 0.1 to 0.

4.

14. A secondary battery, characterized in that: The invention comprises the negative electrode sheet according to any one of claims 1 to 11 or the negative electrode sheet prepared by the preparation method according to any one of claims 12 to 13.

15. An electrical device, characterized in that: Includes the secondary battery as claimed in claim 14.

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