Secondary battery and electronic device

Abstract

The invention provides a secondary battery and an electronic device, the secondary battery comprises a negative electrode plate, the negative electrode plate comprises a negative electrode current collector and a negative electrode material layer arranged on at least one surface of the negative electrode current collector, the negative electrode material layer comprises a first negative electrode material layer and a second negative electrode material layer, and the first negative electrode material layer and the second negative electrode material layer are arranged along the thickness direction of the negative electrode plate. The first negative electrode material layer is arranged between the negative electrode current collector and the second negative electrode material layer, the first negative electrode material layer comprises a first negative electrode active material, and the second negative electrode material layer comprises a solid electrolyte and a second negative electrode active material; the diaphragm resistance of the first negative electrode material layer is RA Omega, the diaphragm resistance of the second negative electrode material layer is RB Omega, RB is more than or equal to 0.005 and less than or equal to 1, and RB / RA is more than or equal to 2 and less than or equal to 450. By regulating and controlling the secondary battery to meet the characteristics, the secondary battery has relatively good conductivity, and meanwhile, the nail penetrating test passing rate of the secondary battery can be improved, so that the safety performance of the secondary battery is improved while relatively good rate capability is considered.

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] Since its commercial application, lithium-ion batteries have stood out from various energy storage technologies due to their advantages in energy density, voltage platform, cycle life, no memory effect, green environmental protection, self-discharge and stability, and have been widely used in various consumer electronics products, electric vehicles, energy storage power stations and other applications.

[0003] However, with the widespread adoption of lithium-ion batteries, while rechargeable batteries offer good rate performance, further improving their safety performance has become increasingly important. Currently, the safety performance of lithium-ion batteries is typically characterized by a pin-through test. During this test, a short circuit occurs when the positive and negative electrodes come into contact. Because the impedance of the negative electrode material layer is uniform, the instantaneous current generated by the short circuit can easily cause thermal runaway in the lithium-ion battery. Therefore, there is an urgent need for a negative electrode structure with a high pin-through test pass rate to improve the safety performance of lithium-ion batteries. Summary of the Invention

[0004] The purpose of this application is to provide a secondary battery and electronic device that, while taking into account the rate performance of the secondary battery, improves the puncture performance of the secondary battery, thereby giving the secondary battery good rate performance and safety performance.

[0005] It should be noted that while this application uses lithium-ion batteries as an example of secondary batteries to explain the invention, the secondary batteries in this application are not limited to lithium-ion batteries. The specific technical solution is as follows:

[0006] A first aspect of this application provides a secondary battery, including a negative electrode sheet. The negative electrode sheet includes a negative current collector and a negative electrode material layer disposed on at least one surface of the negative current collector. The negative electrode material layer includes a first negative electrode material layer and a second negative electrode material layer. Along the thickness direction of the negative electrode sheet, the first negative electrode material layer is disposed between the negative current collector and the second negative electrode layer. The first negative electrode material layer includes a first negative electrode active material, and the second negative electrode layer includes a solid electrolyte and a second negative electrode active material. The film resistance of the first negative electrode material layer is R. A Ω, the film resistance of the second negative electrode material layer is R B Ω, 0.005≤R B ≤1, 2≤R B / R A ≤450. By using R B / R ABy controlling the values ​​within the aforementioned range and differentiating the film resistances of the first and second negative electrode material layers to achieve appropriate values, the impedance of the negative electrode surface can be increased, effectively mitigating heat accumulation during short circuits and reducing the risk of thermal runaway in the secondary battery. Furthermore, the negative electrode exhibits good conductivity, resulting in good cycle capacity retention for the secondary battery. Therefore, by adjusting the secondary battery to meet these characteristics, it achieves good conductivity and improved penetration resistance, thereby enhancing both rate performance and safety.

[0007] In one or more embodiments of this application, 0.05 ≤ R B ≤0.2. By adjusting R B The value satisfies the above range, so that the film resistance of the second negative electrode material layer has a more suitable range, which is conducive to better reducing heat during the short circuit process, better mitigating heat accumulation during the short circuit process, further reducing the risk of thermal runaway of the secondary battery, further improving the pin penetration performance of the secondary battery, and the secondary battery has better conductivity, thereby further improving the safety performance of the secondary battery while taking into account better rate performance.

[0008] In one or more embodiments of this application, the mass percentage of the solid electrolyte is W based on the mass of the second negative electrode material layer. A 1%≤W A ≤30%. By adjusting W A When the value is within the above range, the second negative electrode material layer contains a suitable mass percentage of solid electrolyte, which can reduce the preparation cost and make the second negative electrode material layer have a suitable film resistance. This can better reduce the risk of the secondary battery's rate performance being reduced due to excessive film resistance, and at the same time help improve the secondary battery's penetration performance, thereby improving the safety performance of the secondary battery while taking into account good rate performance.

[0009] In one or more embodiments of this application, the second negative electrode active material includes at least one of graphite or silicon-based materials, and the solid electrolyte includes at least one of oxide-based solid electrolyte, sulfide-based solid electrolyte, or polymer-based solid electrolyte. By controlling the types of each component in the second negative electrode material layer within the above-mentioned range, the negative electrode sheet can have good conductivity, while the second negative electrode material layer has suitable film resistance. This effectively reduces the risk of reduced rate performance of the secondary battery due to excessive film resistance, improves the penetration performance of the secondary battery, and thus improves the safety performance of the secondary battery while maintaining good rate performance.

[0010] In one or more embodiments of this application, the solid electrolyte includes at least one of oxide-based solid electrolyte, sulfide-based solid electrolyte, or polymer-based solid electrolyte. Specifically, the oxide-based solid electrolyte may include at least one of lithium lanthanum titanate (LLTO), lithium lanthanum zirconium oxide (LLZO), or aluminum-doped lithium lanthanum zirconium oxide (Al-LLZO); the sulfide-based solid electrolyte may include at least one of lithium silicon phosphide sulfide, lithium germanium phosphide sulfide, or lithium tin phosphide sulfide; and the polymer-based solid electrolyte may include at least one of polyethylene oxide, polyacrylonitrile, or polyvinylidene fluoride. By controlling the type of solid electrolyte within the above range, the solid electrolyte can have better lithium-ion transport performance and better electronic insulation performance, better reducing the risk of internal short circuits, improving the penetration performance of the secondary battery, and thus improving the safety performance of the secondary battery.

[0011] In one or more embodiments of this application, the first negative electrode material layer includes a first conductive agent, and the second negative electrode material layer includes a second conductive agent. The first and second conductive agents each independently include at least one of carbon nanotubes, conductive carbon black (SuperP), or graphene. By controlling the types of the first and second conductive agents within the aforementioned range, the negative electrode material layer exhibits good conductivity, enabling the secondary battery to have good rate performance.

[0012] In one or more embodiments of this application, the mass percentage of the first conductive agent is W1 based on the mass of the first negative electrode material layer, and the mass percentage of the second conductive agent is W2 based on the mass of the second negative electrode material layer, with 0.4% ≤ W1 ≤ 2%. 0 ≤ W2 / W1 ≤ 0.7, and correspondingly, 0 ≤ W2 ≤ 1.4%. By adjusting the values ​​of W1 and W2 / W1 to satisfy the above characteristics, the film resistance of the first and second negative electrode material layers can be further distinguished, allowing the film resistance of the second negative electrode material layer to have a more suitable range. The first negative electrode material layer has better conductivity, which is beneficial for the secondary battery to have better rate performance. Simultaneously, it helps to better reduce heat during short circuits, better mitigate heat accumulation during short circuits, further reduce the risk of thermal runaway in the secondary battery, and further improve the battery's penetration performance. Thus, while maintaining good rate performance, the safety performance of the secondary battery is further improved.

