Separator and preparation method thereof, wound electrode assembly, battery cell, battery and electrical equipment

Abstract

The present application relates to a separator and a preparation method thereof, a wound electrode assembly, a battery cell, a battery and an electrical device, and belongs to the technical field of secondary batteries. The separator is used to prepare a wound electrode assembly, and the wound electrode assembly has a plurality of straight areas and a plurality of bending areas arranged alternately in sequence along the winding direction. The separator includes a plurality of first areas for winding in the straight areas and a plurality of second areas for winding in the bending areas. The porosity of the first area is 5% to 40% greater than that of the second area. It can ensure that the battery's cycle capacity retention rate is high; at the same time, the amount of lithium ions passing through the separator at the bending area can be reduced, and the lithium plating problem at the bending area can be improved, thereby improving the battery's performance.

Description

Technical Field

[0001] The present application relates to the technical field of secondary batteries, and in particular to an isolation membrane and a preparation method thereof, a wound electrode assembly, a battery cell, a battery and an electrical device. Background Art

[0002] After winding and hot pressing, the wound electrode assembly will form straight areas and bent areas. During charging, lithium deposition is prone to occur in the bent areas, thereby reducing the electrical performance of the lithium-ion battery. Summary of the invention

[0003] In view of the deficiencies in the prior art, the present application provides an isolation membrane and a preparation method thereof, a wound electrode assembly, a battery cell, a battery and an electrical device to improve the lithium plating problem at the bending area of ​​the wound electrode assembly.

[0004] In the first aspect, an embodiment of the present application provides an isolation membrane for preparing a wound electrode assembly, the wound electrode assembly having a plurality of straight regions and a plurality of bending regions alternately arranged in sequence along a winding direction, the isolation membrane comprising a plurality of first regions for winding in the straight regions and a plurality of second regions for winding in the bending regions, the porosity of the first regions being 5% to 40% greater than the porosity of the second regions.

[0005] In the technical solution of the embodiment of the present application, after the winding electrode assembly is formed by the isolation membrane, the first area of ​​the isolation membrane corresponds to the straight area of ​​the winding electrode assembly, and the second area of ​​the isolation membrane corresponds to the bending area of ​​the winding electrode assembly. The porosity of the isolation membrane in the straight area is greater than the porosity of the isolation membrane in the bending area, and the difference is between 5% and 40%. This can make the amount of lithium ions passing through the isolation membrane in the straight area higher, ensuring a high cycle capacity retention rate of the battery; at the same time, the amount of lithium ions passing through the isolation membrane in the bending area can be lowered, improving the lithium plating problem in the bending area, thereby improving the battery performance.

[0006] In some embodiments, the porosity of the first region is 10% to 25% greater than the porosity of the second region. While ensuring the cycle capacity retention rate of the battery, the lithium plating problem at the bending area of ​​the wound electrode assembly can be further improved.

[0007] In some embodiments, the porosity of the first region is 40% to 70%, and the porosity of the second region is 20% to 40%. The straight region of the wound electrode assembly can have more lithium ions passing through, so that the cycle capacity retention rate of the battery is higher, and the lithium ion content passing through the straight region is more reasonable, which can avoid the problem of lithium deposition in the straight region to a certain extent; at the same time, the bending region of the wound electrode assembly also has a certain amount of lithium ions passing through, which can ensure the cycle capacity retention rate of the battery to a certain extent, and can also better improve the lithium deposition problem in the bending region.

[0008] In some embodiments, the porosity of the first region is 40% to 50%, and the porosity of the second region is 20% to 31%. The porosity of the separator in the straight region and the bending region of the wound electrode assembly is more reasonable, which can improve the lithium deposition problem in the bending region while maintaining a higher cycle capacity of the battery.

[0009] In some embodiments, the thickness of the isolation film is 5 μm to 60 μm. The thickness of the isolation film is relatively reasonable, which can avoid the problem of short circuit caused by puncture of the isolation film to a certain extent, and can also make the volume energy density of the battery higher.

[0010] In some embodiments, the thickness of the separator is 7 μm to 20 μm, which can further prevent the separator from being punctured while ensuring a higher volume energy density of the battery.

[0011] In some embodiments, the material of the isolation membrane includes polyolefin. Using the polyolefin material as the main material of the isolation membrane can easily achieve a porosity of the first region that is 5% to 40% greater than that of the second region.

[0012] In the second aspect, the present application provides a wound electrode assembly, the wound electrode assembly having a plurality of straight regions and a plurality of bending regions alternately arranged in sequence along the winding direction, the wound electrode assembly comprising a positive electrode sheet stacked and wound in sequence, any one of the isolation films provided in the first aspect, and a negative electrode sheet, the plurality of first regions of the isolation film being arranged correspondingly to the plurality of straight regions, and the plurality of second regions of the isolation film being arranged correspondingly to the plurality of bending regions.

[0013] In the technical solution of the embodiment of the present application, the first area of ​​the isolation membrane corresponds to the straight area of ​​the wound electrode assembly, and the second area of ​​the isolation membrane corresponds to the bending area of ​​the wound electrode assembly. The porosity of the isolation membrane in the straight area is greater than the porosity of the isolation membrane in the bending area, and the difference is between 5% and 40%. This can make the amount of lithium ions passing through the isolation membrane in the straight area higher, thereby ensuring a high cycle capacity retention rate of the battery; at the same time, the amount of lithium ions passing through the isolation membrane in the bending area can be lowered, thereby improving the lithium plating problem in the bending area, thereby improving the battery performance.

[0014] In a third aspect, the present application provides a battery cell, comprising the wound electrode assembly provided in the second aspect.

[0015] In a fourth aspect, the present application provides a battery, comprising the battery cell provided in the third aspect.

[0016] In a fifth aspect, the present application provides an electrical device, comprising the battery provided in the fourth aspect.

[0017] In a sixth aspect, the present application provides a method for preparing an isolation membrane, wherein the isolation membrane has a plurality of first regions and a plurality of second regions which are alternately arranged in sequence along the length direction, and the preparation method comprises: subjecting the mixture to sequential extrusion, stretching, extraction and shaping. The mixture comprises a pore-forming agent, and the pore-forming agent is removed in the extraction step; in the stretching step, the stretching multiple of the first segment of the casting corresponding to the first region is greater than the stretching multiple of the second segment of the casting corresponding to the second region, so that the porosity of the first region is 5% to 40% greater than the porosity of the second region.

[0018] In the technical solution of the embodiment of the present application, the mixture contains a pore-forming agent, which can form a certain porosity in the isolation membrane; and along the length direction of the casting, the stretching multiple of the first section of the casting is different from the stretching multiple of the second section of the casting. After removing the pore-forming agent, the porosity of the first area of ​​the corresponding isolation membrane can be 5% to 40% greater than the porosity of the second area. After using the isolation membrane to prepare a wound electrode assembly, the battery's cycle capacity retention rate can be guaranteed while improving the occurrence of lithium plating problems.

[0019] In some embodiments, the stretching multiple of the first segment of the cast sheet in the width direction is consistent with the stretching multiple of the second segment of the cast sheet in the width direction, and the stretching multiple of the first segment of the cast sheet in the length direction is 1 to 10 times greater than the stretching multiple of the second segment of the cast sheet in the length direction. After the separator is formed, the width of the separator is basically consistent, so as to facilitate the subsequent preparation of the wound electrode assembly. At the same time, the difference in the stretching multiple in the length direction is limited to the above range, which makes it easy to make the porosity of the first region of the separator 5% to 40% greater than the porosity of the second region.

[0020] In some embodiments, the stretching ratio of the first segment of the casting in the longitudinal direction is 2 to 4 times greater than the stretching ratio of the second segment of the casting in the longitudinal direction. The porosity of the first region of the separator can be 10% to 25% greater than the porosity of the second region.

[0021] In some embodiments, the mixture includes 30 wt% to 65 wt% of polyolefin, 30 wt% to 60 wt% of pore former and 5 wt% to 10 wt% of antioxidant. The first segment of the cast sheet has a stretching multiple of 4 to 8 times in the length direction, and the second segment of the cast sheet has a stretching multiple of 2 to 4 times in the length direction. The content of the pore former is limited to the above range, and in combination with the stretching multiple, the porosity of the first region of the isolation membrane can be 40% to 70%, and the porosity of the second region can be 20% to 40%.

