A secondary battery and an electronic device

Abstract

This application provides a secondary battery and an electronic device. The secondary battery includes an electrode assembly comprising electrode sheets and a first separator. The electrode sheets include a positive electrode sheet and a negative electrode sheet. The negative electrode sheet includes a negative electrode material layer comprising silicon. Based on the mass of the negative electrode material layer, the mass percentage of silicon is W, where W ≥ 3%. The first separator includes a base film, a first coating, and a second coating. Along the thickness direction of the first separator, the base film includes a first surface and a second surface. The first surface faces the negative electrode sheet, and the second surface faces the positive electrode sheet. The first coating is disposed on the first surface, and the second coating is disposed on the second surface. The porosity of the first coating is P1, and the porosity of the second coating is P2, where P1 > P2. This configuration reduces the risk of negative electrode interface deterioration due to insufficient electrolyte and improves the cycle performance of the secondary battery.

Description

Technical Field

[0001] This application relates to the field of electrochemical technology, and in particular to a secondary battery and electronic device. Background Technology

[0002] Secondary batteries, such as lithium-ion batteries, have the characteristics of high specific energy, high operating voltage, low self-discharge rate, small size, and light weight, and are widely used in the consumer electronics field.

[0003] With the increasing demand for higher energy density in rechargeable batteries, silicon, as a material with a high specific capacity of 4200 mAh / g, relatively low cost, and environmental friendliness, is being used more and more in rechargeable battery systems. Currently, due to the ultra-high specific capacity of silicon materials, silicon-based anode material layers, especially when the silicon content in the anode material layer is high, have a lower coating weight during coating, only about one-quarter that of traditional graphite materials, and a thickness less than half that of traditional graphite electrodes. In this case, the electrolyte storage capacity of the anode electrode is relatively poor. Furthermore, because silicon-based materials have a higher electrolyte requirement during rechargeable battery cycling than conventional graphite, higher demands are placed on the electrolyte storage capacity of silicon-containing anode electrodes. Summary of the Invention

[0004] The purpose of this application is to provide a secondary battery and electronic device to improve the liquid storage capacity of the separator on the negative electrode side, improve lithium plating at the negative electrode interface, and improve the cycle performance of the secondary battery.

[0005] It should be noted that the invention description in this application uses lithium-ion batteries as an example of secondary batteries to explain this application, but the secondary batteries in this application are not limited to lithium-ion batteries.

[0006] Existing technologies address the electrolyte storage problem of silicon-containing negative electrode sheets primarily by retaining more electrolyte inside the secondary battery casing to increase the electrolyte storage capacity. However, excess electrolyte stored inside the casing exists in a free state and cannot be promptly supplied to the silicon-based material of the negative electrode sheet. This excess free electrolyte increases the slippage between layers within the electrode assembly, affecting the drop performance of the secondary battery. Furthermore, excessive excess electrolyte can cause the secondary battery to swell, potentially leading to bulging, deformation, or cracking of the casing. Therefore, this application provides a secondary battery that can promptly supply electrolyte to the silicon-containing negative electrode sheet when needed. This reduces the risk of insufficient electrolyte causing ion conduction bridging at the negative electrode interface during later stages of cycling, and the inability to replenish the electrolyte in time after the SEI film is damaged due to silicon particle expansion during cycling, preventing the SEI film from regenerating and thus leading to lithium plating or even cycle failure. The specific technical solution is as follows:

[0007] The first aspect of this application provides a secondary battery, which includes an electrode assembly comprising electrode sheets and a first separator. The electrode sheets include a positive electrode sheet and a negative electrode sheet. The negative electrode sheet includes a negative electrode material layer comprising silicon. Based on the mass of the negative electrode material layer, the mass percentage of silicon is W, where W ≥ 3%. The first separator includes a base film, a first coating, and a second coating. Along the thickness direction of the first separator, the base film includes a first surface and a second surface. The first surface faces the negative electrode sheet, and the second surface faces the positive electrode sheet. The first coating is disposed on the first surface, and the second coating is disposed on the second surface. The porosity of the first coating is P1, and the porosity of the second coating is P2, where P1 > P2. By applying coatings with different porosities to the positive and negative electrodes on the separator side of a silicon-containing secondary battery, the coating on the negative electrode side has a larger porosity. This larger porosity coating has more interparticle gaps, allowing the separator side to absorb more electrolyte, thus improving its electrolyte storage capacity. This ensures timely supply of electrolyte when the silicon-containing negative electrode requires it, reducing the risk of ion conduction bridging at the negative electrode interface due to insufficient electrolyte in the later stages of cycling, and the inability to replenish electrolyte in time after the SEI film is damaged by the cyclic expansion of silicon particles, preventing the SEI film from regenerating and leading to lithium plating or even cycle failure. This improves the cycle performance of the secondary battery.

[0008] In some embodiments of this application, 3% ≤ W ≤ 80%. By adjusting the value of W within the above range, the risk of negative electrode interface deterioration due to insufficient electrolyte is reduced more significantly, further improving the cycle performance of the secondary battery.

[0009] In some embodiments of this application, the first coating includes a first ceramic layer, and the second coating includes a second ceramic layer. This configuration reduces the risk of negative electrode interface deterioration due to insufficient electrolyte, thereby improving the cycle performance of the secondary battery. In some embodiments of this application, 1 < P1 / P2 ≤ 5, and 30% ≤ P1 ≤ 70%. By adjusting the values ​​of P1 / P2 and P1 within the above ranges, the risk of negative electrode interface deterioration due to insufficient electrolyte is reduced while maintaining the mechanical strength of the first separator, thus improving the cycle performance of the secondary battery.

[0010] In some embodiments of this application, 0.6P1 / P2 < 0.04W×100 + 0.98 ≤ 1.5P1 / P2. By adjusting the value of 0.04W×100 + 0.98 within the above range, the risk of negative electrode interface deterioration due to insufficient electrolyte is reduced while maintaining the energy density of the secondary battery, thereby improving the cycle performance of the secondary battery.

[0011] In some embodiments of this application, 3% ≤ W ≤ 10%, 1 < P1 / P2 ≤ 1.5, and 30% ≤ P1 ≤ 40%. When 3% ≤ W ≤ 10%, by adjusting the values ​​of P1 / P2 and P1 within the above ranges, the silicon content and the porosity between different coatings work synergistically. This reduces the risk of negative electrode interface deterioration due to insufficient electrolyte while maintaining the energy density of the secondary battery, thereby improving the cycle performance of the secondary battery.

[0012] In some embodiments of this application, 10% < W ≤ 30%, 1.5 < P1 / P2 ≤ 2.5, and 40% < P1 ≤ 55%. When 10% < W ≤ 30%, by adjusting the values ​​of P1 / P2 and P1 within the above ranges, the risk of negative electrode interface deterioration due to insufficient electrolyte is reduced while taking into account the energy density of the secondary battery, thereby improving the cycle performance of the secondary battery.

[0013] In some embodiments of this application, 30% < W ≤ 80%, 2.5 < P1 / P2 ≤ 5, and 55% < P1 ≤ 70%. When 30% < W ≤ 80%, by adjusting the values ​​of P1 / P2 and P1 within the above ranges, the risk of negative electrode interface deterioration due to insufficient electrolyte is reduced while taking into account the energy density of the secondary battery, thus improving the cycle performance of the secondary battery.

[0014] In some embodiments of this application, the first ceramic layer comprises first ceramic particles, and the second ceramic layer comprises second ceramic particles. The first and second ceramic particles are each independently selected from at least one of alumina, magnesium oxide, aluminum hydroxide, magnesium hydroxide, or boehmite. The average particle size of the first ceramic particles is D1 μm, 0.5 ≤ D1 ≤ 3; the average particle size of the second ceramic particles is D2 μm, 0.25 ≤ D2 ≤ 0.45. By selecting the aforementioned types of first and second ceramic particles and controlling their average particle sizes within the above ranges, it is beneficial to adjust the porosity of the first and second coatings, reduce the risk of negative electrode interface deterioration due to insufficient electrolyte, and improve the cycle performance of the secondary battery.

[0015] In some embodiments of this application, the first coating further includes a first adhesive layer, and the first adhesive layer and the first ceramic layer are sequentially stacked on the first surface, with the first ceramic layer located between the first adhesive layer and the base film. The second coating further includes a second adhesive layer, and the second adhesive layer and the second ceramic layer are sequentially stacked on the second surface, with the second ceramic layer located between the second adhesive layer and the base film. The first adhesive layer includes a first adhesive, and the second adhesive layer includes a second adhesive. The first adhesive and the second adhesive are each independently selected from at least one of styrene-butadiene latex, styrene-acrylic latex, polymethyl methacrylate, polybutyl methacrylate, ethyl acrylate, polyvinyl alcohol, ethylene-vinyl acetate copolymer, polyvinyl acetate, polyurethane, polyvinylidene fluoride, or a copolymer of vinylidene fluoride and hexafluoropropylene. By providing the first adhesive layer and the second adhesive layer, and selecting the above-mentioned types of first adhesives and second adhesives, better electrode interface adhesion is achieved while maintaining cycle performance, thereby improving the safety performance of the secondary battery.

[0016] In some embodiments of this application, the thickness of the first adhesive layer is from 0.5 μm to 3 μm. By adjusting the thickness T1 of the first adhesive layer within the above range, the safety performance and cycle performance of the secondary battery are further improved.

[0017] In some embodiments of this application, the first binder includes at least one selected from styrene-butadiene latex, styrene-acrylic latex, polymethyl methacrylate, polybutyl methacrylate, polyethyl methacrylate, polyvinyl alcohol, ethylene-vinyl acetate copolymer, polyvinyl acetate, or polyurethane; and the second binder includes at least one selected from polyvinylidene fluoride or a copolymer of polyvinylidene fluoride and hexafluoropropylene. By selecting the above-mentioned first and second binders, the adhesion of the second adhesive layer is stronger than that of the first adhesive layer, and the ion conduction performance of the first adhesive layer is better than that of the second adhesive layer, resulting in better electrode interface adhesion and further optimizing interfacial ion conduction. While ensuring the safety performance of the secondary battery, the risk of negative electrode interface deterioration due to insufficient electrolyte is reduced, thus improving the cycle performance of the secondary battery.

