A diaphragm and a secondary battery

Abstract

The application belongs to the technical field of secondary batteries, and mainly relates to a separator suitable for a wound electrode assembly, the electrode assembly having a corner region in a curved state, the separator comprising: a base film, in a flattened state, along the length direction of the base film, the base film comprising straight sections and to-be-bent sections connected alternately in sequence, the to-be-bent sections corresponding to the corner region; a first coating layer arranged on both surfaces of the base film along the thickness direction thereof respectively; and a second coating layer arranged on at least one surface of the base film along the thickness direction thereof, the second coating layer being arranged between the base film and the first coating layer; the second coating layer comprises a plurality of polymer layers, the plurality of polymer layers being arranged at intervals along the length direction of the base film; the polymer layers are arranged correspondingly to the to-be-bent sections; wherein at least one polymer layer is provided with at least two grooves; the application relieves the extrusion force suffered by the separator by intermittently coating the polymer layers; in addition, the application designs a secondary battery comprising the above separator, and further improves the stability of the secondary battery.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of secondary batteries, and mainly relates to a separator and a secondary battery. BACKGROUND

[0002] Lithium ion batteries have been widely used in many fields such as consumer electronics, energy storage systems and new energy vehicles due to their excellent energy density, long stable cycle life and environmental protection characteristics. At present, most of the battery cells adopt a winding structure, and many improvements have been made to the battery electrode and separator according to the problem of lithium precipitation in the corner during the winding process of the electrode.

[0003] Under the current technical background, in view of the technical problem of "lithium precipitation in the corner", experts in the industry generally tend to coat a material with a specific compression ratio on the surface of the cell separator to reduce the pressure on the electrode during the winding process. However, it is worth noting that the above solution does not fully consider the chain reaction caused by the expansion of the anode and cathode inside the cell during the cycle. Specifically, the expansion will increase the tension and extrusion force of the coating layer on the separator in the corner area. At this time, the coating design originally intended to alleviate the pressure may backfire and adversely affect the separator, which may cause the separator to rupture, the ion transmission path to break, and other serious consequences, forming a "black spot lithium precipitation" phenomenon, and ultimately damaging the cycle performance of the battery.

[0004] Therefore, it is urgent to improve the existing separator and secondary battery to solve the defects in the prior art. SUMMARY

[0005] One of the purposes of the present application is to overcome the technical problem that the prior art coats both sides of the separator with a polymer layer and a first coating layer, ignoring the fact that the polymer layer and the first coating layer will also cause a certain degree of extrusion in the corner area during the cycle of the battery. The coating structure of the battery separator is improved.

[0006] In order to achieve the above technical purpose, the following technical solutions are adopted in the present application:

[0007] A separator suitable for a winding type electrode assembly, the electrode assembly having a corner area in a curved state, the separator comprising:

[0008] a base film, in a flattened state, along the length direction of the base film, the base film comprises a straight segment and a to-be-bent segment connected alternately in sequence, the to-be-bent segment corresponding to the corner area;

[0009] a first coating layer arranged on both surfaces of the base film along the thickness direction thereof;

[0010] a second coating layer arranged on at least one surface of the base film along the thickness direction thereof, the second coating layer 4 being arranged between the base film and the first coating layer;

[0011] The second coating layer comprises a plurality of polymer layers, which are arranged at intervals along the length direction of the base film; the polymer layers are arranged corresponding to the bending section;

[0012] At least one of the polymer layers is provided with at least two grooves.

[0013] Through the above technical solutions, the following technical effects are achieved:

[0014] Firstly, the present application only coats the base film in the corner area of the winding electrode assembly with polymer layers, and each polymer layer in the corner area is intermittently coated, and there are many grooves. This design not only effectively reduces the loss of coating polymer layers in the non-corner area of the winding electrode, but also avoids the compression or stretching of the polymer layer during the winding process of the winding electrode, which causes the polymer layer to generate other forces on the separator and the electrode in the corner area, thereby further effectively relieving the risk of damage to the separator due to extrusion during the winding process of the electrode assembly, and ensuring the working stability of the separator.

[0015] As a further improvement of the separator of the present application, the electrode assembly comprises electrode pieces in a winding structure, the electrode pieces being in contact with the separator; the length of the polymer layer corresponding to the bending section is L1, the separator has a recessed area and a raised area in the bending section, and the arc length of the raised area is L2, wherein L1 and L2 satisfy: L2≥L1≥0.8L2.

[0016] As a further improvement of the separator of the present application, the diameter of the circular arc of the raised area of the innermost layer of the separator in the bending section is Ds0, and the arc length L 21 satisfies L 21 =π(Ds0) / 2.

