Secondary battery and electronic device

Abstract

The invention provides a secondary battery and an electronic device, the secondary battery comprises a positive pole piece, the positive pole piece comprises a positive current collector and a coating arranged on at least one surface of the positive current collector, the coating comprises a first coating and a second coating, and the first coating is arranged between the positive current collector and the second coating; the first coating comprises an inorganic material, a conductive agent, a binder and a dispersant; based on the total mass of the first coating, the mass percentage content of the inorganic material is w1, 15% < = w1 < = 50%, the mass percentage content of the conductive agent is w2, 15% < w2 < = 40%, the mass percentage content of the binder is w3, 30% < = w3 < = 50%, and 0 < = w4 < = 5%. According to the secondary battery, while the direct current resistance, the cycle performance and the rate capability of the secondary battery are considered, the bonding force between the second coating and the positive electrode current collector is improved, the falling performance, the impact performance and the puncture performance of the secondary battery are improved, and particularly, the puncture performance of the secondary battery is greatly improved.

Description

TECHNICAL FIELD

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

[0002] Lithium ion batteries (LIB) are widely used in the fields of electric vehicles (such as EV, HEV, PHEV, etc.), electric motorcycles and energy storage (such as home storage, industrial and commercial storage, etc.) due to their high energy density, stable voltage, low self-discharge rate, no memory effect, strong temperature adaptability, green environmental protection and other advantages. However, as the market demand for the service life, charging and discharging speed and safety of electrical equipment is continuously improved, higher requirements are put forward for the cycle life, rate performance and safety performance of LIB.

[0003] The mainstream scheme on the market at present is to coat a layer of primer material on the surface of the positive current collector to improve the adhesion between the active material and the positive current collector and to improve the conductive network, although it can achieve the purpose of reducing the direct current resistance of the secondary battery, improving the cycle performance and rate performance of the secondary battery, but the performance in the aspects of drop performance, impact performance and puncture performance is poor.

[0004] Therefore, how to improve the drop performance, impact performance and puncture performance of the battery under the premise of ensuring that the battery has low direct current resistance, high cycle performance and high rate performance is a technical problem to be solved by those skilled in the art. SUMMARY

[0005] The purpose of the present application is to provide a secondary battery and an electronic device, which improve the adhesion between the second coating and the positive current collector while taking into account the direct current resistance, cycle performance and rate performance of the secondary battery, improve the drop performance, impact performance and puncture performance of the secondary battery, and especially the puncture performance of the secondary battery is greatly improved.

[0006] It should be noted that the present application is explained by taking lithium ion batteries as examples of secondary batteries in the summary of the present application, but the secondary batteries of the present application are not limited to lithium ion batteries. The specific technical solutions are as follows:

[0007] The first aspect of the present application provides a secondary battery, comprising a positive electrode sheet, the positive electrode sheet comprising a positive electrode current collector and a coating layer arranged on at least one surface of the positive electrode current collector, the coating layer comprising a first coating layer and a second coating layer, the first coating layer being arranged between the positive electrode current collector and the second coating layer along the thickness direction of the positive electrode sheet; the first coating layer comprising an inorganic material, a conductive agent, a binder and a dispersant; the mass percentage of the inorganic material in the first coating layer is w1, 15%≤w1≤50%, the mass percentage of the conductive agent is w2, 15%<w2≤40%, the mass percentage of the binder is w3, 30%≤w3≤50%, and the mass percentage of the dispersant is w4, 0≤w4≤5%, based on the total mass of the first coating layer. By arranging the first coating layer between the positive electrode current collector and the second coating layer, and adjusting the mass percentages of the inorganic material, the conductive agent, the binder and the dispersant in the first coating layer to be within the above ranges, on the one hand, the positive electrode sheet can have good electrical conductivity, ensuring the electrical performance of the secondary battery, including the direct current resistance (DCR), the rate performance and the cycle performance of the secondary battery; on the other hand, the thermal stability of the positive electrode sheet can be effectively improved, delaying thermal runaway, especially reducing the risk of internal short circuit when the secondary battery is punctured, thereby making the secondary battery have higher drop performance, impact performance and puncture performance, especially improving the puncture performance; on the other hand, the adhesion between the positive electrode current collector and the second coating layer in the positive electrode sheet can be improved, so that the second coating layer is not easy to peel off from the surface of the positive electrode current collector, ensuring the electronic path, further improving the drop performance, impact performance and puncture performance of the secondary battery, and also enabling the secondary battery to have lower direct current resistance and better rate performance and cycle performance.

[0008] In one or more embodiments of the present application, the sum of the resistances of the positive electrode current collector and the first coating layer is R1Ω / mm 2 , 0.001≤R1≤0.5, optionally 0.1≤R1≤0.3; and / or the resistance of the positive electrode sheet is R2Ω / mm 2 , 0.1≤R2≤4, optionally 1≤R2≤3. Adjusting R1 and R2 to be within the above preferred ranges is conducive to further reducing the direct current resistance of the secondary battery and improving the rate performance and cycle performance of the secondary battery, while also having better drop performance, impact performance and puncture performance, so as to better balance the electrical performance and safety performance of the battery.

[0009] In one or more embodiments of the present application, 35%≤w1≤50%. Adjusting the mass percentage w1 of the inorganic material in the first coating layer to be within the above preferred range is conducive to further improving the drop performance, impact performance and puncture performance of the secondary battery on the premise of ensuring that the secondary battery has lower direct current resistance and better rate performance and cycle performance.

[0010] In one or more embodiments of this application, 15% < w2 ≤ 25%. Adjusting the mass percentage w2 of the conductive agent in the first coating to the above-mentioned preferred range is beneficial to further improve the electrical performance of the secondary battery while ensuring its safety performance.

[0011] In one or more embodiments of this application, the sum of the thermal conductivity of the positive electrode current collector and the first coating is λW / (m·K), where 0.55≤λ≤0.8. By adjusting the thermal conductivity λ to the above range, it is beneficial to quickly conduct the generated heat to the surrounding air when the secondary battery is subjected to mechanical abuse, especially during puncture, reducing the risk of temperature runaway caused by local overheating and effectively improving the drop performance, impact performance, and puncture performance of the secondary battery.

[0012] In one or more embodiments of this application, the sum of the stiffness of the positive electrode current collector and the first coating is S1 mN·m, where 0.03 ≤ S1 ≤ 0.25. By adjusting the stiffness S1 to the above range, it is beneficial to delay the direct contact between the positive electrode current collector and the negative electrode active material when the secondary battery is punctured, thereby improving the drop performance, impact performance, and puncture performance of the secondary battery.

[0013] In one or more embodiments of this application, the stiffness of the positive electrode sheet is S2 mN·m, where 0.01 ≤ S2 ≤ 0.7. By adjusting the stiffness S2 of the positive electrode sheet to the above range, it is beneficial to improve the mechanical strength of the positive electrode sheet, reduce the risk of internal short circuits, and reduce the generation of cracks and fragments when puncture occurs, thus delaying thermal runaway. At the same time, it is beneficial to ensure the dynamic performance of the positive electrode sheet and its structural stability during charging and discharging, so that the secondary battery has high electrical performance and safety performance.

[0014] In one or more embodiments of this application, the peel strength of the second coating is PN / m, where 10 ≤ P ≤ 20. By adjusting the peel strength P of the second coating to the above range, it is beneficial to further improve the adhesion between the second coating and the positive electrode current collector, so that the secondary battery has lower DC resistance, better rate performance and cycle performance, and better drop performance, impact performance and puncture performance.

[0015] In one or more embodiments of this application, the positive electrode sheet satisfies at least one of the following characteristics: (1) the inorganic material includes at least one of boehmite, alumina, aluminosilicate, silica, hydrotalcite, kaolin, or titanate nanoparticles; (2) the conductive agent includes at least one of carbon black or carbon nanotubes; (3) the binder includes at least one of polyacrylic acid, sodium polyacrylate, lithium polyacrylate, polyvinyl alcohol, polyethylene oxide, styrene-butadiene resin, sodium alginate, or chitosan; and (4) the dispersant includes at least one of sodium carboxymethyl cellulose or lithium carboxymethyl cellulose.

[0016] In one or more embodiments of this application, the weight-average molecular weight of the adhesive is Mw g / mol, 1×10 4 ≤Mw≤5×10 5 By controlling the weight-average molecular weight (Mw) of the binder within the aforementioned range, it is beneficial to improve the interfacial bonding force between the first coating and the positive electrode current collector and the second coating, and to ensure the uniformity of the first coating. At the same time, it has a smaller negative impact on the electronic conductivity and lithium-ion transport performance of the positive electrode sheet, resulting in a secondary battery with lower DC resistance, better rate performance and cycle performance, as well as better drop performance, impact performance and puncture performance.

[0017] In one or more embodiments of this application, the average particle size of the carbon black is D1 μm, where 0.5 ≤ D1 ≤ 4. By controlling D1 within the above range, the average particle size of the carbon black is relatively moderate, which is beneficial for reducing the DC resistance of the secondary battery and improving rate performance and cycle performance.

[0018] In one or more embodiments of this application, the average diameter of the carbon nanotubes is D2 nm, where 2 ≤ D2 ≤ 5, and the average length of the carbon nanotubes is L μm, where 1 ≤ L ≤ 2. By controlling D2 and L within the above ranges, the aspect ratio of the carbon nanotubes is moderate, which is beneficial for reducing the DC resistance of the secondary battery and improving rate performance and cycle performance.

[0019] In one or more embodiments of this application, the specific surface area of ​​the conductive agent is B m. 2 / g, 60≤B≤90. By controlling B within the above range, the specific surface area of ​​the conductive agent is moderate, which is conducive to the formation of a dense conductive network and ensures efficient ion transport, enabling the secondary battery to have low DC resistance and good rate performance and cycle performance.

[0020] In one or more embodiments of this application, the thickness of the first coating on one side is T1 μm, where 0.5 ≤ T1 ≤ 1.5. By controlling T1 within the above range, it is beneficial to better balance the physical protective effect of the first coating with the relationship between ion and electron transport, enabling the secondary battery to have lower DC resistance and better rate performance and cycle performance, while also having higher drop performance, impact performance and puncture performance, thereby better balancing the electrical performance and safety performance of the secondary battery.

[0021] In one or more embodiments of this application, the thickness of the second coating on one side is T2 μm, where 40 ≤ T2 ≤ 180. By controlling T2 within the above range, it is beneficial to further improve the rate performance and cycle performance of the secondary battery, and also beneficial to improve the drop performance, impact performance and puncture performance of the secondary battery.

[0022] In one or more embodiments of this application, the average particle size of the inorganic material is D3 μm, where 0.5 ≤ D3 ≤ 2. By controlling the average particle size D3 of the inorganic material within the above range, it is beneficial to reduce the shedding and pulverization of active material while maintaining the barrier effect of the first coating; simultaneously, it also helps to shorten the electron conduction path and ion diffusion path. Therefore, when D3 is within the above range, it is beneficial to enable the secondary battery to have lower DC resistance, better rate performance and cycle performance, and better drop performance, impact performance and puncture resistance.

[0023] In one or more embodiments of this application, the thickness of the first coating on one side is T1 μm, the average particle size of the inorganic material is D3 μm, and 0.8 ≤ T1 / D3 ≤ 1.5. By controlling T1 and D3 to satisfy the above relationship, it is beneficial to improve the protective effect of the first coating while ensuring the volumetric energy density of the secondary battery, effectively delay thermal runaway, and enable the positive electrode sheet as a whole to have high ion transport efficiency and electronic conductivity, thereby giving the secondary battery high safety and electrical performance.

[0024] In one or more embodiments of this application, along the thickness direction of the positive electrode sheet, the projected area of ​​the first coating on the surface of the positive electrode current collector is A1, and the projected area of ​​the second coating on the surface of the positive electrode current collector is A2, with A1 / A2 being 0.9 to 1. By controlling A1 / A2 within the above range, it is beneficial to improve the protective effect of the first coating while ensuring the electrical performance of the secondary battery, and to improve the structural stability of the positive electrode sheet, so that the secondary battery has better drop performance, impact performance, and puncture performance.

