Secondary battery

Abstract

The invention relates to the technical field of secondary batteries, and discloses a secondary battery which comprises a positive plate, a diaphragm and a negative plate, a positive conductive agent in the positive plate comprises conductive carbon black, and the total content of metal elements in the positive conductive agent is Alt; 500 ppm; the diaphragm comprises a base film layer and a functional coating, wherein the functional coating comprises a nitrogen-containing compound; the median particle size of the conductive carbon black is denoted as D1, the median particle size of the nitrogen-containing compound is denoted as D2, and D2 / D1 is more than or equal to 0.015 and less than or equal to 30, D1 is more than or equal to 0.1 mu m and less than or equal to 10 mu m, and D2 is more than or equal to 0.1 mu m and less than or equal to 4.0 mu m. According to the secondary battery provided by the invention, the conductive carbon black containing trace metal elements is adopted as the conductive agent and is matched with the nitrogen-containing compound in the diaphragm, so that the problem of poor K value of the battery can be relieved, the secondary battery has high energy density, the cycle performance of the secondary battery is improved, and the temperature rise of the battery under high rate is reduced.

Description

Technical Field

[0001] This application relates to the field of secondary batteries, specifically to a secondary battery. Background Technology

[0002] Lithium-ion batteries are devices that convert electrical energy into chemical energy. With advancements in science and technology, the demand for and application scenarios of lithium-ion batteries with high energy density and high rate performance are constantly increasing. Currently, the industry primarily uses technologies such as high-nickel cathodes, silicon-based anodes, and thick cathode sheets to improve battery energy density. However, while these methods can increase energy density, they also lead to problems such as high internal resistance, excessive heat generation, and severe self-discharge during cycling, resulting in poor K-values. These issues, along with high internal resistance, excessive heat generation, and poor K-values, pose serious challenges to rechargeable batteries in practical use, including insufficient effective energy release, limited fast-charging capabilities, shortened cycle life, and increased risk of thermal runaway. Consequently, rechargeable batteries cannot simultaneously achieve high energy density, good cycle performance, low temperature rise at high rates, and reliable safety. Summary of the Invention

[0003] In view of this, this application provides a secondary battery to solve the problems of large fluctuations in the K value of secondary batteries, which leads to insufficient effective energy release, limited fast charging capability, shortened cycle life and increased risk of thermal runaway.

[0004] According to an embodiment of this application, in a first aspect, this application provides a secondary battery, including a positive electrode sheet, a separator and a negative electrode sheet stacked together, wherein the positive electrode sheet includes a positive electrode current collector and a positive electrode active layer disposed on at least one surface of the positive electrode current collector in the thickness direction, and the positive electrode active layer includes a positive electrode active material, a positive electrode binder and a positive electrode conductive agent. The positive electrode conductive agent includes conductive carbon black, which includes carbon and metal elements, and the metal elements include at least one of Fe, Cr and Cu. Based on the total mass of the positive electrode conductive agent, the total content of the metal elements is denoted as A, which satisfies: A < 500 ppm. The diaphragm includes a base film layer and a functional coating disposed on at least one side surface of the base film layer in the thickness direction, the functional coating including a nitrogen-containing compound; The median particle size of the conductive carbon black is denoted as D1, and the median particle size of the nitrogen-containing compound is denoted as D2. Both satisfy the following conditions: 0.015≤D2 / D1≤30, D1 satisfies: 0.1μm≤D1≤10μm, and D2 satisfies: 0.1μm≤D2≤4.0μm.

[0005] Furthermore, in an optional implementation, the following condition is satisfied: 0.5μm≤D1≤8μm.

[0006] Furthermore, in an optional implementation, the following condition is satisfied: 0.2μm≤D2≤1.5μm.

[0007] In an optional embodiment, based on the total mass of the positive electrode conductive agent, the Fe content is less than 150 ppm, the Cr content is less than 150 ppm, and the Cu content is less than 200 ppm.

[0008] In an optional implementation, the total content A of the metal elements is satisfied, based on the total mass of the positive electrode conductive agent: A < 200 ppm.

[0009] In an optional embodiment, the nitrogen-containing compound includes at least one of melamine cyanurate, melamine polyphosphate, melamine thiocyanate, melamine, 2,4,6-tris(aminohexanoic acid)-1,3,5-triazine, 2-(4-bromophenyl)-4,6-dimethyl-1,3,5-triazine, 1-(4,6-diamino-1,3,5-triazin-2-yl)guanidine, 2,4-diamino-6-dimethylamino-1,3,5-triazine, melamine chloride, 2,4,6-tris(2-pyridyl)triazine, 2,4,6-triphenyl-1,3,5-triazine, tris(tribromophenoxy)triazine, and 2-amino-4,6-dimethoxy-1,3,5-triazine.

[0010] In an optional implementation, the mass content θ of the conductive carbon black, based on the total mass of the positive electrode active layer, satisfies: 0.1% ≤ θ ≤ 4%.

[0011] In one optional embodiment, the thickness H of the positive electrode active layer on any side surface along the thickness direction of the positive electrode current collector is 0.03 mm to 0.16 mm.

[0012] Furthermore, in an optional embodiment, the thickness H of the positive electrode active layer on any side surface along the thickness direction of the positive electrode current collector is 0.04 mm to 0.15 mm.

[0013] In an optional embodiment, the secondary battery further includes an electrolyte comprising additives, the additives comprising nitrile substances, wherein the mass content N of the nitrile substances, based on the total mass of the electrolyte, satisfies: 0.5% ≤ N ≤ 8%, and the mass content θ of the conductive carbon black and the mass content N of the nitrile substances satisfy: 0.0125 ≤ θ / N ≤ 8.

[0014] In an optional embodiment, the nitrile substance includes at least one selected from succinic anionyl, adiponitrile, 1,2-bis(cyanoethoxy)ethane, glutaronitrile, ethylene glycol bis(propionitrile) ether, 1,3,6-hexanetrionitrile, 1,3,5-pentanetricarboxylon, 1,2,3-propanetricarboxylon, 1,2,6-hexanetrionitrile, 1,2,3-tris(2-cyanoethoxy)propane, 1,2,4-tris(2-cyanoethoxy)butane, 1,1,1-tris(cyanoethoxymethylene)ethane, 1,1,1-tris(cyanoethoxymethylene)propane, 3-methyl-1,3,5-tris(cyanoethoxy)pentane, 1,2,7-tris(cyanoethoxy)heptane, 1,2,6-tris(cyanoethoxy)hexane, glyceryl trionitrile, and 1,2,5-tris(cyanoethoxy)pentane.

[0015] In an optional embodiment, the puncture strength M of the diaphragm satisfies: 100gf ≤ M ≤ 800gf.

[0016] Furthermore, in an optional implementation, the following condition is satisfied: 200gf≤M≤600gf.

[0017] In an optional embodiment, the porosity P of the diaphragm satisfies: 15% ≤ P ≤ 65%.

[0018] Furthermore, in an optional implementation, the following condition is satisfied: 20% ≤ P ≤ 40%.

[0019] In an optional embodiment, the oil absorption value Q of the conductive carbon black satisfies: 150mL / 100g≤Q≤400mL / 100g.

[0020] Furthermore, in an optional implementation, the following condition is satisfied: 200mL / 100g≤Q≤300mL / 100g.

[0021] In an optional embodiment, the specific surface area S of the conductive carbon black satisfies: 100 m² / s². 2 / g≤S≤1400m 2 / g.

[0022] Furthermore, in an optional implementation, the following condition is met: 200m 2 / g≤S≤800m 2 / g.

[0023] In one optional embodiment, the conductive carbon black includes at least one of chain-like conductive carbon black and grape-like spherical conductive carbon black.

[0024] In an optional embodiment, the diaphragm further includes an adhesive layer comprising polymer particles, the adhesive layer being disposed on the surface of the base film layer and opposite to the functional coating, and / or the adhesive layer being disposed on the surface of the functional coating on the side away from the base film layer; the median particle size of the polymer particles is denoted as D3, satisfying: 0.2μm≤D3≤3μm.

[0025] Furthermore, in an optional implementation, the following condition is satisfied: 0.5μm≤D3≤1.5μm.

[0026] In an optional embodiment, the polymer particles include at least one of ester-containing polymer particles and fluoropolymer particles.

[0027] In an optional embodiment, the ester-containing polymer particles include at least one of polymethyl methacrylate, polyethylhexyl acrylate, polybutyl acrylate, acrylate-acrylonitrile copolymer, acrylate-ethylene copolymer, acrylate-acrylonitrile-ethylene copolymer, styrene-acrylate-acrylonitrile copolymer, ethylhexyl acrylate-methyl methacrylate copolymer, butyl acrylate-methyl methacrylate copolymer, methyl acrylate-N,N-dimethylacrylamide copolymer, ethyl acrylate-2-(diethylamino)ethyl acrylate copolymer, and ethyl acrylate-N,N-diethylacrylamide copolymer.

[0028] In one optional embodiment, the fluoropolymer particles include at least one of polyvinylidene fluoride, polytetrafluoroethylene, polyhexafluoropropylene, vinylidene fluoride-hexafluoropropylene copolymer, and tetrafluoroethylene-hexafluoropropylene copolymer.

[0029] In an optional embodiment, the median particle size of the positive electrode active material is denoted as D4, which satisfies: 2.0 μm ≤ D4 ≤ 15 μm.

[0030] Furthermore, in an optional implementation, the following condition is satisfied: 3.0μm≤D4≤8μm.

[0031] In one optional embodiment, the positive electrode active material includes at least one of lithium cobalt oxide, lithium nickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxide.

[0032] In an optional embodiment, the negative electrode sheet includes a negative current collector and a negative active layer disposed on at least one side surface of the negative current collector in the thickness direction, the negative active layer including a negative active material, the negative active material including a silicon-based material.

[0033] In an optional embodiment, the liquid retention σ of the positive electrode active layer per unit thickness satisfies: 0.37 g / mm ≤ σ ≤ 1.7 g / mm.

