A secondary battery and an electronic device

Abstract

The application provides a secondary battery and an electronic device. The secondary battery comprises a shell and an electrode assembly of a lamination structure. The electrode assembly comprises a first pole piece, a second pole piece and a separator which are arranged in layers. The first pole piece comprises two outer first pole pieces, two secondary outer first pole pieces and an inner first pole piece. The second pole piece comprises two outer second pole pieces and an inner second pole piece. The two outer first pole pieces are respectively located at the two outermost sides of the electrode assembly. At least one outer first pole piece is a single-sided first pole piece. The separator comprises a first separator and a second separator. The first separator is arranged between the single-sided first pole piece and the outer second pole piece. The second separator is arranged between the inner first pole piece and the inner second pole piece adjacent to the inner first pole piece. The porosity of the first separator is P1, and the porosity of the second separator is P2. P1>P2. Through the above arrangement, the risk of black spots and lithium precipitation of the outer pole piece of the electrode assembly is reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electrochemistry, in particular to a secondary battery and an electronic device. BACKGROUND

[0002] Secondary batteries, such as lithium ion batteries, have the characteristics of high specific energy, high working voltage, low self-discharge rate, small volume, light weight, etc., and have a wide range of applications in the consumer electronics field.

[0003] At present, the electrode assembly in the lithium ion battery often adopts a multi-tab and laminated structure to reduce the impedance of the battery. In order to ensure the performance of the ultra-thin and high energy density of the lithium ion battery, the outermost tab of the electrode assembly usually adopts a single-sided coated tab. However, due to the manufacturing process and the particularity of the structure of the single-sided coated tab, black spots or lithium precipitation problems are prone to occur during the cycle process, which reduces the performance of the lithium ion battery. SUMMARY

[0004] The purpose of the present application is to provide a secondary battery and an electronic device to reduce the risk of black spots and lithium precipitation on the outer tab of the electrode assembly and improve the cycle performance of the secondary battery.

[0005] It should be noted that the lithium ion battery is used as an example of a secondary battery in the summary of the present application to explain the present application, but the secondary battery of the present application is not limited to a lithium ion battery.

[0006] The production of the electrode assembly with a laminated structure in the secondary battery is to cut and form the positive tab, the negative tab and the separator by a cutter, and then stack and form by hot pressing. The conventional soft package laminated secondary battery structure is packaged by N layers of inner second tabs, N-1 layers of inner first tabs and 2 layers of outer first tabs. Due to the existence of the curling effect, the thickness of the current collector of the outer first tab is usually 2 to 3 times the thickness of the current collector of the inner first tab, which results in that the outer first tab has small resistance, large current density, and the ability to consume and transport electrolyte is greater than that of the inner tab. In the long-term cycle process, the outer electrolyte of the electrode assembly is more likely to be insufficient, which leads to the problem of lithium precipitation at the interface of the outer tab. With the continuous cycle, lithium dendrites are continuously formed and intensified, which eventually pierces the separator to cause the direct contact of the positive and negative tabs and short circuit, affecting the safety performance of the secondary battery. Based on this, the present application provides a secondary battery which can reduce the risk of lithium precipitation of the outer tab in the electrode assembly due to insufficient outer electrolyte, and improve the cycle performance of the secondary battery while considering the energy density of the secondary battery. The specific technical solutions are as follows:

[0007] The first aspect of the present application provides a secondary battery, comprising a shell and an electrode assembly of a laminated structure, the electrode assembly comprising a first electrode sheet, a second electrode sheet and a separator arranged between the first electrode sheet and the second electrode sheet in the thickness direction of the electrode assembly, the first electrode sheet comprising two outer first electrode sheets, two secondary outer first electrode sheets and an inner first electrode sheet arranged between the two secondary outer first electrode sheets, the second electrode sheet comprising two outer second electrode sheets and an inner second electrode sheet arranged between the two outer second electrode sheets, the two outer first electrode sheets being respectively arranged at the two outermost sides of the electrode assembly, at least one outer first electrode sheet being a single-sided first electrode sheet, the two outer second electrode sheets being respectively arranged at the sides of the two outer first electrode sheets away from the shell and adjacent to the two outer first electrode sheets, and the two secondary outer first electrode sheets being respectively arranged at the sides of the two outer second electrode sheets away from the shell and adjacent to the two outer second electrode sheets. The separator comprises a first separator and a second separator, the first separator being arranged between the single-sided first electrode sheet and the outer second electrode sheet, and the second separator being arranged between the inner first electrode sheet and the inner second electrode sheet adjacent thereto. The porosity of the first separator is P1, and the porosity of the second separator is P2, P1>P2. By arranging separators with different porosities in the electrode assembly of the laminated structure and making the porosity of the first separator greater than the porosity of the second separator, the liquid storage capacity of the outer side of the electrode assembly can be improved, and the risk of black spots and lithium precipitation of the outer electrode sheet caused by insufficient electrolyte on the outer side of the electrode assembly during the cycle process can be reduced, and the secondary battery has good cycle performance.

[0008] In some embodiments of the present application, 1.30≤P1 / P2≤2.17, 30%≤P2≤50%. By adjusting P1 / P2 and P2 to be within the above range, the cycle performance of the secondary battery is good while the safety performance is considered.

[0009] In some embodiments of the present application, the first electrode sheet is a positive electrode sheet. When the positive electrode sheet is selected as the first electrode sheet, the effect of reducing the risk of black spots and lithium precipitation of the outer electrode sheet caused by insufficient electrolyte on the outer side of the electrode assembly during the cycle process is more obvious, thereby further improving the cycle performance of the secondary battery.

[0010] In some embodiments of the present application, the first separator comprises a first base film and a first ceramic coating layer on at least one surface of the first base film, the first ceramic coating layer comprises first ceramic particles, the second separator comprises a second base film and a second ceramic coating layer on at least one surface of the second base film, the second ceramic coating layer comprises second ceramic particles, the first ceramic particles and the second ceramic particles are each independently selected from at least one of boehmite, Al2O3, ZrO2 or TiO2; the average particle size of the first ceramic particles is D1 μm, 0.10≤D1≤1.50; the average particle size of the second ceramic particles is D2 μm, 0.50≤D2≤5.00. By selecting the first ceramic particles and the second ceramic particles of the above type and adjusting the average particle size of the first ceramic particles and the second ceramic particles within the above range, the specific surface area of the first ceramic coating layer and the second ceramic coating layer can be adjusted, thereby reducing the risk of black spots and lithium precipitation on the outer electrode sheet caused by insufficient electrolyte on the outside of the electrode assembly during the cycle process, and improving the cycle performance of the secondary battery.

[0011] In some embodiments of the present application, 0.02≤D1 / D2≤1.00. By adjusting the value of D1 / D2 within the above range, the particle size of the first ceramic particles is less than or equal to the particle size of the second ceramic particles, which is conducive to adjusting the specific surface area of the first ceramic coating layer and the second ceramic coating layer, thereby reducing the risk of black spots and lithium precipitation on the outer electrode sheet caused by insufficient electrolyte on the outside of the electrode assembly during the cycle process, and improving the cycle performance of the secondary battery.

[0012] In some embodiments of the present application, the thickness of the first ceramic coating layer is 1.0 μm to 8.0 μm, and the thickness of the second ceramic coating layer is 1.0 μm to 5.0 μm. By adjusting the thickness T1 of the first ceramic coating layer and the thickness T2 of the second ceramic coating layer within the above range, the accommodation space of the electrolyte of the separator, especially the outermost separator of the electrode assembly, can be further increased, and the liquid storage capacity on the outside of the electrode assembly is improved, thereby reducing the risk of black spots and lithium precipitation on the outer electrode sheet caused by insufficient electrolyte on the outside of the electrode assembly during the cycle process, while the energy density of the secondary battery is taken into account, and the cycle performance of the secondary battery is improved.

[0013] In some embodiments of the present application, the material of the first base film and the material of the second base film are each independently selected from at least one of polyethylene or polypropylene; the porosity of the first base film is p1, 50%≤p1≤65%, and the porosity of the second base film is p2, 30%≤p2≤50%. By selecting the first base film and the second base film of the above type and adjusting the porosity of the first base film and the second base film within the above range, the risk of black spots and lithium precipitation on the outer electrode sheet caused by insufficient electrolyte on the outside of the electrode assembly during the cycle process is reduced, and the cycle performance of the secondary battery is improved. At the same time, the mechanical strength of the first separator and the second separator is high, which is conducive to further taking into account the safety performance of the secondary battery.

[0014] In some embodiments of the present application, 1.30≤p1 / p2≤2.17. By regulating the value of p1 / p2 within the above range, the first base film has higher porosity than the second base film, reducing the risk of black spots and lithium precipitation on the outside of the electrode assembly caused by insufficient electrolyte on the outside during the cycle process, while taking into account the mechanical strength of the first and second separators, the cycle performance of the secondary battery is improved.

[0015] In some embodiments of the present application, the thickness of the first base film is 4.0-12.0 μm, and the thickness of the second base film is 4.0-9.0 μm. By regulating the thickness T'1 of the first base film and the thickness T'2 of the second base film within the above range, while taking into account the energy density of the secondary battery, the cycle performance of the secondary battery is improved.

[0016] In some embodiments of the present application, the first ceramic coating further comprises a first binder, and the second ceramic coating further comprises a second binder, the first binder and the second binder are each independently selected from at least one of polyvinylidene fluoride, polyacrylic acid, carboxymethyl cellulose or sodium alginate; the mass percentage of the first binder is 1.0-10.0% based on the mass of the first ceramic coating; the mass percentage of the second binder is 1.0-10.0% based on the mass of the second ceramic coating. By selecting the first binder and the second binder of the above type, and regulating the mass percentage W1 of the first binder and the mass percentage W2 of the second binder within the above range, while taking into account the cycle performance, the safety performance of the secondary battery is improved.

[0017] In some embodiments of the present application, the shell is an aluminum plastic film.

[0018] In some embodiments of the present application, the thickness of the first separator is H1 μm, the thickness of the second separator is H2 μm, 1.0≤H1 / H2≤5.8, and 5.0≤H2≤19.0. By regulating the values of H1 and H2 within the above range, the risk of black spots and lithium precipitation on the outside of the electrode assembly caused by insufficient electrolyte on the outside during the cycle process is reduced, while taking into account the energy density of the secondary battery, the cycle performance and safety performance of the secondary battery are improved.