[0025] In one or more embodiments of this application, the area of ​​the positive current collector along the thickness direction of the positive electrode sheet is A3, and the ratio of A1 / A3 is 0.6 to 0.9. By controlling A1 / A3 within the above range, it is beneficial to reduce the contact between the positive current collector and the negative electrode during mechanical abuse of the secondary battery while ensuring the electrical performance of the secondary battery, and to better disperse mechanical stress, thereby further improving the battery's drop performance, impact performance, and puncture performance.

[0026] In one or more embodiments of this application, the areal density of the first coating is ρmg / cm³. 2 , 0.01≤ρ≤0.05. By controlling the areal density ρ of the first coating within the above range, it is beneficial to further enable the secondary battery to have lower DC resistance, better rate performance and cycle performance, as well as better drop performance, impact performance and puncture performance.

[0027] A second aspect of this application provides an electronic device that includes a secondary battery as described in any of the foregoing embodiments. Therefore, the electronic device has good performance characteristics.

[0028] Beneficial effects of the embodiments in this application:

[0029] This application provides a secondary battery and an electronic device. The secondary battery includes a positive electrode sheet. A first coating is provided between the positive current collector and the second coating of the positive electrode sheet. At the same time, the mass percentage content of inorganic materials, conductive agents, binders and dispersants in the first coating is controlled within the above-mentioned range. On the one hand, it enables the positive electrode sheet to have good conductivity, ensuring that the secondary battery has low DC resistance and good rate performance and cycle performance. On the other hand, it can effectively improve the thermal stability of the positive electrode sheet, playing a role in delaying thermal runaway when the secondary battery is subjected to mechanical abuse. In particular, it can reduce the risk of internal short circuit when the secondary battery is punctured, thereby giving the secondary battery high drop performance, impact performance and puncture performance, especially improving its puncture performance. Furthermore, it can improve the adhesion between the positive current collector and the second coating in the positive electrode sheet, making it less likely for the second coating to peel off from the surface of the positive current collector, forming a good interface contact, ensuring electronic pathways. This not only reduces the risk of internal short circuit when the secondary battery is subjected to mechanical abuse, further improving the battery's drop performance, impact performance and puncture performance, but also gives the battery low DC resistance and good rate performance and cycle performance.

[0030] 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

[0031] 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.

[0032] Figure 1 This is a schematic cross-sectional view of the positive electrode sheet in one embodiment of this application;

[0033] Figure 2 An optical microscope image of the dotted first coating with an A1 / A3 ratio of 0.74 in Embodiment 30 of this application;

[0034] Figure 3 An optical microscope image of the dotted first coating with an A1 / A3 ratio of 0.84 in Embodiment 31 of this application;

[0035] Figure 4 This is an optical microscope image of the dotted first coating with an A1 / A3 ratio of 0.88 in Embodiment 1 of this application.

[0036] Reference numerals: Positive current collector 10, first coating 11, second coating 12, coating 20. Detailed Implementation

[0037] 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. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0038] 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:

[0039] A first aspect of this application provides a secondary battery, including a positive electrode sheet, the positive electrode sheet including a positive current collector and a coating disposed on at least one surface of the positive current collector, the coating including a first coating and a second coating, the first coating being disposed between the positive current collector and the second coating along the thickness direction of the positive electrode sheet; the first coating including an inorganic material, a conductive agent, a binder and a dispersant; based on the total mass of the first coating, the mass percentage of the inorganic material is w1, 15% ≤ w1 ≤ 50%, the mass percentage of the conductive agent is w2, 15% < w2 ≤ 40%, the mass percentage of the binder is w3, 30% ≤ w3 ≤ 50%, and the mass percentage of the dispersant is w4, 0 ≤ w4 ≤ 5%.

[0040] For example, the value of w1 can be 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or a range consisting of any two of these values. The range of w1 can be 15% to 50%, 15% to 45%, 20% to 40%, 25% to 35%, and all of these ranges, as well as subranges. The value of w2 can be 16%, 20%, 25%, 30%, 35%, 40%, or a range consisting of any two of these values. The range of w2 can be 15% to 40% (excluding the endpoint value of 15%), 16% to 35%, 16% to 30%, 20% to 25%, and so on. The values ​​of w3 can be 30%, 35%, 40%, 45%, 50%, or any two of these values. The range of w3 can be 30% to 50%, 30% to 45%, 35% to 45%, 30% to 40%, 40% to 50%, or any two of these values. The values ​​of w4 can be 0%, 1%, 2%, 3%, 4%, 5%, or any two of these values. The range of w4 can be 0 to 5%, 1% to 4%, 1% to 3%, 2% to 4%, 2% to 5%, or any two of these values.

[0041] For ease of understanding, in this application, the length direction of the positive electrode sheet is defined as X, and the thickness direction is defined as Z. It should be understood that the above definitions of direction 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 elements. Furthermore, the length and thickness directions of the positive current collector, coating, first coating, and second coating are the same as those of the positive electrode sheet.

[0042] The aforementioned "coating disposed on at least one surface of the positive electrode current collector" means that the coating (including the first coating and the second coating) can be disposed on one surface of the positive electrode current collector along its own thickness direction, or it can be disposed on two surfaces of the positive electrode current collector along its own thickness direction. It should be noted that the "surface" here can be the entire area of ​​the surface of the positive electrode current collector, or it can be a part of the surface of the positive electrode current collector. This application has no particular limitation, as long as the purpose of this application can be achieved.

[0043] like Figure 1 As shown, the positive electrode includes a positive current collector 10 and a coating 20. The coating 20 is disposed on two surfaces of the positive current collector 10. The coating 20 includes a first coating 11 and a second coating 12, with the first coating 11 disposed between the positive current collector 10 and the second coating 12. Of course, it is understood that in some embodiments of this application, the coating 20 is disposed on only one surface of the positive current collector 10.

[0044] The secondary battery of this application includes a positive electrode sheet, which includes a positive current collector and a coating disposed on at least one surface of the positive current collector. The coating includes a second coating and a first coating disposed between the second coating and the positive current collector. By controlling w1, w2, w3, and w4 within the aforementioned range, this application can better balance the electrical and safety performance of the secondary battery. Specifically, it enables the positive electrode sheet to have good conductivity and thermal stability. This ensures that the secondary battery has low DC resistance and good rate and cycle performance. On the other hand, it can delay thermal runaway when the secondary battery is subjected to mechanical abuse, especially reducing the risk of internal short circuits when the secondary battery is punctured. This results in high drop performance, impact performance, and puncture performance, especially puncture performance. At the same time, it can also effectively improve the adhesion between the second coating and the positive current collector. When mechanical abuse occurs, the second coating is not easy to fall off the surface of the positive current collector, ensuring the electronic path between the second coating and the positive current collector. This helps to reduce the risk of internal short circuits when the secondary battery is subjected to mechanical abuse. Furthermore, the strong adhesion between the second coating and the positive electrode current collector helps to further reduce the DC resistance of the secondary battery and improve its rate performance and cycle performance. Specifically, this is reflected in the following aspects: 1) It helps to reduce the contact resistance between the coating and the positive electrode current collector, thus reducing DC resistance; 2) It can reduce the local detachment of the second coating during high-rate charge and discharge, ensuring the continuity of the electron conduction path, reducing polarization, and reducing capacity decay; 3) It can reduce the shedding of active material during cyclic charge and discharge, reducing the loss of effective active material and reducing capacity decay. Therefore, the secondary battery of this application, while taking into account low DC resistance, good rate performance and cycle performance, improves the adhesion between the second coating and the positive electrode current collector, enhances the drop performance, impact performance and puncture performance of the secondary battery, thereby improving the electrical performance and safety performance of the secondary battery.

[0045] In one or more embodiments of this application, the sum of the resistances of the positive current collector and the first coating is R1Ω / mm. 2 0.001≤R1≤0.5, optionally, 0.1≤R1≤0.3; and / or, the resistance of the positive electrode is R2Ω / mm. 2, 0.1≤R2≤4, optionally, 1≤R2≤3. For example, the value of R1 can be 0.001, 0.005, 0.01, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, or any range of any two values. The value range of R1 can be 0.001 to 0.5, 0.01 to 0.45, 0.1 to 0.4, 0.2 to 0.4, 0.3 to 0.4, 0.1 to 0.3, and all of these ranges, as well as sub-ranges. The value of R2 can be 0.1, 0.2, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, or any range of any two values. The value range of R2 can be 0.1 to 4, 0.5 to 4, 1 to 4, 2 to 4, 3 to 4, 1 to 3, and all of these ranges, as well as sub-ranges. By adjusting R1 and R2 to the aforementioned range, it is beneficial to further improve the conductivity and thermal stability of the positive electrode sheet. Simultaneously, it more effectively delays thermal runaway and suppresses internal short-circuit propagation during mechanical abuse of the secondary battery. Furthermore, it enhances the adhesion between the second coating and the positive electrode current collector, further reducing the contact resistance between them and reducing the shedding of the second coating or active material during cyclic charging and discharging. This results in a secondary battery with lower DC internal resistance, higher cycle performance, and higher rate performance. Therefore, when R1 and R2 are within the aforementioned range, it is beneficial to further reduce the DC internal resistance of the secondary battery and improve its cycle performance and rate performance, while also further enhancing its drop resistance, impact resistance, and puncture resistance, thus better balancing the battery's electrical performance and safety performance.

[0046] In this application, it is understood that when the coating 20 (including the first coating 11 and the second coating 12) is provided only on one surface of the positive current collector, R1 refers to the sum of the resistance of the positive current collector 10 and the resistance of the first coating 11 provided on one surface of the positive current collector 10, and R2 refers to the sum of the resistance of the positive current collector 10 and the resistance of the coating 20 (including the first coating 11 and the second coating 12) provided on one surface of the positive current collector 10; when the coating 20 (including the first coating 11 and the second coating 12) is provided on both surfaces of the positive current collector 10, R1 refers to the sum of the resistance of the positive current collector 10 and the resistance of the first coating 11 provided on both surfaces of the positive current collector, and R2 refers to the sum of the resistance of the positive current collector 10 and the resistance of the coating 20 (including the first coating 11 and the second coating 12) provided on both surfaces of the positive current collector.

[0047] This application does not impose any special restrictions on the method of adjusting R1 and R2, as long as the purpose of this application can be achieved. For example, R1 and R2 can be adjusted by adjusting the slurry formulation of the first coating, the thickness of the first coating, the areal density of the first coating, and the ratio of the projected area of ​​the first coating on the surface of the positive electrode current collector to the area of ​​the positive electrode current collector. For example, when preparing the first coating slurry, w3 and w4, as well as other conditions, are kept constant. Increasing w1 and decreasing w2 can improve R1 and R2, and vice versa. Alternatively, when coating the first coating slurry onto the surface of the positive electrode current collector, other conditions are kept constant. Increasing the coating thickness of the first coating can improve R1 and R2, and vice versa. Alternatively, other conditions are kept constant. Increasing the areal density of the first coating can improve R1 and R2, and vice versa. Alternatively, other conditions are kept constant. Increasing the ratio of the projected area of ​​the first coating on the surface of the positive electrode current collector to the area of ​​the positive electrode current collector can decrease R2. As the area ratio increases, R2 tends to stabilize while R1 remains essentially unchanged. Conversely, R2 increases while R1 remains essentially unchanged. When the area ratio is too low, R1 increases significantly.

[0048] In one or more embodiments of this application, 35% ≤ w1 ≤ 50%. By adjusting the mass percentage w1 of the inorganic material in the first coating to the above-mentioned preferred range, it is beneficial to further improve the drop performance, impact performance and puncture performance of the secondary battery while ensuring that the secondary battery has low DC resistance and high cycle performance and rate performance, thereby giving the secondary battery higher electrical performance and safety performance.

