Negative electrode sheet, battery cell, battery, and electric device

Abstract

A negative electrode sheet (231), a battery cell (20), a battery (100), and an electric device (1000), which belong to the technical field of batteries. By means of making the volumetric capacity density of a first negative electrode active material layer (2312a), which is provided on the inner layer, greater than the volumetric capacity density of a second negative electrode active material layer (2312b), which is provided on the outer layer, i.e., the addition amount of a high-capacity material of the inner layer being higher than that of the outer layer, the probability of contact between the high-capacity material and an electrolyte can be reduced, so that side reactions are further reduced, which is conducive to prolonging a battery life; in addition, the probability of an intercalation reaction or an alloying reaction between the high-capacity material and active ions can be reduced, a loading capacity of the high-capacity material to the active ions can be reduced, and the probability of volume expansion of the high-capacity material in a use process can be reduced, which is also conducive to prolonging the battery life.

Description

Negative electrode sheet, battery cell, battery and power-consuming device

[0001] Cross-references

[0002] This application claims priority to Chinese invention patent application No. 2024102692867 filed on March 8, 2024, entitled “A negative electrode sheet, a battery cell, a battery and an electrical device,” the contents of which are incorporated herein by reference. Technical Field

[0003] The present application relates to the field of battery technology, and in particular to a negative electrode sheet, a battery cell, a battery, and an electrical device. Background Art

[0004] At present, the energy density of graphite negative electrodes in secondary batteries is getting closer and closer to the limit, while the gram capacity of silicon-based materials, tin-based materials and aluminum-based materials is much higher than that of graphite. Therefore, silicon-based materials, tin-based materials and aluminum-based materials can be added to the negative electrode active material layer to increase the energy density of the negative electrode sheet. However, high-energy-density materials such as silicon-based materials, tin-based materials and aluminum-based materials have the problem of cyclic expansion, which leads to the deterioration of the battery's service life.

[0005] Summary of the Invention

[0006] In view of the above problems, the present application provides a negative electrode sheet, a battery cell, a battery and an electrical device, which can improve the problem of deterioration of battery service life caused by cyclic expansion of high energy density materials.

[0007] In the first aspect, the present application provides a negative electrode sheet, which includes a negative electrode collector and a negative electrode active material layer, the negative electrode active material layer is arranged on at least one surface of the negative electrode collector, the negative electrode active material layer includes a first negative electrode active material layer and a second negative electrode active material layer, the first negative electrode active material layer is arranged between the second negative electrode active material layer and the negative electrode collector, and the volume capacity density ρ1 of the first negative electrode active material layer and the volume capacity density ρ2 of the second negative electrode active material layer satisfy the following relationship: 1.6≤ρ1 / ρ2≤2.

[0008] In the technical solutions of the embodiments of the present application, the volume capacity density of the negative electrode active material layer is generally positively correlated with the amount of high-capacity material added. By making the volume capacity density of the first negative electrode active material layer located in the inner layer greater than the volume capacity density of the second negative electrode active material layer located in the outer layer, that is, the amount of high-capacity material added in the inner layer is higher than that in the outer layer, the probability of contact between the high-capacity material and the electrolyte when used in a battery is reduced, thereby reducing side reactions and improving battery life performance. At the same time, it can reduce the probability of intercalation or alloying reactions between the high-capacity material and the active ions, reduce the load of the high-capacity material on the active ions, and reduce the probability of volume expansion during use, which is also beneficial to battery life performance.

[0009] In some embodiments, the volume capacity density ρ1 of the first negative electrode active material layer is 0.65 to 2.6 Ah / cm 3 The volume capacity density ρ2 of the second negative electrode active material layer is 0.4 to 1.3 Ah / cm 3 .

[0010] In the above implementation process, the volume capacity density ρ1 of the first negative electrode active material layer is controlled to be 0.65-2.6 Ah / cm 3 The volume capacity density ρ2 of the second negative electrode active material layer is 0.4 to 1.3 Ah / cm 3 This, to a certain extent, is equivalent to controlling the amount of high-capacity material added to the first and second negative electrode active material layers, allowing the second negative electrode active material layer to contain a certain amount of high-capacity material, but at a lower content than the first negative electrode active material layer. This effectively reduces the probability of contact between the high-capacity material and the electrolyte and reduces the load of active ions on the high-capacity material, thereby extending the battery life. At the same time, the surface of the negative electrode active material layer of the negative electrode sheet has a better receptivity to lithium ions, making it suitable for higher current densities, thereby taking into account the battery's kinetic performance.

[0011] In some embodiments, the negative electrode active materials of the first negative electrode active material layer and the second negative electrode active material layer both include a first active material and a second active material, and the capacity of the first active material is greater than the capacity of the second active material; in the cross-section in the thickness direction of the negative electrode sheet, the relationship between the cross-sectional area ratio N1 of the first active material in the first negative electrode active material layer and the cross-sectional area ratio N2 of the first active material in the second negative electrode active material layer satisfies: N1>N2.

[0012] In the above implementation process, the proportion of the cross-sectional area of ​​the first active material in the thickness direction of the negative electrode sheet is positively correlated with the amount of the first active material to a certain extent. By making the relationship between the cross-sectional area proportion N1 of the first active material in the first negative electrode active material layer and the cross-sectional area proportion N2 of the first active material in the second negative electrode active material layer satisfy: N1>N2, that is, the proportion of the high-capacity first active material in the first negative electrode active material layer is greater than the proportion in the second negative electrode active material layer, the probability of contact between the high-capacity material and the electrolyte and the load of the high-capacity material on the active ions can be effectively reduced, thereby making the battery have a better life.

[0013] In some embodiments, in a cross section in the thickness direction of the negative electrode sheet, a cross-sectional area ratio N1 of the first active material in the first negative electrode active material layer satisfies: 29%≤N1≤83%; and / or

[0014] In a cross section along the thickness direction of the negative electrode sheet, a cross-sectional area ratio N2 of the first active material in the second negative electrode active material layer satisfies: 0≤N2≤20%.

[0015] In the above implementation process, by controlling the cross-sectional area ratio of the first active material in the first negative electrode active material layer and the second negative electrode active material layer to be 29%≤N1≤83% and 0≤N2≤20% respectively, the probability of contact between the high-capacity material and the electrolyte and the load of the high-capacity material on the active ions can be better reduced, thereby making the battery have a better life.

[0016] In some embodiments, the negative electrode active materials of the first negative electrode active material layer and the second negative electrode active material layer each include a first active material and a second active material, the capacity of the first active material is greater than the capacity of the second active material; and the mass of the first active material satisfies at least one of the following conditions (a1) to (a4):

[0017] (a1) the mass proportion M1 of the first active material in the first negative electrode active material layer is 30% to 85%;

[0018] (a2) the mass proportion M1 of the first active material in the first negative electrode active material layer is 50% to 70%;

[0019] (a3) the mass proportion M2 of the first active material in the second negative electrode active material layer is 0% to 20%;

[0020] (a4) The mass proportion M2 of the first active material in the second negative electrode active material layer is 3% to 10%.

[0021] In the above implementation process, by controlling the mass proportion of the first active material in the first negative electrode active material layer and the second negative electrode active material layer to be 30% to 85% and 0% to 15% respectively, the probability of contact between the high-capacity material and the electrolyte and the load of the high-capacity material on the active ions can be better reduced, thereby making the battery have a better life.

[0022] In some embodiments, the relationship between the thickness H1 of the first negative electrode active material layer and the thickness H2 of the second negative electrode active material layer satisfies: 2H1<H2.

[0023] In the above implementation process, by controlling the thickness of the first negative electrode active material layer containing a relatively large amount of high-capacity material that is easily expanded to H1<H2 / 2, it is beneficial to control the expansion of the entire negative electrode sheet.

[0024] In some embodiments, the thickness H1 of the first negative electrode active material layer satisfies: 5 μm≤H1≤20 μm; and / or

[0025] The thickness H2 of the second negative electrode active material layer satisfies: 20 μm≤H2≤90 μm.

[0026] In the above implementation process, by controlling the thickness H1 of the first negative electrode active material layer and the thickness H2 of the second negative electrode active material layer to satisfy: 5μm≤H1≤20μm, 20μm≤H2≤90μm respectively, the entire negative electrode sheet can have a lower volume expansion, which is beneficial to the life performance of the battery.

[0027] In some embodiments, the average particle size of the negative electrode active material in the first negative electrode active material layer is 1 to 10 μm; and / or

[0028] The average particle size of the negative electrode active material in the second negative electrode active material layer is 8 to 20 μm.

[0029] In the above implementation process, by controlling the average particle size of the negative electrode active material in the first negative electrode active material layer to 1-10 μm and the average particle size of the negative electrode active material in the second negative electrode active material layer to 8-20 μm, it is beneficial to the mass production coating of the first negative electrode active material layer and the second negative electrode active material layer, and reduces the risk of scratching the substrate during the preparation process.

[0030] In some embodiments, the second negative electrode active material layer includes a first sublayer and a second sublayer, the first sublayer is arranged between the second sublayer and the first negative electrode active material layer, and the relationship between the volume capacity density ρ2a of the first sublayer and the volume capacity density ρ2b of the second sublayer satisfies: ρ2a<ρ2b.

[0031] In the above implementation, by dividing the second negative electrode active material layer into a first sublayer and a second sublayer, and making the volume capacity density of the first sublayer adjacent to the first negative electrode active material layer smaller than that of the second sublayer, that is, the content of high-capacity material in the first sublayer is smaller than that in the second sublayer, the volume expansion of the first sublayer during use is smaller. During charging, the first sublayer has more space to store electrolyte, which can provide more channels for the transmission of lithium ions and is beneficial to the battery's dynamic performance. The second sublayer containing more high-capacity material can enhance the absorption capacity of the negative electrode active material layer surface for lithium ions, further improving the battery's dynamic performance.

[0032] In some embodiments, the first negative electrode active material layer and the second negative electrode active material layer both include a first active material and a second active material, and the capacity of the first active material is greater than the capacity of the second active material; in the cross-section in the thickness direction of the negative electrode sheet, the relationship between the cross-sectional area ratio N2a of the first active material in the first sublayer and the cross-sectional area ratio N2b of the first active material in the second sublayer satisfies: N2a<N2b.

[0033] In the above implementation process, the proportion of the cross-sectional area of ​​the first active material in the thickness direction of the negative electrode sheet is positively correlated with the amount of the first active material used to a certain extent. By making the relationship between the cross-sectional area proportion N2a of the first active material in the first sublayer and the cross-sectional area proportion N2b of the first active material in the second sublayer satisfy: N2a<N2b, that is, the proportion of the high-capacity first active material in the second sublayer is greater than that in the first sublayer, it can provide more lithium ion channels and enhance the absorption capacity of the surface of the negative electrode active material layer for lithium ions, which is beneficial to the kinetic performance of the battery.