[0019] In some embodiments of the present application, the separator further comprises a third separator arranged between the outer second tab and the next outer first tab, the third separator having a porosity P3, P3>P2. In some embodiments of the present application, the separator further comprises a fourth separator arranged between the next outer first tab and the inner second tab adjacent thereto, the fourth separator having a porosity P4, P4>P2. By the above regulation, the porosity of the next outer separator and / or the next next outer separator, and the outermost separator of the electrode assembly, and the accommodation space of the electrolyte are larger, further reducing the risk of black spot and lithium precipitation of the outer tab of the electrode assembly due to insufficient electrolyte on the outside during the cycle, thereby further improving the cycle performance of the secondary battery.

[0020] The second aspect of the present application provides an electronic device comprising the secondary battery of any of the above embodiments. The secondary battery of the present application has good cycle performance, and therefore the electronic device of the present application has a longer service life.

[0021] The beneficial effects of the embodiments of the present application are:

[0022] The present application provides a secondary battery and an electronic device, by setting the porosity of the first separator to be greater than the porosity of the second separator, the porosity of the outermost separator of the electrode assembly and the accommodation space of the electrolyte are larger at this time, improving the liquid storage capacity of the outer side of the electrode assembly, reducing the risk of black spot and lithium precipitation of the outer tab of the electrode assembly during the cycle, improving the cycle performance of the secondary battery.

[0023] Of course, implementing any product or method of the present application does not necessarily require all the advantages described above to be achieved at the same time. BRIEF DESCRIPTION OF DRAWINGS

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or prior art description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other embodiments can also be obtained by those skilled in the art based on these drawings.

[0025] Figure 1 The cross-sectional structure schematic diagram of the secondary battery of one embodiment of the present application is shown along the length direction of the secondary battery;

[0026] Figure 2 The cross-sectional structure schematic diagram of the secondary battery of another embodiment of the present application is shown along the length direction of the secondary battery;

[0027] Figure 3 The cross-sectional structure schematic diagram of the secondary battery of another embodiment of the present application is shown along the length direction of the secondary battery;

[0028] Figure 4 A cross-sectional structure schematic diagram of a secondary battery according to another embodiment of the present application, viewed in the length direction of the secondary battery.

[0029] Reference signs: secondary battery 001; electrode assembly 01; case 02; first electrode sheet 10; outer first electrode sheet 101; sub-outer first electrode sheet 102; inner first electrode sheet 103; second electrode sheet 20; outer second electrode sheet 201; inner second electrode sheet 202; separator 30; first separator 301; first base film 3011; first ceramic coating layer 3012; second separator 302; second base film 3021; second ceramic coating layer 3022; third separator 303; fourth separator 304. DETAILED DESCRIPTION

[0030] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. All other embodiments obtained by a person skilled in the art based on the present application belong to the scope of protection of the present application.

[0031] It should be noted that, in the specific embodiments of the present application, lithium ion batteries are taken as examples of secondary batteries to explain the present application, but the secondary batteries of the present application are not limited to lithium ion batteries. The specific technical solutions are as follows:

[0032] A first aspect of the present application provides a secondary battery, comprising a case and an electrode assembly of a stacked structure, along the thickness direction of the electrode assembly, the electrode assembly comprises a first electrode sheet, a second electrode sheet and a separator arranged between the first electrode sheet and the second electrode sheet, the first electrode sheet comprises two outer first electrode sheets, two sub-outer first electrode sheets and an inner first electrode sheet located between the two sub-outer first electrode sheets, the second electrode sheet comprises two outer second electrode sheets and an inner second electrode sheet located between the two outer second electrode sheets, the two outer first electrode sheets are respectively located at the two outermost sides of the electrode assembly, at least one outer first electrode sheet is a single-sided first electrode sheet, the two outer second electrode sheets are respectively located at the sides of the two outer first electrode sheets away from the case and adjacent to the two outer first electrode sheets, the two sub-outer first electrode sheets are respectively located at the sides of the two outer second electrode sheets away from the case and adjacent to the two outer second electrode sheets. The separator comprises a first separator and a second separator, the first separator is arranged between the single-sided first electrode sheet and the outer second electrode sheet, and the second separator is arranged between the inner first electrode sheet and the inner second electrode sheet adjacent thereto. The porosity of the first separator is P1, the porosity of the second separator is P2, and P1>P2.

[0033] In the present application, the electrode assembly of the stack structure is defined, the self-width direction is X direction, the self-length direction is Y direction, and the self-thickness direction is Z direction. It can be understood that the self-length direction, the self-width direction and the self-thickness direction of the separator, the first electrode sheet and the second electrode sheet are the same as those of the electrode assembly. Exemplarily, as shown in Figure 1 The secondary battery 001 includes an electrode assembly 01 of a stack structure and a shell 02, and the electrode assembly 01 includes a first electrode sheet 10, a second electrode sheet 20 and a separator 30 arranged between the first electrode sheet 10 and the second electrode sheet 20 in the thickness direction Z of the electrode assembly 01. The first electrode sheet 10 includes two outer first electrode sheets 101, two secondary outer first electrode sheets 102 and an inner first electrode sheet 103 between the two secondary outer first electrode sheets 102. The second electrode sheet 20 includes two outer second electrode sheets 201 and an inner second electrode sheet 202 between the two outer second electrode sheets 201. The two outer first electrode sheets 101 are respectively located at the outermost sides of the electrode assembly 01, and the two outer first electrode sheets 101 are single-sided first electrode sheets. The two outer second electrode sheets 201 are respectively located at the sides of the two outer first electrode sheets 101 away from the shell 02 and are respectively adjacent to the two outer first electrode sheets 101. The two secondary outer first electrode sheets 102 are respectively located at the sides of the two outer second electrode sheets 201 away from the shell 02 and are respectively adjacent to the two outer second electrode sheets 201. The separator 30 includes a first separator 301 arranged between the single-sided first electrode sheet 101 and the outer second electrode sheet 201, and a second separator 302 arranged between the inner first electrode sheet 103 and the inner second electrode sheet 202 adjacent thereto.

[0034] The inventors discovered that by using a stacked, layered separator in the stacked electrode assembly, the porosity of the first separator is greater than that of the second separator, meaning the porosity of the outermost separator is greater than that of the inner separator (P1 > P2). This results in a larger porosity of the outermost separator and a larger electrolyte storage space, which improves the electrolyte storage capacity on the outside of the electrode assembly. This reduces the risk of black spots and lithium plating on the outer electrode due to insufficient electrolyte on the outside of the electrode assembly during cycling, thus improving the cycle performance of the secondary battery. When the porosity of the first separator is less than or equal to that of the second separator (i.e., the porosity of the outermost separator is less than or equal to that of the inner separator, P1≤P2), the porosity of the outermost separator of the electrode assembly and the electrolyte capacity are relatively small. During long-term cycling, insufficient electrolyte on the outer side of the electrode assembly can easily lead to black spots and lithium plating on the outer electrode. Furthermore, as cycling continues, lithium dendrites are continuously generated, posing a high risk of puncturing the separator and causing direct contact between the positive and negative electrodes, resulting in a short circuit. This reduces the cycle performance and safety of the secondary battery. When the electrode assembly uses the first separator, i.e., a high-porosity separator, there is a large amount of free electrolyte in the secondary battery casing. During a drop, the entire electrode assembly shakes violently, resulting in poor drop performance. Simultaneously, the free electrolyte impacts the outer separator of the electrode assembly during a drop, increasing the risk of a short circuit in the secondary battery. Therefore, by setting membranes with different porosities in the stacked electrode assembly, and making the porosity of the first membrane greater than that of the second membrane, the electrolyte storage capacity on the outside of the electrode assembly can be improved, reducing the risk of black spots and lithium plating on the outer electrode due to insufficient electrolyte on the outside of the electrode assembly during cycling. This improves the cycle performance of the secondary battery while taking into account both drop performance and safety performance.

[0035] In this application, it is understood that in a stacked electrode assembly, when the total number of layers of the remaining first and second electrodes, excluding the two outer first electrodes, is less than seven, the diaphragm includes only the first and second diaphragms; or, the diaphragm includes only the first, second, and third diaphragms. For example... Figure 2 As shown, in the stacked electrode assembly 01, the first electrode 10 includes two outer first electrode 101 and an inner first electrode 103 located between the two outer first electrode 101. Both outer first electrode 101 are single-sided first electrode 101. The second electrode 20 only includes two outer second electrode 201. At this time, apart from the two outer first electrode 101, the total number of layers of the remaining first electrode 10 and second electrode 20 is 3 layers. The second electrode 20 does not include the inner second electrode. Then, the first diaphragm 301 is disposed between the single-sided first electrode 101 and the outer second electrode 201, and the second diaphragm 302 is disposed between the inner first electrode 103 and the adjacent second electrode 20, i.e., the outer second electrode 201. At this time, the diaphragm 30 only includes the first diaphragm 301 and the second diaphragm 302.

[0036] In some embodiments of the present application, 1.30≤P1 / P2≤2.17, 30%≤P2≤50%. For example, the value of P1 / P2 can be 1.30, 1.35, 1.40, 1.45, 1.50, 1.55, 1.60, 1.65, 1.70, 1.75, 1.80, 1.85, 1.90, 1.95, 2.0, 2.05, 2.1, 2.13, 2.15, 2.17, or a range defined by any two of them; the value of P2 can be 30%, 32%, 35%, 38%, 40%, 42%, 45%, 48%, 50%, or a range defined by any two of them. By adjusting the values of P1 / P2 and P2 within the above ranges, the outermost separator of the electrode assembly has a larger space for accommodating electrolyte while the mechanical strength of the first and second separators is taken into account, which improves the liquid storage capacity of the outer side of the electrode assembly and the ion transmission efficiency during the cycle process, and reduces the risk of black spots and lithium precipitation on the outer side of the electrode caused by insufficient electrolyte on the outer side of the electrode assembly during the cycle process. While the safety performance is taken into account, the secondary battery has good cycle performance.