[0049] In one or more embodiments of this application, 15% < w2 ≤ 25%. By adjusting the mass percentage w2 of the conductive agent in the first coating to the above-mentioned preferred range, it is beneficial to further reduce the DC resistance of the secondary battery and improve its cycle performance and rate performance while ensuring that the secondary battery has high drop performance, impact performance and puncture performance, thereby giving the secondary battery higher electrical performance and safety performance.

[0050] In one or more embodiments of this application, the sum of the thermal conductivity of the positive electrode current collector and the first coating is λW / (m·K), where 0.55≤λ≤0.8. For example, the value of λ can be 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, or a range consisting of any two of these values. The range of λ can be 0.55 to 0.8, 0.6 to 0.8, 0.7 to 0.8, 0.55 to 0.75, 0.55 to 0.65, 0.6 to 0.7, 0.65 to 0.75, and all such ranges and sub-ranges. By adjusting the thermal conductivity λ to the above range, it is beneficial to rapidly dissipate heat from localized heating points (such as puncture points) when the secondary battery is subjected to mechanical abuse, especially during puncture, to prevent a sudden rise in local temperature, reduce the risk of short circuits and thermal runaway, delay secondary battery failure, and effectively improve the drop performance, impact performance, and puncture performance of the secondary battery, thereby giving the secondary battery higher safety performance.

[0051] In this application, when the coating 20 (including the first coating 11 and the second coating 12) is provided only on one surface of the positive electrode current collector, λ refers to the sum of the thermal conductivity of the positive electrode current collector 10 and the first coating 11 provided on one surface of the positive electrode current collector; when the coating 20 (including the first coating 11 and the second coating 12) is provided on both surfaces of the positive electrode current collector, λ refers to the sum of the thermal conductivity of the positive electrode current collector 10 and the first coating 11 provided on both surfaces of the positive electrode current collector.

[0052] This application does not impose any particular restrictions on the method of controlling the thermal conductivity λ, as long as the purpose of this application can be achieved. For example, the thermal conductivity λ can be controlled by adjusting the slurry formulation of the first coating, the areal density of the first coating, etc. For example, when preparing the first coating slurry, w3 and w4, as well as other conditions, are kept constant, and w1 is decreased while w2 is increased to increase λ, and vice versa; or, when the first coating slurry is coated on the surface of the positive electrode current collector, other conditions are kept constant, and the areal density of the first coating is increased to increase λ, and vice versa.

[0053] In one or more embodiments of this application, the sum of the stiffness of the positive current collector 10 and the first coating 11 is S1mN·m, where 0.03≤S1≤0.25. For example, the value of S1 can be 0.03, 0.05, 0.10, 0.15, 0.20, 0.25, or a range consisting of any two of these values. The range of S1 can be 0.03 to 0.25, 0.05 to 0.25, 0.10 to 0.25, 0.15 to 0.25, 0.03 to 0.20, 0.03 to 0.15, 0.03 to 0.10, 0.05 to 0.20, and all such ranges and sub-ranges. Stiffness reflects the material's ability to resist bending or deformation. By adjusting S1 to the above range, the first coating acts as a skeleton to provide support. During puncture, it helps to delay the direct contact between the positive and negative electrodes, improving the drop performance, impact performance, and puncture performance of the secondary battery, thereby giving the secondary battery higher safety performance.

[0054] In this application, when the coating 20 (including the first coating 11 and the second coating 12) is provided only on one surface of the positive current collector, S1 refers to the sum of the stiffness of the positive current collector 10 and the first coating 11 provided on one surface of the positive current collector; when the coating 20 (including the first coating 11 and the second coating 12) is provided on both surfaces of the positive current collector, S1 refers to the sum of the stiffness of the positive current collector 10 and the first coating 11 provided on both surfaces of the positive current collector.

[0055] This application does not impose any particular restrictions on the method of controlling the stiffness S1 mentioned above, as long as the purpose of this application can be achieved. For example, the stiffness S1 can be controlled by adjusting the slurry formulation of the first coating, the thickness of the first coating, the areal density of the first coating, etc. For example, when preparing the first coating slurry, w3 and w4, as well as other conditions, are kept constant, increasing w1 and decreasing w2 can improve S1, and vice versa; or, other conditions are kept constant, increasing the areal density of the first coating can improve S1, and vice versa; or, other conditions are kept constant, increasing the thickness of the first coating can improve S1, and vice versa.

[0056] In one or more embodiments of this application, the stiffness of the positive electrode sheet is S2 mN·m, where 0.01 ≤ S2 ≤ 0.7. For example, the value of S2 can be 0.01, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, or a range consisting of any two of these values. The value range of S2 can be 0.01 to 0.7, 0.01 to 0.5, 0.01 to 0.4, 0.01 to 0.3, 0.01 to 0.2, 0.1 to 0.7, 0.2 to 0.5, 0.3 to 0.4, and all such ranges and sub-ranges. By adjusting the stiffness S2 of the positive electrode sheet to the above range, the stiffness S2 of the positive electrode sheet is relatively moderate. On the one hand, it helps resist external pressure, making the positive electrode less prone to breakage, delaying the occurrence of internal short circuits, and reducing the risk of active material detachment during drops. Simultaneously, it produces fewer cracks or fragments during punctures, reducing the risk of separator punctures or direct contact between electrodes due to deformation, thus delaying thermal runaway. On the other hand, moderate stiffness helps ensure the stability of the positive electrode during electrolyte wetting and lithium-ion diffusion, thereby ensuring the kinetic performance of the positive electrode. It also helps improve the structural stability of the positive electrode during charge and discharge, extending the cycle life of the secondary battery. Therefore, when the stiffness S2 of the positive electrode is within the above range, it helps to further reduce the DC resistance of the secondary battery and improve rate performance and cycle performance, while also improving the drop performance, impact performance, and puncture performance of the secondary battery, thus giving the secondary battery higher electrical performance and safety performance.

[0057] In this application, when the coating 20 (including the first coating 11 and the second coating 12) is provided only on one surface of the positive current collector 10, S2 refers to the sum of the stiffness of the positive current collector 10 and the coating 20 provided on one surface of the positive current collector; when the coating 20 (including the first coating 11 and the second coating 12) is provided on both surfaces of the positive current collector, S2 refers to the sum of the stiffness of the positive current collector 10 and the coating 20 provided on both surfaces of the positive current collector.

[0058] This application does not impose any particular limitation on the method of adjusting the stiffness S2 mentioned above, as long as the purpose of this application can be achieved. For example, the stiffness S2 can be adjusted by adjusting the thickness of the second coating, the areal density of the second coating, and the orthographic projection area of ​​the second coating on the surface of the positive electrode current collector. For example, when the second coating slurry is applied to the surface of the first coating away from the positive electrode current collector, with other conditions remaining unchanged, decreasing the coating thickness of the second coating will decrease S2, and vice versa; or, with other conditions remaining unchanged, decreasing the areal density of the second coating will increase S2, and vice versa; or, with other conditions remaining unchanged, decreasing the orthographic projection area of ​​the second coating on the surface of the positive electrode current collector will increase S2, and vice versa.

[0059] In one or more embodiments of this application, the peel strength of the second coating 12 is PN / m, where 10 ≤ P ≤ 20. For example, the value of P can be 10, 12, 14, 16, 18, 20, or a range consisting of any two of these values. The range of P can be 10 to 20, 12 to 18, 10 to 18, 10 to 16, 12 to 20, 14 to 20, 16 to 20, and all of these ranges, as well as sub-ranges. By adjusting the peel strength P of the second coating to the above range, it is beneficial to make the second coating bond more firmly with the positive electrode current collector, making it less prone to local peeling during the charging and discharging process of the secondary battery, ensuring the stability of the electron conduction path, reducing the DC resistance of the secondary battery, and improving the rate performance and cycle performance. At the same time, when the secondary battery is subjected to mechanical abuse, the second coating is also less prone to peeling, reducing the risk of internal short circuits and thermal runaway, and improving the drop performance, impact performance, and puncture performance of the secondary battery, thereby giving the secondary battery higher electrical performance and safety performance.

[0060] This application does not impose any particular limitation on the method of controlling the peel strength P, as long as the purpose of this application can be achieved. For example, the peel strength P can be controlled by adjusting the mass percentage of the binder in the second coating, the type of binder in the second coating, the content of the conductive agent in the second coating, the thickness of the second coating, etc. For example, when preparing the first coating slurry, with other conditions kept constant, increasing the mass percentage of the binder in the second coating increases P, and vice versa.

[0061] In one or more embodiments of this application, the inorganic material includes at least one of boehmite, alumina, aluminosilicate, silica, hydrotalcite, kaolin, or titanate nanoparticles. Selecting the above-mentioned inorganic materials is beneficial in increasing the surface roughness of the positive electrode current collector to a certain extent, providing more mechanical anchoring points, and enhancing the adhesion of the binder. Simultaneously, the above-mentioned inorganic materials can also form a stronger bond with the positive electrode current collector, thereby improving the peel strength of the second coating, which is beneficial in reducing the DC resistance of the secondary battery and improving its rate performance and cycle performance. It also improves the drop performance, impact performance, and puncture performance of the secondary battery, thus giving the secondary battery higher electrical performance and safety performance.

[0062] In one or more embodiments of this application, the conductive agent includes at least one of carbon black or carbon nanotubes. The carbon black may include, but is not limited to, acetylene black and / or Ketjen black, and the carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. Using the above-mentioned conductive agent is beneficial for improving the electronic conductivity of the first coating, ensuring charge transfer between the second coating and the positive electrode current collector, reducing the DC resistance of the secondary battery, and improving rate performance and cycle performance, thereby giving the secondary battery higher electrical performance.

[0063] In one or more embodiments of this application, the binder includes at least one selected from polyacrylic acid, sodium polyacrylate, lithium polyacrylate, polyvinyl alcohol, polyethylene oxide, styrene-butadiene resin, sodium alginate, or chitosan. The aforementioned binder helps to improve the adhesion between the second coating and the positive electrode current collector, preventing the coating from peeling or detaching during cyclic charging and discharging or mechanical abuse of the secondary battery. This results in better structural stability of the positive electrode sheet, thereby reducing the DC resistance of the secondary battery and improving its rate performance and cycle performance. Simultaneously, it enhances the drop performance, impact performance, and puncture resistance of the secondary battery, thus providing higher electrical performance and safety performance.

[0064] In one or more embodiments of this application, the dispersant includes at least one of sodium carboxymethyl cellulose or lithium carboxymethyl cellulose. The aforementioned dispersant is beneficial for improving the coating quality of the first coating, reducing defects such as cracks and voids after the positive electrode sheet has dried, and improving the consistency of the first coating.

[0065] In one or more embodiments of this application, the weight-average molecular weight of the adhesive is Mw g / mol, 1×10 4 ≤Mw≤5×10 5 For example, the value of Mw can be 1×10. 4 5×10 4 1×10 5 1.5×10 5 2×10 5 2.5×10 5 3×10 5 3.5×10 5 4×10 5 4.5×10 5 5×10 5 The value of Mw can be any two of these values, and the range of Mw can be 1 × 10. 4 Up to 5×10 5 5×10 4 Up to 4×10 5 1×10 5 Up to 3×10 5 1×10 5 Up to 4×10 5 2×10 5 Up to 5×10 5 3×10 5 Up to 5×10 5And all of these ranges, and sub-ranges. When Mw is within the above range, the weight-average molecular weight of the binder is relatively moderate, which not only helps to improve the interfacial bonding force between the first coating and the positive electrode current collector and the second coating, reducing the risk of delamination or peeling of the coating during mechanical abuse, but also helps to ensure the stability and flowability of the first coating slurry, improve the uniformity of the first coating slurry coating, and reduce brittle fracture caused by local stress concentration; at the same time, it will not affect the electronic conductivity and lithium-ion transport performance of the positive electrode sheet. Therefore, when the weight-average molecular weight Mw of the binder is within the above range, the secondary battery has lower DC resistance and higher rate performance and cycle performance, while also having better drop performance, impact performance and puncture performance, thus giving the secondary battery higher electrical performance and safety performance.

