Secondary battery and electronic device

Abstract

A secondary battery and an electronic device. The secondary battery comprises an electrode assembly, the electrode assembly comprises electrode sheets and a first separator, the electrode sheets comprise a positive electrode sheet and a negative electrode sheet, the negative electrode sheet comprises a negative electrode material layer, and the negative electrode material layer comprises silicon. On the basis of the mass of the negative electrode material layer, the mass percentage content of silicon is W, where W is greater than or equal to 3%. The first separator comprises a base film, a first coating and a second coating, wherein the base film comprises a first surface and a second surface in the thickness direction of the first separator, with the first surface facing the negative electrode sheet and the second surface facing the positive electrode sheet; the first coating is arranged on the first surface; and the second coating is arranged on the second surface. The porosity of the first coating is P1, the porosity of the second coating is P2, and P1 is greater than P2. By means of the arrangement, the risk of negative electrode interface deterioration caused by insufficient electrolyte is reduced, thereby improving the cycle performance of the secondary battery.

Description

Secondary battery and electronic device

[0001] The present application claims priority to the Chinese patent application No. 202411389197.2, filed on September 30, 2024, and entitled "Secondary battery and electronic device", the content of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0002] The present application relates to the technical field of electrochemistry, and in particular, to a secondary battery and an electronic device. BACKGROUND

[0003] Secondary batteries, such as lithium ion batteries, have characteristics of high specific energy, high working voltage, low self-discharge rate, small size, light weight, etc., and are widely used in the field of consumer electronics.

[0004] With the increasing demand for the energy density of secondary batteries, silicon, as a material with a relatively high gravimetric capacity of 4200mAh / g, a relatively low cost, and an environmentally friendly type, is increasingly used in secondary battery systems. Due to the ultra-high gravimetric capacity of the current silicon material, the negative electrode material layer containing silicon-based material, especially when the proportion of silicon element in the negative electrode material layer is high, the coating weight is low, only about one fourth of the traditional graphite material, and the thickness is also less than half of the traditional graphite electrode sheet. At this time, the negative electrode sheet has relatively poor electrolyte storage capacity, and since the demand for electrolyte of silicon-based material is higher than that of conventional graphite during the cycle process of the secondary battery, the liquid storage capacity of the negative electrode sheet containing silicon material is required to be higher. SUMMARY

[0005] The purpose of the present application is to provide a secondary battery and an electronic device to improve the liquid storage capacity of the separator facing the negative electrode side, improve the lithium precipitation of the negative electrode interface, and improve the cycle performance of the secondary battery.

[0006] It should be noted that the lithium ion battery is taken as an example to explain the present application in the summary of the present application, but the secondary battery of the present application is not limited to the lithium ion battery.

[0007] The prior art mainly retains more electrolyte inside the secondary battery shell to increase the liquid storage capacity of the negative electrode sheet for the problem of liquid storage of the negative electrode sheet containing silicon material. However, the excess electrolyte inside the shell is stored in a free state and cannot be timely supplied to the silicon-based material of the negative electrode sheet. At this time, the excess free electrolyte increases the sliding property between the layers inside the electrode assembly, affecting the drop performance of the secondary battery. When there is too much excess electrolyte, the secondary battery swells, and the shell may bulge, deform, or crack. Based on this, the present application provides a secondary battery that can supply electrolyte in time when the negative electrode sheet containing silicon elements needs electrolyte, reduces the risk of ion conduction discontinuity of the negative electrode interface in the later stage of the cycle due to insufficient electrolyte, and the electrolyte cannot be supplemented in time after the SEI film is damaged due to the cycle expansion of silicon particles, the SEI film cannot be regenerated, and further leads to the risk of lithium precipitation and even cycle diving, and improves the cycle performance of the secondary battery. The specific technical solutions are as follows:

[0008] The first aspect of the present application provides a secondary battery, the secondary battery comprising an electrode assembly, the electrode assembly comprising a sheet and a first separator, the sheet comprising a positive electrode sheet and a negative electrode sheet, the negative electrode sheet comprising a negative electrode material layer, the negative electrode material layer comprising a silicon element, the mass percentage of the silicon element being W based on the mass of the negative electrode material layer, W≥3%. The first separator comprises a base film, a first coating layer and a second coating layer. In the thickness direction of the first separator, the base film comprises a first surface and a second surface, the first surface facing the negative electrode sheet, and the second surface facing the positive electrode sheet. The first coating layer is arranged on the first surface, and the second coating layer is arranged on the second surface. The porosity of the first coating layer is P1, and the porosity of the second coating layer is P2, P1>P2. By using coating layers with different porosities on the two sides of the separator in the secondary battery containing silicon elements, the porosity of the coating layer facing the negative electrode side of the separator is larger. At this time, the coating layer with larger porosity has more inter-particle gaps, and the coating layer facing the negative electrode side of the separator can absorb more electrolyte, thereby improving the liquid storage capacity of the coating layer facing the negative electrode side of the separator, supplying electrolyte in time when the negative electrode sheet containing silicon elements needs electrolyte, reducing the risk of ion conduction discontinuity of the negative electrode interface in the later stage of the cycle due to insufficient electrolyte, and the electrolyte cannot be supplemented in time after the SEI film is damaged due to the cycle expansion of silicon particles, the SEI film cannot be regenerated, and further leads to the risk of lithium precipitation and even cycle diving, thereby improving the cycle performance of the secondary battery.

[0009] In some embodiments of the present application, 3%≤W≤80%. By adjusting the value of W within the above range, the effect of reducing the risk of negative electrode interface deterioration due to insufficient electrolyte is more obvious, and the cycle performance of the secondary battery is further improved.

[0010] In some embodiments of the present application, the first coating layer comprises a first ceramic layer, and the second coating layer comprises a second ceramic layer. Through the above arrangement, the risk of deterioration of the negative electrode interface due to insufficient electrolyte is reduced, and the cycle performance of the secondary battery is improved. In some embodiments of the present application, 1

[0011] In some embodiments of the present application, 0.6P1 / P2<0.04Wx100+0.98≤1.5P1 / P2. By regulating the value of 0.04Wx100+0.98 in the above range, the cycle performance of the secondary battery is improved while the energy density of the secondary battery is taken into account.

[0012] In some embodiments of the present application, 3%≤W≤10%, 1

[0013] In some embodiments of the present application, 10%<W≤30%, 1.5

[0014] In some embodiments of the present application, 30%<W≤80%, 2.5

[0015] In some embodiments of the present application, the first ceramic layer comprises first ceramic particles, the second ceramic layer comprises second ceramic particles, and the first ceramic particles and the second ceramic particles are each independently selected from at least one of alumina, magnesia, aluminum hydroxide, magnesium hydroxide, or boehmite. The average particle size of the first ceramic particles is D1 μm, and 0.5≤D1≤3; the average particle size of the second ceramic particles is D2 μm, and 0.25≤D2≤0.45. By selecting the first ceramic particles and the second ceramic particles of the above types and adjusting the average particle sizes of the first ceramic particles and the second ceramic particles to be within the above ranges, the porosities of the first coating layer and the second coating layer can be adjusted, the risk of deterioration of the negative electrode interface due to insufficient electrolyte is reduced, and the cycle performance of the secondary battery is improved.

[0016] In some embodiments of the present application, the first coating layer further comprises a first adhesive layer, the first adhesive layer and the first ceramic layer are sequentially stacked on the first surface, and the first ceramic layer is located between the first adhesive layer and the base film. The second coating layer further comprises a second adhesive layer, the second adhesive layer and the second ceramic layer are sequentially stacked on the second surface, and the second ceramic layer is located between the second adhesive layer and the base film. The first adhesive layer comprises a first adhesive, and the second adhesive layer comprises a second adhesive, and the first adhesive and the second adhesive are each independently selected from at least one of styrene-butadiene latex, styrene-acrylate latex, polymethyl methacrylate, polybutyl methacrylate, polyethyl acrylate, polyvinyl alcohol, ethylene-vinyl acetate copolymer, polyvinyl acetate, polyurethane, polyvinylidene fluoride, or a polyvinylidene fluoride-hexafluoropropylene copolymer. By providing the first adhesive layer and the second adhesive layer and selecting the first adhesive and the second adhesive of the above types, better tab interface adhesion is achieved while the cycle performance is taken into account, and the safety performance of the secondary battery is improved.

[0017] In some embodiments of the present application, the thickness of the first adhesive layer is 0.5 μm to 3 μm. By adjusting the thickness T1 of the first adhesive layer to be within the above range, the safety performance and the cycle performance of the secondary battery are further improved.

[0018] In some embodiments of the present application, the first adhesive comprises at least one of styrene-butadiene latex, styrene-acrylate latex, polymethyl methacrylate, polybutyl methacrylate, polyethyl acrylate, polyvinyl alcohol, ethylene-vinyl acetate copolymer, polyvinyl acetate, or polyurethane, and the second adhesive comprises at least one of polyvinylidene fluoride or a polyvinylidene fluoride-hexafluoropropylene copolymer. By selecting the first adhesive and the second adhesive of the above types, the adhesion of the second adhesive layer is stronger than that of the first adhesive layer, the ion conduction performance of the first adhesive layer is better than that of the second adhesive layer, and the ion conduction between the interfaces is further optimized. While the safety performance of the secondary battery is taken into account, the risk of deterioration of the negative electrode interface due to insufficient electrolyte is reduced, and the cycle performance of the secondary battery is improved.

[0019] In some embodiments of the present application, the electrode assembly is a stack structure. In some embodiments of the present application, the electrode assembly further comprises a second separator, the electrode stack comprises two outer electrode sheets and a plurality of inner electrode sheets, the two outer electrode sheets are respectively located at the outermost two sides of the electrode assembly, the first separator is arranged between at least one outer electrode sheet and an inner electrode sheet adjacent to the outer electrode sheet, and the second separator is arranged between two adjacent inner electrode sheets. In some embodiments of the present application, the number of layers of the first separator is m, the total number of layers of the first separator and the second separator is n, and 1 / 5≤m / n≤1. Through the above arrangement, while reducing the risk of deterioration of the negative electrode interface caused by insufficient electrolyte and improving the cycle performance of the secondary battery, the operation difficulty and processing cost in the actual production process are also taken into account, and the energy density of the secondary battery is further improved.