[0018] In some embodiments of this application, the electrode assembly is a stacked structure. In some embodiments of this application, the electrode assembly further includes a second separator. The electrode includes two outer electrodes and multiple inner electrodes. The two outer electrodes are located on the outermost sides of the electrode assembly. The first separator is disposed at least between the outer electrodes and the adjacent inner electrodes, and the second separator is disposed between two adjacent inner electrodes. In some embodiments of this application, the number of layers in the first separator is m, and the total number of layers in the first and second separators is n, where 1 / 5 ≤ m / n ≤ 1. Through the above configuration, the risk of negative electrode interface deterioration due to insufficient electrolyte is reduced, improving the cycle performance of the secondary battery. Simultaneously, the operational difficulty and processing cost in actual production are considered, further increasing the energy density of the secondary battery.

[0019] In some embodiments of this application, the first separator has a Z-shaped folded structure in the electrode assembly, and the Z-shaped first separator separates adjacent positive and negative electrode plates. This arrangement further improves the stability of the electrode assembly stack structure, thereby enhancing both the cycle performance and safety performance of the secondary battery.

[0020] A second aspect of this application provides an electronic device comprising a secondary battery as described in any of the foregoing embodiments. The secondary battery of this application exhibits excellent cycle performance; therefore, the electronic device of this application has a long service life.

[0021] The beneficial effects of the embodiments of this application are as follows:

[0022] This application provides a secondary battery and electronic device. By using coatings with different porosities on both the positive and negative electrode sides of the separator in the secondary battery, the separator side has a coating with greater porosity on the negative electrode side. This is beneficial for the separator side to absorb more electrolyte on the negative electrode side, improving the electrolyte storage capacity of the separator side on the negative electrode side. When the negative electrode sheet containing silicon elements needs electrolyte, it can supply it in time. This reduces the risk of ion conduction bridging at the negative electrode interface due to insufficient electrolyte in the later stages of the cycle, as well as the risk of electrolyte not being replenished in time after the SEI film is damaged due to the cyclic expansion of silicon particles, which prevents the SEI film from regenerating and thus leads to lithium plating or even cycle failure. This improves the cycle performance of the secondary battery.

[0023] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other embodiments can be obtained based on these drawings.

[0025] Figure 1 This is a schematic diagram of a partial cross-sectional structure of the electrode assembly along its thickness direction after being unfolded in one embodiment of this application.

[0026] Figure 2 This is a schematic diagram of a partial cross-sectional structure of the electrode assembly along its thickness direction after being unfolded in another embodiment of this application.

[0027] Figure 3 This is a schematic cross-sectional view of the secondary battery along its own length in another embodiment of this application.

[0028] Figure 4 This is a schematic cross-sectional view of the secondary battery along its length in another embodiment of this application.

[0029] Reference numerals: Secondary battery 001; Electrode assembly 01; Housing 02; Electrode 10; First separator 20; Base film 201; First coating 202; Second coating 203; First ceramic layer 2021; First adhesive layer 2022; Second ceramic layer 2031; Second adhesive layer 2032; Positive electrode 11; Positive current collector 111; Positive material layer 112; Negative electrode 12; Negative current collector 121; Negative material layer 122; Second separator 21; Outer electrode 101; Inner electrode 102. Detailed Implementation

[0030] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0031] It should be noted that, in the specific embodiments of this application, a lithium-ion battery is used as an example of a secondary battery to explain this application; however, the secondary battery in this application is not limited to lithium-ion batteries. The specific technical solution is as follows:

[0032] The first aspect of this application provides a secondary battery, which includes an electrode assembly comprising electrode sheets and a first separator. The electrode sheets include a positive electrode sheet and a negative electrode sheet. The negative electrode sheet includes a negative electrode material layer comprising silicon. Based on the mass of the negative electrode material layer, the mass percentage of silicon is W, where W ≥ 3%. The first separator includes a base film, a first coating, and a second coating. Along the thickness direction of the first separator, the base film includes a first surface and a second surface. The first surface faces the negative electrode sheet, and the second surface faces the positive electrode sheet. The first coating is disposed on the first surface, and the second coating is disposed on the second surface. The porosity of the first coating is P1, and the porosity of the second coating is P2, where P1 > P2.

[0033] In this application, the length direction of the electrode assembly in its unfolded state is defined as the X direction, the width direction as the Y direction, and the thickness direction as the Z direction. It is understood that the negative electrode, positive electrode, and separator in their unfolded state have the same thickness direction as the electrode assembly. For example, as shown... Figure 1As shown, the electrode assembly 01 includes an electrode 10 and a first separator 20. The electrode 10 includes a positive electrode 11 and a negative electrode 12, and the negative electrode 12 includes a negative electrode material layer 122. The first separator 20 includes a base film 201, a first coating 202, and a second coating 203. Along the thickness direction Z of the first separator 20, the base film 201 includes a first surface (not shown) and a second surface (not shown). The first surface faces the negative electrode 12, and the second surface faces the positive electrode 11. The first coating 202 is disposed on the first surface, and the second coating 203 is disposed on the second surface.

[0034] The inventors discovered that in existing technologies, the coating of the separator is generally symmetrically designed. In this case, 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. In this case, the electrolyte storage capacity of the separator facing the negative electrode is poor. When the negative electrode contains silicon, the electrolyte requirement during the cycle of the secondary battery is higher than that of conventional graphite. In the later stages of the cycle, insufficient electrolyte can easily cause the ion conduction of the negative electrode interface to break down. In addition, the expansion of silicon particles during the cycle can cause the SEI film to be damaged, and the electrolyte cannot be replenished in time. The SEI film cannot be regenerated, which leads to the risk of lithium plating or even cycle failure, affecting the cycle life of the secondary battery and reducing its cycle performance. Furthermore, when both surfaces of the separator in the electrode assembly are coated with a high-porosity coating, the separator may store too much electrolyte. The excess electrolyte inside the casing will be stored in a free state and cannot be supplied to the silicon-based material of the negative electrode in time. At this time, the excess free electrolyte will increase the sliding between the layers inside the electrode assembly, affecting the drop performance and safety performance of the secondary battery. At the same time, when there is too much excess electrolyte, the secondary battery will swell, and the casing may bulge, deform or crack. This application employs coatings with different porosities on the positive and negative electrode sides of the separator in a silicon-containing secondary battery. The coating on the negative electrode side has a higher porosity, resulting in more interparticle gaps. This allows the separator to absorb more electrolyte on the negative electrode side, improving its electrolyte storage capacity. When the silicon-containing negative electrode requires electrolyte, it can supply it promptly, reducing the risk of ion conduction bridging at the negative electrode interface due to insufficient electrolyte in the later stages of cycling, and the inability to replenish electrolyte in time after the SEI film is damaged by the cyclic expansion of silicon particles, preventing the SEI film from regenerating and thus leading to lithium plating or even cycle failure. This approach improves the cycle performance of the secondary battery while balancing drop performance and safety.

[0035] In some embodiments of this 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 of any two of these values. By adjusting the value of W within the above range, the energy density of the secondary battery is improved while maintaining the structural stability of the negative electrode during charging and discharging. Simultaneously, the first separator in the secondary battery of this application can absorb more electrolyte on the negative electrode side, improving the separator's electrolyte storage capacity on the negative electrode side, and ensuring timely supply of electrolyte when the silicon-containing negative electrode requires it. As the mass percentage of silicon increases, the demand for electrolyte in the negative electrode increases accordingly. Therefore, the effect of reducing the risk of lithium plating or even cycle failure caused by insufficient electrolyte leading to ion conduction bridging at the negative electrode interface in the later stages of cycling, as well as the inability to replenish electrolyte in time after the SEI film is damaged due to the expansion of silicon particles during cycling, and the inability of the SEI film to regenerate, is more obvious, further improving the cycle performance of secondary batteries.

[0036] In some embodiments of this application, the first coating includes a first ceramic layer, and the second coating includes a second ceramic layer. For example, as shown... Figure 1 As shown, the first coating 202 includes a first ceramic layer 2021, and the second coating 203 includes a second ceramic layer 2031. Through this configuration, in a silicon-containing secondary battery, coatings with different porosities are applied to the separator side of the secondary battery on both the positive and negative electrode sides. This results in a larger porosity on the separator side of the negative electrode. The larger porosity of the coating provides more interparticle gaps, allowing the separator side to absorb more electrolyte, thus improving its electrolyte storage capacity. This ensures timely supply of electrolyte when the silicon-containing negative electrode requires it, reducing the risk of electrolyte breakdown at the negative electrode interface due to insufficient electrolyte in the later stages of cycling, and the risk of electrolyte not being replenished in time after the SEI film is damaged due to silicon particle expansion during cycling, preventing the SEI film from regenerating and leading to lithium plating or even cycle failure. This improves the cycle performance of the secondary battery while balancing drop performance and safety performance.

[0037] In some embodiments of this application, 1 < P1 / P2 ≤ 5, and 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 of any two of these values; 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 of any two of these values. By adjusting the values ​​of P1 / P2 and P1 within the aforementioned range, while maintaining the mechanical strength of the first separator, the porosity of the first coating on the negative electrode side of the first separator is increased. The first separator can absorb more electrolyte on the negative electrode side, improving its electrolyte storage capacity. When the negative electrode containing silicon requires electrolyte, it can supply it in a timely manner, reducing the risk of ion conduction bridging at the negative electrode interface due to insufficient electrolyte in the later stages of cycling, and the risk of electrolyte not being replenished in time after the SEI film is damaged due to the cyclic expansion of silicon particles, which prevents the SEI film from regenerating and thus leads to lithium plating or even cycle failure. This improves the cycle performance of the secondary battery.

[0038] In one embodiment of this 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 of any two of these values. By adjusting the value of P2 within the above range, the electrolyte storage capacity of the first separator on the negative electrode side is improved while the electrolyte storage capacity of the first separator on the positive electrode side is also taken into account. When the positive and negative electrode plates require electrolyte, it can be supplied in time, reducing the risk of ion conduction bridge breakage at the electrode interface in the later stage of the cycle due to insufficient electrolyte, and the risk of electrolyte not being replenished in time after the SEI film is damaged due to the cyclic expansion of silicon particles, which prevents the SEI film from regenerating and thus leads to lithium plating or even cycle failure. This improves the cycle performance of the secondary battery.