[0066] In one or more embodiments of this application, the average particle size of the carbon black is D1 μm, where 0.5 ≤ D1 ≤ 4. For example, the value of D1 can be 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, or a range consisting of any two of these values. The range of D1 can be 0.5 to 4, 1 to 4, 2 to 4, 3 to 4, 1 to 3, 1.5 to 3.5, 0.5 to 3, 0.5 to 2, and all such ranges and sub-ranges. By controlling D1 within the above range, the average particle size of the carbon black is more moderate, which is beneficial for forming a dense conductive network and leaving sufficient pores in the first coating to ensure the smooth transport of lithium ions. This results in a secondary battery with lower DC resistance and higher rate performance and cycle performance, thereby giving the secondary battery higher electrical performance.

[0067] In one or more embodiments of this application, the average diameter of the carbon nanotubes is D² nm, where 2 ≤ D² ≤ 5, and the average length of the carbon nanotubes is L μm, where 1 ≤ L ≤ 2. For example, the value of D² can be 2, 2.5, 3, 3.5, 4, 4.5, 5, or any range of two values ​​therein. The range of D² can be 2 to 5, 2 to 4, 3 to 5, 3 to 4, and all such ranges and sub-ranges. The value of L can be 1, 1.2, 1.4, 1.6, 1.8, 2, or any range of two values ​​therein. The range of L can be 1 to 2, 1 to 1.6, 1.2 to 1.8, 1.4 to 2, and all such ranges and sub-ranges. When D² and L are within the above ranges, the aspect ratio of the carbon nanotubes is moderate, which is beneficial for forming a denser conductive network, reducing the contact resistance and DC resistance of the electrodes, improving electron transport efficiency, and giving the secondary battery lower DC resistance and higher rate performance and cycle performance, thereby giving the secondary battery higher electrical performance.

[0068] In one or more embodiments of this application, the specific surface area of ​​the conductive agent is B m. 2 / g, 60≤B≤90. For example, the value of B can be 60, 65, 70, 75, 80, 85, 90, or any two of these values. The range of B values ​​can be 60 to 90, 65 to 85, 70 to 80, 60 to 80, 60 to 70, 70 to 90, 80 to 90, and all of these ranges, as well as sub-ranges. When B is within the above range, it is beneficial to form a dense conductive network, shorten the electron conduction path, and leave enough porosity to ensure efficient ion transport, giving the secondary battery lower DC resistance and higher rate performance and cycle performance, thus resulting in higher electrical performance.

[0069] In one or more embodiments of this application, the thickness of one side of the first coating 11 is T1 μm, where 0.5 ≤ T1 ≤ 1.5. For example, the value of T1 can be 0.5, 0.7, 0.9, 1.1, 1.3, 1.5, or a range consisting of any two of these values. The range of T1 can be 0.5 to 1.5, 0.7 to 1.3, 0.9 to 1.3, 0.5 to 1.1, 0.9 to 1.5, and all such ranges and sub-ranges. By controlling T1 within the above range, it is not only beneficial to provide better protection against mechanical abuse of the secondary battery and suppress thermal runaway, but also to avoid hindering the transport of active ions (such as lithium ions) and the conduction of electrons. This results in the secondary battery having lower DC resistance, higher rate performance and cycle performance, and better drop performance, impact performance, and puncture performance. Simultaneously, it also helps to reduce the loss of volumetric energy density in the secondary battery. Therefore, the secondary battery has higher electrical performance and safety performance.

[0070] In one or more embodiments of this application, the thickness of one side of the second coating 12 is T2 μm, where 40 ≤ T2 ≤ 180. For example, the value of T2 can be 40, 60, 80, 100, 120, 140, 160, 180, or a range consisting of any two of these values. The range of T2 can be 40 to 180, 60 to 160, 80 to 140, 100 to 180, 140 to 180, 40 to 140, and all of these ranges, as well as sub-ranges. By adjusting T2 within the above range, it is beneficial to ensure the diffusion of active ions (such as lithium ions) and electron conduction, and to reduce stress concentration caused by volume changes in the positive electrode sheet during cycling, thereby improving the rate performance and cycle performance of the secondary battery. Meanwhile, when T2 is within the aforementioned range, the interfacial stress between the second coating and the positive electrode current collector is relatively low, making it less prone to peeling under mechanical abuse, thus reducing the occurrence of internal short circuits. Furthermore, it results in less heat release during thermal runaway of the secondary battery, which is beneficial for improving the drop performance, impact performance, and puncture resistance of the secondary battery. In addition, when T2 is within the aforementioned range, it is conducive to forming a more uniform second coating thickness, reducing the risk of lithium deposition.

[0071] In this application, the unilateral thickness refers to the thickness of the first coating 11 or the second coating 12 located on either side of the positive current collector 10.

[0072] In one or more embodiments of this application, the average particle size of the inorganic material is D3 μm, where 0.5 ≤ D3 ≤ 2. For example, the value of D3 can be 0.5, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2, or a range consisting of any two of these values. The range of D3 can be 0.5 to 2, 0.6 to 1.8, 0.8 to 1.4, 0.5 to 1.6, 0.5 to 1.2, 0.8 to 2, 1.2 to 2, and all such ranges and sub-ranges. By controlling D3 within the above range, firstly, it is beneficial to form a denser protective structure, delaying electrolyte penetration and heat conduction. In the early stages of thermal runaway caused by mechanical abuse, it can more effectively block heat transfer and reduce the rate of thermal runaway propagation. Secondly, the inorganic material particles have a larger contact area with the positive electrode current collector and the second coating, resulting in stronger interfacial adhesion. This makes it less prone to delamination during mechanical abuse, maintaining the barrier function of the first coating. Third, the low contact resistance between inorganic material particles results in shorter and more continuous electron conduction paths, which helps reduce the DC resistance of the secondary battery. Fourth, it helps shorten the ion diffusion path, leading to higher ion transport efficiency during high-rate discharge and improving the rate performance of the secondary battery. Fifth, it helps improve the interfacial compatibility between the inorganic material particles and the second coating, reducing the shedding and pulverization of active materials and improving the cycle performance of the secondary battery.

[0073] In one or more embodiments of this application, the thickness of the first coating 11 on one side is T1 μm, the average particle size of the inorganic material is D3 μm, and 0.8 ≤ T1 / D3 ≤ 1.5. For example, the value of T1 / D3 can be 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, or a range consisting of any two of these values. The range of T1 / D3 can be 0.8 to 1.5, 0.9 to 1.4, 1 to 1.3, 0.8 to 1.2, 0.8 to 1, 1 to 1.5, 1.2 to 1.5, and all such ranges and sub-ranges. By controlling T1 and D3 to satisfy the above relationship, it is beneficial to enable the first coating to play a better protective role without affecting the overall volumetric energy density of the secondary battery, effectively delaying thermal runaway, and ensuring high ion transport efficiency and electronic conductivity. This results in the secondary battery having lower DC resistance, higher rate performance and cycle performance, as well as better drop performance, impact performance and puncture performance, thereby further improving the electrical performance and safety performance of the secondary battery.

[0074] In one or more embodiments of this application, along the thickness direction of the positive electrode sheet, the projected area of ​​the first coating 11 on the surface of the positive current collector is A1, and the projected area of ​​the second coating 12 on the surface of the positive current collector is A2, with A1 / A2 being 0.9 to 1. For example, the value of A1 / A2 can be 0.9, 0.92, 0.94, 0.96, 0.98, 1, or a range consisting of any two of these values. The range of A1 / A2 can be 0.9 to 1, 0.94 to 1, 0.96 to 1, 0.92 to 0.98, 0.9 to 0.96, 0.9 to 0.94, and all such ranges and sub-ranges. Within this range, it is beneficial to enhance the mechanical protection of the first coating while ensuring the electrical performance of the secondary battery, reduce the direct contact between the positive current collector and the negative electrode when the secondary battery is subjected to mechanical abuse, and reduce the risk of internal short circuit; at the same time, it is beneficial to uniformly disperse mechanical stress, improve structural stability, reduce the peeling of the second coating from the positive current collector, and delay the occurrence of thermal runaway.

[0075] In one or more embodiments of this application, the area of ​​the positive current collector 10 along the thickness direction of the positive electrode sheet is A3, and A1 / A3 is 0.6 to 0.9. For example, the value of A1 / A3 can be 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, or a range consisting of any two of these values. The value range of A1 / A3 can be 0.6 to 0.9, 0.65 to 0.85, 0.7 to 0.8, 0.6 to 0.8, 0.6 to 0.7, 0.7 to 0.9, 0.8 to 0.9, and all such ranges and sub-ranges. Within this range, it is beneficial to reduce the contact between the positive current collector and the negative electrode during mechanical abuse while ensuring that the secondary battery has low DC resistance and high cycle performance and rate performance, thereby reducing the risk of short circuit and better dispersing mechanical stress. When the battery is subjected to external forces such as compression, the second coating and the positive current collector are more firmly bonded, reducing the peeling between the two, thereby improving the structural stability of the battery under mechanical abuse and delaying the occurrence of thermal runaway.

[0076] In one or more embodiments of this application, the areal density of the first coating 11 is ρmg / cm³. 20.01≤ρ≤0.05. For example, the value of ρ can be 0.01, 0.02, 0.03, 0.04, 0.05, or any two of these values. The range of ρ can be 0.01 to 0.05, 0.02 to 0.04, 0.03 to 0.04, 0.02 to 0.03, and all of these ranges, as well as sub-ranges. By controlling ρ within the above range, on the one hand, the flexibility of the first coating can be ensured while maintaining a good physical barrier effect. This not only helps the secondary battery to have good puncture performance, but also makes the first coating less likely to peel off when the secondary battery is impacted. On the other hand, it can also effectively delay heat conduction, prolong the thermal runaway trigger time, and improve the drop performance, impact performance, and puncture performance of the secondary battery. Furthermore, the transport resistance of active ions and the conduction resistance of electrons are relatively small, which helps to further reduce the DC resistance of the secondary battery and improve its rate performance and cycle performance, thereby giving the secondary battery higher electrical performance and safety performance.

[0077] This application does not impose any particular limitation on the method of controlling the above-mentioned areal density ρ, as long as it can achieve the purpose of this application. For example, during the preparation of the positive electrode sheet, the areal density ρ can be controlled by adjusting the solid content of the first coating slurry, the coating thickness of the first coating, A1 / A3, etc. For example, when the first coating slurry is coated on the surface of the positive electrode current collector, with other conditions remaining constant, increasing the solid content of the first coating slurry can increase the areal density ρ of the first coating, and vice versa; or, with other conditions remaining constant, increasing the coating thickness of the first coating can increase the areal density ρ of the first coating, and vice versa; or, with other conditions remaining constant, increasing A1 / A3 can increase the areal density ρ of the first coating, and vice versa.

[0078] 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).

[0079] The second coating includes a positive electrode active material. This application does not have any particular limitation 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.

[0080] The second coating 12 may further include a positive electrode conductive agent and a positive electrode binder. This application does not impose any particular limitation on the types of positive electrode conductive agents and positive electrode binders, as long as they can achieve the purpose of this application. For example, the positive electrode conductive agent may include, but is not limited to, at least one of carbon black, carbon nanotubes, carbon fibers, flake graphite, graphene, metallic materials, or conductive polymers. Among them, carbon fibers may include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or carbon nanofibers; metallic materials may include, but are not limited to, metal powders and / or metal fibers; specifically, metals may include, but are not limited to, at least one of copper, nickel, aluminum, or silver; and the aforementioned conductive polymers may include, but are not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole. The positive 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.

[0081] This application does not impose any particular restrictions on the mass ratio of the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder in the second coating 12. Those skilled in the art can choose according to actual needs, as long as the purpose of this application can be achieved.

[0082] This application does not impose any particular limitation on the thickness of the positive electrode current collector 10, as long as it achieves the purpose of this application. For example, the thickness of the positive electrode current collector can be from 5 μm to 20 μm.