[0022] In some embodiments, the stretching multiples in the width direction of the first section of the casting sheet and the stretching multiples in the width direction of the second section of the casting sheet are both 4 to 6 times. The stretching multiples in the width direction and the stretching multiples in the length direction can be matched to make the porosity of the first region 40% to 50% and the porosity of the second region 20% to 31%.

[0023] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Various other advantages and benefits will become apparent to those of ordinary skill in the art by reading the detailed description of the preferred embodiments below. The accompanying drawings are only for the purpose of illustrating the preferred embodiments and are not to be considered as limiting the present application. Moreover, the same reference numerals are used throughout the drawings to represent the same components. In the drawings:

[0025] Figure 1 A schematic diagram of the structure of a wound electrode assembly provided by the prior art;

[0026] Figure 2 A schematic diagram of the structure of a vehicle provided in some embodiments of the present application;

[0027] Figure 3 A schematic diagram of the exploded structure of a battery provided in some embodiments of the present application;

[0028] Figure 4 A schematic diagram of the structure of a battery cell provided in some embodiments of the present application;

[0029] Figure 5 An exploded view of a battery cell provided for some embodiments of the present application;

[0030] Figure 6 A schematic diagram of the structure of a wound electrode assembly provided in some embodiments of the present application;

[0031] Figure 7 A schematic diagram of the structure between the bending area and the straight area provided in some embodiments of the present application;

[0032] Figure 8 A flow chart of the preparation process of the isolation membrane provided in some embodiments of the present application.

[0033] Icons: 1000-vehicle; 100-battery; 10-casing; 11-accommodation space; 12-first part; 13-second part; 20-battery cell; 21-casing; 211-opening; 22-end cover assembly; 221-end cover; 222-electrode terminal; 23-wound electrode assembly; 231-positive electrode sheet; 232-negative electrode sheet; 233-separation film; 2331-first separation film; 2332-second separation film; 233a-first region; 233b-second region; 234-straight region; 235-bending region; 24-current collecting member; 25-insulating protection member; 200-controller; 300-motor. DETAILED DESCRIPTION

[0034] The following embodiments of the technical solution of the present application are described in detail in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present application, and are therefore only used as examples, and cannot be used to limit the scope of protection of the present application.

[0035] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by technicians in the technical field to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" in the specification and claims of this application and the above-mentioned figure descriptions and any variations thereof are intended to cover non-exclusive inclusions.

[0036] In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "multiple" is more than two, unless otherwise clearly and specifically defined.

[0037] Reference to "embodiments" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0038] In the description of the embodiments of the present application, the term "and / or" is only a description of the association relationship of the associated objects, indicating that there may be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship.

[0039] In the description of the embodiments of the present application, the term "multiple" refers to more than two (including two). Similarly, "multiple groups" refers to more than two groups (including two groups), and "multiple pieces" refers to more than two pieces (including two pieces).

[0040] In the description of the embodiments of the present application, the technical terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc. indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, which are only for the convenience of describing the embodiments of the present application and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the embodiments of the present application.

[0041] In the description of the embodiments of the present application, unless otherwise clearly specified and limited, technical terms such as "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, and it can be the internal connection of two elements or the interaction relationship between two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the embodiments of the present application can be understood according to the specific circumstances.

[0042] At present, from the perspective of market development, the application of power batteries is becoming more and more extensive. Power batteries are not only used in energy storage power systems such as hydropower, thermal power, wind power and solar power stations, but also widely used in electric vehicles such as electric bicycles, electric motorcycles, electric cars, as well as military equipment and aerospace and other fields. With the continuous expansion of the application field of power batteries, the market demand is also constantly expanding.

[0043] The power battery can be a lithium-ion battery. During the charging process of the lithium-ion battery, lithium ions are released from the positive electrode active material, transmitted through the electrolyte, through the isolation membrane, and embedded in the negative electrode active material. + When the Li+ is removed from the positive electrode too quickly but cannot be inserted into the negative electrode in equal amounts, the Li+ cannot be inserted into the negative electrode. + Electrons can only be obtained on the surface of the negative electrode, thus forming a silvery white metallic lithium, which is the so-called lithium deposition. As the precipitated lithium dendrites grow, they may pierce the separator, causing the positive and negative electrodes to overlap, causing a micro short circuit inside the battery, and further causing thermal runaway.

[0044] How to reduce or avoid lithium plating and improve the safety performance of lithium-ion batteries has become a technical problem that needs to be solved urgently. The inventors have found that for a wound square lithium-ion battery, one of its core components is a wound electrode assembly. The wound electrode assembly is usually a first separator, a positive electrode sheet, a second separator and a negative electrode sheet stacked up, and then hot-pressed after winding to form a wound electrode assembly. After hot pressing, the wound electrode assembly has a straight area and a bent area.

[0045] Figure 1 For a structural schematic diagram of a wound electrode assembly 23 provided in the prior art, please refer to Figure 1 , Figure 1 The direction A indicated by the middle arrow is the winding direction. The wound electrode assembly 23 has a plurality of straight regions 234 and a plurality of bending regions 235 (such as Figure 1 , the area between the dotted line b1 and the dotted line b2 is the straight area 234, and the area outside the dotted line b2 is the bending area 235). The surface of the positive electrode sheet 231 has a positive electrode active material, and the surface of the negative electrode sheet 232 has a negative electrode active material. For the bending area 235 of the wound electrode assembly 23, it is easy to cause the positive electrode sheet 231 or / and the negative electrode sheet 232 to have active material shedding (also known as powder falling phenomenon). Due to the shedding of active materials, especially the shedding of negative active materials on the negative electrode sheet 232, the number of lithium insertion sites of the negative electrode active material layer of the negative electrode sheet 232 may be less than the number of lithium ions provided by the positive electrode active material layer of the positive electrode sheet 231 opposite thereto, and therefore, lithium precipitation problems are prone to occur during charging.

[0046] Based on the above considerations, in order to solve the problem of lithium plating in the bending area of ​​the wound electrode assembly, the inventors have designed an isolation membrane for preparing the wound electrode assembly after in-depth research. The wound electrode assembly has a plurality of straight areas and a plurality of bending areas alternately arranged in sequence along the winding direction. The isolation membrane includes a plurality of first areas for winding in the straight areas and a plurality of second areas for winding in the bending areas. The porosity of the first area is 5% to 40% greater than the porosity of the second area.

[0047] In such an isolation membrane, after a wound electrode assembly is formed by the isolation membrane, the first area of ​​the isolation membrane corresponds to the straight area of ​​the wound electrode assembly, and the second area of ​​the isolation membrane corresponds to the bending area of ​​the wound electrode assembly. The porosity of the isolation membrane in the straight area is greater than the porosity of the isolation membrane in the bending area, and the difference is between 5% and 40%. This can make the amount of lithium ions passing through the isolation membrane in the straight area higher, ensuring a high cycle capacity retention rate of the battery; at the same time, the amount of lithium ions passing through the isolation membrane in the bending area can be lowered, improving the lithium plating problem in the bending area, thereby improving the battery performance.

[0048] The wound electrode assembly disclosed in the embodiment of the present application can be used in, but not limited to, electrical equipment such as vehicles, ships or aircraft. The power supply system of the electrical equipment can be composed of the battery cells and batteries disclosed in the present application, which is conducive to slow decomposition of lithium and improves the safety of electricity use.

[0049] The embodiment of the present application provides an electric device that uses a battery as a power source. The electric device can be a vehicle, a mobile phone, a portable device, a laptop computer, a ship, a spacecraft, an electric toy, an electric tool, and the like. The vehicle can be a fuel vehicle, a gas vehicle, or a new energy vehicle. The new energy vehicle can be a pure electric vehicle, a hybrid vehicle, or an extended-range vehicle, and the like; the spacecraft includes an airplane, a rocket, a space shuttle, and a spacecraft, and the like; the electric toy includes a fixed or mobile electric toy, such as a game console, an electric car toy, an electric ship toy, and an electric airplane toy, and the like; the electric tool includes a metal cutting electric tool, a grinding electric tool, an assembly electric tool, and an electric tool for railways, such as an electric drill, an electric grinder, an electric wrench, an electric screwdriver, an electric hammer, an impact drill, a concrete vibrator, and an electric planer, and the like. The embodiment of the present application does not impose any special restrictions on the above-mentioned electric devices.