[0017] As a further improvement of the separator of the present application, the number of winding layers of the separator is n, and the arc length L 2n satisfies L 2n =π(Ds0+(N-1)△t) / 2.

[0018] Wherein, for each additional winding layer based on the innermost winding layer, the increase of the diameter of the circular arc corresponding to the raised area of the separator in the bending section relative to the diameter of the circular arc corresponding to the raised area of the separator in the bending section of the adjacent inner layer is △t.

[0019] As a further improvement of the separator of the present application, along the length direction of the polymer layer, the width of the groove first increases and then decreases, and the width of the groove corresponding to the center of the length of the bending section is the largest.

[0020] As a further improvement of the separator of the present application, the width of the groove is D1, and D1 satisfies: 0.9mm-1.1mm.

[0021] As a further improvement of the diaphragm of the application, the depth of the groove increases first and then decreases along the length direction of the polymer layer, and the depth of the groove corresponding to the length center of the bending section is the largest.

[0022] As a further improvement of the diaphragm of the application, the depth of the groove is D2, and D2 satisfies: 0.5 microns-10 microns.

[0023] As a further improvement of the diaphragm of the application, the polymer layer comprises an emulsion of polymer particles, a dispersion auxiliary agent, and an adhesive;

[0024] The emulsion of polymer particles is one or more of polyethylene emulsion, polyethylene micro-wax emulsion, polyvinylidene fluoride emulsion, polytetrafluoroethylene emulsion, polymethyl methacrylate emulsion, polyimide emulsion, polystyrene emulsion, and polyacrylamide emulsion;

[0025] The dispersion auxiliary agent is one or more of polyacrylic acid and its sodium salt, polyacrylamide, polyvinyl alcohol, sodium citrate, sodium ethylenediaminetetraacetate, sodium diacetate, sodium hexametaphosphate, sodium silicate, and carboxymethyl cellulose;

[0026] The adhesive is a polypropylene-based adhesive.

[0027] As the second technical purpose of the application, in order to overcome the defects of the prior art, a secondary battery with higher working stability is provided.

[0028] In order to achieve the above technical purpose, the application adopts the following technical scheme:

[0029] A secondary battery comprises the diaphragm shown in any of the above.

[0030] Through the above technical scheme, the following technical effects are produced:

[0031] Thanks to the diaphragm provided by the application, by intermittently coating the polymer layer in each corner area of the electrode assembly, the diaphragm and the pole piece in the corner area will not be affected by the polymer layer force due to the compression or stretching of the polymer layer during the winding process of the winding type battery cell or when the battery is subjected to external force, and at the same time, the external force acting on the diaphragm and the pole piece is effectively alleviated, effectively avoiding the occurrence of the "black spot lithium precipitation" phenomenon, thereby improving the cycle stability of the battery and prolonging the service life thereof. BRIEF DESCRIPTION OF DRAWINGS

[0032] The drawings described herein are used to provide further understanding of the application, and form a part of the application. The illustrative embodiments of the application and their descriptions serve to explain the application, and do not constitute an improper limitation on the application. In the drawings:

[0033] Figure 1A schematic diagram of the diaphragm structure in the present application;

[0034] Figure 2 A schematic diagram of the electrode assembly structure in the present application;

[0035] Figure 3 A schematic diagram of the diaphragm structure in the present application; Figure 2 An enlarged view of A in the present application;

[0036] Figure 4 A schematic diagram of the structure of the pole piece and the diaphragm in the present application;

[0037] Figure 5 A curve graph of the cycle capacity retention rate of the battery at normal temperature in Example 1 and Comparative Example 1 of the present application;

[0038] Figure 6 A curve graph of the cycle thickness expansion rate of the battery at normal temperature in Example 1 and Comparative Example 1 of the present application.

[0039] Wherein:

[0040] 1-base film;

[0041] 11-straight section;

[0042] 12-bending section;

[0043] 2-wound electrode assembly;

[0044] 21-corner area;

[0045] 22-pole piece;

[0046] 3-first coating layer;

[0047] 4-second coating layer;

[0048] 41-polymer layer;

[0049] 411-groove. DETAILED DESCRIPTION

[0050] The technical solutions in the embodiments of the present application will be described clearly and completely below in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments of the present application. Based on the embodiments in the present application, all the other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present application. Unless otherwise defined, all the technical and scientific terms used in the present application have the same meanings as those commonly understood by those skilled in the art to which the present application belongs. The terms used in the specification of the present application are only for the purpose of describing the specific embodiments of the present application, and are not intended to limit the present application.