[0083] This application does not impose any particular limitation on the preparation method of the positive electrode sheet, as long as it can achieve the purpose of this application. For example, in some embodiments, the preparation method of the positive electrode sheet includes, but is not limited to, the following steps: (1) preparing a first coating slurry and a second coating slurry; (2) coating the first coating slurry onto one surface of the positive current collector, drying it to obtain a positive electrode sheet with a first coating on one side, coating the second coating slurry onto the surface of the first coating away from the positive current collector, drying it to obtain a positive electrode sheet with a first coating and a second coating on one side; (3) repeating step (2) on the other surface of the positive current collector to obtain a positive electrode sheet with a first coating and a second coating on both sides; (4) obtaining the positive electrode sheet after cold pressing and slitting.

[0084] This application does not impose any particular limitation on the coating method of the first coating 11 slurry and the second coating 12 slurry, as long as the purpose of this application can be achieved. In one or more embodiments of this application, the first coating slurry can be coated on at least one surface of the positive electrode current collector by methods such as spraying or transfer printing to form a dotted first coating. The dotted first coating is beneficial to reducing the negative impact of the first coating on the flexibility and energy density of the electrode sheet, while providing sufficient anchoring effect to balance the adhesion between the positive electrode current collector and the second coating, the conductivity of the positive electrode sheet, and the interfacial impedance.

[0085] For example, applying the first coating slurry by spraying includes, but is not limited to, the following steps: using a spraying device to atomize the first coating slurry into tiny droplets, and spraying the droplets onto the surface of the positive electrode current collector. The spraying device can be a spray gun or an ultrasonic spraying device, etc.

[0086] This application does not impose any particular restrictions on the process parameters for applying the first coating slurry by spraying, as long as the purpose of this application can be achieved. For example, for a spray gun, the atomizing gas pressure can be 0.5MPa to 2MPa, the nozzle diameter can be 0.15mm to 0.35mm, the slurry flow rate can be 200mL / min to 1000mL / min, the spraying distance can be 1.5mm to 5mm, the spray gun moving speed can be 0.5cm / s to 3cm / s, and the spray gun angle can be 10° to 30°; for ultrasonic spraying equipment, the vibration frequency can be 50kHz to 200kHz, the feeding rate can be 300mL / min to 800mL / min, the spraying distance can be 2mm to 6mm, and the stage moving speed can be 1cm / s to 5cm / s, wherein the stage is used to fix the current collector.

[0087] In this application, for the spray gun, the ratio A1 / A3 of the projected area of ​​the first coating on the surface of the positive electrode current collector to the area of ​​the positive electrode current collector can be controlled by adjusting the atomizing air pressure, nozzle diameter, spray gun moving speed, solid content and viscosity of the first coating slurry, slurry flow rate, spraying distance, etc. For example, when other parameters remain constant, A1 / A3 first increases and then decreases with the increase of atomizing air pressure; when other parameters remain constant, A1 / A3 first increases and then tends to level off with the increase of nozzle diameter; when other parameters remain constant, the faster the spray gun moves, the lower A1 / A3, and vice versa; when other parameters remain constant, A1 / A3 first increases and then decreases with the increase of solid content of the first coating slurry; when other parameters remain constant, A1 / A3 first increases and then decreases with the increase of viscosity of the first coating slurry; when other parameters remain constant, A1 / A3 first increases rapidly and then tends to level off (or decreases slightly) with the increase of slurry flow rate; when other parameters remain constant, A1 / A3 first increases and then decreases with the increase of spraying distance. For ultrasonic spraying equipment, the ratio A1 / A3 of the projected area of ​​the first coating on the surface of the positive electrode current collector to the area of ​​the positive electrode current collector can be controlled by adjusting the vibration frequency, feeding rate, spraying distance, and the moving speed of the stage. For example, when other parameters are constant, A1 / A3 first increases and then decreases with increasing vibration frequency; when other parameters are constant, A1 / A3 first increases and then tends to level off with increasing feeding rate (A1 / A3 may decrease slightly when the feeding rate is too high); when other parameters are constant, A1 / A3 first increases and then decreases with increasing spraying distance; when other parameters are constant, the faster the moving speed of the stage, the lower A1 / A3, and vice versa.

[0088] This application does not impose any particular limitation on the solid content and viscosity of the first and second coating slurries, as long as the purpose of this application can be achieved. In one or more embodiments of this application, the solid content of the first coating slurry is 8% to 12%, and the viscosity of the first coating slurry is 100 mPa·s to 2000 mPa·s. For example, the solid content of the first coating slurry can be 8%, 9%, 10%, 11%, 12%, or a range consisting of any two of these values. The range of solid content can be 8% to 12%, 9% to 11%, 8% to 10%, 10% to 12%, and all such ranges and sub-ranges. The viscosity of the first coating slurry can be 100 mPa·s, 200 mPa·s, 400 mPa·s, 600 mPa·s, 800 mPa·s, 1000 mPa·s, 1200 mPa·s, 1400 mPa·s, 1600 mPa·s, 1800 mPa·s, 2000 mPa·s, or any two of these values. The viscosity range can be 100 mPa·s to 2000 mPa·s, 400 mPa·s to 1600 mPa·s, 800 mPa·s to 1200 mPa·s, 100 mPa·s to 1000 mPa·s, 1000 mPa·s to 2000 mPa·s, and all such ranges and sub-ranges. This application does not impose any particular restrictions on the above-mentioned drying, cold pressing, and slitting process parameters, as long as they achieve the purpose of this application.

[0089] In one or more embodiments of this application, the secondary battery includes a casing, an electrode assembly, and an electrolyte, with the electrode assembly and electrolyte housed within the casing. This application does not impose any particular limitation on the structure of the electrode assembly, as long as it achieves the purpose of this application. For example, the electrode assembly can be a stacked structure or a wound structure. The electrode assembly includes a negative electrode, a separator, and the aforementioned positive electrode. The separator is disposed between the positive and negative electrode to separate them, prevent internal short circuits in the secondary battery, allow electrolyte ions to pass freely, and does not affect the electrochemical charging and discharging process.

[0090] This application does not impose any particular limitation on the negative electrode sheet, as long as the purpose of this application can be achieved. For example, 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 aforementioned "negative electrode material layer disposed on at least one surface of the negative current collector" means that the negative electrode material layer can be disposed on one surface of the negative current collector along its own thickness direction, or it can be disposed on two surfaces of the negative current collector along its own thickness direction. It should be noted that the "surface" here can be the entire surface area of ​​the negative current collector, or it can be a partial surface area of ​​the negative current collector; this application does not impose any particular limitation, as long as the purpose of this application can be achieved.

[0091] 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, the negative electrode current collector may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, or copper foam, etc.

[0092] The negative electrode material layer of this application includes a negative electrode active material. This application does not impose any particular limitation on the type of negative electrode active material, as long as it can achieve the purpose of this application. For example, the negative electrode active material may include natural graphite, artificial graphite, mesophase microcarbon spheres (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composites, SiO2, etc. x (0 < x < 2), Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel-structured lithium titanate Li4Ti5O 12 At least one of Li-Al alloy or metallic lithium.

[0093] In this application, there are no particular limitations on the thickness of the negative electrode current collector and the negative electrode material layer, as long as the purpose of this application can be achieved. For example, the thickness of the negative electrode current collector is 4 μm to 15 μm, and the thickness of the negative electrode material layer is 30 μm to 170 μm.

[0094] Optionally, the negative electrode material layer may further include at least one of a negative electrode conductive agent, a negative electrode binder, and a thickener. This application does not impose any particular limitation on the types of negative electrode conductive agents, negative electrode binders, and thickeners in the negative electrode material layer, as long as they achieve the purpose of this application. For example, the negative electrode binder and negative electrode conductive agent may include, but are not limited to, at least one of the substances selected for the second coating layer described above, and the thickener may include, but is not limited to, at least one of sodium carboxymethyl cellulose or lithium carboxymethyl cellulose.

[0095] This application does not impose any particular restrictions on the mass ratio of negative electrode active material, negative electrode conductive agent, negative electrode 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.

[0096] 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 conductive layer commonly used in the art. For example, the conductive layer may include 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.

[0097] This application does not impose any particular limitation on the diaphragm, as long as it can achieve the purpose of this application. For example, the diaphragm material may include, but is not limited to, at least one of polyethylene (PE), polyolefin (PO) based on polypropylene (PP), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid. The type of diaphragm may include at least one of woven membrane, nonwoven membrane, microporous membrane, composite membrane, rolled membrane, or spun membrane.

[0098] 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, membrane, 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.

[0099] Optionally, a surface treatment layer is disposed 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 a mixture of polymer and inorganic material. For example, the inorganic layer includes inorganic particles and a binder. This application does not particularly limit the inorganic particles; for example, the inorganic particles may include at least one of 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. The polymer layer includes a polymer, and the polymer material includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, or polyvinylidene fluoride or poly(vinylidene fluoride-hexafluoropropylene).

[0100] 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.

[0101] This application does not impose any particular restrictions on the electrolyte, as long as it can achieve the purpose of this application. For example, the electrolyte includes lithium salts and non-aqueous solvents.

[0102] This application does not impose any particular limitation on the type of 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 lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium bis(fluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(oxalateborate)borate (LiBOB), or lithium difluorooxalateborate (LiDFOB). This application does not limit the content of lithium salt in the electrolyte, as long as it achieves the purpose of this application.

[0103] This application does not impose any particular limitation on the types of non-aqueous solvents mentioned above, 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. The aforementioned carbonate compounds may include, but are not limited to, at least one of chain carbonate compounds, cyclic carbonate compounds, or fluorinated carbonate compounds. The aforementioned chain carbonate compounds may include, but are not limited to, at least one of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, or methyl ethyl carbonate. The aforementioned cyclic carbonate compounds may include, but are not limited to, at least one of ethylene carbonate, propylene carbonate, butylene carbonate, or ethylene ethylene carbonate. The aforementioned 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 ethylene glycol dimethyl ether, 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, 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.

[0104] This application does not impose any particular restrictions on the outer casing; any casing known in the art can be used, as long as it achieves the purpose of this application. For example, the outer casing can be a rigid casing or a flexible casing; wherein, the material of the rigid casing can be metal or rigid plastic, and this application does not limit the type of metal, and any metal casing known in the art can be used, as long as it achieves the purpose of this application; the flexible casing can be a metal plastic film, such as aluminum-plastic film, steel-plastic film, etc.

[0105] 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. For example, secondary batteries may include, but are not limited to: lithium metal secondary batteries, lithium-ion secondary batteries (lithium-ion batteries), sodium-ion secondary batteries (sodium-ion batteries), lithium polymer secondary batteries, and lithium-ion polymer secondary batteries.

[0106] This application does not impose any particular limitation on the preparation method of the secondary battery; any preparation method known in the art can be used, as long as it achieves the purpose of this application. For example, the preparation method of the secondary battery includes, but is not limited to, the following steps:

[0107] A secondary battery is obtained by stacking the positive electrode, separator, and negative electrode in sequence, and then winding and folding them as needed to obtain a wound electrode assembly. The electrode assembly is placed inside a housing, electrolyte is injected into the housing, and the housing is sealed. Alternatively, a secondary battery is obtained by stacking the positive electrode, separator, and negative electrode in sequence, fixing the four corners of the entire stacked structure, placing the electrode assembly inside a housing, electrolyte is injected into the housing, and the housing is sealed.

[0108] A third aspect of this application provides an electronic device comprising the secondary battery described in any of the foregoing embodiments. Therefore, the electronic device has good performance characteristics.

[0109] 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: laptop computers, 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.

[0110] Example

[0111] 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.

[0112] Test method and equipment:

[0113] Sampling method for positive electrode:

[0114] The lithium-ion batteries prepared in each embodiment and comparative example were completely discharged, the batteries were disassembled to obtain the positive electrode sheet, the positive electrode sheet was soaked in dimethyl carbonate (DMC) for 20 minutes, and then rinsed with dimethyl carbonate and acetone in sequence. After that, it was placed in an oven and baked at 80°C for 12 hours to obtain the dried positive electrode sheet.