[0034] In some embodiments, the negative electrode active materials of the first negative electrode active material layer and the second negative electrode active material layer each include a first active material and a second active material, the capacity of the first active material is greater than the capacity of the second active material; and the mass of the first active material satisfies at least one of the following conditions (b1) to (b2):

[0035] (b1) the mass proportion M2a of the first active material in the first sublayer is 0% to 5%;

[0036] (b2) The mass proportion M2b of the first active material in the second sub-layer is 5% to 25%.

[0037] In the above implementation process, by controlling the mass proportion of the first active material in the first sublayer and the second sublayer to be 0% to 5% and 5% to 25% respectively, it is possible to better provide lithium ion channels and enhance the absorption capacity of the surface of the negative electrode active material layer for lithium ions, thereby being beneficial to the kinetic performance of the battery.

[0038] In some embodiments, the thickness H2a of the first sub-layer and the thickness H2b of the second sub-layer satisfy the following relationship: 0.5≤H2a / H2b≤2.

[0039] In the above implementation process, by controlling the thickness ratio H2a / H2b of the first sublayer and the second sublayer to be 0.5-2, it is beneficial to take into account both the transmission and absorption of lithium ions by the negative electrode sheet, which is more beneficial to the dynamic performance of the battery.

[0040] In some embodiments, the first negative active material layer includes a flexible binder, and the flexible binder satisfies at least one of the following conditions (c1) to (c2):

[0041] (c1) The glass transition temperature of the flexible adhesive is lower than 25°C;

[0042] (c2) The flexible binder has a mass content of 5% to 20% in the first negative electrode active material layer.

[0043] In the above implementation, by setting the glass transition temperature of the flexible binder below 25°C, the first negative electrode active material layer can be formed at room temperature and pressure, reducing the risk of cracking and demolding of the negative electrode sheet. By controlling the weight content of the flexible binder in the first negative electrode active material layer to 5% to 20%, the expansion of the high-capacity material in the first negative electrode active material layer is restrained, thereby reducing the volume expansion of the first negative electrode active material layer.

[0044] In some embodiments, the first negative electrode active material layer includes carbon nanotubes, and the carbon nanotubes satisfy at least one of the following conditions (d1) to (d2):

[0045] (d1) The aspect ratio of carbon nanotubes is 1000-10000;

[0046] (d2) The mass proportion of the carbon nanotubes in the first negative electrode active material layer is 0.05% to 1.2%.

[0047] In the above implementation process, carbon nanotubes are linear materials with properties such as high tensile strength, strong compressive resistance, excellent electrical conductivity, and a large aspect ratio. Adding carbon nanotubes to the first negative electrode active material layer helps improve the electronic conductivity of the first negative electrode active material layer. Furthermore, when carbon nanotubes are distributed on the surface of the high-capacity material, their volume expansion can be reduced, thereby facilitating the control of the volume expansion of the negative electrode sheet. By controlling the mass fraction of carbon nanotubes in the first negative electrode active material layer to 0.05% to 1.2%, the probability of gelation in the slurry during the preparation process can be reduced, facilitating the preparation of the negative electrode sheet.

[0048] In some embodiments, the first negative electrode active material layer includes a surfactant, and the surfactant accounts for 1% to 5% by mass in the first negative electrode active material layer.

[0049] In the above implementation process, the addition of a surfactant is beneficial to the dispersion of the carbon nanotubes, and further beneficial to the uniformity of the carbon nanotubes in the first negative electrode active material.

[0050] In some embodiments, the mass areal density of the negative electrode active material layer is 5.19 to 14.26 mg / cm 2 and / or

[0051] The compaction density of the negative electrode active material layer is 1.4 to 1.85 g / cm 3 .

[0052] In some embodiments, the negative electrode active materials of the first negative electrode active material layer and the second negative electrode active material layer both include a first active material and a second active material, the capacity of the first active material is greater than the capacity of the second active material, and the first active material includes at least one of a silicon-based material, a tin-based material, and an aluminum-based material.

[0053] In the second aspect, the present application provides a negative electrode sheet, which includes a negative electrode collector and a negative electrode active material layer, the negative electrode active material layer is arranged on at least one surface of the negative electrode collector, the negative electrode active material layer includes a first negative electrode active material layer and a second negative electrode active material layer, the first negative electrode active material layer is arranged between the second negative electrode active material layer and the negative electrode collector, the volume capacity density ρ1 of the first negative electrode active material layer and the volume capacity density ρ2 of the second negative electrode active material layer satisfy the following relationship: 1.6≤ρ1 / ρ2≤2, the relationship between the thickness H1 of the first negative electrode active material layer and the thickness H2 of the second negative electrode active material layer satisfies: 2H1<H2; the second negative electrode active material layer includes a first sublayer and a second sublayer, the first sublayer is arranged between the second sublayer and the first negative electrode active material layer, the relationship between the volume capacity density ρ2a of the first sublayer and the volume capacity density ρ2b of the second sublayer satisfies: ρ2a<ρ2b, the thickness H2a of the first sublayer and the thickness H2b of the second sublayer satisfy the following relationship: 0.5≤H2a / H2b≤2.

[0054] In the technical solution of the embodiment of the present application, the volume capacity density of the negative electrode active material layer is generally positively correlated with the amount of high-capacity material added. By making the volume capacity density of the first negative electrode active material layer provided in the inner layer greater than the volume capacity density of the second negative electrode active material layer provided in the outer layer, that is, the amount of high-capacity material added in the inner layer is higher than that in the outer layer, thereby reducing the probability of contact between the high-capacity material and the electrolyte when used as a battery, thereby reducing side reactions, which is beneficial to battery life performance. At the same time, it can reduce the probability of intercalation or alloying reactions between the high-capacity material and the active ions, reduce the load of the high-capacity material on the active ions, and reduce the probability of volume expansion during use, which is also beneficial to battery life performance. In addition, the second negative electrode active material layer is divided into a first sublayer and a second sublayer, and the volume capacity density of the first sublayer close to the first negative electrode active material layer is smaller than that of the second sublayer, that is, the content of the high-capacity material in the first sublayer is less than that of the second sublayer, so the volume expansion of the first sublayer during use will be smaller. During the charging process, the first sublayer will have more space to store electrolyte, which can provide more channels for the transmission of lithium ions, which is beneficial to the kinetic performance of the battery. The second sublayer, which contains more high-capacity materials, can enhance the surface absorption capacity of the negative electrode active material layer for lithium ions, further improving the battery's kinetic performance. In summary, batteries containing this negative electrode sheet can achieve both lifespan performance and kinetic performance.

[0055] In a third aspect, the present application provides a battery cell, which includes the negative electrode sheet provided in the first aspect or the second aspect.

[0056] In a fourth aspect, the present application provides a battery, which includes the battery cell provided in the third aspect.

[0057] In a fifth aspect, the present application provides an electrical device, which includes the battery cell provided in the third aspect or the battery provided in the fourth aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Various other advantages and benefits will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiment below. The accompanying drawings are for illustration purposes only and are not to be considered as limiting the present application. The same reference numerals are used throughout the drawings to represent the same components. In the drawings:

[0059] FIG1 is a schematic structural diagram of a vehicle provided in some embodiments of the present application;

[0060] FIG2 is a schematic diagram of the exploded structure of a secondary battery provided in some embodiments of the present application;

[0061] FIG3 is a schematic structural diagram of a battery cell provided in some embodiments of the present application;

[0062] FIG4 is an exploded view of a battery cell provided in some embodiments of the present application;

[0063] FIG5 is a schematic diagram of a first structure of a negative electrode sheet provided in some embodiments of the present application;

[0064] FIG6 is a second structural schematic diagram of a negative electrode sheet provided in some embodiments of the present application;

[0065] FIG7 is a cross-sectional electron microscope image of a negative electrode active material layer provided in some embodiments of the present application;

[0066] FIG8 is a third structural schematic diagram of a negative electrode sheet provided in some embodiments of the present application;

[0067] FIG9 is a flow chart of a method for preparing a negative electrode sheet provided in some embodiments of the present application.

[0068] The figure numbers in the specific embodiment are as follows: 1000-vehicle; 100-secondary battery; 200-motor; 300-controller; 10-housing; 11-accommodating space; 12-first part; 13-second part; 20-battery cell; 21-housing; 211-opening; 22-end cover assembly; 221-end cover; 222-electrode terminal; 23-electrode assembly; 231-negative electrode sheet; 2311-negative current collector; 2312-negative active material layer; 2312a-first negative active material layer; 2312b-second negative active material layer; 2312c-first sublayer; 2312d-second sublayer; 24-current collecting component; 25-insulating protection part. DETAILED DESCRIPTION

[0069] The following embodiments of the technical solution of the present application will be described in detail with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present application and are therefore only examples and are not intended to limit the scope of protection of the present application.

[0070] 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.

[0071] In the description of the embodiments of this application, the technical terms "first" and "second" are used only to distinguish different objects and should not be understood to indicate or imply relative importance or implicitly specify the quantity, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, the meaning of "plurality" is more than two, unless otherwise clearly and specifically defined.

[0072] References herein to "embodiments" mean that a particular feature, structure, or characteristic described in connection with the embodiments may be included in at least one embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute an independent or alternative embodiment that is mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0073] In the description of the embodiments of this application, the term "and / or" is simply a description of the association relationship between associated objects, indicating that three relationships can exist. For example, A and / or B can represent the following three situations: A exists alone, A and B exist simultaneously, and B exists alone. In addition, the character " / " in this document generally indicates that the associated objects are in an "or" relationship.

[0074] In the description of the embodiments of the present application, the term "multiple" refers to more than two (including two). Similarly, "multiple groups" refers to more than two groups (including two groups), and "multiple pieces" refers to more than two pieces (including two pieces).

[0075] 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.

[0076] In the description of the embodiments of the present application, unless otherwise expressly specified or limited, technical terms such as "installed," "connected," "connected," and "fixed" should be understood in a broad sense. For example, they can refer to fixed connections, detachable connections, or integration; mechanical connections or electrical connections; direct connections or indirect connections through an intermediate medium; internal connections between two components or interactions between two components. Those skilled in the art can understand the specific meanings of the above terms in the embodiments of the present application based on specific circumstances.

[0077] Currently, market developments indicate that power batteries are becoming increasingly widely used. They are not only used in energy storage systems such as hydropower, thermal, wind, and solar power plants, but are also widely used in electric vehicles like electric bicycles, electric motorcycles, and electric vehicles, as well as in military equipment and aerospace. As power battery applications continue to expand, market demand is also growing.