[0051] In the description of the present application, unless specifically defined and limited otherwise, the terms "mounting", "connected", "connection", "fixed", should be understood broadly, for example, can be fixed connection, can also be detachable connection, or integral; can be mechanical connection, can also be electrical connection; can be directly connected, can also be indirectly connected through an intermediate medium, can be the internal communication of two elements or the interaction relationship between two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0052] Although the above is disclosed in the preferred embodiment of the present application, it is not intended to limit the claims, and any person skilled in the art can make several possible changes and modifications without departing from the concept of the present application, therefore the protection scope of the present application should be limited by the scope defined by the claims of the present application.

[0053] It is known that the existing winding method of the winding electrode assembly 2 is Z winding and J winding, no matter what winding method, the essence of winding is to wind the electrode sheet 22 and the separator in the winding electrode assembly 2 from the winding center to the outside layer by layer. In the winding process, the two sides of the winding electrode assembly will form a bending area, and the separator and the electrode sheet 22 in this area exhibit a specific curvature bending state. In the process of forming this bending state, the corner area 21 of the winding electrode assembly 2 becomes the focus of the bending force, especially the electrode sheet 22 and the separator. And the non-corner area 21 of the winding electrode assembly 2 has a small bending amplitude (in this application, the non-corner area 21 is approximately set as a flat area), so the bending curvature has little effect on the battery. Further, in the process of battery charging and discharging cycle, the electrode sheet 22 and the separator in the corner area 21 will experience expansion and contraction, further aggravating the accumulation of stress, thereby increasing the stress load at the corner. Especially the negative electrode of the battery, due to the embedding and stripping of lithium ions during the cycle, will swell in volume, which makes the separator more prone to damage and deformation, and even may cause lithium precipitation phenomenon (i.e. lithium metal deposition at the corner, affecting the performance and safety of the battery).

[0054] Therefore, those skilled in the art propose to coat the polymer layer 3 and the first coating layer 3 on both sides of the base film 1, so as to alleviate the force acting on the separator and the pole piece 22 during the bending process of the winding type battery. However, those skilled in the art have not considered that even if the polymer material with a compression ratio of 80% is bent, the polymer layer 3 located in the concave area of the separator to be bent section 12 will generate outward thrust due to excessive extrusion, and this thrust will directly act on the separator and affect the performance of the separator, and even cause damage to the separator; and the polymer layer 3 coated on the convex area of the separator to be bent section 12 will generate a back-stretching force (due to the nature of the polymer layer 3 itself) due to excessive stretching, which further acts on the outside of the separator and affects the performance of the separator. In addition, due to the different directions or the same direction of the force generated by the concave area and the convex area of the separator, and the two forces can act on both sides of the separator, the separator is subjected to uneven internal and external forces, thereby further increasing the risk of damage to the separator.

[0055] The inventors of the present application observed the above technical problems in experiments, and further generated the inspiration to solve the above technical problems, that is, each polymer layer 3 coated in the bending area of the winding type battery is intermittently treated, and a certain position is reserved so that the polymer layer 3 will not be excessively extruded or stretched during the winding process of the winding type battery, thereby causing damage to the separator and the pole piece 22.

[0056] As shown in Figures 1-4 In order to alleviate the force acting on the separator and the pole piece 22 during the bending process in the prior art, and to alleviate the negative effects of the coating layer on the separator and the pole piece 22, the present application improves the separator,

[0057] Specifically, the separator of the present application is suitable for a winding type electrode assembly 2, the winding type electrode assembly 2 has a corner area 21 in a bent state, and the separator comprises:

[0058] The application is suitable for a winding electrode assembly, and the electrode assembly has a corner area 21 in a bent state. The separator comprises a base film 1, which, in a flattened state, comprises, along the length direction of the base film 1, flat sections 11 and to-be-bent sections 12 connected alternately in sequence, the to-be-bent sections 12 corresponding to the corner area 21; a first coating layer 3 arranged on both surfaces of the base film 1 along the thickness direction thereof respectively; and a second coating layer 4 arranged on at least one surface of the base film 1 along the thickness direction thereof, the second coating layer 4 being arranged between the base film and the first coating layer 3. The second coating layer 4 comprises a plurality of polymer layers 41, which are arranged at intervals along the length direction of the base film 1. The polymer layers 41 are arranged correspondingly to the to-be-bent sections 12. The polymer layer 3 is coated only between the base film and the first coating layer 3 corresponding to the corner area 21, thereby saving the consumption of coating material and avoiding the increase in the thickness of the winding electrode assembly due to the excessive coating of the polymer layer 3 on the flat sections 11 of the base film 1.