[0039] In some embodiments of this 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 of any two of these values. When the silicon content in the negative electrode material layer is high, the demand for electrolyte during cycling is correspondingly higher. By adjusting the value of 0.04W×100+0.98 within the aforementioned range, negative electrode sheets with different silicon contents are paired with two coatings with different porosity ratios. This allows for a synergistic effect between the silicon content and the porosity of the different coatings. While maintaining the mechanical strength of the first separator, the first separator can absorb more electrolyte on the negative electrode side, improving its electrolyte storage capacity. This ensures timely supply of electrolyte when the silicon-containing negative electrode sheet requires it. While maintaining the energy density of the secondary battery, this reduces the risk of ion conduction bridging at the negative electrode interface due to insufficient electrolyte in the later stages of cycling, as well as the risk of insufficient electrolyte replenishment after the SEI film is damaged due to silicon particle cyclic expansion, preventing the SEI film from regenerating and leading to lithium plating or even cycle failure. This improves the cycle performance of the secondary battery.

[0040] In some embodiments of this application, 3% ≤ W ≤ 10%, 1 < P1 / P2 ≤ 1.5, and 30% ≤ P1 ≤ 40%. For example, when 3% ≤ W ≤ 10%, the value of P1 / P2 can be 1.1, 1.15, 1.2, 1.25, 1.3, 1.35, 1.4, 1.45, 1.5, or a range of any two of these values; the value of P1 can be 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, or a range of any two of these values. When 3% ≤ W ≤ 10%, by adjusting the values ​​of P1 / P2 and P1 within the above range, the silicon content in the negative electrode material layer is relatively low, resulting in relatively good electrolyte retention capacity of the negative electrode sheet. Combined with smaller P1 / P2 and P1 values, the silicon content and the porosity between different coatings work synergistically. While maintaining the mechanical strength of the first separator, the first separator can absorb more electrolyte on the negative electrode side, improving its electrolyte storage capacity. This allows for timely supply of electrolyte when the silicon-containing negative electrode sheet requires it. While maintaining the energy density of the secondary battery, this reduces the risk of ion conduction bridging at the negative electrode interface due to insufficient electrolyte in the later stages of cycling, and the risk of electrolyte not being replenished in time after the SEI film is damaged due to silicon particle cyclic expansion, preventing the SEI film from regenerating and leading to lithium plating or even cycle failure. This improves the cycle performance of the secondary battery.

[0041] In some embodiments of this application, 10% < W ≤ 30%, 1.5 < P1 / P2 ≤ 2.5, and 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 of any two of these values; the value of P1 can be 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, or a range of any two of these values. When 10% < W ≤ 30%, by adjusting the values ​​of P1 / P2 and P1 within the above range, the silicon content in the negative electrode material layer is relatively high. This results in a correspondingly higher demand for electrolyte during cycling, increasing the porosity ratio of different coatings and consequently increasing the porosity of the first coating. This synergistic effect between the silicon content and the porosity of different coatings allows the first separator to absorb more electrolyte on the negative electrode side, further improving its electrolyte storage capacity. This ensures timely electrolyte supply when the silicon-containing negative electrode requires electrolyte. While maintaining the energy density of the secondary battery, this reduces the risk of ion conduction bridging at the negative electrode interface due to insufficient electrolyte in the later stages of cycling, as well as the risk of insufficient electrolyte replenishment after the SEI film is damaged due to silicon particle expansion during cycling, preventing the SEI film from regenerating and leading to lithium plating or even cycle failure. This improves the cycle performance of the secondary battery.

[0042] In some embodiments of this application, 30% < W ≤ 80%, 2.5 < P1 / P2 ≤ 5, and 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 consisting of any two of these values; the value of P1 can be 56%, 57%, 58%, 59%, 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, 70%, or a range consisting of any two of these values. When 30% < W ≤ 80%, by adjusting the values ​​of P1 / P2 and P1 within the above range, the silicon content in the negative electrode material layer is high. Combined with a larger ratio of porosity between different coatings and a larger porosity of the first coating, the silicon content and the porosity between different coatings work synergistically. This better meets the electrolyte requirements during cycling. The first separator can absorb more electrolyte on the negative electrode side, further improving the separator's electrolyte storage capacity on the negative electrode side. This ensures timely supply of electrolyte when the silicon-containing negative electrode sheet requires it. While maintaining the energy density of the secondary battery, this reduces the risk of ion conduction bridging at the negative electrode interface due to insufficient electrolyte in the later stages of cycling, and the risk of insufficient electrolyte replenishment after the SEI film is damaged due to silicon particle cyclic expansion, preventing the SEI film from regenerating and leading to lithium plating or even cycle failure. This improves the cycle performance of the secondary battery.

[0043] In some embodiments of this application, the first ceramic layer comprises first ceramic particles, and the second ceramic layer comprises second ceramic particles. The first and second ceramic particles are each independently selected from at least one of alumina, magnesium oxide, 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 consisting of any two of these values. 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 consisting of any two of these values. By selecting the aforementioned types of first and second ceramic particles and controlling their average particle size within the aforementioned range, it is beneficial to adjust the porosity of the first and second coatings, making the porosity of the first coating greater than that of the second coating. The porosity of the first coating on the negative electrode side of the first separator is larger, allowing the first separator to absorb more electrolyte on the negative electrode side, thus improving its electrolyte storage capacity. When the negative electrode sheet containing silicon requires electrolyte, it can be supplied in a timely manner, reducing the risk of ion conduction bridging at the negative electrode interface due to insufficient electrolyte in the later stages of cycling, and the risk of electrolyte not being replenished in time after the SEI film is damaged due to the cyclic expansion of silicon particles, preventing the SEI film from regenerating and leading to lithium plating or even cycle failure. This improves the cycle performance of the secondary battery.

[0044] This application does not impose any particular restrictions on the method of controlling the average particle size of ceramic particles, as long as the purpose of this application can be achieved. For example, the average particle size of ceramic particles can be controlled by classifying or grinding the particles. For example, when other conditions remain unchanged, extending the grinding time reduces the average particle size of the ceramic particles, while shortening the grinding time increases the average particle size of the ceramic particles.

[0045] In some embodiments of this application, the first coating further includes a first adhesive layer, the first adhesive layer and the first coating being sequentially stacked on a first surface, with a first ceramic layer located between the first adhesive layer and the base film. The second coating further includes a second adhesive layer, the second adhesive layer and the second ceramic layer being sequentially stacked on a second surface, with the second ceramic layer located between the second adhesive layer and the base film. For example, as... Figure 2As shown, the first coating 202 further includes a first adhesive layer 2022, and the first adhesive layer 2022 and the first ceramic layer 2021 are sequentially stacked on the surface of the base film 201 facing the negative electrode 12, with the first ceramic layer 2021 located between the first adhesive layer 2022 and the base film 201; the second coating 203 further includes a second adhesive layer 2032, and the second adhesive layer 2032 and the second ceramic layer 2031 are sequentially stacked on the surface of the base film 201 facing the positive electrode 11, with the second ceramic layer 2031 located between the second adhesive layer 2032 and the base film 201. The first adhesive layer includes a first adhesive, and the second adhesive layer includes a second adhesive. The first adhesive and the second adhesive are each independently selected from at least one of styrene-butadiene latex, styrene-acrylic latex, polymethyl methacrylate, polybutyl methacrylate, ethyl acrylate, polyvinyl alcohol, ethylene-vinyl acetate copolymer, polyvinyl acetate, polyurethane, polyvinylidene fluoride, or a copolymer of vinylidene fluoride and hexafluoropropylene. By setting a first adhesive layer and a second adhesive layer, and selecting the aforementioned types of first and second adhesives, the adhesion between the first separator and the positive electrode sheet, and between the first separator and the negative electrode sheet, is improved. This strengthens the interface between the first separator and the positive electrode sheet, and between the first separator and the negative electrode sheet, which is beneficial for lithium-ion transport. Simultaneously, it reduces the possibility of the first and second ceramic layers detaching. Applying the first separator to a secondary battery, while maintaining cycle performance, provides better electrode interface adhesion, thus improving the safety performance of the secondary battery.

[0046] In some embodiments of this application, the thickness T1 of the first adhesive layer is from 0.5 μm to 3 μm. For example, the thickness of the first adhesive 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 of any two of these values. By adjusting the thickness T1 of the first adhesive layer within the above range, while the first coating has high porosity, there is also high adhesion between the first separator and the negative electrode, strengthening the interface between the first separator and the negative electrode, which is beneficial for lithium ion transport during cycling, and at the same time reduces the possibility of the first ceramic layer detaching from the base film, thereby further improving the safety and cycle performance of the secondary battery.

[0047] In one embodiment of this application, the thickness T2 of the second adhesive layer is from 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 of any two of these values. By adjusting the thickness T2 of the second adhesive layer within the above range, while ensuring that the porosity of the first coating is greater than that of the second coating, the first separator and the positive electrode have a higher adhesion, strengthening the interface between the first separator and the positive electrode, which is beneficial for lithium ion transport during cycling, and at the same time reduces the possibility of the second ceramic layer detaching from the base film, thereby further improving the safety and cycle performance of the secondary battery.

[0048] In this application, the thickness of the first adhesive layer and the thickness of the second adhesive layer can be controlled by means known to those skilled in the art. For example, when the first adhesive layer slurry is applied to the surface of the first ceramic layer away from the base film, the coating amount of the first adhesive layer slurry can be increased to increase the thickness of the first adhesive layer, while maintaining a certain solid content. Similarly, when the second adhesive layer slurry is applied to the surface of the second ceramic layer away from the base film, the coating amount of the second adhesive layer slurry can be increased to increase the thickness of the first adhesive layer, while maintaining a certain solid content. This application does not impose any particular limitations, as long as the purpose of this application can be achieved.