[0078] Power batteries can be lithium-ion or sodium-ion secondary batteries, which are widely used in portable electronic devices, electric vehicles, and other fields. Currently, the energy density of graphite anodes in secondary batteries is approaching its limit, while the specific capacity of silicon-, tin-, and aluminum-based materials is much higher than that of graphite. Therefore, the energy density of the negative electrode sheet can be increased by adding silicon-, tin-, and aluminum-based materials to the negative electrode active material layer. However, these high-energy-density materials, such as silicon-, tin-, and aluminum-based materials, suffer from cyclic expansion, which shortens the battery's service life.

[0079] To improve the problem of deteriorating battery life caused by cyclic expansion of high-energy-density materials, the present application proposes a negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer, wherein the negative electrode active material layer is disposed on at least one surface of the negative electrode current collector, the negative electrode active material layer comprising a first negative electrode active material layer and a second negative electrode active material layer, wherein the first negative electrode active material layer is disposed between the second negative electrode active material layer and the negative electrode current collector, and wherein the volume capacity density ρ1 of the first negative electrode active material layer and the volume capacity density ρ2 of the second negative electrode active material layer satisfy the following relationship: 1.6≤ρ1 / ρ2≤2.

[0080] The volumetric capacity density of the negative electrode active material layer is generally positively correlated with the amount of high-capacity material added. In such a negative electrode sheet, by making the volumetric capacity density of the first negative electrode active material layer located in the inner layer greater than the volumetric capacity density of the second negative electrode active material layer located in the outer layer, that is, by adding more high-capacity material to the inner layer than to the outer layer, the probability of contact between the high-capacity material and the electrolyte during battery use is reduced, thereby minimizing side reactions and improving battery life performance. This can also reduce the probability of intercalation or alloying reactions between the high-capacity material and active ions, reducing the load of active ions on the high-capacity material and the probability of volume expansion during use, which is also beneficial to battery life performance.

[0081] The negative electrode sheet can be used to prepare an electrode assembly, which can be used, but not limited to, in electrical devices such as vehicles, ships, or aircraft. A power supply system comprising the battery cells and secondary batteries disclosed in this application can be used to form the electrical device.

[0082] The present invention provides an electric device that uses a battery as a power source. The electric device may be, but is not limited to, 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.

[0083] For the convenience of description, the following embodiments are described by taking a vehicle 1000 as an example of an electrical device according to an embodiment of the present application.

[0084] Please refer to Figure 1, which is a structural schematic diagram of a vehicle 1000 provided in some embodiments of the present application. The vehicle 1000 can be a fuel vehicle, a gas vehicle or a new energy vehicle. The new energy vehicle can be a pure electric vehicle, a hybrid vehicle or an extended-range vehicle, etc. A secondary battery 100 is provided inside the vehicle 1000, and the secondary battery 100 can be provided at the bottom, head or tail of the vehicle 1000. The secondary battery 100 can be used to power the vehicle 1000. For example, the secondary battery 100 can serve as an operating power source for the vehicle 1000. The vehicle 1000 may also include a controller 300 and a motor 200. The controller 300 is used to control the secondary battery 100 to power the motor 200, for example, for starting, navigating and driving the vehicle 1000.

[0085] In some embodiments of the present application, the secondary battery 100 can serve not only as an operating power source for the vehicle 1000 , but also as a driving power source for the vehicle 1000 , replacing or partially replacing fuel or natural gas to provide driving power for the vehicle 1000 .

[0086] In this application, a secondary battery 100 refers to a single physical module that includes multiple battery cells 20 to provide higher voltage and capacity. It can be in the form of a battery pack, a battery module, etc. The secondary battery 100 can include a housing 10 for enclosing the multiple battery cells 20. The housing 10 can prevent liquids or other foreign matter from affecting the charging or discharging of the battery cells 20.

[0087] FIG2 is a schematic diagram of the exploded structure of a secondary battery 100 provided in some embodiments of the present application. Referring to FIG2 , the secondary battery 100 includes a housing 10 and a battery cell 20 , wherein the battery cell 20 is housed in the housing 10 .

[0088] The housing 10 is used to provide a storage space 11 for the battery cells 20. In some embodiments, the housing 10 may include a first portion 12 and a second portion 13, which overlap to define the storage space 11 for accommodating the battery cells 20. Of course, the connection between the first portion 12 and the second portion 13 can be sealed by a seal (not shown), such as a sealing ring, sealant, or the like.

[0089] The first portion 12 and the second portion 13 can have various shapes, such as a rectangular parallelepiped, a cylinder, etc. The first portion 12 can be a hollow structure with an opening on one side to form a receiving cavity for accommodating the battery cell 20. The second portion 13 can also be a hollow structure with an opening on one side to form a receiving cavity for accommodating the battery cell 20. The open side of the second portion 13 covers the open side of the first portion 12, thereby forming the box 10 with the receiving space 11. Of course, as shown in Figure 2, the first portion 12 can also be a hollow structure with an opening on one side, and the second portion 13 can be a plate-like structure. The second portion 13 covers the open side of the first portion 12, thereby forming the box 10 with the receiving space 11.

[0090] In the secondary battery 100, there are multiple battery cells 20. These multiple battery cells 20 can be connected in series, in parallel, or in a hybrid configuration. A hybrid configuration refers to a combination of series and parallel connections among the multiple battery cells 20. Multiple battery cells 20 can be directly connected in series, in parallel, or in a hybrid configuration, and then the entire battery cell 20 is housed within the housing 10. Alternatively, multiple battery cells 20 can be first connected in series, in parallel, or in a hybrid configuration to form a battery module, and then the battery modules are further connected in series, in parallel, or in a hybrid configuration to form a single unit housed within the housing 10. The battery cells 20 can be cylindrical, flat, rectangular, or other shapes. Figure 2 illustrates a case where the battery cells 20 are square.

[0091] In some embodiments, the secondary battery 100 may further include a busbar component (not shown), and the multiple battery cells 20 may be electrically connected via the busbar component to achieve series connection, parallel connection, or hybrid connection of the multiple battery cells 20 .

[0092] Figure 3 is a schematic diagram of the structure of a battery cell 20 provided in some embodiments of the present application, and Figure 4 is an exploded view of a battery cell 20 provided in some embodiments of the present application. Referring to Figures 3 and 4, the battery cell 20 may include a housing 21, an end cap assembly 22, and an electrode assembly 23. The housing 21 has an opening 211, and the electrode assembly 23 is accommodated within the housing 21. The end cap assembly 22 is used to seal the opening 211.

[0093] The shape of the housing 21 can be determined according to the specific shape of the electrode assembly 23. For example, if the electrode assembly 23 is a rectangular parallelepiped structure, the housing 21 can be a rectangular parallelepiped structure. Figures 3 and 4 exemplarily show the case where the housing 21 and the electrode assembly 23 are square.

[0094] The shell 21 may be made of a variety of materials, such as copper, iron, aluminum, stainless steel, aluminum alloy, etc., and the embodiment of the present application does not impose any special restrictions on this.

[0095] The end cap assembly 22 includes an end cap 221 and an electrode terminal 222. The end cap assembly 22 is used to seal the opening 211 of the outer shell 21 to form a sealed installation space (not shown), which is used to accommodate the electrode assembly 23. The installation space is also used to accommodate an electrolyte, such as an electrolyte. The end cap assembly 22 serves as a component for outputting the electrical energy of the electrode assembly 23. The electrode terminal 222 in the end cap assembly 22 is used to electrically connect to the electrode assembly 23, that is, the electrode terminal 222 is electrically connected to the tab of the electrode assembly 23. For example, the electrode terminal 222 is connected to the tab through the current collecting member 24 to achieve electrical connection between the electrode terminal 222 and the tab.

[0096] It should be noted that the opening 211 of the shell 21 can be one or two. If the opening 211 of the shell 21 is one, the end cap assembly 22 can also be one, and two electrode terminals 222 can be provided in the end cap assembly 22, and the two electrode terminals 222 are respectively used to electrically connect to the positive electrode tab and the negative electrode tab of the electrode assembly 23. If the opening 211 of the shell 21 is two, for example, the two openings 211 are provided on opposite sides of the shell 21, the end cap assembly 22 can also be two, and the two end cap assemblies 22 are respectively covered at the two openings 211 of the shell 21. In this case, the electrode terminal 222 in one end cap assembly 22 can be a positive electrode terminal, which is used to electrically connect to the positive electrode tab of the electrode assembly 23; and the electrode terminal 222 in the other end cap assembly 22 can be a negative electrode terminal, which is used to electrically connect to the negative electrode sheet of the electrode assembly 23.

[0097] In some embodiments, as shown in FIG4 , the battery cell 20 may further include an insulating protective member 25 secured to the periphery of the electrode assembly 23. The insulating protective member 25 is used to insulate and isolate the electrode assembly 23 from the housing 21. Exemplarily, the insulating protective member 25 is a tape adhered to the periphery of the electrode assembly 23. In some embodiments, there are multiple electrode assemblies 23, and the insulating protective member 25 is disposed around the periphery of the multiple electrode assemblies 23, forming the multiple electrode assemblies 23 into a single integrated structure to maintain structural stability.

[0098] The electrode assembly 23 includes a positive electrode sheet, a negative electrode sheet, and a separator. The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer. The positive electrode active material layer is coated on the surface of the positive electrode current collector. The positive electrode current collector not coated with the positive electrode active material layer protrudes from the positive electrode current collector coated with the positive electrode active material layer. The positive electrode current collector not coated with the positive electrode active material layer serves as the positive electrode tab.

[0099] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer. The negative electrode active material layer is coated on the surface of the negative electrode current collector. The negative electrode current collector not coated with the negative electrode active material layer protrudes from the negative electrode current collector coated with the negative electrode active material layer. The negative electrode current collector not coated with the negative electrode active material layer serves as a negative electrode tab. The material of the negative electrode current collector may be copper, and the negative electrode active material may be carbon or silicon, etc. In order to ensure that a large current passes without melting, the number of positive electrode tabs is multiple and stacked together, and the number of negative electrode tabs is multiple and stacked together. The material of the isolation membrane may be PP (polypropylene, polypropylene) or PE (polyethylene, polyethylene), etc. In addition, the electrode assembly 23 may be a wound electrode assembly or a laminated electrode assembly, and the embodiments of the present application are not limited to this.