[0034] The technical solution of this application has the following advantages: This application provides a secondary battery comprising a positive electrode sheet, a separator, and a negative electrode sheet stacked together. The positive electrode sheet includes a positive current collector and a positive active layer disposed on at least one surface of the positive current collector in the thickness direction. The positive active layer includes a positive active material, a positive binder, and a positive conductive agent. The positive conductive agent includes conductive carbon black, which comprises carbon and a metal element, wherein the metal element includes at least one selected from Fe, Cr, and Cu. The total mass of the positive conductive agent is used as a basis. The total content of the metal elements is denoted as A, and satisfies: A < 500 ppm; the diaphragm includes a base film layer and a functional coating disposed on at least one side surface of the base film layer in the thickness direction, the functional coating including a nitrogen-containing compound; the median particle size of the conductive carbon black is denoted as D1, the median particle size of the nitrogen-containing compound is denoted as D2, and both satisfy: 0.015 ≤ D2 / D1 ≤ 30, D1 satisfies: 0.1 μm ≤ D1 ≤ 10 μm, and D2 satisfies: 0.1 μm ≤ D2 ≤ 4.0 μm. In this application, conductive carbon black containing trace amounts of metal elements is selected as the positive electrode conductive agent in the positive electrode active layer of the secondary battery. This enhances the conductivity of the positive electrode, thereby reducing its internal resistance and decreasing heat generation. However, during charging and discharging, the positive electrode potential is high, and the aforementioned metal elements in the conductive carbon black are easily oxidized into metal ions and dissolved into the electrolyte. These metal ions diffuse to the negative electrode side along the concentration gradient. At the low negative electrode potential, the metal ions are reduced to elemental metals (e.g., Fe). 2+ +2e - →Fe), first forming nanoscale "metal islands" on the surface of the negative electrode material, which then continue to grow, damaging the solid electrolyte interphase (SEI) film on the negative electrode side, triggering local side reactions, increasing the risk of self-discharge of the battery, and may even penetrate the separator, forming permanent or intermittent micro-short circuit channels, causing the battery's open circuit voltage (OCV) to drop faster during the resting stage, resulting in poor K-value of the battery. Therefore, this application also controls the total content A of metal elements in the positive electrode conductive agent to reduce the content of metal elements in the positive electrode conductive agent that are oxidized into ions during battery cycling, further reducing the content of metal ions deposited on the negative electrode sheet. Secondly, this application provides a functional coating containing nitrogen-containing compounds on the surface of the base film layer on the side of the separator facing the positive electrode sheet. Nitrogen-containing compounds have lone pairs of electrons, which can react with Cu dissolved in the electrolyte. 2+ Fe 2+ Fe 3+ Cr 3+Complexing or forming coordination bonds to adsorb metal ions dissolved from the surface of the positive electrode during battery cycling, improving the poor battery K-value caused by the precipitation of trace metal elements in conductive carbon black, and improving the problems caused by the continuous loss of active lithium in the battery due to poor battery K-value, increased side reactions and heat generation, continuous growth and thickening of the solid electrolyte interphase (SEI) film on the surface of the negative electrode, increased difficulty and resistance for lithium ions to pass through this SEI film, resulting in decreased battery cycle performance and increased battery temperature rise at high rates.

[0035] Meanwhile, this application further regulates the ratio D2 / D1 of the median particle size D2 of the nitrogen-containing compound to the median particle size D1 of the conductive carbon black within the range of 0.015 to 30. This means that the particle sizes of the conductive carbon black and the nitrogen-containing compound are matched to ensure that a suitable and continuous electronic pathway is formed between the conductive carbon black particles and the nitrogen-containing compound particles, thereby reducing the interfacial internal resistance of the battery and solving the problem of metal element dissolution in the conductive carbon black, reducing the battery impedance, reducing the heat generation during the battery charging and discharging process, improving the battery K-value problem, optimizing the battery cycle performance, and reducing the temperature rise of the battery at high rates. Secondly, this application controls the median particle size D1 of the conductive carbon black within the range of 0.01 μm to 1.0 μm, which ensures the formation of a good conductive network within the positive electrode active layer and between the positive electrode active materials. This enhances the conductivity of the positive electrode sheet while increasing the precipitation pathways of metal elements in the conductive carbon black, thus reducing the precipitation probability of metal elements. Furthermore, the particle size of the conductive carbon black within the above range also improves its dispersibility, reducing the risk of poor local conductivity and increased heat generation in the battery. On the other hand, this application also regulates the median particle size D2 of the nitrogen-containing compound within the range of 0.1 μm to 4.0 μm, which provides more adsorption sites for metal elements, reducing the risk of metal element dissolution in the conductive carbon black, reducing the occurrence of side reactions in the battery, improving the poor K-value of the battery, and ensuring that the separator coating has suitable porosity, providing sufficient storage space for the electrolyte, and ensuring suitable "liquid phase ion channels" between the separator and the positive electrode sheet, thereby further reducing the interfacial impedance of the battery and improving the cycle performance of the battery.

[0036] In summary, this application introduces conductive carbon black containing trace amounts of metal elements into the positive electrode as a conductive agent. This agent works in conjunction with nitrogen-containing compounds in the separator. By controlling the median particle size D2 of the nitrogen-containing compounds and the median particle size D1 of the conductive carbon black, as well as their ratio D2 / D1, the conductivity of the positive electrode can be improved. This reduces the internal resistance of the battery, decreases the heat generated during battery charging and discharging, alleviates the problem of poor battery K-value caused by the precipitation of metal elements in the conductive carbon black, and effectively suppresses the occurrence of side reactions in the battery. As a result, the secondary battery has high energy density while improving its cycle performance and reducing the battery temperature rise at high rates. Attached Figure Description

[0037] To more clearly illustrate the technical solutions in the specific embodiments of this application or the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0038] Figure 1 This is a schematic diagram of the structure of the positive electrode sheet provided in Embodiment 1 of this application.

[0039] Figure 1 In the middle, 11 is the positive current collector; 12 is the positive active layer.

[0040] Figure 2 This is a schematic diagram of the negative electrode sheet provided in Embodiment 2 of this application.

[0041] Figure 2 In the middle, 21 is the negative electrode current collector; 22 is the negative electrode active layer. Detailed Implementation

[0042] The following embodiments are provided to better understand this application and are not limited to the preferred embodiments described herein. They do not constitute a limitation on the content and scope of protection of this application. Any product that is the same as or similar to this application, derived by anyone under the guidance of this application or by combining features of this application with other prior art, falls within the scope of protection of this application.

[0043] It should be noted in the description of this application that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance. Furthermore, the technical features involved in the different embodiments of this application described below may be combined with each other as long as they do not conflict with each other.

[0044] According to an embodiment of this application, in a first aspect, this application provides a secondary battery, including a positive electrode sheet, a separator and a negative electrode sheet stacked together, wherein the positive electrode sheet includes a positive electrode current collector and a positive electrode active layer disposed on at least one surface of the positive electrode current collector in the thickness direction, and the positive electrode active layer includes a positive electrode active material, a positive electrode binder and a positive electrode conductive agent. The positive electrode conductive agent includes conductive carbon black, which includes carbon and metal elements, and the metal elements include at least one of Fe, Cr and Cu. Based on the total mass of the positive electrode conductive agent, the total content of the metal elements is denoted as A, which satisfies the following condition: A < 500 ppm. The diaphragm includes a base film layer and a functional coating disposed on at least one side surface of the base film layer in the thickness direction, the functional coating including a nitrogen-containing compound; The median particle size of the conductive carbon black is denoted as D1, and the median particle size of the nitrogen-containing compound is denoted as D2. Both satisfy the following conditions: 0.015≤D2 / D1≤30, D1 satisfies: 0.1μm≤D1≤10μm, and D2 satisfies: 0.1μm≤D2≤4.0μm.

[0045] It should be noted that in this application, the "median particle size" refers to the particle size value corresponding to 50% of the cumulative volume on the cumulative distribution curve from smallest to largest volume in the volume-based particle size distribution.

[0046] In this application, conductive carbon black containing trace amounts of metal elements is selected as the positive electrode conductive agent in the positive electrode active layer of the secondary battery. This enhances the conductivity of the positive electrode, thereby reducing its internal resistance and decreasing heat generation. However, during charging and discharging, the positive electrode potential is high, and the aforementioned metal elements in the conductive carbon black are easily oxidized into metal ions and dissolved into the electrolyte. These metal ions diffuse to the negative electrode side along the concentration gradient. At the low negative electrode potential, the metal ions are reduced to elemental metals (e.g., Fe). 2+ +2e - →Fe), first forming nanoscale "metal islands" on the surface of the negative electrode material, which then continue to grow, damaging the solid electrolyte interphase (SEI) film on the negative electrode side, triggering local side reactions, increasing the risk of self-discharge of the battery, and may even penetrate the separator, forming permanent or intermittent micro-short circuit channels, causing the battery's open circuit voltage (OCV) to drop faster during the resting stage, resulting in poor K value of the battery. Therefore, this application also controls the total content A of metal elements in the positive electrode conductive agent to reduce the content of metal elements in the positive electrode conductive agent that are oxidized into ions during battery cycling, further reducing the content of metal ions deposited on the negative electrode sheet; secondly, this application provides a functional coating containing nitrogen-containing compounds on the surface of the base film layer on the side of the separator facing the positive electrode sheet. Nitrogen-containing compounds have lone pairs of electrons, which can react with Cu dissolved in the electrolyte. 2+ Fe 2+ Fe 3+ Cr 3+ Complexing or forming coordination bonds to adsorb metal ions dissolved from the surface of the positive electrode during battery cycling, improving the poor battery K-value caused by the precipitation of trace metal elements in conductive carbon black, and improving the problems caused by the continuous loss of active lithium in the battery due to poor battery K-value, increased side reactions and heat generation, continuous growth and thickening of the solid electrolyte interphase (SEI) film on the surface of the negative electrode, and increased difficulty and resistance for lithium ions to pass through this SEI film, resulting in a decrease in battery cycle performance and rate performance.