[0020] In some embodiments of the present application, the first separator is folded in a Z-shaped structure in the electrode assembly, and the Z-shaped first separator separates the adjacent positive electrode sheet and the negative electrode sheet. Through the above arrangement, the stability of the electrode assembly stack structure is further improved, and the safety performance of the secondary battery is further improved while improving the cycle performance of the secondary battery.

[0021] The second aspect of the present application provides an electronic device comprising the secondary battery of any one of the preceding 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.

[0022] The beneficial effects of the embodiments of the present application are as follows:

[0023] The present application provides a secondary battery and an electronic device. By using different porosity coatings on the two sides of the separator facing the positive and negative electrodes in the secondary battery, the side of the separator facing the negative electrode has a coating with a larger porosity, which is beneficial to the side of the separator facing the negative electrode to absorb more electrolyte, thereby improving the liquid storage capacity of the side of the separator facing the negative electrode, and when the negative electrode sheet containing silicon elements needs electrolyte, it can be supplied in time, thereby reducing the risk of ion conduction bridge breakage of the negative electrode interface in the later stage of the cycle due to insufficient electrolyte, and the electrolyte cannot be supplemented in time after the SEI film is destroyed by the cycle expansion of silicon particles, the SEI film cannot be regenerated, and further lead to lithium precipitation and even cycle diving, thereby improving the cycle performance of the secondary battery.

[0024] 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

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings in the following description only represent some of the embodiments of the present application, and all other embodiments obtained by those of ordinary skill in the art based on these drawings also belong to the scope of protection of the present application.

[0026] FIG. 1 is a schematic diagram of a partial cross-sectional structure of an electrode assembly along the thickness direction of the electrode assembly in an embodiment of the present application;

[0027] FIG. 2 is a schematic diagram of a partial cross-sectional structure of an electrode assembly along the thickness direction of the electrode assembly in another embodiment of the present application;

[0028] FIG. 3 is a schematic diagram of a cross-sectional structure of a secondary battery along the length direction of the secondary battery in another embodiment of the present application;

[0029] FIG. 4 is a schematic diagram of a cross-sectional structure of a secondary battery along the length direction of the secondary battery in another embodiment of the present application.

[0030] Reference signs: secondary battery 001; electrode assembly 01; housing 02; electrode sheet 10; first separator 20; base film 201; first coating layer 202; second coating layer 203; first ceramic layer 2021; first adhesive layer 2022; second ceramic layer 2031; second adhesive layer 2032; positive electrode sheet 11; positive electrode current collector 111; positive electrode material layer 112; negative electrode sheet 12; negative electrode current collector 121; negative electrode material layer 122; second separator 21; outer electrode sheet 101; inner electrode sheet 102. DETAILED DESCRIPTION

[0031] The technical solutions in the embodiments of the present application will be described clearly and completely below in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments only represent some of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the present application also belong to the scope of protection of the present application.

[0032] It should be noted that in the specific embodiments of the present application, a lithium ion battery is taken as an example of a secondary battery to explain the present application, but the secondary battery of the present application is not limited to a lithium ion battery. The specific technical solutions are as follows:

[0033] The first aspect of the present application provides a secondary battery, the secondary battery comprising an electrode assembly, the electrode assembly comprising a pole piece and a first separator, the pole piece comprising a positive pole piece and a negative pole piece, the negative pole piece comprising a negative pole material layer, the negative pole material layer comprising a silicon element, a mass percentage content of the silicon element being W based on a mass of the negative pole material layer, W≥3%. The first separator comprises a base film, a first coating layer and a second coating layer, along a thickness direction of the first separator, the base film comprises a first surface and a second surface, the first surface facing the negative pole piece, the second surface facing the positive pole piece, the first coating layer being arranged on the first surface, the second coating layer being arranged on the second surface. A porosity of the first coating layer is P1, a porosity of the second coating layer is P2, P1>P2.

[0034] In the present application, it is defined that, in a state of being unfolded, a length direction of the electrode assembly is X direction, a width direction of the electrode assembly is Y direction, and a thickness direction of the electrode assembly is Z direction. It can be understood that, in the state of being unfolded, the thickness direction of the negative pole piece, the positive pole piece and the separator is the same as the thickness direction of the electrode assembly. Exemplarily, as shown in FIG. 1, the electrode assembly 01 comprises a pole piece 10 and a first separator 20, the pole piece 10 comprises a positive pole piece 11 and a negative pole piece 12, the negative pole piece 12 comprises a negative pole material layer 122. The first separator 20 comprises a base film 201, a first coating layer 202 and a second coating layer 203, along a thickness direction Z direction of the first separator 20, the base film 201 comprises a first surface (not shown in the figure) and a second surface (not shown in the figure), the first surface facing the negative pole piece 12, the second surface facing the positive pole piece 11, the first coating layer 202 being arranged on the first surface, the second coating layer 203 being arranged on the second surface.

[0035] The inventors found that the coating of the separator in the prior art is generally designed symmetrically, at this time the porosity of the two coatings on the two surfaces of the separator is equal, or only the surface of the separator facing the positive electrode has a coating, at this time the liquid storage capacity of the surface of the separator facing the negative electrode is poor, and when the negative electrode tab contains silicon elements, the demand for electrolyte during the cycle of the secondary battery is higher than that of the conventional graphite, and in the later cycle, the ion conduction bridge of the negative electrode interface is easily broken due to the lack of electrolyte, and the electrolyte cannot be supplemented in time after the SEI film is destroyed by the cycle expansion of silicon particles, the SEI film cannot be regenerated, thereby causing the risk of lithium precipitation and even cycle diving, affecting the cycle life of the secondary battery and reducing the cycle performance of the secondary battery. In addition, when the two surfaces of the separator in the electrode assembly are coated with a coating with high porosity, at this time the separator stores too much electrolyte, the excess electrolyte in the shell is stored in the form of free state and cannot be supplied to the silicon-based material on the negative electrode tab in time, at this time the excess free electrolyte increases the sliding property between the layers in the electrode assembly, affecting the drop performance and safety performance of the secondary battery, and when the excess electrolyte is too much, the secondary battery swells, and the shell may bulge, deform or crack. In the secondary battery containing silicon elements, the present application uses different porosity coatings on the two sides of the separator in the secondary battery facing the positive and negative electrodes, so that the porosity of the coating on the side of the separator facing the negative electrode is larger, at this time the coating with larger porosity has more inter-particle gaps, the side of the separator facing the negative electrode can absorb more electrolyte, and the liquid storage capacity of the side of the separator facing the negative electrode is improved, the demand for electrolyte of the negative electrode tab containing silicon elements can be supplied in time, the risk of lithium precipitation and even cycle diving caused by the lack of electrolyte in the later cycle of the negative electrode interface ion conduction bridge, and the electrolyte cannot be supplemented in time after the SEI film is destroyed by the cycle expansion of silicon particles, and the SEI film cannot be regenerated, thereby causing the risk of lithium precipitation and even cycle diving, while taking into account the drop performance and safety performance, the cycle performance of the secondary battery is improved.

[0036] In some embodiments of the present application, 3%≤W≤80%. For example, the value of W can be 3%, 5%, 8%, 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, 30%, 32%, 35%, 38%, 40%, 42%, 45%, 48%, 50%, 52%, 55%, 58%, 60%, 62%, 65%, 68%, 70%, 72%, 75%, 78%, 80%, or a range between any two of them. By adjusting the value of W within the above range, the structural stability of the negative electrode sheet during charging and discharging is taken into account, while the energy density of the secondary battery is improved. At the same time, the first separator in the secondary battery of the present application can absorb more electrolyte on the side facing the negative electrode, improving the liquid storage capacity of the side of the separator facing the negative electrode, and can supply electrolyte in time when the negative electrode sheet containing silicon elements needs electrolyte. As the mass percentage of silicon elements increases, the demand for electrolyte by the negative electrode sheet also increases, so the effect of reducing the risk of broken bridges of ion conduction at the negative electrode interface in the later stage of the cycle due to insufficient electrolyte, and the destruction of the SEI film after the cycle expansion of silicon particles, the electrolyte cannot be supplemented in time, and the SEI film cannot be regenerated, thereby leading to lithium precipitation and even cycle diving, is more obvious, further improving the cycle performance of the secondary battery.

[0037] In some embodiments of the present application, the first coating layer comprises a first ceramic layer, and the second coating layer comprises a second ceramic layer. For example, as shown in FIG. 1, the first coating layer 202 comprises a first ceramic layer 2021, and the second coating layer 203 comprises a second ceramic layer 2031. Through the above arrangement, in a secondary battery containing silicon elements, different porosities of coating layers are used on both sides of the separator facing the positive and negative electrodes in the secondary battery, so that the porosity of the coating layer on the side of the separator facing the negative electrode is larger. At this time, the coating layer with a larger porosity has more inter-particle gaps, and the side of the separator facing the negative electrode can absorb more electrolyte, improving the liquid storage capacity of the side of the separator facing the negative electrode, and can supply electrolyte in time when the negative electrode sheet containing silicon elements needs electrolyte, reducing the risk of broken bridges of ion conduction at the negative electrode interface in the later stage of the cycle due to insufficient electrolyte, and the destruction of the SEI film after the cycle expansion of silicon particles, the electrolyte cannot be supplemented in time, and the SEI film cannot be regenerated, thereby leading to lithium precipitation and even cycle diving, while taking into account the drop performance and safety performance, the cycle performance of the secondary battery is improved.

[0038] In some embodiments of the present application, 1 < P1 / P2≤5, 30%≤P1≤70%. For example, the value of P1 / P2 can be 1.1, 1.2, 1.5, 1.8, 2, 2.2, 2.5, 2.8, 3, 3.2, 3.5, 3.8, 4, 4.2, 4.5, 4.8, 5, or a range between any two of them; the value of P1 can be 30%, 32%, 35%, 38%, 40%, 42%, 45%, 48%, 50%, 52%, 55%, 58%, 60%, 62%, 65%, 68%, 70%, or a range between any two of them. By adjusting the values of P1 / P2 and P1 within the above ranges, the porosity of the first coating on the side of the first separator facing the negative electrode is larger while the mechanical strength of the first separator is taken into account, the side of the first separator facing the negative electrode can absorb more electrolyte, and the liquid storage capacity of the side of the first separator facing the negative electrode is improved, which can timely supply the electrolyte when the negative electrode containing silicon elements needs electrolyte, reduce the risk of ion conduction broken bridge at the interface of the negative electrode in the later stage of the cycle due to insufficient electrolyte, and the electrolyte cannot be supplemented in time after the SEI film is destroyed by the cyclic expansion of silicon particles, the SEI film cannot be regenerated, and thus lithium precipitation and even cycle diving, and improve the cycle performance of the secondary battery.