[0100] Figure 5 is a schematic diagram of the first structure of the negative electrode sheet provided in some embodiments of the present application, and Figure 6 is a schematic diagram of the second structure of the negative electrode sheet provided in some embodiments of the present application; please refer to Figures 5 and 6, the embodiments of the present application provide a negative electrode sheet 231, the negative electrode sheet 231 includes a negative electrode collector 2311 and a negative electrode active material layer 2312, the negative electrode active material layer 2312 is arranged on at least one surface of the negative electrode collector 2311, the negative electrode active material layer 2312 includes a first negative electrode active material layer 2312a and a second negative electrode active material layer 2312b, the first negative electrode active material layer 2312a is arranged between the second negative electrode active material layer 2312b and the negative electrode collector 2311, the volume capacity density ρ1 of the first negative electrode active material layer 2312a and the volume capacity density ρ2 of the second negative electrode active material layer 2312b satisfy the following relationship: 1.6≤ρ1 / ρ2≤2.

[0101] The negative electrode current collector 2311 may be made of one or more of copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. Referring to FIG5 , in one embodiment, a first negative electrode active material layer 2312a and a second negative electrode active material layer 2312b are sequentially disposed on one surface of the negative electrode current collector 2311. Referring to FIG6 , in another embodiment, the first negative electrode active material layer 2312a and the second negative electrode active material layer 2312b are sequentially disposed on both surfaces of the negative electrode current collector 2311.

[0102] The negative electrode active material in the first negative electrode active material layer 2312a and the second negative electrode active material layer 2312b can be a carbon material (graphite, soft carbon, hard carbon, mesocarbon microbeads, carbon fiber, carbon nanotubes, graphene, etc.), a titanium oxide-based material (lithium titanate, titanium dioxide, etc.), an alloyed negative electrode material (silicon-based material, tin-based material, germanium-based material, etc.), or a conversion-type negative electrode material (transition metal oxide, phosphide, sulfide, nitride, etc.). The graphite can be selected from one or a combination of artificial graphite, natural graphite, and modified graphite. The graphite can be further modified. There is no specific limitation on the method of modifying the graphite, for example, coating the graphite surface.

[0103] Volume capacity density refers to the ratio of capacity to volume. The volume capacity density of each negative electrode active material layer 2312 in the negative electrode sheet 231 can be obtained by the following method: Use a scanning electron microscope (SEM) (such as ZEISS Sigma 300) to take a picture of the cross section of the negative electrode sheet 231. If the interface between the first negative electrode active material layer 2312a and the second negative electrode active material layer 2312b can be distinguished, then record the thickness of the first negative electrode active material layer 2312a and the second negative electrode active material layer 2312b. Take a double-sided negative electrode sheet 231 of the same type, clean the active material layer on any side of the double-sided negative electrode sheet 231 with DI water to expose the bare copper foil, use a mold to punch the single-sided negative electrode sheet 231 into small discs with a radius of 7 mm, whose area S = 0.49π, use a micrometer to measure the thickness h of the active material layer (excluding the thickness of the current collector), and obtain the volume according to the formula V = Sh. The volume is V The single-sided negative electrode sheet 231 is dried and transferred to a glove box to make a pair of lithium half-cells. The capacity Q of the above wafers is then tested using Wuhan Blue Electric testing equipment. The total volume capacity density of the negative electrode sheet 231 is obtained according to the formula ρ=Q / V. The second negative electrode active material layer 2312b is then peeled off according to the recorded thickness of each layer. The volume and capacity are tested again to obtain the volume V1 and capacity Q1 of the first negative electrode active material layer 2312a. The volume capacity density ρ1 of the first negative electrode active material layer 2312a is obtained according to the formula ρ1=Q1 / V1, and the volume capacity density ρ2 of the second negative electrode active material layer 2312b is obtained by the formula ρ2=(Q-Q1) / (V-V1). If the interface between the first negative electrode active material layer 2312a and the second negative electrode active material layer 2312b is indistinguishable, 70% of the active material layer thickness h is used as the second negative electrode active material layer 2312b for stripping, and the remaining 30% of the active material layer thickness h is used as the first negative electrode active material layer 2312a. The remaining testing steps are performed as described above. It should be noted that if the negative electrode sheet 231 is a single-sided negative electrode sheet 231, the cleaning step is not required.

[0104] The volumetric capacity density of the negative electrode active material layer 2312 is generally positively correlated with the amount of high-capacity material added. This negative electrode sheet 231 employs a design where the volumetric capacity density of the inner first negative electrode active material layer 2312a is greater than the volumetric capacity density of the outer second negative electrode active material layer 2312b. This means that the amount of high-capacity material added to the inner layer is higher than that to the outer layer. This reduces the probability of contact between the high-capacity material and the electrolyte during battery use, thereby minimizing side reactions and improving battery life. This also reduces the probability of intercalation or alloying reactions between the high-capacity material and active ions, reducing the active ion loading of the high-capacity material and the probability of volume expansion during use, which also improves battery life.

[0105] Illustratively, the ratio ρ1 / ρ2 of the volume capacity density ρ1 of the first negative electrode active material layer 2312a and the volume capacity density ρ2 of the second negative electrode active material layer 2312b can be 1.6, 1.7, 1.8, 1.9, 2, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3, 3.1, 3.2, 3.3, 3.4, 3.5 or 3.6, etc., and it can also be any value in the range of 1.6 to 3.6.

[0106] In some embodiments of the present application, the volume capacity density ρ1 of the first negative electrode active material layer 2312a is 0.65-2.6 Ah / cm 3 The volume capacity density ρ2 of the second negative electrode active material layer 2312b is 0.4 to 1.3 Ah / cm 3 By controlling the volume capacity density ρ1 of the first negative electrode active material layer 2312a to be 0.65 to 2.6 Ah / cm 3 The volume capacity density ρ2 of the second negative electrode active material layer 2312b is 0.4 to 1.3 Ah / cm 3 This, to a certain extent, controls the amount of high-capacity material added to the first and second negative active material layers 2312a, 2312b. This ensures that the second negative active material layer 2312b contains a certain amount of high-capacity material, but at a lower level than the first negative active material layer 2312a. This effectively reduces the probability of contact between the high-capacity material and the electrolyte, and reduces the load of active ions on the high-capacity material, thereby extending the battery life. Furthermore, the surface of the negative active material layer on the negative electrode sheet 231 has a better receptivity to lithium ions, making it suitable for higher current densities, thereby maintaining a balanced battery dynamics.

[0107] For example, the volume capacity density ρ1 of the first negative electrode active material layer 2312a may be 0.65 Ah / cm 3 , 1Ah / cm 3 , 1.5Ah / cm3 , 2Ah / cm 3 , 2.5Ah / cm 3 or 2.6Ah / cm 3 etc., which can also be 0.65~2.6Ah / cm 3 The volume capacity density ρ2 of the second negative electrode active material layer 2312b can be 0.4 Ah / cm 3 , 0.6Ah / cm 3 , 0.8Ah / cm 3 , 1Ah / cm 3 , 1.2Ah / cm 3 or 1.3Ah / cm 3 etc., which can also be 0.4~1.3Ah / cm 3 Any value in the range.

[0108] In the technical solutions of some embodiments of the present application, the negative active materials of the first negative active material layer 2312a and the second negative active material layer 2312b both include a first active material and a second active material, and the capacity of the first active material is greater than the capacity of the second active material; in the cross-section in the thickness direction of the negative electrode sheet 231, the relationship between the cross-sectional area ratio N1 of the first active material in the first negative active material layer 2312a and the cross-sectional area ratio N2 of the first active material in the second negative active material layer 2312b satisfies: N1>N2.

[0109] The cross-sectional area ratio N1 of the first active material in the first negative electrode active material layer 2312a and the cross-sectional area ratio N2 of the first active material in the second negative electrode active material layer 2312b can be obtained by performing cross-sectional electron microscope scanning on the negative electrode active material layer 2312. Usually, the cross-sectional area ratio is obtained by calculating the average value after multiple cross-sections, for example, the ratio of 5 cross-sections is obtained at equal intervals for calculation, as shown in Figure 7, which is a cross-sectional electron microscope scan of the negative electrode active material layer 2312 provided in some embodiments of the present application.

[0110] The proportion of the cross-sectional area of ​​the first active material in the thickness direction of the negative electrode sheet 231 is positively correlated to the amount of the first active material used to a certain extent. By making the relationship between the cross-sectional area proportion N1 of the first active material in the first negative electrode active material layer 2312a and the cross-sectional area proportion N2 of the first active material in the second negative electrode active material layer 2312b satisfy: N1>N2, even if the proportion of the high-capacity first active material in the first negative electrode active material layer 2312a is greater than the proportion in the second negative electrode active material layer 2312b, the probability of contact between the high-capacity material and the electrolyte and the load of the high-capacity material on the active ions can be effectively reduced, thereby making the battery have a better life.

[0111] In the technical solutions of some embodiments of the present application, the cross-sectional area percentage N1 of the first active material in the first negative electrode active material layer 2312a, measured along the thickness of the negative electrode sheet 231, satisfies the following conditions: 29% ≤ N1 ≤ 83%; and the cross-sectional area percentage N2 of the first active material in the second negative electrode active material layer 2312b, measured along the thickness of the negative electrode sheet 231, satisfies the following conditions: 0 ≤ N2 ≤ 20%. By controlling the cross-sectional area percentages of the first active material in the first negative electrode active material layer 2312a and the second negative electrode active material layer 2312b to be 29% ≤ N1 ≤ 83% and 0 ≤ N2 ≤ 20%, respectively, the probability of contact between the high-capacity material and the electrolyte and the loading of active ions on the high-capacity material can be reduced, thereby extending the battery life.

[0112] For example, the cross-sectional area ratio N1 of the first active material in the first negative electrode active material layer 2312a may be 29%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, or 83%, or any value within the range of 29% to 83%. The cross-sectional area ratio N2 of the first active material in the second negative electrode active material layer 2312b may be 0%, 5%, 10%, 15%, or 20%, or any value within the range of 0% to 20%.

[0113] In the technical solutions of some embodiments of the present application, the negative electrode active materials of the first negative electrode active material layer 2312a and the second negative electrode active material layer 2312b both include a first active material and a second active material, and the capacity of the first active material is greater than the capacity of the second active material; the mass proportion of the first active material in the first negative electrode active material layer 2312a is 30% to 85%; the mass proportion of the first active material in the second negative electrode active material layer 2312b is 0% to 20%.

[0114] By controlling the mass proportion of the first active material in the first negative electrode active material layer 2312a and the second negative electrode active material layer 2312b to be 30% to 85% and 0% to 20% respectively, the probability of contact between the high-capacity material and the electrolyte and the load of the high-capacity material on the active ions can be better reduced, thereby making the battery have a better life.

[0115] Furthermore, the first active material accounts for 50% to 70% by mass in the first negative electrode active material layer 2312a; and the first active material accounts for 3% to 10% by mass in the second negative electrode active material layer 2312b.