[0047] Meanwhile, this application also found that the median particle size D1 of the conductive carbon black in the positive electrode and the median particle size D2 of the nitrogen-containing compound in the separator affect the battery's conductivity and K-value fluctuations, thereby affecting the battery's energy density release, fast charging capability, cycle life, and thermal safety. A smaller median particle size D1 of the conductive carbon black in the positive electrode helps improve the conductivity of the positive electrode; however, an excessively small median particle size D1 can affect the probability of trace metal element precipitation and the dispersion of the conductive carbon black. While a smaller median particle size D2 of the nitrogen-containing compound in the separator provides more adsorption sites for metal elements, an excessively small median particle size D2 of the nitrogen-containing compound can affect the electrolyte storage space of the separator, compress the "liquid phase ion channel" between the separator and the positive electrode, and increase interfacial impedance. In this regard, this application controls the ratio D2 / D1 of the median particle size D2 of the nitrogen-containing compound to the median particle size D1 of the conductive carbon black within the range of 0.015 to 30. This means that it is possible to provide conductive carbon black and nitrogen-containing compounds with mutually compatible particle sizes, so as to ensure that a suitable and continuous electronic pathway is formed between the conductive carbon black particles and the nitrogen-containing compound particles, reduce the interfacial internal resistance of the battery and the problem of metal element dissolution in the conductive carbon black, reduce the battery impedance, reduce the heat generation during the battery charging and discharging process, improve the battery K-value problem, optimize the battery cycle performance, and reduce the temperature rise of the battery at high rates. Secondly, this application controls the median particle size D1 of the conductive carbon black within the range of 0.1 μm to 10 μm. This ensures the formation of a good conductive network within the positive electrode active layer and between the positive electrode active materials, thereby enhancing the conductivity of the positive electrode sheet. Simultaneously, it increases the precipitation pathways of metal elements in the conductive carbon black, which helps reduce the precipitation probability of metal elements. Furthermore, the particle size of the conductive carbon black within the aforementioned range also improves its dispersibility, reducing the risk of poor local conductivity and increased heat generation in the battery. On the other hand, this application also regulates the median particle size D2 of the nitrogen-containing compounds within the range of 0.1 μm to 4.0 μm. This provides more adsorption sites for metal elements, reducing the risk of metal element dissolution in the conductive carbon black, minimizing side reactions in the battery, improving the battery's K-value, and ensuring that the separator coating has suitable porosity. This provides sufficient storage space for the electrolyte and ensures suitable "liquid phase ion channels" between the separator and the positive electrode sheet, thereby further reducing the battery's interfacial impedance and improving its cycle performance.

[0048] In summary, this application introduces conductive carbon black containing trace amounts of metal elements into the positive electrode as a conductive agent. This agent works in conjunction with nitrogen-containing compounds in the separator. By controlling the median particle size D2 of the nitrogen-containing compounds and the median particle size D1 of the conductive carbon black, as well as their ratio D2 / D1, the conductivity of the positive electrode can be improved. This reduces the internal resistance of the battery, decreases the heat generated during battery charging and discharging, alleviates the problem of poor battery K-value caused by the precipitation of metal elements in the conductive carbon black, and effectively suppresses the occurrence of side reactions in the battery. As a result, the secondary battery has high energy density while improving its cycle performance and reducing the battery temperature rise at high rates.

[0049] This application also found that if the total content A of metal elements in the positive electrode conductive agent is above 500 ppm, and the median particle size D1 of the conductive carbon black and the median particle size D2 of the nitrogen-containing compound exceed the above range, it will increase the oxidation of metal elements in the conductive carbon black into ions during battery cycling, which will dissolve into the electrolyte, diffuse to the negative electrode side of the battery, precipitate on the surface of the negative electrode material, damage the SEI film of the negative electrode sheet, cause local side reactions, increase the probability of membrane penetration, lead to serious problems with the K value of the secondary battery, deteriorate the cycle performance of the battery, and increase the temperature rise of the battery at high rates.

[0050] Further research in this application revealed that if the ratio of the median particle size D2 of the nitrogen-containing compound in the separator to the median particle size D1 of the conductive carbon black in the positive electrode is less than 0.015, it means that the median particle size D2 of the nitrogen-containing compound is too small relative to the median particle size D1 of the conductive carbon black. Too many small nitrogen-containing compound particles will preferentially fill the gaps between larger nitrogen-containing compound particles, significantly reducing the porosity of the functional coating in the separator, decreasing the electrolyte storage space, compressing the "liquid phase ion channel" between the separator and the positive electrode, and increasing the interfacial impedance of the battery. Conversely, the relatively large median particle size D1 of the conductive carbon black will result in poor contact between it and the positive electrode material in the positive electrode active layer, thus increasing the impedance of the positive electrode, decreasing conductivity, and affecting the cycle performance of the battery. Furthermore, if the ratio of the median particle size D2 of the nitrogen-containing compound in the separator to the median particle size D1 of the conductive carbon black in the positive electrode is less than 0.015, then... The ratio D2 / D1 of particle size D1 is higher than 30, indicating that the median particle size D1 of conductive carbon black is too small relative to the median particle size D2 of nitrogen-containing compounds. Too many small particle sizes in conductive carbon black will shorten the precipitation path of metal elements inside, increase the precipitation probability, and increase the risk of poor K value in the battery. Furthermore, conductive carbon black with a median particle size D1 is prone to agglomeration, which will increase the problem of poor local conductivity, increased local impedance, and increased heat generation in the battery, reduce the cycle performance of the battery, and increase the temperature rise of the battery at high rates. On the other hand, nitrogen-containing compound particles with a median particle size D2 that is too large relative to the median particle size D1 of conductive carbon black will reduce the adsorption sites of metal elements on the separator, which is not conducive to the adsorption of metal ions precipitated from the positive electrode, thereby increasing the risk of poor K value in the battery and resulting in a deterioration in the overall electrochemical performance of the battery.

[0051] This application also found that if the median particle size D1 of conductive carbon black is less than 0.1 μm, it can easily lead to a shortened precipitation path for metal elements, an increased precipitation probability, and an increased risk of poor K-value in the battery. Furthermore, conductive carbon black with a median particle size D1 that is too low is prone to agglomeration, which can increase the problem of poor local conductivity, increased local impedance, and increased heat generation in the battery, resulting in poor cycle performance and increased battery temperature rise at high rates. If the median particle size D1 of conductive carbon black is higher than 10 μm, the contact effect between conductive carbon black with an excessively large median particle size D1 and the positive active material in the positive electrode active layer will be poor, resulting in increased impedance of the positive electrode sheet, decreased conductivity, and affecting the cycle performance and battery temperature rise performance at high rates. If the median particle size D2 of the nitrogen-containing compounds in the separator is less than 0.1 μm, too many small nitrogen-containing compounds will preferentially fill the gaps between large nitrogen-containing compounds, significantly reducing the porosity of the functional coating in the separator, reducing the electrolyte storage space, compressing the "liquid phase ion channel" between the separator and the positive electrode, and increasing the interfacial impedance of the battery. If the median particle size D2 of the nitrogen-containing compounds is greater than 4 μm, it will lead to a reduction in the adsorption sites provided on the separator surface by the excessively large nitrogen-containing compound particles, resulting in a poorer adsorption effect of the separator on metal ions in conductive carbon black. This is not conducive to improving the battery's poor K-value, and cannot effectively improve the battery's cycle performance or suppress the temperature rise of the battery during high-rate cycling.

[0052] It should be noted that the total content of metal elements in conductive carbon black can be obtained by testing and calculating the sum of individual metal elements using inductively coupled plasma atomic emission spectrometry (ICP). For example, the total content A of metal elements in conductive carbon black can be 499ppm, 450ppm, 400ppm, 350ppm, 300ppm, 250ppm, 200ppm, 150ppm, 100ppm, 50ppm, etc., or a value within any range of two of the above values.

[0053] It should be noted that the median particle size D1 of the conductive carbon black in the positive electrode sheet can be obtained by the following method: First, take the positive electrode sheet and scrape off the positive active layer powder on the surface of the positive electrode sheet. At this time, the scraped positive active layer coating contains positive active material, conductive agent and binder. Heat the powder to 400~500℃ and keep it for a period of time to completely decompose and volatilize the binder and thoroughly remove the binder. The powder still contains positive active material and conductive agent. Taking advantage of the fact that the density of positive active particles (positive active material) is much higher than that of conductive agent, dissolve the powder after removing the binder in a bromoalkane reagent and then centrifuge it at high speed. After high-speed centrifugation, stratification occurs. The higher density of the positive electrode active material is located in the lower layer of the solution, while the lower density of the conductive agent is located in the upper layer. After removing the upper layer of solution and drying it, conductive agent powder (conductive carbon black) is obtained. The obtained conductive carbon black is randomly sampled, dissolved in anhydrous ethanol, and dispersed using an instrument to prepare a suspension. Next, the dispersed suspension is slowly added to the circulation tank of the laser particle size analyzer while maintaining continuous stirring or circulation to ensure that the particles do not settle and the concentration is uniform. The instrument will display the laser shading degree or concentration, which needs to be controlled within an optimal range. If the shading degree is too low, the signal is weak and the error is large; if the shading degree is too high, multiple scattering will occur, resulting in an underestimation of the value. Once the shading degree stabilizes within a suitable range, measurement begins. The instrument performs multiple automatic measurements until the results stabilize, and then outputs the particle size data of the conductive carbon black in the volume particle size distribution curve, accumulating to 50% from the smallest particle size side. The median particle size D2 of nitrogen-containing compounds in the diaphragm can be obtained by the following method: The diaphragm (coating side up) is fixed on a flat surface. Using a clean, sharp scraper or scalpel blade, the coating is gently and evenly scraped off at a small angle to the surface. Next, the mixed powder is placed in a muffle furnace and calcined in air at about 300°C for 2-4 hours. Other organic components of the diaphragm (such as polymer particles and diaphragm substrate fragments) will be burned off, leaving pure nitrogen-containing compound particles. Then, a laser particle size analyzer is used to measure the particle size distribution curve, which accumulates to 50% of the particle size data from the smallest particle size side.

[0054] For example, the ratio D2 / D1 of the median particle size D2 of the nitrogen-containing compound in the separator to the median particle size D1 of the conductive carbon black in the positive electrode sheet can be, for example, 0.015, 0.05, 0.1, 0.5, 1, 5, 10, 15, 20, 25, 30, or a value within any two of the above values; the median particle size D1 of the conductive carbon black in the positive electrode sheet can be, for example, 0.1μm, 1μm, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, or a value within any two of the above values; the median particle size D2 of the nitrogen-containing compound can be, for example, 0.1μm, 0.5μm, 1.0μm, 1.5μm, 2.0μm, 2.5μm, 3.0μm, 3.5μm, 4.0μm, or a value within any two of the above values.