[0013] In one or more embodiments of this application, the first negative electrode material layer includes a first binder, and the second negative electrode material layer includes a second binder. The first and second binders each independently include at least one of styrene-butadiene rubber, polyacrylic acid, sodium carboxymethyl cellulose, polyvinylidene fluoride, polyethylene oxide, or polyimide. By controlling the types of the first and second binders within the aforementioned range, the first and second negative electrode material layers exhibit better adhesion properties, which helps to reduce the risk of negative electrode material layer detachment, thereby improving the safety performance of the secondary battery.

[0014] In one or more embodiments of this application, the mass percentage of the first binder is W3 based on the mass of the first negative electrode material layer; the mass percentage of the second binder is W4 based on the mass of the second negative electrode material layer, where 1% ≤ W4 ≤ 5%, 1.5 ≤ W3 / W4 ≤ 3, and correspondingly, 1.5% ≤ W3 ≤ 15%. By adjusting the values ​​of W3 and W4 to satisfy the above characteristics, the first negative electrode material layer has a large adhesive force, which is beneficial to improving the bonding performance of the first negative electrode material layer, enabling the components of the negative electrode sheet to produce a better bonding effect, reducing the risk of the negative electrode material layer falling off, improving manufacturability, and thus improving the safety performance of the secondary battery.

[0015] In one or more embodiments of this application, the first negative electrode material layer further includes a solid electrolyte, and the mass percentage of the solid electrolyte in the first negative electrode material layer is W based on the mass of the first negative electrode material layer. C W C <W A 0%≤W C ≤10%. When the first negative electrode material layer includes a solid electrolyte, the content of the solid electrolyte in the first negative electrode material layer is less than that in the second negative electrode material layer. By adjusting the first negative electrode material layer to meet the above characteristics, the first negative electrode material layer can also include a solid electrolyte, so that the first negative electrode material layer has better lithium-ion transport efficiency while reducing the risk of internal short circuit. It can also distinguish the content of solid electrolyte in the first negative electrode material layer and the second negative electrode material layer, so that the film resistance of the second negative electrode material layer is greater than that of the first negative electrode material layer. This can better mitigate the heat accumulation during the short circuit process and reduce the risk of thermal runaway of the secondary battery, improve the penetration performance of the secondary battery, and thus improve the safety performance of the secondary battery.

[0016] A second aspect of this application provides an electronic device that includes the secondary battery found in any of the foregoing embodiments. Therefore, the electronic device provided by this application has good performance.

[0017] The beneficial effects of this application are:

[0018] This application provides a secondary battery and electronic device, including a negative electrode sheet. The negative electrode sheet includes a negative current collector and a negative electrode material layer disposed on at least one surface of the negative current collector. The negative electrode material layer includes a first negative electrode material layer and a second negative electrode material layer. Along the thickness direction of the negative electrode sheet, the first negative electrode material layer is disposed between the negative current collector and the second negative electrode layer. The first negative electrode material layer includes a first negative electrode active material, and the second negative electrode layer includes a solid electrolyte and a second negative electrode active material. The film resistance of the first negative electrode material layer is R. A Ω, the film resistance of the second negative electrode material layer is R B Ω, 0.005≤R B ≤1, 2≤R B / R A ≤450. The structure of the negative electrode sheet meets the characteristics of this application, and the impedance range of the second negative electrode material layer and the ratio of the impedance of the second negative electrode material layer to the impedance of the first negative electrode material layer are within the above range. The secondary battery has good conductivity and can improve the pass rate of the secondary battery's nail penetration test. By adjusting the secondary battery to meet the above characteristics, the safety performance of the secondary battery can be improved while maintaining good rate performance.

[0019] 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

[0020] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other embodiments can be obtained based on these accompanying drawings.

[0021] Figure 1 This is a cross-sectional view of the negative electrode sheet in the thickness direction according to one embodiment of this application.

[0022] Reference numerals: negative electrode 001, negative current collector 11, negative electrode material layer 12, first negative electrode material layer 121, second negative electrode material layer 122. Detailed Implementation

[0023] The technical solutions of this application will be clearly and completely described below with reference to the embodiments of this application. 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.

[0024] 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:

[0025] This application provides a secondary battery, including a negative electrode sheet. The negative electrode sheet includes a negative current collector and a negative electrode material layer disposed on at least one surface of the negative current collector. The negative electrode material layer includes a first negative electrode material layer and a second negative electrode material layer. Along the thickness direction of the negative electrode sheet, the first negative electrode material layer is disposed between the negative current collector and the second negative electrode layer. The first negative electrode material layer includes a first negative electrode active material, and the second negative electrode layer includes a solid electrolyte and a second negative electrode active material. The film resistance of the first negative electrode material layer is R. A Ω, the film resistance of the second negative electrode material layer is R B Ω, 0.005≤R B ≤1. For example, R B The value of R can be 0.005, 0.01, 0.03, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, or a range consisting of any two of the above values. B The value range of can be 0.005 to 1, 0.01 to 0.9, 0.03 to 0.8, 0.05 to 0.7, 0.1 to 0.6, and all of these ranges, as well as subranges. 2≤R B / R A ≤450. For example, R B / R A The value can be 2, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 200, 300, 400, 450, or a range of any two of the above values, R. B / R A The value range can be 2 to 450, 5 to 400, 10 to 300, 20 to 200, 30 to 100, and all of these ranges, as well as subranges.

[0026] For ease of understanding, in this application, the length direction of the negative electrode sheet is defined as the X direction, and the thickness direction is defined as the Z direction. It should be understood that the above definitions of directions are for the purpose of conveniently describing this application, and the directions defined in this application can be understood based on the relative positions of the elements in the accompanying drawings and actual product. Furthermore, the length and thickness directions of the negative electrode current collector, the negative electrode material layer, the first negative electrode material layer, and the second negative electrode material layer are the same as those of the negative electrode sheet. Figure 1As shown, the negative electrode sheet 001 includes a negative electrode current collector 11 and a negative electrode material layer 12 disposed on two surfaces of the negative electrode current collector 11. The negative electrode material layer 12 includes a first negative electrode material layer 121 and a second negative electrode material layer 122. Along the thickness direction Z of the negative electrode sheet 001, the first negative electrode material layer 121 is disposed between the negative electrode current collector 11 and the second negative electrode material layer 122. Of course, it is understood that in some embodiments of this application, the first negative electrode material layer 121 and the second negative electrode material layer 122 may be disposed on only one surface of the negative electrode current collector 11.