[0116] For example, the mass percentage of the first active material in the first negative electrode active material layer 2312a may be 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, or 85%, or any value within the range of 30% to 85%. The mass percentage of the first active material in the second negative electrode active material layer 2312b may be 0%, 2%, 4%, 6%, 8%, 10%, 12%, 14%, 15%, 16%, 18%, or 20%, or any value within the range of 0% to 20%.

[0117] In some embodiments of the present application, the thickness H1 of the first negative electrode active material layer 2312a and the thickness H2 of the second negative electrode active material layer 2312b satisfy the following relationship: 2H1 < H2. By controlling the thickness H1 of the first negative electrode active material layer 2312a, which contains a relatively high amount of easily expandable high-capacity material, to < H2 / 2, the expansion of the entire negative electrode sheet 231 can be controlled.

[0118] Illustratively, the ratio H1 / H2 of the thickness H1 of the first negative electrode active material layer 2312a to the thickness H2 of the second negative electrode active material layer 2312b may be 1 / 26, 1 / 20, 1 / 15, 1 / 10, 1 / 8, 1 / 5, 1 / 3 or 1 / 2, or any value less than 1 / 2.

[0119] In some embodiments of the present application, the thickness H1 of the first negative electrode active material layer 2312a satisfies the following conditions: 5 μm ≤ H1 ≤ 20 μm; the thickness H2 of the second negative electrode active material layer 2312b satisfies the following conditions: 20 μm ≤ H2 ≤ 90 μm. By controlling the thickness H1 of the first negative electrode active material layer 2312a and the thickness H2 of the second negative electrode active material layer 2312b to meet the following conditions: 5 μm ≤ H1 ≤ 20 μm and 20 μm ≤ H2 ≤ 90 μm, respectively, the volume expansion of the entire negative electrode sheet 231 can be reduced, thereby improving the battery's lifespan.

[0120] For example, the thickness H1 of the first negative electrode active material layer 2312a may be 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, or 20 μm, or any value within the range of 5 to 20 μm. The thickness H2 of the second negative electrode active material layer 2312b may be 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 75 μm, 80 μm, 85 μm, or 90 μm, or any value within the range of 20 to 90 μm.

[0121] In the technical solutions of some embodiments of the present application, the average particle size of the negative electrode active material in the first negative electrode active material layer 2312a is 1 to 10 μm, and the average particle size of the negative electrode active material in the second negative electrode active material layer 2312b is 8 to 20 μm. By controlling the average particle size of the negative electrode active material in the first negative electrode active material layer 2312a to 10 μm and the average particle size of the negative electrode active material in the second negative electrode active material layer 2312b to 8 to 20 μm, the mass production coating of the first and second negative electrode active material layers 2312a, 2312b is facilitated, reducing the risk of substrate scraping during the preparation process.

[0122] For example, the average particle size of the negative electrode active material in the first negative electrode active material layer 2312a can be 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, or 10 μm, or any value within the range of 1 to 10 μm. The average particle size of the negative electrode active material in the second negative electrode active material layer 2312b can be 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, or 20 μm, or any value within the range of 8 to 20 μm.

[0123] In the technical solutions of some embodiments of the present application, please refer to Figure 8, which is a third structural schematic diagram of the negative electrode sheet 231 provided in some embodiments of the present application; the second negative electrode active material layer 2312b includes a first sublayer 2312c and a second sublayer 2312d, and the first sublayer 2312c is arranged between the second sublayer 2312d and the first negative electrode active material layer 2312a, and the relationship between the volume capacity density ρ2a of the first sublayer 2312c and the volume capacity density ρ2b of the second sublayer 2312d satisfies: ρ2a<ρ2b.

[0124] The volumetric capacity density of the first sublayer 2312c and the second sublayer 2312d can be tested using the same method as described for the volumetric capacity density of the first negative electrode active material layer 2312a and the second negative electrode active material layer 2312b. It should be noted that when the interface between the first sublayer 2312c and the second sublayer 2312d is difficult to distinguish, half of the thickness of the second negative electrode active material layer 2312b is used as the first sublayer 2312c, and the other half as the second sublayer 2312d for peeling and testing.

[0125] By dividing the second negative electrode active material layer 2312b into a first sublayer 2312c and a second sublayer 2312d, and ensuring that the volume capacity density of the first sublayer 2312c, which is closer to the first negative electrode active material layer 2312a, is lower than that of the second sublayer 2312d, that is, the content of high-capacity material in the first sublayer 2312c is lower than that in the second sublayer 2312d, the volume expansion of the first sublayer 2312c during use is reduced. During charging, the first sublayer 2312c has more space to store electrolyte, providing more channels for lithium ion transmission, which is beneficial to the battery's dynamic performance. The second sublayer 2312d, which contains more high-capacity material, can enhance the absorption capacity of lithium ions on the surface of the negative electrode active material layer 2312, further improving the battery's dynamic performance.

[0126] In the technical solutions of some embodiments of the present application, the first negative electrode active material layer 2312a and the second negative electrode active material layer 2312b both include a first active material and a second active material, and the capacity of the first active material is greater than the capacity of the second active material; in the cross-section in the thickness direction of the negative electrode sheet 231, the relationship between the cross-sectional area ratio N2a of the first active material in the first sub-layer 2312c and the cross-sectional area ratio N2b of the first active material in the second sub-layer 2312d satisfies: N2a<N2b. The proportion of the cross-sectional area of ​​the first active material in the thickness direction of the negative electrode sheet 231 is positively correlated to the amount of the first active material used to a certain extent. By making the relationship between the cross-sectional area proportion N2a of the first active material in the first sub-layer 2312c and the cross-sectional area proportion N2b of the first active material in the second sub-layer 2312d satisfy: N2a<N2b, even if the proportion of the high-capacity first active material in the second sub-layer 2312d is greater than that in the first sub-layer 2312c, it can provide more lithium ion channels and enhance the absorption capacity of the surface of the negative electrode active material layer 2312 for lithium ions, which is beneficial to the battery's kinetic performance.

[0127] In the technical solutions of some embodiments of the present application, the first active material accounts for 0% to 5% by mass in the first sublayer 2312c, and the first active material accounts for 5% to 25% by mass in the second sublayer 2312d. By controlling the first active material to account for 0% to 5% by mass in the first sublayer 2312c and 5% to 25% by mass in the second sublayer 2312d, respectively, better lithium ion channels can be provided and the lithium ion absorption capacity of the surface of the negative electrode active material layer 2312 can be enhanced, thereby improving the dynamic performance of the battery.

[0128] For example, the mass percentage of the first active material in the first sublayer 2312c may be 0%, 1%, 2%, 3%, 4%, or 5%, or any value within the range of 0% to 5%. The mass percentage of the first active material in the second sublayer 2312d may be 5%, 7%, 9%, 11%, 13%, 15%, 17%, 19%, 21%, 23%, or 25%, or any value within the range of 5% to 25%.

[0129] In the technical solutions of some embodiments of the present application, the thickness H2a of the first sublayer 2312c and the thickness H2b of the second sublayer 2312d satisfy the following relationship: 0.5 ≤ H2a / H2b ≤ 2. By controlling the thickness ratio H2a / H2b of the first sublayer 2312c and the second sublayer 2312d to be between 0.5 and 2, it is beneficial to balance the transmission and absorption of lithium ions by the negative electrode sheet 231, thereby improving the dynamic performance of the battery.

[0130] Illustratively, the ratio H2a / H2b of the thickness H2a of the first sublayer 2312c to the thickness H2b of the second sublayer 2312d can be 0.5, 0.7, 0.9, 1.1, 1.3, 1.5, 1.7, 1.9 or 2, etc., and can also be any value within the range of 0.5 to 2.

[0131] In the technical solutions of some embodiments of the present application, the first negative electrode active material layer 2312a includes a flexible binder, and the glass transition temperature of the flexible binder is lower than 25°C.

[0132] Glass transition temperature refers to the temperature at which a substance finally changes from solid to liquid when the temperature is gradually increased.

[0133] By making the glass transition temperature of the flexible binder lower than 25° C., the first negative electrode active material layer 2312 a is advantageously formed at room temperature and pressure, thereby reducing the occurrence of cracking and demolding of the negative electrode sheet 231 .

[0134] In some embodiments of the present application, the first negative electrode active material layer 2312a includes a flexible binder, and the flexible binder content in the first negative electrode active material layer 2312a is 5% to 20% by weight. By controlling the flexible binder content in the first negative electrode active material layer 2312a to 5% to 20% by weight, the expansion of the high-capacity material in the first negative electrode active material layer 2312a is restrained, thereby reducing the volume expansion of the first negative electrode active material layer 2312a.

[0135] In some embodiments of the present application, the first negative electrode active material layer 2312a includes carbon nanotubes (CNTs) with an aspect ratio of 1,000 to 10,000. Carbon nanotubes are linear materials with properties such as high tensile strength, strong compressive resistance, excellent electrical conductivity, and a large aspect ratio. Adding CNTs to the first negative electrode active material layer 2312a improves the electronic conductivity of the first negative electrode active material layer 2312a. Furthermore, when CNTs are distributed on the surface of high-capacity materials, they can reduce their volume expansion, thereby facilitating volumetric expansion control of the negative electrode sheet 231.

[0136] In some embodiments of the present application, the first negative electrode active material layer 2312a includes carbon nanotubes, which may comprise 0.05% to 1.2% by weight of the first negative electrode active material layer 2312a. Specifically, the carbon nanotubes may be selected from single-arm carbon nanotubes. By controlling the carbon nanotube composition of the first negative electrode active material layer 2312a to 0.05% to 1.2% by weight, the probability of gelation in the slurry during the preparation process can be reduced, facilitating the preparation of the negative electrode sheet 231.

[0137] In the technical solutions of some embodiments of the present application, the first negative electrode active material layer 2312 a includes a surfactant, and the mass proportion of the surfactant in the first negative electrode active material layer 2312 a is 1% to 5%.

[0138] Surfactants, also known as interfacial active agents, are compounds that significantly reduce surface tension or interfacial tension between two liquids, between a liquid and a gas, or between a liquid and a solid. Specifically, they can be selected from CMC-Na, PTFE, PAA, and the like.

[0139] The addition of surfactant is beneficial to the dispersion of carbon nanotubes, and further beneficial to the uniformity of carbon nanotubes in the first negative electrode active material.

[0140] In the technical solutions of some embodiments of the present application, the mass area density of the negative electrode active material layer 2312 is 5.19 to 14.26 mg / cm 2 The compaction density of the negative electrode active material layer 2312 is 1.4 to 1.85 g / cm 3 .