[0037] In some embodiments of the present application, the first electrode tab is a positive electrode tab. When a positive electrode tab is selected as the first electrode tab, the outer first electrode tab of the electrode assembly is a positive electrode tab, and the volume change of the positive electrode tab during charging and discharging is relatively small, which is conducive to maintaining the stability of the internal structure of the secondary battery and slowing down the consumption speed of the electrolyte on the outer side of the electrode assembly during the cycle process, thereby reducing the risk of black spots and lithium precipitation on the outer side of the electrode caused by insufficient electrolyte on the outer side of the electrode assembly during the cycle process, and further improving the cycle performance of the secondary battery.

[0038] In some embodiments of the present application, the first separator comprises a first base film and a first ceramic coating layer on at least one surface of the first base film, the first ceramic coating layer comprising first ceramic particles, and the second separator comprises a second base film and a second ceramic coating layer on at least one surface of the second base film, the second ceramic coating layer comprising second ceramic particles; for example, the first ceramic particles and the second ceramic particles are both microporous ceramic particles, and the first ceramic coating layer and the second ceramic coating layer are both microporous ceramic coating layers. Figure 1As shown, the first diaphragm 301 comprises a first base film 3011 and a first ceramic coating 3012 on both surfaces of the first base film 3011, the second diaphragm 302 comprises a second base film 3021 and a second ceramic coating 3022 on both surfaces of the second base film 3021, the first ceramic particles and the second ceramic particles are each independently at least one of boehmite, AI2O3, ZrO2 or TiO2; the average particle size of the first ceramic particles is D1 μm, 0.10≤D1≤1.50, for example, the value of D1 can be 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.70, 0.80, 0.90, 1.00, 1.10, 1.20, 1.30, 1.40, 1.50 or a range formed by any two of them; the average particle size of the second ceramic particles is D2 μm, 0.50≤D2≤5.00, for example, the value of D2 can be 0.50, 0.80, 1.00, 1.30, 1.50, 1.80, 2.00, 2.30, 2.50, 2.80, 3.00, 3.30, 3.50, 3.80, 4.00, 4.30, 4.50, 4.80, 5.00 or a range formed by any two of them. By selecting the above types of first ceramic particles and second ceramic particles, and adjusting the average particle size of the first ceramic particles and the second ceramic particles within the above range, the specific surface area of the first ceramic coating and the second ceramic coating can be adjusted, and then the porosity of the first diaphragm and the porosity of the second diaphragm can be controlled, so that the porosity of the first diaphragm is less than the porosity of the second diaphragm, the porosity of the outermost diaphragm of the electrode assembly and the accommodation space of the electrolyte are larger, the liquid storage capacity of the outside of the electrode assembly is improved, thereby reducing the risk of black spots and lithium precipitation of the outside of the electrode assembly caused by insufficient electrolyte on the outside of the electrode assembly during the cycle process, and improving the cycle performance of the secondary battery.

[0039] In some embodiments of the present application, 0.02≤D1 / D2≤1.00. For example, the value of D1 / D2 can be 0.02, 0.05, 0.08, 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.70, 0.80, 0.90, 1.00 or a range formed by any two of them. By adjusting the value of D1 / D2 within the above range, the average particle size of the first ceramic particles is less than or equal to the average particle size of the second ceramic particles, which is conducive to adjusting the specific surface area of the first ceramic coating and the second ceramic coating, and the specific surface area of the first ceramic coating is larger, and then the porosity of the first diaphragm is greater than the porosity of the second diaphragm, the porosity of the outermost diaphragm of the electrode assembly and the accommodation space of the electrolyte are larger, the liquid storage capacity of the outside of the electrode assembly is improved, thereby reducing the risk of black spots and lithium precipitation of the outside of the electrode assembly caused by insufficient electrolyte on the outside of the electrode assembly during the cycle process, and improving the cycle performance of the secondary battery.

[0040] The control method of the average particle size of the ceramic particles is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the average particle size of the ceramic particles can be controlled by grading, grinding, etc. of the particles of the ceramic particles. Illustratively, when other conditions are unchanged, the average particle size of the ceramic particles decreases as the grinding time is prolonged, and the average particle size of the ceramic particles increases as the grinding time is shortened.

[0041] In some embodiments of the present application, the thickness T1 of the first ceramic coating layer is 1.0 μm to 8.0 μm, and the thickness T2 of the second ceramic coating layer is 1.0 μm to 5.0 μm. For example, the thickness T1 of the first ceramic coating layer can be 1.0 μm, 2.0 μm, 3.0 μm, 4.0 μm, 5.0 μm, 6.0 μm, 7.0 μm, 8.0 μm, or a range formed by any two of the above values; and the thickness T2 of the second ceramic coating layer can be 1.0 μm, 1.5 μm, 2.0 μm, 2.5 μm, 3.0 μm, 3.5 μm, 4.0 μm, 4.5 μm, 5.0 μm, or a range formed by any two of the above values. By controlling the thickness T1 of the first ceramic coating layer and the thickness T2 of the second ceramic coating layer within the above range, the accommodation space of the separator, especially the outermost separator of the electrode assembly, for the electrolyte is further increased, the liquid storage capacity on the outside of the electrode assembly is improved, thereby reducing the risk of black spots and lithium precipitation on the outside of the electrode assembly due to insufficient electrolyte on the outside of the electrode assembly during the cycle process, while the energy density of the secondary battery is taken into account, the cycle performance of the secondary battery is improved.

[0042] In the present application, the thickness of the first ceramic coating layer and the thickness of the second ceramic coating layer can be controlled by means known to those skilled in the art, for example, when the first ceramic coating layer slurry is coated on the surface of the first base film, the coating amount of the first ceramic coating layer slurry is increased to increase the thickness of the first ceramic coating layer on the basis of a certain solid content of the first ceramic coating layer slurry; when the second ceramic coating layer slurry is coated on the surface of the second base film, the coating amount of the second ceramic coating layer slurry is increased to increase the thickness of the second ceramic coating layer on the basis of a certain solid content of the second ceramic coating layer slurry, which is not particularly limited in the present application, as long as the purpose of the present application can be achieved.

[0043] In some embodiments of the present application, the material of the first base film, the material of the second base film are each independently selected from at least one of polyethylene (PE) or polypropylene (PP); the porosity of the first base film is p1, 50%≤p1≤65%, for example, the value of p1 can be 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59%, 60%, 61%, 62%, 63%, 64%, 65%, or a range formed by any two of them; the porosity of the second base film is p2, 30%≤p2≤50%, for example, the value of p2 can be 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, or a range formed by any two of them. By selecting the above types of first base film and second base film, and adjusting the porosity of the first base film and the second base film within the above range, the porosity of the first separator and the porosity of the second separator are adjusted, so that the porosity of the first separator is less than the porosity of the second separator, the porosity of the outermost separator of the electrode assembly and the accommodation space of the electrolyte are larger, the liquid storage capacity of the outside of the electrode assembly is improved, thereby reducing the risk of black spots and lithium precipitation of the outside electrode sheet caused by insufficient electrolyte outside the electrode assembly during the cycle process, and improving the cycle performance of the secondary battery. At the same time, by adjusting the porosity p1 of the first base film and the porosity p2 of the second base film within the above range, the mechanical strength of the first separator and the second separator is higher, which is beneficial to further consider the safety performance of the secondary battery.

[0044] In some embodiments of the present application, 1.30≤p1 / p2≤2.17, for example, the value of p1 / p2 can be 1.30, 1.35, 1.40, 1.45, 1.50, 1.55, 1.60, 1.65, 1.70, 1.75, 1.80, 1.85, 1.90, 1.95, 2.0, 2.05, 2.1, 2.13, 2.15, 2.17, or a range formed by any two of them. By adjusting the value of p1 / p2 within the above range, the first base film has a higher porosity than the second base film, which is beneficial to adjust the porosity of the first ceramic coating and the second ceramic coating, so that the porosity of the first separator is greater than the porosity of the second separator, the accommodation space of the electrolyte of the outermost separator of the electrode assembly is increased, the liquid storage capacity of the outside of the electrode assembly is improved, thereby reducing the risk of black spots and lithium precipitation of the outside electrode sheet caused by insufficient electrolyte outside the electrode assembly during the cycle process, while considering the mechanical strength of the first separator and the second separator, the cycle performance of the secondary battery is improved.

[0045] In the present application, the first base film with different porosities can be purchased, and the first base film with the desired porosity can be selected by combining the test method of "Porosity test of the first base film and the second base film" in the present application. In the present application, the second base film with different porosities can be purchased, and the second base film with the desired porosity can be selected by combining the test method of "Porosity test of the first base film and the second base film" in the present application.

[0046] In some embodiments of the present application, the thickness T'1 of the first base film is 4.0 μm to 12.0 μm, and the thickness T'2 of the second base film is 4.0 μm to 9.0 μm. For example, the thickness T'1 of the first base film can be 4.0 μm, 5.0 μm, 6.0 μm, 7.0 μm, 8.0 μm, 9.0 μm, 10.0 μm, 11.0 μm, 12.0 μm, or a range defined by any two of the above values; the thickness T'2 of the second base film can be 4.0 μm, 4.5 μm, 5.0 μm, 5.5 μm, 6.0 μm, 6.5 μm, 7.0 μm, 7.5 μm, 8.0 μm, 8.5 μm, 9.0 μm, or a range defined by any two of the above values. By adjusting the thickness T'1 of the first base film and the thickness T'2 of the second base film within the above range, the accommodation space of the separator, especially the outermost separator of the electrode assembly, for the electrolyte can be further increased, the liquid storage capacity on the outside of the electrode assembly is improved, thereby reducing the risk of black spots and lithium precipitation on the outside of the electrode caused by insufficient electrolyte on the outside of the electrode assembly during the cycle process. While the energy density of the secondary battery is taken into account, the cycle performance of the secondary battery is improved.

[0047] In the present application, the first base film with different thicknesses can be purchased, and the first base film with the desired thickness can be selected by combining the test method of "Thickness test of the first ceramic coating, the second ceramic coating, the first base film, the second base film, the first separator, and the second separator" in the present application. In the present application, the second base film with different thicknesses can be purchased, and the second base film with the desired thickness can be selected by combining the test method of "Thickness test of the first ceramic coating, the second ceramic coating, the first base film, the second base film, the first separator, and the second separator" in the present application.