[0027] With the widespread adoption of lithium-ion batteries, their safety performance has become increasingly important. Among the various safety tests for lithium-ion batteries, the nail penetration test, which simulates the extreme short-circuit conditions, effectively assesses their safety performance. During the nail penetration test, the needle punctures the battery, causing a short circuit. The short-circuit current passes through the needle and the battery's internal structure, generating ohmic heat. This accumulated heat causes a rapid rise in the secondary battery's temperature, potentially leading to thermal runaway. Furthermore, because conventional negative electrode materials have low impedance and the active graphite material has slow heat dissipation, the more heat generated by the nail penetration and the worse the heat dissipation, the higher the risk of thermal runaway. Therefore, this application designs the negative electrode sheet by dividing the negative electrode material layer into a first negative electrode material layer and a second negative electrode material layer. The first negative electrode material layer is disposed between the negative electrode current collector and the second negative electrode material layer. A solid electrolyte is added to the second negative electrode material layer. The solid electrolyte has good lithium-ion transport performance and electronic insulation performance, enabling the second negative electrode material layer to have good lithium-ion transport efficiency while effectively reducing the risk of internal short circuits, thereby reducing the film resistance R of the second negative electrode material layer. B The film resistance R is greater than that of the first negative electrode material layer. A , specifically, 0.005≤R B ≤1, 2≤R B / R A ≤450. A negative electrode sheet meeting the above characteristics exhibits high contact resistance at the junction of the steel needle and the second negative electrode material layer with high diaphragm resistance during the pin-through test. This effectively reduces heat generated during short circuits. The diaphragm resistance R of the second negative electrode material layer... B Within the above range, the effect on reducing heat during short circuit is significant, when R B When R is too large, the impedance of the secondary battery increases, resulting in poor conductivity and a decrease in rate performance; when R... B When the voltage is too low, the instantaneous current of the secondary battery is large during the short circuit process through the pin, resulting in poor safety performance. By adjusting R... B / R ABy controlling the value within the aforementioned range, the film resistance of the first and second negative electrode material layers is differentiated and appropriately sized. This increases the surface impedance of the negative electrode, effectively mitigating heat accumulation during short circuits and reducing the risk of thermal runaway in the secondary battery. Furthermore, the negative electrode exhibits good conductivity, resulting in excellent rate performance for the secondary battery. Therefore, by adjusting the secondary battery to meet these characteristics, it achieves good conductivity while simultaneously improving its resistance to pin penetration, thus enhancing both rate performance and safety.

[0028] In one or more embodiments of this application, 0.05 ≤ R B ≤0.2. For example, R B The value of R can be 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.12, 0.14, 0.16, 0.18, 0.2, or a range consisting of any two of the above values. B The value range can be 0.05 to 0.2, 0.06 to 0.18, 0.07 to 0.16, 0.09 to 0.14, and all of these ranges, as well as sub-ranges. This can be achieved by adjusting R... B The value satisfies the above range, so that the film resistance of the second negative electrode material layer has a more suitable range, which is conducive to better reducing heat during the short circuit process, better mitigating heat accumulation during the short circuit process, further reducing the risk of thermal runaway of the secondary battery, further improving the pin penetration performance of the secondary battery, and the secondary battery has better conductivity, thereby further improving the safety performance of the secondary battery while taking into account better rate performance.

[0029] In one or more embodiments of this application, 0.0015 ≤ R A ≤0.12. For example, R A The value of R can be 0.0015, 0.002, 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.12, or a range consisting of any two of the above values. A The value range can be 0.0015 to 0.12, 0.002 to 0.1, 0.005 to 0.09, 0.01 to 0.08, 0.02 to 0.07, and all ranges and subranges thereof. (This is related to R...) B Correspondingly, R A The value satisfies the above range, the film resistance of the first negative electrode material layer has a suitable range, the negative electrode sheet has good conductivity, the secondary battery has good rate performance, and it is also beneficial to further limit R. BThis range further reduces the risk of thermal runaway in secondary batteries and improves the penetration performance of secondary batteries, thereby enhancing the safety performance of secondary batteries while also achieving better rate performance.

[0030] In one or more embodiments of this application, the mass percentage of the solid electrolyte is W based on the mass of the second negative electrode material layer. A 1%≤W A ≤30%. For example, W A The value can be 1%, 3%, 5%, 7%, 9%, 10%, 15%, 20%, 25%, 30%, or a range of any two of the above values, W. A The value range can be 1% to 30%, 3% to 25%, 5% to 20%, 7% to 15%, and all of these ranges, as well as sub-ranges. This is achieved by adjusting W... A When the value is within the above range, the second negative electrode material layer contains a suitable mass percentage of solid electrolyte, which can reduce the preparation cost and make the second negative electrode material layer have a suitable film resistance. This can better reduce the risk of the secondary battery's rate performance being reduced due to excessive film resistance, and at the same time help improve the secondary battery's penetration performance, thereby improving the safety performance of the secondary battery while taking into account good rate performance.

[0031] In one or more embodiments of this application, the second negative electrode active material includes at least one of graphite or silicon-based materials, and the solid electrolyte includes at least one of oxide-based solid electrolyte, sulfide-based solid electrolyte, or polymer-based solid electrolyte. By controlling the types of each component in the second negative electrode material layer within the above-mentioned range, the negative electrode sheet can have good conductivity, while the second negative electrode material layer has suitable film resistance. This effectively reduces the risk of reduced rate performance of the secondary battery due to excessive film resistance, improves the penetration performance of the secondary battery, and thus improves the safety performance of the secondary battery while maintaining good rate performance.

[0032] In one or more embodiments of this application, the solid electrolyte includes at least one of oxide-based solid electrolyte, sulfide-based solid electrolyte, or polymer-based solid electrolyte. Specifically, the oxide-based solid electrolyte may include at least one of lithium lanthanum titanate, lithium lanthanum zirconium oxide, or aluminum-doped lithium lanthanum zirconium oxide; the sulfide-based solid electrolyte may include at least one of lithium silicon phosphide sulfide, lithium germanium phosphide sulfide, or lithium tin phosphide sulfide; and the polymer-based solid electrolyte may include at least one of polyethylene oxide, polyacrylonitrile, or polyvinylidene fluoride. By controlling the type of solid electrolyte within the above range, the solid electrolyte can have better lithium-ion transport performance and better electronic insulation performance, better reducing the risk of internal short circuits, improving the penetration performance of the secondary battery, and thus improving the safety performance of the secondary battery.

[0033] In one or more embodiments of this application, the first negative electrode material layer includes a first conductive agent, and the second negative electrode material layer includes a second conductive agent. The first and second conductive agents each independently include at least one of carbon nanotubes, conductive carbon black (SuperP), or graphene. By controlling the types of the first and second conductive agents within the aforementioned range, the negative electrode material layer exhibits good conductivity, enabling the secondary battery to have good rate performance.

