Secondary battery and preparation method therefor, negative electrode sheet and preparation method therefor, and electrical device

Abstract

The present application relates to the technical field of batteries, and particularly relates to a secondary battery and a preparation method therefor, a negative electrode sheet and a preparation method therefor, and an electrical device. The secondary battery comprises a positive electrode sheet, a negative electrode sheet, and a separator. The negative electrode sheet comprises a negative electrode current collector and a buffer layer disposed on at least one surface of the negative electrode current collector. The buffer layer is disposed on at least part of the area of one surface of the negative electrode current collector. The buffer layer can provide pressure to the negative electrode sheet inside the secondary battery, thereby facilitating dense metal deposition on the negative electrode side during cycling of the secondary battery. Furthermore, the buffer layer can provide deposition space for the deposited layer during metal deposition, thereby improving the cycling performance of the battery.

Description

Secondary batteries and their preparation methods, negative electrode sheets and their preparation methods, and electrical devices.

[0001] Related applications

[0002] This application claims priority to Chinese patent application No. 202411599086.4, filed on November 8, 2024, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This application relates to the field of battery technology, and in particular to a secondary battery and its preparation method, a negative electrode sheet and its preparation method, and an electrical device. Background Technology

[0004] Batteries play a vital role as indispensable energy storage and conversion devices in modern life.

[0005] Insufficient pressure inside the battery cell during battery cycling can lead to a decline in battery cycle performance. Summary of the Invention

[0006] The main objective of this application is to provide a secondary battery that aims to improve the cycle performance of the secondary battery.

[0007] To achieve the above objectives, this application proposes a secondary battery, which includes a positive electrode, a negative electrode, and a separator.

[0008] The negative electrode sheet includes a negative current collector and a buffer layer disposed on at least one surface of the negative current collector, wherein the buffer layer is disposed on at least a portion of the surface of the negative current collector.

[0009] The negative electrode sheet of this application includes a negative current collector and a buffer layer disposed on at least one surface of the negative current collector. The buffer layer is disposed on at least a portion of the surface of the negative current collector, which helps to make the metal deposition on the negative side dense during the secondary battery cycle. During the metal deposition process, the buffer layer can provide deposition space for the deposition layer, thereby providing expansion space for the expansion of the negative electrode and improving the cycle performance of the battery.

[0010] Understandably, the buffer layer applies pressure to the negative electrode plate inside the cell during secondary battery cycling, which helps to denser metal deposition on the negative electrode side. Taking lithium metal batteries as an example, dense lithium deposition on the negative electrode side reduces the overall specific surface area of ​​the lithium deposition layer, which helps to reduce side reactions between the lithium deposition layer and the electrolyte, and reduces electrolyte consumption. Furthermore, the elasticity of the buffer layer also helps to provide space for metal deposition on the negative electrode and provides expansion space for the expansion of the negative electrode. At the same time, during discharge, the pressure on the negative electrode plate can reduce interfacial impedance, which helps to uniformly dissolve lithium, reduce lithium ion concentration polarization, reduce battery polarization, reduce battery capacity decay, and thus improve battery cycle performance.

[0011] Optionally, the buffer layer includes at least two spaced buffer bodies located on either side of the center point of one surface of the negative electrode current collector.

[0012] The buffer layer includes at least two spaced buffer bodies located on either side of the center point of one surface of the negative electrode current collector. This helps to improve the stability of the buffer layer in supporting the electrode, which in turn helps to improve the uniformity of stress distribution on the negative electrode and thus contributes to the uniformity of metal deposition.

[0013] Optionally, the two buffer bodies are symmetrically distributed along the center point of one surface of the negative electrode current collector.

[0014] The two buffer bodies are symmetrically distributed along the center point of one surface of the negative electrode current collector, which helps to improve the stability of the buffer layer supporting the electrode, improves the uniformity of stress distribution on the negative electrode, and thus helps to improve the uniformity of metal deposition.

[0015] Optionally, the two buffer bodies have the same area.

[0016] Having two buffer bodies with the same area helps improve the stability of the buffer layer in supporting the electrode, helps the buffer layer to distribute the compression and rebound force evenly, reduces stress concentration, and makes the metal deposition uniform.

[0017] Optionally, the two buffer bodies are located on opposite sides of one surface of the negative electrode current collector.

[0018] The two buffer bodies are located on opposite sides of one surface of the negative electrode current collector, which helps to distribute the stress evenly during compression and rebound of the buffer layer, reduce stress concentration, and make the metal deposition uniform.

[0019] In addition, the two buffers are located on opposite sides of one surface of the negative electrode current collector, which helps to deposit metal in the negative electrode current collector near the middle. The buffers on both sides help to limit the deposited metal, restrict the drift of pulverized lithium, and confine the pulverized lithium between the two sides of the buffer, reducing the pulverized lithium from sticking to the tab and reducing the risk of battery short circuit.

[0020] Optionally, the lengths of the two buffer bodies are the same as the length of the negative electrode current collector.

[0021] Understandably, in a structure where the negative electrode current collector has long and short sides, the buffer can be located on either of the two opposite long sides or the two opposite short sides. When located on the two opposite long sides, the length of the buffer is the same as the length of the long side of the negative electrode current collector; when located on the two opposite short sides, the length of the buffer is the same as the length of the short side of the negative electrode current collector. This helps to confine the deposited metal, restrict the drift of pulverized lithium, and limit the pulverized lithium to the two sides of the buffer, reducing the risk of pulverized lithium overlapping with the tab and reducing the risk of battery short circuit.

[0022] Optionally, the width of the two buffer bodies accounts for 3%-50% of the width of the negative electrode current collector.

[0023] When the two buffer bodies are located on opposite sides of the negative electrode current collector, the width of the two buffer bodies accounts for 3%-50% of the width of the negative electrode current collector. Meeting the above range helps the buffer layer to provide stretchable elasticity, that is, the buffer layer provides support for compression and rebound between the electrodes, provides pressure for the cell during the cycle, improves the uniformity of metal deposition and dissolution, and thus improves the cycle performance of the battery.

[0024] Optionally, the elastic stress of the buffer layer is 2MPa-6.5MPa.

[0025] The elastic stress of the buffer layer is 2MPa-6.5MPa, which means that the buffer layer has the ability to resist elastic deformation under the extrusion pressure inside the battery. In other words, the elastic stress of the buffer layer meets the above range, which helps to maintain the stable compression and rebound ability of the buffer layer and reduce the problem of elastic expansion and contraction failure of the buffer layer.

[0026] Optionally, the area of ​​the buffer layer on one surface of the negative electrode current collector accounts for 3%-50% of the area of ​​one surface of the negative electrode current collector.

[0027] In a scheme where a buffer layer is provided on a portion of the surface of at least one of the negative electrode current collectors, metal can be deposited on other areas of that surface where no buffer layer is provided. In this case, the area of ​​the buffer layer accounts for 3%-50% of the area of ​​one surface of the negative electrode current collector. It is understood that meeting the above range helps the buffer layer provide stretchable elasticity, that is, the buffer layer provides support for compression and rebound between the electrodes, provides pressure for the cell during cycling, improves the uniformity of metal deposition and dissolution, and thus improves the cycle performance of the battery. Of course, meeting the upper limit of the above range helps to reduce the battery volume and reduce the risk of a decrease in the volumetric energy density of the battery. It is understood that it is difficult to further improve the cycle performance of the battery by continuing to increase the coverage area of ​​the buffer layer on the negative electrode current collector, and further increasing the area will increase the battery size and reduce the volumetric energy density. Therefore, meeting the above range helps to improve the cycle performance and volumetric energy density of the battery.

[0028] Optionally, the material of the buffer layer includes at least one of polypropylene, polyethylene, and polyimide;

[0029] And / or, the buffer layer includes a porous structure.

[0030] In one embodiment, the buffer layer is made of at least one of polypropylene, polyethylene, and polyimide. In another embodiment, the buffer layer has a porous structure that facilitates the absorption of electrolyte by the buffer layer, and the absorbed electrolyte is released during charge and discharge, thereby improving electrolyte wetting during battery cycling.

[0031] Optionally, the positive electrode includes a positive current collector and a positive active layer disposed on at least one surface of the positive current collector;

[0032] The negative electrode current collector includes a buffer zone and an active material zone on the same surface, with the buffer layer located in the buffer zone and the projection of the positive electrode active layer located in the active material zone.

[0033] The positive electrode includes a positive current collector and a positive active layer disposed on at least one surface of the positive current collector; the same surface of the negative current collector includes a buffer zone and an active material zone, with the buffer layer located in the buffer zone and the projection of the positive active layer located in the active material zone, thus facilitating the effective deposition of active ions from the positive electrode side to the active material zone from the negative electrode side.