[0050] For the convenience of description, the following embodiments are described by taking the electric device as a vehicle as an example.

[0051] Please refer to Figure 2 , Figure 2 The schematic diagram of the structure of the vehicle 1000 provided in some embodiments of the present application. The vehicle 1000 is provided with a battery 100 inside, and the battery 100 can be provided at the bottom, head or tail of the vehicle 1000. The battery 100 can be used to power the vehicle 1000, for example, the battery 100 can be used as an operating power source of the vehicle 1000.

[0052] The vehicle 1000 may further include a controller 200 and a motor 300 , wherein the controller 200 is used to control the battery 100 to supply power to the motor 300 , for example, to meet the power requirements of starting, navigating, and driving the vehicle 1000 .

[0053] In some embodiments of the present application, the battery 100 can not only serve as an operating power source for the vehicle 1000, but also serve as a driving power source for the vehicle 1000, replacing or partially replacing fuel or natural gas to provide driving power for the vehicle 1000.

[0054] Figure 3 This is a schematic diagram of the exploded structure of the battery 100 provided in some embodiments of the present application. Figure 3 The battery 100 includes a housing 10 and a battery cell 20 , and the battery cell 20 is accommodated in the housing 10 .

[0055] The box body 10 is used to provide a storage space 11 for the battery cell 20. In some embodiments, the box body 10 may include a first portion 12 and a second portion 13, and the first portion 12 and the second portion 13 cover each other to define the storage space 11 for accommodating the battery cell 20. Of course, the connection between the first portion 12 and the second portion 13 can be sealed by a sealing member (not shown in the figure), and the sealing member can be a sealing ring, a sealant, etc.

[0056] The first part 12 and the second part 13 can be in various shapes, such as a cuboid, a cylinder, etc. The first part 12 can be a hollow structure with one side open to form a receiving cavity for receiving the battery cell 20, and the second part 13 can also be a hollow structure with one side open to form a receiving cavity for receiving the battery cell 20. The opening side of the second part 13 covers the opening side of the first part 12, thereby forming a box body 10 with a receiving space 11. Of course, if Figure 2 As shown, the first part 12 may be a hollow structure with one side open, and the second part 13 may be a plate-like structure. The second part 13 covers the open side of the first part 12 to form a box body 10 with an accommodating space 11.

[0057] In the battery 100, there can be one or more battery cells 20. If there are more than one battery cell 20, the battery cells 20 can be connected in series, in parallel or in a mixed connection. A mixed connection means that the battery cells 20 are connected in series and in parallel. The battery cells 20 can be directly connected in series, in parallel or in a mixed connection, and then the whole formed by the battery cells 20 can be accommodated in the box 10; of course, the battery cells 20 can be connected in series, in parallel or in a mixed connection to form a battery module, and then the battery modules can be connected in series, in parallel or in a mixed connection to form a whole, and then accommodated in the box 10. The battery cell 20 can be cylindrical, flat, rectangular or in other shapes. Figure 2 The example shows that the battery cell 20 is in a square shape.

[0058] In some embodiments, the battery 100 may further include a busbar component (not shown), and the multiple battery cells 20 may be electrically connected via the busbar component to achieve series connection, parallel connection, or mixed connection of the multiple battery cells 20 .

[0059] Figure 4 This is a schematic diagram of the structure of a battery cell 20 provided in some embodiments of the present application. Figure 5 This is an exploded view of a battery cell 20 provided in some embodiments of the present application. Figure 4 and Figure 5 The battery cell 20 may include a housing 21 , an end cap assembly 22 and a wound electrode assembly 23 . The housing 21 has an opening 211 , the wound electrode assembly 23 is accommodated in the housing 21 , and the end cap assembly 22 is used to cover the opening 211 .

[0060] The shape of the outer shell 21 can be determined according to the specific shape of the wound electrode assembly 23. For example, if the wound electrode assembly 23 is a rectangular parallelepiped structure, the outer shell 21 can be a rectangular parallelepiped structure. Figure 3 and Figure 4 The case where the housing 21 and the wound electrode assembly 23 are square is exemplarily shown.

[0061] The shell 21 may also be made of a variety of materials, such as copper, iron, aluminum, stainless steel, aluminum alloy, etc., and the embodiment of the present application does not impose any special restrictions on this.

[0062] The end cap assembly 22 includes an end cap 221 and an electrode terminal 222. The end cap assembly 22 is used to seal the opening 211 of the outer shell 21 to form a closed installation space (not shown), and the installation space is used to accommodate the wound electrode assembly 23. The installation space is also used to accommodate an electrolyte, such as an electrolyte. The end cap assembly 22 is a component for outputting the electrical energy of the wound electrode assembly 23. The electrode terminal 222 in the end cap assembly 22 is used to be electrically connected to the wound electrode assembly 23, that is, the electrode terminal 222 is electrically connected to the pole ear of the wound electrode assembly 23. For example, the electrode terminal 222 is connected to the pole ear through the current collecting member 24 to achieve electrical connection between the electrode terminal 222 and the pole ear.

[0063] It should be noted that the opening 211 of the shell 21 can be one or two. If the opening 211 of the shell 21 is one, the end cap assembly 22 can also be one, and two electrode terminals 222 can be provided in the end cap assembly 22, and the two electrode terminals 222 are respectively used to electrically connect to the positive pole ear and the negative pole ear of the wound electrode assembly 23. If the opening 211 of the shell 21 is two, for example, the two openings 211 are provided on opposite sides of the shell 21, the end cap assembly 22 can also be two, and the two end cap assemblies 22 are respectively covered at the two openings 211 of the shell 21. In this case, the electrode terminal 222 in one end cap assembly 22 can be a positive electrode terminal, which is used to electrically connect to the positive pole ear of the wound electrode assembly 23; the electrode terminal 222 in the other end cap assembly 22 can be a negative electrode terminal, which is used to electrically connect to the negative electrode sheet 232 of the wound electrode assembly 23.

[0064] In some embodiments, Figure 5As shown, the battery cell 20 may further include an insulating protective member 25 fixed to the outer periphery of the wound electrode assembly 23, and the insulating protective member 25 is used to insulate and isolate the wound electrode assembly 23 from the housing 21. Exemplarily, the insulating protective member 25 is a tape bonded to the outer periphery of the wound electrode assembly 23. In some embodiments, there are multiple wound electrode assemblies 23, and the insulating protective member 25 is arranged around the outer periphery of the multiple wound electrode assemblies 23, and the multiple wound electrode assemblies 23 are formed into an integral structure to keep the structure of the wound electrode assembly 23 stable.

[0065] Figure 6 This is a schematic diagram of the structure of a wound electrode assembly 23 provided in some embodiments of the present application. Figure 5 and Figure 6 The wound electrode assembly 23 may include a positive electrode sheet 231, a negative electrode sheet 232 and a separator 233. The wound electrode assembly 23 is a wound structure formed by winding and hot pressing the positive electrode sheet 231, the separator 233 and the negative electrode sheet 232.

[0066] The positive electrode sheet 231 includes a positive electrode current collector and a positive electrode active material layer. The positive electrode active material layer is coated on the surface of the positive electrode current collector. The positive electrode current collector not coated with the positive electrode active material layer protrudes from the positive electrode current collector coated with the positive electrode active material layer. The positive electrode current collector not coated with the positive electrode active material layer serves as a positive electrode tab. The positive electrode active material layer can be coated on one side of the positive electrode current collector or on both sides of the positive electrode current collector. Taking a lithium-ion battery as an example, the material of the positive electrode current collector can be aluminum, and the positive electrode active material can be lithium cobalt oxide, lithium iron phosphate, ternary lithium or lithium manganese oxide, etc.