[0059] Further, in the polymer layer 3 of the separator of the application, at least one polymer layer 3 is provided with at least two grooves 411. The grooves 411 are formed in the process of coating the polymer layer 3 due to intermittent coating. The depth of the grooves 411 can be the same as or slightly smaller than the thickness of the polymer layer 3. That is, the intermittent coating of the polymer layer 3 can be selected to be directly free of the polymer layer 3 in the groove 411 area, or a small amount of coating layer can be intermittently coated in the groove 411 area, thereby forming grooves 411 with a depth smaller than the thickness of the polymer layer 3. Through the above design, the separator prepared in the application is more reasonably distributed and stress relieved in the recessed area and the raised area of the bent section due to the presence of the grooves 411 in the winding process of the winding electrode assembly. In the recessed area, the design of the grooves 411 makes the polymer layer 3 in this area not easy to generate outward thrust due to excessive extrusion, thereby reducing the negative impact on the performance of the separator. In the raised area, although the polymer layer 3 will be deformed due to stretching, the presence of the grooves 411 helps to disperse the stretching force and prevent the direct action of the back-stretching force generated by excessive stretching on the outside of the separator. At the same time, the setting of the grooves 411 makes the stress on the inner and outer sides of the separator more uniform during the bending process, thereby reducing the risk of damage caused by uneven stress.

[0060] Further, by setting the grooves 411 in the polymer layer 3, not only the stress between the separator and the electrode tab 22 in the bending process is effectively relieved, but also the increase in the thickness of the electrode assembly caused by the excessive coating of the polymer layer 3 in the unnecessary area is avoided. This design not only ensures the safety of the battery, but also optimizes the thickness of the separator of the winding electrode assembly, thereby further relieving the bending force in the bending area.

[0061] Further, the pole assembly includes pole pieces in a winding structure, the pole pieces being in contact with the separator; a length of the polymer layer corresponding to the to-be-bent segment is L1, the separator has a recessed area and a raised area at the to-be-bent segment, and an arc length of the raised area is L2, wherein L1 and L2 satisfy: L2≥L1≥0.8L2. By controlling the arc length of the raised area corresponding to the to-be-bent segment 12 and the length of the corresponding polymer layer 3, the polymer layer 3 can be coated on the corner area 21 only, so that this setting not only ensures that the polymer layer 3 can fully cover and protect the corner area 21 of the pole piece 22 which is most vulnerable to damage during winding, but also avoids unnecessary excessive coating. Specifically, under this proportion control, the polymer layer 3 is limited to the key position that needs to be enhanced, that is, the part corresponding to the arc length of the raised area. This targeted coating method not only effectively improves the durability of the separator in a complex bending environment, but also significantly reduces the weight and material cost of the entire battery cell. Because the amount of polymer in the non-critical area is reduced, the battery cell can be more compact, and the energy density is also improved accordingly.

[0062] As Figures 1-4 shown, in order to further improve the utilization rate of the polymer layer of the present application, specifically, the diameter of the circular arc of the raised area of the to-be-bent segment of the innermost layer of the separator is Ds0, and the arc length L 21 of the raised area of the to-be-bent segment of the innermost layer of the separator satisfies L 21 =π(Ds0) / 2. Assuming that Ds0 is 10 mm, L 11 is about 15.7 mm. As the winding process proceeds, the diameter of the circular arc formed by the separator in the subsequent winding will gradually increase, but the corresponding arc length L1 still needs to satisfy the condition of L2≥L1≥0.8L2 to ensure effective coverage of the polymer layer 3 and avoid excess. This design detail ensures that the polymer layer 3 can provide just the right protection at different winding stages, neither too much nor too little.

[0063] Specifically, the number of winding layers of the separator is set to N, and the arc length L 2n of the raised area of the to-be-bent segment of the separator at the Nth winding layer satisfies L 2n= π (Ds0+ (N-1)△t) / 2; wherein, for each additional layer of winding of the diaphragm on the basis of the innermost layer winding, the increase of the diaphragm in the convex area of the to-be-bent segment corresponding to the circular arc diameter relative to the circular arc diameter of the adjacent inner layer diaphragm in the convex area of the to-be-bent segment is△t. Taking a battery with 18 layers of winding as an example for calculation, the thickness of the positive electrode sheet 22 is 0.088 mm, the thickness of the negative electrode sheet 22 is 0.105 mm, the thickness of the base film 1 is 0.004 mm, the thickness of the oil coating layer (the first coating layer 3 in the present application) is 0.0015 mm (double-sided), the thickness of the intermittent polymer layer 3 is 0.001 mm, the total thickness of the diaphragm is 0.009 mm, the circular arc diameter Ds0of the diaphragm at the corner of the first layer of this model is 1.2561 mm, and the increase of the diaphragm circular arc diameter at the corner△t is 0.211 mm. The design parameters of the polymer layer 3 coated at the corner of the diaphragm are shown in Table 1:

[0064]

[0065]