[0115] Tests for T1 and T2:

[0116] The dried positive electrode sheet was prepared by ion polishing to obtain an electrode sheet with a longitudinal section (parallel to the thickness direction of the positive electrode sheet). A ZEIS-SEM (Sigma-02-33) scanning electron microscope was used to measure the thickness T1 of the first coating and the thickness T2 of the second coating in the positive electrode sheet. Each characteristic quantity was measured at five locations on the positive electrode sheet, with adjacent measurement points spaced 2 mm to 3 mm apart. The average of the five measurements was taken to obtain T1 and T2, with units of μm.

[0117] Tests for D1, D2, D3, and L:

[0118] The dried positive electrode sheet was then ion-polished to obtain an electrode sheet with a longitudinal section (parallel to the thickness direction of the positive electrode sheet). It is particularly important to note that, in the positive electrode sheet, inorganic materials, conductive agents, binders, and dispersants are used in combination. To facilitate more accurate characterization of the geometric dimensions of the inorganic materials and conductive agents in the positive electrode sheet, the following definitions are used to test and characterize their geometric dimensions:

[0119] The longitudinal section of the electrode prepared above was tested using a scanning electron microscope (ZEISS Sigma-02-33). Elemental analysis was performed using the SEM to select 100 particles containing marker elements of inorganic materials (such as Al, Si, Ti, Mg, Zn, Ni, etc.) and 100 particles containing marker element (C) of the conductive agent. For spherical particles, the longest distance between two points on each particle was used as the particle diameter. For elongated particles, the longest distance along the length of each particle was used as the particle length, and the diameter of the cross-section (perpendicular to the length direction) of each particle (when the conductive agent is a hollow cylindrical tube particle, the diameter refers to the outer diameter of the cross-section) was used as the particle's radial length. The dimensions of the selected inorganic material particles and conductive agent particles were measured and statistically analyzed using image analysis software (such as ImageJ). Based on the measured dimensions of all inorganic material particles and all conductive agent particles, their corresponding average values ​​were calculated. This yielded the average diameter, average length, or average particle size of the conductive agent, and the average particle size D3 of the inorganic material. When the conductive agent was carbon black, the average particle size was D1 μm; when the conductive agent was carbon nanotubes, the average tube diameter was D2 nm and the average tube length was L μm.

[0120] Measurements of w1, w2, w3, w4, B, and Mw:

[0121] The lithium-ion batteries prepared in each embodiment and comparative example were completely discharged. The batteries were disassembled to obtain the positive electrode sheet. The second coating on the positive electrode sheet was scraped off to obtain the electrode sheet coated with the first coating. The positive electrode sheet coated only with the first coating was immersed in liquid nitrogen for 5 minutes to embrittle it. The first coating was then scraped off with a blade to obtain the first coating powder. The first coating powder was passed through a 400-mesh sieve (38 μm pore size) to remove positive electrode current collector fragments. The sieved first coating powder was then vacuum dried at 60°C for 12 hours to remove electrolyte residue. Subsequently, bromoform (CHBr3, p = 2.89 g / cm³) was used. 3 ) and n-hexane (C6H 14 ρ = 0.66 g / cm³ 3Mixed solutions of different densities were obtained by mixing in ratios of 9:1, 7:3, 4:6, and 2:8. The mixed solutions of different densities were then injected into different centrifuge tubes. 20 mg of the dried first coating powder was dispersed in 1 mL of isopropanol and sonicated for 10 min (300 W) to obtain a suspension of the first coating powder. The suspension of the first coating powder was then injected into centrifuge tubes. Using a syringe, slowly inject the 9:1 mixture into a centrifuge tube containing the first coating powder suspension. After centrifuging at 2000 rpm for 120 minutes, based on the principle of density difference, the inorganic material will accumulate in the lower layer of the centrifuge tube. Use a pipette to remove the lower layer containing the inorganic material. The remaining supernatant A (containing dispersant, binder, and conductive agent) is used for subsequent separation. Then, inject the 7:3 mixture into a centrifuge tube containing supernatant A. After centrifuging at 2000 rpm for 120 minutes, based on the principle of density difference, the dispersant will accumulate in the lower layer of the centrifuge tube. Use a pipette to remove the lower layer containing the dispersant. The remaining supernatant B (containing binder and conductive agent) is used for subsequent separation. Then, inject the 7:3 mixture into a centrifuge tube containing supernatant A. A 2:6 mixture of solutions was injected into a centrifuge tube containing supernatant B. After centrifugation at 2000 rpm for 120 min, based on the principle of density difference, the binder would accumulate in the lower layer of the centrifuge tube. The lower layer containing the binder was removed with a pipette, and the remaining supernatant C (containing the conductive agent) was used for subsequent separation. Finally, a 2:8 mixture of solutions was injected into a centrifuge tube containing supernatant C. After centrifugation at 2000 rpm for 120 min, based on the principle of density difference, the conductive agent would accumulate in the lower layer of the centrifuge tube. The lower layer containing the conductive agent was removed with a pipette. This process yielded solutions containing inorganic materials, dispersants, binders, and conductive agents, respectively. After vacuum drying, solid powders of the conductive agent, binder, dispersant, and inorganic materials were obtained. The masses of the inorganic material were measured as m1 mg, the conductive agent as m2 mg, the binder as m3 mg, and the dispersant as m4 mg. Then w1 = (m1 / 20) × 100%, w2 = (m2 / 20) × 100%, w3 = (m3 / 20) × 100%, w4 = (m4 / 20) × 100%.

[0122] According to the national standard "Determination of Specific Surface Area of ​​Solid Substances by Gas Adsorption BET Method" (GB / T 19587-2017), a specific surface area analyzer (model TristarⅡ3020M) was used to test the specific surface area of ​​the conductive agent obtained above by nitrogen adsorption method, and the specific surface area B of the conductive agent was obtained, with the unit being m². 2 / g.

[0123] Refer to the national standard GB / T 21863-2008 Gel permeation chromatography (GPC) using tetrahydrofuran as the eluent; the weight-average molecular weight (Mw) of the binder is determined by gel permeation chromatography (instrument model: ACQUITY APC), with the unit being g / mol.

[0124] Measurement of ρ:

[0125] The second coating on the dried positive electrode sheet is scraped off to obtain the electrode sheet to be tested coated with the first coating. The cross-sectional area is X cm². 2 The sample of the electrode to be tested is weighed on a balance and recorded as m1, in mg. Then the first coating on the sample of the electrode to be tested is removed, and the positive current collector is weighed on a balance and recorded as m2, in mg.

[0126] For a negative electrode sheet with a first coating on only one side, ρ = (m1 - m2) / X, with units of mg / cm³. 2 ;

[0127] For a negative electrode sheet with a first coating on both sides, ρ=(m1-m2) / 2X, the unit is mg / cm³. 2 .

[0128] Measurement of P:

[0129] The peel strength of the second coating was measured using a 180° peel test. The coating (including the first and second coatings) on one side of the dried positive electrode sheet was scraped off to obtain a single-sided coated electrode sheet. The electrode sheet was cut into 15mm × 54mm test strips. A 15mm × 55mm double-sided adhesive tape (NITTO.NO5000NS) was attached to a steel plate, and the test strip was then attached to the tape with the positive current collector facing down. A 15mm × 70mm paper strip was connected to the test strip using the double-sided adhesive tape, adhering to the surface of the second coating away from the positive current collector. A 2kg roller was manually pushed across the test strip 8 times to obtain the test sample. A tensile testing machine was used. The test sample was fixed on the test table, the paper strip was folded upwards 180° and secured with clamps, and then the tensile testing machine was used to pull the paper strip at a speed of 50mm / min until the second coating on the double-sided adhesive surface separated from the positive current collector. The test data was then saved. The peel strength P of the second coating is calculated based on the tensile force and tensile displacement during the separation of the second coating and the positive current collector, with units of N / m.

[0130] Measurement of R1 and R2:

[0131] The second coating on the dried positive electrode sheet was scraped off, leaving only the first coating on the electrode sheet to be tested. This electrode sheet was then cut into 60mm x 80mm pieces. The thickness of the cut electrode sheet was then measured using a Yuaneng Technology BER1200 electrode resistance meter. The test pressure was 400kPa, and the test time was 15s. The thickness of the cut electrode sheet was input, and 12 values ​​were measured. A test report was output, and the average of the 12 film resistances was taken to obtain the resistance R1 of the first coating, in Ω / mm². 2 .

[0132] Similarly, the dried positive electrode sheet was cut into 60mm × 80mm pieces to obtain the electrode sheet to be tested. The electrode sheet to be tested was then tested according to the method described for resistance R1, to obtain the resistance R2 of the positive electrode sheet, in Ω / mm. 2 .

[0133] It is particularly important to note that when a coating (including a first coating and a second coating) is provided on only one surface of the positive current collector, R1 refers to the sum of the resistances of the positive current collector and the first coating provided on one surface of the positive current collector, and R2 refers to the sum of the resistances of the positive current collector and the coating provided on one surface of the positive current collector; when a coating is provided on both surfaces of the positive current collector, R1 refers to the sum of the resistances of the positive current collector and the first coating provided on both surfaces of the positive current collector, and R2 refers to the sum of the resistances of the positive current collector and the coating provided on both surfaces of the positive current collector.

[0134] Measurements of S1 and S2:

[0135] The second coating on the dried positive electrode sheet is scraped off, leaving only the first coating on the electrode sheet to be tested. The electrode sheet is cut to 38mm × 80mm and clamped on a stiffness tester (Guangdong Beidou Precision Instruments Co., Ltd., PT-208), with one end fixed and the other end suspended, and a bending force applied by the instrument. The bending length of the sample (i.e., the distance from the fixed end to the end of the electrode sheet) is set to 50mm. The loading head is pressed down at a constant speed of 0.3m / min, and the stiffness of the electrode sheet is measured when it bends from 0° to 90°. The stiffness value is obtained using the instrument's built-in algorithm. Four parallel samples are measured in each group, and the average value is taken to obtain the stiffness S1 of the positive current collector with the first coating, in mN·m.

[0136] Similarly, the dried positive electrode sheet was cut into 38mm × 80mm pieces to obtain the electrode sheet to be tested. The electrode sheet to be tested was tested according to the above-mentioned stiffness S1 test method. Four parallel samples were tested in each group, and the average value was taken to obtain the stiffness S2 of the positive electrode sheet, in mN·m.

[0137] It is particularly important to note that when a coating is provided on only one surface of the positive electrode current collector, S1 refers to the sum of the stiffness of the positive electrode current collector and the first coating provided on one surface of the positive electrode current collector, and S2 refers to the sum of the stiffness of the positive electrode current collector and the coating provided on one surface of the positive electrode current collector; when a coating is provided on both surfaces of the positive electrode current collector, S1 refers to the sum of the stiffness of the positive electrode current collector and the first coating provided on both surfaces of the positive electrode current collector, and S2 refers to the sum of the stiffness of the positive electrode current collector and the coating provided on both surfaces of the positive electrode current collector.

[0138] Measurement of λ:

[0139] The second coating on the dried positive electrode sheet is scraped off, leaving only the first coating on the electrode sheet to be tested. This electrode sheet is then cut into 30mm x 30mm pieces. The cut electrode sheets are assembled into a sample in the following order: heating plate - electrode sheet - cooling plate. The sample is pre-compressed to a contact pressure of 0.02MPa, and the electrode sheet is heated to a set temperature difference ΔT (where ΔT refers to the temperature difference between the two sides of the electrode sheet, in K) using a heating plate at a heating rate ≤2℃ / min. The temperature of the electrode sheet is continuously monitored until it satisfies the relationship: ΔT / T-average < 0.5%, and this monitoring continues for 30 minutes. During this period, the electrode sheet must satisfy the aforementioned relationship; where T-average refers to the average temperature of the surface of the electrode sheet near the heating plate during the heating process and over the duration of the monitoring. Subsequently, the heat flux value Q of the electrode sheet after temperature stabilization is measured using a heat flux sensor, in W. According to Fourier's law, the thermal conductivity λ = (Q × Δx) / (A × ΔT), with units of W / (m·K); where A is the effective heat transfer area (i.e., the actual contact area between the electrode under test and the heating plate), with units of m². 2 Δx is the sample thickness, in meters (m).