[0049] In some embodiments of this application, the first binder includes at least one selected from styrene-butadiene latex, styrene-acrylic latex, polymethyl methacrylate, polybutyl methacrylate, polyethyl acrylate, polyvinyl alcohol, ethylene-vinyl acetate copolymer, polyvinyl acetate, or polyurethane; the second binder includes at least one selected from polyvinylidene fluoride or a copolymer of vinylidene fluoride and hexafluoropropylene. By selecting the above-mentioned first and second binders, the adhesion of the second adhesive layer is stronger than that of the first adhesive layer, and the ion conduction performance of the first adhesive layer is better than that of the second adhesive layer, resulting in better interfacial adhesion between the electrodes and further optimizing the ion conduction between the interfaces. While strengthening the interfaces between the first separator and the positive electrode and the first separator and the negative electrode, the first separator can absorb more electrolyte on the negative electrode side, improving the electrolyte storage capacity of the separator on the negative electrode side, and ensuring timely supply of electrolyte when the negative electrode containing silicon requires it. While ensuring the safety performance of secondary batteries, it reduces the risk of ion conduction bridging at the negative electrode interface caused by insufficient electrolyte in the later stages of cycling, and the risk of electrolyte not being replenished in time after the SEI film is damaged due to the expansion of silicon particles during cycling, which prevents the SEI film from regenerating and thus leads to lithium plating or even cycle failure, thereby improving the cycle performance of secondary batteries.

[0050] In some embodiments of this application, the electrode assembly is a stacked structure. When the electrode assembly has the above-mentioned structure, since the stacked structure has no interface corner region and the positive and negative electrode sheets are evenly distributed, it helps to reduce the risk of excessive corner stress caused by the volume expansion of the silicon-containing negative electrode sheet in the wound electrode assembly, which could lead to deformation of the overall structure of the electrode assembly or even internal tearing of the electrode sheet. The first membrane face has a larger porosity on the negative electrode side, and the first membrane face can absorb more electrolyte on the negative electrode side, improving the electrolyte storage capacity of the membrane face on the negative electrode side. When the silicon-containing negative electrode sheet needs electrolyte, it can supply it in time, reducing the risk of ion conduction bridging at the negative electrode interface due to insufficient electrolyte in the later stages of the cycle, and the risk of electrolyte not being replenished in time after the SEI film is damaged due to the cyclic expansion of silicon particles, which would prevent the SEI film from regenerating and thus lead to lithium plating or even cycle failure. This improves the cycle performance of the secondary battery.

[0051] In some embodiments of this application, the electrode assembly further includes a second diaphragm. The electrode includes two outer electrode plates and a plurality of inner electrode plates. The two outer electrode plates are respectively located on the outermost sides of the electrode assembly. The first diaphragm is disposed at least between the outer electrode plates and the inner electrode plates adjacent to the outer electrode plates. The second diaphragm is disposed between two adjacent inner electrode plates. For example, as shown... Figure 3 As shown, the electrode assembly 01 also includes a second diaphragm 21. The electrode 10 includes two outer electrode plates 101 and an inner electrode plate 102. The two outer electrode plates 101 are located on the outermost two sides of the electrode assembly 01, respectively. The first diaphragm 20 is disposed between the outer electrode plate 101 and the inner electrode plate 102 adjacent to the outer electrode plate 101, and the second diaphragm 21 is disposed between two adjacent inner electrode plates 102. In conventional stacked electrode assemblies, the outer electrode plates have low resistance and high current density, and their ability to consume and transfer electrolyte is greater than that of the inner electrode plates. By implementing the above settings, the electrolyte storage capacity of the separator on the negative electrode side is improved, ensuring timely supply of electrolyte when the silicon-containing negative electrode sheet requires it. At the same time, the electrolyte storage capacity on the outside of the electrode assembly is also improved. This reduces the risk of lithium plating on the outer electrode sheet due to insufficient electrolyte on the outside of the electrode assembly during cycling, as well as the risk of ion conduction bridging at the negative electrode interface caused by insufficient electrolyte in the later stages of cycling, and the risk of electrolyte not being replenished in time after the SEI film is damaged due to the cyclic expansion of silicon particles, which prevents the SEI film from regenerating and thus leads to lithium plating or even cycle failure. This improves the cycle performance of the secondary battery.

[0052] In some embodiments of this application, the number of layers in the first separator is m, and the total number of layers in the first and second separators is n, where 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 of any two of these values. By adjusting the value of m / n within the above range, the risk of ion conduction bridging at the negative electrode interface due to insufficient electrolyte in the later stages of cycling, and the inability to replenish the electrolyte in time after the SEI film is damaged due to the cyclic expansion of silicon particles, leading to the inability to regenerate the SEI film and thus resulting in lithium plating or even cycle failure, is reduced. This improves the cycle performance of the secondary battery while taking into account the operational difficulty and processing cost in the actual production process, and further improves the energy density of the secondary battery.

[0053] This application does not impose any particular restrictions on the material of the base film, as long as it achieves the purpose of this application. For example, the material of the base film may include at least one of polyimide, polyamide, polysulfone, polyacrylonitrile, cellulose, polyetheretherketone, polyphenylene sulfide, polyacrylate, polyethylene terephthalate, poly(p-phenyleneamide), polyarylethersulfone ketone, aramid, aramid sulfone, or polyolefin. The polymerizing 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 this application, commercially available base films made of the desired material can be selected. This application has no particular restrictions, as long as it achieves the purpose of this application.

[0054] This application does not impose any particular limitation on the second diaphragm, as long as it can achieve the purpose of this application. For example, the second diaphragm includes a base membrane. Optionally, the second diaphragm may also include a third coating, which may be disposed on one surface or both surfaces of the base membrane. The second diaphragm may also be selected from commercially available diaphragms. For example, the second diaphragm may be selected from polypropylene porous membranes, polyethylene porous membranes, polypropylene nonwoven fabrics, polyethylene nonwoven fabrics, or polypropylene-polyethylene-polypropylene porous composite membranes, etc.

[0055] In some embodiments of this application, the first separator has a Z-shaped folded structure in the electrode assembly, and the Z-shaped first separator separates adjacent positive and negative electrode plates. For example, as shown... Figure 4 As shown, the first diaphragm 20 has a Z-shaped folded structure in the electrode assembly 01, and the Z-shaped first diaphragm 20 separates the adjacent positive electrode 11 and negative electrode 12. Through the above arrangement, the stability of the electrode assembly stack structure is further improved, while taking into account the operational difficulty and processing cost in the actual production process, and achieving higher stacking efficiency.

[0056] In one embodiment of this application, the negative electrode material layer includes a negative electrode active material, which includes a silicon-based material. The silicon-based material includes at least one of silicon-carbon composite materials, silicon-oxygen composite materials, or pure silicon materials. By selecting the above-mentioned types of silicon-based materials, the energy density of the secondary battery is improved while maintaining the cycle performance of the secondary battery.

[0057] In one embodiment of this application, the negative electrode active material further includes at least one of artificial graphite, natural graphite, hard carbon, or MCMB mesophase carbon microspheres. By selecting the above-mentioned types of negative electrode active materials, it is beneficial to improve the structural stability of the negative electrode sheet, so that the volume expansion of the negative electrode sheet during charging and discharging is moderate.

[0058] In one embodiment of this application, the first ceramic layer further includes a first ceramic layer adhesive, and the second ceramic layer further includes a second ceramic layer adhesive. This application does not impose any particular limitation on the types of the first and second ceramic layer adhesives, as long as they achieve the purpose of this application. For example, the first and second ceramic layer adhesives are each independently selected from at least one of styrene-butadiene rubber, polyvinyl alcohol, polyvinylidene fluoride, polyacrylic acid, polymethyl methacrylate, polybutyl acrylate, or polyacrylonitrile. This application does not impose any particular limitation on the content of the first ceramic particles and the first ceramic layer adhesive in the first ceramic layer, or the content of the second ceramic particles and the second ceramic layer adhesive in the second ceramic layer. Those skilled in the art can select according to actual needs, as long as the purpose of this application is achieved.

[0059] In one embodiment of this application, the first adhesive layer may further include a first thickener, and the second adhesive layer may further include a second thickener. Applying the first thickener to the first adhesive layer and the second thickener to the second adhesive layer helps to increase the stability of the first and second adhesive layer slurries and prevents the sedimentation of the components in the first and second adhesive layer slurries. This application does not particularly limit the types of the first and second thickeners, as long as they can achieve the purpose of this application. For example, the first and second thickeners are each independently selected from at least one of hydroxyethyl cellulose, methyl hydroxyethyl cellulose, sodium carboxymethyl cellulose, polyacrylamide, or sodium alginate. This application does not particularly limit the content of the first thickener in the first adhesive layer and the content of the second thickener in the second adhesive layer; those skilled in the art can select them according to actual needs, as long as the purpose of this application can be achieved.

[0060] This application does not impose any particular limitation on the preparation method of the first diaphragm, as long as it can achieve the purpose of this application. For example, the preparation method of the first diaphragm includes, but is not limited to, the following steps: (1) adding the first ceramic particles and the first ceramic layer binder to a solvent and mixing them evenly to obtain a first ceramic layer slurry, adding the second ceramic particles and the second ceramic layer binder to a solvent and mixing them evenly to obtain a second ceramic layer slurry; (2) coating the first ceramic layer slurry on one surface of the base membrane, drying it, and forming a first coating layer on one surface of the base membrane; (3) coating the second ceramic layer slurry on the other surface of the base membrane, drying it, and forming a second coating layer on one surface of the first base membrane, thereby obtaining the first diaphragm.

[0061] In another embodiment of this application, the preparation method of the first diaphragm may include, but is not limited to, the following steps: (1) adding the first ceramic particles and the first ceramic layer binder to a solvent and mixing them evenly to obtain a first ceramic layer slurry; adding the second ceramic particles and the second ceramic layer binder to a solvent and mixing them evenly to obtain a second ceramic layer slurry; (2) mixing the first binder and the first thickener evenly to obtain a first adhesive layer slurry; mixing the second binder and the second thickener evenly to obtain a second adhesive layer slurry; (3) coating the first ceramic layer slurry on one surface of the base film, drying it, forming a first ceramic layer on one surface of the base film, coating the first adhesive layer slurry on the surface of the first ceramic layer away from the base film, drying it, and obtaining a first coating including the first ceramic coating and the first adhesive layer; (4) coating the second ceramic layer slurry on the other surface of the base film, drying it, forming a second ceramic layer on one surface of the base film, coating the second adhesive layer slurry on the surface of the second ceramic layer away from the base film, drying it, and obtaining a second coating including the second ceramic coating and the second adhesive layer, thus obtaining the first diaphragm.