[0048] In some embodiments of the present application, the first ceramic coating further comprises a first binder, the second ceramic coating further comprises a second binder, the first binder and the second binder are each independently selected from at least one of polyvinylidene fluoride, polyacrylic acid, carboxymethyl cellulose or sodium alginate; the mass percentage of the first binder W1 is 1.0% to 10.0% based on the mass of the first ceramic coating, for example, the mass percentage of the first binder W1 can be 1.0%, 2.0%, 3.0%, 4.0%, 5.0%, 6.0%, 7.0%, 8.0%, 9.0%, 10.0% or a range between any two of them; the mass percentage of the second binder W2 is 1.0% to 10.0% based on the mass of the second ceramic coating, for example, the mass percentage of the second binder W2 can be 1.0%, 2.0%, 3.0%, 4.0%, 5.0%, 6.0%, 7.0%, 8.0%, 9.0%, 10.0% or a range between any two of them. By selecting the above types of first binder and second binder and adjusting the mass percentage of the first binder W1 and the mass percentage of the second binder W2 within the above range, the first ceramic particles in the first ceramic coating are fixed on the first base film, the second ceramic particles in the second ceramic coating are fixed on the second base film, and the first and second separators are applied to the secondary battery, which improves the safety performance of the secondary battery while taking into account the cycle performance.

[0049] In an embodiment of the present application, the mass percentage of the first ceramic particles W'1 is 90.0% to 99.0% based on the mass of the first ceramic coating, for example, the mass percentage of the first ceramic particles W'1 can be 90.0%, 91.0%, 92.0%, 93.0%, 94.0%, 95.0%, 96.0%, 97.0%, 98.0%, 99.0% or a range between any two of them; the mass percentage of the second ceramic particles W'2 is 90.0% to 99.0% based on the mass of the second ceramic coating, for example, the mass percentage of the second ceramic particles W'2 can be 90.0%, 91.0%, 92.0%, 93.0%, 94.0%, 95.0%, 96.0%, 97.0%, 98.0%, 99.0% or a range between any two of them. By adjusting the mass percentage of the first ceramic particles W'1 and the mass percentage of the second ceramic particles W'2 within the above range, the first ceramic particles and the second ceramic particles can fully play their respective roles, the first and second separators have strong liquid retention capacity, and the first and second separators applied to the secondary battery improve the cycle performance of the secondary battery.

[0050] In some embodiments of the present application, the shell is an aluminum plastic film. When the aluminum plastic film is selected as the shell, and the first tab on the outer side of the electrode assembly is the positive tab, the risk of short circuit of the secondary battery is reduced, and the cycle performance and safety performance of the secondary battery are improved.

[0051] In an embodiment of the present application, the first tab is the negative tab, and the shell is a steel shell. When the steel shell is selected as the shell, and the first tab on the outer side of the electrode assembly is the negative tab, the risk of short circuit of the secondary battery is reduced, and the cycle performance and safety performance of the secondary battery are improved.

[0052] In some embodiments of the present application, the thickness of the first separator is H1 μm, the thickness of the second separator is H2 μm, 1.0≤H1 / H2≤5.8, and 5.0≤H2≤19.0. For example, the value of H1 / H2 can be 1.0, 1.2, 1.5, 1.8, 2.0, 2.2, 2.5, 2.8, 3.0, 3.2, 3.5, 3.8, 4.0, 4.2, 4.5, 4.8, 5.0, 5.2, 5.5, 5.8, or a range formed by any two of the above values, and the value of H2 can be 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0, 13.0, 14.0, 15.0, 16.0, 17.0, 18.0, 19.0, or a range formed by any two of the above values. By adjusting the values of H1 and H2 within the above range, the space for accommodating electrolyte of the separator, especially the outermost separator of the electrode assembly, is further increased, the liquid storage capacity on the outer side of the electrode assembly is improved, the risk of black spots and lithium precipitation of the outer tab due to insufficient electrolyte on the outer side of the electrode assembly during the cycle process is reduced, the cycle performance and safety performance of the secondary battery are improved, and the energy density of the secondary battery is considered.

[0053] In some embodiments of the present application, the separator further comprises a third separator, which is arranged between the outer second tab and the next outer first tab. As shown in Figure 1 、 Figure 3 and Figure 4 , the separator 30 further comprises a third separator 303 arranged between the outer second tab 201 and the next outer first tab 102. The porosity of the third separator is P3, and P3>P2. As shown in Figure 3 , the third separator and the first separator are the same separator. By adjusting the porosity of the third separator to be greater than the porosity of the second separator, the porosity of the outermost separator and the next outer separator of the electrode assembly and the space for accommodating electrolyte are larger, the liquid storage capacity on the outer side of the electrode assembly is further improved, the risk of black spots and lithium precipitation of the outer tab due to insufficient electrolyte on the outer side of the electrode assembly during the cycle process is reduced, and the cycle performance of the secondary battery is further improved.

[0054] In some embodiments of the present application, the separator further comprises a fourth separator, which is arranged between the second inner side tab adjacent to the first outer side tab. As shown in Figure 1 、 Figure 3 and Figure 4 shown, the separator 30 further comprises a fourth separator 304, which is arranged between the second inner side tab 202 adjacent to the first outer side tab 102. The porosity of the fourth separator is P4, P4>P2. By regulating the porosity of the fourth separator to be greater than that of the second separator, the porosity of the outermost separator and the second outermost separator of the electrode assembly and the accommodation space of the electrolyte are larger, which further improves the liquid storage capacity of the outer side of the electrode assembly, reduces the risk of black spots and lithium precipitation of the outer side tab caused by insufficient electrolyte on the outer side of the electrode assembly during the cycle process, and further improves the cycle performance of the secondary battery.

[0055] In an embodiment of the present application, the porosity of the third separator is P3, and the porosity of the fourth separator is P4, P3>P2, P4>P2. Exemplarily, as shown in Figure 4 the third separator, the fourth separator and the first separator use the same separator. By regulating the porosity of the third separator to be greater than that of the second separator and the porosity of the fourth separator to be greater than that of the second separator, the porosity of the outermost separator, the second outermost separator and the second second outermost separator of the electrode assembly and the accommodation space of the electrolyte are larger, which further improves the liquid storage capacity of the outer side of the electrode assembly, reduces the risk of black spots and lithium precipitation of the outer side tab caused by insufficient electrolyte on the outer side of the electrode assembly during the cycle process, and further improves the cycle performance of the secondary battery.

[0056] The preparation method of the first separator is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the preparation method of the first separator includes but is not limited to the following steps: (1) adding the first ceramic particles and the first binder into a solvent and mixing uniformly to obtain a first ceramic coating slurry; (2) coating the first ceramic coating slurry on one surface of the first base film, and after drying, forming a first ceramic coating on one surface of the first base film to obtain a first separator with a single-side first ceramic coating; (3) coating the first ceramic coating slurry on the other surface of the first base film, and after drying, obtaining a first separator with a double-side first ceramic coating. The solvent is not limited in the present application, as long as the purpose of the present application can be achieved.

[0057] The preparation method of the second diaphragm is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the preparation method of the first diaphragm includes but is not limited to the following steps: (1) adding the second ceramic particles and the second binder into a solvent and mixing uniformly to obtain a second ceramic coating slurry; (2) coating the second ceramic coating slurry on one surface of the second base film, and after drying, forming a second ceramic coating on one surface of the second base film to obtain a second diaphragm with a single-sided second ceramic coating; (3) coating the second ceramic coating slurry on the other surface of the second base film, and after drying, obtaining a second diaphragm with a double-sided second ceramic coating. The above-mentioned solvent is not limited in the present application, as long as the purpose of the present application can be achieved.

[0058] In the present application, when the first pole piece is a positive pole piece, the second pole piece is a negative pole piece. When the first pole piece is a negative pole piece, the second pole piece is a positive pole piece.

[0059] The positive pole piece is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the positive pole piece includes a positive current collector and a positive material layer, the positive current collector includes a first surface and a second surface oppositely arranged along the thickness direction of the electrode assembly, and the first surface is closer to the shell than the second surface. When the first pole piece is a positive pole piece, the positive pole piece includes two outer positive pole pieces, two secondary outer positive pole pieces, and an inner positive pole piece located between the two secondary outer positive pole pieces, at least one outer positive pole piece is a single-sided positive pole piece, and the first surface of the single-sided positive pole piece is not provided with a positive material layer, and the second surface of the single-sided positive pole piece is provided with a positive material layer. It should be noted that the "surface" here can be the entire area of the positive current collector surface, or part of the area of the positive current collector surface, which is not particularly limited in the present application, as long as the purpose of the present application can be achieved.

[0060] The positive current collector is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the positive current collector can include an aluminum foil, an aluminum alloy foil, or a composite current collector (such as an aluminum-carbon composite current collector), etc. The positive material layer of the present application contains a positive active material, and the type of positive active material is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the positive active material can include lithium nickel cobalt manganese oxide (LiNi 0.90 Co 0.05 Mn 0.05at least one of O2 (NCM955), NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminate, lithium iron phosphate, lithium-rich manganese-based material, lithium cobaltate (LiCoO2), lithium manganate, lithium manganese iron phosphate, or lithium titanate, etc. In the present application, the positive electrode active material can also include a non-metal element, for example, the non-metal element includes at least one of fluorine, phosphorus, boron, chlorine, silicon, or sulfur. In the present application, the thickness of the positive electrode current collector and the positive electrode material layer is not particularly limited as long as the purpose of the present application can be achieved. In the present application, the positive electrode material layer can also include a positive electrode binder and a conductive agent. The present application does not particularly limit the type of the positive electrode binder in the positive electrode material layer as long as the purpose of the present application can be achieved, for example, the positive electrode binder can include but is not limited to at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride-hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene. The present application does not particularly limit the type of the conductive agent in the positive electrode material layer as long as the purpose of the present application can be achieved, for example, the conductive agent can include but is not limited to at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, ketjen black, graphene, metal materials, or conductive polymers. The above-mentioned carbon nanotubes can include but are not limited to single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The above-mentioned carbon fibers can include but are not limited to vapor-grown carbon fibers (VGCF) and / or nanocarbon fibers. The above-mentioned metal materials can include but are not limited to metal powder and / or metal fibers, and specifically, the metal can include but is not limited to at least one of copper, nickel, aluminum, or silver. The above-mentioned conductive polymers can include but are not limited to at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole. The present application does not particularly limit the mass ratio of the positive electrode active material, the conductive agent, and the positive electrode binder in the positive electrode material layer, and a person skilled in the art can select according to actual needs as long as the purpose of the present application can be achieved.