[0034] Furthermore, the projection of the positive electrode active layer is located in the active material region of the negative electrode, reducing the deposition of active ions on the buffer layer from the positive electrode side. When the buffer layer is located on both sides of the negative electrode current collector, it helps to limit the deposited metal in the middle of the negative electrode current collector, which can restrict the drift of pulverized lithium to a certain extent, confining the pulverized lithium between the two sides of the buffer layer and reducing the risk of short circuit. It can be understood that what is deposited on the negative electrode current collector is lithium, and what is lost from electrical connection with the negative electrode current collector is dead lithium / pulverized lithium. Pulverized lithium is in a free state inside the cell, and when it comes into contact with the positive and negative electrode tabs, it will cause a short circuit. The above-mentioned projection of the positive electrode active layer in the active material region helps to reduce the risk of battery short circuit.

[0035] Optionally, the region of the positive current collector corresponding to the buffer layer is provided with an insulating layer.

[0036] The insulation layer and the buffer layer are positioned opposite each other, which can improve the support of the insulation layer for the buffer layer in the battery and help the buffer layer to undergo elastic deformation.

[0037] Optionally, at least one surface of the negative electrode current collector is provided with an active material replenishment layer.

[0038] The active material replenishment layer is used to replenish the lost active material. For example, in a lithium metal battery, the active material replenishment layer is a lithium replenishment layer. The lithium replenishment layer can replenish the lithium consumption per cycle because during the deposition and dissolution process of lithium metal, an SEI (Solid Electrolyte Interphase) is formed each time. The lithium replenishment layer can replenish the lithium consumed during the formation of the SEI and improve the cycle performance of the battery.

[0039] Optionally, the same surface of the negative electrode current collector includes a buffer zone and an active material zone, with the buffer layer located in the buffer zone and the active material replenishment layer located in the active material zone.

[0040] The same surface of the negative electrode current collector includes a buffer zone and an active material zone, with the buffer layer located in the buffer zone and the active material replenishment layer located in the active material zone. For example, in a lithium metal battery, the active material replenishment layer is a lithium replenishment layer, which facilitates metal deposition on the surface of the lithium replenishment layer.

[0041] Optionally, the thickness L of the buffer layer is greater than the thickness T of the active substance replenishment layer.

[0042] The thickness L of the buffer layer refers to its thickness in its natural state, that is, its thickness when not compressed. During battery assembly, the buffer layer is compressed to the same thickness as the active material replenishment layer, giving it an expansion and rebound force that compresses the negative electrode sheet. When the thickness L of the buffer layer is greater than the thickness T of the active material replenishment layer, after compressing the buffer layer to the same thickness as the active material replenishment layer, the buffer layer also exhibits an expansion and rebound force, compressing the negative electrode sheet.

[0043] For example, taking lithium metal batteries as an example, during battery assembly, the buffer layer is compressed to the same thickness as the lithium replenishment layer. During the lithium metal deposition process, the lithium metal layer on the negative electrode side becomes thicker, and the buffer layer expands and rebounds, providing space for the deposition of lithium metal. After the lithium metal deposition layer disappears, the thickness of the buffer layer is compressed again. In this way, the buffer layer is repeatedly compressed and is always in a compressed state, constantly squeezing the negative electrode sheet, so that the lithium metal battery always has a certain pressure during charging and discharging, maintaining dense lithium deposition on the negative electrode side.

[0044] Optionally, the relationship between the thickness L of the buffer layer and the thickness T of the active substance replenishment layer is: 1.5T≤L≤1.64T.

[0045] The thickness L of the buffer layer and the thickness T of the active material replenishment layer satisfy the above relationship, which helps the buffer layer to be in a compressed state. During the battery charging and discharging process, the buffer layer provides pressure to the negative electrode sheet, which helps to deposit metal uniformly.

[0046] Optionally, the thickness of the active substance replenishment layer is 2μm-100μm;

[0047] And / or, the thickness of the buffer layer is 10μm-200μm.

[0048] The thickness of the active material replenishment layer meets the above-mentioned range and can compensate for the consumption of active material during battery cycling. It is understood that as the battery cycles, the active material will gradually be consumed. To improve the battery's cycle performance, this solution adds an excessive amount of active material replenishment layer; that is, the active material replenishment layer is maintained throughout the entire battery cycle. Thus, when the buffer layer is located on opposite sides of the negative electrode current collector, the active material replenishment layer is located in the middle of the negative electrode current collector. The active material replenishment layer has a certain thickness, and during the expansion and contraction of the buffer layer, it can provide support between the electrodes, reducing the problem of uneven stress on the negative electrode.

[0049] The thickness of the buffer layer, within the aforementioned range, helps it provide a certain amount of compression and expansion, allowing it to provide pressure to the negative electrode during battery cycling. For example, during battery assembly, the buffer layer is initially compressed within the battery. As metal is deposited on the negative electrode side, the thickness of the metal layer on the negative electrode side increases, causing the initially compressed buffer layer to expand. As the metal on the negative electrode side dissolves, the thickness of the metal layer on the negative electrode side decreases, and the expanded buffer layer is compressed again. In this way, it provides pressure to the negative electrode during battery cycling.

[0050] Optionally, a negative electrode tab is connected to the negative electrode current collector, and the buffer layer is close to the negative electrode tab.

[0051] The negative electrode current collector is connected to the negative electrode tab, and the buffer layer is close to the negative electrode tab, which can reduce the contact between the negative electrode tab and the negative electrode metal layer and reduce the risk of battery short circuit.

[0052] Optionally, the diaphragm includes a base membrane and a coating disposed on the surface of the base membrane, the coating including an adhesive and / or ceramic particles;

[0053] And / or, the secondary battery includes stacked batteries, square batteries, cylindrical batteries, and pouch batteries.

[0054] To ensure good contact between the buffer layer and the diaphragm during elastic expansion and contraction, and to ensure the diaphragm's ability to resist elastic forces, the diaphragm includes a base membrane and a coating on the surface of the base membrane. The coating includes an adhesive and / or ceramic particles. The adhesive provides good adhesion between the diaphragm and the buffer layer, reducing displacement of the buffer layer and the diaphragm during expansion and contraction. The ceramic particles can improve the mechanical strength of the diaphragm, giving it better strength to resist the elastic stress of the buffer layer.

[0055] The secondary batteries in this application include stacked batteries, prismatic batteries, cylindrical batteries, and pouch batteries.

[0056] Optionally, this application also provides a negative electrode sheet, the negative electrode sheet including a negative current collector and a buffer layer disposed on at least one surface of the negative current collector, wherein the buffer layer is disposed on at least a portion of one surface of the negative current collector.

[0057] Optionally, the buffer layer includes at least two spaced buffer bodies, which are symmetrically distributed along the center point of one surface of the negative electrode current collector.

[0058] Optionally, the width of the two buffer bodies accounts for 3%-50% of the width of the negative electrode current collector.

[0059] Optionally, the elastic stress of the buffer layer is 2MPa-6.5MPa.

[0060] Optionally, at least one surface of the negative electrode current collector is provided with an active material replenishment layer.

[0061] Optionally, the same surface of the negative electrode current collector includes a buffer zone and an active material zone, the buffer layer is located in the buffer zone, and the active material replenishment layer is located in the active material zone;

[0062] The thickness L of the buffer layer is greater than the thickness T of the active substance replenishment layer.

[0063] Optionally, the relationship between the thickness L of the buffer layer and the thickness T of the active substance replenishment layer is: 1.5T≤L≤1.64T.

[0064] Optionally, the thickness of the active substance replenishment layer is 2μm-100μm;

[0065] And / or, the thickness of the buffer layer is 10μm-200μm.

[0066] Optionally, a negative electrode tab is connected to the negative electrode current collector, and the buffer layer is close to the negative electrode tab.

[0067] Optionally, this application also provides a method for preparing a secondary battery, comprising:

[0068] Prepare a positive electrode, a negative electrode, and a separator. The negative electrode includes a negative current collector and a buffer layer disposed on at least one surface of the negative current collector.

[0069] The positive electrode, the separator, and the negative electrode are alternately stacked or wound, and the separator is used to wind the outermost part to obtain a battery cell;

[0070] A clamp is placed on the outside of the battery cell to compress the battery cell. The compression of the buffer layer is 38% to 80%, resulting in a secondary battery.

[0071] In the secondary battery manufacturing process, after obtaining the cell, a clamp is set on the outside of the cell to compress it. The compression of the buffer layer is 38% to 80%. This helps the buffer layer to always be in a compressed state in the battery. The compressed buffer layer in the battery has a rebound force that can squeeze the negative electrode sheet. At the same time, the compression of the buffer layer is 38% to 80%, so that after metal is deposited on the negative electrode side, the buffer layer will rebound to a certain extent. After rebounding, the buffer layer is still in a compressed state and still exerts a certain amount of pressure on the negative electrode sheet. This helps the buffer layer to continuously squeeze the negative electrode sheet during battery cycling, which helps the dense deposition of metal and improves the cycle performance of the battery.

[0072] Optionally, the same surface of the negative electrode current collector includes a buffer zone and an active material zone, the buffer layer is located in the buffer zone, and the active material zone is provided with an active material replenishment layer;

[0073] The step of setting a clamp on the outside of the battery cell to compress the battery cell includes compressing the buffer layer to the same thickness as the active material replenishment layer.