[0067] The negative electrode sheet 232 includes a negative electrode current collector and a negative electrode active material layer. The negative electrode active material layer is coated on the surface of the negative electrode current collector. The negative electrode current collector not coated with the negative electrode active material layer protrudes from the negative electrode current collector coated with the negative electrode active material layer. The negative electrode current collector not coated with the negative electrode active material layer serves as a negative electrode tab. The negative electrode active material layer can be coated on one side or both sides of the negative electrode current collector. The material of the negative electrode current collector can be copper, and the negative electrode active material can be carbon or silicon, etc. In order to ensure that a large current passes without melting, the number of positive electrode tabs is multiple and stacked together, and the number of negative electrode tabs is multiple and stacked together.

[0068] If a positive electrode active material layer is provided on one surface of the positive electrode collector to form a positive electrode sheet 231, and a negative electrode active material layer is provided on one surface of the negative electrode collector to form a negative electrode sheet 232, then the positive electrode sheet 231, the isolation film 233 and the negative electrode sheet 232 are stacked in sequence to form an electrode unit, and when stacking, the two sides of the isolation film 233 are respectively the positive electrode active material layer and the negative electrode active material layer, and the electrode unit is wound and hot-pressed to form a wound electrode assembly 23.

[0069] If the positive electrode current collector is provided with positive electrode active material layers on both surfaces to form a positive electrode sheet 231, and the negative electrode current collector is provided with negative electrode active material layers on both surfaces to form a negative electrode sheet 232, then the positive electrode sheet 231, the first separator 2331, the negative electrode sheet 232, and the second separator 2332 are sequentially stacked to form an electrode unit, or the first separator 2331, the positive electrode sheet 231, the second separator 2332, and the negative electrode sheet 232 are sequentially stacked to form an electrode unit, and the electrode unit is wound and hot-pressed to form a wound electrode assembly 23. The following is an example of stacking the first separator 2331, the positive electrode sheet 231, the second separator 2332, and the negative electrode sheet 232 to form an electrode unit, and winding and hot-pressing the electrode unit to form an electrode assembly.

[0070] Figure 7 For a schematic diagram of the structure between the bending area 235 and the straight area 234 provided in some embodiments of the present application, please refer to Figure 6 and Figure 7 The wound electrode assembly 23 provided in the embodiment of the present application has a plurality of straight regions 234 and a plurality of bending regions 235 (such as Figure 6 , the direction of arrow A is the winding direction, the area between the dotted line b1 and the dotted line b2 is the straight area 234, and the area outside the dotted line b2 is the bending area 235), the isolation film 233 includes a plurality of first areas 233a for winding in the straight area 234 and a plurality of second areas 233b for winding in the bending area 235, and the porosity of the first area 233a is 5% to 40% greater than the porosity of the second area 233b.

[0071] For the wound electrode assembly 23, each straight area 234 and each bending area 235 include a first isolation film 2331, a positive electrode sheet 231, a second isolation film 2332 and a negative electrode sheet 232 stacked in sequence; multiple first areas 233a of the isolation film 233 are correspondingly arranged in the multiple straight areas 234 of the wound electrode assembly 23, and multiple second areas 233b of the isolation film 233 are correspondingly arranged in the multiple bending areas 235 of the wound electrode assembly 23.

[0072] The first isolation film 2331 , the positive electrode sheet 231 , the second isolation film 2332 and the negative electrode sheet 232 stacked in sequence are wound to form a flat structure, and then hot pressed to form a relatively dense flat structure.

[0073] The straight region 234 of the wound electrode assembly 23 refers to the straight region of the flat structure; the bent region 235 of the wound electrode assembly 23 refers to the bent region of the flat structure. Figure 6As shown, from the outer circle of the wound electrode assembly 23 to the inner circle of the wound electrode assembly 23, the wound electrode assembly 23 includes a plurality of straight areas 234 from top to bottom; from the outer circle of the wound electrode assembly 23 to the inner circle of the wound electrode assembly 23, the wound electrode assembly 23 includes a plurality of bending areas 235 from left to right.

[0074] The positional relationship between the first isolation film 2331 and the second isolation film 2332 and the straight area 234 and the bending area 235 is basically the same, and the isolation film 233 is directly used for the following description. Figure 6 and Figure 7 , along the winding direction of the wound electrode assembly 23 ( Figure 6 The wound electrode assembly 23 is arranged in sequence along the straight area 234, the bending area 235, the straight area 234, the bending area 235, the straight area 234, the bending area 235, and so on; the isolation membrane 233 is arranged in sequence along the first area 233a, the second area 233b, the first area 233a, the second area 233b, and so on.

[0075] Since an inner circle and an outer circle appear after the wound electrode assembly 23 is wound, along the winding direction of the wound electrode assembly 23, the length of the straight area 234 close to the inner circle is substantially the same as the length of the straight area 234 far from the inner circle, and the length of the first area 233a of the isolation membrane 233 close to the inner circle is substantially the same as the length of the first area 233a of the isolation membrane 233 close to the outer circle; at the same time, the length of the bending area 235 close to the inner circle is substantially different from the length of the bending area 235 far from the inner circle, and the length of the second area 233b of the isolation membrane 233 close to the inner circle is substantially different from the length of the second area 233b of the isolation membrane 233 close to the outer circle.

[0076] Normally, there is no obvious boundary between the straight area 234 and the bending area 235 of the wound electrode assembly 23. The second area 233b of the isolation film 233 provided in the present application can correspond one-to-one with the bending area 235 of the wound electrode assembly 23 (along the winding direction of the wound electrode assembly 23, the length of the second area 233b is consistent with the length of the bending area 235); the second area 233b of the isolation film 233 can also partially correspond to the bending area 235 of the wound electrode assembly 23 (along the winding direction of the wound electrode assembly 23). In the winding direction of the wound electrode assembly 23, at least 80% of the length of the second region 233b corresponds to the bending region 235, and a small portion of the second region 233b corresponds to the straight region 234); the second region 233b of the isolation membrane 233 may also partially exceed the bending region 235 of the wound electrode assembly 23 (along the winding direction of the wound electrode assembly 23, the bending region 235 all corresponds to the second region 233b, and a small portion of the second region 233b exceeds the bending region 235 and corresponds to the straight region 234 of the wound electrode assembly 23).

[0077] The following description is made by taking the example that the length of the second region 233b is consistent with the length of the bending region 235 and the length of the first region 233a is consistent with the length of the straight region 234 along the winding direction of the wound electrode assembly 23. Figure 7 The first region 233a of the isolation membrane 233 corresponds to the straight region 234 of the wound electrode assembly 23, and the second region 233b of the isolation membrane 233 corresponds to the bending region 235 of the wound electrode assembly 23. The porosity of the isolation membrane 233 at the straight region 234 is greater than the porosity of the isolation membrane 233 at the bending region 235, and the difference is between 5% and 40%. This can make the amount of lithium ions passing through the isolation membrane 233 at the straight region 234 higher, thereby ensuring a higher cycle capacity retention rate of the battery 100; at the same time, the amount of lithium ions passing through the isolation membrane 233 at the bending region 235 can be lower, so that a small amount of lithium ions passing through the isolation membrane 233 can be more completely embedded in the negative electrode at the bending region 235, thereby improving the lithium plating problem at the bending region 235, thereby improving the performance of the battery 100.

[0078] Optionally, the porosity of the first region 233a of the isolation film 233 is X%, the porosity of the second region 233b of the isolation film 233 is Y%, and the difference between X% and Y% is 5% to 40%. As an example, the difference between X% and Y% is 5%, 10%, 15%, 20%, 25%, 30%, 35% or 40%, which can also be any value within the above range.

[0079] In some embodiments, the porosity of the first region 233a is 10% to 25% greater than the porosity of the second region 233b. The difference between X% and Y% is 10% to 25%. While ensuring the cycle capacity retention rate of the battery 100, the lithium deposition problem at the bending area 235 of the wound electrode assembly 23 can be further improved.

[0080] In some embodiments, the porosity of the first region 233a is 40% to 70%, and the porosity of the second region 233b is 20% to 40%. The straight region 234 of the wound electrode assembly 23 can have more lithium ions passing through, so that the cycle capacity retention rate of the battery 100 is higher, and the lithium ion content passing through the straight region 234 is more reasonable, which can avoid the lithium precipitation problem in the straight region 234 to a certain extent; at the same time, the bending region 235 of the wound electrode assembly 23 also has a certain amount of lithium ions passing through, which can ensure the cycle capacity retention rate of the battery 100 to a certain extent, and can also better improve the lithium precipitation problem in the bending region 235.