[0061] The negative electrode tab is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the negative electrode tab includes a negative current collector and a negative material layer, the negative current collector includes a third surface and a fourth surface oppositely arranged along the thickness direction of the electrode assembly, and the third surface is closer to the shell than the fourth surface. When the first electrode tab is a negative electrode tab, the negative electrode tab includes two outer negative electrode tabs, two secondary outer negative electrode tabs, and an inner negative electrode tab located between the two secondary outer negative electrode tabs, at least one outer negative electrode tab is a single-sided negative electrode tab, and the third surface of the single-sided negative electrode tab is not provided with a negative material layer, and the second surface of the single-sided negative electrode tab is provided with a negative material layer. It should be noted that the "surface" here can be the entire area of the surface of the negative current collector, or it can be part of the area of the surface of the negative current collector, which is not particularly limited in the present application, as long as the purpose of the present application can be achieved.

[0062] The negative current collector is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the negative current collector can include a copper foil, a copper alloy foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam, a copper foam, or a composite current collector (such as a lithium-copper composite current collector, a carbon-copper composite current collector, a nickel-copper composite current collector, a titanium-copper composite current collector, etc.), etc. The negative material layer of the present application includes a negative active material. The type of negative active material is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the negative active material can include natural graphite, artificial graphite, mesocarbon microbeads (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, SiO x (0 < x < 2), Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, lithium titanate Li4Ti5O 12 , Li-Al alloy, or metallic lithium. In the present application, the thickness of the negative current collector and the negative material layer is not particularly limited, as long as the purpose of the present application can be achieved. Optionally, the negative material layer can further include a conductive agent and a negative binder. The type of conductive agent in the negative material layer is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the conductive agent can be the same as the type of conductive agent in the positive material layer described above. The type of negative binder in the negative material layer is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the negative binder can be the same as the type of positive binder in the positive material layer described above. Optionally, the negative material layer further includes a thickening agent, and the type of thickening agent is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the thickening agent can include at least one of carboxymethyl cellulose or sodium carboxymethyl cellulose. The mass ratio of the negative active material, the conductive agent, the binder, and the thickening agent in the negative material layer is not particularly limited in the present application, and those skilled in the art can select according to actual needs, as long as the purpose of the present application can be achieved.

[0063] In the present application, the secondary battery further includes an electrolyte including a lithium salt and a non-aqueous solvent. The lithium salt is not particularly limited in the present application as long as the object of the present application is achieved. For example, the lithium salt can include, but is not limited to, at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalato)borate (LiBOB), or lithium difluoroborate. The content of the lithium salt in the electrolyte is not particularly limited in the present application as long as the object of the present application is achieved. The non-aqueous solvent is not particularly limited in the present application as long as the object of the present application is achieved, for example, the non-aqueous solvent can include, but is not limited to, at least one of a carbonate compound, a carboxylic acid ester compound, an ether compound, or other organic solvents. The carbonate compound can include, but is not limited to, at least one of a chain carbonate compound, a cyclic carbonate compound, or a fluorinated carbonate compound. The chain carbonate compound can include, but is not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), or methyl ethyl carbonate (MEC). The cyclic carbonate can include, but is not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), or vinyl ethylene carbonate (VEC). The fluorinated carbonate compound can include, but is not limited to, at least one of fluorinated ethylene carbonate (FEC), carbonic acid-1,2-difluoro ethylene ester, carbonic acid-1,1-difluoro ethylene ester, carbonic acid-1,1,2-trifluoro ethylene ester, carbonic acid-1,1,2,2-tetrafluoro ethylene ester, carbonic acid-1-fluoro-2-methyl ethylene ester, carbonic acid-1-fluoro-1-methyl ethylene ester, carbonic acid-1,2-difluoro-1-methyl ethylene ester, carbonic acid-1,1,2-trifluoro-2-methyl ethylene ester, or carbonic acid-trifluoromethyl ethylene ester. The carboxylic acid ester compound can include, but is not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valerolactone, or caprolactone. The ether compound can include, but is not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxy ethane, 1,2-diethoxy ethane, 1-ethoxy-1-methoxy ethane, 2-methyl tetrahydrofuran, or tetrahydrofuran. The other organic solvents can include, but is not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, methyl sulfolane, methyldicyclobutyl sulfoxide, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidinone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate. The content of the non-aqueous solvent in the electrolyte is not particularly limited in the present application as long as the object of the present application is achieved.

[0064] The secondary battery of the present application is not particularly limited, and can include any device in which an electrochemical reaction occurs. In one or more embodiments, the secondary battery can include, but is not limited to, a lithium ion secondary battery (lithium ion battery), a lithium polymer secondary battery, or a lithium ion polymer secondary battery, etc.

[0065] The preparation process of the secondary battery of the present application is well known to those skilled in the art, and the present application is not particularly limited, for example, the preparation process of the secondary battery can include, but is not limited to, the following steps: the outer first pole piece, the first separator, the outer second pole piece, the third separator, the next outer first pole piece, the fourth separator, the inner second pole piece, the second separator, the inner first pole piece, the second separator, the inner second pole piece, the fourth separator, the next outer first pole piece, the third separator, the outer second pole piece, the first separator, the outer first pole piece are stacked in order, then the four corners of the entire stack structure are fixed with adhesive tape to obtain an electrode assembly of the stack structure, the electrode assembly is placed in the shell, the electrolyte is injected into the shell and sealed, and the secondary battery is obtained. In addition, the anti-overcurrent element, the guide plate, etc. can also be placed in the shell to prevent the pressure inside the secondary battery from rising and overcharging and discharging.

[0066] The second aspect of the present application provides an electronic device comprising the secondary battery of any of the above embodiments. The secondary battery of the present application has good cycle performance, and therefore the electronic device of the present application has a longer service life.

[0067] The electronic device of the present application is not particularly limited, and can be any electronic device known in the art. For example, the electronic device can include, but is not limited to, a notebook computer, a pen input computer, a mobile computer, an electronic book player, a portable telephone, a portable facsimile machine, a portable copier, a portable printer, a head-mounted stereo headset, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini disc, a transceiver, an electronic organizer, a calculator, a memory card, a portable recorder, a radio, a backup power supply, an electric motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting appliance, a toy, a game machine, a clock, a power tool, a flashlight, a camera, a household large storage battery, and a lithium ion capacitor.

[0068] Embodiments

[0069] Hereinafter, embodiments and comparative examples are given to more specifically explain the embodiments of the present application. Various tests and evaluations were carried out according to the following methods. In addition, unless otherwise specified, "parts" and "%" are on a mass basis.

[0070] Test methods and apparatuses:

[0071] Sampling method of the first separator, the second separator, the third separator, and the fourth separator:

[0072] The lithium ion batteries in the tested examples and the comparative examples were disassembled, and the first separator, the second separator, the third separator and the fourth separator were taken out, dimethyl carbonate (DMC) was used for soaking for 20 min to remove the residual electrolyte, and then the first separator, the second separator, the third separator and the fourth separator were placed in an oven and dried at 60℃ for 12 h to obtain the first separator sample, the second separator sample, the third separator sample and the fourth separator sample.

[0073] Sampling method of the first base film and the second base film:

[0074] The first separator and the second separator obtained by sampling were respectively placed in a container, N-methyl pyrrolidone (NMP) was added, and ultrasonic treatment was performed in an ultrasonic instrument with heating function, the temperature was controlled at 45℃, and the ultrasonic treatment was performed for 3 h. After the first separator and the second separator became completely transparent, they were taken out to obtain the first base film and the second base film. The following tests were performed using the above method to obtain the first base film and the second base film unless otherwise specified.

[0075] Porosity test of the first separator, the second separator, the third separator and the fourth separator:

[0076] The gas displacement method was used for testing, and the first separator, the second separator, the third separator and the fourth separator were respectively punched to prepare samples using a mold (a person skilled in the art can select a mold with a common size and shape in the art according to factors such as the size and shape of the test object, the requirements of the test equipment, etc.). The true volume V0 of the sample was measured by a true density tester, the apparent volume V of the sample could be calculated by measuring the area and thickness of the sample, and then the percentage of the pore volume of the sample to the total area P = (V-V0) / V x 100%, i.e. the porosity P1 of the first separator, the porosity P2 of the second separator, the porosity P3 of the third separator or the porosity P4 of the fourth separator was obtained.

[0077] Porosity test of the first base film and the second base film:

[0078] The gas displacement method was used for testing, and the first base film and the second base film were respectively punched to prepare samples using a mold (a person skilled in the art can select a mold with a common size and shape in the art according to factors such as the size and shape of the test object, the requirements of the test equipment, etc.). The true volume V0 of the sample was measured by a true density tester, the apparent volume V of the sample could be calculated by measuring the area and thickness of the sample, and then the percentage of the pore volume of the sample to the total area p = (V-V0) / V x 100%, i.e. the porosity p1 of the first base film or the porosity p2 of the second base film was obtained.

[0079] Test of the average particle size of the first ceramic particles and the average particle size of the second ceramic particles:

[0080] A cross section of the first separator and the second separator along the thickness direction is prepared by argon ion polishing, the cross section of the first ceramic coating and the cross section of the second ceramic coating are observed by scanning electron microscopy (SEM) respectively, 10 first ceramic particles and 10 second ceramic particles are selected, the equivalent diameters of the 10 first ceramic particles and the 10 second ceramic particles are measured respectively (i.e. a particle with an irregular cross section is converted into a circle with the same area, and the diameter of the circle is measured), and the average values are obtained respectively to obtain the average particle size D1 of the first ceramic particles and the average particle size D2 of the second ceramic particles.

[0081] Thickness test of the first ceramic coating, the second ceramic coating, the first base film, the second base film, the first separator and the second separator:

[0082] The first separator is subjected to argon ion polishing to obtain a cross section of the first separator, the morphology of the cross section of the first separator along the thickness direction is observed by field emission scanning electron microscopy (Philips, model XL-30), and a scanning electron microscope photograph is taken, the thickness T1 of the first ceramic coating, the thickness T'1 of the first base film and the thickness H1 of the first separator are measured by scanning electron microscopy;

[0083] The second separator is subjected to argon ion polishing to obtain a cross section of the second separator, the morphology of the cross section of the second separator along the thickness direction is observed by field emission scanning electron microscopy (Philips, model XL-30), and a scanning electron microscope photograph is taken, the thickness T2 of the second ceramic coating, the thickness T'2 of the second base film and the thickness H2 of the second separator are measured by scanning electron microscopy.