[0074] After setting a buffer layer and an active material replenishment layer on the same surface of the negative electrode current collector, when compressing the cell, the buffer layer is compressed to the same thickness as the active material replenishment layer. This helps the buffer layer to remain compressed after the metal is deposited on the negative electrode side and the buffer layer rebounds.

[0075] Optionally, the compression of the buffer layer is 50%-64%.

[0076] The compression of the buffer layer meets the above range, which helps the buffer layer to be in a compressed state during battery cycling, and also reduces the problem of the buffer layer overflowing into the metal deposition area when the compression of the buffer layer is too large.

[0077] Optionally, this application also provides a method for preparing a negative electrode sheet, comprising:

[0078] A buffer body is provided on opposite sides of at least one surface of the foil, and the buffer body on at least one side is provided intermittently, forming a negative electrode tab in the intermittent region;

[0079] The negative current collector is cut from the negative current collector foil according to the preset size of the negative current collector. Buffer bodies are provided on opposite sides of one surface of the negative current collector, and the negative current collector tab is connected to the negative current collector.

[0080] In order to simultaneously prepare the negative electrode tab during the preparation of the negative electrode sheet, the negative electrode tab is close to the buffer layer, and buffer bodies are provided on opposite sides of at least one surface of the foil, with the buffer body on at least one side being intermittently provided, forming the negative electrode tab in the intermittent area. The negative electrode current collector is cut out on the foil according to the preset size of the negative electrode current collector, and buffer bodies are provided on opposite sides of the negative electrode current collector, with the negative electrode tab connected to the negative electrode current collector.

[0081] Optionally, the step of providing a buffer layer on opposite sides of at least one surface of the foil includes:

[0082] An active material replenishment layer and two spaced buffer bodies are provided on at least one surface of the foil. The two buffer bodies are located on opposite sides of the active material replenishment layer. The thickness of the buffer bodies is 10 μm to 200 μm, and the thickness of the active material replenishment layer is 5 μm to 100 μm.

[0083] Placing the active material replenishment layer between the two buffer bodies helps to limit the pulverized lithium during battery cycling, confining the pulverized lithium between the two sides of the buffer body, reducing the pulverized lithium from sticking to the tabs, and reducing the risk of battery short circuit.

[0084] To compress the buffer layer, the thickness of the buffer body is greater than the thickness of the active material replenishment layer. The thickness of the buffer body is 10μm to 200μm, and the thickness of the active material replenishment layer is 5μm to 100μm. During the battery manufacturing process, compressing the buffer body to the same thickness as the active material replenishment layer helps the buffer layer to always be in a compressed state, constantly squeezing the negative electrode sheet, so that the lithium metal battery always has a certain pressure during charging and discharging, maintaining dense lithium deposition on the negative electrode side.

[0085] Optionally, this application also provides an electrical device, which includes a secondary battery as described above.

[0086] The negative electrode sheet of this application includes a negative current collector and a buffer layer disposed on at least one surface of the negative current collector. The buffer layer is disposed on at least a portion of the surface of the negative current collector. The buffer layer can provide pressure to the negative electrode sheet inside the secondary battery, which helps to make the metal deposition on the negative side dense during the cycle of the secondary battery. During the metal deposition process, the buffer layer can provide deposition space for the deposition layer and improve the cycle performance of the battery.

[0087] Understandably, the buffer layer applies pressure to the negative electrode plate inside the cell during secondary battery cycling, which helps to denser metal deposition on the negative electrode side. Taking lithium metal batteries as an example, dense lithium deposition on the negative electrode side reduces the overall specific surface area of ​​the lithium deposition layer, which helps to reduce side reactions between the lithium deposition layer and the electrolyte, and reduce electrolyte consumption. Furthermore, the elasticity of the buffer layer also helps to provide space for metal deposition on the negative electrode. At the same time, during discharge, the pressure on the negative electrode plate can reduce interfacial impedance, which helps to uniformly dissolve lithium, reduce lithium ion concentration polarization, reduce battery polarization, reduce battery capacity decay, and thus improve battery cycle performance. Attached Figure Description

[0088] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0089] Figure 1 is a schematic diagram of the structure of a negative electrode sheet according to an embodiment of this application;

[0090] Figure 2 is a schematic diagram of the negative electrode sheet according to another embodiment of this application;

[0091] Figure 3 is a schematic diagram of the negative electrode current collector according to an embodiment of this application;

[0092] Figure 4 is a schematic diagram of the structure of the positive electrode and the negative electrode according to an embodiment of this application;

[0093] Figure 5 is a schematic diagram of the structure of the positive electrode and the negative electrode according to another embodiment of this application;

[0094] Figure 6 is a schematic diagram of the negative electrode sheet in another embodiment;

[0095] Figure 7 is a schematic diagram of the foil material structure during the preparation of the negative electrode sheet in this application;

[0096] Figure 8 is a schematic diagram of a battery cell according to an embodiment of this application;

[0097] Figure 9 is an exploded view of a battery cell according to an embodiment of this application shown in Figure 8;

[0098] Figure 10 is a schematic diagram of a battery module according to an embodiment of this application;

[0099] Figure 11 is a schematic diagram of a battery pack according to an embodiment of this application;

[0100] Figure 12 is an exploded view of a battery pack according to an embodiment of this application, as shown in Figure 11;

[0101] Figure 13 is a schematic diagram of an electrical device in which a single battery cell is used as a power source according to an embodiment of this application.

[0102] Explanation of icon numbers:

[0103] The realization of the purpose, functional features and advantages of this application will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0104] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of the embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0105] The following detailed description, with appropriate reference to the accompanying drawings, discloses the secondary battery and its preparation method, the negative electrode sheet and its preparation method, and the electrical device thereof. However, unnecessary details may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0106] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0107] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0108] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0109] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0110] Insufficient pressure inside the battery cell during battery cycling can lead to a decline in battery cycle performance.

[0111] For example, taking lithium metal batteries as an example, a certain pressure is required inside the cell during the charging and discharging process. If the pressure is insufficient, the lithium metal deposition will be loose, resulting in a large specific surface area of ​​lithium metal deposition and more side reactions, which will affect the cycle performance of the battery.

[0112] To address the aforementioned issues, this application proposes a secondary battery comprising a positive electrode, a negative electrode, and a separator; the negative electrode comprises a negative current collector and a buffer layer disposed on at least one surface of the negative current collector, wherein at least a portion of the surface of the negative current collector is provided with the buffer layer.

[0113] The buffer layer is compressible and can rebound after the compressive force is released.

[0114] The test method for the percentage W of the buffer layer area on one surface of the negative electrode current collector to the total area of ​​the negative electrode current collector is as follows: disassemble the battery to obtain the negative electrode sheet, measure the area S of one surface of the negative electrode current collector, and measure the area s of the buffer layer on that side surface. Then W = s / S × 100%.

[0115] The fact that a buffer layer is provided on at least a portion of the surface of the negative electrode current collector means that a buffer layer can be provided on a portion of the surface of the negative electrode current collector or on the entire surface.

[0116] The negative electrode sheet of this application includes a negative electrode current collector and a buffer layer disposed on at least one surface of the negative electrode current collector. The buffer layer is disposed on at least a portion of the surface of the negative electrode current collector. The buffer layer can provide pressure to the negative electrode sheet inside the secondary battery, which helps to make the metal deposition on the negative electrode side dense during the cycle of the secondary battery. During the metal deposition process, the buffer layer can provide deposition space for the deposition layer, thereby providing expansion space for the expansion of the negative electrode and improving the cycle performance of the battery.

[0117] Understandably, the buffer layer applies pressure to the negative electrode plate inside the cell during secondary battery cycling, which helps to denser metal deposition on the negative electrode side. Taking lithium metal batteries as an example, dense lithium deposition on the negative electrode side reduces the overall specific surface area of ​​the lithium deposition layer, which helps to reduce side reactions between the lithium deposition layer and the electrolyte, and reduces electrolyte consumption. Furthermore, the elasticity of the buffer layer also helps to provide space for metal deposition on the negative electrode and provides expansion space for the expansion of the negative electrode. At the same time, during discharge, the pressure on the negative electrode plate can reduce interfacial impedance, which helps to uniformly dissolve lithium, reduce lithium ion concentration polarization, reduce battery polarization, reduce battery capacity decay, and thus improve battery cycle performance.

[0118] Furthermore, in general, the volume expansion caused by lithium deposition-stripping during cycling usually requires external buffer structures such as elastic pads and springs to provide expansion space and pressure. On the one hand, these additional devices increase the complexity of the system application and limit its scope of application. On the other hand, they reduce the energy density of the system and weaken the high energy density advantage of the lithium metal cell itself.

[0119] By adopting the solution of this application, a buffer layer is directly set on the negative electrode sheet, which can reduce the use of buffer structures outside the battery cell, reduce the setting of these additional devices, and improve the energy density of the battery.