[0034] In one or more embodiments of this application, the mass percentage of the first conductive agent is W1 based on the mass of the first negative electrode material layer, and the mass percentage of the second conductive agent is W2 based on the mass of the second negative electrode material layer, where 0.4% ≤ W1 ≤ 2%. For example, the value of W1 can be 0.4%, 0.56%, 0.72%, 0.88%, 1.04%, 1.2%, 1.36%, 1.52%, 1.68%, 1.84%, 2%, or a range consisting of any two of the above values. The value range of W1 can be 0.4% to 2%, 0.56% to 1.84%, 0.72% to 1.68%, 0.88% to 1.52%, 1.04% to 1.36%, and all such ranges and sub-ranges. 0 ≤ W2 / W1 ≤ 0.7. For example, the value of W2 / W1 can be 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, or a range consisting of any two of the above values. The range of W2 / W1 can be 0 to 0.7, 0.1 to 0.6, 0.2 to 0.5, 0.3 to 0.4, and all such ranges and sub-ranges. When the mass percentage of the second conductive agent is less than the mass percentage of the first conductive agent, or even zero, the conductivity of the second negative electrode material layer is weak, and the film resistance of the second negative electrode material layer is relatively high. By adjusting the values ​​of W1 and W2 / W1 to meet the above characteristics, on the one hand, the conductive agent in the first negative electrode material layer can have a suitable mass percentage content, resulting in better conductivity. This is beneficial for the secondary battery to have better rate performance and can effectively alleviate the lithium plating problem caused by poor kinetic performance due to insufficient conductive agent in the negative electrode sheet. At the same time, it helps to reduce the risk of low energy density in the secondary battery due to excessive conductive agent. On the other hand, it can further distinguish the film resistance of the first and second negative electrode material layers, allowing the film resistance of the second negative electrode material layer to have a more suitable range. This helps to better reduce heat during short circuits, better mitigate heat accumulation during short circuits, further reduce the risk of thermal runaway in the secondary battery, and further improve the penetration performance of the secondary battery. Thus, while ensuring good rate performance, it further improves the safety performance of the secondary battery.

[0035] In one or more embodiments of this application, 0 ≤ W2 ≤ 1.4%, preferably, 0 ≤ W2 ≤ 0.4%. For example, the value of W2 can be 0, 0.2%, 0.4%, 0.6%, 0.8%, 1.0%, 1.2%, 1.4%, or a range consisting of any two of the above values. The value range of W2 can be 0 to 1.4%, 0.2 to 1.2%, 0.4 to 1.0%, 0.6% to 0.8%, and all such ranges and sub-ranges. Corresponding to W1, the value of W2 satisfies the above range. The conductive agent in the second negative electrode material layer has a suitable mass percentage content, the second negative electrode material layer has good conductivity, and the secondary battery has good rate performance. This is beneficial for the secondary battery to have good kinetic performance, alleviate the lithium plating problem of the negative electrode sheet, and reduce the risk of low energy density of the secondary battery due to excessive conductive agent. At the same time, the film resistance of the second negative electrode material layer has a more suitable range, which is beneficial for better reducing heat during the short circuit process, better mitigating heat accumulation during the short circuit process, further reducing the risk of thermal runaway of the secondary battery, and further improving the penetration performance of the secondary battery. Thus, while taking into account good rate performance, the safety performance of the secondary battery is further improved.

[0036] In one or more embodiments of this application, the first negative electrode material layer includes a first binder, and the second negative electrode material layer includes a second binder. The first and second binders each independently include at least one of styrene-butadiene rubber, polyacrylic acid, sodium carboxymethyl cellulose, polyvinylidene fluoride, polyethylene oxide, or polyimide. By controlling the types of the first and second binders within the aforementioned range, the first and second negative electrode material layers exhibit better adhesion properties, which helps to reduce the risk of negative electrode material layer detachment, thereby improving the safety performance of the secondary battery.

[0037] In one or more embodiments of this application, the mass percentage of the first binder is W3 based on the mass of the first negative electrode material layer; the mass percentage of the second binder is W4 based on the mass of the second negative electrode material layer, where 1% ≤ W4 ≤ 5%. For example, the value of W4 can be 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or a range consisting of any two of the above values. The range of W4 can be 1% to 5%, 1.5% to 4.5%, 2% to 4%, 2.5% to 3.5%, and all such ranges and sub-ranges. 1.5 ≤ W3 / W4 ≤ 3. For example, the value of W3 / W4 can be 1.5, 1.65, 1.8, 1.95, 2.1, 2.25, 2.4, 2.55, 2.7, 2.85, 3, or a range consisting of any two of the above values. The range of W3 / W4 values ​​can be 1.5 to 3, 1.65 to 2.85, 1.8 to 2.7, 1.95 to 2.55, 2.25 to 2.4, and all such ranges and sub-ranges. The mass percentage content of the first adhesive is greater than that of the second adhesive, meaning that the first negative electrode material layer has greater adhesion compared to the second negative electrode material layer. By adjusting the values ​​of W4 and W3 / W4 to meet the above characteristics, on the one hand, the first negative electrode material layer can have a more suitable adhesive force, which is beneficial to improve the adhesive performance of the first negative electrode material layer, so that the components of the negative electrode sheet can produce a better adhesive effect, and the risk of the negative electrode material layer falling off can be reduced. At the same time, the risk of low energy density of secondary battery due to excessive binder can be reduced. On the other hand, manufacturability can be improved, thereby improving the safety performance of secondary battery.

[0038] In one or more embodiments of this application, 1.5% ≤ W3 ≤ 15%, and optionally, 2% ≤ W3 ≤ 5%. For example, the value of W3 can be 1.5%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, or a range consisting of any two of the above values. The value range of W3 can be 1.5% to 15%, 2% to 14%, 3% to 13%, 4% to 14%, and all of these ranges, as well as sub-ranges. Corresponding to W4, the value of W3 satisfies the above range, and the second negative electrode material layer has suitable adhesion, which is beneficial to the formation of a better adhesion effect among the components of the negative electrode sheet, and can better reduce the risk of the negative electrode material layer falling off. At the same time, it can reduce the risk of low energy density of the secondary battery due to excessive binder, thereby improving the safety performance of the secondary battery.

[0039] In one or more embodiments of this application, the first negative electrode material layer further includes a solid electrolyte, and the mass percentage of the solid electrolyte in the first negative electrode material layer is W based on the mass of the first negative electrode material layer. C W C <W A0%≤W C ≤10%. For example, W C The value can be 0%, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a range consisting of any two of the above values, W. C The value range can be 0% to 10%, 0.1% to 9%, 0.5% to 8%, 1% to 7%, 2% to 6%, and all of these ranges, as well as sub-ranges. When the first negative electrode material layer includes a solid electrolyte, the content of the solid electrolyte in the first negative electrode material layer is less than that in the second negative electrode material layer. By adjusting the first negative electrode material layer to meet the above characteristics, the first negative electrode material layer can also include a solid electrolyte, so that the first negative electrode material layer has better lithium-ion transport efficiency while better reducing the risk of internal short circuits. It can also distinguish the content of solid electrolytes in the first and second negative electrode material layers, so that the film resistance of the second negative electrode material layer is greater than that of the first negative electrode material layer. This can better mitigate the heat accumulation during the short circuit process and reduce the risk of thermal runaway of the secondary battery, improve the penetration performance of the secondary battery, and thus improve the safety performance of the secondary battery.

[0040] In this application, the first negative electrode material layer may further include a solid electrolyte, which may include at least one of an oxide-based solid electrolyte, a sulfide-based solid electrolyte, or a polymer-based solid electrolyte. Specifically, the oxide-based solid electrolyte includes at least one of lithium lanthanum titanate, lithium lanthanum zirconium oxide, or aluminum-doped lithium lanthanum zirconium oxide; the sulfide-based solid electrolyte includes at least one of lithium silicon phosphide sulfide, lithium germanium phosphide sulfide, or lithium tin phosphide sulfide.

[0041] The polymer-based solid electrolyte includes at least one of polyethylene oxide, polyacrylonitrile, or polyvinylidene fluoride. The type of solid electrolyte in the first negative electrode material layer may be the same as or different from the type of solid electrolyte in the second negative electrode material layer.