[0039] In an embodiment of the present application, 6%≤P2<70%. For example, the value of P2 can be 6%, 8%, 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, 30%, 32%, 35%, 38%, 40%, 42%, 45%, 48%, 50%, 52%, 55%, 58%, 60%, 62%, 65%, 68%, 69%, or a range between any two of them. By adjusting the value of P2 within the above range, the liquid storage capacity of the side of the first separator facing the negative electrode is improved while the liquid storage capacity of the side of the first separator facing the positive electrode is taken into account, which can timely supply the electrolyte when the positive and negative electrodes need electrolyte, reduce the risk of ion conduction broken bridge at the interface of the electrode in the later stage of the cycle due to insufficient electrolyte, and the electrolyte cannot be supplemented in time after the SEI film is destroyed by the cyclic expansion of silicon particles, the SEI film cannot be regenerated, and thus lithium precipitation and even cycle diving, and improve the cycle performance of the secondary battery.

[0040] In some embodiments of the present application, 0.6P1 / P2≤0.04W×100+0.98≤1.5P1 / P2. For example, the value of 0.04W×100+0.98 / (P1 / P2) can be 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, or a range between any two of them. When the content of silicon element in the negative electrode material layer is relatively high, the demand for electrolyte during the cycle process is correspondingly higher. By regulating the value of 0.04W×100+0.98 within the above range, and matching the negative electrode sheet with different silicon element contents with two coating layers with different porosity ratios, the content of silicon element and the porosity between different coating layers synergize, while taking into account the mechanical strength of the first separator, the first separator facing the negative electrode side can absorb more electrolyte, improving the liquid storage capacity of the separator facing the negative electrode side, and can supply electrolyte in time when the negative electrode sheet containing silicon element needs electrolyte. While taking into account the energy density of the secondary battery, the risk of ion conduction bridge breakage at the negative electrode interface in the later stage of the cycle due to insufficient electrolyte, and the electrolyte being unable to supplement in time after the SEI film is destroyed by the cycle expansion of silicon particles, the SEI film being unable to regenerate, and further leading to lithium precipitation and even cycle diving, is reduced, improving the cycle performance of the secondary battery.

[0041] In some embodiments of the present application, 3%≤W≤10%, 1

[0042] In some embodiments of the present application, 10% < W≤ 30%, 1.5 < P1 / P2≤ 2.5, 40% < P1≤ 55%. For example, when 10% < W≤ 30%, the value of P1 / P2 can be 1.6, 1.7, 1.8, 1.9, 2, 2.1, 2.2, 2.3, 2.4, 2.5 or a range between any two of them; the value of P1 can be 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55% or a range between any two of them. When 10% < W≤ 30%, by regulating the value of P1 / P2 and P1 within the above range, the content of silicon element in the negative electrode material layer is relatively high, and the demand for electrolyte during the cycle process is correspondingly higher, and the ratio of the porosities of different coating layers is correspondingly increased, and the porosity of the first coating layer is also correspondingly increased, so that the content of silicon element and the porosity between different coating layers synergize, the first diaphragm facing the negative side can absorb more electrolyte, further improving the liquid storage capacity of the diaphragm facing the negative side, and can supply in time when the negative electrode sheet containing silicon element needs electrolyte. While taking into account the energy density of the secondary battery, the risk of ion conduction bridge breakage at the negative electrode interface in the later stage of the cycle due to insufficient electrolyte, and the destruction of the SEI film by the cycle expansion of silicon particles, the inability of the electrolyte to supplement in time after the SEI film is destroyed, and the inability of the SEI film to regenerate, thereby leading to lithium precipitation and even cycle diving, is reduced, and the cycle performance of the secondary battery is improved.

[0043] In some embodiments of the present application, 30% < W≤ 80%, 2.5 < P1 / P2≤ 5, 55% < P1≤ 70%. For example, when 30% < W≤ 80%, the value of P1 / P2 can be 2.6, 2.7, 2.8, 2.9, 3, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, 5, or a range between any two of them; the value of P1 can be 56%, 57%, 58%, 59%, 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, 70%, or a range between any two of them. When 30% < W≤ 80%, by regulating the value of P1 / P2 and P1 within the above range, the content of silicon element in the negative electrode material layer is high, the ratio of the porosities of different coating layers is large, and the porosity of the first coating layer is large, so that the content of silicon element and the porosity between different coating layers synergize, which can better meet the demand for electrolyte during the cycle process, and the first diaphragm facing the negative electrode side can absorb more electrolyte, further improving the liquid storage capacity of the diaphragm facing the negative electrode side, and can supply electrolyte in time when the negative electrode sheet containing silicon element needs electrolyte. While taking into account the energy density of the secondary battery, the risk of ion conduction bridge breakage at the negative electrode interface in the later cycle due to insufficient electrolyte, and the destruction of the SEI film by the cycle expansion of silicon particles, the inability of the electrolyte to replenish in time after the SEI film is destroyed, and the inability of the SEI film to regenerate, thereby leading to lithium precipitation and even cycle diving, is reduced, and the cycle performance of the secondary battery is improved.

[0044] In some embodiments of the present application, the first ceramic layer comprises first ceramic particles, the second ceramic layer comprises second ceramic particles, and the first ceramic particles, the second ceramic particles are each independently selected from at least one of alumina, magnesia, aluminum hydroxide, magnesium hydroxide, or boehmite. The average particle size of the first ceramic particles is D1 μm, 0.5≤D1≤3, for example, the value of D1 can be 0.5, 0.8, 1, 1.2, 1.5, 1.8, 2, 2.2, 2.5, 2.8, 3, or a range between any two of them; the average particle size of the second ceramic particles is D2 μm, 0.25≤D2≤0.45, for example, the value of D2 can be 0.25, 0.28, 0.3, 0.32, 0.35, 0.38, 0.4, 0.42, 0.45, or a range between 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 ranges, the porosity of the first coating layer and the porosity of the second coating layer can be adjusted, so that the porosity of the first coating layer is greater than the porosity of the second coating layer, the porosity of the first coating layer on the side of the first separator facing the negative electrode is greater, and the side of the first separator facing the negative electrode can absorb more electrolyte, thereby improving the liquid storage capacity of the side of the first separator facing the negative electrode, and when the negative electrode containing silicon requires electrolyte, it can be supplied in time, reducing the risk of broken bridges of ion conduction at the negative electrode interface in the later stage of the cycle due to insufficient electrolyte, and the electrolyte cannot be supplemented in time after the SEI film is damaged by the cyclic expansion of silicon particles, and the SEI film cannot be regenerated, thereby improving the cycle performance of the secondary battery.

[0045] The present application does not have a particular limitation on the way of adjusting the average particle size of the ceramic particles, as long as the purpose of the present application can be achieved. For example, the average particle size of the ceramic particles can be adjusted by classifying, 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.

[0046] In some embodiments of the present application, the first coating further comprises a first bonding layer, the first bonding layer and the first coating are sequentially stacked on the first surface, and the first ceramic layer is located between the first bonding layer and the base film. The second coating further comprises a second bonding layer, the second bonding layer and the second ceramic layer are sequentially stacked on the second surface, and the second ceramic layer is located between the second bonding layer and the base film. For example, as shown in FIG. 2, the first coating 202 further comprises a first bonding layer 2022, the first bonding layer 2022 and the first ceramic layer 2021 are sequentially stacked on the surface of the base film 201 facing the negative electrode tab 12, and the first ceramic layer 2021 is located between the first bonding layer 2022 and the base film 201; the second coating 203 further comprises a second bonding layer 2032, the second bonding layer 2032 and the second ceramic layer 2031 are sequentially stacked on the surface of the base film 201 facing the positive electrode tab 11, and the second ceramic layer 2031 is located between the second bonding layer 2032 and the base film 201. The first bonding layer comprises a first bonding agent, the second bonding layer comprises a second bonding agent, and the first bonding agent and the second bonding agent are each independently selected from at least one of a styrene-butadiene latex, a styrene-acrylate latex, polymethyl methacrylate, polybutyl methacrylate, polyethyl acrylate, polyvinyl alcohol, ethylene-vinyl acetate copolymer, polyvinyl acetate, polyurethane, polyvinylidene fluoride, or a polyvinylidene fluoride-hexafluoropropylene copolymer. By arranging the first bonding layer and the second bonding layer and selecting the first bonding agent and the second bonding agent from the above-mentioned types, the adhesion between the first separator and the positive electrode tab and the adhesion between the first separator and the negative electrode tab can be improved, the interface between the first separator and the positive electrode tab and the interface between the first separator and the negative electrode tab can be strengthened, the transmission of lithium ions can be facilitated, and the possibility of the first ceramic layer and the second ceramic layer falling off can be reduced. When the first separator is applied to a secondary battery, the cycle performance is considered, the interface adhesion between the tabs is better, and the safety performance of the secondary battery is improved.

[0047] In some embodiments of the present application, the thickness T1 of the first bonding layer is 0.5 μm to 3 μm. For example, the thickness of the first bonding layer can be 0.5 μm, 0.8 μm, 1 μm, 1.2 μm, 1.5 μm, 1.8 μm, 2 μm, 2.2 μm, 2.5 μm, 2.8 μm, 3 μm, or a range formed by any two of the above-mentioned values. By adjusting the thickness T1 of the first bonding layer within the above-mentioned range, the first coating has a higher porosity, the adhesion between the first separator and the negative electrode tab is higher, the interface between the first separator and the negative electrode tab is strengthened, the transmission of lithium ions during the cycle process is facilitated, the possibility of the first ceramic layer falling off from the base film is reduced, and thus the safety performance and the cycle performance of the secondary battery are further improved.