[0120] It is understood that at least a portion of the surface of the negative electrode current collector is provided with a buffer layer. That is, the buffer layer can be provided on all of the surface of at least one negative electrode current collector. In this case, the area of ​​the buffer layer on at least one surface of the negative electrode current collector accounts for 100% of the area of ​​the surface of the negative electrode current collector. As shown in Figure 1, the surface of the negative electrode current collector 10 is provided with a buffer layer 20. Taking a lithium metal battery as an example, during the lithium metal deposition process, lithium metal can be deposited on the surface of the buffer layer 20.

[0121] Of course, a buffer layer can be partially provided on at least one surface of the negative electrode current collector. In this case, the area of ​​the buffer layer on at least one surface of the negative electrode current collector accounts for more than 3% and less than 100% of the area of ​​one surface of the negative electrode current collector. As shown in Figure 2, a buffer layer 20 is provided on part of the surface of the negative electrode current collector 10. Taking a lithium metal battery as an example, during the lithium metal deposition process, lithium metal can be deposited on the surface of the buffer layer 20, or deposited on the part of the negative electrode current collector where no buffer layer is provided.

[0122] In one embodiment, the buffer layer includes at least two spaced buffer bodies located on either side of the center point of one surface of the negative electrode current collector.

[0123] The center point of one surface of the negative electrode current collector refers to the geometric center point of one surface of the negative electrode current collector. As shown in Figure 3, the intersection of the two diagonal lines in the negative electrode current collector 10 is the center point 30 of the negative electrode current collector.

[0124] The buffer layer includes at least two spaced buffer bodies located on either side of the center point of one surface of the negative electrode current collector. This helps improve the stability of the buffer layer in supporting the electrode sheet, which in turn improves the uniformity of stress distribution on the negative electrode sheet and thus contributes to the uniformity of metal deposition. As shown in Figure 2, two buffer bodies 201 are provided on either side of the center point of the surface of the negative electrode current collector 10.

[0125] In one embodiment, the two buffer bodies are symmetrically distributed along the center point of one surface of the negative electrode current collector.

[0126] The two buffer bodies are symmetrically distributed along the center point of one surface of the negative electrode current collector, which helps to improve the stability of the buffer layer supporting the electrode, improves the uniformity of stress distribution on the negative electrode, and thus helps to improve the uniformity of metal deposition.

[0127] In one embodiment, the two buffer bodies have the same area.

[0128] Having two buffer bodies with the same area helps improve the stability of the buffer layer in supporting the electrode, helps the buffer layer to distribute the compression and rebound force evenly, reduces stress concentration, and makes the metal deposition uniform.

[0129] In one embodiment, the two buffer bodies are located on opposite sides of one surface of the negative electrode current collector.

[0130] The two buffer bodies are located on opposite sides of one surface of the negative electrode current collector, which helps to distribute the stress evenly during compression and rebound of the buffer layer, reduce stress concentration, and make the metal deposition uniform.

[0131] In addition, the two buffers are located on opposite sides of one surface of the negative electrode current collector, which helps to deposit metal in the negative electrode current collector near the middle. The buffers on both sides help to limit the deposited metal, restrict the drift of pulverized lithium, and confine the pulverized lithium between the two sides of the buffer, reducing the pulverized lithium from sticking to the tab and reducing the risk of battery short circuit.

[0132] In one embodiment, the lengths of the two buffer bodies are the same as the length of the negative electrode current collector.

[0133] Understandably, in a structure where the negative electrode current collector has long and short sides, the buffer can be located on either of the two opposite long sides or the two opposite short sides. When located on the two opposite long sides, the length of the buffer is the same as the length of the long side of the negative electrode current collector; when located on the two opposite short sides, the length of the buffer is the same as the length of the short side of the negative electrode current collector. This helps to confine the deposited metal, restrict the drift of pulverized lithium, and limit the pulverized lithium to the two sides of the buffer, reducing the risk of pulverized lithium overlapping with the tab and reducing the risk of battery short circuit.

[0134] In one embodiment, the width of the two buffer bodies accounts for 3%-50% of the width of the negative electrode current collector.

[0135] When the two buffer bodies are located on opposite sides of the negative electrode current collector, the width of the two buffer bodies accounts for 3%-50% of the width of the negative electrode current collector. Meeting the above range helps the buffer layer to provide stretchable elasticity, that is, the buffer layer provides support for compression and rebound between the electrodes, provides pressure for the cell during the cycle, improves the uniformity of metal deposition and dissolution, and thus improves the cycle performance of the battery.

[0136] The values ​​in the range of 3%-50% include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, as well as 3%, 5%, 10%, 25%, 50%, etc., and the range values ​​between any two of the above point values.

[0137] In one embodiment, the elastic stress of the buffer layer is 2MPa-6.5MPa.

[0138] Elastic stress refers to the internal stress generated in a material when it is subjected to external forces.

[0139] The testing procedure for the elastic stress of a buffer layer is as follows: Prepare the buffer layer, and determine its stress by compressing it. For example, the static method involves applying a known static force to the material sample and then measuring the resulting deformation to determine the elastic stress. Alternatively, a universal testing machine can be used to compress the sample, and the stress data of the buffer layer can be recorded.

[0140] The elastic stress of the buffer layer is 2MPa-6.5MPa, which means that the buffer layer has the ability to resist elastic deformation under the extrusion pressure inside the battery. In other words, the elastic stress of the buffer layer meets the above range, which helps to maintain the stable compression and rebound ability of the buffer layer and reduce the problem of elastic expansion and contraction failure of the buffer layer.

[0141] The values ​​in the range of 2MPa-6.5MPa include the minimum and maximum values ​​of this range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, as well as 2MPa, 3MPa, 4MPa, 5MPa, 6MPa, 6.5MPa, etc., and the range values ​​between any two of the above point values.

[0142] In one embodiment, the area of ​​the buffer layer on one surface of the negative electrode current collector accounts for 3%-50% of the area of ​​one surface of the negative electrode current collector.

[0143] In a scheme where a buffer layer is provided on a portion of the surface of at least one of the negative electrode current collectors, metal can be deposited on other areas of that surface where no buffer layer is provided. In this case, the area of ​​the buffer layer accounts for 3%-50% of the area of ​​one surface of the negative electrode current collector. It is understood that meeting the above range helps the buffer layer provide stretchable elasticity, that is, the buffer layer provides support for compression and rebound between the electrodes, provides pressure for the cell during cycling, improves the uniformity of metal deposition and dissolution, and thus improves the cycle performance of the battery. Of course, meeting the upper limit of the above range helps to reduce the battery volume and reduce the risk of a decrease in the volumetric energy density of the battery. It is understood that it is difficult to further improve the cycle performance of the battery by continuing to increase the coverage area of ​​the buffer layer on the negative electrode current collector, and further increasing the area will increase the battery size and reduce the volumetric energy density. Therefore, meeting the above range helps to improve the cycle performance and volumetric energy density of the battery.

[0144] For example, as shown in Figure 2, taking a lithium metal battery as an example, during the lithium metal deposition process, lithium metal can be deposited in the area between the two buffer bodies 201.

[0145] It is understandable that with the external buffer structure of the battery cell, direct contact and compression between the internal electrodes can easily increase localized stress unevenness, leading to increased polarization and accelerated capacity decay, especially when there is uneven electrode surface thickness. In this application, metal can be deposited on other areas of the current collector surface where no buffer layer is provided. The buffer layer mainly serves to compress adjacent electrodes. During the compression process, the thickness change of the buffer layer is relatively consistent, making it difficult for uneven buffer layer thickness to occur. Providing a buffer layer on the negative electrode can reduce the problem of uneven stress distribution caused by direct contact and compression of the negative electrode, reduce increased polarization, and decrease battery capacity decay.

[0146] The values ​​in the range of 3%-50% include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, as well as 3%, 5%, 10%, 25%, 50%, etc., and the range values ​​between any two of the above point values.

[0147] In one embodiment, the material of the buffer layer includes at least one of polypropylene, polyethylene, and polyimide.

[0148] In one embodiment, the buffer layer includes a porous structure.

[0149] In one embodiment, the buffer layer is made of at least one of polypropylene, polyethylene, and polyimide. In another embodiment, the buffer layer has a porous structure that facilitates the absorption of electrolyte by the buffer layer, and the absorbed electrolyte is released during charge and discharge, thereby improving electrolyte wetting during battery cycling.

[0150] In one embodiment, the positive electrode sheet includes a positive current collector and a positive active layer disposed on at least one surface of the positive current collector; the same surface of the negative current collector includes a buffer zone and an active material zone, the buffer layer is located in the buffer zone, and the projection of the positive active layer is located in the active material zone.

[0151] The positive electrode includes a positive current collector and a positive active layer disposed on at least one surface of the positive current collector; the same surface of the negative current collector includes a buffer zone and an active material zone, with the buffer layer located in the buffer zone and the projection of the positive active layer located in the active material zone, thus facilitating the effective deposition of active ions from the positive electrode side to the active material zone from the negative electrode side.