[0141] For example, the mass areal density of the negative electrode active material layer 2312 may be 5.19 mg / cm 2 , 6mg / cm 2 , 7mg / cm 2 , 8mg / cm 2 , 9mg / cm 2 、10mg / cm 2、11mg / cm 2 , 12mg / cm 2 、13mg / cm 2 or 14.26 mg / cm 2 etc., which can also be 5.19 to 14.26 mg / cm 2 The compaction density of the negative electrode active material layer 2312 can be 1.4 g / cm 3 , 1.5g / cm 3 , 1.6g / cm 3 , 1.7g / cm 3 , 1.8g / cm 3 or 1.85g / cm 3 etc., which can also be 1.4 to 1.85 g / cm 3 Any value in the range.

[0142] In the technical solutions of some embodiments of the present application, the negative electrode active materials of the first negative electrode active material layer 2312a and the second negative electrode active material layer 2312b both include a first active material and a second active material, the capacity of the first active material is greater than the capacity of the second active material, and the first active material includes at least one of a silicon-based material, a tin-based material, and an aluminum-based material.

[0143] For example, the silicon-based material can be selected from any one or more of elemental silicon, silicon oxide, and silicon carbon. The tin-based material can be selected from any one or more of elemental tin, tin oxide compounds, and tin alloys. The aluminum-based material can be selected from aluminum and / or aluminum alloys.

[0144] After the above introduction to the materials and structure of the negative electrode sheet, the preparation method of the negative electrode sheet will be specifically introduced below.

[0145] The negative electrode sheet preparation method includes the following steps: applying a first negative electrode active slurry to the surface of a negative electrode current collector to form a first negative electrode active material layer 2312a. Applying a second negative electrode active slurry to the surface of the first negative electrode active material layer 2312a to form a second negative electrode active material layer 2312b. The volumetric capacity density ρ1 of the first negative electrode active material layer 2312a and the volumetric capacity density ρ2 of the second negative electrode active material layer 2312b satisfy the following relationship: 1.6 ≤ ρ1 / ρ2 ≤ 3.6.

[0146] The volumetric capacity density of the negative electrode active material layer 2312 is generally positively correlated with the amount of high-capacity material added. This method increases the volumetric capacity density of the inner first negative electrode active material layer 2312a relative to the outer second negative electrode active material layer 2312b. This increases the amount of high-capacity material added to the inner layer compared to the outer layer. This reduces the probability of contact between the high-capacity material and the electrolyte during battery use, thereby minimizing side reactions and improving battery life. This also reduces the probability of intercalation or alloying reactions between the high-capacity material and active ions, reducing the active ion loading of the high-capacity material and the probability of volume expansion during use, which also improves battery life.

[0147] When the second negative electrode active material layer 2312b of the aforementioned negative electrode sheet includes a first sublayer 2312c and a second sublayer 2312d, the preparation of the second negative electrode active material layer 2312b includes the following steps: applying a slurry of the first sublayer 2312c to the surface of the first negative electrode active material layer 2312a to form the first sublayer 2312c, and then applying a slurry of the second sublayer 2312d to the surface of the first sublayer 2312c to form the second sublayer 2312d, thereby forming the second negative electrode active material layer 2312b. The relationship between the volume capacity density ρ2a of the first sublayer 2312c and the volume capacity density ρ2b of the second sublayer 2312d satisfies the following: ρ2a<ρ2b.

[0148] This method divides the second negative electrode active material layer 2312b into a first sublayer 2312c and a second sublayer 2312d. The volume capacity density of the first sublayer 2312c, which is closer to the first negative electrode active material layer 2312a, is lower than that of the second sublayer 2312d. This means that the content of high-capacity material in the first sublayer 2312c is lower than that in the second sublayer 2312d. As a result, the volume expansion of the first sublayer 2312c during use is smaller. During charging, the first sublayer 2312c has more space to store electrolyte, providing more channels for lithium ion transmission and improving the battery's kinetic performance. The second sublayer 2312d, which contains more high-capacity material, can enhance the surface absorption capacity of the negative electrode active material layer 2312 for lithium ions, further improving the battery's kinetic performance.

[0149] The following is an example of a method for preparing a negative electrode sheet containing only a first negative electrode active material layer and a second negative electrode active material layer, wherein the second negative electrode active material layer includes a first sublayer and a second sublayer.

[0150] FIG9 is a flow chart of a method for preparing a negative electrode sheet according to some embodiments of the present application. Referring to FIG9 , an embodiment of the present application provides a method for preparing a negative electrode sheet, the method comprising:

[0151] S110, preparing a first negative electrode active slurry: Dispersing a first active material, a second active material, a binder, and a conductive agent in a solvent to form a first negative electrode active slurry. The first active material may be at least one of the aforementioned silicon-based materials, tin-based materials, and aluminum-based materials, such as silicon monoxide. The second active material may be artificial graphite.

[0152] The binder may be one or more of styrene-butadiene rubber, water-based acrylic resin, carboxymethyl cellulose, polyvinylidene fluoride, polytetrafluoroethylene, ethylene-vinyl acetate copolymer, polyvinyl alcohol, and polyvinyl butyral. The conductive agent may be at least one of conductive carbon black, carbon fiber, carbon nanotubes, Ketjen black, graphene, or acetylene black. The solvent may be one or more of dimethyl glutarate and N-methylpyrrolidone. A leveling agent, dispersant, etc. may also be added to the first negative electrode active slurry.

[0153] S120, preparing a second negative electrode active slurry: ① Preparing a first sublayer slurry: Dispersing the first active material, the second active material, a binder, and a conductive agent in a solvent to form a first sublayer slurry. ② Preparing a second sublayer slurry: Dispersing the first active material, the second active material, a binder, and a conductive agent in a solvent to form a second sublayer slurry. The first active material can be at least one of the aforementioned silicon-based materials, tin-based materials, and aluminum-based materials, such as silicon monoxide. The second active material can be artificial graphite.

[0154] The binder, conductive agent, and solvent may be the binder, conductive agent, and solvent in the first negative electrode active slurry. The binder in the first negative electrode active slurry and the binder in the second negative electrode active material may be the same or different; the conductive agent in the first negative electrode active slurry and the conductive agent in the second negative electrode active material may be the same or different; and the solvent in the first negative electrode active slurry and the solvent in the second negative electrode active material may be the same or different. Furthermore, a leveling agent, dispersant, and the like may also be added to the second negative electrode active slurry, but this application does not limit these.

[0155] S130, preparing a first negative electrode active material layer: coating the first negative electrode active slurry on the surface of the negative electrode current collector, and then drying to form the first negative electrode active material layer. The coating can be applied to one or both surfaces of the negative electrode current collector as required.

[0156] The coating method may be: blade coating, roller coating, slit coating, etc., which is not limited in this application. It should be noted that step S120 and step S130 can be interchanged or performed simultaneously, which is not limited in this application.

[0157] S140, preparing a second negative electrode active material layer: coating the second negative electrode active material slurry on the surface of the first negative electrode active material layer (specifically, first coating the first sublayer slurry on the surface of the first negative electrode active material layer to form a first sublayer, and then coating the second sublayer slurry on the surface of the first sublayer to form a second sublayer). The first sublayer and the second sublayer together form the second negative electrode active material layer. During coating, the second negative electrode active material layer can be formed on the surface of the first negative electrode active material layer according to the condition of the first negative electrode active material layer.

[0158] S150 , rolling the second negative electrode active material layer to obtain a negative electrode sheet.

[0159] It should be noted that when there is only one second negative electrode active material layer, the first sublayer and the second sublayer can be prepared in the same ratio.

[0160] After the negative electrode sheet is prepared, the first isolation membrane, the positive electrode sheet, the second isolation membrane and the negative electrode sheet 231 are stacked in sequence, wound to form a wound flat structure, and then hot pressed to obtain a wound electrode assembly; or, after the negative electrode sheet is prepared, the positive electrode sheet, isolation membrane, negative electrode sheet 231, isolation membrane, and so on are stacked in sequence to form a laminated electrode assembly.

[0161] The electrode assembly 23 can be used to prepare a battery cell 20 , and the battery cell 20 can be used to prepare a secondary battery 100 and provide electrical energy to an electrical device.

[0162] Next, one or more embodiments will be described in more detail with reference to the following examples. Of course, these examples do not limit the scope of one or more embodiments.

[0163] Examples and Comparative Examples

[0164] Example 1

[0165]

Preparation of positive electrode sheet

[0166] Lithium nickel cobalt manganese oxide (LiNi0.8Co0.1Mn0.1O2) (NCM811), conductive carbon black (Super P), and binder polyvinylidene fluoride (PVDF) were thoroughly mixed in an appropriate amount of N-methylpyrrolidone (NMP) at a weight ratio of 97.5:1.5:1 to form a uniform positive electrode slurry. The slurry was then applied to the surface of the positive electrode current collector aluminum foil. The cathode sheet was then dried, cold-pressed, slit, and cut. The sheet had a compacted density of 3.5 g / cm3 and a surface density of 18.04 mg / cm2.

[0167]

Preparation of negative electrode sheet

[0168] The negative electrode active material (including 30% by mass of the first active material 1200mAh / g silicon oxide and 57.2% by mass of the second active material artificial graphite), the conductive agent (including 1% by mass of conductive carbon black and 0.3% by mass of single-arm carbon nanotubes), the binder styrene-butadiene rubber (SBR), and the surfactant sodium hydroxymethyl cellulose (CMC-Na) are dissolved in the solvent deionized water, wherein the mass proportion of the negative electrode active material is 87.2%, the mass proportion of the conductive agent is 1.3%, the mass proportion of the binder is 10%, and the mass proportion of the surfactant is 1.5%. After uniform mixing, a first negative electrode active slurry is prepared; the first negative electrode active slurry is uniformly coated on the negative electrode current collector copper foil once or multiple times to obtain a first negative electrode active material layer.

[0169] The negative electrode active material (including 0% by mass of the first active material silicon oxide and 96.2% by mass of the second active material artificial graphite), the conductive agent conductive carbon black, the binder styrene butadiene rubber (SBR), and the dispersant sodium hydroxymethyl cellulose (CMC-Na) are dissolved in the solvent deionized water, wherein the mass proportion of the negative electrode active material is 96.2%, the mass proportion of the conductive agent is 0.8%, the mass proportion of the binder is 1.8%, and the mass proportion of the dispersant is 1.2%. After uniform mixing, a second negative electrode active slurry is prepared. The second negative electrode active slurry is coated on the surface of the first negative electrode active material layer to form a second negative electrode active material layer. After drying, cold pressing, and slitting, the negative electrode sheet is obtained.

[0170] Note: The total silicon content in its negative electrode active material layer is about 3%.