[0062] This application does not limit the solvents mentioned above, as long as they can achieve the purpose of this application.

[0063] In this application, the negative electrode sheet further includes a negative electrode current collector, and a negative electrode material layer is disposed on at least one surface of the negative electrode current collector. The phrase "the negative electrode material layer is disposed on at least one surface of the negative electrode current collector" means that the negative electrode material layer can be disposed on one surface of the negative electrode current collector along its thickness direction, or on two surfaces of the negative electrode current collector along its thickness direction. It should be noted that the "surface" here can be the entire surface area of ​​the negative electrode current collector, or it can be a partial surface area. This application has no particular limitation, as long as the purpose of this application is achieved. Figure 1 As shown, the negative electrode sheet 12 also includes a negative electrode current collector 121, and a negative electrode material layer 122 is disposed on two surfaces of the negative electrode current collector 121 along its thickness direction Z.

[0064] This application does not impose any particular limitation on the negative electrode current collector, as long as it can achieve the purpose of this application. For example, it may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or composite current collectors. For example, the composite current collector may 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, etc. In this application, there is no particular limitation on the thickness of the negative electrode current collector or the negative electrode material layer, as long as it can achieve the purpose of this application. Optionally, the negative electrode material layer may also include a conductive agent and a negative electrode binder. This application does not impose any particular limitation on the type of conductive agent in the negative electrode material layer, as long as it can achieve the purpose of this application. For example, the conductive agent may include, but is not limited to, at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, graphene, metallic materials, or conductive polymers. The conductive carbon black may include, but is not limited to, at least one of acetylene black or Ketjen black. The aforementioned carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The aforementioned carbon fibers may include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or carbon nanofibers. The aforementioned metallic materials may include, but are not limited to, metal powders and / or metal fibers; specifically, the metal may include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The aforementioned conductive polymer may include, but is not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole. This application does not impose any particular limitation on the type of negative electrode binder in the negative electrode material layer, as long as it achieves the purpose of this application. For example, the negative electrode binder may 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, styrene-butadiene rubber, or polyvinylidene fluoride. Optionally, the negative electrode material layer may also include a thickener. This application does not impose any particular limitation on the type of thickener, as long as it achieves the purpose of this application. For example, the thickener may include at least one of carboxymethyl cellulose or sodium carboxymethyl cellulose. This application does not impose any particular restrictions on the mass ratio of negative electrode active material, conductive agent, binder and thickener in the negative electrode material layer. Those skilled in the art can choose according to actual needs, as long as the purpose of this application can be achieved.

[0065] This application does not impose any particular limitation on the positive electrode sheet, as long as the purpose of this application can be achieved. For example, the positive electrode sheet includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. The aforementioned "positive electrode material layer disposed on at least one surface of the positive current collector" means that the positive electrode material layer can be disposed on one surface of the positive current collector along its own thickness direction, or it can be disposed on two surfaces of the positive current collector along its own thickness direction. It should be noted that the "surface" here can be the entire surface area of ​​the positive current collector, or it can be a partial surface area of ​​the positive current collector; this application does not impose any particular limitation, as long as the purpose of this application can be achieved. Figure 1 As shown, the positive electrode 11 includes a positive current collector 111 and a positive electrode material layer 112 disposed on two surfaces of the positive current collector 111 along its thickness direction Z.

[0066] This application does not impose any particular limitation on the positive electrode current collector, as long as it can achieve the purpose of this application. For example, the positive electrode current collector may include aluminum foil, aluminum alloy foil, or composite current collectors (such as aluminum-carbon composite current collectors). The positive electrode material layer of this application includes a positive electrode active material. This application does not impose any particular limitation on the type of positive electrode active material, as long as it can achieve the purpose of this application. For example, the positive electrode active material may include lithium nickel cobalt manganese oxide (LiNi). 0.90 Co 0.05 Mn 0.05 At least one of the following: O2 (NCM955), NCM811, NCM622, NCM523, NCM111, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide (LiCoO2), lithium manganese oxide, or lithium manganese iron phosphate. In this application, the positive electrode active material may also contain non-metallic elements, such as at least one of fluorine, phosphorus, boron, chlorine, silicon, or sulfur. In this application, there are no particular limitations on the thickness of the positive electrode current collector and the positive electrode material layer, as long as the purpose of this application is achieved. In this application, the positive electrode material layer may also include a positive electrode binder and a conductive agent. In this application, there are no particular limitations on the type of positive electrode binder in the positive electrode material layer, as long as the purpose of this application is achieved; for example, the positive electrode binder may be the same type as the negative electrode binder in the aforementioned negative electrode material layer. In this application, there are no particular limitations on the type of conductive agent in the positive electrode material layer, as long as the purpose of this application is achieved; for example, the conductive agent may be the same type as the conductive agent in the aforementioned negative electrode material layer. This application does not impose any particular restrictions on the mass ratio of positive electrode active material, conductive agent, and positive electrode binder in the positive electrode material layer. Those skilled in the art can choose according to actual needs, as long as the purpose of this application can be achieved.

[0067] In this application, the secondary battery also includes an electrolyte, which comprises lithium salts and non-aqueous solvents. This application does not impose any particular limitation on the lithium salt, as long as it achieves the purpose of this application. For example, the lithium salt may 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. This application does not impose any particular limitation on the content of lithium salts in the electrolyte, as long as it achieves the purpose of this application. This application does not impose any particular limitation on the non-aqueous solvent, as long as it achieves the purpose of this application. For example, the non-aqueous solvent may include, but is not limited to, at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents. The aforementioned carbonate compounds may include, but are not limited to, at least one of chain carbonate compounds, cyclic carbonate compounds, or fluorocarbonate compounds. The aforementioned chain carbonate compounds may include, but are 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 aforementioned cyclic carbonates may include, but are not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), or vinyl ethylene carbonate (VEC). Fluorinated carbonate compounds may include, but are not limited to, at least one of fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, or trifluoromethylethylene carbonate. The aforementioned carboxylic acid ester compounds may include, but are not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valproic acid lactone, or caprolactone. The aforementioned ether compounds may include, but are not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. The aforementioned other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate. This application does not impose any particular limitation on the content of non-aqueous solvents in the electrolyte, as long as the purpose of this application is achieved.

[0068] For example, such as Figure 3 and Figure 4 As shown, the secondary battery 001 also includes a casing 02 for accommodating the positive electrode, a first separator or a first and second separator, a negative electrode, and an electrolyte, as well as other components known in the field of secondary batteries. This application does not limit the scope of these other components. This application does not impose any particular limitation on the casing; it can be a casing known in the art, as long as it achieves the purpose of this application. For example, the casing can be a rigid casing or a flexible casing. The material of the rigid casing can be metal; this application does not limit the type of metal and can use known metal rigid casings, as long as they achieve the purpose of this application. The flexible casing can be a metal plastic film, such as aluminum-plastic film, steel-plastic film, etc.

[0069] The secondary battery described in this application is not particularly limited and may include any device in which an electrochemical reaction occurs. In some embodiments of this application, the secondary battery may include, but is not limited to, lithium-ion secondary batteries (lithium-ion batteries), lithium polymer secondary batteries, or lithium-ion polymer secondary batteries.

[0070] The fabrication process of the secondary battery described in this application is well known to those skilled in the art, and this application does not impose any particular limitations. For example, the fabrication process of the secondary battery may include, but is not limited to, the following steps: stacking the positive electrode sheet, the first separator, and the negative electrode sheet in sequence, and performing operations such as winding and folding as needed to obtain a wound electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain the secondary battery. Alternatively, stacking the positive electrode sheet, the first separator or the first and second separators, and the negative electrode sheet in sequence, and then fixing the four corners of the entire stacked structure with tape to obtain a stacked electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain the secondary battery. In addition, overcurrent protection elements, conductive plates, etc., may be placed in the housing as needed to prevent the internal pressure of the secondary battery from rising and overcharging / discharging.

[0071] A second aspect of this application provides an electronic device comprising a secondary battery as described in any of the foregoing embodiments. The secondary battery of this application exhibits excellent cycle performance; therefore, the electronic device of this application has a long service life.

[0072] The electronic device described in this application is not particularly limited and can be any electronic device known in the prior art. For example, the electronic device may include, but is not limited to, laptops, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, household large-capacity batteries, and lithium-ion capacitors.

[0073] Example

[0074] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0075] Test methods and equipment:

[0076] Sampling method for the first diaphragm:

[0077] The lithium-ion batteries in the tested examples and comparative examples were disassembled, and the first separator was removed. It was then soaked in dimethyl carbonate (DMC) for 20 minutes to remove electrolyte residue. Afterward, the first separator was placed in an oven and dried at 60°C for 12 hours to obtain the first separator sample. The ceramic layer on the surface of the separator facing the negative electrode is the first ceramic layer, and the ceramic layer on the surface of the separator facing the positive electrode is the second ceramic layer. If there is only one first coating layer, then the first coating layer is the first ceramic layer; if there is only one second coating layer, then the second coating layer is the second ceramic layer. If an adhesive layer is provided on the surface of the ceramic layers, then the adhesive layer on the surface of the first ceramic layer is the first adhesive layer, and the adhesive layer on the surface of the second ceramic layer is the second adhesive layer.

[0078] Test for the mass percentage of silicon:

[0079] A lithium-ion battery discharged at 0.5C to 3.0V was disassembled, and the negative electrode sheet was removed. It was then soaked in dimethyl carbonate (DMC) for 20 minutes, followed by rinsing with DMC and acetone respectively. The negative electrode sheet was then placed in an oven and baked at 80℃ for 12 hours to obtain the negative electrode sheet. The negative electrode sheet was then placed in a vacuum oven and dried at 100℃ for 24 hours. One gram of powdered negative electrode material layer was scraped off the negative electrode sheet with a blade, and the mass percentage (W) of silicon in the negative electrode material layer was measured using an inductively coupled plasma (ICP) analyzer.