[0152] Furthermore, the projection of the positive electrode active layer is located in the active material region of the negative electrode, reducing the deposition of active ions on the buffer layer from the positive electrode side. When the buffer layer is located on both sides of the negative electrode current collector, it helps to limit the deposited metal in the middle of the negative electrode current collector, which can restrict the drift of pulverized lithium to a certain extent, confining the pulverized lithium between the two sides of the buffer layer and reducing the risk of short circuit. It can be understood that what is deposited on the negative electrode current collector is lithium, and what is lost from electrical connection with the negative electrode current collector is dead lithium / pulverized lithium. Pulverized lithium is in a free state inside the cell, and when it comes into contact with the positive and negative electrode tabs, it will cause a short circuit. The above-mentioned projection of the positive electrode active layer in the active material region helps to reduce the risk of battery short circuit.

[0153] As shown in Figure 4, when the buffer body 201 is located on opposite sides of the negative electrode current collector 10, the active material region is located between the two buffer bodies 201 of the negative electrode current collector 10. The positive electrode current collector 40 is provided with a positive electrode active layer 50, and the projection of the positive electrode active layer 50 is located in the active material region of the negative electrode current collector 10.

[0154] In one embodiment, an insulating layer is provided in the region of the buffer layer corresponding to the positive current collector.

[0155] The insulation layer and the buffer layer are positioned opposite each other, which can improve the support of the insulation layer for the buffer layer in the battery and help the buffer layer to undergo elastic deformation.

[0156] As shown in Figure 5, when the buffer body 201 is located on opposite sides of the negative current collector 10, the positive current collector is provided with an insulating layer 60 at the part corresponding to the buffer body 201.

[0157] In one embodiment, at least one surface of the negative electrode current collector is provided with an active material replenishment layer.

[0158] The active material replenishment layer is used to replenish the lost active material. For example, in a lithium metal battery, the active material replenishment layer is a lithium replenishment layer. The lithium replenishment layer can replenish the lithium consumption per cycle because during the deposition and dissolution process of lithium metal, an SEI (Solid Electrolyte Interphase) is formed each time. The lithium replenishment layer can replenish the lithium consumed during the formation of the SEI and improve the cycle performance of the battery.

[0159] In one embodiment, the same surface of the negative electrode current collector includes a buffer zone and an active material zone, with the buffer layer located in the buffer zone and the active material replenishment layer located in the active material zone.

[0160] The same surface of the negative electrode current collector includes a buffer zone and an active material zone, with the buffer layer located in the buffer zone and the active material replenishment layer located in the active material zone. For example, in a lithium metal battery, the active material replenishment layer is a lithium replenishment layer, which facilitates metal deposition on the surface of the lithium replenishment layer.

[0161] As shown in Figure 5, when the buffer body 201 is located on opposite sides of the negative electrode current collector 10, the active material region is located between the two buffer bodies 201 of the negative electrode current collector 10. At this time, the active material supplement layer 70 is located between the two buffer bodies 201 of the negative electrode current collector 10.

[0162] In one embodiment, the thickness L of the buffer layer is greater than the thickness T of the active material replenishment layer.

[0163] The thickness L of the buffer layer refers to its thickness in its natural state, that is, its thickness when not compressed. During battery assembly, the buffer layer is compressed to the same thickness as the active material replenishment layer, giving it an expansion and rebound force that compresses the negative electrode sheet. When the thickness L of the buffer layer is greater than the thickness T of the active material replenishment layer, after compressing the buffer layer to the same thickness as the active material replenishment layer, the buffer layer also exhibits an expansion and rebound force, compressing the negative electrode sheet.

[0164] The buffer layer in the battery is initially compressed, providing pressure to the inside of the cell. During cycling, the deposited layer thickens as metal is deposited, and the thickness of the buffer layer is released, providing space for the expansion of the negative electrode. After the deposited layer dissolves and disappears, the buffer layer is compressed again.

[0165] For example, taking lithium metal batteries as an example, during battery assembly, the buffer layer is compressed to the same thickness as the lithium replenishment layer. During the lithium metal deposition process, the lithium metal layer on the negative electrode side becomes thicker, and the buffer layer expands and rebounds, providing space for the deposition of lithium metal. After the lithium metal deposition layer disappears, the thickness of the buffer layer is compressed again. In this way, the buffer layer is repeatedly compressed and is always in a compressed state, constantly squeezing the negative electrode sheet, so that the lithium metal battery always has a certain pressure during charging and discharging, maintaining dense lithium deposition on the negative electrode side.

[0166] In one embodiment, the relationship between the thickness L of the buffer layer and the thickness T of the active material replenishment layer is: 1.5T≤L≤1.64T.

[0167] Measurement steps for the thickness L of the buffer layer and the thickness T of the active material replenishment layer: During the preparation of the negative electrode sheet, a micrometer can be used to measure the thickness of the buffer layer and the active material replenishment layer separately. In the battery, the battery can be disassembled, and the thickness of the buffer layer and the active material replenishment layer can be measured separately using a micrometer.

[0168] The thickness L of the buffer layer and the thickness T of the active material replenishment layer satisfy the above relationship, which helps the buffer layer to be in a compressed state. During the battery charging and discharging process, the buffer layer provides pressure to the negative electrode sheet, which helps to deposit metal uniformly.

[0169] In the above 1.5T≤L≤1.64T, the values ​​include the minimum and maximum values ​​of this range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, and L=1.5T, L=1.51T, L=1.55T, L=1.57T, L=1.6T, L=1.61T, L=1.64T, etc., as well as the range values ​​between any two of the above point values.

[0170] In one embodiment, the thickness of the active substance replenishment layer is 2 μm-100 μm.

[0171] In one embodiment, the thickness of the buffer layer is 10 μm-200 μm.

[0172] The thickness of the active material replenishment layer meets the above-mentioned range and can compensate for the consumption of active material during battery cycling. It is understood that as the battery cycles, the active material will gradually be consumed. To improve the battery's cycle performance, this solution adds an excessive amount of active material replenishment layer; that is, the active material replenishment layer is maintained throughout the entire battery cycle. Thus, when the buffer layer is located on opposite sides of the negative electrode current collector, the active material replenishment layer is located in the middle of the negative electrode current collector. The active material replenishment layer has a certain thickness, and during the expansion and contraction of the buffer layer, it can provide support between the electrodes, reducing the problem of uneven stress on the negative electrode.

[0173] The thickness of the buffer layer, within the aforementioned range, helps it provide a certain amount of compression and expansion, allowing it to provide pressure to the negative electrode during battery cycling. For example, during battery assembly, the buffer layer is initially compressed within the battery. As metal is deposited on the negative electrode side, the thickness of the metal layer on the negative electrode side increases, causing the initially compressed buffer layer to expand. As the metal on the negative electrode side dissolves, the thickness of the metal layer on the negative electrode side decreases, and the expanded buffer layer is compressed again. In this way, it provides pressure to the negative electrode during battery cycling.

[0174] The values ​​in the range of 2μm-100μm include the minimum and maximum values ​​of this range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, as well as 2μm, 3μm, 5μm, 10μm, 20μm, 50μm, 80μm, 100μm, etc., and the range values ​​between any two of the above point values.

[0175] The values ​​in the range of 1μm-200μm include the minimum and maximum values ​​of this range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, and 10μm, 12μm, 13μm, 15μm, 20μm, 40μm, 80μm, 100μm, 150μm, 180μm, 200μm, etc., as well as the range values ​​between any two of the above point values.

[0176] In one embodiment, a negative electrode tab is connected to the negative electrode current collector, and a buffer layer is close to the negative electrode tab.

[0177] A negative electrode tab is connected to the negative current collector, and a buffer layer is close to the negative electrode tab, as shown in Figure 7. The negative electrode tab 80 is connected to the negative current collector 10 near the buffer body 201, which can reduce the contact between the negative electrode tab and the negative electrode metal layer and reduce the risk of battery short circuit.

[0178] In one embodiment, the diaphragm includes a base membrane and a coating disposed on the surface of the base membrane, the coating including an adhesive and / or ceramic particles.

[0179] In one embodiment, the secondary battery includes a stacked battery, a square battery, a cylindrical battery, and a pouch battery.

[0180] To ensure good contact between the buffer layer and the diaphragm during elastic expansion and contraction, and to ensure the diaphragm's ability to resist elastic forces, the diaphragm includes a base membrane and a coating on the surface of the base membrane. The coating includes an adhesive and / or ceramic particles. The adhesive provides good adhesion between the diaphragm and the buffer layer, reducing displacement of the buffer layer and the diaphragm during expansion and contraction. The ceramic particles can improve the mechanical strength of the diaphragm, giving it better strength to resist the elastic stress of the buffer layer.

[0181] The secondary batteries in this application include stacked batteries, prismatic batteries, cylindrical batteries, and pouch batteries.