[0171] Preparation of electrolyte

[0172] In an argon atmosphere glove box (H2O<0.1ppm, O2<0.1ppm), the organic solvents ethylene carbonate (EC) / diethyl carbonate (DEC) / ethyl methyl carbonate (EMC) were mixed in a volume ratio of 1 / 1 / 1, 1 mol / L LiPF6 lithium salt was added and dispersed evenly, and then 5% fluoroethylene carbonate was dissolved in the above organic solvents and stirred evenly to obtain an electrolyte.

[0173]

Isolation film

[0174] A polyethylene film was used as the separator, and its thickness was 12 μm.

[0175]

Preparation of secondary batteries

[0176] The positive electrode sheet, separator and negative electrode sheet are stacked in order, with the separator placed between the positive electrode sheet and the negative electrode sheet to serve as an isolation. The electrode assembly is then wound to obtain an electrode assembly, which is placed in an outer packaging shell. After drying, the electrolyte is injected, and a secondary battery is obtained through processes such as vacuum packaging, standing, formation, and shaping.

[0177] Example 2

[0178] The difference between Example 2 and Example 1 is that:

[0179] The first negative electrode active material layer includes 40% by mass of a first active material of 1700 mAh / g silicon carbon and 47.2% by mass of a second active material of artificial graphite.

[0180] The second negative electrode active material layer includes 3% by mass of a first active material of silicon carbon with a capacity of 1700 mAh / g and 93.2% by mass of a second active material of artificial graphite.

[0181] Note: The total silicon content in its negative electrode active material layer is about 8%.

[0182] Example 3

[0183] The difference between Example 3 and Example 1 is that:

[0184] The first negative electrode active material layer includes 60% by mass of a first active material, silicon carbon, with a capacity of 1700 mAh / g, and 27.2% by mass of a second active material, artificial graphite.

[0185] The second negative electrode active material layer includes 10% by mass of a first active material of 1700 mAh / g silicon carbon and 86.2% by mass of a second active material of artificial graphite.

[0186] Note: The total silicon content in its negative electrode active material layer is about 19%.

[0187] Example 4

[0188] The difference between Example 4 and Example 1 is that:

[0189] The first negative electrode active material layer includes 70% by mass of a first active material, silicon carbon with a capacity of 2000 mAh / g, and 17.2% by mass of a second active material, artificial graphite.

[0190] The second negative electrode active material layer includes 15% by mass of a first active material of 2000 mAh / g silicon carbon and 81.2% by mass of a second active material of artificial graphite.

[0191] Note: The total silicon content in its negative electrode active material layer is about 28%.

[0192] Example 5

[0193] The difference between Example 5 and Example 1 is that:

[0194] The first negative electrode active material layer includes 85% by mass of a first active material, silicon carbon, with a capacity of 2500 mAh / g, and 2.2% by mass of a second active material, artificial graphite.

[0195] The second negative electrode active material layer includes 20% by mass of a first active material, silicon carbon with a capacity of 2500 mAh / g, and 76.2% by mass of a second active material, artificial graphite.

[0196] Note: The total silicon content in its active material layer is approximately 32%.

[0197] Example 6

[0198] This embodiment is the same as Example 3 except that the second negative electrode active material layer in the negative electrode sheet is divided into a first sub-layer and a second sub-layer. Other contents are the same as Example 3.

[0199] The total amount of the first active material in the first and second sublayers in this embodiment is the same as the total amount of the first active material in the second negative electrode active material layer in Example 3, and the amount of the first active material in the second sublayer is 3 / 4 of the total amount of the first active material in the second negative electrode active material layer in Example 3. The first and second sublayers have the same thickness.

[0200] The preparation process of the second negative electrode active material layer is as follows:

[0201] The negative electrode active material (including 10% by mass of the first active material 1700mAh / g silicon carbon and 86.2% by mass of the second active material artificial graphite), the conductive agent conductive carbon black, the binder styrene-butadiene rubber (SBR), and the dispersant sodium hydroxymethyl cellulose (CMC-Na) are dissolved in the solvent deionized water, wherein the mass proportion of the negative electrode active material is 96.2%, the mass proportion of the conductive agent is 0.8%, the mass proportion of the binder is 1.8%, and the mass proportion of the dispersant is 1.2%. The first sub-layer slurry and the second sub-layer slurry are obtained by adjusting the proportion of the first active material. The first sub-layer slurry is coated on the surface of the first negative electrode active material layer to form a first sub-layer, and then the second sub-layer slurry is coated on the surface of the first sub-layer to form a second sub-layer, and then cold pressed and cut to obtain the negative electrode sheet.

[0202] Note: The total silicon content in its negative electrode active material layer is about 19%.

[0203] Example 7

[0204] This embodiment is the same as Example 3 except that the second negative electrode active material layer in the negative electrode sheet is divided into a first sub-layer and a second sub-layer.

[0205] The total amount of the first active material in the first sublayer and the second sublayer in this embodiment is the same as the total amount of the first active material in the second negative electrode active material layer in Example 3, and the amount of the first active material in the second sublayer is equal to the total amount of the first active material in the second negative electrode active material layer in Example 3. The first sublayer and the second sublayer have the same thickness.

[0206] The preparation process of the second negative electrode active material layer is as follows:

[0207] The negative electrode active material (including 10% by mass of the first active material 1700mAh / g silicon carbon and 86.2% by mass of the second active material artificial graphite), the conductive agent conductive carbon black, the binder styrene-butadiene rubber (SBR), and the dispersant sodium hydroxymethyl cellulose (CMC-Na) are dissolved in the solvent deionized water, wherein the mass proportion of the negative electrode active material is 96.2%, the mass proportion of the conductive agent is 0.8%, the mass proportion of the binder is 1.8%, and the mass proportion of the dispersant is 1.2%. The first sub-layer slurry and the second sub-layer slurry are obtained by adjusting the proportion of the first active material. The first sub-layer slurry is coated on the surface of the first negative electrode active material layer to form a first sub-layer, and then the second sub-layer slurry is coated on the surface of the first sub-layer to form a second sub-layer, and then cold pressed and cut to obtain the negative electrode sheet.

[0208] Note: The total silicon content in its negative electrode active material layer is about 19%.

[0209] Comparative Example 1

[0210] The difference between Comparative Example 1 and Example 1 is:

[0211] The first and second negative electrode active material layers both adopt the second negative electrode active material layer formula in Example 1, which includes 3% by mass of the first active material 1200mAh / g silicon dioxide and 93.2% by mass of the second active material artificial graphite.

[0212] Note: The total silicon content in its negative electrode active material layer is 3%.

[0213] Comparative Example 2

[0214] The difference between Comparative Example 2 and Example 1 is:

[0215] The second negative electrode active material layer includes 2% by mass of a first active material of 1200 mAh / g silicon oxide and 94.2% by mass of a second active material of artificial graphite.

[0216] Note: The total silicon content in its negative electrode active material layer is about 3%.

[0217] Comparative Example 2

[0218] The difference between Comparative Example 3 and Example 2 is:

[0219] The first and second negative electrode active material layers both adopt the second negative electrode active material layer formula in Example 2, which includes 8% by mass of the first active material 1700mAh / g silicon carbon and 88.2% by mass of the second active material artificial graphite.

[0220] Note: The total silicon content in its negative electrode active material layer is 8%.

[0221] Comparative Example 4

[0222] The difference between Comparative Example 4 and Example 2 is:

[0223] The second negative electrode active material layer includes 5% by mass of a first active material of 1700 mAh / g silicon carbon and 91.2% by mass of a second active material of artificial graphite.

[0224] Note: The total silicon content in its negative electrode active material layer is about 8%.

[0225] Comparative Example 5

[0226] The difference between Comparative Example 5 and Example 3 is:

[0227] The first and second negative electrode active material layers both adopt the second negative electrode active material layer formula in Example 2, which includes 19% by mass of the first active material 1700mAh / g silicon carbon and 77.2% by mass of the second active material artificial graphite.

[0228] Note: The total silicon content in its negative electrode active material layer is 19%.

[0229] Comparative Example 6

[0230] The difference between Comparative Example 6 and Example 3 is that:

[0231] The second negative electrode active material layer includes 17% by mass of a first active material, silicon carbon, with a capacity of 1700 mAh / g, and 89.2% by mass of a second active material, artificial graphite.

[0232] Note: The total silicon content in its negative electrode active material layer is about 19%.

[0233] Comparative Example 7

[0234] The difference between Comparative Example 7 and Example 3 is that:

[0235] The second negative electrode active material layer includes 0% by mass of a first active material of 1700 mAh / g silicon carbon and 96.2% by mass of a second active material of artificial graphite.

[0236] Note: The total silicon content in its negative electrode active material layer is about 19%.

[0237] Comparative Example 8

[0238] The difference between Comparative Example 8 and Example 4 is that:

[0239] The first and second negative electrode active material layers both adopt the second negative electrode active material layer formula in Example 2, which includes 28% by mass of the first active material 2000mAh / g silicon carbon and 68.2% by mass of the second active material artificial graphite.

[0240] Note: The total silicon content in its negative electrode active material layer is 28%.

[0241] Comparative Example 8

[0242] The difference between Comparative Example 9 and Example 5 is:

[0243] The first and second negative electrode active material layers both adopt the second negative electrode active material layer formula in Example 2, which includes 32% by mass of the first active material 2500mAh / g silicon carbon and 64.2% by mass of the second active material artificial graphite.

[0244] Note: The total silicon content in its negative electrode active material layer is 32%.

[0245] The main parameters of each embodiment and comparative example are shown in the following table:

[0246] The performance test of the batteries provided in each embodiment and comparative example was performed, and the performance test specifically included:

[0247] Battery first cycle capacity test:

[0248] The prepared secondary battery was charged to 3.4 V at a rate of 0.02 C at 45 ° C, then charged to 3.75 V at a rate of 0.1 C, then charged to 4.25 V at a rate of 0.33 C at 25 ° C, charged to 0.05 C at a constant voltage of 4.25 V, and finally discharged to 2.5 V at 0.33 C. The measured capacity is marked as C0.

[0249] Cycle test:

[0250] The secondary battery was charged to 4.25 V at a gradient rate of 2C0 to 0.1C0 at a constant temperature of 25°C, then allowed to stand for 10 min, and then discharged at a constant current rate of 0.5C0 to a voltage of 2.5 V (the capacity of this step is marked as C1), allowed to stand for 10 min, and the secondary battery was cycled 1000 times according to the above method (the discharge capacity of the 1000th cycle is marked as C1000), and the cycle capacity retention rate of the secondary battery was calculated as R1 = C1000 ÷ C1 × 100%.