[0081] As an example, the porosity of the first region 233a of the isolation membrane 233 is X%, which is 40%, 45%, 50%, 55%, 60%, 65% or 70%, which can also be any value in the above range; the porosity of the second region 233b of the isolation membrane 233 is Y%, which is 20%, 25%, 30%, 35% or 40%, which can also be any value in the above range; and the difference between X% and Y% is between 5% and 40%.

[0082] In some embodiments, the porosity of the first region 233a is 40% to 70%, and the porosity of different positions of the first region 233a of the isolation film 233 may be the same or different, and may be any value within the range of 40% to 70%; the porosity of the second region 233b is 20% to 40%, and the porosity of different positions of the second region 233b of the isolation film 233 may be the same or different, and may be any value within the range of 40% to 70%. Optionally, in the same isolation film 233, the difference between the minimum porosity of the first region 233a and the maximum porosity of the second region 233b is ≥5%; the difference between the maximum porosity of the first region 233a and the minimum porosity of the second region 233b is ≤40%.

[0083] In some embodiments, the porosity of the first region 233a is 40% to 50%, and the porosity of the second region 233b is 20% to 31%. The porosity of the separator 233 at the straight region 234 and the bending region 235 of the wound electrode assembly 23 is more reasonable, which can improve the lithium deposition problem at the bending region 235 while maintaining a higher cycle capacity of the battery 100.

[0084] In some embodiments, the thickness of the isolation film 233 is 5 μm to 60 μm. The thickness of the isolation film 233 is relatively reasonable, which can avoid the short circuit problem caused by the puncture of the isolation film 233 to a certain extent, and can also make the volume energy density of the battery 100 higher.

[0085] As an example, the thickness of the isolation film 233 is 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 45 μm, 45 μm, 50 μm, 55 μm or 60 μm, which can also be any value within the above range.

[0086] In some embodiments, the thickness of the isolation film 233 is 5 μm to 60 μm. The thickness of the isolation film 233 at different positions may be the same or different, and may be any value within the range of 5 μm to 60 μm.

[0087] In some embodiments, the thickness of the isolation film 233 is 7 μm to 20 μm, which can further prevent the isolation film 233 from being punctured while ensuring that the volume energy density of the battery 100 is higher.

[0088] In some embodiments, the material of the isolation film 233 includes polyolefin. Polyolefin is a thermoplastic resin obtained by polymerizing or copolymerizing α-olefins such as ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 4-methyl-1-pentene, and some cyclic olefins. Using polyolefin as the main material of the isolation film 233, it is easy to achieve that the porosity of the first region 233a is 5% to 40% greater than the porosity of the second region 233b.

[0089] In some embodiments, the polyolefin includes polyethylene and / or polypropylene. The polyolefin is polyethylene, or the polyolefin is polypropylene, or the polyolefin is a copolymer of ethylene and propylene.

[0090] In some embodiments, the weight average molecular weight of the polyolefin is 800,000 to 1.6 million. As an example, the weight average molecular weight of the polyolefin is 800,000, 900,000, 1,000,000, 1,100,000, 1,200,000, 1,300,000, 1,400,000, 1,500,000 or 1,600,000, which can also be any value in the above range.

[0091] In some embodiments, the material of the isolation film 233 further includes an antioxidant. The antioxidant and the polyolefin are evenly distributed, which can extend the service life of the isolation film 233.

[0092] In some embodiments, the antioxidant includes at least one of 4,4-thiobis(6-tert-butyl-m-cresol), dibutylhydroxytoluene, phosphite, tert-butylhydroquinone, β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 2-tert-butyl-6-methylphenol, N,N'-di-β-naphthyl-p-phenylenediamine, dilauryl thiodipropionate, tris(nonylphenyl) phosphite, and triphenyl phosphite.

[0093] Please continue reading Figure 6 and Figure 7 The embodiment of the present application provides an isolation film 233 for preparing a wound electrode assembly 23. The wound electrode assembly 23 has a plurality of straight regions 234 and a plurality of bending regions 235 which are alternately arranged in sequence along the winding direction. The isolation film 233 includes a plurality of first regions 233a for winding in the straight regions 234 and a plurality of second regions 233b for winding in the bending regions 235. The porosity of the first regions 233a is 40% to 70%, and the porosity of the second regions 233b is 20% to 40%. The porosity of the first regions 233a is 5% to 40% greater than that of the second regions 233b.

[0094] Please continue reading Figure 6 and Figure 7 The wound electrode assembly 23 provided in the embodiment of the present application has a plurality of straight areas 234 and a plurality of bending areas 235 alternately arranged in sequence along the winding direction. The wound electrode assembly 23 includes a first isolation film 2331, a positive electrode sheet 231, a second isolation film 2332 and a negative electrode sheet 232 which are stacked and wound in sequence. The first isolation film 2331 and the second isolation film 2332 both include a first region 233a and a plurality of second regions 233b alternately arranged in sequence along the winding direction. The plurality of first regions 233a are correspondingly arranged in the plurality of straight areas 234, and the plurality of second regions 233b are correspondingly arranged in the plurality of bending areas 235. The porosity of the first region 233a is 40% to 70%, and the porosity of the second region 233b is 20% to 40%. The porosity of the first region 233a is 5% to 40% greater than that of the second region 233b.

[0095] After the above introduction to the structures of the isolation membrane 233 and the corresponding wound electrode assembly 23 , the preparation method of the isolation membrane 233 will be specifically introduced below.

[0096] The isolation film 233 has a plurality of first regions 233a and a plurality of second regions 233b which are alternately arranged in sequence along the length direction. The preparation method of the isolation film 233 comprises: subjecting the mixture to sequential extrusion, stretching, extraction and shaping. The mixture comprises a pore-forming agent, and the pore-forming agent is removed in the extraction step; in the stretching step, the stretching multiple of the first section of the casting corresponding to the first region 233a is greater than the stretching multiple of the second section of the casting corresponding to the second region 233b, so that the porosity of the first region 233a is 5% to 40% greater than the porosity of the second region 233b.

[0097] After the mixture is extruded, a film structure is formed. In order to distinguish it from the film after stretching, the film before stretching is referred to as a cast sheet, and the film after stretching is referred to as a film. After the first section of the cast sheet is stretched, the length of the first section of the film is consistent with the length of the first area 233a of the isolation film 233; after the second section of the cast sheet is stretched, the length of the second section of the film is consistent with the length of the second area 233b of the isolation film 233.

[0098] The stretching ratio means: if the length of the first section of the cast sheet before stretching is L1, and the length of the first section of the film after stretching is 2×L1, then the stretching ratio of the first section of the cast sheet in the length direction is 2 times; if the length of the first section of the cast sheet before stretching is L1, and the length of the first section of the film after stretching is 3×L1, then the stretching ratio of the first section of the cast sheet in the length direction is 3 times; if the width of the first section of the cast sheet before stretching is D1, and the width of the first section of the film after stretching is 2×D1, then the stretching ratio of the first section of the cast sheet in the width direction is 2 times; if the width of the first section of the cast sheet before stretching is D1, and the width of the first section of the film after stretching is 3×D1, then the stretching ratio of the first section of the cast sheet in the width direction is 3 times, and so on.

[0099] The stretching ratio of the first section of the casting corresponding to the first area 233a is greater than the stretching ratio of the second section of the casting corresponding to the second area 233b: the stretching ratio of the first section of the casting in the width direction can be greater than the stretching ratio of the second section of the casting in the width direction, and the stretching ratio of the first section of the casting in the length direction is consistent with the stretching ratio of the second section of the casting in the length direction; the stretching ratio of the first section of the casting in the width direction can be consistent with the stretching ratio of the second section of the casting in the width direction, and the stretching ratio of the first section of the casting in the length direction is greater than the stretching ratio of the second section of the casting in the length direction; the stretching ratio of the first section of the casting in the width direction can be greater than the stretching ratio of the second section of the casting in the width direction, and the stretching ratio of the first section of the casting in the length direction is greater than the stretching ratio of the second section of the casting in the length direction.