[0140] It is particularly important to note that when a coating is provided on only one surface of the positive electrode current collector, λ refers to the sum of the thermal conductivity of the positive electrode current collector and the first coating provided on one surface of the positive electrode current collector; when a coating is provided on both surfaces of the positive electrode current collector, λ refers to the sum of the thermal conductivity of the positive electrode current collector and the first coating provided on both surfaces of the positive electrode current collector.

[0141] A1 / A3 test:

[0142] The second coating on the dried positive electrode sheet was scraped off to obtain the electrode sheet to be tested, which was coated only with the first coating. The electrode sheet to be tested was cut into 1cm×1cm samples. The sample surface was gently wiped with a lint-free cloth soaked in anhydrous ethanol to remove dust and electrostatically adsorbed particles, and then dried. The dried sample was then flatly pasted onto a glass slide, ensuring that there were no wrinkles. Using a Keyence optical microscope (model: VHX-S750E), the pixel size was calibrated with a standard ruler. The magnification was set to 20x to 50x, the light source intensity to 30% to 50%, and the image resolution to 1920 pixels × 1080 pixels. Five images were taken. The images were analyzed to obtain the coverage of the first coating in the five images. The average value was taken as the coverage of the sample, i.e., A1 / A3.

[0143] A1 / A2 test:

[0144] The length and width of the second coating of the dried positive electrode sheet are measured using an OMM (Optical Measuring Machine) to calculate A1. Then the second coating is scraped off to obtain the electrode sheet to be tested coated with the first coating. The length and width of the first coating are measured to calculate A2. Then A1 / A2 is calculated.

[0145] DC resistance testing:

[0146] At 25℃, the lithium-ion battery was charged at a constant current of 0.5C to 3.3V, then charged at a constant voltage of 3.3V to the cutoff current of 0.2C, and left to stand for 30 minutes. It was then discharged at 0.1C for 1 second, and the voltage value was recorded as U1. Finally, it was discharged at 1C for 360 seconds, and the voltage value was recorded as U2. This charging and discharging process was repeated 5 times. The DC resistance of the lithium-ion battery at 25℃ was calculated using the following formula: R = (U2 - U1) / (1C - 0.1C), where "1C" refers to the current value required to completely discharge the lithium-ion battery within 1 hour.

[0147] Ratio capacity retention test:

[0148] At 25℃, the lithium-ion battery was discharged at a constant current of 0.5C to 2.5V, allowed to stand for 5 minutes, charged at a constant current of 0.5C to 3.3V, and then charged at a constant voltage of 3.3V to the cutoff current of 0.2C, followed by a 30-minute rest. This charging and discharging process was repeated, adjusting the discharge rate, and discharge tests were conducted at 0.5C and 1C respectively to obtain the corresponding discharge capacity. The percentage ratio of the discharge capacity at 0.5C to 1C was used as the basis for evaluating rate performance, yielding the rate capacity retention rate, expressed as a percentage.

[0149] Cyclic performance testing:

[0150] At 25°C, the lithium-ion battery was charged at a constant current of 0.5C to 3.3V, then charged at a constant voltage of 3.3V until the cutoff current of 0.2C, and finally discharged at a constant current of 0.5C to 2.5V. This discharge capacity was recorded as the first cycle discharge capacity. The lithium-ion battery was subjected to 500 cycles under the above conditions, and the discharge capacity after 500 cycles was recorded. The cycle capacity retention rate characterizes the cycle performance of the lithium-ion battery; the higher the cycle capacity retention rate, the better the cycle performance of the lithium-ion battery. Cycle capacity retention rate (%) = (Discharge capacity after 500 cycles / Discharge capacity of the first cycle) × 100%.

[0151] Puncture performance test:

[0152] Before and after testing, inspect the appearance and take photos. Place the lithium-ion battery in a 25℃ constant temperature chamber and let it stand for 30 minutes to allow it to reach a constant temperature. Charge the battery at a constant current of 0.5C until the voltage reaches 3.5V, then charge at a constant voltage of 3.5V until the current reaches the cutoff current of 0.2C. Let it stand for 60 minutes; at this point, the lithium-ion battery is fully charged. Transfer the fully charged lithium-ion battery to a nail penetration tester. Maintain the testing environment temperature at 25℃. Use a 5mm diameter steel nail with a 45° taper to penetrate the center of the largest surface of the lithium-ion battery at a uniform speed of 25mm / s, holding it for 10 minutes. The entire penetration test must be video recorded. The nail is for single use only; a new steel nail must be used for each lithium-ion battery test. A lithium-ion battery that does not catch fire or explode is considered to have passed. Each group tests 20 lithium-ion batteries.

[0153] Puncture test pass rate = number of lithium-ion batteries that pass the puncture test / 20. The higher the puncture test pass rate, the better the safety performance of the lithium-ion battery.

[0154] Drop performance test:

[0155] The lithium-ion battery under test was placed in an environment of 25°C and charged at a constant current of 0.5C to a voltage of 3.5V. Then, it was charged at a constant voltage of 3.5V to a cutoff current of 0.2C and allowed to stand for 60 minutes. At this point, the lithium-ion battery was fully charged. The fully charged lithium-ion battery was placed in a fixed fixture, and its appearance was inspected and photographed before and after the test. Subsequently, the fully charged lithium-ion battery was dropped from a height of 1.8m, once from each of the two sides (head and tail) and once from each of the four corners, for a total of 7 rounds of testing, with 6 drops per round. The drop sequence was: head -> tail -> head right corner -> tail right corner -> head left corner -> tail left corner (angle: 45±15 degrees). The voltage and internal resistance of the lithium-ion battery were measured after the drop test and 2 hours after the test. The criteria for passing the drop test were: no fire, no explosion, no smoke, no leakage, and voltage drop <100mV. Twenty lithium-ion batteries were tested for each example or comparative example.

[0156] Drop test pass rate = number of lithium-ion batteries that pass the drop test / 20. The higher the drop test pass rate, the better the safety performance of the lithium-ion battery.

[0157] Impact performance test:

[0158] At 25°C, the lithium-ion battery was charged to 3.5V at a current of 0.5C, and then charged to a cutoff current of 0.2C at 3.5V. After standing for 60 minutes, the lithium-ion battery was considered fully charged. Two relatively larger opposing surfaces of the fully charged lithium-ion battery were designated as surface A and surface B. Surface B has a first direction and a second direction perpendicular to each other, with the dimension of surface B in the first direction being larger than its dimension in the second direction. At 25°C, the fully charged lithium-ion battery was placed on a test platform with surface A in direct contact with the platform. A 15.8mm diameter rod was placed at the center of surface B, with the rod's axis perpendicular to the first direction and parallel to the second direction (forming a cross with surface B). The midpoint of the rod's axis coincided with the center of surface B. A 10kg hammer was dropped vertically from a height of 500mm, striking the midpoint of the rod. Judgment criteria: If the battery does not catch fire, explode, smoke, leak, crack, leak gas, or disintegrate, it is considered to have passed the impact test. Each group of 20 lithium-ion batteries is tested.

[0159] Impact test pass rate = number of lithium-ion batteries that pass the impact test / 20. The higher the impact test pass rate, the better the safety performance of the lithium-ion battery.

[0160] Example 1

[0161] <Preparation of the positive electrode>

[0162] The mixture includes inorganic material boehmite (average particle size D3 is 1 μm) and conductive agent conductive carbon black (average particle size D1 is 2 μm, specific surface area B is 70 m²). 2 / g), binder sodium polyacrylate (PAA-Na, weight-average molecular weight Mw is 4×10⁻⁶). 5 The raw materials (g / mol) and dispersant sodium carboxymethyl cellulose (CMC-Na) were added to water at a solid mass ratio of 40:16:42:2 and mixed. The mixture was stirred evenly under the action of a vacuum mixer to obtain a first coating slurry with a solid content of 10% and a viscosity of 200 mPa·s.

[0163] The positive electrode active material includes lithium iron manganese phosphate (chemical formula LiFe). 0.7 Mn 0.3 PO4) and ternary materials (chemical formula LiNi) 0.8 Co 0.1 Mn 0.1The raw materials of O2, conductive agent acetylene black and binder polyvinylidene fluoride are added to N-methylpyrrolidone (NMP) in a solid mass ratio of 67.9:29.1:2:1 and stirred evenly under vacuum to obtain a second coating slurry with a solid content of 70%.

[0164] An aluminum foil with a surface tension of 40 Dyne / cm and a thickness of 13 μm was used as the positive electrode current collector. A dotted first coating was formed by spraying. The ratio of the projected area of ​​the first coating on the surface of the positive electrode current collector to the area of ​​the positive electrode current collector, A1 / A3, was 0.88. The specific steps included: atomizing the first coating slurry into tiny droplets using a spray gun, spraying the droplets onto the surface of the positive electrode current collector, with an atomizing gas pressure of 2 MPa, a nozzle diameter of 0.25 mm, a slurry flow rate of 800 mL / min, a spraying distance of 3 mm, a spray gun moving speed of 2.5 cm / s, and a spray gun angle of 15°. Subsequently, the same second coating slurry was applied to the surface of the first coating. The ratio of the projected area A1 of the first coating on the surface of the positive electrode current collector to the projected area A2 of the second coating on the surface of the positive electrode current collector, A1 / A2, was 1. The foil was then dried. The above steps were then repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with both the first and second coatings on both sides. After cold pressing, cutting, and welding of tabs, the positive electrode sheet of this embodiment is obtained. Specifically, by controlling the coating amounts of the first and second coating slurries, the areal density ρ of the first coating is made to be 0.03 mg / cm³. 2 The thickness of the first coating on one side, T1, is 1 μm, and the thickness of the second coating on one side, T2, is 60 μm. Therefore, T1 / D3 is 1.

[0165] Based on the total mass of the first coating, the mass percentage of boehmite (w1) is 40%, the mass percentage of conductive carbon black (w2) is 16%, the mass percentage of sodium polyacrylate (w3) is 42%, and the mass percentage of sodium carboxymethyl cellulose (w4) is 2%. Based on the total mass of the second coating, the mass percentage of lithium iron manganese phosphate (LMP) is 67.9%, the mass percentage of ternary materials is 29.1%, the mass percentage of conductive agent is 2%, and the mass percentage of binder is 1%.

[0166] <Preparation of Negative Electrode Sheets>

[0167] Raw materials including artificial graphite (anode active material), styrene-butadiene rubber (SBR) (binder), and carboxymethyl cellulose (CMC) (dispersant) were added to deionized water at a solid mass ratio of 96:1.5:2.5 and mixed evenly under the action of a vacuum mixer to obtain a cathode material slurry with a solid content of 55%.

[0168] A 10μm thick copper foil was used as the negative electrode current collector. The aforementioned negative electrode material slurry was uniformly coated onto one surface of the copper foil and dried to obtain a negative electrode sheet with a single-sided negative electrode material layer. The above steps were then repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided negative electrode material layer. After cold pressing, cutting, and welding of tabs, the negative electrode sheet of this embodiment was obtained.

[0169] Of these, based on the total mass of the negative electrode material layer, the mass percentage of artificial graphite is 96%, the mass percentage of styrene-butadiene rubber is 1.5%, and the mass percentage of carboxymethyl cellulose is 2.5%.

[0170] <Preparation of Electrolyte>

[0171] In an environment with a water content of less than 10 ppm, ethylene carbonate (EC) and propylene carbonate (PC) are mixed at a mass ratio of 3:1 to obtain an organic solvent. Lithium salt LiPF6 is then added to the organic solvent and mixed thoroughly to obtain the electrolyte. The electrolyte comprises 30% lithium salt by mass, with the remainder being organic solvent.