[0251] Storage test:

[0252] The secondary battery was charged at a constant current of 0.33C0 to a voltage of 4.25V at 25°C, then charged at a constant voltage of 4.25V to a current of less than or equal to 0.05C0, then allowed to stand for 10 minutes, and then discharged at a constant current of 0.1C0 to a voltage of 2.5V (the capacity of this step is marked as C2). The secondary battery is then charged at a constant current rate of 0.33C0 to a voltage of 4.25V, and then charged at a constant voltage at a voltage of 4.25V to a current less than or equal to 0.05C0. At this time, the secondary battery is fully charged and is transferred to a 60°C environment for storage for 90 days. The secondary battery is then transferred to a 25°C environment and discharged at a constant current rate of 0.33C0 to a voltage of 2.5V. Finally, the secondary battery is charged at a constant current rate of 0.1C0 to a voltage of 4.25V, and then charged at a constant voltage of 4.25V to a current less than or equal to 0.05C0. After that, it is allowed to stand for 10 minutes, and then discharged at a constant current rate of 0.1C0 to a voltage of 2.5V (the capacity of this step is marked as C90). The storage reversible capacity retention rate of the secondary battery is calculated as R2 = C90 ÷ C2 × 100%.

[0253] The test results are shown in the following table:

[0254] As can be seen from the above table, the solution provided in the embodiments of the present application can improve the problem of deterioration of battery service life caused by cyclic expansion of high energy density materials, so that the battery has better comprehensive life performance. Among them, the comprehensive life performance of Example 1 is improved by 2.2% compared with Comparative Example 1; the comprehensive life performance of Example 2 is improved by 3.8% compared with Comparative Example 3; the comprehensive life performance of Example 3 is improved by 5.1% compared with Comparative Example 5; and the comprehensive life performance of Example 5 is improved by 2.2% compared with Comparative Example 9.

[0255] By comparing the data of Example 1 and Comparative Example 1 and Comparative Example 2, Example 2 and Comparative Example 3 and Comparative Example 4, Example 3 and Comparative Example 5 and Comparative Example 6, Example 4 and Comparative Example 8, and Example 5 and Comparative Example 9, it can be obtained that when the value of ρ1 / ρ2 is above 1.6, the comprehensive life performance of the battery can be significantly improved. Among them, the comprehensive life performance of Example 1 is improved by 2.2% and 2.1% relative to Comparative Example 1 and Comparative Example 2, respectively; the comprehensive life performance of Example 2 is improved by 3.8% and 3.6% relative to Comparative Example 3 and Comparative Example 4, respectively; the comprehensive life performance of Example 3 is improved by 5.1% and 5.0% relative to Comparative Example 5 and Comparative Example 6, respectively; the comprehensive life performance of Example 4 is improved by 3.0% relative to Comparative Example 8; and the comprehensive life performance of Example 5 is improved by 2.2% relative to Comparative Example 9.

[0256] By comparing the data of Example 3 and Comparative Example 7, it can be seen that when the value of ρ1 / ρ2 is below 2.55, it is more conducive to improving the comprehensive life performance of the battery. Among them, the comprehensive life performance of Example 3 is improved by 8.6% compared with Comparative Example 7.

[0257] By comparing the data of Example 3 with those of Example 6 and Example 7, it can be seen that by dividing the second negative electrode active material layer into a first sublayer and a second sublayer, and controlling the volume capacity density of the first sublayer to be smaller than that of the second sublayer, the comprehensive life performance of the battery can be further improved.

[0258] The above are merely specific embodiments of the present application and are not intended to limit the present application. Those skilled in the art will readily appreciate that various modifications and variations are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present application shall be included within the scope of protection of the present application.

Claims

1. A negative electrode sheet, characterized in that the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer, the negative electrode active material layer is provided on at least one surface of the negative electrode current collector, the negative electrode active material layer includes a first negative electrode active material layer and a second negative electrode active material layer, the first negative electrode active material layer is provided between the second negative electrode active material layer and the negative electrode current collector, and the volume capacity density ρ1 of the first negative electrode active material layer and the volume capacity density ρ2 of the second negative electrode active material layer satisfy the following relationship: 1.6≤ρ1 / ρ2≤2.

2. The negative electrode sheet according to claim 1, characterized in that: The volume capacity density ρ1 of the first negative electrode active material layer is 0.65 to 2.6 Ah / cm 3 The volume capacity density ρ2 of the second negative electrode active material layer is 0.4 to 1.3 Ah / cm 3 .

3. The negative electrode sheet according to any one of claims 1 to 2, characterized in that: The negative electrode active materials of the first negative electrode active material layer and the second negative electrode active material layer both include a first active material and a second active material, and the capacity of the first active material is greater than the capacity of the second active material; in the cross-section in the thickness direction of the negative electrode sheet, the relationship between the cross-sectional area ratio N1 of the first active material in the first negative electrode active material layer and the cross-sectional area ratio N2 of the first active material in the second negative electrode active material layer satisfies: N1>N2.

4. The negative electrode sheet according to claim 3, characterized in that: In a cross section along the thickness direction of the negative electrode sheet, a cross-sectional area ratio N1 of the first active material in the first negative electrode active material layer satisfies: 29%≤N1≤83%; and / or In a cross section along the thickness direction of the negative electrode sheet, a cross-sectional area ratio N2 of the first active material in the second negative electrode active material layer satisfies: 0≤N2≤20%.

5. The negative electrode sheet according to any one of claims 1 to 2, characterized in that: The negative electrode active materials of the first negative electrode active material layer and the second negative electrode active material layer each include a first active material and a second active material, the first active material includes a silicon-based material; the second active material includes graphite; and the mass of the first active material satisfies at least one of the following conditions (a1) to (a4): (a1) the mass proportion M1 of the first active material in the first negative electrode active material layer is 30% to 85%; (a2) the mass proportion M1 of the first active material in the first negative electrode active material layer is 50% to 70%; (a3) the mass proportion M2 of the first active material in the second negative electrode active material layer is 0% to 20%; (a4) The mass ratio M2 of the first active material in the second negative electrode active material layer is 3% to 10%.

6. The negative electrode sheet according to any one of claims 1 to 5, characterized in that: The relationship between the thickness H1 of the first negative electrode active material layer and the thickness H2 of the second negative electrode active material layer satisfies: 2H1<H2.

7. The negative electrode sheet according to claim 6, characterized in that: The thickness H1 of the first negative electrode active material layer satisfies: 5 μm≤H1≤20 μm; and / or The thickness H2 of the second negative electrode active material layer satisfies: 20 μm≤H2≤90 μm.

8. The negative electrode sheet according to any one of claims 1 to 7, characterized in that: The average particle size of the negative electrode active material in the first negative electrode active material layer is 1 to 10 μm; and / or The average particle size of the negative electrode active material in the second negative electrode active material layer is 8 to 20 μm.

9. The negative electrode sheet according to any one of claims 1 to 8, characterized in that: The second negative electrode active material layer includes a first sublayer and a second sublayer, the first sublayer is arranged between the second sublayer and the first negative electrode active material layer, and the relationship between the volume capacity density ρ2a of the first sublayer and the volume capacity density ρ2b of the second sublayer satisfies: ρ2a<ρ2b.

10. The negative electrode sheet according to claim 9, characterized in that: The first negative electrode active material layer and the second negative electrode active material layer both include a first active material and a second active material, and the capacity of the first active material is greater than the capacity of the second active material; in the cross-section in the thickness direction of the negative electrode sheet, the relationship between the cross-sectional area ratio N2a of the first active material in the first sublayer and the cross-sectional area ratio N2b of the first active material in the second sublayer satisfies: N2a<N2b.

11. The negative electrode sheet according to claim 9, characterized in that: The negative electrode active materials of the first negative electrode active material layer and the second negative electrode active material layer each include a first active material and a second active material, the capacity of the first active material is greater than the capacity of the second active material; and the mass of the first active material satisfies at least one of the following conditions (b1) to (b2): (b1) the mass proportion M2a of the first active material in the first sublayer is 0% to 5%; (b2) The mass proportion M2b of the first active material in the second sub-layer is 5% to 25%.

12. The negative electrode sheet according to any one of claims 9 to 11, characterized in that: The thickness H2a of the first sub-layer and the thickness H2b of the second sub-layer satisfy the following relationship: 0.5≤H2a / H2b≤2.

13. The negative electrode sheet according to any one of claims 1 to 12, characterized in that: The first negative electrode active material layer includes a flexible binder, and the flexible binder satisfies at least one of the following conditions (c1) to (c3): (c1) the glass transition temperature of the flexible adhesive is lower than 25°C; (c2) the flexible binder has a mass content of 5% to 20% in the first negative electrode active material layer; (c3) The flexible adhesive comprises styrene-butadiene rubber.

14. The negative electrode sheet according to any one of claims 1 to 13, characterized in that: The first negative electrode active material layer includes carbon nanotubes, and the carbon nanotubes meet at least one of the following conditions (d1) to (d2): (d1) the aspect ratio of the carbon nanotubes is 1000 to 10000; (d2) The carbon nanotubes account for 0.05% to 1.2% by mass in the first negative electrode active material layer.

15. The negative electrode sheet according to claim 14, characterized in that: The first negative electrode active material layer includes a surfactant, and the surfactant accounts for 1% to 5% by mass in the first negative electrode active material layer.

16. The negative electrode sheet according to any one of claims 1 to 15, characterized in that: The mass area density of the negative electrode active material layer is 5.19 to 14.26 mg / cm 2 and / or The compaction density of the negative electrode active material layer is 1.4 to 1.85 g / cm 3 .

17. A negative electrode sheet, characterized in that: The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer, wherein the negative electrode active material layer is provided on at least one surface of the negative electrode current collector, and the negative electrode active material layer includes a first negative electrode active material layer and a second negative electrode active material layer, wherein the first negative electrode active material layer is provided between the second negative electrode active material layer and the negative electrode current collector, and the volume capacity density ρ1 of the first negative electrode active material layer and the volume capacity density ρ2 of the second negative electrode active material layer satisfy the following relationship: 1.6≤ρ1 / ρ2≤2, the relationship between the thickness H1 of the first negative electrode active material layer and the thickness H2 of the second negative electrode active material layer satisfies: 2H1<H2; the second negative electrode active material layer includes a first sublayer and a second sublayer, the first sublayer is arranged between the second sublayer and the first negative electrode active material layer, the relationship between the volume capacity density ρ2a of the first sublayer and the volume capacity density ρ2b of the second sublayer satisfies: ρ2a<ρ2b, the thickness H2a of the first sublayer and the thickness H2b of the second sublayer satisfy the following relationship: 0.5≤H2a / H2b≤2.

18. A battery cell, characterized in that: The battery cell includes the negative electrode sheet according to any one of claims 1 to 17.

19. A battery, characterized in that: The battery comprises the battery cell according to claim 18 .

20. An electrical device, characterized in that: The electrical device comprises the battery cell according to claim 18 or the battery according to claim 19.

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