[0048] In an embodiment of the present application, the thickness T2 of the second adhesive layer is 0.25 μm to 5 μm. For example, the thickness T2 of the second adhesive layer can be 0.25 μm, 0.4 μm, 0.6 μm, 0.8 μm, 1.0 μm, 1.2 μm, 1.5 μm, 1.8 μm, 2.0 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, or a range defined by any two of the above values. By adjusting the thickness T2 of the second adhesive layer within the above range, the first separator has a higher adhesive force with the positive electrode tab while the porosity of the first coating layer is greater than that of the second coating layer, which strengthens the interface between the first separator and the positive electrode tab, facilitates the transmission of lithium ions during the cycle process, and reduces the possibility of the second ceramic layer falling off from the base film, thereby further improving the safety performance and cycle performance of the secondary battery.

[0049] In the present application, the thickness of the first adhesive layer and the thickness of the second adhesive layer can be adjusted by means known to those skilled in the art, for example, when the first adhesive layer slurry is coated on the surface of the first ceramic layer away from the base film, the thickness of the first adhesive layer is increased by increasing the coating amount of the first adhesive layer slurry on the basis of a certain solid content of the first adhesive layer slurry; when the second adhesive layer slurry is coated on the surface of the second ceramic layer away from the base film, the thickness of the first adhesive layer is increased by increasing the coating amount of the second adhesive layer slurry on the basis of a certain solid content of the second adhesive layer slurry. The present application is not particularly limited as long as the purpose of the present application can be achieved.

[0050] In some embodiments of the present application, the first adhesive includes at least one of styrene-butadiene latex, styrene-acrylate latex, polymethyl methacrylate, polybutyl methacrylate, polyethyl acrylate, polyvinyl alcohol, ethylene-vinyl acetate copolymer, polyvinyl acetate, or polyurethane, and the second adhesive includes at least one of polyvinylidene fluoride or a copolymer of vinylidene fluoride and hexafluoropropylene. By selecting the first adhesive and the second adhesive of the above types, the adhesive force of the second adhesive layer is stronger than that of the first adhesive layer, the ion-conducting performance of the first adhesive layer is better than that of the second adhesive layer, and the ion conduction between the interfaces is further optimized. While the interfaces between the first separator and the positive electrode tab and the first separator and the negative electrode tab are strengthened, the first separator facing the negative electrode side can absorb more electrolyte, improving the liquid storage capacity of the separator facing the negative electrode side, and timely supplying electrolyte when the negative electrode tab containing silicon elements needs electrolyte. While the safety performance of the secondary battery is considered, the risk of ion conduction discontinuity at the negative electrode interface in the later cycle stage due to insufficient electrolyte, the destruction of the SEI film after the cycle expansion of silicon particles, the inability of the electrolyte to be replenished in time, and the inability of the SEI film to be regenerated, thereby leading to lithium precipitation and even cycle diving, is reduced, and the cycle performance of the secondary battery is improved.

[0051] In some embodiments of the present application, the electrode assembly is a laminated structure. When the electrode assembly is the above structure, since the laminated structure has no interface corner area, the positive and negative electrode plates are uniformly distributed, which is conducive to reducing the excessive corner stress caused by the volume expansion of the negative electrode plate containing silicon elements in the electrode assembly of the winding structure, and thus the overall structure of the electrode assembly is deformed or even the risk of internal tearing of the electrode plate. The porosity of the first coating on the side of the first separator facing the negative electrode is larger, and the side of the first separator facing the negative electrode can absorb more electrolyte, thereby improving the liquid storage capacity of the side of the first separator facing the negative electrode, and when the negative electrode plate containing silicon elements needs electrolyte, it can be supplied in time, thereby reducing the risk of broken bridge of ion conduction at the negative electrode interface in the later stage of the cycle due to insufficient electrolyte, and the electrolyte cannot be supplemented in time after the SEI film is destroyed by the cyclic expansion of silicon particles, and the SEI film cannot be regenerated, thereby causing lithium precipitation and even cycle diving, thereby improving the cycle performance of the secondary battery.

[0052] In some embodiments of the present application, the electrode assembly further comprises a second separator, and the electrode plate comprises two outer electrode plates and a plurality of inner electrode plates, the two outer electrode plates are respectively located at the outermost two sides of the electrode assembly, the first separator is arranged at least between the outer electrode plate and the inner electrode plate adjacent to the outer electrode plate, and the second separator is arranged between two adjacent inner electrode plates. For example, as shown in FIG. 3, the electrode assembly 01 further comprises a second separator 21, and the electrode plate 10 comprises two outer electrode plates 101 and inner electrode plates 102, the two outer electrode plates 101 are respectively located at the outermost two sides of the electrode assembly 01, the first separator 20 is arranged between the outer electrode plate 101 and the inner electrode plate 102 adjacent to the outer electrode plate 101, and the second separator 21 is arranged between two adjacent inner electrode plates 102. In a conventional laminated electrode assembly, the outer electrode plate has small resistance, large current density, and greater ability to consume and transport electrolyte than the inner electrode plate. Through the above arrangement, while improving the liquid storage capacity of the side of the first separator facing the negative electrode and supplying electrolyte in time when the negative electrode plate containing silicon elements needs electrolyte, the liquid storage capacity of the outer side of the electrode assembly is also improved, thereby reducing the risk of lithium precipitation of the outer electrode plate due to insufficient electrolyte of the outer side of the electrode assembly during the cycle process, and the risk of lithium precipitation and even cycle diving caused by the broken bridge of ion conduction at the negative electrode interface in the later stage of the cycle due to insufficient electrolyte, and the electrolyte cannot be supplemented in time after the SEI film is destroyed by the cyclic expansion of silicon particles, and the SEI film cannot be regenerated, thereby improving the cycle performance of the secondary battery.

[0053] In some embodiments of the present application, the number of layers of the first separator is m, the total number of layers of the first separator and the second separator is n, and 1 / 5≤m / n≤1. For example, the value of m / n can be 1 / 5, 3 / 10, 2 / 5, 1 / 2, 3 / 5, 7 / 10, 4 / 5, 9 / 10, 1, or a range between any two of the above values. By adjusting the value of m / n within the above range, the risk of the electrolyte being unable to supplement in time after the ion conduction bridge of the negative electrode interface is broken due to insufficient electrolyte in the later stage of the cycle, and the SEI film is destroyed by the cycle expansion of the silicon particles, and the SEI film cannot be regenerated, thereby causing lithium precipitation and even cycle diving, is reduced, the cycle performance of the secondary battery is improved, the operation difficulty and processing cost in the actual production process are considered, and the energy density of the secondary battery is further improved.

[0054] The material of the base film is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the material of the base film can include at least one of polyimide, polyamide, polysulfone, polyacrylonitrile, cellulose, polyether ether ketone, polyphenylene sulfide, polyacrylate, polyethylene terephthalate, poly-p-phenylene terephthalamide, polyarylether sulfone ketone, aramid, arnosulfone, or polyolefin, and the polymerized monomer of the polyolefin includes at least one of ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 4-methyl-1-pentene, cyclobutene, cyclopentene, or cyclohexene. In the present application, a commercially available base film of a desired material can be selected. The present application is not particularly limited, as long as the purpose of the present application can be achieved.

[0055] The second separator is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the second separator includes a base film. Optionally, the second separator can further include a third coating layer, which can be disposed on one surface of the base film or on both surfaces of the base film. The second separator can also be selected from commercially available separators. For example, the second separator can be selected from a polypropylene porous film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric, or a polypropylene-polyethylene-polypropylene porous composite film, etc.

[0056] In some embodiments of the present application, the first separator is in a Z-shaped folded structure in the electrode assembly, and the Z-shaped first separator separates adjacent positive electrode sheets and negative electrode sheets. For example, as shown in FIG. 4, the first separator 20 is in a Z-shaped folded structure in the electrode assembly 01, and the Z-shaped first separator 20 separates adjacent positive electrode sheets 11 and negative electrode sheets 12. Through the above arrangement, the stability of the electrode assembly sheet structure is further improved, the operation difficulty and processing cost in the actual production process are considered, and the sheeting efficiency is higher.

[0057] In an embodiment of the present application, the negative electrode material layer comprises a negative electrode active material, and the negative electrode active material comprises a silicon-based material, and the silicon-based material comprises at least one of a silicon-carbon composite material, a silicon-oxygen composite material or a pure silicon material. By selecting the above-mentioned silicon-based material, the energy density of the secondary battery is improved while the cycle performance of the secondary battery is taken into account.

[0058] In an embodiment of the present application, the negative electrode active material further comprises at least one of artificial graphite, natural graphite, hard carbon or MCMB mesophase carbon microbeads. By selecting the above-mentioned negative electrode active material, the structural stability of the negative electrode sheet is improved, so that the volume expansion of the negative electrode sheet in the charging and discharging process is moderate.

[0059] In an embodiment of the present application, the first ceramic layer further comprises a first ceramic layer binder, and the second ceramic layer further comprises a second ceramic layer binder. The present application does not have a particular limitation on the types of the first ceramic layer binder and the second ceramic layer binder, as long as the purpose of the present application can be achieved. For example, the first ceramic layer binder and the second ceramic layer binder are each independently selected from at least one of styrene-butadiene rubber, polyvinyl alcohol, polyvinylidene fluoride, polyacrylic acid, polymethyl methacrylate, polybutyl acrylate or polyacrylonitrile. The present application does not have a particular limitation on the contents of the first ceramic particles and the first ceramic layer binder in the first ceramic layer, and the contents of the second ceramic particles and the second ceramic layer binder in the second ceramic 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.