[0055] Furthermore, in some implementations, the following condition is satisfied: 0.5μm≤D1≤8μm.

[0056] Furthermore, in some implementations, the following condition is satisfied: 0.2μm≤D2≤1.5μm.

[0057] In some embodiments, based on the total mass of the positive electrode conductive agent, the Fe content is below 150 ppm, the Cr content is below 150 ppm, and the Cu content is below 200 ppm. This further enhances the conductivity of the positive electrode, reduces its internal resistance, and also reduces the amount of metal elements in the positive electrode conductive agent (conductive carbon black) oxidized into ions during battery cycling. This reduces the amount of metal ions deposited on the negative electrode, thus better mitigating K-value fluctuations in the secondary battery and improving its cycle performance and safety.

[0058] It should be noted that the mass content of Fe, Cr, and Cu elements in conductive carbon black can all be obtained by inductively coupled plasma atomic emission spectrometry (ICP). For example, the Fe content in conductive carbon black can be, for example, 150 ppm, 140 ppm, 130 ppm, 120 ppm, 110 ppm, 100 ppm, 90 ppm, 80 ppm, 70 ppm, 60 ppm, 50 ppm, 40 ppm, 30 ppm, 20 ppm, 10 ppm, 5 ppm, etc., or values ​​within any range of two of the above values; the Cr content in conductive carbon black can be, for example, 150 ppm, 140 ppm, 130 ppm, 120 ppm, 110 ppm, 100 ppm, 90 ppm, 80 ppm, 70 ppm, 60 ppm, 50 ppm, 40 ppm, 30 ppm, 20 ppm, 10 ppm, 5 ppm, etc., or values ​​within any range of two of the above values; The values ​​are 0ppm, 90ppm, 80ppm, 70ppm, 60ppm, 50ppm, 40ppm, 30ppm, 20ppm, 10ppm, 5ppm, etc., or values ​​within any two of the above ranges; the Cu content in conductive carbon black can be, for example, 200ppm, 180ppm, 160ppm, 140ppm, 120ppm, 100ppm, 80ppm, 60ppm, 40ppm, 20ppm, 10ppm, etc., or values ​​within any two of the above ranges.

[0059] Furthermore, in some embodiments, based on the total mass of the positive electrode conductive agent, the total content A of the metal elements satisfies: A < 200 ppm. This improves the conductivity of the conductive agent while further reducing the precipitation of metal elements in the positive electrode, thereby better mitigating K-value fluctuations in the secondary battery and improving its cycle performance and safety.

[0060] In an optional embodiment, the nitrogen-containing compound includes at least one of melamine cyanurate, melamine polyphosphate, melamine thiocyanate, melamine, 2,4,6-tris(aminohexanoic acid)-1,3,5-triazine, 2-(4-bromophenyl)-4,6-dimethyl-1,3,5-triazine, 1-(4,6-diamino-1,3,5-triazin-2-yl)guanidine, 2,4-diamino-6-dimethylamino-1,3,5-triazine, melamine chloride, 2,4,6-tris(2-pyridyl)triazine, 2,4,6-triphenyl-1,3,5-triazine, tris(tribromophenoxy)triazine, and 2-amino-4,6-dimethoxy-1,3,5-triazine.

[0061] In some embodiments, based on the total mass of the positive electrode active layer, the mass content θ of the conductive carbon black satisfies: 0.1% ≤ θ ≤ 4%. This ensures a continuous conductive network structure on the positive electrode, which is more conducive to electron transport, increases the utilization rate of the active material, and reduces polarization. Simultaneously, it further ensures that the secondary battery still has high energy density, low battery gas production, and suitable slurry viscosity, which is beneficial to the electrode processing performance. This further reduces battery impedance, improves the cycle performance of the secondary battery, and further reduces battery heat generation. Ultimately, this allows the secondary battery to achieve superior energy density, cycle stability, and safety performance, as well as lower battery temperature rise during high-rate cycling.

[0062] This study found that if the mass content θ of conductive carbon black in the positive electrode is less than 0.1%, a continuous conductive network cannot be effectively constructed on the positive electrode, resulting in obstructed electron transport, low utilization of active materials, deterioration of rate performance, increased polarization, and excessively high local current density, which also accelerates capacity decay and causes battery heating. If the mass content θ of conductive carbon black in the positive electrode is greater than 4%, it will lead to a decrease in the mass content of positive electrode active material in the positive electrode, resulting in a decrease in battery energy density. Furthermore, due to excessive conductive carbon black, its huge specific surface area will catalyze the violent decomposition of electrolyte, causing gas production, which will increase battery impedance and rapidly reduce cycle life. At the same time, excessive conductive carbon black will also greatly increase the viscosity of the positive electrode slurry and make processing difficult.

[0063] It should be noted that the mass content θ of the conductive carbon black was determined using a thermogravimetric analyzer. The battery was discharged to 0% SOC, the positive electrode sheet was removed, and the positive active layer was scraped off from the positive current collector with a knife to obtain the sample to be tested. The mass of the positive active layer was 15~25g. The sample to be tested was placed in a tube furnace or muffle furnace, and the mass content of conductive carbon black was determined by precisely controlling the heating program and the atmosphere: First, the sample to be tested was heated in nitrogen to 400~500℃ and held for a period of time to allow the binder to completely decompose and volatilize. Then, the atmosphere was switched to an oxidizing atmosphere, and the conductive carbon black was completely burned to generate CO2 in the range of 500~600℃. Finally, by calculating the percentage of mass loss of the sample in the corresponding temperature range, the mass fraction of conductive carbon black in the positive active layer can be accurately obtained. For example, the mass content of the conductive carbon black θ can be, for example, 0.1%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, or a value within the range of any two of the above values.

[0064] In some embodiments, the thickness H of the positive electrode active layer on any side of the positive electrode current collector's thickness direction is 0.03 mm to 0.16 mm. This effectively achieves the desired performance of the positive electrode sheet, providing a prerequisite for achieving higher energy density, superior cycle performance, and safety performance in secondary batteries.

[0065] It should be noted that the thickness H of the positive electrode active layer on any side surface of the positive electrode current collector along the thickness direction is obtained by scanning electron microscopy (SEM). The specific testing method for the thickness H of the positive electrode sheet includes: discharging the battery to 0% SOC, removing the positive electrode sheet, cutting off the edge of the positive electrode sheet sample, preparing a cross-section sample using an argon ion polishing (CP) instrument, observing the cross-section using a scanning electron microscope (SEM), and sequentially measuring the thickness of the positive electrode active layer on any side surface of the positive electrode current collector at 10 points. The average value is taken as the single-sided thickness H of the positive electrode active layer. For example, the thickness H of the positive electrode active layer on any side surface along the thickness direction of the positive electrode current collector can be, for example, 0.03 mm, 0.04 mm, 0.05 mm, 0.06 mm, 0.07 mm, 0.08 mm, 0.09 mm, 0.10 mm, 0.11 mm, 0.12 mm, 0.13 mm, 0.14 mm, 0.15 mm, 0.16 mm, or a value within the range of any two of the above values.

[0066] Furthermore, in some embodiments, the thickness H of the positive electrode active layer on any side surface of the positive electrode current collector in the thickness direction is 0.04 mm to 0.15 mm.

[0067] In some embodiments, the secondary battery further includes an electrolyte comprising additives, including nitrile substances. The mass content N of the nitrile substances, based on the total mass of the electrolyte, satisfies the following condition: 0.5% ≤ N ≤ 8%. Thus, by introducing nitrile substances as additives into the electrolyte, the nitrile groups in the nitrile substances can undergo complexation reactions with the metal elements in the positive electrode conductive agent (conductive carbon black), further reducing the amount of metal elements deposited from the positive electrode and improving the battery's poor K-value. Secondly, nitrile substances, as additives, can preferentially oxidize on the high-voltage positive electrode surface, promoting the formation of a relatively dense solid electrolyte interphase (CEI) film with ion channels on the positive electrode surface, and reducing heat generation and heat concentration during cycling. Furthermore, nitrile substances can broaden the electrochemical window of the electrolyte due to their higher antioxidant potential. However, this application found that when the mass content of nitrile substances in the electrolyte is too low, the above-mentioned effects cannot be achieved; conversely, when the mass content of nitrile substances in the electrolyte is too high, excessive nitrile substances will generate gas and acidic substances on the positive electrode surface, causing gas expansion and safety issues. Furthermore, excessive nitrile substances affect the stable formation of the solid electrolyte interface (SEI) film on the surface of the negative electrode and affect lithium-ion transport, resulting in significant fluctuations in the battery's K-value and affecting battery cycle performance. To address this, this application further controls the mass content N of nitrile substances to ensure the inhibitory effect of nitrile substances on metal element precipitation and alleviate the heat generation and heat concentration phenomenon of the positive electrode, while avoiding the impact of excessive nitrile substances on battery performance. This is more conducive to improving the battery's poor K-value, enhancing cycle stability, and reducing battery temperature rise during high-rate cycling.

[0068] It should be noted that the mass content N of the nitrile substances can be determined by separating and quantifying the electrolyte sample using gas chromatography-mass spectrometry and by using a standard curve method.

[0069] For example, the mass content N of the nitrile substance may be 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, 5.0%, 5.5%, 6.0%, 6.5%, 7.0%, 7.5%, 8.0%, or a value within the range of any two of the above values.

[0070] In some embodiments, the mass content θ of the conductive carbon black and the mass content N of the nitrile material satisfy the following ratio: 0.0125 ≤ θ / N ≤ 8. Thus, by controlling the range of the ratio of the mass content θ of the conductive carbon black to the mass content N of the nitrile material, sufficient complexation reaction occurs between the nitrile groups in the nitrile material and the metal elements in the conductive carbon black. This ensures that the metal elements enhance the conductivity of the conductive carbon black while further reducing the amount of metal elements deposited from the positive electrode, improving the K-value problem of the secondary battery, and ensuring sufficient nitrile material to more effectively promote the formation of the solid electrolyte interface film on the positive electrode, blocking electrolyte oxidation, reducing side reactions under high-temperature conditions, and further reducing heat generation of the positive electrode during cycling. Simultaneously, it ensures a sufficient amount of conductive carbon black in the positive electrode to construct a continuous and efficient electronic conductivity network, ensuring low-resistance electron transport channels between active material particles, reducing battery polarization, effectively alleviating or even suppressing the K-value problem, and allowing the capacity of the positive electrode material to be fully released. The synergistic interaction between conductive carbon black and nitriles within a specific ratio range is more conducive to achieving a dynamic balance between "interface passivation and electronic conduction" at high voltages, resulting in lower battery temperature rise and excellent cycle stability at high rates.