[0042] In this application, R can be controlled by adjusting the content of solid electrolyte, conductive agent, or binder in the first negative electrode material layer. A For example, when other conditions remain unchanged, as the content of solid electrolyte increases, R... A Increase; the content of solid electrolyte decreases, R A Decrease. When other conditions remain constant, an increase in the content of the conductive agent results in a decrease in R. A Decrease; decrease in the content of conductive agent, R A Increase. When other conditions remain unchanged, as the binder content increases, R... A Increase; binder content decreases, R A Decrease.

[0043] In this application, R can be controlled by adjusting the content of solid electrolyte, conductive agent, or binder in the second negative electrode material layer. B For example, when other conditions remain unchanged, as the content of solid electrolyte increases, R... B Increase; the content of solid electrolyte decreases, R B Decrease. When other conditions remain constant, an increase in the content of the conductive agent results in a decrease in R. B Decrease; decrease in the content of conductive agent, R B Increase. When other conditions remain unchanged, as the binder content increases, R... B Increase; binder content decreases, R B Decrease.

[0044] This application does not impose any particular restrictions 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 collector. For example, the composite current collector may be lithium copper composite current collector, carbon copper composite current collector, nickel copper composite current collector, titanium copper composite current collector, etc.

[0045] In this application, there is no particular limitation on the type of silicon-based material, as long as it can achieve the purpose of this application. For example, silicon-based materials include at least one of elemental silicon, silicon boride, silicon nitride, silicon carbide, silicon oxide, and silicon alloys. Among them, silicon carbide includes silicon-carbon composites (Si / C) and / or silicon carbide (SiC). This application does not have a particular limitation on silicon-carbon composites, as long as they can achieve the purpose of this application; for example, silicon-carbon composites can be composites obtained by deposition; exemplaryly, silicon-carbon composites can be silicon materials deposited on a carbon skeleton, or carbon materials deposited on a silicon skeleton. Silicon oxides include SiOx, where 0 < x < 2; exemplaryly, silicon oxides can include silicon suboxide (SiO, where the molar ratio of silicon to oxygen is 1:1). Silicon alloys include at least one of silicon-tin alloys, silicon-germanium alloys, silicon-magnesium alloys, and silicon-titanium alloys.

[0046] In this application, the first negative electrode material layer includes a first negative electrode active material. This application does not impose any particular limitation on the first negative electrode active material, as long as it can achieve the purpose of this application. For example, the first negative electrode active material may include, but is not limited to, natural graphite, artificial graphite, mesophase microcarbon spheres, hard carbon, soft carbon, silicon, silicon-carbon composites, Li-Sn alloys, Li-Sn-O alloys, Sn, SnO, SnO2, and spinel-structured lithiated TiO2-Li4Ti5O. 12 Or at least one of Li-Al alloys.

[0047] This application does not impose any particular limitation on the thickness of the negative electrode current collector, as long as it can achieve the purpose of this application. For example, the thickness of the negative electrode current collector can be 3 μm to 15 μm.

[0048] Optionally, the negative electrode sheet may further include a conductive layer located between the negative electrode current collector and the negative electrode material layer. This application does not impose any particular limitation on the composition of the conductive layer, which can be a commonly used conductive layer in the art. For example, the conductive layer includes a conductive agent and a binder. This application does not impose any particular limitation on the conductive agent and binder in the conductive layer. For example, the binder may include at least one of polyacrylate, polyimide, polyamide, polyamide-imide, polyvinylidene fluoride, polystyrene-butadiene copolymer (styrene-butadiene rubber), sodium alginate, polyvinyl alcohol, polytetrafluoroethylene, polyacrylonitrile, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, sodium carboxymethyl cellulose, or potassium carboxymethyl cellulose. The conductive agent may include at least one of conductive carbon black, carbon nanotubes (CNTs), carbon fibers, flake graphite, or graphene. The conductive carbon black may be at least one of acetylene black, Super P, or Ketjen black. The carbon nanotubes may be at least one of single-walled carbon nanotubes or multi-walled carbon nanotubes. The carbon fibers may be at least one of vapor-grown carbon fibers (VGCF) or carbon nanofibers.

[0049] In this application, the secondary battery further includes a positive electrode sheet, which includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. The phrase "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 thickness direction, or on two surfaces of the positive current collector along its thickness direction. It should be noted that the term "surface" here can refer to the entire surface area of ​​the positive current collector, or only a portion thereof; this application does not impose any particular limitation, as long as the purpose of this application is achieved.

[0050] This application does not impose any particular restrictions on the positive electrode current collector, as long as it can achieve the purpose of this application. For example, it may include aluminum foil, aluminum alloy foil, or composite current collector (such as aluminum-carbon composite current collector).

[0051] The positive electrode material layer includes a positive electrode active material. This application does not impose any particular restrictions on the positive electrode active material, as long as it can achieve the purpose of this application. For example, the positive electrode active material may include, but is not limited to, lithium nickel cobalt manganese oxide (e.g., NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide (LiCoO2), lithium manganese oxide, lithium manganese iron phosphate, or lithium titanate.

[0052] The positive electrode material layer may also include a conductive agent and a binder. This application does not impose any particular restrictions on the types of conductive agents and binders, as long as they achieve the purpose of this application. For example, it may include at least one of the aforementioned conductive agents and binders. This application does not impose any particular restrictions on the mass ratio of the positive electrode active material, conductive agent, and binder in the positive electrode material layer. Those skilled in the art can select according to actual needs, as long as the purpose of this application is achieved.

[0053] This application does not impose any 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 can be achieved. For example, the thickness of the positive electrode current collector can be 5 μm to 20 μm, and the thickness of the single-sided positive electrode material layer can be 30 μm to 120 μm.

[0054] Optionally, the positive electrode may further include a conductive layer located between the positive current collector and the positive electrode material layer. The composition of the conductive layer is not particularly limited and can be any conductive layer commonly used in the art. The conductive layer includes a conductive agent and a binder. This application does not impose any particular limitation on the conductive agent and binder in the conductive layer; for example, it can be at least one of the aforementioned conductive agents and binders.

[0055] In this application, the secondary battery also includes an electrolyte, which includes lithium salts and non-aqueous solvents.

[0056] 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 also does not impose any particular limitation on the content of the lithium salt in the electrolyte, as long as it achieves the purpose of this application.

[0057] This application does not impose any particular restrictions on non-aqueous solvents, as long as they can achieve the purpose of this application. For example, non-aqueous solvents may include, but are not limited to, at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents.

[0058] 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). Fluorocarbonate 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. Other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methylsulfolane, 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.

[0059] In this application, the secondary battery also includes a separator. This application does not impose any particular limitation on the separator, as long as it achieves the purpose of this application. For example, the separator material may include, but is not limited to, at least one of polyethylene (PE), polyolefins (PO) primarily composed of polypropylene (PP), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid. The separator type may include at least one of woven membrane, nonwoven membrane, microporous membrane, composite membrane, rolled membrane, or spun membrane.

[0060] In one or more embodiments of this application, the diaphragm may include a substrate layer and a surface treatment layer. The substrate layer may be a nonwoven fabric or composite membrane with a porous structure, and the material of the substrate layer may include at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Optionally, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane may be used.

[0061] Optionally, a surface treatment layer is provided on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by mixing polymers and inorganic substances.