[0066] Table 1

[0067] Further, the polymer layer 3 in the present application exhibits a trend of first increasing and then decreasing in the groove 411 width along the length direction (i.e. the X direction) of the polymer layer 3. Specifically, the groove 411 width corresponding to the length (i.e. the X direction) center of the to-be-bent segment 12 of the polymer layer 3 reaches the maximum value. It should be pointed out that if the length center of the to-be-bent segment 12 is not directly provided with a groove 411, i.e. taking the length center as the axis, the width of the closest and symmetric grooves 411 on both sides will be considered as the maximum. Furthermore, if the length center of the to-be-bent segment 12 is neither directly provided with a groove 411 nor provided with a symmetrically designed groove 411 along the center axis on both sides, the width of the closest groove 411 to the length center axis of the to-be-bent segment 12 will be considered as the maximum in this region. Through this design, the outward pushing force of the polymer layer 3 at the recessed area of the diaphragm to-be-bent segment 12 due to excessive extrusion and the back-stretching force of the polymer layer 3 at the convex area of the diaphragm to-be-bent segment 12 due to excessive stretching can be further relieved.

[0068] Specifically, the width of the groove 411 is D1, and D1 satisfies: 0.9 mm-1.1 mm. In the specific implementation process, the width of the groove 411 can be: 0.9 mm, 0.91 mm, 0.92 mm, 0.93 mm, 0.94 mm, 0.95 mm, 0.96 mm, 0.97 mm, 0.98 mm, 0.99 mm, 1.00 mm, 1.01 mm, 1.02 mm, 1.03 mm, 1.04 mm, 1.05 mm, 1.06 mm, 1.07 mm, 1.08 mm, 1.09 mm, 1.10 mm.

[0069] Further, the groove 411 of the polymer layer 3 in the present application has a trend of first increasing and then decreasing along the length direction (i.e. the X direction) of the polymer layer 3. Specifically, the groove 411 corresponding to the center of the length direction of the to-be-bent segment 12 has the maximum depth. It should be noted that if the to-be-bent segment 12 has no groove 411 directly corresponding to the center of the length direction, the depth of the groove 411 closest to and symmetric to the center of the length direction will be considered as the maximum. In addition, if the to-be-bent segment 12 has no groove 411 directly corresponding to the center of the length direction and no groove 411 symmetrically designed along the center of the length direction, the depth of the groove 411 closest to the center of the length direction will be considered as the maximum.

[0070] Through this design, the thickness of the polymer layer 3 in the recessed area or the raised area of the to-be-bent segment 12 of the diaphragm can be reduced. According to the experiment, it is found that when the thickness of the polymer layer 3 corresponding to the groove 411 is small, the protection of the diaphragm by the polymer layer 3 is ensured, and at the same time, the polymer protrusion is thin, so that when the diaphragm and the pole piece 22 are bent during winding, the polymer layer 3 at the groove 411 corresponding to the recessed area will not produce significant extrusion force, thereby avoiding the large outward thrust caused by excessive extrusion of the polymer layer 3.

[0071] At the same time, when the depth of the groove 411 corresponding to the recessed area is the maximum, the surface area is also increased, which is sufficient to disperse the generated outward thrust, thereby reducing the damage to the diaphragm. In the present application, when the diaphragm and the pole piece 22 are bent during winding, the polymer layer 3 at the groove 411 corresponding to the raised area will not produce significant back tension. At the same time, when the depth of the groove 411 corresponding to the raised area is the maximum, the thickness of the polymer layer 3 corresponding to the groove 411 is also reduced, thereby reducing the tensile force. Further, when the thickness of the polymer layer 3 in the recessed area or the raised area of the to-be-bent segment 12 of the diaphragm is thin, the internal stress generated during the charging and discharging process of the battery can be effectively reduced, thereby improving the cycle life and safety of the battery.