[0071] This study found that if the ratio of the mass content θ of conductive carbon black to the mass content N of nitrile substances is less than 0.0125, the mass content N of nitrile substances is too high relative to the mass content θ of conductive carbon black. This affects the formation of the CEI film in the positive electrode, resulting in an excessively thick CEI film that hinders lithium-ion migration, reduces battery kinetics, causes a surge in charge transfer impedance, and degrades battery cycle performance. Furthermore, excessive nitrile substances cause the electrolyte to undergo severe oxidation and gas production (CO2, O2) under high pressure, leading to battery swelling and increased internal pressure. Simultaneously, excessive nitrile substances affect the stable formation of the SEI film in the negative electrode, accelerating the depletion of lithium reserves. Conversely, a relatively low mass content of conductive carbon black leads to an incomplete electronic conductivity network in the positive electrode. If the positive electrode active material is intact, some of it becomes a "dead zone" because it cannot participate in the electrochemical reaction, affecting the cycle performance of the secondary battery. Conversely, if the ratio between the mass content θ of conductive carbon black and the mass content N of nitrile substances is higher than 8, it means that the content θ of conductive carbon black in the positive electrode is too high relative to the mass content N of nitrile substances. The high specific surface area of ​​conductive carbon black will catalyze the occurrence of side reactions, further consuming active lithium, increasing the battery interface impedance, resulting in a decrease in battery energy density and an increase in the risk of thermal runaway. On the other hand, if the content of nitrile substances is relatively low, it will lead to insufficient protection of the positive electrode interface and will not be able to effectively inhibit the dissolution of metal elements inside the conductive carbon black in the positive electrode, thereby increasing the fluctuation of the battery's K value and affecting the battery's cycle performance.

[0072] For example, the ratio θ / N of the mass content of the conductive carbon black to the mass content of the nitrile substance can be, for example, 0.0125, 0.05, 0.1, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, or a value within the range of any two of the above values.

[0073] In some embodiments, the nitrile substances include at least one selected from succinic anionyl nitrile, adiponitrile, 1,2-bis(cyanoethoxy)ethane, glutaronitrile, ethylene glycol bis(propionitrile) ether, 1,3,6-hexanetrionitrile, 1,3,5-pentanetricarboxynitrile, 1,2,3-propanetricarboxynitrile, 1,2,6-hexanetrionitrile, 1,2,3-tris(2-cyanoethoxy)propane, 1,2,4-tris(2-cyanoethoxy)butane, 1,1,1-tris(cyanoethoxymethylene)ethane, 1,1,1-tris(cyanoethoxymethylene)propane, 3-methyl-1,3,5-tris(cyanoethoxy)pentane, 1,2,7-tris(cyanoethoxy)heptane, 1,2,6-tris(cyanoethoxy)hexane, glyceryl trinitrile, and 1,2,5-tris(cyanoethoxy)pentane.

[0074] In some embodiments, the additives in the electrolyte also include at least one of vinyl sulfate, lithium tetrafluoroborate, propylene sulfite, and lithium difluorophosphate.

[0075] In some embodiments, the puncture strength M of the separator satisfies: 100gf ≤ M ≤ 800gf. This results in a separator with an appropriately dense microfiber structure and strong mechanical toughness, which significantly suppresses the risk of dendrite or electrode debris penetration, avoiding localized current density anomalies and self-discharge caused by micro-short circuits, thereby improving the battery's poor K-value. Simultaneously, a separator with an appropriate puncture strength M can maintain a more stable electrode-separator interface distance during battery assembly and cycling, reducing interface contact resistance fluctuations caused by separator deformation, thereby lowering the overall ohmic impedance of the battery and improving its cycle performance.

[0076] This study found that if the puncture strength M of the separator is less than 100 gf, the above effect cannot be achieved; if the puncture strength M of the separator is greater than 800 gf, the porosity of the separator will be extremely low, which will hinder the transport of lithium ions and increase the polarization of the battery, thereby making the cycle performance of the battery worse. Furthermore, when the separator with excessively high puncture strength is too dense, it may also lead to poor heat dissipation of the battery, affecting the thermal safety of the battery.

[0077] It should be noted that the puncture strength M of the diaphragm can be tested according to the method defined in GB / T 36363-2018. For example, the puncture strength M of the diaphragm can be 100gf, 200gf, 300gf, 400gf, 500gf, 600gf, 700gf, 800gf, etc., or a value within the range of any two of the above values.

[0078] Furthermore, in some embodiments, the following condition is satisfied: 200gf ≤ M ≤ 600gf. In some embodiments, the porosity P of the separator satisfies: 15% ≤ P ≤ 65%. This allows for the formation of a permeable porous structure within the separator, which promotes rapid and uniform electrolyte penetration and distribution through capillary action, thereby significantly improving the overall wetting efficiency of the separator. Simultaneously, the suitable pore structure provides lithium ions with low-torsion migration channels, effectively reducing ion transport resistance and maintaining a uniform current density distribution to reduce interfacial polarization. This optimized microstructure synergistically improves ion conductivity and interfacial charge transfer efficiency, thereby jointly reducing the ohmic impedance and polarization impedance of the battery, achieving a comprehensive improvement in the battery's K-value and internal resistance.

[0079] This study found that if the porosity P of the separator is less than 15%, the internal pore structure of the separator becomes smaller, which is not conducive to the wetting effect of the electrolyte on the separator and will also hinder the transport of lithium ions, resulting in an increase in the ohmic impedance and polarization impedance of the battery, an increase in the internal resistance of the battery, and a deterioration in cycle performance. If the porosity P of the separator is greater than 65%, it will lead to a decrease in the mechanical strength of the separator, making the separator easier to be punctured by lithium dendrites or electrode debris, causing an internal short circuit in the battery, affecting the improvement of the battery's K-value stability. Furthermore, excessive porosity will also lead to a decrease in the electrolyte retention capacity of the separator and uneven distribution of electrolyte, thereby leading to a decrease in the cycle stability of the battery.

[0080] It should be noted that the porosity P of the membrane can be tested according to the method in GB / T 36363-2018. For example, the porosity P of the membrane can be 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, etc., or a value within the range of any two of the above values.

[0081] Furthermore, in some implementations, the following condition is met: 20% ≤ P ≤ 40%.

[0082] In some embodiments, the oil absorption value Q of the conductive carbon black satisfies: 150mL / 100g ≤ Q ≤ 400mL / 100g. This gives the conductive carbon black a certain branched structure, enabling it to form certain pores in the positive electrode active layer and adsorb a large amount of electrolyte. While ensuring good contact between the conductive agent and the positive electrode active material, it improves the cycle stability of the battery, effectively reducing the formation of failure areas caused by poor contact between the conductive agent and the positive electrode active material. This suppresses problems of uneven local processes or reactions on the electrode, alleviates battery capacity decay, and simultaneously improves the liquid retention capacity of the positive electrode active material, thus improving the battery's cycle performance and thermal safety. When the oil absorption value of the conductive carbon black is too low, it indicates that the conductive carbon black particles lack the ability to connect the active particles and binder through abundant branches, resulting in a sparse conductive network, reduced electrical connections between materials, insufficient liquid absorption, increased battery impedance, and poorer battery cycle performance. When the oil absorption value of the conductive carbon black is too high, it will adsorb a large amount of binder and solvent, making dispersion difficult, increasing local resistance, and causing excessive liquid retention, leading to an increase in side reactions.

[0083] It should be noted that the oil absorption value Q of the conductive carbon black is based on the detailed test procedures of ASTM D2414 standard (American Society for Testing and Materials standard): Under constant stirring conditions, DBP (dibutyl phthalate) oil is added dropwise to a quantitative amount of conductive carbon black sample at a constant rate; the structure of the conductive carbon black itself will gradually absorb DBP and fill the gaps between the aggregates; with the addition of DBP, the mixture gradually changes from a free-flowing powder state to a semi-dry agglomerate, eventually reaching a point where the torque increases sharply; at this point, all gaps have been filled, the conductive carbon black aggregates are lubricated by DBP, and the system viscosity is at its maximum; the volume of DBP consumed at this point can be recorded to calculate the oil absorption value. For example, the oil absorption value Q of the conductive carbon black can be, for example, 150mL / 100g, 175mL / 100g, 200mL / 100g, 225mL / 100g, 250mL / 100g, 275mL / 100g, 300mL / 100g, 325mL / 100g, 350mL / 100g, 375mL / 100g, 400mL / 100g, or a value within the range of any two of the above values.

[0084] Furthermore, in some implementations, the following condition is met: 200mL / 100g≤Q≤300mL / 100g.

[0085] In some embodiments, the specific surface area S of the conductive carbon black satisfies: 100 m² 2 / g≤S≤1400m 2 / g. This allows the conductive carbon black to provide superior density and continuity in three-dimensional space, directly reducing the electrode resistance and ohmic impedance, thereby improving electron transport efficiency under high current. During cycling, the conductive agent with a moderate specific surface area can provide a stable conductive network, ensuring continuous and good electrical contact between active material particles. Even after high rates or long-term charge and discharge, it can effectively reduce the generation of "dead zones" caused by contact failure, suppress local overcharging or uneven reaction, and thus slow down capacity decay, thereby simultaneously improving the rate performance and cycle life of the battery. This study found that when the specific surface area S of conductive carbon black is too small, the particle size is large and the aggregates are simple, making it impossible to form sufficient "point-line-network" contact with the active material. This results in fewer electron transport channels, increased electrode resistance, increased heat generation, decreased battery cycle performance, and increased battery temperature rise at high rates. Conversely, when the specific surface area S of conductive carbon black is too large, its surface energy is high and van der Waals forces are strong, making it easy to form "hard agglomerates" in the positive electrode slurry. Even with high shear dispersion, these agglomerates are difficult to break up, resulting in an uneven conductive network in the positive electrode active layer. This leads to an increase in electrode resistance, which in turn worsens battery cycle performance and increases heat generation.