[0062] In one or more embodiments of this application, the inorganic layer comprises inorganic particles and a binder. This application does not particularly limit the inorganic particles; for example, the inorganic particles may include at least one selected from alumina, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. This application does not particularly limit the binder; for example, the binder may be at least one of the binders described above. In one or more embodiments of this application, the polymer layer comprises a polymer, the polymer material of which includes at least one selected from polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, or polyvinylidene fluoride or poly(vinylidene fluoride-hexafluoropropylene).

[0063] In this application, there is no particular limitation on the thickness of the diaphragm, as long as it can achieve the purpose of this application. For example, the thickness of the diaphragm can be from 3 μm to 30 μm.

[0064] The secondary battery of this application also includes a packaging bag for containing the positive electrode, separator, negative electrode, and electrolyte, as well as other components known in the art for secondary batteries. This application does not limit the aforementioned other components. This application does not have any particular limitation on the packaging bag; it can be any packaging bag known in the art, as long as it can achieve the purpose of this application.

[0065] This application does not impose any particular limitation on the type of secondary battery, which may include any device in which an electrochemical reaction occurs. In this application, the secondary battery may include, but is not limited to: lithium metal secondary batteries, lithium-ion secondary batteries (lithium-ion batteries), lithium polymer secondary batteries, or lithium-ion polymer secondary batteries (lithium-ion polymer batteries), etc.

[0066] The preparation process of the secondary battery described in this application is well known to those skilled in the art, and this application has no particular limitations. For example, it may include, but is not limited to, the following steps: stacking the positive electrode, separator, and negative electrode in sequence, and performing operations such as winding and folding as needed to obtain a wound electrode assembly; placing the electrode assembly in a packaging bag; injecting electrolyte into the packaging bag and sealing it to obtain a secondary battery; or stacking the positive electrode, separator, and negative electrode 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 in a packaging bag; injecting electrolyte into the packaging bag and sealing it to obtain a secondary battery. In addition, overcurrent protection components, conductive plates, etc., may be placed in the packaging bag as needed to prevent the internal pressure of the secondary battery from rising and overcharging / discharging.

[0067] A second aspect of this application provides an electronic device that includes the secondary battery found in any of the foregoing embodiments. Therefore, the electronic device provided by this application has good performance.

[0068] This application does not specifically limit the type of electronic device; it can be any electronic device known in the prior art. In one or more embodiments of this application, 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, large household batteries, and lithium-ion capacitors, etc.

[0069] Example

[0070] 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.

[0071] Test methods and equipment:

[0072] Solid electrolyte content test

[0073] The lithium-ion battery was discharged at a constant current of 0.5C to the discharge cutoff voltage at 25℃. The battery was then disassembled under an argon atmosphere, and the negative electrode sheet was removed. The negative electrode sheet was then soaked in dimethyl carbonate solvent for 2 hours and dried at 60℃ for 1 hour to obtain the negative electrode sheet. Conductive carbon adhesive was used to fix the negative electrode sheet onto the sample stage, which was then placed on a support. The support and sample stage were placed in a cross-section polishing apparatus (CP), and a vacuum of 10... -4 After Pa, argon gas was used to cut along the thickness direction of the negative electrode sheet. The cut sample was placed cross-section-up on the SEM stage and observed using a Philips XL-30 field emission scanning electron microscope (SEM). The accelerating voltage was 10 kV, the emission current was 10 mA, and the magnification was 3000x. The mass percentage of the corresponding element in the solid electrolyte in each negative electrode material layer was measured using the energy dispersive X-ray spectroscopy (EDS) provided with the SEM. The mass percentage W of the solid electrolyte in the first negative electrode material layer was obtained. A The mass percentage of solid electrolyte in the second negative electrode material layer, W C .

[0074] Negative electrode material film resistance test

[0075] Take the prepared negative electrode sheet and cut it into test samples with dimensions of 60mm × 80mm. Using a film resistance meter (model Yuaneng Technology BER2500), test pressure of 0.4T and holding time of 10s, the area of ​​the second negative electrode material layer is measured to be 1540mm². 2 After testing a circular area, the test sample was moved and tested again with the same parameters on another area of ​​1540 mm². 2 The resistance R of the second negative electrode material film layer was obtained by repeating the process 5 times in a circular region. B After scraping away the second negative electrode material layer, a negative electrode sample with the first negative electrode material layer is obtained. The film resistance R of the first negative electrode material layer is then tested according to the above testing steps. A .

[0076] Piercing test

[0077] Ten lithium-ion batteries were taken from each group prepared in the examples or comparative examples and fully charged at 25°C. The specific steps were as follows: constant current charging at 0.5C to 4.2V, and then constant voltage charging at 4.2V to 0.05C current cutoff.

[0078] Then, a nail penetration test was conducted on a fully charged lithium-ion battery at 25℃ using a 5mm diameter carbon steel nail with a 45° cone angle and a total length of 100mm. The penetration speed was set to 15mm / s, and the penetration depth was determined by the nail's cone angle penetrating the lithium-ion battery. The nail was retained and observed for 1 hour. The state of the lithium-ion battery was observed during the test, with the criterion being that the lithium-ion battery did not burn or explode. The nail penetration test pass rate = number of nail penetration tests passed / 10.

[0079] 2C Rate Capacity Retention Test

[0080] At 25℃, the lithium-ion battery was charged at a constant current of 0.5C to 4.2V, then charged at a constant voltage of 4.2V to 0.05C, allowed to stand for 30 minutes, and then discharged at a constant current of 0.2C to 2.8V. After standing for 2 hours, the discharge capacity C0 was recorded. The lithium-ion battery was then charged at a constant current of 0.5C to 4.2V, then charged at a constant voltage of 4.2V to 0.05C, allowed to stand for 30 minutes, and then discharged at a constant current of 2C to 2.8V. After standing for 2 hours, the discharge capacity C1 was recorded. The 2C rate capacity retention rate (%) = (C1 - C0) × 100%.

[0081] Example 1

[0082] <Preparation of the positive electrode>

[0083] The positive electrode active materials LiFePO4 and LiNi 0.8 Co 0.1 Mn 0.1 O2, conductive agent Super P, conductive agent carbon nanotubes, and binder polyvinylidene fluoride were mixed in a mass ratio of 19:78:0.8:0.7:1.5. N-methylpyrrolidone (NMP) was added as a solvent to prepare a slurry with a solid content of 75 wt%. After vacuum stirring, a positive electrode slurry was obtained. The positive electrode slurry was uniformly coated onto one surface of a 10 μm thick aluminum foil used as a positive electrode current collector and dried at 120°C to obtain a single-sided coated positive electrode sheet. The coating weight of the positive electrode material layer was 267.8 mg / 1540 mm². 2 The above steps are then repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of positive electrode material. After drying at 120℃, it is cold-pressed, then cut and welded with tabs to obtain a positive electrode sheet with a size of 74mm×867mm for later use. The thickness of the single-sided positive electrode material layer is 42μm.

[0084] <Preparation of Negative Electrode Sheets>

[0085] The first negative electrode active material, artificial graphite, binder sodium carboxymethyl cellulose, binder styrene-butadiene rubber, and conductive agent Super P were mixed in a mass ratio of 94.2:2.4:2.4:1. Deionized water was added as a solvent to prepare a slurry with a solid content of 45wt%. After being stirred evenly in a vacuum mixer, the first negative electrode material layer slurry was obtained.