[0060] In an embodiment of the present application, the first adhesive layer can further comprise a first thickening agent, and the second adhesive layer can further comprise a second thickening agent. The application of the first thickening agent to the first adhesive layer and the second thickening agent to the second adhesive layer is beneficial to increase the stability of the first adhesive layer slurry and the second adhesive layer slurry, and prevent the sedimentation of the components in the first adhesive layer slurry and the second adhesive layer slurry. The present application does not have a particular limitation on the types of the first thickening agent and the second thickening agent, as long as the purpose of the present application can be achieved. For example, the first thickening agent and the second thickening agent are each independently selected from at least one of hydroxyethyl cellulose, methylhydroxyethyl cellulose, sodium carboxymethyl cellulose, polyacrylamide or sodium alginate. The present application does not have a particular limitation on the content of the first thickening agent in the first adhesive layer and the content of the second thickening agent in the second adhesive 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 preparation method of the first 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) uniformly mixing the first ceramic particles and the first ceramic layer binder in the solvent to obtain a first ceramic layer slurry, and uniformly mixing the second ceramic particles and the second ceramic layer binder in the solvent to obtain a second ceramic layer slurry; (2) coating the first ceramic layer slurry on one surface of the base film, and after drying, forming a first coating layer on one surface of the base film; (3) coating the second ceramic layer slurry on the other surface of the base film, and after drying, forming a second coating layer on one surface of the first base film, i.e. obtaining the first diaphragm.

[0062] In another embodiment of the present application, the preparation method of the first diaphragm can include but is not limited to the following steps: (1) uniformly mixing the first ceramic particles and the first ceramic layer binder in the solvent to obtain a first ceramic layer slurry, and uniformly mixing the second ceramic particles and the second ceramic layer binder in the solvent to obtain a second ceramic layer slurry; (2) uniformly mixing the first binder and the first thickening agent to obtain a first bonding layer slurry, and uniformly mixing the second binder and the second thickening agent to obtain a second bonding layer slurry; (3) coating the first ceramic layer slurry on one surface of the base film, and after drying, forming a first ceramic layer on one surface of the base film, coating the first bonding layer slurry on the surface of the first ceramic layer away from the base film, and after drying, obtaining a first coating layer including the first ceramic coating layer and the first bonding layer; (4) coating the second ceramic layer slurry on the other surface of the base film, and after drying, forming a second ceramic layer on one surface of the base film, coating the second bonding layer slurry on the surface of the second ceramic layer away from the base film, and after drying, obtaining a second coating layer including the second ceramic coating layer and the second bonding layer, i.e. obtaining the first diaphragm.

[0063] The above-mentioned solvent is not limited in the present application, as long as the purpose of the present application can be achieved.

[0064] In the present application, the negative electrode sheet further includes a negative electrode current collector, and the negative electrode material layer is arranged on at least one surface of the negative electrode current collector. The "negative electrode material layer arranged on at least one surface of the negative electrode current collector" means that the negative electrode material layer can be arranged on one surface of the negative electrode current collector along the thickness direction of the negative electrode current collector, or can be arranged on two surfaces of the negative electrode current collector along the thickness direction of the negative electrode current collector. It should be noted that the "surface" here can be the entire area of the surface of the negative electrode current collector, or can be part of the area of the surface of the negative electrode current collector, which is not particularly limited in the present application, as long as the purpose of the present application can be achieved. As shown in FIG. 1, the negative electrode sheet 12 further includes a negative electrode current collector 121, and the negative electrode material layer 122 is arranged on two surfaces of the negative electrode current collector 121 along the thickness direction Z of the negative electrode current collector 121.

[0065] The negative current collector is not particularly limited in the present application, as long as the object of the present application can be achieved, for example, it 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, exemplarily, the composite current collector can be a lithium-copper composite current collector, a carbon-copper composite current collector, a nickel-copper composite current collector, a titanium-copper composite current collector, or the like. In the present application, the thickness of the negative current collector and the negative material layer is not particularly limited, as long as the object 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 the conductive agent in the negative material layer is not particularly limited in the present application, as long as the object 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, graphene, a metal material, or a conductive polymer, and the conductive carbon black can include, but is not limited to, at least one of acetylene black or Ketjen black. 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 material can include, but is 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 polymer can include, but is not limited to, at least one of polyphenylene derivative, polyaniline, polythiophene, polyacetylene, or polypyrrole. The type of the negative binder in the negative material layer is not particularly limited in the present application, as long as the object of the present application can be achieved, for example, the negative binder can include, but is not limited to, at least one of polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyimide, polyvinyl alcohol, carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, polyimide, polyamide-imide, butadiene-styrene rubber, or polyvinylidene fluoride. Optionally, the negative material layer further includes a thickening agent, and the type of the thickening agent is not particularly limited in the present application, as long as the object 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 a person skilled in the art can select according to the actual needs, as long as the object of the present application can be achieved.

[0066] The positive 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 positive electrode tab includes a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector. The "positive electrode material layer disposed on at least one surface of the positive electrode current collector" means that the positive electrode material layer can be disposed on one surface of the positive electrode current collector in the thickness direction of the positive electrode current collector, or can be disposed on both surfaces of the positive electrode current collector in the thickness direction of the positive electrode current collector. It should be noted that the "surface" herein can be the entire area of the surface of the positive electrode current collector, or can be part of the area of the surface of the positive electrode current collector, which is not particularly limited in the present application, as long as the purpose of the present application can be achieved. As shown in FIG. 1, the positive electrode tab 11 includes a positive electrode current collector 111 and a positive electrode material layer 112 disposed on both surfaces of the positive electrode current collector 111 in the thickness direction Z of the positive electrode current collector 111.

[0067] The positive electrode 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 electrode current collector can include an aluminum foil, an aluminum alloy foil, or a composite current collector (e.g., an aluminum-carbon composite current collector), etc. The positive electrode material layer of the present application includes a positive electrode active material, and the type of the positive electrode 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 electrode active material can include at least one of lithium nickel cobalt manganese oxide (LiNi 0.90 Co 0.05 Mn 0.05 O2(NCM955), NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based material, lithium cobalt oxide (LiCoO2), lithium manganese oxide, or lithium iron manganese phosphate, etc. In the present application, the positive electrode active material can also include a non-metallic element, for example, the non-metallic 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 type of the positive electrode binder in the positive electrode 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 positive electrode binder can be the same as the type of the negative electrode binder in the negative electrode material layer described above. The type of the conductive agent in the positive electrode 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 the conductive agent in the negative electrode material layer described above. The mass ratio of the positive electrode active material, the conductive agent, and the positive electrode binder in the positive electrode material layer is not particularly limited in the present application, which can be selected by a person skilled in the art according to actual needs, as long as the purpose of the present application can be achieved.

[0068] 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, methyldicyclohexyl 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.

[0069] Exemplarily, as shown in FIG. 3 and FIG. 4, the secondary battery 001 further comprises a housing 02 for accommodating the positive electrode sheet, the first separator or the first separator and the second separator, the negative electrode sheet, and the electrolyte, and other components known in the art of secondary batteries, which are not limited in the present application. The housing is not particularly limited in the present application, and can be a housing known in the art as long as the purpose of the present application can be achieved. For example, the housing can be a hard-shell housing or a flexible housing. The material of the hard-shell housing can be metal, and the type of metal is not limited in the present application, and a metal hard-shell housing known in the art can be used as long as the purpose of the present application can be achieved. The flexible housing can be a metal plastic film, such as an aluminum plastic film, a steel plastic film, etc.

[0070] The secondary battery of the present application is not particularly limited, and can include any device that undergoes an electrochemical reaction. In some embodiments of the present application, 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.

[0071] 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: stacking the positive electrode sheet, the first separator, and the negative electrode sheet in order, and winding, folding, etc. as needed to obtain an electrode assembly with a winding structure, placing the electrode assembly into the housing, injecting the electrolyte into the housing and sealing, to obtain the secondary battery. Alternatively, the positive electrode sheet, the first separator or the first separator and the second separator, and the negative electrode sheet are stacked in order, and then the four corners of the entire stack structure are fixed with adhesive tape to obtain an electrode assembly with a stack structure, the electrode assembly is placed into the housing, the electrolyte is injected into the housing and sealed, to obtain the secondary battery. In addition, the overcurrent prevention element, the guide plate, etc. can also be placed in the housing as needed, so as to prevent the pressure inside the secondary battery from rising and overcharging and discharging.

[0072] The second aspect of the present application provides an electronic device comprising the secondary battery of any of the preceding 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.

[0073] The electronic device of the present application is not particularly limited, and it 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 type computer, a mobile computer, an electronic book player, a portable telephone, a portable facsimile, a portable copying machine, a portable printer, a head-mounted stereo headphone, 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 audio 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 timepiece, an electric tool, a flashlight, a camera, a household large storage battery, and a lithium ion capacitor.

[0074] Embodiment

[0075] Hereinafter, examples and comparative examples are cited to more specifically describe the embodiments of the present application. Various tests and evaluations were performed according to the following methods. In addition, unless otherwise specified, "parts" and "%" are on a mass basis.

[0076] Test method and equipment:

[0077] Sampling method of the first separator:

[0078] The lithium ion battery in the measured example and the comparative example was disassembled, and the first separator was taken out, soaked in dimethyl carbonate (DMC) for 20 min to remove electrolyte residues, and then placed in an oven, dried at 60°C for 12 h to obtain a first separator sample. The ceramic layer on the surface of the separator facing the negative electrode tab is the first ceramic layer, and the ceramic layer on the surface of the separator facing the positive electrode tab is the second ceramic layer. If the first coating layer has only one layer, the first coating layer is the first ceramic layer, and if the second coating layer has only one layer, the second coating layer is the second ceramic layer. If an adhesive layer is provided on the surface of the ceramic layer, the adhesive layer provided on the surface of the first ceramic layer is the first adhesive layer, and the adhesive layer provided on the surface of the second ceramic layer is the second adhesive layer.

[0079] Test of mass percentage content of silicon element:

[0080] The lithium ion battery discharged at 0.5C to 3.0V was disassembled, and the negative electrode tab was taken out, soaked in dimethyl carbonate (DMC) for 20 min, and then rinsed with DMC and acetone in turn. Then the negative electrode tab was placed in an oven and baked at 80°C for 12 hours to obtain a negative electrode tab. The negative electrode tab was placed in a vacuum oven at 100°C for 24 hours, and 1 g of powder sample of the negative electrode material layer on the negative electrode tab was scraped off with a blade, and then the mass percentage content W of silicon element in the negative electrode material layer was tested using an inductively coupled plasma (ICP) analyzer.