[0086] It should be noted that the specific surface area S of the conductive carbon black can be tested using the BET nitrogen adsorption method, specifically as follows: At liquid nitrogen temperature (-196℃), the conductive carbon black sample is exposed to nitrogen gas at different partial pressures; nitrogen molecules are physically adsorbed onto the surface of the conductive carbon black (including the outer surface and the inner pore surface); the amount of nitrogen adsorbed by the sample under different relative pressures is measured, and an adsorption-desorption isotherm is plotted. Within a suitable relative pressure range (typically P / P0 = 0.05-0.35), the adsorption data conforms to the BET equation; through linear regression, the volume of nitrogen gas required to form a monolayer can be calculated; and the specific surface area of ​​a single nitrogen molecule (0.162 nm) is determined. 2 The total specific surface area of ​​the sample is calculated. For example, the specific surface area S of the conductive carbon black can be, for example, 100 m². 2 / g、200m 2 / g、400m 2 / g、600m 2 / g、800m 2 / g, 1000m 2 / g、1200m 2 / g, 1400m 2 / g or values ​​within the range of any two of the above values.

[0087] Furthermore, in some implementations, the following condition is met: 200m 2 / g≤S≤800m 2 / g.

[0088] In one optional embodiment, the conductive carbon black includes at least one of chain-like conductive carbon black and grape-like spherical conductive carbon black.

[0089] In some embodiments, the separator further includes an adhesive layer comprising polymer particles, disposed on the surface of the base film layer and opposite to the functional coating, and / or disposed on the surface of the functional coating on the side away from the base film layer; the median particle size of the polymer particles is denoted as D3, satisfying: 0.2μm≤D3≤3μm. Thus, introducing an adhesive layer into the separator in this application can improve the adhesion between the separator and the electrode, improve the interfacial contact between the separator and the electrode, reduce the interfacial contact resistance, and ensure a uniform distribution of lithium-ion flow at the interface, reducing polarization caused by excessively high local current density. When the adhesive layer is disposed on the surface of the functional coating, it can also protect the functional coating and improve its durability, thereby improving the battery's cycle stability while addressing battery defects. Meanwhile, this application further controls the median particle size D3 of the polymer particles, which can further improve the contact effect between the separator and the electrode sheet, reduce the interfacial contact resistance, and at the same time, the median particle size D3 of the polymer particles within the above range can match the particle size of the active material particles on the surface of the electrode sheet, increase the adhesion stability between the separator and the electrode sheet, thereby reducing the shedding of active material from the surface of the electrode sheet, improving the powder shedding phenomenon of active material particles, alleviating the occurrence of local micro-short circuits and side reactions, and thus being more conducive to alleviating the problem of poor battery K value.

[0090] It should be noted that the median particle size D3 of the polymer particles can be obtained by the following method: First, the diaphragm (adhesive layer facing upwards) is fixed on a flat surface. Using a clean, sharp scraper or scalpel blade, the adhesive layer is gently and evenly scraped off at a small angle to the surface. The scraped sample is then subjected to plasma etching (e.g., oxygen plasma) to selectively etch away the organic binder and part of the polymer particle surface layer, making the particles protrude. The treated sample is then directly observed using a high-resolution field scanning electron microscope (SEM). The exposed particles and their distribution and aggregation within the coating can be clearly seen on the SEM image. Finally, the equivalent circle diameter of hundreds of particles is measured manually or semi-automatically using image analysis software (such as ImageJ) to obtain the statistical distribution of particle size in the coating. The particle size data accumulating to 50% from the smallest particle size side in the volumetric particle size distribution curve are then taken. For example, the median particle size D3 of the polymer particles may be, for example, 0.2μm, 0.4μm, 0.6μm, 0.8μm, 1.0μm, 1.2μm, 1.4μm, 1.6μm, 1.8μm, 2.0μm, 2.2μm, 2.4μm, 2.6μm, 2.8μm, 3.0μm, or a value within the range of any two of the above values.

[0091] Furthermore, in some implementations, the following condition is satisfied: 0.5μm≤D3≤1.5μm.

[0092] In some embodiments, the polymer particles include at least one of ester-containing polymer particles and fluoropolymer particles.

[0093] In some embodiments, the ester-containing polymer particles include at least one of polymethyl methacrylate, polyethylhexyl acrylate, polybutyl acrylate, acrylate-acrylonitrile copolymer, acrylate-ethylene copolymer, acrylate-acrylonitrile-ethylene copolymer, styrene-acrylate-acrylonitrile copolymer, ethylhexyl acrylate-methyl methacrylate copolymer, butyl acrylate-methyl methacrylate copolymer, methyl acrylate-N,N-dimethylacrylamide copolymer, ethyl acrylate-2-(diethylamino)ethyl acrylate copolymer, and ethyl acrylate-N,N-diethylacrylamide copolymer.

[0094] In some embodiments, the fluoropolymer particles include at least one of polyvinylidene fluoride, polytetrafluoroethylene, polyhexafluoropropylene, vinylidene fluoride-hexafluoropropylene copolymer, and tetrafluoroethylene-hexafluoropropylene copolymer.

[0095] In some embodiments, the median particle size of the positive electrode active material is denoted as D4, satisfying: 2.0 μm ≤ D4 ≤ 15 μm. This improves the particle size matching between the positive electrode active material and the separator surface particles, enabling effective connections between them. Simultaneously, it provides a certain surface roughness, increasing interfacial friction and suppressing particle shedding due to shear stress during winding. This mitigates the phenomenon of solid particles migrating with the electrolyte and embedding into the separator, forming localized micro-short-circuit channels or persistent side reactions caused by positive electrode active particle shedding, thus avoiding additional leakage current and helping to address the problem of increased K-value in the battery. Furthermore, by controlling the median particle size of the positive electrode active material within the aforementioned range, this application also increases the contact area between the positive electrode active material and the separator, improving their contact effect, reducing contact resistance, enhancing the uniform distribution of lithium ions at the interface, reducing battery polarization, and improving the battery's cycle stability.

[0096] It should be noted that the average particle size D4 of the positive electrode active material can be obtained by laser particle size analyzer. For example, the average particle size D4 of the positive electrode active material can be 2μm, 4μm, 6μm, 8μm, 10μm, 12μm, 14μm, 15μm, etc., or a value within any two of the above values.

[0097] Furthermore, in some implementations, the following condition is satisfied: 3.0μm≤D4≤8μm.

[0098] In some embodiments, the positive electrode active material includes at least one of lithium cobalt oxide, lithium nickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxide.

[0099] In some embodiments, the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer disposed on at least one surface of the negative electrode current collector in the thickness direction. The negative electrode active layer includes a negative electrode active material, which is a silicon-based material. Based on the total mass of the negative electrode active layer, the silicon content is 2%-40% by mass. The mass content of silicon in the negative electrode active layer can be adjusted by adjusting the proportion of silicon-carbon composite material in the negative electrode slurry and / or the silicon content in the silicon-based material.

[0100] In some embodiments, the electrolyte retention σ per unit thickness of the positive electrode active layer satisfies: 0.37 g / mm ≤ σ ≤ 1.7 g / mm. This allows for the formation of a continuous and stable electrolyte distribution network within the electrode, ensuring sufficient wetting of the active material interface to reduce charge transfer resistance, while maintaining a suitable liquid lithium-ion concentration and migration channels to reduce diffusion polarization. Simultaneously, satisfying the aforementioned range for the electrolyte retention σ per unit thickness also prevents the formation of high-resistance points in localized "dry areas" due to insufficient electrolyte retention, or avoids tortuous ion migration paths caused by excessive electrolyte retention. This balances interfacial reaction kinetics and ion transport efficiency, ultimately synergistically reducing the ohmic impedance and electrochemical polarization impedance of the battery.

[0101] It should be noted that the liquid retention capacity σ of the positive electrode active layer per unit thickness can be obtained through the following testing process: After discharging the battery to 0 SOC%, disassemble the battery and cut a positive electrode sheet per unit area (e.g., cut a positive electrode sheet of 2cm×2cm size). Soak the cut electrode sheet in dimethyl carbonate for 24 hours, and then place the above positive electrode sheet in an oven for complete drying before taking it out. Measure its thickness with a micrometer in the drying room, lay it flat on the test bench, weigh the electrode sheet, and record the data. Then, drop electrolyte onto the positive electrode sheet, and after the electrolyte is fully absorbed (there is no electrolyte flowing on the electrode sheet surface), weigh the electrode sheet and record the data. Calculate this value using the formula: Liquid retention capacity of the active layer per unit thickness = (Weight of the positive electrode sheet per unit area after absorbing electrolyte - Weight of the positive electrode sheet per unit area after drying) / Thickness of the positive electrode active layer in the positive electrode sheet per unit area. For example, the liquid retention capacity σ of the positive electrode active layer per unit thickness can be, for example, 0.37 g / mm, 0.4 g / mm, 0.5 g / mm, 0.6 g / mm, 0.7 g / mm, 0.8 g / mm, 0.9 g / mm, 1.0 g / mm, 1.1 g / mm, 1.2 g / mm, 1.3 g / mm, 1.4 g / mm, 1.5 g / mm, 1.6 g / mm, 1.7 g / mm, or a value within the range of any two of the above values.

[0102] In some embodiments, the mass ratio of the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder in the positive electrode active layer is (93~98):(0.1~4):(0.1~3).

[0103] For example, in the positive electrode active layer, the mass fraction of the positive electrode active material can be, for example, 93, 94, 95, 96, 97, 98, or a value within the range of any two of the above values; the mass fraction of the positive electrode conductive agent can be, for example, 0.1, 0.5, 1, 2, 3, 4, or a value within the range of any two of the above values; the mass fraction of the positive electrode binder can be, for example, 0.1, 0.3, 0.5, 1, 1.5, 2, 2.5, 3, or a value within the range of any two of the above values.

[0104] In some embodiments, the functional coating further includes a first adhesive, which includes one or more of polyvinyl alcohol, styrene-butadiene rubber, ethylene-vinyl acetate copolymer, polyvinylpyrrolidone, styrene-acrylic latex, polyacrylonitrile, ethyl polyacrylate, polyvinyl acetate, polyacrylic acid, polyurethane, fluoropolymers, and acrylate adhesives.