[0182] The solution proposed in this application utilizes a buffer layer on the negative electrode. The compression and expansion of this buffer layer exerts pressure on the negative electrode, simplifying external components and increasing the system's energy density. It is understood that, generally, to provide pressure to the negative electrode, external buffer materials such as elastic pads or springs are required. These additional devices increase the complexity of the battery system, limit its application range, and reduce its energy density. The solution proposed in this application, by setting a buffer layer on the negative electrode, simplifies the battery structure and increases its energy density. Furthermore, by placing buffer layers on opposite sides of the negative electrode, the compression and rebound force distribution of the buffer layer is made more uniform, reducing stress concentration. This helps the buffer layer provide some of the pressure and expansion space required for metal deposition during charging and discharging, thus improving the battery's cycle performance.

[0183] In one embodiment, this application also provides a negative electrode sheet, which includes a negative current collector and a buffer layer disposed on at least one surface of the negative current collector, wherein the buffer layer is disposed on at least a portion of the surface of the negative current collector.

[0184] In one embodiment, the buffer layer includes at least two spaced buffer bodies located on either side of the center point of one surface of the negative electrode current collector.

[0185] In one embodiment, the two buffer bodies are symmetrically distributed along the center point of one surface of the negative electrode current collector.

[0186] In one embodiment, the two buffer bodies have the same area.

[0187] In one embodiment, the two buffer bodies are located on opposite sides of one surface of the negative electrode current collector.

[0188] In one embodiment, the lengths of the two buffer bodies are the same as the length of the negative electrode current collector.

[0189] In one embodiment, the width of the two buffer bodies accounts for 3%-50% of the width of the negative electrode current collector.

[0190] In one embodiment, the elastic stress of the buffer layer is 2MPa-6.5MPa.

[0191] In one embodiment, the material of the buffer layer includes at least one of polypropylene, polyethylene, and polyimide.

[0192] In one embodiment, the buffer layer includes a porous structure.

[0193] In one embodiment, at least one surface of the negative electrode current collector is provided with an active material replenishment layer.

[0194] In one embodiment, the same surface of the negative electrode current collector includes a buffer zone and an active material zone, with the buffer layer located in the buffer zone and the active material replenishment layer located in the active material zone.

[0195] In one embodiment, the thickness L of the buffer layer is greater than the thickness T of the active material replenishment layer.

[0196] In one embodiment, the relationship between the thickness L of the buffer layer and the thickness T of the active material replenishment layer is: 1.5T≤L≤1.64T.

[0197] In one embodiment, the thickness of the active substance replenishment layer is 2 μm-100 μm.

[0198] In one embodiment, the thickness of the buffer layer is 10 μm-200 μm.

[0199] In one embodiment, a negative electrode tab is connected to the negative electrode current collector, and a buffer layer is close to the negative electrode tab.

[0200] In one embodiment, this application also provides a method for preparing a secondary battery, comprising: preparing a positive electrode sheet, a negative electrode sheet, and a separator, wherein the negative electrode sheet includes a negative current collector and a buffer layer disposed on at least one surface of the negative current collector; alternately stacking or winding the positive electrode sheet, the separator, and the negative electrode sheet, and winding the outermost layer with the separator to obtain a battery cell; and setting a clamp on the outside of the battery cell to compress the battery cell, wherein the compression amount of the buffer layer is 38% to 80%, thereby obtaining a secondary battery.

[0201] In the secondary battery manufacturing process, after obtaining the cell, a clamp is set on the outside of the cell to compress it. The compression of the buffer layer is 38% to 80%. This helps the buffer layer to always be in a compressed state in the battery. The compressed buffer layer in the battery has a rebound force that can squeeze the negative electrode sheet. At the same time, the compression of the buffer layer is 38% to 80%, so that after metal is deposited on the negative electrode side, the buffer layer will rebound to a certain extent. After rebounding, the buffer layer is still in a compressed state and still exerts a certain amount of pressure on the negative electrode sheet. This helps the buffer layer to continuously squeeze the negative electrode sheet during battery cycling, which helps the dense deposition of metal and improves the cycle performance of the battery.

[0202] The compression amount of the buffer layer is calculated as (thickness before compression - thickness after compression) / thickness before compression × 100%.

[0203] The values ​​in the range of 38% to 80% include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments and 38%, 40%, 50%, 60%, 70%, 80%, etc., as well as the range values ​​between any two of the above point values.

[0204] In one embodiment, the same surface of the negative electrode current collector includes a buffer zone and an active material zone, the buffer layer is located in the buffer zone, and the active material zone is provided with an active material supplement layer; the step of providing a clamp on the outside of the cell to compress the cell includes compressing the buffer layer to the same thickness as the active material supplement layer.

[0205] After setting a buffer layer and an active material replenishment layer on the same surface of the negative electrode current collector, when compressing the cell, the buffer layer is compressed to the same thickness as the active material replenishment layer. This helps the buffer layer to remain compressed after the metal is deposited on the negative electrode side and the buffer layer rebounds.

[0206] Test method for the compressibility of the buffer layer: Test the thickness D1 of the buffer layer when it is not under stress, and the thickness D2 of the active material supplement layer. The compressibility of the buffer layer is (D1-D2) / D1×100%.

[0207] In one embodiment, the compression of the buffer layer is 50%-64%.

[0208] The compression of the buffer layer meets the above range, which helps the buffer layer to be in a compressed state during battery cycling, and also reduces the problem of the buffer layer overflowing into the metal deposition area when the compression of the buffer layer is too large.

[0209] The values ​​in the range of 38% to 64% include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments and 38%, 40%, 50%, 60%, 64%, etc., as well as the range values ​​between any two of the above point values.

[0210] In one embodiment, this application also provides a method for preparing a negative electrode sheet, comprising: providing buffer bodies on opposite sides of at least one surface of a foil, wherein the buffer bodies on at least one side are intermittently provided, and forming a negative electrode tab in the intermittent region; cutting out (e.g., laser cutting or die cutting) a negative electrode current collector from the negative electrode current collector foil according to a preset size of the negative electrode current collector, wherein buffer bodies are provided on opposite sides of the negative electrode current collector, and a negative electrode tab is connected to the negative electrode current collector.

[0211] In order to simultaneously prepare the negative electrode tab during the preparation of the negative electrode sheet, the negative electrode tab is close to the buffer layer, and buffer bodies are provided on opposite sides of at least one surface of the foil, wherein the buffer body on one side is continuously provided and the buffer body on the other side is intermittently provided, forming the negative electrode tab in the intermittent area. The negative electrode current collector is cut out on the foil according to the preset size of the negative electrode current collector, and buffer bodies are provided on opposite sides of the negative electrode current collector, and the negative electrode tab is connected to the negative electrode current collector.

[0212] In one embodiment, buffer bodies are provided on opposite sides of one surface of the foil, wherein the buffer bodies on one side are continuously provided and the buffer bodies on the other side are intermittently provided. A negative electrode current collector is cut out on the foil according to the preset size of the negative electrode current collector, and a negative electrode tab is provided on the negative electrode current collector.

[0213] In another embodiment, buffer bodies are provided on opposite sides of one surface of the foil, and the buffer bodies on both sides are intermittently provided. The negative current collector is cut out on the foil according to the preset size of the negative current collector, and two negative electrode tabs are provided on the negative current collector.

[0214] As shown in Figure 9, on one side of the foil material, there is a continuous buffer body 201 and a spaced buffer body 201 on the other side. The foil material is exposed at the spaced interval. The foil material at the spaced interval is used as a negative electrode tab 80. The negative electrode current collector is cut on the foil material according to the preset size of the negative electrode current collector, so that the negative electrode tab is provided on the negative electrode current collector. The negative electrode tab is close to the buffer layer.

[0215] In one embodiment, the step of providing a buffer layer on opposite sides of at least one surface of the foil includes: providing an active material replenishment layer and two spaced buffer bodies on at least one surface of the foil, the two buffer bodies being located on opposite sides of the active material replenishment layer, the thickness of the buffer bodies being 10 μm to 200 μm, and the thickness of the active material replenishment layer being 5 μm to 100 μm.

[0216] Placing the active material replenishment layer between the two buffer bodies helps to limit the pulverized lithium during battery cycling, confining the pulverized lithium between the two sides of the buffer body, reducing the pulverized lithium from sticking to the tabs, and reducing the risk of battery short circuit.

[0217] To compress the buffer layer, the thickness of the buffer body is greater than the thickness of the active material replenishment layer. The thickness of the buffer body is 10μm to 200μm, and the thickness of the active material replenishment layer is 5μm to 100μm. During the battery manufacturing process, compressing the buffer body to the same thickness as the active material replenishment layer helps the buffer layer to always be in a compressed state, constantly squeezing the negative electrode sheet, so that the lithium metal battery always has a certain pressure during charging and discharging, maintaining dense lithium deposition on the negative electrode side.

[0218] In one embodiment, this application also provides an electrical device, which includes a secondary battery, such as a rechargeable battery.