[0072] Specifically, the depth of the groove 411 is D2, and the depth of the groove 411 is D2, D2 satisfies: 0.5 microns-10 microns. In the specific implementation process, the depth of the groove 411 can be: 0.5 microns, 0.6 microns, 0.7 microns, 0.8 microns, 0.9 microns, 1.0 microns, 1.1 microns, 1.2 microns, 1.3 microns, 1.4 microns, 1.5 microns, 1.6 microns, 1.7 microns, 1.8 microns, 1.9 microns, 2.0 microns, 2.1 microns, 2.2 microns, 2.3 microns, 2.4 microns, 2.5 microns, 2.6 microns, 2.7 microns, 2.8 microns, 2.9 microns, 3.0 microns, 3.1 microns, 3.2 microns, 3.3 microns, 3.4 microns, 3.5 microns, 3.6 microns, 3.7 microns, 3.8 microns, 3.9 microns, 4.0 microns, 4.1 microns, 4.2 microns, 4.3 microns, 4.4 microns, 4.5 microns, 4.6 microns, 4.7 microns, 4.8 microns, 4.9 microns, 5.0 microns, 5.1 microns, 5.2 microns, 5.3 microns, 5.4 microns, 5.5 microns, 5.6 microns, 5.7 microns, 5.8 microns, 5.9 microns, 6.0 microns, 6.1 microns, 6.2 microns, 6.3 microns, 6.4 microns, 6.5 microns, 6.6 microns, 6.7 microns, 6.8 microns, 6.9 microns, 7.0 microns, 7.1 microns, 7.2 microns, 7.3 microns, 7.4 microns, 7.5 microns, 7.6 microns, 7.7 microns, 7.8 microns, 7.9 microns, 8.0 microns, 8.1 microns, 8.2 microns, 8.3 microns, 8.4 microns, 8.5 microns, 8.6 microns, 8.7 microns, 8.8 microns, 8.9 microns, 9.0 microns, 9.1 microns, 9.2 microns, 9.3 microns, 9.4 microns, 9.5 microns, 9.6 microns, 9.7 microns, 9.8 microns, 9.9 microns, 10.0 microns. Through the design of controlling the width and depth of the groove 411, the adaptability and stability of the polymer layer 3 in the bending area of the diaphragm can be ensured, thereby improving the performance and safety of the battery as a whole. However, it should be noted that the depth of the groove 411 of the present application should not be greater than the thickness of the polymer layer 3, which is determined by the principle that the groove 411 is essentially a control of the local thickness of the polymer layer 3.

[0073] Further, the first coating layer 3 in the present application is an oily coating layer, which includes PVDF and ceramic material. Specifically, in consideration of the thickness requirement of the jelly-roll electrode assembly 2, the present application only coats the first coating layer 3 on one side of the base film 1 in the thickness direction, which can realize the adhesion between the separator and the electrode sheet 22, reduce the impact on the battery thickness, and further help to maintain the stability of the jelly-roll electrode assembly 2 during the charging and discharging process of the battery. The number of the first coating layer 3 is less than or equal to the number of the flat section 11 of the base film 1. When the number is equal, each flat section 11 is coated with the first coating layer 3 to ensure good adhesion between the electrode sheet 22 and the separator, while not significantly increasing the overall thickness of the battery. In the specific implementation process, the separator of the present application can be used with various types of positive and negative electrode materials, such as lithium cobalt oxide (Li CoO2), lithium nickel cobalt manganese oxide (NCM), lithium iron phosphorus (LFP), etc. Specifically, by optimizing the coating distribution of the separator, the present application not only improves the cycle stability of the jelly-roll electrode assembly 2, but also can adapt to different winding forms of the jelly-roll electrode assembly 2.

[0074] Further, the polymer layer 3 in the present application refers to a coating layer including an emulsion of polymer particles, a dispersion auxiliary agent, and an adhesive. In the specific implementation process, the polymer layer includes an emulsion of polymer particles, a dispersion auxiliary agent, and an adhesive; the emulsion of polymer particles is one or more of polyethylene emulsion, polyethylene micro-wax emulsion, polyvinylidene fluoride emulsion, polytetrafluoroethylene emulsion, polymethyl methacrylate emulsion, polyimide emulsion, polystyrene emulsion, and polyacrylamide emulsion; the dispersion auxiliary agent is one or more of polyacrylic acid and its sodium salt, polyacrylamide, polyvinyl alcohol, sodium citrate, sodium ethylenediaminetetraacetate, sodium diacetate, sodium hexametaphosphate, sodium silicate, and carboxymethyl cellulose; and the adhesive is a polypropylene-based adhesive. The above-mentioned materials have a certain compressibility, and the material compression ratio is 20% to 90%.

[0075] In order to further embody the improvement of the separator structure by the above technical solution, the present application designs the separator materials shown in Examples 1-5 and the separator materials shown in Comparative Examples 1-3 by controlling variables. The jelly-roll electrode assembly is obtained by winding the separator materials of Examples 1-5 and Comparative Examples 1-2 and the positive and negative electrode sheets 22 after sheeting. The finished product battery is obtained after the jelly-roll electrode assembly is packaged, baked, injected, formed, two-sealed, and divided. The capacity retention rate and thickness expansion rate of the batteries prepared by Examples 1-5 and Comparative Examples 1-2 are detected, which can more clearly understand the performance difference of the batteries prepared by the separator of the present application and the existing separator.