[0105] Further, in some embodiments, the acrylate adhesive includes at least one of polymethyl methacrylate, polybutyl acrylate, acrylate monomer-acrylonitrile copolymer, acrylate monomer-ethylene copolymer, acrylate monomer-acrylonitrile-ethylene copolymer, styrene-acrylate monomer-acrylonitrile copolymer, ethylhexyl acrylate-methyl methacrylate copolymer, butyl acrylate-methyl methacrylate copolymer, methyl acrylate-N,N-dimethylacrylamide copolymer, ethyl acrylate-2-(diethylamino)ethyl acrylate copolymer, ethyl acrylate-N,N-diethylacrylamide copolymer, and ethyl acrylate-2-(diethylamino)ethyl acrylate.

[0106] Furthermore, in some embodiments, the acrylate monomers include at least one of methyl acrylate, methyl methacrylate, ethyl acrylate, butyl acrylate, and ethyl methacrylate.

[0107] In this application, the fluoropolymer includes polymers formed by copolymerization of at least one monomer selected from vinylidene fluoride, tetrafluoroethylene, hexafluoroethylene, and hexafluoropropylene.

[0108] In some embodiments, the base film layer is made of at least one of polyolefin, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyvinyl chloride, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyethylene terephthalate, polybutylene terephthalate, poly(p-phenylene terephthalamide), poly(m-phenylene isophthalamide), polybenzimidazole, polysulfone, polyketone, or derivatives of the above polymers.

[0109] In some embodiments, the thickness of the base film layer is 3 μm to 9 μm.

[0110] For example, the thickness of the base film layer may be 3.0 μm, 3.5 μm, 4.0 μm, 4.5 μm, 5.0 μm, 5.5 μm, 6.0 μm, 6.5 μm, 7.0 μm, 7.5 μm, 8.0 μm, 8.5 μm, 9.0 μm, or a value within the range of any two of the above values.

[0111] In some embodiments, the polyolefin may include at least one of polyethylene and polypropylene.

[0112] In some embodiments, the thickness of the functional coating on one side is 0.3 μm to 10 μm.

[0113] The present application will be further described in detail below with reference to specific embodiments. These embodiments should not be construed as limiting the scope of protection claimed in this application. Where specific experimental steps or conditions are not specified in the embodiments and comparative examples, they can be performed according to the conventional experimental steps or conditions described in the literature in the art. Reagents or instruments used, unless otherwise specified, are all commercially available conventional reagent products.

[0114] Example 1 (1) Preparation of positive electrode The positive electrode active material, lithium nickel cobalt manganese oxide (chemical formula LiNi), is dispersed using N-methylpyrrolidone. 0.8 Co 0.1 Mn 0.1 O2 (median particle size D4 of 4.9 μm), conductive agent (conductive carbon black), and binder (polyvinylidene fluoride) form a positive electrode slurry. The positive electrode slurry is uniformly coated onto both sides of the positive electrode current collector using a coating method. After baking and rolling processes, a positive electrode sheet including a positive electrode active layer is obtained. The mass ratio of the positive electrode active material, conductive agent, and binder is 97:1:2. The positive electrode current collector is aluminum foil, and the conductive agent uses conductive carbon black containing carbon and metal elements, including Fe. Based on the total mass of the positive electrode conductive agent, the total content of metal elements A is 157 ppm, Fe is 52 ppm, Cr is 43 ppm, and Cu is 62 ppm. The median particle size D1 of the conductive carbon black is 5 μm. Based on the total mass of the positive electrode active layer, the mass content θ of the conductive carbon black is 1%, the oil absorption value Q of the conductive carbon black is 300 mL / 100 g, and the specific surface area S of the conductive carbon black is 600 m². 2 / g, the conductive carbon black is grape-shaped conductive carbon black, and the thickness of the positive electrode active layer on each side is 0.04mm; the liquid retention capacity σ of the positive electrode active layer per unit thickness is 1.43g / mm.

[0115] The structural schematic diagram of the obtained positive electrode is as follows: Figure 1 As shown in the figure, the positive electrode consists of a positive current collector 11 and a positive active layer 12 disposed on both sides of the positive current collector 11 along the thickness direction.

[0116] (2) Preparation of negative electrode A negative electrode slurry is formed by dispersing the negative electrode active material, conductive agent (conductive carbon black), and binder (sodium carboxymethyl cellulose) with deionized water. Then, a negative electrode active layer is formed on both sides of the negative electrode current collector containing the first coating through coating, baking, and rolling processes to obtain the negative electrode sheet. The mass ratio of artificial graphite, silicon-carbon composite material, lithium carboxymethyl cellulose, polyacrylic acid, and conductive carbon black is 65:31:1.5:1.0:1.5. Based on the total mass of the negative electrode active layer, the mass content of silicon element is 15.5 wt%, and the negative electrode current collector is copper foil.

[0117] The structural schematic diagram of the obtained negative electrode is as follows: Figure 2 As shown in the figure, the negative electrode sheet consists of a negative electrode current collector 21 and a negative electrode active layer 22 disposed on both sides of the negative electrode current collector 21 along the thickness direction.

[0118] (3) Electrolyte preparation Ethyl carbonate (EC), ethyl propionate (EP), and propyl propionate (PP) solvents were mixed uniformly in a mass ratio of 20:10:40. Lithium hexafluorophosphate (LiPF6) was slowly added to the mixed solution at a mass of 14 wt% of the total electrolyte. Additives were then added, and the mixture was stirred until homogeneous. After passing tests for moisture and free acid, the electrolyte was obtained. The additives included nitrile substances and ethylene sulfate. Based on the total mass of the electrolyte, the mass content of nitrile substances (N) was 4%, the mass content of ethylene sulfate was 8%, and the nitrile substance was adiponitrile.

[0119] (4) Preparation of the diaphragm ① Preparation of functional coating: A nitrogen-containing compound is mixed with polyvinylidene fluoride and deionized water. The nitrogen-containing compound is melamine cyanurate, and the median particle size D2 of the nitrogen-containing compound is 1 μm. After thorough stirring, a first mixed slurry with a solid content of 30% is obtained. Based on the solid mass of the first mixed slurry as 100 wt%, the mass ratio of melamine cyanurate to polyvinylidene fluoride is 30:5. The first mixed slurry is coated on the surface of the base film layer on one side of the thickness direction by a gravure roller. After passing through a multi-section oven at 60°C, it is dried to form a functional coating on the surface of the base film layer. The thickness of the functional coating is 3 μm, and the thickness of the base film layer is 9 μm. The material of the base film layer is polyethylene.

[0120] ② Preparation of adhesive layer: Polymethyl methacrylate (with a mean particle size D3 of 0.7 μm) is mixed with water and stirred thoroughly to form a second mixed slurry with a solid content of 8%. The second mixed slurry is coated onto the surface of the functional coating away from the base film layer and onto the other surface of the base film layer, which is opposite to the functional coating layer, using a gravure roller. After drying in a multi-section oven at 60°C, an adhesive layer is formed. The total thickness of the two adhesive layers is 1.5 μm.

[0121] The puncture strength M of the formed diaphragm is 500gf, and the porosity of the diaphragm is 30%.

[0122] (5) Preparation of lithium-ion batteries The positive electrode, separator, and negative electrode are stacked sequentially after fabrication, with the functional coating in the separator facing the positive electrode. Then, they are wound to obtain a bare cell. The bare cell is placed in an aluminum-plastic film shell, and electrolyte is injected into the bare cell. After vacuum sealing, settling, formation, shaping, and capacity testing, a lithium-ion battery is obtained.

[0123] The preparation methods and parameter settings of the remaining embodiments and comparative examples are basically the same as those of Example 1. The differences are shown in Tables 1 and 2. D1 represents the median particle size of conductive carbon black; D2 represents the median particle size of nitrogen-containing compounds; D2 / D1 represents the ratio of the median particle size of nitrogen-containing compounds to the median particle size of conductive carbon black; θ represents the mass content of conductive carbon black based on the total mass of the positive electrode active layer; N represents the mass content of nitrile substances based on the total mass of the electrolyte; θ / N represents the ratio of the mass content of conductive carbon black in the positive electrode active layer to the mass content of nitrile substances in the electrolyte; M represents the puncture strength of the separator; P represents the porosity of the separator; Q represents the oil absorption value of conductive carbon black; S represents the specific surface area of ​​conductive carbon black; D3 represents the median particle size of polymer particles in the adhesive layer; D4 represents the median particle size of the positive electrode active material; σ represents the liquid retention capacity per unit thickness of the positive electrode active layer.

[0124] Table 1

[0125] Table 2

[0126] In Tables 1 and 2, compared with Example 1, when the mass content of nitriles in Examples 21-28 changed, the mass content of solvent in the electrolyte also changed accordingly, while the mass contents of lithium hexafluorophosphate (LiPF6) and vinyl sulfate remained unchanged.

[0127] Test example: The secondary batteries provided in the above embodiments and comparative examples were tested as follows: (1) K-value defect rate test: Charge the battery to the specified state of charge (usually 50% SOC) under standard conditions (e.g., 25℃). Then let it stand for 1 hour to allow the battery voltage to stabilize fully and eliminate the effect of transient polarization. Apply a short-duration high-current pulse I (1C, lasting 10 seconds). Record the instantaneous voltage V2 before the current pulse is turned off and the rebound voltage V1 immediately after the pulse ends. Calculate the K-value using the formula K=(V1-V2) / I, where the acceptable range of the K-value is 0.001~0.01. Batteries outside the calculated K-value range are considered K-value defective batteries. Test the K-value of a batch of 1000 batteries. Calculate the K-value defect rate of 1000 batteries using the formula: K-value defect rate = number of K-value defective batteries / total number of batteries.

[0128] (2) DCIR at 25℃: First, let the battery stand at 25℃ for 60 minutes; second, discharge it at 0.5C to 2.5V and let it stand for 15 minutes; third, charge it at 0.5C to 4.3V, keep the voltage constant at 0.05C, and let it stand for 120 minutes; fourth, discharge it at 1.5C rate to 50% SOC, and record the voltage of the battery at this time as V1. Then discharge it at 3C for 15s (sampling every 200ms), and record the voltage at the end of the discharge as V2; use the discharge DCIR of the battery at 50% SOC to represent the DCIR of the battery, and the DCIR calculation formula is = (V1-V2) / 3C.