[0219] In addition, the following description of the battery (cell battery, battery module, battery pack) and electrical device of this application will be made with appropriate reference to the accompanying drawings.

[0220] In one embodiment of this application, a battery cell is provided.

[0221] Typically, a battery cell includes a positive electrode, a negative electrode, an electrolyte, and a separator. The electrolyte acts as a conductor of ions between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through. The separator described above is the improved separator of this application.

[0222] The positive electrode includes a positive current collector and a positive coating disposed on at least one surface of the positive current collector.

[0223] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive coating is disposed on either or both of the two opposite surfaces of the positive current collector.

[0224] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0225] In some embodiments, when the secondary battery is a lithium-ion battery, the positive electrode active material may be a positive electrode active material known in the art for lithium-ion batteries. As an example, the positive electrode active material may include at least one of the following materials: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, this application is not limited to these materials, and other conventional materials that can be used as battery positive electrode active materials may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as LiNi). 1 / 3 Co 1 / 3 Mn1 / 3 O2 (also known as NCM) 333 LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811 ), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.15 Al 0.05 At least one of O2 and its modified compounds. Examples of lithium phosphates with an olivine structure include, but are not limited to, lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0226] During the charging and discharging process of a battery, Li undergoes insertion / extraction and consumption, resulting in varying molar Li content at different discharge states. In the examples of cathode materials in this application, the molar Li content refers to the initial state of the material, i.e., the state before feeding. When the cathode material is applied to the battery system, the molar Li content changes after charge-discharge cycles.

[0227] In the examples of cathode materials in this application, the molar content of O is only a theoretical value. Oxygen release from the crystal lattice will cause changes in the molar content of oxygen, and the actual molar content of O will fluctuate.

[0228] In some embodiments, the positive electrode coating may optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0229] In some embodiments, the positive electrode coating may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0230] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0231] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not specify any particular type of electrolyte; it can be selected according to requirements.

[0232] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0233] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0234] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0235] In some embodiments, the battery cell may include an outer packaging. This outer packaging can be used to encapsulate the electrode assembly and electrolyte described above.

[0236] In some embodiments, the outer packaging of the battery cell can be a rigid shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the battery cell can also be a flexible package, such as a pouch. The material of the flexible package can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0237] This application does not impose any particular limitation on the shape of the battery cell; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 8 shows a square battery cell 5 as an example.

[0238] In some embodiments, referring to FIG9, the outer packaging may include a housing 51 and a cover plate 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be placed over the opening to close the receiving cavity. The positive electrode, negative electrode, and separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the battery cell 5 may be one or more, which can be selected by those skilled in the art according to specific practical needs.

[0239] In some implementations, individual battery cells can be assembled into a battery module. The number of individual battery cells contained in a battery module can be one or more, and the specific number can be selected by those skilled in the art based on the application and capacity of the battery module.

[0240] Figure 10 shows a battery module 4 as an example. Referring to Figure 10, in the battery module 4, multiple battery cells 5 can be arranged sequentially along the length of the battery module 4. Of course, they can also be arranged in any other arbitrary way. Furthermore, the multiple battery cells 5 can be fixed in place by fasteners.

[0241] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0242] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0243] Figures 11 and 12 show a battery pack 1 as an example. Referring to Figures 11 and 12, the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper box 2 and a lower box 3, with the upper box 2 covering the lower box 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0244] In addition, this application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided in this application. The secondary battery, battery module, or battery pack can be used as the power source of the electrical device or as the energy storage unit of the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0245] As an electrical device, you can choose individual battery cells, battery modules, or battery packs according to your usage requirements.

[0246] Figure 13 shows an example of an electrical device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of the individual battery cells, a battery pack or battery module can be used.

[0247] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a single battery cell as their power source.

[0248] Example

[0249] Example 1

[0250] Preparation of negative electrode sheet

[0251] The negative electrode current collector (copper foil, 12μm) has a buffer layer (made of polypropylene) attached to both sides. That is, the buffer layer is located on opposite sides of the copper foil, with one side being a continuous buffer and the other side being a discontinuous buffer. The long side of the electrode sheet is made to have a buffer layer by die cutting, and the discontinuous part is cut to serve as the negative electrode tab.

[0252] The length of the negative electrode current collector is 203 μm, the length of the buffer layer is 203 μm, and the width of the negative electrode current collector is the sum of the width of the metal deposition layer (141 μm) and the width of the two buffer bodies.

[0253] Preparation of the positive electrode sheet: Lithium nickel cobalt manganese oxide, conductive agent acetylene black, and binder PVDF were mixed at a mass ratio of 98:1:1. NMP solvent was added and stirred until the system was homogeneous to obtain the positive electrode slurry. The positive electrode slurry was uniformly coated on both sides of the positive electrode current collector aluminum foil, air-dried at room temperature, and then transferred to an oven for further drying. It was then cut into rectangles of 207mm*138mm to form the positive electrode sheet, with a positive electrode surface capacity of 3.5mAh / cm². 2 .

[0254] The diaphragm is a porous polyethylene membrane with a surface coating consisting of an adhesive and ceramic particles. The diaphragm width is 147 mm.

[0255] Preparation of electrolyte: Take 1.51g of lithium hexafluorophosphate, add 3g of solvent EC and 7g of EMC, stir thoroughly to form a colorless and transparent electrolyte with a concentration of 1M.

[0256] Battery Assembly: The positive electrode, separator, and negative electrode are stacked layer by layer, with 11 layers of negative electrode and 10 layers of positive electrode. An extra layer of separator is wrapped around the outermost layer of the stack to finish the assembly. A clamp is placed on the outside of the dry cell to compress it, and it is then wrapped in an aluminum-plastic film bag to form a stacked dry cell. 41.4g of the previously prepared electrolyte is injected, and the aluminum-plastic film bag is vacuum-sealed using heat sealing. After standing at room temperature for at least 6 hours, cycle testing can begin. The rated capacity of the stacked battery prepared in this way is 23Ah.

[0257] Example 2

[0258] Based on Example 1, after lithium replenishment, buffer layers are attached to both sides of the negative electrode current collector (copper foil, 12μm). That is, the buffer layers are located on opposite sides of the copper foil, and the lithium replenishment layer is located between the two buffer layers. One side is a continuous buffer body, and the other side is an intermittent buffer body. The long side of the electrode sheet is made to have a buffer layer by die cutting, and the intermittent cut is used as the negative electrode tab.

[0259] The negative electrode current collector has a length of 203 μm, the buffer layer has a length of 203 μm, the lithium replenishment layer has a thickness of 50 μm, and the width of the negative electrode current collector is the sum of the width of the lithium replenishment layer (141 μm) and the width of the two buffer layers.

[0260] Example 3

[0261] Based on Example 1, a buffer layer is provided on the entire surface of one side of the negative electrode current collector.

[0262] Examples 4 to 10

[0263] Based on Example 2, the percentage of buffer layer area, the thickness of lithium replenishment layer, and the thickness of buffer layer on the negative electrode current collector side were adjusted, as shown in Table 1 below.

[0264] Comparative Example 1

[0265] Based on Example 1, no buffer layer is set.

[0266] Comparative Example 2

[0267] Based on Comparative Example 1, a buffer layer is not set, but a lithium replenishment layer is set, as shown in Table 1 below.

[0268] Performance testing

[0269] First-week full-charge lithium deposition thickness: After the first week of full charging of the battery cell, the residual electrolyte on the surface of the negative electrode was cleaned with DMC (dimethyl carbonate). The negative electrode was then subjected to ion beam interface polishing to expose the cross-section, and the deposition morphology and thickness of the negative electrode were observed using a scanning electron microscope (SME).

[0270] Cell Cycling Performance: The initial preload during cell cycling was based on compressing the buffer layer to the thickness of the lithium replenishment layer. The ambient temperature was set at 25℃, and charge-discharge cycles were performed using a rate of 0.2C-1C (i.e., 4.6A-23A). The cutoff voltages for charge and discharge were set at 4.3V and 2.8V, respectively. The battery life was considered to have ended when the discharge capacity decayed to 80% of the first cycle's discharge capacity. Specifically, the cells were charged at a constant current of 0.5C to a voltage of 4.3V, then charged at a constant voltage of 4.3V to 0.05C, and allowed to rest for 10 minutes. The cells were then discharged at a constant current of 0.5C to a voltage of 2.8V, and allowed to rest for 10 minutes. This process was repeated for less than 100 cycles. The capacity retention rate was statistically analyzed, and the cells were disassembled to observe the distribution of pulverized lithium.

[0271] Table 1. List of Experimental Data

[0272] As can be seen from Table 1 above, compared with Comparative Example 1, Example 1 and Example 3 have a buffer layer. The lithium deposition thickness of Example 1 and Example 3 is smaller than that of Comparative Example 1. This indicates that the buffer layer can provide pressure to the negative electrode plate inside the secondary battery, which helps to make the metal deposition on the negative electrode side dense during the cycling process of the secondary battery.