[0100] The mixture contains a pore-forming agent, which can form a certain porosity in the isolation membrane 233; and along the length direction of the casting, the stretching multiple of the first section of the casting is different from the stretching multiple of the second section of the casting. After removing the pore-forming agent, the porosity of the first area 233a of the corresponding isolation membrane 233 can be 5% to 40% larger than the porosity of the second area 233b. After using the isolation membrane 233 to prepare the wound electrode assembly 23, the cycle capacity retention rate of the battery 100 can be guaranteed while improving the occurrence of lithium plating problems. Figure 8 For a flowchart of the preparation process of the isolation film 233 provided in some embodiments of the present application, please refer to Figure 8 The method for preparing the isolation film 233 provided in the embodiment of the present application includes the following steps:

[0101] S110, feeding step: uniformly mixing the polyolefin, pore former and antioxidant to obtain a mixed material, and then conveying the mixed material to an extrusion system.

[0102] Among them, polyolefins are thermoplastic resins obtained by polymerizing or copolymerizing α-olefins such as ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 4-methyl-1-pentene, and certain cycloolefins. For example, polyolefins are polyethylene and / or polypropylene, and the weight average molecular weight of the polyolefins is 800,000 to 1.6 million.

[0103] The pore former includes one or more of white oil, paraffin oil, kerosene or dioctyl phthalate.

[0104] The antioxidant includes at least one of 4,4-thiobis(6-tert-butyl-m-cresol), dibutylhydroxytoluene, phosphite, tert-butylhydroquinone, β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate, 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 2-tert-butyl-6-methylphenol, N,N'-di-β-naphthyl-p-phenylenediamine, dilauryl thiodipropionate, tris(nonylphenyl) phosphite, and triphenyl phosphite.

[0105] In the embodiments of the present application, the mixture is not limited to the above three components, and other components may be added, such as: plasticizers, extractants, and additives to improve the uniformity of pore formation, etc., and antioxidants may not be added, or polyolefins may not be used, and other materials (such as: polyester resin, cellulose non-woven fabric, polyimide electrospun porous membrane and its modified products, etc.) may be used to prepare the isolation membrane 233, etc.

[0106] S120, extrusion step: the mixed material in step S110 is passed through a twin-screw extruder at a temperature of 100°C to 200°C to obtain a high-temperature melt, which is then accurately metered by a melt pump and fed into a die head to be extruded from the slit of the die head. The extruded high-temperature melt is treated by a chilled roller to obtain an extruded cast sheet.

[0107] Optionally, the extrusion temperature of the twin-screw extruder is 100°C, 110°C, 120°C, 130°C, 140°C, 150°C, 160°C, 170°C, 180°C, 190°C or 200°C, which can also be any value within the above range. If the polyolefin is polyethylene, the extrusion temperature of the twin-screw extruder is 100°C to 140°C; if the polyolefin is polypropylene, the extrusion temperature of the twin-screw extruder is 180°C to 200°C.

[0108] S130, stretching step: the extruded cast sheet is placed in a synchronous stretching machine or an asynchronous stretching machine, and is synchronously or asynchronously stretched in the longitudinal direction (MD, length direction) and the transverse direction (TD, width direction) to obtain a film containing a pore-forming agent.

[0109] During stretching, the stretching ratio of the first section of the cast sheet corresponding to the first region 233 a of the isolation film 233 is greater than the stretching ratio of the second section of the cast sheet corresponding to the second region 233 b of the isolation film 233 .

[0110] S140, extraction step: soaking the film containing the pore-forming agent in an organic solvent for 10 minutes to 20 minutes to remove the pore-forming agent in the film, and then drying at 35° C. to 45° C. to volatilize the organic solvent to form a film containing micropores.

[0111] Optionally, the organic solvent includes at least one of dichloromethane, carbon tetrachloride or ether. The time for soaking in the organic solvent can be 10 min, 12 min, 14 min, 16 min, 18 min or 20 min, which can also be any value in the above range; the drying temperature can be 35° C., 37° C., 39° C., 41° C., 43° C. or 45° C., which can also be any value in the above range.

[0112] S150, shaping step: the microporous film obtained in step S140 is placed in a high-temperature device 5°C to 60°C below the melting temperature of polyolefin to fully eliminate the internal stress of the film to obtain an isolation film 233; in the isolation film 233, the porosity of the first region 233a is 5% to 40% greater than the porosity of the second region 233b.

[0113] Optionally, the temperature for relieving film stress may be 5° C., 10° C., 20° C., 30° C., 40° C. or 50° C. lower than the melting temperature of polyolefin, and may also be any value within the above range.

[0114] S160, cutting: cutting the separator 233 into finished separators 233 according to the specification requirements of the wound electrode assembly.

[0115] In some embodiments, the stretching multiple of the first segment of the casting in the width direction is consistent with the stretching multiple of the second segment of the casting in the width direction, and the stretching multiple of the first segment of the casting in the length direction is 1 to 10 times greater than the stretching multiple of the second segment of the casting in the length direction. After the separation film 233 is formed, the width of the separation film 233 is basically consistent, so as to facilitate the subsequent preparation of the wound electrode assembly. At the same time, the difference in the stretching multiple in the length direction is limited to the above range, which makes it easy to make the porosity of the first region 233a of the separation film 233 greater than the porosity of the second region 233b by 5% to 40%.

[0116] Generally speaking, when other conditions are the same, the smaller the difference between the stretching ratio of the first section of the casting in the length direction and the stretching ratio of the second section of the casting in the length direction, the smaller the difference between the porosity of the corresponding first region 233a and the porosity of the second region 233b; the larger the difference between the stretching ratio of the first section of the casting in the length direction and the stretching ratio of the second section of the casting in the length direction, the larger the difference between the porosity of the corresponding first region 233a and the porosity of the second region 233b.

[0117] As an example, the stretching ratio of the first segment of the cast sheet in the length direction is 1 times, 2 times, 3 times, 4 times, 5 times, 6 times, 7 times, 8 times, 9 times or 10 times greater than the stretching ratio of the second segment of the cast sheet in the length direction, and it can also be any value within the above range.

[0118] In some embodiments, the stretching ratio of the first segment of the casting in the length direction is 2 to 4 times greater than that of the second segment of the casting in the length direction, so that the porosity of the first region 233a of the isolation membrane 233 is 10% to 25% greater than that of the second region 233b.

[0119] The relationship between the stretching ratio, the amount of pore-forming agent added, and the final porosity of the isolation film 233 is described in detail below.

[0120] Optionally, in the mixture, the mass percentage of polyolefin is 30wt% to 65wt%, the mass percentage of pore-forming agent is 30wt% to 60wt%, and the mass percentage of antioxidant is 5wt% to 10wt%.

[0121] As an example, the mass percentage of polyolefin is 30wt%, 35wt%, 40wt%, 45wt%, 50wt%, 55wt%, 60wt% or 65wt%, which can also be any value in the above range; the mass percentage of pore-forming agent is 30wt%, 35wt%, 40wt%, 45wt%, 50wt%, 55wt% or 60wt%, which can also be any value in the above range; the mass percentage of antioxidant is 5wt%, 6wt%, 7wt%, 8wt%, 9wt% or 10wt%, which can also be any value in the above range, and the sum of the mass fractions of the above components is 100%.

[0122] In some embodiments, the first segment of the casting is stretched 4 to 8 times in the length direction, and the second segment of the casting is stretched 2 to 4 times in the length direction. The content of the pore-forming agent is limited to the above range, and in combination with the stretching ratio, the porosity of the first region 233a of the isolation membrane 233 can be 40% to 70%, and the porosity of the second region 233b can be 20% to 40%.

[0123] As an example, the stretching ratio of the first section of the cast sheet in the length direction is 4 times, 5 times, 6 times, 7 times or 8 times, which can also be any value in the above range; the stretching ratio of the second section of the cast sheet in the length direction is 2 times, 3 times or 4 times, which can also be any value in the above range; and the difference between the stretching ratio of the first section of the cast sheet in the length direction and the stretching ratio of the second section of the cast sheet in the length direction is ≥ 1 times.