[0081] Test of porosity P1 of the first coating and porosity P2 of the second coating:

[0082] Take the first separator sample, use N-methyl pyrrolidone (NMP) solvent to wipe off the second coating of the first separator, to obtain a single-sided first separator sample with only the base film and the first coating, first test the porosity Pa of the single-sided sample; wipe off the first coating again to obtain a sample with only the base film, test the porosity Pb of the base film sample; at the same time, obtain the cross section of the separator by argon ion polishing, measure the thickness Ta of the single-sided sample, the thickness Tb of the base film sample, and the thickness Tc of the first coating sample by field emission scanning electron microscope, Ta = Tb + Tc; it is easy to obtain Tb / Ta x Pb + Tc / Ta x P1 = Pa, P1 = (Tb x Pa - Ta x Pb) / Tc.

[0083] Use NMP solvent to wipe off the first coating of the first separator, to obtain a single-sided separator sample with only the base film and the second coating, refer to the above test steps and calculation logic, to obtain the porosity P2 of the second coating.

[0084] Test of average particle size of the first ceramic particles and average particle size of the second ceramic particles:

[0085] Prepare the cross section of the first separator along the thickness direction by argon ion polishing, observe the cross sections of the first ceramic layer and the second ceramic layer by scanning electron microscope (SEM) respectively, select 10 first ceramic particles and 10 second ceramic particles, measure the equivalent diameters of the 10 first ceramic particles and the 10 second ceramic particles (i.e. convert a particle with irregular cross section into a circle with the same area, the diameter of the circle), and obtain the average particle size D1 of the first ceramic particles and the average particle size D2 of the second ceramic particles.

[0086] Test of thickness of the first adhesive layer and thickness of the second adhesive layer:

[0087] Obtain the cross section of the first separator by argon ion polishing, observe the morphology of the cross section of the first separator along the thickness direction by field emission scanning electron microscope (Philips, XL-30 type), and take scanning electron microscope photos, and measure the thickness T1 of the first adhesive layer and the thickness T2 of the second adhesive layer by scanning electron microscope.

[0088] Test of lithium analysis performance:

[0089] The lithium ion battery in the example and comparative example was placed in a thermostat at 10°C, and after 60 minutes, charged at 2C constant current to 4.45V, charged at 4.45V constant voltage to a current of 0.025C, and after 5 minutes of standing, discharged at 0.5C constant current to 2.75V, which was one cycle. After 100 cycles according to the above charging and discharging process, the lithium ion battery was charged at 2C constant current to 4.45V, charged at 4.45V constant voltage to a current of 0.025C, and after 5 minutes of standing, the lithium ion battery was disassembled, and the negative electrode sheet in the electrode assembly was taken out, and the lithium precipitation state on the surface of the negative electrode sheet was observed. The area of the negative electrode sheet surface without lithium precipitation was golden yellow, and the area of the lithium precipitation was off-white.

[0090] The judgment standard of the degree of lithium precipitation of the lithium ion battery is as follows: the lithium precipitation area is 0% for no lithium precipitation, i.e., the degree of lithium precipitation is none, the lithium precipitation area is greater than 0% and less than or equal to 2% for mild lithium precipitation, i.e., the degree of lithium precipitation is mild, the lithium precipitation area is greater than 2% and less than or equal to 20% for moderate lithium precipitation, i.e., the degree of lithium precipitation is moderate, and the lithium precipitation area is greater than 20% and less than or equal to 100% for 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 negative electrode sheet.

[0091] Cycle performance test:

[0092] The lithium ion battery was placed in a constant temperature test box at 25°C, and after 30 minutes of standing, the lithium ion battery reached a constant temperature state of 25°C. The lithium ion battery was charged at 1C constant current to 4.45V, charged at 4.45V constant voltage to a current of 0.025C, and after 5 minutes of standing, discharged at 0.2C constant current to 2.75V, which was the first cycle. The initial discharge capacity C0was recorded. The lithium ion battery was cycled according to the above process, and when the cycle reached 500 cycles (cls), the test was stopped, and the discharge capacity after 500 cycles (cls) was recorded as C1. The 500 cls capacity retention rate was calculated as an index for evaluating the cycle performance of the lithium ion battery.

[0093] 500 cls capacity retention rate (%) = C1 / C0x 100%.

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

[0095] Drop test:

[0096] The lithium ion batteries in the examples and comparative examples were placed in a 25°C environment for 30 minutes, and then charged in the following steps: constant current charging to 4.53V at 0.5C, constant voltage charging to 0.05C, standing for 60min, and then testing the voltage of the lithium ion battery before the drop test; the lithium ion battery was loaded into a fixture and dropped freely from a distance of 1.5m from the ground in the following order: head-tail-head right corner-tail right corner-head left corner-tail left corner (angle: 45±15°), repeated for 6 rounds. After the drop test, the lithium ion battery was allowed to stand at room temperature for 24h, and the voltage of the lithium ion battery was measured and recorded. The appearance of the lithium ion battery was checked before and after the test and photographed. The pass criteria for the drop test: voltage drop <30mV (20 lithium ion batteries were prepared for each example or comparative example and tested, and the number of lithium ion batteries that passed the test was X, and the test pass rate was X / 20x100%).

[0097] The higher the drop test pass rate, the better the drop performance of the lithium ion battery in the group.

[0098] Example 1-1

[0099] Preparation of the first separator

[0100] A polyethylene (PE) film with a thickness of 10μm was used as the base film, and the porosity of the first base film was 60%.

[0101] The first ceramic particles boehmite and the first ceramic layer binder polyvinylidene fluoride were mixed in a mass ratio of 95:5, and then deionized water was added. After mixing uniformly, the first ceramic layer slurry was obtained.

[0102] The second ceramic particles boehmite and the second ceramic layer binder polyvinylidene fluoride were mixed in a mass ratio of 95:5, and then deionized water was added. After mixing uniformly, the second ceramic layer slurry was obtained.

[0103] The first ceramic layer slurry was coated on one surface of the base film, and after drying at 60°C, a first coating layer was formed on one surface of the first base film; the second ceramic layer slurry was coated on the other surface of the base film, and after drying at 60°C, a second coating layer was formed on the other surface of the first base film, thereby obtaining the first separator.

[0104] The average particle size of the first ceramic particles was 1.5μm, i.e., D1 was 1.5μm, and the average particle size of the second ceramic particles was 0.33μm, i.e., D2 was 0.33μm. The coating weight of the first ceramic layer was 0.75mg / 1540.25mm 2 , and the coating weight of the second ceramic layer was 0.75mg / 1540.25mm 2 . The porosity of the first coating layer P1 was 55%, and the porosity of the second coating layer P2 was 22%.

[0105] Preparation of the positive electrode sheet

[0106] 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 97:1.5:1.5, 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℃ 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×40mm for use. The compaction density of the positive electrode material layer was 4.23g / cm 3 .

[0107] Preparation of the negative electrode sheet

[0108] The negative electrode active material artificial graphite, the silicon-carbon composite material SiC, the conductive agent carbon nanotube and the negative electrode binder polyacrylic acid were mixed in a mass ratio of 41.1:42.9:1:5, and then deionized water was added as a solvent. After stirring and mixing uniformly, a negative electrode slurry with a solid content of 35wt% was obtained. The negative electrode slurry was coated on one surface of a negative electrode current collector copper foil with a thickness of 10μm, and dried at 110℃ to obtain a negative electrode sheet with a negative electrode material layer coated with a thickness of 40μm. 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 90μm was obtained. The coated negative electrode sheet was cold-pressed and then cut into a specification of 74mm×42mm for use. The compaction density of the negative electrode material layer was 1.5g / cm 3 , and the mass percentage of silicon element W was 30% based on the mass of the negative electrode material layer.

[0109] Preparation of the electrolyte

[0110] In a glove box filled with a dry argon atmosphere, 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 added to the above base solvent to dissolve and mix uniformly to obtain an electrolyte. The mass percentage of LiPF6 was 4.5% based on the mass of the electrolyte, and the balance was the base solvent.

[0111] Preparation of the lithium ion battery

[0112] The single-sided positive electrode sheet in the preparation of the positive electrode sheet is placed on the outermost side of the electrode assembly as the outermost positive electrode sheet of the electrode assembly. The remaining positive electrode sheets in the electrode assembly are all double-sided positive electrode sheets.

[0113] The outer positive electrode sheet, the first separator, the outer negative electrode sheet, the first separator, the second outer positive electrode sheet, the first separator, the inner negative electrode sheet, the first separator, the inner positive electrode sheet, the first separator, the inner negative electrode sheet, the first separator, the inner positive electrode sheet, the first separator, the inner negative electrode sheet, the first separator, the inner positive electrode sheet, the first separator, the inner negative electrode sheet, the first separator, the inner positive electrode sheet, the first separator, the inner negative electrode sheet, the first separator, the second outer positive electrode sheet, the first separator, the outer negative electrode sheet, the first separator, the outer positive electrode sheet prepared above are sequentially stacked in order, wherein the first separator is continuous and uninterrupted, and has a Z-shaped folding structure in the electrode assembly. Then, the four corners of the entire stack structure are fixed with adhesive tape to obtain an electrode assembly with a stack structure. The electrode assembly is placed in an aluminum plastic film packaging bag and dried in a vacuum oven at 80°C for 12 hours to remove water. The electrolyte prepared above is injected, and the lithium ion battery is obtained after vacuum packaging, standing, formation, degassing, and edge cutting processes. The design potential interval of the lithium ion battery is 2.75V to 4.45V.

[0114] Examples 1-2 to 1-13

[0115] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1. When the average particle size of the same ceramic particles changes, the grinding time is adjusted so that the average particle size value is as shown in Table 1. When the mass percentage content of silicon changes, the mass percentage contents of the conductive agent carbon nanotube and the negative electrode binder remain unchanged, and the mass ratio of artificial graphite and silicon carbon composite SiO is adjusted so that the mass percentage content of silicon is as shown in Table 1. In Example 1-7, Example 1-8, and Example 1-11, the silicon carbon composite SiC is replaced with pure silicon, the mass percentage contents of the conductive agent carbon nanotube and the negative electrode binder remain unchanged, and the mass ratio of artificial graphite and pure silicon is adjusted so that the mass percentage content of silicon is as shown in Table 1.

[0116] Example 1-14

[0117] Except for preparing the first separator according to the following steps, the rest is the same as Example 1-1.

[0118] A polyethylene (PE) film with a thickness of 10μm is used as the base film, and the porosity of the first base film is 60%.