[0084] Lithium precipitation performance test:

[0085] The lithium ion battery in the examples and comparative examples is placed in a thermostat at 10°C, and after 60 minutes, it is charged at 2C constant current to 4.5V, and charged at 4.5V constant voltage until the current is 0.025C, and after 5 minutes of standing, it is discharged at 0.5C constant current to 3.0V, which is one cycle. After 100 cycles according to the above charging and discharging process, the lithium ion battery is charged at 2C constant current to 4.5V, and charged at 4.5V constant voltage until the current is 0.025C, and after 5 minutes of standing, the lithium ion battery is disassembled, the outer negative electrode sheet of the electrode assembly is taken out, and the lithium precipitation state on the surface of the outer negative electrode sheet is observed. The area of the outer negative electrode sheet that does not precipitate lithium is golden yellow, and the area that precipitates lithium is grayish white.

[0086] The judgment standard of the degree of lithium precipitation of the lithium ion battery is as follows: 0% of the lithium precipitation area is no lithium precipitation, i.e. the degree of lithium precipitation is none, more than 0% and less than or equal to 2% of the lithium precipitation area is mild lithium precipitation, i.e. the degree of lithium precipitation is mild, more than 2% and less than or equal to 20% of the lithium precipitation area is moderate lithium precipitation, i.e. the degree of lithium precipitation is moderate, and more than 20% and less than or equal to 100% of the lithium precipitation area is severe lithium precipitation, i.e. the degree of lithium precipitation is severe, wherein the percentage of the lithium precipitation area is calculated based on the total area of the negative electrode material layer of the outer negative electrode sheet.

[0087] Black Spot Test:

[0088] The lithium ion battery was placed in a 0 °C constant temperature oven and rested for 60 min to allow the lithium ion battery to reach a constant temperature. The lithium ion battery that reached a constant temperature was charged at 1C constant current to 4.5V full charge voltage at 0 °C, then charged at 4.5V constant voltage to 0.025C, rested for 5 min, and then discharged at 1C constant current to 3.0V. This was one charge-discharge cycle. After 500 cycles, the lithium ion battery was charged at 1C constant current to 4.5V full charge voltage, and then charged at 4.5V constant voltage to 0.025C to obtain a full charged battery after 500 cycles. The lithium ion battery was disassembled in a dry room with a humidity of less than 5%, and the surface of the negative electrode sheet on the outside was photographed to record whether black spots appeared.

[0089] The judgment standard for the degree of black spots of the lithium ion battery is as follows: a black spot area of 0% means that no black spots have appeared, i.e., the degree of black spots is none, a black spot area greater than 0% and less than or equal to 2% means light black spots, i.e., the degree of black spots is light, a black spot area of 2% and less than or equal to 20% means moderate black spots, i.e., the degree of black spots is moderate, and a black spot area greater than 20% and less than or equal to 100% means severe black spots, i.e., the degree of black spots is severe. The percentage of the black spot area is calculated based on the total area of the negative electrode material layer of the negative electrode sheet on the outside.

[0090] Cycle Performance Test:

[0091] The lithium ion battery was placed in a 25 °C constant temperature test box and rested for 30 min to allow the lithium ion battery to reach a constant temperature of 25 °C. The lithium ion battery was charged at 1C constant current to 4.5V, charged at 4.5V constant voltage to a current of 0.025C, rested for 5 min, and then discharged at 0.2C constant current to 3.0V. This was the first cycle, and the initial discharge capacity was recorded as C0. The lithium ion battery was subjected to charge-discharge cycles according to the above process, and the test was stopped when the cycle reached 500 cycles (cls). The discharge capacity after 500 cycles (cls) was recorded as C1. The 500 cls capacity retention rate and the thickness growth rate were calculated as indicators for evaluating the cycle performance of the lithium ion battery.

[0092] 500 cls capacity retention rate (%) = C1 / C0 x 100%.

[0093] 500 cls thickness growth rate (%) = (T1-T0) / C0 x 100%.

[0094] The higher the 500 cls capacity retention rate and the smaller the thickness growth rate, the better the cycle performance of the lithium ion battery.

[0095] Drop Performance Test:

[0096] (1) The lithium ion battery in the examples and comparative examples was placed in an environment of 25±2°C and humidity ≤60% for 30 minutes, and then charged in the following steps: constant current charging to 3.908V, and constant voltage charging to 0.025C, and standing for 30 min.

[0097] (2) The lithium ion battery was loaded into a clamp, and high-definition photos were taken to confirm that the lithium ion battery was normal in appearance before falling. The voltage and volatile organic compounds (VOC) before falling were tested, and the voltage range was required to be 3.853V to 3.933V, and the VOC range was required to be 0 to 500ppm, to ensure that the lithium ion battery before falling had no damage, no voltage abnormality, and the falling performance test was arranged.

[0098] (3) The automatic falling tower falling equipment was used to perform 18 rounds of free falling in different directions from a position 1.5m away from the ground in the following order (front right edge, top front edge, bottom left edge, bottom right edge, bottom front edge, front, front left edge, top left edge, back right edge, left side, right side, bottom back edge, back, top back edge, top right edge, back left edge, top side, bottom side). After falling, the lithium ion battery was placed at room temperature for 24h, and high-definition photos were taken of the lithium ion battery.

[0099] (4) After falling, the appearance of the lithium ion battery was checked for damage and liquid leakage, and the voltage and VOC of the lithium ion battery were measured and recorded. The falling test passed the following judgment criteria: the lithium ion battery had no damage and no liquid leakage; the voltage drop was <10mV; the VOC organic gas detection was <500ppm (30 lithium ion batteries were prepared for each example or comparative example and tested, and the number of lithium ion batteries that failed the test was X, and the falling test failure rate of the lithium ion battery in this group was X / 30).

[0100] The higher the falling test failure rate, the worse the falling performance of the lithium ion battery in this group.

[0101] Example 1-1

[0102] Preparation of the first separator

[0103] A polyethylene (PE) film with a thickness of T'1=10μm was used as the first base film, and the porosity p1 of the first base film was 60%.

[0104] The first ceramic particles Al2O3 and the first binder polyvinylidene fluoride were mixed in a mass ratio of 95:5, and then deionized water was added as a solvent. After uniform mixing, a first ceramic coating slurry was obtained.

[0105] The first ceramic coating slurry is coated on one surface of the first base film, and after drying at 60°C, a first ceramic coating is formed on one surface of the first base film; the first ceramic coating slurry is coated on the other surface of the first base film, and after drying at 60°C, a first ceramic coating is formed on the other surface of the first base film, thereby obtaining the first separator.

[0106] The average particle size of the first ceramic particles is 0.9 μm, i.e., D1 is 0.9 μm. The mass percentage content of the first binder W1 is 5% and the mass percentage content of the first ceramic particles W'1 is 95% based on the mass of the first ceramic coating. The thickness T1 of the first ceramic coating is 5 μm, the thickness H1 of the first separator is 20 μm, and the porosity P1 of the first separator is 63.0%.

[0107] <Preparation of the second separator>

[0108] The second base film is a polyethylene (PE) film with a thickness of T'2 = 6 μm, and the porosity p2 of the second base film is 40%.

[0109] The second ceramic particles Al2O3 and the second binder polyvinylidene fluoride are mixed in a mass ratio of 95:5, and then deionized water is added. After uniform mixing, the second ceramic coating slurry is obtained.

[0110] The second ceramic coating slurry is coated on one surface of the second base film, and after drying at 60°C, a second ceramic coating is formed on one surface of the second base film; the second ceramic coating slurry is coated on the other surface of the second base film, and after drying at 60°C, a second ceramic coating is formed on the other surface of the second base film, thereby obtaining the second separator.

[0111] The average particle size of the second ceramic particles is 3 μm, i.e., D2 is 3 μm. The mass percentage content of the second binder W2 is 5% and the mass percentage content of the first ceramic particles W'2 is 95% based on the mass of the second ceramic coating. The thickness T2 of the second ceramic coating is 3 μm, the thickness H2 of the second separator is 12 μm, and the porosity P2 of the second separator is 44.0%.

[0112] <Preparation of the third separator> The same as the above <Preparation of the second separator>.

[0113] <Preparation of the fourth separator> The same as the above <Preparation of the second separator>.

[0114] <Preparation of the positive electrode sheet>

[0115] The positive electrode active material lithium cobaltate, the conductive agent conductive carbon black and the positive electrode binder polyvinylidene fluoride were mixed in a mass ratio of 97.0:1.5:1.5, N-methyl pyrrolidone (NMP) was added as a solvent, and the mixture was stirred and mixed uniformly to prepare a positive electrode slurry with a solid content of 75wt%. The positive electrode slurry was uniformly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 10μm, and dried at 110°C to obtain a positive electrode sheet with a single-sided coated positive electrode material layer with a thickness of 55μm. The positive electrode slurry was coated on the other surface of the positive electrode current collector aluminum foil, and after drying, a positive electrode sheet with a total thickness of 120μm was obtained. The coated positive electrode sheet was cold-pressed, and then cut into a specification of 70mm×800mm for use. The compaction density of the positive electrode material layer was 4.23g / cm 3 .

[0116] The positive electrode active material lithium cobaltate, the conductive agent conductive carbon black and the positive electrode binder polyvinylidene fluoride were mixed in a mass ratio of 97.0:1.5:1.5, N-methyl pyrrolidone (NMP) was added as a solvent, and the mixture was stirred and mixed uniformly to prepare a positive electrode slurry with a solid content of 75wt%. The positive electrode slurry was uniformly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 10μm, and dried at 110°C to obtain a positive electrode sheet with a single-sided coated positive electrode material layer with a thickness of 55μm. The coated positive electrode sheet was cold-pressed, and then cut into a specification of 70mm×800mm for use. The compaction density of the positive electrode material layer was 4.23g / cm 3 .