[0086] The second negative electrode active material is artificial graphite, and the solid electrolyte is lithium lanthanum titanate (LLTO, Li). 0.33 La 0.57 TiO3), sodium carboxymethyl cellulose binder, styrene-butadiene rubber binder, and Super P conductive agent were mixed in a mass ratio of 87.2:10:1.2:1.2:0.4. Deionized water was added as a solvent to prepare a slurry with a solid content of 45wt%. After being stirred evenly in a vacuum mixer, the second negative electrode material layer slurry was obtained.

[0087] The first negative electrode material layer slurry was uniformly coated onto one surface of a 6μm thick copper foil used as a negative electrode current collector, and dried at 120℃ to obtain a semi-finished negative electrode sheet with a single-sided coating of the first negative electrode material layer. The second negative electrode material layer slurry was then uniformly coated onto the first negative electrode material layer and dried at 120℃ to obtain a semi-finished negative electrode sheet with a single-sided double-layer coating. The coating weight of the first negative electrode material layer was 142mg / 1540mm². 2 The coating weight of the second negative electrode material layer is 142 mg / 1540 mm². 2 The above steps are then repeated on the other surface of the copper foil to obtain a double-sided, double-layer coated negative electrode sheet. After drying at 120℃, it is cold-pressed, then cut and welded with tabs to obtain a negative electrode sheet with dimensions of 78mm × 875mm for later use. The thickness of the first negative electrode material layer on one side is 27.2μm, and the thickness of the second negative electrode material layer on one side is 27.2μm.

[0088] <Preparation of Electrolyte>

[0089] In an environment with a water content of less than 10 ppm, dimethyl carbonate, diethyl carbonate, and ethylene carbonate were mixed in a mass ratio of 1:1:1 to obtain an organic solvent. Then, the electrolyte salt LiPF6 was added to the organic solvent and mixed thoroughly to obtain the electrolyte. Based on the mass of the electrolyte, the electrolyte salt comprised 12.5% ​​by mass, with the remainder being the organic solvent.

[0090] <Septum>

[0091] A porous polyethylene film with a thickness of 7μm (provided by Celgard) was used as the separator.

[0092] <Preparation of Lithium-ion Batteries>

[0093] The prepared positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes to provide insulation. The electrode assembly is then wound to obtain the electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag and dehydrated at 80°C. The electrolyte prepared above is then injected, and the battery undergoes vacuum sealing, settling, formation, degassing, and edge trimming to obtain a lithium-ion battery. The formation upper limit voltage is 4.15V, the formation temperature is 70°C, and the formation settling time is 2 hours.

[0094] Examples 2 to 27

[0095] Except for adjusting the relevant preparation parameters according to Table 1, everything else is the same as in Example 1.

[0096] Comparative Example 1

[0097] Except for adjusting the <Preparation of Negative Electrode Sheet> process according to the following steps, the rest is the same as in Example 1.

[0098] <Preparation of Negative Electrode Sheets>

[0099] Artificial graphite (negative electrode active material), sodium carboxymethyl cellulose (binder), styrene-butadiene rubber (binder), and Super P (conductive agent) were mixed in a mass ratio of 94.2:2.4:2.4:1. Deionized water was added as a solvent to prepare a slurry with a solid content of 45 wt%. The slurry was stirred evenly in a vacuum mixer to obtain the negative electrode material layer slurry. The negative electrode material layer slurry was uniformly coated onto one surface of a 6 μm thick copper foil used as a negative electrode current collector and dried at 120°C to obtain a semi-finished negative electrode sheet with a single-sided negative electrode material layer. The coating weight of the negative electrode material layer was 142 mg / 1540 mm². 2 Then, the above steps are repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided coating of negative electrode material. After drying at 120℃, it is cold-pressed, then cut and welded with tabs to obtain a negative electrode sheet with a size of 78mm×875mm for use. The thickness of the single-sided negative electrode material layer is 54.5μm.

[0100] Comparative Examples 2 to 4

[0101] Except for adjusting the relevant preparation parameters according to Table 1, everything else is the same as in Example 1.

[0102] Table 1

[0103] Table 2

[0104] As can be seen from Examples 1 to 27 and Comparative Examples 1 to 4, by setting a first negative electrode material layer and a second negative electrode material layer in the negative electrode sheet, and adjusting the impedance range of the second negative electrode material layer and the ratio of the impedance of the second negative electrode material layer to the impedance of the first negative electrode material layer, the lithium-ion battery exhibits a high pass rate in the pin penetration test and a high 2C rate capacity retention rate within the scope of this application. This indicates that the pin penetration performance of the embodiments of this application is good, and the secondary battery has good safety performance. In Comparative Example 1, a conventional negative electrode material layer is coated on the negative electrode current collector in a single layer. The impedance of the conventional negative electrode material layer is relatively small, the instantaneous current in the pin penetration test is large, and the pass rate in the pin penetration test is low. The impedance R of the second negative electrode material layer in Comparative Example 2 is relatively high. B The value range of R is not within the scope of this application, and the second negative electrode material layer B If the impedance ratio is too small, the instantaneous current in the nail penetration test will be large, resulting in a low pass rate. In Comparative Examples 3 and 4, the ratio of the impedance of the second negative electrode material layer to the impedance of the first negative electrode material layer is outside the range of this application. Specifically, in Comparative Example 3, the ratio is too small, resulting in a large instantaneous current in the nail penetration test and a low pass rate. In Comparative Example 4, the ratio is too large, indicating an excessively high solid electrolyte content, leading to poor conductivity of the second negative electrode material layer, low 2C rate capacity retention, and poor rate performance. This shows that embodiments not meeting the requirements of this application have poor safety performance, while embodiments of this application simultaneously have a high nail penetration test pass rate and a high 2C rate capacity retention rate, demonstrating that embodiments meeting the features of this application improve the safety performance of lithium-ion batteries while maintaining good rate performance.

[0105] W A The value of W affects the rate performance and safety performance of lithium-ion batteries. As can be seen from Examples 1, 2 to 6, when W is adjusted... A When the value is within the range of this application, the impedance R of the second negative electrode material layer can be made so that B With appropriate values, the instantaneous current in the nail penetration test is small, and it also has good conductivity. The lithium-ion battery has both a high nail penetration test pass rate and a high 2C rate capacity retention rate, indicating that the lithium-ion battery provided in this application has good safety performance while taking into account good rate performance.

[0106] The type of solid electrolyte affects the safety performance of lithium-ion batteries. As can be seen from Examples 1, 6 to 10, when the type of solid electrolyte is controlled within the scope of this application, the lithium-ion battery can simultaneously have a high pass rate in the nail penetration test and a high 2C rate capacity retention rate, indicating that the lithium-ion battery provided by this application has good safety performance while taking into account good rate performance.