[0119] The first ceramic particles boehmite and the first ceramic layer binder polyvinylidene fluoride are mixed in a mass ratio of 95:5, then solvent deionized water is added, and after uniform mixing, the first ceramic layer slurry is obtained.

[0120] The second ceramic particles boehmite and the second ceramic layer binder polyvinylidene fluoride are mixed in a mass ratio of 95:5, then solvent deionized water is added, and after uniform mixing, the second ceramic layer slurry is obtained.

[0121] The first binder styrene butadiene emulsion and the first thickening agent sodium carboxymethyl cellulose are mixed in a mass ratio of 98.5:1.5, then solvent deionized water is added, and after uniform mixing, the first adhesive layer slurry is obtained.

[0122] The second binder polyvinylidene fluoride homopolymer and the second thickening agent sodium carboxymethyl cellulose are mixed in a mass ratio of 98.5:1.5, then solvent deionized water is added, and after uniform mixing, the second adhesive layer slurry is obtained.

[0123] The first ceramic layer slurry is coated on one surface of the base film, and after drying at 60°C, the first ceramic layer is formed on one surface of the first base film. The first adhesive layer slurry is coated on the surface of the first ceramic layer away from the base film, and after drying at 60°C, the first coating layer including the first ceramic coating layer and the first adhesive layer is obtained. The second ceramic layer slurry is coated on the other surface of the base film, and after drying at 60°C, the second ceramic layer is formed on the other surface of the base film. The second adhesive layer slurry is coated on the surface of the second ceramic layer away from the base film, and after drying at 60°C, the second coating layer including the second ceramic coating layer and the second adhesive layer is obtained, that is, the first separator is prepared.

[0124] Wherein, the average particle size of the first ceramic particles is 1.5 μm, that is, D1 is 1.5 μm, and the average particle size of the second ceramic particles is 0.33 μm, that is, D2 is 0.33 μm. The coating weight of the first ceramic layer is 0.75 mg / 1540.25 mm 2 , and the coating weight of the second ceramic layer is 0.75 mg / 1540.25 mm 2 . The porosity P1 of the first coating layer is 55%, and the porosity P2 of the second coating layer is 22%. The thickness T1 of the first adhesive layer is 1 μm, and the thickness T2 of the second adhesive layer is 1.2 μm.

[0125] Example 1-15 to Example 1-20

[0126] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1. Among them, when the thickness of the adhesive layer changes, the coating amount is adjusted so that the thickness of the adhesive layer is as shown in Table 1.

[0127] Example 2-1

[0128] The rest is the same as Example 1-1 except that the positive electrode sheet is cut into a specification of 70 mm x 800 mm in the <preparation of the positive electrode sheet>, the negative electrode sheet is cut into a specification of 74 mm x 824 mm in the <preparation of the negative electrode sheet>, and a lithium ion battery is prepared according to the following steps.

[0129] <Preparation of the lithium ion battery>

[0130] The first separator, the negative electrode sheet, the first separator, and the positive electrode sheet prepared above are sequentially stacked in order, with the first coating of the first separator facing the negative electrode sheet and the second coating of the first separator facing the positive electrode sheet, and are sequentially wound to obtain an electrode assembly. The electrode assembly is placed in an aluminum plastic film packaging bag, dried, and then injected with the electrolyte prepared above, and is subjected to processes such as vacuum packaging, standing, formation, degassing, and edge cutting to obtain a lithium ion battery.

[0131] Example 2-2

[0132] The rest is the same as Example 1-1 except that a lithium ion battery and a second separator are prepared according to the following steps.

[0133] <Preparation of the second separator>

[0134] A polyethylene (PE) film with a thickness of 10 μm is used as the base film, and the porosity of the base film is 60%.

[0135] The ceramic particles boehmite and the ceramic layer binder polyvinylidene fluoride are mixed in a mass ratio of 95:5, and then deionized water is added. After uniform mixing, a third ceramic layer slurry is obtained.

[0136] The third ceramic layer slurry is coated on one surface of the base film, and after drying at 60°C, a third ceramic layer is formed on one surface of the base film. The above operation is repeated on the other surface of the base film to obtain the second separator.

[0137] The average particle size of the ceramic particles is 0.33 μm, the porosity of the third coating layer is 22%, and the coating weight of the third ceramic layer is 0.75 mg / 1540.25 mm 2 .

[0138] <Preparation of the lithium ion battery>

[0139] The single-sided positive electrode sheet in the <preparation of the positive electrode sheet> is placed on the outermost side of the electrode assembly as the outermost sheet of the electrode assembly. The rest of the positive electrode sheets in the electrode assembly are all double-sided positive electrode sheets.

[0140] The outer side positive electrode tab, the first separator, the outer side negative electrode tab, the first separator, the secondary outer side positive electrode tab, the second separator, the inner side negative electrode tab, the second separator, the inner side positive electrode tab, the second separator, the inner side negative electrode tab, the second separator, the inner side positive electrode tab, the second separator, the inner side negative electrode tab, the second separator, the inner side positive electrode tab, the second separator, the inner side negative electrode tab, the second separator, the inner side positive electrode tab, the second separator, the inner side negative electrode tab, the second separator, the inner side positive electrode tab, the second separator, the inner side negative electrode tab, the second separator, the secondary outer side positive electrode tab, the first separator, the outer side negative electrode tab, the first separator, the outer side positive electrode tab prepared above are sequentially stacked in order. Then the four corners of the whole stack structure are fixed by using adhesive tape to obtain an electrode assembly of the stack structure. The electrode assembly is placed in an aluminum-plastic film packaging bag, and is placed in a vacuum oven at 80°C for 12 hours to remove water, and is injected with the electrolyte prepared above. The lithium ion battery is obtained through processes of vacuum packaging, standing, formation, degassing, and edge cutting. The design potential interval of the lithium ion battery is 2.75V to 4.45V.

[0141] Example 2-3

[0142] Except that the lithium ion battery is prepared according to the following steps, the rest is the same as Example 2-2.

[0143] <Preparation of lithium ion battery>

[0144] The single-sided positive electrode tab in the <Preparation of positive electrode tab> is placed at the outermost side of the electrode assembly as the outermost tab of the electrode assembly. The rest of the positive electrode tabs in the electrode assembly are all double-sided positive electrode tabs.

[0145] The outer side positive electrode tab, the first separator, the outer side negative electrode tab, the first separator, the secondary outer side positive electrode tab, the first separator, the inner side negative electrode tab, the first separator, the inner side positive electrode tab, the second separator, the inner side negative electrode tab, the second separator, the inner side positive electrode tab, the second separator, the inner side negative electrode tab, the second separator, the inner side positive electrode tab, the second separator, the inner side negative electrode tab, the second separator, the inner side positive electrode tab, the first separator, the inner side negative electrode tab, the first separator, the secondary outer side positive electrode tab, the first separator, the outer side negative electrode tab, the first separator, the outer side positive electrode tab prepared above are sequentially stacked in order. Then the four corners of the whole stack structure are fixed by using adhesive tape to obtain an electrode assembly of the stack structure. The electrode assembly is placed in an aluminum-plastic film packaging bag, and is placed in a vacuum oven at 80°C for 12 hours to remove water, and is injected with the electrolyte prepared above. The lithium ion battery is obtained through processes of vacuum packaging, standing, formation, degassing, and edge cutting. The design potential interval of the lithium ion battery is 2.75V to 4.45V.

[0146] Example 2-4

[0147] The first separator is used in the preparation of the lithium ion battery, and the rest is the same as Example 2-2.

[0148] Comparative Example 1

[0149] The preparation steps of the first coating layer are completely the same as the preparation steps of the second coating layer in Example 1-1, and the rest is the same as Example 1-1.

[0150] Comparative Example 2

[0151] The preparation steps of the second coating layer are completely the same as the preparation steps of the first coating layer in Example 1-1, and the rest is the same as Example 1-1.

[0152] Comparative Example 3

[0153] The preparation steps of the first coating layer are completely the same as the preparation steps of the second coating layer in Example 1-1, and the rest is the same as Example 1-1.

[0154] The preparation parameters and performance parameters of each example and comparative example are shown in Tables 1-2.

[0155] As can be seen from Example 1-1 to Example 1-20 and Comparative Examples 1-3, by adjusting the porosity of the first coating layer to be greater than the porosity of the second coating layer, the degree of lithium precipitation of the negative electrode sheet in the electrode assembly is lighter, the 500cls capacity retention of the lithium ion battery is improved, and the drop test pass rate is higher, indicating that the lithium ion battery of the present application can reduce the risk of negative electrode interface deterioration caused by insufficient electrolyte, while taking into account the drop performance and safety performance, the lithium ion battery has good cycle performance. The porosities of the first coating layer and the second coating layer of the first separator of the electrode assembly in Comparative Examples 1 and 2 are the same; the parameters of the first coating layer and the second coating layer in Comparative Example 3 are exactly opposite to those of the first coating layer and the second coating layer in Example 1-1; the degree of lithium precipitation of the negative electrode sheet in the lithium ion battery in Comparative Examples 1-3 is heavier; the 500cls capacity retention is lower; and / or, the drop test pass rate is lower. While the degree of lithium precipitation of the negative electrode sheet in the lithium ion battery in Examples 1-1 to 1-20 is lighter, the 500cls capacity retention is higher, and the drop test pass rate is higher, indicating that the risk of negative electrode sheet deterioration due to insufficient electrolyte during the cycle process is lower, while taking into account the drop performance and safety performance, the lithium ion battery has good cycle performance.

[0156] The value of W generally affects the cycle performance of the lithium ion battery. As can be seen from Example 1-1 to Example 1-11, when the value of W is within the range of the present application, the extent of lithium precipitation of the negative electrode sheet in the lithium ion battery is lighter, the 500 cls capacity retention is higher, and the drop test pass rate is higher, indicating that the risk of deterioration of the negative electrode sheet due to insufficient electrolyte during the cycle is lower, and the lithium ion battery has good cycle performance while taking into account the drop performance and safety performance.