[0117] <Preparation of a negative electrode sheet>

[0118] The negative electrode active material artificial graphite, styrene-butadiene rubber and sodium carboxymethyl cellulose were mixed in a mass ratio of 97.0:1.5:1.5, and then deionized water was added as a solvent. After stirring uniformly, a negative electrode slurry with a solid content of 48wt% was obtained. The negative electrode slurry was coated on one surface of a negative electrode current collector copper foil with a thickness of 6μm, and dried at 110°C to obtain a negative electrode sheet with a negative electrode material layer coated with a thickness of 50μm. The negative electrode slurry was coated on the other surface of the negative electrode current collector copper foil, and after drying, a negative electrode sheet with a total thickness of 106μm was obtained. The coated negative electrode sheet was cold-pressed, and then cut into a specification of 74mm×824mm for use. The compaction density of the negative electrode material layer was 1.735g / cm 3 .

[0119] <Preparation of an electrolyte>

[0120] In a glove box filled with dry argon gas, propylene carbonate (PC), diethyl carbonate (DEC), and ethylene carbonate (EC) were mixed in a mass ratio of 1:1:1 to obtain a base solvent, and then lithium hexafluorophosphate (LiPF6) lithium salt was dissolved in the above-mentioned base solvent and mixed uniformly to obtain an electrolyte. The mass percentage of LiPF6 in the electrolyte was 4.5% based on the mass of the electrolyte, and the balance was the base solvent.

[0121] <Preparation of lithium ion battery>

[0122] The single-sided positive electrode sheet in the <Preparation of positive electrode sheet> was placed on the outermost side of the electrode assembly as the outermost electrode sheet of the electrode assembly. The remaining positive electrode sheets in the electrode assembly were all double-sided positive electrode sheets.

[0123] The outer positive electrode sheet, the first separator, the outer negative electrode sheet, the third separator, the next outer positive electrode sheet, the fourth separator, the inner negative electrode sheet, the second separator, the inner positive electrode sheet, the second separator, the inner negative electrode sheet, the fourth separator, the next outer positive electrode sheet, the third separator, the outer negative electrode sheet, the first separator, and the outer positive electrode sheet were sequentially stacked in order, and then the four corners of the entire stack structure were fixed with adhesive tape to obtain an electrode assembly with a stack structure. The electrode assembly was placed in a steel shell and dried in a vacuum oven at 80°C for 12 hours to remove water. The electrolyte prepared above was injected, and the lithium ion battery was obtained after vacuum packaging, standing, formation, degassing, and edge cutting. The design potential interval of the lithium ion battery was 3.0V to 4.5V.

[0124] Examples 1-2 to 1-18

[0125] Except for adjusting the relevant preparation parameters according to Table 1, the rest was the same as Example 1-1. When the average particle size of the same ceramic particles changed, the grinding time was adjusted so that the average particle size value was as shown in Table 1. When the thickness of the ceramic coating changed, the coating amount was adjusted so that the thickness of the ceramic coating was as shown in Table 1.

[0126] Example 1-19

[0127] In the <Preparation of second separator>, except that the second ceramic coating slurry was only coated on one surface of the second base film, the other surface of the second base film was not coated with the second ceramic coating slurry; in the <Preparation of lithium ion battery>, the surface of the second separator coated with the second ceramic coating slurry faced the positive electrode sheet, and except for adjusting the relevant preparation parameters according to Table 1, the rest was the same as Example 1-1. When the thickness of the ceramic coating changed, the coating amount was adjusted so that the thickness of the ceramic coating was as shown in Table 1.

[0128] Examples 1-20 to 1-22

[0129] The rest was the same as Example 1-1 except that the related preparation parameters were adjusted according to Table 1. Among them, when the mass percentage of the first binder and the mass percentage of the second binder changed, the mass ratio of the first ceramic particles to the first binder and the mass ratio of the second ceramic particles to the second binder were adjusted so that the values of W1 and W2 were as shown in Table 1.

[0130] Example 2-1

[0131] The rest was the same as Example 1-1 except that the <preparation of the third separator> was the same as the <preparation of the first separator> in the example.

[0132] Example 2-2

[0133] The rest was the same as Example 1-1 except that the <preparation of the fourth separator> was the same as the <preparation of the first separator> in the example.

[0134] Example 2-3

[0135] The rest was the same as Example 1-1 except that the <preparation of the third separator> and the <preparation of the fourth separator> were the same as the <preparation of the first separator> in the example.

[0136] Example 2-4

[0137] The rest was the same as Example 1-1 except that the negative electrode sheet was selected as the first electrode sheet, the corresponding positive electrode sheet was selected as the second electrode sheet, and in the <preparation of the lithium ion battery>, the single-sided negative electrode sheet was placed on the outermost side of the electrode assembly as the outermost electrode sheet of the electrode assembly. The rest of the negative electrode sheets in the electrode assembly were double-sided negative electrode sheets, and the positive electrode sheets in the electrode assembly were double-sided positive electrode sheets. Among them, the preparation steps of the single-sided negative electrode sheet were as follows.

[0138] <Preparation of a single-sided negative electrode sheet>

[0139] The negative electrode active material artificial graphite, styrene-butadiene rubber, and sodium carboxymethyl cellulose were mixed in a mass ratio of 97.0:1.5:1.5, then deionized water was added as a solvent, and after stirring uniformly, a negative electrode slurry with a solid content of 48wt% was obtained. The negative electrode slurry was coated on one surface of a negative electrode current collector copper foil with a thickness of 6μm, and dried at 110℃ to obtain a single-sided negative electrode sheet with a negative electrode material layer coating thickness of 50μm. The coated negative electrode sheet was cold-pressed and then cut into a specification of 74mm×824mm for use. Among them, the compacted density of the negative electrode material layer was 1.735g / cm 3 .

[0140] Comparative Example 1

[0141] Example 1-1 except that <Preparation of the second separator> was the same as <Preparation of the first separator> in Example 1-4, and <Preparation of the first separator> was the same as <Preparation of the second separator> in Example 1-1.

[0142] Comparative Example 2

[0143] Example 1-1 except that <Preparation of the second separator> was the same as <Preparation of the first separator> in Example 1-4, and <Preparation of the first separator> was the same as <Preparation of the second separator> in Example 1-1.

[0144] Comparative Example 3

[0145] Example 1-1 except that <Preparation of the second separator> was the same as <Preparation of the first separator> in Example 1-4, and <Preparation of the first separator> was the same as <Preparation of the second separator> in Example 1-1.

[0146] The preparation parameters and performance parameters of each example and comparative example are shown in Tables 1 to 3.

[0147]

[0148]

[0149] Table 2

[0150]

[0151] As can be seen from Examples 1-1 to 1-22, Comparative Examples 1 to 3, by adjusting the porosity of the first separator to be greater than the porosity of the second separator, the lithium ion battery has smaller lithium precipitation and black spot area of the electrode assembly outer tab, higher 500 cls capacity retention rate, lower 500 cls thickness growth rate, and lower failure rate in drop test, indicating that the lithium ion battery of the present application can reduce the risk of lithium precipitation and black spot of the electrode assembly outer tab during the cycle process, while taking into account the drop performance, the lithium ion battery has good cycle performance. In Comparative Examples 1 and 2, the inner and outer side separators of the electrode assembly are the same; in Comparative Example 3, the parameters of the first and second separators are exactly opposite to those of the first and second separators of Example 1-1; the lithium ion batteries in Comparative Examples 1 and 3 have higher failure rates in drop tests, and while improving the cycle performance of the lithium ion battery, they cannot take into account the drop performance, and the safety performance of the lithium ion battery is poorer. In Comparative Example 2, the lithium ion battery has larger lithium precipitation and black spot area, lower 500 cls capacity retention rate, and larger 500 cls thickness growth rate, indicating that the lithium ion battery has a higher risk of lithium precipitation and black spot of the electrode assembly outer tab during the cycle process, and the cycle performance of the lithium ion battery is poorer. While in the lithium ion batteries in Examples 1-1 to 1-21, the area of lithium precipitation and black spot of the outer first tab is smaller, and the 500 cls capacity retention rate is higher, the 500 cls thickness growth rate is lower, and the failure rate in drop test is lower, indicating that the risk of lithium precipitation and black spot of the electrode assembly outer tab during the cycle process is lower, while taking into account the drop performance, the lithium ion battery has good cycle performance.

[0152] The values of P1 / P2 and P2 will generally affect the cycle performance of the lithium ion battery. As can be seen from Examples 1-1 to 1-22, when the values of P1 / P2 and P2 are adjusted within the range of the present application, the lithium ion battery has smaller lithium precipitation and black spot area of the outer first tab, and higher 500 cls capacity retention rate, lower 500 cls thickness growth rate, and lower failure rate in drop test, indicating that the risk of lithium precipitation and black spot of the electrode assembly outer tab during the cycle process is lower, while taking into account the drop performance, the lithium ion battery has good cycle performance.

[0153] The average particle size of the first ceramic particles and the average particle size of the second ceramic particles will generally affect the cycle performance of the lithium ion battery. As can be seen from Examples 1-1 to 1-5 and 1-10, when the average particle size of the first ceramic particles and the average particle size of the second ceramic particles are within the range of the present application, the lithium ion battery has smaller lithium precipitation and black spot area of the outer first tab, and higher 500 cls capacity retention rate, lower 500 cls thickness growth rate, and lower failure rate in drop test, indicating that the risk of lithium precipitation and black spot of the electrode assembly outer tab during the cycle process is lower, while taking into account the drop performance, the lithium ion battery has good cycle performance.

[0154] The type of the first ceramic particles and the type of the second ceramic particles generally affect the cycle performance of the lithium ion battery. As can be seen from Example 1-1 and Example 1-6, when the first ceramic particles and the second ceramic particles within the scope of the application are selected, the lithium ion battery has a smaller lithium precipitation and black spot area of the outer first electrode sheet, a higher 500 cls capacity retention rate, a lower 500 cls thickness growth rate, and a lower failure rate in the drop test, indicating that the risk of black spot and lithium precipitation of the outer electrode sheet of the electrode assembly during the cycle process is lower. While taking into account the drop performance, the lithium ion battery has good cycle performance.

[0155] The porosity of the first base film and the porosity of the second base film generally affect the cycle performance of the lithium ion battery. As can be seen from Example 1-1, Example 1-7 to Example 1-11, when the porosity of the first base film and the porosity of the second base film are within the scope of the application, the lithium ion battery has a smaller lithium precipitation and black spot area of the outer first electrode sheet, a higher 500 cls capacity retention rate, a lower 500 cls thickness growth rate, and a lower failure rate in the drop test, indicating that the risk of black spot and lithium precipitation of the outer electrode sheet of the electrode assembly during the cycle process is lower. While taking into account the drop performance, the lithium ion battery has good cycle performance.