[0129] (3) High-rate discharge temperature rise test: At 45℃, firstly, after the battery is left to stand for 2 hours, it is discharged to 2.5V at a rate of 0.5C; secondly, after the battery is left to stand for 60 minutes, it is charged to 4.3V at 1.8C and kept constant at 0.05C; then, after standing for 30 minutes, the battery is discharged to 2.5V at a high rate of 3C (and 4C). The entire process is repeated. When each rate of discharge is completed, the surface temperature of the cell is monitored and recorded as T1℃. The discharge temperature rise (℃) at 45℃ = T1℃ - 45℃.

[0130] (4) Cyclic test at 45℃: First, at 45℃, test and record the battery voltage, internal resistance, thickness, and discharge capacity C0 under full charge (100% SOC); Second, at 45℃, discharge to 2.5V at 0.5C, let stand for 30 minutes, charge to 4.3V at 1.8C, keep constant voltage to 0.05C, let stand for 30 minutes, discharge to 2.5V at 3C, let stand for 30 minutes, charge to the upper limit voltage at 1.8C, keep constant voltage to 0.05C; Third, let stand for 30 minutes, discharge the battery to 2.5V at 3C. Step 4: After resting for 60 minutes, charge the battery at 1.8C to 4.3V, maintain the voltage at 0.05C, and after exceeding 57℃, switch to 1C charging. Step 5: After resting for 60 minutes, discharge the battery at 3C to 2.5V, and after exceeding 82℃, switch to 2C discharging. Step 6: After resting for 60 minutes, charge the battery at 1.8C to 4.3V, maintain the voltage at 0.05C, and after exceeding 57℃, switch to 1C charging. Repeat steps 3 to 6. After each 500 cycles, test the battery's discharge capacity C1 and calculate the capacity retention rate = C1 / C0 × 100%.

[0131] The test results are shown in Table 3: Table 3

[0132] As can be seen from Tables 1 to 3, this application introduces conductive carbon black containing trace amounts of metal elements into the positive electrode as a conductive agent. This agent works in conjunction with nitrogen-containing compounds in the separator. By controlling the median particle size D2 of the nitrogen-containing compounds and the median particle size D1 of the conductive carbon black, as well as their ratio D2 / D1, the conductivity of the positive electrode can be improved. This reduces the internal resistance of the battery, decreases the heat generated during battery charging and discharging, alleviates the problem of poor battery K-value caused by the precipitation of metal elements in the conductive carbon black, and effectively suppresses the occurrence of side reactions in the battery. As a result, the secondary battery has high energy density while improving its cycle performance and reducing the battery temperature rise at high rates.

[0133] A comparison of Example 1 with Comparative Examples 1-6 shows that when the median particle size of the conductive carbon black and the nitrogen-containing compound does not meet a specific range, it increases the likelihood of metal elements in the conductive carbon black being oxidized into ions and dissolved into the electrolyte during battery cycling. These ions diffuse to the negative electrode side of the battery, precipitate on the surface of the negative electrode material, damage the SEI film of the negative electrode, lead to local side reactions, increase the probability of membrane penetration, increase the problem of poor K-value in the secondary battery, deteriorate the battery's cycle performance, and increase the temperature rise of the battery at high rates. Conversely, when the ratio of the median particle size of the conductive carbon black to the nitrogen-containing compound is too large or too small, it affects the compatibility between the conductive carbon black and the nitrogen-containing compound, fails to effectively improve the conductivity of the positive electrode, increases the battery's internal resistance, results in poor heat dissipation during charging and discharging, fails to effectively alleviate the precipitation problem of metal elements in the conductive carbon black, and causes large fluctuations in the battery's K-value. This is detrimental to improving the cycle performance and safety of the secondary battery while simultaneously increasing its energy density.

[0134] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A secondary battery, comprising a positive electrode sheet, a separator, and a negative electrode sheet stacked together, wherein the positive electrode sheet includes a positive current collector and a positive active layer disposed on at least one surface of the positive current collector in the thickness direction, the positive active layer comprising a positive active material, a positive binder, and a positive conductive agent, characterized in that, The positive electrode conductive agent includes conductive carbon black, which includes carbon and metal elements, and the metal elements include at least one of Fe, Cr and Cu. Based on the total mass of the positive electrode conductive agent, the total content of the metal elements is denoted as A, which satisfies: A < 500 ppm. The diaphragm includes a base film layer and a functional coating disposed on at least one side surface of the base film layer in the thickness direction, the functional coating including a nitrogen-containing compound; The median particle size of the conductive carbon black is denoted as D1, and the median particle size of the nitrogen-containing compound is denoted as D2. Both satisfy the following conditions: 0.015≤D2 / D1≤30, D1 satisfies: 0.1μm≤D1≤10μm, and D2 satisfies: 0.1μm≤D2≤4.0μm.

2. The secondary battery according to claim 1, characterized in that, Based on the total mass of the positive electrode conductive agent, the Fe content is less than 150 ppm, the Cr content is less than 150 ppm, and the Cu content is less than 200 ppm; Based on the total mass of the positive electrode conductive agent, the total content A of the metal element satisfies: A < 200 ppm; The nitrogen-containing compound includes at least one of melamine cyanurate, melamine polyphosphate, melamine thiocyanate, melamine, 2,4,6-tris(aminohexanoic acid)-1,3,5-triazine, 2-(4-bromophenyl)-4,6-dimethyl-1,3,5-triazine, 1-(4,6-diamino-1,3,5-triazin-2-yl)guanidine, 2,4-diamino-6-dimethylamino-1,3,5-triazine, melamine chloride, 2,4,6-tris(2-pyridyl)triazine, 2,4,6-triphenyl-1,3,5-triazine, tris(tribromophenoxy)triazine, and 2-amino-4,6-dimethoxy-1,3,5-triazine.

3. The secondary battery according to claim 1, characterized in that, 0.5μm≤D1≤8μm; And / or, 0.2μm≤D2≤1.5μm.

4. The secondary battery according to claim 1, characterized in that, Based on the total mass of the positive electrode active layer, the mass content θ of the conductive carbon black satisfies: 0.1% ≤ θ ≤ 4%; And / or, the thickness H of the positive electrode active layer on any side surface along the thickness direction of the positive electrode current collector is 0.03mm~0.16mm, preferably 0.04mm~0.15mm.

5. The secondary battery according to claim 4, characterized in that, The secondary battery further includes an electrolyte, which includes additives, including nitrile substances. Based on the total mass of the electrolyte, the mass content N of the nitrile substances satisfies: 0.5%≤N≤8%, and the mass content θ of the conductive carbon black and the mass content N of the nitrile substances satisfy: 0.0125≤θ / N≤8%. And / or, the nitrile substances include at least one selected from succinic anhydride, adiponitrile, 1,2-bis(cyanoethoxy)ethane, glutaronitrile, ethylene glycol bis(propionitrile) ether, 1,3,6-hexanetrionitrile, 1,3,5-pentanetricarboxylon, 1,2,3-propanetricarboxylon, 1,2,6-hexanetrionitrile, 1,2,3-tris(2-cyanoethoxy)propane, 1,2,4-tris(2-cyanoethoxy)butane, 1,1,1-tris(cyanoethoxymethylene)ethane, 1,1,1-tris(cyanoethoxymethylene)propane, 3-methyl-1,3,5-tris(cyanoethoxy)pentane, 1,2,7-tris(cyanoethoxy)heptane, 1,2,6-tris(cyanoethoxy)hexane, glyceryl trionitrile, and 1,2,5-tris(cyanoethoxy)pentane.

6. The secondary battery according to claim 1, characterized in that, The puncture strength M of the diaphragm satisfies: 100gf≤M≤800gf, preferably 200gf≤M≤600gf; And / or, the porosity P of the diaphragm satisfies: 15%≤P≤65%, preferably, 20%≤P≤40%.

7. The secondary battery according to claim 1, characterized in that, The oil absorption value Q of the conductive carbon black satisfies: 150mL / 100g≤Q≤400mL / 100g, preferably, 200mL / 100g≤Q≤300mL / 100g; And / or, the specific surface area S of the conductive carbon black satisfies: 100 m² 2 / g≤S≤1400m 2 / g, preferably, 200m 2 / g≤S≤800m 2 / g; And / or, the conductive carbon black includes at least one of chain-like conductive carbon black and grape-like spherical conductive carbon black.

8. The secondary battery according to any one of claims 1-7, characterized in that, The diaphragm further includes an adhesive layer comprising polymer particles. The adhesive layer is disposed on the surface of the base film layer and opposite to the functional coating, and / or the adhesive layer is disposed on the surface of the functional coating on the side away from the base film layer. The median particle size of the polymer particles is denoted as D3, and satisfies: 0.2μm≤D3≤3μm, preferably 0.5μm≤D3≤1.5μm. The polymer particles include at least one of ester-containing polymer particles and fluoropolymer particles; The ester-containing polymer particles include at least one of polymethyl methacrylate, polyethylhexyl acrylate, polybutyl acrylate, acrylate-acrylonitrile copolymer, acrylate-ethylene copolymer, acrylate-acrylonitrile-ethylene copolymer, styrene-acrylate-acrylonitrile copolymer, ethylhexyl acrylate-methyl methacrylate copolymer, butyl acrylate-methyl methacrylate copolymer, methyl acrylate-N,N-dimethylacrylamide copolymer, ethyl acrylate-2-(diethylamino)ethyl acrylate copolymer, and ethyl acrylate-N,N-diethylacrylamide copolymer. The fluoropolymer particles include at least one of polyvinylidene fluoride, polytetrafluoroethylene, polyhexafluoropropylene, vinylidene fluoride-hexafluoropropylene copolymer, and tetrafluoroethylene-hexafluoropropylene copolymer.

9. The secondary battery according to claim 8, characterized in that, The median particle size of the positive electrode active material is denoted as D4, and satisfies: 2.0μm≤D4≤15μm, preferably 3.0μm≤D4≤8μm; The positive electrode active material includes at least one of lithium cobalt oxide, lithium nickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxide; The negative electrode sheet includes a negative current collector and a negative active layer disposed on at least one side surface of the negative current collector in the thickness direction. The negative active layer includes a negative active material, and the negative active material includes a silicon-based material.

10. The secondary battery according to any one of claims 1-7, characterized in that, The liquid retention capacity σ of the positive electrode active layer per unit thickness satisfies: 0.37g / mm≤σ≤1.7g / mm.

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