[0076] Example 1

[0077] Taking the battery cell with the core layer number N = 18 layers as an example, the positive electrode sheet 22 has a thickness of 0.088 mm, the negative electrode sheet 22 has a thickness of 0.105 mm, the base film 1 has a thickness of 0.004 mm, the first coating layer 3 (the oily coating layer) has a double-sided coating thickness of 0.0015 mm, the polymer layer 3 has a double-sided coating thickness of 0.001 mm, the polymer layer 41 is coated in an intermittent coating manner, the maximum width of the groove 411 in the intermittent coating is 0.9 mm, the maximum depth of the groove 411 in the intermittent coating is 0.0008 mm, and the total thickness of the separator is 0.009 mm. The model has a separator arc diameter Ds0 = 1.2561 mm at the corner of the first layer, and the increase range of the corner of the separator arc diameter is △Ds = 0.0001 mm.

[0078] t = 0.211 mm.

[0079] Example 2

[0080] Different from example 1, the battery separator in this embodiment has a double-sided coating thickness of 0.0001 mm for the polymer layer 3, wherein the polymer layer 41 is coated in an intermittent coating manner, and the maximum depth of the groove 411 in the intermittent coating is 0.009 mm.

[0081] Example 3

[0082] Different from example 1, the battery separator in this embodiment has a double-sided coating thickness of 0.0002 mm for the polymer layer 3, wherein the polymer layer 41 is coated in an intermittent coating manner, and the maximum depth of the groove 411 in the intermittent coating is 0.0015 mm.

[0083] Example 4

[0084] Different from example 1, the battery separator in this embodiment has a double-sided coating thickness of 0.001 mm for the polymer layer 3, wherein the polymer layer 41 is coated in an intermittent coating manner, and the maximum width of the groove 411 in the intermittent coating is 0.8 mm.

[0085] Example 5

[0086] Different from example 1, the battery separator in this embodiment has a double-sided coating thickness of 0.001 mm for the polymer layer 3, wherein the polymer layer 41 is coated in an intermittent coating manner, and the maximum width of the groove 411 in the intermittent coating is 1.0 mm.

[0087] Comparative Example 1

[0088] Different from example 1, the base film 1 in this application has a thickness of 0.004 mm, the first coating layer 3 (the oily coating layer) has a double-sided coating thickness of 0.0015 mm, and the entire separator area has only the double-sided first coating layer 3 (coated with the oily coating layer) and no polymer layer 41 coating.

[0089] Comparative Example 2

[0090] Different from Example 1, the base film 1 of the present application has a thickness of 0.004 mm, the first coating layer 3 (oily coating layer) has a double-sided coating thickness of 0.0015 mm, and the entire separator area is only double-sided first coating layer 3 (coated with oily coating layer), wherein the polymer layer 41 has the same coating length as the first coating layer 3, and is distributed in both the flat section 11 and the curved section.

[0091] The detection method is as follows:

[0092] The battery cells prepared in Examples 1-5 and Comparative Examples 1-2 were respectively subjected to normal temperature cycling, and the test steps were as follows:

[0093] The charging mode is: 1.2C CC to 4.18V, CV to 0.7C or 0.7C CC to 4.50V, CV to 0.05C; wherein CC represents constant current charging, and CV represents constant voltage charging.

[0094] The discharging mode is: 0.5C DC to 3.0V, wherein DC represents constant current discharging.

[0095] In the experimental data of Tables 2 and 3 described below, the capacity retention rate = weekly discharge capacity / initial discharge capacity*100%, and the thickness expansion rate = thickness per 100 full charge / initial full charge thickness*100%.

[0096]

[0097] Table 2 Test capacity retention rate experimental data

[0098]

[0099]

[0100] Table 3 Test thickness expansion rate experimental data

[0101] From the above table data and the data of the batteries prepared by the separators of Examples 1-5 and Comparative Examples 1-2, Figure 5 the capacity retention rate curves of the batteries in Example 1 and Comparative Example 1 at normal temperature cycling, and Figure 6The curve of thickness expansion rate of the battery at room temperature in Example 1 and Comparative Example 1 of the present application. It can be seen that the battery prepared by the separator of the present application is superior to the prior art in terms of capacity retention rate and thickness expansion rate. Specifically, the capacity retention rate of the battery of Examples 1 to 5 is higher than 90% after 1000 cycles, while the capacity retention rate of the batteries of Comparative Examples 1 and 2 decreases to 84.2% and 86.7%, respectively. In terms of thickness expansion rate, the expansion rate of the batteries of Examples 1 to 5 is less than 7% after 1000 cycles, while the expansion rate of the batteries of Comparative Examples 1 and 2 reaches 8.11% and 7.82%, respectively. This indicates that the separator preparation technology of the present application has a significant advantage in the long-term performance of the battery.