[0124] In some embodiments, the stretching multiples in the width direction of the first segment of the casting sheet and the stretching multiples in the width direction of the second segment of the casting sheet are both 4 to 6 times. The stretching multiples in the width direction and the stretching multiples in the length direction can be matched to make the porosity of the first region 233a 40% to 50%, and the porosity of the second region 233b 20% to 31%.

[0125] As an example, the stretching ratio of the first section of the cast sheet in the width direction and the stretching ratio of the second section of the cast sheet in the width direction are both 4 times, 5 times or 6 times, which can also be any value in the above range.

[0126] In other embodiments, the stretching multiple of the first segment of the casting in the width direction may be 1 to 10 times greater than the stretching multiple of the second segment of the casting in the width direction, and the stretching multiple of the first segment of the casting in the length direction is consistent with the stretching multiple of the second segment of the casting in the length direction. For the same isolation film 233, its width is different, but it can still meet the purpose of the porosity of the corresponding first area 233a being 5% to 40% greater than the porosity of the second area 233b.

[0127] For example: the stretching multiple of the first section of the casting in the width direction is 2 to 8 times, the stretching multiple of the second section of the casting in the width direction is 1 to 4 times, the stretching multiple of the first section of the casting in the length direction and the stretching multiple of the second section of the casting in the length direction are both 2 to 8 times; and the difference between the stretching multiples of the first section of the casting in the width direction and the stretching multiples of the second section of the casting in the width direction is ≥ 1 times.

[0128] In other embodiments, the stretching multiple of the first segment of the casting in the width direction may be 1 to 10 times greater than the stretching multiple of the second segment of the casting in the width direction, and the stretching multiple of the first segment of the casting in the length direction may be 1 to 10 times greater than the stretching multiple of the second segment of the casting in the length direction. For the same isolation film 233, its width is different, but it can still meet the purpose of the porosity of the corresponding first area 233a being 5% to 40% greater than the porosity of the second area 233b.

[0129] For example: the stretching ratio of the first section of the casting in the width direction is 1 to 4 times, the stretching ratio of the first section of the casting in the length direction is 2 to 6 times, the stretching ratio of the second section of the casting in the width direction is 1 to 3 times, and the stretching ratio of the second section of the casting in the length direction is 2 to 6 times; and the difference between the stretching ratios of the first section of the casting in the width direction and the stretching ratios of the second section of the casting in the width direction is ≥ 1 times, or / and the difference between the stretching ratios of the first section of the casting in the length direction and the stretching ratios of the second section of the casting in the length direction is ≥ 1 times.

[0130] In some embodiments, the isolation film 233 has a plurality of first regions 233a and a plurality of second regions 233b arranged alternately in the length direction, and the preparation method of the isolation film 233 includes: subjecting the mixture to extrusion, stretching, extraction and shaping in sequence. The mixture includes 30wt% to 60wt% of a pore-forming agent, and the pore-forming agent is removed in the extraction step; in the stretching step, the stretching multiples in the width direction of the first segment of the casting and the stretching multiples in the width direction of the second segment of the casting are both 4 to 6 times, the stretching multiples in the length direction of the first segment of the casting are 4 to 8 times, and the stretching multiples in the length direction of the second segment of the casting are 2 to 4 times; and the stretching multiples in the length direction of the first segment of the casting are 2 to 4 times greater than the stretching multiples in the length direction of the second segment of the casting. The porosity of different regions of the same isolation film 233 can be controlled by adjusting the stretching multiple, and the adjustment is more convenient.

[0131] After introducing the method for preparing the isolation film 233 above, the method for preparing the wound electrode assembly will be specifically introduced below. Before introducing the method for preparing the wound electrode assembly, the method for preparing the isolation film 233 will be supplemented with an explanation.

[0132] Please continue reading Figure 6Based on the structure of the wound electrode assembly, the length of each section of the separator 233 (each first region 233a and each second region 233b) can be preset. For example, from the inner circle to the outer circle, the length of the separator 233 at the straight region 234 is basically the same, that is, the length of the first region 233a of the separator 233 is basically the same, and the length of the first section of the cast sheet after stretching is set as the target length of the first region 233a of the separator 233. From the inner circle to the outer circle, the length of the separator 233 at the bending region 235 is different, that is, the length of the second region 233b of the separator 233 close to the inner circle is greater than the length of the second region 233b of the separator 233 far from the outer circle, and the length of the second section of the cast sheet after stretching is set as the target length of the second region 233b of the separator 233.

[0133] Based on the target size of the wound electrode assembly 23, the length of each first region 233a and each second region 233b of the isolation membrane 233 is first preset, and then based on the setting of the stretching multiple, the length of each first region 233a and the length of each second region 233b of the prepared isolation membrane 233 are made to be basically consistent with the preset length, thereby obtaining the target isolation membrane 233.

[0134] After preparing the separator 233, the wound electrode assembly 23 is prepared. First, the first separator 2331, the positive electrode sheet 231, the second separator 2332 and the negative electrode sheet 232 are stacked in sequence. When stacking, the first first region 233a of the first separator 2331 and the first first region 233a of the second separator 2332 are aligned, and then wound in sequence to form a wound flat structure, and then hot pressed to make the flat structure more compact. At this time, the first region 233a of the first separator 2331 and the first region 233a of the second separator 2332 are both located in the straight region 234 of the flat structure.

[0135] The wound electrode assembly 23 can be used to prepare a battery cell 20 , and the battery cell 20 can be used to prepare a battery 100 and provide electrical energy to electrical devices.

[0136] Next, one or more embodiments are described in more detail with reference to the following examples. Of course, these examples do not limit the scope of one or more embodiments.

[0137] Example

[0138] Preparation of the first isolation film 2331 and the second isolation film 2332:

[0139] Step 1: Based on the target size of the wound electrode assembly 23 , preset the length of each first region 233a and each second region 233b of the first isolation film 2331 ; and the length of each first region 233a and each second region 233b of the second isolation film 2332 .

[0140] Step 2: Weigh the following raw materials in weight ratio and mix them thoroughly to obtain a mixture: 47% polyethylene with an average molecular weight of 1.2 million, 45% paraffin oil, and 8% phosphite.

[0141] The third step: the mixed raw materials are passed through a twin-screw extruder at 180°C to obtain a high-temperature melt, the high-temperature melt is accurately measured by a melt pump and then fed into the die head, the high-temperature melt fed into the die head flows out from the slit of the die head, and the high-temperature melt flowing out from the slit of the die head passes through a chilled roller to obtain an extruded casting sheet.

[0142] Step 4: The extruded cast sheet enters a bidirectional synchronous stretching machine, and the stretching multiples in the length direction (longitudinal direction, MD) and the width direction (transverse direction, TD) of the first section of the cast sheet and the stretching multiples in the length direction (longitudinal direction, MD) and the width direction (transverse direction, TD) of the second section of the cast sheet are set respectively, so that the length of the film after the first section of the cast sheet is stretched is consistent with the length of the first region 233a, the length of the film after the second section of the cast sheet is stretched is consistent with the length of the second region 233b, the width of the film after the first section of the cast sheet is stretched is consistent with the width of the first region 233a, and the width of the film after the second section of the cast sheet is stretched is consistent with the width of the second region 233b, so as to obtain a film containing a pore-forming agent.

[0143] Step 5: After the film containing the pore-forming agent is fully soaked in dichloromethane for 20 minutes, the pore-forming agent in the film is removed, and then the organic solvent is evaporated by drying at 40° C. to form a film containing micropores.

[0144] Step 6: The microporous film obtained in step 5 is kept in a high temperature shaping device at 130°C for 6 hours to fully remove the stress of the diaphragm, and finally a microporous isolation membrane 233 with a thickness of 10 μm is obtained, which is then cut into finished membranes according to the specifications of the battery cell.

[0145] Preparation of negative electrode sheet 232:

[0146] The first step: preparing the negative electrode material, wherein the negative electrode material comprises 80 wt % of negative electrode active material particle graphite, 15 wt % of conductive agent Super P, 3 wt % of thickener sodium carboxymethyl cellulose and 2 wt % of binder styrene-butadiene rubber.