[0157] The values of P1 / P2 and P1 generally affect the cycle performance of the lithium ion battery. As can be seen from Example 1-1 to Example 1-11, when the values of P1 / P2 and P1 are within the range of the present application, the extent of lithium precipitation of the negative electrode sheet in the lithium ion battery is lighter, the 500 cls capacity retention is higher, and the drop test pass rate is higher, indicating that the risk of deterioration of the negative electrode sheet due to insufficient electrolyte during the cycle is lower, and the lithium ion battery has good cycle performance while taking into account the drop performance and safety performance. Among them, the values of P1 / P2 and P1 in Example 1-11 are larger, at this time the liquid storage capacity of the separator on the positive electrode side is relatively weak, the liquid storage capacity of the separator on the negative electrode side is relatively strong, and the liquid storage amount on both sides is large, resulting in a decrease in the 500 cls capacity retention of the lithium ion battery, and there is more free electrolyte in the shell, which reduces the drop test pass rate of the lithium ion battery, and the cycle performance and drop performance of the lithium ion battery are affected.

[0158] The corresponding relationship between P1 / P2 and W generally affects the cycle performance of the lithium ion battery. As can be seen from Example 1-1 to Example 1-10, when the corresponding relationship between P1 / P2 and W meets the present application, the extent of lithium precipitation of the negative electrode sheet in the lithium ion battery is lighter, the 500 cls capacity retention is higher, and the drop test pass rate is higher, indicating that the risk of deterioration of the negative electrode sheet due to insufficient electrolyte during the cycle is lower, and the lithium ion battery has good cycle performance while taking into account the drop performance and safety performance. Among them, in Example 1-10, the values of P1 / P2 and P1 are larger relative to the value of W, at this time the liquid storage capacity of the separator on the positive electrode side is relatively weak, and the liquid storage amount on both sides is large, resulting in a decrease in the 500 cls capacity retention of the lithium ion battery, and the cycle performance of the lithium ion battery is affected.

[0159] The average particle size of the first ceramic particles and the average particle size of the second ceramic particles generally affect the cycle performance of the lithium ion battery. As can be seen from Example 1-1 to Example 1-11, 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 extent of lithium precipitation of the negative electrode sheet in the lithium ion battery is lighter, the 500 cls capacity retention is higher, and the drop test pass rate is higher, indicating that the risk of deterioration of the negative electrode sheet due to insufficient electrolyte during the cycle is lower, and the lithium ion battery has good cycle performance while taking into account the drop performance and safety performance.

[0160] 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, Example 1-12 and Example 1-13, when the first ceramic particles and the second ceramic particles within the scope of the present application are selected, the lithium ion battery has a lighter degree of lithium precipitation in the negative electrode sheet, a higher capacity retention rate at 500 cls, and a higher pass rate of drop test, indicating that the risk of deterioration of the negative electrode sheet due to insufficient electrolyte during the cycle process is lower, and the lithium ion battery has good cycle performance while taking into account the drop performance and safety performance.

[0161] The type of the first binder and the type of the second binder generally affect the cycle performance of the lithium ion battery. As can be seen from Example 1-1, Example 1-14 and Example 1-17, when the first binder and the second binder within the scope of the present application are selected, the lithium ion battery has a lighter degree of lithium precipitation in the negative electrode sheet, a higher capacity retention rate at 500 cls, and a higher pass rate of drop test, indicating that the risk of deterioration of the negative electrode sheet due to insufficient electrolyte during the cycle process is lower, and the lithium ion battery has good cycle performance while taking into account the drop performance and safety performance.

[0162] The thickness T1 of the first adhesive layer generally affects the cycle performance of the lithium ion battery. As can be seen from Example 1-1, Example 1-18 to Example 1-20, when the thickness T1 of the first adhesive layer is within the scope of the present application, the lithium ion battery has a lighter degree of lithium precipitation in the negative electrode sheet, a higher capacity retention rate at 500 cls, and a higher pass rate of drop test, indicating that the risk of deterioration of the negative electrode sheet due to insufficient electrolyte during the cycle process is lower, and the lithium ion battery has good cycle performance while taking into account the drop performance and safety performance. Among them, the thickness of the first adhesive layer in Example 1-20 is larger, and the porosity of the first coating layer is reduced. At this time, the resistance during the lithium ion transmission process increases, the ion transmission capacity is affected, resulting in a decrease in the 500 cls capacity retention rate of the lithium ion battery, and the cycle performance of the lithium ion battery is affected.

[0163] Table 2

[0164] Note: " / " in Table 2 means no relevant preparation parameters.

[0165] The structure of the electrode assembly generally affects the cycle performance of the lithium ion battery. As can be seen from Example 1-1, Example 2-1 and Example 2-4, when the structure of the electrode assembly meets the present application, the lithium ion battery has a lighter degree of lithium precipitation in the negative electrode sheet, a higher capacity retention rate at 500 cls, and a higher pass rate of drop test, indicating that the risk of deterioration of the negative electrode sheet due to insufficient electrolyte during the cycle process is lower, and the lithium ion battery has good cycle performance while taking into account the drop performance and safety performance.

[0166] The value of m / n usually affects the cycle performance of the lithium ion battery. As can be seen from Example 1-1, Example 2-2 and Example 2-4, when the value of m / n meets the present application, the extent of lithium precipitation of the negative electrode sheet in the lithium ion battery is lighter, the capacity retention rate at 500 cls is higher, and the pass rate of the drop test is higher, indicating that the risk of deterioration of the negative electrode sheet due to insufficient electrolyte during the cycle is lower, the lithium ion battery has good cycle performance while taking into account the drop performance and safety performance.

[0167] It should be noted that, in this document, relational terms such as first and second and the like can be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Moreover, the terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but can include other elements not expressly listed or inherent to such process, method, article, or apparatus.

[0168] Each of the embodiments in the specification is described in a relevant manner, and the same or similar parts between the embodiments can be referred to each other. Each embodiment focuses on the difference from other embodiments.

[0169] 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 an electrode assembly, the electrode assembly comprising a tab and a first separator, the tab comprising a positive tab and a negative tab, the negative tab comprising a negative material layer, the negative material layer comprising a silicon element, a mass percentage content of the silicon element being W based on a mass of the negative material layer, W ≥ 3%; the first separator comprising a base film, a first coating layer and a second coating layer, the base film comprising a first surface and a second surface along a thickness direction of the first separator, the first surface facing the negative tab, the second surface facing the positive tab, the first coating layer being disposed on the first surface, the second coating layer being disposed on the second surface; a porosity of the first coating layer being P1, a porosity of the second coating layer being P2, P1 > P2.

2. The secondary battery according to claim 1, wherein 3％≤W≤80％。 3. The secondary battery according to claim 1, wherein the first coating layer comprising a first ceramic layer, the second coating layer comprising a second ceramic layer.

4. The secondary battery according to claim 3, wherein 1 < P1 / P2 ≤ 5, 30%≤ P1 ≤ 70%.

5. The secondary battery according to claim 4, wherein 0.6P1 / P2 ≤ 0.04W × 100 + 0.98 ≤ 1.5P1 / P2.

6. The secondary battery according to claim 4, wherein 3%≤ W ≤ 10%, 1 < P1 / P2 ≤ 1.5, 30%≤ P1 ≤ 40%.

7. The secondary battery according to claim 4, wherein 10% < W ≤ 30%, 1.5 < P1 / P2 ≤ 2.5, 40% < P1 ≤ 55%.

8. The secondary battery according to claim 4, wherein 30% < W ≤ 80%, 2.5 < P1 / P2 ≤ 5, 55% < P1 ≤ 70%.

9. The secondary battery according to claim 3, wherein the first ceramic layer comprising first ceramic particles, the second ceramic layer comprising second ceramic particles, the first ceramic particles, the second ceramic particles each independently being selected from at least one of alumina, magnesia, aluminum hydroxide, magnesium hydroxide or boehmite; an average particle size of the first ceramic particles being D1 μm, 0.5 ≤ D1 ≤ 3; an average particle size of the second ceramic particles being D2 μm, 0.25 ≤ D2 ≤ 0.

45.

10. The secondary battery according to claim 3, wherein the first coating layer further comprising a first adhesive layer, the first adhesive layer and the first ceramic layer being sequentially stacked on the first surface, the first ceramic layer being located between the first adhesive layer and the base film; the second coating layer further comprising a second adhesive layer, the second adhesive layer and the second ceramic layer being sequentially stacked on the second surface, the second ceramic layer being located between the second adhesive layer and the base film; the first adhesive layer comprising a first adhesive, the second adhesive layer comprising a second adhesive, the first adhesive, the second adhesive each independently being selected from at least one of styrene butadiene latex, styrene-acrylate latex, polymethyl methacrylate, polybutyl methacrylate, polyethyl acrylate, polyvinyl alcohol, ethylene-vinyl acetate copolymer, polyvinyl acetate, polyurethane, polyvinylidene fluoride or vinylidene fluoride-hexafluoropropylene copolymer.

11. The secondary battery according to claim 10, wherein a thickness of the first adhesive layer being 0.5 μm to 3 μm.

12. The secondary battery according to claim 10, wherein the first adhesive comprising at least one of styrene butadiene latex, styrene-acrylate latex, polymethyl methacrylate, polybutyl methacrylate, polyethyl acrylate, polyvinyl alcohol, ethylene-vinyl acetate copolymer, polyvinyl acetate or polyurethane, the second adhesive comprising at least one of polyvinylidene fluoride or vinylidene fluoride-hexafluoropropylene copolymer.

13. The secondary battery according to claim 1, wherein The electrode assembly is a laminated structure.

14. The secondary battery according to claim 13, wherein The electrode assembly further comprises a second separator, the electrode plate comprises two outer electrode plates and a plurality of inner electrode plates, the two outer electrode plates are respectively located at the two outermost sides of the electrode assembly, the first separator is arranged at least between the outer electrode plate and the inner electrode plate adjacent to the outer electrode plate, and the second separator is arranged between two adjacent inner electrode plates.

15. The secondary battery according to claim 14, wherein The number of layers of the first separator is m, the total number of layers of the first separator and the second separator is n, and 1 / 5≤m / n≤1.

16. The secondary battery according to claim 13, wherein The first separator is folded in a Z-shaped structure in the electrode assembly, and the Z-shaped first separator separates the adjacent positive electrode plate and the negative electrode plate. 17.An electronic device comprising the secondary battery of any one of claims 1 to 16.

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