[0102] Further analysis, the batteries of Examples 1 to 5 show good stability during the cycle process, which is mainly due to the optimized coating of the polymer layer 3 in the separator. By adjusting the coating thickness and coating method of the polymer layer 3, the porosity and mechanical strength of the separator can be effectively controlled, thereby improving the cycle life and safety of the battery. For example, the coating thickness of the polymer layer 3 in Example 2 is thinner, but the groove 411 depth is larger, which also helps to improve the air permeability of the separator, thereby reducing the gas pressure inside the battery and reducing the thickness expansion.

[0103] In addition, the coating thickness of the polymer layer 3 in Examples 4 and 5 is the same, but the groove 411 width is different, which shows that adjusting the coating method within a certain range has an effect on the performance of the battery. The groove 411 width of Example 4 is smaller, which may help to improve the mechanical strength of the separator, while the groove 411 width of Example 5 is larger, which may be more conducive to the penetration and ion transport of the electrolyte.

[0104] In summary, the separator preparation technology of the present application significantly improves the cycle stability and safety of the battery by optimizing the coating thickness and coating method of the polymer layer 3.

[0105] The above is only a preferred embodiment of the present application and is not intended to limit the present application. Those skilled in the art can make various modifications and changes to the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A separator suitable for use in a jelly-roll electrode assembly, the electrode assembly (2) having a corner region (21) in a curved state, characterized in that, The diaphragm comprises: a base film (1) comprising, in a flattened state, straight sections (11) and bending sections (12) connected alternately in sequence along the length direction of the base film (1), the bending sections (12) corresponding to the corner regions (21); a first coating layer (3) arranged on both surfaces of the base film (1) along the thickness direction thereof respectively; a second coating layer (4) arranged on at least one surface of the base film (1) along the thickness direction thereof, the second coating layer (4) being arranged between the base film (1) and the first coating layer (3); the second coating layer (4) comprises a plurality of polymer layers (41) arranged at intervals along the length direction of the base film (1); the polymer layers (41) are arranged correspondingly to the bending sections (12); at least one of the polymer layers (41) is provided with at least two grooves (411); along the length direction of the polymer layer (41), the width of the groove (411) first increases and then decreases, and the width of the groove (411) corresponding to the length center of the bending section (12) is the largest; along the length direction of the polymer layer (41), the depth of the groove (411) first increases and then decreases, and the depth of the groove (411) corresponding to the length center of the bending section (12) is the largest.

2. The separator according to claim 1, characterized in that The electrode assembly (2) comprises a pole piece (22) in a winding structure, the pole piece (22) being in contact with the diaphragm; the length of the polymer layer (41) corresponding to the bending section (12) is L1, the diaphragm has a recessed region and a raised region in the bending section (12), and the arc length of the raised region is L2, wherein L1 and L2 satisfy: L2≥L1≥0.8L2.

3. The diaphragm of claim 2, wherein The circular arc diameter of the raised area of the to-be-bent section (12) of the innermost layer of the diaphragm is Ds0, and the arc length L corresponding to the raised area of the to-be-bent section (12) of the innermost layer of the diaphragm 21 satisfies L 21 = π(Ds0) / 2.

4. The diaphragm of claim 3, wherein The winding layer number of the diaphragm is n, and the diaphragm is in the arc length L of the corresponding convex area of the winding n layer in the to-be-bent section (12) 2n Satisfies L 2n =π(Ds0+(n-1)△t) / 2; Wherein, for each additional layer of winding on the basis of the innermost layer of winding, the increase of the diameter of the circular arc corresponding to the raised region of the bending section (12) of the diaphragm relative to the diameter of the circular arc corresponding to the raised region of the bending section (12) of the adjacent inner layer diaphragm is Δt.

5. The separator of claim 1, wherein The width of the groove (411) is D1, and D1 satisfies: 0.9 millimeters-1.1 millimeters.

6. The separator of claim 1, wherein The depth of the groove (411) is D2, and D2 satisfies: 0.5 microns-10 microns.

7. The separator of claim 1, wherein The polymer layer (41) comprises an emulsion of polymer particles, a dispersion auxiliary agent, and an adhesive; The emulsion of polymer particles is one or more of polyethylene emulsion, polyethylene micro-wax emulsion, polyvinylidene fluoride emulsion, polytetrafluoroethylene emulsion, polymethyl methacrylate emulsion, polyimide emulsion, polystyrene emulsion, and polyacrylamide emulsion; The dispersion auxiliary agent is one or more of polyacrylic acid and its sodium salt, polyacrylamide, polyvinyl alcohol, sodium citrate, sodium ethylenediaminetetraacetate, sodium diacetate, sodium hexametaphosphate, sodium silicate, and carboxymethyl cellulose; The adhesive is a polypropylene-based adhesive.

8. A secondary battery characterized by comprising: The diaphragm comprises the diaphragm according to any one of claims 1-7.

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