[0080] Testing of the porosity P1 of the first coating and the porosity P2 of the second coating:

[0081] Take the first diaphragm sample and use N-methylpyrrolidone (NMP) solvent to wipe off the second coating of the first diaphragm, obtaining a single-sided first diaphragm sample containing only the base film and the first coating. First, test the porosity Pa of this single-sided sample. Then wipe off the first coating again to obtain a sample containing only the base film, and test the porosity Pb ​​of the base film sample. At the same time, use argon ion polishing to obtain the diaphragm cross section, and use field emission scanning electron microscopy to measure the thickness Ta of the single-sided sample, the sample thickness Tb of the base film, and the sample thickness Tc of the first coating, where Ta = Tb + Tc. It is easy to obtain Tb / Ta×Pb + Tc / Ta×P1 = Pa, P1 = (Tb×Pa - Ta×Pb) / Tc.

[0082] The first coating of the first membrane was removed by wiping it off with NMP solvent, resulting in a single-sided membrane sample containing only the base membrane and the second coating. The porosity P2 of the second coating can be obtained by referring to the above test steps and calculation logic.

[0083] Tests of the average particle size of the first ceramic particles and the average particle size of the second ceramic particles:

[0084] The cross-section of the first diaphragm along the thickness direction was prepared by argon ion polishing. The cross-sections of the first ceramic layer and the second ceramic layer were observed by scanning electron microscopy (SEM). Ten first ceramic particles and ten second ceramic particles were randomly selected, and the equivalent diameters of the ten first ceramic particles and ten second ceramic particles were measured respectively (that is, the diameter of a circle with an irregular cross-section converted into a circle with an equal area). The average values ​​were calculated to obtain the average particle size D1 of the first ceramic particles and the average particle size D2 of the second ceramic particles.

[0085] Testing the thickness of the first adhesive layer and the thickness of the second adhesive layer:

[0086] The first diaphragm was polished with argon ions to obtain the cross-section of the first diaphragm. The morphology of the cross-section of the first diaphragm along the thickness direction was observed and scanned electron micrographs were taken using a field emission scanning electron microscope (Philips, XL-30). The thickness T1 of the first adhesive layer and the thickness T2 of the second adhesive layer were measured by scanning electron microscopy.

[0087] Lithium plating performance test:

[0088] The lithium-ion batteries used in the examples and comparative examples were placed in a constant temperature chamber at 10°C. After 60 minutes, they were charged at a constant current of 2C to 4.45V, then charged at a constant voltage of 4.45V to a current of 0.025C. After resting for 5 minutes, they were discharged at a constant current of 0.5C to 2.75V. This constitutes one cycle. After 100 cycles of the above charge-discharge process, the batteries were charged at a constant current of 2C to 4.45V, then charged at a constant voltage of 4.45V to a current of 0.025C. After resting for 5 minutes, the lithium-ion batteries were disassembled, and the negative electrode was removed from the electrode assembly. The lithium plating state on the surface of the negative electrode was observed. The non-lithium-plated areas on the surface of the negative electrode were golden yellow, while the lithium-plated areas were grayish-white.

[0089] The criteria for judging the degree of lithium plating in lithium-ion batteries are as follows: 0% lithium plating area is considered no lithium plating, i.e., the degree of lithium plating is zero; lithium plating area greater than 0 and less than or equal to 2% is considered mild lithium plating; lithium plating area greater than 2% and less than or equal to 20% is considered moderate lithium plating; and lithium plating area greater than 20% and less than or equal to 100% is considered severe lithium plating. The percentage of lithium plating area is calculated based on the total area of ​​the negative electrode material layer of the negative electrode sheet.

[0090] Cyclic performance test:

[0091] The lithium-ion battery was placed in a 25°C constant-temperature test chamber and allowed to stand for 30 minutes to reach a constant temperature of 25°C. It was then charged at a constant current of 1C to 4.45V, followed by constant voltage charging at 4.45V to a current of 0.025C. After standing for 5 minutes, it was discharged at a constant current of 0.2C to 2.75V. This was the first cycle, and the initial discharge capacity was recorded as C0. The lithium-ion battery was cycled according to the above process. The test was stopped after 500 cycles (cls), and the discharge capacity after 500 cycles (cls) was recorded as C1. The capacity retention rate at 500cls was calculated as an indicator of the lithium-ion battery's cycle performance.

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

[0093] A higher capacity retention rate (500cls) indicates better cycle performance of the lithium-ion battery.

[0094] Drop test:

[0095] The lithium-ion batteries in the examples and comparative examples were placed in an environment of 25°C and left to stand for 30 minutes. Then, they were charged as follows: constant current charging at 0.5C to 4.53V, followed by constant voltage charging to 0.05C. After standing for 60 minutes, the voltage of the lithium-ion batteries before the drop test was measured. The lithium-ion batteries were placed in a fixture and dropped freely from a height of 1.5m above the ground using a drop device in the following sequence: head-tail-right corner of head-right corner of tail-left corner of head-left corner of tail (angle: 45±15°), repeated 6 times. After the drop test, the batteries were left to stand at room temperature for 24 hours, and the voltage of the lithium-ion batteries was measured and recorded. The appearance of the lithium-ion batteries was checked and photographed before and after the test. The drop test pass criterion was: voltage drop <30mV (20 lithium-ion batteries were prepared and tested for each example or comparative example; the number of lithium-ion batteries that passed the test was X, and the pass rate was X / 20×100%).

[0096] A higher drop test pass rate indicates better drop performance of the lithium-ion battery group.

[0097] Example 1-1

[0098] <Preparation of the first diaphragm>

[0099] A polyethylene (PE) film with a thickness of 10 μm was used as the base film, and the porosity of the first base film was 60%.

[0100] The first ceramic particles, boehmite, and the first ceramic layer binder, polyvinylidene fluoride, were mixed at a mass ratio of 95:5, and then deionized water was added as a solvent. After mixing evenly, the first ceramic layer slurry was obtained.

[0101] The second ceramic particles (boehmite) and the second ceramic layer binder (polyvinylidene fluoride) were mixed at a mass ratio of 95:5, and then deionized water was added as a solvent. After mixing evenly, the second ceramic layer slurry was obtained.

[0102] A first ceramic layer slurry is coated on one surface of a base membrane and dried at 60°C to form a first coating on one surface of the first base membrane; a second ceramic layer slurry is coated on the other surface of the base membrane and dried at 60°C to form a second coating on the other surface of the first base membrane, thus obtaining the first diaphragm.

[0103] The first ceramic particle has an average particle size of 1.5 μm (D1 = 1.5 μm), and the second ceramic particle has an average particle size of 0.33 μm (D2 = 0.33 μm). The coating weight of the first ceramic layer is 0.75 mg / 1540.25 mm. 2 The coating weight of the second ceramic layer is 0.75 mg / 1540.25 mm. 2 The porosity P1 of the first coating is 55%, and the porosity P2 of the second coating is 22%.

[0104] <Preparation of the positive electrode>

[0105] Lithium cobalt oxide (positive electrode active material), conductive carbon black (conductive agent), and polyvinylidene fluoride (PVDF) (positive electrode binder) were mixed in a mass ratio of 97:1.5:1.5. N-methylpyrrolidone (NMP) was added as a solvent, and the mixture was thoroughly stirred to form a positive electrode slurry with a solid content of 75 wt%. This slurry was uniformly coated onto one surface of a 10 μm thick aluminum foil used as a positive electrode current collector and dried at 110°C to obtain a positive electrode sheet with a single-sided coating thickness of 55 μm. The positive electrode slurry was then coated onto the other surface of the aluminum foil and dried to obtain a positive electrode sheet with a total thickness of 120 μm. The coated positive electrode sheets were cold-pressed and then cut into 70 mm × 40 mm pieces for later use. The compaction density of the positive electrode material layer was 4.23 g / cm³. 3 .

[0106] <Preparation of Negative Electrode Sheets>

[0107] Artificial graphite (anode active material), silicon-carbon composite material (SiC), conductive agent carbon nanotubes, and anode binder polyacrylic acid were mixed in a mass ratio of 41.1:42.9:1:5. Deionized water was then added as a solvent, and the mixture was stirred until homogeneous, yielding a negative electrode slurry with a solid content of 35 wt%. This slurry was coated onto one surface of a 10 μm thick copper foil current collector and dried at 110 °C to obtain a negative electrode sheet with a 40 μm thick coating. The slurry was then coated onto the other surface of the copper foil current collector and dried to obtain a negative electrode sheet with a total thickness of 90 μm. The coated negative electrode sheets were cold-pressed and then cut into 74 mm × 42 mm pieces for later use. The compaction density of the negative electrode material layer was 1.5 g / cm³. 3 Based on the mass of the negative electrode material layer, the mass percentage of silicon element W is 30%.

[0108] <Preparation of Electrolyte>

[0109] 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. Lithium hexafluorophosphate (LiPF6) was then added to the base solvent, dissolved, and thoroughly mixed to obtain the electrolyte. The mass percentage of LiPF6 in the electrolyte was 4.5%, with the remainder being the base solvent.

[0110] <Preparation of Lithium-ion Batteries>

[0111] The single-sided positive electrode sheet from the "Preparation of Positive Electrode Sheet" section is placed on the outermost side of the electrode assembly, serving as the outermost electrode sheet of the electrode assembly. All other positive electrode sheets in the electrode assembly are double-sided positive electrode sheets.

[0112] The outer positive electrode, the first separator, the outer negative electrode, the first separator, the second outer positive electrode, the first separator, the inner negative electrode, the first separator, the inner positive electrode, the first separator, the inner negative electrode, the first separator, the inner positive electrode, the first separator, the inner negative electrode, the first separator, the inner positive electrode, the first separator, the inner negative electrode, the first separator, the inner positive electrode, the first separator, the inner negative electrode, the first separator, the inner positive electrode, the first separator, the inner negative electrode, the first separator, the inner positive electrode, the first separator, the inner negative electrode, the first separator, the second outer positive electrode, the first separator, the outer negative electrode, the first separator, and the outer positive electrode are stacked in sequence, wherein the first separator is continuous and uninterrupted, and has a Z-shaped folded structure in the electrode assembly. Then, the four corners of the entire stacked structure are fixed with tape to obtain the electrode assembly of the stacked structure. The electrode assembly is placed in an aluminum-plastic film packaging bag and dried in an 80℃ vacuum oven for 12 hours to remove moisture. The prepared electrolyte is then injected, and after vacuum sealing, settling, formation, degassing, and edge trimming, a lithium-ion battery is obtained. The design potential range of the lithium-ion battery is 2.75V to 4.45V.