[0172] <Isolation membrane>

[0173] A porous polyethylene film with a thickness of 7μm (provided by Celgard) was used as the separator.

[0174] <Preparation of Lithium-ion Batteries>

[0175] The prepared positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. 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.

[0176] Example 2

[0177] Except for adjusting the relevant parameters according to Table 1, everything else is the same as in Example 1.

[0178] Examples 3 to 6

[0179] Except for adjusting the relevant parameters according to Table 1, everything else is the same as in Example 2.

[0180] Example 7

[0181] Except for adjusting the relevant parameters according to Table 1, everything else is the same as in Example 1.

[0182] Example 8

[0183] Boehmite was replaced with alumina, carbon black with carbon nanotubes, PAA-Na with polyvinyl alcohol, and CMC-Na with CMC-Li, with the rest being the same as in Example 1.

[0184] Example 9

[0185] Replace boehmite with kaolin, carbon black with carbon nanotubes, PAA-Na with polyethylene oxide, and CMC-Na with CMC-Li, otherwise the same as in Example 1.

[0186] Examples 10 to 23

[0187] Except for adjusting the relevant parameters according to Table 1, everything else is the same as in Example 1.

[0188] Examples 24 to 26

[0189] Except for adjusting the relevant parameters according to Table 1, everything else is the same as in Example 23.

[0190] Examples 27 to 32

[0191] Except for adjusting the relevant parameters according to Table 1, the rest is the same as in Example 1. In Examples 29 to 32, A1 / A2 is further controlled by adjusting the solid content and flow rate of the first coating slurry during the spraying process of the first coating.

[0192] Comparative Example 1

[0193] Except for adjusting the relevant preparation parameters according to Table 1, everything else is the same as in Example 1.

[0194] Comparative Example 2

[0195] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Comparative Example 1.

[0196] Comparative Example 3

[0197] Except for adjusting the relevant preparation parameters according to Table 1, everything else is the same as in Example 6.

[0198] Comparative Example 4

[0199] Except for adjusting the relevant preparation parameters according to Table 1, everything else is the same as in Example 1.

[0200] The parameters and test results of each embodiment and comparative example are shown in Table 1 and Table 2.

[0201]

[0202]

[0203]

[0204]

[0205] As can be seen from Examples 1 to 32 and Comparative Examples 1 to 4, by setting a first coating between the positive current collector and the second coating of the positive electrode sheet, and simultaneously controlling the mass percentage content of inorganic materials, conductive agents, binders, and dispersants in the first coating within the range of this application, the safety performance (including puncture performance, drop performance, and impact performance) and electrical performance (including DC resistance, rate performance, and cycle performance) of the lithium-ion battery are high, especially the puncture performance. At the same time, the adhesion between the second coating and the positive current collector is improved. In Comparative Example 1, the mass percentage content of inorganic materials and conductive agents in the first coating is not within the range of this application; in Comparative Example 2, the mass percentage content of conductive agents in the first coating is not within the range of this application; in Comparative Example 3, the mass percentage content of inorganic materials in the first coating is not within the range of this application; and in Comparative Example 4, the mass percentage content of binders and dispersants in the first coating is not within the range of this application. This shows that by controlling the mass percentage content of inorganic materials, conductive agents, binders, and dispersants in the first coating within the range of this application, the electrical and safety performance of the lithium-ion battery can be effectively improved. The lithium-ion batteries in Examples 1 to 32 exhibited lower DC resistance and higher rates of capacity retention, cycle capacity retention, puncture test pass rate, drop test pass rate, and impact test pass rate. The puncture test pass rate, in particular, indicates improved electrical and safety performance of the lithium-ion batteries. In Comparative Example 2, the first coating contained a high content of inorganic materials and a low content of conductive agent, resulting in poor conductivity of the positive electrode and consequently, a lower cycle capacity retention rate for the lithium-ion battery.

[0206] The T1 / D3 value typically affects the safety and electrical performance of lithium-ion batteries. As can be seen from Examples 10 to 15, when the T1 / D3 value is within the range specified in this application, the lithium-ion battery exhibits lower DC resistance and higher rate capacity retention, cycle capacity retention, puncture test pass rate, drop test pass rate, and impact test pass rate. The puncture test pass rate is particularly noteworthy, indicating that the lithium-ion battery of this application possesses high electrical and safety performance. However, in Examples 10 and 15, the thickness of the first coating is relatively thin, which reduces the barrier effect of the first coating, resulting in a lower puncture test pass rate for the lithium-ion battery.

[0207] The value of T2 typically affects the safety and electrical performance of lithium-ion batteries. As can be seen from Examples 1, 16, and 17, when the value of T2 is within the range of this application, the DC resistance of the lithium-ion battery is low, and the rate capacity retention rate, cycle capacity retention rate, puncture test pass rate, drop test pass rate, and impact test pass rate are all high. In particular, the puncture test pass rate indicates that the lithium-ion battery of this application has high electrical and safety performance.

[0208] The value of Mw typically affects the safety and electrical performance of lithium-ion batteries. As can be seen from Examples 1, 18, and 19, when the value of Mw is within the range specified in this application, the lithium-ion battery exhibits lower DC resistance and higher rate capacity retention, cycle capacity retention, puncture test pass rate, drop test pass rate, and impact test pass rate. The puncture test pass rate, in particular, indicates that the lithium-ion battery of this application possesses high electrical and safety performance. In Example 19, the value of Mw is relatively large, corresponding to an increase in the value of P. However, this also reduces the stability of the first coating slurry and worsens the uniformity of the conductive agent's dispersion in the slurry, thereby slightly decreasing the rate capacity retention and cycle capacity retention.

[0209] The size and structure of the conductive agent, such as the values ​​of D1, D2, L, and B, typically affect the safety and electrical performance of lithium-ion batteries. Examples 1, 20, and 22 show that when the conductive agent is carbon black, and the average particle size D1 of the carbon black is within the range specified in this application, the lithium-ion battery exhibits lower DC resistance and higher rate capacity retention, cycle capacity retention, puncture test pass rate, drop test pass rate, and impact test pass rate, especially the puncture test pass rate. Examples 23 and 26 show that when the conductive agent is carbon nanotubes, and the average tube diameter D2 and average tube length L of the carbon nanotubes are within the range specified in this application, the lithium-ion battery exhibits lower DC resistance and higher rate capacity retention, cycle capacity retention, puncture test pass rate, drop test pass rate, and impact test pass rate, especially the puncture test pass rate, indicating that the lithium-ion battery of this application possesses high electrical and safety performance.

[0210] The values ​​of A1 / A2 and A1 / A3 typically affect the safety and electrical performance of lithium-ion batteries. As can be seen from Examples 1, 27 to 32, when the values ​​of A1 / A2 and A1 / A3 are within the range of this application, the lithium-ion battery exhibits lower DC resistance and higher rate capacity retention, cycle capacity retention, puncture test pass rate, drop test pass rate, and impact test pass rate. The puncture test pass rate, in particular, indicates that the lithium-ion battery of this application possesses high electrical and safety performance. In Example 32, the positive electrode has a larger A1 / A3 value, which has a smaller impact on the electrical and safety performance of the lithium-ion battery, but increases production costs.

[0211] like Figures 2 to 4 In the figure, the white area is the exposed surface of the positive electrode current collector, and the black area is the surface of the first coating. Figure 2 The ratio of A1 to A3 is 0.74, corresponding to a peel strength of 14.32 N / m for the second coating; Figure 3 The ratio of A1 to A3 is 0.84, corresponding to a peel strength of 15.16 N / m for the second coating. Figure 4 In Example 32, A1 / A3 is 0.88, corresponding to a peel strength of 15.61 N / m for the second coating. Referring to Table 2, in Example 32, A1 / A3 is 1, corresponding to a peel strength of 11.36 N / m for the second coating. Therefore, controlling A1 / A3 within the scope of this application is beneficial for improving the peel strength of the second coating.

[0212] 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.

[0213] 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.

[0214] 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 positive electrode sheet, the positive electrode sheet comprising a positive current collector and a coating disposed on at least one surface of the positive current collector, the coating comprising a first coating and a second coating, wherein the first coating is disposed between the positive current collector and the second coating along the thickness direction of the positive electrode sheet; The first coating comprises inorganic materials, a conductive agent, a binder, and a dispersant; based on the total mass of the first coating, the mass percentage of the inorganic materials is w1, 15% ≤ w1 ≤ 50%, the mass percentage of the conductive agent is w2, 15% < w2 ≤ 40%, the mass percentage of the binder is w3, 30% ≤ w3 ≤ 50%, and the mass percentage of the dispersant is w4, 0 ≤ w4 ≤ 5%.

2. The secondary battery according to claim 1, wherein, The sum of the resistances of the positive current collector and the first coating is R1Ω / mm. 2 , 0.001≤R1≤0.5; and / or, the resistance of the positive electrode is R2Ω / mm 2 , 0.1≤R2≤4.

3. The secondary battery according to claim 2, wherein, 0.1≤R1≤0.3; and / or, 1≤R2≤3.

4. The secondary battery according to claim 1, wherein, The positive electrode sheet satisfies at least one of the following characteristics: (1)35％≤w1≤50％； (2)15％＜w2≤25％； (3) The sum of the thermal conductivity of the positive current collector and the first coating is λW / (m·K), 0.55≤λ≤0.8; (4) The sum of the stiffness of the positive current collector and the first coating is S1 mN·m, 0.03≤S1≤0.25; (5) The stiffness of the positive electrode sheet is S2 mN·m, 0.01≤S2≤0.7; (6) The peel strength of the second coating is PN / m, 10≤P≤20.

5. The secondary battery according to claim 1, wherein, The positive electrode sheet satisfies at least one of the following characteristics: (1) The inorganic material includes at least one of boehmite, alumina, aluminosilicate, silica, hydrotalcite, kaolin or titanate nanoparticles; (2) The conductive agent includes at least one of carbon black or carbon nanotubes; (3) The adhesive includes at least one of polyacrylic acid, sodium polyacrylate, lithium polyacrylate, polyvinyl alcohol, polyethylene oxide, styrene-butadiene resin, sodium alginate or chitosan; (4) The dispersant includes at least one of sodium carboxymethyl cellulose or lithium carboxymethyl cellulose.

6. The secondary battery according to claim 5, wherein, The weight-average molecular weight of the binder is Mw g / mol, 1×10 4 ≤Mw≤5×10 5 .

7. The secondary battery according to claim 5, wherein, The conductive agent satisfies at least one of the following characteristics: (1) The average particle size of the carbon black is D1μm, 0.5≤D1≤4; (2) The average diameter of the carbon nanotube is D2 nm, 2≤D2≤5, and the average length of the carbon nanotube is Lμm, 1≤L≤2. (3) The specific surface area of ​​the conductive agent is B m 2 / g, 60≤B≤90.

8. The secondary battery according to claim 1, wherein, The positive electrode sheet satisfies at least one of the following characteristics: (1) The thickness of the first coating on one side is T1μm, 0.5≤T1≤1.5; (2) The thickness of the second coating on one side is T2μm, 40≤T2≤180; (3) The average particle size of the inorganic material is D3μm, and 0.5≤D3≤2.

9. The secondary battery according to claim 1, wherein, The thickness of the first coating on one side is T1 μm, the average particle size of the inorganic material is D3 μm, and 0.8 ≤ T1 / D3 ≤ 1.

5.

10. The secondary battery according to claim 1, wherein, Along the thickness direction of the positive electrode sheet, the projected area of ​​the first coating on the surface of the positive current collector is A1, and the projected area of ​​the second coating on the surface of the positive current collector is A2, with A1 / A2 being 0.9 to 1.

11. The secondary battery according to claim 10, wherein, Along the thickness direction of the positive electrode sheet, the area of ​​the positive current collector is A3, and A1 / A3 is 0.6 to 0.

9.

12. The secondary battery according to claim 1, wherein, The areal density of the first coating is ρmg / cm³. 2 , 0.01≤ρ≤0.

05.

13. An electronic device comprising a secondary battery according to any one of claims 1 to 12.

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