[0107] The type of conductive agent affects the rate performance and safety performance of lithium-ion batteries. As can be seen from Examples 1 and 11, when the type of conductive agent is controlled within the scope of this application, the negative electrode sheet has suitable conductivity, enabling the secondary battery to have both a high pass rate in the pin penetration test and a high 2C rate capacity retention rate. This indicates that the lithium-ion battery provided by this application has good safety performance while maintaining good rate performance. The value of W1 affects the rate performance and safety performance of lithium-ion batteries. As can be seen from Examples 1, 12 to 14, when the value of W1 is controlled within the scope of this application, the first negative electrode material layer has suitable conductivity, enabling the lithium-ion battery to simultaneously have a high pass rate in the pin penetration test and a high 2C rate capacity retention rate. When W1 is outside the scope of this application, for example, when W1 is too large, the conductive agent content of the first negative electrode material layer is high, resulting in a low energy density of the lithium-ion battery; when W1 is too small, the conductive agent content of the first negative electrode material layer is low, resulting in poor kinetic performance of the lithium-ion battery and an increased risk of lithium plating during cycling. This indicates that the lithium-ion battery provided in this application has good safety performance while maintaining good rate performance.

[0108] The value of W2 / W1 affects the rate performance and safety performance of lithium-ion batteries. As can be seen from Examples 1, 15, and 16, when the value of W2 / W1 is adjusted within the range of this application, the negative electrode sheet has suitable conductivity, enabling the lithium-ion battery to simultaneously achieve a high pass rate in the nail penetration test and a high 2C rate capacity retention rate. When W2 / W1 is outside the range of this application, for example, when W2 / W1 is too large, the conductive agent content of the second negative electrode material layer is greater than that of the first negative electrode material layer, resulting in the impedance of the second negative electrode material being close to that of the first negative electrode material layer. This leads to a larger instantaneous current in the nail penetration test and poorer puncture performance of the lithium-ion battery. This indicates that the lithium-ion battery provided by this application achieves good safety performance while maintaining good rate performance.

[0109] The type of binder affects the safety performance of lithium-ion batteries. As can be seen from Examples 1 and 17, when the type of binder is controlled within the scope of this application, the negative electrode sheet has suitable adhesion and good processing performance, enabling the secondary battery to simultaneously have a high pass rate in the pin penetration test and a high 2C rate capacity retention rate. This indicates that the lithium-ion battery provided by this application has good safety performance while taking into account good rate performance.

[0110] The value of W4 affects the safety performance of lithium-ion batteries. As can be seen from Examples 1, 18 to 20, when the value of W4 is adjusted within the range of this application, the second negative electrode material layer has suitable adhesion and good processing performance, enabling the lithium-ion battery to simultaneously achieve a high pass rate in the pin penetration test and a high 2C rate capacity retention rate. When W4 is outside the range of this application, for example, when W4 is too large, the binder content of the second negative electrode material layer is high, resulting in a lower energy density of the lithium-ion battery; when W4 is too small, the binder content of the second negative electrode material layer is low, the adhesion of the second negative electrode material layer is poor, and the processing performance is poor. This indicates that the lithium-ion battery provided by this application has good safety performance while maintaining good rate performance.

[0111] The W3 / W4 ratio affects the safety performance of lithium-ion batteries. As can be seen from Examples 1, 21 to 23, when the W3 / W4 ratio is adjusted within the range of this application, the negative electrode sheet has suitable adhesion, enabling the lithium-ion battery to simultaneously achieve a high pass rate in the pin penetration test and a high 2C rate capacity retention rate. When W3 / W4 is outside the range of this application, for example, when W3 / W4 is too small, the binder content of the first negative electrode material layer is low, the adhesion of the first negative electrode material is poor, and the processing performance is poor; when W3 / W4 is too large, the binder content of the first negative electrode material layer is high, resulting in a low energy density of the lithium-ion battery. This indicates that the lithium-ion battery provided by this application has good safety performance while maintaining good rate performance.

[0112] W C The value of W affects the rate performance and safety performance of lithium-ion batteries. As can be seen from Examples 1, 24 to 27, when W is adjusted... C When the value is within the scope of this application, the first negative electrode material layer also has a solid electrolyte, which can reduce the impedance R of the first negative electrode material layer. A With appropriate values, the instantaneous current in the nail penetration test is small, and it also has good conductivity. The lithium-ion battery has both a high nail penetration test pass rate and a high 2C rate capacity retention rate, indicating that the lithium-ion battery provided in this application has good safety performance while taking into account good rate performance.

[0113] 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.

[0114] 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 a negative electrode sheet, the negative electrode sheet including a negative current collector and a negative electrode material layer disposed on at least one surface of the negative current collector, the negative electrode material layer including a first negative electrode material layer and a second negative electrode material layer, wherein along the thickness direction of the negative electrode sheet, the first negative electrode material layer is disposed between the negative current collector and the second negative electrode material layer, the first negative electrode material layer including a first negative electrode active material, and the second negative electrode layer including a solid electrolyte and a second negative electrode active material; the film resistance of the first negative electrode material layer is R. A Ω, the film resistance of the second negative electrode material layer is R B Ω, 0.005≤R B ≤1, 2≤R B / R A ≤450.

2. The secondary battery according to claim 1, wherein, 0.05≤R B ≤0.2。 3. The secondary battery according to claim 1, wherein, Based on the mass of the second negative electrode material layer, the mass percentage of the solid electrolyte is W. A 1%≤W A ≤30%.

4. The secondary battery according to claim 1, wherein, The second negative electrode active material includes at least one of graphite or silicon-based materials, and the solid electrolyte includes at least one of oxide-based solid electrolyte, sulfide-based solid electrolyte, or polymer-based solid electrolyte.

5. The secondary battery according to claim 4, wherein it satisfies at least one of the following characteristics: (1) The oxide-based solid electrolyte includes at least one of lithium lanthanum titanate, lithium lanthanum zirconium oxide, or aluminum-doped lithium lanthanum zirconium oxide; (2) The sulfide-based solid electrolyte includes at least one of lithium phosphine sulfide, lithium phosphium germanium sulfide, or lithium phosphium tin sulfide; (3) The polymer-based solid electrolyte includes at least one of polyethylene oxide, polyacrylonitrile or polyvinylidene fluoride.

6. The secondary battery according to claim 1, wherein, The first negative electrode material layer includes a first conductive agent, and the second negative electrode material layer includes a second conductive agent. The first conductive agent and the second conductive agent each independently include at least one of carbon nanotubes, conductive carbon black, or graphene.

7. The secondary battery according to claim 6, wherein, Based on the mass of the first negative electrode material layer, the mass percentage of the first conductive agent is W1, and based on the mass of the second negative electrode material layer, the mass percentage of the second conductive agent is W2, where 0.4%≤W1≤2% and 0≤W2 / W1≤0.

7.

8. The secondary battery according to claim 1, wherein, The first negative electrode material layer includes a first adhesive, and the second negative electrode material layer includes a second adhesive. The first adhesive and the second adhesive each independently include at least one of styrene-butadiene rubber, polyacrylic acid, sodium carboxymethyl cellulose, polyvinylidene fluoride, polyethylene oxide, or polyimide.

9. The secondary battery according to claim 8, wherein, Based on the mass of the first negative electrode material layer, the mass percentage of the first binder is W3; based on the mass of the second negative electrode material layer, the mass percentage of the second binder is W4, 1%≤W4≤5%, 1.5≤W3 / W4≤3.

10. The secondary battery according to any one of claims 1 to 9, wherein, The first negative electrode material layer also includes a solid electrolyte, and based on the mass of the first negative electrode material layer, the mass percentage of the solid electrolyte in the first negative electrode material layer is W. C W C <W A 0%≤W C ≤10%.

11. An electronic device comprising a secondary battery as described in any one of claims 1 to 10.

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