[0113] Examples 1-2 to Examples 1-13

[0114] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Example 1-1. Specifically, when the average particle size of the same type of ceramic particles changes, the grinding time is adjusted to achieve the average particle size value as shown in Table 1. When the mass percentage of silicon element changes, the mass percentage of conductive carbon nanotubes and negative electrode binder remains unchanged, and the mass ratio of artificial graphite and silicon-carbon composite material SiO is adjusted to achieve the mass percentage of silicon element as shown in Table 1. In Examples 1-7, 1-8, and 1-11, the silicon-carbon composite material SiC is replaced with pure silicon, the mass percentage of conductive carbon nanotubes and negative electrode binder remains unchanged, and the mass ratio of artificial graphite and pure silicon is adjusted to achieve the mass percentage of silicon element as shown in Table 1.

[0115] Examples 1-14

[0116] Except for the preparation of the first diaphragm according to the following steps, the rest is the same as in Example 1-1.

[0117] A polyethylene (PE) film with a thickness of 10 μm was used as the base film, and the porosity of the first base film was 60%.

[0118] The first ceramic particles, boehmite, and the first ceramic layer binder, polyvinylidene fluoride, were mixed at a mass ratio of 95:5, and then deionized water was added as a solvent. After mixing evenly, the first ceramic layer slurry was obtained.

[0119] The second ceramic particles (boehmite) and the second ceramic layer binder (polyvinylidene fluoride) were mixed at a mass ratio of 95:5, and then deionized water was added as a solvent. After mixing evenly, the second ceramic layer slurry was obtained.

[0120] The first binder, styrene-butadiene latex, and the first thickener, sodium carboxymethyl cellulose, were mixed at a mass ratio of 98.5:1.5, and then deionized water was added as a solvent. After mixing evenly, the first adhesive layer slurry was obtained.

[0121] The second binder, vinylidene fluoride homopolymer, and the second thickener, sodium carboxymethyl cellulose, were mixed at a mass ratio of 98.5:1.5, and then deionized water was added as a solvent. After mixing evenly, the second adhesive layer slurry was obtained.

[0122] A first ceramic layer slurry is coated on one surface of a base membrane and dried at 60°C to form a first ceramic layer on that surface. A first adhesive layer slurry is then coated on the surface of the first ceramic layer away from the base membrane and dried at 60°C to obtain a first coating comprising a first ceramic coating and a first adhesive layer. A second ceramic layer slurry is coated on the other surface of the base membrane and dried at 60°C to form a second ceramic layer on that surface. A second adhesive layer slurry is then coated on the surface of the second ceramic layer away from the base membrane and dried at 60°C to obtain a second coating comprising a second ceramic coating and a second adhesive layer, thus forming the first separator.

[0123] The first ceramic particle has an average particle size of 1.5 μm (D1 = 1.5 μm), and the second ceramic particle has an average particle size of 0.33 μm (D2 = 0.33 μm). The coating weight of the first ceramic layer is 0.75 mg / 1540.25 mm. 2 The coating weight of the second ceramic layer is 0.75 mg / 1540.25 mm. 2 The porosity P1 of the first coating is 55%, and the porosity P2 of the second coating 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.

[0124] Examples 1-15 to Examples 1-20

[0125] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Example 1-1. Specifically, 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.

[0126] Example 2-1

[0127] Except for cutting the positive electrode sheet to a size of 70mm×800mm in the "Preparation of Positive Electrode Sheet" section and cutting the negative electrode sheet to a size of 74mm×824mm in the "Preparation of Negative Electrode Sheet" section, and preparing the lithium-ion battery according to the following steps, the rest is the same as in Example 1-1.

[0128] <Preparation of Lithium-ion Batteries>

[0129] The first separator, negative electrode, and positive electrode prepared above are stacked sequentially, ensuring that the first coating of the first separator faces the negative electrode and the second coating of the first separator faces the positive electrode. The electrode assembly is then wound sequentially to obtain the electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag, dried, and then injected with the electrolyte prepared above. After vacuum sealing, settling, formation, degassing, and edge trimming, a lithium-ion battery is obtained.

[0130] Example 2-2

[0131] Except for the preparation of the lithium-ion battery and the second separator according to the following steps, the rest is the same as in Example 1-1.

[0132] <Preparation of the Second Diaphragm>

[0133] A 10 μm thick polyethylene (PE) membrane was used as the base membrane, with a porosity of 60%.

[0134] The ceramic particles boehmite and the ceramic layer binder polyvinylidene fluoride were mixed at a mass ratio of 95:5, and then deionized water was added as a solvent. After mixing evenly, the third ceramic layer slurry was obtained.

[0135] A third ceramic layer slurry is coated on one surface of the base membrane and dried at 60°C to form a third ceramic layer on one surface of the base membrane. The above operation is repeated on the other surface of the base membrane to obtain a second diaphragm.

[0136] The average particle size of the ceramic particles is 0.33 μm, the porosity of the third coating is 22%, and the coating weight of the third ceramic layer is 0.75 mg / 1540.25 mm. 2 .

[0137] <Preparation of Lithium-ion Batteries>

[0138] The single-sided positive electrode sheet from the "Preparation of Positive Electrode Sheet" section is placed on the outermost side of the electrode assembly, serving as the outermost electrode sheet of the electrode assembly. All other positive electrode sheets in the electrode assembly are double-sided positive electrode sheets.

[0139] The outer positive electrode, the first diaphragm, the outer negative electrode, the first diaphragm, the second outer positive electrode, the second diaphragm, the inner negative electrode, the second diaphragm, the inner positive electrode, the second diaphragm, the inner negative electrode, the second diaphragm, the inner positive electrode, the second diaphragm, the inner negative electrode, the second diaphragm, the inner positive electrode, the second diaphragm, the inner negative electrode, the second diaphragm, the inner positive electrode, the second diaphragm, the inner negative electrode, the second diaphragm, the inner positive electrode, the second diaphragm, the inner negative electrode, the second diaphragm, the second outer positive electrode, the first diaphragm, the outer negative electrode, the first diaphragm, and the outer positive electrode are stacked in sequence. Then, the four corners of the entire stacked structure are fixed with tape to obtain the electrode assembly of the stacked structure. The electrode assembly is placed in an aluminum-plastic film packaging bag and dried in an 80℃ vacuum oven for 12 hours to remove moisture. The prepared electrolyte is then injected, and after vacuum sealing, settling, formation, degassing, and edge trimming, a lithium-ion battery is obtained. The design potential range of the lithium-ion battery is 2.75V to 4.45V.

[0140] Example 2-3

[0141] Except for the preparation of lithium-ion batteries according to the following steps, the rest is the same as in Example 2-2.

[0142] <Preparation of Lithium-ion Batteries>

[0143] The single-sided positive electrode sheet from the "Preparation of Positive Electrode Sheet" section is placed on the outermost side of the electrode assembly, serving as the outermost electrode sheet of the electrode assembly. All other positive electrode sheets in the electrode assembly are double-sided positive electrode sheets.

[0144] The outer positive electrode, the first diaphragm, the outer negative electrode, the first diaphragm, the second outer positive electrode, the first diaphragm, the inner negative electrode, the first diaphragm, the inner positive electrode, the second diaphragm, the inner negative electrode, the second diaphragm, the inner positive electrode, the second diaphragm, the inner negative electrode, the second diaphragm, the inner positive electrode, the second diaphragm, the inner negative electrode, the second diaphragm, the inner positive electrode, the second diaphragm, the inner negative electrode, the second diaphragm, the inner positive electrode, the first diaphragm, the inner negative electrode, the first diaphragm, the second outer positive electrode, the first diaphragm, the outer negative electrode, the first diaphragm, and the outer positive electrode are stacked in sequence. Then, the four corners of the entire stacked structure are fixed with tape to obtain the electrode assembly of the stacked structure. The electrode assembly is placed in an aluminum-plastic film packaging bag and dried in an 80℃ vacuum oven for 12 hours to remove moisture. The prepared electrolyte is then injected, and after vacuum sealing, settling, formation, degassing, and edge trimming, a lithium-ion battery is obtained. The design potential range of the lithium-ion battery is 2.75V to 4.45V.

[0145] Examples 2-4

[0146] Except for the use of the first separator in the <Preparation of Lithium-ion Batteries>, the rest is the same as in Examples 2-2.

[0147] Comparative Example 1

[0148] Except for the preparation steps of the first coating, which are exactly the same as those of the second coating in Example 1-1, the rest are the same as in Example 1-1.

[0149] Comparative Example 2

[0150] Except for the preparation steps of the second coating, which are exactly the same as those of the first coating in Example 1-1, the rest are the same as in Example 1-1.

[0151] Comparative Example 3

[0152] Except that the preparation steps of the first coating are exactly the same as those of the second coating in Example 1-1, and the preparation steps of the second coating are exactly the same as those of the first coating in Example 1-1, everything else is the same as in Example 1-1.

[0153] The preparation and performance parameters of each embodiment and comparative example are shown in Tables 1 and 2.

[0154]

[0155]

[0156] As can be seen from Examples 1-1 to 1-20 and Comparative Examples 1 to 3, by adjusting the porosity of the first coating to be greater than that of the second coating, the degree of lithium plating on the negative electrode in the electrode assembly is less, the 500cls capacity retention rate of the lithium-ion battery is improved, and the drop test pass rate is higher. This indicates that the lithium-ion battery of this application can reduce the risk of negative electrode interface deterioration due to insufficient electrolyte. While taking into account drop performance and safety performance, the lithium-ion battery has good cycle performance. In Comparative Examples 1 and 2, the porosity of the first and second coatings of the first separator of the electrode assembly is the same; in Comparative Example 3, the parameters of the first and second coatings are exactly opposite to those of the first and second coatings in Examples 1-1; the degree of lithium plating on the negative electrode in the lithium-ion batteries of Comparative Examples 1 to 3 is more severe; the 500cls capacity retention rate is lower; and / or, the drop test pass rate is lower. In the lithium-ion batteries of Examples 1-1 to 1-20, the negative electrode plate showed less lithium deposition, a higher 500cls capacity retention rate, and a higher drop test pass rate. This indicates that the risk of the negative electrode plate deteriorating due to insufficient electrolyte during cycling is low. While taking into account drop performance and safety performance, the lithium-ion battery has good cycle performance.