[0273] Compared to Comparative Example 2, Example 2 includes a buffer layer and a lithium replenishment layer, while Comparative Example 2 does not have a buffer layer but does have a lithium replenishment layer. Therefore, the lithium deposition thickness in Example 2 is less than that in Comparative Example 2. Furthermore, Comparative Example 2 does not have a buffer layer on the side of the negative electrode current collector, resulting in significant overflow of pulverized lithium from the negative electrode to the edge. In Example 2, the two buffer bodies are located on opposite sides of one surface of the negative electrode current collector, which helps to deposit metal near the center of the current collector. The buffer bodies on both sides help to confine the deposited metal, limiting the drift of pulverized lithium and confining it between the two sides of the buffer bodies. This reduces the risk of pulverized lithium overlapping with the electrode tabs and decreases the risk of battery short circuits.

[0274] Examples 4 to 10 all include a buffer layer and a lithium replenishment layer. When the compression of the buffer layer under preload is 38% to 80%, the lithium deposition thickness in Examples 4 to 10 is less than that in the comparative example. Furthermore, when the compression of the buffer layer under preload is 50% to 64%, the lithium deposition is more compact and the thickness is smaller.

[0275] In Examples 9 and 10, the compression of the buffer layer was too large, causing it to overflow into the metal deposition area, which was detrimental to lithium deposition. When the compression of the buffer layer under preload is 50% to 64%, it helps the buffer layer to remain in a compressed state during battery cycling, and also reduces the problem of the buffer layer overflowing into the metal deposition area when the compression is too large.

[0276] The buffer layer can provide pressure to the negative electrode plate inside the secondary battery, which helps to make the metal deposition on the negative electrode side dense during the cycle of the secondary battery. In the process of metal deposition, the buffer layer can provide deposition space for the deposition layer and improve the cycle performance of the battery.

[0277] Furthermore, the buffer layer is located on the side of the negative electrode current collector, which helps to limit the pulverized lithium.

[0278] The above are merely preferred embodiments of this application and do not limit the scope of the patent application. Any equivalent structural transformations made based on the inventive concept of this application and the contents of the specification and drawings of this application, or direct / indirect applications in other related technical fields, are included within the scope of patent protection of this application.

Claims

1. A secondary battery, wherein, The secondary battery includes a positive electrode, a negative electrode, and a separator; The negative electrode sheet includes a negative current collector and a buffer layer disposed on at least one surface of the negative current collector, wherein the buffer layer is disposed on at least a portion of the surface of the negative current collector.

2. The secondary battery as described in claim 1, wherein, The buffer layer includes at least two spaced buffer bodies, which are located on either side of the center point of one surface of the negative electrode current collector.

3. The secondary battery as described in claim 2, wherein, The two buffer bodies are symmetrically distributed along the center point of one surface of the negative electrode current collector.

4. The secondary battery as described in claim 2 or 3, wherein, The two buffer bodies have the same area.

5. The secondary battery as described in any one of claims 2 to 4, wherein, The two buffer bodies are located on opposite sides of one surface of the negative electrode current collector.

6. The secondary battery as described in claim 5, wherein, The lengths of the two buffer bodies are the same as the length of the negative electrode current collector.

7. The secondary battery as described in claim 5 or 6, wherein, The width of the two buffer bodies accounts for 3%-50% of the width of the negative electrode current collector.

8. The secondary battery according to any one of claims 1 to 7, wherein, The elastic stress of the buffer layer is 2MPa-6.5MPa.

9. The secondary battery according to any one of claims 1 to 8, wherein, The area of ​​the buffer layer on one surface of the negative electrode current collector accounts for 3%-50% of the area of ​​one surface of the negative electrode current collector.

10. The secondary battery according to any one of claims 1 to 9, wherein, The material of the buffer layer includes at least one of polypropylene, polyethylene, and polyimide; And / or, the buffer layer includes a porous structure.

11. The secondary battery according to any one of claims 1 to 10, wherein, The positive electrode sheet includes a positive current collector and a positive active layer disposed on at least one surface of the positive current collector; The negative electrode current collector includes a buffer zone and an active material zone on the same surface, with the buffer layer located in the buffer zone and the projection of the positive electrode active layer located in the active material zone.

12. The secondary battery as claimed in claim 11, wherein, The positive current collector has an insulating layer in the region corresponding to the buffer layer.

13. The secondary battery according to any one of claims 1 to 12, wherein, At least one surface of the negative electrode current collector is provided with an active material replenishment layer.

14. The secondary battery as claimed in claim 13, wherein, The negative electrode current collector includes a buffer zone and an active material zone on the same surface, with the buffer layer located in the buffer zone and the active material replenishment layer located in the active material zone.

15. The secondary battery as described in claim 14, wherein, The thickness L of the buffer layer is greater than the thickness T of the active substance replenishment layer.

16. The secondary battery as claimed in claim 15, wherein, The relationship between the thickness L of the buffer layer and the thickness T of the active substance replenishment layer is: 1.5T≤L≤1.64T.

17. The secondary battery according to any one of claims 14 to 16, wherein, The thickness of the active substance replenishment layer is 2μm-100μm; And / or, the thickness of the buffer layer is 10μm-200μm.

18. The secondary battery according to any one of claims 1 to 17, wherein, The negative electrode current collector is connected to a negative electrode tab, and the buffer layer is close to the negative electrode tab.

19. The secondary battery according to any one of claims 1 to 18, wherein, The diaphragm includes a base membrane and a coating disposed on the surface of the base membrane, the coating including an adhesive and / or ceramic particles; And / or, the secondary battery includes stacked batteries, square batteries, cylindrical batteries, and pouch batteries.

20. A negative electrode plate, wherein, The negative electrode sheet includes a negative current collector and a buffer layer disposed on at least one surface of the negative current collector, wherein the buffer layer is disposed on at least a portion of the surface of the negative current collector.

21. The negative electrode sheet as described in claim 20, wherein, The buffer layer includes at least two spaced buffer bodies, which are located on opposite sides of one surface of the negative electrode current collector.

22. The negative electrode sheet as described in claim 21, wherein, The width of the two buffer bodies accounts for 3%-50% of the width of the negative electrode current collector.

23. The negative electrode sheet according to any one of claims 20 to 22, wherein, The elastic stress of the buffer layer is 2MPa-6.5MPa.

24. The negative electrode sheet according to any one of claims 20 to 23, wherein, At least one surface of the negative electrode current collector is provided with an active material replenishment layer.

25. The negative electrode sheet as described in claim 24, wherein, The negative electrode current collector includes a buffer zone and an active material zone on the same surface, with the buffer layer located in the buffer zone and the active material replenishment layer located in the active material zone; The thickness L of the buffer layer is greater than the thickness T of the active substance replenishment layer.

26. The negative electrode sheet as described in claim 25, wherein, The relationship between the thickness L of the buffer layer and the thickness T of the active substance replenishment layer is: 1.5T≤L≤1.64T.

27. The negative electrode sheet as described in claim 25 or 26, wherein, The thickness of the active substance replenishment layer is 2μm-100μm; And / or, the thickness of the buffer layer is 10μm-200μm.

28. A method for preparing a secondary battery, wherein, include: Prepare a positive electrode, a negative electrode, and a separator. The negative electrode includes a negative current collector and a buffer layer disposed on at least one surface of the negative current collector. The positive electrode, the separator, and the negative electrode are alternately stacked or wound, and the separator is used to wind the outermost part to obtain a battery cell; A clamp is placed on the outside of the battery cell to compress the battery cell. The compression of the buffer layer is 38% to 80%, resulting in a secondary battery.

29. The method for preparing a secondary battery as described in claim 28, wherein, The negative electrode current collector includes a buffer zone and an active material zone on the same surface, the buffer layer is located in the buffer zone, and the active material zone is provided with an active material replenishment layer; The step of setting a clamp on the outside of the battery cell to compress the battery cell includes compressing the buffer layer to the same thickness as the active material replenishment layer.

30. The method for preparing a secondary battery as described in claim 28 or 29, wherein, The compression of the buffer layer is 50%-64%.

31. A method for preparing a negative electrode sheet, wherein, include: A buffer body is provided on opposite sides of at least one surface of the foil, and the buffer body on at least one side is provided intermittently, forming a negative electrode tab in the intermittent region; The negative electrode current collector is cut out from the foil according to the preset size of the negative electrode current collector. Buffer bodies are provided on opposite sides of one surface of the negative electrode current collector, and the negative electrode tab is connected to the negative electrode current collector.

32. The method for preparing the negative electrode sheet as described in claim 31, wherein, The step of providing a buffer layer on opposite sides of at least one surface of the foil includes: An active material replenishment layer and two spaced buffer bodies are provided on at least one surface of the foil. The two buffer bodies are located on opposite sides of the active material replenishment layer. The thickness of the buffer bodies is 10 μm to 200 μm, and the thickness of the active material replenishment layer is 5 μm to 100 μm.

33. An electrical appliance, wherein, The electrical device includes a secondary battery as described in any one of claims 1 to 19.

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