[0156] The material of the first base film and the material of the second base film generally affect the cycle performance of the lithium ion battery. As can be seen from Example 1-1 and Example 1-12, when the first base film and the second base film within the scope of the application are selected, the lithium ion battery has a smaller lithium precipitation and black spot area of the outer first electrode sheet, a higher 500 cls capacity retention rate, a lower 500 cls thickness growth rate, and a lower failure rate in the drop test, indicating that the risk of black spot and lithium precipitation of the outer electrode sheet of the electrode assembly during the cycle process is lower. While taking into account the drop performance, the lithium ion battery has good cycle performance.

[0157] The thickness of the first ceramic coating and the thickness of the second ceramic coating generally affect the cycle performance of the lithium ion battery. As can be seen from Example 1-1, Example 1-13 to Example 1-14, Example 1-17 to Example 1-19, when the thickness of the first ceramic coating and the thickness of the second ceramic coating are within the scope of the application, the lithium ion battery has a smaller lithium precipitation and black spot area of the outer first electrode sheet, a higher 500 cls capacity retention rate, a lower 500 cls thickness growth rate, and a lower failure rate in the drop test, indicating that the risk of black spot and lithium precipitation of the outer electrode sheet of the electrode assembly during the cycle process is lower. While taking into account the drop performance, the lithium ion battery has good cycle performance.

[0158] The thickness of the first base film and the thickness of the second base film generally affect the cycle performance of the lithium ion battery. As can be seen from Example 1-1, Example 1-15 to Example 1-19, when the thickness of the first base film and the thickness of the second base film are within the scope of the present application, the lithium ion battery has a smaller lithium precipitation and black spot area of the outer first electrode sheet, a higher 500 cls capacity retention rate, a lower 500 cls thickness growth rate, and a lower failure rate in the drop test, indicating that the risk of black spot and lithium precipitation of the outer electrode sheet of the electrode assembly during the cycle process is lower. While taking into account the drop performance, the lithium ion battery has good cycle performance.

[0159] The thickness H2 of the second separator and the value of H1 / H2 generally affect the cycle performance of the lithium ion battery. As can be seen from Example 1-1, Example 1-10, Example 1-13 to Example 1-19, when the thickness H2 of the second separator and the value of H1 / H2 are within the scope of the present application, the lithium ion battery has a smaller lithium precipitation and black spot area of the outer first electrode sheet, a higher 500 cls capacity retention rate, a lower 500 cls thickness growth rate, and a lower failure rate in the drop test, indicating that the risk of black spot and lithium precipitation of the outer electrode sheet of the electrode assembly during the cycle process is lower. While taking into account the drop performance, the lithium ion battery has good cycle performance.

[0160] The type and content of the first binder and the type and content of the second binder generally affect the cycle performance of the lithium ion battery. As can be seen from Example 1-1, Example 1-20 to Example 1-22, when the first binder and the second binder within the scope of the present application are selected, and the content of the first binder and the content of the second binder are controlled within the scope of the present application, the lithium ion battery has a smaller lithium precipitation and black spot area of the outer first electrode sheet, a higher 500 cls capacity retention rate, a lower 500 cls thickness growth rate, and a lower failure rate in the drop test, indicating that the risk of black spot and lithium precipitation of the outer electrode sheet of the electrode assembly during the cycle process is lower. While taking into account the drop performance, the lithium ion battery has good cycle performance.

[0161] Table 3

[0162]

[0163] The relationship between the porosity of the third separator and the porosity of the second separator generally affects the cycle performance of the lithium ion battery. As can be seen from Example 1-1, Example 2-1 and Example 2-3, when the relationship between the porosity of the third separator and the porosity of the second separator meets the present application, the lithium ion battery has a smaller lithium precipitation and black spot area of the outer first electrode sheet, a higher 500 cls capacity retention rate, a lower 500 cls thickness growth rate, and a lower failure rate in the drop test, indicating that the risk of black spot and lithium precipitation of the outer electrode sheet of the electrode assembly during the cycle process is lower. While taking into account the drop performance, the lithium ion battery has good cycle performance.

[0164] The relationship between the porosity of the fourth separator and the second separator generally affects the cycle performance of the lithium ion battery. As can be seen from Example 1-1, Example 2-2 and Example 2-3, when the relationship between the porosity of the fourth separator and the second separator meets the present application, the lithium ion battery has smaller lithium precipitation and black spot area of the outer first electrode sheet, higher 500cls capacity retention rate, lower 500cls thickness growth rate, and lower failure rate in drop test, indicating that the risk of black spot and lithium precipitation of the outer electrode sheet of the electrode assembly during the cycle process is lower, and the lithium ion battery has good cycle performance while taking into account the drop performance.

[0165] The type of the first electrode sheet generally affects the cycle performance of the lithium ion battery. As can be seen from Example 1-1 and Example 2-4, when the first electrode sheet is a positive electrode sheet or a negative electrode sheet, the lithium ion battery has smaller lithium precipitation and black spot area of the outer first electrode sheet, higher 500cls capacity retention rate, lower 500cls thickness growth rate, and lower failure rate in drop test, indicating that the risk of black spot and lithium precipitation of the outer electrode sheet of the electrode assembly during the cycle process is lower, and the lithium ion battery has good cycle performance while taking into account the drop performance.

[0166] It should be noted that in this document, relational terms such as first and second and the like can only be used to distinguish one entity or action from another entity or action, without necessarily requiring or implying that there is any such actual relationship or order between these entities or actions. Moreover, the terms "comprises", "comprising", or any other variations thereof are intended to cover non-exclusive inclusions, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements, but can also include other elements not expressly listed or inherent to such process, method, article, or apparatus.

[0167] Each of the embodiments in the specification is described in a related manner, and the same or similar parts between the embodiments can be referred to each other, and each embodiment mainly explains the difference from other embodiments.

[0168] The above description is only the preferred embodiment of the present application, and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the scope of protection of the present application.

Claims

1. A secondary battery, comprising a casing and an electrode assembly of a laminated structure, the electrode assembly comprising a first electrode tab, a second electrode tab, and a separator disposed between the first electrode tab and the second electrode tab in a thickness direction of the electrode assembly, the first electrode tab comprising two outer first electrode tabs, two sub-outer first electrode tabs, and an inner first electrode tab disposed between the two sub-outer first electrode tabs, the second electrode tab comprising two outer second electrode tabs and an inner second electrode tab disposed between the two outer second electrode tabs, the two outer first electrode tabs being respectively disposed at two outermost sides of the electrode assembly, at least one of the outer first electrode tabs being a single-sided first electrode tab, the two outer second electrode tabs being respectively disposed at sides of the two outer first electrode tabs away from the casing and adjacent to the two outer first electrode tabs, the two sub-outer first electrode tabs being respectively disposed at sides of the two outer second electrode tabs away from the casing and adjacent to the two outer second electrode tabs; the separator comprising a first separator and a second separator, the first separator being disposed between the single-sided first electrode tab and the outer second electrode tab, the second separator being disposed between the inner first electrode tab and the inner second electrode tab adjacent to the inner first electrode tab; a porosity of the first separator being P1, a porosity of the second separator being P2, P1>P2. 1.30≤P1 / P2≤2.17, 30%≤P2≤50%. The first electrode tab is a positive electrode tab.

2. The secondary battery according to claim 1, wherein The first separator comprises a first base film and a first ceramic coating layer disposed on at least one surface of the first base film, the first ceramic coating layer comprising first ceramic particles, the second separator comprises a second base film and a second ceramic coating layer disposed on at least one surface of the second base film, the second ceramic coating layer comprising second ceramic particles, the first ceramic particles and the second ceramic particles are each independently selected from at least one of boehmite, Al2O3, ZrO2, or TiO2; an average particle size of the first ceramic particles being D1 μm, 0.10≤D1≤1.50; an average particle size of the second ceramic particles being D2 μm, 0.50≤D2≤5.

00.

3. The secondary battery according to claim 1 or 2, wherein 0.02≤D1 / D2≤1.

00.

4. The secondary battery according to claim 3, wherein A thickness of the first ceramic coating layer is 1.0 μm to 8.0 μm, a thickness of the second ceramic coating layer is 1.0 μm to 5.0 μm.

5. The secondary battery according to claim 4, wherein Materials of the first base film and the second base film are each independently selected from at least one of polyethylene or polypropylene; a porosity of the first base film being p1, 50%≤p1≤65%, a porosity of the second base film being p2, 30%≤p2≤50%.

6. The secondary battery according to claim 4, wherein 1.30≤p1 / p2≤2.

17.

7. The secondary battery according to claim 4, wherein A thickness of the first base film is 4.0 μm to 12.0 μm, a thickness of the second base film is 4.0 μm to 9.0 μm.

8. The secondary battery according to claim 7, wherein ​ 9. The secondary battery according to claim 7, wherein ​ 10. The secondary battery according to claim 4, wherein The first ceramic coating further comprises a first binder, the second ceramic coating further comprises a second binder, the first binder, the second binder are each independently selected from at least one of polyvinylidene fluoride, polyacrylic acid, carboxymethyl cellulose or sodium alginate; the mass percentage content of the first binder is 1.0% to 10.0% based on the mass of the first ceramic coating; the mass percentage content of the second binder is 1.0% to 10.0% based on the mass of the second ceramic coating.

11. The secondary battery according to claim 3, wherein The shell is an aluminum plastic film.

12. The secondary battery according to claim 4, wherein The thickness of the first diaphragm is H1 μm, the thickness of the second diaphragm is H2 μm, 1.0≤H1 / H2≤5.8, 5.0≤H2≤19.

0.

13. The secondary battery according to claim 1, wherein The diaphragm further comprises a third diaphragm, the third diaphragm is arranged between the outer second pole piece and the secondary outer first pole piece, the porosity of the third diaphragm is P3, P3>P2.

14. The secondary battery according to claim 1, wherein The diaphragm further comprises a fourth diaphragm, the fourth diaphragm is arranged between the secondary outer first pole piece and the adjacent inner second pole piece, the porosity of the fourth diaphragm is P4, P4>P2. 15.An electronic device comprising the secondary battery of any one of claims 1 to 14.

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