[0147] Step 2: Use a 4μm copper foil current collector to coat the negative electrode active material of the first step on both surfaces of the copper foil current collector to obtain a negative electrode active material layer, then roll-press to obtain a negative electrode sheet 232, and then cut into strips to obtain a finished negative electrode sheet 232.

[0148] Preparation of positive electrode sheet 231:

[0149] The first step: preparing the positive electrode material, wherein the positive electrode material includes 80 wt % of positive electrode active material particles LiCoO2, 10 wt % of conductive agent Super P and 10 wt % of binder polyvinylidene fluoride.

[0150] Step 2: Use a 9μm aluminum foil current collector to coat the positive electrode active material of the first step on both surfaces of the aluminum foil current collector to obtain a positive electrode active material layer, then roll-press to obtain a positive electrode sheet 231, and then cut into strips to obtain a finished positive electrode sheet 231.

[0151] Preparation of wound electrode assembly:

[0152] The first separator 2331, the positive electrode sheet 231, the second separator 2332 and the negative electrode sheet 232 are stacked in sequence. When stacking, the first first region 233a of the first separator 2331 and the first first region 233a of the second separator 2332 are aligned, and then wound in sequence to form a wound flat structure, and then hot pressed. At this time, the first region 233a of the first separator 2331 and the first region 233a of the second separator 2332 are both located in the straight region 234 of the wound flat structure.

[0153] Preparation of battery cell 20:

[0154] The wound electrode assembly 23 is placed in a housing and injected with electrolyte to form a battery cell 20 .

[0155] The stretching multiples and porosities of the first isolation film 2331 and the second isolation film 2332 are shown in Table 1:

[0156] Table 1 Stretching ratio and porosity of the separator

[0157]

[0158] The method for detecting the porosity of the first area 233a and the second area 233b of the isolation membrane 233 is as follows: knead the isolation membrane 233 test sample of the first area 233a or the isolation membrane 233 test sample of the second area 233b into a ball and stuff it into a sample cup, place the sample cup containing the isolation membrane 233 sample in a true density tester, close the test system, and introduce helium according to the program, detect the pressure of the gas in the sample chamber and the expansion chamber, and then calculate the real volume according to Bohr's law (PV=nRT), so as to obtain the porosity of the isolation membrane 233 sample to be tested. Sample cup volume: 3.5cm 3 , analysis gas: helium.

[0159] Three battery cells 20 were selected from each of the battery cells 20 of Examples 1 to 8 and Comparative Examples 1 to 4, and were placed in a 25°C constant temperature box for a constant current 2C charge / 1C discharge cycle for 1000 cycles. The battery cells 20 were then disassembled, and the anode pole pieces at the corners were taken to count the number of corners where lithium deposition occurred. The statistical results are shown in Table 2:

[0160] The test results of the cycle capacity retention rate of the battery cells 20 of Examples 1 to 8 and Comparative Examples 1 to 4 are shown in Table 2, wherein the voltage range is: 2.5 to 4.35V, charging: 1 / 3C constant current charging to 4.35V, 4.35V constant voltage charging to current <0.05C; after standing for 30 minutes, discharging: 1 / 3C constant current discharge to 2.5V, the measured discharge capacity. 1000-cycle cycle capacity retention rate = 1000th cycle discharge capacity / first cycle discharge capacity × 100%.

[0161] Table 2 Performance of wound electrode assembly

[0162]

[0163] It can be seen from Tables 1 and 2 that the wound electrode assembly 23 provided in the embodiment of the present application can not only improve the lithium plating problem in the bending area, but also make the cycle capacity retention rate of the battery cell 20 higher, because the porosity of the isolation membrane 233 in the straight area is 5% to 40% greater than the porosity of the isolation membrane 233 in the bending area.

[0164] From the comparison of Examples 1 to 8, it can be seen that the lithium deposition problem of the wound electrode assembly prepared by the separator 233 provided in Examples 2 to 4 is significantly improved (the number of lithium deposition corners is not more than 12), and the cycle capacity retention rate is also very high (more than 82%). It can be seen that when the porosity of the first region 233a is 40% to 50%, the porosity of the second region 233b is 25% to 31%, and the porosity of the first region 233a is 10% to 25% greater than the porosity of the second region 233b, the lithium deposition problem of the wound electrode assembly can be significantly improved, and the cycle capacity retention rate of the battery 100 is also very high.

[0165] The embodiments described above are part of the embodiments of the present application, rather than all of the embodiments. The detailed description of the embodiments of the present application is not intended to limit the scope of the present application for protection, but merely represents the selected embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present application.

Claims

1. A separation film, characterized in that: Used to prepare a wound electrode assembly, the wound electrode assembly has a plurality of straight areas and a plurality of bending areas alternately arranged in sequence along the winding direction, the isolation film includes a plurality of first areas for winding in the straight areas and a plurality of second areas for winding in the bending areas, the porosity of the first areas is 5% to 40% greater than the porosity of the second areas.

2. The isolation film according to claim 1, characterized in that: The porosity of the first region is 10% to 25% greater than the porosity of the second region.

3. The isolation film according to claim 2, characterized in that: The porosity of the first region is 40% to 70%, and the porosity of the second region is 20% to 40%.

4. The isolation membrane according to claim 3, characterized in that: The porosity of the first region is 40% to 50%, and the porosity of the second region is 20% to 31%.

5. The isolation film according to any one of claims 1 to 4, characterized in that: The isolation film has a thickness of 5 μm to 60 μm.

6. The isolation film according to claim 5, characterized in that: The isolation film has a thickness of 7 μm to 20 μm.

7. The isolation film according to any one of claims 1 to 4, characterized in that: The material of the isolation film includes polyolefin.

8. A wound electrode assembly, characterized in that: The wound electrode assembly has a plurality of straight regions and a plurality of bending regions alternately arranged in sequence along the winding direction, and the wound electrode assembly includes a positive electrode sheet, an isolation film as described in any one of claims 1 to 7, and a negative electrode sheet stacked and wound in sequence, and the plurality of the first regions of the isolation film are correspondingly arranged in the plurality of the straight regions, and the plurality of the second regions of the isolation film are correspondingly arranged in the plurality of the bending regions.

9. A battery cell, characterized in that: Comprising the wound electrode assembly as described in claim 8.

10. A battery, characterized in that: Comprising the battery cell according to claim 9.

11. An electrical device, characterized in that: Comprising the battery as claimed in claim 10.

12. A method for preparing an isolation film, characterized in that: The isolation film has a plurality of first regions and a plurality of second regions alternately arranged in sequence along the length direction, and the preparation method comprises: subjecting the mixed material to sequential extrusion, stretching, extraction and shaping; Wherein, the mixture includes a pore-forming agent, and the extraction step removes the pore-forming agent; in the stretching step, the stretching multiple of the first section of the cast sheet corresponding to the first region is greater than the stretching multiple of the second section of the cast sheet corresponding to the second region, so that the porosity of the first region is 5% to 40% greater than the porosity of the second region.

13. The preparation method according to claim 12, characterized in that: The stretching ratio of the first section of the cast sheet in the width direction is consistent with the stretching ratio of the second section of the cast sheet in the width direction, and the stretching ratio of the first section of the cast sheet in the length direction is 1 to 10 times greater than the stretching ratio of the second section of the cast sheet in the length direction.

14. The preparation method according to claim 13, characterized in that: The stretching ratio of the first section of the cast sheet in the length direction is 2 to 4 times greater than the stretching ratio of the second section of the cast sheet in the length direction.

15. The preparation method according to claim 13, characterized in that: The mixture comprises 30 wt% to 65 wt% of polyolefin, 30 wt% to 60 wt% of the pore former and 5 wt% to 10 wt% of an antioxidant; The stretching ratio of the first section of the cast sheet in the length direction is 4 to 8 times, and the stretching ratio of the second section of the cast sheet in the length direction is 2 to 4 times.

16. The preparation method according to claim 15, characterized in that: The stretching ratio of the first section of the cast sheet in the width direction and the stretching ratio of the second section of the cast sheet in the width direction are both 4 to 6 times.

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