[0157] The value of W typically affects the cycle performance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-11, when the value of W is within the range of this application, the lithium-ion battery exhibits less lithium plating on the negative electrode, a higher 500cls capacity retention rate, and a higher drop test pass rate. This indicates that the risk of the negative electrode deteriorating due to insufficient electrolyte during cycling is low. While balancing drop performance and safety performance, the lithium-ion battery demonstrates excellent cycle performance.

[0158] The values ​​of P1 / P2 and P1 typically affect the cycle performance of lithium-ion batteries. As seen in Examples 1-1 to 1-11, when the values ​​of P1 / P2 and P1 are within the range specified in this application, the lithium-ion battery exhibits less lithium plating on the negative electrode, a higher 500cls capacity retention rate, and a higher drop test pass rate. This indicates a lower risk of deterioration of the negative electrode due to insufficient electrolyte during cycling. The lithium-ion battery demonstrates good cycle performance while balancing drop performance and safety. However, in Examples 1-11, the values ​​of P1 / P2 and P1 are relatively large. In this case, the separator's electrolyte storage capacity on the positive electrode side is relatively weak, while its electrolyte storage capacity on the negative electrode side is relatively strong. The significant difference in electrolyte storage on both sides leads to a decrease in the 500cls capacity retention rate of the lithium-ion battery. Simultaneously, the increased free electrolyte within the casing reduces the drop test pass rate, thus affecting both the cycle performance and drop performance of the lithium-ion battery.

[0159] The relationship between P1 / P2 and W typically affects the cycle performance of lithium-ion batteries. As seen in Examples 1-1 to 1-10, when the relationship between P1 / P2 and W meets the requirements of this application, the lithium-ion battery exhibits less lithium plating on the negative electrode, a higher 500cls capacity retention rate, and a higher drop test pass rate. This indicates a lower risk of deterioration of the negative electrode due to insufficient electrolyte during cycling. The lithium-ion battery demonstrates good cycle performance while balancing drop performance and safety. However, in Examples 1-10, the values ​​of P1 / P2 and P1 are relatively large compared to the value of W. In this case, the separator's ability to store electrolyte on the positive electrode side is relatively weak, resulting in a significant difference in electrolyte levels on both sides. This leads to a decrease in the 500cls capacity retention rate of the lithium-ion battery, thus affecting its cycle performance.

[0160] The average particle size of the first ceramic particles and the average particle size of the second ceramic particles typically affect the cycle performance of lithium-ion batteries. As can be seen from Examples 1-1 to 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 this application, the lithium deposition on the negative electrode in the lithium-ion battery is less severe, the 500cls capacity retention rate is higher, and the drop test pass rate is higher. This indicates that the risk of the negative electrode deteriorating due to insufficient electrolyte during cycling is lower. While balancing drop performance and safety performance, the lithium-ion battery exhibits good cycle performance.

[0161] The types of the first and second ceramic particles typically affect the cycle performance of lithium-ion batteries. As can be seen from Examples 1-1, 1-12, and 1-13, when the first and second ceramic particles within the scope of this application are used, the lithium plating on the negative electrode of the lithium-ion battery is less severe, the 500cls capacity retention rate is higher, and the drop test pass rate is higher. This indicates that the risk of the negative electrode deteriorating due to insufficient electrolyte during cycling is lower. While balancing drop performance and safety performance, the lithium-ion battery exhibits good cycle performance.

[0162] The types of the first and second binders typically affect the cycle performance of lithium-ion batteries. As can be seen from Examples 1-1, 1-14, and 1-17, when the first and second binders within the scope of this application are used, the lithium plating on the negative electrode of the lithium-ion battery is less severe, the 500cls capacity retention rate is higher, and the drop test pass rate is higher. This indicates that the risk of the negative electrode deteriorating due to insufficient electrolyte during cycling is lower. While balancing drop performance and safety performance, the lithium-ion battery exhibits good cycle performance.

[0163] The thickness T1 of the first adhesive layer typically affects the cycle performance of lithium-ion batteries. As seen in Examples 1-1, 1-18 to 1-20, when the thickness T1 of the first adhesive layer is within the range specified in this application, the lithium plating on the negative electrode of the lithium-ion battery is less severe, the 500cls capacity retention rate is higher, and the drop test pass rate is higher. This indicates a lower risk of deterioration of the negative electrode due to insufficient electrolyte during cycling. While balancing drop performance and safety performance, the lithium-ion battery exhibits good cycle performance. However, in Examples 1-20, the thickness of the first adhesive layer is larger, reducing the porosity of the first coating. This increases the resistance to lithium-ion transport during cycling, affecting ion transport capacity and leading to a decrease in the 500cls capacity retention rate of the lithium-ion battery, thus impacting its cycle performance.

[0164] Table 2

[0165] Electrode assembly structure n m m / n Lithium plating degree 500cls volume retention rate (%) Drop test pass rate (%) Example 1-1 Stacked pieces / / 1 Non-lithium plating 85 100 Example 2-1 winding / / 1 Non-lithium plating 76 100 Example 2-2 Stacked pieces 16 4 1 / 4 Moderate lithium plating 75 100 Example 2-3 Stacked pieces 16 8 1 / 2 Mild lithium plating 80 100 Examples 2-4 Stacked pieces 16 16 1 Non-lithium plating 85 95

[0166] Note: " / " in Table 2 indicates that there are no relevant preparation parameters.

[0167] The structure of the electrode assembly typically affects the cycle performance of lithium-ion batteries. As can be seen from Examples 1-1, 2-1, and 2-4, when the electrode assembly structure meets the requirements of this application, the lithium-ion battery exhibits less lithium plating on the negative electrode, a higher 500cls capacity retention rate, and a higher drop test pass rate. This indicates a lower risk of deterioration of the negative electrode due to insufficient electrolyte during cycling. Thus, the lithium-ion battery demonstrates good cycle performance while balancing drop performance and safety.

[0168] The m / n value typically affects the cycle performance of lithium-ion batteries. As can be seen from Examples 1-1, 2-2, and 2-4, when the m / n value meets the requirements of this application, the lithium-ion battery exhibits less lithium plating on the negative electrode, a higher 500cls capacity retention rate, and a higher drop test pass rate. This indicates a lower risk of deterioration of the negative electrode due to insufficient electrolyte during cycling. Therefore, the lithium-ion battery demonstrates good cycle performance while balancing drop performance and safety.

[0169] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or article that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, or article.

[0170] The various embodiments in this specification are described in a related manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

[0171] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A secondary battery, comprising an electrode assembly, the electrode assembly comprising an electrode sheet and a first separator, the electrode 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 silicon, and the mass percentage of silicon, based on the mass of the negative electrode material layer, being W, wherein 3% ≤ W ≤ 80%; The first separator includes a base film, a first coating, and a second coating. Along the thickness direction of the first separator, the base film includes a first surface and a second surface. The first surface faces the negative electrode sheet, and the second surface faces the positive electrode sheet. The first coating is disposed on the first surface, and the second coating is disposed on the second surface. The porosity of the first coating is P1, the porosity of the second coating is P2, P1>P2, 0.6P1 / P2≤0.04W×100+0.98≤1.5P1 / P2.

2. The secondary battery according to claim 1, wherein, The first coating includes a first ceramic layer, and the second coating includes a second ceramic layer.

3. The secondary battery according to claim 2, wherein, 1<P1 / P2≤5, 30%≤P1≤70%.

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

5. The secondary battery according to claim 3, wherein, 10%<W≤30%, 1.5<P1 / P2≤2.5, 40%<P1≤55%.

6. The secondary battery according to claim 3, wherein, 30%<W≤80%, 2.5<P1 / P2≤5, 55%<P1≤70%.

7. The secondary battery according to claim 2, wherein, The first ceramic layer includes first ceramic particles, and the second ceramic layer includes second ceramic particles. The first ceramic particles and the second ceramic particles are each independently selected from at least one of alumina, magnesium oxide, 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, where 0.25≤D2≤0.

45.

8. The secondary battery according to claim 2, wherein, The first coating further includes a first adhesive layer, wherein 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 further includes a second adhesive layer, wherein 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 includes a first adhesive, and the second adhesive layer includes a second adhesive. The first adhesive and the second adhesive are each independently selected from at least one of styrene-butadiene latex, styrene-acrylic latex, polymethyl methacrylate, polybutyl methacrylate, polyethyl methacrylate, polyvinyl alcohol, ethylene-vinyl acetate copolymer, polyvinyl acetate, polyurethane, polyvinylidene fluoride, or a copolymer of vinylidene fluoride and hexafluoropropylene.

9. The secondary battery according to claim 8, wherein, The thickness of the first adhesive layer is 0.5 μm to 3 μm.

10. The secondary battery according to claim 8, wherein, The first adhesive includes at least one of styrene-butadiene latex, styrene-acrylic 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 polyvinylidene fluoride and hexafluoropropylene.

11. The secondary battery according to claim 1, wherein, The electrode assembly has a stacked structure.

12. The secondary battery according to claim 11, wherein, The electrode assembly further includes a second diaphragm. The electrode includes two outer electrode plates and a plurality of inner electrode plates. The two outer electrode plates are respectively located on the outermost two sides of the electrode assembly. The first diaphragm is disposed at least between the outer electrode plates and the inner electrode plates adjacent to the outer electrode plates. The second diaphragm is disposed between two adjacent inner electrode plates.

13. The secondary battery according to claim 12, wherein, The first diaphragm has m layers, and the total number of layers of the first and second diaphragms is n, where 1 / 5 ≤ m / n ≤ 1.

14. The secondary battery according to claim 11, wherein, The first separator has a Z-shaped folded structure in the electrode assembly, and the Z-shaped first separator separates the adjacent positive electrode and the negative electrode.

15. An electronic device comprising a secondary battery as claimed in any one of claims 1 to 14.

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